Central to this review are the methods to compute the scattering of light by single particles, with emphasis on the properties relevant to seed particles in Phase Doppler Anemometry (PDA). The Lorenz-Mie solution for spheres is summarized and its simplifications in certain limiting cases explained. A number of modern developments is mentioned, as well as the possibility to generalize this solution to convergent or to gaussian beams. Three methods developed to compute scattering by particles of arbitrary shapes are discussed. They work well but are in practice limited to size parameters <15. The paper opens with a simple example taken from the PDA literature, suggesting simpler equations than usually given. It closes with a concise review of multiple scattering. Only a small fraction of the vast body of publications on multiple scattering may be directly relevant in the PDA context.

By determining velocity field inside fresh gases of premixed flame submitted at a sinusoidal perturbation of both high intensity and high frequency, it has been observed a measurement bias by processing the information coming from the analog output of LDV counters. After the evidence is put in this velocity bias by different measurements: average, RMS, FFT and histograms, the attention is focused on instantaneous velocity signal and particularly on the seeding behavior. An explanation of the bias is proposed and also some ideas to control it.

Laser Doppler anemometry is in wide-spread use in research laboratories of industry and university. Based on innumerable applications in the past, there is no doubt that laser Doppler anemometry is one of the most accurate flow measuring techniques. However, recent investigations have shown that the period lengths of LDA signal bursts are not as constant as one might expect within the individual burst. Since a finite number of cycles within each burst is evaluated for frequency determination, this induces an additional scatter in the measurement of flow velocity. The paper describes experimental investigations which show quantitatively that the particle passage through the laser beams in the vicinity of the point of superposition, i.e. the LDA measuring volume, yields a distorted LDA fringe pattern which results in a scatter of the measured velocity data for those particles, passing the center of the measuring volume at the same time. As a consequence, turbulence is determined slightly higher than in reality.

Accurate measurement of the velocity field and associated flow properties, using LDV, demands the use of an optimized signal processing system with the ability to extract signals from noisy photodetector output. Although, ostensibly, both auto-correlation and spectrum analysis techniques convey the same information, the unique nature of the Doppler signal provides a special situation where auto-correlation has certain advantages. The theoretical approach to examine the analysis techniques and the potential errors that can be caused by some simplifying assumptions are discussed. To overcome the limitations of the theoretical approach, results from both computer simulation and experimental measurements are provided. Special characteristics of the Doppler signal make the sampling process and the signal detection to be crucial to getting accurate measurements. It is shown that in the DFT approaches used partial fill of data block occurs resulting in a strong impact on the lowest measurable signal quality and the associated measurement accuracy. The ability of the auto- correlation approach to overcome these problems and the robustness of the processing scheme are discussed. In addition, the auto-correlation based processors (IFA 750 & 650) provide the ability to optimize the sampling rate for each burst and the system in real time to provide higher measurement rate, better accuracy and the ability to measure lower turbulence. These capabilities are borne out by results from actual measurements.

The acquisition and digital storage of analog turbulent signals is optimized with respect to minimal data handling. A best anti-aliasing filter and optimum settings of the A/D converter are found by simulating a turbulent signal digitally and calculation of the distortion under different circumstances.

With the interface of simple and cheap micro-computers, even low-cost LDA hardware can be made to produce very high quality and large sets of reliable data. With such interfaces, many of the operator demanding tasks of LDA instrument setting and data collection can be automated in a manner approaching an expert system. In this paper, the structure plans of three software routines are presented as elementary modules for such a system. They would suit a LDA signal processing system based on Frequency Tracking. With proper implementation of the ideas presented here, the computer can help establish a non-ambiguous velocity sign convention, optimize the amount of frequency shift, tune the Tracker and automatically collect data.

Sophisticated optical metrology is conquering more and more dimensions of complex flow fields. A survey of more-dimensional techniques from particle image velocimetry to stereo- photography and particle image holography is presented including some special emphasis on stereoscopic back-projection and multiple light sheet holography.

A novel frequency estimation method for the evaluation of LDA burst signals is presented. The new method is based on the measurement of time series of phase angles defined by a pair of quadrature signals (sine/cosine signal pair). Up to now quadrature techniques have preponderantly been used in the two beam interferometry for carrying out direction sensitive and high resolution measurements as well as in radio frequency applications. The quadrature demodulation technique can be considered as a straightforward development of the counter technique with the advantages of a higher resolution for a single burst evaluation and of directional discrimination. Compared with conventional signal processing techniques the application of the quadrature demodulation technique additionally allows a directional discrimination and the analysis of velocity fluctuations within single Doppler signal bursts with a higher accuracy by evaluating the measured time series of phase angle values.

For stratified flows it is important to know the mass transport from the denser salt water layer through the turbulent mixing layer into the upper fresh water layer. Therefore it is necessary that both the velocity and salinity fluctuations are measured in the turbulent mixing layer. To realize these measurements with minimum disturbance of the flow in the measurement volume, (optical) 'non-contact' methods are needed. When adding a very low concentration of the fluorescent dye uranine to the salt water as a tracer and measuring the local intensity of the fluorescent light local salinity fluctuations can be measured. Thus an optical instrument has been developed that is a combination of an earlier developed immersible Laser Doppler Anemometer (LDA) and a new developed immersible Laser Induced Fluorometer (LIF). The optical path length in the water is only a few cm. So it is possible to measure in water with variations in the refractive index as is the case in turbulent mixing layers. With this instrument measurements were carried out during different phases of a stratified tidal flow in the so called 'Tidal Flume', a special large installation at DELFT HYDRAULICS. Detailed information about the instrument is presented, together with results of the first measurements.

One of the main parameters governing diesel spray formation is the fuel's velocity just beneath the nozzle. The high density of the injected liquid within the first few millimeters under the injector prohibits accurate measurements of this velocity. The liquid's velocity in this region has been mainly measured using intrusive methods and has been numerically calculated without considering the complex flow fields in the nozzle. A new optical method based on laser induced delayed fluorescence allowing the measurement of the fuel's velocity close to the nozzle is reported. The results are accurate to about 14% and represent the velocities of heavy oils within the first 2 - 5 mm beneath the nozzle. The development of the velocity over the injection period showed a drastic deceleration of the fuel within the first 3 mm beneath the nozzle. This is assumed to be due to the complex interaction of cavitation in the injection hole and pressure waves in the injection system which causes the start of atomization in the nozzle hole.

An experimental study using Laser Doppler Anemometry (LDA) has been carried out to investigate the developing laminar flow inside a 180 degree(s) U-tube. A previous experimental loop was modified for the purpose of measuring both the axial and secondary velocities. The uncertainties of velocity, the Reynolds number, and the Dean number were found to be under 1.52%, 2.37% and 2.60%, respectively. .

Techniques have been developed for making 3D coincident LDA measurements within an annular S-shaped duct of complex geometry. In order to illustrate these methods aerodynamic data is presented relating to measurements of the axisymmetric wall boundary layers at inlet to the duct. Particular attention is given to the accuracy and limitations of the measurements produced using these techniques. A variety of information is presented including mean velocity components. Reynolds stresses and triple correlations of the fluctuating velocities. The results show that many of the terms in the Reynolds stress transport equations can be determined within a specified accuracy. Such measurements are becoming increasingly important for the validation of computational codes and the further development of turbulence models.

Detailed velocity data of the unsteady flow downstream of a sphere model were analyzed using a combination of a two channel Laser Doppler system and a hot wire anemometer. This set-up yields mean values at constant phase of the quasiperiodic vortex shedding process and thus delivers new insights into the instability mechanisms of a free shear layer and the forming of large scale vortices. The results show that the shearlayer instability shortly after flow separation induces an amplification of the longitudinal Reynolds normal stress component while the extremum in the transverse Reynolds normal stress is initiated by a rotating helical vortex structure which emanates from the rear end of the separation zone. The coherence length of the vortex structure is five sphere diameters D.

The recent years have seen an ever increasing amount of work in the field of brush seals. Fundamental and prototypal industrial research have attested to the improved fluidic and dynamic performance of such seals when compared to other classes of seals. This paper presents a continuation of the basic research performed by Braun et al. and further explores the nature of the flow and pressure patterns in configurations of dense array of cylinders simulating brush seal behavior. The investigation follows through the method of Full Field Flow Tracking (FFFT). The images supplied by the FFFT system are processed by means of a computer-integrated image quantification method (CIIQ) into quantitative information for flow trajectories and velocities. The tracking method permits identification and tracking of the same particle in a lagrangian frame of reference.

If Phase-Doppler-Anemometry (PDA) is applied to analyze a spray of an optically inhomogeneous liquid a broad particle size distribution is measured. PDA is unsuitable for many applications. To overcome this problem we propose a deconvolution method using the measured size distribution from a droplet generator. The method is described and measurements are conducted on a milk spray and a water spray. The deconvoluted milk distribution compares well with the water distribution.

Nozzles are utilized in atomizing liquids in many industrial and domestic applications. Examples include domestic and industrial heating units (furnaces and boilers), internal combustion engines, agricultural spraying, spray painting, etc. To improve atomizer design, it is necessary to obtain spray characteristics which include the simultaneous droplet size and velocity information. Also droplet interactions in the dense spray region affect the characteristics. Interacting spray configurations are also encountered in many industrial applications and lead to direct interactions among the droplets. The Particle Dynamic Analyzer which utilizes a combination of Laser Doppler Anemometry and Phase Doppler Interferometry to simultaneously measure droplet velocity and diameter was used to study (1) the spray characteristics of Delavan nozzles and (2) the interaction between two sprays. The results of the tests are discussed in this paper.

A 2-Dimensional Laser Doppler Velocimeter was used to investigate particle flow in the sampling chamber of a particle analyzer. Results were compared with those obtained from a Computational Fluid Dynamics model of the particle delivery nozzle.

Lasers are used in a wide variety of medical applications. While laser catheters have been developed for highly accurate velocity measurements these are invasive; noninvasive techniques are more desirable but not as precise. The laser is, however, a great tool for in vitro measurements. Several groups internationally are using the laser in the study of local velocity distribution in microscopic areas of specially constructed models. Laser Doppler anemometry is widely used to measure the local, time-dependent velocities, while phase Doppler anemometry has been developed to measure particle size, distribution and velocity. Most recently, laser analyzer techniques have been developed for analyzing the particle size of two phase flow systems. It has become increasingly important for physicians to visualize blood flow. In addition to the techniques mentioned above, several laser sheet techniques have been developed for precise measurements. This paper presents a short review of laser techniques and shows some applications especially for the laser-Doppler anemometer.

A velocimeter, consisting of a semiconductor laser, coupled to a glass fiber, to be inserted in the flow, and applying self-mixing as the detection technique, is described. A special application is the measurement of blood velocity in veins and arteries. Technical aspects, including flow profile calculations and measurements, and in-vivo and in-vitro velocity measurements are described and discussed.

The lateral migration of platelets and red blood cells in highly diluted suspensions was studied using a laser-Doppler velocimeter with microscopic resolution. The experiments were performed in a 100 micrometers width, rectangular flow channel of high aspect ratio. It was found that platelets tend to migrate towards an equilibrium position well away from the channel wall. This migration effect turns out to be identical to the so-called tubular pinch effect that is well- known for rigid spheres. Red blood cells, on the other hand, eventually accumulate in the center of the flow similar to deformable liquid droplets.

In order to measure the local velocity field in opaque fluid flows like blood flow, a new laser Doppler velocimeter having a pickup consisting of a small distributed index lens attached to the tips of two fibers which are joined side by side in parallel has been newly developed. The distributed index lens is the shape of a truncated cone. The flow field around this sensor has been measured very precisely by means of an ordinary LDV. The effect of turbidity of fluid on the quality of the laser Doppler signal from this sensor has been examined by experiments. As a result, it has been shown that this LDV sensor has a high signal-to-noise ratio, and that the disturbance against flow by the sensor is very small, and it is very promising as a velocity sensor for opaque or semi-opaque fluid flow like blood flow.

The Spiral Vortex (SV) ventricular assist device (VAD), a pulsatile blood pump that was designed to minimize both blood stasis and trauma to red cells, has been evaluated using flow visualization and laser Doppler anemometry. The flow visualization provided qualitative and some quantitative data on velocity distribution, shear stress, turbulence, and flow recirculation, during key phases in the pump cycle. Compared to a VAD of conventional design, the SV exhibit less turbulence, less recirculation, and better washout of critical surfaces. The laser anemometer has provided much more detailed data on the velocity distribution in the SV, and also an indication of the extent of cycle-to-cycle variations in the flow.

PIV has been applied to characterize the flow around static solid bodies in an attempt to link flow structure with blood cot formation: a simplification of the flows found around mechanical heart valve prostheses. The technique has been used to study both steady and pulsatile flows around these bodies and can be extended to investigate the flow patterns around valves where computer simulation is difficult. The study has shown that shear stress is readily calculable over the investigation plane and for the test bodies is below the level postulated to damage blood cells. The velocity fields determined support a link between areas of stasis and clot formation. A preliminary investigation of the residence time distribution of cells in the recirculation zone for one of the test bodies allows a reasonable estimate for the rate of deposition of the clot to be made.

In order to gain quantitative information of the velocity fields in non-stenosed and stenosed models of the carotid artery bifurcation. Laser Doppler Anemometer (LDA) experiments have been performed. For this purpose a two component backscatter LDA system has been used. The experiments have been conducted in a 1:2.5 enlarged plexiglass model of the carotid artery bifurcation. Both axial and secondary velocities were measured as a function of time at locations of interest. The data were ensemble averaged and analyzed in the frequency domain in order to find characteristic flow phenomena. For the frequency analyses, the transfer functions between velocities at specific sites in the bifurcation and the input flow signal have been calculated for both the non-stenosed and the stenosed bifurcation. Both from the results of the velocity fields and the transfer functions, it can be concluded that the main differences between the flow fields in the non-stenosed and the stenosed bifurcation can be found in an area with high velocity and in a shear layer, which is located at the border between a region with low shear rates at the non-divider wall and a region with high shear rates at the divider wall. The values of the transfer function at these locations seem to be useful for the characterization of the influence of the stenosis.

A model symmetric junction is used to simulate steady expiratory flow in the upper part of human central airways. Velocity profiles were measured with a two color, two component laser Doppler anemometer at three Reynolds number levels of 518, 1036 and 2089. The test section is a symmetric junction of constant cross-sectional area with a branching angle of 70 degrees. The inlet flow rate into the two bronchi was the same for all Reynolds numbers studied. Results show that in the junction plane, velocity profiles in the bronchi are skewed towards the inner walls. In the trachea, the biconcave shape of the profile (i.e., a dip at the center of the profile) just downstream of the carina is rapidly transformed into a velocity spike. In a plane transverse to the junction plane, parabolic velocity distribution was conserved through the daughter branches. In the parent tube, following merging, the transverse profiles become flat as the flow proceeds downstream of the junction point. Further downstream towards the termination of the trunk of the junction the velocity profile develops a defect at the center. The velocity defect, nonetheless, is confined to a small region only in the vicinity of the centerline. Strong secondary helical motion is chiefly responsible for this type of velocity field in the junction.

The use of laser anemometry blood flow systems, in vivo, and especially LA fiberscopes is gaining momentum. The systems are usually based on the reflected laser Doppler system, where the beams propagate along the fiber or bundle of fibers. The systems are often equipped with a specially designed steering mechanism to enable the positioning of the fiber probe in several points across the measured site. The optical design allows for a very small measuring volume. The systems are becoming a powerful and important tool for the in vivo analysis of blood flow.

Experimental and computational investigations were carried out to study base flows behind afterbodies embedded in a supersonic freestream. Experiments were conducted in a blow down wind tunnel at a nominal Mach number of 2.06 and for an angle of attack of zero degrees. Afterbody models are interchangeable and sting mounted to avoid interference on the base from usual support systems. The experimental study consisted of two axisymmetric afterbodies, a cylindrical afterbody and a tapered afterbody with a conical boattail of 6 degree(s) and a boattail length of one diameter. Results indicate flow visualization, static wall pressure distributions and turbulent flow properties obtained by means of a two-dimensional laser Doppler velocimeter (LDV). Computations were carried out using a multi-dimensional Navier- Stokes code based upon a fully implicit, combined finite volume/flux element discretization approach with a standard k-(epsilon) turbulence model. Computed solutions show some good agreement with experiment as far as the flow field structure, surface and base pressures but show some deficiencies with velocities and shear stress correlations.

Laser transit velocimetry has been used at DRA Pyestock to investigate the internal flow fields of high-speed turbomachinery test rigs. Measurements have been made on a highly loaded core compressor and a multi-stage transonic fan. The data acquisition and analysis systems are described and their use on the rigs detailed. A number of test results are presented along with comparisons between other measurement techniques and the DRA S1-S2 flow calculation method. Various problems encountered are discussed along with ways of overcoming them in the future.

High speed liquid flows within glass nozzles of very small geometry and the resulting jets leaving those nozzles were studied. Due to the high flow velocity cavitation films are generated at the sharp inlet of the nozzles, they collapse and break up into bubbles further downstream. These bubbles can serve as tracers for a velocimeter based on a cross-correlation method. Using this technique we succeeded to measure velocities inside and outside the nozzle of up to several hundred meters per second.

The authors describe the behavior of particles (connected with Laser Doppler Anemometry measurements) in high velocity flows. The different flow regimes are exposed, as well as the associated usual drag laws for a sphere. A unique relation is proposed for high velocities and for all flow regimes. Calculated values obtained with this law and experimental results are compared in the case of particle motion through an oblique shock wave. They are found in good agreement. Different effects due to rarefaction, compressibility and high relative Reynolds number are discussed.

As in many other branches of the raw material industry, hardcoal preparation uses froth flotation to clean its ultrafine coal fractions in the < 500 im size range (fig. 1). To this end air is fed to a suspension containing both coal and mineral particles. The separation results from the selective adhesion of the hydrophobic coal particles to air bubbles. The coallbubble aggregates having formed rise to the surface whence they are withdrawn as foam. The hydrophilic mineral particles instead remain in the sluny.1 From the dynamic point ofview there are thus two mechanisms occurring during flotation, i.e. the elementary step of coal particle/air bubbles aUachment and, secondly, the rise of the coal-laden air bubbles through the suspension towards the surface.

From thrust augmenters to material movers Coanda ejectors have many applications in industry. Despite these widespread potential uses, the operating principles of these ejectors are not clearly understood. Hence it is often difficult to design these ejectors for specific applications. The purpose of this study was to experimentally elucidate the key physics of Coanda ejectors. The measurement of the velocity profiles inside the ejector were successfully accomplished by angling the beams of a laser anemometer with respect to the axis of the ejector. Also turbulence intensities, turbulence spectrum, correlation function and wall pressure distributions were measured. The results from this study indicate that there exists a universal velocity distribution for all velocity profiles at different locations, operating conditions and ejectors. Also based on the experimental results we postulated a mechanism for the operation of the Coanda ejectors that is different from classic ejectors.

Optical techniques such as birefringence and Laser Doppler velocimetry can be used to characterize the flow behavior of polymeric liquids in complex flow geometries. This use of optical techniques is called rheo-optics. In the study reported here, rheo-optical measurements were carried out on a low density polyethylene (LDPE) melt in a converging rectangular slit die. Using Laser Doppler velocity measurements as input data, extensional stresses along the center line of the converging slit were calculated using two different constitutive equations, the Wagner equation and the Phan-Thien and Tanner (PTT) equation. The calculated curves were compared with experimental stress data as obtained from birefringence experiments. It was found that calculations based on the PTT model give a better agreement with the experimental data than Wagner's equation. The application of rheo-optical measurements has the great advantage that local information is obtained on both the stress field and the velocity field in non-homogeneous flows. However, a number of possible causes of errors are introduced that should carefully be considered. Some of the problems involved in rheo-optical experiments are discussed.

Two component, phase averaged mean velocity data have been obtained with an LDV system, on the upper surface, near the leading edge of an oscillating airfoil undergoing compressible dynamic stall. In particular, the effect of oscillation amplitude has been studied. The results show that at an oscillation amplitude of 10 degrees, a separation bubble forms, that eventually bursts on the upstroke, well beyond the static stall angle. At 2 degrees amplitude, the bubble forms on the upstroke, but dynamic stall occurs on the downstroke. The results reveal new flow physics and the data sets serve as valuable quantitative information for validation of unsteady flow codes at transitional Reynolds numbers. The maximum velocity seen in the flow is about 1.6 times the free stream value and it occurs slightly downstream of the suction peak location. Some of the measurement difficulties are also discussed.

This paper presents a comparison among three of the most used velocity measurement techniques applied in experimental fluid dynamics: Hot Wire Anemometry (HWA), Laser Doppler Velocimetry (LDV) and Particle Image Velocimetry (PIV). The comparison is done on a fully developed turbulent jet. The measurements are processed in time and in space, according to the different nature of the measurement techniques. Results expressed in terms of velocity plots, turbulent fluctuations and integral length, scales are compared. The different capabilities of the three techniques are discussed, their complementarity is outlined, also related to the uncertainty and resolution of the three measurement techniques.

The commonly used techniques for directional discrimination in velocity measurements involve a frequency shift of the LDA light beams which are focussed into the measuring volume. The paper presents a new technique for the generation of a frequency shift be using the optical frequency difference of two current and temperature stabilized monomode laser diodes. By focussing the output beams of the two frequency stabilized laser diodes into the measuring volume it is possible (1) to save the commonly required components for the beam splitting and frequency shifting and (2) to double the light intensity in the measuring volume. It will be shown that using conventional laser diodes, the influence of frequency shift fluctuations and large band widths on the measuring information in the carrier frequency signal can be eliminated by appropriate heterodyne signal processing techniques. The concept of the described new frequency shift technique will be discussed critically and first experimental results will be presented.

A model is set up for predicting the performance of a PIV recording system using simple relations for light scattering, particle imaging and recording. The scattering efficiencies for various seed materials are measured experimentally.

This paper presents some measurements of wind speed and direction obtained by a LDA system. The case was a squall-line event on June 2, 1992 at Lichtenau, a location 50 km approximately southwest of Munich. The measurements were taken during the pass of a squall-line and after.

Based on laser Doppler anemometry, the mean and fluctuating components of the flow of a non-Newtonian shear thinning fluid in the transition regime in a vessel agitated either by a narrow or a wide blade axial flow hydrofoil impeller are presented for the first time. Comparison with similar measurements for water (fully turbulent flow) shows that the flow is confined much more to the impeller region and that the flow numbers are some 15 to 35% lower. The fluctuating velocity components were 20 to 50% lower. The total radial-axial circulation flow was also estimated and used together with the impeller power to show that the efficiency of the narrow blade hydrofoil was significantly higher.

This paper presents fundamental research work on jet dissipation in pipes and closed chambers in order to study the details of flow pattern, velocity distribution, and energy reduction. The effects of chamber dimensions and jet heights are also investigated. Experiments were carried out with air and water jets being discharged into a pipe. The ratio of pipe to nozzle diameter (B/D) was varied between 3 and 12. The axial maximum velocity and the lateral distribution of velocity across the pipe were measured up to a distance of 20 diameters downstream. The results should have industrial applications in all fields on engineering.

A fiber optics laser knife and a fiber optics laser Doppler anemometer (LDA) with a varying fringe spacing were used for investigation of movement of gas bubbles which appear in liquid under the action of ultrasound field. A possibility of measurement of bubbles sizes, velocities and vibrations under the action of acoustic wind, an ultrasonic field and the Archimedes' force is shown.

The buoyant countercurrent exchange flow through a vertical tube in a horizontal partition of an enclosure was measured by densimetry and laser Doppler velocimetry. In this study, brine and water were used as the two fluids, and the length of diameter ratio of the vent tube was one. Unsteady mixing was observed inside the vent tube, and the less dense water rose in a plume from the vent into the brine in the upper compartment. An equation for exchange flow rate over time was developed based on densimetric measurements, and the flow coefficient was found to be a constant. A two-component forward-scatter laser Doppler velocimetry (LDV) system was configured to study the flow. A motorized X-Y positioning system was built to move the LDV measuring volume efficiently through a horizontal cross section of the water plume. Volumetric flow estimated from the integral of positive velocity measured throughout a plane cross section of the plume was about 1.77 times the exchange flow.

A differential LDA system is described, which employs twin front lens transmitting optics and provides a very small probe volume of 9 microns diameter combined with relatively large working distance of 135 mm. The system incorporates optical fibers and frequency shifting using either a Bragg cell or rotating diffraction grating. The advantages of the twin front lens approach for systems with a high ratio of working distance to size of probe volume are discussed and the positional tolerances required when using a rotating diffraction grating in conjunction with single mode fibers are analyzed. Finally, some experimental data are presented to confirm the optical properties and performance of the system.

Heat releases from directly or indirectly heated buildings to the ambient air induces buoyancy effects. The superposition of atmospheric approach flow and buoyancy changes the flow field when compared with the non-heated case. This can affect, e.g., the spreading of pollutants which are either transported by the oncoming flow or released from the building. When problems of local air pollution, pollutant transport, air ventilation and air condition around buildings are of interest, the superposition of approach flow and buoyancy-driven flow has not been considered in detail so far. Within this paper, flow field analyses around non-heated and heated model buildings are given which were carried out in an atmospheric boundary layer wind tunnel. To measure the flow field around the buildings, a 2-d-LDA-system was used. The aim of the investigations was to show the phenomenological changes in the flow field of heated buildings when compared with the non-heated case.

Atmospheric structures can be measured with incoherent optical radars (lidars). Because these structures drift with the wind, they can serve as a tracer for remote sensing of the wind vector. For this purpose, a dual monostatic-scanning lidar system is available to measure the atmosphere simultaneously in two different directions over a maximum range of about 1 km. The transit time of identified patterns between two sensing points in the horizontal plane, in combination with the geometry of the lidar, provides sufficient information to derive the horizontal wind vector. The method is based on cross-correlation techniques. To determine the spatial wind vector as a function of altitude it is sufficient to measure in three different slant directions. This can be realized with a triple lidar or with a single lidar by measuring consecutively in three different directions and using a similar but more extensive inversion method. A selection of experimental results is presented.

Higher buildings and pre-fabricated walls have resulted in an increase in the rate of occurrence of rain penetration. The forces that cause this water infiltration through modern walls are wind induced and can only be altered at the design stage. This paper describes an experimental and computational study of the processes of water ingress and proposes design features to minimize or eliminate them.

This paper describes flow measurements in a rotating annular flume, that is an annular flume with a rotating top lid as well as a rotating bottom. Annular flumes are used for studying erosion and/or deposition of cohesive sediments in the laboratory, because these flumes have advantages over straight recirculating flumes in that effects of inflow and outflow conditions are avoided and that there are no pumps to break down suspended flocculated sediment. A disadvantage of annular flumes is that secondary flow velocity components in vertical and radial direction are generated, yielding a complex 3-D flow field that is not well known up to now.

A method was developed for burst type laser velocimeter systems to be used for the highly unsteady but periodic velocity fields found in multi-component turbomachines, in which the components rotate at different speeds. Velocity signals are measured in the stationary frame, translated into the rotating frame, and organized by the relative angular positions of all independent components of the turbomachine. This method requires the angular position of all rotating components of the turbomachine to be measured at the instant a velocity signal is processed by the LV system. Because the flow field is periodic, large sample sizes can be obtained for every instantaneous registration of the components of the turbomachine. By properly 'sorting' the recorded data, one is able to correlate unsteady interaction of two or more components. The method was applied to an automotive torque converter turbine inlet. A torque converter has three components that rotate at different speeds: pump, turbine, and stator. Tests were run for a turbine speed of 881 rpm and a pump speed of 1100 rpm. Results from the tests were used to produce a 'movie' with 37 'frames' of the time dependent three- directional velocity vector fields at a rotating turbine inlet passage as the upstream pump blade passed. A strong influence of the pump on the turbine inlet velocity field is shown. Namely, the jet-wake region generated by the pump is identified as an unsteady inlet condition to the turbine.

The development of coherent structures formed between coflowing vertical jets at moderate Reynolds numbers has been studied experimentally. By means of phase-locked LDV and flow visualization it has been possible to produce data enough to describe the flow subjected to the effects of axial forcing. Flow mapping includes axial and radial phase averaged velocity profiles, vorticity profiles, streamlines and streak-lines. Vorticity dynamics inferred from these data as well as streak-lines shows a good agreement with flow visualization. Mixing between the streams is also analyzed, improving with higher levels of forcing amplitude.

Quantitative velocity and turbulence measurements which were obtained using laser Doppler velocimetry during the course of the physical modeling of the molten aluminum flow in a tundish are presented. The experimental findings are compared to the results obtained by the computational simulation of the flow, showing favorable agreement between the two approaches. The measurements revealed asymmetric velocity and non-isotropic turbulence field near the nozzle exit. In the vicinity of the nozzle, the interaction of the impinging jet and the inclined walls gives rise to recirculating vortex structures. Downstream of the nozzle, two counter-rotating secondary cells diminish in strength.

This paper describes an experimental study of highly turbulent cross-flow through a staggered array of finned tubes in sub-critical flow. Measurements of the mean and root-mean-square fluctuating components of tangential velocity within the inter-fin region were obtained about a cylinder in fully developed conditions. Measurements are compared with the results of an earlier study within a bank of plain tubes.

While organizing this conference, we realized that it would be appropriate to pay attention to laser safety hazards. After all, every user of LA instrumentation may someday accidentally be exposed to an overdose of laser radiation. In the days of LDA systems with predominantly low power HeNe lasers, no body really worried about it. Nowadays, that we use 5 W Art-ion lasers, a power 10000 times the safety limit, this possibility is far from being imaginary. Of the safety hazards, such as . radiation damage to the eyes or skin . fire risk . exposure to high voltage or toxic chemicals especially the risk of radiation damage to the eyes has to be emphasized. Spurious reflections caused by moving objects (wedding rings!) and imperfect beam alignment procedures can yield dangerous situations

Classification schemes to identify laser hazards may give a quick insight into the hazards of radiation. However the most serious accidents involve exposure to high voltages. Besides toxic chemicals may present hazards in laser work stations. In this paper an overview will be given of hazards associated with working with lasers. Special attention will be given to radiation hazards of laser-doppler anenometry and laser particle imaging techniques using argon and copper lasers.

Safe laser exposure limits use to be stated, rather than argued. As a result their uncertainties often remain obscure. In this paper a few of these are discussed.

As particle image velocimetry has been developed as a useful experimental method for measuring instantaneous velocity vector fields in turbulent fluid flows, a systematic and comprehensive study of the procedure to assess its accuracy, spatial resolution, reliability and success rate for known velocity fields, experimental parameters and interrogation procedures has been undertaken. A systematic study of the parameters which affect the measurement of velocity vector fields in fluid mechanics experiments using the particle image velocimetry procedure has been carried out for a range of alternative methods of PIV recording and analysis. This study has developed an analytic model and a Monte Carlo simulation to analyze experimental and interrogation procedures for a range of high image-density PIV systems. The study has provided a number of criteria which are used to determine which experimental and interrogation parameters are most important for a range of fluid motions and a range of alternative methods of PIV recording and interrogation. In addition, it has been shown that velocity gradients within the flow field result in biased experimental measurements. The extent of the bias has been quantified and some recommendations to minimize bias have been made.

A numerical simulation of particle image velocimetry (PIV) is developed in order to evaluate the effectiveness of different methods for the analysis of a multi-exposed image (Fourier transform, auto-correlation and tracking). The trajectory of each particle is obtained by solving a modified Langevin equation in a three-dimensional limited field in which the statistical characteristics of the flow are assigned. In this way it is possible to obtain the position of the baricenters of the seeding particles at prefixed time intervals. A plane section through the field simulates a light sheet on which is obtained, by numerical methods, a multi- exposed image. The analysis is performed with a different number of seeding particles and with different noise levels in order to obtain the effectiveness threshold of each tested method.

This paper compares, for a periodic wake flow, the results of flow visualization with those of particle-image velocimetry to ascertain the extent to which the dye and vorticity structures coincide.

This work is a part of a larger study devoted to the understanding of the way a siphon works, in particular the final stage, in order to predict the true levels of the liquids in the tanks at the end of the liquid transfer. Experimental measurements (pressure, levels, velocities) during the transient flows in siphons were carried out and were compared with results obtained by using the computer code CATHARE. The liquid velocity profiles were measured by PIV technique which was the only available measurement technique for this kind of experiment. We show here that the computer curves closely follow the experimental curves during the priming and the liquid transfer stages but significantly move away during unpriming.

This paper describes particle image velocimetry measurements obtained in a Mach 6 flow field. The measurements were performed in the Langley Mach 6 High Reynolds Number Tunnel facility. A wedge model oriented at an angle-of-attack of -15 degree(s) was used to generate an oblique 22.7 degree(s) bow shock. Using 1.0-micrometer aluminum oxide powder as the seed material, PIV photographs in the vicinity of the bow shock region were taken on the centerline of the model at a location 110 mm from the leading edge. Using the two- dimensional velocity fields obtained from analysis of the photographs, normal and tangential components of velocity with respect to the shock angle were obtained. These velocity components were then used to infer the aerodynamic particle sizes present in the tunnel. Results indicated that the 1.0 micrometer seed material introduced into the tunnel had an aerodynamic size of approximately 1.0 - 2.0 micrometers. Differences were noted between the measured and predicted normal component of velocity downstream of the shock after full particle relaxation had occurred. Using qualitative flow visualization, it was determined that due to cavity flow along the test section walls, reflected shocks in the vicinity of the PIV measurements could account for these differences.

The instantaneous flow field above a NACA 0012 airfoil has been investigated by means of Particle Image Velocimetry (PIV). Two different flow conditions were realized in the high speed blow down wind tunnel of DLR Gottingen in order to obtain a flow field similar to that above a rotor blade of a helicopter. In the first part of the experiment the angle of incidence varied from (alpha) equals 5 degree(s) to (alpha) equals 20 degree(s) at a freestream velocity of Ma equals 0.25 in order to simulate the flow above the retreating blade of the rotor. Typical values for the second measuring sequence were (alpha) equals 0 degree(s) to (alpha) equals 5 degree(s) and Ma equals 0.75 (advancing blade). Due to some technical improvements of the DLR PIV-System, such as a high speed rotating mirror system and an optimized seeding device, it was possible to obtain the velocity data of these instantaneous flow fields with an accuracy high enough to allow a later comparison with data of numerical calculations.

Three ways of implementing the 2-dimensional autocorrelation function (ACF) used for the extraction of displacement information in particle image velocimetry (PIV) records have been compared. The three methods are: (1) Numerical computation by means of two consecutive FFT algorithms directly from the digitized image, (2) a hybrid method by which an optical Fourier transform first forms the spatial power spectrum of the image, after which the ACF is computed by a numerical FFT (this method is usually known as the Young's fringe method) and (2) an all-optical correlator. These three methods are compared in terms of resolution and 'quality of data', a term covering signal-to-noise and detectability considerations. In spite of the fact that three methods in principle carry out the same underlying mathematical operation the resulting ACFs may look very different. However, as it turns out, the differences may be explained by artifacts characteristic of the practical implementations, and when these differences are taken into account, the three methods are seen to perform quite similarly.

An optical processor for generating a two-dimensional squared autocorrelation function is presented for post processing Particle Image Velocimetry (PIV) photographs of fluid flows. The incoherent-to-coherent conversion is performed by an optically addressed spatial light modulator. The optical processor presents a fast and comparable alternative to the numerical processors, which typically are used for this purpose. Results of analysis of synthetic tests and real fluid flow PIV images are present.

In the past 20 years, considerable progress has been made in the physical understanding and theoretical prediction of complex turbulent flow which can now be computed by solving the full time averaged Navier-Stokes equations. However, there is still a need to validate these calculations before their routine use in design procedure. The advent of LDV has allowed to obtain the detailed experimental information required to an in depth validation of Navier- Stokes calculations. The contribution of LDV to the development of advanced models is illustrated by presenting experiments performed in high speed flows. Two-dimensional transonic and supersonic shock wave/boundary layer interactions are first considered. Then axisymmetric base flows are examined. A three-dimensional configuration is discussed by considering a transonic channel. In each case, the LDV measurements are compared to computed results to illustrate the strong coupling that must exist between experimental and theoretical approaches.

Experimental (LDA) and computational (CFD) investigations of steady, laminar natural convection in a horizontally vented chamber containing a vertical heated cylinder at the center are reported. The heated cylinder was located on the lower adiabatic wall of the chamber and was 2/3 the height of the chamber. The cylinder surfaces were maintained at a constant temperature. The chamber inlet and outlet were located at the bottom and top of facing vertical walls, respectively. In this study, experiments were conducted using laser-Doppler anemometry (LDA) with a two-component Ar-Ion laser connected to two burst spectrum analyzers (BSA). Velocity profiles (of both vertical and horizontal velocity components) were measured at the inlet, outlet and at other locations inside the chamber. For the numerical simulation (3-D, steady-state, laminar) the governing equations were solved using the computational fluid dynamics code PHOENICS. Comparison of the experimental data with computational results showed satisfactory agreement and the importance that LDA measurements have in verifying CFD simulations.

Laser Doppler anemometry (LDA) is a useful experimental tool which is capable of providing high quality data for validating Computational Fluid Dynamic (CFD) simulations. Previous work with a mixed-flow pump had shown that there is a considerable discrepancy between the LDA data and the CFD simulation if the standard turbulence model is not modified to allow for the additional effect of the centrifugal action. Prior to the investigation of a cyclone separator in which the flow has a very high centrifugal action, it was decided to investigate a simpler flow consisting of an annular jet impinging on a flat plate in which the curvature of the jet can be controlled by the distance between the jet exit and the impinging plate. By comparing the results of LDA and simulation of the annular jet with different amounts of curvature it was expected that a transition level would be found at which a modified model would be required. The flow from an annular jet impinging on a plate can be simply analyzed in terms of the internal pressure which is generated by the rate of change of momentum due to the jet being turned through 90 degrees. However the detailed velocity distribution and variation of pressure across the jet cannot be obtained from simple analysis and require the use of a CFD code. The commercial code PHOENICS has been used in this investigation and the experimental data was obtained using a DANTEC single component fiber optic LDA system.

Modern instruments such as laser anemometers connected to automatic traversing and data collection systems generate large amounts of data which can be difficult to interpret. An interface has been developed which makes use of existing commercial software to display the data in multi-dimensional vector format.

Two points velocity measurements in a fully developed turbulent duct flow are performed using LDA technique. Color separation is used to discriminate signals from close positions. High spatial (about 0.1 mm) and temporal (about 10^-3 s) resolutions are obtained by careful selection of optical and electronic parameters. The signals are analyzed using classical auto-correlation and power spectral functions, but also time cross-correlation, transfer and coherence functions. From time-space quantities, information on the evolution in a reference frame moving with the mean flow velocity can be derived and compared with Lagrangian correlation; the results agree at least over one integral time scale. Different Reynolds numbers are tested. Taylor and advection hypotheses are also investigated.

The integral laser Doppler anemometry (IDA) is considered. In IDA a thin and extended measuring volume is formed across the flow, and Doppler shifts from all particles in this volume are registered simultaneously, giving an integral Doppler spectrum. The theory of IDA is presented for the differential optical arrangement of LDA. It is shown, that under certain conditions easily met in the measurements, the solution of the inverse problem is possible, namely, the reconstruction of the flow velocity profile, or the particles concentration profile from the shape of the measured integral Doppler spectrum. The transverse concentrational profiles of particles in dilute suspension flow in a flat channel of 110 (mu) width are measured using IDA technique.

Using two-point LDA systems, estimates of the integral length scales of turbulent flowfields can be performed by forming the cross-correlation function of the two velocity records for various measurement volume separations. Several estimators for the cross-correlation function are reviewed and their bias and variance are examined as a function of particle density and of chosen processing parameters. The investigations were performed using numerical simulations.

Difficulties of LDA for the near-wall turbulence measurement are well known. The measuring error increase at drawing near to a wall. Joint use of laser anemometry in a flow and measurings on a wall remove it. Existent methods permit the measurement of average values and pulsations of friction and pressure on a wall with good accuracy. The offered method of their processing is based on the harmonical analysis of experimental values of amplitudes and phases. The near-wall flow is calculated on the basis of simplified equations of Navier-Stokes. The obtained solution may be grown together with the values determined on the basis of the LDA measurings, which are carried out with a high accuracy in to distance from a wall.

The flow above an F/A-18 model, set to 25-degrees angle of attack, was measured using a Doppler global velocimeter, (DGV). The investigation indicated that the complex flow contained many similarities to the vortical flow above a simple delta wing set to a high-angle of attack, including flow standard deviations greater than 30-percent of free-stream. These standard deviation levels were also comparable to results found during a previous investigation of the vortical flow above a YF-17 using fringe-type laser velocimetry. The global measurement capability of the DGV provided the first evidence that the burst vortices above the model were structured. These structures were found to maintain their spatial coherence while the flow varied in an overall sense.

The use of laser anemometry as a technique for flow investigation within turbomachines is now well established. Difficulties in using this technique, however, vary from machine to machine and depend upon the nature of flow involved. Various parameters such as optical access, temperature and turbulence level influence the difficulties. In this paper we explain how laser anemometry was used to investigate the flow downstream the two radial inflow turbines which variously involve such difficulties. Small radial inflow turbines have various applications in industry and are successfully used as major components of small gas turbines and turbochargers. Units were provided by american (USA) industry. Flow conditions involved in the two units were very different as one unit was operated at 400 degree(s)C inlet temperature and the other at near ambient inlet conditions but which involved subzero turbine outlet temperatures. The paper presents a comparative study of laser anemometry techniques used to measure flow in these two very different situations. Problems such as seeding, optical access have been detailed. Finally, typical results are presented.

Two channel laser Doppler and single channel constant current (cold wire) thermal anemometry was used to obtain simultaneous velocity and temperature data for the cross flow development of heated turbulent air flow through a staggered tube bundle. The model examined is based upon the UK Advanced Gas Reactor boiler tube assemblies but is more generally representative of a much wider range of cross-flow tube bundle arrangements. Distributions of the time mean velocities and temperatures, the rms velocity and temperature fluctuations, the Reynolds stresses and the turbulent heat flux terms were measured for two different Reynolds numbers. The results are sufficiently reliable and detailed that they may be used for the validation of computer predictions of this important class of problem.

This paper describes an experimental study in which the LDA technique has been applied to investigate the supersonic vortical flow generated by a lifting delta wing at a free stream Mach number of 2.3. A three color system was employed to measure the complex three-dimensional flow field around the delta. The data have been transposed onto a convenient grid and presented as velocity vector and vorticity information so that the effect of changing the angle of incidence is revealed. The results are in good agreement with previous findings from flow visualization and theoretical studies.

Wind turbine performance is critically dependent on the geometry of the rotor wake. Acquisition of detailed wake data is of major importance for further development of wind turbine studies. This paper presents the results of a preliminary experimental study of the flow past a model turbine using Particle Image Velocimetry (PIV), a whole field velocimetry method, with a scanning beam system of flow illumination. The results strongly validate the PIV technique applied to studying wind turbine wakes. PIV provides the means whereby detailed full-field data can be obtained and an experimental data base for wake velocities and structure established.

We demonstrate the design of a fiber optic beam delivery system for particle image velocimetry (PIV) applications. The system is capable of transmitting pulse energies of about 10 mJ in 7 ns pulses from a frequency doubled and Q-switched Nd:YAG laser; it is hence suitable for use with high speed air flows. The fiber used has a fused silica core of diameter 400 micrometers , and is of the step index multimode type. The output beam quality is 80 mm- mrad. Through exploitation of intermodal dispersion of the fiber, intensity variations in the output from the fiber due to speckle are suppressed to a visibility of < 7%.

Whole field velocity measurements of the flow past a NACA 0012 airfoil at high angle of attack are obtained by means of a particle image velocimetry, in both static and dynamic conditions. The 'On-Line' Velocimeter used for the measurements features a solid state imager, a microcomputer workstation and dedicated electronic bands. The Velocimeter is capable of acquiring, displaying and processing the multiple exposed frames. Using this novel approach the fully separated flow over the upper surface of the airfoil at high subsonic free- stream velocities (approximately 150 m/sec) was mapped.

This presentation describes the application of laser techniques to large flames, diesel sprays, fires and internal combustion engines. The 'thinking' behind such technique applications is discussed and results are presented--as can realistically be achieved through good setting up and optimization of the basic techniques. The presentation may be viewed as a brief though practical guide to the feasibility and methods of LDA, PIV, sizing, holography, schlieren/shadowgraphy, and others, applied to the more demanding of applications.

The objective of this paper is to investigate how the low and high frequency parts of the flow at different locations close to the spark plug influence the early combustion at individual cycles in a SI-engine. 2-components cycle-resolved laser doppler velocimetry (LDV) measurements have been done both inside the spark gap, and around the spark tip to extract velocity information. The pressure in the cylinder was measured with a piezo-electric transducer connected to an A/D-card in a standard PC. The velocity information was filtered to get 'mean velocity' and 'turbulence'. The pressure signal was used in an one-zone heat-release model to get different levels of mass fraction burned etc. The results show a significant correlation between the velocity and the early combustion when the velocity was measured close to the spark plug tip. The correlation decreased when the velocity was measured at some distance from the electrodes for both a pancake and a high squish combustion chamber. The correlation between the velocity close to the spark plug and the early flame development showed no dependence on the air-fuel ratio.

A numerical simulator of a laser Doppler system has been developed to have a computer aided design tool for system analysis. The simulator uses Mie optics to compute the light scattering intensity of the particles crossing the measuring volume. The simulated Doppler signal can be analyzed by numerical simulators of the Doppler processors. The simulators of Burst Spectrum Analyzer (BSA), Intelligent Flow Analyzer (IFA 550) and Counter have been developed. The processor simulators have been experimentally compared with the real processors using a special device that allows the Doppler signals computed by the simulator to be generated. Under well known conditions some numerical experiments have been performed to investigate the accuracy of the LDA technique. In this paper results about measurement errors due to the ambiguity phase noise of the Doppler signal are presented.

The paper describes a new alignment technique for three-dimensional Laser Doppler Anemometers (LDAs), which has enabled accurate velocity measurements to be obtained within very small flow regions. This technique uses a quantitative, rather than the more usual qualitative approach, to give greater consistency and reliability, resulting in greatly improved alignment of the laser beams. This enables much higher data rates and validity to be achieved, even in the critical cross-coupled mode, which in turn allows a significantly smaller measurement volume, thereby increasing the spatial resolution of the system. Examples are given which illustrate the type of small scale velocity surveys now possible as a result of this technique. These show 3D velocity surveys taken within the wake behind a 0.8 mm diameter surface particle in a boundary layer, and examples of three-component boundary layer velocity profiles in the order of 0.2 mm thick, recorded on a horizontally mounted cylinder rotating about a central vertical axis.

As part of research on swirler stabilized combustors, swirling flows in an expanding and, after a length L, contracting tube are investigated. If the flow is given sufficient rotation in a flow pattern with central recirculation area is measured in a tube with expansion ratio 0.69. In a larger tube with expansion ratio 0.23 a toroidal recirculation area is found. The time dependence of the flow is investigated by determining the autocorrelation function of the velocity signal. In the range of rotational velocities of the flow the flow field is shown to be periodic in time. This is a very interesting phenomenon that occurs only in sufficiently large tubes, with expansion ratio normally encountered in industrial furnaces. Numerical simulation of this flow will be difficult as it is periodically oscillating and probably not axisymmetric.

A method is described to measure the total velocity field and the complete Reynolds stress tensor using a two component LDV system. The method is applied in a turbulent pipe flow. To reduce the beam refraction to a negligible amount, pipe sections made of transparent film of 80 micrometers thickness are used. It proved necessary to calibrate the frequency trackers in order to obtain an acceptable accuracy. Results are given for fully developed pipe flow and a flow containing a strong swirl.

The flow through the inlet valve gap of a Diesel engine cylinder head has been measured under steady flow and motored engine conditions. The excellent agreement between the swirling flows generated at maximum valve lift justifies the use of steady flow data as inlet conditions for the computational fluid dynamic modelling of engine cylinder flows.

The velocity field of a BANKI water turbine was studied by employing an one component LDV system. More emphasis was given in the area around the guide vane of the turbine, the entrance to its runner and the region between its two stages. A comparison with respective numerical data proved to be quite satisfactory.