Presented are the results of in-situ computational fluid dynamics (CFD) modeling of the air knife assembly, associated grid-convergence study, and factory acceptance test results. The air knife assembly, a key thermal systems component of the Daniel K. Inouye Solar Telescope (DKIST), serves the dual purpose of ventilating the ceiling of the Coudé laboratory and as the interface between laboratory and ambient environments using an air curtain. During factory acceptance testing, flow visualization of the air curtain revealed that the flow was deflected upwards, suggesting that an unexpected pressure gradient had developed. Installed filter pressure sensors between the bottom of the air knife and the factory floor, indicated that a significant positive pressure had developed, 0.77 in w.c. above ambient. Outflow from the air knife, measured using a hot wire anemometer, exceeded the expected flow rate by a factor of approximately 1.5. It is hypothesized that large-scale eddies, generated by air knife outflow reversing at the floor, may have induced false downward velocity readings causing the increased pressure and flow rate. In-situ CFD modeling suggests that the air knife will meet or exceed all performance requirements. Outflow is predicted to be unidirectional, with an average velocity of 0.28 m/s and temperature within the 20 ± 0.5 C˚ margins. Maximum wavefront error introduced at the interface is predicted to be 84.3 nm root mean square (RMS).
Implementation of an air curtain at the thermal boundary between conditioned and ambient spaces allows for observation over wavelength ranges not practical when using optical glass as a window. The air knife model of the Daniel K. Inouye Solar Telescope (DKIST) project, a 4-meter solar observatory that will be built on Haleakalā, Hawai’i, deploys such an air curtain while also supplying ventilation through the ceiling of the coudé laboratory. The findings of computational fluid dynamics (CFD) analysis and subsequent changes to the air knife model are presented. Major design constraints include adherence to the Interface Control Document (ICD), separation of ambient and conditioned air, unidirectional outflow into the coudé laboratory, integration of a deployable glass window, and maintenance and accessibility requirements. Optimized design of the air knife successfully holds full 12 Pa backpressure under temperature gradients of up to 20°C while maintaining unidirectional outflow. This is a significant improvement upon the .25 Pa pressure differential that the initial configuration, tested by Linden and Phelps, indicated the curtain could hold. CFD post- processing, developed by Vogiatzis, is validated against interferometry results of initial air knife seeing evaluation, performed by Hubbard and Schoening. This is done by developing a CFD simulation of the initial experiment and using Vogiatzis’ method to calculate error introduced along the optical path. Seeing error, for both temperature differentials tested in the initial experiment, match well with seeing results obtained from the CFD analysis and thus validate the post-processing model. Application of this model to the realizable air knife assembly yields seeing errors that are well within the error budget under which the air knife interface falls, even with a temperature differential of 20°C between laboratory and ambient spaces. With ambient temperature set to 0°C and conditioned temperature set to 20°C, representing the worst-case temperature gradient, the spatial rms wavefront error in units of wavelength is 0.178 (88.69 nm at λ = 500 nm).