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This paper presents a new class of compliant surfaces, dubbed
tensegrity fabrics, for the problem of reducing the drag induced by
near-wall turbulent flows. The substructure upon which this compliant
surface is built is based on the "tensegrity" structural paradigm, and
is formed as a stable pretensioned network of compressive members
("bars") interconnected by tensile members ("tendons"). Compared with
existing compliant surface studies, most of which are based on
spring-supported plates or membranes, tensegrity fabrics appear to be
better configured to respond to the shear stress fluctuations (in
addition to the pressure fluctuations) generated by near-wall
turbulence. As a result, once the several parameters affecting the
compliance characteristics of the structure are tuned appropriately,
the tensegrity fabric might exhibit an improved capacity for dampening
the fluctuations of near-wall turbulence, thereby reducing drag.
This paper improves our previous work (SPIE Paper 5049-57) and uses a
3D time-dependent coordinate transformation in the flow simulations to
account for the motion of the channel walls, and the Cartesian
components of the velocity are used as the flow variables. For the
spatial discretization, a dealiased pseudospectral scheme is used in
the homogeneous directions and a second-order finite difference scheme
is used in the wall-normal direction. The code is first validated
with several benchmark results that are available in the published
literature for flows past both stationary and nonstationary walls.
Direct numerical simulations of turbulent flows at Re_tau=150 over the
compliant tensegrity fabric are then presented. It is found that,
when the stiffness, mass, damping, and orientation of the members of
the the unit cell defining the tensegrity fabric are selected
appropriately, the near-wall statistics of the turbulence are altered
significantly. The flow/structure interface is found to form
streamwise-travelling waves reminiscent of those found at air-water
interfaces, but traveling at a faster phase velocity. Under certain
conditions, the coupled flow/structure system is found to resonate,
exhibiting a synchronized, almost sinusoidal interfacial motion with
relatively long streamwise correlation.
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