Effect of compliant wall motion on turbulent boundary layers

Abstract

A critical analysis of available compliant wall data which indicated drag reduction under turbulent boundary layers is presented. Detailed structural dynamic calculations suggest that the surfaces responded in a resonant, rather than a compliant, manner. Alternate explanations are given for drag reductions observed in two classes of experiments: (1) flexible pipe flows and (2) water−backed membranes in air. Analysis indicates that the wall motion for the remaining data is typified by short wavelengths in agreement with the requirements of a possible compliant wall drag reduction mechanism recently suggested by Langley.

Obtaining Extremely High Lift to Drag Ratios with Flexible-Wall Turbulence Control

“…The precise matching of fluid and wall motion needed for this form of control has only been realized in experiments where controlled disturbances were introduced and subsequently annulled by the wall motion. Hence earlier experiments involving compliant or externally actuated flexible walls on attached and usually zero-pressure-gradient boundary layers have however failed to show appreciable repeatable drag reduction in real flows 9 . Additionally, devising compliant walls for lowspeed air flows has been considered impractical due to material property constraints 11 .…”

“…Even though the Deturbulator (Fig 1) appears to be a manifestation of a "compliant wall", it is significantly different. Earlier compliant walls were aimed at damping instabilities in attached laminar boundary layers to delay transition or counteracting wall-normal velocity components in turbulent boundary layers [9][10][11] . The precise matching of fluid and wall motion needed for this form of control has only been realized in experiments where controlled disturbances were introduced and subsequently annulled by the wall motion.…”

Verification of drag-reduction capabilities of stiff compliant coatings in air flow at moderate speeds

“…The phenomenal drag reduction as much as 60% by Kramer (1957Kramer ( , 1962 has never been reproduced afterwards. Those attempts were reviewed by Bushnell et al (1977) and Gad-el-Hak (1996), who focused on the experiments employing the "soft" coatings, i.e., either porous material covered by thin film or gel-like substance. Those coatings are soft enough to form λ-shaped wrinkles on the surface and scarcely associated with drag reduction (Gad-elHak et al 1984).…”

Flow stabilization by subsurface phonons

“…Numerous investigations were conducted on the ensuing effects on phenomena as complex as laminar-to-turbulent transition and skin-friction drag (e.g. [11,12]). A compliant surface predominantly admits Rayleigh elastic waves along the surface [15], 1 and due to its low stiffness allows for the possibility of large surface motion and hence significant interaction with the flow.…”

“…into the flow [5], addition of ribs on the surface [6], suction and blowing [7][8][9] and coating of the surface with a compliant material [10][11][12], among others.…”