Modulation of Turbulent Channel Flow by Bristling Shark Denticles

Shark denticles possess a flexible base anchored in the dermis, enabling pivoting and scale bristling. Direct numerical simulations were conducted to investigate the behavior of channel flows over an array of shark denticle replicas. The simulations were performed at $Re_b = U_b H/ν = 8000$. Without the presence of the denticles, the friction Reynolds number was around $Re_τ = 450$. The shark denticle replicas that had a height of 0.098$H$ are represented by an immersed boundary method. The array was assigned a sinusoidal variation in the angle of inclination, ranging from 0 to 15 and 30 degrees, mimicking the bristling motion. The bristling period was 4.0$H/U_b$, comparable to the bursting time scale of the boundary layer at this Reynolds number. Comparisons were made with the non-bristled denticles to explore the effects of the motion. The bristling significantly increases the total drag. The equivalent sand grain roughness height of the denticle increases from $k_s^+$ = 158 (no-bristling) to $466$ (30-degree bristling). The mean shear over the crown of the denticle is increased (decreased) when the denticle is bristling up (back down). Streamwise-elongated vortices formed between the ridges on the crown are periodically shed off by the bristling and alter the turbulence in the near-wall region.