Direct Numerical Simulations of Bristled Shark Denticles in Turbulent Channel Flows

The mechanisms by which dermal scales (i.e., `denticles') reduce drag for sharks are still under debate. Recently, reduction of form drag and maneuver load by bristled denticles has been observed in a few experiments using real shark skin samples. To investigate how bristled denticles can change the dynamics of a turbulent boundary layer, we performed DNS of open channel flows over arrays of shark denticle replicas. Representative denticles from Isurus oxyrinchus are modeled using an immersed-boundary method. The complete 3D denticle features including the enameloid crown, neck, and the base at which the scale is anchored are considered. The denticle replicas are inclined at 0, 15, and 30 degrees in this study. Both forward and reversed flow directions of the open channel are examined given that bristling may be caused by bending of the fish body and/or local flow reversal. The total drag is increased for all the simulated arrangements despite that the width between keels on top of the crown matches the optimum value for drag-reduction riblets. Complex vortical structures are observed as the bristled denticles interact with the boundary layer. The region under the crown of the denticle acts as a porous media with a variable porosity determined by the bristling and flow direction. The authors acknowledge support from NSF Grant 2039433.