A wind tunnel study of flow structure adjustment on deformable sand beds containing a surface‐mounted obstacle

Abstract

AbstractRoughness elements of varied scale and geometry commonly appear on the surfaces of sedimentary deposits in a wide range of planetary environments. They perturb the local fluid flow so that the entrainment, transport, and deposition of particles surrounding each element are fundamentally altered. Fluid dynamists have expended much effort in examining the flow structures surrounding idealized elements mounted on fixed, planar walls. However self‐regulation occurs in sedimentary systems as a result of the bed surface undergoing rapid topographic modification with sediment transport, until it reaches a stable form that enhances the net physical roughness. The present wind tunnel study examines how the flow pattern surrounding an isolated cylinder, a problem extensively studied in classical fluid mechanics, is altered through morphodynamic development of a deep well that envelopes the windward face and sidewalls of the roughness element. Spatial patterns in the fluid velocity, turbulence intensity, and Reynolds stress obtained from laser Doppler anemometer measurements suggest that the flow structures surrounding such a cylinder are fundamentally altered through self‐regulation of the bed topography as it reaches steady state. For example, flow stagnation and the turbulent dissipation of momentum are substantially increased at selected points surrounding the upwind face and sidewalls of the cylinder, respectively. Along the center line of the wake flow to the rear of the cylinder, several structures arising from flow separation are annihilated by strong upwelling of the airflow exhausted from the terminus of the well. Feedback plays a complex, time‐dependent role in this system.