The Effects of Flexible Cylinder Structural Dynamics to the near Wake Turbulence

Abstract

The utilization of a rigid and projecting surface, coupled with an agitator and vortex generator, frequently results in the dissipation of more energy than the production of turbulence that meets the required criteria. By contrast, a passively oscillating flexible protruding surface can generate a greater turbulence level. In the current study, a circular finite cylinder (cantilever) was used as the geometry of the rigid and protruding surface. Both the material and the aspect ratio were varied. Also, a local Reynolds number within the subcritical flow range (102 < ReD < 105) was considered. The results from the rigid protruding surface (finite cylinder) serve as a validation of the published results and a benchmark for the improvement of the turbulence generated by the flexible protruding surface. The results obtained via an ultrasonic velocity profiler have further demonstrated that the flexible cylinder is capable of generating greater turbulence by examining the turbulence intensity, the turbulence production term and the Reynolds stress. All the flexible cylinders that oscillate show an increase in turbulence production but at different percentages. The cylinders studied in this work ranged from the least structural stiffness (EVA) to moderate (aluminum) and the highest structural stiffness (carbon steel). Through studying the normalized amplitude responses graph for the flexible cylinders, it is found that the oscillating motion does indeed contribute to the increment. A further examination of the results shows that the increase is due to the structural velocity instead of just the oscillating motion. Keywords: flexible circular cylinder; Reynolds stress; turbulence production; ultrasonic velocity profiler; wakes