Hydrodynamics of flat plates in cross-flow near the free surface

Resumen

Although water-based transportation has played a very important role in the development of human civilization, their designs are not as good as that observed in nature. This is mainly due to our lack of understanding of the physics involved. The effect of free surface on the forces generated being one such phenomenon. It is expected that only through a better understanding of the fluidstructure interaction involving the free surface can help us achieve not only huge improvements in current marine structural and naval designs through drag reduction but also develop newer modes of underwater propulsors for Autonomous Underwater Vehicles (AUV), and devices for harvesting of energy from renewable sources, like tides and surface waves. Owing to the fact that limited amount of studies has been conducted pertaining to the effect of free surface and on bluff bodies, this work focuses on understanding its effect on normal rigid plates experimentally. The shape/orientation of the plates, and the effect of Reynolds number has been looked at in order to understand its behavior, over a range of submergence depths measured from the free surface. Qualitative as well as quantitative flow visualization techniques have been used to analyze the wake structure and correlate the forces with flow. It was found that the drag increases abruptly prior subsiding with increasing submergence depth, with this jump in drag being more prominent in low aspect ratio plates. The abrupt rise in the drag is due to the existence of a gap-flow at the free surface resulting in the formation of a recirculating flow in close proximity to the base region of plate. Overall, the trends are Reynolds number independent, except when the aspect ratios are in the range from 0.75 to 1.33, and the plate was near the free surface. This has been followed up with experiments involving strategically located pores to understand their effect on drag, and finally, experiments involving flexible structures have been pursued to characterize the effect of bending rigidity on drag, all within the realm of understanding the effect of free surface on drag force. The flexible models showed the existence of a critical Cauchy number post which drag was depth independent, a large range of Vogel exponents, and another critical Cauchy number which rendered Vogel exponents submergence depth independent. Meanwhile, the porous models presented trends in drag similar to rigid plates but confirmed that the location of the holes as well as their angle of orientation plays a major role in drag reduction, besides the magnitude of porosity. Both concepts have resulted in significant drag reduction, especially near the free surface. The mechanism of drag reduction for porous models has been shown to be due to the interaction between the jets formed at the holes, the shear layers and the gap-flow. With structural flexibility, reconfiguration leads to drag reduction.