Sukruth Satheesh’s research while affiliated with Universidad Rovira i Virgili and other places

Flat plate models of fixed aspect ratio incorporating either serrations or holes at the upper and lower plate edges, were towed normal to the flow and in proximity to the free surface. The hydrodynamics and the drag generated by the plates has been studied in detail for several configurations. In general, the models with holes presented greater drag reduction than those with serrations, with the hole angle playing an important role on the total drag measured. The fluid dynamic sources for drag reduction at low submergence depths have been investigated using quantitative flow visualization, and they have been related to the changes in the separating shear layers, as a result of the different edge features imposed to the plates.

Experiments were conducted on 2 splitter plates of different structural rigidities, attached to a rigid cylinder and exposed to cross-flow at different submergence depths in order to study the influence of free surface. It was found that the presence of flexible splitter plate results in drag reduction at all submergence depths when compared to a plain cylinder, with the reduction being dependent on the rigidity of the splitter plate. The lift force was also found to increase abruptly at low depths for the cases involving splitter plates. The results from the analyses of tip kinematics on flexible splitter plate indicated that it performs vertical oscillations symmetrically about its mounting position at high submergence depths, but is displaced in one direction up to an order of cylinder diameter and undergoes minimal oscillations when located near the free surface.

Rectangular foils of different flexural rigidities were towed normal to the flow at a fixed speed in a quiescent fluid in order to study the effect of the proximity of the upper edge of the models to free surface. It was found that flexibility ensured drag reduction due to the reconfiguration process at all submergence depths, with certain foils exhibiting depth-independent behavior. The study of Vogel exponents also showed that a sublinear or even a depth independent relationship between drag and velocity can be attained at specific flexural rigidity values. A modified classical beam theory model using a power-law based load distribution was utilized to obtain an empirical relationship between the loading exponent and Cauchy numbers and to identify the foil tip location. Particle image velocimetry was also undertaken to interpret and further understand the force results. The experiments showed the existence of 2 ranges of Cauchy numbers with a small degree of overlap in ranges wherein the drag coefficient and Vogel exponents are independent of submergence depth.

Flat rectangular plates of different aspect ratios were towed normal to the flow at various speeds in a quiescent fluid, with the focus being the study of the effect of the plate edge proximity to the free surface on drag force. The submergence depth was measured from the plate upper edge to the free surface and varied from zero to the centre of the tank. 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. Flow visualization was also employed on specific plates and at certain depths to understand the flow features and link them to the observed force trends.

A series of experiments were carried out with a flat plate towed normal to the flow in quiescent fluid. The focus was given to the analysis of the drag force seen by the plate as a function of its aspect ratio and hydraulic diameter. The effect of towing the plate near the water free surface was also investigated thoroughly. Plates of aspect ratio ranging from 0.25 to 4 were towed in a still water tank at different Reynolds numbers in the range from 15000 to 60000. Submergence depth was measured from the upper edge to the free surface and varied from zero to the centre of the tank. Forces on the plates were measured using a submersible bending beam load cell and the carriage motion was monitored by a rotary potentiometer. It was found that the drag increases abruptly prior subsiding with increasing submergence depth, with this effect being more dominant in lower aspect ratio plates. The abrupt rise in the drag is due to the interaction of the upper edge of the plate with the free surface resulting in a large shrinkage of the recirculation zone. The non-unit low aspect ratio plates also showed another drag peak around 50% depth, especially at lower speeds. Overall, the trends were Reynolds number independent, except when the aspect ratios was in the range from 0.75 to 1.33 and the plate was near the free surface.