Power loss minimization for drag reduction and self propulsion using surface mass transpiration
Speaker 
Pritam Giri, Department of Mechanical Engineering, Indian Institute of Science, Bangalore


When 
Mar 31, 2016
from 02:00 PM to 03:00 PM 
Where  LH 006 
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Abstract: The remarkable control authority of surface mass transpiration (blowing and suction) in altering a given base flow to achieve a desired predefined objective has motivated several investigations on drag reduction, selfpropulsion and suppression of separation and wake unsteadiness in bluff body flows. However, the energetic efficiency, a critical parameter that determines the actual efficacy and in particular practical feasibility of this control strategy, has received significantly less attention. In this work, we aim to determine optimal zero net mass transpiration blowing and suction profiles that minimize net power consumption while reducing drag or enabling selfpropulsion in typical bluff body flows. We begin by establishing the influence of prescribed blowing and suction profiles on the hydrodynamic loads and net power consumption for a representative bluff body flow involving flow past a stationary twodimensional circular cylinder. Using analysis based on Oseens equations we find that all the symmetric modes, except for the first one, lead to an increase in the net power consumption without affecting hydrodynamic drag. The optimal blowing and suction profile that achieves minimum power consumption is such that the normal stress acting on the cylinder surface vanishes identically. Furthermore, we show that a selfpropelling state corresponding to zero net drag force is obtained when the first mode of blowing and suction profile is such that the flow field becomes irrotational. Based on these findings we employ direct numerical simulation tools to decipher the Reynolds number dependence of the optimal profiles and the associated power consumption for both drag reduction and selfpropulsion. For a typical Reynolds number, the mean drag coefficient first decreases due to vortex shedding suppression, then increases and eventually decreases again after attaining a local maximum as the strength of the first mode is increased. The net power consumption continues to decrease with the increase in strength before reaching a minima after which it rises continuously. For a Reynolds number of 1000 over fifteen fold reduction in drag is achieved for an optimal blowing and suction profile with a maximum radial surface velocity that is nearly 1.97 times the freestream velocity.
Next, to establish whether or not higher modes play a role in decreasing net power consumption at finite Reynolds number we perform theoretical analysis of a configuration similar to the one described above for a spherical body. At zero Reynolds number, as a result of mode independence, we find that surface blowing and suction of any form that involves second or higher order axisymmetric or nonaxisymmetric modes does not contribute to drag and only leads to an increase in total power consumption. However, at finite Reynolds number, using analysis based on Oseens equations, we find that the second and higher modes contribute substantially to the optimal profiles. Finally to understand the effects of a change in shape we consider generalization of the above analysis to asymmetric selfpropelling prolate and oblate spheroidal bodies. We find that for a general axisymmetric body with nonconstant curvature the optimal drag reducing and selfpropelling blowing and suction profiles for minimum power consumption contain second and higherorder modes along with the first mode even when the Reynolds number is zero. The net decrease in power consumption with the use of second and higher order modes exceeds 33% for a disklike high aspect ratio selfpropelling oblate spheroid. Moreover, we perform comparisons between blowing and suction and surface treadmilling based boundary deformation propulsion mechanisms. Below an aspect ratio of 0.56 we find blowing and suction mechanism to be more ef ficient for selfpropulsion of an oblate spheroid. In contrast, for a selfpropelling prolate spheroidal microswimmer, we show that the tangential surface treadmilling consumes less power irrespective of the aspect ratio.