03.3 - Basic Fluid Dynamics and Flow Control
EFFECTS OF ADVERSE PRESSURE GRADIENT ON THE BEHAVIOURS OF RESOLVENT MODE
C. Chen, China; J.-T. Huang, China; X. Chen, China
With the purpose of investigating the effect mechanism of adverse pressure gradient (APG) on the coherent structure in turbulent boundary layer (TBL), we have used the model-based approach ‘resolvent analysis’, proposed by Professor McKeon at Caltech, to obtain the resolvent mode for pure turbulent Couette flow corresponding to zero pressure gradient (case C) and turbulent Couette-Poiseuille flow with zero mean shear at the moving wall (case SL), which is somewhat similar to the case of APG TBL at the verge of separation. The results of resolvent analysis show that for the near-wall mode (?x+=700, ?z+=100, c=10.35), the APG applied for case SL leads to a small decrease in singular value (mode amplification rate). Specifically, this APG yields a 6% decrease in gain (i.e. the ratio of SL to C singular value is ?k,SL/?k,C = 0.94). Under the premise that the two cases have near-identical forcing strengths for this mode as a result of almost the same mean shear near the stationary wall, which plays a primary role in the mechanisms of turbulence production, this observation is consistent with the statement on inner/outer layer interactions in turbulent CP flows by Pirozzoli et al. (2011), who found that the coherent streaky structure near the stationary wall is suppressed in terms of velocity fluctuations in the cross-stream components and concluded that the imprinting and amplitude modulation imparted by large-scale events in the channel core onto the near-wall motions increase gradually from flow case P (turbulent Poiseuille flow) to case SL to case C. Further, the symmetry of the distribution of streamwise energy density is broken by APG in the (y,?x) and (y,?z) plane and the energetic structures seem to be ‘squeezed’ towards the stationary wall by APG.