Graduate Seminar in Fluid Mechanics
4:10 pm Presentation
“Transition Delay in High-Speed Boundary Layers by Thermal Texture”
Presented by REZA JAHANBAKHSHI (Adviser: Prof. Zaki)
Delay of transition to turbulence in the boundary layer that forms on hypervelocity vehicles remains one of the key difficulties in achieving reliable long-range manned hypersonic flight. Since the environmental conditions are often uncertain, the main challenge is how we can guarantee robust flow design in such applications. Previous attempts to attenuate the flow disturbances and sustain the laminar state in the boundary layer have been ad hoc and their robustness unconfirmed. In the present effort, the surface heat-flux in a transitional zero-pressure-gradient high-speed boundary layer is optimized using an ensemble-variational (EnVar) algorithm in order to guarantee transition delay, when the environmental disturbance is the most dangerous condition from the standpoint of transition. The control vector is parametrized using a two-dimensional basis function on the wall which allows to target specific parts of the flow-field upstream of the fully turbulent region. The EnVar algorithm is able to identify regions of the flow that are most sensitive to thermal treatment and apply most of the heating and cooling to these regions. This leads to appreciable power saving as a result of reducing the drag forces associated with turbulent region. The results indicate that in order to attenuate instability waves using heating and cooling, the thermal roughness must be placed upstream of the synchronization point of the slow and the fast modes. On the other hand, the first-mode instability wave and the second-mode instability whose synchronization point is downstream of the thermal roughness, are minimally affected in our computational setup.
4:35 pm Presentation
“Experimental Evidence of Amplitude Modulation in Permeable-Wall Turbulence”
Presented by TAEHOON KIM (Adviser: Prof. Ni)
The dynamic interplay between surface and subsurface flow in the presence of a permeable boundary layer was investigated using low and high frame-rate particle-image velocimetry (PIV) measurements in a refractive-index matching environment. Two idealized permeable wall models were considered. Both models contained five layers of cubically-packed spheres, but one exhibited a smooth interface with the flow, while the other embodied a hemispherical surface topography. The relationship between the large-scale and within the walls was explored using instantaneous and statistical analyses. Although previous studies have indirectly identified the potential existence of amplitude modulation in permeable-wall turbulence (a phenomenon previously reported in impermeable-wall turbulence), the present effort provides direct evidence of its existence in flow over both permeable walls. The spatio-temporal signatures of amplitude modulation were also characterized using conditional averaging based on zero-crossing events. This analysis highlights the connection between large-scale regions of high and low streamwise momentum in the surface flow, downwelling/upwelling across the permeable interface and enhancement/suppression of small-scale turbulence, respectively, just above and within the permeable walls.