Seo Yoon Jung

Turbulence and skin friction modification in channel flow with streamwise-aligned superhydrophobic surface texture

Direct numerical simulations of turbulent flow in a channel with superhydrophobic surfaces (SHS) were performed, and the effects of the surface texture on the turbulence and skin-friction coefficient were examined. The SHS is modeled as a planar boundary comprised of spanwise-alternating regions of no-slip and free-slip boundary conditions. Relative to the reference no-slip channel flow at the same bulk Reynolds number, the overall mean skin-friction coefficient is reduced by 21.6%. A detailed analysis of the turbulence kinetic energy budget demonstrates a reduction in production over the no-slip phases, which is explained by aid of quadrant analysis of the Reynolds shear stresses and statistical analysis of the turbulence structures. The results demonstrate a significant reduction in the strength of streamwise vortical structures in the presence of the SHS texture and a decrease in the Reynolds shear-stress component ⟨R12⟩ which has a favorable influence on drag over the no-slip phases. A secondary flow ...

Characterization of the three-dimensional turbulent boundary layer in a concentric annulus with a rotating inner cylinder

Direct numerical simulations of fluid flow through a concentric annulus with a rotating inner wall were performed at ReDh=8900. To elucidate the modifications of the near-wall turbulent structure induced by rotation of the inner wall, we compared data obtained at rotation rates of N=0.0 and 0.429 for a system with a radius ratio (R*) of 0.5. Conditional quadrant/octant analysis and probability density functions of the velocity fluctuations revealed distinctive features of the three-dimensional turbulent boundary layer (3DTBL) in the concentric annulus with a rotating inner wall. Coherent structures near the inner wall were identified by a λ2-based eduction scheme to give the detailed information on the activated near-wall turbulent structures. The ensemble-averaging of the educed coherent vortices showed that enhanced ejections near the vortices were primarily responsible for the augmented turbulent structures. The alteration of the turbulent structures was attributed to the centrifugal force arising from rotation of the inner wall. The assumption of Littell and Eaton on the cause of the altered turbulent structures in 3DTBLs was invalid in the present study. Taken together, the present results showed that the 3DTBL in a rotating concentric annulus has features different from those observed in other types of 3DTBL due to the transverse curvature.Direct numerical simulations of fluid flow through a concentric annulus with a rotating inner wall were performed at ReDh=8900. To elucidate the modifications of the near-wall turbulent structure induced by rotation of the inner wall, we compared data obtained at rotation rates of N=0.0 and 0.429 for a system with a radius ratio (R*) of 0.5. Conditional quadrant/octant analysis and probability density functions of the velocity fluctuations revealed distinctive features of the three-dimensional turbulent boundary layer (3DTBL) in the concentric annulus with a rotating inner wall. Coherent structures near the inner wall were identified by a λ2-based eduction scheme to give the detailed information on the activated near-wall turbulent structures. The ensemble-averaging of the educed coherent vortices showed that enhanced ejections near the vortices were primarily responsible for the augmented turbulent structures. The alteration of the turbulent structures was attributed to the centrifugal force arising from...

Large-eddy simulation of turbulent mixed convection in a vertical annulus with a rotating inner cylinder

Large-eddy simulations of mixed convection in an upward turbulent flow inside a vertical annular pipe with a rotating inner cylinder were performed at Re Dh = 8900 and Pr = 0.71. The principal objective of this study was to investigate the combined effects of buoyancy and inner wall rotation on the turbulent structures. The incompressible filtered Navier–Stokes equations and energy equations were solved using a mixed fourth and second-order accurate finite difference method. Isoflux wall boundary conditions with low and high heating (buoyancy numbers, Bo = 0.18 and 0.5) were imposed. Two rotation rates (N = 0.1716 and 0.2574) were used and the radius ratio (R*) was 0.5. Various turbulent statistics were obtained to analyze the near-wall turbulent structures. The results show that strong heating of the system with a stationary inner wall causes distortions of the flow structure resulting in reduced turbulent intensities, shear stress, and turbulent heat flux, particularly near the wall. In contrast, rotati...