Bulletin of the American Physical Society
67th Annual Meeting of the APS Division of Fluid Dynamics
Volume 59, Number 20
Sunday–Tuesday, November 23–25, 2014; San Francisco, California
Session G27: Turbulent Boundary Layers IV |
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Chair: Stefano Leonardi, University of Texas at Dallas Room: 2009 |
Monday, November 24, 2014 8:00AM - 8:13AM |
G27.00001: Unsteady boundary layer detachment in planar flows at large Reynolds number Romain Nguyen van yen, Marie Farge, Matthias Waidmann, Rupert Klein, Kai Schneider We study a vortex dipole impinging onto a wall with two different models: Navier-Stokes equations and Euler's equation coupled with Prandtl's boundary layer equation. The solutions in the limit of large Reynolds number $Re$ are computed by DNS performed using a high-order compact finite differences scheme with no-slip boundary conditions. For both models we first observe the formation on the wall of two opposite-sign boundary layers whose thickness scales in $Re^{-1/2}$, as predicted by Prandtl in 1904. At a later time $t_D$ the solution of the Navier-Stokes equation shows that the boundary layers suddenly collapse down to thickness as fine as $Re^{-1}$, as predicted by Kato in 1984, then detach from the wall and roll up into strongly dissipative new dipoles that are ejected away from the wall into the bulk flow. In contrast, at the same time $t_D$ Prandtl's solution becomes singular and the boundary layers can no more be computed, while Euler's solution gives two opposite sign-vortices that slip along the wall in opposite directions without detaching. [Preview Abstract] |
Monday, November 24, 2014 8:13AM - 8:26AM |
G27.00002: Direct Numerical Simulation of an Adverse Pressure Gradient Turbulent Boundary Layer at the Verge of Separation Vassili Kitsios, Callum Atkinson, Juan Sillero, Borrell Guillem, Ayse Gungor, Javier Jimen\'ez, Julio Soria We investigate the structure of an adverse pressure gradient (APG) turbulent boundary layer (TBL) at the verge of separation. The intended flow is generated via direct numerical simulation (DNS). The adopted DNS code was previously developed for a zero pressure gradient TBL. Here the farfield boundary condition (BC) is modified to generate the desired APG flow. The input parameters required for the APG BC are initially estimated from a series of Reynolds Averaged Navier-Stokes simulations. The BC is implemented into the DNS code with further refinement of the BC performed. The behaviour of the large scale dynamics is illustrated via the extraction of coherent structures from the DNS using analysis of the velocity gradient tensor and vortex clustering techniques. [Preview Abstract] |
Monday, November 24, 2014 8:26AM - 8:39AM |
G27.00003: Evolution of the Reynolds shear stresses in highly accelerated turbulent boundary layers Guillermo Araya, Luciano Castillo, Fazle Hussain Turbulent boundary layers subjected to severe acceleration or strong Favorable Pressure Gradients (FPG) are of great fundamental and technological importance; examples of the latter include nozzle design, underwater bodies and drag reduction applications. Scientifically, they pose great interest from the point of view of scaling laws, the complex interaction between the outer and inner regions, and relaminarization phenomena. Direct Numerical Simulations (DNS) of highly accelerated turbulent boundary layers are performed by means of the Dynamic Multi-scale Approach (DMA) recently developed by [Araya et al. JFM, vol. 670, pp. 581-605, 2011]. It is shown that the Reynolds shear stress monotonically decreases and exhibits a logarithmic layer in the meso-layer region during the laminarization process. In addition, the local maxima of streamwise velocity fluctuations in wall units remain almost constant in the very strong FPG region, which prevents the flow to become completely laminar. Furthermore, the re-distribution of Reynolds shear stresses due to sweeps and ejections in the FPG region is performed and a physical mechanism is proposed. [Preview Abstract] |
Monday, November 24, 2014 8:39AM - 8:52AM |
G27.00004: Structure of relaminarizing turbulent boundary layers O. Ramesh, Saurabh Patwardhan Relaminarization of a turbulent boundary layer in a strongly accelerated flow has received a great attention in recent times. It has been found that such relaminarization is a general and regularly occurring phenomenon in the leading-edge region of a swept wing of an airplane (van Dam et. al., 1993). In this work, we investigate the effect of initial Reynolds number on the process of relaminarization in turbulent boundary layers. The experimental and numerical investigation of relaminarizing turbulent boundary layers undergoing same history reveals that the boundary layer with higher initial Reynolds number relaminarizes at a lower pressure gradient value compared to the one with lower Reynolds number. This effect can be explained on the inviscid theory proposed earlier in the literature. Further, various parameter criteria proposed to predict relaminarization, are assessed and the structure of relaminarizing boundary layers is investigated. A mechanism for stabilization of near-wall low speed streaks is proposed. [Preview Abstract] |
Monday, November 24, 2014 8:52AM - 9:05AM |
G27.00005: Pressure measurements in a rapidly sheared turbulent wall layer Sourabh Diwan, Jonathan Morrison The aim of the present work is to improve understanding of the role of pressure fluctuations in the generation of coherent structures in wall-bounded turbulent flows, with particular regard to the rapid and slow source terms. The work is in part motivated by the recent numerical simulations of Sharma et al. (Phy. Fluids, 23, 2011), which showed the importance of pressure fluctuations (and their spatial gradients) in the dynamics of large-scale turbulent motions. Our experimental design consists of first generating a shearless boundary layer in a wind tunnel by passing a grid-generated turbulent flow over a moving floor whose speed is matched to the freestream velocity, and then shearing it rapidly by passing it over a stationary floor further downstream. Close to the leading edge of the stationary floor, the resulting flow is expected to satisfy the approximations of the Rapid Distortion Theory and therefore would be an ideal candidate for studying linear processes in wall turbulence. We carry out pressure measurements on the wall as well as within the flow -- the former using surface mounted pressure transducers and the latter using a static pressure probe similar in design to that used by Tsuji et al. (J. Fluid. Mech. 585, 2007). We also present a comparison between the rapidly sheared flow and a more conventional boundary layer subjected to a turbulent free stream. [Preview Abstract] |
Monday, November 24, 2014 9:05AM - 9:18AM |
G27.00006: The effects of localized blowing on pressure-velocity correlation Can Liu, Guillermo Araya, Luciano Castillo, Stefano Leonardi It is well known that wall pressure fluctuations are footprints of the large coherent motions existent in the outer region of the boundary layer. In this investigation, spatial-temporal correlations of the pressure and velocity fields are computed in a spatially-developing turbulent channel flow with five-blowing jets located at the bottom wall and along the spanwise direction. Direct numerical simulations are performed at a friction Reynolds number of 394. The main purpose behind the present study is to assess the influence of perturbing blowing jets on the large scale structures of the turbulent channel flow. Furthermore, the key role of pressure fluctuations on the energy redistribution among the velocity components is scrutinized by computing the energy budgets of tke and Reynolds stresses, and a physical mechanism is proposed to explain the outer peak on turbulence production due to localized blowing. [Preview Abstract] |
Monday, November 24, 2014 9:18AM - 9:31AM |
G27.00007: Quantifying Hairpin Vortex Generation Rijan Maharjan, Daniel Sabatino Hairpin vortices are artificially generated via fluid injection through a streamwise oriented slot into an otherwise laminar boundary layer in a free-surface water channel. Injection through the 32:1 aspect ratio slot is intended to approximate the behavior of a low speed streak along with its neighboring streamwise vortices that spawn naturally occurring hairpins in fully turbulent boundary layers. A parametric study is performed by varying the slot streamwise location, the average injection flow rate and injection duration. Hairpins are examined for boundary layer conditions between $485 < Re_{\delta^*}< 600$ and blowing ratios up to 0.2. Cross-stream 2D-PIV is primarily used to characterize the injection profile and the strength of the initial streamwise vortices as well as establish the strength and structure of the resulting hairpin for each condition. The role of the streamwise vorticity in the generation of the hairpin is examined. Threshold conditions which will yield hairpins that have sufficient strength to autogenerate secondary hairpins are also considered. [Preview Abstract] |
Monday, November 24, 2014 9:31AM - 9:44AM |
G27.00008: Evolution of Lagrangian structures in the K-type temporal transition in channel flow Yaomin Zhao, Yue Yang, Shiyi Chen We report a Lagrangian study on the evolution of hairpin vortices in the K-type temporal transition in a channel flow. Based on the Eulerian velocity field from the direct numerical simulation, a backward-particle-tracking method is used to solve the Lagrangian scalar transport equation, and Lagrangian material surfaces are extracted as isosurfaces of the Lagrangian scalar. As an approximation of the Helmholtz vorticity theorem, a Lagrangian surface, which is initially a vortex surface, can be approximately as a vortex surface before significant vortex reconnections in a time evolution (Yang and Pullin, J. Fluid. Mech., 2010). Thus, by tracking the evolution of Lagrangian material surfaces in the early transitional phase, the dynamics of hairpin vortices can be studied in a Lagrangian framework. In the present study, the Lagrangian surface evolves from a streamwise-spanwise vortex sheet to a $\Lambda$-shaped bulge, and then rolls up into a hairpin-shaped structure. The dynamical evolution of the Lagrangian hairpin vortex is analysed in consecutive times. With the comparison of the coherent structures identified by the Eulerian criteria (e.g., `$\lambda_2$-criterion'), differences between Lagrangian and Eulerian structures are discussed. [Preview Abstract] |
Monday, November 24, 2014 9:44AM - 9:57AM |
G27.00009: DNS of the flow around a wall-mounted square cylinder under various inflow conditions Ricardo Vinuesa, Philipp Schlatter, Johan Malm, Dan S. Henningson, Catherine Mavriplis The flow around a wall-mounted square cylinder is investigated by means of DNS. The effect of inflow conditions is assessed by considering two different cases with matching $Re_{\theta} \simeq 1000$ at the obstacle: the first case is a fully-turbulent zero pressure gradient boundary layer, and the second one is a laminar boundary layer with prescribed Blasius inflow profile. An auxiliary simulation carried out with the pseudo-spectral code SIMSON is used to obtain time-dependent inflow conditions which are then fed into the main simulation where the actual flow around the cylinder is computed. This main simulation is performed, for both laminar and turbulent inflows, with the spectral element code Nek5000. Transition to turbulence is observed in the laminar case, induced by the recirculation bubble produced at the obstacle. In both cases we find the same Strouhal number $St=0.1$, in good agreement with available experimental measurements, although the two wakes exhibit structural differences associated with turbulent wall-normal transport and spanwise fluctuations. In the turbulent case the streamwise fluctuations modulate the horseshoe vortex formed around the cylinder. Additional insight on the differences between both wakes is achieved by means of a POD study of the flow. [Preview Abstract] |
Monday, November 24, 2014 9:57AM - 10:10AM |
G27.00010: Turbulent structures in a wall jet Shibani Bhatt, Sravan Artham, Reda Mankbadi, Ebenezer Gnanamanickam Wall Jets are special shear layers, in that they have two shear layers which arise from two different instability mechanisms, to form a single turbulent layer. The three-dimensional wall jet in addition has an inherent secondary flow which adds to the complexity of the flow physics. These jets finds wide use in heat transfer applications such as film cooling. However, the wall jet has received little attention when compared to the turbulent boundary layer, fully developed channel flow or a free jet. As part of this study hot-wire measurements were carried out in a three-dimensional wall jet. The eventual goal of this work is to study inner-outer interactions using the wall jet as the two shear layers affords the possibility of independent control. Velocity statistics as well as spectra derived from velocity measurements are presented. The wall jet is shown to have structures of longer wavelength in the outer free-jet region which arise from the inviscid instability of the free-jet layer. The near wall region of the wall jet, which arises from a viscous instability, is populated with finer structures similar to that seen in a turbulent boundary layer. The implications of these observations towards studying inner-outer interactions is also discussed. [Preview Abstract] |
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