Bulletin of the American Physical Society
71st Annual Meeting of the APS Division of Fluid Dynamics
Volume 63, Number 13
Sunday–Tuesday, November 18–20, 2018; Atlanta, Georgia
Session A21: Boundary Layers: High Speed/ Compressible |
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Chair: Guillermo (Juan) Araya, University of Puerto Rico at Mayaguez Room: Georgia World Congress Center B309 |
Sunday, November 18, 2018 8:00AM - 8:13AM |
A21.00001: A joint experimental-numerical study of turbulent structures in supersonic spatially-developing turbulent boundary layers Miguel A. Ramirez, Ezequiel F. Medici, Guillermo Araya The evolution of spatially-developing turbulent boundary layers (SDTBL) is studied experimentally and numerically at a Mach number of 1.25. Experiments are performed on SDTBL over a zero-pressure gradient isothermal flat plate, generated by means of a shock tube. The primary experimental observation includes flow visualization using a high speed shadowgraph system at 30,000 frame per second (fps). The shadowgraph image sequences allow us to track and assess the different turbulent structures and events inside the boundary layer by detecting density changes across the field of view. Direct Numerical Simulation (DNS) of compressible SDTBL is also carried out at similar experimental conditions. The obtained high spatial/temporal resolution numerical information is employed for evaluating the wall-normal thermal transport phenomena and the Strong Reynolds Analogy (SRA). Focus is given to the assessment of flow compressibility on the dynamics of turbulent coherent structures. This presentation will emphasize the calibration effort of the numerical model in order to capture the observed flow structure. |
Sunday, November 18, 2018 8:13AM - 8:26AM |
A21.00002: Effect of wall temperature on the growth of Gortler vortices in high-speed boundary layers Adrian Sescu, Safae El Amrani, Mohammed Z Afsar Boundary layer flows over concave surfaces are subject to a centrifugal instability that leads to spatially growing longitudinal vortices known as Gortler vortices. From the practical standpoint, this phenomenon can be encountered, for example, in flows evolving over the concave part of a wing or turbomachinery blade, or on the walls of diverging-converging nozzles such as those utilized in supersonic/hypersonic wind tunnels. Depending on the curvature, the Reynolds number of the flow, and the level of environmental disturbances, these vortices can first lead to secondary instabilities, potential vortex breakdown, and eventual transition to turbulence. Here, we investigate the effect of varying the wall temperature on the development of Gortler vortices in high-speed boundary layer flows (with free-stream Mach number ranging from 1.5 to 7), using direct numerical simulations of the Navier-Stokes equations. The results reveal that the cooling of the wall can reduce the wall skin friction commensurately with the decrease in wall temperature, but at the same time increases the energy of the Gortler vortices. |
Sunday, November 18, 2018 8:26AM - 8:39AM |
A21.00003: Reynolds number dependency in supersonic spatially-developing turbulent boundary layers via DNS Guillermo Araya, Kenneth E Jansen Direct Numerical Simulation (DNS) of compressible spatially-developing turbulent boundary layers (SDTBL) is performed at a Mach number of 2.5 and low/high Reynolds numbers over isothermal Zero-Pressure Gradient (ZPG) flat plates. Turbulent inflow information is generated by a dynamic rescaling-recycling approach (J. Fluid Mech., 670, pp. 581-605, 2011), which avoids the use of empirical correlations in the computation of inlet turbulent scales. The range of the low Reynolds number case is approximately 400-800, based on the momentum thickness, freestream velocity and wall viscosity. DNS at higher Reynolds numbers (about six times larger) is also carried out with the purpose of analyzing the effect of Reynolds number on the thermal transport and on the Morkovin’s Strong Reynolds Analogy (SRA) in the supersonic regime. Additionally, low/high order flow statistics are compared with DNS of an incompressible isothermal ZPG boundary layer at similar low Reynolds numbers and temperature considered as a passive scalar. Focus is given to the assessment of Reynolds number and compressibility effects on the dynamics of turbulent coherent structures. |
Sunday, November 18, 2018 8:39AM - 8:52AM |
A21.00004: Analysis of the equilibrium wall model for wall-bounded high-speed turbulent flows Prahladh S Iyer, Mujeeb R Malik A priori and a posteriori analysis of the equilibrium wall model is performed for wall-bounded high-speed compressible turbulent flows. Available supersonic/hypersonic turbulent channel and boundary layer DNS databases are used for the a priori analysis. Two mixing-length models, and three damping function scalings are considered in this study. Sensitivity of the wall model results to different mixing-length models, damping function scalings, and model constants is assessed. While previous studies indicate that the semi-local scaling for the damping function is most accurate for compressible flows, our results suggest that this is true only for very cold thermal wall boundary conditions. For moderately cold to adiabatic wall conditions, a new mixed scaling appears to work better. Wall-modeled Large Eddy Simulations of a Mach 3 turbulent channel flow with cold wall boundary condition corresponding to the DNS of Coleman, Kim & Moser (JFM 1995), and an axisymmetric Mach 2.85 boundary layer at adiabatic wall conditions interacting with a shock corresponding to the experiments of Dunagan et al. (NASA TM 1986) are consistent with the trends obtained from the a priori analysis of DNS data. |
Sunday, November 18, 2018 8:52AM - 9:05AM |
A21.00005: Wall-Modelled Large-Eddy Simulation of a Mach 7.5 Hypersonic Boundary Layer Romain Buttay, Wan Cheng, Ravi Samtaney Large-Eddy Simulation (LES) has proven to be a computationally valuable tool to simulate unsteady turbulent flows. However, restrictive resolution requirements high-Reynolds number wall-bounded flows necessitate the use of wall models or approximate wall boundary conditions in addition to a subgrid-scale (SGS) model. Inoue and Pullin~(J. Fluid Mech. 2011) developed a virtual wall model for incompressible boundary layer flows. This model dynamically couples the outer resolved region with the wall region, and imposes a slip velocity boundary condition for the filtered velocity field on a ``virtual'' wall. An extension of the previous wall model is presented for LES of compressible turbulent boundary layer flows. The new virtual wall model is combined with the stretched spiral vortex sub-grid scale model in a self-consistent framework. Both models are incorporated in a fourth order finite volume compressible Navier-Stokes solver. A wall-modeled LES (WMLES) of a Mach 7.5 turbulent boundary layer is performed. We present results of mean profiles, turbulence intensity and Reynolds shear stress and compare these with the experimental results of Williams et al. (J. Fluid Mech, 2018). |
Sunday, November 18, 2018 9:05AM - 9:18AM |
A21.00006: Large streamwise-elongated motions in high Re wall-bounded turbulence Myoungkyu Lee, Robert D Moser We revisit the spectral analysis of the turbulent kinetic energy and its transport in high Re wall-bounded turbulent flow. To do so, we introduce a log-polar representation of two-dimensional spectra that clearly visualizes anisotropy and the contributions of very-large-scale motions. This overcomes difficulties in interpreting one-dimensional spectra and log-log representations of two-dimensional spectra. Results of this analysis show that outer-layer turbulence is dominated by modes with wavelengths λ≥1000, while the inner layer is dominated by λ≤1000. Far from the wall, energy and its production are concentrated in streamwise-elongated modes with spanwise wavelengths that grow with wall-normal distance through the log region. In contrast, energy is dissipated isotropically away from the wall, requiring that energy is transferred from the streamwise elongated modes to modes with a range of orientations, as well as from large to small-scales. In one-dimensional spectra, this misleadingly appears as a transfer from small to large scales, but it is not. These dominant streamwise elongated modes also modulate the dynamics of the near-wall turbulence. |
Sunday, November 18, 2018 9:18AM - 9:31AM |
A21.00007: A symmetry approach to quantify wall turbulence: Reynolds stresses Xi Chen, Fazle Hussain, Zhen-Su She We (Chen, Hussain & She, JFM 2018) present new scaling expressions, including high-Reynolds-number (Re) predictions, for all Reynolds stress components in the entire flow domain of turbulent channel and pipe flows. Here, by extending our previous symmetry approach, we apply random dilations on the second-order balance equations for all the Reynolds stresses (-u'v', u'u', v'v', w'w'), and obtain four-layer formulae (similar to that for l12 in She, Chen & Hussain, JFM 2017) of the corresponding stress length functions l11, l22, l33. Direct numerical simulation (DNS) data are shown to agree well with our l12 and l22. However, data show an invariant peak location for w'w', implying an anomalous scaling in l33 in the log layer only. Furthermore, another meso-layer modification of l11 yields the experimentally observed location and magnitude of the outer peak of u'u'. The resulting Reynolds stresses are all in good agreement with DNS and experimental data in the entire flow domain. Our additional results include the location of peak -u'v'p has a scaling transition from Re1/3τ to Re1/2τ at Reτ≈3000; the peak value w'w'+p≈0.84Re0.14τ (1-48/Reτ ); and an alternative derivation of the log law of Townsend, namely, u'u'+≈-1.25lny +1.63 and w'w'+≈-0.41 lny+1.00 in the bulk. |
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