Showing papers by "Parviz Moin published in 2013"

Direct numerical simulation of complete H-type and K-type transitions with implications for the dynamics of turbulent boundary layers

Abstract: The onset and development of turbulence from controlled disturbances in compressible ( ), flat-plate boundary layers is studied by direct numerical simulation. We have validated the initial disturbance development, confirmed that H- and K-regime transitions were reproduced and, from these starting points, we carried these simulations beyond breakdown, past the skin-friction maximum and to higher Reynolds numbers than investigated before to evaluate how these two flow regimes converge towards turbulence and what transitional flow structures embody the statistics and mean dynamics of developed turbulence. We show that H- and K-type breakdowns both relax toward the same statistical structure typical of developed turbulence at high Reynolds number immediately after the skin-friction maximum. This threshold marks the onset of self-sustaining mechanisms of near-wall turbulence. At this point, computed power spectra exhibit a decade of Kolmogorov inertial subrange; this is further evidence of convergence to equilibrium turbulence at the late stage of transition. Here, visualization of the instantaneous flow structure shows numerous, tightly packed hairpin vortices (Adrian, Phys. Fluids, vol. 19, 2007, 041301). Strongly organized coherent hairpin structures are less perceptible farther downstream (at higher Reynolds numbers), but the flow statistics and near-wall dynamics are the same. These structurally simple hairpin-packet solutions found in the very late stages of H- and K-type transitions obey the statistical measurements of higher-Reynolds-number turbulence. Comparison with the bypass transition of Wu & Moin (Phys. Fluids, vol. 22, 2010, pp. 85–105) extends these observations to a wider class of transitional flows. In contrast to bypass transition, the (time- and spanwise-averaged) skin-friction maximum in both H- and K-type transitions overshoots the turbulent correlation. Downstream of these friction maxima, all three skin-friction profiles collapse when plotted versus the momentum-thickness Reynolds number, . Mean velocities, turbulence intensities and integral parameters collapse generally beyond in each transition scenario. Skin-friction maxima, organized hairpin vortices and the onset of self-sustaining turbulence found in controlled H- and K-type transitions are, in many dynamically important respects, similar to development of turbulent spots seen by Park et al. (Phys. Fluids, vol. 24, 2012, 035105). A detailed statistical comparison demonstrates that each of these different transition scenarios evolve into a unique force balance characteristic of higher-Reynolds-number turbulence (Klewicki, Ebner & Wu, J. Fluid Mech., vol. 682, 2011, pp. 617–651). We postulate that these dynamics of late-stage transition as manifested by hairpin packets can serve as a reduced-order model of high-Reynolds-number turbulent boundary layers.

On the Use of the Ffowcs Williams-Hawkings Equation to Predict Far-Field Jet Noise from Large-Eddy Simulations

Abstract: This study presents best practices for the use of the permeable-surface Ffowcs Williams-Hawkings equations to calculate far-field sound from large-eddy simulations of high-speed turbulent jets. A parametric study of the Ffowcs Williams-Hawkings equations is performed by post-processing existing large-eddy simulations at different operating conditions gathering subsonic, supersonic, cold, isothermal, and heated jets. It is concluded that using the pressure formulations of the Ffowcs Williams-Hawkings equations yields better results than the density formulation, especially for the heated jet. In terms of surface closure, best results are obtained with closed surfaces, in conjunction with outflow disk averaging, which confirms the results obtained by Spalart and Shur in 2009. This is different from most previous studies, which recommend using open surfaces. In addition, detailed implementation information and quantified technical recommendations are presented as a guideline for post-processing large-eddy sim...

Large eddy simulation of high-lift devices

Abstract: Accurate predictions of flow around high lift devices is of interest for airframe noise predictions as well as wing design with or without active flow control. In particular, prediction of the maximum lift coefficient remains challenging using RANS solvers. We discuss how large eddy simulation can be used in this context, together with wall-modeling to reach flight Reynolds numbers. The study is focused on the flow around the McDonnellDouglas 30P/30N airfoil at several angles of attack and at Reynolds number (based on the stowed chord) of Rec = 9 · 10.

Application of vortex identification schemes to direct numerical simulation data of a transitional boundary layer

Abstract: We have demonstrated how various vortex identification and visualization criteria perform using direct numerical simulation data from a transitional and turbulent boundary layer by Sayadi, Hamman, and Moin [“Direct numerical simulation of complete transition to turbulence via h-type and k-type secondary instabilities,” Technical Report, Stanford University, CTR Annual Research Briefs, 2011]. The presence of well-known Λ vortices in the transitional region provides a well defined and yet realistic benchmark for evaluation of various criteria. We investigate the impact of changing the threshold used for iso-surface plotting.

An improved dynamic non-equilibrium wall-model for large eddy simulation

Abstract: A non-equilibrium wall-model based on unsteady 3D Reynolds-averaged Navier-Stokes (RANS) equations has been implemented in an unstructured mesh environment. The method is similar to that of the wall-model for structured mesh described by Wang and Moin [Phys. Fluids 14, 2043–2051 (2002)], but is supplemented by a new dynamic eddy viscosity/conductivity model that corrects the effect of the resolved Reynolds stress (resolved turbulent heat flux) on the skin friction (wall heat flux). This correction is crucial in predicting the correct level of the skin friction. Unlike earlier models, this eddy viscosity/conductivity model does not have a stress-matching procedure or a tunable free parameter, and it shows consistent performance over a wide range of Reynolds numbers. The wall-model is validated against canonical (attached) transitional and fully turbulent flows at moderate to very high Reynolds numbers: a turbulent channel flow at Reτ = 2000, an H-type transitional boundary layer up to Reθ = 3300, and a hig...

Backscatter of turbulent kinetic energy in chemically-reacting compressible flows

Abstract: This study addresses the dynamics of kinetic-energy backscatter in the context of large-eddy simulations (LES) of turbulent chemically-reacting compressible flows. As a case study, a-priori analyses of direct numerical simulations (DNS) of reacting and inert supersonic, time-developing, hydrogen-air turbulent mixing layers with complex chemistry and multi-component diffusion are conducted herein to examine the effects of compressibility and combustion on subgrid-scale (SGS) backscatter of kinetic energy. General formulations of SGS backscatter with dilatation are provided, including an LESbased energy-transfer diagram that illustrates the conversion dynamics. Lastly, influences of SGS backscatter on the Boussinesq eddy viscosity are analyzed. 2. Background The energy-cascade hypothesis predicts that turbulent kinetic energy is generated at the largest scales in a flow and then transferred to progressively smaller and smaller scales until it is dissipated by molecular viscosity. This energy cascade typically holds in a statistically-averaged sense, but it does not always describe the local behavior of a turbulent flow. The turbulent dissipation associated with the smallest, viscous scales, is actually the difference between two energy fluxes, namely, the forwardscatter, corresponding to the classical energy cascade, and the backscatter, a reversal of this process in which energy is transferred from the small scales back to the large scales (Lesieur & �

Preliminary visualization study on the flat-plate boundary layer with continuous freestream turbulence

Abstract: We report our most recent visualization study using very-large-scale direct numerical simulations (VLS DNS) with over one billion mesh points on a set of three zero pressure-gradient, smooth, incompressible, flat-plate boundary layers (ZPGFPBL). The boundary layers are beneath a continuous flow of freestream grid turbulence whose inlet strength is varied systematically from 3% (weak), 1.3% (very- weak) to 0.01% (infinitesimal) of the mean velocity. The momentum-thickness Reynolds number develops continuously starting from 80 (an exact laminar Blasius layer) at the inlet of the computational domain; and it grows to 2000 (fully turbulent) in the 3% weak perturbation case. The very-weak disturbance layer is still transitional by the exit of the computational domain, whereas the infinitesimally disturbed boundary layer remains essentially Blasius throughout. The companion thermal boundary layer is also simulated at unit molecular Prandtl number for all the three cases. The visualization results demonstrate that certain subcategory of bypass transition might be loosely considered as the secondary instability of natural transition with merely the Tollmien- Schlichting wave being circumvented.