Showing papers by "Parviz Moin published in 2014"

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...

A dynamic slip boundary condition for wall-modeled large-eddy simulation

Abstract: Wall models for large-eddy simulation (LES) are a necessity to remove the prohibitive resolution requirements of near-wall turbulence in high Reynolds turbulent flows. Traditional wall models often rely on assumptions about the local state of the boundary layer (e.g., logarithmic velocity profiles) and require a priori prescription of tunable model coefficients. In the present study, a slip velocity boundary condition for the filtered velocity field is obtained from the derivation of the LES equations using a differential filter. A dynamic procedure for the local slip length is additionally formulated making the slip velocity wall model free of any a priori specified coefficients. The accuracy of the dynamic slip velocity wall model is tested in a series of turbulent channel flows at varying Reynolds numbers and in the LES of a National Advisory Committee for Aeronautics (NACA) 4412 airfoil at near-stall conditions. The wall-modeled simulations are able to accurately predict mean flow characteristics, including the formation of a trailing-edge separation bubble in NACA 4412 configuration. The validation cases demonstrate the effectiveness of this wall-modeling approach in both attached and separated flow regimes.

Reduced-order representation of near-wall structures in the late transitional boundary layer

Abstract: Direct numerical simulations (DNS) of controlled H- and K-type transitions to turbulence in an M=0.2 (where M is the Mach number) nominally zero-pressure-gradient and spatially developing flat-plate boundary layer are considered. Sayadi, Hamman & Moin (J. Fluid Mech., vol. 724, 2013, pp. 480-509) showed that with the start of the transition process, the skin-friction profiles of these controlled transitions diverge abruptly from the laminar value and overshoot the turbulent estimation. The objective of this work is to identify the structures of dynamical importance throughout the transitional region. Dynamic mode decomposition (DMD) (Schmid, J. Fluid Mech., vol. 656, 2010, pp. 5-28) as an optimal phase-averaging process, together with triple decomposition (Reynolds & Hussain, J. Fluid Mech., vol. 54 (02), 1972, pp. 263-288), is employed to assess the contribution of each coherent structure to the total Reynolds shear stress. This analysis shows that low-frequency modes, corresponding to the legs of hairpin vortices, contribute most to the total Reynolds shear stress. The use of composite DMD of the vortical structures together with the skin-friction coefficient allows the assessment of the coupling between near-wall structures captured by the low-frequency modes and their contribution to the total skin-friction coefficient. We are able to show that the low-frequency modes provide an accurate estimate of the skin-friction coefficient through the transition process. This is of interest since large-eddy simulation (LES) of the same configuration fails to provide a good prediction of the rise to this overshoot. The reduced-order representation of the flow is used to compare the LES and the DNS results within this region. Application of this methodology to the LES of the H-type transition illustrates the effect of the grid resolution and the subgrid-scale model on the estimated shear stress of these low-frequency modes. The analysis shows that although the shapes and frequencies of the low-frequency modes are independent of the resolution, the amplitudes are underpredicted in the LES, resulting in underprediction of the Reynolds shear stress.

Subgrid-scale backscatter in reacting and inert supersonic hydrogen–air turbulent mixing layers

Abstract: This study addresses the dynamics of backscatter of kinetic energy in the context of large-eddy simulations (LES) of high-speed turbulent reacting flows. A priori analyses of direct numerical simulations (DNS) of reacting and inert supersonic, time-developing, hydrogen–air turbulent mixing layers with complex chemistry and multicomponent diffusion are conducted here in order to examine the effects of compressibility and combustion on subgrid-scale (SGS) backscatter of kinetic energy. The main characteristics of the aerothermochemical field in the mixing layer are outlined. A selfsimilar period is identified in which some of the turbulent quantities grow in a quasi-linear manner. A differential filter is applied to the DNS flow field to extract filtered quantities of relevance for the large-scale kinetic-energy budget. Spatiotemporal analyses of the flow-field statistics in the selfsimilar regime are performed, which reveal the presence of considerable amounts of SGS backscatter. The dilatation field becomes spatially intermittent as a result of the high-speed compressibility effect. In addition, the large-scale pressure-dilatation work is observed to be an essential mechanism for the local conversion of thermal and kinetic energies. A joint probability density function (PDF) of SGS dissipation and large-scale pressure-dilatation work is provided, which shows that backscatter occurs primarily in regions undergoing volumetric expansion; this implies the existence of an underlying physical mechanism that enhances the reverse energy cascade. Furthermore, effects of SGS backscatter on the Boussinesq eddy viscosity are studied, and a regime diagram demonstrating the relationship between the different energy-conversion modes and the sign of the eddy viscosity is provided along with a detailed budget of the volume fraction in each mode. A joint PDF of SGS dissipation and SGS dynamic-pressure dilatation work is calculated, which shows that high-speed compressibility effects lead to a decorrelation between SGS backscatter and negative eddy viscosities, which increases for increasingly large values of the SGS Mach number and filter width. Finally, it is found that the combustion dynamics have a marginal impact on the backscatter and flow-dilatation distributions, which are mainly dominated by the high-Mach-number effects.

Space-time characteristics of wall-pressure fluctuation in wall-modeled large-eddy simulation

Abstract: Pressure and shear stress fluctuations at the wall are of great importance in external hydroand aerodynamics, since they are directly related to the structural vibration and noise generation from immersed bodies. Often the space-time characteristics of wallpressure fluctuations are required for low-frequency sound propagation and vibration models. In this brief, we focus on the prediction of fluctuating wall pressure and shear stress from wall-modeled large-eddy simulation (WMLES). WMLES is a technique to circumvent the prohibitive grid-resolution requirement in LES of high Reynolds number wallbounded flows. In WMLES, the dynamically important but very small near-wall eddies are not directly resolved by the LES grid, but their effect is modeled by a wall model. In such an approach, one aims to compute the outer-layer using a coarse LES, while modeling the effect of momentum and heat transfer from the inner layer to the outer layer. Assessment of WMLES has always been based on the predictive quality of the mean velocity and Reynolds stresses. Secondary quantities from WMLES such as wall-pressure fluctuations and their spectra have received little attention, and are not reported. Here, the r.m.s. and wavenumber-frequency spectra of wall-pressure fluctuations are documented and analyzed. These supplementary data will elucidate to what extent the near-wall pressure field from WMLES can be utilized for modeling sound propagation and vibrations. A high Reynolds number turbulent channel flow is considered for this purpose.