Showing papers by "Parviz Moin published in 2016"

Direct numerical simulation of a turbulent hydraulic jump: turbulence statistics and air entrainment

Abstract: We present direct numerical simulation (DNS) of a stationary turbulent hydraulic jump with inflow Froude number of 2, Weber number of 1820 and density ratio of 831, consistent with ambient water–air systems, all based on the inlet height and inlet velocity A non-dissipative geometric volume of fluid (VOF) method is used to track the detailed interactions between turbulent flow structures and the nonlinear interface dynamics Level set equations are also solved concurrent with VOF in order to calculate the interface curvature and surface tension forces The mesh resolution is set to resolve a wide range of interfacial scales including the Hinze scale Calculations are compared against experimental data of void fraction and interfacial scales indicating, reasonable agreement despite a Reynolds number mismatch Multiple calculations are performed confirming weak sensitivity of low-order statistics and void fraction on the Reynolds number The presented results provide, for the first time, a comprehensive quantitative data for a wide range of phenomena in a turbulent breaking wave using DNS These include mean velocity fields, Reynolds stresses, turbulence production and dissipation, velocity spectra and air entrainment data In addition, we present the energy budget as a function of streamwise location by keeping track of various energy exchange processes in the wake of the jump The kinetic energy is mostly transferred to pressure work, potential energy and dissipation while surface energy plays a less significant role Our results indicate that the rate associated with various energy exchange processes peak at different streamwise locations, with exchange to pressure work flux peaking first, followed by potential energy flux and then dissipation The energy exchange process spans a streamwise length of order jump heights Furthermore, we report statistics associated with bubble transport downstream of the jump The bubble formation is found to have a periodic nature Meaning that the bubbles are generated in patches with a specific frequency associated with the roll-up frequency of the roller at the toe of the jump, with its footprint apparent in the velocity energy spectrum Our study also provides the ensemble-averaged statistics of the flow which we present in this paper These results are useful for the development and validation of reduced-order models such as dissipation models in wave dynamics simulations, Reynolds-averaged Navier–Stokes models and air entrainment models

Constant-energetics physical-space forcing methods for improved convergence to homogeneous-isotropic turbulence with application to particle-laden flows

Abstract: This study investigates control-based forcing methods for incompressible homogeneous-isotropic turbulence forced linearly in physical space which result in constant turbulent kinetic energy, constant turbulent dissipation (also constant enstrophy), or a combination of the two based on a least-squares error minimization. The methods consist of proportional controllers embedded in the forcing coefficients. During the transient, the controllers adjust the forcing coefficients such that the controlled quantity achieves very early a minimal relative error with respect to its target stationary value. Comparisons of these forcing methods are made with the non-controlled approaches of Rosales and Meneveau [“Linear forcing in numerical simulations of isotropic turbulence: Physical space implementations and convergence properties,” Phys. Fluids 17, 095106 (2005)] and Carroll and Blanquart [“A proposed modification to Lundgren’s physical space velocity forcing method for isotropic turbulence,” Phys. Fluids 25, 105114 (2013)], using direct numerical simulations (DNS) and large-eddy simulations(LES). The results indicate that the proposed constant-energetics forcing methods shorten the transient period from a user-defined artificial flow field to Navier-Stokes turbulence while maintaining steadier statistics. Additionally, the proposed method of constant kinetic-energy forcing behaves more robustly in coarse LES when initial conditions are employed that favor the occurrence of subgrid-scale backscatter, whereas the other approaches fail to provide physical turbulent flow fields. For illustration, the proposed forcing methods are applied to dilute particle-laden homogeneous-isotropic turbulent flows; the results serve to highlight the influences of the forcing strategies on the disperse-phase statistics.

Minimum-dissipation scalar transport model for large-eddy simulation of turbulent flows

Abstract: The anisotropic minimum dissipation (AMD) subfilter eddy-viscosity model for large-eddy simulations is tested, and results show good agreement with well-established empirical correlations and theoretical predictions of the resolved flow statistics. In particular, the AMD model can accurately predict the expected surface-layer similarity profiles and power spectra for both velocity and scalar concentration.

Space-time characteristics of wall-pressure and wall shear-stress fluctuations in wall-modeled large eddy simulation.

Abstract: We report the space-time characteristics of the wall-pressure fluctuations and wall shear-stress fluctuations from wall-modeled large eddy simulation (WMLES) of a turbulent channel flow at Re τ = 2000. Two standard zonal wall models (equilibrium stress model and nonequilibrium model based on unsteady RANS) are employed, and it is shown that they yield similar results in predicting these quantities. The wall-pressure and wall shear-stress fields from WMLES are analyzed in terms of their r.m.s. fluctuations, spectra, two-point correlations, and convection velocities. It is demonstrated that the resolution requirement for predicting the wall-pressure fluctuations is more stringent than that for predicting the velocity. At least δ/Δx > 20 and δ/Δz > 30 are required to marginally resolve the integral length scales of the pressure-producing eddies near the wall. Otherwise, the pressure field is potentially aliased. Spurious high wave number modes dominate in the streamwise direction, and they contaminate the pressure spectra leading to significant overprediction of the second-order pressure statistics. When these conditions are met, the pressure statistics and spectra at low wave number or low frequency agree well with the DNS and experimental data. On the contrary, the wall shear-stress fluctuations, modeled entirely through the RANS-based wall models, are largely underpredicted and relatively insensitive to the grid resolution. The short-time, small-scale near-wall eddies, which are neither resolved nor modeled adequately in the wall models, seem to be important for accurate prediction of the wall shear-stress fluctuations.

On the suitability of second-order accurate discretizations for turbulent flow simulations

Abstract: This contribution is a tribute to the foresight of Paolo Orlandi who demonstrated and advocated the suitability of second-order discretization methods for turbulent flow simulations. His pioneering work has shown how to combine cost effective and relatively easy to implement, central, second-order accurate finite difference schemes with the appropriate variable discretization and time integration methods to efficiently solve complex turbulent flows. In this communication, we first compare the cost and accuracy of Fourier-spectral methods and second order finite difference discretization for the solution of a simple variable coefficient convection problem. We then extend our analysis to the inviscid Taylor Green vortex. We conclude with a discussion of related findings from the literature including the accuracy requirements for large eddy simulation (LES) of turbulent flows.

Wall-Modeling in Complex Turbulent Flows

Abstract: Resolution of wall layer turbulent structures in large eddy simulation of high Reynolds number flows of aeronautical interest requires inordinate computational resources. LES with wall models is therefore employed in engineering applications. We report on recent advances at the Center for Turbulence Research (CTR) in the development of wall boundary conditions for complex turbulent flows computed on unstructured grids. We begin by describing a novel application of wall modeled LES to a high lift airfoil system. This flow field is very complex involving boundary layers, free shear flows, separation and laminar/turbulence transition. We then describe a non-equilibrium model that requires the solution of the full 3D RANS equations in the near wall region. This model is successfully applied to a spatially evolving transitional and a high Reynolds number flat plate boundary layer. Finally we describe a new approach to LES using differential filters. An important byproduct of this approach is the derivation of slip velocity boundary conditions for wall modeled LES. This methodology is successfully applied to flow over NACA4412 airfoil at near stall conditions.

LES-based characterization of a suction and oscillatory blowing fluidic actuator

Abstract: Recently, a novel fluidic actuator using steady suction and oscillatory blowing was developed for active control of high-speed turbulent flows (Arwatz et al. 2008). The suction and oscillatory blowing (SaOB) actuator converts compressed air input into pulsed bistable oscillatory blowing at the actuator outlets. The mechanism for generating bi-stable oscillatory blowing is based upon the Coandă effect and use of a feedback tube. Also, actuator geometry was found to be crucial in producing robust control outputs (Arwatz et al. 2008). The SaOB actuator has been developed and tested for several canonical flow configurations as well as for external aerodynamic control problems (Wilson et al. 2013; Schatzman et al. 2014; Shtendel & Seifert 2014; Lubinsky & Seifert 2014, 2015; Schatzman et al. 2015). These recent studies showed that the addition of steady suction in close proximity to the pulsed blowing is important in increasing the efficacy and efficiency of this flow control approach. The SaOB actuator is particularly interesting in two respects. First, generating oscillatory blowing does not involve any moving parts. Instead, a feedback tube is used to stably sustain the oscillation without additional external inputs. The length of the feedback tube is a key parameter determining oscillation frequency. Second, a suction system is employed to further increase total flow rates for a given inlet pressure. These two respects are based upon physical principles of fluid dynamics and are essential to efficiently and effectively producing oscillatory blowing. The basic mechanisms of the SaOB actuator and some of the important flow features were examined experimentally (Arwatz et al. 2008; Wassermann et al. 2013). However, detailed characteristics of unsteady flows within the actuator are not completely understood. Geometric complexity, compressibility, and the strongly turbulent nature of the internal flows make diagnostics and characterization difficult. An objective of this study is to predict the internal turbulent flows of the SaOB actuator and gain more understanding of the flow physics. A challenging part for prediction is to accurately resolve turbulent fluctuations as well as geometry of engineering complexity, both of which are important to correctly characterize the actuator. Large-eddy simulation (LES) based upon a novel unstructured-grid technique is applied to compute and characterize the internal flows within the SaOB actuator. The simulation tools are well validated for turbulent flows with multi-physics and tested to scale very well up to O(10) cores. In addition to prediction, this study targets the development of reducedorder modeling techniques for fluidic oscillators, a process which is useful if not essential for integrated simulation of aerodynamic flow control system where actuator arrays are used on complex geometries with possible great variability of important scales.