One major drawback of the eddy viscosity subgrid‐scale stress models used in large‐eddy simulations is their inability to represent correctly with a single universal constant different turbulent fields in rotating or sheared flows, near solid walls, or in transitional regimes. In the present work a new eddy viscosity model is presented which alleviates many of these drawbacks. The model coefficient is computed dynamically as the calculation progresses rather than input a priori. The model is based on an algebraic identity between the subgrid‐scale stresses at two different filtered levels and the resolved turbulent stresses. The subgrid‐scale stresses obtained using the proposed model vanish in laminar flow and at a solid boundary, and have the correct asymptotic behavior in the near‐wall region of a turbulent boundary layer. The results of large‐eddy simulations of transitional and turbulent channel flow that use the proposed model are in good agreement with the direct simulation data.

A dynamic subgrid‐scale eddy viscosity model

We present an overview of the lattice Boltzmann method (LBM), a parallel and efficient algorithm for simulating single-phase and multiphase fluid flows and for incorporating additional physical complexities. The LBM is especially useful for modeling complicated boundary conditions and multiphase interfaces. Recent extensions of this method are described, including simulations of fluid turbulence, suspension flows, and reaction diffusion systems.

Lattice boltzmann method for fluid flows

Considerable confusion surrounds the longstanding question of what constitutes a vortex, especially in a turbulent flow. This question, frequently misunderstood as academic, has recently acquired particular significance since coherent structures (CS) in turbulent flows are now commonly regarded as vortices. An objective definition of a vortex should permit the use of vortex dynamics concepts to educe CS, to explain formation and evolutionary dynamics of CS, to explore the role of CS in turbulence phenomena, and to develop viable turbulence models and control strategies for turbulence phenomena. We propose a definition of a vortex in an incompressible flow in terms of the eigenvalues of the symmetric tensor ${\bm {\cal S}}^2 + {\bm \Omega}^2$ are respectively the symmetric and antisymmetric parts of the velocity gradient tensor ${\bm \Delta}{\bm u}$. This definition captures the pressure minimum in a plane perpendicular to the vortex axis at high Reynolds numbers, and also accurately defines vortex cores at low Reynolds numbers, unlike a pressure-minimum criterion. We compare our definition with prior schemes/definitions using exact and numerical solutions of the Euler and Navier–Stokes equations for a variety of laminar and turbulent flows. In contrast to definitions based on the positive second invariant of ${\bm \Delta}{\bm u}$ or the complex eigenvalues of ${\bm \Delta}{\bm u}$, our definition accurately identifies the vortex core in flows where the vortex geometry is intuitively clear.

https://www.cambridge.org/core/services/aop-cambridge-core/content/view/S0022112095000462

On the identification of a vortex

http://home.eps.hw.ac.uk/~ab226/tmp/implicit/lele_92.pdf

Compact finite difference schemes with spectral-like resolution

/pdf/turbulence-and-the-dynamics-of-coherent-structures-i-1o6o6m60fm.pdf

Turbulence and the dynamics of coherent structures. I. Coherent structures

The sound generated by vortex pairing in a two-dimensional mixing layer is studied by solving the N avier-Stokes equations (DNS) for the layer and a portion of its acoustic field, and by solving acoustic analogies with source terms determined from the DNS. Predictions for the acoustic field based on Lilleys equation are in excellent agreement with the DNS results giving detailed verification of Lilleys acoustic analogy for the first time. We show that parts of the full source term which arise when the left-hand-side of Lilleys equation is linearized should not be neglected solely because they are attributable to refraction and scattering, nor because they are proportional to the dilatation. Lilleys source, -2Ui,jUj,kUk,i, appears to be mainly responsible for the overall directivity of the acoustic field produced by the vortex pairings, which is highly focused at shallow angles to the streamwise axis. Scattering of the waves by the flow appears also to be significant, causing the directivity to be more omnidirectional than the Lilley source alone would predict. We also show how small errors in determining the sources, especially those due to scattering, can sometimes lead to large errors in the predictions.

The Sound Generated by a Two-Dimensional Shear Layer: A Comparison of Direct Computations and Acoustic Analogies

The tip-leakage flow dynamics in axial turbomachines has been studied using large-eddy simulation (LES) with particular emphasis on understanding the underlying mechanisms for viscous losses, low-pressure fluctuations, and tip-leakage vortex oscillations. Such an understanding is essential to predicting and eventually controlling cavitation, noise, and vibration in liquid handling systems such as pumps and ducted propellers, and improving their performance. An overview of the computational challenges and major accomplishments of this Challenge Project is presented. These include: (i) improvements of parallel performance, optimization, and portability of the LES code, (ii) LES of the tip-leakage flow in a linear cascade, (iii) detailed analysis of flow statistics, vortex dynamics, and pressure fluctuations, (iv) exploration of tip-leakage flow control strategies by modifying the tip-gap size and end-wall shape, and (v) extension of the flow solver for multiphase simulations of cavitating flow in a rotating twisted blade.

Large-Eddy Simulation of Rotor Tip-Clearance Flows: Computational Challenges and Accomplishments

: A review of progress in large eddy simulation of complex reacting flows is presented. Topics include: the governing equations for low Mach number combustion, assumed PDF subgrid-scale models, subgrid-scale modeling for the variance and dissipation rate of a conserved scalar, inflow and exit boundary conditions for confined swirling flows, simulation results for isothermal swirling flow with experimental validation, and results from a preliminary reacting flow simulation.

Large Eddy Simulation of a Coaxial Jet Combustor.

Simulations of high-Mach-number compressible flows, and for high Reynolds numbers, require an accurate and stable discontinuity-capturing method. In this work, we propose a novel, entropy-consistent, and stable localized artificial-viscosity/diffusivity (LAD)-based method for capturing shock and contact discontinuities in compressible flows. Using an analogy between the Lax-Friedrichs (LF) flux and the artificial-viscosity methods, a discrete LF-type flux formulation is proposed for the LAD method. The proposed method satisfies the discrete kinetic energy– and entropy-consistency conditions presented by Jain & Moin (2020), hence the name entropy-consistent localized artificial diffusivity (EC-LAD). We also propose new sensors that localize where the artificial viscosity is acting, and show that the proposed method is suitable for large-eddy simulation (LES) of compressible turbulent flows with shocks. The sensors are designed in a way that the resulting method does not require tuning coefficients, depending on the problem being solved, that are typical of LAD methods. At the end, the extension of the proposed method to compressible two-phase flows is also presented.

Stable, entropy-consistent, and localized artificial-viscosity method for capturing shocks and contact discontinuities

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.

Parviz Moin

Papers

The Sound Generated by a Two-Dimensional Shear Layer: A Comparison of Direct Computations and Acoustic Analogies

Large-Eddy Simulation of Rotor Tip-Clearance Flows: Computational Challenges and Accomplishments

Large Eddy Simulation of a Coaxial Jet Combustor.

Stable, entropy-consistent, and localized artificial-viscosity method for capturing shocks and contact discontinuities

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