Inflow Conditions For Large Eddy Simulation Research Articles

Adaptation and evaluation of a weak recycling strategy for inflow boundary layers in large-eddy simulation

This article presents a detailed analysis of an improved weak recycling method used to generate turbulent inflow conditions for large-eddy simulations. Answering some of the points raised in the literature, various flat-plate cases are considered in order to evaluate the fidelity and the robustness of the method. It is shown that the distance it takes for the canonical compressible boundary layer to forget its weak recycling inflow turbulence is rather short (< 10 δ, for Reθ up to 2 340). Yet, the study also reveals that the optimal distance between the inlet and the source plane location used to extract the fluctuations depends on the case at hand with a tendency of compressible simulations to require a sightly larger distance. Cases involving compressibility effects (M=0.8) and 3D effects (wall in translation perpendicular to the main flow) show that the recycling method remains effective in more complex flows. When using limited resolution inlet flow profiles coming from experiments as an input, spurious fluctuations are observed whose main wavelength equals the recycling distance. This study proposes a strategy to circumvent this drawback by limiting the amplitude of recycled velocity fluctuations with respect to the mean velocity at the inlet.

Optimizing turbulent inflow conditions for large-eddy simulations of the atmospheric boundary layer

Large-eddy simulations (LES) of the atmospheric boundary layer (ABL) require the specification of a turbulent inflow condition with appropriate turbulence intensities and length scales. When using a synthetic turbulence generator, the statistics obtained downstream of the inlet might deviate considerably from the intended values. In the present work we propose a fully automated approach to modify the input parameters for the turbulence generator such that the desired turbulence statistics are obtained at the downstream location of interest. The method employs a gradient-based optimization in combination with the divergence-free version of the digital filter method developed by Xie and Castro [1, 2]. A sensitivity analysis showed that the spanwise and vertical Reynolds stresses and length scales are the most influential input parameters. Hence, the optimization adjusts these parameters until the desired turbulence statistics are obtained downstream in the domain. The results demonstrate the promising capabilities of the method: the mean velocity profile is correctly maintained using an appropriate wall function, while the optimization results in Reynolds stresses, integral length-scales and turbulence spectra that compare well to ABL wind tunnel measurements.

Overview of Turbulent Inflow Boundary Conditions for Large-Eddy Simulations

Generating realistic turbulent inflow conditions for large-eddy simulations and other large-scale-resolving approaches is essential to fully exploit the ever-increasing capabilities of modern computers in solving fluid flow problems of engineering interest. This research area has received much attention during the last two decades and will continue to grow as large-eddy simulation is applied to an even wider range of flows. It is deceptively difficult to generate turbulent inflow fluctuations that are realistic for a specific problem, and various methods have been designed to improve this realism. A review of such turbulent inflow boundary conditions is presented, broadly classified into transition-inducing approaches, turbulence library-based methods, recycling–rescaling-based methods, and synthetic turbulence generators. The last category is given more attention to cover a variety of creative approaches that are found in the literature as well as due to its practicality for problems of industrial interest. Selection of a particular method for a particular application is a compromise between the required fidelity and the allowable complexity and computational expense. Lack of quantitative knowledge about turbulence to be fed into the simulation is recognized as a limiting factor in the effective use of such inflow generators for problems of practical interest.

Generation of unsteady inflow conditions for compressible LES with a modified vortex method

This article is to explore and modify vortex method to generate large eddy simulation( LES)inlet condition. In the modification,a density distribution of vortex which can avoid unreasonable vortex quantity in local place was used. The movement of vortex was controlled by Langevin equation,which mimics the active grid turbulence so as to generate an unsteady flow field which is more like the real turbulence. Knowing the results of Reynolds-averaged Navier-Stokes equations( RANS) and reconstructing the turbulence by vortex method( VM),the method was tested in the experimental channel. The statistical results of both inlet and outlet of the channel were given,which include the mean velocity,the vorticity and the Reynolds stress. By analyzing the generated field with the reference of direct numerical simulation( DNS) results and comparing with the results of original method,during the development of turbulence in the channel,it shows that the modified vortex method is viable and cost-effective for generating unsteady inflow condition for LES.

A stochastic perturbation method to generate inflow turbulence in large-eddy simulation models: Application to neutrally stratified atmospheric boundary layers

Despite the variety of existing methods, efficient generation of turbulent inflow conditions for large-eddy simulation (LES) models remains a challenging and active research area. Herein, we extend our previous research on the cell perturbation method, which uses a novel stochastic approach based upon finite amplitude perturbations of the potential temperature field applied within a region near the inflow boundaries of the LES domain [Muñoz-Esparza et al., “Bridging the transition from mesoscale to microscale turbulence in numerical weather prediction models,” Boundary-Layer Meteorol., 153, 409–440 (2014)]. The objective was twofold: (i) to identify the governing parameters of the method and their optimum values and (ii) to generalize the results over a broad range of atmospheric large-scale forcing conditions, Ug = 5 − 25 m s−1, where Ug is the geostrophic wind. We identified the perturbation Eckert number, Ec=Ug2/ρcpθ̃pm, to be the parameter governing the flow transition to turbulence in neutrally stratified boundary layers. Here, θ̃pm is the maximum perturbation amplitude applied, cp is the specific heat capacity at constant pressure, and ρ is the density. The optimal Eckert number was found for nonlinear perturbations allowed by Ec ≈ 0.16, which instigate formation of hairpin-like vortices that most rapidly transition to a developed turbulent state. Larger Ec numbers (linear small-amplitude perturbations) result in streaky structures requiring larger fetches to reach the quasi-equilibrium solution, while smaller Ec numbers lead to buoyancy dominated perturbations exhibiting difficulties for hairpin-like vortices to emerge. Cell perturbations with wavelengths within the inertial range of three-dimensional turbulence achieved identical quasi-equilibrium values of resolved turbulent kinetic energy, q, and Reynolds-shear stress, 〈w′u′〉. In contrast, large-scale perturbations acting at the production range exhibited reduced levels of 〈w′u′〉, due to the formation of coherent streamwise structures, while q was maintained, requiring larger fetches for the turbulent solution to stabilize. Additionally, the cell perturbation method was compared to a synthetic turbulence generator. The proposed stochastic approach provided at least the same efficiency in developing realistic turbulence, while accelerating the formation of large-scales associated with production of turbulent kinetic energy. Also, it is computationally inexpensive and does not require any turbulent information.

Divergence-free turbulence inflow conditions for large-eddy simulations with incompressible flow solvers

Synthetic turbulence for inflow conditions formulated on a 2-D plane generally produces unphysically large pressure fluctuations in direct numerical and large-eddy simulations. To reduce such artificial fluctuations a divergence-free method is developed with incompressible flow solvers. The procedure of the velocity–pressure solvers is slightly modified on a vertical plane near (rather than at) the inlet by inserting the synthetic turbulence on that plane during the procedure. Simple analytic and numerical error estimations are used to show that the impact of the modified solvers on solution accuracy is small. The final synthetic turbulence satisfies the divergence-free condition. No additional CPU time is required to achieve this condition. The method was tested via simulations of a plane channel flow with Reτ=395. Reynolds stresses, wall skin friction and power spectra of velocity fluctuations are compared with those obtained from using periodic inlet–outlet boundary conditions. In particular, the variances and power spectra of pressure fluctuations are shown to be accurately predicted only when the divergence-free inlet condition is used.

Large-eddy simulation for flow and dispersion in urban streets

Large-eddy simulations (LES) with our recently developed inflow approach (Xie, Z.-T., Castro, I.P., 2008a. Efficient generation of inflow conditions for large-eddy simulation of street-scale flows. Flow Turbul. Combust., vol. 81(3), pp. 449–470.) have been used for flow and dispersion within a genuine city area – the DAPPLE site, located at the intersection of Marylebone Rd and Gloucester Pl in Central London. Numerical results up to second-order statistics are reported for a computational domain of 1.2 km (streamwise) × 0.8 km (lateral) × 0.2 km (in full scale), with a resolution down to approximately one meter in space and one second in time. They are in reasonable agreement with the experimental data. Such a comprehensive urban geometry is often, as here, composed of staggered, aligned, square arrays of blocks with non-uniform height and non-uniform base, street canyons and intersections. Both the integrative and local effect of flow and dispersion to these geometrical patterns were investigated. For example, it was found that the peaks of spatially averaged u rms , v rms , w rms and < u′ w′> occurred neither at the mean height nor at the maximum height, but at the height of large and tall buildings. It was also found that the mean and fluctuating concentrations in the near-source field is highly dependent on the source location and the local geometry pattern, whereas in the far field (e.g. >0.1 km) they are not. In summary, it is demonstrated that full-scale resolution of around one meter is sufficient to yield accurate prediction of the flow and mean dispersion characteristics and to provide reasonable estimation of concentration fluctuations.

Turbulent inflow conditions for large-eddy simulation based on low-order empirical model

Generation of turbulent inflow boundary conditions is performed by interfacing an experimental database acquired by particle image velocimetry to a computational code. The proposed method ensures that the velocity fields introduced as inlet conditions in the computational code present correct one- and two-point spatial statistics and a realistic temporal dynamics. This approach is based on the use of the proper orthogonal decomposition (POD) to interpolate and extrapolate the experimental data onto the numerical mesh and to model both the temporal dynamics and the spatial organization of the flow in the inlet section. Realistic representation of the flow is achieved by extracting and modeling independently its coherent and incoherent parts. A low-order dynamical model is derived from the experimental database in order to provide the temporal evolution of the most energetic structures. The incoherent motion is modeled by employing time series of Gaussian random numbers to mimic the temporal evolution of higher order POD modes. Validation of the proposed method is provided by performing a large-eddy simulation of a turbulent plane mixing layer, which is compared to experimental results.

A synthetic-eddy-method for generating inflow conditions for large-eddy simulations

The generation of inflow data for spatially developing turbulent flows is one of the challenges that must be addressed prior to the application of LES to industrial flows and complex geometries. A new method of generation of synthetic turbulence, suitable for complex geometries and unstructured meshes, is presented herein. The method is based on the classical view of turbulence as a superposition of coherent structures. It is able to reproduce prescribed first and second order one point statistics, characteristic length and time scales, and the shape of coherent structures. The ability of the method to produce realistic inflow conditions in the test cases of a spatially decaying homogeneous isotropic turbulence and of a fully developed turbulent channel flow is presented. The method is systematically compared to other methods of generation of inflow conditions (precursor simulation, spectral methods and algebraic methods).

Synthetic turbulence inflow conditions for large-eddy simulation

Direct numerical or large-eddy simulations of the majority of spatially inhomogeneous turbulent flows require turbulent inflow boundary conditions. A potential implication is that any results computed may be strongly influenced by the prescribed instantaneous inlet velocity profiles. Such profiles are practically never available, and a usual practice is to generate synthetic inflow data satisfying certain statistical properties, which may, for example, be known from experimental data or empirical correlations. The present paper describes a new method for generating turbulent inflow data based on digital filters that is capable of reproducing specified statistical data. Two variants of the approach are presented: a simple method in which the Reynolds stresses and a single length scale are prescribed, and a more detailed approach that is able to reproduce the complete Reynolds-stress tensor as well as any given, locally defined, spatial and temporal correlation functions. The application of the methods to a plane jet flow and to a developing wall boundary layer serve to demonstrate the applicability of the approach.

A priori and a posteriori tests of inflow conditions for large-eddy simulation

Comparisons of inflow conditions for large-eddy simulations of turbulent, wall-bounded flows are carried out. Consistent with previous investigations, it is found that the spectral content of the inflow velocity is important. Inflow conditions based on random-noise, or small-scale eddies only, dissipate quickly. Temporal and spatial filtering of a time series obtained from a separate calculation indicates that it is important to capture eddies of dimensions equal to or larger than the integral length scale of the flow. Three methods for generating inflow velocity fields are tested in a simulation of spatially developing turbulent channel flow. Synthetic turbulence generation methods that introduce realistic length scales are more suitable than uncorrelated random noise, but still require fairly long development lengths before realistic turbulence is established. A recycling method based on the use of turbulent data obtained from a separate calculation, in different flow conditions, was found to result in more rapid transition. A forcing method that includes a control loop also appears to be effective by generating turbulence with the correct Reynolds stresses and correlations within less than ten channel half heights.

Turbulent Inflow Conditions for Large-Eddy-Simulation of Compressible Wall-Bounded Flows

The issue of turbulent inflow conditions for large-eddy simulation (LES) is addressed through three representative examples recently treated at ONERA. First, the performance of an extension to compressible flows of the rescaling method of Lund et al. is assessed. The second example, the flow above a deep cavity shows that the imposition of turbulent fluctuations has nearly no influence on the accuracy of the simulation on this acoustically driven flow. The third case demonstrates that particular attention must he paid to the response of the inflow condition to acoustic perturbations when local hybrid Reynolds-averaged Navier-Stokes/LES approaches are considered

Inflow conditions for large-eddy simulations of mixing layers

A method to generate inflow boundary conditions for large-eddy simulations (LES) of turbulent free shear layers is presented. A time series of instantaneous velocity planes, with duration approximately equal to the integral time scale of the flow, is extracted from a periodic simulation and saved on disk. This signal is transformed into a periodic one, by using a conventional windowing technique, and is re-used in the actual simulation as many times as required to obtain converged statistics. The method is applied in a LES of a spatially developing turbulent mixing layer. It is shown that the periodicity induced by the inflow signal decays rapidly in about 25% of the domain, and that the length of the period has small effect on the statistics, which agree well with the reference experimental data. The method appears to be a cost-effective strategy in the generation of inflow data in a large variety of flows.