Smagorinsky-Lilly Sub-grid Scale Research Articles

Flow over a 25°Ahmed body at a Reynolds number characteristic of suburban driving speeds

Four turbulence models were used to predict the flow over a 25°Ahmed body at a Reynolds number characteristic of suburban driving speeds in the United States: the k-ω shear stress transport (SST) model, large eddy simulation (LES) with the Smagorinsky-Lilly subgrid scale model, LES with the wall-adapting local eddy viscosity (WALE) subgrid scale model, and improved delayed detached eddy simulation (IDDES). The Reynolds number based on body height was ReH=2.65×105 - this corresponds to a freestream velocity of 13.44m/s, or approximately 30mph. The k-ω SST model only showed promise in predicting the drag coefficient; otherwise, it significantly underpredicted the lift coefficient and predicted an incorrect wake structure in terms of flow separation and the size of the separation bubble behind the body. All three scale-resolving models predicted a wake structure in good agreement with experimental data, with the WALE model and IDDES providing the best agreement in terms of separation bubble size, streamwise velocity magnitude, and turbulent kinetic energy in the wake. The best predictions of the aerodynamic forces were those of the WALE model, with overpredictions of only 5.45% for drag and 5.65% for lift. Overall, both the WALE model and IDDES demonstrated promise in predicting the wake structure and aerodynamic forces, but the best agreements were achieved with the WALE model. Therefore, the WALE model showed the lowest dependence on Reynolds number when comparing lower Reynolds number simulations to higher Reynolds number experiments for this situation.

PIV and dynamic LES of the turbulent stream and mixing induced by a V-grooved blade axial agitator

The hydrodynamic behavior of a turbulent flow and the mixing characteristics generated by a V-grooved axial impeller inside an agitated tank reactor were investigated both experimentally and numerically. Angle resolved Particle Image Velocimetry (PIV) techniques with an angular displacement Δθ = 5° have been applied and two aerodynamic planes along the blades were considered. PIV-based results were compared to those obtained by Large Eddy Simulation (LES), used with the dynamic Smagorinsky-Lilly sub-grid scale (SGS) model. Results showed the existence of distinctive recirculation zones in the aerodynamic planes, and new additional frequencies in the impeller stream, induced by the grooves. A decrease of mixing time of about 11% was obtained experimentally, consequence of the better suction induced by the grooved blades in the early stages of mixing. Mean velocities, vorticity, TKE obtained from LES showed a good agreement with the PIV-based results. The distributions of turbulence dissipation rate ε were similar to those obtained from PIV, however showing high under-predicted magnitudes.

Simulating marine propellers with vortex particle method

The vortex particle method is applied to compute the open water characteristics of marine propellers. It is based on the large-eddy simulation technique, and the Smagorinsky-Lilly sub-grid scale model is implemented for the eddy viscosity. The vortex particle method is combined with the boundary element method, in the sense that the body is modelled with boundary elements and the slipstream is modelled with vortex particles. Rotational periodic boundaries are adopted, which leads to a cylindrical sector domain for the slipstream. The particle redistribution scheme and the fast multipole method are modified to consider the rotational periodic boundaries. Open water characteristics of three propellers with different skew angles are calculated with the proposed method. The results are compared with the ones obtained with boundary element method and experiments. It is found that the proposed method predicts the open water characteristics more accurately than the boundary element method, especially for high loading condition and high skew propeller. The influence of the Smagorinsky constant is also studied, which shows the results have a low sensitivity to it.

Three-Dimensional Numerical Simulation of the Flow around Two Side-by-Side Cylinders of Different Diameters

This paper presents the calculation of hydrodynamic characteristics of two side-by-side cylinders of different diameters in three dimensional incompressible uniform cross flow by using Large-eddy simulation method based on dynamical Smagorinsky-Lilly sub-grid scale model. Solution of the three dimensional N-S equations were obtained by the finite volume method. The numerical simulation focused on investigating the characteristic of the pressure distribution (drag and lift force), vorticity field and turbulence Re=. Results shows that, the asymmetry of the time –averaged velocity distribution in the flow direction behind the two cylinders is very obvious; the frequency of eddy shedding of the small cylinder is about twice of the large one. The turbulence of cylinders is more obvious.

The Computational Modeling of Transitional Flow through a Transonic Linear Turbine: Comparative Performance of Various Turbulence Models

Aero-thermal characteristics of transitional flow through a highly loaded transonic linear turbine are studied at the design incidence using computational methods. The three-dimensional compressible turbulent flow through a turbine inlet guide vane is simulated using finite-volume method-based fluid flow solutions using both Reynolds-averaged Navier Stokes (RANS) equations-based turbulence models, and a dynamic large eddy simulation (LES) approach which is based on a Smagorinsky-Lilly subgrid scale (SGS) model. The calculations are made for exit isentropic Mach numbers ranging from 0.7 to 1.1, and for Reynolds numbers ranging from 5 × 105 to 2 × 106. The numerical results obtained from different turbulence model simulations are compared with each other, and with the experimental data available in the open literature in terms of integrated flow parameters, such as pressure and heat transfer coefficients for varying exit Mach and Reynolds numbers. The present numerical approach indicates a guide line that the present LES approach can be a reliable choice for turbomachinery flows.

A Numerical Investigation of the Flow Past a Generic Side Mirror and its Impact on Sound Generation

The case investigated is the flow past a generic side mirror mounted on a flat plate at the Reynolds number of ReD=5.2×105 based on the mirror diameter. The present work studies both flow and acoustic sources by evaluating two second-order advection schemes, different levels of turbulence modeling, and three different grids. The advection schemes discussed in the present study are a second-order upwind scheme and a monotonic central scheme. The turbulence models investigated cover three levels of modeling. These are the original formulation of the detached eddy simulation (DES) model, the Smagorinsky–Lilly sub-grid scale (SGS) model with near-wall damping, and a dynamic Smagorinsky model. The different grids are as follows: a primary grid where all parameter studies are conducted and a second grid with significantly higher wake resolution and to some extent also increased plate resolution, while maintaining the resolution at the front side of the mirror. The final grid uses a significantly higher plate resolution and a wake resolution similar to that of grid two, but a comparably lower mirror front side resolution as compared with the two other grids. The general outcome of this work is that the estimation of the grid cutoff frequency through a relation of the velocity fluctuation and the grid size matches both the experimental results and trend lines perfectly. Findings from the flow field show that the horseshoe vortex in front of the mirror causes pressure fluctuations with a magnitude exceeding the maximum levels at the rear side of the mirror. Its location and unsteady properties are perfectly captured in the final simulation as compared with the experiments conducted by Daimler–Chrysler. A laminar separation at the front side of the mirror is more or less found for all wall resolved cases except the DES simulation. The third grid fails to predict this flow feature, but it is shown that this effect has no significant effect on either the static pressure sensors at the mirror surface or at the dynamic sensors located downstream of the mirror. The simulation also supports the fundamental frequency based on the eddy convection in the mirror shear layer, which is shown to be twice as high as the frequency peak found in the lateral force spectra.

Large-Eddy Simulation of a Swirling Diffusion Flame Using a SOM SGS Combustion Model

Swirling flows and methane–air swirling diffusion combustion are studied by large-eddy simulation (LES) using a Smagorinsky-Lilly subgrid-scale (SGS) turbulence model, a second-order moment (SOM) SGS combustion model, proposed by the present authors, and an eddy-break-up (EBU) combustion model used by some investigators. The LES statistical results are compared with the experimental results and the Reynolds-averaged Navier-Stokes (RANS) modeling results using the Reynolds stress equation model and the SOM combustion model. For swirling flows, the LES statistical results give better agreement with the experimental results than the RANS modeling, indicating that the adopted SGS turbulence model is suitable for swirling flows. For swirling combustion, both the SOM SGS combustion model and the RANS-SOM model give results in good agreement with the experimental results, but the LES-EBU modeling results are not in agreement with the experimental results. The LES instantaneous results show large vortices formed in the shear layer and a complex vortex shedding pattern in the downstream region. It is shown that combustion weakens the vortex structures, combustion is intensified by the coherent structures, and swirl thickens the flame front.

Large Eddy Simulations of Mixing Time in a Stirred Tank

Abstract Large eddy simulations (LES) of mixing process in a stirred tank of 0.476m diameter with a 3-narrow blade hydrofoil CBY impeller were reported. The turbulent flow field and mixing time were calculated using LES with Smagorinsky-Lilly subgrid scale model. The impeller rotation was modeled using the sliding mesh technique. Better agreement of power demand and mixing time was obtained between the experimental and the LES prediction than that by the traditional Reynolds-averaged Navier-Stokes (RANS) approach. The curve of tracer response predicted by LES was in good agreement with the experimental. The results show that LES is a reliable tool to investigate the unsteady and quasi-periodic behavior of the turbulent flow in stirred tanks.

Large-eddy structures of turbulent swirling flows and methane-air swirling diffusion combustion

Turbulent swirling flows and methane-air swirling diffusion combustion are studied by large-eddy simulation (LES) using a Smagorinsky-Lilly subgrid scale turbulence model and a second-order moment (SOM) SGS combustion model, and also by RANS modeling using the Reynolds Stress equation model with the IPCM+wall and IPCM pressure-strain models and SOM combustion model. The LES statistical results for swirling flows give good agreement with the experimental results, indicating that the adopted subgrid-scale turbulence model is suitable for swirling flows. The LES instantaneous results show the complex vortex shedding pattern in swirling flows. The initially formed large vortex structures soon break up in swirling flows. The LES statistical results of combustion modeling are near the experimental results and are as good as the RANS-SOM modeling results. The LES results show that the size and range of large vortex structures in swirling combustion are different from those of isothermal swirling flows, and the chemical reaction is intensified by the large-eddy vortex structures.