Dynamic Smagorinsky Subgrid Scale Research Articles

Experimental and numerical investigation of a three-dimensional vertical-axis wind turbine with variable-pitch

A combined experimental and numerical investigation is carried out to study the performance of a micro vertical-axis wind turbine (VAWT) with variable-pitch. Three-dimensional numerical simulations are essentially employed, for the VAWT involves a low aspect ratio (AR) three straight blades with struts. The performance of the VAWT is experimentally measured using a wind tunnel, while large eddy simulation (LES) with dynamic smagorinsky subgrid scale (SGS) model is employed to help understand the associated flow structure. The effects of wind speed, turbulence intensity, airfoil shape, and strut mechanism with and without variable-pitch on the performance of the turbine are carefully assessed, both experimentally and numerically. The accuracy of the SGS model in predicting the laminar–turbulent transition is also examined.

Performance of Transition-Sensitive Models in Predicting Flow Structures over Delta Wings

Laminar-turbulent transition may have considerable influence on the performance of moderate and high-speed aerodynamic vehicles. The two recently developed transition-sensitive models, SST-TR and , are employed to assess their abilities in resolving the complex underlying flow physics associated with flow structures over delta wings. In particular, their capabilities in capturing the aerodynamic characteristics of three-dimensional flow associated with vortex breakdown, shear-layer instabilities, and boundary-layer transition are assessed. These closures are employed to simulate vortex breakdown and near-wall flow structures over a delta wing planform with a moderate sweep angle of 50 deg at a Reynolds number of , where a combination of laminar, transitional, and turbulent flowfields coexist. Accuracy of these models in predicting boundary-layer transition adjacent to a delta wing surface with a high-sweep angle of 70 deg and Reynolds numbers of and is investigated. Large-eddy simulation with dynamic Smagorinsky subgrid scale model is also employed to assess its capability in predicting transition on delta wing planforms. All numerical simulations are compared with available experimental data found in the literature.

Evaluation of Turbulent Models for Natural Convection of Compressible Air in a Tall Cavity

Numerical simulation of turbulent natural convection of compressible air in a tall cavity is carried out. In order to evaluate the accuracy of turbulent models, various turbulent models are applied to solve the natural convection in a tall cavity that has different temperatures imposed on two opposing vertical walls. For the large-eddy simulation (LES) model, Smagorinsky subgrid scale (SGS) and dynamic Smagorinsky SGS are also applied to the same cases in order to investigate the differences in temperature and velocity caused by different turbulent models. It is found that the k–ϵ model has a high accuracy of predicting velocity distribution at various sampled lines by comparing with experimental data at Rayleigh number of 2.03 × 1010 and 3.37 × 1010, while the LES model has good performance in predicting temperature distributions.

Large eddy simulation of flow around two side-by-side spheres

Large eddy simulation, using a dynamic Smagorinsky sub-grid scale, is used for the prediction of flow structures around two identical spheres fixed side-by-side at a subcritical flow regime with the Reynolds number equal to 5,000. The QUICK discretization method is applied to discretize the convection terms of the Navier-Stokes equation by means of the finite volume approach. This work focuses mainly on the wake structures downstream of the two interactive spheres located at three various dimensionless separation distances between spheres such as G/D = 1.50, 2.00, and 3.00. The obtained results revealed that the interaction between wakes affects the flow structures downstream of spheres. The rate of this interaction is strongly altered as a function of separation distance. On the other hand, some flow data such as mean drag and mean lift coefficients are affected due to the wake interactions. Finally, examination of the Reynolds stress variation along the different lateral axis, L/D, revealed that the nozzle effect does not play a significant role on the turbulence characteristics beyond the G/D = 3.00.

J0501-1-1 航空用エンジン低圧タービンの高負荷比に関する研究 : 翼負荷分布の効果に関するEFD/CFD([J0501-1]流体機械の研究開発におけるEFD/CFD(1))

This paper deals with studies on the flow field around three types of linear cascades of low pressure turbine (LPT) airfoils with different chordwise loading distributions, while keeping the aerodynamic loading index almost the same. Cylindrical bars on the timing belts work as wake generator to emulate upstream stator wakes that impact the boundary layer on the airfoil suction surface. Hot-wire probe measurement is conducted over the blade suction surface to understand to what extent and how the interaction of incoming wakes as well as freestream turbulence affect the boundary layer and separation bubble. Computational Fluid Dynamics (CFD) analyses are also applied to the flow fields around the cascades, mainly using Large-Eddy Simulation (LES) with dynamic Smagorinsky subgrid scale model.

Large-eddy simulation of combined forced and natural convection in a vertical plane channel

A combined forced and natural convective flow between two vertical plates with different temperatures is studied using large eddy simulation. The numerical simulations were performed with a Grashof number of Gr = 9.6 × 10 5 and Reynolds number of Re τ = 150 (based on the wall friction velocity and half channel width). Two sets of dynamic subgrid-scale (SGS) models were tested in the simulation; namely, the set of linear SGS models consisting of the dynamic Smagorinsky SGS stress model (DM) and dynamic eddy diffusivity SGS heat flux model (DEDM-HF), and the set of nonlinear SGS models consisting of the dynamic nonlinear SGS stress model (DNM) and dynamic tensor diffusivity SGS heat flux model (DTDM-HF). The numerical results are compared with the reported direct numerical simulation data. It is found that the resolved and SGS quantities related to the temperature field are noticeably influenced by the choice of SGS models. In general, the set of dynamic nonlinear SGS models yields better prediction of the flow than the set of dynamic linear SGS models.

A dynamic finite volume scheme for large-eddy simulation on unstructured grids

In recent years there has been considerable progress in the application of large-eddy simulation (LES) to increasingly complex flow configurations. Nevertheless a lot of fundamental problems still need to be solved in order to apply LES in a reliable way to real engineering problems, where typically finite-volume codes on unstructured meshes are used. A self-adaptive discretisation scheme, in the context of an unstructured finite-volume flow solver, is investigated in the case of isotropic turbulence at infinite Reynolds number. The Smagorinsky and dynamic Smagorinsky sub-grid scale models are considered. A discrete interpolation filter is used for the dynamic model. It is one of the first applications of a filter based on the approach presented by Marsden et al. In this work, an original procedure to impose the filter shape through a specific selection process of the basic filters is also proposed. Satisfactory results are obtained using the self-adaptive scheme for implicit LES. When the scheme is coupled with the sub-grid scale models, the numerical dissipation is shown to be dominant over the sub-grid scale component. Nevertheless the effect of the sub-grid scale models appears to be important and beneficial, improving in particular the energy spectra. A test on fully developed channel flow at Re τ = 395 is also performed, comparing the non-limited scheme with the self-adaptive scheme for implicit LES. Once again the introduction of the limiter proves to be beneficial.

A general optimal formulation for the dynamic Smagorinsky subgrid-scale stress model

SUMMARY In this paper, a general optimal formulation for the dynamic Smagorinsky subgrid-scale (SGS) stress model is reported. The Smagorinsky constitutive relation has been revisited from the perspective of functional variation and optimization. The local error density of the dynamic Smagorinsky SGS model has been minimized directly to determine the model coecient CS. A sucient and necessary condi- tion for optimizing the SGS model is obtained and an orthogonal condition (OC), which governs the instantaneous spatial distribution of the optimal dynamic model coecient, is formulated. The OC is a useful general optimization condition, which unies several classical dynamic SGS modelling formula- tions reported in the literature. In addition, the OC also results in a new dynamic model in the form of a Picard's integral equation. The approximation tensorial space for the projected Leonard stress is identied and the physical meaning for several basic grid and test-grid level tensors is systematically discussed. Numerical simulations of turbulent Couetteow are used to validate the new model formula- tion as represented by the Picard's integral equation for Reynolds numbers ranging from 1500 to 7050 (based on one half of the velocity dierence of the two plates and the channel height). The relative magnitudes of the Smagorinsky constitutive parameters have been investigated, including the model coecient, SGS viscosity andltered strain rate tensor. In general, this paper focuses on investigation of fundamental mathematical and physical properties of the popular Smagorinsky constitutive relation and its related dynamic modelling optimization procedure. Copyright ? 2005 John Wiley & Sons Ltd.

Large eddy simulation of the effects of mild swirl on the near field of a round free jet

The near fields of turbulent round free jets with swirl numbers of S=0.0 and 0.24 are modeled with large eddy simulations that use a dynamic Smagorinsky subgrid scale model. The time-averaged results are in accordance with other numerical and experimental data. Instantaneous plots of vorticity are used to examine the evolution of and interaction between coherent structures. Ring structures aligned with the plane normal to the flow form downstream of the jet shear layer and collide with the streamwise braid structures. The resulting interaction cause the rings to break apart into smaller, less organized turbulence structures. The addition of swirl increases the number of streamwise braids, which enhances the breakdown mechanism of the rings. The results suggest that the increased entrainment observed in swirling flows is due to the action of the braids rather than the rings.