Search

In this study, we proposed an idea for an advanced switching parameter used in a hybrid approach connecting large eddy simulation (LES) with Reynolds-averaged Navier-Stokes (RANS) modeling (the hybrid LES/RANS (HLR) model). Although the HLR model is promising way to predict engineering turbulent flows, an important problem is that RANS is always adopted in the near-wall region, even if the grid resolution is fine enough for LES. To overcome this difficulty, the switching parameter proposed here introduced knowledge of the Kolmogorov microscale that is thought to be reasonable for representing the near-wall turbulence. This parameter enabled the present HLR model to be smoothly replaced by a full LES if a grid resolution was fine enough in the near-wall region. To confirm model performance, the present HLR model was applied to numerical simulations of a periodic hill flow as well as fundamental plane channel flows. The model generally provided reasonable predictions for these test cases that include complex turbulence with massive flow separation.

The "double-buffer problem" has been regarded as a crucial concern for the strategy behind the hybrid large eddy simulation (LES)/Reynolds-veraged Navier-Stokes (RANS) model (or HLR model, for short). Such models are likely to show unphysical mean-velocity distributions in the LES-RANS interface region, where "super-streak structures" also appear that look like low-speed streaks generated in the near-wall region of wall turbulence. To overcome this difficulty, the stochastic backscatter model, in which the vortex structures in the interface region are divided into smaller scales, holds promise due to the effect of random source term imposed in the momentum equation. Although this method is effective, several parameters must be prescribed and their specification process is arbitrary and ambiguous. An alternative advanced HLR model has been proposed, in which an anisotropy-resolving subgrid scale (SGS) model was adopted in the LES region as well as a one-equation non-linear eddy-viscosity model in the RANS region. Previous investigations indicated that this HLR model did not exhibit or, at least, largely reduced the "double-buffer problem" in the mean-velocity distribution, with no special treatment being applied. The main purpose of the present study is to reveal why this HLR model improves the predictive performance in the LES-RANS interface region. Specifically, we focus on the role of the extra anisotropic term introduced in the SGS model, finding that it plays an important role in enhancing vortex structures in the interface region, leading to a considerable improvement in model performance.

An improved subgrid-scale (SGS) model was proposed by combining an isotropic linear eddy-viscosity term with an extra anisotropic term. In the present study, primary attention was given to maintaining the computational stability while improving the predictive performance particularly for coarse grid resolution in the near-wall region. For the extra anisotropic term used for this purpose, the present study introduced a residual term after subtracting an eddy-viscosity form from the Bardina SGS-Reynolds-stress model (Bardina et al., AIAA Paper, 80-1357, 1980). The resultant extra term yields no undesirable extra energy transfer between the grid-scale and SGS components that could cause numerical instability under coarse grid conditions. Therefore, this extra term is not expected to have any serious negative effects on the computational stability. In order to assess the performance, the proposed model was applied to the numerical simulation of fully-developed plane channel flows with various grid resolutions and at various Reynolds numbers. The computational results were considerably improved by the present SGS model and detailed investigations of the obtained results indicated the usefulness of the present model for engineering applications.

In large eddy simulation (LES), the mean-velocity distribution in wall turbulence depends strongly on the distribution of the ensembleaveraged Reynolds (Re) shear stress, which consists of two parts: the resolved grid-scale (GS) and unresolved subgrid-scale (SGS) components. As the grid resolution becomes coarser, the GS component decreases and thus the SGS component must increase to compensate for this. The GS decrease is originally caused by filtering, through which the power spectrum is cut off mainly in the high-wavenumber region. Therefore, the SGS model has been discussed mostly in terms of the energy transfer between the GS and SGS components. Recently, however, some studies have found that the SGS-stress anisotropy directly influences instantaneous GS vortex motions. This also means that the SGS stress may have a large effect on the ensemble-averaged GS Re stress because the instantaneous fluctuation of the SGS stress correlates with that of the velocity gradient in the GS budget. In this study, we investigate in detail the effect of the SGS stress on predicting the resolved GS Re shear stress through its budget. For this purpose, we perform a priori tests with highly resolved LES data of a plane channel flow. The knowledge obtained is then confirmed by a posteriori tests for various grid resolutions and Reynolds numbers. It is found that the SGS-stress anisotropy is very important for providing a reasonable trend of the GS Re shear stress, leading to more accurate prediction of the mean velocity for coarse-grid resolutions.

An anisotropy-resolving subgrid-scale (SGS) model for large eddy simulation was investigated. Primary attention was given to the predictive performance of the SGS model in the case of complex turbulence with flow impingement and/or flow separation. The SGS model was constructed by combining an isotropic linear eddy-viscosity model with an extra anisotropic term. Since the extra anisotropic term was modeled to prevent undesirable energy transfer between the grid-scale and SGS parts, the model is expected not to seriously affect computational stability. To validate the model performance for complex turbulent flow fields, the SGS model was applied to numerical simulations of a plane impinging jet and 3-D diffuser flow as well as fundamental plane channel flows. The SGS model provided reasonable predictions for these test cases. Furthermore, the predicted SGS stress components were decomposed into linear and anisotropic parts and their roles were investigated in detail. The usefulness of the present anisotropy-resolving SGS model in practical engineering applications was thus described.

In this study, a three-dimensional simulation of a fish-like body swimming in a channel with non-slip walls was carried out to investigate the effects of kinematics on swimming performance. Selfpropelled swimming in an inertial coordinate system was examined by using the direct forcing immersed boundary method. The fish body consisted of several rigid bodies and behaved analogously to a multi-segment robotic fish. The computational program was first validated by simulating fluid flow around a circular cylinder at Reynolds number (Re) =100 and Re = 1000, as well as around a settling particle. The results were compared with experimental and numerical results from past research in the area. A virtual parametric study of the tail-beat frequency, phase difference between neighboring body segments, and body amplitude was then conducted. The effect of the lateral and vertical distance between the model body and walls on swimming performance is also discussed. The results for the velocity and vorticity fields around the model body provided evidence for the mechanism of thrust generation and highlighted the effects of kinematics on swimming performance.