Shia-Hui Peng

Large Eddy Simulation for Turbulent Buoyant Flow in a Confined Cavity

Abstract A turbulent natural convection flow ( Ra =1.58×10 9 ) in a confined cavity with two differentially heated side walls was numerically investigated by means of large eddy simulation (LES). The mean flow in the cavity is characterized by stable thermal stratification and a relatively low turbulence level. The LES results for the mean flow quantities show good agreement with the experiment. This is particularly the case when the dynamic model is used. Nevertheless, there are some discrepancies in the prediction of turbulence statistics, particularly in the outer region of the near-wall flow where the boundary layer interacts with the recirculating core region. In the viscous/conductive sublayer of the boundary layer close to the heated/cooled vertical walls, the flow tends to form streak-like structures, which do not however emerge in the near-wall flow along the horizontal top and bottom walls. To resolve the flow structure near the vertical walls, sufficient grid resolution is required.

Computation of turbulent buoyant flows in enclosures with low-Reynolds-number k-ω models

This work deals with the computation of turbulent buoyant convection flows with thermal stratification using the low-Reynoldsnumber (LRN) k-x model. When applying the k-e model to buoyancy-driven cavity flows induced by diAerentially heated side walls, a problem commonly encountered at moderate Rayleigh numbers (Raa 10 10 ‐1 0 12 ) is that the model is not capable of giving gridindependent predictions owing to the transition regime along the vertical walls. It was found that the buoyancy source term for the turbulence kinetic energy, Gk, exhibits strong grid sensitivity, as this term is modelled with the Standard Gradient DiAusion Hypothesis (SGDH). By introducing a damping function into this term, the above grid-dependence problem is eliminated and, additionally, the modelled Gk renders correct asymptotic behavior near the vertical wall. The mechanism held in the k-x model for describing the onset of transition is analyzed. The present approach is simple for practical use and gives reasonable predictions. ” 1999 Elsevier Science Inc. All rights reserved.

On a subgrid-scale heat flux model for large eddy simulation of turbulent thermal flow

Abstract A non-linear subgrid-scale (SGS) heat flux model is introduced in large eddy simulation for turbulent thermal flows. Unlike the linear isotropic eddy diffusivity model, the proposed model accounts for the SGS heat flux in terms of the large-scale strain-rate tensor and the temperature gradients. This is equivalent to using a tensor diffusivity. The model is to some extent similar to a scale-similarity model subjected to a Taylor expansion for the filtering operation. The formulation leading to the present proposal is discussed. The model is examined in LES for a buoyant flow in an infinite vertical channel with two differentially heated side walls. It is shown that the proposed model reproduces reasonable results as compared with the isotropic SGS diffusivity model and DNS data.

A Modified Low-Reynolds-Number k-ω Model for Recirculating Flows

A modified form of Wilcoxs low-Reynolds-number k-ω model (Wilcox) is proposed for predicting recirculating flows. The turbulent diffusion for the specific dissipation rate, ω, is modeled with two parts: a second-order diffusion term and a first-order cross-diffusion term. The model constants are re-established. The damping functions are redevised, which reproduce correct near-wall asymptotic behaviors, and retain the mechanism describing transition as in the original model. The new model is applied to channel flow, backward-facing step flow with a large expansion ratio (H/h = 6), and recirculating flow in a ventilation enclosure. The predictions are considerably improved

Large-eddy simulation and deduced scaling analysis of Rayleigh-Benard convection up to Ra=10^9

Large-eddy simulation of turbulent Rayleigh–Bénard (RB) convection has been performed for a 6:1:6 open-ended domain for Rayleigh numbers ranging from 6.3 × 105 to 109 at Prandtl number of Pr = 0.71. The scaling analysis based on the LES data shows that the heat transfer follows a single relation of Nu = 0.162 Ra 0.286, which is consistent with the scaling law for the hard turbulence regime reported in several earlier experimental and DNS studies. The present LES also supports some earlier experimental and DNS findings that most of characteristic parameters can be scaled reasonably well with Ra number in the considered Ra number range using a single relation. Nonetheless, it is found that the scaling of several quantities shows a sensible offset from a single relation, and could be fitted better with the separate scaling relations for the soft and hard convective turbulence transitioned at about Ra = 4 × 107. It has been argued that the transition, reflected in the scaling relation, may be attributed to the increasing ‘containing effect’ of the plume leaving the horizontal wall on the plume approaching the wall at large Ra numbers in the near-wall region. **This paper is a modified version from the paper presented at the Forth International Symposium of Turbulence and Shear Flow Phenomena (Williamsburg, Virginia, 27–29 June 2005).

An improved k−ω turbulence model applied to recirculating flows

In this paper an improved k−ω turbulence model is proposed, which brings the asymptotic boundary value for ω into accord with direct numerical simulation (DNS) data. In the new ω-equation both a turbulent and a viscous cross-diffusion term are included, justified by an analogy to the Yap-correction and the pressure-diffusion process, respectively. The importance of cross-diffusion terms in removing the freestream sensitivity with respect to ω for free shear flows is shown. The performance of the model is evaluated and compared with DNS and with other turbulence models in a channel flow, a backward-facing step flow and a rib-roughened channel flow with heat transfer. The model requires neither wall-function nor wall-distance information and is fully integrable over the near-wall region.

Embedded Large-Eddy Simulation Using the Partially Averaged Navier–Stokes Model

An embedded large-eddy-simulation modeling approach is explored and verified using the partially averaged Navier-Stokes model as a platform. With the same base model, the turbulence-resolving large-eddy simulation region is embedded by setting the partially averaged Navier-Stokes model coefficient to f(k) < 1 as distinguished from its neighboring Reynolds-averaged Navier-Stokes region, where f(k) = 1 is specified. The embedded large-eddy simulation approach is verified in computations of a turbulent channel flow and a turbulent flow over a hump. Emphasis is placed on the impact of turbulent conditions at the Reynolds-averaged Navier-Stokes/large-eddy simulation interface using anisotropic velocity fluctuations generated from synthetic turbulence. The effect of the spanwise size of the computational domain is investigated. It is shown that the embedded large-eddy-simulation method based on the partially averaged Navier-Stokes modeling approach is computationally feasible and able to provide reasonable turbulence-resolving predictions in the embedded large-eddy simulation region. The wall-adapting local eddy-viscosity model is also evaluated for the hump flow and it is found that its performance is worse than that of the the low-Reynolds-number partially averaged Navier-Stokes model when the results are compared with experiments.

On the assessment of ventilation performance with the aid of numerical simulations

Abstract The assessment of ventilation performance is discussed. New local indices are developed with the aid of numerical simulations to quantify air diffusion and contaminant dispersion. The local purging effectiveness, Asp, is an index for evaluating the contribution of each inlet in a multi-inlet system. The local specific contaminant-accumulating index, α, can be used to indicate the tolerance of a ventilation flow to contaminants. Asp and α can be derived from transport equations. A method based on age-variation analysis is used to define Asp and the Expected Contaminant Dispersion Index (ECDI). The latter is an index for forecasting contaminant dispersion emitted at a specific location with unknown source strength. These new scales and methods can be used to assess ventilation performance.

Drag Prediction for the DLR-F6 Wing-Body Configuration Using the Edge Solver

Numerical investigations are reported on the DLR-F6 wing-body configuration with and without fairing. The configurations have been adopted as test cases for the Third AIAA Drag Prediction Workshop. The addition of the fairing is to eliminate the flow separation bubble in the junction between the wing trailing edge and the fuselage. The computations have been carried out using two groups of unstructured grids with different sizes. In addition to the effect of incidences, studies of grid convergence have also been performed. The computational fluid dynamics solver Edge is used for the investigation. The calculations confirm that the flow separation can be removed in the wing-fuselage junction with the fairing. For this configuration, the results obtained with the two groups of grid are very similar. Without fairing, however, one group of grids has pronounced a lower lift and produced a more extended trailing-edge separation. Because no experimental data are available for the flow condition as swerved in Drag Prediction Workshop-3, additional calculations have been carried out with the clean wing-body configuration at the Drag Prediction Workshop-2 Reynolds number to validate the numerical results against available experimental data. Very good agreement is obtained, in particular, for the global forces and moments. The calculation indicates that, as compared with experimental data, the grid which predicts a relatively large separation region provides improved predictions.

Towards the determination of regional purging flow rate

Abstract This paper deals with the description and determination of the purging flow rate, U p , for ventilation systems or equivalent flow systems. The regional purging flow rate and its use are discussed and proposed. By using the mass conservation principle, U p is embodied in various accessible mathematical expressions in terms of the transfer probability. Some U p -related parameters are described. A Markov chain model is proposed for determining the transfer probability and exploring several useful ventilation indices. An effective CN method is proposed for calculating the interchanging flow rates between various regions. The application of these proposals is demonstrated, and they appear to be promising for analyzing and assessing ventilation performance.