Shian Gao

Numerical study of heat transfer from an impinging jet

Abstract A computational study of the impingement of a thermally inhomogeneous turbulent jet on a solid plate, using large-eddy simulation, is reported. We investigate the case of a plane jet of water issuing from a plane channel into an enclosed pool and impinging normally on a perspex plate 1.8 jet-widths downstream. It is shown that the dynamics of the turbulence in this particular geometry results in the temperature variations at the plate surface having very high lateral correlation, so that lateral conduction of heat within the plate fails to have any significant effect on the transmission of thermal fluctuations from the fluid into the plate. By this means a simple one-dimensional model of the thermal, interaction between the media may be justified.

Large-eddy simulation of turbulent heat transport in enclosed impinging jets

The results of large-eddy simulation (LES) of thermally inhomogeneous jets issuing into an enclosed pool and impinging on a plate are presented. The LES has been performed by strongly conservative linear finite-volume techniques, with a simple subgrid-scale model. Several cases have been simulated with different geometries of the outflows at either side of the plate, or with different fluid properties. The mechanisms by which thermal eddies are formed and transported into the impingement zone have been fully elucidated for one of the simulated cases through graphic and video output of the thermal field. The reason for the very high lateral correlations found in the experiments and confirmed by LES results is explained convincingly in terms of the altered shape of thermal eddies as they convect toward the plate.

Investigation of Coherent Structures in Turbulent Mixing Layers using Large Eddy Simulation

Large Eddy Simulation (LES) of two- and three-dimensional spatially-developing mixing layers are presented. The maximum Reynolds number obtained in the simulations is Re ∼ 70, 000 based on the visual thickness of the layer and the velocity difference across it. The purpose of this research is to assess the capability of two- and three-dimensional LES to replicate the features found in the plane mixing layer that originates from initially laminar conditions. Two dimensional simulations offer a poor representation of the real flow; the obtained velocity fluctuation statistics do not achieve similarity and the lack of spanwise dimension prevents the formation of streamwise vortex structures. This prevents the flow from undergoing transition to turbulence and the flow remains in an unsteady laminar state. Three-dimensional simulations produce flow statistics that agree very well with experimental data, and replicate many of the salient features of the laboratory flow. Streamwise vortex structure is observed to develop in the flow, and a transition to turbulence in the flow is recorded. Careful inspection of flow visualisation reveals the presence of quasi-two-dimensional coherent structures embedded within the turbulent flow, the topography of which bears remarkable resemblance to comparable experimental flow visualisation. In order to produce a credible numerical representation of the flow, it is essential to perform well-resolved, fully three-dimensional simulations that are initialised from boundary conditions that mimic those found in the real flow.

Turbulent Simulation of a Flat Plate Boundary Layer and Near Wake

Extensive LES studies have been made of the low-Reynolds number turbulent boundary layer following a trip, and the near wake behind a thin flat plate. The numerical results are compared with those from parallel experiments of identical flow conditions conducted in conjunction with the simulations. Apart from the integral parameters and profiles of mean quantities and fluctuations, the balance analyses of Reynolds stress, kinetic energy and dissipation have also been derived. These linked studies allow us to understand in greater depth the complex dynamics that operates in both wall-bounded and spatially developing turbulent flows.

The effect of surface roughness on rotor-stator cavity flows

We are concerned with the CFD simulation of annular rotor-stator cavities using the general purpose second-order finite volume method (FVM) solver OpenFOAM® and Large Eddy Simulation (LES) methods. Simulations of cavities with smooth surfaces are conducted at various Reynolds numbers, and the properties of the mean turbulent flows are validated against experimental and numerical data available in the literature. Comparisons show that second-order accurate FVM approaches can produce high-fidelity simulations of rotor-stator cavities to an acceptable accuracy and are therefore a viable alternative to the computationally intensive high-order methods. Our validated second-order FVM model is then combined with the parametric force approach of Busse and Sandham [“Parametric forcing approach to rough-wall turbulent channel flow,” J. Fluid Mech. 712, 169–202 (2012)] to simulate cavities with a rough rotor surface. Detailed flow visualisations suggest that roughness-induced disturbances propagate in the downstream direction of the rotor flow toward the outer wall of the cavity. The outer wall subsequently provides a passage to transport said roughness effects from the rough rotor layer to the smooth stator layer. We demonstrate that rotor-stator cavity flows are sensitive to even small roughness levels on the rotor surface alone.We are concerned with the CFD simulation of annular rotor-stator cavities using the general purpose second-order finite volume method (FVM) solver OpenFOAM® and Large Eddy Simulation (LES) methods. Simulations of cavities with smooth surfaces are conducted at various Reynolds numbers, and the properties of the mean turbulent flows are validated against experimental and numerical data available in the literature. Comparisons show that second-order accurate FVM approaches can produce high-fidelity simulations of rotor-stator cavities to an acceptable accuracy and are therefore a viable alternative to the computationally intensive high-order methods. Our validated second-order FVM model is then combined with the parametric force approach of Busse and Sandham [“Parametric forcing approach to rough-wall turbulent channel flow,” J. Fluid Mech. 712, 169–202 (2012)] to simulate cavities with a rough rotor surface. Detailed flow visualisations suggest that roughness-induced disturbances propagate in the downstream...

Mixed convection heat transfer enhancement in a cubic lid-driven cavity containing a rotating cylinder through the introduction of artificial roughness on the heated wall

The aim of the present numerical investigation is to comprehensively analyse and understand the heat transfer enhancement process using a roughened, heated bottom wall with two artificial rib types (R-s and R-c) due to unsteady mixed convection heat transfer in a 3D moving top wall enclosure that has a central rotating cylinder, and to compare these cases with the smooth bottom wall case. These different cases (roughened and smooth bottom walls) are considered at various clockwise and anticlockwise rotational speeds, −5 ≤ Ω ≤ 5, and Reynolds numbers of 5000 and 10 000. The top and bottom walls of the lid-driven cavity are differentially heated, whilst the remaining cavity walls are assumed to be stationary and adiabatic. A standard k-e model for the Unsteady Reynolds-Averaged Navier-Stokes equations is used to deal with the turbulent flow. The heat transfer improvement is carefully considered and analysed through the detailed examinations of the flow and thermal fields, the turbulent kinetic energy, the m...

On the heat transfer effects of nanofluids within rotor-stator cavities

Owing to the rapid development of a number of technological and industrial sectors, high-performance electronic devices are now ubiquitous in modern engineering and industrial applications. Effective heat management is crucial to the smooth operation of such devices, and sometimes conventional methods of heat transfer fail to deliver the required performance. Recent advances in the field of nanofluids are a promising route to improve heat-transfer performance, and this is our motivation. We propose two computational fluid dynamics models for a rotor-stator cavity operating at Reω = 1.0 × 105 and filled with a fluid that consists of different volume fractions of Al2O3 nanoparticles. The first model simulates the nanofluid mixture using a single-phase transport model, and the second approach uses a two-phase transport model that allows for the relative velocity between the particle and fluid phases. All simulations are conducted using the second-order accurate solver, OpenFOAM®, that is based on the finite volume method and using Large eddy simulation methods. Our results show that the higher volume fractions of Al2O3 nanoparticles can achieve higher heat transfer rates, and at the same time, dilute nanoparticle concentrations have subtle effects on the momentum transport of the system. This is an advantage over micro-particle dispersion. Furthermore, we consider the effects of particle forces in the two-phase model, such as Brownian and thermophoresis forces, and suggest that the thermophoresis forces are the dominant effect within the cavity geometry.Owing to the rapid development of a number of technological and industrial sectors, high-performance electronic devices are now ubiquitous in modern engineering and industrial applications. Effective heat management is crucial to the smooth operation of such devices, and sometimes conventional methods of heat transfer fail to deliver the required performance. Recent advances in the field of nanofluids are a promising route to improve heat-transfer performance, and this is our motivation. We propose two computational fluid dynamics models for a rotor-stator cavity operating at Reω = 1.0 × 105 and filled with a fluid that consists of different volume fractions of Al2O3 nanoparticles. The first model simulates the nanofluid mixture using a single-phase transport model, and the second approach uses a two-phase transport model that allows for the relative velocity between the particle and fluid phases. All simulations are conducted using the second-order accurate solver, OpenFOAM®, that is based on the finite ...