No description available.

A computational investigation was undertaken to examine techniques for predicting supersonic dynamic flows, involving unsteadiness over fixed and moving surfaces. The fixed geometries examined were cylinder-flares and compression ramps, and the moving body geometries a pitching aerofoil and a rapidly deployed flap. Investigation into the characteristics of incipient separation of a supersonic cylinder-flare flow revealed that the separated length varied with a power of the flare angle and that the variation in height of the separated region varies in a bi-modal manner with flare angle. For small-scale separations (flare angles less than those which would traditionally have been expected to induce separation) the height of the separated region was seen to vary slowly with flare angle. For larger flare angles, the separation bubble was found to grow rapidly in height and length with increasing flare angle and produce significant deflection of the external flow. Computations of a Mach 5, compression ramp induced unsteady shock boundary layer interaction exhibited self-sustained oscillations at frequencies and amplitudes consistent with experimental data. Large dynamic structures (up to 1.7 boundary layer thicknesses in extent) were observed, and their production, propagation and deformation illustrated. By modifying the turbulent viscosities produced by a non-dimensional implementation of the Baldwin-Lomax turbulence model (using under- relaxation) a turbulence model was produced which accurately predicted separation lengths for a series of Mach 6.85 compression ramp flows encompassing laminar, transitional and turbulent flow regimes (dependent on ramp angle). A technique was developed to enable efficient computation of dynamically moving and/or deforming body flows. This technique was based on hierarchical, adaptive mesh refinement coupled with automatic generation of body surfaces, in which mesh adaption was used to capture the body geometry to within a specified accuracy. This, in conjunction with automatic cell creation and destruction, enabled the derivation of steady and unsteady, time accurate, conservative boundary conditions. This algorithm was used to compute a quasi-steady laminar supersonic pitching aerofoil flow, and an unsteady turbulent supersonic flap deployment. In both cases agreement with experiment was found to be good.

A new formulation of Delayed Detached-Eddy Simulation (DDES) based upon elliptic relaxation is derived and implemented within a finite-volume framework. This new formulation is based upon the φ − f RANS model which has previously demonstrated both improved modelling of the near-wall physics and numerical robustness for industrial applications. The φ − f DDES model is calibrated and validated using Decaying Isotropic Turbulence (DIT) to establish the validity of the derivation and to calibrate the model constants. In light of the numeri- cal scheme requirements for DDES, a hybrid numerical scheme is proposed and implemented, which is shown to perform in the intended manner.Initially, three DDES formulations (SA-DDES, SST-DDES and φ − f DDES) are compared on the 2D periodic hills test case at Re = 10590 and Re = 37000. This test case primarily serves as a validation case to evaluate whether the im- plementation and calibration were correct. The flow over a NACA0021 airfoil post-stall at 60o incidence is then evaluated; a test case that DDES was origi- nally devised for (i.e massive separation from an airfoil). The three formulations are then evaluated on a 2D wall-mounted hump which exhibits largely geometry induced separation, but is still sensitive to the modelling of the initial separated shear layer and upstream turbulence levels. The final case is the Ahmed car body which combines both geometry and pressure-induced separation from a 3D surface. This complex flow is challenging for any turbulence modelling approach and is sensitive to the underlying RANS model.A general sensitivity to the underlying RANS model is demonstrated for the majority of the test cases investigated. The φ − f DDES model is shown to have encouraging performance on these wide range of test cases compared to the established SST-DDES and SA-DDES models. Whilst the φ − f DDES model is not a fix for the shortcomings of DDES, it is shown to be a practical and robust alternative to the established SST-DDES and SA-DDES variants that have become the de facto choice for many DDES users.

No description available.

In this thesis, the flow and heat transfer in dimpled channels have been investigated using the Reynolds-averaged Navier-Stokes (RANS) approach. The primary objective of this investigation is to identify the capabilities of RANS models to reproduce the characteristics of the flow and heat transfer in dimples. The flow in dimpled channels has been chosen as the test case due to their relevance to gas turbine cooling applications, as well as the fairly complex flow features over dimples, which poses a challenge to turbulence modelling. Five turbulence models have been tested in the present work. These include: the Launder and Sharma k-epsilon model, both the Craft et al. (1996) and (2000) cubic k-epsilon models, the Hanjalic and Jakirlic Reynolds stress model (RSM), as well as the Craft (1998) two-component limit (TCL) RSM. The models have been chosen such that all three classes of RANS closure were tested. The tested models have been applied to two dimpled channel configurations with increasing complexity. In the first, the flow over a single dimple in a channel has been considered, while in the second, the case of a staggered array of dimples has been examined. Moreover, across these two configurations, the effect of the dimple depth, the channel height and the Reynolds number have also been investigated. The results show that all models produce a physically viable solution for the problem of the flow in dimpled channels. Nevertheless, the Craft et al. (1996) and (2000) cubic k-ε models, as well as the Craft (1998) TCL RSM, predicted dimple flow structures that deviate from the expected state. In general, the main flow characteristics are reproduced by the RANS models, and the predicted mean velocity profiles are in good agreement with the data. All models report an overall enhancement in heat transfer levels when using dimples in comparison to those of a plane channel.

No description available.

The thesis introduces a new version of an elliptic-blending low-Reynolds-number eddy-viscosity Reynolds-averaged Navier Stokes model. It is a model intended to be implemented in an industrial solver. It will be argued that there is still room for such a simple model, though eddy-viscosity models must rely on developments specificallymade for higher order formulations. It is the aim of the v2-f model to integrate elements of Reynolds-stress modelling developments into a simpler formulation, but the former paradoxically suffers from numerical stiffness, which kept it out of reachof industry researchers everyday simulations. The v2-f formulation endeavours to reproduce the near-wall asymptotic behaviour of the turbulent quantities, as sounder alternative to empirical damping functions, and the required near-wall balance of small terms represents a numerical challenge. The present work first provides a comprehensive review of v2-f developments proposed over the past twenty years, and the different remedies for the numericalstiffness linked to the original formulation. The review focuses on ten v2-f variants, proposed between 1991 and 2006, whose behaviour is compared in some fundamental flows: the channel flow for five different Reynolds numbers, the asymptotic case of the logarithmic layer at infinite Reynolds number and the case of a flow with homogeneous sheared turbulence. Based on the conclusions of the review, the thesis proposes new developments. Firstly, the derivation of a new model, namely the φ - α model, is introduced. It relies on the resolution of two non-dimensional variables: φ represents the wall-normal anisotropy and α is a wall-proximity sensor. It is argued that only this formulation can address the numerical problems already mentioned without altering the predictions. Secondly, additional upgrades of the φ - α model are proposed to correct the dissipation rate equation. The aim is to improve the model behaviour in some specific regions of a boundary layer, by isolating some viscous terms and by improving the representation of turbulent transport at the edge of a boundary layer. Final developments are combined in a new model, the BL-v2/k model. The φ - α and BL-v2/k models are then validated for a set of two pressure induced separated flows and two buoyant flows, and beneficial effects of the proposed developments on the predictions are demonstrated. The numerical properties of the convergency of the BL-v2/k model are also reported at the end of this work.

No description available.

Turbulent buoyant jets are a major feature in fire hazards. The solution of the Reynolds Averaged Navier-Stokes (RANS) equations through computational fluid dynamic (CFD) techniques allow such flows to be simulated. The use of Reynolds averaging requires an empirical model to close the set of equations, this is known as the turbulence model. This thesis undertakes to investigate linear and nonlinear approaches to turbulence modelling and to apply the knowledge gained to the simulation of compartment fires. The principle contribution of this work is the reanalysis of the standard k- ε turbulence model and the implementation and application of more sophisticated models as applied to thermal plumes. Validation in this work, of the standard k- ε model against the most recent experimental data, counters the established view that the model is inadequate for the simulation of buoyant flows. Examination of previous experimental data suggests that the measurements were not taken in the self-similar region resulting in misleading comparisons with published numerical solutions. This is a significant conclusion that impacts of the general approach taken to modelling turbulence in this field. A number of methods for modelling the Reynolds stresses and the turbulent scalar fluxes have been considered and, in some cases for the first time, are applied to nonisothermal flows. The relative influence of each model has been assessed enabling its performance to be gauged. The results from this have made a valuable contribution to the knowledge in the field and have enabled the acquired experience to be applied to the simulation of compartment fires. The overall conclusion drawn from this thesis is that for the simulation of compartment fires, the most appropriate approach with current computational resources, is still the buoyancy corrected standard k- ε model. However, the turbulence scalar flux should be modelled by the generalised gradient diffusion hypothesis (GGDH) rather than the eddy-diffusivity assumption.

In hybrid RANS/LES, Reynolds-averaged Navier-Stokes (RANS) equations method is used to treat the near-wall region and large-eddy simulation (LES) is applied to the core turbulent region. Owing to the incompatibility of these two numerical modelling approaches, an artificial (i.e., non-physical) buffer layer forms along the interface where the model switches from RANS to LES. In this thesis, a novel dynamic forcing scheme incorporating backscatter is proposed in order to remove the artificial buffer layer. In contrast to previous forcing techniques, the proposed forcing is determined dynamically from the flow field itself, and does not require any extraction of turbulent fields from reference direct numerical simulation (DNS) or high-resolution LES databases. The proposed forcing model has been tested on three types of wall-bounded turbulent flows, namely, turbulent flow in a plane channel; turbulent flow in a spanwise rotating channel; and turbulent flow in a spanwise rotating rib-roughened channel. In order to validate the present hybrid approach, turbulence statistics obtained from hybrid RANS/LES simulations are thoroughly compared with the available DNS results and laboratory measurement data. Based on the study of a plane channel flow, transport equations for the resolved turbulent stresses and kinetic energy are introduced to investigate the effects of dynamic forcing on reduction of the thickness and impact of the artificial buffer layer. As long as the dynamic forcing is in use, the artificial buffer layer have been successfully removed, indicating that the proposed hybrid approach is insensitive to the choices of the forcing region or interface location. The predictive performance of the dynamic forcing scheme has been further evaluated by considering turbulent flows subjected to a special type of body force, i.e., the non-inertial and non-conservative Coriolis force. Due to the effects of system rotation, turbulence level is enhanced on the pressure side and suppressed on the suction side of the rotating channel. Furthermore, it is reported in this thesis that the classification of the roughness type now relies not only on the pitch ratio, but also on the rotation number in the context of rotating rib-roughened flows. / February 2016