David Apsley

UK-ADMS: A new approach to modelling dispersion in the earth's atmospheric boundary layer

The UK atmospheric dispersion modelling system is a computer code for modelling the dispersion of buoyant or neutrally buoyant gaseous and particulate emissions to the atmosphere. It comprises a number of individual modules, each dealing with either one aspect of the dispersion process or data input and output. Each module can be modified independently to keep it, and hence the whole model, scientifically up-to-date. Emissions can be of any duration. The effects of plume rise, wet and dry deposition, radio-active decay, hilly and variable roughness terrain, coastal regions and large buildings are allowed for. Fluctuations in concentration about the calculated mean are also modelled for time scales less than one hour. Output includes mean values, variances and percentiles of air concentration, dosage (time integrated concentration raised to some power), and deposition to the ground. For emissions of radio-active isotopes estimates can be made of the ground level gamma-radiation dose rate beneath the plume. A user interacts with the model via a system of menus based on Microsoft Windows so that it is easy to use.

A new low-Reynolds-number nonlinear two-equation turbulence model for complex flows

A new nonlinear, low-Reynolds-number k–e turbulence model is proposed. The stress–strain relationship is formed by successive iterative approximations to an algebraic Reynolds-stress model. Truncation of the process at the third iteration yields an explicit expression for the Reynolds stresses that is cubic in the mean velocity gradients and circumvents the singular behaviour that afflicts the exact solution at large strains. Free coefficients are calibrated – as functions of y∗ – by reference to direct numerical simulation (DNS) data for a channel flow. By using the nonlinear stress–strain relationship, the sublayer behaviour of all turbulent stresses is reproduced. The extension to nonequilibrium conditions is achieved by sensitising the model coefficients to strain and vorticity invariants on the basis of formal relations derived from the algebraic Reynolds-stress model. The new model has been applied to a number of complex two dimensional (2-D) flows, and its performance is compared to that of other linear and nonlinear eddy-viscosity closures.

Advanced Turbulence Modelling of Separated Flow in a Diffuser

The paper describes an investigation into the predictive performance of linear and non-linear eddy-viscosity models and differential stress-transport closures for separated flow in a nominally two-dimensional, asymmetric diffuser. The test case forms part of a broader collaborative exercise between academic and industrial partners. It is demonstrated that advanced turbulence models using strain-dependent coefficients and anisotropy-resolving closure offer tangible advantages in predictive capability, although the quality of their performance can vary significantly, depending on the details of closure approximations adopted. Certain features of the flow defy resolution by any of the closures investigated. In particular, no model resolves correctly the flow near the diffusers inclined wall immediately downstream of the inlet corner, which may reflect the presence of a “flapping” motion associated with a highly-localised process of unsteady separation and reattachment.

A LIMITED-LENGTH-SCALE k-ε MODEL FOR THE NEUTRAL AND STABLY-STRATIFIED ATMOSPHERIC BOUNDARY LAYER

Analytical and numerical models of the neutral and stably-stratifiedatmospheric boundary layer are reviewed. Theoretical arguments andcomputational models suggest that a quasi-steady state is attainable in aboundary layer cooled from below and it is shown how this may be incorporatedwithin a time-steady, one-dimensional model. A new length-scale-limitedk-ε model is proposed for flows where a global maximum mixing length isimposed by the finite boundary-layer depth or, in stably-stratifiedconditions, by the Obukhov length, whilst still reducing to a form consistentwith the logarithmic law in the surface layer. Simulations compare favourablywith data from the Leipzig experiment and from Cardington airfield inEngland.

Flow and dispersion over topography: A comparison between numerical and laboratory data for two-dimensional flows

Abstract Computations of the flow and dispersion over two-dimensional hills of various slope and submerged in a neutrally stable boundary layer are described. The results are compared with those of corresponding laboratory experiments undertaken by the U.S. Environmental Protection Agency (Khurshudyan et al., 1981, Report EPA-600/4-81-067). It is shown that a suitably modified k-v turbulence model generally produces reasonable agreement for the mean flow behaviour, but somewhat lower values for the turbulent kinetic energy and the lateral plume spread. This latter deficiency can be offset by normalising ground-level concentration values by the maximum value obtained in the absence of the hill. The resulting “terrain amplification factors” are in good agreement with the laboratory data. For a hill slope large enough to generate steady separation, the impact of the recirculating flow on concentration levels is also well predicted although, for somewhat lower slopes when separation is intermittent, results are less satisfactory.

Investigation of advanced turbulence models for the flow in a generic wing-body junction

The predictive performance of several turbulence models, among them formulations based on non-linear stress-strain relationships and on stress-transport equations, is examined in a collaborative university-industry study directed towards a generic wing-body junction. The geometry consists of a variation of the symmetric NACA 0020 aerofoil mounted on a flat plate, with the oncoming stream aligned with the aerofoils symmetry plane. The dominant feature of this flow is a pronounced horseshoe vortex evolving in the junction region following separation ahead of the aerofoils leading edge. This case is one of 6 forming a broad programme of turbulence-model validation by UMIST, Loughborough University, BAE Systems, Aircraft Research Association, Rolls-Royce plc and DERA. Key aspects of this collaboration were a high level of interaction between the partners, the use of common grids and boundary conditions, and numerical verifications aimed at maximizing confidence in the validity of the computational solutions. In total, 12 turbulence models were studied by four partners. Model performance is judged by comparing solutions with experimental data for pressure fields on the plane wall and around the aerofoil; for velocity, turbulence energy, shear stress and streamwise normal stress in the upstream symmetry plane; and for velocity, turbulence energy and shear stress in cross-flow planes downstream of the aerofoil leading edge. The emphasis of the study is on the structure of the horseshoe vortex and its effects on the forward flow. The main finding of the study is that, for this particular 3D flow, second-moment closure offers predictive advantages over the other models examined, especially in terms of the far-field structure of the horse-shoe vortex, although no model achieves close agreement with the experimental data in respect of both mean flow and turbulence quantities.

Simulation of turbulent flows around a circular cylinder using nonlinear eddy-viscosity modelling: steady and oscillatory ambient flows

Abstract Steady incident flow past a circular cylinder for sub- to supercritical Reynolds number has been simulated as an unsteady Reynolds-averaged Navier–Stokes (RANS) equation problem using nonlinear eddy-viscosity modelling assuming two-dimensional flow. The model of Craft et al. (Int. J. Heat Fluid Flow 17 (1996) 108), with adjustment of the coefficients of the ‘cubic’ terms, predicts the drag crisis at a Reynolds number of about 2×105 due to the onset of turbulence upstream of separation and associated changes in Strouhal number and separation positions. Slightly above this value, at critical Reynolds numbers, drag is overestimated because attached separation bubbles are not simulated. These do not occur at supercritical Reynolds numbers and drag coefficient, Strouhal number and separation positions are in approximate agreement with experimental measurements (which show considerable scatter). Fluctuating lift predictions are similar to sectional values measured experimentally for subcritical Reynolds numbers but corresponding measurements have not been made at supercritical Reynolds numbers. For oscillatory ambient flow, in-line forces, as defined by drag and inertia coefficients, have been compared with the experimental values of Sarpkaya (J. Fluid Mech. 165 (1986) 61) for values of the frequency parameter, β=D2/νT, equal to 1035 and 11240 and Keulegan–Carpenter numbers, KC=U0T/D, between 0.2 and 15 (D is cylinder diameter, ν is kinematic viscosity, T is oscillation period, and U0 is the amplitude of oscillating velocity). Variations with KC are qualitatively reproduced and magnitudes show best agreement when there is separation with a large-scale wake, for which the turbulence model is intended. Lift coefficients, frequency and transverse vortex shedding patterns for β=1035 are consistent with available experimental information for β≈250−500. For β=11240, it is predicted that separation is delayed due to more prominent turbulence effects, reducing drag and lift coefficients and causing the wake to be more in line with the flow direction than transverse to it. While these oscillatory flows are highly complex, attached separation bubbles are unlikely and the flows probably two dimensional.

A wall-distance-free low-Re k ε turbulence model

Abstract A low Reynolds number k — ϵ turbulence model is presented. The model is based on the following three attributes: (1) it involves neither explicit wall distance nor normal-to-wall directionality, except to the first grid point off walls; (2) it enforces time scale realisability; (3) it invokes a simple wall boundary condition for ϵ. Several computed results are shown, indicating the validity of the current approach.

Flow and dispersion over hills: Comparison between numerical predictions and experimental data

A finite-volume solver (SWIFT) for numerical prediction of flow and dispersion over hills is described. Comparison of predictions with experiment are made for (i) adiabatic flow over 2-D hills (RUSHIL experiment) and (ii) stably-stratified flow over real terrain (Cinder Cone Butte). Suitably-modified k−e turbulence models are shown to yield satisfactory wind speed profiles (windpower) and terrain-amplification factors (dispersion applications).

Non-linear eddy-viscosity modelling of separated flows

The paper reviews some recent developments in the area of non-linear eddy-viscosity modelling and investigates the performance of several model variants when these are applied to flows with complex strain. The formulation of this type of model has been motivated by the desire to combine the advantageous numerical properties and economy of linear Boussinesq-viscosity models with the superior predictive performance of second-moment closure which is mathematically complex and numerically challenging. The rationale, fundamental principles and inherent properties of non-linear stress-strain relationships are considered first in general terms, with particular emphasis being placed on the mechanisms by which normal-stress anisotropy and sensitivity to curvature strain are represented. This is followed by a review of several particular model forms and the principal elements of their derivation. The models are then applied to four two-dimensional flows featuring, inter alia, strong curvature, irrotational strainin...