T. Glyn Thomas

Wind-direction effects on urban-type flows

Practically all extant work on flows over obstacle arrays, whether laboratory experiments or numerical modelling, is for cases where the oncoming wind is normal to salient faces of the obstacles. In the field, however, this is rarely the case. Here, simulations of flows at various directions over arrays of cubes representing typical urban canopy regions are presented and discussed. The computations are of both direct numerical simulation and large-eddy simulation type. Attention is concentrated on the differences in the mean flow within the canopy region arising from the different wind directions and the consequent effects on global properties such as the total surface drag, which can change very significantly—by up to a factor of three in some circumstances. It is shown that for a given Reynolds number the typical viscous forces are generally a rather larger fraction of the pressure forces (principally the drag) for non-normal than for normal wind directions and that, dependent on the surface morphology, the average flow direction deep within the canopy can be largely independent of the oncoming wind direction. Even for regular arrays of regular obstacles, a wind direction not normal to the obstacle faces can in general generate a lateral lift force (in the direction normal to the oncoming flow). The results demonstrate this and it is shown how computations in a finite domain with the oncoming flow generated by an appropriate forcing term (e.g. a pressure gradient) then lead inevitably to an oncoming wind direction aloft that is not aligned with the forcing term vector.

Extending helicopter operations to meet future integrated transportation needs

Helicopters have the potential to be an integral part of the future transport system. They offer a means of rapid transit in an overly populated transport environment. However, one of the biggest limitations on rotary wing flight is their inability to fly in degraded visual conditions in the critical phases of approach and landing. This paper presents a study that developed and evaluated a Head up Display (HUD) to assist rotary wing pilots by extending landing to degraded visual conditions. The HUD was developed with the assistance of the Cognitive Work Analysis method as an approach for analysing the cognitive work of landing the helicopter. The HUD was tested in a fixed based flight simulator with qualified helicopter pilots. A qualitative analysis to assess situation awareness and workload found that the HUD enabled safe landing in degraded conditions whilst simultaneously enhancing situation awareness and reducing workload. Continued development in this area has the potential to extend the operational capability of helicopters in the future.

Numerical study of turbulent manoeuvring-body wakes: interaction with a non-deformable free surface

Direct numerical simulation (DNS) is used to investigate the development of a turbulent wake created by an impulsively accelerating axisymmetric self-propelled body below a non-deformable free surface. The manoeuvring body is represented by the combination of an immersed boundary method and a body force. The Reynolds number based on either the diameter of the virtual body or the jet forcing intensity is relatively high (O(1000)), corresponding to the fully turbulent case. The vertical growth of the coherent structure behind the body is restricted by the upper and lower stress-free layers, and the wake signatures are observed to penetrate to the free surface. The late-time behaviour of the dipole induced due to vertical confinement can be predicted by scaling laws, also relevant to a stratified fluid.

A numerical investigation of impulsively generated vortical structures in deep and shallow fluid layers

The evolution and formation of large-scale turbulent coherent structures induced by an impulsive jet between non-deformable stress-free layers are investigated via direct numerical simulation at a jet Reynolds number of 1250. The ratio of the initial size of the vortex to the domain depth is varied to study the influence of the bounding surface confinement. A non-conservative body force is applied to the governing equations to represent the momentum source. During the forcing period, the coherent structure appears in the form of a leading vortex ring together with a trailing jet, and breaks down to turbulence due to an instability very similar to the Widnall instability before interacting with the free surface. The input parameters (the momentum flux J, the forcing period Δtf, and the domain depth h) can be grouped together as the confinement number C = J1/2Δtf/h2 to parameterise the intensity and strength of the eddy signature at the free surface. Increasing the confinement number corresponds to reducing...

On Subgrid-Scale Model Implementation for a Lee-Wave Turbulent Patch in a Stratified Flow above an Obstacle

Results of application of a subgrid-scale (SGS) model to describe turbulence arising in a stably stratified flow above an obstacle are shown for high Schmidt/Prandtl numbers. An appropriate SGS Schmidt/Prandtl number is 0.3 for the case of weakly unstable stratification in the internal wave breaking region. Use of the SGS model allows us to remove numerical noise and obtain adequate spectra, as well as fine details of secondary instabilities during the transition to turbulence.

Internal Wave Breaking in Stratified Flows Past Obstacles

Turbulent patches occurring in environmental flows can arise from breaking internal waves generated topographically. The convective overturning of waves leads to shear instability and then to turbulence which develops into the fully mixed region in the place of initial wave breaking. The objective of the present studies is to explore the steady internal wave breaking observed in stably stratified flows past obstacles [1]. We use a numerical approach based on well-resolved Navier–Stokes DNS/LES methods using parallel multi-block architecture [2] with the Boussinesq approximation and sponge layer treatment to avoid wave reflection from upstream/downstream boundaries.