T.G. Thomas

Structure of turbulent flow over regular arrays of cubical roughness.

The structure of turbulent flow over large roughness consisting of regular arrays of cubical obstacles is investigated numerically under constant pressure gradient conditions. Results are analysed in terms of first- and second-order statistics, by visualization of instantaneous flow fields and by conditional averaging. The accuracy of the simulations is established by detailed comparisons of first- and second-order statistics with wind-tunnel measurements. Coherent structures in the log region are investigated. Structure angles are computed from two-point correlations, and quadrant analysis is performed to determine the relative importance of Q2 and Q4 events (ejections and sweeps) as a function of height above the roughness. Flow visualization shows the existence of low-momentum regions (LMRs) as well as vortical structures throughout the log layer. Filtering techniques are used to reveal instantaneous examples of the association of the vortices with the LMRs, and linear stochastic estimation and conditional averaging are employed to deduce their statistical properties. The conditional averaging results reveal the presence of LMRs and regions of Q2 and Q4 events that appear to be associated with hairpin-like vortices, but a quantitative correspondence between the sizes of the vortices and those of the LMRs is difficult to establish; a simple estimate of the ratio of the vortex width to the LMR width gives a value that is several times larger than the corresponding ratio over smooth walls. The shape and inclination of the vortices and their spatial organization are compared to recent findings over smooth walls. Characteristic length scales are shown to scale linearly with height in the log region. Whilst there are striking qualitative similarities with smooth walls, there are also important differences in detail regarding: (i) structure angles and sizes and their dependence on distance from the rough surface; (ii) the flow structure close to the roughness; (iii) the roles of inflows into and outflows from cavities within the roughness; (iv) larger vortices on the rough wall compared to the smooth wall; (v) the effect of the different generation mechanism at the wall in setting the scales of structures.

Large eddy simulation of turbulent flow in an asymmetric compound open channel

A Large Eddy Simulation of turbulent flow in a compound open channel with one floodplain is reported for a Reynolds number of approximately 42000. The results are in good agreement with experimental measurements and previous numerical calculations. The mean velocity field, secondary circulation field, bed stress distribution, and lateral stress distribution are presented in detail.

Direct numerical simulation of vortex ring evolution from the laminar to the early turbulent regime

Direct numerical simulation is used to study the temporal development of single vortex rings at various Reynolds numbers and core thicknesses. Qualitative differences between the evolution of thin- and thick-core rings are observed leading to a correction factor to the classical equation for the ring translational velocity. We compare the obtained linear modal growth rates with previous work, highlighting the role of the wake in triply periodic numerical simulations. The transition from a laminar to a turbulent ring is marked by the rearrangement of the outer core vorticity into a clearly defined secondary structure. The onset of the fully turbulent state is associated with shedding of the structure in a series of hairpin vortices. A Lagrangian particle analysis was performed to determine the ring entrainment and detrainment properties and to investigate the possibility of an axial flow being generated around the circumference of the core region prior to the onset of turbulence.

Development of a parallel code to simulate skewed flow over a bluff body

This paper outlines a new complex geometry large-eddy simulation code using finite differences and a multigrid Poisson solver written for a parallel computer. The flow domain may be constructed from an arbitrary arrangement of rectangular blocks thus permitting flow in regions with complicated shapes, and may be mapped to any number of processors up to the number of blocks. The code is used to simulate turbulent flow at a Reynolds number of 3000 past a cube placed at ground level in rough terrain with its sides set 45° to mean flow direction. Preliminary results are presented which reproduce the conical vortices on the top of the cube.

Large-eddy simulation of flow in a rectangular open channel

Turbulent flow in a narrow open channel is investigated using the large-eddy simulation (LES) technique in which the surface is allowed to freely deform. A relatively large Reynolds number of 90,40...

Simulation of skewed turbulent flow past a surface mounted cube

The flow past a cubic ground-mounted obstacle placed in a turbulent wind environment is studied using the large eddy simulation technique. The wind environment is taken from a pre-computed database containing the time-dependent inflow boundary conditions and representing a typical full-scale urban wind environment (Jenson number J=60). The Reynolds number R=10 000 is high enough for viscous scaling effects to be ignored, the turbulence intensity is about 15% at the cube height, and the integral length Lux is about 1.1 times the cube height h. The cube is aligned with one corner pointing upstream so that a pair of conical roof vortices are created. The computational grid used is effectively 362×226×98 in the streamwise, spanwise, and vertical directions, i.e. about 3×107 degrees of freedom, and uses 32 grid points along the sides of the cube. Two simulations are performed: (a) the flow with the cube absent so that the reference wind environment can be assessed; and (b) the flow past the cube for that wind environment. We present the flow topology as given by the mean streamlines, the roof pressures, the mean and fluctuating velocity and pressure field, and flow visualisation of the unsteady vortex shedding. A new shedding mechanism is identified which explains the turbulence statistics found in the wake.

Free-Surface Effects in Open Channel Flow at Moderate Froude and Reynold's Numbers

A large-eddy simulation of flow in an open channel has been carried out at the moderate Froude number, Fr = 0.66, and Reynolds number. Re = 20,800. A resolution has been used that allowed the simulation to be carried out on a workstation; thus enabling the results obtained to be viewed within an engineering context. The code used treats the free-surface exactly and allows it to freely deform with the underlying turbulence. The turbulent features found in this study are consistent with those found both by experimental observations and direct numerical simulations at lower Reynolds numbers where the free-surface has been treated as a stress free rigid lid.

The instability of a vortex ring impinging on a free surface

Direct numerical simulation is used to study the development of a single laminar vortex ring as it impinges on a free surface directly from below. We consider the limiting case in which the Froude number approaches zero and the surface can be modelled with a stress-free rigid and impermeable boundary. We find that as the ring expands in the radial direction close to the surface, the natural Tsai–Widnall–Moore–Saffman (TWMS) instability is superseded by the development of the Crow instability. The Crow instability is able to further amplify the residual perturbations left by the TWMS instability despite being of differing radial structure and alignment. This occurs through realignment of the instability structure and shedding of a portion of its outer vorticity profile. As a result, the dominant wavenumber of the Crow instability reflects that of the TWMS instability, and is dependent upon the initial slenderness ratio of the ring. At higher Reynolds number a short-wavelength instability develops on the long-wavelength Crow instability. The wavelength of the short waves is found to vary around the ring dependent on the local displacement of the long waves

Large Eddy Simulation of a symmetric trapezoidal channel at a Reynolds number of 430,000

This paper dcsribes a Large Eddy Simulation of steady uniform flow in a symmetric compound channel of trapezoidal cross-section with flood plains at a Reynolds number of 430.000. The simulation captures the complex interaction between the main channel and the flood plains and predicts the bed stress distribution, velocity distribution, and the secondary circulation across the floodplain. The results are compared with experimental data from the SERC Flood Channel Facility at Hydaulics Research Ltd, Wallingford, England.

Turbulent simulation of open-channel flow at low Reynolds number

Abstract A numerical technique for simulating turbulent flows in which the free surface is allowed to undergo arbitrarily large deformations and is subject only to a maximum slope limit is applied to turbulent open channel flow at a Reynolds number of approximately 3000 based on the surface velocity and depth. The test problem has been extensively studied in the literature and allows detailed comparisons to be made. It is found that the method is in general agreement with published results and can be used for a more extensive examination of turbulent fluid mechanics at a free surface.