In this paper we consider the introduction of a body force, in the incompressible limit, into the lattice Boltzmann model. A number of methods are considered and their suitability to our objectives determined. When there is no density variation across the fluid, gravity can be introduced in the form of an altered pressure gradient. This method correctly satisfies the Navier-Stokes equation; however, if there is a non-negligible density variation present (produced by the body force or otherwise) this method becomes less accurate as the density variation increases and the constant density approximation becomes less valid. Three other methods are also considered for application when there is a non-negligible density variation. The equations of motion satisfied by these models are found up to second order in the Knudsen number and it is seen that only one of these methods satisfies the true Navier-Stokes equation. Numerical simulations are performed to compare the different models and to assess the range of application of each.

The three components of the velocity field in a plane can be measured simultaneously by combining particle image velocimetry (PIV) and stereoscopy. A set-up has been devised to take two stereoscopic images of the flow simultaneously with only one camera, thus making automatic recording of three-dimensional (3D) velocity fields quite straightforward. Theoretical and practical considerations in implementing this stereoscopic PIV system are presented. The performance of the technique has been investigated by measuring several known displacements produced on a solid surface, and the application of the system to the measurement of the 3D velocity field in an acoustic streaming flow is presented.

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We consider the application of the lattice Boltzmann BGK model to simulate sound waves in situations where the density variation is small compared to the mean density. Linear sound waves are simulated in two different situations: a plane wave propagating in an unbound region; and a wave in a tube. For both cases the behaviour of the simulated waves is found to be in good agreement with analytic expressions. Non-linear sound waves are also simulated and are seen to display the expected features.

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The effect of polyethyleneoxide (Polyox grade WSR 301) on grid-generated turbulence was investigated using laser anemometry and flow-visualization techniques. It was found that the polymer additive reduced both the turbulent intensity and the rate of decay behind the grid. At typical drag-reducing concentrations, turbulent energy spectra were qualitatively the same as those in water, in agreement with the results of other investigations. However, at higher additive concentrations, the dissipation-range spectra showed noticeable attenuation. This seemed to be a threshold effect with onset at a polymer concentration between 100 and 250 ppm. This result was supported by photographs of dye-injection tracer but in this case the onset concentration for small-eddy suppression was between 50 and 100 ppm.