Clive A. Greated

Stereoscopic particle image velocimetry

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.

Review of LDA and PIV applied to the measurement of sound and acoustic streaming

Abstract A review of laser Doppler anemometry (LDA) and particle image velocimetry (PIV) with their application to the measurement of sound is presented. The fundamental principles behind LDA and PIV are discussed and extended to the application of sound measurement. Special attention is paid to analysis of LDA signals including the Hilbert transform, which enables amplitude information to be obtained about various frequency components of a signal and wavelet analysis, which allows non-stationary signals to be accurately analysed. The influence of the refractive index variations in a medium due to a sound wave on the laser beams of an LDA signal is discussed. Attention is also paid to acoustic streaming which arises due to high-intensity sound, and PIV results are presented to demonstrate the effect.

An analysis of the scanning beam PIV illumination system

The scanning beam illumination method provides a highly efficient method of illuminating flow fields for recording PIV (particle image velocimetry) images so that the instantaneous velocity profile of the whole flow field can be determined. The authors analyse the scanning beam technique and discuss its advantages over the more conventional pulsed illumination methods. The versatility of the scanning beam system is demonstrated with examples of its application to the study of both breaking water waves and pneumatic particle transport.

Application of the lattice Boltzmann method to transition in oscillatory channel flow

In this study the applicability of the lattice Boltzmann method to oscillatory channel flow with a zero mean velocity has been evaluated. The model has been compared to exact analytical solutions in the laminar case (Reδ < 100, where Reδ is the Reynolds number based on the Stokes layer) for the Womersley parameter 1 < α < 31. In this regime, there was good agreement between numerical and exact analytical solutions. The model was then applied to study the primary instability of oscillatory channel flow with a zero mean velocity. For these transitionary flows the parameters were varied in the range 400 < Reδ < 1000 and 4 < α < 16. Disturbances superimposed on the numerical solution triggered the two-dimensional primary instability. This phenomenon has not been numerically evaluated over the range of α or Reδ currently investigated. The results are consistent with quasi-steady linear stability theories and previous numerical investigations.

The measurement of flow velocity and acoustic particle velocity using particle-image velocimetry

This paper outlines the method by which the flow velocity and acoustic particle velocity can be measured instantaneously over an area using particle-image velocimetry techniques. Theoretical expressions for the power spectrum and autocorrelation function of the intensity distribution in the plane of the interrogation area when a flow and sound field is present are derived and it is shown that the flow velocity and the acoustic particle velocity can be measured. The technique is used to measure the velocities in a standing wave tube with a monotonic sound field of sufficient strength such that acoustic streaming is set up. The limitations of this technique are also discussed.

Fibre optic beam delivery system for high peak power laser PIV illumination

Diffractive optical elements (DOEs) are used to couple Q-switched and frequency-doubled Nd:YAG laser beams into optical fibres to achieve significantly increased damage thresholds, enabling fibre optic beam delivery for high-speed particle image velocimetry (PIV) measurements. For single fibre delivery systems, the maximum pulse energy density that can be transmitted is increased by a factor of 5-10 compared with the best that can be achieved using conventional optics, of up to 10 mJ in a m core diameter fibre at 532 nm. We also applied the DOE arrangement to our previously developed bundle delivery system, comprising nineteen m core diameter fibres. On testing, over 1000 pulses with energies of 30 mJ were successfully transmitted, with no indication of damage. This allows fibre delivery to become a practical option for many air-flow PIV applications, as demonstrated here with measurements of flow in a square duct.

Particle image velocimetry for predictions of acceleration fields and force within fluid flows

We describe an experimental investigation of various methods of using the non-intrusive particle image velocimetry (PIV) technology to obtain the acceleration field of fluid flows and particularly the force being exchanged between a fluid flow and its boundaries. Methods based on the Lagrangian and the Eulerian specifications respectively have been developed, applied to experiments and compared. The experiments were performed by using a four-CCD-camera system designed to acquire between two and four frames in a fast time sequence, to provide single- or multiple-exposure PIV images of general fluid flows. The experiments involved various types of surface waves impinging upon a vertical wall at the end of a flume. Simultaneous monitoring with various transducers provided the experimental evaluation of the force predictions, whereas numerical simulations and PIV measurements have been used to validate the techniques for determining accelerations.

On the measurement of strain rate in an oscillatory baffled column using particle image velocimetry

Abstract We report, for the first time, our direct experimental measurement of velocity vectors and strain-rate distributions in an oscillatory baffled column (OBC) using time-resolved particle image velocimetry (PIV). The technique allowed a time series of spatial velocity maps to be obtained for several phases per oscillation cycle and the results, comprising several thousand such maps, illustrate in detail the variations of velocity and strain rate in a baffled region over a range of oscillation amplitudes and oscillation frequencies and provide a deep insight into the mixing, vortex convection and transport mechanism in such a device. The results show that an OBC not only provides enhanced mixing, but also offers low strain rates, which are lower than those in stirred tank vessels. We have also reported for the first time the quantitative correlation between the strain rate and the power dissipation in an OBC. Using the PIV technique we are able to quantify the strain rates experienced in an OBC.

Effect of polymer additives on the small‐scale structure of grid‐generated turbulence

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.

A stochastic model for photon correlation measurements in sound fields

A model is presented for the measurement of periodic acoustic fields using laser Doppler anemometry utilising the photon counting correlation method of signal analysis. The model agrees well with previous experimental work and yields, as limiting cases, earlier results (published previously) and results obtained from considering the process to be one of simple frequency modulation.