J. A. Cosgrove

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.

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.

Application of the lattice Boltzmann model to simulated stenosis growth in a two-dimensional carotid artery

The lattice Boltzmann model is used to observe changes in the velocity flow and shear stress in a carotid artery model during a simulated stenosis growth. Near wall shear stress in the unstenosed artery is found to agree with literature values. The model also shows regions of low velocity, rotational flow and low near wall shear stress along parts of the walls of the carotid artery that have been identified as being prone to atherosclerosis. These regions persist during the simulated stenosis growth, suggesting that atherosclerotic plaque build-up creates regions of flow with properties that favour atherosclerotic progression.

Investigation of A Lattice Boltzmann Model with a Variable Speed of Sound

A lattice Boltzmann model is considered in which the speed of sound can be varied independently of the other parameters. The range over which the speed of sound can be varied is investigated and good agreement is found between simulations and theory. The onset of nonlinear effects due to variations in the speed of sound is also investigated and good agreement is again found with theory. It is also shown that the fluid viscosity is not altered by changing the speed of sound.

PIV applied to Eckart streaming produced by a medical ultrasound transducer

Particle image velocimetry (PIV) is applied for the first time to study Eckart streaming induced by a medical ultrasonic transducer operating at a frequency of 3.3 MHz and effective acoustic intensities of 0.25 and 3 Wcm(-2). A temporal series of velocities in a two-dimensional plane were recorded resulting in an experimental set comprising over half a million velocity data points. These enabled average and fluctuating properties to be determined and clearly indicated the quasi-steady nature of the flow. The average large scale velocity fluctuations along the axis caused by this quasi-steady property were calculated to be 2 and 20 m ms(-1) at effective intensities of 0.25 and 3 Wcm(-2) respectively corresponding to approximately 25% of the peak flow velocity in both cases. Furthermore averaged shear rates were calculated with peak values of 1 and 8 s(-1) for the low and high intensities respectively. The present investigation indicates the usefulness of PIV for such studies and serves as a prelude to investigations of streaming in biological type fluids.

Application of the acousto-optic effect to pressure measurements in ultrasound fields in water using a laser vibrometer

A non-intrusive measuring technique, applied to sensing and measuring acoustic waves at ultrasonic frequencies is considered. The method is optically based and so does not interfere with the ultrasound field. The measurement procedure relies on the acousto-optic effect, that is the change in refractive index which occurs with changing pressure in the ultrasound field. This change in refractive index is detected through the change in the path length of a laser beam propagating through the region of interest. Typically these changes are small corresponding to a physical change of the order of 10−6 m. Fourier analysis is used to separate the component of the signal corresponding to the pressure variation from background noise and vibrations which can be dominant. Application of the technique is illustrated for an underwater ultrasound transducer. Measurements are made using the optical technique and compared to measurements taken with a hydrophone. The effectiveness of the optical measuring technique is disc...

Gravity-capillary internal wave simulation using a binary fluid lattice Boltzmann model

The simulation of internal waves using a lattice Boltzmann model is considered. Two different situations are considered: a sharp interface between two fluids of different densities; and a continuous density change over a finite depth. Both situations are examined for pure gravity waves and for gravity-capillary driven waves. When a sharp interface is applied the wave motion is that of interfacial waves. When a wide interface is applied interfacial waves are simulated when surface tension effects are significant. When the surface tension effects are small enough that they can be neglected, internal waves on a continuously varying density distribution are modelled.

The accuracy of blood velocity measurement using ultrasound

In this study we explore the use of colour flow imaging to quantify blood velocity and wall shear rate. A known velocity profile was obtained using a straight tube flow rig. Colour Doppler ultrasound images were obtained and velocity profiles extracted off-line. There was a small discrepancy (2.6%) in maximum velocity, which was not present after convolution with the colour flow point spread function, hence is likely to be due to velocity gradient broadening within the sample volume. There was disagreement at the edge of the vessel due to loss of velocities below the clutter filter. At a typical clinical colour flow scale (12.8 cms/sup -1/) the error in wall shear measurement is 62 % and at a lower scale setting (4.4 cms/sup -1/) the error is 11%. This improvement seems to be due to the lower level of clutter filter that is used as the velocity scale decreases.

Characterization of the bubble cluster and velocity field in the focal region of a lithotripter

The cavitation bubble cluster and the velocity field in the focal region of a lithotripter are investigated. Images of the bubble cluster were captured which show the bubbles increasing in size over approximately 500 μs before collapsing rapidly. Velocity measurements were obtained using particle image velocimetry. A few milliseconds after the inception of cavitation the velocity field consisted of localized flows with maximum velocities and shear rates of the order of 100 mm s−1 and 50 s−1 respectively. At times 40 ms after cavitation an average uni-directional flow of the order of 10 mm s−1 was observed. The mechanism by which the flow is generated is also considered.