William J. Easson

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.

A Method to Estimate Wall Shear Rate with a Clinical Ultrasound Scanner

A simple technique to estimate the wall shear rate in healthy arteries using a clinical ultrasound scanner has been developed. This method uses the theory of fully developed oscillatory flow together with a spectral Doppler trace and an estimate of mean arterial diameter. A method using color flow imaging was compared with the spectral Doppler method in vascular phantoms and found to have errors that were on average 35% greater. Differences from the theoretic value for the time averaged wall shear rate using the spectral Doppler method varied by artery: brachial -9 (1) %; carotid -7 (1) %; femoral -22 (4) %; and fetal aorta -17 (10) %. Test measurements obtained from one healthy volunteer demonstrated the feasibility of the technique in vivo.

Particle image velocimetry: simultaneous two-phase flow measurements

A method using particle image velocimetry to study pneumatic conveyance of a solid phase is presented. The technology enables simultaneous velocity measurements of both phases by isolating the measurements of each phase. The image processing which implements the phase separation is simple, but has limitations. The method is restricted to low solid densities to avoid a quality drop in the measurements, and cross talk between measurements of the two phases. A discussion based on numerical and experimental results concludes on the working range of the method and demonstrates measurements of slip velocity.

Experimental study of three-dimensional breaking wave kinematics

Abstract Particle image velocimetry has been used to examine three-dimensional breaking wave kinematics. Two cases of wave breaking were studied. In the first case, the wave field contains a single frequency with a uniform angular spreading within a given range {{ — α, α.}}. The wave field of the second case consists of a number of frequencies with a uniform angular spreading applied to each frequency. In both cases, the waves are designed such that the wave energy is focused at a given point. The degree of angular spreading has been found to have great effects on the breaking characteristics and kinematics. Two types of breaker were observed, the first being plunging and the second being spilling. Increasing the angular spreading had the effect of making the velocities within the extreme waves larger. The ratio of the crest velocity to the breaking wave speed was approximately unity under both single and multiple frequency conditions, regardless of the angular spreading.

Fluid—structure interaction in axially symmetric models of abdominal aortic aneurysms

Abstract Abdominal aortic aneurysm disease progression is probably influenced by tissue stresses and blood flow conditions and so accurate estimation of these will increase understanding of the disease and may lead to improved clinical practice. In this work the blood flow and tissue stresses in axially symmetric aneurysms are calculated using a complete fluid—structure interaction as a benchmark for calculating the error introduced by simpler calculations: rigid walled for the blood flow, homogeneous pressure for the tissue stress, as well as one-way-coupled interactions. The error in the peak von Mises stress in a homogeneous pressure calculation compared with a fluid—structure interaction calculation was less than 3.5 per cent for aneurysm diameters up to 7 cm. The error in the mean wall shear stress, in a rigid-walled calculation compared with a fluid—structure interaction calculation, varied from 30 per cent to 60 per cent with increasing aneurysm diameter. These results suggest that incorporation of the fluid—structure interaction is unnecessary for purely mechanical modelling, with the aim of evaluating the current rupture probability. However, for more complex biological modelling, perhaps with the aim of predicting the progress of the disease, where accurate estimation of the wall shear stress is essential, some form of fluid—structure interaction is necessary.

Acquisition of 3-D Arterial Geometries and Integration with Computational Fluid Dynamics

A system for acquisition of 3-D arterial ultrasound geometries and integration with computational fluid dynamics (CFD) is described. The 3-D ultrasound is based on freehand B-mode imaging with positional information obtained using an optical tracking system. A processing chain was established, allowing acquisition of cardiac-gated 3-D data and segmentation of arterial geometries using a manual method and a semi-automated method, 3D meshing and CFD. The use of CFD allowed visualization of flow streamlines, 2-D velocity contours and 3-D wall shear stress. Three-dimensional positional accuracy was 0.17-1.8mm, precision was 0.06-0.47mm and volume accuracy was 4.4-15%. Patients with disease and volunteers were scanned, with data collection from one or more of the carotid bifurcation, femoral bifurcation and abdominal aorta. An initial comparison between a manual segmentation method and a semi-automated method suggested some advantages to the semi-automated method, including reduced operator time and the production of smooth surfaces suitable for CFD, but at the expense of over-smoothing in the diseased region. There were considerable difficulties with artefacts and poor image quality, resulting in 3-D geometry data that was unsuitable for CFD. These artefacts were exacerbated in disease, which may mean that future effort, in the integration of 3-D arterial geometry and CFD for clinical use, may best be served using alternative 3-D imaging modalities such as magnetic resonance imaging and computed tomography.

On the prediction of monocyte deposition in abdominal aortic aneurysms using computational fluid dynamics

In abdominal aortic aneurysm disease, the aortic wall is exposed to intense biological activity involving inflammation and matrix metalloproteinase–mediated degradation of the extracellular matrix. These processes are orchestrated by monocytes and rather than affecting the aorta uniformly, damage and weaken focal areas of the wall leaving it vulnerable to rupture. This study attempts to model numerically the deposition of monocytes using large eddy simulation, discrete phase modelling and near-wall particle residence time. The model was first applied to idealised aneurysms and then to three patient-specific lumen geometries using three-component inlet velocities derived from phase-contrast magnetic resonance imaging. The use of a novel, variable wall shear stress-limiter based on previous experimental data significantly improved the results. Simulations identified a critical diameter (1.8 times the inlet diameter) beyond which significant monocyte deposition is expected to occur. Monocyte adhesion occurred proximally in smaller abdominal aortic aneurysms and distally as the sac expands. The near-wall particle residence time observed in each of the patient-specific models was markedly different. Discrete hotspots of monocyte residence time were detected, suggesting that the monocyte infiltration responsible for the breakdown of the abdominal aortic aneurysm wall occurs heterogeneously. Peak monocyte residence time was found to increase with aneurysm sac size. Further work addressing certain limitations is needed in a larger cohort to determine clinical significance.

A dual-phantom system for validation of velocity measurements in stenosis models under steady flow.

A dual-phantom system is developed for validation of velocity measurements in stenosis models. Pairs of phantoms with identical geometry and flow conditions are manufactured, one for ultrasound and one for particle image velocimetry (PIV). The PIV model is made from silicone rubber, and a new PIV fluid is made that matches the refractive index of 1.41 of silicone. Dynamic scaling was performed to correct for the increased viscosity of the PIV fluid compared with that of the ultrasound blood mimic. The degree of stenosis in the models pairs agreed to less than 1%. The velocities in the laminar flow region up to the peak velocity location agreed to within 15%, and the difference could be explained by errors in ultrasound velocity estimation. At low flow rates and in mild stenoses, good agreement was observed in the distal flow fields, excepting the maximum velocities. At high flow rates, there was considerable difference in velocities in the poststenosis flow field (maximum centreline differences of 30%), which would seem to represent real differences in hydrodynamic behavior between the two models. Sources of error included: variation of viscosity because of temperature (random error, which could account for differences of up to 7%); ultrasound velocity estimation errors (systematic errors); and geometry effects in each model, particularly because of imperfect connectors and corners (systematic errors, potentially affecting the inlet length and flow stability). The current system is best placed to investigate measurement errors in the laminar flow region rather than the poststenosis turbulent flow region.

Fiber‐optic‐bundle delivery system for high peak power laser particle image velocimetry illumination

We demonstrate the design and implementation of a fiber‐optic‐bundle beam delivery system for particle image velocimetry (PIV) applications. The system is designed for the transmission of high peak power pulses (≳20 mJ) from a Q‐switched and frequency doubled Nd:YAG laser. A fiber bundle offers advantages over a single fiber in beam delivery systems for light sheet formation. The damage‐limit‐maximum power that can be transmitted is greater for the bundle than for any of its component fibers, and the quality of the derived light sheet is higher than that obtainable from a single large core fiber of power handling capacity equivalent to that of the bundle. The beam delivery system was demonstrated in PIV measurements on a premixed propane‐air flame.

Breaking wave forces and velocity fields

Abstract A new technique for measuring velocities under breaking wave crests using laser-doppler anemometry has been developed. Results may be obtained at positions up to 4 millimeters from the crest. A wave field is presented for a vertically fronted wave and comparison is made with an appropriate numerical study. The velocities obtained are far in excess of the predictions of linear theory. Measurements have also been made of the forces produced by this wave on a horizontal cylinder. Comparisons with large regular waves indicate that forces due to breaking waves can be up to five times greater for similar wave heights.