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Hemodialysis vascular access dysfunction is still considered as a major cause of morbidity and mortality in hemodialysis patients. The most common cause of this vascular access dysfunction is venous stenosis secondary to the progression of venous intimal hyperplasia at the graft-vein junction or in the draining vein. The geometry of the vessels and the local hemodynamics are believed to be contributory factors in the development of intimal hyperplasia. In the present study, the hemodynamic parameters in two types of vascular access grafts of different diameters: 6 mm and 8 mm tapered to 6 mm at the arterial site were compared. A three-dimensional CFD model was developed to simulate blood flow in the two graft types. The boundary conditions applied were a pulsatile velocity waveform at the arterial inlet and a pulsatile pressure waveform at the venous outlet. Computational fluid dynamics software was used to solve the time dependent Navier-Stokes equations under physiological conditions in both graft types. Since the cross section area and the diameter of the venous side of the tapered graft are larger than those of the straight one, the flow aligns well with the vein downstream of the graft and hence the smaller vortices are generated. Comparing the parameters at the two venous anastomosis regions also indicates that in spite of the higher flow rate in 6–8 mm tapered graft, the velocity and the wall shear stress values are lower than the values of the 6 mm straight graft.Copyright © 2010 by ASME

A numerical code to simulate mass transfer effects on spherical cavitation bubble collapse in an acoustic pressure domain in quiescent water has been developed. Gilmore equation is used to simulate bubble dynamics, with considering mass diffusion and heat transfer. Bubbles with different initial radii were considered in quiescent infinite water in interaction with sinusoidal shock waves with different magnitudes of amplitude and frequency. Simulations were done in two cases; with and without considering mass transfer. Good agreement with reference data was achieved. For bubbles with small radii in high frequency pressure field with low amplitude, mass transfer causes larger maximum radii and growth time, and more violent resultant collapse. Decreasing pressure frequency or increasing its amplitude causes larger maximum radii, longer collapse time, and more violent collapse. But, in cases with mass transfer because at the last moments of collapse stage a large amount of water vapor is trapped inside the bubble, the collapse will become less violent. For larger bubbles collapse becomes more violent for the cases without mass transfer in all pressure amplitudes and higher frequencies. But decreasing pressure frequency makes the collapse of the bubbles with mass transfer more violent. However, mass transfer effects decreases with increasing initial bubble radius.Copyright © 2010 by ASME

In the present study collapse stage of cavitation bubbles in the vicinity of a rigid boundary in quiescent water has been studied numerically with concentrating on initial bubble radius and stand-off parameter (the ratio between the distance of bubble center and the wall, and maximum bubble radius γ). Navier-Stokes and energy equations in an axisymmetric domain were hired to simulate the flow, and VOF method was used in order to track the interface between liquid and gas phases. Surrounding liquid was assumed to be incompressible water, and water vapor as ideal gas was considered for the bubble contents. Simulations were done in a wide range of bubble radius and stand-off parameter. Good agreements with reference experimental data were achieved. Results show that for the same bubble radii maximum jet velocities increase and collapse time decreases with increasing stand-off. However, increasing initial bubble radius causes the differences between maximum jet velocities, bubble volume at the moment of jet impact to the other side of the bubble, and collapse time of the cases with different values of stand-off to decrease. For the bubbles with smaller values of stand-off liquid jet forms sooner and its length with respect to bubble size is greater. Also, for the same standoff parameters liquid jet is wider and its length with respect to bubble size is smaller for smaller bubbles. As bubble manner tends to spherical with increasing stand-off parameter, calculated velocities in present study become larger than those of physical values.Copyright © 2010 by ASME