Immersed boundary-finite element model of fluid-structure interaction in the aortic root

It has long been recognized that aortic root elasticity helps to ensure efficient aortic valve closure, but our understanding of the functional importance of the elasticity and geometry of the aortic root continues to evolve as increasingly detailed in vivo imaging data become available. Herein, we describe fluid-structure interaction models of the aortic root, including the aortic valve leaflets, the sinuses of Valsalva, the aortic annulus, and the sinotubular junction, that employ a version of Peskin's immersed boundary (IB) method with a finite element (FE) description of the structural elasticity. We develop both an idealized model of the root with three-fold symmetry of the aortic sinuses and valve leaflets, and a more realistic model that accounts for the differences in the sizes of the left, right, and noncoronary sinuses and corresponding valve cusps. As in earlier work, we use fiber-based models of the valve leaflets, but this study extends earlier IB models of the aortic root by employing incompressible hyperelastic models of the mechanics of the sinuses and ascending aorta using a constitutive law fit to experimental data from human aortic root tissue. In vivo pressure loading is accounted for by a backwards displacement method that determines the unloaded configurations of the root models. Our models yield realistic cardiac output at physiological pressures, with low transvalvular pressure differences during forward flow, minimal regurgitation during valve closure, and realistic pressure loads when the valve is closed during diastole. Further, results from high-resolution computations demonstrate that IB models of the aortic valve are able to produce essentially grid-converged dynamics at practical grid spacings for the high-Reynolds number flows of the aortic root

An Efficient Implicit Finite-Difference Scheme for Transonic Flow

In this paper, transonic flow, where the interaction of a shock wave and a boundary layer often leads to extremely complicated flow phenomena, has been studied numerically. A unique method of implicit formulation is proposed by way of analyzing a model equation, which can separately deal with convection, diffusion and source terms. By combining this idea with Flux Vector Splitting, a Modified Implicit Flux Vector Splitting of the Navier-Stokes equations, which can avoid approximate factorization or block-bidiagonal, has been developed. So it has obvious superiority over the conventional FVS in terms of accuracy, robustness, convergence and computing cost. In the cases of two and three dimensional transonic and separated flows through a double throated nozzle and D’elery Case C, the numerical results agree with the published calculation results and measured data for both shock positions, separation/attachment points and overall flow field

Dispersion in oscillatory flows

The enhanced axial mixing which is caused by dispersion in oscillatory flows in some mass transfer devices may limit the reactor performance. This effect has provided the motivation for the present study in which oscillatory flow dispersion in a flat channel of large aspect ratio is investigated. The rate of spreading of a uniform slug of some passive tracer has been predicted using numerical and analytical techniques and the results have been verified experimentally. The numerical approach has used a finite difference time-marching method to obtain predictions for the channel concentrations. From the results, the dispersion coefficient (D) has been evaluated for Strouhal numbers of O.O1→0.2 and for mean Reynolds numbers of O.4→2OO at Schmidt numbers (Sc) O(1O³) . It has been concluded that under these conditions D varies as stroke squared. Unless the flow is not quasi-steady (i.e. if pulsatile Reynolds number α²O(l)) D is only a weak function of frequency. These predictions for the dispersion coefficient have been in excellent agreement with those of Watson (256). It has also been concluded from the numerical study that the phase of the velocity sinusoid at the instant of injection has a critical effect upon the form of the concentration evolution. An approximate analytical technique has been developed in which weighted mean cross-channel concentrations are defined. The wall concentration is expressed approximately using a Fourier series. This procedure leads to ordinary differential equations for the axial moments. When the axial variance of mean concentration and the dispersion coefficient were computed in this way for quasi-steady flows good agreement was obtained with the numerical work. Simple opto-electronic gauges have been developed to measure mean cross-channel concentrations. The sensors have been used to obtain experimental data for the dispersion coefficient of a furrowed channel mass transfer device using slug stimulus techniques. Experimental investigations of dispersion in oscillatory flows in a flat channel using these gauges has produced values for D which are in agreement with the theoretical predictions for quasi-steady flows.</p

A Novel Membrane Bioreactor for Microbial-Growth

A novel membrane bioreactor, previously assessed for its gas transfer characteristics, was used in various size and membrane configurations for the growth of the strictly aerobic bacterium Pseudomonas aeruginosa. The bioreactor was found to readily support growth, and the initial growth rates showed the previously demonstrated enhanced effect in gas O-2 mass transfer of the dimpled membrane bioreactor over flat membrane bioreactors. The production of a secondary metabolite by a Pseudomonas sp. following growth was demonstrated, as was the biotransformation of a nitrile by nocardia rhodochrous with the removal of the biotransformation products across a membrane. The potential of the bioreactor, in terms of other applications in the field of biotechnology, is discussed