Effect Of Wall Deformation Research Articles

INVESTIGATION OF FLOW IN AN IDEALIZED CURVED ARTERY: COMPARATIVE STUDY USING CFD AND FSI WITH NEWTONIAN AND NON-NEWTONIAN FLUIDS

Artery curvatures, where disturbed flow patterns are expected, are preferred sites of formation of atherosclerosis. Experimental studies have shown that low and oscillating wall shear stress (WSS) plays an important role in the development and progression of atherosclerosis. Accurate estimation of these biomechanical parameters is important to assess the risk of atherosclerosis formation. The coupled effects of non-Newtonian behavior of blood and artery wall flexibility for the transient blood flow through an idealized curved coronary artery are investigated using computational fluid dynamics (CFD) as well as fluid–structure interaction (FSI) simulations. The choice of fluid model, Carreau and Newtonian, was found to impact the time averaged and minimum WSS values. The effects of wall deformation on time averaged wall shear tress were negligible. However, a comparison of temporal minima of WSS along the curvature showed significant variations between CFD and FSI simulations. Since low WSS values are crucial in the prediction of atherosclerosis development, it is concluded that both the non-Newtonian behavior of blood and the wall flexibility should be considered for computational studies.

Effect of Wall Deformation on Dewatering-Induced Ground Surface Settlement

In practice, dewatering for pressure relief is commonly undertaken during ongoing excavation to secure bottom stability against basal upheaval. Simultaneously, through unloading, wall deflection is obviously observed. Noticing that both cause soil deformations, this research is to study the effect of wall deformation on dewatering-induced settlement. A coupled numerical analysis of finite-difference software is employed to model Shanghai soft soils under multi-aquifer-aquitard systems (MAASs) by analyzing the results in association with an empirical approach. Consequently, through gradual force reduction, shear strength at soil-wall interface is significantly diminished. As wall deformation increases instantaneously upon lower loading, wall surface becomes deformedly bending; this condition causes the challenge to workability of shear strength. Moreover, wall deformation caused by unloading affects dewatering-induced settlement substantially. Under smaller loading, large wall deflection is observed; soil plane of failure caused by both sliding and compression occurs along slip curve, with weaker shear-strength soils at rD = 0.4 and stronger shear-strength soils between rD = 0.4 and rD = 0.65, where rD is the distance from the wall that is normalized by the depth measured from ground surface. During dewatering, stronger soils tend to drag weaker soils upward to reduce large differential settlements caused by additional compression. Consequently, settlement becomes larger at rD = 0.4 and smaller at rD = 0.65. Remarkably, at rD > 2.3, both settlement curves that result from numerical analysis and empirical method show overlapping; this indicates that the unloading effect on dewatering-induced settlement at rD > 2.3 is insignificant. Furthermore, as wall reaches maximum allowable wall deflection by 67% applied force, additional compression caused by dewatering after loading remains smaller than that under 70% applied force, contributing to smaller dewatering-induced settlement.

Effect of wall deformability on the stability of shear-imposed film flow past an inclined plane

The linear stability of shear-imposed liquid film flow past an inclined plane is examined when a soft, deformable solid layer is attached to the inclined plane. A liquid film flowing past a rigid inclined plane exhibits a long-wave gas–liquid (GL) interfacial instability which is modified by the presence of an imposed shear stress at gas–liquid interface. This GL interfacial mode does not become unstable in creeping flow limit whether a GL interfacial shear is present or not. We demonstrate that the GL interface becomes unstable even at zero Reynolds number when the shear-imposed liquid film flows past an inclined plane which is coated with soft solid layer. This GL mode instability exists only when both shear and a deformable liquid–solid (LS) interface are simultaneously present. For non-zero Reynolds number, we show that there exists multiple unstable modes originating because of the presence of deformable LS interface. These unstable LS modes become important and practically realizable only for shear-imposed liquid film flow and become irrelevant for film flows in absence of imposed shear. We also show that this LS interfacial instability dominate the stability behavior of the composite fluid film-solid system in low Reynolds number regime. Our results suggest that the shear-imposed film flow can be made unstable by using a deformable solid coating in parameter regime where the film flow otherwise remains stable in rigid wall limit. Finally, we show that a deformable solid layer can be used to obtain a stable film flow configuration for the parameter regime where the GL interfacial mode is unstable for shear imposed film flow over a rigid incline. Thus, we demonstrate the capability of a soft solid layer in manipulation and control of instabilities for shear-imposed film flows.

Effect of wall deformability on the stability of surfactant-laden visco-elastic liquid film falling down an inclined plane

The stability behavior of a visco-elastic (Upper Convected Maxwell) liquid film flowing down an inclined plane is examined using the standard normal mode linear stability analysis in creeping flow limit. The inclined plane is coated with a deformable solid layer, and the gas-liquid interface is covered with a monolayer of an insoluble surfactant. There exist three modes of instabilities for this composite fluid-solid system: (i) the free surface gas-liquid (GL) mode, (ii) the surfactant or Marangoni mode, and (iii) the liquid-solid (LS) mode. Previous studies have shown that the Marangoni mode remains stable for film flowing down a rigid inclined plane. We show that when the rigid wall is replaced by a deformable wall, the Marangoni mode becomes unstable when a non-dimensional deformability parameter, defined as G=μV/EsR, increases above a threshold value (μ, V, Es, and R are fluid viscosity, free-surface velocity, shear modulus of soft solid layer, and fluid thickness, respectively). It is well known that the GL mode remains stable for Newtonian liquid film at zero Reynolds number, but becomes unstable for a visco-elastic film flowing past a rigid inclined plane even in creeping flow limit purely because of fluid elasticity. Previous studies have shown that the presence of soft solid layer can suppress this free surface GL mode instability. The presence of surfactant also has a stabilizing effect on this GL mode instability in rigid limit. Here, we evaluate whether a soft solid coating can be used to obtain a stable flow configuration for a surfactant-covered visco-elastic film as well particularly (i) in view of the above mentioned Marangoni mode destabilization observed solely due to wall deformability, and (ii) when the surfactant stabilizing contribution remains insufficient to suppress the free-surface instability. We demonstrate that there exists a window in terms of parameter G where the free surface GL mode instability is suppressed without exciting any other mode. We also identify the parameter regimes where it is not possible to achieve stable film flow, however, in these cases, we show that the Marangoni mode can be selectively destabilized by appropriately selecting the thickness and shear modulus of solid layer.

Wave Propagation in Tapered Vessels: New Analytic Solutions That Account for Vessel Distensibility and Fluid Compressibility

The central aim of this paper is to contribute to the theoretical analysis and understanding of the effect of vessel tapering on the propagation of pressure and velocity wave forms. To this end, it presents new analytic expressions for the temporal and spatial variation of these two variables that account for weak fluid compressibility. It extends previous work in which only the effect of wall deformation (i.e., vessel distensibility) was taken into account. The solutions are derived in the frequency domain and can account for the steady solution component (d.c. component) obtained by taking the asymptotic limit for very low frequencies. It is shown that the effect of compressibility makes the equations more complex but it is still possible to derive closed form analytic solutions in terms of Bessel functions of orders 1/3 and 4/3. The analytical solutions are compared with full 3D fluid structure interaction (FSI) simulations for the case of propagation of a step pressure variation at the inlet of a tapered vessel. Good agreement is observed between the 1D analytical and 3D numerical solutions.

Response of half–full horizontal cylinders under transverse excitation

The present work investigates the response of half–full horizontal cylindrical vessels under external excitation in the transverse direction. A two dimensional mathematical model is developed to describe sloshing effects in rigid vessels. The velocity potential is expressed in a series form, where each term is the product of a time function and the associated spatial function. In this geometrical configuration the spatial functions are not orthogonal and the problem is not separable. Application of the boundary conditions results in a system of ordinary linear differential equations, which are solved numerically. Sloshing frequencies of half–full horizontal cylinders are computed, and hydrodynamic forces are calculated. Under harmonic excitation, the formulation results in a system of linear equations, allowing for a semi-analytical solution. A simplified version of the mathematical model is also developed, which considers the first two terms of the series and results in an elegant solution. Furthermore, assuming a beam-type deformation of the container, the simplified formulation can be extended to approximate the coupled response of the container-liquid system. Using this formulation, the response of a typical pressure vessel under ground-motion excitation is calculated and the effects of wall deformation are demonstrated.

Distribution of Earth Pressure on Sheet-Pile Walls

All theoretical solutions of the sheet pile problem used at present assume linear earth pressure distribution above dredge level and disregard the effect of wall deformation on the pressure distribution. The distribution coefficient method as evolved herein provided a solution in which wall deformation and earth pressure distribution are interrelated. The variation of the coefficient of earth pressure is presented as a function of sheet-pile flexural rigidity number, penetration depth, tie-rod position, tie-rod yielding and soil characteristics. The obtained earth pressure distribution curves exhibit the effect of arching in the soil mass. Both the pressure distribution curves and the computed maximum bending moment and tie-rod load values are in close agreement with test results.