Browsing by Subject "residence time"

Now showing 1 - 2 of 2

Open Access
Computational and Analytic Perspectives on the Drift Paradox
(2010) Pasour, VB; Ellner, SP
The fact that many small aquatic and marine organisms manage to persist in their native environments in the presence of constant advection into unfavorable habitat is known as the "drift paradox." Although advection may determine large scale biological patterns, individual behavior such as predation or vertical/horizontal migration can dominate at smaller scales. Using both computational and analytical methods to model flow in an idealized channel, we explore the extent to which biological processes can counteract physical drivers. In particular, we investigate how different zooplankton migration behaviors affect biological retention time under a variety of flow regimes and whether a combination of physical/biological regimes exists that can resolve the drift paradox, i.e., allow the zooplankton to avoid washout for time periods much greater than the hydrologic retention time. The computational model is a three-dimensional semi-implicit hydrodynamic model which is coupled with an individual-based model for zooplankton behavior, while the analytical model is a simple partial differential equation containing both advective and behavioral components. The only behavior exhibited by the zooplankton is diel vertical migration. Our studies show that the interaction of zooplankton behavior and exchange flow can significantly influence zooplankton residence time. For a channel without vegetation, the analytical methods give biological residence times that vary by at most a day from the computational results.
Open Access
Fluid Dynamics of a Centrifugal Left Ventricular Assist Device
(2010) Selgrade, Brian Paul
High shear stresses and shear rates in left ventricular assist devices (LVADs) make endothelialization of the LVAD difficult and likely contribute to cleavage of large von Willebrand factor multimers and resulting bleeding problems in patients. To better understand shear in a centrifugal LVAD, flow was simulated using finite volume and computational fluid dynamics (CFD) analysis. The k-ω model simulated turbulence and sliding meshes were used to model the movement of the impeller. CFD results showed high-shear backflows in the radial gap between the impeller and the volute wall, but residence times in this region were under 5ms. It is unclear if this is sufficient to cleave VWF, and more study is necessary to determine if other areas in the LVAD have potential for VWF cleavage. Although the walls near the outlet experience low shear stress and may be good candidates for endothelialization, shear stresses above 20-30Pa on all other walls of the pump make the possibility of endothelial cell growth elsewhere in the LVAD unlikely. An LVAD designed specifically to have low shear may be a better candidate for endothelialization.