Analysis of temporal shear stress gradients during the onset phase of flowover a backward-facing step

Endothelial cells in blood vessels are exposed to bloodflow and thus fluid shear stress. In arterial bifurcations and stenoses, disturbed flow causes zones of recirculation and stagnation, which are associated with both spati al and temporal gradients of shear stress. Such gradients have been linked to the generation of atherosclerotic plaques. For in-vitro studies of endot helial cell responses, the sudden-expansion flow chamber has been widely us ed and described. A two-dimensional numerical simulation of the onset phase of flow through the chamber was performed. The wall shear stress action on the bottom plate was computed as a function of time and distance from the sudden expansion. The results showed that depending on the time for the flo w to be established, significant temporal gradients occurred close to the s econd stagnation point of flow. Slowly ramping the flow over 15 s instead o f 200 ms reduces the temporal gradients by a factor of 300, while spatial g radients are reduced by 23 percent. Thus, the effects of spatial and tempor al gradients can be observed separately. In experiments on endothelial cell s, disturbed flow stimulated cell proliferation only when flow onset was su dden. The spatial patterns of proliferation rate match the exposure to temp oral gradients. This study provides information on the dynamics of spatial and temporal gradients to which the cells are exposed in a sudden-expansion flow chamber and relates them to changes in the onset phase of flow.