To analyze flow effects in some pathological coditions, such as a stenosis, a sudden-expansion flow chamber is often used to simulate those flow conditions. In such a flow chamber, flow is led over a step expansion. Right after the expansion point, the flow divides and causes a forward flow zone and a recirculating zone -- an eddy.
The size of the recirculation zone depends on several parameters, primarily flow rate. Higher flow velocity leads to a larger eddy. In addition, there are dynamic events at the onset of flow: The eddy needs several milliseconds to fully develop.
This movie shows the onset phase of flow in a sudden-expansion flow chamber. Blue colors indicate a flow to the right, while a red tint shows flow with a main direction to the left (recirculation). The white bars represent the approximate direction of flow at a certain location. The flow rate rises during the first 200 ms from 0.1 to 3.5 ml/sec and remains constant thereafter. A steady state is reached 300 ms after the onset of flow. The development of the primary eddy below the sudden expansion, and the secondary eddy at the top downstream of the reattachment point, can clearly be seen. Both the increase of flow and the growth of the primary eddy cause strong temporal gradients of shear stress at the bottom plate. Significant temporal gradients can also be observed downstream of the reattachment point, where a stagnation zone forms. At even higher flow rates, this stagnation zone would eventually lead to the formation of a tertiary eddy.
Temporal gradients in shear, but not spatial gradients, stimulate
endothelial cell proliferation
Haidekker MA, White CR, Bao X, Frangos JA.
Circulation. 2001 May 22;103(20):2508-13.
Analysis of temporal shear stress gradients during the onset phase of flow over a backward-facing step.
Haidekker MA, White CR, Frangos JA.
J Biomech Eng. 2001 Oct;123(5):455-63.