Apparent stresses in disturbed pulsatile flows.

Traditional attempts at decomposing measured velocities into repeatable and random components are examined for a set of velocity data measured under pulsatile flow conditions distal to a 90% axisymmetric constriction. The Reynolds numbers, which are typical of those found in the human carotid artery, are such that transitional phenomena occur during portions of the pulsatile cycle at several axial stations. The implications of the method selected for velocity decomposition upon the computation of fluctuating or 'apparent' stresses is a point of major focus. It is shown that the usual estimation of Reynolds stresses in a pulsatile flow by subtracting the ensemble-averaged velocity from the instantaneous velocity leads to an underestimation of the apparent stress when coherent or repeatable disturbances exist in the flow. An alternative decomposition using a frequency domain approach is presented which combines both random and coherent stresses into a single apparent stress, and it is proposed that this approach is preferable to the traditional ensemble averaging method when estimating fluctuating stresses in arterial flows.