DETERMINATION OF PRINCIPAL REYNOLDS STRESSES IN PULSATILE FLOWS AFTERELLIPTIC FILTERING OF DISCRETE VELOCITY-MEASUREMENTS

The purpose of this study was to develop a method to accurately determ ine mean velocities and Reynolds stresses in pulsatile flows. The puls atile flow used to develop this method was produced within a transpare nt model of a left ventricular assist device (LVAD). Velocity measurem ents were taken at locations within the LVAD using a two-component las er Doppler anemometry (LDA) system. At each measurement location, as m any as 4096 realizations of two coincident orthogonal velocity compone nts were collected during preselected time windows over the pump cycle . The number of realizations was varied to determine how the number of data points collected affects the accuracy of the results. The durati on of the time windows was varied to determine the maximum window size consistent with an assumption of pseudostationary flow. Erroneous vel ocity realizations were discarded from individual data sets by impleme nting successive elliptical filters on the velocity components. The me an velocities and principal Reynolds stresses were determined for each of the filtered data sets. The filtering technique, while eliminating less than 5 percent of the original data points, significantly reduce d the computed Reynolds stresses. The results indicate that, with prop er filtering, reasonable accuracy can be achieved using a velocity dat a set of 250 points, provided the time window is small enough to ensur e pseudostationary flow (typically 20 to 40 ms). The results also reve al that the time window which is required to assume pseudostationary f low varies with location and cycle time and can range from 100 ms to l ess than 20 ms. Rotation of the coordinate system to the principal str ess axes can lead to large variations in the computed Reynolds stresse s, up to 2440 dynes/cm2 for the normal stress and 7620 dynes/cm2 for t he shear stress.