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PubMed1985

Estimation of Reynolds shear stresses during pulsatile flow in the region of aortic valves.

Some investigators have attempted to estimate the Reynolds shear stress on the basis of a single component of velocity. The purpose of this investigation is to determine the validity of such estimates in a complex flow field, such as occurs in the cardiovascular system in the region of the aortic valve. Turbulent velocities were obtained in an in vitro pulse duplicating system with a two-channel laser Doppler anemometer. Velocities were measured in the region of two stenotic natural aortic valves and a normal stent mounted porcine bioprosthetic valve. Constants of proportionality between the Reynolds shear stress, averaged over ejection, and the Reynolds normal stress were determined. The constants of proportionality depended upon the local conditions, namely, whether the valves were stenotic or normal bioprosthetic. There was wide scatter of data. This suggests that any estimate of the Reynolds shear stress, based upon a single axial velocity in a complex flow field, such as occurs in the cardiovascular system, is likely to be inaccurate.

Aortic Valve↗
PubMed1983

An experimental evaluation of the use of an ensemble average for the calculation of turbulence in pulsatile flow.

A comparison was made between turbulence calculated by subtracting an ensemble average from the instantaneous velocity and calculations made with a high pass digital filter. Velocity was measured with a laser Doppler anemometer in vitro in the region of a normal porcine aortic valve and in patients with a hot film anemometer in the region of normal aortic valves. From the velocity obtained in patients, the absolute turbulence intensity calculated using an ensemble average of 50 beats was nearly twice the turbulence intensity calculated using a digital filter. Individual beats sometimes showed differences of 150% compared to calculations based upon the use of a digital filter. Inspection showed that the ensemble average varied widely from the actual nonfluctuating velocity. Studies in vitro showed less beat to beat variation than occurred in patients. The absolute turbulence intensity measured in vitro, when calculated using an ensemble average, was only 20% greater than calculations using a digital filter. The differences were due primarily to beat-to-beat variations of the nonfluctuating velocity, but these beat-to-beat variations were less prominent than occurred in patients. These observations suggest that ensemble averaging may not be appropriate for the calculation of turbulence, particularly in patients.

Biomedical Engineering↗