TRANSITION TO TURBULENCE IN PULSATILE FLOW-THROUGH HEART-VALVES - A MODIFIED STABILITY APPROACH

The presence of turbulence in the cardiovascular system is generally a n indication of some type of abnormality. Most cardiologists agree tha t turbulence near a valve indicates either valvular stenosis or regurg itation, depending on the phase of its occurrence during the cardiac c ycle. As no satisfying analytical solutions of the stability of turbul ent pulsatile flow exist, accurate, unbiased flow stability criteria a re needed for the identification of turbulence initiation. The traditi onal approach uses a stability diagram based upon the stability of a p lane Stokes layer where alpha (the Womersley parameter) is defined by the fundamental heart rate. We suggest a modified approach that involv es The decomposition of alpha into ifs frequency components, where alp ha is derived from the preferred modes induced on the flow by interact ion between flow pulsation and the value. Transition to turbulence in pulsatile flow through heart values was investigated in a pulse duplic ator system using three polymer aortic valve models representing a nor mal aortic valve, a 65 percent stenosed and Heat Transfer, value and a 90 percent severely stenosed valve, and two mitral valve models repre senting a normal mitral valve and a 65 percent stenosed valve. Valve c haracteristics were closely simulated as to mimic the conditions that alter flow stability and initiate turbulent flow conditions. Valvular velocity waveforms were measured by laser Doppler anemometry (LDA). Sp ectral analysis was performed on velocity signals at selected spatial and temporal points to produce the power density spectra, in which the preferred frequency modes were identified. The spectra obtained durin g the rapid closure stage of the valves were found to be governed by t he stenosis geometry. A shift toward higher dominant frequencies was c orrelated with the severity of the stenosis. According to the modified approach, stability of the flow is represented by a cluster of points , each corresponding to a specific dominant made apparent in the flow. In order to compare our results with those obtained by the traditiona l approach, the cluster of points was averaged to collapse into a sing le point that represents the flow stability. The comparison demonstrat es the bias of the traditional stability diagram that leads to unrelia ble stability criteria. Our approach derives the stability information from measured flow phenomena known to initiate flow instabilities. If differentiates between stabilizing and destabilizing modes and depict s an unbiased and explicit stability diagram of the flow, thus offerin g a more reliable stability criteria.