8/19/2023 0 Comments Turbulent flow vs laminar flow![]() ![]() The blood which was at rest in the poststenotic region during diastole is accelerated through the sample volume. In the post-stenotic region of a diseased artery the Doppler waveform is more complex ( Fig. Within the sample volume the blood velocity magnitude and direction are similar for all of the red cells, hence the spectral width is low and the waveform outline is smooth. Doppler spectra are shown from the normal femoral artery in Figure 2-2A in this case flow is laminar. The effect of the flow state on the Doppler waveform is illustrated in Figure 2-2. During pulsatile flow, vortex shedding may occur for only a portion of the cardiac cycle. Vortices which are shed travel a few diameters downstream and eventually die out as their energy is absorbed through viscous losses. At even higher Re values, the vortex shedding will be combined with the random flow patterns of true turbulence. Again this is not strictly turbulence, as the velocity magnitude and direction at any location is not random, but follows a regular pattern. At higher Re values there will be vortex shedding at regular time intervals. At low velocity the fluid flow within the vortex is actually laminar. During steady flow at low Re values the vortex will be stable and limited to the region immediately behind the stenosis. The pattern of vortex production will change as the degree of stenosis and blood velocity increase. ![]() The most important example of this is seen with the phenomenon of vortices, which are regions of circulating flow often produced when there is some obvious change in vessel geometry such as a stenosis, or the normal carotid bulb. There is also a third flow state called disturbed flow, which refers to variations in velocity magnitude and direction which occur at low values of Re. During pulsatile flow it is therefore possible for the flow to be laminar at values of Re higher than the critical value, because turbulence does not have time to develop before the blood velocity has decreased. ![]() When velocity is increased so that the Re is above the critical value, turbulence will take a small amount of time to develop. For flow in which Re is about 2000 the fluid flow will alternate between turbulent and laminar. For a wide variety of fluids the transition to turbulence takes place at a value of Re of about 2000. Where ρ is the fluid density, L is the vessel diameter, V is the mean velocity and μ is the fluid viscosity. It was found that a dimensionless number called the Reynolds number ( Re) was useful in characterising the fluid flow. (B) The velocity vector magnitude and direction in turbulent flow have random components the time-averaged profile is blunt.Ĭoncepts of laminar and turbulent flow first arose from consideration of flow in long straight tubes. FIGURE 2-1 (A) Laminar flow consists of flow along well-defined streamlines the velocity profile in a long straight tube under conditions of steady flow is parabolic. ![]()
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