I am using Laser Doppler Velocity (LDV) meter for measuring the velocity profile along the radial direction in pipe flow (ID 25mm). I am finding difficulty near the wall region. Near the wall, i am getting abrupt value of velocity and i also observed that the turbulent intensity (%) reaches up to 100 or more than that. Please anybody suggest me how to measure correct value of velocity near the pipe wall.
For such a small pipe it will be impossible to measure representive velocity and turbulence data, because the dimensions of the mesuring volume compared to the thickness of teh boundary layer is to big. Depending on Re-number and surface roughness you will be in an unstable region of the boundary layer where you will have very high turbulence at high Re numbers. Perhaps heat film or heat wire measuring technique is more usefull.
How close are you trying to get to the wall, and is this gas or liquid? Consider that as the velocity gets lower, the gradients get higher, and the wall gets closer, it will be unlikely that the particle velocities are matching the flow velocities.
Another thing to think about is the size of the measurements domain (where both laser overlap). Near wall, if the gradient is important, particles in the small (but not small enough) volume can travel at different speed. This can lead to high turbulence intensity just because of the disparity of the particles velocity in the domain.
It is very difficult to obtain velocity data from LDV in the vicinity of a wall, mainly due to the number of particles that vanishes close to the wall where the velocity is reaching very low values
Thank you for reply. I am measuring the velocity for single phase flow (medium is water) using LDV. There is one doubt, at low Reynolds number (in the range of laminar flow) can LDV measure the velocity profile which is exactly match with the analytical or empirical study? If yes than in what accuracy it measures? Suppose at the center point of the pipe can i get maximum value which is equal to twice to the average value of velocity by using LDV? I am using LDV which has a lens of 250 mm and it covers 2 to 3 mm measuring volume (fringe volume).
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