Jarek Dekowski
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 Posted: Fri Dec 12, 2003 4:54 pm Post subject: Re: wall shear stress measurments |
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karnaf1@netvision.net.il (Ze>ev) wrote in message news:<58725783.0312090227.49c3640@posting.google.com>...
[quote]I>m looking for sources about the latest methodes to measure wall
shear stress on vascular modles, and explanations about the current
limitations of regular PIV or LDA measurment techniques.
thanks,
Ze>ev
[/quote]
I am working with PIV method for about 5 years. The most important
problem with measuring velocity near the wall is reflection of the
laser light from the wall. This will blind your images in vivinity of
the wall. Another problem with this method is seeding. You must have
particles in the flow which will follow the flow and will scatter the
light. In some cases this is big problem. One more thing is very
important. The velocity measured by PIV method is the average velocity
of particles in some area(interrogation area from which is calculated
one vector) and in time (time between two images, or laser pulses), it
is especially well seen whan you measure velocity with shock waves.
Error of the method is also the problem. The error of the component
perpendicular to the main flow direction can be very large. The error
is proportional to total velocity not only one of its components. In
the boundary layers it gives very large error of velocity
perpendicular to the wall. Also when you have large gradients of the
velocity in the measurement area you will also have problems with
error of the small velocities.
I am not specialist with LDA method, but it has much smaller
measurement error, but you measure only one point not whole 2D area.
Mayby one more thing, you must be carful with measurements with very
low velocities, becouse laser light can introduce disturbances, and
cause the flow.
If you have any other questions connected with PIV just e-mail me. |
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