Dear all,
PIV is in general equivalent to the averaged flow velocity.
But what can be expected in presence of strong fluctuations/noise ? Could PIV be significantly different from the flow velocity in the case strong fluctuations/noise exist in the motion of the fluid ?
The system is a culture of a cell layer plated on a dish.
Fluctuations can be measured using PIV if you have a camera with a sufficiently high image recording frequency. However, if the frequency of changes exceeds 1000 Hz, this may be difficult or impossible for technical reasons.
Although PIV is a well established technique which has reached great success in last three decades, it is still an indirect approach to measure flow velocity. Technically, it measures locally averaged velocity of the particles in interrogation window to evaluate local flow velocity. Therefore, it may not be applicable for a flow with a large velocity gradient and large dynamic range, e.g. shock wave. Physically, the difference between the density of particle and fluid could also be a problem for accurate measurement of flow velocity in a highly fluctuated flow. However, the actual error is hard to estimate and depends on various factors.
Any tool for experimental measurements has a proper filter cut-off kc and you have to assess the Kolmogorov frequency of your flow problem. If kc is in the range between the Taylor and the Kolmogorov frequencies you have a quite good resolution. Otherwise, the effect of the filter acts on the inertial range and you have a loss of resolution.
That must be intended in wide spatio-temporal range.
There are several main factors to consider:
1) Seed particle characteristic response time which is related to Stokes number. This should always be explicitly calculated for any PIV investigation and is one good gauge for how well the resultant cross-correlated particle velocity represents the actual flow velocity
2) Overlap ratio. In regions with sharp gradients you must pay more attention to using the correct/most advantageous overlap ratio between iterations to best capture the gradients
3) The relative magnitude of your velocity fluctuations to your mean flow. Like any measurement instrument, you have a limited bandwidth. If your fluctuations are on the order of 1/100th or above of full scale/mean you should be able to get reliable measurements without resorting to specialized approaches. Fluctuations below that may still be possible with PIV but require more novel approaches.
4) Based on your physical and assumed temporal scales, you will need to be rigorous in your application of optical distortion corrections as they may be on the order of the magnitude of your fluctuations
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