In a PIV system due to low velocity dynamic range (VDR), velocities outside the range will result in wrong vectors being output. This is especially true for low speed regions of the flow as a PIV system is configured to resolve high speed regions of the flow.

 

A good way to counter this limitation is to use multiple (or two) time steps - a longer time interval would have resulted in a larger displacement in slower regions of the flow resulting in the velocities to enter VDR of the PIV system. Of course, using, a larger time step results in faster flow regions to go outside the VDR. 

 

To fix this and to consolidate the two vector fields, our group has proposed a normalised residual median which is able to detect erroneous vectors. Using this normalised residual median, we are able to detect erroneous vectors and output a consolidated vector map at an instant. 

 

We have shown that using this method we are able to resolve the ambient region of a jet. We were able to correctly resolve the radial velocity and hence a better quantification of entrainment rates. The entrainment coefficient as a function of axial distance was shown to match DNS data.