It depends on the number of your grids.You can only extract the flow near the wall by different extraction filters in order to reduce the computation time.
Use the calculator to create a vector, then use it in the Gradient filter.
Hello,
I am also facing difficulty to get Q-criteria in 3d, could you please guide me where I can find the filter Gradient of Unstructured Data set?
In recent versions of ParaView, there is just the Gradient filter, and it handles unstructured grids as input.