  The following is a report on an experimental technique allowing to quantify
and map the velocity field with a very high resolution and a simple equipment
in large 2D devices. A simple Shlieren technique is proposed to reinforce the
contrast in the images and allow you to detect seeded particles that are
pixel-sized or even inferior to it. The velocimetry technique that we have
reported on is based on auto-correlation functions of the pixel intensity,
which we have shown are directly related to the magnitude of the local average
velocity. The characteristic time involved in the decorrelation of the signal
is proportional to the tracer size and inversely proportional to the average
velocity. We have reported on a detailed discussion about the optimization of
relevant involved parameters, the spatial resolution and the accuracy of the
method. The technique is then applied to a model porous media made of a random
channel network. We show that it is highly efficient to determine the magnitude
of the flow in each of the channels of the network, opening the road to the
fundamental study of the flows of complex fluids. The latter is illustrated
with yield stress fluid, in which the flow becomes highly heterogeneous at
small flow rates.
