First lets talk Doppler effect (shift in the pitch of the train whissle as
it approaches and recedes ).  It is my understanding that when one reference
point is stationary and another is moving that there will always be a
Doppler effect (abet, not always measurable).  It does not require high
speed (i.e. high % of C) of  the measured particle, e.g.:  Measuring the
flow of RBC in a neck vessel by Doppler ultra sound.  Laser flow cytometry
is a striaght through measurement and a bounce back (to the original frame
of reference) reading.  I imagine the speed flow of particles by Doppler
ultra sound might be used in some flow cell sensors, but not to obtain
results.

Secondly, what does have relevancy to flow cytometry is Reynold's
formulation, where one wishes to have laminar flow with the sheath fluid.
Mean velocity of fluid, tube diameter, density of fluid and viscosity.  When
the Reynold's formulation is >2300 turbulence will prevent laminar flow and
flow cytometry counting is not possible.  So one can't just go increasing
the speed of flow, as laminar conditions must prevail.

Don't take me as a relible reference, as there is much good reading on the
subject to be found.  The reason I indulge in the above discussion is in the
hopes that an expert source my correct any errors in my above statements.