Laser doppler velocity measurement
At transitions in the diameter of the tube the flow profile will generally be disturbed over some distance: the flow needs a certain distance to stabilise again.This effect can also occur due to disturbances such as by a non-smooth wall of the tube or a bend in the tube.
The distance over which the disturbances can be observed depends on the velocity of the flow, the diameter of the tube and the initial profile. Empirically the following formula has been established for the distance over which the flow profile can be disturbed.
For a laminar flow with in a tube with the radius R = 1 cm one may thus expect a disturbance over a distance of 2 meter. Notice that the formula does not give hard criteria but rather represents an crude estimate .
Transition between laminar and turbulent
With increasing Reynolds number it takes more time for small disturbances to die off. Above a certain flow velocity, characterised by the critical Reynolds number , a transition to turbulence will take place: disturbances are longer damped but rather do reinforced one another. Usually one finds 2300<
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