Calculation of Reynolds number for a pipe
There is no reason to abbreviate parameter names to three letters.  That just makes the code unnecessarily cryptic.  (Using names like rho instead of density could be an acceptable alternative, if brevity is essential.)  Furthermore, consider naming the function more specifically, so that it is obvious that it applies to cylindrical pipes.

As per PEP 8, binary operators should have have spaces for readability.

There is no good reason to catch ZeroDivisionError.  If the viscosity is 0, then this function would return None, and would likely cause other problems elsewhere in your calling code.  Just let the ZeroDivisionError propagate naturally, and your code would be easier to debug.  (Furthermore, your two comments are pointless and redundant.)

Watch your spelling ("dimentionless" → "dimensionless", "charakterize" → "characterize").

def reynolds_pipe(density, velocity, diameter, viscosity):
    """
    Calculate the Reynolds number for a given pipe flow.
    The resulting Reynolds number is dimensionless. 
    It is used to characterize flow patterns.

    Arguments:
    density -- density of the fluid in [kg/m3]
    velocity -- the mean velocity of the fluid in [m/s]
    viscosity -- dynamic viscosity of the fluid in [Pa*s]
    diameter -- inside diameter of the pipe in [m]
    """
    return density * velocity * diameter / viscosity