averaged velocity profiles at 43 in addition the parameters used for the ultrasound doppler method are input at 418 specifically the doppler angle at 411 acoustic information at 412 the initial depth at 413 the channel distance at 414 the measuring window at 415 the pulse repetition rate at 416 and the used beam geometry at 417 the standard deviation for each velocity channel of the velocity profile is determined at 44 and then compared with a predetermined limiting value at 45 the actual initial depth the actual penetration depth and the actual channel distance are then determined from this at 46 47 and 48 respectively proceeding from these three values reliable velocity data are then selected at 49 for subsequent calculations which are finally prepared for the curve adjustment at 410 fig5 a and 5b diagrammatically illustrate the process for selecting a suitable model choosing reliable data and checking the boundary conditions for the flowing fluid the velocity data for curve adjustment are prepared at 51 the determined standard deviation smd is compared with a predetermined limiting value smdl for standard deviation at 52 if smd is less than smdl a decision is made at 53 to use the smd to adjust the data curve adjustment in this case a curve adjustment is performed by method of least error squares at 54 this is used at 55 to monitor the axial symmetry of the flow profile and at 56 to determine the maximum flow velocity if smd is greater than smdl the process of solving the boundary value problem is initiated at 518 and a warning signal is output at 520 the warning signal indicates that the boundary conditions have not been satisfied it is determined whether the wall velocity is zero at 57 if it.