Transit time ultrasonic method is based on a well known principle: let A and B be 2 locations of transducers on each side of a pipe, then the apparent difference of the sound speed on the path AB and on the path BA is proportional to the fluid velocity averaged over the path. The estimation of the flow rate needs the conversion of this path velocity to a velocity averaged over the entire cross-section of the pipe containing the flowing fluid under investigation. Two phenomena have a particularly important impact on flowmeter performance: on the one hand, swirl which is the whole of nonflowing transverse velocities and on the other hand, the fluid velocity profile which can be asymmetric downstream an elbow for example. To date, the correct estimation of the fluid velocity profile and the compensation of swirl is not satisfactory solved. For these purposes we investigated two main directions.