what's difference of sound velocity in a stationary fluid compared to a moving fluid?
No difference at all if you are measuring relative to the fluid.
“Ultrasonic flow meters utilize sound waves to determine the flow rate of fluids. Pulses from a piezoelectric transducer travel through a moving fluid at the speed of sound and provide an indication of fluid velocity. Two different methods are currently employed to establish this velocity measurement; transit-time and Doppler methods.
Ultrasonic meters have several advantages, including freedom from obstructions in the pipe and negligible cost sensitivity with respect to pipe diameter. However, their performance is sensitive to flow conditions.
The transit-time method uses two opposing transducers which are mounted at a 45 degree angle to the direction of flow. The speed of sound from the upstream transducer to the downstream transducer represent the inherent speed of sound plus a contribution from the fluid velocity. A simultaneous measurement is taken in the opposite direction which represents the speed of sound minus the fluid velocity. The difference between these two values is representative of the fluid velocity which is linearly proportional to the flow rate.
This method works well where the fluid is free of entrained gas or solids which can scatter the sound waves traveling between the transducers.
This type of ultrasonic meter uses two transducers mounted in the same case on one side of the pipe. An ultrasonic sound wave of constant frequency is transmitted into the fluid by one transducer. Solids or bubbles within the fluid reflect the sound back to the receiver element. The Doppler principle states that there will be a shift in apparent frequency or wavelength when there is relative motion between transmitter and receiver. Within the Doppler flow meter, the relative motion of the reflecting bodies suspended within the fluid tends to compress the sound into shorter wave length (high frequency). This new frequency measured at the receiving element is electronically compared with the transmitted frequency to provide a frequency difference which is directly proportional to the flow velocity in the pipe. In contrast to the transit-time method, Doppler ultrasonic meters required entrained gases or suspended solids within the flow to function correctly.”
Separate names with a comma.