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Ultrasound is advantageous for gas flow velocity measurements, owing to its high sensitivity to all kinds of turbulences in the streaming fluid. The cross-correlation method and vortex measurement behind a bluff body have been proven to be good. The most important and difficult problem is ultrasound signal processing. A complex modulated signal has be demodulated by undersampling. The real and imaginary parts of the complex signal can be determined by Hilbert transformation. The demodulated phase or amplitude signal can be applied to cross-correlation functions for the detection of flow velocity. The vortex frequency can be detected by a simple analog signal processing device. The combination of vortex and cross-correlation measurements results in a self-monitoring system. The pattern of a group of vortices is used to determine the transition time of the fluid between two sensor pairs. Measurement results show that the relative error of gas flow velocity with the bluff body is evidently smaller than that without the bluff body.