Fig. 8.

Relation between Doppler phase shift and speed. (a) Fitting of estimated axial speed and averaged Doppler phase shift in the center of the tube; the blue line is the linear polynomial fitting [root measure square error $(\mathrm{RMSE})=0.19$], and the red line is the quadratic polynomial fitting with $\mathrm{RMSE}=0.17$. (b) Transformation between Doppler phase shift and speed.