Image Number 11 for United States Patent #6536291.
A fiber optic strain gauge based flow rate measurement system includes two measurement regions located an average axial distance .DELTA.X apart along the pipe, the first measurement region having two fiber optic strain gauges located a distance X.sub.1 apart, and the second measurement region having two other fiber optic strain gauges located a distance X.sub.2 apart, each capable of measuring the unsteady pressure in the pipe 12. Signals from each pair of fiber optic strain gauges are differenced by summers to form spatial wavelength filters. Each spatial filter filters out acoustic pressure disturbances P.sub.acoustic and other long wavelength pressure disturbances in the pipe and passes short-wavelength low-frequency vortical pressure disturbances P.sub.vortical associated with the vortical flow field. The spatial filters provide signals to band pass filters that filter out high frequency signals. The P.sub.vortical -dominated filtered signals from the two sensing regions are cross-correlated to determine a time delay .tau. between the two sensing locations which is divided into the distance .DELTA.X to obtain a convection velocity U.sub.c (t) that is related to an average flow rate of the fluid (i.e., one or more liquids and/or gases) flowing in the pipe 12. The invention may also be configured detect the velocity of any desired inhomogeneous pressure field in the flow.