The hydraulic pressure fluctuations (pseudo sound) inside a straight pipe and a curved-pipe with a 90° bend were studied.Firstly, the Poisson equation of the pressure field related to incompressible flow was derived and compared with the wave equation.Then, by solving the incompressible Navier-Stokes equations together with the SST k-ω turbulence model, the unsteady flows inside the two pipes with a sinusoidal pressure boundary condition being applied at the pipe inlet were obtained.Finally, the characteristics of the mean and fluctuating flow fields in the straight and curved pipes were analyzed.The results show that the amplitude of pressure fluctuation decreases linearly along the pipe downstream direction with 1D propagation pattern, while the velocity fluctuation is very chaotic.The 90° bend has obvious influence on the local flow causing the increase of pressure and velocity fluctuation therein.Due to the sinusoidal pressure pulsations at the pipe inlet, the flow fluctuations inside the pipes are dominated by the fundamental frequency and its low-order harmonics, and the 90° bend contributes the broadband fluctuation.Owing to the adoption of the elliptic Poisson equation of pressure fluctuation, the propagation speed of the disturbance is infinite, and there is no phase difference in the pressure fluctuation signals at different locations inside the pipes.