Wall Shear Stress in Pulsating Pipe Flow at Resonance Conditions

Hydraulic vibrations in liquid-filled pipelines may cause unwanted operational problems. Wall shear stress and consequential damping can reduce the harmful effects of vibrations close to resonance. A large-scale pipeline apparatus at Deltares, Delft, The Netherlands, has been used for pulsating pipe flow tests. The apparatus consists of a constant-head tank, a horizontal 206 mm diameter 49 meters long steel pipe and an oscillating valve at the downstream end. Wall shear stress has been measured by a number of hot-film sensors. Tests have been performed with an average flow Reynolds number of about 22,000. Results of a hydraulic resonance case with oscillating frequency f_ex = 5 Hz are presented. Pipe wall vibrations for this case are small. The shape of the velocity profile at resonance conditions is a typical unsteady-state velocity profile with reverse flow near the pipe wall. The CFD study in an axisymmetric domain was conducted to better understand the pulsating flow phenomena. Different settings of boundary conditions, based on the experimental investigations, were used. The CFD results show the unsteady character of wall shear stress at resonance. This phenomenon has not been observed in the measured results to such an extent. The measured shear stress resembles quasi-steady behaviour.