Wall shear stress in transient turbulent pipe flow

Experimental measurements of the wall shear stress combined with velocity profiles via the electrochemical technique and Ultrasonic pulsed Doppler Velocimetry, are used to analyze the flow behavior in transient flow caused by a downstream short pipe valve closure. The Reynolds number of the steady flow based on the pipe diameter is Re=121700. The results show that the quasi-steady approach of representing unsteady friction is valid during the initial phase for relatively large decelerations. For higher decelerations, the unsteady wall shear stress is consistently higher than the quasi-steady values obtained from the velocity profiles. An examination of the range of applicability of the instantaneous-acceleration model shows that the empirical coefficient of unsteady friction is closely linked to the deceleration intensity. This study is made possible owing to the repeatability of different valve closures allowing data to be averaged over numerous tests.