Paper presented at the 6th International Conference on Heat Transfer, Fluid Mechanics and Thermodynamics, South Africa, 30 June - 2 July, 2008.
Experimental measurements of the wall shear stress
combined to those of the velocity profiles via the
electrochemical technique and Ultrasonic pulsed
Doppler Velocimetry, are used to analyse the flow
behaviour 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.
