Affiliation:
1. Department of Mechanical Engineering, University of Manchester.
2. Constantine College of Technology, Middlesbrough.
Abstract
The importance of rapidly changing velocity profiles in turbulent ducted flow systems is discussed. A review of the available experimental data is then made. Various methods of generating an artificial velocity profile are described and, where possible, comparison is made between experiment and theory. It is shown that two velocity profiles which apparently have similar characteristics, namely peakiness and shape, may have widely differing decay rates. This is determined by the turbulence introduced into the flow at the time of profile generation. A correlation is discussed which relates the decay rate of a particular velocity profile to the profile peakiness and the loss coefficient of the generating spoiler. Some evidence is produced which gives a physical foundation for the correlation. A method of calculating the maximum pressure rise associated with changes in velocity profile shape is described. It is then shown that this maximum pressure rise will be significantly reduced by turbulent mixing and wall skin friction. The changes are discussed in relation to pipe velocity profiles of the power law type. Estimates are obtained for the mean transverse velocities leading to the decay. These are compared with some recent experimental jet flow data. Finally, the paper discusses the effect which velocity profile decay may have on turbulent ducted flow. In particular, the improvement in performance of a diffuser and the changes in the drag on a cylindrical strut when the upstream flow is in a state of decay are commented upon.