Thesis: R. Marwood

Doctor of Philosophy, Oxford University, Trinity Term 1996

An Investigation of Conical Roof Edge Vortices

Summary

This thesis describes experiments carried out using laser Doppler anemometry (LDA) to measure velocities within conical roof edge vortices. These roof edge vortices produce high surface suctions and are a primary cause of damage to roofs, and therefore warrant investigation. Time averaged mean velocities in the vortex are initially considered and in later work instantaneous velocity events are related to maximum and minimum suctions exerted on the roof under the vortex.

The work relating surface pressures to velocity events was made possible by the development of a simultaneous pressure-velocity measuring technique and a conditional statistical technique to analyse the resulting data.
The conditional sampling technique has allowed a point measurement technique (LDA) to be used to determine pressure-conditional instantaneous velocity events over an entire 2-D flow field.

In addition, the relationship between velocity vectors in the flow upstream of the building and the suctions on the model roof is assessed. These results have been used to show that quasi-steady theory is a good basis for the prediction of conical vortex induced loading. Quasi-steady theory predicts that instantaneous flow patterns, and hence surface pressures, produced by short duration wind gusts are the same as those produced by a time averaged mean flows with the same magnitude and direction.

(no thesis available)