Mean Characteristics of Conical Vortices Above Roof Eaves of Low-Rise Cubic Buildings Using Particle Image Velocimetry

Abstract

FLUCTUATING LOW PRESSURES NEAR THE EDGES OF FLAT ROOFS ARE OF TEN CAUSED WHEN THE WIND IMPINGES ON ONE CORNER OF THE BUILDING SO THAT CONICAL VORTICES FORM ABOVE THE DIAGONAL ROOF EDGES. IN TURBULENT FLOW, THE SE VORTICES VARY IN POSITION AND STRENGTH AND THE UNDERLYING SURFACE PRESSURES FLUCTUATE ACCORDINGLY. A PRELIMINARY APPROACH TO THE STUDY OF THE MECHANISM LINKING INSTANTANEOUS ROOF EDGE PRESSURES WITH THE WIND VORTICAL STRUCTURES INVOLVES THE EVALUATION OF MEAN CHARACTERISTICS AND POSITIONS OF THE LATTER. HOWEVER THE FLOW EXAMINATION HAS SO FAR BEEN SEVERELY LIMITED BY THE RESTRICTION OF AVAILABLE ANEMOMETERS TO SINGLE-POINT SAMPLING. IN THIS EXPERIMENTAL STUDY, A 200MM CUBE HAS BEEN USED TO MODEL A BUILDING WITH A FLAT SQUARE ROOF SET AT AN ANGLE OF 45° TO THE ON COMING FLOW DIRECTION, AND A PARTICLE IMAGE VELOCIMETRY SYSTEM WAS USED TO CAPTURE INSTANTANEOUS TWO-DIMENSIONAL VELOCITY VECTOR IMAGES OF ENTIRE FLOW CROSS-SECTIONS, BOTH NOR MAL TO THE VORTEX AXIS AND IN PLANES PARALLEL TO THAT AXIS. THE SE VECTOR MAPS WE RE USED TO ESTIMATE THE MEAN CHARACTERISTICS OF THE VORTICES AND APPROPRIATE OBSERVATION-PLANE DIRECTIONS TO MEASURE WIND VELOCITIES IN THE STUDY OF THE INSTANTANEOUS PROBLEM.