The wakes and the drag forces of canopy patches with different densities, immersed in turbulent boundary layers, are investigated experimentally. The patches are circular (with outer diameter DD ) and are made of several identical circular cylinders (height, HH , and diameter, dd ). The bulk aspect ratio of all of the patches ( AR=H/DAR=H/D ) was fixed at 1 and the patch density ( ϕ=Ncd2/D2ϕ=Ncd2/D2 , also referred to as the solidity) is altered by varying the number of cylinders ( NcNc ) in the patch. Drag measurements show that the patch drag coefficient increases with increasing density. However, the drag coefficient of the highest investigated density ( ϕ≈0.25ϕ≈0.25 ) is greater than the drag coefficient of a solid patch (i.e. ϕ=1ϕ=1 , which is a solid cylinder with AR=1AR=1 ). Particle image velocimetry (PIV) measurements were carried out along the streamwise–wall-normal ( xx – yy ) plane along the centreline of patch and in the streamwise–spanwise ( xx – zz ) plane at its mid height (i.e. y=H/2y=H/2 ). Mean velocity fields show that the porosity of the patch promotes bleeding along all directions. It was observed that bleeding along the vertical/horizontal direction increases/decreases with increasing ϕϕ . Furthermore, it was also observed that bleeding along the lateral direction dictates the point of flow separation along the sides of the patch and makes it independent of ϕϕ . All of these aspects make wakes for porous patches markedly different from their solid counterpart and provide a rational basis to explain the observed trends in the drag coefficient.
|Titolo:||Characterisation of drag and wake properties of canopy patches immersed in turbulent boundary layers|
|Data di pubblicazione:||2016|
|Digital Object Identifier (DOI):||10.1017/jfm.2016.312|
|Appare nelle tipologie:||1.1 Articolo in rivista|