Doctor of Philosophy
Civil and Environmental Engineering
Dr. Gregory Kopp
This thesis is concerned with the wind-induced pressure distribution acting on air permeable, double-layer roof systems placed on low-rise buildings. Because of the pressure transmission into the cavity between layers, the pressure difference in the double-layer systems differs significantly from that in a single layer. The interior pressures (in the cavity between layers) are highly correlated with external pressures at the openings on the external surface of the outer layer and thereby, the net wind load for the design of the outer layer is considerably reduced by the pressure equalization between the external and internal surfaces of the outer layer. Examples of the outer layer in practice are loose-laid roof pavers and solar panel arrays on roofs, and rainscreen walls. The objective of this thesis is to characterize and model how the cavity pressures between layers are related to the external pressures and the outer layer and cavity geometries.
First, experimental studies were conducted to investigate the detailed cavity pressure distributions, the correlation with external pressures and effects of geometric parameters within the double-layer system. Wind tunnel experimental data on a model of a low-rise building in an open country terrain were used and in total, 39 configurations consisting of six different heights of the panels above the roof surface (H which is the gap between layers), six different gaps between panels (G) and three additional configurations with a larger panel size (L) were examined. It was found in these experiments that the pressure distributions on the internal surface of panels are significantly affected by the geometric parameters.
An analytical model was developed to simulate one-dimensional pressure distributions in the cavity, given external pressure data and the geometric parameters. The model was derived considering the pressure drops associated with the flow through the gaps between panels, which is like an orifice flow, and the cavity flow between parallel plates. Thus, the model uses two primary equations: the unsteady discharge equation and the equation for unsteady flow between two parallel plates. The model accounts for several geometric parameters including the gap (G) between the panels, the height (H) of the cavity between the layers, the length (L) of the panels and the thickness (lo) of the panels, as well as the loss coefficients for the orifice and cavity flows. The proposed model is able to capture the fluctuations of cavity pressures and a good agreement is found between the numerical and experimental results for the mean, RMS and peak coefficients, to a great extent, when spanwise-averaged external pressure coefficients are used as input.
The analytical model was further validated with wind tunnel test data on a more practical problem which consists of 12 rows of photovoltaic panels placed within all roof zones, from the separation region, the reattachment point, up to the leeward region of the roof. The model captures fluctuations of the cavity pressure, although there are slight differences between the numerical and experimental data, in particular, on panels at the leading edge and in the reattachment regions. Lastly, the thesis discusses how the analytical model can be used as a tool to design compartments for such air permeable, double-layer systems.
Oh, Jeong Hee, "Wind-Induced Pressures in Air Permeable, Double-Layer Roof Systems in Regions of Separated Flow" (2014). Electronic Thesis and Dissertation Repository. 2344.