On the applicability of the Forchheimer equation in simulating flow through woven screens

Abstract

Flow through woven screens is of continuing interest in many fields, including protected horticulture, in which such screens are used mainly to protect crops against insects. Screens may significantly affect crop microclimate, therefore computational fluid dynamics (CFD) simulations have been used by researchers in recent years to predict their effects on the distribution of microclimate parameters within the structures where the crop is grown. In such simulations screens are usually represented by porous media, through which the pressure drop is estimated by means of the Forchheimer equation. The applicability of the Forchheimer equation to estimate pressure drops through woven screens was investigated. Firstly, the results of CFD simulations of flow through realistic models of woven screens were validated by means of wind tunnel tests and by means of a published correlation for calculating pressure drops through such screens. The CFD tool was then used in conjunction with the wind tunnel tests and published correlation results to examine whether the coefficients in the Forchheimer equation are constant over a wide range of Reynolds numbers, and to explore the flow through the screens. It was found that the values of the Forchheimer coefficients varied with Reynolds number, and that the changes in their values can be assumed to be relatively small and not very significant but only up to a Reynolds number, Rei ≈ 130–160. Moreover, presenting the Forchheimer coefficients as functions of only porosity may lead to erroneous predictions of pressure drops.