Methods to assess drag force in flow through irregularly arranged roughness elements

Canopy flow models require the spatial distribution of drag force as an input. Such detailed information is scarce. As a result, models often rely on data that is based on ideal, fully developed flow conditions. However, real canopy systems have physical boundaries, where the flow enters and leaves patches of vegetation, generating a complex pressure field and velocity variations. This study focuses on methods to obtain drag with high spatial resolution. In a collaborative effort of the Technion in Haifa, Israel, and the LeichtweiB-Insitut fur Wasserbau (LWI) at TU Braunschweig, Germany, we used a canopy model constructed of randomly distributed thin plates. The flow field was measured with Particle Image Velocimetry (PIV) to calculate drag with the DANS equations. The results are compared with drag forces directly measured by a drag balance method and drag force sensor. The results show very good agreement between the measured and calculated forces.