Discussion on “Experimental issues in atmospheric boundary layer simulations: roughness length and integral length scale determination” by A.K.S. Iyengar, C. Farell

In their paper, Iyengar and Farell, Experimental issues in atmospheric boundary layer simulations: roughness length and integral length scale determination, J. Wind Eng. Ind. Aerodyn. 89 (2001) 1059, compare Reynolds stress measurements in a boundary layer developed over an array of cubes with the drag force on the cube array, expecting these two forms of measurement to be equivalent, but finding that they differ by more than 15%. This leads to inconsistencies in the estimates of aerodynamic roughness and zero-plane displacement from the measurements. It is suggested in this discussion that not all the drag on the elements is mobilised into the boundary layer as Reynolds stresses when the aerodynamic zero plane is a significant proportion of the roughness height. Accordingly, balance measurements exaggerate the equivalent surface shear stress and direct measurement of the Reynolds stress should be used instead.