The velocity profile shape and boundary layer thickness of an equilibrium boundary layer grown over a long fetch of roughness are closely matched with those of a boundary layer artificially thickened using spires (by adjusting the shape and height of the spires). Other turbulent characteristics of these two wind tunnel simula tions of the atmospheric wind are then compared. At the same time, more information on rough wall boundary layers is obtained to allow for a rational choice of the shape and spacing of roughness elements required to produce a particular simulation of the full scale boundary layer.
A technique for calculating the shape of boundary layers in exact equilibrium with the roughness beneath, using a data correlation for the wall stress associated with very rough boundaries and a semi-empirical calculation method, is examined experimentally. Wall shear stress, measured directly from a drag plate, i combined with boundary layer integral properties to show that the shear stress formula is reasonably accurate and that the boundary layer grown over a long fetch of roughness is close to equilibrium after passing over a streamwise distance equal to about 350 times the roughness
element height.
The boundary layer quickly generated using spires proved to be a fair approximation to that grown over a long fetch of roughness, but did not accurately represent the longitudinal turbulence intensity of the full scale atmospheric wind or the naturally grown boundary layer.
The boundary layer produced here by spires showed little change in gross characteristics after travelling
about eight spire heights downstream of the spires. A distance of six or seven such heights has been advised by other workers in the past.Applied Science, Faculty ofMechanical Engineering, Department ofGraduat
