Roughness Element Geometry Required for Wind Tunnel Simulations of the Atmospheric Wind

Using a data correlation for the wall stress associated with very rough boundaries and a semi-empirical calculation method, the shape of boundary layers in exact equilibrium with the roughness beneath them is calculated. A wide range of roughness geometries (two- and three-dimensional elements) is included by the use of equivalent surfaces of equal drag per unit area. Results can be summarized in a single figure which relates the shape factor of the boundary layer (its exponent if it has a power law velocity profile) to the height of the roughness elements and their spacing. New data for one turbulent boundary layer developing over a long fetch of uniform roughness is presented. Wall shear stress, measured directly from a drag plate is combined with boundary layer integral properties to show that the shear stress correlation adopted is reasonably accurate and that the boundary layer is close to equilibrium after passing over a streamwise roughness fetch equal to about 350 times the roughness element height. An example is given of the way in which roughness geometry may be chosen from calculated equilibrium results, for one particular boundary layer thickness and a shape useful for simulating strong atmospheric winds in a wind tunnel.

A Drag Plate Flowmeter for Pulsating Flow

A survey of recent articles shows that drag plate flowmeters, in one form or another, have been about the most popular devices for indicating transient flows. Design principles are given and features discussed, which affect performance in transient or pulsating flow. Where a transient flowmeter has an electrical output it is quîte easy to process the output signal so as to obtain the average flow under pulsating conditions of flow. Thus the drag plate flowmeter is of interest for this important application. Up to the present the transient performance of these flowmeters has been inferred from the examination of the output waveform and from the comparison of the indicated flow with true average flow under pulsating conditions. Details of frequency analysis tests of a drag flowmeter are given.

The aerodynamic drag of grassland

An improvised drag-plate apparatus, on the principle of that used by Sheppard (1947) on a concrete surface, but suitably modified in design, has been used for exploratory measurements of the aerodynamic drag of grassland. The grass cover was variable (1 to 15 cm. in height), and measurements were made at a number of positions (not simultaneously) in order to obtain an approximate representative value of the drag over a considerable area. Wind velocities were in the region of 500 cm./sec. and, judged in terms of the Richardson number, effectively adiabatic conditions of flow prevailed. The drag (r 0 ) and the simultaneous vertical distribution of wind velocity (u z ) up to a height of 2 m. were found to be expressible in terms of the law well established in the laboratory, i.e. with k (von Kármán’s constant) = 0.37, d (the zero plane displacement) = 8 cm. and z 0 (the roughness parameter) = 0.66 cm. For reasons which are discussed the drag measurements are regarded as approximate, and the close agreement of the numerical value of k with the laboratory value of 0.4 is probably fortuitous. However, the general consistency achieved in this preliminary application suggests that the technique could be profitably developed for a critical investigation of the relation between drag and wind profile and its dependence on atmospheric stability. In a brief discussion of previous work some evidence is now provided for the validity of the Reynolds formulation of the turbulent shearing stress. Attention is drawn to the application of the present results in treatments of the diffusion of matter in the lower atmosphere.