Wind flow over ridges in simulated atmospheric boundary layers

Abstract

The flows over four two-dimensional triangular hills and three two-dimensional bell-shaped hills have been investigated in a simulated rural atmospheric boundary layer modelled to a scale of 1 : 300: Further measurements were made over two of the triangular hills in a simulated rural boundary layer of 1 : 3000 scale and in a simulated urban boundary layer modelled to a scale of 1 : 400. The effect of the model hill surface roughness was also investigated. Flow measurements were restricted to the mean velocity 8, RMS velocity fluctuations u' and the energy spectra for the streamwise velocity component Measurements were made at a number of longitudinal positions in the approach Row, over the model hills and downstream of the model hills. For each model hill, the crest was the region of largest mean velocity and smallest velocity fluctuations. The largest mean velocities over the model hills occurred for hills of intermediate slope rather than for the steepest hills. A decrease in the scale of the simulated atmospheric boundary layer led to a reduction in the amplification factors at the hill crests, whereas an increase in the surface roughness of the approach flow resulted in increased amplification factors at the hill crests. List of SymbolsAmplification factor defined as the mean wind speed at a height r above level ground divided by the reference mean wind speed measured at the same height above ground level but in the undisturbed flow over flat ground, upstream of the hill, zero-plane displacement height, model hill height, distance from the hill crest to where the hill height is half its maximum value, scaling length in the model atmospheric spectra, frequency, power spectral density function, fluctuating component of streamwise velocity, streamwise RMS velocity fluctuation, streamwise mean velocity, mean velocity at a height z above the surface, mean velocity 13 hill heights above the ground in the upstream reference measurement position, 10 hill heights upstream of the start of the model hills, mean streamwise velocity at the boundary-layer height, streamwise direction, vertical direction, roughness length, boundary-layer height.Subscript p refers to spectral peak values. Subscript ref refers to the upstream reference measurement ppsition, 10 hill heights upstream of the start of the models hills.

Linear dunes on Titan and earth: Initial remote sensing comparisons

“…Studies of wind flow over obstacles indicate wind flow is reduced on the downwind side, interrupting normal processes of dune formation (Pearse et al, 1981). Although wind conditions and sand supply may preclude the formation of dunes behind obstacles, sands may still be transported over these regions.…”

Dynamics of beach-dune systems

“…It is also worth while remembering that a long, straight slope is not aerodynamically efficient. Laboratory 424 studies in wind tunnels demonstrate that there are zones of reduced stress at the base and crest of triangular 'hills', whereas bell-shaped 'hills' have a much more even distribution of surface shear stress (Pearse et al, 1981). Aeolian forces might therefore be expected to alter both a planar or a complex wave-created profile into one that is more aerodynamically 'correct' (i.e., convexo-concave).…”

Types of Aeolian Sand Dunes and Their Formation

“…Any such change in wind velocity has a significant influence on surface shear stress and, hence, on sand transport rates and dune morphology. On every hillock patch there is an increase in surface shear stress up the windward slope toward the crest and a decrease on the lee side [37,38,39,40]. Sand dunes, as a dynamic geomorphic system, have the attribute of negative feedback, selfregularity bedform which subjects them to periodic changes in energy (wind velocity and direction) and material (sand).…”