Some effects of escarpments on the atmospheric boundary-layer.
Thesis DisciplineMechanical Engineering
Degree GrantorUniversity of Canterbury
Degree NameDoctor of Philosophy
The project described in this thesis was undertaken in order to contribute to the understanding of the wind flow behaviour over complex terrain by making field and model measurements of the flow over two-dimensional, forward facing escarpments. In achieving these objectives, two field studies over a low 2:1 and cliff escarpment are reported which used orthogonal arrays of fast response propeller anemometers to record the wind flow data. The development and performance of these anemometers together with the digital data acquisition system are discussed in detail and are shown to cope adequately with the longitudinal turbulence data. Both escarpments were situated in open rural terrain at oblique incidence angles to the recorded strong winds. Mean wind speeds and direction, turbulence intensities, Reynolds stresses, gust factors and some longitudinal power spectra are reported for positions at 1/2 and 4 escarpment heights behind the crest, up to a height of nearly two escarpment heights. The data are compared with the conditions measured upstream in the undisturbed boundary-layer which are shown to be affected by the slightly non-neutral atmospheric conditions prevailing at the time. Wind tunnel tests using accurate models of the two full scale situations and wind directions are also reported. In addition, two model test series using plain 2D escarpments are described which illustrate the effects of upwind hill slope and wind incidence on the flow over rough and smooth escarpments. A comparison of the accumulated results from the model and field tests with the literature is made. The mean flow and turbulence close to the ground were dominated by the turbulent wakes generated by the sharp crests. Significant increases in mean wind speed were evident below 3 escarpment heights above local ground level. However little change in the turbulence characteristics was noticed outside the wake regions. No significant trends in gust factor were evident due to the presence of the escarpment. The variation of the recorded gust factor was fully explained in terms of instrument response, gust averaging time, record length and turbulence intensity using established methods.