Diploma Thesis

Flow over hills - comparison of SODAR-RASS measurements and WAsP modelling

Silke Oldenburg (05/2005-10/2005)

Support: Thomas Foken, Christoph Thomas

In the last decades flow over a hill experienced growing interest in research, because of its relation to wind energy industry. Due to large-scale experiments, simulation models have been developed to estimate wind flow more easily. They are based on theories concerning the Froude Number FL and Richardson Number Ri. The stratification of the lower boundary layer plays a major role on these considerations. Risø National Laboratory constructed the modelling program WAsP Engineering based on the studies on Askervein Hill in 1982 and 1983. In this study definitions of the inner and outer li and hm layer’s height are given and the fractional speed-up ratio s is determined. Furthermore, this work compares wind velocities and wind directions of data measured during HOHPEX 2004 and calculated data of the modelling program WAsP Engineering. The compared datasets show no correlation, especially the com-parison of different wind directions is poor, and that is why WAsP Engineering 1.2 has no legitimacy to be utilised for predictions for flow over a hill. While the simulated flow is based mainly on the roughness length z0, complex topography is not taken under consideration in WAsP Engineering. Changing weather conditions are not taken into account either, since WAsP assumes always neutrally stable atmospheric conditions. Still, the modelling program is able to simulate flows over flat terrain or smooth and gentle hills, using rapid distortion theory and power laws to calculate wind velocities and directions. Flow over a hill is barely described and relates only to compression and extension of streamlines at the crest and the downwind side of the hill.

Related links:
  • Experiment: HOHPEX: Hohenpeissenberg OH-Interkomparison and Photochemistry EXperiment
  • Project: SALSA: Beitrag von Salpetriger Säure zur atmosphärischen OH-Konzentration

last modified 2005-11-25