Measuring systems to determine the velocity field in and close to the roughness sublayer

Abstract

River beds are often characterized by marked surface textures. To extend the applicability of the model suggested by Shields (1936) to natural river beds with marked surface textures, research was carried out to develop a physical model that allows the stability and instability of river beds to be determined more precisely than previous ones (see Dittrich (1992)). The model is based on the exact knowledge of the turbulence characteristics of the liquid phase in combination with parameters describing the solid phase. For a good description of the velocity field it is not only necessary to know the mean velocity but also the deviations of the mean values up to the fourth moment close to the roughness elements. Until now, the only method to measure the turbulence in the roughness sublayer is Laser Doppler Anemometry. As this method attains its range of validity close to the roughness elements, three different measuring systems were tested. The received data show system dependent relationships. Thus, it is important to consider errors and failures that can be related to the utilized LDA-system. A description of the three different LDA-systems as well as a list of errors and failures and a short summary of main results will be the content of this paper.