Turbulent Boundary Layers
Turbulent boundary layer with zero pressure gradient
In the last year, new concepts for the evaluation of PIV recordings were developed, qualified and established with the aim of enhancing the spatial resolution from the millimeter to the range of a few micrometers. The resolution improvement of more than two orders of magnitude makes it possible to estimate high resolution flow fields without bias errors, which are typical for conventional cross-correlation PIV analysis but also for other measurement techniques such as LDV or hotwire. For Re = 0.4·106 with a boundary layer thickness of about 0.5 m, a resolution of 0.1 wall units was achieved. Thus 50 independent data points were present in the viscous sub-layer and the wall shear stress was determined directly by a fit function. Within the project the analysis of the scaling of the mean velocity, the Reynolds stresses and the probability density function of the velocity fluctuations in the range below 50 wall units will be analyzed at high Reynolds numbers. In addition the interface between the turbulent / non-turbulent interface at the outer edge of the boundary layer is examined with enhanced spatial resolution and measurement precisions. Here the resolution of the weak flow gradients is the challenging problem.
Funded by the Deutsche Forschungsgemeinschaft DFG.
Contacts: Dipl.-Phys. Scharnowski, Dr. Cierpka
Turbulent boundary layers
The question of whether a direct correlation exists between the large scale turbulent superstructures in a boundary layer's outer part and the small near-wall coherent turbulent structures comes up in the experimental investigation as well as how the correlation depends on the pressure gradient and the Reynolds Number. Because of the large range of scales which must be resolved in order to answer this question, a measurement technique concept is required which allows for the simultaneous resolution of the small scales near the wall and the large scale superstructures that elongate over several boundary layer thicknesses. The answer to the scientific question will enhance the physical understanding of the interaction between the different turbulent scales. Furthermore, a data basis will be developed which will serve to validate numerical flow simulation techniques and to develop enhanced turbulence models for flows with pressure gradients. While many investigations have been performed at low Reynolds numbers in the past, the technical relevant flow conditions will be of interest. Since this is only possible in wind tunnels with long test sections, the experiments are performed in the 22 meter long test section of the atmospheric wind tunnel at the Bundeswehr University Munich.
Funded by the Deutsche Forschungsgemeinschaft DFG (KA 1808/14-1).
Contacts: N.N., Prof. Kähler