Modification of the SSG/LRR-omega RSM for adverse pressure gradients using turbulent boundary layer experiments at high Re

A modification of the SSG/LRR-omega model for turbulent boundary layers in adverse pressure gradient is presented. The modification is based on a new wall law for the mean velocity at adverse pressure gradient. The wall law is found from two new joint DLR/UniBw experiments and from the analysis of a data base from the literature. The mean velocity profile in the inner layer is found to consist of a log-law region, which is thinner than its zero pressure gradient counterpart, and a half-power law region above the log law. An empirical correlation for the wall-distance of the transition from the log-law to the half-power law is presented. Then a modification of the omega-equation to account for a half-power law behaviour of the mean velocity is described. The modified SSG/LRR-omega model is then applied to the two joint DLR/UniBw experiments. The modification leads to a reduction of the mean velocity in the inner part of the boundary layer and makes the model more susceptible for flow separation, which is in good agreement with the experimental data

Modification of turbulence models for pressure-induced separation on smooth surfaces using the DLR VicToria experiment

A new experiment of a turbulent boundary layer flow at a large adverse pressure gradient at a high Reynolds number is presented. The strong pressure gradient leads to pressure-induced separation on the smooth surface of the geometry model with a thin separation bubble. The experiment was performed within the DLR internal project VicToria. First, the design of the test case, the set-up in the wind tunnel, and the measurement technique using both large-scale and high-magnification particle imaging and Lagrangian particle tracking are described. Then the experimental results for the mean velocity are described as the flow evolves downstream from the zero-pressure gradient region into the adverse pressure gradient region. From the measurement data a wall law for the mean velocity with a thin log-law region and a half-power law region above the log-law is observed in the adverse pressure gradient region. Then the differential Reynolds stress transport model SSG/LRR-omega is considered. Based on the observation that the length-scale equation is not consistent with the assumed wall laws at adverse-pressure gradient, a modification of the equation for the dissipation rate omega in the model is proposed, so that the modified model can predict the observed wall law at adverse-pressure gradient. Finally, the numerical results using the modified SSG/LRR-omega model are shown. The modification causes a reduction of the mean velocity in the inner part of the boundary layer at adverse-pressure gradients, making the modified model more susceptible for flow separation. The numerical predictions of the modified model are found to be in good agreement with the experimental data

Impact of spatially correlated pore-scale heterogeneity on drying porous media

We study the effect of spatially-correlated heterogeneity on isothermal drying of porous media. We combine a minimal pore-scale model with microfluidic experiments with the same pore geometry. Our simulated drying behavior compares favorably with experiments, considering the large sensitivity of the emergent behavior to the uncertainty associated with even small manufacturing errors. We show that increasing the correlation length in particle sizes promotes preferential drying of clusters of large pores, prolonging liquid connectivity and surface wetness and thus higher drying rates for longer periods. Our findings improve our quantitative understanding of how pore-scale heterogeneity impacts drying, which plays a role in a wide range of processes ranging from fuel cells to curing of paints and cements to global budgets of energy, water and solutes in soils

Automatisierte Auswertung der optischen Wandschubspannungsmessung

Prinzip der Ölfilminterferometrie, bisherige Anwendungen, Stärken und Probleme, weiteres Vorgehe

Bestimmung der Wandschubspannung mittels 3-Farben -Ölfilminterferometrie

Masterarbeit Entwicklung und Anwendung eines Computerprogramms zur automatisierten Auswertung der Ölfilminterferometri

Experimental and numerical investigation of turbulent boundary layers with strong pressure gradients

A detailled investigation of a turbulent boundary-layer flow subjected to a strong adverse pressure gradient (APG) is presented. The main goal is to define a test case for the validation and improvement of RANS-turbulence models from wind-tunnel measurement data collected over the course of multiple measurement campaigns, including volumetric Lagrangian Particle Tracking (LPT) and stereoscopic PIV (SPIV), and oil-film interferometry. The boundary layer at a zero-pressure gradient (ZPG) reference position upstream of the pressure gradient region is found to exhibit a mild deviation from a canonical flow in the sense that the boundary layer thickness and hence the Reynolds number based on the momentum loss thickness Reθ are larger than for a canonical flow. Moreover a mild deviation in skin-friction coefficient and shape factor is found. The experimental data using LPT and SPIV in a spanwise domain around the centerplane show an increase of the boundary layer thickness compared to a canonical flow and a spanwise variability. This can possibly be attributed to the wake flow of the turning vanes upstream of the nozzle and the test-section. For the mean velocity profiles, this leads to a deviation in the law-of-the-wake region compared to canonical flows. The inner region, which is essential for the turbulence modelling and validation, is largely unaffected and agrees well with canonical flows. The Reynolds stresses are also in good agreement with canonical flows. Regarding the ultimate aim to define the computational set-up for RANS simulations, a pragmatic approach is pursued. The inlet length of the test-section is increased to account for the larger boundary layer thickness, corresponding to an adjustment of the virtual origin of the boundary layer. This leads to a good matching with the experimental mean velocity profile and the boundary layer parameters at the ZPG reference position. Downstream, in the pressure gradient region, which is the focus region for the improvement and validation of RANS turbulence models, the deviation between the RANS results and the experimental data is found to be almost insensitive with respect to minor changes in the computational set-up. In the strong APG region, the clearly most important deviation between the numerical predictions and the experimental data is due to the RANS turbulence models used