The present paper tests the capability of a standard Reynolds-Averaged Navier-Stokes (RANS) turbulence model for predicting the turbulent heat transfer in a generic trailing-edge situation with a cutback on the pressure side of the blade. The model investigated uses a gradient-diffusion assumption with a scalar turbulent-diffusivity and constant turbulent Prandtl number. High-fidelity Large-Eddy Simulations (LES) were performed for three blowing ratios to provide reliable target data and the mean velocity and eddy viscosity as input for the heat transfer model testing. Reasonably good agreement between the LES and recent experiments was achieved for mean flow and turbulence statistics. The LES yielded coherent structures which were analysed, in particular with respect to their effect on the turbulent heat transfer. For increasing blowing ratio, the LES replicated an also experimentally observed counter-intuitive decrease of the cooling effectiveness caused by the coherent structures becoming stronger. In contrast, the RANS turbulent heat transfer model failed in predicting this behaviour and yielded significantly too high cooling effectiveness. It is shown that the model cannot predict the strong upstream and wall-directed turbulent heat fluxes caused by large coherent structures, which were found to be responsible for the counter-intuitive decrease of the cooling effectiveness.@en

For high-pressure turbine blades, an efficient cooling of the trailing edge can be achieved by ejecting a film into the flow over a cutback on the pressure-side of the blade. Here, results of well-resolved Large-Eddy Simulations (LES) are reported that match an existing experimental setup. A low blowing ratio M=0.5 was chosen and compared to results for an engine-typical value of 1.1. LES and experiments agreed reasonably well for mean and r.m.s. velocity profiles and adiabatic film-cooling effectiveness ηaw. By imposing different flow conditions in the coolant channel, the LES data show that the flow regime of the coolant at ejection has a significant impact on the performance of the resulting cooling film for both blowing ratios. There are two surprising results: (1) a counter-intuitive behavior causing an also experimentally observed increase in ηaw for a reduced blowing ratio. (2) For M=0.5, a turbulent coolant sustains a higher cooling effectiveness farther downstream compared to a laminar coolant, whereas, for M=1.1, the opposite is observed. It is shown that both phenomena are related to a change in type and strength of the dominant coherent structures that are formed behind the cutback lip and convected downstream along the trailing edge.@en

Film cooling is a key technology for improving the thermal efficiency and power output of gas turbines. The trailing-edge section of high-pressure turbine blades can be efficiently cooled by ejecting a film over a cutback on the pressure side of the blade. In this paper, results of Large-Eddy Simulations (LES) are presented that match an existing experimental setup. Altogether, eight simulations with the blowing ratio M varying as the only parameter were performed over a range from M = 0.35 to 1.4. Reasonably good agreement between LES and experiments were obtained for flow field statistics and adiabatic film-cooling effectiveness η aw. Within a limited range of blowing ratios, an increase in the blowing ratio results in a counter-intuitive decrease of the cooling effectiveness. The present work suggests a mechanism that can explain this behavior. The visualization and analysis of large coherent structures showed that there exists dominant clockwise-rotating structures that can give rise to a combined upstream- and wall-directed turbulent heat flux. This turbulent heat flux represents the main contribution of the total heat flux and causes a significantly intensified thermal mixing process, which in turn results in the counter-intuitive decrease of the cooling effectiveness.@en

The turbulent separated flow through an asymmetric diffuser with and without manipulation of incoming turbulence-driven mean secondary vortices (MSVs) from a rectangular duct is investigated by large-eddy simulations. The simulations carried out for two diffuser geometries reveal that by introducing a small amount of mean-flow kinetic energy via the MSVs into the flow, the complex three-dimensional separation behaviour and pressure recovery can be effectively controlled. Manipulated MSVs were found to enhance cross-sectional transport of high-momentum fluid, which determined the location, shape, and size of the separation bubble. The integral effect was a delay or expedition in the onset of separation. This change strongly affected the conversion of mean-flow kinetic energy to pressure, in particular for the front part of the diffuser. In addition, a substantial reduction in total pressure loss could be achieved. The manipulation of the MSVs is an efficient mechanism for performance enhancement in the cases investigated. The results have important implications for both control and statistical modelling of turbulent separated flow in rectangular diffusers.@en

In the present study a tandem cylinder setup is simulated by means of embedded DNS. The influence of wake turbulence on the heat transfer in the stagnation region of the rear cylinder is investigated. The oncoming flow is varied by increasing the distance between the two cylinders, causing a change of the turbulent wake characteristics and the heat transfer. The data of both simulations show good agreement with an existing experimental correlation in the literature. For the small wake generator distance, a clear shift of the maximum heat transfer is observed away from the stagnation line. This shift is less pronounced for the larger distance.@en

In the present study the flow and heat transfer in a tandem cylinder setup are simulated by means of embedded direct numerical simulation (DNS). The influence of wake turbulence on the heat transfer in the stagnation region of the rear cylinder is investigated. The oncoming flow is varied by increasing the distance between the two cylinders, causing a change of the turbulent wake characteristics and the heat transfer. The data of both simulations show good agreement with an existing experimental correlation in the literature. For the small wake generator distance, a clear shift of the maximum heat transfer away from the stagnation line is observed. This shift is less pronounced for the larger distance.@en

Large-eddy simulations (LES) and Reynolds-averaged Navier-Stokes (RANS) calculations of the flow in two asymmetric three-dimensional diffusers were performed. The setup was chosen to match an existing experiment with separation. Both diffusers possess the same expansion ratio but differ in performance. The aim of the present study is to find the least expensive method to reliably and with reasonable accuracy account for the impact of the change in geometry. RANS calculations failed to predict both the extent and location of the separation. In contrast, LES with wall-functions delivered results within the accuracy of the experimental data.@en

In rectangular ducts with fully-developed turbulent flow, mean vortices in the corners form secondary flow patterns whose energy contents is orders of magnitude lower than that of the flow in the streamwise direction. In the present numerical experiments, it is demonstrated using Large Eddy Simulations (LES) that these Mean Secondary Vortices (MSV) exert a profound influence on flow separation in three-dimensional asymmetric diffusers following such a duct. By removing, enhancing or switching the sense of rotation of the MSV in the inlet duct of two diffusers the shape, location and extent of separation zones farther downstream were drastically altered and, hence, the performance of the device. These results provide an explanation why eddy-viscosity based Reynolds-Averaged Navier-Stokes (RANS) models, that inherently cannot account for MSV, fail in predicting even the location of the separated flow in such diffusers.@en