Hayder Schneider

Reliable and Accurate Prediction of Three-Dimensional Separation in Asymmetric Diffusers Using Large-Eddy Simulation

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.

Diffusers with Three-Dimensional Separation as Test Bed for Hybrid LES/RANS Methods

The turbulent flow in two asymmetric diffusers with complex three-dimensional separation was computed employing Large-Eddy Simulations (LES) and Reynolds-Averaged Navier–Stokes (RANS) calculations. The computational setup matches existing experiments in the literature. The objective of the present study is to obtain reference data to be used for assessing the performance of newly developed hybrid LES/RANS techniques.

Coherent Structures in Trailing-Edge Cooling and the Challenge for Turbulent Heat Transfer Modelling

In the present paper, we test the capability of a standard Reynolds-Averaged Navier-Stokes (RANS) turbulence model to predict 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 reference data. Reasonably good agreement between the LES and recent experiments was achieved for mean flow and turbulence statistics. For increasing blowing ratio, the LES replicated an also experimentally observed counter-intuitive decrease of the cooling effectiveness. In contrast, the model failed in predicting this behavior and yielded significant overpredictions. It is shown that the model cannot predict the strong upstream and wall-directed turbulent heat fluxes, which were found to cause the counter-intuitive decrease of the cooling effectiveness.Copyright

Impact of Secondary Vortices on Separation Dynamics in 3D Asymmetric Diffusers

The flow in two three-dimensional (3D) asymmetric diffusers with the same expansion but different aspect ratios was recently measured (Cherry et al., 2008). The results revealed complex 3D separation patterns with a severe sensitivity to the geometric variation. The setup served as a test case for two ERCOFTAC workshops (Jakirlic et al., 2010) that aimed at assessing the predictive capabilities of various turbulence modeling approaches. Reynolds-Averaged Navier–Stokes (RANS) models based on the eddy-viscosity assumption yielded qualitatively wrong results. These models cannot reproduce secondary vortices (SV) in the inlet duct. Methods that account for SV or even resolve these structures fared better. In particular Large-Eddy Simulation (LES) was able to compute the flow in both diffuser geometries within measurement uncertainty (Schneider et al., 2010). The hypothesis that SV have a strong impact on the separation dynamics was further corroborated by recent experiments (Grundmann et al., 2010). At the inlet of one of the diffusers, localized (steady and unsteady) perturbations were introduced. The authors conjectured that the forcing generated streamwise vortices and that these SV were responsible for the observed change in pressure recovery by up to 14%. In the present paper, the hypothesis is tested by controlled numerical experiments using LES and manipulation of (mean) SV in the inlet duct for both diffuser geometries.

The Impact of Secondary Mean Vortices on Turbulent Separation in 3D Diffusers

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.