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Jakirlic S.,Institute of Fluid Mechanics and Aerodynamics SLA | Maduta R.,TU Darmstadt
52nd AIAA Aerospace Sciences Meeting - AIAA Science and Technology Forum and Exposition, SciTech 2014 | Year: 2014

It is well-known that the separation process is inherently a highly unsteady phenomenon. To capture it correctly LES-relevant models - conventional LES and hybrid LES/RANS models (DES schemes, PITM, PANS) - have to be applied. Because of their high spatial and temporal requirements the application of these methods is not straightforwardly affordable for the flow configurations of industrial relevance. On the other hand, apart of the backward-facing step flow geometry characterized by the sharpe-edge separation of a flat plate boundary layer which can be reasonably well solved by an advanced steady RANS model, the flows involving separation are in general beyond the reach of the conventional RANS method independent of the modeling level. Typical outcome is a low level of turbulence activity in the separated shear layer and a correspondingly long recirculation zone. The latter issues motivated the present work demonstrating the possibility to appropriately improve the computational results pertinent to the flow configurations featured by wall-bounded separation in the "Steady RANS" framework. An appropriately designed term modeled in terms of the von Karman length scale (adopted from the SAS modeling strategy for "unsteady" flow computations, Menter and Egorov, 2010) was introduced into the scale-supplying equation governing the homogeneous part of the inverse time scale (ωh = e{open}h/k). This term (denoted by) being active only in the narrow area of the separation region acts towards an appropriate enhancement of the (fully-modeled) turbulence in the separated shear layer resulting in a correct mean velocity development and proper size of the recirculation zone. Predictive performances of the proposed ωh model equation solved in conjunction with the Jakirlic and Hanjalic's Reynolds stress model equation (2002) were illustrated by computing several configurations featured by boundary layer separation including the flow over a periodical arrangement of smoothly contoured 2- D hills in a range of Reynolds numbers, flow over a wall-mounted fence and in a 3D diffuser.

Maden I.,TU Darmstadt | Maden I.,Institute of Fluid Mechanics and Aerodynamics SLA | Maduta R.,TU Darmstadt | Maduta R.,Institute of Fluid Mechanics and Aerodynamics SLA | And 10 more authors.
International Journal of Heat and Fluid Flow | Year: 2013

A complementary experimental and computational study of the flow field evoked by a plasma actuator mounted on a flat plate was in focus of the present work. The main objective of the experimental investigation was the determination of the vector force imparted by the plasma actuator to the fluid flow. The force distribution was presently extracted from the Navier-Stokes equations directly by feeding them with the velocity field measured by a PIV technique. Assuming a steady-in-mean, two-dimensional flow with zero-pressure gradient, the imbalance between the convective term and the momentum equation's right-hand-side terms reveals the desired resulting force. This force-distribution database was used afterwards as the source term in the momentum equation. Furthermore, an empirical model formulation for the volume-force determination parameterized by the underlying PIV-based model is derived. The model is tested within the RANS framework in order to predict a wall jet-like flow induced by a plasma actuator. The Reynolds equations are closed by a near-wall second-moment closure model based on the homogeneous dissipation rate of the kinetic energy of turbulence. The computationally obtained velocity field is analysed along with the experimental data focussing on the wall jet flow region in proximity of the plasma actuator. For comparison purposes, different existing phenomenological models were applied to evaluate the new model's accuracy. The comparative analysis of all applied models demonstrates the strength of the new empirical model, particularly within the plasma domain. In addition, the presently formulated empirical model was applied to the flow in a three-dimensional diffuser whose inflow was modulated by a pair of streamwise vortices generated by the present plasma actuator. The direct comparison with existing experimental data of Grundmann et al. (2011) demonstrated that the specific decrease of the diffuser pressure corresponding to the continuous forcing was predicted correctly. © 2013 Elsevier Inc.

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