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    Restatement of the Spalart-Allmaras eddy-viscosity model in strain-adaptive formulation
    (2003)
    Rung, Thomas
    ;
    Bunge, U.
    ;
    Schatz, M.
    ;
    Thiele, Frank
    An overview is given of the strain-adaptive linear Spalart-Allmaras model which provides improved results for nonequilibrium flows while grossly retaining the performance of the Edwards Spalart-Allmaras model in simple boundary-layer and free-shear flows. Although the model cannot be considered universal, it might serve as a good extension of the one-equation modeling framework toward nonequilibrium.
    Publicationtype: Journal Article
      48
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    Item type:Publication, Access status: without files ,
      68
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    Universal wall-boundary conditions for turbulence-transport models
    (2001)
    Rung, Thomas
    ;
    Lübcke, H.
    ;
    Thiele, Frank
    In the industrial design process of fluids engineering devices, the use of numerical simulation is of ever increasing importance. The predictive quality of such simulations is often governed by the representation of turbulence. Virtually all industrial simulations mimic the influence of turbulence by a closure model based on transport equations for statistical turbulence properties. Besides the derivation of such transport‐equation models, the adequate formulation of wall‐boundary conditions has come into the focus of attention. Conventional boundary conditions rely on the validity of specific flow conditions pertaining to the wall‐shear stress and the resolution properties of the computational grid in the wall‐adjacent region. Since the shear stress is part of the simulation result, this approach—strictly speaking—requires the anticipation of the solution. Moreover, it significantly affects the efficiency and flexibility of the simulation due to the associated mesh constraints. The principal aim of this research is the development of a universal boundary condition. Examples included show encouraging results for attached and separated flows.
    Publicationtype: Journal Article
      48
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    Comparison of LES and RANS in bluff-body flows
    (2001)
    Lübcke, H.
    ;
    Schmidt, S.
    ;
    Rung, Thomas
    ;
    Thiele, Frank
    The turbulent flow around bluff-bodies features a variety of complex phenomena, e.g. streamline curvature, separation and the formation of large unsteady vortical structures. In particular, an accurate representation of the interaction between mean transient motion and residual turbulence poses a challenge to numerical simulation procedures. In virtually all commercial simulation packages, the representation of turbulence relies on Reynolds-averaged Navier-Stokes (RANS) equations in conjunction with Boussinesq-viscosity models (BVM). Various studies have demonstrated the inability of established RANS methodologies to render the fundamental physics when applied to transient flows. In contrast to this, the computationally more demanding large eddy simulation (LES) is known to be a viable approach to simulate unsteady turbulent flows. The present study aims to assess the predictive prospects of advanced recent RANS practices, i.e. explicit algebraic stress models (EASM), in unsteady bluff-body flows. Results are reported in comparison to conventional BVM, LES and measurements. Examples included refer to three different cylinder flows, which indicate that the predictive accuracy obtained from an EASM is in close proximity to LES results, whereas the computational surplus remains moderate in comparison with a linear BVM.
    Publicationtype: Journal Article
      28
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    Turbulence closure model constraint derived from stress-induced secondary flow
    (2000)
    Rung, Thomas
    ;
    Lübcke, H.
    ;
    Thiele, Frank H.
    ;
    Fu, Song
    ;
    Wang, Chen
    The crucial part of the turbulence closure in flows exposed to stress-induced longitudinal vorticity is identified. An anisotropic modeling practice is far from being a panacea. The predictive failures can be traced back to the pressure strain (PS) model rooted in the Reynolds stress transport model (RSTM) and its respective analog in the nonlinear stress strain relation. The related analysis employs a turbulence closure based on an explicit algebraic Reynolds stress model (EASM).
    Publicationtype: Journal Article
      27