Hybrid LES-RANS: Inlet Boundary Conditions for Flows with Recirculation
The paper evaluates a new method for prescribing synthesized turbulent inlet boundary conditions. When making LES, DES or hybrid LES-RANS, a precursor channel DNS is often used. The disadvantage of this method is that it is difficult to re-scale the DNS f
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Abstract The paper evaluates a new method for prescribing synthesized turbulent inlet boundary conditions. When making LES, DES or hybrid LES-RANS, a precursor channel DNS is often used. The disadvantage of this method is that it is difficult to rescale the DNS fluctuations to higher Reynolds numbers. In the present work, synthesized isotropic turbulent fluctuations are generated at the inlet plane with a prescribed turbulent length scale and energy spectrum. A large number of independent realizations are generated. A correlation in time between these realizations is introduced via an asymmetric, non-truncated time filter. In this way the turbulent time scale of the synthesized isotropic turbulent fluctuations is prescribed. The method has previously been validated for DNS at Reτ=500 (Davidson, 2007). In that study it was found that the present approach is at least as good as using inlet boundary conditions from a pre-cursor DNS. This method has also been employed using hybrid LES-RANS for channel flow at Reτ=2000 (Davidson, 2007).
1 Introduction Isotropic synthesized fluctuations based on the method of Kraichnan et al (1970) are often used to generate turbulent fluctuations. This method prescribes an energy spectrum that yields the amplitude of the fluctuations as a function of wave number. Non-isotropic fluctuations have been investigated (Batten et al, 2004; Billson, 2004; Billson et al, 2004; Le and Moin, 1994; Smirnov et al, 2001) where the fluctuations were scaled so that the time-averaged synthesized fluctuations match a prescribed Reynolds stress tensor. A disadvantage of this kind of scaling is that the prescribed spectrum, and hence the two-point correlation, are modified if – as is always the case in real flows – the Reynolds stress tensor is non-homogeneous. To achieve correlation in time, Fourier series were applied in time in the same way as in space in most of the work cited above. In Lee at al (1992) a method was also investigated where a three-dimensional box with generated fluctuations was convected across the inlet plane; in this way fluctuation correlations in the streamwise directions were transformed into correlations in time. In the work by Billson et al (2004) correlation in time is defined by an asymmetric infinite time filter. The method offers a convenient way to prescribe turbulent length and time scales independently. This method is adopted in the present work. A method based partly on synthesized fluctuations was recently presented and is called the vortex method (Jarrrin et al, 2006). It is based on a superposition of coherent eddies where each eddy is described by a shape function that is localized in S.-H. Peng and W. Haase (Eds.): Adv. in Hybrid RANS-LES Modelling, NNFM 97, pp. 55–66, 2008. © Springer-Verlag Berlin Heidelberg 2008 springerlink.com
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space. The eddies are generated randomly in the inflow plane and then convected through it. The method is able to reproduce first and second-order statistics as well as two-point correlations.
2 Synthesized Turbulence
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