Abstract
Different variable resolution turbulence modelling approaches (Hybrid, Bridging models and LES) are evaluated for turbulent channel flow at Reτ=395, for cases using either streamwise periodic boundary conditions or a synthetic turbulence generator. The effect of iterative, statistical and discretisation errors is investigated. For LES, little difference between the different sub-filter modelling approaches is found on the finer grids, while on coarser grids ILES deviates from explicit LES approaches. The results for Hybrid models are strongly dependent on their formulation, and the corresponding blending between the RANS and LES regions. The application of PANS with different ratios of modelled-to-total kinetic energy, fk, shows that there is no smooth transition in the results between RANS and DNS. Instead a case-dependent threshold which separates two solution regimes is observed: fk values below 0.2 yield a proper turbulent solution, similar to LES results; higher fk values lead to a laminar flow due to filtering of the smallest scales in the inverse energy cascade. The application of a synthetic turbulence generator is observed to yield similar performance for all models. The reduced computational cost and increased flexibility makes it a suitable approach to enable the usage of SRS for industrial flow cases which depend on the development of a turbulent boundary layer. It ensures that sufficient large-scale structures develop over the full boundary layer height, thereby negating the problem of relying on the inverse energy cascade for the development of turbulence. Both LES and PANS with turbulence generator yield a better match with the reference data than Hybrid models; of these methods PANS is preferable due to the separation of modelling and discretisation errors.
Highlights
The application of Computational Fluid Dynamics (CFD) in the maritime sector is moving towards more complex problems, such as massively separated flows, blunt bodies, off-design conditions, cavitation and noise predictions
The results indicate that for cases where the instantaneous near wall flow field is of importance, Hybrid models are less suitable than Large Eddy Simulation (LES) or Partially Averaged Navier-Stokes (PANS)
This is beneficial for Scale-Resolving Simulations (SRS) techniques such as LES and PANS which yield unphysical laminar and/or separating flows without proper inflow conditions [10], and for Hybrid models where the problems tend to be more concealed. While both precursor and synthetic turbulence generation methods are independent of the turbulence simulation approach, it should be emphasised that recycling in combination with PANS using insufficiently low fk values does not lead to a turbulent flow
Summary
The application of Computational Fluid Dynamics (CFD) in the maritime sector is moving towards more complex problems, such as massively separated flows, blunt bodies, off-design conditions, cavitation and noise predictions. When performing a computation with a high physical resolution on a coarse grid, the discretisation error will become dominant Due to these properties, Bridging models are becoming attractive for industrial CFD, where extensive grid refinement studies are often unaffordable while an estimate of the numerical error is still required [6,7]. At the interface between RANS and LES regions not all the modelled turbulence is transferred directly into resolved turbulence, leading to an overly laminar flow field In such cases, inflow turbulence might still be necessary.
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