UFR 2-12 Best Practice Advice

Turbulent Flow Past Two-Body Configurations

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Best Practice Advice

NOTE: the BPA formulated below are overall well in line with conclusions based on the outcome of BANC-I [5] and not yet published results of BANC-II.

Key Physics

The key physical features of this UFR are separation of the turbulent shear layer from the upstream cylinder, free shear layer roll-up and chaotization, interaction of the essentially unsteady wake of the upstream cylinder with the downstream one, and massively separated wake of the downstream cylinder. It is found that it is necessary to capture these challenging features in a simulation claiming a reliable prediction of all the UFR characteristics. Whether this is reached or not in a simulation should be checked by:

• Obtaining a visual impression of the unsteadiness of the shear layer separated from the upstream cylinder and of a range of spatial scales present in its wake and in the wake of the downstream cylinder using e.g. a snapshot of isosurface of λ₂ ("swirl") or Q-criterion (see Figure 4 for an example of the former) and vorticity contours (Figure 11).
• Confirming a mixed tonal and broadband nature of the pressure signals on the surface of the cylinders by their spectral analysis (Figure 16).

Numerical Issues

In terms of numerics, based on experience accumulated in the course of the ATAAC and related projects, the following advice can be given:

• In the Focus Region of simulation (see Figure 2), use numerical schemes with as low numerical dissipation as possible, particularly, pure or close to pure CDS for convective fluxes with the order of accuracy not less than 2. Acceptability of the level of numerical dissipation may be assessed by examining snapshots of e.g. vorticity in the focus region: the size of the smallest resolved eddies should not be noticeably larger than the local grid spacing.
• In the Euler and Departure Regions (see Figure 2) use a scheme with sufficient numerical dissipation to prevent grid oscillations or "wiggles" in these regions.
• Use a minimum second order accurate temporal integration scheme.
• Use a time step sufficiently fine to capture the motion of the turbulent eddies resolved by the grid in the Focus Region. This corresponds to the approximate guideline CFL_max ≈ 1.

In terms of grids, although no systematic grid-sensitivity studies for the considered UFR have been carried out, indirect evidence allows the following recommendations:

• In the Focus Region, use nearly isotropic grids with sizes not larger than around 0.02D, although even smaller values are desirable.
• Outside the Focus Region, expand the grid cell size gradually towards the inflow/outflow boundaries, avoiding sudden jumps.

Size of computational domain should not be less than about ±20D in the streamwise and about ±10D in the lateral direction. For the spanwise direction, the domain size should not be less than 3D, but larger domains are strongly recommended provided that available computer resources allow this.

Physical Modelling Issues

• Turbulence modelling
• Transition modelling
• Near-wall modelling
• Other modelling

Application Uncertainties

Summarise any aspects of the UFR model set-up which are subject to uncertainty and to which the assessment parameters are particularly sensitive (e.g location and nature of transition to turbulence; specification of turbulence quantities at inlet; flow leakage through gaps etc.)

Recommendations for Future Work

Propose further studies which will improve the quality or scope of the BPA and perhaps bring it up to date. For example, perhaps further calculations of the test-case should be performed employing more recent, highly promising models of turbulence (e.g Spalart and Allmaras, Durbin's v2f, etc.). Or perhaps new experiments should be undertaken for which the values of key parameters (e.g. pressure gradient or streamline curvature) are much closer to those encountered in real application challenges.

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Contributed by: A. Garbaruk, M. Shur and M. Strelets — New Technologies and Services LLC (NTS) and St.-Petersburg State Polytechnic University

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Description

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Best Practice Advice

References

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