High Reynolds Number Flow around Airfoil in Deep Stall

Flows Around Bodies

Underlying Flow Regime 2-11

Best Practice Advice

Key Physics

The key physics of this UFR is predominantly characterised by the unsteady, three-dimensional, massively-separated wake region. This takes the form of a nominally periodic shedding of large scale, coherent vortices in a vortex street pattern, which is overlaid with finer random turbulent fluctuations at higher frequencies and random modulation and intermittency at frequencies lower than the vortex shedding frequency. It has been found that it is necessary to capture these key physical features in a simulation in order to reliably predict the assessment parameters. Whether this is achieved or not in a simulation should be checked by:

Obtaining a visual impression of the range of spatial scales present in the wake using e.g. a snapshot of the vorticity magnitude.

Confirming the mixed tonal and broadband nature of the force coefficient time histories by e.g. visual inspection or spectral analysis of time histories.

Numerical Modelling

Discretisation method

Use a numerical scheme with low numerical dissipation (e.g. pure CDS for convective fluxes) in the region of resolved turbulence near the airfoil.

Check for low numerical dissipation by examining instantaneous snapshots (e.g. of vorticity magnitude) in the turbulent wake region: the smallest resolved turbulent scales should have nearly the same size as the local grid spacing (if they are noticeably larger, this is an indication of excessive numerical dissipation).

Use a numerical scheme with sufficient numerical dissipation to prevent grid oscillations or wiggles in the coarse grid and/or irrotational flow regions.

Use a minimum of second order accurate spatial discretisation. No evident benefit of higher spatial accuracy has been proven for this UFR.

Use a minimum of 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 region of resolved turbulence near the airfoil. This corresponds to the approximate guideline CFL_max ≈ 1 in this region.

Grids and grid resolution

Note: These BPA items regarding grid & resolution are not backed up by any specific studies on grid sensitivity, but describe recommendations from experience.

Use a minimum grid spacing of around 1/32 of the chord length in the near wake region.

Use roughly isotropic cells (as far as possible) in the near wake region.

Expand the grid cell size gradually towards the outflow boundary, avoiding sudden jumps where possible and beginning the expansion roughly two chord lengths downstream of the airfoil.

Physical Modelling

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.

Contributed by: Charles Mockett; Misha Strelets — CFD Software GmbH and Technische Universitaet Berlin; New Technologies and Services LLC (NTS) and Saint-Petersburg State University