Revision as of 10:47, 15 September 2011

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 CFLmax ? 1 in this region.

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