UFR 3-33 Best Practice Advice

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Turbulent flow past a smooth and rigid wall-mounted hemisphere

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Description

Test Case Studies

Evaluation

Best Practice Advice

References

Semi-confined flows

Underlying Flow Regime 3-33

Best Practice Advice

This section should be structured around the six subsections below.

Wherever possible, the advice should be in the form of an instruction rather than a conclusion. If appropriate, the conclusion can included after the "instruction" in order to provide context. Thus, for example:

"The aerodynamic coefficients can be accurately predicted with algebraic turbulence models. However these fail to predict the detailed dynamics of the wake boundary layer interaction. Such detail can, however, be predicted with reasonable accuracy using Spalart and Allmaras"

is a conclusion. The BPA advice flowing from this conclusion is:

  • "Use algebraic turbulence models if the requirement is to predict accurately just the aerodynamic coefficents"
  • "Use the Spalart Allmaras turbulence model if the requirement is to predict the detailed dynamics of the wake-boundary layer interaction as well as the aerodynamic coefficients".

It is generally easier to draw conclusions than to convert these into clear statements of advice. Thus it may be helpful to first set down your conclusions at the end of the Evaluation section and then work on these to develop the BPA.

Be extremely careful to ensure that your BPA is strongly supported by the evidence examined in the Evaluation section. Do not offer advice based upon your own experience or prejudices or upon published/unpublished evidence which is not fully examined in the UFR document (e.g. you may have read a recent paper which concludes Spalart and Allmaras is the best for this test case. You cannot base BPA on this if you have not discussed the calculations here).

Key Physics

Summarise the key flow physics which characterise the UFR and which must be captured for accurate prediction of the assessment parameters.

Numerical Modelling

  • Discretization accuracy: In order to perform LES predictions it is required that spatial and temporal discretization are both at least of second-order accuracy. It is also important that the numerical schemes applied possesses low numerical diffusion (and dispersion) properties in order to resolve all the scales and not to dampen them out. A predictor-corrector scheme (projection method) of second-order accuracy forms the kernel of the fluid solver. In the predictor step an explicit Runge-Kutta scheme advances the momentum equation in time. This explicit method is chosen because of its accuracy, speed and low memory consumption. The discretization in space is done with second order central discretization scheme with a flux blending including 5% of a first-order upwind scheme.
  • Grid resolution: The second critical issue to perform LES is the grid resolution. The mesh near the wall, in the free-shear layers and also in the interior flow domain has to be fine enough. For wall-resolved LES the recommendations given by Piomelli and Chasnov (1996) should be followed or outperformed, e.g., y^+ < 2, \; \Delta x^+ < 50, \; \Delta z^+ < 50-150. In the present investigation the grid possesses about 30 million CVs. The first cell center is positioned at a distance of Δz/D=5 x 10 − 5. It was found to be sufficient to resolve the flow accurately at walls as well as in the free shear layers. Similar to the classical flow around a cylinder it is important to resolve the region close to the separation point and the evolving shear layer region adequately.
  • Grid quality: The third point is the quality of the grid. Smoothness and orthogonality is a very important issue for LES computations. In order to capture separations and reattachments on the hemisphere reliably, the orthogonality of the curvilinear grid in the vicinity of the walls has to be high.
  • Inlet boundary condition: At the inlet a 1/7 power law with δ / D = 0.5 and without any perturbation is applied. However, to mimic the targeted boundary layer, perturbations generated by a synthetic turbulence inflow generator are injected as source terms upstream of the hemisphere. These additional perturbations are important to reach a good agreement between experimental and LES. Indeed, as demonstrate in Wood et al. (2016), they directly affect the size of the horseshoe vortex, the position of the separation line and consequently the recirculation area.
  • Outlet boundary condition: A mix of convective and non-convective outflow boundary condition is applied. The convective outlet boundary condition favored allowing vortices to leave the integration domain without significant disturbances (Breuer, 2002). The convection velocity is set to the 1/7 power law without perturbation.

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: Jens Nikolas Wood, Guillaume De Nayer, Stephan Schmidt, Michael Breuer — Helmut-Schmidt Universität Hamburg

Front Page

Description

Test Case Studies

Evaluation

Best Practice Advice

References


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