Thoughts of a poor old sod

Flummery & Flannery

Application Challenge AC7-01

Best Practice Advice

The Best Practice Advice (BPA) for Application Challenges (ACs) follows the same format as that adopted for UFR BPA . However, the advice should concentrate on the prediction of the design or assessment parameters (DOAPs) since, by definition, these are the quantities of prime interest to the analyst. This does not preclude consideration of the detailed flow structure (i.e higher order parameters) if this is both possible and deemed to add value to the advice. The BPA should constitute a synthesis of the data contained within this AC document with that in the associated UFRs (as formalised in the UFR BPAs ). Guidance on how to make this synthesis is set out below. It is important to stress that the advice set down should be supported by the evidence presented in these documents. Personal prejudice and judgements based on personal experience must be avoided.

The DOAPs may not be well predicted in the AC study because the UFR-BPA, based on strong high quality evidence, has not been followed in the AC calculations (e.g. insufficient grid, low order numerical scheme, incompetent turbulence models etc.). Under such circumstances the AC-BPA advice should be based upon the UFR-BPA and appropriate recommendations made for further AC studies.

The evidence embodied within the AC-Document may not be consistent with the associated UFR-BPA. There may be various reasons for this:

There are no UFRs presently within the knowledge base which are relevant to this AC.
The UFR test cases which have been studied are not sufficiently well aligned with the flow conditions encountered in the AC. For example the flow parameters/conditions controlling the UFR test-case may not be as severe as the AC case (e.g. pressure gradient, level of streamline curvature, Grashof number etc.), or perhaps the UFR test case features several interacting flow regimes of which only one is relevant to the AC.

Under such conditions, you should base the BPA solely on the AC evidence (provided this is of sufficient detail and quality) and then make appropriate recommendations for the identification and analysis of UFR test cases.

The reasons for the (marked) inconsistency may be none of the above and may not be easily identifiable. The inconsistency could be due to AC application uncertainties. Once again, under such circumstances you should base your AC-BPA solely on the AC evidence (providing this is of sufficient detail and quality) whilst embodying appropriate caveats.

If, in the last analysis, the detail and quality of the AC data is not sufficient for drawing out reasonably well-founded BPA, then this should be stated, the BPA left open, and recommendations made for further remedial work.

Key Fluid Physics

Briefly describe the key fluid physics/flow regimes which exert an influence on the DOAPs. Ideally this should draw together into a coherent picture the associated UFR descriptions together with any important interactions which are AC specific. Mention the UFRs associated with this AC that you have considered in drafting your best practice advice. Access the Knowledge Base to find the UFRs associated with your AC.

Application Uncertainties

List any uncertainties which make a high fidelity CFD model difficult to assemble. Typical examples might include:

a gas leakage between two components which is difficult to resolve on practical meshes, and even if it is resolved, the leakage flow conditions may not be known.

flow conditions at inlet to the AC (or indeed other boundaries) which may be complex and not precisely known.

fine details of the geometry are imprecise.

Briefly discuss the sensitivity of the DOAP predictions to these uncertainties and their impact on the BPA. In particular, can clear, unequivocal BPA be given or is it necessary to introduce appropriate caveats.

Computational Domain and Boundary Conditions

State any restrictions on simplifications to the geometry (e.g. three dimensional effects are important and thus 2-D idealisation must not be used; wind tunnel blockage effects are significant and thus the wind tunnel geometry must be modelled; the flow displays transient un-symmetric behaviour and thus a geometric symmetry element cannot be used etc.).

Advise on the extent of the computational domain in order to capture all flow features and minimise uncertainties in the setting of boundary conditions.

At each boundary of the computational domain provide advice on which boundary condition to use and how to set it up.

Discretisation and Grid Resolution

Provide advice on the order of the numerical scheme which is necessary to resolve the main flow features (controlling the DOAPs) on practical grids.

Provide clear advice on the level of grid resolution (at walls and across shear layers and dominant flow structures) which is required in order to predict the DOAPs with reasonable accuracy.

If possible, provide advice on the level of grid resolution which is required to capture more detailed (higher order) aspects of the flow (e.g. velocity profiles, turbulent stresses etc.). This is not mandatory.

The advice should be consistent with the BPA for the associated UFRs whilst not contradicting the evidence in the AC document. If this is not the case give reasons why.

Physical Modelling

Provide advice and recommendations on which models are capable of delivering accurate predictions of the DOAPs, provided the advice in subsections 2.3 and 2.4 above is heeded.

If possible, provide advice and recommendations on which models are capable of delivering accurate predictions of higher order flow parameters (this is not mandatory).

The advice should be consistent with the BPA for the associated UFRs whilst not contradicting the evidence in the AC document. If this is not the case give reasons why.