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 Do not offer advice based upon your own experience or prejudices or upon published/unpublished evidence which is not fully examined in the UFR document (e.g. you may have read a recent paper which concludes Spalart and Allmaras is the best for this test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

FSI-PfS-1a consists of a flexible thin structure with a distinct thickness clamped behind a fixed rigid non-rotating cylinder installed in a water channel.

At an inflow velocity of $u_{\text{inflow}}=1.385$ m/s the displacements are quasi two-dimensional, symmetrical, reasonably large and well reproducible. The rubber plate deforms in the first swiveling mode. This FSI phenomenon is dominated by an instability-induced excitation (IIE) (Naudascher and Rockwell (1994)). IIE is provoked by flow instability which gives rise to flow fluctuations if a specific flow velocity is reached. These fluctuations and the resulting forces become well correlated and their frequency is close to a natural frequency of the flexible structure (``lock-in phenomenon). In this case oscillations with large amplitudes are expected.

Based on the inflow velocity chosen ( $u_{\text{inflow}}=1.385$ m/s) and the cylinder diameter the Reynolds number of the experiment is equal to ${\text{Re}}=30,470$ . Regarding the flow around the front cylinder, at this inflow velocity the flow is in the sub-critical regime. That means the boundary layers are still laminar, but transition to turbulence takes place in the free shear layers evolving from the separated boundary layers behind the apex of the cylinder. Transition to turbulence means that from that point onwards the flow is three-dimensional and chaotic, and consists of a variety of different length and time scales. The low-frequency components of the turbulent flow dominate the coupled FSI problem, whereas the high-frequency contributions are visible in the fluid forces but are filtered out by the flexible structure. That is the reason why the signals for the deflections show the quasi-periodic signals without high-frequency fluctuations.

Numerical Modelling

Discretisation method
Grids and grid resolution

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: G. De Nayer, A. Kalmbach, M. Breuer — Helmut-Schmidt Universität Hamburg (with support by S. Sicklinger and R. Wüchner from Technische Universität München)