Trans RINA, Vol 153, Part B2, Intl J Small Craft Tech, 2011 Jul-Dec


in the normal and tangential direction to the walls. Flow gradients should be well resolved. This may be a difficult task on typical sails because of thickness and the subsequent


the zero leading-edge pressure


gradient when there is no mast and the angle of attack is not ideal. Based on these constraints, hybrid mesh technology may be a critical issue for high-fidelity RANS simulations [19].


In fact, results are never totally independent of the chosen mesh. The relevant question when interpreting RANS results on sails is: how bounded is the mesh influence on physical quantities of


interest and the


required precision? This should be investigated on a simplified geometry through validation with wind- tunnel results [19].


To illustrate the mesh convergence, Figure 1, the lift-to- drag ratio (Cl/Cd) convergence with mesh nodes on a typical sail (f/c = 12.5%, Reynolds number Re = 1.4 x 106) calculated on four meshes have been shown. On this particular example, a good convergence on a critical physical quantity may be observed.


10% 15% 20%


0% 5%


0 100 200 300 400


Figure 1: lift-to-drag ratio normalized by the finest mesh value convergence with mesh refinement (nodes number divided by 1000)


Another important feature of meshes is their flexibility to be used with different kinds of sail geometries and trim angles. A critical point for yacht rig aerodynamic studies is the necessity to generate meshes on multiple bodies (mast, mainsail, jib, etc…) which interact and may be displaced relative to each other. The challenge is to generate good quality meshes in the boundary layer regions of each body without using too high aspect ratio cells which may generate numerical scheme instability and too many grid points for computational efficiency reasons. To respect


these topologic


constraints and obtain good mesh control, hybrid meshes (Figure 2) is a useful technology. For more flexibility, it may be completed by non conformal interface between the inner structured region around masts and sails and the outer unstructured region around all interacting structured domains (Figure 2) as was done with Gambit [25]. The mast


with link to the zero-thickness sail is a region of difficulty for the structured mesh part and needs much more attention and some tricks.


Hence all simulations presented in this paper are based on the use of the Spalart-Allmaras turbulence model. Concerning mesh considerations, the model is used in its low Reynolds number form for two-dimensional RANS simulations (y+<5) or with wall functions with y+~30 for


y+~300 for optimizations.


three-dimensional RANS simulations three-dimensional


RANS or or FSI


It has been shown that RANS modelling with a careful application of best practices is able to predict qualitatively main flow features of these two- dimensional flows. A similar validation process of RANS modelling


on three-dimensional STRUCTURE MODEL sail


configurations is challenging and still needs to be done. 3.


trailing-edge


The structural modelling is based on the software RELAX. It is an interactive, fully non-linear finite- element code to analyse fabric structures using a state of the art


relaxation method. RELAX’s special sail


analysis features enables it to be used to predict the behaviour of almost any large-displacement structures [29]. It is a good candidate for sail analysis through a fluid-structure loop.


Figure 2: hybrid mesh example 2.3 TURBULENCE MODELING


Sail aerodynamics is highly concerned with separation bubble, turbulent transition


and the turbulent


reattachment process and it is well known that these phenomenon and their associated pressure losses may have a critical influence on pressure and friction distribution on sails. Also an accurate representation of laminar and turbulent separated flow regions is critical when we are concerned with drag prediction.


In [19] detail flow analysis with separation bubble, turbulent transition and turbulent reattachment process on various mast and mainsail configurations have been computed for validation purposes. Comparisons were made with wind-tunnel results of Wilkinson [26-28]. It has been shown that the one equation Spalart-Allmaras turbulence model may have coherent


qualitative


behaviour on mast-sail geometries and may show to be better than more sophisticated turbulence models based on two transport equations.


flow


©2011: The Royal Institution of Naval Architects


B-105


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