Above: the pressure fields on the sails and streamlines in way of PRB’s Code 0… using K-FSI-RANS a simplified hull form is added to capture the end-plate effect of the deck on the lower portion of the sails. Here the Code 0 sheet could be eased to reduce the flow recirculation aft of the foot of the sail. Capturing flow separation and recirculation behaviour accurately is only possible with CFD based simulation approaches.When Michel Desjoyeaux won the 2008 Vendée Globe at a canter (left), after restarting days after the fleet, most eyes focused on an innovative Farr Imoca and above all a brilliant skipper. What others saw, however, was how far ahead Desjoyeaux was with his Incidence wardrobe – particularly his downwind sails which were a step ahead of anything else that year
modelling of the fibre layout permits the designer to optimise the fibre placement to produce a light but durable sail. Such an optimisation process is only possible by seamless interoperability between the sail CAD model in SailPack and the structure model constructed for K-Struct. The visu- alisation of the analysis results is simplified by directly viewing them in SailPack such that a flying shape can easily be compared to both the moulded shape and prior flying shapes of other candidate designs. But modelling the structure is not
enough, as it is the loads acting on the structure that deform it and, in any case, it is the generation of aerodynamic forces that is the purpose of the sails. To compute the aerodynamic problem two distinct options are available. In the first method the equations of fluid
motion are simplified to what is known as potential flow. Panel codes solve the potential flow problem by partitioning one or more sails into discrete panels for which the influence of the wake and all other panels is computed. Panel codes have the distinct advantage of being very quick to compute a flow solution, taking only a few seconds to run on an ordinary computer. K-FSI-inviscid, also developed by K-
Epsilon, computes the inviscid flow solution and then transmits the resulting pressure distribution to K-Struct. The sails and rig deform under the influence of the pressure field and the resulting displacements are then passed back to K-FSI-inviscid.
The circular process of computing the
fluid and then the structure while updating the flying shape is continued until the sails converge to a final flying shape, all in a mat- ter of seconds. This rapidity is perhaps the main reason panel codes have enjoyed such widespread use by sail designers as they can naturally be incorporated into the design process for relatively modest financial out- lay. The combined solution of K-FSI-invis- cid and SailPack was developed over five years ago and is used for upwind sail analy- sis by a number of sailmakers including Incidence, Doyle, Quantum and One Sails. An inherent assumption of potential
flow is that the fluid is inviscid – that is to say that the effect of viscosity is neglected. This assumption is both one of the reasons why panel codes are so rapid and the cause of their greatest deficit: the inability to capture flow separation. In a potential flow solution we must
impose the point on the sail at which the flow leaves it, at the leech, head and foot. Imposing the point of flow detachment like this is reasonable when the flow really does do this, as is the case sailing upwind. Even then care must be taken not to over- trim the sails as the flow at the luff will stay attached regardless of whether this is physically realistic. Unfortunately, for other points of sail the flow may separate at points other than the edges, making a potential flow solution unsuitable. To capture flow separation now a viscous flow solution is necessary. Compu-
tational fluid dynamics (CFD) divides the space surrounding the sails into small vol- umes, or ‘cells’, in a mesh in order to solve the equations of fluid motion. The resulting fluid solution includes all the complexities of a real fluid and hence can account for both flow separation and effects like blan- keting of a large headsail by the mainsail. So if CFD is the perfect tool for assess-
ing downwind sails and is just as capable for upwind sails, why isn’t every sail designer using it? Three obstacles have inhibited it from becoming the tool of choice in sail aerodynamics: difficulty, nec- essary expertise and cost. First, recall that sails are flexible struc-
tures whose flying shapes can be quite dif- ferent from their original moulded shape. When the sails deform the cells in the volume mesh surrounding them must be deformed to follow the sails, while remain- ing of good quality and resolution to ensure the accuracy and stability of the computa- tion. This is challenging as sails like spin- nakers can undergo very large changes to their shape; specialised tools are needed to both transfer the fluid loads to the sail struc- ture model and then deform the fluid mesh once the sail’s deformation is computed. Constructing meshes and running CFD
software have traditionally required both significant engineering time and specialised expertise. While great progress has been made to simplify the process, having a sail designer performing this process takes them away from doing what they are
SEAHORSE 61
Page 1 |
Page 2 |
Page 3 |
Page 4 |
Page 5 |
Page 6 |
Page 7 |
Page 8 |
Page 9 |
Page 10 |
Page 11 |
Page 12 |
Page 13 |
Page 14 |
Page 15 |
Page 16 |
Page 17 |
Page 18 |
Page 19 |
Page 20 |
Page 21 |
Page 22 |
Page 23 |
Page 24 |
Page 25 |
Page 26 |
Page 27 |
Page 28 |
Page 29 |
Page 30 |
Page 31 |
Page 32 |
Page 33 |
Page 34 |
Page 35 |
Page 36 |
Page 37 |
Page 38 |
Page 39 |
Page 40 |
Page 41 |
Page 42 |
Page 43 |
Page 44 |
Page 45 |
Page 46 |
Page 47 |
Page 48 |
Page 49 |
Page 50 |
Page 51 |
Page 52 |
Page 53 |
Page 54 |
Page 55 |
Page 56 |
Page 57 |
Page 58 |
Page 59 |
Page 60 |
Page 61 |
Page 62 |
Page 63 |
Page 64 |
Page 65 |
Page 66 |
Page 67 |
Page 68 |
Page 69 |
Page 70 |
Page 71 |
Page 72 |
Page 73 |
Page 74 |
Page 75 |
Page 76 |
Page 77 |
Page 78 |
Page 79 |
Page 80 |
Page 81 |
Page 82 |
Page 83 |
Page 84 |
Page 85 |
Page 86 |
Page 87 |
Page 88 |
Page 89 |
Page 90 |
Page 91 |
Page 92 |
Page 93 |
Page 94 |
Page 95 |
Page 96 |
Page 97 |
Page 98 |
Page 99 |
Page 100 |
Page 101 |
Page 102 |
Page 103 |
Page 104 |
Page 105 |
Page 106 |
Page 107 |
Page 108 |
Page 109 |
Page 110 |
Page 111 |
Page 112 |
Page 113 |
Page 114 |
Page 115 |
Page 116 |
Page 117 |
Page 118 |
Page 119 |
Page 120 |
Page 121 |
Page 122 |
Page 123 |
Page 124
