CFD可以提供定性和定量结果。除了用力，速度和压力的硬数据，我们可以使用许多可视化来指示它们随时间的分布或变化。了解显示流线和压力分布让我们深入明白流动的行为，并指出几何形状的改变可以改善力或流动特性 – 例如失速角或流动分离。
边界条件应用于模型的感兴趣的表面 – 龙骨，灯泡或船体等 – 然后模型被网格化。网格化生成大量的节点和元素面，用于表示流体及其中的能量，方向和速度。我们通常建模的网格尺寸在50mm和100mm之间的船体，而我们的CFD模型的范围从大约2百万元素到高达800万，取决于网格的大小和密度。
Aeronautical and Automotive
The investigation into the helicopter airframe was to provide two key pieces of data – the total drag of the airframe and pressures on various panels, all at a range of yaw angles and speeds. The pressure – and its distribution – over the glazing/door panels can be used to specify glazing thickness and the supporting structure. With accurate loads defined the structural aspects can be designed more efficiently, saving weight – always a benefit in an aircraft. An automotive company, making a replica E-type Jaguar, wanted to explore the addition of a rear diffuser to help improve the down-force and hence road holding of this classic 1960’s sports car. With over double the original’s horsepower installed, speeds of over 200kmh were being attained, so some updating of the aerodynamics was in order.
Amphibious craft launch simulation
One analysis that sits squarely in between quantitative and qualitative is the investigation in to launching the ARC600 Amphibious Rescue Craft. Exploring a range of gradients and entry speeds to determine when the bow is inundated allows the envelope of safe operations to be entered in to the owner’s manual. While this study was done retrospectively, the ability to define the safe limits for slope and speed for another amphibious craft will inform the design at the early stages and help define sheerline/bow freeboards early in the process. We started with a 2D study to ensure the range of motions was as expected, then moved to a 3D study. The last image is a screen grab from a video of the craft launching as modelled.
Headseas – behaviour in waves
The ability to model a vessel’s behaviour in headseas is important for both predicting the added resistance in waves but also the design of the forward sections and, in the case of a multihull, the wing-deck clearance and extents forward. The increase in drag due to pitching in a seaway can be a large proportion of the calm water resistance, requiring additional horsepower to be installed. The amount of bow flare, as well as chine height and width, can have a bearing on the pitch response and thus both comfort and resistance. The CFD analysis can indicate panel pressures for engineers to incorporate in their structural calculations, and the output of pitch and heave over time allows accelerations to be calculated to assess passenger/crew comfort levels. While we cannot model irregular waves, we can define a sinusoidal wave form, specifying the wave amplitude, wave length and period of encounter, so an approximation to WMO sea states can be modelled. Animations of a 19m ocean rowing skiff in a chop can be seen here and a 56ft power cat here.
Free Surface – the wave pattern
One of the more interesting outputs for hull design is the free surface – or a representation of the wash or wake pattern. Along with the drag forces (which determine the amount of power required) the free surface helps us with the less numerical aspects such as locating chines at the most effective positions according to the way the bow wave is formed. We can look at the flow with a pressure map, vectors or streamlines to understand ‘where it is going’ and refine the design of prop tunnels, or the energy in the wash as it meets the beach or bank to minimise erosion and so on. The hull can be analysed in a fixed orientation – such as a yaw angle that might be used for a towing tank simulation for a yacht sailing upwind – or it could be free trimming so the vessel will change it attitude as the forces evolve, giving us information for a powerboat about running trim and sinkage or stern squat at the transition from displacement to planing modes
Keels, bulbs and rudders.
The first two images here show the keel and bulb analysis from an IRC 42ft design, followed by a new rudder design for a 30m sloop. While a racing yacht’s keel and bulb might seem to be the obvious candidates for CFD analysis – and they are and area where large gains can be made – we have also carried out some less straight forward investigations. Our 56ft cruising yacht had a twin rudder and twin engine configuration, and we wanted to be certain that the prop shafts and p-brackets would not interfere with the flow leading on to the rudders, especially upwind at higher yaw angles. We also carried out a study into the design of skegs for a 40m dumb barge that yawed excessively under tow. Originally fitted with only one skeg on centre, we found the bluff stern was preventing the skeg from seeing any lateral flow and so couldn’t correct the yaw. By placing two skegs outboard and investigating the effects of splay angles we were able to remove the yaw issue completely.
Aerofoils – wings and things
Slotted and multi-element wingsails are becoming more common place, and analysing them is something we can undertake. The images show a slotted leading edge design intended to increase lift/drag ratio and the stall angle , and a conventional hinged two-element wingsail against which it was compared. Other studies have included the design of a cowling and straightening vanes for a frost fan. They are intended for drawing down warmer air from above an inversion layer, and blowing it over several hundred metres along a vineyard to prevent frost damage to the crop. The cowling was improved to remove the ‘dead air’ at the tips of the impellor blades, and the vanes had an element of twist introduced to account for local variations in flow direction across the prop disc.
Aero studies of superstructures
The Elite 10m was an exercise in determining the total aero drag at a number of speeds as the speed requirements were only marginally meet when calculated using the standard formulations for aero drag. This investigation gave us the confidence to state the contract speeds were attainable. The second image shows the 2D investigation into a visor for the Global 80 cruiser. The client wished to settle the argument as to whether a vertical, forward raked or aft raked visor would afford the best protection from a head wind when lying at anchor. (Vertical was the winner). The owner of the Origami 560 wanted to be sure that windscreen would deflect the oncoming airflow at 30knots over his 1.9m head height and would not be unduly affected by the bimini hard top.