Ocean Racing Magazine - #2 - April & May 2007 - (Page 56)

LAB While simulations using digital calculations are increasingly used in the design of racing yachts (VPP,CAO,CFD),these programmes need to be checked to ensure the data is correct in tank tests for hull design using wind tunnels for the sails and appendages.Guillaume Verdier,designer of the IMOCA monohulls Safran and Bel with the design studio VPLP,and member of AREVA Challenge's design team, explains what type of data can be obtained using wind tunnels. OUT OF BREATH has at his disposal the software allowing him to define the theoretical polars for a yacht (VPP : Velocity Prediction Program), curves, which give the boat’s speed at each angle and wind strength. This allows them to understand the interaction between the sail design and the hull. They have to define the power of their craft (the sail plan) and determine the “brake” (the hull) in order to understand the balance of forces. With the VPP, every sail configuration with every wind strength is tested for each hull, which also provides them with the ideal heel angle when sailing. The software offers them the balance between the drag on the hull calculated in CFD (Computational Fluid Dynamics), with the thrust from the sails and the side force of ©F. Socha/Areva Challenge Every designer the rig, while indicating the boat’s speed according to the sail hoisted and the configuration of the appendages. "From the virtual simulation of hulls being tested, we can extract several hulls, in addition to a reference boat, to carry out tank tests. The aim is to measure and confirm the resistance that is predicted by the software, as in CFD, it is difficult to obtain data about the turbulence of fluids. If there are any special elements on the hull, there can be a large difference between the virtual and real data in dynamics. With tank tests, we are able to pay particular attention to the shape of the hull and the secondary effects of the keel and rudder are less important. We carry out tests on the various models with the same appendages to get as close as possible to reality, but to understand the precise flow past a keel profile, we need to test it in another tank and in a wind tunnel," explained Guillaume Verdier. Appreciating the flows A wind tunnel used to see the flow past a car, a speeding skier… or a yacht, consists of a huge fan, generating an air-flow, either in a closed circuit (ring tunnel), or in an open circuit. The key element to the wind tunnel can reach up to 6 metres in diameter or more, and in certain places, by restricting the tunnel, you can accelerate the flow just before it reaches the model. This is fixed on a round table, which is used to modify the angle of the apparent wind, and this table is itself placed on a dynamometric measuring unit to measure the forces. The deformation on the side face gives the thrust moment, while the force of the thrust is seen from the shearing. It then becomes possible to find the position of the centre of pressure: moment divided by force equals lever. The measuring unit also gives the longitudinal stresses to give us the drag. All of these measures are taken in the axis of the boat. Windtunnel tests also allow us to visualise with smoke, what happens to the profile, which is not the case in a tank test. We also require hydrodynamic tunnels, where it is water that creates the flow and the fixed keel is observed through a window, but that costs a lot… The size of the model is a function of the diameter of the wind tunnel. Just as with a tank test, there are blocking effects because of the walls: you therefore need to ensure there is sufficient space around the model for the flow to be as regular as possible. However, the air heats up through friction and the wind tunnel needs to be turned off to cool down. A series of tests can therefore last half an hour at the most. Temperature indeed modifies the viscosity of the fluid. The wind tunnel test is carried out on a large scale, because the model is 1/2 for a Class America keel, 1/12th for a sail plan. However, you need to compare a fluid density for a given speed: the number of Reynolds is the speed of the fluid in m/s multiplied by the profile in metres Ocean Racing - april 2007

Table of Contents for the Digital Edition of Ocean Racing Magazine - #2 - April & May 2007

Contents
What they said
News
Portfolio
Report
Records
America's Cup
Sponsoring
Ocean Planet
Class 40
Imoca
New products
Launch
Lab
Portrait
Practical tips
Profession
Mini Class
Design
Thirty years on
New launches
Equipment
Chronicle

Ocean Racing Magazine - #2 - April & May 2007

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