Computational Fluid Dynamics

Computational Fluid Dynamics – or CFD for short- is a widespread technique used for rapid prototyping and design development and evolution. Unlike wind tunnel testing or data collection from instrumented models it relies instead on massive computer power to test the interaction of the vehicle with the elements around it. 

The vehicle “model” exists only on the computer as a CAD model which in our case will be a more complex evolution of the one created for us by Addqual by high resolution scanning of the original wooden Railton model. Minor or major changes to a CAD model can be made relatively quickly followed by further CFD analysis to test the results. This method is used extensively (and at great expense) by all F1 teams to produce a season long flow of modified parts used to update their cars race by race.

Although is requires large scale investment, the major benefit of CFD is the speed with which developments can take place. Comparative testing also takes place using wind tunnels but back in the days when wind tunnels were all that designers had, testing could be laborious and a bit hit and miss – science meets black art. Perhaps the best illustration of this is was the work carried out by The National Physical Laboratory at Teddington by Eric Bannister and Professor Dalby for Nigel Gresley while finalising the shape of his Bugatti inspired streamlined steam locomotive Mallard. For over 40 years designers had struggled to direct smoke away from the locomotive so that the driver could see ahead. Many weird and wonderful versions of smoke deflectors had been produced giving not very successful results, so using a 1:12 scale wooden model with a plasticine chimney, the NPL ran test after test with no luck. Then, suddenly and without any apparent change being made, the smoke flowed away from the model matching exactly what they were looking for. Unable to believe it they took the model in and out of the chamber to repeat the tests and still getting the same results. Problem solved – but how?  Inspecting the model carefully, they noticed a small indentation made by a finger mark at the rear of the plascticine chimney made by chance when placing the model in the wind tunnel.

You still need clever and experienced designers to make sense of things but the use of CFD modelling and testing techniques means the whole process can go though many evolutions (some elements in parallel if necessary)  very quickly to get to the result needed. We feel sure that both Nigel Gresley and Reid Railton would have jumped at the chance of using such techniques so it will be fantastic to do this on their behalf.

For boats, the wind tunnel tests would only have taken place after extensive tank testing of the basic hull shape to make sure that it would plane successfully. This hydrodynamic testing would then have been followed up with wind tunnel testing to make sure the upper surfaces enabled stability and control of a craft now skimming across the surface of the water rather than ploughing through it. With modern technology this can be achieved with CFD.