The difficulty for any aerodynamicist is to make their test results match those on the track. This correlation is far from straight forward, both CFD and tunnels are from true representations of the real world.
Honda (Nee B.A.R) have run their original tunnel for many years, the open jet design had it draw backs but under Geoff Willis and Willem toet, the team got the test results matching those on track. They now have a new tunnel and this uses a different test section (adaptive wall..?) and measuring equipment. Getting this matching the original tunnels tests and to that on track will take time. Williams have suffered similar issues with their second tunnel. Its not surprising that Toyota have planned a new tunnel identical to that of the old one, this should limit differences.
Aside from the issue of calibrating the tunnel, Both tunnels and CFD can sometimes give results different to those on track. When Toyota started in f1 they struggled at tracks with bumps and high kerbs. Of course the team looked deeply into the suspension set up, the problem eventually was found to be an unexpected vortex coming off the front wing endplate when going over kerbs and big bumps, causing a huge loss in downforce. This wasn’t shown up on the test results, as that specific attitude never gets tested.
I think Honda have found a aerodynamic concept around their front end that is fundamentally flawed. The teams cars for a few years have struggled with their front ends. So in some respect this problem pre-dates the new tunnel. I have spoken to Jacky Eeckelaert and the team are working through the issue systematically, testing the model and full size car in the tunnel, as well as instrumented run with the car on track. Finding the problem could take months or moments, Simply going off and redesigning a new front end without finding the root cause could come back to haunt them if the same design mistake is repeated.
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