The water tank method of testing wing airflows
When one thinks of aerodynamic testing, CFD, wind tunnel and track testing are the forms that most readily spring to mind. None of these are foolproof, however, and Formula One teams sometimes need to go to unusual lengths in order to validate data gathered using these methods. In one case, the Honda F1 team opted to use a water tank – more normally home to boat hulls – to test some characteristics of rear wing flows.
The reason for this unusual choice was that data obtained during track test had indicated the possibility that normalised downforce levels fell when the vehicle was decelerating. In particular, the team thought it was possible that the normalised downforce was declining because of the development of a boundary layer at the rear wing as the wake from the rear wing overtook the wing section as the car decelerated. The team’s then current simulation methods could not be relied on to accurately predict such subtle transient flows, so they decided to use a full-sized rear wing in a water towing tank. This would allow analysis of the occurrence, or non-occurrence, of flow separation and the changes in loadings as the vehicle was decelerated.
The most important factor in such a test was to account for the differing densities of air and water. The key benefit of such a test is the fact that water’s higher density effectively allows flows to be simulated in slow motion. Roughly speaking, for the same dynamic pressure across the wing, the speed of water flow over the wing element need only be 1/30th of that if it were in air. Provided the differing factors such as viscosity are properly accounted for – the acceleration rate needed to generate representative forces were about 1/1000th of those in air – it is therefore feasible to collect data in a water tank that can be directly related to operation in air.
Given this, it is possible to conduct slow-motion tests in water, and assess transient aerodynamic loads during deceleration – which are challenging to measure when the vehicle is actually running on a race track – to be measured with a high degree of accuracy. During Honda’s specific rear wing assessment programme, deceleration tests from 0.005-0.05 g and fixed-speed tests from 1.02-2.94 m/s (corresponding to 106-307 kph in air) were conducted.
The tank used for the test was 200 m long, 10 m wide and 5 m deep. The rear wing assembly was placed in the tank, inverted and attached to a six-component load cell, 1 m below the surface. To prevent waves forming on the surface of the water that could potentially skew the results, a flat acrylic plate was placed near the water surface.
Honda conducted tests with this system to assess the rear wing flows under deceleration, and was able to conclude that the flow was not reversing over the wing and that the wing was operating as it should at all times. The tests also showed that such methods were a useful additional testing tool, particularly for physically assessing hard-to-measure behaviours such as aero elasticity.
Whether other teams have undertaken such tests is not known, but it raises an interesting question – would such methods fall outside the CFD and wind tunnel limits set by the FIA, therefore providing a valuable extra source of unrestricted aerodynamic data?
Written by Lawrence Butcher