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Originally Posted by topwelshman
Yeah that's what I thought, we have too much angle, the unnderfloor is not flat, we do have an adjustable twin element rear wing up high on the back running between 15-40 degrees and the diffuser is quite long, extends under the diff and we plan to panel in as much as we can around it and around the wheels but by no means is it going to be optimised as you say so need to look at changing the angles
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Take care. With a rear wing such as you have the steeper angle on the diffuser may well be correctly designed; I recently (like two days ago) discovered I know someone who is an aero man at Red Bull Technology/Racing working under Adrian Newey and in talking about the downforce on our car (see another thread) he pointed out that at the rear of our car we have a void, more or less the full width of the car and extending back about a metre behind the (transverse) engine & box. This void is in fact acting as a diffuser and will allow the floor area of the car to give downforce. A correctly designed diffuser 'tidies up' this area and matches the pressure under the car more accurately to the pressure behind the car. The angle in the diffuser is to prevent seperation and the resulting turbulence and drag.
But a rear wing such as you have can change dramatically what is required of the diffuser, rather than one simply stuck under the boot of a hatchback, for example. This is because the area under the wing is low pressure too and quite different from the rear of an 'ordinary' car. The aero effects interact and that is why you will see F1/CART cars and LMPs of course with much steeper angles on their diffusers.
To summarise, a diffuser stuck under a car with no other aero devices will work withouth creating turbulence at 7 to 7.5 degrees angle. More angle MIGHT be possible, but only time in a windtunnel will allow that judgement to be made, so 7 degrees is 'safe' for everyone.
If you have a rear wing, then all that is still true, but the likelyhood is, if the package was designed together and a wind tunnel and/or accurate modelling used to come up with the design, then the greater angle you have may work without seperation and going to a lower angle may reduce your downforce from the underbody, and may even upset the downforce generated by the rear wing.