5 replies to this topic

[Boro]

Doing a CFD study on a wing design for my uncle's club racing car; and noticing something that I can't wrap my head around.  

 

Don't know its naca designation because searching airfoiltools database came up empty.  This is for a single element spoiler. 

 

 

Aspect ratio is 5.  The width of the car's roof is 1300mm.  

 

If I use a single surface / profile, at the wing's peak AoA (in relation to the air off the roof, not to the ground plane), with a gurney (roughly 2% chord length) produces about 105kg of negative lift.  

 

Why am I seeing a decrease in performance (in terms of neg lift)  when I switch it over to a multi-surface / multi-profile but w/o a gurney?  There is an approx 5 deg difference between the air that's coming over the roof vs the air that's coming past the side window.  The outer profile's AoA difference is exactly that - 5 deg; everything is pretty much identical incl. endplates.  Only difference is I didn't CAD up a gurney for this thing.  Does the gurney play that big of a role in a wing's efficiency in that even in the areas where the air hitting it isn't at its optimal AoA, it can still make the thing outperform on a design that is letting it see the air at an optimal angle across the span but w/o the lip?  The performance decrease is roughly 4.7%; so it's not exactly chump change considering there's very little to begin with.  

 

 

[gruntguru]

Depends what you mean by "performance" and "efficiency". If it's L/D you want - no Gurney. If you just want lift and high AOA without stall the Gurney will always outperform.

[Boro]

Thanks for the answer.

[gruntguru]

You will get some better responses. There are some aero experts here.

[Fat Boy]

I'm not an aero expert, but I'll give you my take.

 

1. Simulations make all sorts of assumptions. A single element and multi-element model will give you different results due to their different assumptions, boundary conditions, calculation methods, etc. It doesn't surprise me that they're different.

 

2. Gurneys are a big deal. Most people completely under estimate their effect. The overall effect on the car will be larger than you'll be able to simulate...numbers don't do them justice. 105 KgF for a gurney that's 5-10mm tall? Sounds completely reasonable.

[Boro]

I'm not an aero expert, but I'll give you my take.

 

1. Simulations make all sorts of assumptions. A single element and multi-element model will give you different results due to their different assumptions, boundary conditions, calculation methods, etc. It doesn't surprise me that they're different.

 

2. Gurneys are a big deal. Most people completely under estimate their effect. The overall effect on the car will be larger than you'll be able to simulate...numbers don't do them justice. 105 KgF for a gurney that's 5-10mm tall? Sounds completely reasonable.

 

I tried it with a bunch of different heights as well after I saw the gain.  It was...enlightening  

 

Always hear people talking about its effects.  I've never worked with a car that has pushrod strain gauges so the real world effects I've never seen, but to see some sort of numbers (even in simulation) was very interesting.