Aerodynamics - Diffuser

[tornado33]

Hi all,

I have a little confusion with the modelling principles behind the terms found in:

DiffuserBase=(-0.646, -4.8, 68)

As I understand from the very helpful posts found on this forum and others. CoL is calculated by "CoL = DiffuserBase_1 + DiffuserBase_2 x Rear_RideHeight + DiffuserBase_3 x Rear_RideHeight^2". This is where my confusion begins, I have CoL & CoD values for a diffuser as well as load values, front and planform areas, this has all been taken from CFD and CAD models. However I am completely lost by the meaning and the calculation of the 1st, 2nd and 3rd order values in this parameter.

Additionally, "DiffuserFrontHeight=(1.8)", how is this value calculated, being associated with height I would assume that it would be a measured height from a datum. However clearly the value is to large useless a monster truck with an optimistic aero package ^^.

Any help with explaining these would be much appreciated.

 

[Guy Moulton]

I was looking at the aero for the GP79 Ligier today and stumbled on this ausfactor.com post about the HDV file. It's very helpful. Also, realize that though the Diffuser settings area of the HDV is called the Diffuser section, it really is more than the diffuser- it is the area that defines the settings for the whole of the underside of the car and how ground effect downforce is calculated.

 

[tornado33]

Thanks for the reply, I have already seen all of the incredibly helpful posts from Bristow and I understand that overall principle of how rF is representing a "Diffuser". What I really do not understand is how to calculated the values for Diffuser Base 1, 2 and 3, from normal data.

I am sure I am missing something really obvious, but I would be surprised if this is such an ambiguous part of rF as is pretty much the foundation for the diffuser.

 

[silente]

Thanks for the reply, I have already seen all of the incredibly helpful posts from Bristow and I understand that overall principle of how rF is representing a "Diffuser". What I really do not understand is how to calculated the values for Diffuser Base 1, 2 and 3, from normal data.

I am sure I am missing something really obvious, but I would be surprised if this is such an ambiguous part of rF as is pretty much the foundation for the diffuser.

Tornado,

why do you want to only reproduce the aero contribution from the diffuser?

If you think to how aero is modeled into rF, at the end you have several objects that can create downforce: front and rear wings, body, diffuser etc. Some of them, produce downforce in a way that depends only from ride heights, some of them in a way that also depends on rake (see diffuser).

But at the end of the story, you should aim to reproduce your complete aeromap in the closest possible way, not only the diffuser contribution...

So for example, if you know that downforce change, overall, by x% with a certain angle change on wings, but also by a y% with a certain ride height change front or rear, then if i was you i would try to reproduce the best i can all these features. To do so, you will have to explore the possibilities given by devices which are only ride height sensitive and by devices that are also rake sensitive...

In your place i would not want to only reproduce the downforce coming from the diffuser. Also from a real physics perspective, it doesn´t make so much sense in my opinion.

 

[tornado33]

Tornado,

why do you want to only reproduce the aero contribution from the diffuser?

If you think to how aero is modeled into rF, at the end you have several objects that can create downforce: front and rear wings, body, diffuser etc. Some of them, produce downforce in a way that depends only from ride heights, some of them in a way that also depends on rake (see diffuser).

But at the end of the story, you should aim to reproduce your complete aeromap in the closest possible way, not only the diffuser contribution...

So for example, if you know that downforce change, overall, by x% with a certain angle change on wings, but also by a y% with a certain ride height change front or rear, then if i was you i would try to reproduce the best i can all these features. To do so, you will have to explore the possibilities given by devices which are only ride height sensitive and by devices that are also rake sensitive...

In your place i would not want to only reproduce the downforce coming from the diffuser. Also from a real physics perspective, it doesn´t make so much sense in my opinion.

Thanks for the reply,

I understand and agree with your point, however I am not trying to exclusively account for the effect of the diffuser to encompass the whole of the vehicles aero map. However I am trying to represent the contribution of aero on the vehicle model where applicable. I have data for all other aerodynamic devices, in this case a front and rear wing are non existent on the vehicle, however it does have quite a significant diffuser. I understand and can quantify the effect on performance that the diffuser has to the vehicle, in for the format of change in lift and drag on the overall vehicle package, CoP etc etc. I can simply represent the aero contribution for the body in the relevant section, however I do not understand how to represent the diffuser with the given parameters.

 

[silente]

Thanks for the reply,

I understand and agree with your point, however I am not trying to exclusively account for the effect of the diffuser to encompass the whole of the vehicles aero map. However I am trying to represent the contribution of aero on the vehicle model where applicable. I have data for all other aerodynamic devices, in this case a front and rear wing are non existent on the vehicle, however it does have quite a significant diffuser. I understand and can quantify the effect on performance that the diffuser has to the vehicle, in for the format of change in lift and drag on the overall vehicle package, CoP etc etc. I can simply represent the aero contribution for the body in the relevant section, however I do not understand how to represent the diffuser with the given parameters.

Tornado,

following the equations you mentioned you are able to calculate coefficients that, multiplied by speed^2 give Aero forces in Newtons. From your CoL or CoD, you can basically do the same (assuming a certain air density). At the end, you will have to change your diffuser parameters so that the aero forces you get in rF matches as close as possible the forces you get with your real world data.

I don´t know if it was what you were asking, anyway this is all described in Bristow guide, someone in this forum has already posted a link to it.

Basically, rF works like: Force = rf Coeff*Speed^2 instead of the normal Force = 0.5*rho*S*Speed^2.

you just have to create a match with the two formulas.

was this what you were asking?

 

[tornado33]

Tornado,

following the equations you mentioned you are able to calculate coefficients that, multiplied by speed^2 give Aero forces in Newtons. From your CoL or CoD, you can basically do the same (assuming a certain air density). At the end, you will have to change your diffuser parameters so that the aero forces you get in rF matches as close as possible the forces you get with your real world data.

I don´t know if it was what you were asking, anyway this is all described in Bristow guide, someone in this forum has already posted a link to it.

Basically, rF works like: Force = rf Coeff*Speed^2 instead of the normal Force = 0.5*rho*S*Speed^2.

you just have to create a match with the two formulas.

was this what you were asking?

Thanks for your reply again,

I am surprised that there isn't a definitive method for calculating a,b and c, other than trial and error. Which is fair enough I will use that as my first method, although as I have a large pool of data, of a CoL and Ride Heights in at least 3 different instances I will try and solve for a,b and c using simultaneous equations.

 

[silente]

Thanks for your reply again,

I am surprised that there isn't a definitive method for calculating a,b and c, other than trial and error. Which is fair enough I will use that as my first method, although as I have a large pool of data, of a CoL and Ride Heights in at least 3 different instances I will try and solve for a,b and c using simultaneous equations.

Tornado,

of course there are other ways. That´s up to you!

You could also use very sophisticated optimization algorithms or simply excel solver....

I am just trying to explain how the physics modeling side work, then it´s up to you how to come to the right coefficients values.

 

[tornado33]

Tornado,

of course there are other ways. That´s up to you!

You could also use very sophisticated optimization algorithms or simply excel solver....

I am just trying to explain how the physics modeling side work, then it´s up to you how to come to the right coefficients values.

Cheers for your help silente,

Funnily enough I found an archived thread between yourself, niels and Lieu (I think), regarding suspension representation with motion ratios which was very helpful so thanks for that as well ^^. I am still a little unsure about the diffuser front height, regarding the units as typically most heights would be in meters. Clearly this is not, i am not sure if it is calculated via diffuserheight(m)*diffuserbase1 or a similar calculation? Or is this a similar case of adjustment to fit and replicate the desired normal load on the tire?