Diffusers...and aero in general.
05-01-2011, 03:10 AM
#1
Rennlist Member
Thread Starter
Diffusers...and aero in general.
Big can of worms and possibly should deal with each item separately but as they're so interconnected...
So you see diffusers on virtually all cars nowdays. Even Econoboxes. In terms of them there seems to be two distinct types.
1) What I would term as more integrated and perhaps more 'Streetable'.
2) DTM type 'Extendo-trays', for want of a better expression.
I guess we have to assume that the eTrays are more effective don't we?
If we can't stretch to this type I have to also assume that a more integrated version is still going to be helpful rather than nothing? From what you read there is varying support of a diffuser without flat bottom car in front of it...but still, you'd have to think it can only help, no? How beneficial are they and do they pretty much work in with the rear wing? Would adding one increase the need for more front downforce?
Also, how about the exhaust pipe and where that exits the car?
So you see diffusers on virtually all cars nowdays. Even Econoboxes. In terms of them there seems to be two distinct types.
1) What I would term as more integrated and perhaps more 'Streetable'.
2) DTM type 'Extendo-trays', for want of a better expression.
I guess we have to assume that the eTrays are more effective don't we?
If we can't stretch to this type I have to also assume that a more integrated version is still going to be helpful rather than nothing? From what you read there is varying support of a diffuser without flat bottom car in front of it...but still, you'd have to think it can only help, no? How beneficial are they and do they pretty much work in with the rear wing? Would adding one increase the need for more front downforce?
Also, how about the exhaust pipe and where that exits the car?
05-01-2011, 03:49 AM
#2
Rennlist Member
venturi tunnels are more for increasing downforce, rear diffusers are primarily used to reduce drag.
You ask, is it worth it? it really depends on the track you are running and the speeds you reach as well as the hp you have.
You ask, is it worth it? it really depends on the track you are running and the speeds you reach as well as the hp you have.
05-01-2011, 04:03 AM
#3
I know the double rear diffusers in F1 increased overall downforce and allowed the cars to corner faster. I don't recall reading about reduced drag at all. That may have been realized in the design, but clearly that was not the most important result.
I suppose you are going to tell us that everyone else but you is using the wrong terminology....
Scott
05-01-2011, 05:10 AM
#4
Rennlist Member
Thread Starter
Respectfully, guys can we keep this to a tech discussion. I am happy for opinions to be posted as well as hard evidence.
My assumption that something is better than nothing may be baseless. As for tracks and speeds, well I only know of the tracks so far as my car is undergoing a lot of changes. Currently we hit about 150mph down the main straight but with a lot more grip, power, less weight, chassis stiffness, aero etc I imagine it will be substantially more than this.
The options will be quite open as I'm currently building the car up to compete in this years World Time Attack. Some seriously fast cars will be entered from all corners of the globe. To put them into perspective they run faster than the Supercar V8s around Eastern Creek....on R spec rubber, not slicks!
http://www.worldtimeattack.com/
http://www.youtube.com/watch?v=8vPn0...layer_embedded
My assumption that something is better than nothing may be baseless. As for tracks and speeds, well I only know of the tracks so far as my car is undergoing a lot of changes. Currently we hit about 150mph down the main straight but with a lot more grip, power, less weight, chassis stiffness, aero etc I imagine it will be substantially more than this.
The options will be quite open as I'm currently building the car up to compete in this years World Time Attack. Some seriously fast cars will be entered from all corners of the globe. To put them into perspective they run faster than the Supercar V8s around Eastern Creek....on R spec rubber, not slicks!
http://www.worldtimeattack.com/
http://www.youtube.com/watch?v=8vPn0...layer_embedded
05-01-2011, 07:18 AM
#5
Three Wheelin'
One of my favorite topics ...
Integrated and the more separate looking diffusers that you see these days are really just different versions of the same thing. The ones you see on econoboxes and the like don't do too much and are probably more for looks, though they may decrease drag. The purpose of diffusers on race cars is pretty much always to increase downforce. These "real" diffusers often look more like add-ons (less integrated), since they essentially are.
From what I know, the extendo-thingees that you see on some race cars are usually a result of sactioning body rules. It's a common rule that diffusers have to start at or behind the rear wheel centerline, so some builders choose to make the largest diffuser that they can fit within the rules (for more downforce), hence the extension beyond the back of the car.
As for exhaust placement. you'll see all different ways of doing it. I've read that there is evidence that the added flow provided by exhaust can improve flow through the diffuser and increase downforce. Some designers take advantage of this by putting the exhaust outlets just above the diffuser. I believe some put the outlets right in the roof of the diffuser, exiting the exhaust gases inside the diffuser. The only thing you wouldn't want to do is put the actual exhaust pipes (or anything else for that matter) inside the diffuser, because it would disrupt the airflow inside the diffuser and reduce the effectiveness. I've noticed that some F1 constructors have gone away from this entirely and put exhaust outlets above the bodywork right above the engine.
One thing to keep in mind is that race car designers will almost always put the largest diffuser possible on the car. Before these regs, back in the Group C and IMSA GTP days, they had the diffuser start right behind the cockpit. These days most sactioning bodies put very specific limits on the size of diffusers, where they can start, end, etc.
If you want to read about diffusers and racing aero in general, have a look at Joseph Katz's book Race Car Aerodynamics, and also mulsannescorner.com. This site has lots of pictures of diffusers on current and older prototypes.
Scott
Integrated and the more separate looking diffusers that you see these days are really just different versions of the same thing. The ones you see on econoboxes and the like don't do too much and are probably more for looks, though they may decrease drag. The purpose of diffusers on race cars is pretty much always to increase downforce. These "real" diffusers often look more like add-ons (less integrated), since they essentially are.
From what I know, the extendo-thingees that you see on some race cars are usually a result of sactioning body rules. It's a common rule that diffusers have to start at or behind the rear wheel centerline, so some builders choose to make the largest diffuser that they can fit within the rules (for more downforce), hence the extension beyond the back of the car.
As for exhaust placement. you'll see all different ways of doing it. I've read that there is evidence that the added flow provided by exhaust can improve flow through the diffuser and increase downforce. Some designers take advantage of this by putting the exhaust outlets just above the diffuser. I believe some put the outlets right in the roof of the diffuser, exiting the exhaust gases inside the diffuser. The only thing you wouldn't want to do is put the actual exhaust pipes (or anything else for that matter) inside the diffuser, because it would disrupt the airflow inside the diffuser and reduce the effectiveness. I've noticed that some F1 constructors have gone away from this entirely and put exhaust outlets above the bodywork right above the engine.
One thing to keep in mind is that race car designers will almost always put the largest diffuser possible on the car. Before these regs, back in the Group C and IMSA GTP days, they had the diffuser start right behind the cockpit. These days most sactioning bodies put very specific limits on the size of diffusers, where they can start, end, etc.
If you want to read about diffusers and racing aero in general, have a look at Joseph Katz's book Race Car Aerodynamics, and also mulsannescorner.com. This site has lots of pictures of diffusers on current and older prototypes.
Scott
05-01-2011, 03:26 PM
#6
Rennlist Member
Scott, what is your issue? You dont have to be a total jerk, first of all.
Second, the terminology is "diffuser" . By definition, it will take a flow and spread it out, creating a higher pressure zone, usually gradually to keep the flow more laminar as the air from under the car exits the path under the car.
the confustion with diffusers is again, like I mentionted when used in conjuction with venturi tunnels where the air is accelerated under the car, thus lowering the pressure. (this is the part where it creates downforce). then, what to do with the air as it exits the car, is where the diffuser comes into play.
with out it, the air would scatter violently, the diffuser, guides the air flow that is rapidly exiting the bottom of the car and spreads out the flow, raising its pressure in a controled fashion, and guiding it upward to meet the flow from over the car
The pictures of the diffuser here are not F1. F1 is a combination of diffuser and venturi tunnels, allong with accelerated air flow under the car. our cars, for the most part are too hgh and dont have lower pressure flow under it. the flow we have under the car is generally ambient pressure, and turbulent, becuase of no underbody flat plates . in these cases, the diiffuser, acts mainly to reduce drag.
Second, the terminology is "diffuser" . By definition, it will take a flow and spread it out, creating a higher pressure zone, usually gradually to keep the flow more laminar as the air from under the car exits the path under the car.
the confustion with diffusers is again, like I mentionted when used in conjuction with venturi tunnels where the air is accelerated under the car, thus lowering the pressure. (this is the part where it creates downforce). then, what to do with the air as it exits the car, is where the diffuser comes into play.
with out it, the air would scatter violently, the diffuser, guides the air flow that is rapidly exiting the bottom of the car and spreads out the flow, raising its pressure in a controled fashion, and guiding it upward to meet the flow from over the car
The pictures of the diffuser here are not F1. F1 is a combination of diffuser and venturi tunnels, allong with accelerated air flow under the car. our cars, for the most part are too hgh and dont have lower pressure flow under it. the flow we have under the car is generally ambient pressure, and turbulent, becuase of no underbody flat plates . in these cases, the diiffuser, acts mainly to reduce drag.
Most all references I see to rear diffuser function talk about reduced lift as the primary function. Some claim reduced drag while others claim slight drag increases.
I know the double rear diffusers in F1 increased overall downforce and allowed the cars to corner faster. I don't recall reading about reduced drag at all. That may have been realized in the design, but clearly that was not the most important result.
I suppose you are going to tell us that everyone else but you is using the wrong terminology....
Scott
I know the double rear diffusers in F1 increased overall downforce and allowed the cars to corner faster. I don't recall reading about reduced drag at all. That may have been realized in the design, but clearly that was not the most important result.
I suppose you are going to tell us that everyone else but you is using the wrong terminology....
Scott
05-01-2011, 03:55 PM
#7
Rennlist Member
Respectfully, guys can we keep this to a tech discussion. I am happy for opinions to be posted as well as hard evidence.
My assumption that something is better than nothing may be baseless. As for tracks and speeds, well I only know of the tracks so far as my car is undergoing a lot of changes. Currently we hit about 150mph down the main straight but with a lot more grip, power, less weight, chassis stiffness, aero etc I imagine it will be substantially more than this.
The options will be quite open as I'm currently building the car up to compete in this years World Time Attack. Some seriously fast cars will be entered from all corners of the globe. To put them into perspective they run faster than the Supercar V8s around Eastern Creek....on R spec rubber, not slicks!
My assumption that something is better than nothing may be baseless. As for tracks and speeds, well I only know of the tracks so far as my car is undergoing a lot of changes. Currently we hit about 150mph down the main straight but with a lot more grip, power, less weight, chassis stiffness, aero etc I imagine it will be substantially more than this.
The options will be quite open as I'm currently building the car up to compete in this years World Time Attack. Some seriously fast cars will be entered from all corners of the globe. To put them into perspective they run faster than the Supercar V8s around Eastern Creek....on R spec rubber, not slicks!
If you need me to fly in and help...I'd love to get out of Texas in August!!!
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05-01-2011, 04:05 PM
#8
Mark,
http://www.racecar-engineering.com/t...-aerodynamics/
Read this quote:
‘The key role of the diffuser on a modern racecar is to accelerate the flow of air under the car, creating an area of low pressure, thus increasing downforce.’
Then look at Figure 2 in the article. Note that this article is not F1-specific.
So, are you saying that Racecar Engineering is using incorrect terminology?
Scott
http://www.racecar-engineering.com/t...-aerodynamics/
Read this quote:
‘The key role of the diffuser on a modern racecar is to accelerate the flow of air under the car, creating an area of low pressure, thus increasing downforce.’
Then look at Figure 2 in the article. Note that this article is not F1-specific.
So, are you saying that Racecar Engineering is using incorrect terminology?
Scott
05-01-2011, 04:23 PM
#9
Rennlist Member
Good article
There could be somethings left for the reader to think for himself. basically, if you go to figure 2, you can see the defuser doing its job. the downforce is not in the routing of the air rearward where the pressue is rapidly raising. its to help the acceleration of the air flow from nose to rear under the car and vent it in an efficient fashion as it meets the upper air flow. in F1 and DTM cars, there is accelerated flow under the car due to underbody designs. under most of our races cars you see in club racing, thats not the case. So, the diffuser has a few different functions, its part of the system. its not responsible for accelerating the flow under the car on its own. Its the underbody design and ride height.
so yes, there could be a little error in terminology. rememeber, these are writers trying to get a boad message in only a few words. actuallly, a diffuser, raises the pressure, in a controled fashion. its how a diffuser works. as the air flow spreads out, pressure rises over that device. the article also says this too. (fig 3)
however, it acts to assist in the acceleration of the flow before hand,under the car, if the car is able to have accelerated flow under it.
as a side note, it really is designed to keep the higher pressure to the sides , fron neutralizing the lower pressure zones created by the underbody flow. without it, this would raise the pressure and disturb flow as the accelerated flow exits the underbody tray area. thats why there are tunnels and routed chambers to vent the air rearward.
There could be somethings left for the reader to think for himself. basically, if you go to figure 2, you can see the defuser doing its job. the downforce is not in the routing of the air rearward where the pressue is rapidly raising. its to help the acceleration of the air flow from nose to rear under the car and vent it in an efficient fashion as it meets the upper air flow. in F1 and DTM cars, there is accelerated flow under the car due to underbody designs. under most of our races cars you see in club racing, thats not the case. So, the diffuser has a few different functions, its part of the system. its not responsible for accelerating the flow under the car on its own. Its the underbody design and ride height.
so yes, there could be a little error in terminology. rememeber, these are writers trying to get a boad message in only a few words. actuallly, a diffuser, raises the pressure, in a controled fashion. its how a diffuser works. as the air flow spreads out, pressure rises over that device. the article also says this too. (fig 3)
however, it acts to assist in the acceleration of the flow before hand,under the car, if the car is able to have accelerated flow under it. as a side note, it really is designed to keep the higher pressure to the sides , fron neutralizing the lower pressure zones created by the underbody flow. without it, this would raise the pressure and disturb flow as the accelerated flow exits the underbody tray area. thats why there are tunnels and routed chambers to vent the air rearward.
Mark,
http://www.racecar-engineering.com/t...-aerodynamics/
Read this quote:
‘The key role of the diffuser on a modern racecar is to accelerate the flow of air under the car, creating an area of low pressure, thus increasing downforce.’
Then look at Figure 2 in the article. Note that this article is not F1-specific.
So, are you saying that Racecar Engineering is using incorrect terminology?
Scott
http://www.racecar-engineering.com/t...-aerodynamics/
Read this quote:
‘The key role of the diffuser on a modern racecar is to accelerate the flow of air under the car, creating an area of low pressure, thus increasing downforce.’
Then look at Figure 2 in the article. Note that this article is not F1-specific.
So, are you saying that Racecar Engineering is using incorrect terminology?
Scott
05-01-2011, 04:37 PM
#10
Addict
Rennlist Member
Rennlist Member
The trick is keeping the pressure when no throttle is applied and you will hear an unusual engine note during broadcasts when that is happening. MB F1 is particularly loud.
05-01-2011, 04:54 PM
#12
Mark,
The bottom line is that a diffusers job is to reduce lift and possibly create downforce with very little increase in drag. It is not to reduce drag.
Look here:
http://www.symscape.com/blog/secrets_of_diffusers
Scott
The bottom line is that a diffusers job is to reduce lift and possibly create downforce with very little increase in drag. It is not to reduce drag.
Look here:
http://www.symscape.com/blog/secrets_of_diffusers
Scott
05-01-2011, 06:36 PM
#13
Rennlist Member
Look, im not going to get into a symantec discussion.
a diffuser is an area of divergent flow. in this area, the pressure rises as the flow speed decreases. its a nessary element to increasing downforce of a race car with underbody panels designed for low pressure, higher speed flow.
Its part of the system.
Just bolting one on your car, will not increase downforce, but will decrease drag. a significant part of its purpose is to decrease the separation of fow as the upper and lower flows meet. this results in lower drag. the longer versions maintain downforce and delays the separation of the flow until the flow is beyond the race cars chasssis. (as seen on DTM cars) this increases downfoce as well.
This arguement is simular to one about why wings produce lift. differential pressure, or change in momentium.
If you dont have a high speed air flow underbody, ( diffuser tunnels, venturi tunnels, flat bottom, low to the ground) the diffuser will only produce decreased drag.
a diffuser is an area of divergent flow. in this area, the pressure rises as the flow speed decreases. its a nessary element to increasing downforce of a race car with underbody panels designed for low pressure, higher speed flow.
Its part of the system.
Just bolting one on your car, will not increase downforce, but will decrease drag. a significant part of its purpose is to decrease the separation of fow as the upper and lower flows meet. this results in lower drag. the longer versions maintain downforce and delays the separation of the flow until the flow is beyond the race cars chasssis. (as seen on DTM cars) this increases downfoce as well.
This arguement is simular to one about why wings produce lift. differential pressure, or change in momentium.
If you dont have a high speed air flow underbody, ( diffuser tunnels, venturi tunnels, flat bottom, low to the ground) the diffuser will only produce decreased drag.
Mark,
The bottom line is that a diffusers job is to reduce lift and possibly create downforce with very little increase in drag. It is not to reduce drag.
Look here:
http://www.symscape.com/blog/secrets_of_diffusers
Scott
The bottom line is that a diffusers job is to reduce lift and possibly create downforce with very little increase in drag. It is not to reduce drag.
Look here:
http://www.symscape.com/blog/secrets_of_diffusers
Scott
05-01-2011, 11:19 PM
#14
Rennlist Member
Every April NASAaz has the national tuner time attack shootout here in Phoenix. There is nothing mild about the strap on aero that gets stuck on these cars. Many forgo the sublties of splitters and slap on big wings front and rear. Almost all are turbo/supercharged and many run nitrous. 6-850hp is nothing special.
I don't know how they class as the run the superunlimiteds here. But I would say you're going to have a challenge getting a 951 in the same run group as some of these subies, lancers and evo's...they make easy power that can overcome the "exuberant" aero packages...with 500?hp max out of a 951 you're going to have to have a better balance between downforce and drag. Many of the aero on these cars are bolt on for particular tracks as well you may want to start there...
I don't know how they class as the run the superunlimiteds here. But I would say you're going to have a challenge getting a 951 in the same run group as some of these subies, lancers and evo's...they make easy power that can overcome the "exuberant" aero packages...with 500?hp max out of a 951 you're going to have to have a better balance between downforce and drag. Many of the aero on these cars are bolt on for particular tracks as well you may want to start there...
