let's talk downforce
10-15-2016, 10:39 AM
#106
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Well, sometimes knowing what is possible is half the battle!
As planned, Peter Argetsinger was my best upgrade
It was amazing that when he got in the car again he put down controlled laps, each one second faster than the next. 2:12, 2:11, 2:10, and then on pace for a 2:09 but we hit traffic. Still have no idea how fast he can make that particular car go.
Since I still am finding time and setting PBs at most events I guess I shouldn't be in a hurry.
As planned, Peter Argetsinger was my best upgrade
It was amazing that when he got in the car again he put down controlled laps, each one second faster than the next. 2:12, 2:11, 2:10, and then on pace for a 2:09 but we hit traffic. Still have no idea how fast he can make that particular car go.
Since I still am finding time and setting PBs at most events I guess I shouldn't be in a hurry.
__________________
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www.peterkrause.net
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-Peter Krause
www.peterkrause.net
www.gofasternow.com
"Combining the Art and Science of Driving Fast!"
Specializing in Professional, Private Driver Performance Evaluation and Optimization
Consultation Available Remotely and at VIRginia International Raceway
10-16-2016, 01:25 AM
#107
Rennlist Member
This has been an amazing thread. The dialogue produced by debating a seemingly trivial minute detail has turned into a heck of a primer on aero. If there had been a stream of insult strewn the through the whole thing, there is no way I could have read it. But because it remained relatively clean, I now have a better understanding of how little I know.
One clarifying question that may prove I learned nothing, does the underbody aero (including diffuser) create the downforce or does it just create a low pressure zone that increases the downforce produced by the topside aero?
One clarifying question that may prove I learned nothing, does the underbody aero (including diffuser) create the downforce or does it just create a low pressure zone that increases the downforce produced by the topside aero?
To keep it simple, if there was zero lift on the top of the car, the air above the car would push with an average of ~14.5 pounds per square inch of surface area looking down from the sky. If your aerodynamic design has reduced the average upward pressure on the bottom of the car by 0.1 psi at a certain speed, then the air below the car will push back up with 14.4 pounds per square inch under those conditions. The net pressure on the car is 14.5 pounds per square inch down from the top of the car minus 14.4 pounds per square inch up from the bottom of the car, which for the complete car nets that 0.1 pound per square inch acting downward. A car 12 ft long x 6 feet wide has a surface area of 144 in * 72 in = 10,368 in^2. Using round numbers, that hypothetical 0.1 pounds per square inch taken over 10,000 square inches would produce 1000 lbs of downforce.
You may already know this as well, but:
-If your downforce isn't right at the center of gravity of the car, your handling changes from low speeds (little aero effect) to high speeds (strong aero effect).
-The distance between the underbody and the ground can have a big impact on how much downforce is produced by the underbody aero system, and this distance will change rapidly when you hit bumps and such.
On this topic, the Lotus 88 interesting: http://www.f1fanatic.co.uk/2011/07/0...f1-car-banned/
10-16-2016, 01:38 AM
#108
Rennlist Member
Got it! Thanks for taking the time to explain.
10-16-2016, 02:45 PM
#109
Rennlist Member
Great points and usually missed. the point of that if you can just rece the pressure by .1psi, that .1psi is a change in downforce of like you said, 1000lbs (over 10,000square") also noted and ill reiterate, the center of pressure is important. if you have it all up front, the car will behave very strangely and dangerously at speed, as would it if it was to the rear . (just slapping on a rear wing can do this)
there is also a blend of not only Bernoulli here , but as im finding with my small experiments, Newton comes into play too. Ill explain later. but it's one of the reasons that the air dams work so well and rake is so critically important.
there is also a blend of not only Bernoulli here , but as im finding with my small experiments, Newton comes into play too. Ill explain later. but it's one of the reasons that the air dams work so well and rake is so critically important.
The main objective of the underbody aero is to reduce the pressure under the body of the car without adding a drag or other penalty. Because we are in the atmosphere, ambient pressure is around 14.5 psi and that acts to compress things in all directions (though it changes with temperature, altitude). This air pressure pushes the top, bottom, and sides of your car just like the top, bottom, and sides of everything else. With lift/downforce, we are concerning ourselves with the up and down part of that pressure.
To keep it simple, if there was zero lift on the top of the car, the air above the car would push with an average of ~14.5 pounds per square inch of surface area looking down from the sky. If your aerodynamic design has reduced the average upward pressure on the bottom of the car by 0.1 psi at a certain speed, then the air below the car will push back up with 14.4 pounds per square inch under those conditions. The net pressure on the car is 14.5 pounds per square inch down from the top of the car minus 14.4 pounds per square inch up from the bottom of the car, which for the complete car nets that 0.1 pound per square inch acting downward. A car 12 ft long x 6 feet wide has a surface area of 144 in * 72 in = 10,368 in^2. Using round numbers, that hypothetical 0.1 pounds per square inch taken over 10,000 square inches would produce 1000 lbs of downforce.
You may already know this as well, but:
-If your downforce isn't right at the center of gravity of the car, your handling changes from low speeds (little aero effect) to high speeds (strong aero effect).
-The distance between the underbody and the ground can have a big impact on how much downforce is produced by the underbody aero system, and this distance will change rapidly when you hit bumps and such.
On this topic, the Lotus 88 interesting: http://www.f1fanatic.co.uk/2011/07/0...f1-car-banned/
To keep it simple, if there was zero lift on the top of the car, the air above the car would push with an average of ~14.5 pounds per square inch of surface area looking down from the sky. If your aerodynamic design has reduced the average upward pressure on the bottom of the car by 0.1 psi at a certain speed, then the air below the car will push back up with 14.4 pounds per square inch under those conditions. The net pressure on the car is 14.5 pounds per square inch down from the top of the car minus 14.4 pounds per square inch up from the bottom of the car, which for the complete car nets that 0.1 pound per square inch acting downward. A car 12 ft long x 6 feet wide has a surface area of 144 in * 72 in = 10,368 in^2. Using round numbers, that hypothetical 0.1 pounds per square inch taken over 10,000 square inches would produce 1000 lbs of downforce.
You may already know this as well, but:
-If your downforce isn't right at the center of gravity of the car, your handling changes from low speeds (little aero effect) to high speeds (strong aero effect).
-The distance between the underbody and the ground can have a big impact on how much downforce is produced by the underbody aero system, and this distance will change rapidly when you hit bumps and such.
On this topic, the Lotus 88 interesting: http://www.f1fanatic.co.uk/2011/07/0...f1-car-banned/
10-16-2016, 03:26 PM
#110
Rennlist Member
This has been an amazing thread. The dialogue produced by debating a seemingly trivial minute detail has turned into a heck of a primer on aero. If there had been a stream of insult strewn the through the whole thing, there is no way I could have read it. But because it remained relatively clean, I now have a better understanding of how little I know.
One clarifying question that may prove I learned nothing, does the underbody aero (including diffuser) create the downforce or does it just create a low pressure zone that increases the downforce produced by the topside aero?
One clarifying question that may prove I learned nothing, does the underbody aero (including diffuser) create the downforce or does it just create a low pressure zone that increases the downforce produced by the topside aero?
so, how do you gain downforce? by taking incoming air and venting to it to and out the hood, does two things, it increases the low pressure zone at the front top area of the hood area (raising its pressure) which creases net downforce) and the Newton effect of the change of direction of that air diversion, causes a downward force as well. wings in the rear obviously increase downforce.
underbody air , for most cases, will stay at near ambient. Duane sytem i think had an unexpected result due to parts of the "diffuser" that incorporated bulges that narrowed the space under the car (raduised aras) those areas can accelerate the air, and lower pressure in spot locations. this is good for two reasons, it speeds up the air for downforce, and those downforce areas can be located near the front and rear of the car to balance the forces.
Im doing some model testing right now to see the effects of these underbody forms on flow , pressure and force.
10-16-2016, 06:00 PM
#111
Mark,
My floor is flat, there are no "bulges". Just transitions (two steps) from flat to the diffuser. There is no venturi built into the design, the flared sections you see are just there to better merge with the flat underbody.
Diffusers in the typical and commonly understood definition do create downforce in any application involving a flat floor. It is the transition from the flat floor to the diffuser section that localizes the aft pressure drop due to the acceleration, but the air is accelerated as a function of the diffuser. I think your continuing to parse this definition is it confusing the discussion. Again, there is no venturi built into the design - just a transition from flat floor to the diffuser section.
Mass flow is maintained - but I believe you are mistaken on how the molecular reaction occurs - the air is not accelerated through compression from the front, but pulled through the underbody from the rear by the need to maintain the mass flow in the diffuser. This is how the diffuser effects flow ahead of itself and, in fact, ahead of the entire car.
My result was not unexpected, but completely consistent with theory and engineering. Please stop claiming the success was an accident - it is a bit insulting to the expert that designed it.
Cheers,
My floor is flat, there are no "bulges". Just transitions (two steps) from flat to the diffuser. There is no venturi built into the design, the flared sections you see are just there to better merge with the flat underbody.
Diffusers in the typical and commonly understood definition do create downforce in any application involving a flat floor. It is the transition from the flat floor to the diffuser section that localizes the aft pressure drop due to the acceleration, but the air is accelerated as a function of the diffuser. I think your continuing to parse this definition is it confusing the discussion. Again, there is no venturi built into the design - just a transition from flat floor to the diffuser section.
Mass flow is maintained - but I believe you are mistaken on how the molecular reaction occurs - the air is not accelerated through compression from the front, but pulled through the underbody from the rear by the need to maintain the mass flow in the diffuser. This is how the diffuser effects flow ahead of itself and, in fact, ahead of the entire car.
My result was not unexpected, but completely consistent with theory and engineering. Please stop claiming the success was an accident - it is a bit insulting to the expert that designed it.
Cheers,
Last edited by Krokodil; 10-16-2016 at 07:03 PM.
10-16-2016, 07:16 PM
#112
Rennlist Member
Duane, lets get the definitions straight first. because im not saying anything you did is on accident, im trying to undersand how the structure is working for your car. from front to back .... as i understand it from your description, is the splitter, the "packer" the flat floor, the rear compound structure venturi/diffuser
if the floor is flat, what is the "packer" and what was your engieer saying when he referedced decreasing the radius of the front "packer" so the rear "diffuser" can have more effect?.
(edit) also we cant ignore Newton on this one either... the diffuser changes the direction of the air flow to the rear by tossing it up to better meet over car flow. BUT, this change of dirction is a force that creates downforce too.
1. Diffuser = This any point in the underbody work that the volume increases in reference from ground or/ and sides . this is where the air flow slows as it enters the higher volume area upon exiting the underbody and the pressure rises.
2. Aft =This is the rear of the car. if a diffuser is attached to the rear of the car, the diffuser contributes to ONLY pressure rise, not drop.
3. venturi = this is the lowest volume point in a flow path, causing flow acceleration which creates molecule separation, not attraction to lower pressure in this region. (this would be the " small " area on the picture below)
If we can agree on the above (and maybe we cant at this point yet), we can better understand what is happening. you speak of the transition point of the flat floor to the diffuser. if there are any neck down areas of what you are calling the diffuser, then this is a "venturi" again, a diffuser , in its engineering definition, has nothing to do with speeding up air anywhere. especially upon entering a diffuser. a diffuser is the tail section of a venturi (to use an mechanical analogy) if you notice, there is no point in the ventur that cares or is measured at the rear diffuser area.. it has NO effect. ony the entering air speed , convergent angles and the neck down (venturi) area determines the max speed achieved and therefor pressure reduction. the diffuser is ONLY to return the air flow to the original flow speed without creating flow separation.
Im not saying your engineers have not designed a solution by chance. many of these designs are tested in a wind tunnel to find the best way to achieve the desired outcome. Ive spent a lot of time in scale wind tunnels doing tests like this some intuitive , and some surprising.
help me understand how there are no "bulges when there is a serious radius to the piece that you are calling the "diffuser". Ill post a picture to show what im talking about and asking about. any chance you can take a picture under your car to see how the diffuser structure mates with the flat floor?
Duane said:
now, how i understand the physics, and have always been reminded by instructors. vacuum does nothing, pressure does the work. I understand what you are saying , but i dont think its correct..... lets find out if there are any other engineers here that can validate this . there is no compression with on coming air flow.. if there was , pressure would rise at the venturi and it does the opposite. the air enters an area of lower volume.. its speeds up, pressure goes down. the only reason there is a diffuser, is to return the flow to the initial speed that fed the structure in the first place. there is no interaction with the forward area flow characteristics. mass flow rate is retained by the air speeding up through the "small " area, and slowed through the diffuser. solid venturi theory.
My first impression and small model tests are showing, that IF there is a bulge area at the fornt axle and the rear axle as well, that lowers the aif low path volume in any way, that could be contributing to downforce local to those areas.
if you show me a picture that shows totallly flat, then im not understandng the configuration.
(edit) in looking at the picutre , maybe there is an opical illuseion. the the part before the "diffuser" totally flat? (i point to them as "large radius area)
if so, then still, even laterally, the small area is before the "diffuser" starts which even without a radius would cause acceleration and low pressure. then you would have a venturi.. (same as if it was radiused or not, but just not as much as i first estimated). that neck down is the " venturi" (im calling it "small area". )
if the floor is flat, what is the "packer" and what was your engieer saying when he referedced decreasing the radius of the front "packer" so the rear "diffuser" can have more effect?.
(edit) also we cant ignore Newton on this one either... the diffuser changes the direction of the air flow to the rear by tossing it up to better meet over car flow. BUT, this change of dirction is a force that creates downforce too.
1. Diffuser = This any point in the underbody work that the volume increases in reference from ground or/ and sides . this is where the air flow slows as it enters the higher volume area upon exiting the underbody and the pressure rises.
2. Aft =This is the rear of the car. if a diffuser is attached to the rear of the car, the diffuser contributes to ONLY pressure rise, not drop.
3. venturi = this is the lowest volume point in a flow path, causing flow acceleration which creates molecule separation, not attraction to lower pressure in this region. (this would be the " small " area on the picture below)
If we can agree on the above (and maybe we cant at this point yet), we can better understand what is happening. you speak of the transition point of the flat floor to the diffuser. if there are any neck down areas of what you are calling the diffuser, then this is a "venturi" again, a diffuser , in its engineering definition, has nothing to do with speeding up air anywhere. especially upon entering a diffuser. a diffuser is the tail section of a venturi (to use an mechanical analogy) if you notice, there is no point in the ventur that cares or is measured at the rear diffuser area.. it has NO effect. ony the entering air speed , convergent angles and the neck down (venturi) area determines the max speed achieved and therefor pressure reduction. the diffuser is ONLY to return the air flow to the original flow speed without creating flow separation.
Im not saying your engineers have not designed a solution by chance. many of these designs are tested in a wind tunnel to find the best way to achieve the desired outcome. Ive spent a lot of time in scale wind tunnels doing tests like this some intuitive , and some surprising.
help me understand how there are no "bulges when there is a serious radius to the piece that you are calling the "diffuser". Ill post a picture to show what im talking about and asking about. any chance you can take a picture under your car to see how the diffuser structure mates with the flat floor?
Duane said:
Mass flow is maintained - but I believe you are mistaken on how the molecular reaction occurs - the air is not accelerated through compression from the front, but pulled through the underbody from the rear by the need to maintain the mass flow in the diffuser. This is how the diffuser effects floor ahead of itself and, in fact, ahead of the entire car.
My first impression and small model tests are showing, that IF there is a bulge area at the fornt axle and the rear axle as well, that lowers the aif low path volume in any way, that could be contributing to downforce local to those areas.
if you show me a picture that shows totallly flat, then im not understandng the configuration.
(edit) in looking at the picutre , maybe there is an opical illuseion. the the part before the "diffuser" totally flat? (i point to them as "large radius area)
if so, then still, even laterally, the small area is before the "diffuser" starts which even without a radius would cause acceleration and low pressure. then you would have a venturi.. (same as if it was radiused or not, but just not as much as i first estimated). that neck down is the " venturi" (im calling it "small area". )
Mark,
My floor is flat, there are no "bulges". Just transitions (two steps) from flat to the diffuser. There is no venturi built into the design, the flared sections you see are just there to better merge with the flat underbody.
Diffusers in the typical and commonly understood definition do create downforce in any application involving a flat floor. It is the transition from the flat floor to the diffuser section that localizes the aft pressure drop due to the acceleration, but the air is accelerated as a function of the diffuser. I think your continuing to parse this definition is it confusing the discussion. Again, there is no venturi built into the design - just a transition from flat floor to the diffuser section.
Mass flow is maintained - but I believe you are mistaken on how the molecular reaction occurs - the air is not accelerated through compression from the front, but pulled through the underbody from the rear by the need to maintain the mass flow in the diffuser. This is how the diffuser effects floor ahead of itself and, in fact, ahead of the entire car.
My result was not unexpected, but completely consistent with theory and engineering. Please stop claiming the success was an accident - it is a bit insulting to the expert that designed it.
Cheers,
My floor is flat, there are no "bulges". Just transitions (two steps) from flat to the diffuser. There is no venturi built into the design, the flared sections you see are just there to better merge with the flat underbody.
Diffusers in the typical and commonly understood definition do create downforce in any application involving a flat floor. It is the transition from the flat floor to the diffuser section that localizes the aft pressure drop due to the acceleration, but the air is accelerated as a function of the diffuser. I think your continuing to parse this definition is it confusing the discussion. Again, there is no venturi built into the design - just a transition from flat floor to the diffuser section.
Mass flow is maintained - but I believe you are mistaken on how the molecular reaction occurs - the air is not accelerated through compression from the front, but pulled through the underbody from the rear by the need to maintain the mass flow in the diffuser. This is how the diffuser effects floor ahead of itself and, in fact, ahead of the entire car.
My result was not unexpected, but completely consistent with theory and engineering. Please stop claiming the success was an accident - it is a bit insulting to the expert that designed it.
Cheers,
Last edited by mark kibort; 10-16-2016 at 07:48 PM.
10-17-2016, 01:51 PM
#113
Three Wheelin'
now, how i understand the physics, and have always been reminded by instructors. vacuum does nothing, pressure does the work. I understand what you are saying , but i dont think its correct..... lets find out if there are any other engineers here that can validate this . there is no compression with on coming air flow.. if there was , pressure would rise at the venturi and it does the opposite. the air enters an area of lower volume.. its speeds up, pressure goes down. the only reason there is a diffuser, is to return the flow to the initial speed that fed the structure in the first place. there is no interaction with the forward area flow characteristics. mass flow rate is retained by the air speeding up through the "small " area, and slowed through the diffuser. solid venturi theory.
. So movement of air from the front of the car, or in from the sides, toward the diffuser is not passive, but sucked in there by the low pressure created by the diffuser. You know how I love to reference Katz so have a look at the diagram below from his book showing the high negative pressure peak at the entry of the diffuser.
10-17-2016, 04:32 PM
#114
Rennlist Member
actually, in a venturi, the pressure and the often forgotten momentum, causes the flow. instead of compressing in the convergent region, (under the car or just before the divergent area of the diffuser) the air speeds up. mass stays constant. you dont even need a diffuser for this to happen .
however if you look at the design of the diffusers (rather than just traditional venturi outlets) other things are happening to cause a speed up just before the air is released upward. by the way, this change in momentium from a 7 to 15degree release angle, can cause a downward force too:
Now, what i understand from the flow that you see, is that the air under the car is slightly faster and slightly lower pressure. its not that its attracted to the diffuser, because that flow will be higher than the under body flow. (basic venturi priciple.) you dont need "vacuum" to "instigate" it. it will flow from ambient to low pressure and then out the diffuser at ambient again. If the exit was too restricted or you dint have an opening, the pressure would build at the front of the car and be released to lower pressure area zones. (like under the car , over the car, around the car.)
if there was no low pressure zone under the car. just ambient . the mass flow would still be the same.
think of a venturi... the rear exit cone, diffuser, or whatever has no bearing on he acceleration of the air from the front entrance . how the air is released is only a function of the diffusers angles and how laminar the flow is on the exit as the flow pressure returns to the pressure of the original on coming air. the low pressure zone doesnt "suck" air in from the front... its sped up via the convergence and the molecules (if you want to get to that level) get spaced out and sped up and that's why the pressure drops and the speed increases. you clog the venturi and the pressure just builds to its maximum potential and there is no flow.
one of the most interesting aspects of the diffuser, and this peak of low pressure just at the entry point . Ive seen some flow data that shows there is some non-traditional flow characteristics at that point that causes a drop in pressure. if this is what is happening, it would be interesting to see why.
Guess what two aero engineers do on a rainy day. we table top a windtunnel and start doing some test of venturis, underbody flow with small changes in rake, as well as splitters, side curtains and ride height. over a few hours, we saw some very interesting things . sometimes 1 degree differences can be the difference between lift and downforce. while venturis under the car work 100% of the time. diffuser tests next with more controllable and repeatable tests.
as far as Katz's diagram.... this possibly could be because of the shape of the entire body and the integration of the "diffuser" to the rear of the car, starting at the middle... its analagous to creating come camber under the car. basically, making an upside down wing.. that could be the reason for the peak of low pressure at the transition state. I say transition, because the pressure drop is very isolated, and goes in the opposite values soon after.
EDIT: I dont why i didnt see it earlier... the Katz's model of the "prototype" car shows something beyond just the diffuser being the exit for the air under the car, it has changed the structure to make the car look like an upside down wing. notice how the angle which has a Cp of 1.5 and a Cl of 2.0? why is that?? why the peak where it is? its like you have an upside down wing because the release point is actually higher than the nose of the car. air takes a longer route under than over... this is what makes lift or downforce. ... so yes, adding a diffuser that starts a mid car and ends up higher than your head , will create this kind of downforce. thats the reason for the peak downforce at the Cp point.
Mark
however if you look at the design of the diffusers (rather than just traditional venturi outlets) other things are happening to cause a speed up just before the air is released upward. by the way, this change in momentium from a 7 to 15degree release angle, can cause a downward force too:
Now, what i understand from the flow that you see, is that the air under the car is slightly faster and slightly lower pressure. its not that its attracted to the diffuser, because that flow will be higher than the under body flow. (basic venturi priciple.) you dont need "vacuum" to "instigate" it. it will flow from ambient to low pressure and then out the diffuser at ambient again. If the exit was too restricted or you dint have an opening, the pressure would build at the front of the car and be released to lower pressure area zones. (like under the car , over the car, around the car.)
if there was no low pressure zone under the car. just ambient . the mass flow would still be the same.
think of a venturi... the rear exit cone, diffuser, or whatever has no bearing on he acceleration of the air from the front entrance . how the air is released is only a function of the diffusers angles and how laminar the flow is on the exit as the flow pressure returns to the pressure of the original on coming air. the low pressure zone doesnt "suck" air in from the front... its sped up via the convergence and the molecules (if you want to get to that level) get spaced out and sped up and that's why the pressure drops and the speed increases. you clog the venturi and the pressure just builds to its maximum potential and there is no flow.
one of the most interesting aspects of the diffuser, and this peak of low pressure just at the entry point . Ive seen some flow data that shows there is some non-traditional flow characteristics at that point that causes a drop in pressure. if this is what is happening, it would be interesting to see why.
Guess what two aero engineers do on a rainy day.
we table top a windtunnel and start doing some test of venturis, underbody flow with small changes in rake, as well as splitters, side curtains and ride height. over a few hours, we saw some very interesting things . sometimes 1 degree differences can be the difference between lift and downforce. while venturis under the car work 100% of the time. diffuser tests next with more controllable and repeatable tests.as far as Katz's diagram.... this possibly could be because of the shape of the entire body and the integration of the "diffuser" to the rear of the car, starting at the middle... its analagous to creating come camber under the car. basically, making an upside down wing.. that could be the reason for the peak of low pressure at the transition state. I say transition, because the pressure drop is very isolated, and goes in the opposite values soon after.
EDIT: I dont why i didnt see it earlier... the Katz's model of the "prototype" car shows something beyond just the diffuser being the exit for the air under the car, it has changed the structure to make the car look like an upside down wing. notice how the angle which has a Cp of 1.5 and a Cl of 2.0? why is that?? why the peak where it is? its like you have an upside down wing because the release point is actually higher than the nose of the car. air takes a longer route under than over... this is what makes lift or downforce. ... so yes, adding a diffuser that starts a mid car and ends up higher than your head , will create this kind of downforce. thats the reason for the peak downforce at the Cp point.
Mark
Mark - If I am understanding your point, I'd say the difference in pressure (high to low) causes air to move toward the diffuser (low pressure area). So while pressure is doing the work, vacuum is instigating it . So movement of air from the front of the car, or in from the sides, toward the diffuser is not passive, but sucked in there by the low pressure created by the diffuser. You know how I love to reference Katz so have a look at the diagram below from his book showing the high negative pressure peak at the entry of the diffuser.
. So movement of air from the front of the car, or in from the sides, toward the diffuser is not passive, but sucked in there by the low pressure created by the diffuser. You know how I love to reference Katz so have a look at the diagram below from his book showing the high negative pressure peak at the entry of the diffuser.
10-17-2016, 05:26 PM
#115
Three Wheelin'
EDIT: I dont why i didnt see it earlier... the Katz's model of the "prototype" car shows something beyond just the diffuser being the exit for the air under the car, it has changed the structure to make the car look like an upside down wing. notice how the angle which has a Cp of 1.5 and a Cl of 2.0? why is that?? why the peak where it is? its like you have an upside down wing because the release point is actually higher than the nose of the car. air takes a longer route under than over... this is what makes lift or downforce. ... so yes, adding a diffuser that starts a mid car and ends up higher than your head , will create this kind of downforce. thats the reason for the peak downforce at the Cp point.
Mark
Mark
Exactly ... in profile, purpose-built cars with aero wind up looking like wings. Wings in ground effect, no less. And the wing on the car becomes the flap for the car/wing. No wonder those old GTP cars made such outrageous amounts of downforce.
10-17-2016, 06:04 PM
#116
Rennlist Member
look at the car , with body removed and turned upside down... ............ its like a wing.... longer distance now, going under the car vs over it.... velocity peak will be by the diffuser start.
Back to the chicken and egg" part of the discussion of what causes the higher speed flow, it really goes back to the laws of conservation of momentum, and energy. the flow when it hits the narrow area, it speeds up and gains KE, but loses pressure. the total energy has to be the same at any point in the system. so the higher pressure exit doesn't "instigate" the higher flow rate. it's the other way around .. the KE is exchanged for pressure and the energy in the system stays the same at any point.
Last edited by mark kibort; 10-17-2016 at 07:07 PM.
10-20-2016, 06:49 PM
#117
Rennlist Member
Duane,
What is the cost range for just the piece diffuser you used in the rear of the car? can you release the company name where you got it, and ball park costs?
Was it designed for a 911 or cayman? or a custom fit for a generic diffuser.
In lookng at the effects on your car as well as the drag reduction benefits, its worth looking into a little more closely. im thinking of doing what the 350Z guy did as far as more rake, and let the a diffuser do more of the downforce job, if installed, take some wing out and increase front downforce, lower drag overall. (the 350Z guy, increased rake, had slightly less rear wing, added dive planes and sealed off hood entry at windshield. and went from 0 downforce at the front and 600 to the rear, to near balanced to match its weight disty.)
I think we went over a lot of technology here. I hope the points i made were well taken, and most of which were just definition differences and reasons for the diffuser effects. (some basic aero 101 stuff)
In thinking about it over the past few days, i think your gains were made by loosening up the front end (opening up the splitter) and increasing air flow to the rear diffuser. since the rear diffuser cant effect flow ahead of it. (because of the laws of conservation of energy) the KE increase is localized based on the venturi effect of the convergent air feeders. However, never discount the effects of Newtons second law here either. The change of direction of the air flow at the diffuser influction point, puts downward force on the rear of the car, localizing its force and changing the Center of pressure. Just like the front part of a wing accelerates the air, and then it changes direction downward to take advantage of newtons 2nd law.
Fast moving air is not creating any down-force as it travels under the car. It's just the change in the flow that creates higher and lower pressure zones. (accelerated and slowed speeds). I think your aero engineer has devised two spots under the car to do that, and that is the reason for the results, because like vegas, what happens at the rear, stays at the rear of the car. there is no upward "moleucular interaction". thats how i understand it. because of conservation of energy. all the air flow under the car, has to follow the law. anytime there is lower pressure, the speed of the air increases. when it slows, the pressure rises. (trading potential energy (pressure) for KE (speed).
Nothing happens forward of the diffuser. as far as i have understood the concepts......it cant. it would certanly break a couple of physics laws if it did.
I stilll want to know what the "packers" are and the radius that can change with them to increase or decrease their influence of air flow speed.
Again, im not discounting his design, only the reason and description for it working so that others like me can take that into account and design something that works for me and my car with its own aero issues.
Cheers,
Mark
What is the cost range for just the piece diffuser you used in the rear of the car? can you release the company name where you got it, and ball park costs?
Was it designed for a 911 or cayman? or a custom fit for a generic diffuser.
In lookng at the effects on your car as well as the drag reduction benefits, its worth looking into a little more closely. im thinking of doing what the 350Z guy did as far as more rake, and let the a diffuser do more of the downforce job, if installed, take some wing out and increase front downforce, lower drag overall. (the 350Z guy, increased rake, had slightly less rear wing, added dive planes and sealed off hood entry at windshield. and went from 0 downforce at the front and 600 to the rear, to near balanced to match its weight disty.)
I think we went over a lot of technology here. I hope the points i made were well taken, and most of which were just definition differences and reasons for the diffuser effects. (some basic aero 101 stuff)
In thinking about it over the past few days, i think your gains were made by loosening up the front end (opening up the splitter) and increasing air flow to the rear diffuser. since the rear diffuser cant effect flow ahead of it. (because of the laws of conservation of energy) the KE increase is localized based on the venturi effect of the convergent air feeders. However, never discount the effects of Newtons second law here either. The change of direction of the air flow at the diffuser influction point, puts downward force on the rear of the car, localizing its force and changing the Center of pressure. Just like the front part of a wing accelerates the air, and then it changes direction downward to take advantage of newtons 2nd law.
Fast moving air is not creating any down-force as it travels under the car. It's just the change in the flow that creates higher and lower pressure zones. (accelerated and slowed speeds). I think your aero engineer has devised two spots under the car to do that, and that is the reason for the results, because like vegas, what happens at the rear, stays at the rear of the car. there is no upward "moleucular interaction". thats how i understand it. because of conservation of energy. all the air flow under the car, has to follow the law. anytime there is lower pressure, the speed of the air increases. when it slows, the pressure rises. (trading potential energy (pressure) for KE (speed).
Nothing happens forward of the diffuser. as far as i have understood the concepts......it cant. it would certanly break a couple of physics laws if it did.
I stilll want to know what the "packers" are and the radius that can change with them to increase or decrease their influence of air flow speed.
Again, im not discounting his design, only the reason and description for it working so that others like me can take that into account and design something that works for me and my car with its own aero issues.
Cheers,
Mark
10-20-2016, 07:42 PM
#118
Racer
"In lookng at the effects on your car as well as the drag reduction benefits, its worth looking into a little more closely. im thinking of doing what the 350Z guy did as far as more rake, and let the a diffuser do more of the downforce job, if installed, take some wing out and increase front downforce, lower drag overall. (the 350Z guy, increased rake, had slightly less rear wing, added dive planes and sealed off hood entry at windshield. and went from 0 downforce at the front and 600 to the rear, to near balanced to match its weight disty.)"
Using their components and methods may do nothing more than just add drag to your car. All vehicles are unique in the wind. You need a way to A-B-A test what you are trying. In aero, anything you do affects everything you've done. The 350Z guys came in with a with very well thought out, very race savvy aero package and estimated there was 2 to 300 lbs in the front and 4 to 500 lbs in the rear. They weren't close. There were a number of additional tweaks that didn't show up in the video. Getting the chassis angle of attack right and the splitter angle and positioning right took a while and was critical to the balance. What Sasha did in the UOIT/ACE wind tunnel took less than three hours, total, to get right.
Using their components and methods may do nothing more than just add drag to your car. All vehicles are unique in the wind. You need a way to A-B-A test what you are trying. In aero, anything you do affects everything you've done. The 350Z guys came in with a with very well thought out, very race savvy aero package and estimated there was 2 to 300 lbs in the front and 4 to 500 lbs in the rear. They weren't close. There were a number of additional tweaks that didn't show up in the video. Getting the chassis angle of attack right and the splitter angle and positioning right took a while and was critical to the balance. What Sasha did in the UOIT/ACE wind tunnel took less than three hours, total, to get right.
10-20-2016, 08:06 PM
#119
Rennlist Member
"In lookng at the effects on your car as well as the drag reduction benefits, its worth looking into a little more closely. im thinking of doing what the 350Z guy did as far as more rake, and let the a diffuser do more of the downforce job, if installed, take some wing out and increase front downforce, lower drag overall. (the 350Z guy, increased rake, had slightly less rear wing, added dive planes and sealed off hood entry at windshield. and went from 0 downforce at the front and 600 to the rear, to near balanced to match its weight disty.)"
Using their components and methods may do nothing more than just add drag to your car. All vehicles are unique in the wind. You need a way to A-B-A test what you are trying. In aero, anything you do affects everything you've done. The 350Z guys came in with a with very well thought out, very race savvy aero package and estimated there was 2 to 300 lbs in the front and 4 to 500 lbs in the rear. They weren't close. There were a number of additional tweaks that didn't show up in the video. Getting the chassis angle of attack right and the splitter angle and positioning right took a while and was critical to the balance. What Sasha did in the UOIT/ACE wind tunnel took less than three hours, total, to get right.
Using their components and methods may do nothing more than just add drag to your car. All vehicles are unique in the wind. You need a way to A-B-A test what you are trying. In aero, anything you do affects everything you've done. The 350Z guys came in with a with very well thought out, very race savvy aero package and estimated there was 2 to 300 lbs in the front and 4 to 500 lbs in the rear. They weren't close. There were a number of additional tweaks that didn't show up in the video. Getting the chassis angle of attack right and the splitter angle and positioning right took a while and was critical to the balance. What Sasha did in the UOIT/ACE wind tunnel took less than three hours, total, to get right.
I do think a rear diffuser which helps the air exit the underside of the car is a good thing, with little downside, if i can integrate it . however, you are probably right. ill end up bolting something on that becomes more of a parachute than what i already have!
btw: as far as theory goes, i found some information pointing to the tunnels of the diffuser (the vertical sides of the tunnels, or "Fences") maybe being responsible for narrowing the flow through the tunnels , via the vortexs that are produced when air flow hits them, which speeds the air up momentarily by creating a neck down area or venturi...... this could be the "speed up through the diffuser throat" that Duane was referring too. I'm digging a little deeper with an F1 engineer on this one!
so Duane, you then could be right about downforce being created at a certain point , via this characteristic of a Diffuser. However, i still stand fast on thinking that in no way, can this effect flow well ahead of the diffuser. there is no "Molecular" talk between the rear air and the front air flow, that i can understand.
Last edited by mark kibort; 10-21-2016 at 01:57 PM. Reason: added correct terminology to describe tunnel sides
