drag question - need some aerodynamic experts

Discussion in 'Technical' started by Orange F1LM, Sep 24, 2004.

  1. Regarding a flat bottom with diffuser creating negative pressure under a supercar. (for more downforce)


    Does the above have any direct mathematical relationship to calculating the coefficient of drag of the vehicle using frontal area etc?

    It seems that there would have to be a trade off no matter how you get downforce, as in it would take extra engine power (speed) just to be building and increasing negative pressure under the car.

    can anyone explain how it could be theoretically beneficial to trade all downforce producing wings , spoilers etc for the flat bottom with diffuser instead, and how either car's "Cd" would compare numerically to the other with the same downforce?.
     
  2. #2 Ssa rM, Sep 24, 2004
    Last edited by a moderator: Apr 25, 2016
    This probably isn't the place to ask...try a more technical site.

    www.pro-touring.com might help
     
  3. "can anyone explain how it could be theoretically beneficial to trade all downforce producing wings , spoilers etc for the flat bottom with diffuser instead, and how either car's "Cd" would compare numerically to the other with the same downforce?. "

    If you need downforce, flat underbodies do not replace wings at all. A flat underbody just reduces lift, without using wings or spoilers. Without wings, a car will not produce any positive downforce. The air travels faster above the car than beneath it. If you imagine the side profile of a McLaren F1, the vehicle is shaped as a wing which will still lift at high speeds. The underbody is completely flat, while the upper side itself is curved which ultimately means more air velocity at the cost of pressure as it travels a greater distance along curved surface rather than along a flat surface.

    Drag usually reduces the less wing you use, as in less angle of attack of each wing, or not use any wings at all. Cd isnt the be all and end all of drag, the frontal area has equal importance in overal drag.

     
  4. #4 Orange F1LM, Sep 24, 2004
    Last edited by a moderator: Apr 25, 2016
    http://www.mulsannescorner.com/diffuser.htm

    "As the air enters towards the front of the car it accelerates and reduces pressure. There is a second suction peak at the transition of the flat bottom and diffuser. The diffuser then eases this "high velocity" air back to normal velocity and also helps fill in the area behind the race car making the whole underbody a more efficient downforce producing device by reducing drag and increasing downforce. "


    if your reducing lift arent you actually creating downforce in reality? Apparently the website above says that the underside design of the car can produce downforce, which in racing would be called negative lift.




    The Ferrari F430 and Enzo dont have any wings that I can see and still pruduce downforce greatly by way of a flat bottom and diffuser through negative lift "negative pressure under the car".

    Apparently this design "from what im now reading" gives you more downforce without as large of a drag penalty as wings and spoilers give you.
     
  5. #5 SaabJohan, Sep 24, 2004
    Last edited by a moderator: Apr 25, 2016
    It is possible to create downforce by using a flat bottom alone. Typically the car is angled, increasing flow area at the rear of the car.

    There is something that is called drag-to-lift ratio, negative lift being downforce. To increase downforce more compared to drag you must make use of the ground effect, making downforce by using wings is very inefficient. You also don't want open wheels, the old group C cars had some very great drag-to-lift ratios due to the use of ground effects, underbody tunnels and a closed body.

    But note that ground effects require a flow under the car, if the flow is stopped by for example the car hitting the ground the downforce will be lost, it will also be lost if the ride height i too high. Even F1 cars which use flat underbodies get some of their downforce from their flat underbodies.

    Check out www.mulsannescroner.com
    There you can find some info about aerodynamics and data on a few cars.
     
  6. #6 Cosworth, Sep 24, 2004
    Last edited by a moderator: Apr 25, 2016
    Downforce defined in my typical blunt manner is where the overal pressure above the car is greater than the overal pressure under the car.

    Reducing lift means simply reducing lift, but the tires will still be unloaded to some degree at higher speeds, which means less grip and stability.

    The diffuser reduces drag as it creates higher pressure behind the rear wheels. For minimal drag for a given body, the air should travel along the shortest path to a region of lower pressure (i.e. along the car to behind the car), low pressure behind the wheels under the car body will attract the flowing air away from the most efficient path, attracting the air to the rear arches, past the wheels and any extrusions which creates a longer path.
     
  7. #7 Cosworth, Sep 24, 2004
    Last edited by a moderator: Apr 25, 2016
    "It is possible to create downforce by using a flat bottom alone. Typically the car is angled, increasing flow area at the rear of the car."

    Yeah, that's true. It's called "rake angle" or "raking", but I'm sure that the profile or airfoil envelope of the car must be as flat as its ergonomically feasible in combination with raking to actually get sufficient downforce.
     
  8. #8 Cosworth, Sep 24, 2004
    Last edited by a moderator: Apr 25, 2016
    "It is possible to create downforce by using a flat bottom alone. Typically the car is angled, increasing flow area at the rear of the car."

    Yeah, that's true. It's called "rake angle" or "raking", but I'm sure that the profile or airfoil envelope of the car must be as flat as its ergonomically feasible in combination with raking to actually get sufficient downforce.
     
  9. "It is possible to create downforce by using a flat bottom alone. Typically the car is angled, increasing flow area at the rear of the car."

    Yeah, that's true. It's called "rake angle" or "raking", but I'm sure that the profile or airfoil envelope of the car must be as flat as its ergonomically feasible in combination with raking to actually get sufficient downforce.
     
  10. I found more on the diffuser

    A diffuser allows the air traveling underneath the car a place to expand and decelerate back to road speed as well as providing wake infill.

    One thing to note is that the rear wing interacts with the diffuser "driving" it. The proximity of the low pressure side of the rear wing encourages better flow through for the underbody.

    http://www.mulsannescorner.com/diffuser.htm
     
  11. "The underbody is completely flat, while the upper side itself is curved which ultimately means more air velocity at the cost of pressure as it travels a greater distance along curved surface rather than along a flat surface."

    Let's stop with the high-school explanation of how wings create lift. The 'greater distance, equal time' "theory" doesn't hold water.

    Aerodynamic forces (ie lift and drag) are the simple product of a change in flow direction. Turning the flow creates a force. Lift can be calculated by integrating velocity or pressure over the surface, but simply applying Bernoulli’s equation is a vast oversimplification. Why? Because it isn’t the speed change, it’s the direction change (remember, velocity is a vector…).



    In any case, Orange… the flat bottom is more of an attempt to create low pressure between the car and the ground than it is to create aerodynamic downforce.
     
  12. "The underbody is completely flat, while the upper side itself is curved which ultimately means more air velocity at the cost of pressure as it travels a greater distance along curved surface rather than along a flat surface."

    Let's stop with the high-school explanation of how wings create lift. The 'greater distance, equal time' "theory" doesn't hold water.

    Aerodynamic forces (ie lift and drag) are the simple product of a change in flow direction. Turning the flow creates a force. Lift can be calculated by integrating velocity or pressure over the surface, but simply applying Bernoulli’s equation is a vast oversimplification. Why? Because it isn’t the speed change, it’s the direction change (remember, velocity is a vector…).



    In any case, Orange… a flat bottom with a diffusr is more of an attempt to create low pressure between the car and the ground than it is to create aerodynamic downforce. There are obviously design interactions with aero surfaces as well.
     
  13. Bernoulli's principle applies to the airfoil envelope of a car's body, there was no implication in my previous posts that it's the only way to achieve downforce. The side profile of a car's body causes air to flow faster at above the body than below, but at the expense of pressure, which usually leads to lift.

    The "Change of direction" theory causes a counter reaction which results in a force that reacts to that the the direction air is turned. The angle of attack of wings effects this.

    Forumula 1 wings are built to take advantage of both Bermoullis principle and the theory of chaning direction of air.

     
  14. Bernoulli's equation relates velocity to pressure, not speed.

    Further Bernoulli's equation is a) localized and b) doesn't apply to real flow.
     
  15. wouldnt the negative pressure under the car be the same as negative lift? and wouldnt that be equivalent to achieving downforce(negative lift)in just a different way?

    I am sure this is the reason the Enzo, Ferrari F430, Modena etc all lost rear spoilers and wings, and instead Ferrari really concentrated on using the design of the underside of the car to achieve the same downforce results they were looking for as far as negative lift goes.
     
  16. Enzo, Modena and whatnot all have actual venturies under the car instead of a plain flatbottom, which as said before was used in Group C and still being used in both form of Indy car. Although not exactly to the same degree as those race cars. In most flatbottom car, to my understanding, generally try to inhibit the flow as much as possible under the car, by using a nose splitter which to encourage the flow to go around the side or over the car instead of under it, and by running extremely low rideheight to the ground at the splitter ensures the flow that does get under the car is accelerated and through the rake of the bottom and the rear diffuser rejoins the flow at the rear. While a Venturi car such as 956 actually want to encourage flow under the car for the venturies to work their magic, as noted by the pic of the 956 below, noticed the lack of presence of the splitter as well as the higher section on the middle part of the front. The fact that Ferrari went with a split radiator on the design of the 360 as opposed to the F355 style nose is a proof of such design thinking as to encourage airflow to travel under the car through their carefully shaped venturi. Which is also a reason why Gordon Murray was a big fan of split rad design on the McLaren as well.

    Obviously though in road car design the ride height cannot be lowered to the extend of sealing the side of the tunnel, but then again they do not create the insane amount of downforce as the Group C/IMSA GTP car does neither....

    Forgoing the use of wing is even used in the height of the Ground Effect era in f1 as well as seen in the Brabham BMW's lack of front wing at one point in the 80s and creating the needed downforce simply through the use of Venturi....
     
  17. The result is the same, but 'suction' and aerodynamic lift are different.
     
  18. I like you to explain how a car with a flat underbody which suffers lift is caused by the turning of air. Since you require dominate pressure or some force underneath the car to lift, how could air be turned when the underbody is completely flat? The upper side of the body consists of various curves and contours, plenty of opportunity for the turning of air in favour of downforce. The implication is no car would ever lift if it was purely about turning of air.

    "Bernoulli's equation relates velocity to pressure, not speed."

    Yes, but as air travels along a non-flat surface, the velocity of the air changes. It is the *change* in air velocity which comes at the expense of pressure of that air. It's agreed that velocities above an airplane wing or car is higher than required for an equal-transit time. The change in velocity of air and the resulting change in pressure wasn't supposed to prove that latter theory, it just proves that pressure is ultimately altered on the upper surface ...

    "Further Bernoulli's equation is a) localized"

    True, but the localisation of different pressures across and along the surface of the car all sum up and affect where the center of pressure will be. Just like the distribution of weight across and along the car affects the center of gravity. The force of lift acts on the center of pressure, the point of the lift acts around the center of gravity.

    "b) doesn't apply to real flow."

    Real flow takes account of applying all the principles correctly. Newton's, Bernoulli's and others. An F1 rear wing is profiled to increase air velocity on the under surface and turn the air near the trailing edge. The upper surface purely turns the air.
     
  19. I like you to explain how a car with a flat underbody which suffers lift is caused by the turning of air. Since you require dominate pressure or some force underneath the car to lift, how could air be turned when the underbody is completely flat? The upper side of the body consists of various curves and contours, plenty of opportunity for the turning of air in favour of downforce. The implication is no car would ever lift if it was purely about turning of air.



    No body whose surfaces are all parallel to the flow direction will produce lift. If, however, it induces a net change in the flow-field, it will. A flat plate will lift if placed at an angle of attack because it changes the net direction of flow in the region of the plate.

    As to how you get suction between the ground and the flat underbody, that’s a boundary-layer effect. The freestream actually moves in a slightly curved path over the ground and under the car. In profile, it looks like fluid moving through a nozzle. Also, there is usually a venturi or diffuser section used in conjuncion.

    _____

    "Bernoulli's equation relates velocity to pressure, not speed."

    Yes, but as air travels along a non-flat surface, the velocity of the air changes. It is the *change* in air velocity which comes at the expense of pressure of that air. It's agreed that velocities above an airplane wing or car is higher than required for an equal-transit time. The change in velocity of air and the resulting change in pressure wasn't supposed to prove that latter theory, it just proves that pressure is ultimately altered on the upper surface ...


    So long as you aren’t trying to apply equal transit. Yes, energy remains constant so air in motion must take away from the overall stagnation pressure. I’m not disputing the conservation laws.

    _____


    "Further Bernoulli's equation is a) localized"

    True, but the localisation of different pressures across and along the surface of the car all sum up and affect where the center of pressure will be. Just like the distribution of weight across and along the car affects the center of gravity. The force of lift acts on the center of pressure, the point of the lift acts around the center of gravity.

    Mathematically, yes, aero forces and moments can be treated as acting at and about the aerodynamic center (like CG).

    _____

    "b) doesn't apply to real flow."

    Real flow takes account of applying all the principles correctly. Newton's, Bernoulli's and others. An F1 rear wing is profiled to increase air velocity on the under surface and turn the air near the trailing edge. The upper surface purely turns the air.


    You’re misunderstanding what I’m saying. Bernoulli’s equation does not apply to real flow. To derive Bernoulli’s equation, you have to make a number of assumptions, including irrotational flow, incompressible flow, inviscid flow, and no body force acting upon the flow. Real flow is rotational, compressible (but under automotive circumstances can be treated otherwise), viscous and with body forces, so Bernoulli’s equation doesn’t work. Now, given boundary-layer theory, you can apply Bernoulli’s equation to the freestream, but if you don’t have an overall direction change in the flow vectors, you won’t produce lift.


    Remember, a change in direction = a change in velocity.
     
  20. good info, does anyone here know of any true flat bottom cars that are street legal?
     
  21. McLaren F1 GTR is probably streetlegal if you fit it with neccesary equipment. It is a flat bottom car, the fan used on the street car wasn't allowed, nor underbody tunnels so it used a flat bottom and a rear wing to produce a lift coefficient of about -.6 if I remember correct.
     
  22. basically the LM then...
     
  23. Figures it had to be that car...lol
     
  24. Oh… this thread was about drag. Okay.

    Well, if you are creating aerodynamic lift with a finite surface, you will also create induced drag. This will be minimized by ground effect, but it will still add to the total CD (remember, CD is the sum of many drag coefficients, one of which is CDi for induced).

    Also, a flat bottom will feed more laminar air to the wake, which will increase the form or pressure drag coefficient.

    There is a direct mathematical relationship for CDi that includes aspect ratio and span efficiency factor. Wake drag is much harder to compute. It’s generally done experimentally.


     
  25. #25 MrMortigi, Oct 8, 2004
    Last edited by a moderator: Apr 25, 2016
    The Lotus Elise uses a flat bottom with a venturi.
    http://www.panix.com/~clay/elise/exhaust_rear.jpg
    I believe Lotus says it produces about 30 pounds of downforce at 100mph, but I could be wrong about that.
     

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