Several posts ago, I made one very large posting on flat underbodies, venturi shaped underbodies, underbody tunnels, diffusers, front air dams and front mounted splitters and there effects on vehicles. I have received a lot of E-mails asking more in depth questions about their different aspects and construction, so to address them I will split them up and delve a LOT deeper into each of them over the next few posts. I will start it off with the flat underbody and the rear diffuser because it is the most common and most recognized of all of the different underbodies used for down force generation or to merely decrease underbody drag. The flat underbody is the one that is the easiest to build, but as we will see in some of the later posts I will be making on the venturi underbodies and the tunnel underbodies, the flat floors with diffusers are not the best for down force generation.
A diffuser is next to useless without being attached to a flat underbody, so I will combine them together here. The vehicle underbody will need to be as smooth and flat as you can make it, to have an effective and operational diffuser. This post will cover a vehicle with a flat floor in close ground proximity, a rear mounted diffuser and 4 tires in wheel wells.
United States Air Force pilot Joe Kittinger holds the world record for the highest, longest and fastest sky dive. On August 16, 1960, leaping from a helium balloon at 102,800 feet, he fell for 4 minutes and 36 seconds and attained a maximum speed of 614 miles per hour. During a previous jump from 76,400 feet, Joe lost consciousness due to equipment problems and went into a flat spin at 120rpm’s that generated 22G’s on his extremities, which set another record. Joe wore a pressure suit for all of his high altitude jumps. At 63,100 feet in altitude the air density is very low and the air pressure is one‑sixteenth that of the standard sea level atmospheric pressure and water will boil at the normal temperature of the human body, 98.6 °F. The water wetting a person’s lungs would boil as well as the saliva in the mouth, this is called the Armstrong limit, and it is named after Harry George Armstrong.
If you were to use a front airdam with a splitter and a rear spoiler to generate the same amount of downforce as a flat floor with a rear diffuser, the airdam and rear spoiler would generate 20% to 30% more drag than the flat floor with the rear diffuser. Something I like to say and I repeat over and over, downforce from the underbody is virtually drag free and when properly implemented will actually lower overall vehicle drag and generate downforce at the same time.
For land speed racing the air can be a pretty viscous and downright brutal enemy. You have to hit it and force it out of the way, making an opening to pass through as it pushes back against you, forming a high pressure stagnation point at the leading edge of the vehicle, and just when you think you have made it past it, it grabs ahold of you, wrapping its arms around your throat and tries to hang on, forming a low pressure area behind the vehicle in the wake that will pull back on you. Both the high pressure at the front and the low pressure at the rear will retard your forward movement, hindering acceleration and top speed.
The air is normally stationary and not moving very much close to the ground and if it does move it’s only as fast as the wind blows. The air does not really flow over your vehicle very well, unless it is a streamliner. The air is stationary and a vehicle under acceleration is hitting the air particles and forcing them out of the way, making an opening for your vehicle to pass through. Air has weight and density and there for viscosity that resists movement. At sea level, air pressure is 14.7psi at 60’F; 1 pound of air occupies a volume of 13.1 cubic feet. As air temperature is reduced, the air pressures decreases and the air density increases. The movement of your vehicle through the air causes several unwanted things to happen around it, such as increased skin friction, a high pressure area at the leading edge called a stagnation point and a low pressure area at the trailing edge of the vehicle called a wake, that has an effect similar to a suction pulling back on the vehicle and slowing its acceleration. The power required to run a given speed goes up with the cube of velocity. Increasing the speed of your vehicle from 100mph to 125mph requires double the horsepower and going from 100mph to 200mph requires 8 times the horsepower. Drag increases with the square of the speed.
For a typical 4 wheel vehicle, the rolling resistance and aerodynamic resistance will be equal around 30mph on a hard surface and as the speed increases the aerodynamic drag will become the main source of resistance and the rolling resistance will stay more or less constant on a hard surface. The rolling resistance of a wheel being run on a deformable surface will be greater than that for a hard surface and will have a greater variance.
Aerodynamic down force will tend to improve vehicle directional stability at high speed. The amount of downforce produced by the vehicle underbody is related to the shape of the underbody. As vehicle ground clearance decreases, the underbody airflow velocity increases and in turn the air pressure decreases. Downforce should be generated with as low a drag as possible. The highest aerodynamic down force with the least amount of drag can be generated at the vehicles underbody. The “ground effect” underbody parts of a vehicle are very efficient; they will contribute less drag with much more downforce than any other part that can be added to the vehicle, such as wings, rear spoilers, splitters or dive plates.
The rear low pressure wake, pressure drag, will help to drive the airflow through the rear diffuser and the diffuser will have an influence on the lower region of the rear wake, near the ground. If there were a rear deck mounted spoiler or wing it would influence the upper part of the rear low pressure wake. Higher static pressure in the wake will reduce the drag of the vehicle somewhat, but also it will reduce the driving force of the diffuser slightly, leaving it less efficient. The diffuser will influence the lower part of the wake behind the vehicle, since that area of the wake will help to drive the flow through the diffuser. The low pressure in the wake will help to pull airflow through the diffuser.
There will be 3 main things to consider when designing and building a proper diffuser, the inlet to outlet area ratio of the diffuser, the ramp angle inside the diffuser and the vehicle ride height.
The role that the diffuser will serve in the generation of downforce, will be to take the accelerating airflow that is moving under the vehicle from the front to the rear, slow it down, which will cause an increase in its pressure. As the air in the diffuser slows and gains pressure it will cause the air particles to move much closer together and in doing so will cause a pumping effect within the diffuser, pulling air from under the flat underbody, which will cause the airflow underneath the vehicle to speed up and lower its pressure even more, reducing the pressure along the path of the airflow on the underbody, resulting in a decrease of the static pressure underneath the vehicle.
By decreasing the forward angle of the vehicle “rake” it will have the same effect as an increase in the effective angle of the diffuser.
If there is a contoured inlet leading into the vehicle underbody, the downforce will increase as the inlet angle increases. The underbody downforce can be moved slightly forwards or backwards with the angle of the inlet but it will have a much smaller influence than the angle of the rear diffuser has on the point of maximum downforce. An increase in the inlet angle will bring some of the downforce closer to the inlet due to the steeper inlet angle allowing the floor area to be larger and the steeper inlet will allow the low pressure in the underbody to begin at a more forward location and thus generate downforce at a more forward location than for a shallower inlet angle.
The rough underbody will contribute about 20% of the total aerodynamic drag of the average passenger vehicle with no modifications done to it. The flat floor can be used in several ways to reduce drag or generate downforce on a vehicle. One example is to seal the gap between the sides of the vehicle and the ground entirely, leaving only the rear portion open, allowing the low pressure in the wake behind the vehicle to generate the low pressure under the vehicle, thereby generating downforce or you can add a rear diffuser to the flat floor to generate more downforce, you would normally want the highest diffuser angle without causing internal flow separation, to generate maximum downforce. Another way of using the flat floor is to seal the vehicle underside off 360° to the ground and use an active means to lower the air pressure under the vehicle, using a powered fan, there are 2 documented cases of it successfully being used, and both were banned, the last well over 30 years ago in formula 1. With careful construction and placement of a rear wing it can be used to increase the pumping effect on the underbody or to increase the effectiveness of the diffuser, I will cover that in more detail on a later post covering rear wings.
The drag penalty is quite small for a diffuser, compared to the downforce generated from it, so a lot of care should be taken to have the diffuser operating at its best. On a race car the diffuser and flat floor with no side skirts, will contribute much more downforce than a 2 tiered high camber rear wing with a small fraction of the drag penalty. If rules were not so strict vehicle underbodies could make 100% of the required downforce, 30% more than the rear wing and front wing combined, but then there would be a balance issue to sort out. Formula 1 is actually considering a reversal on a 30 year old moratorium on venturi shaped underbodies so there can be more overtaking during the races and allow the vehicles to be more stable when following closely.
The downforce from the diffuser can be increased by 4% to 10% when the rear outside portion of the rims are covered because the air underneath the vehicle is more enclosed by the closed rims and the flow through the diffuser is increased instead of being pulled out by the pumping effect of the rear wheels if they are not covered. By covering the front and rear rims, will result in a smoother flow as well as prevent the flow underneath the vehicle from spilling out towards the sides and maintain a high flow velocity underneath the vehicle. By blocking the crossflow in the rear rims will result in increased downforce due to an increased air flow rate through the underbody and the diffuser, downforce can be increased when the rear rims are enclosed. The rims, if they are not covered, will generate a pumping effect, pulling the air from under the vehicle and lowering the amount of airflow thru the diffuser and decreasing the downforce. By covering the rear wheels only, this will be most sensitive to yaw regarding lift. In yaw conditions the flow underneath the vehicle will be greatly disturbed. Since the rear diffuser is the most important device to create downforce the lift will be greatly affected by the yaw condition.
The interaction between the vehicle underbody and the track surface is strongly dependent on the vehicle ground clearance. The air flow that goes under a vehicle that does not have a flat underside will be subjected to many obstructions along its path. The air will strike the oil pan, steering rack, front suspension, transmission, tires, driveshaft, exhaust, rear end, and fuel tank and all of these will cause points of stagnation and high pressure, that will increase the vehicle drag and generate lift, both of which should be avoided. The flat underbody will keep the airflow moving fast and at low pressure, decreasing drag and lift.
As air flows underneath the vehicle there will be a boundary layer generated on the underside of the vehicle, its effects can cause everything from generate down force to make the vehicle uncontrollable and become air born. A vehicle with a flat underside will have a minimum lift and drag at around 5in to 6in of ground clearance and downforce will start to be generated from there, as the ground clearance is reduced, and drag will gradually start to increase also. Under vehicle air velocity will increase as ground clearance is decreased. Velocity varies with area; if you reduce the area by 50% you will double the air velocity. At higher ground clearances the air flowing through the underbody will be affected by viscosity less; as the ground clearance is reduced it will be affected more by the viscosity of the air, as the speed of the airflow is increased. The most downforce will be generated from a flat floor, with a ground clearance of between 1.5in and 2.75in.
On a vehicle with a flat floor and rear diffuser, the point where the greatest aerodynamic downforce occurs will be where the flat floor meets the diffuser ramp; this point can be moved forward or backwards by moving the location of the entrance of the vehicle floor to the diffuser. The highest downforce will be generated at the transition from the vehicle floor to the diffuser entrance. The flat floor area leading into the diffuser entrance should be kept as smooth as possible, no rivet heads or other protrusions so the flow will have the least disturbance as this will be the point of maximum low pressure and greatest downforce. The diffuser entrance can be moved by changing the angle of the diffuser ramp. The angle of the diffuser floor can be between 5deg and 13deg, with an angle of 9deg to 10deg being most effective, diffuser angels over 14deg will cause the pressure to become too great and cause the airflow to separate, reducing downforce. The majority of the downforce from the flat underbody and rear diffuser will be produced at the rear of the vehicle. This will cause a stabilizing effect at higher speeds but it will also cause the vehicle to become less responsive to steering input. A vehicle with nothing more than a flat underbody and a slight forward angle will generate downforce with no rear diffuser, but without the rear diffuser the low pressure, high velocity air exiting the underbody at the rear of the vehicle into the turbulent wake, will only add more turbulence to the wake and increase the drag slightly, it will not be easy for the flow exiting the vehicle underbody to merge with the wake due to low pressure mixing with low pressure, it will hinder the underbody downforce production. A plain flat floor with a few degrees forward angle and an up sweep of the floor at the rear of between 5 degrees and 13 degrees will generate more down force. The more elaborate you make the design the more down force will be created. The diffuser will allow the air to slow down and gain pressure, when it exits the underbody at the rear of the vehicle the higher pressure air will help to reduce the turbulence and decrease the drag. The low pressure in the wake region and the higher pressure flow exiting into the low pressure region will have a slight suction effect and increase the flow in the diffuser.
General wisdom would dictate that the diffuser angle should be between 5° and 7° and normally that would be true, but we have an unseen aerodynamic ally on our side that will help the diffuser function in many ways, vortices. Where the flat floor meets the diffuser entrance there will be vortices created, much the same way as the vortices are created and functions inside a NACA duct, which will travel down the length of the diffuser, adding rotation to the velocity which will decrease the pressure along its length, causing an increase in the airflow into the diffuser. Low pressure areas will form in the corners of the diffuser where the vortices originate from. This vortex flow also adds energy to the flow and will delay internal flow separation inside the diffuser allowing for a greater than normal diffuser angle to be used. There will be counter rotating vortices formed at the sides of the diffuser, which will improve the airflow through the diffuser. If the diffuser angle is too steep, the underbody flow will tend to separate at the sharp diffuser entrance only to be reattached by the counter rotating vortices further into the diffuser. The loss of downforce in a high angle diffuser at low ground clearances; will be the result of vortex breakdown causing the flow separation. The low ground clearance destroying the vortices leads to the flow separation, the flow would become reattached in the diffuser as the ground clearance is increased and the vortices regain their strength. Low angle diffusers will experience some loss of downforce at low ground clearances due to the vortex breakdown decreasing the amount of flow through the diffuser but the shallow diffuser angle will not experience internal flow separation as in the high angle diffuser during the period of vortex breakdown. If the vehicle is to be run with extremely low ground clearances, to the point that the vortices cannot function, the diffuser angle will be best at between 5° and 7° angle.
The air entrance is just as important as the air exit. There will be a tradeoff of drag and downforce with the diffuser, as diffuser angle is decreased the drag will decrease and as the vehicle ground clearance is reduced drag will increase. The main job of the rear diffuser is to slow the speed of the under vehicle low pressure, high velocity airflow and decrease the speed and let the pressure rise to that of the external free stream airflow or as close to free stream flow as possible, before exiting into the free stream air at the rear of the vehicle. The greater the cross sectional area ratio between the entrance and the exit of the diffuser, the greater the suction effect will be on the underbody. Some down force will be generated by the diffuser because normally the pressure in the diffuser will be lower than external pressure. Downforce is created not only in the diffuser but also under the entire floor area. The diffuser drives the airflow for the complete vehicle underbody. Because of the angle of the diffuser floor, its internal volume will increase as the aiflow moves to the back, causing the air from the underbody to expand as it passes through the diffuser, pulling air through the underbody. The diffuser is not sealed to the ground; it has gaps around the edges allowing air leakage into the diffuser. If the angle of the diffuser is too great, the airflow will separate, because it will not be able to overcome the pressure in the diffuser, causing pressure to build up under the vehicle. The angled flat floor will generate downforce by its self; the addition of a diffuser will increase the velocity of the air under the vehicle. The job of the diffuser is to convert the airflow’s kinetic energy or dynamic pressure into pressure rise or static pressure. The expansion of the air from the underbody in the diffuser slows the air, increasing its pressure. As the air is slowed down it is forced to become denser as the pressure increases. The diffuser can be a simple upsweep in the flat floor at the rear of the vehicle, but adding side plates that drop close to the ground and forming a tunnel will increase its effectiveness greatly. The side plates will allow the diffuser to generate more downforce at a lower speed. There will be a counter rotating vortice generated at the inside of the diffuser side plates, enhancing airflow through the diffuser. The attached vortices inside the side plates will trail downstream into the wake behind the vehicle and cause a vortex induced suction. At very low ground clearances, the effect of the counter rotating vortices will be reduced as ground clearance is reduced. The counter rotating vortices will help to keep the airflow attached to the diffuser surface longer than it would be expected to at higher angles. The diffuser angle should be between 9deg and 10deg to be most effective. The smooth flat floor of the vehicle leading to the diffuser will allow a larger area over which the low pressure air can act, creating more downforce. The diffuser itself does not create downforce, it is the area in front of the diffuser that the low pressure acts on that creates the downforce. Another benefit of the diffuser is that by it being located at the very rear of the vehicle, allows all of the high pressure mass airflow to help fill the void behind the vehicle, reducing the wake size and the resulting pressure drop and the induced pressure drag. The diffuser will reduce the turbulence in the wake, thereby decreasing the pressure drag.
The rear diffuser and the angle it is set at will have a big influence on the underbody airflow and the wake behind the vehicle and can contribute to reducing the pressure drag in the rear wake of the vehicle. At the lower angles for the diffuser, the downforce will be gradual as the ride height is reduced and at very low ground clearances the falloff of downforce will be more gradual also. At the higher angles for the diffuser, downforce will build more quickly and falloff quicker at reduced ground clearances.
The larger the floor area can be maintained, the more downforce will be created. Efficiency in the inlet and diffuser can be sacrificed somewhat, for the sake of a larger floor area, for the low pressure to act upon. The amount of downforce generated from the flat floor is proportional to the pressure differential and the size of the vehicle floor. Generally for every 1% increase in underbody airflow velocity, generated by the rear diffuser, will generate 3% of extra downforce. The efficiency of the downforce created by the flat floor is only as good as the efficiency of the diffuser.
A vehicle with nothing but a flat smooth underside installed can be neutral, generate lift or generate downforce, depending on the angle of the underside and the effect will be magnified with reduced ground clearance. Everything being equal though, as ground clearance is reduced, drag will increase; because of the viscous effects of the airflow under the vehicle will be increased as ground clearance is reduced due to the airflow velocity increasing. As ground clearances become smaller air velocity will increase, thus increasing its viscous drag. As the ground clearances fall below 5in to 6in, the effects of the ground will begin to be felt and be magnified as the ground clearance is reduced.
If side plates are used with the diffuser, the thickness of the side plates that drop down close to the ground on each side of the diffuser, that makes the outer walls, will have a big effect on the downforce and drag of the diffuser. The thickness of the side plates will have an influence on the amount of airflow entering the diffuser from the sides. The more high pressure airflow that leaks into the diffuser from the sides, the more the pumping effect of the diffuser is decreased. The thinner the side plate is the less outside airflow from the sides will enter the diffuser, thereby maintaining a lower pressure between the side plate and the diffuser ramp. If airflow thru the flat underbody is maintained and flow in the diffuser is not allowed to separate, the boundary layer on the underbody will remain thin with a high friction drag at the underbody surface.
Diffuser efficiency can be increased with the addition of longitudinal vanes to the diffuser. The longitudinal vanes can be straight or curved in the diffuser. Adding straight longitudinal vanes in the diffuser will increase the friction drag but they will generate an extra mixing effect that will increase the airflow in the diffuser and cause the air flow to remain attached, you can think of the extra longitudinal vanes as turbulators, increasing the airflow in the diffuser. If highly curved vanes are used in the diffuser they maximize the mixing effect inside the diffuser and allow a steeper diffuser angle to be used. With the addition of longitudinal center vanes in the diffuser, it will maintain a higher flow velocity and help to prevent the rear wheel wakes from extending towards the center of the vehicle and decreasing the diffuser efficiency, if the rear wheels do not have covers to prevent the crossflow. This will help to maintain a higher flow rate thru the diffuser and increase the downforce. There will be a lower pressure generated in the diffuser with the center vanes than without them due to the higher flow velocity. The position of the center vanes can be tuned to achieve even better performance from the diffuser. The increased flow rate from the diffuser will decrease the wake at the rear of the vehicle. The center vanes channel the flow and will generate more downforce with only a minor increase in drag. The diffuser with the center vanes will be influenced more by small yaw angle movements of the vehicle than a diffuser with no vanes but the diffuser with vanes will generate more downforce regardless of yaw than with no vanes. At angles where the flow starts to become separated in normal open diffusers, the multiple-channel diffuser will have large improvements in downforce with a minimal increase in the drag, allowing for increased diffuser angles and a decrease in the ride height sensitivity. The improvements in the diffuser will occur through improved diffuser pumping and pressure recovery in the inner and the outer channels of the diffuser. Diffusers with multiple-channels will generate greater downforce per unit of area than conventional open diffusers.
The rear downforce will be most affected by large increases in yaw, because the rear diffuser will lose efficiency in yaw conditions. As vehicle yaw increases the airflow underneath the vehicle is significantly disturbed. Since the rear diffuser is the most important device to create vehicle downforce, as the yaw increases the downforce will decrease.
In an ideal diffuser the pressure at the diffuser exit would be that of the freestream flow. If the design of the diffuser were to cause the airflow pressure inside the diffuser, to reach atmospheric pressure before the diffuser exit, it would hinder the performance of the diffuser due to making the remaining portion of the diffuser behind the point of where atmospheric pressure was reached, to be a waste, because the job of the diffuser is to increase the airflow pressure to that of the freestream. If freestream pressure is reached inside the diffuser too early, before the diffuser exit, it may be due to two reasons, either the diffuser ramp angle is too large or the inlet to outlet area ratio is causing the expansion of the airflow to be completed too early. As the ratio of the inlet to outlet area increases this will generate greater pressure recovery, decreasing the pressure at the inlet.
If a rear wing is mounted to the vehicle, it will influence the upper regions of the base wake whereas the diffuser influence the lower regions of the base wake and since the base wake drives the flow through the diffuser, the suction force created by the vacuum in the wake behind the vehicle will be far more detrimental to the vehicle performance than the high pressure at the leading edge stagnation point.
It will be much harder to construct, but a diffuser with a curved roof will generate more downforce and can be operated at a more extreme angle than the traditional flat roof diffuser. A curved diffuser roof will increase the rate of expansion more gradually as the diffuser cross section area is increased inside the diffuser. As the expanding cross section of a diffuser with a flat roof slows the incoming airflow, the air pressure will increase at a much faster rate as it moves towards the rear of the diffuser. If the diffuser angle is to steep, the airflow velocity in the diffuser will be greatly reduced, to the point of becoming nearly stagnant at the diffuser outlet, thus leading to airflow separation and a dramatic increase in turbulence inside the diffuser. The separation in the diffuser can be decreased and the proper diffuser expansion rate can be maintained with the use of a curved diffuser roof. The use of a curved diffuser roof will have a more gradual and controlled change in the buildup of the air pressure inside the diffuser, reducing the separation and turbulence of a flat ramp high angle diffuser. Optimal diffuser roof angle will be dependent on the vehicle ride height due to the diffuser pressure recovery being highly influenced by the ride height. The diffuser pressure recovery coefficient and diffuser downforce performance will be controlled by the diffuser inlet to outlet area ratio. As the vehicle ride height is decreased the initial expansion rate inside the diffuser can be decreased, so shallower diffuser angles will be required and increased ride heights can use larger diffuser angles.
There are 2 types of vortex generators that can be used on the vehicle underbody; they are classified by their height. The first one is the smallest in height and is referred to as a Sub-Boundary Layer Vortex Generator and it will normally be about 60% to 80% of the height of the boundary layer it is placed in and the vortices it will generate will not be very strong but they will be generated directly in the boundary layer, the Sub-Boundary Layer Vortex Generator does not extend above the boundary layer so its drag will be very low. The Sub-Boundary Layer Vortex Generator will generally have a greater angle than the regular vortex generator has, normally the Sub-Boundary Layer Vortex Generator angle will be around 20° to 25°. The second type of vortex generator is much taller than the Sub-Boundary Layer Vortex Generators and there height extends above the boundary layer and will generate much larger and stronger vortices that will be generated in the airflow above the boundary layer and there drag will be much greater than for the Sub-Boundary Layer Vortex Generators, these type of vortex generators will have shallower angles than the Sub-Boundary Layer Vortex Generators, normally around 10° to 15°. A stable and long vortex will reduce the pressure along its trail, and if used in the vehicle underbody with a flat floor and adequate ground clearance, will increase the downforce generated by the flat floor. If vortex generators are installed on the vehicle flat underbody it would be best to install them very close to the front of the vehicle, so the vortices will have time and the distance for their trails to generate the low pressure in and around them. The size and strength of the vortices will be mainly dependent on the size of the vortex generator. So a very strong and stable vortex can be generated, the vortex generator needs to be taller than the boundary layer it is placed in, on the vehicle underbody. The vortices will increase in strength as the ground clearance is reduced, increasing the suction effect and low pressure but at the same time drag will increase also. The underbody vortex generators can create 15% to 30% more drag but contribute from 30% to 75% more downforce. Vortex Generators increase downforce by increasing the dynamic loads by creating strong vortices near solid surfaces. Unbound vortices trapped in ground effect can be used to increase the downforce, because it increases the vorticity of the airflow. The bottom surfaces of a racing vehicle with a flat underbody, generally moves very close to the ground at almost-zero angle of attack, which makes stability of trapped vortices less of an issue, increasing downforce. If there are openings or breaks in the flat underbody that can make the airflow become detached from the surface of the underbody, such as openings for suspension arms to have vertical movement or frame connectors, vortex generators can be used to delay the separation or can make the airflow reattach to the underbody. Most underbody vortex generators will have wing profile shapes. If single vortex generators are used at trouble areas to help the airflow they should be angled between 10° to 25° with respect to the vehicle’s longitudinal axis.
If multiple vortex generators are placed close together, the optimum vortex generator spacing should be 2 times their height in lateral spacing and the optimum orientation of the pair of inner and outer vortex generator should be between 10° and 20° with respect to the vehicle’s longitudinal axis. The vortices from the vortex generators will breakdown and disappear right after they enter the diffuser inlet. The vortices will remain stable throughout the center section of the vehicle underbody until the moment that they enter the diffuser inlet. The vortex breakdown is due to the vortices not being able to handle the adverse gradient inside the diffuser. A vortex created close to the diffuser will be very strong and will cause a disruption in the diffuser flow. The area behind the vortex generators will produce a very strong suction on the vehicle underbody. As the vortex generators are placed closer to the diffuser inlet they will become less effective due to the suction peak in the diffuser inlet. As the vortex generators are placed closer to the diffuser inlet they will begin to interfere with the diffuser function and cause underbody downforce to begin to drop off. As a rule vortex generators should not be placed on the underbody past the rear edge of the front door when a rear diffuser is used with a flat underbody. Despite the fact that the Vortex Generators are highly cambered the air flow does not separate on the vortex generators. At the Bottom Rear of the underbody, the boundary layer is not able to cope with the diffuser’s adverse pressure gradient, causing it to separate. This severe separation will cause underbody downforce to drop off quickly, causing lift to increase and downforce on the back of the vehicle to decrease. If the vehicle has a rear wing or rear spoiler attached, the separation of flow in the diffuser can result in a decrease of downforce from the rear wing or spoiler also, creating a cascade effect that will increase the vehicle lift even more. If a vortice is created on the vehicle underbody it will be stretched and distorted due to the accelerating flow in the underbody, decreasing the radius of the vortice.
If vortex generators are used at the middle to rear part of the vehicle underbody that has a rear diffuser installed it will lead to a very inefficient way of increasing the downforce of the vehicle, because the drag of the vehicle will increase and downforce will decrease to such a level that the lift-to-drag ratio will be below that of the original vehicle before the vortex generators were installed. Vortex generators should be placed away from the front tires, centered between them if possible and not be placed directly behind the front wheels either as they will have a negative influence on the drag of the Front wheel wells. If cambered vortex generators have to be placed close to the diffuser, decreasing the amount of camber will help to reduce the separation in the diffuser and help to maintain its downforce.
Vortex generators on the vehicle underbody can produce a significant amount of drag, but they can provide the vehicle with a lot of downforce as well. In very high horsepower applications or in situations of extreme loss of rear grip, the drag, unlike the downforce, is not very important. In times of low tire grip, high downforce levels can be achieved from the underbody mounted with vortex generators, the drag coefficient is not much of an issue in high horsepower situations. Vortex generators can be used as a tuning aid, they can be easily modified so there camber, locations and length can be adjusted, as well as removed and installed easily.
