I searched both the forum and Google and couldn't quite find what I'm after, but forgive me if I've missed something.
Essentially I've been looking into vortex generators and want to clear some stuff up. As far as I can understand, they energise the airflow over a surface so to delay flow separation and reduce drag. Or to explain it to a 5 year old, they create mini tornados which suck slower moving air into them and keep it all moving quickly. Or something like that.
So what's the disadvantage? We see them on a few cars, but not many. Why is this? One would assume they'd be all over the place on cars trying to increase performance and fuel efficiency. Is it something to do with the difference between surface drag and pressure drag? I assume they reduce pressure drag by maintaining a good boundary layer and decreasing the low pressure area behind the car, but the physical shape of them creates surface drag with the air hitting them?
The disadvantage is that generating that vortex takes energy. That energy has to come from somewhere. Where it comes from is slowing the car down (drag).
Due to the high pressure and low pressure areas that form above and below the car respectively, vortices are an inevitable product of when these two air masses converge. Because they create drag, vortices tend to be a pain in an aerodynamicist’s side which is why teams have developed ways of reducing their impact to increase top speed.
Take the rear wing endplates as an example. Endplates are placed on the tips of the wings to prevent the high pressure air above the wing (remember, we have now flipped the aerofoil over to create downforce rather than lift) spiralling its way to the low pressure below. However this cannot be totally managed as vortices still form at the very tips of the wing where the two pressures converge, causing drag and lowering top speed.
Vortices come in a number of forms - they all obey the same fundamental rules and are technically the exact same but they can be produced in a number of ways and the way in which they are produced gives us some insight into their function/usefulness. I stress again, ALL VORTICES ARE THE SAME PHYSICAL THING, but some are just more useful than others.
The primary distinction between a useful and a useless vortex is how tight the vortex is: a huge vortex moving lots of air but rather slowly takes a lot of energy to produce (and energy taken to generate this vortex is energy lost from our car) but doesn't do us much good, as dynamic pressure, the pressure force which allows us to generate lift, is proportional to velocity squared - these vortices are undesirable; a small but very tight vortex can rotate extremely quickly, producing an area of intense low pressure without actually requiring a great deal of energy to generate - these vortices are extremely useful so long as they remain tight and do not burst.
vortex 1 - a wingtip vortex: this is the kind of vortex turbof1 is referring to in his quote and they are always considered undesirable but there aren't a lot of ways for F1 to get rid of them. For any finite plate, if there is a difference in pressures on the two main surfaces then there is going to be a force encouraging air to spill from one side to the other, causing something called vortex induced drag. The best way to negate these vortices is to either have a very very long wing, like you see on gliders, or to alter your loading so that the wingtips are producing less lift (and so there is a smaller pressure difference between the two sides of the plate). The width of our F1 wing is restricted by the rules and we pretty much want to generate as much downforce as we can so we don't really want to significantly reduce our loading. Endplates try to manage the wingtip vortices, amongst other things, but don't really do an amazing job.
vortex 2 - a 'sealing' vortex: you will see on many modern F1 cars designers intentionally try to seal the underbody, around the back end of the car, with a vortex or more likely a few vortices. The successful application of these vortices is difficult, as if they burst they do much more harm than good, however the idea is to create an area of intense low pressure as a skirt to stop the higher pressure air above your floor spilling over the sides and mixing with the low pressure under your car.
vortex 3 - a lift/downforce vortex: okay so I couldn't think of a good name for this one, they all probably have long, official, technical names but I am trying to explain this in layman's terms so this will have to do. If you look at fighter jets, and Concorde, and any other supersonic aircraft, you will notice that the wings don't really look that much like aerofoils - thats because they don't operate like a normal aerofoil, they are known as a delta wing. Delta wings attempt to generate a few very tight, extremely low pressure vortices to produce their lift - its actually an extremely effective method. F1 uses these underneath the car to generate the underbody lift. You will generate them whenever you have a sharp edge so you will see under wings etc little thin slats which are designed to generate you a vortex. They are particularly interesting for F1 because they generate lots of downforce without really spoiling the air for the following cars, so you would get better racing if a larger percentage of a car's downforce was produced this way. Food for thought...
vortex 4 - a sheet vortex: the viscosity of the boundary layer causes the flow to produce vortices with a regular frequency, as there is a velocity difference between the boundary layer and the free flow. This velocity difference, when viscosity is considered, causes the flow to roll over on itself (imagine what would happen if you turned a treadmill up really fast, too fast for you to run on, then jumped on - airflows undergo these comically hilarious situations all the time). They are generated across the wing, not along it, and they are extremely important for lift, although only when you start talking in technicalities so don't worry too much about why that is. They are identical to what you would see if you stood on a bridge over a fast moving stream and watched the little whirlpools form off of the bridge structures in the water, and in this bridge case they generate every few seconds. On an F1 car in air, they are generated at a much higher frequency.
tl;dr vortices are really really important for both lift and drag; they are also really complicated; some of them are good, some of them are bad - try make the bad ones good if you can.
EDIT: I read this through again and realised just how bad the English was; I have tried to correct it.
Last edited by thepowerofnone on 30 Apr 2014, 00:11, edited 2 times in total.
Something not mentioned here and worth reading about. 3 types of drag
Profile drag - simple put your hand outside a car window and feel the force
Friction drag - simple as the air passes over a surface it creates friction expressed as drag
Induced drag - drag created as high and low pressure dynamic flows meet each other.
Interestingly induced drag decreases with speed whereas profile and friction increase with speed
beelsebob wrote:The disadvantage is that generating that vortex takes energy. That energy has to come from somewhere. Where it comes from is slowing the car down (drag).
I thought the whole point of them was to reduce drag?
Del Boy wrote:Something not mentioned here and worth reading about. 3 types of drag
Profile drag - simple put your hand outside a car window and feel the force
Friction drag - simple as the air passes over a surface it creates friction expressed as drag
Induced drag - drag created as high and low pressure dynamic flows meet each other.
Interestingly induced drag decreases with speed whereas profile and friction increase with speed
I see. So I'm guessing vortex generators reduce the induced drag at the back of the car but create profile and friction drag due to the physical size and shape of them? Meaning you need to find the correct balance between reducing the induced drag and minimising profile/friction drag?
Del Boy wrote:
Interestingly induced drag decreases with speed whereas profile and friction increase with speed
what ?????
btw the OP seems to be looking at boundary layer vortex generation (to prevent seperation of the BL)
usefuel to avoid the alternative, eliminating the seperation by other means eg reduce AoA, increase camber, add flap etc
vortices are a function of wing inefficiency, increased by design rules or convenience ie brief operation at very high AoA
geese shed rather large (tip) vortices which get part used to benefit their neighbours
the F1 car's front wing does the same to benefit its own aerodynamic mechanisms elsewhere (the diffuser)
Last edited by Tommy Cookers on 30 Apr 2014, 09:25, edited 2 times in total.
Del Boy wrote:
Interestingly induced drag decreases with speed whereas profile and friction increase with speed
what ?????
He's not wrong, its just he hasn't applied this to a car. On an aircraft as you increase speed you can decrease your C_L to maintain the same lift, and since induced drag is a function of C_L your induced drag decreases, whereas your skin and profile drag continue to increase as velocity increases.
It’s very true that there are vortices and then there are vortices. But the question at least in part concerned vortices in boundary layers. The boundary layer is nominally a laminar layer of fluid between the static boundary and the free stream flow, i.e. with a velocity profile of zero at the boundary surface and through friction and viscosity, increasing to the free flow velocity thereat through shear. But, when the viscosity is unable to maintain the laminar flow status in the boundary layer, the boundary layer may shear apart internally to from vortices in the boundary layer proper, i.e. a turbulent boundary layer. These vortices internal of the boundary layer act as rollers, as it were, within the boundary layer keeping it intact and attached when the shear forces are too great for laminar flow.
In many instances attached flow is desirable lower drag, lift or whatever –though a clean detachment can sometimes be preferable.
ACJJ619 wrote:So we're saying that they're not all over road cars because sometimes designers don't want the flow attached?
No, they're not all over road cars because they cause drag, and at the speeds a road car travels they are not needed. Vortices in F1 are being used as "invisible bodywork", keeping airflows separated - best example is the floor / diffusor. Because of these vortices, little to no air from outside the tires can get under the car , so there is a higher pressure difference and downforce is increased. If you wanted this effect on a street car, you could simply imstall bodywork to do so - it is fail-safe, compared to a fluid/gas (air).
“Strange women lying in ponds distributing swords is no basis for a system of government. Supreme executive power derives from a mandate from the masses, not from some farcical aquatic ceremony!” Monty Python and the Holy Grail
ACJJ619 wrote:So we're saying that they're not all over road cars because sometimes designers don't want the flow attached?
No, far simpler than that - you don't see them on road cars because they don't need them. F1cars are governed by rules that prohibit bodywork in many of the places that it would useful to place bodywork. Vortices can be used to affect the airflow as if bodywork were present. They're used in other ways too of course.
Don't forget that road cars are designed to be fuel efficient and quiet. Smooth airflow is good in this case (even if it results in lift - most road cars generate lift (except some supercars)) because it's quiet and fuel efficient.
Some road cars have had vortex generators on them but they're generally homologation specials used to keep within the rules requiring road car based racers etc.
If you are more fortunate than others, build a larger table not a taller fence.
thepowerofnone wrote:Vortices come in a number of forms - they all obey the same fundamental rules and are technically the exact same but they can be produced in a number of ways and the way in which they are produced gives us some insight into their function/usefulness. I stress again, ALL VORTICES ARE THE SAME PHYSICAL THING, but some are just more useful than others.
The primary distinction between a useful and a useless vortex is how tight the vortex is: a huge vortex moving lots of air but rather slowly takes a lot of energy to produce (and energy taken to generate this vortex is energy lost from our car) but doesn't do us much good, as dynamic pressure, the pressure force which allows us to generate lift, is proportional to velocity squared - these vortices are undesirable; a small but very tight vortex can rotate extremely quickly, producing an area of intense low pressure without actually requiring a great deal of energy to generate - these vortices are extremely useful so long as they remain tight and do not burst.
vortex 1 - a wingtip vortex: this is the kind of vortex turbof1 is referring to in his quote and they are always considered undesirable but there aren't a lot of ways for F1 to get rid of them. For any finite plate, if there is a difference in pressures on the two main surfaces then there is going to be a force encouraging air to spill from one side to the other, causing something called vortex induced drag. The best way to negate these vortices is to either have a very very long wing, like you see on gliders, or to alter your loading so that the wingtips are producing less lift (and so there is a smaller pressure difference between the two sides of the plate). The width of our F1 wing is restricted by the rules and we pretty much want to generate as much downforce as we can so we don't really want to significantly reduce our loading. Endplates try to manage the wingtip vortices, amongst other things, but don't really do an amazing job.
vortex 2 - a 'sealing' vortex: you will see on many modern F1 cars designers intentionally try to seal the underbody, around the back end of the car, with a vortex or more likely a few vortices. The successful application of these vortices is difficult, as if they burst they do much more harm than good, however the idea is to create an area of intense low pressure as a skirt to stop the higher pressure air above your floor spilling over the sides and mixing with the low pressure under your car.
vortex 3 - a lift/downforce vortex: okay so I couldn't think of a good name for this one, they all probably have long, official, technical names but I am trying to explain this in layman's terms so this will have to do. If you look at fighter jets, and Concorde, and any other supersonic aircraft, you will notice that the wings don't really look that much like aerofoils - thats because they don't operate like a normal aerofoil, they are known as a delta wing. Delta wings attempt to generate a few very tight, extremely low pressure vortices to produce their lift - its actually an extremely effective method. F1 uses these underneath the car to generate the underbody lift. You will generate them whenever you have a sharp edge so you will see under wings etc little thin slats which are designed to generate you a vortex. They are particularly interesting for F1 because they generate lots of downforce without really spoiling the air for the following cars, so you would get better racing if a larger percentage of a car's downforce was produced this way. Food for thought...
vortex 4 - a sheet vortex: the viscosity of the boundary layer causes the flow to produce vortices with a regular frequency, as there is a velocity difference between the boundary layer and the free flow. This velocity difference, when viscosity is considered, causes the flow to roll over on itself (imagine what would happen if you turned a treadmill up really fast, too fast for you to run on, then jumped on - airflows undergo these comically hilarious situations all the time). They are generated across the wing, not along it, and they are extremely important for lift, although only when you start talking in technicalities so don't worry too much about why that is. They are identical to what you would see if you stood on a bridge over a fast moving stream and watched the little whirlpools form off of the bridge structures in the water, and in this bridge case they generate every few seconds. On an F1 car in air, they are generated at a much higher frequency.
tl;dr vortices are really really important for both lift and drag; they are also really complicated; some of them are good, some of them are bad - try make the bad ones good if you can.
EDIT: I read this through again and realised just how bad the English was; I have tried to correct it.
While largely correct there is one slight error. Vortices don't just "burst" unless they hit something else that causes them to. They will start to expand and lose energy the moment that they are created. They progressively lose energy until at some point they just become turbulent air.
I think what is confusing him about the drag is situational. Bodywork has angle changes, if the air can flow around these angle changes and still stay attached to the body then all adding a vortex generator would do is increase drag. If however the angle change in the bodywork, such as over the top of some F1 sidepods, is great enough to cause the flow to become detached and turbulent, in most cases, the Cd (coefficient of drag) will increase substantially. This is where a vortex generator can help reduce drag. By placing the vortex generator there it keeps the flow attached to the bodywork and reduces the drag that would have been created had the air become detached from the bodywork. Not down to levels had the angle been low enough for flow to stay attached on its own, but lower than with no vortex generator.
I realise this is an F1 forum but I am primarily talking about road cars.
I would imagine there is a large amount of flow separation at the back of most road cars as the air goes over the roof and doesn't stay attached as it suddenly drops into the rear window. As seen here:
So why don't more cars have generators such as these?
