What is aerodynamics function of front active lip spoiler?

[Jurgen M]

This is question for aerodynamicist, so I put it here in aerospace department.
(Mechanical engineers don't learn aerodynamics at university)

What is aerodynamic function of active front lip spoiler (on video) and what is function of flexibile plate infront of front tyers(picture)?
Why reduce amount of air that goes under the car?

 

[pbuk]

Why reduce amount of air that goes under the car?

Do you see these things on aeroplanes?

 

[jack action]

Mechanical engineers don't learn aerodynamics at university

Yes, they do.

Why reduce amount of air that goes under the car?

The more air that goes under the car, the higher the pressure under the car. This tends to lift the car, which is the opposite of what we want at high speed (i.e. downforce).

Also, if the floor of the car is rough (lots of mechanical components exposed), it creates a lot of turbulence which may add aerodynamic drag. Better direct the airflow over the car which is a better design for drag.

Images source: https://www.buildyourownracecar.com/race-car-aerodynamics-basics-and-design/​

What is aerodynamic function of active front lip spoiler (on video)

As previously said, blocking the airflow is mostly relevant at high speed to help aerodynamic downforce.

This is a rear-engine Porsche 911, so I suspect the floor is mostly smooth, thus turbulence drag is not as much of an issue. All things considered, it's probably more harmful drag-wise to block the airflow and this is why it is removed when not necessary.

what is function of flexibile plate infront of front tyers(picture)?

Exposed rotating tires are one of the worst parts of drag. Covering them up as much as possible reduces drag. The little spoiler in front of them diverts the airflow from the tires.

Mirrors and wheels can account for as much as a 25% increase in aerodynamic drag​

According to Tesla rear-view mirrors are said to increase aerodynamic drag between 3 to 6 percent. Palin goes on to say that wheels can generate an additional 20% of drag as seen in the following flow field diagram from Exa’s PowerFLOW software.

Areas marked in red indicate the highest areas of drag. (Image credit: SAE.org)

“One of the objectives with the Model S was to minimize the airflow around the front wheels and to line up the airflow so it hits the front wheels head on. We needed to avoid air hitting at an angle, as often the side of the tire acts like a bucket that catches the air, producing significant drag. This was an area where we made a huge improvement from the initial concept designs to the final design.”, said Palin.

 

[FactChecker]

As previously said, blocking the airflow is mostly relevant at high speed to help aerodynamic downforce.

Yes, but in these days of fuel efficiency, automakers work to get a small coefficient of friction at all speeds, as you say below.

This is a rear-engine Porsche 911, so I suspect the floor is mostly smooth, thus turbulence drag is not as much of an issue. All things considered, it's probably more harmful drag-wise to block the airflow and this is why it is removed when not necessary.
Exposed rotating tires are one of the worst parts of drag. Covering them up as much as possible reduces drag. The little spoiler in front of them diverts the airflow from the tires.

Good points.

 

[jrmichler]

Why reduce amount of air that goes under the car?

Two reasons - reducing aerodynamic drag and reducing aerodynamic lift. Reducing drag is important if you are interested in either gas mileage or top speed. Reducing lift is important if you want the vehicle to stay on the road at high speed. Both are affected by the front spoiler, also known as an air dam. An excellent source for detailed information is Aerodynamics of Road Vehicles, Fourth Edition, by Wolf-Heinrich Hucho: https://www.amazon.com/dp/0768000297/?tag=pfamazon01-20. It has 918 pages of more than you ever wanted to know about the subject. A sample plot from page 190 in the section on front spoilers:

There is a group of people who are not satisfied with vehicles as designed by the manufacturer, so we modify them. Photo of a truck with a modified front spoiler, grille block, and aerodynamic topper:

If you are curious about the effect of these, and other, modifications on the gas mileage, here's the thread: https://ecomodder.com/forum/showthread.php/modding-06-gmc-canyon-17070.html.

 

[Jurgen M]

View attachment 312910

In aerodynamics there is no free lunch, if you increase downforce you also increase drag.

I don't understand how this simple vertical spoiler reduce drag or reduce lift,because it is just flat vertical plate,there is no flat undertray behind,like on race car that have horizontal front spoiler like on pcture below.
Do you know how it works?

This vertical plate must produce flow separation behind ..

Horizontal flat make sense to produce downforce using high stagnation pressure above .

 

[Jurgen M]

The more air that goes under the car, the higher the pressure under the car. This tends to lift the car, which is the opposite of what we want at high speed (i.e. downforce).

Not completely true, point is in air speed(acceleration).

You can sleep under this car and has downforce in tons.

 

[Motore]

You can sleep under this car.

The front has a flap which increases downforce. Enough that any air that escapes under the car doesn't produce much lift:

What's actually bothering you?

 

[Jurgen M]

The front has a flap which increases downforce. Enough that any air that escapes under the car doesn't produce much lift:

View attachment 312948

What's actually bothering you?

Wrong, 80% of downforce produce tunnels.

 

[russ_watters]

In aerodynamics there is no free lunch, if you increase downforce you also increase drag.

That is only true for an airfoil when changing angle attack. When changing the shape, all bets are off; you can increase or decrease either independently. That's what a spoiler is for.

Wrong, 80% of downforce produce tunnels.

That isn't a sentence.

Where is this going? You got some truly exceptional replies, but you don't seem to be responding to them except to look for exceptions in scenarios that aren't the one you started with.

 

[jack action]

There are 2 ways to look at the airflow under a vehicle. The designs you are showing in posts #6 & #7 will create a low-pressure zone by using a venturi effect. This usually requires careful design of the underbody:

Venturi Tunnels​

Venturi tunnels, much the like a venturi tube observed in a laboratory, use the constriction of a flow to generate high speed, low air pressure areas under the race car. In diagram AD8 below, we show a car with a venturi tunnel, and below that a similar venturi tube you might see in a lab.

Diagram AD8. The Venturi tunnel shape increases the velocity of the mass of air flowing through it, lowering the pressure and generating downforce.​

On race vehicles, the venturi is usually formed by making the undertray of the vehicle shaped like an inverted wing. The distance between the undertray and the road forms a constriction and then expands to enable the low pressure created by the constriction to act along the middle and rear and of the vehicle. Venturis are very efficient devices but are susceptible to changes in vehicle ride height.

Having the front horizontal plate will set the low-pressure zone more towards the front of the vehicle, therefore being able to adjust the front/rear distribution of the downforce.

With the simple vertical plate, there might be turbulence, but there is also a low-pressure zone. On the plus side, the design is a lot less complicated:

Front Air Dam​

A Front air dam is used to prevent air from flowing underneath a vehicle. It does this by creating a “dam” or wall across the front of the vehicle that extends close down to the road and usually along the sides to some extent. This creates an area of vacuum or low pressure underneath the car as shown in diagram AD4 below. This low pressure area, in combination with the higher pressures above the front and top of the vehicle, generates downforce at the front of the vehicle.

Diagram AD4. The front air dam blocks air from going under the vehicle. This creates a low pressure area immediately behind the air dam. Which provides downforce. However, without side skirts, air soon enters from the sides to equalize the pressure underneath the car which diminishes downforce further back.​

If we extend the air dam along the sides of the vehicle to become “skirts”, we can extend the vacuum or low pressure area generated under the vehicle by the air dam as well.

Reducing the ground clearance will help you reduce the underbody pressure just like a front air dam does. Lowering it more at the front axle than at the rear axle will get you closer to a venturi tunnel design.

 

[Jurgen M]

With the simple vertical plate, there might be turbulence, but there is also a low-pressure zone. On the plus side, the design is a lot less complicated:

How low pressure is behind dam, if air velocity is reduced because dam represent blockage to air?

If we completely seal front with air dam and both side with side skirts, air velocity will be close to zero, so here will be freestream pressure.
Isnt it?
Or must be some gap between dam and road, to make dam work correctly?

Isnt the goal to increase air speed as much as you can , not decrease?

 

[russ_watters]

How low pressure is behind dam, if air velocity is reduced because dam represent blockage to air?

There's always a pressure drop across an obstruction. If you're thinking about a Venturi, this is different.

If we completely seal front with air dam and both side with side skirts, air velocity will be close to zero, so here will be freestream pressure.
Isnt it?
Or must be some gap between dam and road, to make dam work correctly?

But leave the back open? Then the pressure is whatever reduced/negative pressure there is at the back.

Isnt the goal to increase air speed as much as you can , not decrease?

No, the goal is to reduce pressure/lift. There's more than one way to do that.

 

[Baluncore]

When the vehicle is being driven in a cross wind, side skirts significantly reduce drag, and so increase fuel economy. Then the side skirt reduces the drag of the under-body and rear wheels.

 

[jack action]

How low pressure is behind dam, if air velocity is reduced because dam represent blockage to air?

Because the air can (and will) escape at the rear faster than it can enter at the front. Reducing the presence of air molecules reduces the pressure.

If we completely seal front with air dam and both side with side skirts, air velocity will be close to zero, so here will be freestream pressure.
Isnt it?

Velocity will be zero relative to the ground but not to the moving vehicle. So if it is perfectly sealed, the air will completely escape from under the vehicle as it moves, leaving a perfect vacuum. Of course, in real life, this vacuum tends to pull the air back in from the rear (and other little openings), hence the presence of those turbulences.

Isnt the goal to increase air speed as much as you can , not decrease?

The goal is to reduce the pressure. Removing air molecules does that too. Here is a good example of that: Seal the underbody of the car and pull the air from underneath with fans.

 

[Jurgen M]

Two reasons - reducing aerodynamic drag

How drag is reduced if we can see (and expect) that air dam will increase pressure at front part of car plus will add higly turbulent airflow behind dam?
Any forward faceing plate will add more drag,I mean this is pure logic.

No, the goal is to reduce pressure/lift. There's more than one way to do that.

Obviusly this is very bad way to make downforce, there is no any raceing car in the world that use this "blockage way" to produce downforce.

 

[russ_watters]

How drag is reduced if we can see (and expect) that air dam will increase pressure at front part of car plus will add higly turbulent airflow behind dam?
Any forward faceing plate will add more drag,I mean this is pure logic.View attachment 312961

Where did you get those photos? What is the car that's being modeled there? The photos are so small it is hard to see the shape of the air dam. Anyway, the logic has already been explained, but you didn't respond to it so it isn't clear to me that you saw it. The drag from an open grill isn't at the grill it's inside the engine compartment. And the drag from air flowing under the car is from obstructions under the car and from the road. It isn't at all clear to me if those are being modeled in those photos.

And again, it's highly dependent on the specifics of the modification if it will increase or decrease drag while increasing downforce/decreasing lift.

Obviusly this is very bad way to make downforce, there is no any raceing car in the world that use this "blockage way" to produce downforce.

That isn't true. NASCAR uses them.

Btw, I don't think anyone answered this:

What is aerodynamic function of active front lip spoiler (on video) and what is function of flexibile plate infront of front tyers(picture)?

It's part of the air dam, but is flexible so you don't damage it if the front of the car bottoms-out.

 

[jack action]

Obviusly this is very bad way to make downforce, there is no any raceing car in the world that use this "blockage way" to produce downforce.

People at Bonneville all do it this way and their only goal is to go as fast as possible!

​

But it is used with other types of race cars too:

 

[berkeman]

Obviusly this is very bad way to make downforce, there is no any raceing car in the world that use this "blockage way" to produce downforce.

Please take care not to make unsubstantiated false claims like this on PF. As the replies above show, you are wrong in making this assertion. You have done this several times already in this thread; please do not do it again. Thank you.

 

[russ_watters]

But it is used with other types of race cars too:

​

While I'm sure it is at least partly for show, it is ridiculously common on street cars as well.

https://vehq.com/do-air-dams-increase-mileage/

 

[jrmichler]

The OP has received several good answers to the original question, but refuses to believe them or to follow the given links. Enough is enough, so this thread is locked.