# Questions involving airflow and pressure around a car

• Matt Poirier
In summary, as the roadster travels, the airflow around its windshield must bend towards the front edge of the driver's side window. This bending airflow creates a low pressure area just outside the front portion of the window. As the airflow continues along the driver's side window, it straightens out and creates a high pressure area just outside the rear portion of the window. This pressure arrangement can cause objects, such as gum, to be blown into the rear portion of the window. The roadster's tail and top surface also play a role in its aerodynamics, with the tail needing to be designed to minimize the size of the turbulent air pocket behind the car and the top surface needing to be shaped to avoid creating an upward aerodynamic force that could
Matt Poirier

## Homework Statement

1.Since the roadster seats only two people, it has only two side windows--one on each side. Each window is aligned squarely on the vehicle, so air can flow straight past the window as the vehicle heads forward. However, the air flowing around the roadster's windshield has to bend toward the front edge of the driver's side window in order to flow toward the back of the car. How does that bending airflow affect the air pressure just outside the front portion of the side window?
Select one:
a. The pressure near the outside front portion of the window must be high, greater than atmospheric.
b. The pressure near the outside front portion of the window is atmospheric pressure because bending the airflow has no effect on its pressure.
c. The pressure near the outside front portion of the window must be low, less than atmospheric.
d. The pressure near the outside front portion of the window is atmospheric pressure because there is nothing separating this airflow from the open air around it.
2.
As the airflow continues along the driver's side window, it stops bending and travels straight past the rest of the window. How does that straight flow of air affect the air pressure just outside the rear portion of the side window?
Select one:
a. The air is inertial, so its pressure is has a gradient across it. Its pressure is lower near the window than it is far from the window.
b. The air is inertial, so its pressure is uniformly atmospheric pressure.
c. The air pressure is higher than atmospheric because fast moving air is always high pressure air.
d. The air pressure is lower than atmospheric because fast moving air is always low pressure air.
3.
Suppose that this pressure arrangement just outside the driver's side window is unchanged by opening the window. You jokingly suggest putting a sign on the window, warning people not to throw their chewing gum out the front portion of the open window because it will reenter the rear portion of the open window. You are correct, of course. Why would gum (or anything else) be blown into the rear portion of window?
Select one:
a. Air flows out of the rear portion of the window because the air pressure outside that rear portion is less than atmospheric. Gum moves opposite the air, so it travels into the rear portion of the window.
b. Air flows out of the front portion of the window because the air pressure outside that front portion is less than atmospheric. Air (and gum) flow into the rear portion of the window to replace the air that left through the front.
c. Air flows out of the front portion of the window because the air pressure outside that front portion is greater than atmospheric. Air (and gum) flow into the rear portion of the window to replace the air that left through the front.
d. Air flows out of the rear portion of the window because the air pressure outside that rear portion is greater than atmospheric. Gum moves opposite the air, so it travels into the rear portion of the window.
4.
The other designers are tinkering with various shapes for the roadster's tail. They aren't worried about how those designs would affect the roadster's air resistance because they assume that air resistance is determined only by the shape of the roadster's nose. You set them straight, pointing out that the roadster's tail does matter. How should the roadster's tail be designed in order to minimize its air resistance.
Select one:
a. The roadster's tail should be as long as possible.
b. The roadster's tail should maximize the size of the turbulent air pocket that forms behind the roadster.
c. The roadster's tail should be as short as possible.
d. The roadster's tail should minimize the size of the turbulent air pocket that forms behind the roadster.
5.
The top surface of the roadster is also important. One of the proposed designs has a top surface that arcs like a rainbow from nose to tail and a bottom surface that is essentially flat. You point out to the design team that the air flowing past that design will tend to lift the roadster off the roadway and ruin its traction. Why would the roadster develop this upward aerodynamic force?
Select one:
a. Air bends away from the top of the vehicle, so the pressure above the roadster is higher than atmospheric. The air pressure below the flat bottom of the roadster is essentially atmospheric, so there is an overall upward pressure force on the roadster
b. Air bends away from the top of the vehicle, so the pressure above the roadster is lower than atmospheric. The air pressure below the flat bottom of the roadster is essentially atmospheric, so there is an overall upward pressure force on the roadster.
c. Air bends toward the top of the vehicle, so the pressure above the roadster is lower than atmospheric. The air pressure below the flat bottom of the roadster is essentially atmospheric, so there is an overall upward pressure force on the roadster.
d. Air bends toward the top of the vehicle, so the pressure above the roadster is higher than atmospheric. The air pressure below the flat bottom of the roadster is essentially atmospheric, so there is an overall upward pressure force on the roadster.
6.
You make several design changes that result in a roadster that experiences a downward lift force several times stronger than its weight. The roadster now hugs the road like a Formula One racecar. Air presses this roadster so strongly against the surface on which it is driven that it can actually be driven upside-down on the ceiling of a tunnel. While the roadster is driving on the ceiling of the tunnel, how does it affect the passing airstream?
Select one:
a. The roadster will leave the passing airstream undeflected and that airstream will remain a single stream.
b. The roadster will leave the passing airstream undeflected, but that airstream will be separated into two halves.
c. If the roadster pushed the airstream upward, the airstream would push the roadster downward.
d. The roadster will push the passing airstream downward (toward the floor of the tunnel).

## The Attempt at a Solution

1. C, the airflow has to be less than atmospheric, the pressure is high when the air hits the front windshield, and therefore decreases along the gradient.
2. B, since the air isn't bending at all, it must be at atmospheric pressure.
3. A, I am really not sure, I am guessing a pressure gradient is involved to make air flow out the rear
4. D, minimizing the turbulent wake minimizes pressure drag forces.
5.C, it acts like a wing, air bending toward the wing is less than atmospheric, pressure at the bottom in at atmospheric,
6. D, the air pushes the roadster up into the ceiling, so the car must also be pushing the air downwards via Newtons 3rd law pair.

Two thoughts...

'Spoilers' may control vortex production, reducing over-all drag and improving handling in rapidly changing cross-winds. Like when overtaking a truck, or passing gaps between buildings...

"Don't throw that apple-core out---"
But, out the *front* of the open window it went, only to return over my passenger's shoulder and land on the back seat...

Nik_2213 said:
Two thoughts...

'Spoilers' may control vortex production, reducing over-all drag and improving handling in rapidly changing cross-winds. Like when overtaking a truck, or passing gaps between buildings...

"Don't throw that apple-core out---"
But, out the *front* of the open window it went, only to return over my passenger's shoulder and land on the back seat...
,
Nik_2213 said:
Two thoughts...

'Spoilers' may control vortex production, reducing over-all drag and improving handling in rapidly changing cross-winds. Like when overtaking a truck, or passing gaps between buildings...

"Don't throw that apple-core out---"
But, out the *front* of the open window it went, only to return over my passenger's shoulder and land on the back seat...
Ok, we're not dealing with spoilers in our class, and I don't really understand what the 3rd question is asking, how did you read that question? And my revised choice then would be B, and wouldn't that mean that the air in question 1 is below atmospheric?

Matt Poirier said:
1. C, the airflow has to be less than atmospheric, the pressure is high when the air hits the front windshield, and therefore decreases along the gradient.
I agree with choosing C, but I do not follow your reasoning. If the pressure at the windscreen is high, then decreases as it slides off the windscreen, it could still be higher than atmospheric. Can you think of a more convincing argument?
Matt Poirier said:
2. B, since the air isn't bending at all, it must be at atmospheric pressure.
I don't see that failing to bend in some locale implies [edit: removing "less than"; I did not mean that] atmospheric there.
Note that each option specifies the reason as well, so you need to match on that.

For 3, air definitely flows out in the forward portion of the window, right? What is the consequence for the mass of air within the vehicle?

Last edited:
Matt Poirier said:
4. D, minimizing the turbulent wake minimizes pressure drag forces.
Are you sure? Where did you get that from?

I think you are right on 5 and 6.

haruspex said:
Are you sure? Where did you get that from?

I think you are right on 5 and 6.
Thank you for your replies, so for 4, I think it could be as long as possible, but the smaller the air pocket behind the car, the better, which is why I chose D, but the point at which air separates from the car should be as far back as possible, which aligns with A, so I am not sure. For 2, I suppose it could be less than atmospheric pressure (D), since it is moving quickly, which would also imply answer B for question 3, as high pressure air in the car exits so lower pressure outside, how does that sound? My argument for 1, is the air has to bend towards the car, not away, which means it's less than atmospheric, air that bends away from the surface is greater than atmospheric.

haruspex said:
Are you sure? Where did you get that from?

I think you are right on 5 and 6.
haruspex said:
I agree with choosing C, but I do not follow your reasoning. If the pressure at the windscreen is high, then decreases as it slides off the windscreen, it could still be higher than atmospheric. Can you think of a more convincing argument?

I don't see that failing to bend in some locale implies less than atmospheric there.
Note that each option specifies the reason as well, so you need to match on that.

For 3, air definitely flows out in the forward portion of the window, right? What is the consequence for the mass of air within the vehicle?
I'm thinking about question 2, what's your reasoning for the air being less than atmospheric? just because the air is fast doesn't mean that air is less than atmospheric pressure, it is based on relative speed along the streamline, so I am actually thinking that it is A, the air is inertial and since it just had lower than atmospheric pressure air as it bent around the window, its pressure starts low but becomes higher as it moves across the window.

Matt Poirier said:
the smaller the air pocket behind the car, the better,
How are you defining the size of that pocket? The car has a certain cross section. In the region behind the car, some of that corresponds to turbulent air and some to laminar flow. The smaller the region of turbulent, the greater the laminar region.
Matt Poirier said:
question 2, what's your reasoning for the air being less than atmospheric?
Sorry, I mistyped that comment. I did not mean to suggest I thought it was less than atmospheric (though I think it might be). I only meant to point out that your answer there did not seem to match any of the options because your argument did not match the reason given in the option you selected. Have now edited that comment.
Here's a question: what does fast mean here, relative to the vehicle or relative to the ambient air? If comparing with atmospheric pressure, which of those is the more interesting?
If an air mass includes slow and fast streams, what is the usual relationship between the pressures in them?

haruspex said:
How are you defining the size of that pocket? The car has a certain cross section. In the region behind the car, some of that corresponds to turbulent air and some to laminar flow. The smaller the region of turbulent, the greater the laminar region.

Sorry, I mistyped that comment. I did not mean to suggest I thought it was less than atmospheric (though I think it might be). I only meant to point out that your answer there did not seem to match any of the options because your argument did not match the reason given in the option you selected. Have now edited that comment.
Here's a question: what does fast mean here, relative to the vehicle or relative to the ambient air? If comparing with atmospheric pressure, which of those is the more interesting?
If an air mass includes slow and fast streams, what is the usual relationship between the pressures in them?
We only care about its speed relative to ambient air, more specifically, air along the same streamline. Faster equals less pressure, but I'm still pretty confused for that question.

Matt Poirier said:
My argument for 1, is the air has to bend towards the car, not away, which means it's less than atmospheric
That is ok, but it's not what you wrote originally.
Matt Poirier said:
for question 3, as high pressure air in the car exits so lower pressure outside
Why is the pressure in the car higher than outside at the trailing part of the window?
You did not answer my question: if air is leaving the car in the forward part of the window, what is the consequence for the air still in the car?

Matt Poirier said:
We only care about its speed relative to ambient air, more specifically, air along the same streamline. Faster equals less pressure, but I'm still pretty confused for that question.
Sounds right to me. It follows from Bernoulli, no?

haruspex said:
Sounds right to me. It follows from Bernoulli, no?
Sure does,

haruspex said:
That is ok, but it's not what you wrote originally.

Why is the pressure in the car higher than outside at the trailing part of the window?
You did not answer my question: if air is leaving the car in the forward part of the window, what is the consequence for the air still in the car?
That means air in the back would have to replace the leaving air in the front, I suppose.

Matt Poirier said:
That means air in the back would have to replace the leaving air in the front, I suppose.
Air from the back of the car? So what replaces that?

haruspex said:
Air from the back of the car? So what replaces that?
Only thing that makes sense would be higher pressure air from somewhere (outside), but the air outside is lower pressure, no?

Matt Poirier said:
Only thing that makes sense would be higher pressure air from somewhere (outside), but the air outside is lower pressure, no?
Lower than what?

haruspex said:
Lower than what?
Lower than the pressure inside the car, unless there's a gradient that forms where the outside air pressure becomes higher as the air moves along the side of the car.

Matt Poirier said:
Lower than the pressure inside the car
How do you know the pressure outside is lower than the pressure inside?
We agree air is leaving the car at the forward part of the window. If no air comes in elsewhere, what will happen?

haruspex said:
How do you know the pressure outside is lower than the pressure inside?
We agree air is leaving the car at the forward part of the window. If no air comes in elsewhere, what will happen?
It becomes a vacuum, so then the pressure outside must be higher than pressure inside, but does that not contradict our findings in question 2, where it was implied fast moving air is lower in pressure?

Matt Poirier said:
does that not contradict our findings in question 2, where it was implied fast moving air is lower in pressure?
That's within the same streamline.

haruspex said:
That's within the same streamline.
Ok, then I am assuming my last assumptions are correct.

Matt Poirier said:
Ok, then I am assuming my last assumptions are correct.
Which were?

## 1. What is aerodynamics and how does it affect a car's performance?

Aerodynamics is the study of how air flows around objects and how it affects their motion. In terms of cars, aerodynamics plays a crucial role in determining a car's performance, particularly in terms of speed, handling, and fuel efficiency. A car with good aerodynamics will experience less drag and resistance, allowing it to move more efficiently and quickly through the air.

## 2. How do you measure the aerodynamic efficiency of a car?

The aerodynamic efficiency of a car is typically measured by its coefficient of drag (Cd). This is a unitless number that represents the amount of resistance a car experiences as it moves through the air. The lower the Cd, the more aerodynamically efficient the car is.

## 3. What factors influence a car's aerodynamics?

Several factors can impact a car's aerodynamics, including its shape, size, weight, and design features such as spoilers, air dams, and side skirts. The angle and placement of these features can also affect the way air flows around the car.

## 4. How does air pressure affect a car's performance?

Air pressure can have a significant impact on a car's performance, particularly in terms of handling and stability. When air flows over a car, it creates an area of high pressure in front and low pressure behind. This difference in pressure can affect the car's steering and can even lift the car off the ground at high speeds, causing it to lose traction and stability.

## 5. What is the difference between laminar and turbulent airflow around a car?

Laminar airflow is smooth and steady, with air moving in parallel layers. This type of airflow creates less drag and is more efficient. On the other hand, turbulent airflow is chaotic and can create more drag and resistance. Cars with good aerodynamics are designed to minimize turbulent airflow and promote laminar airflow for better performance.

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