Calculate pressure from vehicle moving through air

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Discussion Overview

The discussion revolves around calculating the air pressure exerted on a vehicle moving through air, specifically focusing on a car's frontal area and its speed. Participants explore various formulas and concepts related to drag and stagnation pressure, aiming to understand the forces acting on the vehicle while it travels at a constant speed.

Discussion Character

  • Exploratory, Technical explanation, Debate/contested

Main Points Raised

  • One participant seeks to calculate the air pressure against a vehicle based on its weight, speed, and frontal area, expressing a desire for the answer in multiple units.
  • Another participant suggests using drag equations to find the necessary calculations.
  • A participant mentions the concept of stagnation pressure as a simpler method to estimate the pressure on the vehicle's flat surface.
  • There is a discussion about the applicability of stagnation pressure, with one participant clarifying that it applies to any moving fluid, not just pipes.
  • One participant expresses confusion about the relationship between drag coefficients and sectional areas, indicating a need for clarity on how to apply these concepts to their specific scenario.
  • Another participant agrees with the use of stagnation pressure and suggests that it can provide a rough estimate of the pressure on the vehicle's frontal area.
  • There is a mention of using a drag coefficient for a flat plate, with acknowledgment that it may lead to an overestimate of drag.

Areas of Agreement / Disagreement

Participants do not reach a consensus on the best approach to calculate the air pressure, with multiple competing views on the use of stagnation pressure versus drag equations. The discussion remains unresolved regarding the most accurate method to apply.

Contextual Notes

Participants express uncertainty about the definitions and applications of drag coefficients and stagnation pressure, indicating potential limitations in their understanding of these concepts in the context of their calculations.

Cbrown92
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Alright, I've been looking everywhere, and tried to use a few formulas but they don't answer what I'm trying to find. Let's say I have a car that weighs 1815kg )4000 pounds) and its traveling at 90kph (roughly 56 mph) with a flat one square meter (10.76 sq ft.) grill facing the wind as it travels. What is the air pressure (In pascals and psi) I can expect to generate against the vehicle as it moves? Though providing me the answer in Newtons wouldn't hurt either.Also, the vehicle is moving at a constant rate assuming a flat road surface. I'm trying to invent a new kind of engine based on relatively low air pressures
 
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What formulae did you try? It's not to hard to find one.
 
I tried F=MA in relation to P=F/A but the vehicle isn't accelerating, its moving at a constant rate. Unless I'm confused about the definition of acceleration itself? Is there anything I'm missing?
 
You need to look at drag equations...

http://en.wikipedia.org/wiki/Drag_(physics)

eg..

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If the front is flat, he doesn't even need to concern himself with the drag equation. He can get a good enough estimate just using the concept of stagnation pressure.

So, in short, just look up "stagnation pressure."

You might also look up "stagnation flow" if you want a more accurate estimate. That would require an integral of the pressure distribution over your flat surface, but that's pretty trivial.
 
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Sorry for having taken so long to reply, but if I use a drag coefficient, that accounts for the drag in its entirety not a specific sectional area. Stagnation pressure, from what I understand applies to pressure in pipes. I guess what I'm trying to find out is the amount of drag on that single plate of which measures 1sq meter in area at speed, and then how to transition that into pressure and calculate for higher and lower speeds. Think of something about the shape of a Cadillac grille in the front (Similar size even) while ignoring the drag/pressure on the windshield, sides of the car, etc.
 
Stagnation pressure applies to any moving fluid, not just pipes. I wouldn't have suggested it if it didn't make sense to use it. Stagnation pressure is just the pressure that results from isentropically slowing the flow to zero velocity. Just calculating the stagnation pressure at the front of a car moving a given speed is going to be a rough estimate of the pressure on the given size of plate. If you want a better estimate, then like I said before, stagnation flow would give you a more accurate estimate.
 
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Sorry for having taken so long to reply, but if I use a drag coefficient, that accounts for the drag in its entirety not a specific sectional area.

Not sure what you mean but I was proposing to use the drag coefficient for a flat plate not the whole car (I agree that using the drag coefficient for a flat plate would probably give an over estimate).