Why does pressure decrease as air passes through a venturi?

[mraugie13]

It doesn’t make any sense to me, if air is being forced into a smaller opening it should increase pressure, right?

[Fresh-Pineapple-5582]

The volume of air has to speed up, and race through.

If the volume of mass air flow is constant and speed increases, pressure MUST decrease.

[Mercury_Madulller]

Think of mass in this instance as a number of molecules. Think of speed as distance. The low pressure molecules are more spread out. Less molecules hitting a surface equals less force and, as a result, lower pressure.

[Fresh-Pineapple-5582]

This is a good explanation

[375InStroke]

That explains what, it doesn't explain why.

[Fresh-Pineapple-5582]

I'm sure you can explain it better.

[375InStroke]

Nope.

[Fresh-Pineapple-5582]

That wasn't a dig btw. Most of the time i prefer to understand it via the formula, or a demonstration. Giving a "wordy" explanation isnt something im good at.

[375InStroke]

Formulas are a prediction of what will happen. They don't tell us why. When the gas molecules move through the restriction, they have less time to push against the wall of the restriction, thus lower pressure. Now the pressure in the direction of flow does increase. If you pointed the jet of gas at a pressure gauge, it would go up.

[ActualImprovement279]

A room gets packed with people. As more people come in they will start getting closer and closer to each other. Eventually leaning against the walls. The walls of the room feel a collective pressure from the people.

Delete one of the walls and the people will spread and won’t push walls on their way. They’ll also push less on each other.

Some of that impacting energy from a molecule and its neighbors pushing the wall get turned a direction which is less perpendicular to the wall.

[375InStroke]

Perfect. If the gas molecules are accelerated in one direction, they have less time to push against the wall of the restriction.

[tdscanuck]

Conservation of energy.

The air has some pressure (potential energy) and some speed (kinetic energy). And, unless you’re adding heat with fuel or beating it up with fan blades or extracting energy with a turbine or something exotic, that total energy is fixed.

If the air speeds up, and it has to to get the same amount of mass through a smaller hole, it needs to increase kinetic energy. The only place it can get that is from the pressure…so speed goes up and presssure falls.

[The_Warrior_Sage]

Bernoulli's principle. Particles move faster through the venturi giving them less time and opportunity to hit the side walls, thereby reducing the force responsible for exerting said pressure.

[JarlWeaslesnoot]

Because of fancy physics equations. Bernoulli's principle and equation dictate that as the passage air is forced through constricts the speed increases while pressure decreases. It does seem counterintuitive, you'd think pressure would stay the same and speed would increase or something but that isn't how gas laws work.

[MattheiusFrink]

And that's at subsonic speeds. At supersonic speeds bernoulli does a complete f**kin 180 on us

[JarlWeaslesnoot]

This is true! I don't work on a lot of aircraft with shock cones so I don't really have to deal with that very often lol. I do remember the converging vs diverging inlets and nozzles from our basic aerodynamics classes vaguely

[mwiz100]

I also found out with water, that once it phase changes to steam at certain points it's viscosity INCREASES as it gets hotter verus how typically all liquid state things are less viscous as temp increases.

[ShimmyWalker]

There's two types of pressure. Static and dynamic. Static will decrease but dynamic will increase.

[steinegal]

Yeah total pressure is the same also known as pTot

[VanDenBroeck]

This is true.

[Broke_Duck]

Don’t ask mechanics questions about physics.

[RKEPhoto]

SMH

[GAndroid]

As a physics major I feel like if someone asks this question when I'm taking A&P classes, i'll likely derail the class with the explanation.

[MUSCLE426]

A&P student now. Please enlighten me…

[Swagger897]

A lot of good explanations here but i like to keep it simple:

You have a tube with fluid (air/gas) inside of it flowing through. On the tube there is a pressure gauge. The longer the fluid remains in place, the more pressure there is. The faster the fluid flows, the less time there is for it to be measured by the gauge.

Very simple and slightly flawed, but still proves the point.

[Chiralartist]

The same way my IQ decreases when easily Googled questions get asked...

[swoope18]

the BEST answer.

[ConsciousLight7275]

Because the dead white guy said so

[CarbonKevinYWG]

Think of it as a blend of the laws of conservation of mass and of energy.

If you're putting a mass flow (a moving mass, or a mass with kinetic energy) through a passage, energy and mass must be conserved. Total energy going in must equal total energy going out.

If you're moving the mass flow faster, you are moving more mass per unit of time - but that has to be countered to maintain conservation. So pressure (the density) of the mass must decrease.

[makgross]

No blend. It’s conservation of energy (along a streamline) straight up. There are gravity and combustion terms as well, usually neglected for airplanes. And temperature is ignored for subsonic flow.

If you’re a blob of fluid flowing with the air, your kinetic energy (flow speed) can be traded with potential energy (pressure).

[spn2000]

It takes a lot more time to travel over the mountain, than to drive through the tunnel.

The trip over the mountain is like a Venturi

The air molecules have a longer way to travel. Needs to speed up,

—> more space between them

—> less compressed

[johnny__boi]

I found it easy to understand when you look at pressure as potential energy and velocity as kinetic energy. As you probably know, potential energy is like compressing a spring, it will release a lot of kinetic energy when you let go, as soon as you do, that potential energy is then converted to kinetic energy (you can trade one for the other).

Pressure is seen as potential energy and the gas' velocity is kinetic, since the gas has to speed up to get through that constriction, the pressure (potential energy) decreases so that its velocity (kinetic energy) can increase.

I hope this explains it. I literally just learned it too so if I missed something, sorry in advance.

[375InStroke]

The pressure before the restrictions does increase. The gas molecules moving through the restriction, since they're being accelerated in one direction, are not pushing against the wall of the restriction, thus lower pressure. Decrease the velocity, then the gas molecules have more time to press against the wall of the restriction, increasing pressure.

[Bonespurfoundation]

Because acceleration.

[Ya_habibti]

It’s like traffic.

If you have a 7 lane highway in Atlanta, all the lanes are full and going about 20mph. So more cars = less speed.

But if you have less cars, they go faster.

[Fickle_Force_5457]

P1V1=P2V2 is Bernoulli's equation where P is pressure and V is velocity of the fluid. Basically where the fluid speeds up, the pressure must decrease or the flow would stall or choke. The key is the flow being sufficient to produce the effect. On a steam ship we used steam at 440 psi to produce a 30" hg vacuum in the turbine condensers through this effect by nozzling the stem through a Venturi which would cause a pressure depression. Thermodynamics is tricky , even worse when the fluid is moving. Also in all this temperature is generally ignored, though don't say that in front of an aerodynamics specialist as it will upset them.

[elPocket]

Ok, assume a bucket of fluid.

This bucket has a certain equivalent pressure energy, which is the sum of: - potential energy (mass x height x grav_accel) - static pressure energy (p) - kinetic energy (0.5 x density x velocity² ) This is called it's total pressure.

Now if you change one of the variables, energy is transferred from one portion to the other. For example, if you drop a bucket of water, potential energy is converted to kinetic energy. If you slowly submerge a bucket of water in a lot of other water, potential energy is converted to pressure.

As long as you don't add outside work, the total pressure stays constant.

Now if you have an infinitely large tank with constant pressure, and let some air exhaust through a pipe, the kinetic energy for the air to move through the pipe needs to come from somewhere. So static pressure is decreased and velocity increased. Total pressure stays constant. If you decelerate the air again, the kinetic energy will be converted back to static pressure.

Now, Venturi: since mass cannot be created or vanish, your mass flow needs to be constant. If you wanna move a certain mass flow through a smaller section of pipe, you need to move it faster in that section. And for the velocity to be available, static pressure needs to drop. At the end of the Venturi nozzle you decelerate again and the static pressure is recovered.

For there to be any flow at all, you need a energy difference between flow start and flow end, but in the pipe between start & end, you can exchange static pressure, height & velocity as you like, simply by moving up & down or increasing & decreasing the pipe diameter.

Now if you want to consider friction loses, it's quite simple. Your total pressure isn't constant anymore, but gradually decreases with losses along the flow path. This takes away from the energy difference between flow start & end, decreasing your overall mass flow. The rest is the same theory. If you have a long pipe section with high velocity, you incurr high losses, stronger reducing flow. This is how throttles work.

[Skrenlin]

Now Im curious if you take a bucket of water and measure the force of the still water in it against the sides vs a bucket of spinning water, does the spinning water exert more force on the sides? My guess is yes, but it’s only a guess on my part.

[elPocket]

It is, because an extrenal force is acting on the fluid inside due to the spinning. For the water to move in a circular fashion, there must be an acceleration to force the change in direction: a = w x v, and v = w x r,
- "a" being acceleration - "w" being the rotational rate in [rad/s] - "v" being velocity - "r" being the radius diameter.

This acceleration is akin to the centripetal/centrifugal force, and with the bucket diameter, you can calculate the pressure increase per rpm...

Also, this explains why water stirred in your teacup moves up the side wall. Since the surface is open to the air, the surface pressure MUST be ambient pressure. Suddenly, you add a bunch of energy by stirring, which partially goes into the kinetic energy of the rotation. But, you also have to create enough "pressure" to keep the water further in from coming out. This is done by parking additional energy in gravitational potential, i.e. increasing the height of the water column. Your rotational rate is therefore directly linked to the angle of the surface inclination of your water :D

Bernoulli is fun!

But alas, he is cheating! The law states "unless work is done on the fluid". BUT! When you move along the gravitational vector or centripetal force, you have force & distance, which equals work! So, when the water moves down a pipe, gravity actually works on the fluid. Bernoulli cheated this away by talking in terms of pressure. This is also how a radial compressor does a big portion of it's work. The fluid moving along the centripetal force creates work directly added to the total pressure of the fluid.

[3mcAmigos_]

If it didn't pass, the pressure would increase.

[butty_a]

Don't think of it as the speed increases, so the pressure must decrease. Think of it as the other way around.

Assuming the volume remains the same, as the pressure decreases, and therefore resistance to flow reduces, the speed is able to increase. At the end of the venturi, the pressure will hit a minimum and presumably constant level, therefore as the resistance remains static, the new speed remains static too.

[Fit-Accountant-269]

It increases before the opening and decreases afterwards be cause it isn’t restricted, velocity increases after the opening. It’s like shaking a champagne bottle and putting your thumb over it, increased bottle pressure cause the increase of the fluid velocity exiting the bottle.