If you're seeing this message, it means we're having trouble loading external resources on our website.

If you're behind a web filter, please make sure that the domains *.kastatic.org and *.kasandbox.org are unblocked.

### Course: Physics archive>Unit 9

Lesson 3: Fluid Dynamics

# Venturi effect and Pitot tubes

The Venturi effect demonstrates how fluid speed increases and pressure decreases in constricted areas. This video breaks down Bernoulli's equation, revealing the surprising principle that faster fluid equals lower pressure. It also introduces the Pitot tube, a practical tool for measuring fluid velocity.

## Want to join the conversation?

• The Venturi effect and our circulatory/venous system seem to directly defy each other.

When we need to increase our blood pressure our veins constrict, and when we want to decrease blood pressure our veins dilate. This is completely opposite of what the Venturi effect is stating. Can someone shed some light on this please?
• Blood flow in our bodies are turbulent, unsteady (pumping nature of heart) and does not agree with assumptions we have adopted to derive Bernoulli's laws. Hence this behavior. Please correct me if I am wrong.
• At on the top of tube where hole is at right angle, doen't some air flow in by turbulence or something?
• I think the previous answer may be misleading: volume is not a conserved quantity because air is compressible. It is unnecessary for the air to leave at the same rate. The pressure in the tube will rise when turbulent flows enter from the side. These flows, however, are typically very minor because pitot tube is used in laminar flow regimes. In turbulent flows, a correction factor is typically used, as well as signal processing and time averaging of measurements. This works because turbulent flow regimes are oscillatory in nature, and averaging over time will yield net pressure. The correction factors account for the fact that the particle velocity in fluid flow regimes may be faster than the free stream velocity, as they move along curved paths.
• how can you say that air inside the pitot tube(Stagnation chamber ) is now moving.the whole PITOT table has some velocity with respect to ground...........
• and the earth is spinning on an axis, which is orbiting around the sun, which is moving around in the galaxy.... etc.

You pick a frame of reference and set it to zero. So your point of observation is from the airplane, and you are just observing the speed of the air with respect to the airplane. (since the flow of air over the wing is really what is important anyways). so the air in the pitot tube is not moving with respect to the plane
• Why in capillaries do you have much lower pressure and lower velocity as opposed to arteries where you have much higher pressure and higher velocity.
• Just a clarification then. For Venturi's Effect, there will be a decreased pressure at a location of smaller radius, but the volume won't change, because fluids are incompressible.

I'm guessing this would be different for gases though, because then Boyle's Law would come into effect P1V1 = P2V2?
• they are two separate sets of pressures

1. in Venturi's effect (and formulas for fluid in general)
pressures are acting upon the fluid from outside (e.g. from the atmosphere left and right side of the fluid)

2. in Boyle's law (and formulas for gas in general)
pressures are acting by the gas to the surface around it (e.g. the walls of a container or cylinder)

3. actual cases
1) for the first case, volume won't change as you said even though the pressure from outside to the fluid is decreasing (cause they are not "directly" related)
2) for the second case, when volume is constant, pressure is too. no change. and when volume of gas increases, pressure by the molecules inside gas toward the walls decreases. (cause the same number of them should push away a larger area with same force)

in short, two pressures are different. not the fundamental natures of two concepts (fluid and gas) are
• doesn't air from higher pressure flow toward lower pressure regions? Then, how can there be stagnant air at all?
• Can someone solve this question? :
A pitot tube is mounted on an airplane to measure the speed of the plane. The tube contains alcohol and shows a level difference of 40 cm. What is the speed of the plane relative to the air? Given that specific gravity of alcohol=0.8, density of air=1kg/m^3, g=10m/s^2.
• So if we start with the equation P,s = P,1 + 1/2qv^2 where q = roe we can rearange it to sqrt((2*P,s-P,1)/q) then the pressure difference is equal to the change in height of the alochol times its density times gravity. Basically deltaP = 800 kg/m^3 * 10 m/s^2 * 0.4m this is equal to 3200 pascals. We can subsitute this for P,s-P,1 in the firsty equation getting sqrt((2*3200)/airdesnity) = v this resolves as sqrt(2*3200/1000) = 80 m/s
• Is there a reason that the Pitot tube would not be mounted to the object (for which it was to determine velocity) with the cross section in the video's diagram in the horizontal dimension? David said that the height difference from end to end is negligible, but why not minimize it?
(1 vote)
• Good question. You're on the right track - if you want to, you can rotate the tube about its axis all you want. But remember, we want to minimize our impact on the free stream flow, right? So however you're mounting this thing, you should have the second chamber vent to the opposite side. That's more important than the microscopic difference in ambient pressure over an inch or so.