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MCAT Venturi Effect
Bernoulli’s Equation & Pitot Tubes Explained

William Cohen
Published by William Cohen
Last Updated On: November 19, 2021

The MCAT covers many topics, such as biology, general chemistry, organic chemistry, psychology, and more.

One of the areas on the MCAT that students traditionally struggle with is fluid mechanics, and the Venturi effect is part of this topic. This is a medium-yield topic, which means you should pay attention to it, but don’t overdo it.

I've helped countless students get ready for the MCAT, and I’ve successfully covered the Venturi effect as well.

Here’s a comprehensive guide to the Venturi effect. I’ll start with the fluids and ease into the Venturi effect specifics.

How to Solve Venturi Effect on the MCAT?

  • To get ready for the Venturi effect you should start with the Fluid Mechanics.
  • Bernoulli’s equation is important because it helps analyze a fluid flowing up and down through different tubes.
  • The Venturi effect is the result of Bernoulli's equation.
  • You should know the Venturi effect formula, and how to use it on the MCAT.

Fluid Mechanics

Fluid Mechanics formula

Fluid is a liquid, gas, or any other material, that cannot keep a shape when subjected to tangential or shearing stress [1].

The study of fluids is important for many real-world physics applications, such as blood flow.

The flow of fluids can be classified into two types: laminar and turbulent.

Laminar flow happens when a fluid flows in parallel layers. There’s no disruption between the layers.

It happens at low velocities, where there’s no lateral mixing, nor eddies or swirls of fluids. All the particles move in straight lines and are parallel with the pipe walls.

On the other hand, turbulent flow is more chaotic. There are fast changes in pressure and flow velocity. Compared to laminar flow, the liquid doesn’t travel in layers in turbulent flow but mixes all across the tube.

In a laminar flow, even if a minor disturbance happens, it’s quickly corrected, while in a turbulent flow, a disturbance leads to eddies and bigger interruptions that aren’t corrected.

Bernoulli’s Equation

An illustration of Bernoullis Equation

Now that we’ve talked about fluids let’s move onto Bernoulli’s equation.

Bernoulli’s equation is important because it helps us analyze a fluid flowing up and down through different tubes.

Bernoulli says that in a horizontal fluid flow, the points of higher fluid speed will have less pressure than the points of slower fluid speed.

  • P + 12pv2+ pgh = constant
  • p is the pressure
  • ρ is the density
  • V is the velocity
  • h is elevation
  • g is the gravitational acceleration

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It happens at low velocities, where there’s no lateral mixing, nor eddies or swirls of fluids. All the particles move in straight lines and are parallel with the pipe walls.

On the other hand, turbulent flow is more chaotic. There are fast changes in pressure and flow velocity. Compared to laminar flow, the liquid doesn’t travel in layers in turbulent flow but mixes all across the tube.

In a laminar flow, even if a minor disturbance happens, it’s quickly corrected, while in a turbulent flow, a disturbance leads to eddies and bigger interruptions that aren’t corrected.

Venturi Effect

Venturi Effect

The Venturi effect is a result of Bernoulli’s equation.

If there’s a constriction in a pipe, and the fluid passes through it, the velocity will increase, and the pressure will decrease.

The most important thing to keep in mind is: If the velocity goes up, the pressure goes down.

Take a look at the picture. Imagine water flowing through the first half of the diagram when it meets a constriction (the narrow part in the middle).

The water will keep flowing, but it’ll start flowing faster through the constricted region.

Imagine the entire volume of water moving through the first section in one second. That same volume has to make it through each portion of the pipe. Otherwise, the pipe’s going to break.

All the water from the front section will have to change shape, and the water will have to travel faster to get through the constricted region.

Note: The smaller the constricted region is, the faster the fluid will go through.

It is significant because faster-moving fluid means lower pressure. This is called Bernoulli's principle, and it’s what causes the Venturi effect.

To sum up, the Venturi effect happens when you have a tube with a narrow constriction. You’ll have a smaller pressure region, and the pressure will drop. Father moving fluid will cause lower pressure.

A Venturi tube — a tube with a constriction in the middle — determines the velocity of the fluid flowing through it.

Pitot Tube

Pitot tube on plain background

However, we can also determine the velocity of the fluid flowing past the tube.

This is called the pitot tube. Pitot tubes are mainly used on aircrafts as speedometers [2].

A pitot tube is a slender tube with two openings. The front hole measures the stagnation pressure, while the side hole measures the static pressure.

By measuring the difference between these two pressures, we can determine the velocity of the fluid that’s flowing past the tube.

“The stagnation pressure represents the pressure at a point where the fluid is brought to a complete stop.” -NiLTime Youtube Channel

We also apply Bernoulli’s formula here to get the formula for stagnation pressure:

P1 + ρ v22= P2=Pstag

What You Have to Know About the Venturi Effect for MCAT

Fluid mechanics is a medium-yield topic on the MCAT. Yes, it’s worth your time, but don’t go too crazy.

Start with Bernoulli's equation and go from there. You should know that Venturi’s effect is the common utilization of Bernoulli's equation.

For the MCAT, you should be able to explain what’s the Venturi effect and show your understanding of it by applying it to devices such as pitot tubes and diseases that commonly occur in human circulation.

My advice is to go through several Venturi effect practice questions and work on this theory until you can solve practice questions with ease.

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References:

  1. https://www.google.com/url?q=https://www.britannica.com/science/fluid-physics
  2. https://www.google.com/url?q=https://www.grc.nasa.gov/www/k-12

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