Login 


Lesson: Bernoulli's Principle Contributed by: Integrated Teaching and Learning Program and Laboratory, College of Engineering, University of Colorado Boulder
PreReq Knowledge (Return to Contents) algebra and gas laws
Learning Objectives (Return to Contents) After this lesson, students should be able to:
Introduction/Motivation (Return to Contents) (Note that Figures 1 and 2 are also available as PowerPoint slides in the Bernoulli Flow Graphics attachment. They are also suitable to print out as overhead transparencies or student handouts.)
Have you ever wondered how an airplane can fly? The simple answer is that as air flows around the wing, the plane is pushed up by higher pressure air under the wing, compared to lower pressure over the wing. But to understand this phenomenon more deeply, we must look at a branch of physics known as fluid mechanics, and in particular a principle known as the Bernoulli equation. Not only can this equation predict the air pressure around an airplane's wing, but it can also be used to find the force of high winds on a skyscraper, the pressure through a chemical reactor, or even the speed of water coming out of the hose in your backyard.
The Bernoulli equation states that for an ideal fluid (that is, zero viscosity, constant density and steady flow), the sum of its kinetic, potential and thermal energy must not change. This constraint gives rise to a predictable relationship between the velocity (speed) of the fluid, its pressure, and its elevation (relative height). Specifically, given two points along a streamline (an imaginary line tangent to the direction of flow, as shown in Figure 1), the Bernoulli equation states that:
where v is fluid velocity, ρ is fluid density, z is relative height, and P is pressure. Applying this equation to an example helps to make it clearer. Consider a reservoir located up in the mountains with a pipe leading down to a town at a lower elevation. The pipe delivers water to a hydroelectric plant, and we want to know how fast the water will flow into the plant turbines. Figure 2 illustrates this situation.
Now let's get back to how Bernoulli's principle applies to the wing of an airplane. When air flow is split around a wing, the air above the wing moves faster than the air below, due to the wing's shape. Since the velocity of the upper air increases, its pressure must decrease to maintain balance — as described in Bernoulli's equation. This results in greater pressure below the wing than above, which forces the wing upwards, enabling flight!
Lesson Background & Concepts for Teachers (Return to Contents) The Bernoulli equation is an important expression relating pressure, height and velocity of a fluid at one point along its flow. The relationship between these fluid conditions along a streamline always equal the same constant along that streamline in an idealized system. An idealized system refers to a fluid that has a constant density (incompressible), and is inviscid.
Assuming that the fluid is inviscid means that it has no viscosity. Therefore, certain effects of viscosity, such as water sticking to the walls of its pipe or container, do not apply.
Because the Bernoulli equation is equal to a constant at all points along a streamline, we can equate two points on a streamline. Using information on the system at one point, we can solve for information at another.
According to the Venturi effect, a fluid's pressure decreases as its velocity increases. The Bernoulli equation puts this relationship into mathematical terms and includes a term for fluid height. To illustrate this relationship, consider water moving down a water slide. At the top where you load, the water is slow moving, pushed only by the water behind it. When the slide drops, the water rushes down quickly, increasing speed as it falls. Thus, the velocity is also affected by gravity through height. When all these terms are related and scaled for density and gravity, we have the Bernoulli equation, where v is fluid velocity, ρ is fluid density, h is relative height, and P is pressure. Notice the constant has units of pressure as well.
The Bernoulli equation can also be interpreted as an expression of conservation of energy, which can be formulated as
where KE is kinetic energy, PE is potential energy, and W is the work done on the system. Imagine a block of ice sliding down the water slide at some velocity. The block has a kinetic energy equal to onehalf its mass times its velocity squared, or
where KE is kinetic energy and m is mass. The block also has a certain potential energy described by
where PE is the potential energy and h is the height. Finally, the work is being done on the block by the force of the water pressure behind it (with a force F=PA, where P is water pressure, and A is the area of the face of the ice that is getting pushed along). When the block is pushed a distance equivalent to its own width, Δx, then the work done on the block is
where V is volume. The equation for conservation of energy becomes
Dividing this equation by the volume, and recalling that density, ρΔ equals mass divided by volume, it reduces to
which is the Bernoulli equation.
Engineers can set the Bernoulli equation at one point equal to the Bernoulli equation at any other point on the streamline and solve for unknown properties. For example, a civil engineer might want to know how much pressure changes in a pipe between the top of a building and the ground.
Vocabulary/Definitions (Return to Contents)
Associated Activities (Return to Contents)
Lesson Closure (Return to Contents) Bernoulli's equation has a wide application of uses, from wing design to pipe flow. According to the Venturi effect, as fluid velocity increases, the pressure decreases and vice versa. Bernoulli's equation is a mathematical representation of this. Bernoulli's equation can be understood though manipulation of the energy of a flowing fluid. By setting Bernoulli's equation equal at two different points along a streamline, one can calculate the fluid conditions at one point by using information from another point.
Attachments (Return to Contents)
Assessment (Return to Contents) PreLesson Assessment
Matching: To help students with this lesson and its associated activity, create a list of physics equations that they have already studied, such as kinetic energy, potential energy, work, kinematics equations. Randomly write the physics terms on the left side of the board and the matching halves of the equations (out of order) on the right side of the board. As a class, have students match the correct sides together. Examples:
PostIntroduction Assessment
Homework/Independent Practice: Have students complete the Bernoulli Equation Practice Worksheet. Have all students complete the first problem in class and review the answer together. Have students complete the second problem as homework.
Lesson Summary Assessment
Discussion Question: During the next class period, lead a fiveminute discussion asking students what they learned from the homework assignment.
Additional Multimedia Support (Return to Contents) You may want to use PowerPoint slides of Figure 1. Simple Pipe Flow, and Figure 2. Reservoir Example, with the class, as provided in the attached Bernoulli Flow Graphics.
See descriptions of additional reallife applications of Bernoulli's principle at the Science Clarified website: http://www.scienceclarified.com/everyday/RealLifeChemistryVol3PhysicsVol1/BernoullisPrincipleReallifeapplications.html
Show students an interactive animation of the Bernoulli Principle in the form of a cutaway view of a pipe with a shape that you can change using your computer's mouse, resulting changes in the pressure, cross sectional area and velocity of the fluid flowing through the pipe, which are graphed below the pipe. See Mark. Mitchell's website called, "Animated Demonstration of Bernoulli's Principle," at http://home.earthlink.net/~mmc1919/venturi.html
References (Return to Contents) Bernoulli's principle (definition). Last updated February 11, 2010. Wikipedia, The Free Encyclopedia. Accessed February 17, 2010. http://en.wikipedia.org/w/index.php?title=Bernoulli%27s_principle&oldid=343435891 Knight, Randall. Physics for Scientists and Engineers: a Strategic Approach. Second edition. San Francisco, CA: Pearson AddisonWesley, 2008. Munson, B. R., Young, D.F., Okiishi, T.H. Fundamentals of Fluid Mechanics. Fifth edition. New York, NY: John Wiley & Sons, Inc., 2006. Venturi effect (definition). Last updated February 12, 2010. Wikipedia, The Free Encyclopedia. Accessed February 17, 2010. http://en.wikipedia.org/w/index.php?title=Venturi_effect&oldid=343508711 Contributors James Prager, Karen King, Denise W. CarlsonCopyright © 2009 by Regents of the University of Colorado.Supporting Program (Return to Contents) Integrated Teaching and Learning Program and Laboratory, College of Engineering, University of Colorado BoulderAcknowledgements (Return to Contents) The contents of this digital library curriculum were developed under a grant from the Fund for the Improvement of Postsecondary Education (FIPSE), U.S. Department of Education and National Science Foundation GK12 grant no. 0338326. However, these contents do not necessarily represent the policies of the Department of Education or National Science Foundation, and you should not assume endorsement by the federal government.
 
 