Workout: The continuity equation


Read the web pages:  Quantifying fluid flow and The continuity equation.


To give you some practice on working with fluid flow, go to the PhET simulation Fluid Pressure and Flow to perform a few tasks

Set up

Choose the tab "Flow". The simulation displays a pipe carrying 5000 liters of water per second. By default, randomly spaced red dots are distributed through the fluid so you can see it moving.  

Answer these questions

1. Grab the Speed meter from the measuring-devices palette at the top right. Measure the speed at different places along the pipe. How does it change along the pipe and across the pipe? (The motion across the pipe is not correctly displayed. In a real pipe with a uniform flow, the fluid flows more quickly in the center of the pipe and more slowly towards the edges.)

Source: PhET simulation, Fluid Pressure and Flow

2. Turn on the flux meter. As you move it back and forth along the pipe it will show you the flow rate (liters/s), the cross sectional area of the pipe (m2), and the flux (liters/m2s). Confirm that the flow rate is constant all along the pipe. Now expand the back end of the pipe using the handles attached to pull it out. Your screen should look something like the image at the right.

First predict what you think the flow meter will show in the wide portion of the pipe for flow rate, are, and flux. You don't have to give numbers, just say whether they will increase, decrease, or remain the same and explain why you think so. Then check and see if your predictions are confirmed. If they are not, explain what went wrong.

Joe Redish, Fall 2017

Article 547
Last Modified: February 14, 2019