This might be a stretch for young people to use to learn about using computers to do simulations but maybe a teacher could use it to demonstrate using computers to model fluid flows:
What’s My Model?
http://education.ti.com/educationportal/activityexchange/Activity.do?cid=US&aId=8518
“Students will investigate several different regression models and determine which of the models makes the most sense, based upon a real-world situation (cooling a cup of hot chocolate).”
This is geared for TI calculators but the idea could be used for applications on computers also.
Fractal Science Kit
http://www.fractalsciencekit.com/
The Fractal Science Kit is a program to explore a mathematical object called a fractal. The term fractal was coined by Benoit Mandelbrot in 1975 in his book Fractals: Form, Chance, and Dimension. In 1979, while studying the Julia set, Mandelbrot discovered what is now called the Mandelbrot set and inspired a generation of mathematicians and computer programmers in the study of fractals and fractal geometry.
It does cost $49.95 but it looks good!
ElastoLab – simulated physics environment
http://www.simberon.com/elastolab.htm
ElastoLab is a fun program for kids that lets them play with sound, images and motion in a simulated physics environment.
ElastoLab has been released as an open-source free-ware program.
Conway’s Game of Life and Cellular Automata
http://www.math.com/students/wonders/life/life.html
Conway’s game of life is always a fun thing to show kids learning about computer science.
Here is a nice, simple applet to play around with the Game of Life
http://www.ibiblio.org/lifepatterns/
Another good one:
http://vlab.infotech.monash.edu.au/simulations/cellular-automata/game-of-life/
If you really want to keep up with the world of GoL:
http://pentadecathlon.com/lifeNews/index.php
An epidemic simulator
http://vlab.infotech.monash.edu.au/simulations/cellular-automata/epidemic/
Here’s a simple ant colony simulator:
http://vlab.infotech.monash.edu.au/simulations/swarms/ant-sort/
Physics and 3D in Python
http://showmedo.com/videos/video?name=pythonThompsonVPython1&fromSeriesID=30
Erik Thompson has a very nice video showing how you can use VPython to simulate a ball falling under the effects of gravity. it’s reasonably simple. It could be used to show how computers can simulate the physical world.