Trying to model drag can prove somewhat confusing for someone who is not familiar with fluid dynamics. One reason is because there is not just one, but several types of drag. Depending on the object and flow you are simulating, you might need to consider different types of drag.
Another source of difficulty is that a lot of what we know about drag is based on empirical work. The laws of drag were discovered by doing lots of experiments, and not as part of a beautiful universal theory like gravity.
The bottom line is that you will bump into formulas that were developed from experimental results that you just have to accept without argument.
For most of what we'll do in this book, we'll use one of two drag laws: one that holds for low velocities ...