# Fluid Dynamics Summary

1. Assumed that fluid flow is a continuum phenomenon.

2. Fluids, being composed of matter, obey the laws of physics.

To calculate the lift and drag forces, we would like to be able to calculate the pressure acting on the

surface. This depends on the velocity, and also the state of the fluid, which includes knowledge

of the temperature and density.

Thus, we would like to be able to calculate the velocity vector, density, temperature and pressure

everywhere, as functions of space (x,y,z) and time (t). For this we use 3 laws of physics:

Mass is neither created nor destroyed.......... (1)

Rate of change of momentum = Net force .....(2)

Energy can change form, but is neither created nor destroyed ......(3)

In addition, the state of the gas is specified by the “State Equations”. The Thermal Equation of State

for a gas reduces to the Perfect Gas Law, which relates density, temperature and pressure.

To solve very any flow problem, we can specify the boundary conditions and/or initial conditions, and solve all these equations simultaneously all over the flow field.

In the next sections, we will see how to reduce these conservation equations to forms which can be

d t l l t h used to calculate changes in velocity and pressure over space and time, and thus to calculate

flow properties at given points, or over entire vehicles.

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