Gas processes Flashcards
When is a collection of particles a fluid?
When the average distance between two collision (MFP = mean free path) is much less than the macroscopic scales of interest.
What are the two types of collisions?
Collisions between neutral particles (short range) or between charged particles (long range)
What is viscosity
Transport of momentum from low velocity to high velocity
Shock wave properties
- The sound speed depends on local pressure and a steppening occurs at maximum pressure
- Heat is generated and entropy increases
- Therefore a show wave is formed like a wave in the ocean, on a lenghtscale of a few mean free path.
- Strong gradient occur at a shock where dissipation and viscosity become important
What are Rankine Hugoniot relations ?
They are used in the context of an hydro shock. The are composed of the conservation of mass, the conservation of momentum and the conservation of energy. We use them to express the downstream values in function of the upstream values.
They allow us to define the Mach number.
What is the Mach number?
In the context of an hydro shock, M = v1 / cs
For an upstream supersonic flow, M1 > 1.
What are radiative shock ?
Assumption made that the gas is able to radiate energy in a radiative relaxation layer after the shock.
Is the radiative relaxation layer large or small
The radiative relaxation layer is always large compared to the shock front itself because it takes a lot of collisions to cool down the gas.
What happens in the radiative relaxation layer?
The temperature drops and the gas is squeezed (its density increases). A radiative shock can actually increase the density by a very large factor !
What is a collisionless shock?
In a collisionless shock, the interaction happens between charged particles and a magnetic field.
Condition for a collisionless shock to happen?
Low density and a diffuse region.
