We prescribe stagnation pressure and temperature at subsonic inlet (physical BC). Now we extrapolate velocity from inside the solution domain (numerical BC). For calculating internal energy (e) for solving energy equation for inviscid flow, we require velocity values.
Do stagnation properties mean these values of velocity at inlet will be zero ? Numerically, we would get some velocity by extrapolation. What is the physical meaning of taking stagnation properties at inlet?
What you really want here are characteristic boundary conditions.
Compressible flow is physically hyperbolic: you can think of information traveling in discrete waves with known speeds. In 1D, this is an entropy wave traveling at the flow speed and two acoustics, moving at flow speed plus or minus the speed of sound. In 2D and 3D, you add shear waves traveling at the flow speed. For each characteristic, there is a physical quantity which is constant along that wave.
For a compressible, subsonic flow, all but one of these characteristic waves is moving into the domain at an inflow. The outgoing wave is the left-moving acoustic. In practice, this means that for 2D, they stagnation pressure, stagnation temperature and flow direction are generally specified at the inflow, and the fourth piece of information comes from that outgoing acoustic wave.
The velocity at the inlet is calculated, but isn't zero. It also isn't extrapolated. Instead, it's found by solving a system of equations based on the wave characteristics. See Charles Hirsch's compressible CFD books for more details, including all the gory math.
What you really want here are characteristic boundary conditions.
Compressible flow is physically hyperbolic: you can think of information traveling in discrete waves with known speeds. In 1D, this is an entropy wave traveling at the flow speed and two acoustics, moving at flow speed plus or minus the speed of sound. In 2D and 3D, you add shear waves traveling at the flow speed. For each characteristic, there is a physical quantity which is constant along that wave.
For a compressible, subsonic flow, all but one of these characteristic waves is moving into the domain at an inflow. The outgoing wave is the left-moving acoustic. In practice, this means that for 2D, they stagnation pressure, stagnation temperature and flow direction are generally specified at the inflow, and the fourth piece of information comes from that outgoing acoustic wave.
The velocity at the inlet is calculated, but isn't zero. It also isn't extrapolated. Instead, it's found by solving a system of equations based on the wave characteristics. See Charles Hirsch's compressible CFD books for more details, including all the gory math.
Thanks for your reply. I am understanding that the velocity at inlet changes so as to match the stagnation pressure supplied at inlet. Is it right though stagnation pressure is specified at inlet, but the flow will start at inlet and it will not me zero. As the term stagnation implies.
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