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The physical definition for the compressible solvers in SU2 based around the definition of the free-stream. The free-stream values are not only used as boundary conditions for the `MARKER_FAR` option, but also for initialization and non-dimensionalization. That means even if you don't have any farfield BCs in your problem, it might be important to prescribe physically meaningful values for the options.
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### Thermodynamic State ###
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The thermodynamic state of the free-stream is defined by the pressure $$p_{\infty}$$, the density $$\rho_{\infty}$$ and the temperature $$T_{\infty}$$. Since these quantities are not independent, only two of these values have to be described and the third one can be computed by an equation of state, depending on the fluid model used. There are two possible ways implemented that can be set using `FREESTREAM_OPTION`:
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-`TEMPERATURE_FS` (default): Density $$\rho_{\infty}$$ is computed using the specified pressure $$p_{\infty}$$ (`FREESTREAM_PRESSURE`) and temperature $$T_{\infty}$$ (`FREESTREAM_TEMPERATURE`).
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-`DENSITY_FS`: Temperature $$T_{\infty}$$ is computed using the specified pressure $$p_{\infty}$$ (`FREESTREAM_PRESSURE`) and density $$\rho_{\infty}$$ (`FREESTREAM_DENSITY`).
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|`NEMO_EULER`, `NEMO_NAVIER_STOKES`| 7.0.0 |
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### Mach Number and Velocity ###
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The physical definition for the thermochemical nonequilibrium (NEMO) solvers is similar to the compressible solvers, but with additional parameters to specify. The free-stream values are not only used as boundary conditions for the `MARKER_FAR` option, but also for initialization and non-dimensionalization. That means even if you don't have any farfield BCs in your problem, it might be important to prescribe physically meaningful values for the options.
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The free-stream velocity $$v_{\infty}$$ is always computed from the specified Mach number $$Ma_{\infty}$$ (`MACH_NUMBER`) and the computed thermodynamic state. The flow direction is based on the angle of attack (`AOA`) and the side-slip angle (`SIDESLIP_ANGLE`, for 3D).
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### Chemical Composition and Mass Fractions ###
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### Reynolds Number and Viscosity ###
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The NEMO solvers require a specification of thermochemical nonequilibrium library using the `FLUID_MODEL` option, either `SU2_NONEQ` if using the SU2 built-in thermochemical library, or `MUTATIONPP` if using the Mutation++ thermochemical library.
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If it is a viscous computation, by default the pressure $$p_{\infty}$$ will be recomputed from a density $$\rho_{\infty}$$ that is found from the specified Reynolds number $$Re$$ (`REYNOLDS_NUMBER`). Note that for an ideal gas this does not change the Mach number $$Ma_{\infty}$$ as it is only a function of the temperature $$T_{\infty}$$. If you still want to use the thermodynamic state for the free-stream definition, set the option `INIT_OPTION` to `TD_CONDITIONS` (default: `REYNOLDS`). In both cases, the viscosity is computed from the dimensional version of Sutherland's law or the constant viscosity (`FREESTREAM_VISCOSITY`), depending on the `VISCOSITY_MODEL` option.
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### Non-Dimensionalization ###
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A chemistry model, consisting of a set of flow species, thermochemical properties, and chemical reactions, is specified using `GAS_MODEL`. The names of these models are specific to the thermochemical library. If using the `SU2_NONEQ` option the choices are `ARGON`, `N2`, `AIR-5`, and `AIR-7`.
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For all schemes, as reference values for the density and temperature the free-stream values are used, i.e. $$ \rho_{ref} = \rho_{\infty}, T_{ref} = T_{\infty}$$. The reference velocity is based on the speed of sound defined by the reference state: $$v_{ref} = \sqrt{\frac{p_{ref}}{\rho_{ref}}}$$. The dimensionalization scheme can be set using the option `REF_DIMENSIONALIZATION` and defines how the reference pressure $$p_{ref}$$ is computed:
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-`DIMENSIONAL`: All reference values are set to `1.0`, i.e. the computation is dimensional.
-`FREESTREAM_VEL_EQ_MACH`: Reference pressure is chosen such that the non-dimensional free-stream velocity equals the Mach number: $$p_{ref} = \gamma p_{\infty}$$.
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-`FREESTREAM_VEL_EQ_ONE`: Reference pressure is chosen such that the non-dimensional free-stream velocity equals `1.0`: $$p_{ref} = Ma^2_{\infty} \gamma p_{\infty}$$.
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Free-stream mass fractions must also be specified in list using the option `GAS_COMPOSITION`. The mass fractions are specified as decimal values in the order of the species in the gas model. For example, an AIR-5 mixture of 77% oxygen and 23% nitrogen would be expressed as (0.77, 0.23, 0.00, 0.00, 0.00).
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