Material

What this step does

Fluid. Fluid selects dry air or the products of complete combustion of a hydrocarbon fuel (Fuel, H/C atom ratio, Excess-air ratio). The choice sets the molar mass and, with the temperature-dependent heat-capacity model, the heat capacity of every run, and through them the density and the speed of sound in the acoustics.

Temperature and ambient pressure. Temperature is the gas temperature. The ambient pressure $p_{atm}$ is given by an altitude in the standard atmosphere or typed directly (Ambient pressure: mode, Altitude, Atmospheric pressure). Every gauge pressure of the case (outlet pressures, reported pressures) is relative to $p_{atm}$, and the compressible solver's absolute pressure starts from it.

Fluid properties. The density follows the ideal-gas law at $p_{atm}$ (or at the mean pressure, Density reference) and the temperature; the viscosity follows Sutherland's law of the temperature only:

$\rho = (p_{atm} + p_g) M / (\mathcal{R} T)$, $\mu = A_s T^{3/2}/(T + T_s)$, $\mathcal{R} = 8314.47$ J/(kmol K).

For an incompressible run each value can be typed over (Density, Dynamic viscosity). The field then shows a "user" badge and a reset link, and a warning when the value is more than 2 % off the computed one. The compressible solver always computes both itself, so the fields are read-only there.

Compressible gas properties. The heat-capacity model (Heat capacity model) is either the temperature-dependent polynomial of the selected gas or a constant $c_p$ and molar mass (Specific heat capacity cp, Molar mass). Prandtl number and the Sutherland constants (Sutherland coefficient As, Sutherland temperature Ts) complete the set; Reference pressure shows the reference pressure.

These values are used by compressible and heat-transfer runs and by the acoustics (the ratio of heat capacities gives the speed of sound), even when the flow run is incompressible. A live table shows the molar mass, gas constant, density, viscosity, $c_p$, $c_v$, $\gamma$ and the speed of sound at the entered temperature and pressure, with dry air alongside for exhaust gas.

Case files from older versions are converted on loading (the ambient pressure was formerly set on the Boundary Conditions step, and the temperature in kelvin under the incompressible properties); a note says so once.

Source

The standard atmosphere is that of (U.S. Standard Atmosphere, 1976); the viscosity follows Sutherland's law (Sutherland, 1893). The ideal-gas density uses the universal gas constant $\mathcal{R} = 8314.47$ J/(kmol K) throughout the program.

Fields in this step 33

Related 18

• Fluidmaterial.fluid
• Temperaturematerial.temperature_C
• Ambient pressure: modematerial.ambient.mode
• Altitudematerial.ambient.altitude_m
• Atmospheric pressurematerial.ambient.p_atm_Pa
• Density referencematerial.ambient.density_reference
• Densitymaterial.incompressible.rho
• Dynamic viscositymaterial.incompressible.mu
• Heat capacity modelmaterial.compressible.cp_model
• Molar massmaterial.compressible.molar_mass
• Specific heat capacity cpmaterial.compressible.Cp
• Prandtl numbermaterial.compressible.Pr
• Sutherland coefficient Asmaterial.compressible.sutherland.As
• Sutherland temperature Tsmaterial.compressible.sutherland.Ts
• Reference pressurematerial.compressible.p_ref
• Fuelmaterial.exhaust.fuel
• H/C atom ratiomaterial.exhaust.h_to_c
• Excess-air ratiomaterial.exhaust.excess_air_ratio

References 2

1. Sutherland, W. (1893). The viscosity of gases and molecular force. The London, Edinburgh, and Dublin Philosophical Magazine and Journal of Science, 36(223), 507-531. DOI 10.1080/14786449308620508
2. NOAA, NASA and USAF (1976). U.S. Standard Atmosphere, 1976. NOAA-S/T-76-1562, NASA-TM-X-74335. Web page