Atmosphere Incorporation: USatm1976, MIL_SPEC_210A Hot/Cold/Polar/Tropical, and Offsets

[ehariton]

Summary

Incorporates USATM1976 directly into aviary as the default atmosphere so we no longer need to rely on the dymos atmo library.

Incorporates open-source MIL_SPEC_210A atmospheres (hot/cold/polar/tropical) which the user can switch to via options.

Individual atmosphere data is now separated into it's own file, while the core component that imports the data and does the calculations has been named atmosphere.py.

Adds an temperature offset option, allowing the user to shift an atmosphere profile by a specific number of degrees kelvin to mimic hotter / colder days.

Geometric vs. Geopotential input option is preserved.

Removed option to add dsos_dh as an output. This is now always provided. This is only used by solved 2DOF ODE.

Removed drhos_dh output as it was not used in Aviary.

Many tests that relied on atmosphere, including some engine conversion tests, referenced the atmosphere model. Because the original dymos atmosphere model was based on English units and used an English akima spline, the values changed slightly when converted to the SI units and the new akima splines. SI units is what the original 1976 atm standard was published in. There were noticeable small differences, rounding etc, between the dymos atmosphere implementation in imperial units and the values published in SI units in the original standard.

Related Issues

• Resolves Add ability to use hot day (non standard atmosphere) #133

Backwards incompatibilities

None

New Dependencies

None

[jphorton]

Did the expected_density values in AtmDeltaTKelvinTestCase1 come from the atmosphere model itself? They appear to deviate from an online resource I checked on the order of e-6 or so (for kg/m^3). It could be due an error in the density formula in `atmosphere.py'. When I replace line 215:

outputs['rho'] = corrected_density = (raw_density ** (-1) + self._R_air * self._dt * pressure ** (-1)) ** (-1)

with:

outputs['rho'] = corrected_density = raw_density * ((temp - self._dt) / temp)

I get a complete match with the online data. Not saying one or the other is correct, but we should look into this. The second formula makes sense to me but I don't recall how the first one was derived.

[ehariton]

Did the expected_density values in AtmDeltaTKelvinTestCase1 come from the atmosphere model itself? They appear to deviate from an online resource I checked on the order of e-6 or so (for kg/m^3). It could be due an error in the density formula in `atmosphere.py'. When I replace line 215:

outputs['rho'] = corrected_density = (raw_density ** (-1) + self._R_air * self._dt * pressure ** (-1)) ** (-1)

with:

outputs['rho'] = corrected_density = raw_density * ((temp - self._dt) / temp)

I get a complete match with the online data. Not saying one or the other is correct, but we should look into this. The second formula makes sense to me but I don't recall how the first one was derived.

The expected_density values for the AtmDeltaTKelvinTestCase1 were based on the calculations from the component itself. As there are no source tables that describe the effects of the delta_T on density or pressure for us to reference. As such, they serve as "checking against a reference value" incase some one comes along later and modifies the calculations.

Corrected density equation you referenced scales the density multiplicatively based on the difference in size of the original temp vs. the change in temperature. The equation I am using adds a correction factor instead. I suspect both methods are within the error tolerance of "estimating a standard atmosphere" since the atmosphere has a great deal of variability.