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Combustion-instability analysis

What the module computes

For an ideal cylindrical chamber, EnSim calculates longitudinal, tangential and radial acoustic eigenfrequencies. Tangential/radial modes use roots of the appropriate Bessel-function derivative; longitudinal modes use chamber length and frozen sound speed.

This is an acoustic mode finder. A frequency is not, by itself, an instability.

Stability classification

When compatible modal growth and damping rates are supplied, EnSim reports the net rate

\[ \alpha_{net}=\alpha_{drive}-\alpha_{damp}. \]

Positive net rate indicates exponential growth within that supplied linear model; negative net rate indicates decay. If rates are absent, the result remains “not assessed.” The software does not fabricate a universal threshold from chamber pressure, L-star or injector count.

The low-frequency chug estimate uses the explicitly documented feed/compliance inputs. It should be treated as a lumped-system screening result.

Missing physics

Injector admittance, time lag, distributed combustion response, nonlinear limit cycles, entropy/vorticity waves, nozzle damping, baffles, acoustic liners and multiphase propellant dynamics are not derived from geometry automatically.

References