Linear isentropic sound waves
Linearizing mass continuity and inviscid momentum around a uniform stationary fluid with δP=c_s²δρ gives ∂_t²δP=c_s²∇²δP.
Key knowledge
Linearizing mass continuity and inviscid momentum around a uniform stationary fluid with δP=c_s²δρ gives ∂_t²δP=c_s²∇²δP. The thermodynamic sound speed is c_s²=(∂P/∂ρ)_s, and a calorically ideal gas with fixed heat-capacity ratio has c_s²=γP₀/ρ₀.
Values and equations
∂_t²δP=c_s²∇²δP; c_s²=(∂P/∂ρ)_s; c_s²=γP₀/ρ₀ (ideal gas)
- δP,P₀
- Pressure disturbance/equilibrium pressure Pa
- δρ,ρ₀
- Density disturbance/equilibrium density kg m⁻³
- c_s
- Sound speed m s⁻¹
- γ
- Heat-capacity ratio C_P/C_V
- s
- Entropy per mass J kg⁻¹ K⁻¹
- t
- Time s
Conditions and scope
- Small disturbances, positive isentropic bulk modulus, homogeneous equilibrium, local equilibrium and negligible heat conduction, viscosity and background body-force perturbation at the sound scale.
- Use entropy per mass fixed in the derivative. Isothermal or dissipative/relaxing propagation uses a different closure; nonlinear shocks and boundary modes need separate treatment.
Sources · 2
References
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Evidence and review
Classical Mechanics III 8.09 (2014): Chapter 6, fluid mechanics: §6.5 printed pages 123–125, equations 6.91–6.104: perturbation mass/momentum, bulk-modulus closure and wave equation. The isentropic derivative qualification is supplied by fixed-entropy parcel thermodynamics. Thermodynamics and Statistical Mechanics (2016): Heat engines: Equation 6.205: ideal-gas adiabatic TV^(γ−1) constant; combining the ideal-gas equation and fixed parcel mass gives Pρ⁻γ constant and its sound derivative.
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