Draft: Direct and Indirect Noise Generated by Injected Entropic and Compositional Inhomogeneities

Flow disturbances are generated inside a duct via pulsed injection of helium into a flow of air. This leads to the generation of an acoustic pulse (direct noise), as well as the production of entropic and compositional inhomogeneities which are convected with the mean flow. As these inhomogeneities are convected through a choked nozzle, they generate indirect noise. The resulting acoustic pressure fluctuations are measured experimentally using pressure transducers upstream of the nozzle. Insight obtained from theoretical models and a time-delay analysis can be used to isolate and extract the contributions of direct and indirect noise in the experimental signal. These results are directly compared to existing one-dimensional direct and indirect noise models. The experimental measurement of indirect noise is found to be in good agreement with the theoretical models for entropy noise and compositional noise for a compact one-dimensional isentropic nozzles. NOMENCLATURE M Mach number [-] P acoustic wave amplitude [-] R acoustic reflection coefficient amplitude [-] T temperature [K] Y mass fraction [K] Z mixture fraction [-] c sound speed [m.s-1] ∗Address all correspondence to this author: [email protected] cp specific heat capacity [J.K-1 kg-1] g gibbs free energy [J.kg-1] ṁ mass flow rate [g.s-1] p pressure [Pa] s specific entropy [J.K-1 kg-1] u velocity [m.s-1] L acoustic dissipation coefficient [-] γ heat capacity ratio [-] ρ density [kg.m-3] ξ compositional wave amplitude [-] σ entropy wave amplitude [-] Φm mass flux perturbation [kg.m-2.s-1] ΦM momentum flux perturbation [kg.m-1.s-2] Φe energy flux perturbation [J.m-2.s-1] ΦZ mass fraction perturbation [-] φm normalised mass flux perturbation [-] φM normalised momentum flux perturbation [-] φe normalised energy flux perturbation [-] φZ normalised mass fraction perturbation [-] Ψ chemical potential function [-] (·)σ/ξ entropic/compositional noise (·)d/i direct/indirect noise (·)air relating to the air flow (·)He relating to the helium flow ̄ (·) mean component (·) fluctuating component (·)± forward or backward propagating 1 Copyright c © 2017 by ASME