Potential-core closing of temporally developing jets at Mach numbers between 0.3 and 2: Scaling and conditional averaging of flow and sound fields

Christophe Bogey (1) , Pierre Pineau (1)

1 LMFA - Laboratoire de Mecanique des Fluides et d'Acoustique

In a previous work [Bogey, J. Fluid Mech. 859, 1022 (2019)], the potential-core closing of temporally developing isothermal round jets at a Mach number M = 0.9 was shown to generate a strong axisymmetric noise component in the downstream direction. The persistence of this component is investigated in the present work for jets at M = 0.3, 0.6, 1.3, and 2 computed by direct numerical simulation, from a low subsonic to a high supersonic Mach number. The flow and sound fields are presented, and the Mach number scaling of their magnitudes and spectral content is examined. The centerline velocity and hydrodynamic pressure spectra are close to each other using k z r 0 in abscissa, where k z and r 0 are the axial wave number and the initial jet radius, respectively. The sound spectra for M 1.3 collapse well when they are plotted as a function of k z r 0 M −1 and adjusted in amplitude using a M 7.5 power law, whereas a k z r 0 scaling and a lower power-law exponent seem to apply to the spectra for M 1.3. The flow and sound fields are then correlated with each other, and conditionally averaged based on a synchronization of the fields with the minimum values of centerline velocity at potential-core closing. The noise component radiated in the downstream direction for M = 0.9 is clearly identified for M = 0.6, 1.3, 2, and is also detected, albeit with more difficulty, for M = 0.3, indicating the presence of the associated sound source over a wide range of Mach numbers. In all cases, its generation process extracted by the conditional averaging consists in the growth of a spot of low velocity and a high vorticity level in the inner side of the mixing layer, reaching a peak intensity at its arrival on the axis and weakening subsequently. The use of different trigger conditions for the averaging suggests that, for a given Mach number, the amplitude of the acoustic waves radiated during that stage depends linearly on the velocity deficit and the strength of the vortical structures on the jet centerline at the time of potential-core closing.

Domaines: Sciences de l'ingénieur [physics]

Christophe Bogey, Pierre Pineau. Potential-core closing of temporally developing jets at Mach numbers between 0.3 and 2: Scaling and conditional averaging of flow and sound fields. Physical Review Fluids, 2019, 4, ⟨10.1103/PhysRevFluids.4.124601⟩. ⟨hal-02400864⟩