Distributions of Noise Sources in Heated and Cold Jets: Are They Different?

International Journal of Aeroacoustics

2014

The noise emitted by an overexpanded round jet at a Mach number of 3.3 and a Reynolds number of 10 5 , computed in a previous study using large-eddy simulation (LES), is investigated. In a first step, the non-linear sound propagation effects are quantified by performing two far-field wave extrapolations from the LES near-field data. The extrapolations are carried out by solving the linearized Euler equations in one case and the full Euler equations in the other, without atmospheric absorption, up to a distance of 240 radii from the jet nozzle exit. The non-linear effects are shown to be quite significant, resulting in a series of N-shaped waves in the pressure signals, and in weaker mid-frequency components and stronger high-frequency components in the spectra. Close to the peak directivity radiation angle, for instance, they lead to about a 8 dB loss and a 6 dB gain at the Strouhal numbers of 0.2 and 1, respectively. In a second step, noise generation mechanisms are discussed by calculating correlations between far-field pressure fluctuations and turbulent quantities in the jet. High levels of correlation are found with the centerline flow fluctuations at the end of the potential core, with the shear-layer flow fluctuations over a large axial distance, and with the centerline density fluctuations between the 3rd and the 5th shock cells. They are attributed to the intermittent intrusion of low-speed vortical structures in the potential core, to the supersonic convection of turbulent structures, and to the shock motions at the screech tone frequency.

Section: Non-linear Effects Along the Line φ = 60°nmentioning

confidence: 81%

Section: Introductionmentioning

confidence: 99%

Noise of an Overexpanded Mach 3.3 Jet: Non-Linear Propagation Effects and Correlations with Flow

Cacqueray

International Journal of Aeroacoustics

2014

“…Each ILEE computation requires 200 GB of memory, and lasts during between 5000 and 12,000 iterations, resulting to a total number of about 100,000 CPU hours consumed. Pressure is recorded at a distance of 150r 0 from z ¼ r ¼ 0 where far-field acoustic conditions are expected to apply according to measurements, 58,59 as in the experiment of Bridges and Brown, 16 for angles between u ¼ 15 degrees and u ¼ 135 degrees. Pressure spectra are evaluated using overlapping samples of duration 90r 0 =u j , and they are averaged in the azimuthal direction.…”

Section: Jetmentioning

confidence: 99%

Grid sensitivity of flow field and noise of high-Reynolds-number jets computed by large-eddy simulation

International Journal of Aeroacoustics

2018

Three isothermal round jets at a Mach number of 0.9 and a diameter-based Reynolds number of 10 5 are computed by large-eddy simulation using four different meshes in order to investigate the grid sensitivity of the jet flow field and noise. The jets correspond to two initially fully laminar jets and one initially strongly disturbed jet considered in previous numerical studies. At the exit of a pipe nozzle of radius r 0 , they exhibit laminar boundary-layer mean-velocity profiles of thickness 0:2r 0 ; 0:025r 0 and 0:15r 0 , respectively. For the third jet, a peak turbulence intensity close to 9% is also imposed by forcing the boundary layer in the nozzle. The grids contain up to one billion points, and, compared to the grids used in previous simulations, they are finer in the axial direction downstream of the nozzle and in the radial direction on the jet axis and in the outer region of the mixing layers. The main flow field and noise characteristics given by the simulations, including the mixing-layer thickness, the centerline mean velocity, the turbulence intensities on the nozzle lip line and the jet axis, spectra of velocity and far-field pressure obtained from the jet near field by solving the isentropic linearized Euler equations, are presented. With respect to those from previous studies, the results are very similar for the initially laminar jet with thick boundary layers, but they differ significantly for the initially laminar jet with thin boundary layers and for the initially disturbed jet. For the latter two jets, using a finer grid leads to a faster flow development, to higher turbulence intensities in the shear layers and at the end of the potential core, to stronger large-scale structures, and to the generation of more low-frequency noise.

“…Radiation conditions [56,57] are implemented at the outer radial boundary and at the inflow and outflow axial boundaries. After a time t ≃ 120r 0 ∕u j , pressure is recorded at a distance of 120r 0 from z r 0, where far-field acoustic conditions are expected to apply according to experiments [66,67], for angles relative to the jet direction between φ 40 deg and φ 90 deg, during a period of about 200r 0 ∕u j . Pressure spectra are evaluated using overlapping samples of duration 38r 0 ∕u j , and they are averaged in the azimuthal direction.…”

Section: Far-field Extrapolationmentioning

confidence: 99%

Simulations of Initially Highly Disturbed Jets with Experiment-Like Exit Boundary Layers

Marsden

2016

AIAA Journal

Two isothermal round jets at a Mach number of 0.9 and a diameter-based Reynolds number of 2 × 10 5 have been computed by compressible large-eddy simulation using high-order finite differences on a grid of 3.1 billion points. At the exit of a straight pipe nozzle in which a trip forcing is applied, the jet flow velocity parameters, including the momentum thickness and the shape factor of the boundary layer, the momentum-thickness-based Reynolds number, and the peak turbulence intensity, roughly match those found in experiments using two nozzles referred to as the ASME and the conical nozzles. The boundary layer is in a highly disturbed laminar state in the first case and in a turbulent state in the second. The exit flow conditions, the shear-layer and jet flowfields, and the far-field noise provided by the large-eddy simulation are described. The jet with the ASME-like initial conditions develops a little more rapidly, with slightly higher turbulence levels than the other. Overall, however, the results obtained for the two jets are very similar, and they are in good agreement with measurements available for Mach 0.9 jets. In particular, this similarity holds for the far-field spectra. Because the ASME nozzle has been reported to yield higher noise levels than the conical nozzle, this suggests that the nozzle-exit conditions in the large-eddy simulation do not adequately reflect those in the experiments and/or that the link between the noise differences and the jet initial conditions using the two nozzles is not as simple as was first thought, and that other parameters, associated for instance with the nozzle geometry such as the presence of pressure gradients, may also play an important role.