Experimental studies of incompressible elliptic jets of different aspect ratios and initial conditions are summarized along with the effects of excitations at selected frequencies and amplitudes. The experimental facilities and procedures are described and jet spread and decay are discussed. The instability of elliptic shear layers, the behavior of the jet column under controlled excitation, and the time-average measures of unexcited jets are addressed.

Elliptic jets. I - Characteristics of unexcited and excited jets

Round jets originating from a pipe nozzle are computed by large-eddy simulations (LES) to investigate the effects of the nozzle-exit conditions on the flow and sound fields of initially laminar jets. The jets are at Mach number 0.9 and Reynolds number 105, and exhibit exit boundary layers characterized by Blasius velocity profiles, maximum root-mean-square (r.m.s.) axial velocity fluctuations between 0.2 and 1.9% of the jet velocity, and momentum thicknesses varying from 0.003 to 0.023 times the jet radius. The far-field noise is determined from the LES data on a cylindrical surface by solving the acoustic equations. Jets with a thinner boundary layer develop earlier but at a slower rate, yielding longer potential cores and lower centreline turbulent intensities. Adding random pressure disturbances of low magnitude in the nozzle also increases the potential core length and reduces peak r.m.s. radial velocity fluctuations in the shear layer. In all the jets, the shear-layer transition is dominated by vortex rolling-ups and pairings, which generate strong additional acoustic components, but also amplify the downstream-dominant low-frequency noise component when the exit boundary layer is thick. The introduction of inlet noise however results in weaker pairings, thus spectacularly reducing their contributions to the sound field. This high sensitivity to the initial conditions is in good agreement with experimental observations.

/pdf/influence-of-nozzle-exit-boundary-layer-conditions-on-the-497wfykzms.pdf

Influence of nozzle-exit boundary-layer conditions on the flow and acoustic fields of initially laminar jets

Experience gained from previous jet noise studies with the unstructured large-eddy simulation flow solver “Charles” is summarized and put to practice for the predictions of supersonic jets issued f...

Unstructured Large-Eddy Simulations of Supersonic Jets

We present experimental results for the acoustic field of jets with Mach numbers
between 0.35 and 0.6. An azimuthal ring array of six microphones, whose polar
angle, θ, was progressively varied, allows the decomposition of the acoustic pressure
into azimuthal Fourier modes. In agreement with past observations, the sound field for
low polar angles (measured with respect to the jet axis) is found to be dominated by
the axisymmetric mode, particularly at the peak Strouhal number. The axisymmetric
mode of the acoustic field can be clearly associated with an axially non-compact
source, in the form of a wavepacket: the sound pressure level for peak frequencies is
found be superdirective for all Mach numbers considered, with exponential decay as a
function of (1 – M_c cos θ)^2, where M_c is the Mach number based on the phase velocity
U_c of the convected wave. While the mode m = 1 spectrum scales with Strouhal
number, suggesting that its energy content is associated with turbulence scales,
the axisymmetric mode scales with Helmholtz number – the ratio between source
length scale and acoustic wavelength. The axisymmetric radiation has a stronger
velocity dependence than the higher-order azimuthal modes, again in agreement with
predictions of wavepacket models. We estimate the axial extent of the source of
the axisymmetric component of the sound field to be of the order of six to eight
jet diameters. This estimate is obtained in two different ways, using, respectively,
the directivity shape and the velocity exponent of the sound radiation. The analysis
furthermore shows that compressibility plays a significant role in the wavepacket
dynamics, even at this low Mach number. Velocity fluctuations on the jet centreline
are reduced as the Mach number is increased, an effect that must be accounted for
in order to obtain a correct estimation of the velocity dependence of sound radiation.
Finally, the higher-order azimuthal modes of the sound field are considered, and a
model for the low-angle sound radiation by helical wavepackets is developed. The
measured sound for azimuthal modes 1 and 2 at low Strouhal numbers is seen to
correspond closely to the predicted directivity shapes.

/pdf/axisymmetric-superdirectivity-in-subsonic-jets-4ul5veshsp.pdf

Axisymmetric superdirectivity in subsonic jets

Large-eddy simulations (LESs) of isothermal round jets at a Mach number of 09 and a diameter-based Reynolds number ReD of 105 originating from a pipe are performed using low-dissipation schemes in combination with relaxation filtering The aim is to carefully examine the capability of LES to compute the flow and acoustic fields of initially nominally turbulent jets As in experiments on laboratory-scale jets, the boundary layers inside the pipe are tripped in order to obtain laminar mean exit velocity profiles with high perturbation levels At the pipe outlet, their momentum thickness is δθ(0)=0018 times the jet radius, yielding a Reynolds number Reθ=900, and peak turbulence intensities are around 9% of the jet velocity Two methods of boundary-layer tripping and five grids are considered The results are found to vary negligibly with the tripping procedure but appreciably with the grid resolution Based on analyses of the LES quality and on comparisons with measurements at high Reynolds numbers, fine d

/pdf/large-eddy-simulation-of-the-flow-and-acoustic-fields-of-a-7t2v4wl777.pdf

A.K.M.F. Hussain

Papers

Roles of initial condition and vortex pairing in jet noise