Olivier Marsden

ABSTRACT The effects of the Reynolds number on initially highly disturbed isothermal round jets at Mach number M = 0.9 and at diameter-based Reynolds numbers ReD = 2.5 × 10 4, 5 × 10 4, 10 5 and 2 × 10 5 are investigated using Large-Eddy Simulation. The jets originate from a pipe nozzle of radius r0, in which a tripping procedure is applied to the boundary layers. At the nozzle exit, laminar-like mean velocity profiles of momentum thickness δ θ ≃ 0.018 r0, yielding Reynolds numbers Re θ varying from 251 to 1830 depending on Re D, and peak turbulence intensities around 9% of the jet velocity, are thus obtained. With increasing Reynolds number, the turbulence spectra close to the nozzle exit and in the mixing layers broaden, as expected, while remaining dominated by the large-scale components naturally observed in turbulent boundary layers and shear layers, respectively. The mixing layers however develop more slowly, with reduced levels of velocity fluctuations. The axial profiles of turbulence intensities become smoother, showing in particular a clear overshoot two radii downstream of the nozzle exit at Re D = 2.5 × 10 4, but a monotonical growth at Re D = 2 × 10 5. The jet potential core moreover lengthens slightly with ReD, but the flow properties do not change significantly farther downstream. The jets at higher Reynolds numbers are finally found to generate lower sound levels, with a decrease of about 2 dB over the range of Re D considered.

ABSTRACT Large-Eddy Simulations of isothermal round jets at a Mach number of 0.9 are performed in order to investigate the influence of the nozzle-exit boundary-layer thickness on initially highly disturbed subsonic jets at moderate Reynolds numbers. The jets are originating from a pipe nozzle of radius r0, and exhibit, at the exit section, peak disturbance levels of 9 per cent of the jet velocity, and mean velocity profiles similar to laminar boundary-layer profiles of thickness δ0 = 0.09r0, 0.15r0, 0.25r0 or 0.42r0, yielding momentum thicknesses δθ(0) between 0.012r0 and 0.05r0. Four jets at a diameter Reynolds number ReD = 5 × 104, providing momentum-thickness Reynolds numbers Reθ = 304, 486, 782 and 1288 depending on δ0, are first considered. Four jets at Reynolds numbers ReD = 8.3×104, 5×104, 3×104 and 1.8×104, with δ0 = 0.09r0, 0.15r0, 0.25r0 and 0.42r0, respectively, giving Reθ ≃ 480 in all cases, are then examined. The effects of δ0/r0 and Reθ on the jet flow and sound fields can thus be distinguished. At a constant ReD, thickening the initial shear layers mainly results in lower turbulence intensities in the mixing layers and weaker sound levels at all emission angles due to the variations of Reθ. Different trends are therefore obtained at a nearly identical Reθ. Increasing the ratio δ0/r0 in this case leads to a shorter potential core, higher centerline velocity fluctuations, and stronger noise in the downstream direction. © 2012 by the authors. Published by the American Institute of Aeronautics and Astronautics, Inc.

ABSTRACT This paper presents the continuation of an earlier work1 dealing with Large-Eddy Simulations (LES) of a Mach number 0.9 isothermal round jet at a Reynolds number of 105, whose boundary layers are tripped inside a pipe nozzle so as to exhibit, at the pipe exit, a laminar mean velocity profile of momentum thickness δ(0) = 0.018 times the jet radius and peak turbulent intensities around 9%. In order to further assess the validity of the flow and acoustic fields determined by LES, an additional simulation is performed using a finer grid, characterized by minimum mesh spacings of 0.20, 0.34 and 0.40 times δ(0), respectively, in the radial, azimuthal and axial directions. The results are compared to previous LES results, as well as to those obtained using the same grid for a jet with identical exit conditions, except for a double boundary-layer thickness, up to 8 radii downstream of the nozzle exit. The new simulation at higher resolution is shown to provide shear-layer solutions that are practically grid-converged and, more generally, numerically accurate and physically relevant2 data for the present initially nominally turbulent jet, which could serve in future studies.

ABSTRACT The influence of temperature on the flow and acoustic fields of high subsonic jets is investigated by computing one isothermal and three hot circular jets using large-eddy simulation. The jets have an identical velocity yielding a Mach number M = uj/ca = 0.9, and diameter-based Reynolds numbers ReD = ujD/vj between 2.5 × 104 and 105, where subscripts j and a denote inflow and ambient conditions. They are characterized by similar nozzle-exit boundary-layer parameters, including 9% of peak turbulence intensity. The isothermal jet is at a temperature Tj = Ta and at ReD = 105. The next two jets are at Tj = 1.5Ta and Tj = 2.25Ta, and have the same diameter as the isothermal jet, leading to ReD = 5×104 and ReD = 2.5×104, respectively. The last jet is also at Tj = 1.5Ta, but its diameter is doubled in order to obtain ReD = 105. In all cases, with rising temperature, the jets develop more rapidly with higher turbulence levels, and generate more noise at low frequencies and less noise at high frequencies in the flow direction, in agreement with corresponding measurements. The variations of the shear-layer properties and of the farfield pressure levels with Tj are however strongly dependent on the Reynolds number. For the jets at a constant diameter, due to the decrease in ReD, the mixing layers spread more quickly with higher velocity fluctuations and length scales, and the overall sound intensity increases. For the hot jet at ReD = 105, on the contrary, the flow field downstream of the nozzle does not change significantly with respect to the isothermal case, and a noise reduction is found as observed experimentally for high Reynolds number jets at M > 0.7.

ABSTRACT The influence of the nozzle-exit boundary-layer profile on high-subsonic round jets is investigated by performing compressible large-eddy simulations of four jets using lowdissipation numerical schemes. The jets are isothermal, and have a Mach number of 0.9 and a diameter-based Reynolds number of 5 × 104. They originate from a pipe nozzle in which a trip-like forcing is applied. In that way, they exhibit, at the exit section, around 6% of peak turbulence intensity and boundary-layer velocity profiles characterized by a momentum thickness of about 2.8% of the nozzle radius, yielding a Reynolds number around 700, and by shape factors equal to 1.68, 1.77, 2.01 and 2.36. The results from the fourth case with a laminar velocity profile differ significantly from those from the three first cases with transitional profiles, whose accuracy is shown by a grid refinement study. Clear trends are thus identified when the shape of the exit boundary-layer profile changes from laminar to turbulent. Higher azimuthal modes and higher Strouhal numbers are found to predominate, respectively, at the pipe exit close to the wall and early on in the mixing layers. The latter appear to develop more slowly, leading to a longer potential core, and weaker velocity fluctuations are obtained in the shear layers and on the jet axis. Finally, lower noise levels are generated in the acoustic field.

In this paper, Large-Eddy Simulations (LES) of five isothermal round jets at Mach number 0.9 and Reynolds number 10 5 originating from a pipe nozzle are reported. In the pipe, the boundary layers are tripped so that in all jets the mean velocity profiles at the exit agree with a Blasius profile for a laminar boundary layer of momentum thickness �� = 0.018 times the jet radius, and that the peak turbulent intensities are around 9% of the jet velocity. Two means of tripping the boundary layers and four grids are considered. The effects of the tripping method and of the grid resolution on the turbulent development of initially nominally turbulent jets are thus investigated.

ABSTRACT The effects of moderate Reynolds numbers on the flow and acoustic fields of initially highly disturbed isothermal round jets at Mach number M = 0.9 and diameter-based Reynolds numbers ReD between 2.5 × 104 and 2 × 105 are investigated using large-eddy simulation under carefully controlled conditions. To the best of our knowledge, this is the first comprehensive study of its kind. The jets originate at z = 0 from a pipe nozzle of radius r0, in which a tripping procedure is applied to the boundary layers. At the nozzle exit, laminar-like mean velocity profiles of thickness δ ≃ 0.15r0 and momentum thickness δθ ≃ 0.018r0, yielding Reynolds numbers Reθ varying from 256 to 1856 depending on ReD, and peak turbulence intensities around 9% of the jet velocity, are thus obtained. As the Reynolds number increases, the mixing layers develop more slowly, with smaller integral length scales and lower levels of velocity fluctuations. The axial profiles of turbulence intensities become smoother, showing a clear overshoot around z = 2r0 at ReD = 2.5 × 104, but a monotonical growth at ReD = 2 × 105. Velocity spectra downstream of the nozzle exit also broaden with ReD, as expected. Large-scale components usually observed in turbulent boundary layers and shear layers, characterized by Strouhal numbers Stθ ≃ 0.013 around z = r0 and by azimuthal spacings λθ ≃ δ, remain dominant, although the contribution of fine-scale structures with λθ ⩽ δ/2 strengthens. Moreover, with rising ReD, the jet potential core lengthens slightly, but the flow properties do not change significantly farther downstream. Finally, lower sound pressure levels are generated, with a decrease of about 2 dB over the range of ReD considered.