Linear stability and acoustic characteristics of compressible, viscous, subsonic coaxial jet flow

Gloor, Michael; Obrist, Dominik; Kleiser, Leonhard

doi:10.1063/1.4816368

Cited by 24 publications

(21 citation statements)

References 42 publications

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“…This stabilizing effect for small separation lengths between the shear layers was also found in Ref. 18 for a spatial analysis. This physical mechanism based on the interaction of counter-propagating Rossby waves explains for example instabilities in stratified shear flows with statically stable density fields 44 .…”

Section: Physical Mechanism Driving the Absolute Instability In Issupporting

confidence: 54%

“…The coaxial base flow supports two unstable Kelvin-Helmholtz modes, which are coupled to the inner or outer velocity shear layer 18 and are denoted as the inner mode and the outer mode, respectively. Previous investigations on heated single jets considered only one unstable mode, which in our case corresponds to the inner mode for h = 0.…”

Section: A Influence Of the Bypass-velocity Ratio Hmentioning

confidence: 99%

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Absolute and convective instabilities of heated coaxial jet flow

2015

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This study investigates the inviscid, linear spatio-temporal stability of heated, compressible and incompressible coaxial jet flows. The influence of the temperature ratio and the velocity ratio between the core jet and the bypass stream on the transition from convectively to absolutely unstable flows is studied numerically. The investigation shows that for coaxial jets absolute instability can occur for considerably lower core-stream temperatures than for single jets. The reason for this modified stability character is the appearance of an additional unstable mode as a result of the outer velocity shear layer between the bypass stream and the ambient flow. The presence of two shear layers enables the interaction between otherwise free waves to give rise to new instabilities. When the bypass-stream velocity is increased, the classical absolute mode known from single jets (inner mode) is first stabilized and then destabilized for high bypass-stream velocities, whereas the outer mode reaches maximum spatiotemporal growth rates when the core-stream velocity is approximately equal to twice the bypass-stream velocity. Additionally, it is demonstrated that the spatio-temporal character of the modes is very sensitive to the shear-layer thickness and to the distance separating the two layers. Increasing the Mach number strongly dampens the onset of an absolute instability for both modes. a) gioele.balestra@epfl.ch

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Section: Physical Mechanism Driving the Absolute Instability In Issupporting

confidence: 54%

Section: A Influence Of the Bypass-velocity Ratio Hmentioning

confidence: 99%

Absolute and convective instabilities of heated coaxial jet flow

2015

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“…5(a) and (b), respectively. The streamwise disturbance growth in the initially laminar inflow is considerably stronger in the outer shear layer, as predicted by linear stability theory, 32 whereas the peak of the velocity fluctuations can be observed closer to the centerline in the vicinity of the end of the potential core region. As pointed out earlier, it is important to realize that this strong peak is not solely due to the instability mechanism of the inner shear layer but also due to the shedding of vorticity towards the core region from the outer shear layer, which broke down further upstream.…”

Section: Statistical Description Of Fluctuationsmentioning

confidence: 88%

“…The velocity ratio w s /w p acts as an important control parameter for the stability of the inner and outer velocity shear layers as has already been shown by our linear stability analysis of the inflow profiles. 32 The ratio w s /w p not only affects the growth rates of the Kelvin-Helmholtz instabilities that develop at the two interfaces between the core jet and the bypass stream (inner shear layer) and between the bypass stream and the ambient flow (outer shear layer) but also increases or reduces their phase velocities. Both, growth characteristics and phase velocities of the instability waves, are of high relevance for their potential to radiate noise.…”

Section: Statistical Description Of Fluctuationsmentioning

confidence: 99%

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Flow Dynamics and Aeroacoustics of Heated Coaxial Jets at Subsonic Mach Numbers

Gloor

Buehler

Kleiser

2014

20th AIAA/CEAS Aeroacoustics Conference

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The flow field and the noise radiation of coaxial jet flows with a hot primary stream are investigated numerically by Large-Eddy Simulation and Direct Noise Computation (DNC). The core-stream velocity is subsonic based on local conditions but mostly supersonic with respect to the ambient speed of sound. The flow field and the associated acoustic radiation are analyzed systematically for a range of velocity and temperature ratios between the primary (core) and the secondary (bypass) stream. Strong mixing of cold and hot fluid and the intermittent character of the flow field close to the end of the potential core lead to a pronounced noise radiation at an aft angle of approximately 35 • . The unsteady pressure field within the jet flow is analyzed and radiating contributions are extracted using a Fourier-filtering technique. NomenclatureMach number Ma p,0 ≡ Ma 0 primary jet Mach number M a p,∞ = w * 0 /a * ∞ ambient Mach number (core) M a s,∞ = w * s /a * ∞ ambient Mach number (bypass) N number of grid points P power spectral density estimate p pressure ∆r, ∆θ, ∆z grid spacings ∆t simulation time step (r, θ, z) cylindrical coordinates R 1,2 primary / secondary jet radius r mic distance of virtual microphonesradial, azimuthal, axial velocity w 0 core-jet velocity z c length of potential core α axial wavenumber κ heat conductivity (ξ, η, ζ) computational coordinates µ dynamic viscosity ω circular frequency φ radiation angle ρ density Subscripts 0 reference quantity at r = z = 0 ∞ ambient conditions cl centerline quantity (r = 0) p primary (core) stream s secondary (bypass) stream rms root-mean-square Superscripts ′′ Favre fluctuation quantity b base flow * dimensional quantityConventions · temporal and azimuthal average

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