The role of density discontinuity in the inviscid instability of two-phase parallel flows

Behzad, M.

doi:10.1063/1.4864779

Cited by 3 publications

(4 citation statements)

References 18 publications

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“…We refer to such a wave "shear-density wave". Similar wave has also been obtained by Behzad and Ashgriz 4 (they refer to it as "density wave") as well as Yellin-Bergovoy, Heifetz, and…”

Section: 21)supporting

confidence: 81%

“…Parallel flows are often produced when two streams of different velocities and densities flow side by side. For example in fuel injection systems, a fast stream of gas flows over a slow stream of liquid (fuel), making the liquid-gas interface prone to shear instabilities 3,4 . Here density stratification plays a crucial role in the instability dynamics, however gravity does not.…”

Section: Introductionmentioning

confidence: 99%

“…As mentioned previously, many multi-phase systems are incompressible flows having fluids of different densities (not necessarily small) flowing side by side, e.g. air-sea interface 10 , fuel injectors 3,4 , displacement flows 11,12 . Finite density variations in the same fluid due to a stratifying agent (like temperature) is also possible, e.g.…”

Section: Introductionmentioning

confidence: 99%

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On the inertial effects of density variation in stratified shear flows

Guha

Raj

2018

Physics of Fluids

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In this paper, we first revisit the celebrated Boussinesq approximation in stratified flows. Using scaling arguments we show that when the background shear is weak, the Boussinesq approximation yields either (i)is the ratio of density variation to the mean density and F r c is the ratio of the phase speed to the long wave speed. The second clause implies, contrary to the commonly accepted notion, that a flow with large density variations can also be Boussinesq.Indeed, we show that deep water surface gravity waves are Boussinesq while shallow water surface gravity waves arent. However, in the presence of moderate/strong shear, Boussinesq approximation implies the conventionally accepted A t O(1). To understand the inertial effects of density variation, our second objective is to explore various non-Boussinesq shear flows and study different kinds of stably propagating waves that can be present at an interface between two fluids of different background densities and vorticities. Furthermore, three kinds of density interfaces -neutral, stable and unstable -embedded in a background shear layer, are investigated. Instabilities ensuing from these configurations, which includes Kelvin-Helmholtz, Holmboe, Rayleigh-Taylor and triangular-jet, are studied in terms of resonant wave interactions. The effects of density stratification and the shear on the stability of each of these flow configurations are explored. Some of the results, e.g. the destabilizing role of density stratification, stabilizing role of shear, etc. are apparently counter-intuitive, but physical explanations are possible if the instabilities are interpreted from wave interactions perspective.

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“…We refer to such a wave "shear-density wave". Similar wave has also been obtained by Behzad and Ashgriz 4 (they refer to it as "density wave") as well as Yellin-Bergovoy, Heifetz, and…”

Section: 21)supporting

confidence: 81%

Section: Introductionmentioning

confidence: 99%

Section: Introductionmentioning

confidence: 99%

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On the inertial effects of density variation in stratified shear flows

Guha

Raj

2018

Physics of Fluids

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“…In many investigation, the velocity profile between two mixing layers is considered stable. [41][42][43][44] However, the importance of the relaxation of the velocity profile for jets in quiescent air was pointed out by Sterling and Sleicher 45 who argued that profile relaxation has a destabilizing effect just as the aerodynamic interaction and Weber's theory is not quantitatively correct if the liquid jet velocity profile relaxation is not taken into account. The inviscid linear stability analysis of Ibrahim and Marshall 29 also points out that as the profile in the liquid jet becomes uniform downstream, the instability becomes more pronounced.…”

Section: Temporal Developmentmentioning

confidence: 99%

How do liquid fuel physical properties affect liquid jet development in atomisers?

Charalampous

Hardalupas

2016

Physics of Fluids

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The influence of liquid fuel properties on atomisation remains an open question. The droplet sizes in sprays from atomisers operated with different fuels may be modified despite the small changes of the liquid properties. This paper examines experimentally the development of a liquid jet injected from a plain orifice in order to evaluate changes in its behaviour due to modifications of the liquid properties, which may influence the final atomisation characteristics. Two aviation kerosenes with similar, but not identical physical properties are considered, namely standard JP8 kerosene as the reference fuel and bio-derived Hydro-processed Renewable Jet (HRJ) fuel as an alternative biofuel. The corresponding density, dynamic viscosity, kinematic viscosity and surface tension change by about +5%, -5%, -10% and +5% respectively, which are typical for ‘drop-in’ fuel substitution. Three aspects of the liquid jet behaviour are experimentally considered. The pressure losses of the liquid jet through the nozzle are examined in terms of the discharge coefficient for different flowrates. The morphology of the liquid jet is visualised using high magnification Laser Induced Fluorescence (LIF) imaging. Finally, the temporal development of the liquid jet interfacial velocity as a function of distance from the nozzle exit is measured from time-dependent motion analysis of dual-frame LIF imaging measurements of the jet. The results show that for the small changes in the physical properties between the considered liquid fuels, the direct substitution of fuel did not result in a drastic change of the external morphology of the fuel jets. However, the small changes in the physical properties modify the interfacial velocities of the liquid and consequently the internal jet velocity profile. These changes can modify the interaction of the liquid jet with the surroundings, including air flows in coaxial or cross flow atomisation, and influence the atomisation characteristics during changes of liquid fuels

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Azimuthal shear instability of a liquid jet injected into a gaseous cross-flow

Behzad

Mashayek

2015

J. Fluid Mech.

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We investigate azimuthal instabilities which exist on the periphery of a non-turbulent liquid jet injected transversely into a gaseous cross-flow. We predict that the temporal growth of such instabilities may lead to the formation of interface corrugations, which are eventually sheared off of the jet surface (known as the jet 'surface breakup'). In this study we employ temporal linear stability analyses to understand the nature of these instabilities. The analysis is based on a continuous formulation of momentum equations in which the jet and cross-flow are considered to be slightly miscible at the vicinity of the interface. We identify the shear instability as the primary destabilization mechanism in the flow. This inherently inviscid mechanism opposes the previously suggested mechanism of surface breakup (known as 'boundary-layer stripping'), which is based on a viscous interpretation. The results show that the wavelengths of instabilities increase by moving away from the jet windward stagnation point toward the leeward point. We also investigate the influence of the jet-to-cross-flow density ratio on the flow stability and find that a higher ratio leads to formation of instabilities with higher wavenumbers on the jet surface. The results show that the density may have a non-monotonic stabilizing/destabilizing effect on the flow.

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The role of density discontinuity in the inviscid instability of two-phase parallel flows

Cited by 3 publications

References 18 publications

On the inertial effects of density variation in stratified shear flows

On the inertial effects of density variation in stratified shear flows

How do liquid fuel physical properties affect liquid jet development in atomisers?

Azimuthal shear instability of a liquid jet injected into a gaseous cross-flow

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