### Abstract

A study has been made of the instability and the subsequent breakdown of axisymmetric jets of helium/air mixtures emerging into ambient air. Although the density of the nozzle gas is less than that of the ambient fluid, the jet is essentially non-buoyant. Two kinds of instability are observed in the near field, depending upon the mean flow parameters. When the ratio of the exiting nozzle fluid density to ambient fluid density is rho "SUB e" / rho "SUB infinity" 0.6, shear-layer fluctuations evolve in a fashion similar to that observed in constant-density jets: the power spectrum near the nozzle is determined by weak background disturbances whose subsequent spatial amplification agrees closely with the spatial stability theory. When the density ratio is 0.6 an intense oscillatory instability may also arise. The overall behaviour of this latter mode (to be called the 'oscillating' mode) is shown to depend solely upon the density ratio and upon D/ theta, where D is the nozzle diameter and theta is the momentum thickness of the boundary layer at the nozzle exit. The behaviour of this mode is found to be independent of the Reynolds number, within the range covered by the present experiments. This is even true in the immediate vicinity of the nozzle where, unlike in the case of shear-layer modes, the intensity of the oscillating mode is independent of background disturbances. The streamwise growth rate associated with the oscillating mode is not abnormally large, however. The frequency of the oscillating mode compares well with predictions based on a spatio-temporal theory, but not with those of the standard spatial theory. (from Authors)

Original language | English (US) |
---|---|

Pages (from-to) | 619-664 |

Number of pages | 46 |

Journal | Journal of Fluid Mechanics |

Volume | 249 |

State | Published - 1993 |

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### ASJC Scopus subject areas

- Computational Mechanics
- Mechanics of Materials
- Physics and Astronomy(all)
- Condensed Matter Physics

### Cite this

*Journal of Fluid Mechanics*,

*249*, 619-664.

**The instability and breakdown of a round variable-density jet.** / Kyle, D. M.; Sreenivasan, K. R.

Research output: Contribution to journal › Article

*Journal of Fluid Mechanics*, vol. 249, pp. 619-664.

}

TY - JOUR

T1 - The instability and breakdown of a round variable-density jet

AU - Kyle, D. M.

AU - Sreenivasan, K. R.

PY - 1993

Y1 - 1993

N2 - A study has been made of the instability and the subsequent breakdown of axisymmetric jets of helium/air mixtures emerging into ambient air. Although the density of the nozzle gas is less than that of the ambient fluid, the jet is essentially non-buoyant. Two kinds of instability are observed in the near field, depending upon the mean flow parameters. When the ratio of the exiting nozzle fluid density to ambient fluid density is rho "SUB e" / rho "SUB infinity" 0.6, shear-layer fluctuations evolve in a fashion similar to that observed in constant-density jets: the power spectrum near the nozzle is determined by weak background disturbances whose subsequent spatial amplification agrees closely with the spatial stability theory. When the density ratio is 0.6 an intense oscillatory instability may also arise. The overall behaviour of this latter mode (to be called the 'oscillating' mode) is shown to depend solely upon the density ratio and upon D/ theta, where D is the nozzle diameter and theta is the momentum thickness of the boundary layer at the nozzle exit. The behaviour of this mode is found to be independent of the Reynolds number, within the range covered by the present experiments. This is even true in the immediate vicinity of the nozzle where, unlike in the case of shear-layer modes, the intensity of the oscillating mode is independent of background disturbances. The streamwise growth rate associated with the oscillating mode is not abnormally large, however. The frequency of the oscillating mode compares well with predictions based on a spatio-temporal theory, but not with those of the standard spatial theory. (from Authors)

AB - A study has been made of the instability and the subsequent breakdown of axisymmetric jets of helium/air mixtures emerging into ambient air. Although the density of the nozzle gas is less than that of the ambient fluid, the jet is essentially non-buoyant. Two kinds of instability are observed in the near field, depending upon the mean flow parameters. When the ratio of the exiting nozzle fluid density to ambient fluid density is rho "SUB e" / rho "SUB infinity" 0.6, shear-layer fluctuations evolve in a fashion similar to that observed in constant-density jets: the power spectrum near the nozzle is determined by weak background disturbances whose subsequent spatial amplification agrees closely with the spatial stability theory. When the density ratio is 0.6 an intense oscillatory instability may also arise. The overall behaviour of this latter mode (to be called the 'oscillating' mode) is shown to depend solely upon the density ratio and upon D/ theta, where D is the nozzle diameter and theta is the momentum thickness of the boundary layer at the nozzle exit. The behaviour of this mode is found to be independent of the Reynolds number, within the range covered by the present experiments. This is even true in the immediate vicinity of the nozzle where, unlike in the case of shear-layer modes, the intensity of the oscillating mode is independent of background disturbances. The streamwise growth rate associated with the oscillating mode is not abnormally large, however. The frequency of the oscillating mode compares well with predictions based on a spatio-temporal theory, but not with those of the standard spatial theory. (from Authors)

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UR - http://www.scopus.com/inward/citedby.url?scp=0027529710&partnerID=8YFLogxK

M3 - Article

VL - 249

SP - 619

EP - 664

JO - Journal of Fluid Mechanics

JF - Journal of Fluid Mechanics

SN - 0022-1120

ER -