The effects of Reynolds number and Richardson number on the structure of a vertical co-flowing buoyant jet

Subbarao, E. R.

doi:10.2514/6.1989-1800

Cited by 5 publications

(8 citation statements)

References 7 publications

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“…In the latter case the amplitude of the excitation was small enough so that there was no visual difference between the selfexcited flow and the driven flow. Velocity measurements confirm that the velocity fluctuations a t the jet exit were the same for the self-excited and the weakly driven cases (Subbarao 1987). Strong excitation has relatively little effect on flow structure unless the imposed perturbations are very large.…”

Section: Experimental Apparatusmentioning

confidence: 52%

“…Checks on flow uniformity and free-stream turbulence level were made for a wide range of pressures and velocities, and the free-stream turbulence intensity is under 1 % over the range of velocities used. Complete tabulation of the flow quality studies and experimental results may be found in Subbarao (1987).…”

Section: Experimental Apparatusmentioning

confidence: 99%

“…Small deviations from the parabolic shape apparent in the figure are due to fluctuations in the exit velocity induced by unsteady vortex formation just downstream of the exit (which flattens the mean profile slightly) and by acceleration of the exit flow due to buoyancy and the co-flow which causes the exit velocity to slightly exceed the velocity expected for a fully developed pipe flow. Conditional measurements of the exit velocity profile as a function of the phase of the vortex formation process can be found in Subbarao (1987). Checks on axisymmetry of the mean flow a t the jet exit were made and appear in figure 3, which includes measurements in two orthogonal diametric planes.…”

Section: Experimental Apparatusmentioning

confidence: 99%

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Investigation of a co-flowing buoyant jet: experiments on the effect of Reynolds number and Richardson number

Subbarao

Cantwell

1992

J. Fluid Mech.

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Experiments have been carried out on a vertical jet of helium issuing into a co-flow of air at a fixed exit velocity ratio of 2.0. At all the experimental conditions studied, the flow exhibits a strong self-excited periodicity. The natural frequency behaviour of the jet, the underlying flow structure, and the transition to turbulence have been studied over a wide range of flow conditions. The experiments were conducted in a variable-pressure facility which made it possible to vary the Reynolds number and Richardson number independently. A stroboscopic schlieren system was used for flow visualization and single-component laser-Doppler anemometry was used to measure the axial component of velocity. The flow exhibits several interesting features. The presence of co-flow eliminates the random meandering typical of buoyant plumes in a quiescent environment. The periodicity of the helium jet under high-Richardson-number conditions is striking. Under these conditions transition to turbulence consists of a rapid but highly structured and repeatable breakdown and intermingling of jet and free-stream fluid. At Ri = 1.6 the three-dimensional structure of the flow is seen to repeat from cycle to cycle. The point of transition moves closer to the jet exit as either the Reynolds number or the Richardson number increases. The wavelength of the longitudinal instability increases with Richardson number. At low Richardson numbers, the natural frequency scales on an inertial timescale, τ1 = D/Uj where D is the jet diameter and Uj is the mean jet exit velocity. At high Richardson number, the natural frequency scales on a buoyancy timescale, τ2 = [ρjD/g(ρ∞ -ρj)]½ where g is the gravitational acceleration and ρj and ρ∞ are the jet and free-stream densities respectively. The transition from one flow regime to another occurs over a narrow range of Richardson numbers from 0.7 to 1. A buoyancy Strouhal number is used to correlate the high-Richardson-number frequency behaviour.

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Section: Experimental Apparatusmentioning

confidence: 52%

Section: Experimental Apparatusmentioning

confidence: 99%

Section: Experimental Apparatusmentioning

confidence: 99%

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Investigation of a co-flowing buoyant jet: experiments on the effect of Reynolds number and Richardson number

Subbarao

Cantwell

1992

J. Fluid Mech.

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“…The density gradient can also be due simply to the presence of different species in the medium. 1 Examples of situations or problems involving buoyant jets include the emission of pollutant into the atmosphere or oceanic waters, safety and fire hazards associated with leakage of gases such as hydrogen into air 2 , and heating issues associated with an uneven temperature distribution within combustion systems. Of a particular interest in relation to the present work is the case of the Ultra Compact Combustor (UCC).…”

Section: Introductionmentioning

confidence: 99%

“…In 1989, Subbarao studied a vertical buoyant with a co-flow and noted the scarcity of the studies of this combination. 1 Although buoyancy effects have been studied widely in the literature since then, most of the investigations were concerned with jets into ambient air and not into a co-flow of air. The relevance of the present work stems from the need to characterize the effects of buoyancy and its governing parameters on the trajectory of a jet in a co-flow and its mixing properties.…”

Section: Introductionmentioning

confidence: 99%

Time Resolved Filtered Rayleigh Scattering Measurement of a Buoyant Jet In a Co-flow

Benhassen¹,

Polanka²,

Reeder³

2011

49th AIAA Aerospace Sciences Meeting Including the New Horizons Forum and Aerospace Exposition

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The filtered Rayleigh scattering technique is implemented using a continuous wave laser in conjunction with a high speed camera and a molecular filter to obtain near field concentration measurements of a carbon dioxide jet in a co-flow of air with a sampling rate of 400 Hz. The beam from a 12-Watt Coherent Verdi laser was directed through the jet, of diameter ranging from 8.9 mm to 17.1 mm, at two streamwise stations, spaced about one diameter apart, and image sequences were collected with a Phantom high-speed camera for up to 20 seconds. In addition to the mean of the concentration and standard deviation of the scattered light intensity, space-time correlations of multiple points in the flow field are illustrated. The jet trajectory and mixing properties are investigated with and without the co-flow. The results demonstrate the significance the addition of the co-flow has on flattening the jet trajectory and increasing the mixing rate. The effects of various flow parameters such as the jet velocity, the jet-to-co-flow velocity ratio, the relative velocity between the jet and the co-flow, the Froude number, and the Reynolds number on the jet trajectory are studied. The results indicate that, within the range of test conditions, the velocity of the jet has the most significant effect on the jet trajectory. Furthermore, the investigation of the jet trajectory with respect to the Froude number highlighted the need to use a more robust Froude number definition that takes into account the effects of the addition of a co-flow in the flow field.

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Investigation of an excited jet diffusion flame at elevated pressure

Strawa

Cantwell

1989

J. Fluid Mech.

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Experiments have been carried out with the objective of studying the relationship between flow structure, flow excitation and the reaction process in the near field of a low-speed coflowing jet diffusion flame. The effect of axial forcing and increasing pressure on the structure and controllability of the flame has been studied in an attempt to elucidate some of the underlying mechanisms of control. The experiments were conducted in a variable-pressure flow facility which permits the study of reacting flows at pressures ranging from 10 to 1000 kPa (0.1 to 10 atm.). The flame was excited by adding a small-amplitude, periodic fluctuation to the central fuel jet exit velocity. The flow was visualized using an optical scheme which superimposes the luminous image of the flame on its schlieren image, giving a useful picture of the relationship between the luminous soot-laden core flow and the edge of the surrounding hot-gas envelope. Phase-conditioned velocity measurements were made with a one-component laser Doppler anemometer. The excitation frequency was varied, and it was found that a narrow band of frequencies exists in which several of the instabilities of the flow seem to be in coincidence, causing the flame to break up periodically into a series of distinct eddies. Hereafter this will be called the strongly coupled state. Maps of the one-dimensional velocity vector field, viewed in a frame of reference convecting with the large eddies, are used to study the topology of the flow. When the excitation frequency lies above the strongly coupled range, the flow pattern is found to contain stagnation points which straddle the axis of the jet. When the excitation frequency is reduced to a point where strong coupling occurs the stagnation points move onto the axis promoting breakup of the flame. As the pressure is increased, the relative role of diffusion is decreased and the flame becomes highly three-dimensional. In the strongly coupled state, the flow continues to be very periodic, even to the extent that much of the three-dimensional structure is repeatable from cycle to cycle.

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The effects of Reynolds number and Richardson number on the structure of a vertical co-flowing buoyant jet

Cited by 5 publications

References 7 publications

Investigation of a co-flowing buoyant jet: experiments on the effect of Reynolds number and Richardson number

Investigation of a co-flowing buoyant jet: experiments on the effect of Reynolds number and Richardson number

Time Resolved Filtered Rayleigh Scattering Measurement of a Buoyant Jet In a Co-flow

Investigation of an excited jet diffusion flame at elevated pressure

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