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The effect of vortex generators on a jet in a cross-flow (1997)

Zaman, K. B. M. Q. ; Foss, J. K.

[S.l.] : American Institute of Physics (AIP)

Physics of Fluids 9 (1997), S. 106-114

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ISSN: 1089-7666

Source: AIP Digital Archive

Topics: Physics

Notes: The effect of vortex generators in the form of tabs on the penetration and spreading of a jet in a cross-flow has been studied experimentally. It is found that the tab has very little effect when placed on the leeward side, i.e., on the downstream edge of the jet nozzle relative to the free-stream flow. A study of the static pressure distribution reveals significantly lower pressures on the leeward side. Thus, when placed on that side the tab does not produce a "pressure hill'' of sufficient magnitude that is the primary source of streamwise vorticity in the flow field over the tab. This qualitatively explains the ineffectiveness. In comparison, there is a significant effect on the flow field when the tab is placed on the windward side. The sense of vorticity generated by the tab in the latter configuration is opposite to that of the bound vortex pair that otherwise characterizes the flow. Thus, the strength of the bound vortex pair is diminished and the jet penetration is reduced. © 1997 American Institute of Physics.

Type of Medium: Electronic Resource

URL: http://dx.doi.org/10.1063/1.869154

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2

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Initial turbulence effect on jet evolution with and without tonal excitation (1989)

Raman, G. ; Zaman, K. B. M. Q. ; Rice, E. J.

New York, NY : American Institute of Physics (AIP)

Physics of Fluids 1 (1989), S. 1240-1248

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ISSN: 1089-7666

Source: AIP Digital Archive

Topics: Physics

Notes: The effect of the initial turbulence level on the development of an axisymmetric jet was experimentally investigated. The turbulence intensity at the jet exit was varied, over the range 0.15%–5%, by using screens and grids placed upstream of an 8.8 cm diam nozzle. Top-hat initial mean velocity profiles with approximately identical boundary layer states were ensured in all cases; the turbulence was homogeneous and the spectra were broadband. It was found, contrary to earlier reports, that the natural jet evolution remained essentially unchanged for varying initial turbulence intensities. The response of the jet to single frequency, plane wave excitation was then studied over the full range of initial turbulence intensities. It was found that for even the highest turbulence (5%), the jet was quite excitable and could be influenced measurably by a tone of small amplitude. However, the jet "excitability,'' as assessed from the variations of the mean velocity as well as the total and fundamental rms amplitudes on the axis, diminished with increasing initial turbulence. As the initial turbulence was increased, the amplitude of the discrete tone had to be increased in order to achieve the same excited state of the jet. It was also found that there existed a limit to the jet excitability, beyond which no additional effect could be achieved, as the amplitude of the discrete tone was increased. Results are also shown for a case having no grid or boundary layer trip, yielding a nominally laminar boundary layer at the jet exit. This case illustrates the profound effect of the initial boundary layer state on the jet evolution and excitability. This jet decayed the fastest naturally, and consequently, it was the least excitable, in spite of very low initial turbulence intensity (0.15%).

Type of Medium: Electronic Resource

URL: http://dx.doi.org/10.1063/1.857347

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3

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Control of an axisymmetric jet using vortex generators (1994)

Zaman, K. B. M. Q. ; Reeder, M. F. ; Samimy, M.

[S.l.] : American Institute of Physics (AIP)

Physics of Fluids 6 (1994), S. 778-793

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ISSN: 1089-7666

Source: AIP Digital Archive

Topics: Physics

Notes: The results of an experimental investigation on the effect of vortex generators, in the form of small tabs at the nozzle exit on the evolution of a jet, are reported in this paper. Primarily tabs of triangular shape are considered, and the effect is studied up to a jet Mach number of 1.8. Each tab is found to produce a dominant pair of counter-rotating streamwise vortices having a sense of rotation opposite to that expected from the wrapping of the boundary layer. This results in an inward indentation of the mixing layer into the core of the jet. A triangular-shaped tab with its apex leaning downstream, referred to as a delta tab, is found to be the most effective in producing such vortices, with a consequential large influence on the overall jet evolution. Two delta tabs, spaced 180° apart, completely bifurcate the jet. Four delta tabs stretch the mixing layer into four "fingers,'' resulting in a significant increase in the jet mixing downstream. For six delta tabs the mixing layer distortion settles back to a three finger configuration through an interaction of the streamwise vortices. The tabs are found to be equally effective in jets with turbulent or laminar initial boundary layers. Two sources of streamwise vorticity are postulated for the flow under consideration. One is the upstream "pressure hill,'' generated by the tab, which constitutes the main contributor of vorticity to the dominant pair. Another is due to vortex filaments shed from the sides of the tab and reoriented downstream by the mean shear of the mixing layer. Depending on the orientation of the tab, the latter source can produce a vortex pair having a sense of rotation opposite to that of the dominant pair. In the case of the delta tab, vorticity from the two sources add, explaining the strong effect in that configuration.

Type of Medium: Electronic Resource

URL: http://dx.doi.org/10.1063/1.868316

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4

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Asymptotic spreading rate of initially compressible jets—experiment and analysis (1998)

Zaman, K. B. M. Q.

[S.l.] : American Institute of Physics (AIP)

Physics of Fluids 10 (1998), S. 2652-2660

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ISSN: 1089-7666

Source: AIP Digital Archive

Topics: Physics

Notes: Experimental results for the spreading and centerline velocity decay rates for round, compressible jets, from a convergent and a convergent-divergent nozzle, are presented. The spreading rate is determined from the variation of streamwise mass flux obtained from Pitot probe surveys. Results for the far asymptotic region show that both spreading and centerline velocity decay rates, when nondimensionalized by parameters at the nozzle exit, decrease with increasing "jet Mach number" MJ. Dimensional analysis with the assumption of momentum conservation, together with compressible flow calculations for the conditions at the nozzle exit, predict this Mach number dependence well. The analysis also demonstrates that an increase in the "potential core length" of the jet occurring with increasing MJ, a commonly observed trend, is largely accounted for simply by the variations in the density and static pressure at the nozzle exit. The effect of decreasing mixing efficiency with increasing compressibility is inferred to contribute only partially to the latter trend.

Type of Medium: Electronic Resource

URL: http://dx.doi.org/10.1063/1.869778

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Reversal in spreading of a tabbed circular jet under controlled excitation (1997)

Zaman, K. B. M. Q. ; Raman, G.

[S.l.] : American Institute of Physics (AIP)

Physics of Fluids 9 (1997), S. 3733-3741

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ISSN: 1089-7666

Source: AIP Digital Archive

Topics: Physics

Notes: Detailed flow field measurements have been carried out for a turbulent circular jet perturbed by tabs and artificial excitation. Two "delta tabs" were placed at the nozzle exit at diametrically opposite locations. The excitation condition involved subharmonic resonance that manifested in a periodic vortex pairing in the near flow field. While the excitation and the tabs independently increased jet spreading, a combination of the two diminished the effect. The jet spreading was most pronounced with the tabs but was reduced when excitation was applied to the tabbed jet. The tabs generated streamwise vortex pairs that caused a lateral spreading of the jet in a direction perpendicular to the plane containing the tabs. The excitation, on the other hand, organized the azimuthal vorticity into coherent ring structures whose evolution and pairing also increased entrainment by the jet. In the tabbed case, the excitation produced coherent azimuthal structures that were distorted and asymmetric in shape. The self-induction of these structures produced an effect that opposed the tendency for the lateral spreading of the streamwise vortex pairs. The passage of the distorted vortices, and their pairing, also had a cancellation effect on the time-averaged streamwise vorticity field. These led to the reduction in jet spreading. © 1997 American Institute of Physics.

Type of Medium: Electronic Resource

URL: http://dx.doi.org/10.1063/1.869511

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