Conditioned sampling in an unsteady jet

Piatt, Michael; Viets, Hermann

doi:10.2514/6.1979-1857

Cited by 8 publications

(4 citation statements)

References 2 publications

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“…Background Various methods of jet excitation have been investigated widely, usually with a view to stimulating increased rates of mixing. The means of excitation include acoustic (Hill & Greene 1977), fluidic (Piatt & Viets 1979) and mechanical (Simmons, Lai & Platzer 1981) techniques. Described here is the flow within a nozzle that generates a precessing jet flow which has proved beneficial in combustion applications.…”

Section: Introductionmentioning

confidence: 99%

An axisymmetric ‘fluidic’ nozzle to generate jet precession

1998

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A continuously unstable precessing flow within a short cylindrical chamber following a large sudden expansion is described. The investigation relates to a nozzle designed to produce a jet which achieves large-scale mixing in the downstream field. The inlet flow in the plane of the sudden expansion is well defined and free from asymmetry. Qualitative flow visualization in water and semi-quantitative surface flow visualization in air are reported which identify this precession within the chamber. Quantitative simultaneous measurements from fast-response pressure transducers at four tapping points on the internal walls of the nozzle chamber confirm the presence of the precessing field. The investigation focuses on the flow within the nozzle chamber rather than that in the emerging jet, although the emerging flow is also visualized.Two flow modes are identified: a ‘precessing jet’ mode which is instantaneously highly asymmetric, and a quasi-symmetric ‘axial jet’ mode. The precessing jet mode, on which the investigation concentrates, predominates in the geometric configuration investigated here. A topologically consistent flow field, derived from the visualization and from the fluctuating pressure data, which describes a three-dimensional and time-dependent precessing motion of the jet within the chamber is proposed. The surface flow visualization quantifies the axial distances to lines of positive and negative bifurcation allowing comparison with related flows involving large-scale precession or flapping reported by others. The Strouhal numbers (dimensionless frequencies) of these flows are shown to be two orders of magnitude lower than that measured in the shear layer of the jet entering the chamber. The phenomenon is demonstrated to be unrelated to acoustic coupling.

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Section: Introductionmentioning

confidence: 99%

An axisymmetric ‘fluidic’ nozzle to generate jet precession

1998

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“…This has resulted in more complex systems which include features such as swirl (e.g., Syred and Beer, 1 ), bluff body recirculation (e.g., Gutmark et al 2 ) and various types of "excited" jet devices (e.g., Refs. [3][4][5][6].…”

Section: Introductionmentioning

confidence: 99%

Statistical analysis of the velocity field in a mechanical precessing jet flow

Nathan

2004

Physics of Fluids

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An experimental investigation of a precessing jet issuing from a mechanically rotating nozzle directed at an angle of ␣ = 45°relative to the axis of rotation is reported. Both conventional and conditional statistics of the velocity field of the jet were measured using a combined hot-wire and cold-wire (to identify any reverse flow) probe. Three distinct values (Ϸ0.005, 0.01, and 0.02) of the precession Strouhal number St p (ϵ rotation frequency ϫ nozzle diameter / jet exit bulk velocity) were used to assess the effect of varying St p . The measurements reveal that the Strouhal number in general has significant influence on the entire mixing field generated by a precessing jet. The occurrence of precession at all the Strouhal numbers of investigation produces a central recirculation zone at x ഛ 7d, where x is a distance measured from the rotating nozzle exit. A critical Strouhal number, i.e., St p,cr Ϸ 0.008 for the present case, is identified: at St p ജ St p,cr the core jet converges to the axis of rotation while at St p ജ St p,cr it does not. The characteristics of the turbulent flow in the near and intermediate regions are quite different and depend upon the magnitude of St p . The near-field region, x / d ഛ 10-15, is dominated by a regime of global precession of the entire jet. As a result, the large-scale entrainment of the ambient fluid is substantially enhanced while the fine-scale turbulent mixing is suppressed. Under the supercritical regime (i.e., St p ജ St p,cr ), the jet in the far field resembles some features of the nonprecessing counterpart. Nevertheless, significant differences still retain in the statistical properties.

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“…In the case of steady jets, the centerline velocity decay is a monotonically decreasing function of streamwise distance. In the unsteady jet case, the centerline velocity decay (where the centerline is a quasi-sinusoidal shape) has a typical behavior 17 shown in Figure 25. The velocity decays with streamwise distance, reaches a local minimum and starts to increase again.…”

Section: Quantitative Confirmationmentioning

confidence: 99%