2 THEORY OF JETS 2.1 Introduction 2.2 Overview of Jet Flows 2.3 Structure and Development of a Free Jet 2.4 Factors Influencing Jet Spread 2.5 Conclusion 2.1 Introduction (original) (raw)
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Flow, Turbulence and Combustion, 2009
Turbulent free jets issuing from five different nozzle geometries; smooth pipe, contracted circular, rectangular, triangular, and square, are experimentally investigated by using TSI 2-D laser Doppler velocimetry (LDV) to assess the effect of nozzle geometry and quarl (i.e. a cylindrical sudden expansion) on jet entrainment and spreading. The centerline mean velocity decay and the jet half-velocity width, which are indicators of jet entrainment and spreading rates, are determined for each nozzle's flow configuration, i.e. with and without sudden expansion. Furthermore, turbulence quantities, such as the flow mean velocities and their mean fluctuating components, as well as Reynolds shear stresses, are all measured along the centerline plane of the jet to facilitate understanding the extent of the effect of nozzle's geometry (i.e. nozzle's orifice shape and sudden expansion) on jet's entrainment and spreading. The main results show that the jet flow with the presence of sudden expansion exhibits higher rates of entrainment and spreading than without. In addition, these results reveal that sudden expansion exercises a greater effect on the asymmetric jet characteristics, especially for the triangular and rectangular nozzles compared to their axisymmetric counterparts (i.e. circular contracted nozzle).
The Influence of Nozzle Shape Upon the Evolution of Axisymmetric Free Jet
2014
The paper presents the results of research devoted to the experimental verification of the self-sustained oscillation in axisymmetric jet, which were found in numerical experiment of A. Boguslawski et al. (2013). Preliminary experiments and LES calculations reported in the above paper allowed to formulate the idea that in the free axisymmetric jet there is a possibility to obtain the regime of self-sustained oscillations, provided that the boundary layer at the nozzle exit is sufficiently thin and the perturbation level sufficiently low. In particular it was found that the thickness of the exit shear layer is of great importance for the existence of regime of self-sustained oscillations. The main problems with the comparison of experimental and numerical investigations in free jets is the identity of initial conditions, which in the flow considered should be of matter of extreme care. As it was stated in the recent review paper by Ball and Pollard (2012) the round jet is extremely s...
On the role of initial conditions on a turbulent plane jet: The role oif nozzle exit area
The dependence of statistical flow properties of a turbulent plane jet on the nozzle-exit area was investigated experimentally. Jet flows from three plane nozzles were measured using hot wire anemometry at an identical Reynolds number (Re = U co H / ) of 9,000 and nozzle aspect ratio of 36, but with nozzle-exit areas of A = 900 mm 2 , 3600 mm 2 and 14400 mm 2 , whereby U o,c was the mean exit velocity, was the kinematic viscosity of air, and H was the slot opening width. Results demonstrated a longer potential core and a higher decay rate for the case with a larger nozzle-exit area. The centerline turbulence intensity (u*) showed a strong dependence on A in the near field but this dependence weakened progressively as the jet propagated downstream. The jet flow from the nozzle with a larger exit area produced a relatively higher near field turbulence intensity, indicating a significant dependence of near field vortical structures on the nozzle-exit area. The centerline turbulence intensity profiles became selfsimilar, collapsing on a single curve only for x/H 10. Overall, the dependence of the mean and turbulent velocity field on nozzle-exit area was significant, confirming the influence of initial and boundary conditions.
Fluid Dynamics of a High Aspect-Ratio Jet
9th AIAA/CEAS Aeroacoustics Conference and Exhibit, 2003
Circulation control wings are a type of pneumatic high-lift device that have been extensively researched as to their aerodynamic benefits. However, there has been little research into the possible airframe noise reduction benefits of a circulation control wing. The key element of noise is the jet noise associated with the jet sheet emitted from the blowing slot. High aspect-ratio jet acoustic results (aspect-ratios from 100 to 3,000) from a related study showed that the jet noise of this type of jet was proportional to the slot height to the 312 power and slot width to the 112 power. Fluid dynamic experiments were performed in the present study on the high aspect-ratio nozzle to gain understanding of the flow characteristics in an effort to relate the acoustic results to flow parameters. Single hot-wire experiments indicated that the jet exhaust from the high aspect-ratio nozzle was similar to a 2-d turbulent jet. Two-wire space-correlation measurements were performed to attempt to find a relationship between the slot height of the jet and the length-scale of the flow noise generating turbulence structure. The turbulent eddy convection velocity was also calculated, and was found to vary with the local centerline velocity, and also as a function of the frequency of the eddy. Nomenclature A-Area (typically of nozzle) AR-Aspect ratio a-Speed of sound a,,-Ambient speed of sound f-Frequency HARN-High aspect-ratio nozzle h-Slot height or rectangular nozzle height (small dimension) L-Characteristic length L L,-Length scale (streamwise direction) L,-Integral time scale M,-Convection Mach number-Characteristic length for the HARN, Leg = h 3/%1/4 M,-Jet centerline Mach number PIV-Particle image velocimetry p-pressure Re-Reynolds number R,-Space-correlation R,,-Cross-correlation w-width of rectangular nozzle (large dimension) V-Velocity V,-Turbulent eddy convection velocity Vj-Jet exit velocity (fully expanded) V,-Local jet centerline velocity x-Streamwise dimension, typically x = 0 is nozzle exit y-Dimension perpendicular to major axis of nozzle, y = 0 is center of nozzle z-Dimension along span of nozzle (parallel to major axis), z = 0 is center of nozzle Ax-Spacing between hot-wires in x-direction Ay-Spacing between hot-wires in y-direction p-density z-Time shift, used for cross-correlation
Experimental and quantitative investigation of a free round jet
The European Physical Journal Applied Physics, 2010
Flow visualization, basic image processing and statistical treatment are used to investigate the flow characteristics of an axisymmetric free air jet having an initially uniform velocity profile. Vortical structures and their development in the transition zone of the jet are visualized for a range of Reynolds numbers from 1300 to 1700. It is shown the onset and growth of these structures which propagate downstream of the nozzle exit at approximately 0.65 of the velocity of the mean flow. The duration of this development mechanism, before reaching the zone of turbulence, is estimated at 7.6 × 10 −3. Near-field and intermediate-field region lengths, separation distance between two neighboring vortical structures, apparition frequency and jet Strouhal number are measured as functions of the Reynolds number. In particular, it is shown that the Strouhal number, which is estimated at 0.64 in our case, is independent of Reynolds numbers.
Velocity and Passive Scalar Characteristics in a Round Jet with Grids at the Nozzle Exit
Flow, Turbulence and Combustion formerly `Applied Scientific Research', 2000
Velocity and passive scalar (temperature) measurements have been made in the near field of a round jet with and without obstructing grids placed at the jet exit. The Reynolds number Re D (based on the exit centreline velocity and nozzle diameter) is 4.9 × 10 4 and the flow is incompressible, while the temperature rise does not affect the velocity behaviour. The streamwise development and radial spreading of the passive scalar are attenuated, relative to the unobstructed jet. Close to the jet outlet, the spatial similarity of the moments (up to the third-order) of velocity fluctuations is improved, when the jet is perturbed. An explanation, based on the reduced effect of the large coherent structures in the developing region, is provided.
Hydrodynamics During the Transient Evolution of Open Jet Flows from/to Wall Attached Jets
Flow, Turbulence and Combustion, 2016
Swirl stabilized flows are the most widely deployed technology used to stabilize gas turbine combustion systems. However, there are some coherent structures that appear in these flows close to the nozzle whose occurrence and stability are still poorly understood during transition. The external recirculation zone and the Precessing Vortex Core to/from the Coanda effect are some of them. Thus, in this paper the transition of an Open Jet Flow-Medium Swirl flow pattern to/from a Coanda jet flow is studied using various geometries at a fixed Swirl number. Phase Locked Stereo Particle Image Velocimetry and High Speed Photography experiments were conducted to determine fundamental characteristics of the phenomenon. It was observed that the coherent structures in the field experience a complete annihilation during transition, with no dependency between the structures formed in each of the flow states. Moreover, transition occurs at a particular normalized step size whilst some acoustic shifts in the frequencies of the system were noticed, a phenomenon related to the strength of the vortical structures and vortices convection. It is concluded that a transient, precessing, Coanda Vortex Breakdown is formed, changing flow dynamics. The structure progresses to a less coherent Trapped Vortex between the two states. During the phenomenon there are different interactions between structures such as the Central Recirculation Zone, the High Momentum Flow Region and the Precessing Vortex Core that were also documented.
EFFECTS OF THE SHAPE OF A NOZZLE WITH CHEVRONS ON THE DYNAMICS OF TURBULENT IMPINGING JET
This work is a numerical study of a turbulent impinging jet issuing from a nozzle with chevrons. The Reynolds number based on the jet exit velocity and nozzle diameter is equal to 5000 corresponding to a low Mach number of 0.0057 at the nozzle exit. The main objectives of the investigation, inspired by the work of Violato et al.(Int. J. of Heat and Fluid Flow, 37, 2012) , are to highlight, from a fundamental point of view, the effects of the nozzle shape and the nozzle-to-plate distance on the mean parameters characterizing the dynamics of the flow in question. The nozzle configurations considered are a circular nozzle without chevrons and nozzles provided with 4 and 6 chevrons. The nozzle-to-plate distance ranges from 2 to 6 nozzle diameters. All the other flow conditions and geometrical parameters used in the different cases treated are identical. Interesting features of the flow are revealed by the obtained results of averaged three-dimensional fields of velocity and turbulent kinetic energy, particularly close to the wall. An attempt is made to bring additional insight into the phenomena in the free jet, the impingement region and the wall jet when using 4, 6 and no chevrons, for different nozzle-to-plate distances.
Jet diffusion from a circular nozzle above a solid plane
Journal of Fluid Mechanics, 1980
The decay of a jet discharging from a circular nozzle parallel to and displaced from a solid surface is investigated under conditions where the transitional process from circular-jet flow to oblate wall-jet flow begins in the initial, transition or self-preserving regions of the original jet. The influence of displacement of the nozzle from the plane on the developed three-dimensional wall jet downstream is demonstrated and it is found that the transitional interaction with the plane is more extended when the plane interacts first in the initial zone of the circular jet. Measurements of turbulence and Reynolds stress show the transverse mixing parallel to the plane to exceed that perpendicular to the plane, and are generally consistent with the spreading rates in these two directions, the ratio of which approaches 8·5 at large distances from the nozzle. It is shown that the interaction between the plane and jet involves a relatively large-scale coherent motion in which components of...