Structure and mixing of a transverse jet in incompressible flow (original) (raw)

The Time Evolution of the Flow Fields and Mixing Characteristics of Non-Reactive Jets

2010

Numerical simulations are used to study an under-expanded sonic jet injected into a supersonic crossflow and an over-expanded supersonic jet injected into a subsonic crossflow. A finite volume compressible Navier–Stokes solver developed by Park & Mahesh (2007) for unstructured grids is used. The flow conditions are based on Santiago et al.’s (1997) and Beresh et al.’s (2005) experiments for sonic and supersonic injection, respectively. The simulations successfully reproduce experimentally observed flow vortical structures and shock systems such as the barrel shock, Mach disk, horseshoe vortices that wrap up in front of the jet and the counter rotating vortex pair (CVP) downstream of the jet. The time averaged flow fields are compared to the experimental results, and reasonable agreement is observed.

Transverse jet mixing characteristics

This experimental study explores and quantifies mixing characteristics associated with a gaseous round jet injected perpendicularly into cross-flow for a range of flow and injection conditions. The study utilizes acetone planar laser-induced fluorescence imaging to determine mixing metrics in both centreplane and cross-sectional planes of the jet, for a range of jet-to-cross-flow momentum flux ratios (2 J 41), density ratios (0.35 S 1.0) and injector configurations (flush nozzle, flush pipe and elevated nozzle), all at a fixed jet Reynolds number of 1900. For the majority of conditions explored, there is a direct correspondence between the nature of the jet's upstream shear layer instabilities and structure, as documented in detail in Getsinger et al. (J. Fluid Mech., vol. 760, 2014, pp. 342–367), and the jet's mixing characteristics, consistent with diffusion-dominated processes, but with a few notable exceptions. When quantified as a function of distance along the jet trajectory, mixing metrics for jets in cross-flow with an absolutely unstable upstream shear layer and relatively symmetric counter-rotating vortex pair cross-sectional structure tend to show better local molecular mixing than for jets with convectively unstable upstream shear layers and generally asymmetric cross-sectional structures. Yet the spatial evolution of mixing with downstream distance can be greater for a few specific convectively unstable conditions, apparently associated with the initiation and nature of shear layer rollup as a trigger for improved mixing. A notable exception to these trends concerns conditions where the equidensity jet in cross-flow has an upstream shear layer that is already absolutely unstable, and the jet density is then reduced in comparison with that of the cross-flow. Here, density ratios below unity tend to mix less well than for equidensity conditions, demonstrated to result from differences in the nature of higher-density cross-flow entrainment into lower-density shear layer vortices.

An incompressible jet in a weak crossflow

Journal of Fluid Mechanics, 1993

A description of the incipient bending of a round incompressible jet issuing into a weak crossflow is presented. Axial vorticity is shown to appear from the early stages of the jet evolution owing to the distortion and reorientation of the azimuthal vorticity, and it eventually dominates the flow around the jet and determines the shape of its cross-section. The crossflow is considered weak enough for the distortion of the jet to occur downstream of its development region, where diffusion already influences the whole cross-section and the jet can be modelled as a point source of momentum. Axial pressure gradient and axial diffusion are negligible under these conditions, since the jet is a slender structure. Sufficiently near the origin, a finite-length entraining wake is identified on the lee side of the jet, which gradually merges with the main core. A t the same time, the cross-section begins to acquire a characteristic elongated shape, with the jet concentrating in a thin layer. Farther downstream the axial vorticity of the jet rearranges into a couple of large locally two-dimensional contrarotating vortices standing against the wind under the action of their own induced velocity, and a smaller vortex sheet coinciding with the distorted jet.

Effects of controlled vortex generation and interactions in transverse jets

Physical Review Fluids

This experimental study examined the effects of controlled vortex generation and interactions created by axisymmetric excitation of a transverse jet, with a focus on the structural and mixing characteristics of the flow. The excitation consisted of a double-pulse forcing waveform applied to the jet, where two distinct temporal square-wave pulses were prescribed during a single forcing period. The two distinct pulses produced vortex rings of different strength and celerity, the strategic selection of which promoted vortex ring interactions or collisions in the near field to varying degrees. Jet flow conditions corresponding to a transitionally convectively and absolutely unstable upstream shear layer (USL) in the absence of forcing, at a jet-to-cross-flow momentum flux ratio of J = 10, and to an absolutely unstable USL at J = 7, were explored for a jet Reynolds number of 1800. Acetone planar laser-induced fluorescence imaging was utilized to quantify the influence of different prescribed temporal waveforms. All forcing conditions enhanced the spread, penetration, and molecular mixing of the jet as compared to the unforced jet, though to differing degrees. Interestingly, when the jet was convectively unstable, forcing which promoted vortex collisions provided the greatest enhancement in molecular mixing, whereas the absolutely unstable jet produced the greatest enhancement in mixing when the vortex rings did not interact, with important implications for optimized jet control.

An Experimental Analysis of the Mixing Flow Field of a Turbulent Cross Jet

1990

There are indeed many people who deserve my deepest gratitude for their support and assistance during the many months of me writing my thesis. I would fIrst like to thank my supervisor Dr. Diane Enns for her unyielding encouragement, forthrightness, and critical vision. Thank you to the always cheerful and supportive faculty and staff in the Philosophy Department at McMaster University. A special thank you goes out to my many graduate colleagues and friends who have been there with their laughter and support at every step. My deepest debt of gratitude is of course reserved for my parents and family who have through their actions, shown me what love is. Most importantly, thank you to Paul who continuously shows me just how delightful love can be.

VORTEX FORMATION IN INCOMPRESSIBLE AXISYMMETRIC FREE JETS

Free jets have been utilized extensively in many industrial applications. In general jet fluid discharging from a nozzle develops flow oscillations in the shear layer. The oscillations will roll up to eventually become toroidal vortices which increase in size with the axial distance from the nozzle. In the present work, flow visualization as well as hot-film anemometry have been employed in order to study incompressible axisymmetric free jet in moderate Reynolds numbers up to 20,000. The injection of liquid dye or micro particles associated with a laser sheet turns possible to visualize the shear layers and, consequently, the vortex formation. Hot-film measurements into the jet allow determining the flow velocity profile. Flow visualization is a qualitative tool which a broad view of the flow topology. On the other hand, hot-film anemometry is a precise quantitative tool but measurement in only one location at a time. The association of flow visualization and hot-film anemometry shows very adequate for free jet studies.

The velocity field produced by a submerged jet directed upwards at a free surface

International Journal of Heat and Mass Transfer, 1983

This paper considers the flow in a round,turbulent submerged water jet directed normaIly at a free water surface from below. Axial velocity distributions are presented and compared to those of the corresponding free jet. The radial velocity distribution on the surface above the jet was measured and these results are present cd and compared with the distribution for a waIl jet produced by jet impingement on to a rigid surface. The overall pattern of the flow in the test tank was observed by flow visualisation. It is demonstrated that knowledge of these parameters wiIlbe useful in modelling heat and mass transfer in flows of this kind.

Influence of Issued Jet Conditions on Mixing of Confined Flows

The development of the velocity and scalar fields in a coaxial jet mixer has been experimentally investigated applying simultaneously Particle Image Velocimetry (PIV) and Planar Laser Induced Fluorescence (PLIF) methods. Mixing of a turbulent jet (water solution of Rhodamin 6G) issued from a long round nozzle (l/d = 60) with co-flow (water) was studied. Because of the nozzle length the boundary layers of the inner flow already merged upstream of the jet exit. The issued jet conditions were changed installing vortex generators (tabs) at the nozzle exit. The tabs of rectangular and triangular forms with heights of 13 15 % the inner nozzle diameter accelerated mixing significantly but the scalar field developed to the homogeneous state faster than the velocity one. Turbulent characteristics measured downstream of the jet exit gave evidence the mixing specific behind the rectangular and triangular tabs.