Axis-switching Phenomenon Research Articles

Effect of noncircular orifice plates on the near flow field of turbulent free jets

In this paper, we experimentally investigate the near-field flow characteristics of turbulent free jets respectively issued from circular, triangular, diamond, rectangular, and notched-rectangular orifice plates into air surroundings. All the orifice plates have identical opening areas or equivalent diameters (De) and their aspect ratios (AR) range from 1 to 6.5. Planar particle image velocimetry (PIV) is used to measure the velocity field at the same Reynolds number of Re = 5 × 104, where Re = UeDe/v with Ue being the exit bulk velocity and v the kinematic viscosity of fluid. The mean and turbulent velocity fields of all the five jets are compared in detail. Results show that the noncircular jets can enhance the entrainment rate, reflected by the higher acceleration rates of mean velocity decay and spread, shorten the length of the unmixed core, expedite the increase of turbulent intensity compared with the circular counterpart shortened unmixed core, and increase turbulent intensity comparing to the circular counterpart. Among the five jets, the rectangular jet (AR = 6.5) produces the greatest decay rate of the near-field mean velocity, postpones the position at which the `axis-switching' phenomenon occurs. This supports that axis switching phenomenon strongly depends on jet initial conditions. In addition, the hump in the centerline variation of the turbulence intensity is observed in the rectangular and triangular jets, but not in the circular jet, nor in diamond jet nor in notched-rectangular jet.

An experimental investigation on mixing enhancements in non-circular sharp-edged nozzles using the entropy production concept

An experimental study on mixing enhancement in free jets, issuing from sharp-edged nozzles of different geometry, is performed by using particle image velocimetry. The attention is focused on the jet near-field and interaction zones (0 < X/D < 18, where X is the axial coordinate and D the diameter of the equivalent circular jet). The mixing efficiency is evaluated and quantified using the definition of entropy production derived from the velocity field. The effect of Reynolds number is also discussed by performing measurements at Re = 8000 and Re = 35,000. The results are compared to circular nozzle data to evaluate the change in mixing efficiency among axisymmetric and non-symmetric nozzles. While the effect of Reynolds number on mixing is small, at least for the values tested here, the change in geometry is rather crucial. In terms of entropy production, the rectangular and elliptical nozzles show higher mixing for X/D< 7, whereas the other ones attain the best results for larger distances. This behaviour is basically related to the axis-switching phenomenon observed in elongated jets. Different variables are tested and compared as possible velocity and length scales to derive a meaningful non-dimensional entropy production.

Experimental study of elliptical jet from sub to supercritical conditions

The jet mixing at supercritical conditions involves fluid dynamics as well as thermodynamic phenomena. All the jet mixing studies at critical conditions to the present date have focused only on axisymmetric jets. When the liquid jet is injected into supercritical environment, the thermodynamic transition could be well understood by considering one of the important fluid properties such as surface tension since it decides the existence of distinct boundary between the liquid and gaseous phase. It is well known that an elliptical liquid jet undergoes axis-switching phenomena under atmospheric conditions due to the presence of surface tension. The experimental investigations were carried out with low speed elliptical jet under supercritical condition. Investigation of the binary component system with fluoroketone jet and N2 gas as environment shows that the surface tension force dominates for a large downstream distance, indicating delayed thermodynamic transition. The increase in pressure to critical state at supercritical temperature is found to expedite the thermodynamic transition. The ligament like structures has been observed rather than droplets for supercritical pressures. However, for the single component system with fluoroketone jet and fluoroketone environment shows that the jet disintegrates into droplets as it is subjected to the chamber conditions even for the subcritical pressures and no axis switching phenomenon is observed. For a single component system, as the pressure is increased to critical state, the liquid jet exhibits gas-gas like mixing behavior and that too without exhibiting axis-switching behavior.

Breakup of liquid jets from non-circular orifices

The purpose of this investigation is to study the effect of the orifice geometry on liquid breakup. In order to develop a better understanding of the liquid jet breakup, investigations were carried out in two steps—study of low-pressure liquid jet breakup and high-pressure fuel atomization. This paper presents the experimental investigations conducted to study the flow behavior of low-pressure water jets emanating from orifices with non-circular geometries, including rectangular, square, and triangular shapes and draws a comparison with the flow behavior of circular jets. The orifices had approximately same cross-sectional areas and were machined by electro-discharge machining process in stainless steel discs. The liquid jets were discharged in the vertical direction in atmospheric air at room temperature and pressure conditions. The analysis was carried out for gage pressures varying from 0 to 1,000 psi (absolute pressures from 0.10 to 6.99 MPa). The flow behavior was analyzed using high-speed visualization techniques. To draw a comparison between flow behavior from circular and non-circular orifices, jet breakup length and width were measured. The flow characteristics were analyzed from different directions, including looking at the flow from the straight edges of the orifices as well as their sharp corners. The non-circular geometric jets demonstrated enhanced instability as compared to the circular jets. This has been attributed to the axis-switching phenomenon exhibited by them. As a result, the non-circular jets yielded shorter breakup lengths as compared to the circular jets. In order to demonstrate the presence of axis-switching phenomenon in square and triangular jets, the jet widths were plotted along the axial direction. This technique clearly demonstrated the axis switching occurring in square and triangular jets, which was not clearly visible unlike the case of rectangular jets. To conclude, non-circular geometry induces greater instabilities in the liquid jets, thereby leading to faster disintegration. Thus, non-circular orifice geometries can provide a cheaper solution of improving liquid breakup and thus may enhance fuel atomization as compared to the precise manufacturing techniques of drilling smaller orifices or using costly elevated fuel injection pressure systems.

Some observations in the vortex-turning behaviour of noncircular inclined jets

An experimental comparison was conducted for Re = 2,500, free elliptic and rectangular jets inclined at 30° and 60° along major or minor planes. Regardless of the jet base geometry, minor-plane inclined jets produced vortex roll-ups that remain inclined. In contrast, major-plane inclined jets produce significantly stronger vortex-roll-up turning behaviour. Interestingly, major-plane inclined rectangular jets exhibit strong vortex-overturning behaviour, where the vortex-roll-up inclination exceeds the 0° incline angle considerably. Vortex-turning extents and rates are compared between major-plane inclined elliptic and rectangular jets here and support present qualitative observations. Closer inspections reveal that the lack of axis-switching phenomenon in major-plane inclined rectangular jets allows vortex-overturning behaviour. In addition, jet centreline deflection is most sensitive in minor-plane inclined jets, where increasing the incline angle leads to a decrease and an increase in the elliptic and rectangular jet deflection, respectively.

Near-field structure of underexpanded elliptic jets

Quantitative measurements of velocity for a low aspect ratio underexpanded elliptical jet are presented. Four jets at nozzle pressure ratios NPR = [2.2, 2.6, 3.4, 4.2] are studied for a smoothly contoured elliptical nozzle with an aspect ratio a/b = 2. High-resolution planar particle image velocimetry is used to extract information about the velocity fields and turbulent statistics. All four jets display the expected axis switching phenomenon, with the axis switching gaining strength with increasing pressure ratio. Evidence of regular reflection is seen for jets at pressure ratios of 2.6 and above. Measurements of fluctuating velocity indicate an oscillatory flapping mode in the minor axis plane for all but the highest pressure ratio. At the highest pressure ratio of NPR = 4.2, there is evidence of a cut-and-connect vortex bifurcation previously only observed for jets with higher aspect ratio.

Numerical Simulation of the Breakup of Elliptical Liquid Jet in Still Air

The numerical simulation of liquid jets ejecting from a set of elliptical orifices with different aspect ratios between 1 (circular) and 3.85 is performed for several Weber numbers, ranging from 15 to 330. The axis-switching phenomenon and breakup length of the jets are characterized by means of a volume of fluid (VOF) method, together with a dynamic mesh refinement model. This three-dimensional simulation is compared with a recent experimental work and the results agree well. It is concluded that for Weber numbers ranging from 15 to 100, by increasing the Weber number, the breakup length of the liquid jet increases, reaches a peak, and then decreases suddenly.

PIV measurements of turbulent jets issuing from triangular and circular orifice plates

The present study experimentally investigated the near-field flow mixing characteristics of two turbulent jets issuing from equilateral triangular and circular orifice plates into effectively unbounded surroundings, respectively. Planar particle image velocimetry (PIV) was applied to measure the velocity field at the same Reynolds number of Re=50,000, where Re = U e D e/ν with U e being the exit bulk velocity and ν the kinematic viscosity of fluid, D e the equivalent diameters. The instantaneous velocity, mean velocity, Reynolds stresses were obtained. From the mean velocity field, the centreline velocity decay rate and half-velocity width were derived. Comparing the mixing characteristics of the two jets, it is found that the triangular jet has a faster mixing rate than the circular counterpart. The triangular jet entrainments with the ambient fluid at a higher rate in the near field. This is evidenced by a shorter unmixed core, faster Reynolds stress and centreline turbulence intensity growth. The primary coherent structures in the near field are found to break down more rapidly in the triangular jet as compared to the circular jet. Over the entire measurement region, the triangular jet maintained a higher rate of decay and spread. Moreover, all components of Reynolds stress of the triangular jet appear to reach their peaks earlier, and then decay more rapidly than those of the circular jet. In addition, the axis-switching phenomenon is observed in the triangular jet.

Mean and fluctuating velocity fields of a diamond turbulent jet

The present paper reports the first investigation on a turbulent jet issuing from a diamond orifice (hereafter termed a “diamond jet") with an aspect ratio of 1.7. Velocity measurements were conducted in the transitional region, and the exit Reynolds number of the jet was 50000. For comparison, a round jet with identical normalized boundary conditions was also measured. It is shown that the diamond jet decays and spreads faster than the round jet does over the measured flow region. The axis-switching phenomenon is observed in the diamond jet. Although both jets display primary coherent structures in the near field, these structures are found to break down more rapidly in the diamond jet, due to the higher three-dimensionality of the flow. Moreover, the streamwise components of the Reynolds normal stress and all the shear stresses reach their maxima around the location of the maximal mean shear while the maxima of the lateral components of the Reynolds normal stresses occur around the centreline of the jet.

Characteristic study of non-circular incompressible free jet

This paper reports an experimental investigation of bulk properties of turbulent, which is three dimensional, incompressible, air jets issuing into still air surrounding from the nozzles. The jet orifices utilized included circular, hexagonal and cruciform geometries. Experimental results of pertinent mean flow properties such as axis velocity decay, half width growth, potential core and turbulence intensities are reported. Single Hotwire anemometer was used for measurements of the velocity field. The experiment for the three jets was conducted under the same nominal conditions with the exit Reynolds number of 15,400. Consistent with previous investigations of other non circular jets, the cruciform jet is found to have an overall superior mixing capability over the circular counter part. Immediately downstream of the nozzle exit, it entrains, and then mixes with, the surroundings at a higher rate. This jet has a shorter potential core with higher rates of decay and spread than the circular jet. This phenomenon of axis switching is also found to occur in this jet.

NUMERICAL SIMULATION OF FREE CROSS-SHAPED JET

Numerical simulation of a turbulent cross-shaped orifice jet using the SST kω model is compared with measurements. The SST kω model is able to predict the axis-switching phenomenon. The initial boundary layers are lower than reference values and turbulent kinetic energy is over predicted. The flow expansion in the minor plane and flow contraction in the major plane are more pronounced and the potential core length is overestimated generating a delay on the streamwise maximum velocity decay. However, satisfactory predictions of the flow transverse area and volumetric flow rate are observed and lead to good predictions of “mean flow radius” and “mean flow velocity” evolutions. We are interested in optimizing HVAC diffusers using the passive control of the jet induction. The lobed perforated panel is an innovative diffuser among others. The obtained results show that the SST kω model is an efficient tool for quickly optimizing non-circular orifice design.DOI: http://dx.doi.org/10.5755/j01.mech.18.4.2328

LARGE EDDY SIMULATION OF AN ELLIPTIC JET INJECTED INTO A SUPERSONIC CROSSFLOW

A transverse jet issuing from an elliptic injector into a supersonic crossflow has been investigated using large eddy simulation. The complex flow structures and the relevant flow features are analyzed to exhibit the evolution of shock structures, vertical structures and jet shear layer. A horseshoe vortex is formed in the upstream of the jet and the shock structures exhibit small fluctuations due to the flow interaction. The kidney-shaped counter-rotating vortex pair dominates the flow field in the downstream of the jet. The elliptic jet spreads rapidly in the spanwise direction and then the axis-switching phenomenon occurs. Intense turbulent fluctuations are identified behind the Mach disk because of the large velocity gradients.

Effect of Orifice Shape on Flow Behavior and Impingement Heat Transfer

Impingement jets are widely used in industries because they provide a high heat transfer coefficient near the stagnation region. However, few methods exist for controlling impingement heat transfer. Recently, a peculiar diffusion process called for three-dimensional free jet has begun to attract attention, and there is a novel possibility of control diffusion and mixture process using this phenomenon. In this paper, we report on the effect of non-circular polygonal orifice shapes on impingement heat transfer. In addition, we demonstrate axis-switching phenomenon by using flow visualization with hydrogen bubbles. Orifice configurations are the regular polygons with 3 to 6 sides. Heat transfer experiments covered the distance between the orifice-to-target plate is 4 to 8 and Reynolds number is 5 × 10 4 and the heat flux is 600 W/m 2 . The flow was visualized in Reynolds number 1,500. For a free jet emerging from a regular polygonal orifice, the location of axis-switching phenomenon shifts toward the orifice exit as the number of sides on the orifice is increased. The iso-Nusselt number profile tends to take the shape of a concentric circle farther upstream. However, with a decrease in the number of sides of the orifice, the iso-Nusselt number profile after axis switching remains downstream. ing the flow using hydrogen bubbles. We also investigated the heat transfer characteristics of an impinging air jet from a non-circular orifice shapes including triangle, square, penta- gon, and hexagon. Moreover, we examined the possibility of controlling impinging heat transfer by changing the orifice configuration.

On Turbulent Jets Issuing from Notched-Rectangular and Circular Orifice Plates

This paper reports an experimental investigation of the near-field flow characteristics of two air/air turbulent jets issuing respectively from notched-rectangular and circular orifice plates with identical opening areas or equivalent diameters (De). Planar particle image velocimetry (PIV) was used to measure the velocity field at the same Reynolds number, based on De, of Re = 72,000. Consistent with previous work on other noncircular jets, the present study finds that the notched jet has a higher rate of mixing than does the circular counterpart. In particular, this jet in the very near field transfers its momentum to the surroundings at a greater rate, evidenced by a notably shorter unmixed core and faster turbulence intensity growth. The higher rates of overall decay and spread of the notched jet are maintained over the entire measurement region and likely beyond. In addition, the phenomenon of axis switching is also found to occur in this jet.

Magnetic Field Effects on Axis-Switching and Instabilities in Rectangular Plasma Jets

The effect of an external magnetic field on the evolution of rectangular plasma jets is examined. Specifically investigated is the influence of a primarily axial magnetic field on the uniquely characteristic axis-switching phenomenon of rectangular jets and flow instabilities. The results indicate that the magnetic field decelerates the jet (more rapid spreading), prevents axis-switching and inhibits instabilities. The key physical mechanisms underlying the changes are (1) the ability of the magnetic field to reverse the direction of vorticity and (2) transfer of energy from kinetic to magnetic forms. This study has important implications for magneto-hydro-dynamic flow control and propulsion applications.

Studies of magnetic axis switching phenomenon in magnetite

The results of magnetic and structural studies of magnetic easy axis switching in magnetite are presented. This phenomenon was first observed after the easy axis was established by the in-field cooling across the Verwey transition. Although the subsequent application of magnetic field at low temperatures along certain <1 0 0> is almost as effective in establishing the easy axis as field cooling, the repeated easy axis redirection below T V causes the field required to switch the axis to become lower. We have also shown that the change of magnetic easy axis is reflected in the change of crystallographic c direction. Finally, the energy required to switch the axis was calculated and was found comparable to the characteristic energy of the Verwey transition.

LES of turbulent square jet flow using an MRT lattice Boltzmann model

In this paper we consider the application of multiple-relaxation-time (MRT) lattice Boltzmann equation (LBE) for large-eddy simulation (LES) of turbulent flows. The implementation is discussed in the context of 19-velocity (D3Q19) MRT-LBE model in conjunction with the Smagorinsky subgrid closure model. The MRT-LBE-LES is then tested in the turbulent square jet flow case. We compare MRT-LBE-LES results with (a) single-relaxation-time (SRT) or BGK LBE results and (b) experimental data. Computed results include the distribution of centerline mean streamwise velocity, jet spread, and spanwise profiles of mean streamwise velocity in the near-field region. The phenomenon of axis switching is investigated. The advantages of MRT over SRT are demonstrated. Reasonable agreement between our numerical results and experimental data demonstrate that the MRT-LBE is a potentially viable tool for LES of turbulence.

Effect of Delta Tabs on Axis Switching of a Coaxial Square Jet

The effects of delta tabs in the near field of a coaxial square jet have been studied experimentally using a laser Doppler anemometer. The tabs are added to the centers of the four sidewalls of the inner nozzle trailing edge, with the tip of each tab tilted into the inner jet, that is, θ = 45 deg, or into the outer jet, that is, θ = -45 deg. For each tab orientation, three velocity ratios have been considered: γ u = 0, 0.5, and 1.0 for θ = 45 deg and γ u = 1.0, 2.0, and oo for θ = -45 deg. The results illustrate that tabs increase the mixing between the inner jet and outer jet and may promote or suppress the phenomenon of axis switching, depending on the orientation of the tabs. Axis switching is defined as follows: As a noncircular (here square) jet spreads, its cross-section may evolve from the initial shape similar to that the jet exit, to have its axis rotated some angle at some downstream location. In the case of θ =45 deg, the cross-sectional shape of the inner mixing layers is bifurcated into a four-finger structure, which tends to suppress the onset of axis switching. When θ = -45 deg, it exhibits an eightfold mode, and a rapid axis-switching occurs in the near-field. The effect of tabs is briefly discussed using inviscid vortex dynamics.

On the near-field of a square jet with vortex-generating tabs

A square jet with four vortex-generating tabs has been investigated by flow visualizations using Laser Induced Fluorescence technique and by velocity measurements using a Laser Doppler Anemometer. The tabs, which are triangular in shape, are tilted into the jet at either θ=45° or 135° with respect to the main flow direction. The flow visualization results illustrate that with the addition of fourtabs, the cross-section of the jet core is bifurcated into a four-“finger” structure. For θ=135° in particular, there is an additional “mushroom” structure between two neighboring fingers. Secondary velocities show that a pair of streamwise vortices is generated by each tab. These streamwise vortices tend to spread the jet fluids “outward” along the diagonal direction, resulting in a four-finger structure of the jet core. In the case of θ=135°, the additional third pair of streamwise vortices previously proposed by other researchers is confirmed in the present study, which is responsible for the unique mushroom structure. The tab effects on the jet spreading and on the axis-switching phenomenon are also discussed.

Direct numerical simulations of two-phase laminar jet flows with different cross-section injection geometries

Direct numerical simulations are performed of spatial, three-dimensional, laminar jets of different inlet geometric configurations for the purpose of quantifying the characteristics of the flows; both single-phase (SP) and two-phase (TP) free jets are considered. The TP jets consist of gas laden with liquid drops randomly injected at the inlet. Drop evaporation ensues both due to the gaseous flow being initially unvitiated by the vapor species corresponding to the liquid drops, and to drop heating as the initial drop temperature is lower than that of the carrier gas. The conservation equations for the TP flow include complete couplings of mass, momentum, and energy based on thermodynamically self-consistent specification of the vapor enthalpy, internal energy, and latent heat of vaporization. Inlet geometries investigated are circular, elliptic, rectangular, square, and triangular. The results focus both on the different spreading achieved according to the inlet geometry, as well as on the considerable change in the flow field due to the presence of the drops. The most important consequence of the drop interaction with the flow is the production of streamwise vorticity that alters entrainment and species mixing according to the inlet geometry. Similar to their SP equivalent, TP jets are shown to reach steady-state entrainment; examination of the flows at this time station shows that the potential cores of TP jets are shorter by an order of magnitude than their SP counterpart. Moreover, whereas the TP circular jet exhibits a symmetric entrainment pattern well past the streamwise location of the potential core, noncircular jets display at the same location strong departures from symmetry. Furthermore, the SP-jet phenomenon of axis switching is no longer present in TP jets. The distributions of drop-number density, liquid mass, and evaporated species are compared for different inlet cross sections and recommendations are made regarding the optimal choice for different applications.