High Aspect Ratio Jet Research Articles

The influences of jet axis switching and aerodynamic focusing on aerosol deposition in converging–diverging slit impactors

The deposition pattern or “linewidth” of an inertial particle deposit from acceleration of an aerosol through a high aspect ratio slit nozzle-substrate system is influenced by particle size-dependent aerodynamic focusing. In addition to this, high aspect ratio jets will eventually undergo downstream “jet-axis switching”, wherein the jet profile shrinks along its major axis and elongates in the direction of the minor axis. Jet axis switching in an aerosol may also lead to “rotated” particle deposits. In this study, we use a converging–diverging slit nozzle system with a major axis length of 8 mm and a throat width (minor axis) of 200 μm to examine the deposition patterns of monodisperse particles in the 100 nm–5 μm diameter range, in air with an upstream pressure of 252 Torr and variable downstream pressure in the 3–50 Torr range. The nozzle-to-substrate distance L∗ is varied from 90 to 248 times longer than the throat width. We find deposition patterns that are strongly dependent on particle size, downstream pressure, and L∗. Depending on particle diameter and operating conditions, we obtain deposits resembling the nozzle dimensions and orientation, deposits which are completely switched (perpendicular to the nozzle), or deposits which are relatively symmetric and focused at a center point. The latter appear to be the result of the combined effects of jet axis switching and aerodynamic focusing. In general, the influence of jet axis switching is more pronounced on smaller particles at higher downstream pressures, and with larger distances to the substrate. The extent of switching and area of the deposit are also both inversely related to the particle Stokes number.

Investigation of the Normal Blowing Approach to Controlling Wingtip Vortex Using LES

The characteristics and control of a wingtip vortex are of great significance when considering drag reduction and flight safety of transportation aircrafts. The associated aerodynamic phenomenon resulting from rolling up of a wingtip vortex includes boundary layer flow, shear layer separation, and vortex breakdown, while the interaction of a wingtip vortex with the airframe causes induced drag, wingtip noise, etc. This paper studies a normal blowing method utilized to control the wingtip vortex. Large eddy simulation (LES) technique applied to a straight NACA0012 wing having a chord length ( c ) of 0.4 m is adopted for this study. The Reynolds number based on the chord length is 1.6 × 10 6 and the angle of attack is 12°. The computational approach utilized the dynamic Smagorinsky-Lilly subgrid model for 3D simulations. Normal blowing from a high aspect ratio jet from the wingtip lower surface was used to control the wingtip vortex. From 0.05c to 0.30c, the blowing slit width was 1 mm, with the slit exit treated as a velocity inlet boundary condition which supplied the blowing jet with a momentum coefficient of 0.28%. Results of axial velocity and span-wise pressure distribution of the clean airfoil presented good agreement with known experimental data. LES results indicate that normal blowing suppresses the primary vortex strength, while the vortex core radius, maximum induced velocity, axial vorticity flux, and pressure peak of the primary vortex are reduced by 25%, 28%, 46%, and 52%, respectively. Flow field structures before and after blowing show that blowing suppresses the shedding, coiling, and convergence of the free vortex layers near the primary vortex. This study also shows that normal blowing generates a jet-induced vortex at the location of the secondary vortex, while backflow, volume expansion, and spiral burst can be observed in the jet-induced vortex. The bursting jet-induced vortex destroys the jet-like flow structure of the primary vortex at the trailing edge.

Axis-switching and breakup of rectangular liquid jets

The behavior of low-speed liquid jets emerging from rectangular orifices into a quiescent air is studied numerically. After ejection, the rectangular cross-section transforms into an elliptical form along the jet and while axis-switching includes elliptical cross-sections only, the rectangular shape never establishes again. The optimum wavenumber, corresponding to the most dominant wave, is found to be greater in orifices with higher aspect ratios and, as a result, breakup length of the jet will be shorter. The breakup length decreases exponentially with the initial amplitude of disturbances. Moreover, it is observed that the form of final breakup leads to elimination of the satellite droplets at the optimum wavenumber with small and uniform main droplets. It is also shown that high-aspect ratio jets have a more extended range of effective disturbances. The results provide an in-depth insight into the effect of instabilities on axis-switching, breakup, and droplet formation.

Helical Shear-Flow Stabilization of an Astrophysically Relevant Laboratory Plasma Jet.

Astrophysical jets are collimated, high-speed outflows observed to be natural features of celestial objects that spin and accrete matter. From protoplanetary nebula and young stellar objects to active galactic nuclei, common features suggest that universal mechanisms may lead to the remarkable straightness observed in many jets. Here we report observations from a new astrophysically relevant laboratory plasma jet experiment demonstrating the formation of long-lived, collimated, high aspect-ratio jets. The magnetized jets have strong helical shear flows and remain stable to instabilities over many growth times. These observations corroborate theoretical predictions that strong helical shear flows can stabilize current-driven instabilities in magnetically confined plasmas, solar prominences, and magnetically driven astrophysical jets in nature.

Vortex topology of wing tip blowing

An extensive parametric study was conducted to examine the effect of continuous blowing from high aspect ratio jets on the tip vortex in the near wake. It was found that the high aspect ratio jet produces a pair of counter-rotating vortices of unequal strength; their interaction with the tip vortex leads to single or multiple-vortex wakes. The flow topology of the wake depends on a number of parameters such as the blowing configuration, the blowing coefficient and the wing incidence. Vertical and spanwise blowing were examined, from different vertical and longitudinal positions. In most cases, blowing from near the pressure surface led to diffused vortices in the near wake with blowing from near the suction surface producing the opposite effect. The tip shape was also varied; the round tip showed the ability to promote the Coanda effect, thus modifying the vortex interaction. The wing incidence was found to affect not only the tip vortex but also the jet-generated vortices.

Flow-structure interaction effects on a jet emanating from a flexible nozzle

In recent years, a wide variety of applications have been found for the use of pulsed jets in the area of flow control. The goal of the current study was to identify the flow field and mixing characteristics associated with an incompressible elongated jet emitted from a flexible nozzle. The shape of the nozzle was that of a high aspect ratio jet deforming from a fully opened to a completely closed configuration. The jet was characterized by a pulsatile flow that was self-excited by the motion of the flexible tube. The frequency of excitation was found to be between 150 and 175 Hz and the Strouhal number (nondimensional frequency) varied from 0.17 to 0.45. The jet flow was dominated by vortices that were shed from the nozzle with an axis parallel to the major axis. The vortices in the near field were quasi-two-dimensional so that measurements performed at the center plane represented the dynamics of the entire vortex. The nozzle excited two different modes depending on the tension applied to the flexible nozzle and the volumetric flow through it. The first was a flapping mode, which was associated with alternate shedding of vortices. This caused strong steering of the jet to one side or the other. The second mode was a symmetric mode that was associated with the formation of counter-rotating vortex pairs. Turbulence and jet spread in the measured planes were much larger in the first mode than the second one.

Near-field developments of elliptic jets in crossflow

An experimental study has been carried out for aspect ratio AR = 0.3 and 3 elliptic jets in crossflow, with the results being compared to a circular jet in crossflow using the digital particle image velocimetry technique. Results show that for AR = 0.3 and 3 elliptic jets, which are significantly streamlined and blunt with respect to the crossflow respectively, unsteady foci observed earlier in AR = 0.5 and 2 elliptic and circular jet lee-side regions are either non-existent or not well defined. Mean peak vorticity values along leading-edge and lee-side shear layers are also observed to be higher for elliptic jets than for the circular jet, regardless of aspect ratio. Intriguingly, AR = 0.3 elliptic jet had lower vorticity values than its AR = 0.5 counterpart along the shear layers and could be due to rapid vortex entanglements of the leading-edge vortices within short distances away from the AR = 0.3 elliptic jet exit. Velocity plots also show ‘bifurcations’ for the transverse velocity component and demonstrated reduced activities associated with the counter-rotating vortex pair formation as the aspect ratio increases. Analysis of the velocity profile developments downstream of the jets shows multiple velocity maxima and minima associated with various features of the jet in a crossflow phenomenon and that crossflow recovery at the lee-side regions will be slower for high aspect ratio jets. Lastly, results also show that although stronger flow reversals occur for high aspect ratio jets, they do not lead to more intense counter-rotating vortex pair formations over the low aspect ratio elliptic jets.

The use of turbulent inflow conditions for the modelling of a high aspect ratio jet

The aim of this work is to investigate the capabilities of a turbulent inflow method. The application selected for this study is the high aspect ratio jet. The complexities associated to the numerical modelling of a high aspect ratio jet are embedded in its physical complexity. Consequently, the numerical modelling does not only require a high mesh resolution, but furthermore it requires a careful mesh construction, inflow conditions and subgrid-scale modelling to make an accurate computation of the unsteady flow phenomena. The results indicate that increased grid resolution and enhanced turbulence modelling reduce the effect of the imposed flow fluctuations. It is concluded that for a high aspect ratio free-jet turbulent inflow conditions are effective if the mesh resolution is insufficient to trigger shear-layer instabilities. Applied with sufficient mesh resolution the onset of vortex motions will occur in the shear layer, hence there is limited inflow sensitivity.

A flow field study of an elliptic jet in cross flow using DPIV technique

The digital particle image velocimetry (DPIV) technique has been used to investigate the flow fields of an elliptic jet in cross flow (EJICF). Two different jet orientations are considered; one with the major axis of the ellipse aligned with the cross flow (henceforth referred to as a low aspect ratio (AR) jet), and the other with the major axis normal to the cross flow (henceforth referred to as a high aspect ratio jet). Results show that the vortex-pairing phenomenon is prevalent in the low aspect ratio jet when the velocity ratio (VR)≥3, and is absent in the high aspect ratio jet regardless of the velocity ratio. The presence of vortex pairing leads to a substantial increase in the leading-edge peak vorticity compared to the lee-side vorticity, which suggests that vortex pairing may play an important role in the entrainment of ambient fluid into the jet body, at least in the near-field region. In the absence of vortex pairing, both the leading-edge and the lee-side peak vorticity increase monotonically with velocity ratio regardless of the aspect ratio. Moreover, time-averaged velocity fields for both AR=0.5 and AR=2 jets reveal the existence of an “unstable focus” (UF) downstream of the jet, at least for VR≥2. The strength and the location of this focus is a function of both the velocity ratio and aspect ratio. In addition, time-averaged vorticity fields show a consistently higher peak-averaged vorticity in the low aspect ratio jet than in the high aspect ratio jet. This behavior could be due to a higher curvature of the vortex filament facing the cross flow in the low aspect ratio jet, which through mutual interaction may lead to higher vortex stretching, and therefore higher peak-averaged vorticity.