Axisymmetric Jet Research Articles

On the scalar turbulent/turbulent interface of axisymmetric jets

The effect of a zero-mean-flow turbulent ambient on the geometry of the turbulent/turbulent interface (TTI) of an axisymmetric jet is investigated and compared with the traditional turbulent/non-turbulent interface (TNTI). The ambient turbulence is generated using a random jet array. Orthogonal cross-sections of the jet subjected to different levels of ambient turbulence intensities and length scales are captured using planar laser-induced fluorescence. The enhanced radial transport of concentration from the jet core towards the interface and the increased scalar fluctuations within the jet caused by the ambient turbulence result in steeper mean and root-mean-square scalar conditional jumps across the TTI layer compared with those of the TNTI, respectively. When compared with the quiescent ambient, the mean effect of background turbulence is to stretch and corrugate the interface surface area as evident from the wider probability density of the interface radial position, the lowered occurrence of zero-curvature surface elements, the greater misalignment between the radial and normal unit vectors of the interface, the increased tortuosity and the increased magnitude of the fractal exponent.

Structure influence on the operating limit range and mixing performance of non-axisymmetric jet pump

Although cavitation chocked jet pumps guarantee a steady and accurate liquid mixture, the existing pumps have the shortcomings of big energy loss and small cavitation working range. In the current study, aiming at enhancing the performance of the cavitation mixing devices, an innovative non-axisymmetric jet pump design is proposed. The cavitation characteristics and the mixing performance of the new design have been investigated by both computational simulation and experimental testing. Based on the results of computational fluid dynamics (CFD), it is found that the cavitation on the suction tube side is strengthened due to the turbulence caused by the abrupt change in the local flow channel structure, while the cavitation on the opposite side is weakened due to the gradual flow channel structure. Our experimental testing results prove that our new design can provide a steady mixing ratio as long as the non-axisymmetric vapor cloud steadily covers the suction tube outlet. Furthermore, geometric parameters (convergent angle, divergent angle, throat length and area ratio) of the device have been optimized through the orthogonal analysis. The critical pressure ratio of the optimized device ranges from 0.76 to 0.63 when the critical flow ratio is in the range of 0–10%, which indicates that the optimized device has much less energy loss and a wider working range than the current axisymmetric cavitating jet pumps. Through quantitative energy loss analysis, we have found that the cavitation maintenance corresponds to the greatest energy loss in the jet pumps, yet our non-axisymmetric structure design could effectively reduces energy loss. The current research reveals the physical mechanism on how a non-axisymmetric structure affects the cavitation characteristics as well as the performance of jet pumps.

Optimization of thermal characteristics of axisymmetric synthetic air jet impingement on flat surface

The local heat transfer of axisymmetric synthetic air jet impinging on flat surface is investigated experimentally. Acoustic speaker is used for generation of synthetic air jet with cavity of cylindrical shape.The experiments is conducted for actuator frequency ranging from 50Hz to 400Hz, orifice diameter 2mm to 10mm, orifice plate thickness 2mm to 10mm, cavity diameter 50mm to 75mm, cavity depth 30mm to 60mm, jet to plate distance 16mm to 112mm. A steady heat flow is maintained on the flat surface. Local heat transfer characteristics on flat surface is found by thermal images using IR thermal imaging and thin foil technique.The experimental results reveals that the heat transfer is highly effected by frequency, orifice diameter, orifice plate thickness, jet to plate distance. However, cavity depth and cavity diameter has small influence on thermal performance.The higher cavity volume show more influence on heat transfer characteristics. An optimization of synthetic jet performance parameters is studied for maximum thermal performance interms of heat transfer characteristics.

Coupled VOF and Level-Set methods for simulating nozzle geometry effect on axisymmetric water jet breakup

In this work, a numerical investigation of nozzle geometry effect on axisymmetric water jet breakup by coupling the Volume-Of-Fluid (VOF) method with Level-Set (LS) was carried out. Through this coupling, the benefit from the advantages of both approaches in order to get better results is realized. The impact of two nozzle geometries (cone-up and cone-down configurations) on the characteristics of water jets flow downstream of the nozzle has been discussed for pressures below 172 bars. Flow was assumed to be two dimensional, viscous, incompressible, and unsteady. Velocity distributions, turbulence sensitivity, and flow discharge coefficient Cd was examined as a function of Reynolds number and injection pressure. CFD simulations have been validated with the published experimental data of Vahedi et al. and Ghassemieh et al. After simulation, it can be found that the calculated solutions are in good agreement with experiment data. The agreement between the calculations and the experimental data indicates that the model provides a good prediction of the discharge coefficient. The pressure injection and turbulence have a great influence on the velocity profile and the intact length.

Three-dimensional large-scale and very-large-scale coherent structures in a turbulent axisymmetric jet

The direct numerical simulation database from Shin et al. (J. Fluid Mech., vol. 823, 2017, pp. 1–25) is used to study three-dimensional vortical and very-large-scale coherent structures in a turbulent round jet at a Reynolds number of $7300$ . In particular, horseshoe vortices and their role in the formation of very-large-scale coherent structures in the jet near and intermediate fields are assessed. The swirling strength criterion together with conditional averaging are used to visualize volumetric vortical structures. It is shown that, similar to wall-bounded turbulent flows, the turbulent jet is populated with symmetric and asymmetric horseshoe-like vortices, which induce high-momentum and low-momentum regions in the flow. However, unlike what is found for wall-bounded flows, inverse horseshoe-like vortices are common in the turbulent jet. They prevail in the shear region around the potential core in the jet near field and contribute to the mixing of the potential core in the jet. In the jet near field, groups of axially aligned horseshoe structures induce long streaky structures, which are periodic in the azimuthal and streamwise directions. In the jet intermediate field, very-large-scale motions (VLSMs) consisting of high-momentum regions, flanked on either side by low-momentum regions, are found to be associated with groups of horseshoe vortices. Instantaneous three-dimensional flow fields suggest that horseshoe vortices tend to concatenate and form organized spiral as well as axially aligned coherent VLSMs. A detection scheme is introduced to identify and average over these VLSMs. This conditional averaging reveals that spiral VLSMs and axially aligned VLSMs constitute $72\,\%$ and $28\,\%$ of the total VLSMs, respectively.

A similarity scaling model for the axisymmetric turbulent jet based on first principles

Similarity scaling, when it can be justified, is a powerful tool for predicting properties of fluid flows and reducing the computational load when using mathematical models. Numerous publications describe different applications of this method, using often different scaling laws with one or more scaling parameters. The justification for these laws is often based on some assumptions or references to experimental results. In this paper, we base the scaling law on basic physical principles of classical Newtonian physics (Galilei group) and derive some predictions that we apply to a simple model for the axisymmetric turbulent jet. In a companion paper, we compare these predictions to careful measurements on a free jet in the laboratory and evaluate how far our model predictions are borne out by the experimental results. We have succeeded in obtaining such high-measurement quality that we can compute both second- and third-order statistical functions even far downstream and far-off axis. We can already here reveal that we find very good agreement between a simple one-parameter geometric scaling law derived from the model and numerous first-order and higher-order statistical results computed from the experimental data.

Similarity scaling of the axisymmetric turbulent jet

In this work, we find that a free axisymmetric jet in air displays self-similarity in the fully developed part of the jet (≳30 jet exit diameters from the jet exit). We report accurate measurements of first, second, and third order spatially averaged statistical functions of the axial velocity component performed with a laser Doppler anemometer, including the outer (high intensity, ≳20%) regions of the jet. The measurements are compared to predictions derived from a simple jet model, which is described in a separate publication, and we discuss the implications for the further study of self-similarity in a free jet. It appears that all statistical functions included in this study can be scaled with a single geometrical scaling factor—the downstream distance from a common virtual origin, z−z0.

Effect of tab length on supersonic jet mixing

An experimental investigation has been carried out to assess the efficiency of rectangular tabs of 5% blockage with three different aspect ratios of 1, 1.5, and 2, in promoting the mixing of a Mach 1.73 axisymmetric free jet, in the presence of adverse and favorable pressure gradient, by varying the operating nozzle pressure ratio (NPR) from 4 to 8, in steps of 1, covering all the three levels of expansion, namely, the over-, correct-, and underexpaned states at the nozzle exit. For NPR 4, the Mach 1.73 jet is overexpanded with an adverse pressure gradient of about 23%. At NPR 5, the nozzle is almost correctly expanded with an adverse pressure gradient of only about 3%. At NPRs 6, 7, and 8, the nozzle is underexpanded with favorable pressure gradients of 16%, 35%, and 55%, respectively. It is found that as high as about 88% reduction in the core length is achieved by the tab with an aspect ratio of 1.5 at NPR 7. However, the reduction caused by tabs with aspect ratios of 1 and 2 is only 73.3% and 42.3%, respectively. It is to be noted that though the mixing promotion caused by the tab with an aspect ratio of 1.5, at NPR 7, leads to the core length reduction to the tune of about 88%, it is at the cost of momentum thrust loss of about 5% and the drag caused by the tabs. It is also found that except at NPR 4, the mixing caused by the tab with an aspect ratio of 1.5 is better than tabs with aspect ratios of 1 and 2. The shadowgraph pictures for the uncontrolled and controlled jets demonstrate the effectiveness of the tabs in weakening the waves present in the jet core.

Visualization of hydrogen jet using deformation of the laser beam profile

Hydrogen has the widest flammable range, the fastest flame propagation speed, and the lowest ignition energy, so its safety needs special attention before the wide application of hydrogen energy. The main objective of this work is to propose a new method to evaluate hydrogen jet pressure by using a Helium–Neon laser through the jet. A mathematical model was proposed, which describes the deformation of the laser beam profile passing through an axisymmetric circular hydrogen jet pressure flow field in detail. This research attempts to apply the expression of density Gaussian distribution, ideal gas equation, and Gladstone Dale equation to disclose the deformation of laser beam profile under different outlet conditions. The experimental uncertainty is about 3 × 10−3. A non-contact optical experimental system is established to visually measure the density gradient distribution of the gas jet. Our findings show that the hydrogen jet can be regarded as a gas-phase lens, and the deformation of the laser beam profile in the horizontal direction increases linearly with jet pressure. Finally, the preliminary results of calculations of the spot area with the theoretical model were presented and compared with the images of the laser beam profile passing through the jet in the experiment. The theoretical model gave similar results and the overall agreement with the experiment was satisfactory. Our technology exhibits high sensitivity in the measurement of hydrogen leakage pressure, providing a theoretical basis for non-contact, non-damaged, high-response, and high-sensitivity detection of hydrogen leakage.

Analysis of the dynamics of subharmonic flow structures via the harmonic resolvent: Application to vortex pairing in an axisymmetric jet

We demonstrate the use of the harmonic resolvent to study the physics of subharmonic flow structures in the proximity of time-periodic solutions of the Navier-Stokes equation. As an illustrative example, we study the vortex pairing phenomenon in a periodically forced axisymmetric jet. While it is well-known that vortex pairing in jets and mixing layers arises from the interaction between subharmonic perturbations and the fundamental vortex shedding mode, here we provide additional insight. In particular, we uncover the nature of the strong cross-frequency interactions in the flow, and we compute the spatiotemporal mode that drives the pairing mechanism.

Wall Shear Stress Measurements Using A MTV-Based Plenoptic Microscope

In this contribution, we describe the first ever microscopic Molecular Tagging Velocimetry (MTV) measurements using Integral Imaging. This technique permits the measurement of the 3D-2C velocity field in an axi-symmetric stagnation jet and observed profiles agree well with analytical predictions. Standard reconstruction algorithms are compared against more advanced deconvolution algorithms and impressive improvements in reconstructed probe resolution are obtained. Integral imaging is compared with a scanning confocal microscope which serves as a basis of comparison and good agreement is achieved. The flow facility was designed for a low Reynolds number, but with an adjustable viscous length scale comparable with higher Reynolds number flows. Our instrument proves capable of resolving viscous length scales on the order of 30 micrometers which can be improved by increasing the numerical aperture of the microscope objective.

Measurement of turbulent supersonic steam jet flow characteristics using TDLAS

Ejectors have no moving parts and are preferable to mechanical compressors in many applications, but ejectors typically have a relatively low efficiency. To aid in the ejector design process, thorough understanding of the turbulent mixing of multi-phase compressible jets is beneficial.This paper reports experimental results for Tunable Diode Laser Absorption Spectroscopy (TDLAS) measurements derived from an axisymmetric supersonic steam jet apparatus.In this experimental work, a supersonic steam jet nozzle exit of a diameter 13.6 mm was surrounded by a low-speed flow of dry nitrogen. The TDLAS system was traversed through the flow at three different planes downstream from the ejector nozzle exit: 15, 20, and 30 mm distance. At each of the three planes, line-of-sight measurements were made with the laser passing through locations between 0 and 15 mm from the jet centreline.Through the analysis of the TDLAS data and application of the Abel inversion method, the radial distribution of the pressure, temperature, and the concentration of the water-vapour were obtained. The key findings are that it is possible to determine key physical parameters using experimental TDLAS measurements when combined with a suitable numerical optimization approach.

Hybrid Pulse-Burst / Cross-Correlation DGV For High-Speed Flow Measurements At 100 KHz

Hybrid pulse-burst Doppler global velocimetry (PB-DGV)/pulse-burst, cross-correlation DGV (PB-CC-DGV) was demonstrated for the first time in the NASA Langley 4-foot Supersonic Unitary Plan Wind Tunnel, allowing simultaneous acquisition of mean and instantaneous high-repetition-rate velocity fields. The technique was used to make planar velocity measurements across an oblique shockwave generated by a large splitter plate set at a -2° angle of attack in a Mach 2.4 flow. Sequences of >100 consecutive instantaneous velocity images were obtained at a rate of 100 kHz. Velocity fields from both aspects of the hybrid measurement indicated errors less than 1-percent of anticipated velocities. Likewise, all assessments of measurement precision were approximately 1.6-percent of the local velocities. Additional demonstrations in different flowfields including a subsonic axi-symmetric jet and a supersonic Mars reentry vehicle model further illustrate the utility of the new hybrid method.

Dynamics and bifurcations of laminar annular swirling and non-swirling jets

This paper presents bifurcation analyses characterising the nonlinear dynamics of fully developed laminar annular jets with respect to the centrebody diameter${ {d}}$, Reynolds number${ {Re}}$, and swirl ratio${ {S}}$. Similar flows appear in numerous applications and feature a vibrant range of topological and dynamical characteristics associated with phenomena including shear layer separation and vortex breakdown. Our results begin by describing the non-monotonic evolution of the axisymmetric jet's steady topology under varying${ {S}}$. In accord with earlier reports, the jet progresses through a sequence of wake, breakdown and wall jet regimes in a qualitatively similar manner across a wide span of${ {d}}$and${ {Re}}$values. In the wake regime, the non-swirling jet bifurcates to a plane-symmetric, but not axisymmetric, steady flow pattern beyond a${ {d}}$-dependent critical${ {Re}}$value. With further increase in${ {Re}}$, the steady non-swirling jet destabilises subsequently via multiple distinct Hopf bifurcations. Introducing${ {S}}>0$to the jet also induces unsteadiness by twisting the singly azimuthally periodic ($|m|=1$) asymmetric wake structure and causing it to precess periodically in time about the central axis. Intermediate swirl stabilises this unsteady dynamics and restores the jet's axisymmetry. This stabilising effect is then reversed in the breakdown regime at higher${ {S}}$, where a variety of different$|m|=1$and$|m|=2$instabilities bifurcate from the steady flow as${ {S}}$is increased. Several instances of hysteresis and subcritical behaviour are reported and discussed, including one that manifests precessing vortex core oscillations.

Optimizing Oblique Projections for Nonlinear Systems using Trajectories

Reduced-order modeling techniques, including balanced truncation and $\mathcal{H}_2$-optimal model reduction, exploit the structure of linear dynamical systems to produce models that accurately capture the dynamics. For nonlinear systems operating far away from equilibria, on the other hand, current approaches seek low-dimensional representations of the state that often neglect low-energy features that have high dynamical significance. For instance, low-energy features are known to play an important role in fluid dynamics, where they can be a driving mechanism for shear-layer instabilities. Neglecting these features leads to models with poor predictive accuracy despite being able to accurately encode and decode states. In order to improve predictive accuracy, we propose to optimize the reduced-order model to fit a collection of coarsely sampled trajectories from the original system. In particular, we optimize over the product of two Grassmann manifolds defining Petrov--Galerkin projections of the full-order governing equations. We compare our approach with existing methods including proper orthogonal decomposition, balanced truncation-based Petrov--Galerkin projection, quadratic-bilinear balanced truncation, and the quadratic-bilinear iterative rational Krylov algorithm. Our approach demonstrates significantly improved accuracy both on a nonlinear toy model and on an incompressible (nonlinear) axisymmetric jet flow with $10^5$ states.

Numerical study of free surface axisymmetric jet impinging on a heated flat surface utilizing high concentration SiO2 nanofluid

BackgroundA comprehensive numerical study is presented to investigate the heat transfer and fluid flow characteristics between a vertical free surface jet and a horizontal heated plate. MethodsThe jet was made up of water-SiO2 nanofluid with an average particle size of 8 nm that was delivered from a 6 mm nozzle diameter. The numerical model covered a wide range of jet Reynolds numbers up to 31,800, five nanoparticle volume fractions of 0, 2.5, 4.5, 6.5, and 8.5%, five values of the nozzle to plate aspect ratios (z/d = 0.5, 1, 2, 4, and 8), and plate radius to jet diameter ratio (r/d) of 12.5. Two-dimensional continuity, momentum, and energy equations are discretized with commercial finite volume software (ANSYS Fluent 15) under the v2f turbulence model. The numerical model has been verified through the comparison between its predicted results and those obtained from previous experimental published work. Significant findingsThe results were presented graphically in the form of free jet interface thickness, radial velocity profiles, turbulence intensity profiles, and local and average Nusselt numbers on the heated plate. The results showed that the improvement of the average Nusselt number increases with the volume fraction and Reynolds number. Therefore, the utilization of SiO2 nanoparticles can significantly provide the improvement of the average Nusselt number up to 66.02%, for a volume fraction of 8.5% compared to pure water. Also, the average Nusselt number is slightly influenced by the nozzle to plate aspect ratio (0.5 ≤ z/d ≤ 4). Finally, a new heat transfer correlation for the average Nusselt number has been proposed. The new correlation presents the average Nusselt number as a function of Reynolds number, Prandtl number, nanoparticle volume fraction, plate to jet diameter ratio, and nozzle to plate aspect ratio.

Large eddy simulation of a thermal impinging jet using the lattice Boltzmann method

A compressible Hybrid Lattice Boltzmann Method solver is used to perform a wall-resolved Large eddy simulation of an isothermal axisymmetric jet issuing from a pipe and impinging on a heated flat plate at a Reynolds number of 23 000, a Mach number of 0.1, and an impingement distance of two jet diameters. The jet flow field statistics, Nusselt number profile (including the secondary peak), and shear stress profile were well reproduced. The azimuthal coherence of the primary vortical structures was relatively low, leading to no discernible temporal periodicity of the azimuthally averaged Nusselt number at the location of the secondary peak. While local unsteady near-wall flow separation was observed in the wall jet, this flow separation did not exhibit azimuthal coherence and was not found to be the only cause of the thermal spots blue, which lead to the secondary peak in the Nusselt number, as stream-wise oriented structures also played a significant role in increasing the local heat transfer.

Large eddy simulations of single and multiple turbulent round jets

We present results from three large eddy simulations (LES). The first two were those of a single jet at a Reynolds number of 11000 with different cell density distributions. The simulation results are validated with earlier experimental and computational studies at the same Reynolds number and almost the same boundary conditions. We establish the repeatability and reproducibility of the characterisation of a single axisymmetric jet. Additionally, we performed an LES of five round jets using the same discretisation schemes and boundary conditions. The five jets were placed in a cross or plus configuration, with a central jet surrounded by four outer jets. The mass flux, momentum flux and the Reynolds number of the five jet configuration were set to be equal to those of the single jet. Further, we analyse the near-field development of the multiple jets, along with entrainment and symmetry characteristics as the jet evolves. LES's ability to provide information about large-scale motions was used to compute conditional statistics. We, then, present details of an initial attempt to characterise the turbulent non-turbulent interface boundary and the coherent structures in the core of the jet in a unified manner using helicity density as the detector variable.

Hybrid constraint multi-line absorption spectroscopy for non-uniform thermochemical measurements in axisymmetric laminar and jet flames

We propose a novel strategy for the accurate determination of non-uniform thermochemical distributions in axisymmetric flames using line-of-sight laser absorption spectroscopy with the constraints of hybrid information. In this method, the computational fluid dynamics (CFD) simulation or other diagnostic method (e.g., thermocouple measurement) is used to qualitatively characterize the thermochemical properties, whereas the kinetic modelling, chemical equilibrium calculation and thermocouple measurement are used to constrain the range of unknown parameters. Experimental demonstrations were firstly performed in a standard laminar CH4/air premixed flame stabilized on a McKenna burner at different equivalence ratios (Φ = 0.8-1.2). With proper hybrid constraints, multiple solutions are avoided, and the deviation and convergence time can be simultaneously reduced comparing to the conventional method. For further demonstration, this strategy was then applied to a turbulent jet flame to recover the significant thermochemical gradient without the need for tomographic reconstruction techniques.

Experimental study of vortex formation in pulsating jet flow by time-resolved particle image velocimetry

We experimentally investigate the pulsating circular jet flow at moderate Reynolds numbers. By applying time-resolved particle image velocimetry in the axial-radial plane, we measure the near-field velocity fields with the jet source temporally modulated by sinusoidal pulsations. As a baseline, the steady jet flow with the same mean Reynolds number is tested. The direct comparisons of the mean and fluctuating velocity fields show that the whole potential core as well as the axisymmetric shear layer is modulated by the pulsation effect. Meanwhile, larger-scale vortices are formed in the shear layer with phase correlation of the pulsation cycle. As a result, the pulsation increases the turbulent mixing in the latter half of the potential core, and it extends the fluid entrainment further in the radial direction. The increased fluid entrainment of the ambient quiescent fluid is clearly identified by the attracting Lagrangian coherent structures as the bounds of the growing vortices within the shear layer. By analyzing the dynamic modes, we find that the low-frequency off-the-axis helical structures, which are dominant in the steady jet flow, are inhibited. The axisymmetric jet column mode and its harmonics along the axis are strengthened by the pulsation effect. Furthermore, the vortex formation mainly takes place particularly in the deceleration phase, whereas a shock-like wave front is formed during the acceleration, indicating the distinct roles of the pulsation phases in the jet instability.