Jet Nozzle Exit Research Articles

A three-dimensional numerical investigation on drag reduction of a supersonic spherical body with an opposing jet

A three-dimensional numerical simulation of a supersonic free-stream at Mach 2.5 over a spherical body with a sonic opposing jet from its stagnation point is carried out by solving the three-dimensional Navier–Stokes equations coupled with the standard k–ɛ turbulence model. It is aimed to investigate the effects of the jet on the drag reduction on the body and the flow field around the body. The influences of the jet pressure, the nozzle size of the jet, and the angle of attack are systematically studied for the purpose. An unsteady oscillatory motion mode and a nearly steady motion mode are identified depending upon the jet total pressure. There exists a critical jet pressure where the flow mode transition from one to the other happens suddenly and this critical pressure value varies approximately linearly with the jet nozzle exit size inversely. For the zero angle of attack, the results show that there exists a maximum overall drag reduction as the jet pressure changes for each jet nozzle size and the maximum overall drag reduction always happens at the unsteady oscillatory motion mode. The main shock in front of the body is pushed backward by the jet and the displacement of the shock decreases with the increase of the angle of attack, and the drag reduction efficiency also decreases with the angle. Regarding to the mode transition, it is found that the drag rises suddenly when the transition happens for the angle of attack smaller than or equal to 5° but it does not result in the rise for the angle larger than 5°. The results show that the maximum overall drag reduction can be reached as high as 32.6% for the cases studied. The present results provide useful information for drag reduction applications using an opposing jet.

Performance characteristic curve insensitive to feedback fluidic oscillator configurations

The effects of different geometrical factors such as the jet nozzle exit width, feedback channel width, oscillator depth, and the oscillating chamber shape on the feedback fluidic oscillator performance were investigated experimentally. A characteristic curve relating to only the oscillation frequency f and Reynolds number Re was found to be insensitive to the oscillator configurations and is expressed as f (Hz)=6.05exp(−α/3.30)+14.5exp(−α/0.859)+0.669, where α=ln(Re)/f. The characteristic curve can be used to determine the Reynolds number and thereby the flowrate through the oscillators according to the detected oscillation frequency.

A direct numerical simulation study of turbulence and flame structure in transverse jets analysed in jet-trajectory based coordinates

AbstractAn${\mathrm{H} }_{2} / {\mathrm{N} }_{2} $jet in cross-flow (JICF) of air is studied using three-dimensional direct numerical simulation with and without chemical reaction in order to investigate the role of the complex JICF turbulent flow field in the mechanism of fast fuel-oxidant mixing and of aerodynamic flame stabilization in the near field of the jet nozzle. Focus is on delineating the flow/mixing/chemistry conditions that are necessary and/or sufficient to achieve flame anchoring that ultimately enables the formulation of more reliable and precise guidelines for design of fuel injection nozzles. A mixture averaged diffusion formulation that includes the effect of thermal diffusion is used along with a detailed chemical kinetics mechanism for hydrogen–air combustion. A new parametrization technique is used to describe the jet trajectory: solution of Laplace’s equation upon, and then within, an opportune scalar surface anchored by Dirichlet boundary conditions at the jet nozzle and plume exit from the domain provides a smoothly varying field along the jet path. The surface is selected to describe the scalar mixing and reaction associated with a transverse jet. The derived field,$j(\mathbi{x})$, is used as a condition to mark the position along the natural jet trajectory when analysing the variation of relevant flow, mixing and reaction quantities in the present direct numerical simulation (DNS) datasets. Results indicate the presence of a correlation between the flame base location in parameter space and a region of low velocity magnitude, high enstrophy, high mixing rate and high equivalence ratio (flame root region). Instantaneously, a variety of vortical structures, well known from the literature as important contributors to fuel-oxidant mixing, are observed in both inert and reactive cases with a considerable span in length scales. Moreover, instantaneous plots from reactive cases illustrate that the most upstream flame tongues propagate close to the trailing edge of the fuel jet potential core near the jet shear layer vortex shedding position. Some degree of asymmetry with respect to the domain mid-plane in the spanwise direction is observed in the averaged fields, both for the inert and reactive cases.

Mixing due to a heated elliptic air jet

This paper presents measurements of velocity and temperature for a contoured contraction nozzle elliptic air jet with a 2:1 aspect ratio issuing into stagnant unconfined surroundings. Initial conditions at the jet nozzle exit plane were laminar. All measurements of mean and root-mean-square (RMS) velocities and temperatures were carried out using hot-wire and cold-wire anemometers. Flow development was found to be more rapid in the minor axis than major axis due to the thinner initial boundary layer momentum thickness in the minor axis plane. This is clearly demonstrated by the rapid growth of the jet width in the minor axis (compared to the major axis) in the near-field of the jet. Self-similarity in the mean velocity and temperature profiles was attained relatively early. However, RMS quantities have not become self-similar even by the streamwise distance of 38 equivalent jet nozzle diameters (De). One axis-switchover due to the initial difference in jet-width growth rates in the two axes was detected in the current jet. The rates of centerline temperature decay, temperature half-width growths and peak RMS temperatures at all measurement locations were consistently higher than their velocity counterparts. The temperature spreading rate in the major axis was found to be significantly larger vis-à-vis that of velocity, possibly due to the minima in radius of curvature of the elliptic geometry. All the results presented suggest that the use of heated elliptic jets could offer enhanced mixing performance in relevant applications.

LES-based evaluation of a microjet noise reduction concept in static and flight conditions

The Large-Eddy Simulation (LES) numerical system established since 2002 for jet-noise computation is first evaluated in terms of recent gains in accuracy with increased computer resources, and is then used to explore the relatively new “microjet” noise-reduction concept (injection of high-pressure microjets in the vicinity of the main jet nozzle exit), which currently attracts attention in the aeroacoustic community. The simulations, which are carried out with an emulation of the microjets by specially designed distributed sources of mass, momentum, and energy in the governing equations, are found to capture the essential features of the flow/turbulence and the far-field noise alteration by the microjets observed in experiments, and to reveal the subtle flow features responsible for the effect of injection on noise. They also confirm the experimental observation that in static conditions microjets provide a noise reduction comparable with that from chevrons in the low-frequency range, and probably have a less pronounced high-frequency penalty. This positive evaluation of the microjets concept is, however, mitigated by the far less favorable results of simulations in flight conditions, which were never studied experimentally. The latter results, which are awaiting an experimental verification, make a practical use of the concept in its current form rather unlikely.

Swirling and Impinging Effects in an Annular Nonpremixed Jet Flame

The effects of swirl and downstream wall confinement on an annular nonpremixed flame were investigated using direct numerical simulation (DNS). Fully three-dimensional parallel DNS was performed employing high-order numerical methods and high-fidelity boundary conditions to solve governing equations for variable-density flow and finite-rate Arrhenius chemistry. Three swirl numbers have been examined: 0 (without swirl), 0.4 and 0.8, while the effects of downstream wall confinement have been examined for swirl numbers 0 and 0.4. Results have been presented in terms of instantaneous and time-averaged flow quantities, which have also been analysed using energy spectra and proper orthogonal decomposition (POD). Effects of swirl on the fluid dynamic behaviour of the annular nonpremixed flame were found to be significant. The fluid dynamic behaviour of the flame is greatly affected by the interaction between the geometrical recirculation zone (GRZ) near the jet nozzle exit due to the annular configuration, the central recirculation zone (CRZ) associated with swirl, the unsteady vortical structures in the jet column due to the shear instability, and the downstream wall confinement. Depending on the degree of swirl, the GRZ near the burner mouth and the CRZ may co-exist or one zone may be overwhelmed by another. At a moderate swirl number, the co-existence leads to a flame with strong reaction attached to the burner mouth; while at a high swirl number, the CRZ dominates over the GRZ. The precessing vortex core was observed to exist in the swirling flow fields. The Nusselt number distribution of the annular impinging flames differs from that of round impinging jets. The POD analysis revealed that wall effects on the flow field are mainly associated with the higher mode numbers.

Numerical studies of vortex shedding in forced oscillatory non-premixed flames

A comparative study has been performed to nvestigate the flow nstabilities and their nteractions n a non-premixed methane jet flame using highly accurate numerical methods. The nteraction between the jet flapping modes and buoyancy nstability has been examined by comparing a full three-dimensional case and an idealised axisymmetric case, in order to identify the three-dimensional effects on the vortex dynamics. In addition, air co-flow flames have been numericaly simulated to nvestigate the interaction between the shear and buoyancy nstabilities. It was found that three-dimensional effects dominate the flame dynamics at downstream locations, where the flame exhibits transitional behaviour. The axisymmetric simulation is only able to capture the flow dynamics close to the jet nozzle exit. The air co-flow tends to stabilise the flame and to reduce the flame unsteadiness, which may also have an mpact on the flickering frequency. Three-dimensional effects can greatly affect the strength of flame oscillation energy at downstream locations, where the flapping modes enhance the flame unsteadiness. On the contrary, co-flow tends to reduce the strength of flame oscillation, especialy at upstream locations.

Reprint of: LES-Based Evaluation of a Microjet Noise Reduction Concept in Static and Flight Conditions

The Large-Eddy Simulation (LES) numerical system established since 2002 for jet-noise computation is first evaluated in terms of recent gains in accuracy with increased computer resources, and is then used to explore the relatively new “microjet” noisereduction concept (injection of high-pressure microjets in the vicinity of the main jet nozzle exit), which currently attracts significant attention in the aeroacoustic community. The simulations are found to capture the essential features of the flow/turbulence and the far-field noise alteration by the microjets observed in experiments, and to reveal the subtle flow features responsible for the effect of injection on noise. They also confirm the experimental observation that in static conditions microjets provide a noise reduction comparable with that from chevrons in the low-frequency range, and probably have a less pronounced high-frequency penalty. This positive evaluation of the microjets concept is, however, mitigated by results of simulations in flight conditions, which were never studied experimentally. The latter results, which are awaiting an experimental verification, make a practical use of the concept in its current form rather unlikely.

LES-based evaluation of a microjet noise reduction concept in static and flight conditions

The Large-Eddy Simulation (LES) numerical system established since 2002 for jet-noise computation is first evaluated in terms of recent gains in accuracy with increased computer resources, and is then used to explore the relatively new “microjet” noisereduction concept (injection of high-pressure microjets in the vicinity of the main jet nozzle exit), which currently attracts significant attention in the aeroacoustic community. The simulations are found to capture the essential features of the flow/turbulence and the far-field noise alteration by the microjets observed in experiments, and to reveal the subtle flow features responsible for the effect of injection on noise. They also confirm the experimental observation that in static conditions microjets provide a noise reduction comparable with that from chevrons in the low-frequency range, and probably have a less pronounced high-frequency penalty. This positive evaluation of the microjets concept is, however, mitigated by results of simulations in flight conditions, which were never studied experimentally. The latter results, which are awaiting an experimental verification, make a practical use of the concept in its current form rather unlikely.

Direct numerical simulation of the near-field dynamics of annular gas-liquid two-phase jets

Direct numerical simulation has been used to examine the near-field dynamics of annular gas-liquid two-phase jets. Based on an Eulerian approach with mixed fluid treatment, combined with an adapted volume of fluid method and a continuum surface force model, a mathematical formulation for the flow system is presented. The swirl introduced at the jet nozzle exit is based on analytical inflow conditions. Highly accurate numerical methods have been utilized for the solution of the compressible, unsteady, Navier–Stokes equations. Two computational cases of gas-liquid two-phase jets including swirling and nonswirling cases have been performed to investigate the effects of swirl on the flow field. In both cases the flow is more vortical at the downstream locations. The swirling motion enhances both the flow instability resulting in a larger liquid spatial dispersion and the mixing resulting in a more homogeneous flow field with more evenly distributed vorticity at the downstream locations. In the annular nonswirling case, a geometrical recirculation zone adjacent to the jet nozzle exit was observed. It was identified that the swirling motion is responsible for the development of a central recirculation zone, and the geometrical recirculation zone can be overwhelmed by the central recirculation zone leading to the presence of the central recirculation region only in the swirling gas-liquid case. Results from a swirling gas jet simulation were also included to examine the effect of the liquid sheet on the flow physics. The swirling gas jet developed a central recirculation region, but it did not develop a precessing vortex core as the swirling gas-liquid two-phase jet. The results indicate that a precessing vortex core can exist at relatively low swirl numbers in the gas-liquid two-phase flow. It was established that the liquid greatly affects the precession and the swirl number alone is an insufficient criterion for the development of a precessing vortex core.

PIV measurement study of fully developed superfluid turbulent thermal counterflow jet

The particle image velocimetry (PIV) technique was successfully applied for measuring the normal component velocity of a He II thermal counterflow jet. Neutrally buoyant hydrogen-deuterium solid particles were used as tracer particles for the PIV measurement. The jet velocity profiles and the decay properties were investigated for the cases of small and large normal component velocities, and were compared with those of fully developed turbulent round jets of ordinary viscous fluids. The velocity measured near the jet nozzle exit was compared with the theoretical prediction for the normal component velocity, Un,theo. It is found the ratio of the PIV velocity to Un,theo is smaller than unity at any temperatures irrespectively of Un,theo

Application of particle image velocimetry for measuring He II thermal counterflow jets

The particle image velocimetry (PIV) technique was successfully applied for measuring the velocity of a He II thermal counterflow jet. Neutrally buoyant hydrogen–deuterium solid particles were used as tracer particles for PIV measurement. In the application, the normal component velocity was measured. The jet velocity profile and spatial decay of the jet velocity were compared with those of turbulent round jets of ordinary viscous fluids. The velocity measured near the jet nozzle exit was compared with the theoretical prediction for the normal component flow velocity.

Experimental study of parallel and inclined turbulent wall jets

An experimental study of the flow field in a two-dimensional wall jet has been conducted. All measurements were carried out using hot-wire anemometry. The experimental facility has a rectangular slot nozzle of high aspect ratio l/b = 100 (where l and b are the length and height slot, respectively). Mean velocities and Reynolds stresses were determined with three nozzle Reynolds numbers ( Re = 1 × 10 4, 2 × 10 4 and 3 × 10 4) and four different inclination angles between the wall and the flow velocity at the nozzle ( β = 0°, 10°, 20° and 30°). Results indicate that all wall jets are self-preserving in the developed region. Normal to the wall two regions can be identified: one similar to a plane free jet and the other similar to a boundary layer. Downstream the interaction between these two regions creates a mixed or third region. The logarithmic region increases with the distance from the nozzle and with the Reynolds number. For the inclined wall jet, the spreading rate expressed in terms of jet half-width or maximum velocity decay with respect to the streamwise distance, asymptotes to a linear law. The streamwise locations where the jet becomes self-similar are farther from the exit than in parallel wall jet. The slope of both half-width and maximum velocity decay in the developed region are affected by both wall jet inclination angle and nozzle exit Reynolds number.

Noise Reduction Performance of Aerodynamic Tabs in a Supersonic Jet

Noise reduction performance of aerodynamic tabs in an under-expanded supersonic jet is investigated experimentally. From a converging round nozzle, air is injected into the atmosphere (Main jet) at a pressure ratio of 2.25 (Mj=1.14). The nozzle exit diameter D=8.0mm. From small nozzles mounted in the main jet nozzle wall, air is injected in the normal direction to the main jet (Secondary jet). The nozzle exit diameter d=1/8D and 1/16D, and the number of nozzles is varied from 1 to 8. The average total pressure of the main jet decreases with the increase in the total mass flow rate of the secondary jets. The secondary jet penetration is estimated from the momentum flux ratio and the secondary jet nozzle exit diameter. Equivalent secondary jet penetration is introduced to consider the effect of the number of secondary jet nozzles. The screech SPL decreases monotonously with the increase in the equivalent secondary jet penetration. The OASPL, which represents the broadband noise level, also decreases with the increase in the equivalent secondary jet penetration, and takes the minimum value where the screech is removed.It i§ shown that the jet penetration is one of the most important criteria of the noise reduction performance of aerodynamic tabs.

Vortical Structures in a Two-Dimensional Jet Under Flapping Motion Generated by a Wake of a Cylinder Installed in a Nozzle Contraction (2nd Report, Flow Visualization and Image Correlatioin Analysis)

When a plane jet exhibits flapping motion, two distinctive peaks, f1 and f2, appear in the spectrum of fluctuating velocity. The mean frequency is half of the frequency, fv, of vortices shedding from a two-dimensional body that is installed in a jet nozzle contraction. Jet mixing has been enhanced by the flapping motion with a low frequency, f3=f1-f2, which is considered to be caused by an inharmonic excitation. Vortical structures in the flapping jet are investigated by using flow visualization, image correlation and laser Doppler velocimetry. Vortices are seen to roll up asymmetrically at the jet nozzle exit, at a double frequency of that of the vortex shedding from the cylinder. Two types of vortices are formed by coalescence between these vortices and the vortices shedding from the cylinder. The periodicities of these two types of vortices are found to be equal. It is noted that vortices with f1 or f2 do not exist in the flow. The f1 and f2 are caused by the modulation of amplitude of the coalesced vortical structures with half of the flapping frequency, f3.

Control of a submerged jet in a thin rectangular cavity

An innovative method is presented for control of an oscillatory turbulent jet in a thin rectangular cavity with a thickness to width ratio of 0.16. Jet flow control is achieved by mass injection of a secondary jet into the region above the submerged primary jet nozzle exit and perpendicular to the primary nozzle axis. An experimental model, a 2-D and a 3-D computational fluid dynamics (CFD) model are used to investigate the flow characteristics under various secondary injection mass flow rates and injection positions. Two-dimensional laser Doppler anemometry (LDA) measurements are compared with results from the CFD models, which incorporate a standard k – ε turbulence model or a 2-D and 3-D realisable k – ε model. Experimental results show deflection angles up to 23.3° for 24.6% of relative secondary mass flow are possible. The key to high jet control sensitivity is found to be lateral jet momentum with the optimum injection position at 12% of cavity width (31.6% of the primary nozzle length) above the primary nozzle exit. CFD results also show that a standard k – ε turbulence closure with nonequilibrium wall functions provides the best predictions of the flow.

Application of Compact Schemes to Large Eddy Simulation of Turbulent Jets

We present 3-D large eddy simulation (LES) results for a turbulent Mach 0.9 isothermal round jet at a Reynolds number of 100,000 (based on jet nozzle exit conditions and nozzle diameter). Our LES code is part of a Computational Aeroacoustics (CAA) methodology that couples surface integral acoustics techniques such as Kirchhoff's method and the Ffowcs Williams-- Hawkings method with LES for the far field noise estimation of turbulent jets. The LES code employs high-order accurate compact differencing together with implicit spatial filtering and state-of-the-art non-reflecting boundary conditions. A localized dynamic Smagorinsky subgrid-scale (SGS) model is used for representing the effects of the unresolved scales on the resolved scales. A computational grid consisting of 12 million points was used in the present simulation. Mean flow results obtained in our simulation are found to be in very good agreement with the available experimental data of jets at similar flow conditions. Furthermore, the near field data provided by the LES is coupled with the Ffowcs Williams--Hawkings method to compute the far field noise. Far field aeroacoustics results are also presented and comparisons are made with experimental measurements of jets at similar flow conditions. The aeroacoustics results are encouraging and suggest further investigation of the effects of inflow conditions on the jet acoustic field.

Active control of jet mixing

A control law to improve jet flow mixing is presented. The control law employs a pair of actuators at the jet nozzle exit that act on the shear layers near the corners by blowing and subtracting fluid in an anti-symmetric fashion and a sensor downstream or at the nozzle exit with a time delay that measures the pressure difference across the nozzle diameter. A 2-D jet flow is numerically simulated along with massless/mass particles and a passive scalar. The mixing enhancement produced by these controllers is demonstrated visually by snapshots of the vorticity, streaklines, particle distribution and scalar field. Probability function for the particles and the scalar field are constructed, which serve as an index of mixing quality and the effectiveness of the controllers. This closed-loop control law successfully alters the jet flow and improves the mixing of particles with mass and passive scalar.

Development of turbulence in subsonic submerged jets

The development of turbulence in subsonic submerged jets is reviewed. It is shown that the turbulence results from a strong amplification of the weak input noise that is always present in the jet nozzle exit section. At a certain distance from the nozzle the amplification becomes essentially nonlinear. This amplified noise leads to a transition of the system to a qualitatively new state, which depends only slightly on the characteristics of the input noise, such as its power spectrum. Such a transition has much in common with nonequilibrium noise-induced phase transitions in nonlinear oscillators with multiplicative and additive noise. The Krylov–Bogolyubov method for spatially extended systems is used to trace the evolution of the power spectra, the root-mean-square amplitude of the turbulent pulsations, and the mean velocity, with increasing distance from the nozzle. It is shown that, as turbulence develops, its longitudinal and transverse scales increase. The results coincide qualitatively and also, in specific cases, quantitatively, with known experimental data.

Evolution of coherent structures and feedback mechanism of the plane jet impinging on a small cylinder

The dynamics of coherent structures and their instability evolution of the small cylinder impinging plane jet are extensively studied in this paper. The hot-wire measurements are conducted to investigate the physical flow properties and relevant flow field behaviors. The jet exit velocity is operated between 2– 26 m/ s , with the Reynolds number range from 2.1×10 3 to 2.7×10 4 based on the initial jet width. The acoustic excitation is applied to verify the instability evolution mechanism and is introduced at the jet nozzle exit with two arrays of small loudspeakers. A modified feedback mechanism is proposed to successfully explain the interaction between the cylinder and the plane jet. The competition between the jet and wake shear layer instabilities is significantly revealed in comparison with the standing wave number measured in the self-sustained oscillating flow. The augmentation effect of acoustics excited at the resonant frequency on the impinging flow with the small cylinder is also explored in detail to substantiate the feedback mechanism.