Supersonic Air Jet Research Articles

Outflow of a supersonic overexpanded air jet into a water

Numerical simulation of the underwater outflow of a supersonic overexpanded air jet is considered. The calculations are carried out in an unsteady three-dimensional formulation with a hybrid approach using Reynolds-averaged Navier–Stokes equations for nozzle flow and large eddy-simulation for jet flow. The calculation of the interaction of a gas with a liquid is performed with a volume of fluid, which takes into account the surface tension and compressibility of water. The structural features of turbulence when a supersonic air jet exhausted into water are analyzed. An unsteady formation of a supersonic jet and a gas cavity in a water is obtained, the shock-wave structure of the flow is visualized and the level of pressure fluctuations in water is reported. A comparison of the flow pattern when an air jet exhausted into air and water is made on a qualitative level. The results of numerical simulation of the jet velocity field are used to analyze the Kelvin–Helmholtz and Rayleigh–Taylor instability mechanisms and to reveal the dominant instability mechanism in different jet propagation regions. The results obtained are compared with experimental high-speed photographic recording data and numerical calculations At a qualitative level in literature.

High-Frequency Pulsed Coaxial Injectors for High-Speed Flow Mixing and Control

Efficient and controlled mixing of fuel with fast-moving air is a challenging physical problem relevant to hypersonic systems. Although mixing happens at the molecular level through diffusion, the macroscopic phenomenon such as entrainment and vorticity dynamics resulting from the shear layer instabilities of the mixing fluids play a significant role in the overall efficiency of the process. With a focus on improving mixing at extreme flow conditions, this paper presents a fundamental study of a novel, high-speed, pulsed coflow system integrated with ultrahigh-frequency actuators (11–20 kHz). This injection system consists of a supersonic actuation air jet at the inner core that provides large mean and fluctuating velocity profiles in the shear layers of a fluid stream injected surrounding the core through an annular nozzle, with pulsing occurring at a designated frequency. The high-frequency streamwise vortices and shockwaves tailored to the mean flow significantly enhanced supersonic flow mixing between the fluids compared to a steady coaxial configuration operating at the same input pressure. Experiments also indicate a strong connection between the frequency and unsteady amplitude of the actuation jet to the supersonic flow mixing phenomenon. This paper reports the design details of the injector assembly and flow mixing characteristics captured using phase-locked microschlieren and planar laser-induced fluorescence techniques.

Залежність структури електродугових покриттів від параметрів напилення деталей транспортної техніки

The use of special flux-cored wires for electric arc spraying allows for coatings with high wear resistance. However, the insufficient adhesion and cohesion of the resulting coatings does not allow these coatings to be used under increased operating loads. To improve the mechanical characteristics of gas-thermal coatings, a supersonic gas jet is used to transport molten droplets to the sprayed surface, increasing their kinetic energy. It is proposed to apply a supersonic air jet using a Laval nozzle and increasing the air jet pressure from 0.6 to 1.0...1.2 MPa. The aim of the study is to determine the effect of air jet pressure on the structure of electric arc coatings. It has been determined that an increase in the air jet pressure from 0.6 to 1.2 MPa causes a 2-fold increase in the air flow velocity from 300 to 600 m/s. At the same time, the velocity of droplets dispersed by the air jet during the spraying of electrode cored wires increases from 60-90 m/s to 160-220 m/s, and their size decreases. Reducing the flight time of the dispersed droplets from the arc to the sprayed surface ensures their higher temperature when they hit the sprayed surface. Increasing the air jet pressure also reduces the expansion angle of the metal-air jet from 30° to 15°. The microhardness of iron oxide inclusions - magnetite, wustite and hematite - in the coating of unalloyed steel wire sputtered is in the range of 700-800 HV. This ensures a microhardness of the U8 coating of 350-400 HV at a pressure of 0.6 MPa. Sputtering the same coating at a pressure of 1.2 MPa reduces the size of the lamellae and oxides, and their volume content increases, which increases the microhardness to 450...500 HV. The microhardness of coatings made of 90X17R3GS increases from 800 to 910 HV. It was found that an increase in air jet pressure from 0.6 to 1.2 MPa provides an increase in air jet velocity from 300 to 600 m/s, and the velocity of dispersed droplets from 120 to 220 m/s.

Heat Conductivity of the Material Obtained by Melting Steel on Flame Supersonic Jet of Air–Propane Mix

Heat Conductivity of the Material Obtained by Melting Steel on Flame Supersonic Jet of Air–Propane Mix

Investigation of the transient heat transfer to a supersonic air jet impinging on a high-temperature plate based on a discrimination-experiment method.

A discrimination-experiment method is developed to investigate the transient heat transfer of air jet impingement by discretizing the solid domain into mutually adiabatic test cylinders. This method can not only reduce the influence of the transverse heat transfer of a solid domain on the heat transfer characteristics of the jet but can also simplify the two-dimensional or three-dimensional heat conduction problem into a one-dimensional problem. Moreover, the discrimination-experiment method eliminates the embedment of thermocouples into the solid domains, further improving the accuracy and reliability of the proposed method. The transient heat transfer characteristics of a supersonic air jet impinging on a high-temperature target (860°C) and the effects of thermo physical parameters, such as the density, specific heat capacity, thermal conductivity and nozzle-to-target distance (H/D = 3, 4, and 5) are analyzed in detail using the discrimination-experiment method. The results provide important guidance for the thermal design of supersonic air jet impingement.

Using sonic crystals to separate the acoustic from the flow field of a fluidic transducer

Ultrasonic testing is a widely applied measurement method in materials research and medicine. Commonly, a transducer is coupled to the specimen directly or via a liquid coupling agent. While reducing acoustic transmission losses significantly, this procedure is time-consuming and cannot be used for sensitive specimens. Air-coupled ultrasound is a viable alternative in such cases, although suffering from very high acoustic transmission losses between transducer, air and specimen.The recently introduced fluidic transducer (FT) generates ultrasound by utilizing the instability of a supersonic air jet switched inside a fluidic amplifier. Since only air is used as the working medium and no vibrating surfaces are used for ultrasound generation, the transducer is able to efficiently generate large acoustic pressure amplitudes. The resulting acoustic field shares its directivity with the ejected high-velocity air jet. Thus, the acoustic energy needs to be redirected from the jet axis in order to make the fluidic transducer applicable to sensitive specimens.In this study, the effectivity of using sonic crystals (SCs) for this redirection is investigated using acoustic and flow measurements. SCs are air-permeable while being reflective to large acoustic frequency bands. It was shown that both a defect waveguide and a mirroring strategy successfully redirected the acoustic field from the air jet. Furthermore, the interaction of flow and SC showed strong acoustic quenching if the SC was placed too close to the FT outlet. Blockage of the jet entrainment due to the SC may result in slightly higher off-axis flow velocities locally, which should be considered in sensitive applications.

Effect of nozzle pressure ratios on the flow and distribution of abrasive particles in abrasive air jet machining

Effect of nozzle pressure ratio (NPR) on the flow and distribution of abrasive particles was investigated to improve supersonic abrasive air jet machining (AAJM) effect. The supersonic air-particle flow was simulated via CFD-DEM modeling, which was verified by AAJM experiments. It was concluded that NPR determined the acceleration and distribution of particle flow by controlling the structure and turbulent kinetic energy of the air jet free-flow field. When NPR = 1.12, the particle flow field had a longer stable region and a higher particle velocity, and the radial distribution resembled an n-shape. Further, in AAJM experiments, when the material was fixed in the stable region of particle flow field, the impact stresses were relatively high and uniformly distributed. Thus, when NPR = 1.12, the target material is fixed in the stable region of the abrasive particle flow, and the target material can obtain a better machining effect in the AAJM.

Modification of HfB2–30% SiC UHTC with Graphene (1 vol %) and Its Influence on the Behavior in a Supersonic Air Jet

Modification of HfB2–30% SiC UHTC with Graphene (1 vol %) and Its Influence on the Behavior in a Supersonic Air Jet

Shock-wave distortion cancellation using numerical recalculated intensity propagation phase holography

Digital holography is a three-dimensional (3D) measurement technique that can be used to quantitatively determine the size and 3D location of the objects inside a field-of-view. However, in systems where refractive index gradients are present, variations in optical phase due to high-speed shock-waves or low-speed thermal gradients can cause distortions that obscure objects. While techniques like phase-conjugate digital in-line holography and iterative phase measurement techniques have been developed in the past for phase removal or phase measurement, they require either nonlinear four-wave-mixing or iterative algorithms to operate. In this paper, we demonstrate a novel recalculated intensity propagation phase holography (RIPPH) method that captures distorted holograms using low-power continuous lasers, refocuses the hologram to the plane of the distortion, and cancels the phase distortion numerically in a single post-processing step. The resulting hologram can be numerically refocused to provide distortion-free 3D information describing objects that absorb or scatter light. In RIPPH, only the approximate z-locations of the phase distortions are needed, making this method significantly faster to compute than phase retrieval methods. Theoretical simulations are first used to describe and assess the distortion removal process. Experiments are then conducted to demonstrate at least 3× lower edge distortion for RIPPH compared to traditional digital in-line holography. We demonstrate, for the first time, how phase distortions from supersonic air jets, particle-laden spherically expanding shock-waves, and convectively-driven thermal gradients are numerically cancelled and show how objects of interest are accurately recovered.

On the mechanism of the influence of medium density on the supersonic core length of an underexpanded jet

Exhaustion of a supersonic underexpanded air jet into air–helium and air–sulfur hexafluoride mixtures is studied experimentally. The air jet escapes from a convergent circular nozzle in the range of moderate Reynolds numbers covering the laminar and transitional regimes of the jet flow. The experiments are performed with “cold” and heated jets in a low-pressure jet setup, which allows the Reynolds number of the jet exhaustion from the nozzle, the jet pressure ratio, and the density of the gas surrounding the jet to be maintained independently. A Pitot tube is used to measure the supersonic core length of the air jet as a function of the density of the ambient medium whereto the jet exhausts. In addition, the spatial distributions of the amplitude–frequency characteristics of acoustic oscillations generated by the jet in the ambient space are measured by a moving sensor of pressure fluctuations. The effect of the ratio of the underexpanded air jet density to the ambient medium density on the supersonic core length is demonstrated. It is shown that the mechanism of influence of the ambient medium density is associated with a change in the conditions for the global instability of the underexpanded air jet and, in particular, the conditions for generation of the screech tone.

Flow Characteristics of Two-Dimensional Supersonic Under-Expanded Coanda-Reattached Jet

When there is a wall near the jet, it deflects and flows while being attached to the wall owing to the Coanda effect. The flow characteristics of the incompressible and two-imensional (2D) Coanda-reattached jets have been considerably explained. However, 2D supersonic under-expanded jets, reattached to side walls, have not been sufficiently investigated. These jets are used in gas-atomization to produce fine metal powder particles of several micrometers to several tens micrometers. In this case, the supersonic under-expanded jets are issued from an annular nozzle, which is set around a vertically installed circular nozzle for molten metal. The jet flow at the center cross-section of the annular jet resembles a 2D Coanda-reattached jet that deflects and attaches on the central axis. In this study, the flow characteristics of a supersonic under-expanded Coanda air jet from a 2D nozzle that reattaches to an offset side wall are elucidated through experiment and numerical analysis. For numerical analysis, we show how much it can express experimental results. The effects of supply pressure P0 on the flow characteristics such as the flow pattern, size of shock cell, reattachment distance, and velocity and pressure distributions, etc. are examined. The flow pattern was visualized by Schlieren method and the velocity distribution was measured using a Pitot tube. These results will be also useful in understanding the flow characteristics of a gas-atomization annular nozzle approximately.

CALCULATION OF THE FREQUENCY CHARACTERISTICS AT THE POINT OF THE ACOUSTIC FIELD OF A FREE SUPERSONIC JET OF A ROCKET ENGINE

The paper studies the frequency characteristics of the acoustic field of a free supersonic jet of a powder rocket engine. The results were compared with similar results of free supersonic cold air jets by a number of other authors.

The effect of underexpanded jet flow conditions on the supersonic core length

The paper describes the results of measurements of the supersonic core length of underexpanded jets exhausted into a submerged space. Exhaustion of supersonic He, air, CO2, Ar, and SF6 jets into He, air, CO2, Ar, and SF6 at moderate Reynolds numbers for the laminar, transitional, and turbulent flow regimes is considered. These jets outflow from convergent nozzles with diameters of 0.52, 0.72 and 1.06 mm; the measurements are performed by a Pitot tube. The amplitude-frequency spectra of acoustic oscillations generated by the jets in the ambient space are measured. The experiments are performed in a low-pressure jet setup, which allows independently maintaining the Reynolds number of gas exhaustion from the nozzle and the jet pressure ratio. The jet setup is used to study exhaustion of a jet of one gas into the submerged space filled with another gas and the influence of the ratio of densities of these gases on the supersonic core length.

Liquid jet penetration and breakup in a free supersonic gas jet

The interaction of a coherent liquid jet with a free supersonic gas jet is of importance in cooling high-temperature exhaust plumes generated by ground rocket tests. Further, the associated phenomena are distinct from the well-studied jet in supersonic crossflow due to the unbounded nature of the gas jet, its shock train structure, and the relative sizes of the two jets. This work utilizes non-intrusive diagnostics including focused color Schlieren, high-speed diffused backlit imaging, and volume-illuminated imaging to study the interaction process of a high Reynolds number water jet with a free supersonic air jet. The effect of the water jet on the shock train structure and its dependence on water injection location and pressure are studied. The ensuing water jet breakup mode is investigated and found to exhibit distinguishing characteristics from known breakup modes observed at high Reynolds and gas Weber numbers. Finally, the water jet penetration height is quantified, and the ability of existing correlations from the literature to predict the penetration height are investigated.

Study of liquid hydrocarbons atomization by supersonic air or steam jet

In this work, the gas-drop two-phase flow characteristics in a perspective atomizing burner were measured with respect to operating conditions. The liquid fuel (diesel fuel and waste engine oil) was sprayed by a superheated steam or air jet. Data on the main parameters of such a gas-fuel jet (dispersed composition, carrier and dispersed phase velocity, jet opening angle) were obtained for various operating conditions (steam/air and fuel temperatures and flow rates) using modern optical flow diagnostics methods (SP, IPI, PIV, PTV). The flow structure, characteristic of a supersonic submerged jet with an opening angle of 20°, was registered near the nozzle. Characteristic diameter of the identified drops in all investigated modes is 10–20 µm, which is sufficient for efficient fuel combustion that proves the advantages of the investigated method of liquid fuel atomization. It has been established that operating conditions (such as gas or fuel temperature and fuel rates, ratio of flow rates) in the studied range slightly affects the change in the droplet size distribution. The research results showed that the size distribution of fuel droplets depends weakly on the choice of atomizing phase (steam or air).

Experimental and Numerical Investigation of Heat Exchange between Underexpanded High-Enthalpy Air Jets and Cylindrical Models

Experiments on heat transfer in supersonic underexpanded high-enthalpy air jets are conducted on the VGU-4 induction plasmatron at the pressure in the compression chamber of 8.5 hPa. At the air flow rate of 3.6 g/s and the high-frequency generator powers of 45 kW(regime 1) and 64 kW (regime 2) the heat fluxes to the copper surface at the stagnation point of watercooled cylindrical models along the axes of dissociated air jets are measured. The models, 30 mm in diameter, could have a flat face or a hemispherical nose. In the same regimes, the stagnation pressures are measured using the Pitot tube in the shape of a cylinder, 30 mm in diameter, having either a flat face or a hemispherical bluntness with a receiving hole, 14 mm in diameter. For the experimental conditions calculations of flows in the plasmatron discharge channel and supersonic underexpanded jets issuing from the discharge channel are performed within the framework of the Navier–Stokes and Maxwell equations. The heat fluxes to the experimental models are computed and compared with the experimental data.

Flows of Supersonic Underexpanded Jets on the Range of Moderate Reynolds Numbers

The results of an experimental investigation of the gasdynamic structure of supersonic underexpanded air jets flowing out of a sonic nozzle into a low-pressure medium are presented. This setting of the experimentmakes it possible to achieve high values of the nozzle-to-ambient pressure ratio at moderate outflow Reynolds numbers characteristic of underexpanded jets issuing from micronozzles. The data on the supersonic core length, the laminar-turbulent transition location, and the jet flow characteristics are obtained. The results are compared with those obtained in microjets flowing out of sonic nozzles. Emphasis is placed on the earlier discovered effect of inverse transition of a turbulent jet into the laminar flow regime with increase in the Reynolds number.

Stable electron beams from laser wakefield acceleration with few-terawatt driver using a supersonic air jet

The generation of stable electron beams produced by the laser wakefield acceleration mechanism with a few-terawatt laser system (600 mJ, 50 fs) in a supersonic synthetic air jet is reported and the requirements necessary to build such a stable electron source are experimentally investigated in conditions near the bubble regime threshold. The resulting electron beams have stable energies of (17.4 ± 1.1) MeV and an energy spread of (13.5 ± 1.5) MeV (FWHM), which has been achieved by optimizing the properties of the supersonic gas jet target for the given laser system. Due to the availability of few-terawatt laser systems in many laboratories around the world these stable electron beams open possibilities for applications of this type of particle source.

Mechanical properties of poly(ethylene terephthalate) nanofiber three‐dimensional structure prepared by CO2 laser supersonic drawing

ABSTRACTCO2‐laser supersonic drawing method can produce bulky fluffy poly(ethylene terephthalate) (PET) nanofibers (NFs) by only irradiating CO2‐laser to as‐spun PET fibers in the supersonic air jet. Cylindrical PET NF three‐dimensional structure (NF‐3DS) was fabricated by compression‐molding the obtained fluffy PET NFs using the cylindrical metal mold. NF‐3DS mold was completely disordered 3DS without a laminated structure because NFs were disorderly packed in the metal mold. The porosity of NF‐3DS can be changed by varying the filling weight of NF into the metal mold, and the highest porosity was 95.4%. The shape recovery ratio after 50% uniaxial compression in the height of NF‐3DS increases as the porosity increases, and NF‐3DS with a porosity of 95.4% had a shape recovery ratio of 98.1%. NF‐3DS with a desired shape will be produced if the metal mold can be prepared. © 2017 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2018, 135, 45763.

A Feasibility Study of Supersonic Air Flow for Cooling Application

The theoretical study of steady, one-dimensional, isentropic, compressible fluid flow suggests that a convergent-divergent passage is required for accelerating a compressible fluid to supersonic velocities. It also reveals that along with pressure, temperature of the compressible fluid continuously decreases along the flow passage. Air is also a compressible fluid which is freely and abundantly available. Keeping these facts in mind the possibility of obtaining supersonic flow with the help of compressed air is first studied theoretically. A convergent divergent nozzle is designed and developed for obtaining supersonic flow of air. An experiment is carried out for various back pressures for verifying supersonic flow and measuring the exit temperature of air. It is observed that supersonic flow can be obtained with minimum back pressure of 1 bar. Exit temperature of air can be varied by changing the back pressure of the nozzle. Temperature drop of air as high as 18.23 0C could be obtained. These factors prove the potential of supersonic air jet as a good and environmentally friendly cooling medium especially for localized cooling applications.