Subsonic Air Jet Research Articles

Effect of Flow Swirling on the Subsonic Air Jet in the VGU-4 HF Plasmatron

Effect of Flow Swirling on the Subsonic Air Jet in the VGU-4 HF Plasmatron

Hydrogen Micro Jet Diffusion Combustion in a Coaxial Air Jet

In this paper the results of experimental studies of the combustion of a round microjet of hydrogen in a ring subsonic coaxial air jet are presented. It is shown that the combustion scenario for a subsonic hydrogen jet outflow is associated with the presence of a “bottleneck flame region” and nozzle heating, but the initial spherical shape of the flame region is transformed into a cylindrical one. It was found that the combustion of a round microjet of hydrogen in a satellite ring coaxial air jet at a supersonic outflow velocity is accompanied by the presence of supersonic cells.

The Effect of the Geometry of the Discharge Channel in a High-Frequency Plasmatron on Heat Transfer in High-Enthalpy Subsonic Air Jets

The Effect of the Geometry of the Discharge Channel in a High-Frequency Plasmatron on Heat Transfer in High-Enthalpy Subsonic Air Jets

Uncertainty Quantification of Turbulence Measurements in a Fully Pulsed Axisymmetric Subsonic Air Jet

This paper reports the statistical accuracy evaluation of the implementation of ensemble-averaged equation used for the measurement of turbulence quantities in a fully pulsed, axisymmetric-subsonic air jet issuing into the still surrounding air. A constant mass flow rate and frequency of pulsation were used to generate the intermittent free jet, which physically contained a period of no flow between pulses due to a mechanical excitation. Time-history records of the random process were gained from a single normal hot-wire probe by a digital sampling technique, where the probe was operated in a constant temperature mode and placed at a fixed location in the jet field with the probe stems parallel to the mean flow direction. Within the framework of amplitude-domain analysis, the accuracy assessment was performed to which the uncertainties of mean value and higher-order moments were evaluated from the given values of sampling parameters, where the estimate of integral-time scale of the random process was the basis for determining the given sampling parameters. The present results suggest appropriate values of number of samples, sampling frequency, and sampling time of the hot-wire anemometer (HWA) technique for a repeatable turbulence measurement in the unsteady jet within a high degree of accuracy.

An experimental study of the flow of subsonic flat mini and micro air jets

We have experimentally studied subsonic submerged air jets emitted from flat mini and micro nozzles with characteristic dimensions from 22 to 600 μm in a range of Reynolds numbers 70–2600. The point of laminar-turbulent transition (jet penetration range) was determined using the flow visualization technique. It is established that the penetration range of micro jets can reach 100–300 nozzle calibers. The Reynolds number for the transition to turbulence in flat mini and micro jets reaches high values (1000–2600), which are two to three orders of magnitude greater than the Reynolds numbers for the loss of stability (3–10). Available experimental data are summarized and generalized based on the Reynolds number determined for the jet penetration range.

Fundamental Experimental Studies Supporting Active-Jet Acoustic Control of In-Plane Rotor Harmonic Noise

A new approach to reducing low-frequency, in-plane harmonic noise of helicopter rotors is explored experimentally, and results are compared with an extended theoretical model. The active-jet acoustic control methodology employs on-blade, tip-located, unsteady air blowing to produce acoustic waves that reduce the observed noise at targeted positions in the acoustic far field of the rotor system. An extended theoretical model of the subsonic air jet, which is idealized as a source of mass and momentum, is presented. Experimental measurements in the University of Maryland acoustic chamber for an approximately one-seventh-scale rotor, operated at a hover tip Mach number of 0.661, indicate that active-jet acoustic control is a viable option for reducing low-frequency in-plane harmonic noise. Good correlation between theoretical predictions and measured data for three valve control cases are observed. Model-scale limitations of the tip-jet blowing experiment lmited the peak noise level reductions to 30%. However, theory suggests that, if the limitations of the model-scale controller were mitigated, much larger noise reductions are possible.

An experimental study of submerged jets at low reynolds numbers

Results of an experimental study of submerged subsonic air jets exiting from axially symmetric and plane channels in the Reynolds number range of 100–6000 are presented. Data on visualizing the flow picture and zone of the laminar-turbulent transition have been obtained. The measurement results are compared with well-known experimental data for gas macro- and microjets. The coordinate of the transition to the turbulence for microjets is shown to be able to reach high values, and a generalization of the experimental data is presented.

Discrete Frequency Noise Generated by Air Jet between Flat Plate and Wedge

Aerodynamic noise with discrete frequencies is generated in an unexpected way from a slit between a flat plate, which is placed in parallel to the main flow, and a wedge that is placed normal to the flow. To study this phenomenon, we examined the discrete frequency noise generated by a subsonic air jet flow from a slit. It was found that the slit height and the plate length have considerable effects on the noise generation. In addition, the discrete frequency noise was generated only when the values of these parameters are within a limited range. Furthermore, it was found that the noise frequency is proportional to the jet velocity and is inversely proportional to the plate length. Finally, it was made clear from the present study that the interaction of the vortex, which is generated from the shear layer due to a wedge, with the wake of a flat plate is a major cause for the discrete noise due to a slit.

Experimental study of slot jet impingement heat transfer on a wedge-shaped surface

An experimental investigation was conducted to study the convective heat transfer rate from a wedge-shaped surface to a rectangular subsonic air jet impinging onto the apex of the wedge. The jet Reynolds number, nozzle-to-surface distance and the wedge angle were considered as the main parameters. Jet Reynolds number was ranged from 5,000 to 20,000 and two dimensionless nozzle-to-surface distances h/w = 4 and 10 were examined. The apex angle of the wedge ranged from 30° to 180° where the latter case corresponds with that of a flat surface. Velocity profile and turbulence intensity were provided for free jet flow using hot wire anemometer. Local and average Nusselt numbers on the impinged surface are presented for all the configurations. Based on the results presented, the local Nusselt number at the stagnation region increases as the wedge angle is decreased but, it then decreases over the remaining area of the impinged surface. Average Nusselt number over the whole surface is maximum when the wedge angle is 180° (i.e. plane surface) for any jet and nozzle-to-surface configuration.

The spatial-temporal evolution of combustion under conditions of low temperature discharge plasma of liquid alcohol injected into an air stream

Stabilization of the combustion of liquid alcohol injected in a drop phase at a mass flow rate of dm2/dt = 0.5–3 g/s into a subsonic air jet at a mass flow rate of dm1/dt = 10–100 g/s flowing in a submerged space (atmospheric pressure air) was performed under a combined discharge created in a programmable pulse mode.

Optimal geometry design of the melt‐blowing slot die with high stagnation temperature via the orthogonal array method and numerical simulation

In this paper, a method combining the orthogonal array design and the numerical simulation is proposed to optimize the geometry parameters of the melt‐blowing slot die. An index, the stagnation temperature, is used to evaluate the performance of the slot die. The stagnation temperature is obtained by simulating the subsonic compressible air jet from the melt‐blowing slot die, whereas the optimization is accomplished by the orthogonal array method. Three geometry parameters of the slot die: slot width, nose piece width, and slot angle are investigated. The results show that smaller slot angle and larger slot width will result in a higher stagnation temperature, which is beneficial to the air drawing of the polymer melt and thus to reducing fiber diameter, whereas the effect of nose piece width is insignificant. The optimal geometry parameters of the melt‐blowing slot die achieved in this study are: slot width of 1.5 mm, slot angle of 30°, and nose piece width of 2 mm.

Asymptotic properties of the overall sound pressure level of subsonic jet flows using isotropy as a paradigm

Measurements of subsonic air jets show that the peak noise usually occurs when observations are made at small angles to the jet axis. In this paper, we develop further understanding of the mathematical properties of this peak noise by analysing the properties of the overall sound pressure level with an acoustic analogy using isotropy as a paradigm for the turbulence. The analogy is based upon the hyperbolic conservation form of the Euler equations derived by Goldstein (Intl J. Aeroacoust., vol. 1, 2002, p. 1). The mean flow and the turbulence properties are defined by a Reynolds-averaged Navier–Stokes calculation, and we use Green's function based upon a parallel mean flow approximation. Our analysis in this paper shows that the jet noise spectrum can, in fact, be thought of as being composed of two terms, one that is significant at large observation angles and a second term that is especially dominant at small observation angles to the jet axis. This second term can account for the experimentally observed peak jet noise (Lush,J. Fluid Mech., vol. 46, 1971, p. 477) and was first identified by Goldstein (J. Fluid Mech., vol. 70, 1975, p. 595). We discuss the low-frequency asymptotic properties of this second term in order to understand its directional behaviour; we show, for example, that the sound power of this term is proportional to the square of the mean velocity gradient. We also show that this small-angle shear term does not exist if the instantaneous Reynolds stress source strength in the momentum equation itself is assumed to be isotropic for any value of time (as was done previously by Morris & Farrasat,AIAA J., vol. 40, 2002, p. 356). However, it will be significant if the auto-covariance of the Reynolds stress source, when integrated over the vector separation, is taken to be isotropic in all of its tensor suffixes. Although the analysis shows that the sound pressure of this small-angle shear term is sensitive to the statistical properties of the turbulence, this work provides a foundation for a mathematical description of the two-source model of jet noise.

Experimental simulation of heat transfer in an HF-plasmatron with lengthened segmented discharge channel

Subsonic and supersonic air induction plasma flows in a VGU-4 100 kW plasmatron with segmented water-cooled cylindrical nozzle with the outlet cross-section 40 mm in diameter are investigated experimentally. The enthalpy on the axis of flow is measured in subsonic air jets. The heat fluxes are measured at the stagnation flow point on a cylindrical water-cooled model 50 mm in diameter located in subsonic air and nitrogen jets. The effect of the generator power, nozzle length, and pressure in the plasmatron pressure chamber on the distributions of the heat flux and the pressure at the stagnation point on the surface of cylindrical models 20 mm in diameter with a plane and hemispheric nose is investigated along the axis of underexpanded dissociated air jets.

Control of an axisymmetric subsonic air jet by plasma actuator

It is known that surface non-thermal plasma actuators have proved their efficiency for aerodynamics flow control. In this study, a dielectric barrier discharge (DBD) is mounted on the diffuser of an axisymmetric turbulent air jet in order to control the flow separation along a 12-degree diffuser bevel. The momentum created by the actuator is applied to separate an air flow naturally attached to the diffuser for air flow velocity up to 40 m s−1. Laser sheet visualizations and LDV measurements are achieved to characterize the unforced and forced air jet. The flow separation, the induced velocity fluctuations, the jet mixing improvement and vectoring are investigated. The main results of this study demonstrate that DBD actuators are suitable to fully detach the air flow along the bevel for a velocity of 20 m s−1 and that a jet vectoring between 13.5° and 5.5° could be achieved for velocity ranging between 20 and 40 m s−1. Considerations about a potential improvement of the jet mixing are also introduced and the laser sheet visualization attests that induced flow perturbations are highly 3D.

Visualization of air flow using infrared thermography

A technique to visualize airflow using IR thermography is developed. A trace quantity of sulfur hexafluoride (SF6) gas is injected into the flow field to detect intensity patterns using a scanning IR thermography system with a single 8–13 µm bandwidth detector. Equations that relate the intensity patterns to volume-averaged temperature and SF6 mass concentration are presented. The visualization technique involves using a background surface at a known temperature different from that of the flow and image subtraction. The technique is demonstrated on free, impinging, and reattachment subsonic air jets, and is shown to be an effective means of visualizing flows at both elevated and ambient temperatures.

Measurement of vortex ring characteristics during pairing in a forced subsonic air jet

The pairing of vortex rings in an acoustically excited jet is experimentally investigated. The acoustical forcing of the jet allows a series of particle image velocimetry (PIV) measurements to be performed at a frequency slightly lower than the frequency at which the vortex rings are shed in the shear layer. This technique, here called stroboscopic PIV, provides a time-resolved evolution of the velocity and vorticity fields during the threading and merging of the vortex rings, with a view to the prediction of the sound produced by this interaction. The evolution of the characteristics of the cores of the vortex rings during pairing is determined using an automated vortex detection algorithm. Three stages are distinguished in the evolution of the rings, based on the relative acceleration and deformation of their cores. The conservation of the circulation during the merging is addressed. The accuracy of the measurements is estimated, being a crucial issue for the future prediction of the sound generation based on the experimental description of the flow field.

Disturbance generation in a subsonic turbulent jet under the influence of high-intensity sound

The effect of high-intensity sound (L=140–160 dB) on a subsonic turbulent air jet under transverse external irradiation was studied experimentally. It is shown that the onset and development of disturbances in the early stage does not depend on the excitation frequency, while the intensity of the jet mixing is determined in large measure by the distance between the sound-generated vortices and by the degree of interaction between them.

A Study on the Noise Generation Mechanism in a Circular Air Jet

An investigation of the noise generation mechanism of turbulent structures in a subsonic air jet has been done theoretically as well as experimentally. Based on the results obtained, it is proposed that the rate of change of velocities of the jet structures plays an important role in the generation of sound in an air jet.

Velocity field of an axisymmetric pulsed, subsonic air jet

Laser Doppler anemometer measurements in a fuUy pulsed, subsonic air jet with a significant no-flow period between pulses have been conducted and show much higher entrainment than steady or partially pulsed jets of the same mass flow. The mean centerline velocity decay is linearly related to the inverse of the effective distance from exit for some 50 diameters, but centerline velocity decay is much slower than for steady jets due to domination by the periodic component and its associated pressure field, which affects jet momentum. For larger distances, the decay changes to the steady jet rate. Reynolds stresses are considerably larger than for a steady jet and are considered to be responsible for the increased entrainment. Results are, except for a small increase in the constant of proportionality, consistent with Taylor's entrainment hypothesis. Phase averaged results through a cycle show the ratio of shear stress to turbulent kinetic energy to be in the range of 0.2-0.3 for the bulk of the flow.

A gas jet impacting a cavity

A subsonic air jet impinging upon a cavity is studied to explain the resultant heating phenomenon. Flow visualization within the cavity shows a large central vortex dominating the flow pattern. Velocity measurements inside the cavity are made using a hot-wire anemometer. Temperature is measured with a copper-constantan thermocouple. The velocity field within the cavity is described by a modified Rankine combined vortex. An uncommon form of the energy equation is used to account for turbulent heating in adverse pressure gradients. A theoretical solution is developed to model the temperature field in the cavity. There is a good agreement between the calculated and measured temperatures. The heating effect is related to Ranque-Hilsch tubes.