An experimental study of jet noise part II: Shock associated noise

The characteristics of the sound field of shock-containing under-expanded jet flows are studied by measuring the noise from a convergent nozzle operated over an extensive envelope of supercritical jet operating conditions. The measurements were conducted in an anechoic facility. They are complementary to the turbulent mixing noise experiments (described in Part I) for subsonic and fully-expanded (shock-free) supersonic jets. The overall results from shock-containing jets are compared directly with the corresponding results from shock-free jets, and the effects of nozzle pressure ratio and jet exhaust temperature on broadband shock-associated noise are assessed independently. For a supersonic jet, the regimes of jet operating conditions, observer angles, and frequencies over which the sound field is dominated by shock-associated noise are identified. Finally, the spectral results are compared in a preliminary manner with the spectra predicted by an existing theoretical model, and good agreement is obtained in most cases.     

超音喷流啸音发声机理的实验研究

啸音是超音速喷流噪声的三大成分之一,其特点是向上游传播的离散纯音,被认为是飞机部件声疲劳的重要因素之一。但到目前为止,啸音仍是喷流噪声研究中理解最少、预测能力最低的成分,研究表明这主要是因为啸音受环境的影响很大,如啸音的强度就受到喷流马赫数、喷流温度、喷嘴唇口厚度以及形状等的影响。本文通过实验研究了在超音速喷流情况下喷嘴唇口厚度对啸音幅值、模态等的影响。实验结果表明,随着喷嘴厚度的增加,啸音的幅值增加达到了10 db以上。啸音的频率则随着唇口厚度的增加有所减小。     

An experimental study of jet noise part I: Turbulent mixing noise

The characteristics, both spectral and directivity, of turbulent mixing noise in the far field from subsonic and fully-expanded supersonic jet flows have been studied experimentally over an extensive envelope of jet operating conditions (jet exit velocity and temperature). The measurements were conducted in an anechoic room which provides a free-field environment. The results are presented in a systematic manner, and the observed trends and dependencies are discussed in detail. In particular, the changes in detailed jet noise features with varying velocity and exhaust temperature are assessed independently. Empirical prediction schemes or comparisons with recent theoretical investigations are not attempted here. However, the isothermal jet noise results are compared with those predicted by the freely-convecting quadrupole theories (that is, in the absence of any mean flow shrouding effects). The discrepancies between this model and the measurements, many of which have been recently shown to occur due to the presence of mean velocity and temperature gradients surrounding the sources, are obtained accurately over all jet operating conditions of interest.     

Shock associated noise of supersonic jets from convergent-divergent nozzles

Results of experimental and theoretical studies of the characteristics of shock associated noise from imperfectly expanded supersonic jets over an extensive range of underexpanded and overexpanded operating conditions are described. This kind of broadband noise is believed to be generated by the weak but coherent interaction between the downstream propagating large scale turbulent flow structures in the mixing layer of the jet and the nearly periodic shock cell system. Theoretical reasoning based on this mechanism leads to the scaling formula that the intensity of shock associated noise varies as (M_j² ? M_d²)² where M_j and M_d are the fully expanded jet operating Mach number and nozzle design Mach number, respectively. This formula holds for underexpanded as well as overexpanded jet Mach numbers. In addition, a peak frequency formula is also derived from the same model. The noise intensity, directivity and spectra of supersonic jets from a convergent-divergent nozzle of design Mach number 1·67 were measured in an anechoic facility over the Mach number range of 1·1 to 2·0. The effect of jet temperature was investigated by operating the jet at three temperature conditions. These sets of data provide sufficient information for fully assessing the relative importance and characteristics of shock associated noise of supersonic jets from convergent-divergent nozzles. Comparisons between theoretical results and measurements show very favorable agreement.     

Ignition of hexane-air mixtures by highly under-expanded hot jets

We report an experimental study of ignition of flammable mixtures by highly unexpanded, supersonic hot jets. The high-pressure, hot-gas reservoir supplying the jet is created by impacting a projectile on a plunger to rapidly compress and ignite a rich n-hexane/air mixture, resulting in a peak reservoir pressure of more than 20 MPa. A locking mechanism was used to prevent the plunger from rebounding and the jet was created by rupturing a diaphragm covering a nozzle with an exit diameter between 0.25 and 1 mm. The jet development and ignition processes in the main chamber filled with hexane-air mixture were visualized using high-speed schlieren and OH* chemiluminescence imaging. The ignition threshold was determined as a function of composition in the jet and main chamber, the nozzle diameter, and the initial pressure in the main chamber. Unlike the case of subsonic jets in which ignition occurs at the shear layer near the nozzle exit, ignition of combustion in the main chamber was found to take place downstream of the Mach disk terminating the supersonic expansion and within the turbulent mixing region created by the startup of the supersonic jet. The results are interpreted using a constant-pressure, well-stirred reactor model simulating the mixing between the hot jet and cold ambient gas. The critical conditions for ignition are determined by the competition between energy release due to chemical reactions initiated by the hot jet gas and cooling due to mixing with the cold chamber atmosphere. The critical value (maximum for which ignition occurs) of the mixing rate was computed using a detailed chemical reaction model and found to be a useful qualitative guide to our observations.     

Acoustic characteristics of interacting supersonic jets

This study was intended primarily to reveal more information about the noise producing mechanisms of supersonic jets. Two identical, small, cold air, supersonic, overexpanded jets were tested at selected angles, varying from parallel to 90 degrees intersecting, and at various distances apart. Schlieren photographs of the jet structure and far field sound data were obtained. Close spacing of the parallel jets caused acoustic attenuation, which reached a maximum at one diameter centerline spacing, where the sound of two jets nearly equals that of a single jet. In every case the intersecting jets merged into a single supersonic jet.The overall sound power level of intersecting jets is generally higher than that of two independent jets, because of the turbulent mixing of the two jet flows. A maximum level is reached when the jets intersect at a point near the middle of the flow region containing repetitive shocks. For the parallel jets and intersecting jets at large separation, the sound levels are lower in the plane containing the jet centerlines. For intersecting jets at small separation, however, this shielding effect is reversed.     

Research on the mechanics of underwater supersonic gas jets

An experimental research was carried out to study the fluid mechanics of underwater supersonic gas jets. High pressure air was injected into a water tank through converging-diverging nozzles (Laval nozzles). The jets were operated at different conditions of over-, full- and under-expansions. The jet sequences were visualized using a CCD camera. It was found that the injection of supersonic air jets into water is always accompanied by strong flow oscillation, which is related to the phenomenon of shock waves feedback in the gas phase. The shock wave feedback is different from the acoustic feedback when a supersonic gas jet discharges into open air, which causes screech tone. It is a process that the shock waves enclosed in the gas pocket induce a periodic pressure with large amplitude variation in the gas jet. Consequently, the periodic pressure causes the jet oscillation including the large amplitude expansion. Detailed pressure measurements were also conducted to verify the shock wave feedback phenomenon. Three kinds of measuring methods were used, i.e., pressure probe submerged in water, pressure measurements from the side and front walls of the nozzle devices respectively. The results measured by these methods are in a good agreement. They show that every oscillation of the jets causes a sudden increase of pressure and the average frequency of the shock wave feedback is about 5–10 Hz.     

Computation of streamwise vorticity in a compressible flow of a winglet nozzle-based COIL device

Chemical oxygen iodine laser (COIL) is a high-power laser with potential applications in both military as well as in the industry. COIL is the only chemical laser based on electronic transition with a wavelength of 1.315 μm, which falls in the near-infrared (IR) range. Thus, COIL beam can also be transported via optical fibers for remote applications such as dismantling of nuclear reactors. The efficiency of a supersonic COIL is essentially a function of mixing specially in systems employing cross-stream injection of the secondary lasing (I₂) flow in supersonic regime into the primary pumping (O₂¹Δ_g) flow. Streamwise vorticity has been proven to be among the most effective manner of enhancing mixing and has been utilized in jet engines for thrust augmentation, noise reduction, supersonic combustion, etc. Therefore, a computational study of the generation of streamwise vorticity in the supersonic flow field of a COIL device employing a winglet nozzle with various delta wing angles of 5°, 10°, and 22.5° has been carried out. The study predicts a typical Mach number of approximately 1.75 for all the winglet geometries. The analysis also confirms that the winglet geometry doubles up both as a nozzle and as a vortex generator. The region of maximum turbulence and fully developed streamwise vortices is observed to occur close to the exit, at x/λ of 0.5, of the winglets making it the most suitable region for secondary flow injection for achieving efficient mixing. The predicted length scale of the scalloped mixer formed by the winglet nozzle is 4λ. Also, the winglet nozzle with 10° lobe angle is most suitable from the point of view of mixing developing cross-stream velocity of 120 m/s with acceptable pressure drop of 0.7 Torr. The winglet geometry with 5° lobe angle is associated with a low cross-stream velocity of 60 m/s, whereas the one with 22.5° lobe angle is associated with a large static and total pressure drop of 1.87 and 9.37 Torr, respectively, making both the geometries unsuitable for COIL systems. The experimental validation shows a close agreement with the computationally predicted values. The studies for the most suitable 10° lobe angle geometry show an observed Mach number of 1.72 with an improved mixing efficiency of 74% due to the occurrence of predicted streamwise vortices in the flow.     

Use of a plane jet for flow-induced noise reduction of tandem rods

Unsteady wake from upstream components of landing gear impinging on downstream components could be a strong noise source.The use of a plane jet is proposed to reduce this flow-induced noise.Tandem rods with different gap widths were utilized as the test body.Both acoustic and aerodynamic tests were conducted in order to validate this technique.Acoustic test results proved that overall noise emission from tandem rods could be lowered and tonal noise could be removed with use of the plane jet.However,when the plane jet was turned on,in some frequency range it could be the subsequent main contributor instead of tandem rods to total noise emission whilst in some frequency range rods could still be the main contributor.Moreover,aerodynamic tests fundamentally studied explanations for the noise reduction.Specifically,not only impinging speed to rods but speed and turbulence level to the top edge of the rear rod could be diminished by the upstream plane jet.Consequently,the vortex shedding induced by the rear rod was reduced,which was confirmed by the speed,Reynolds stress as well as the velocity fluctuation spectral measured in its wake.This study confirmed the potential use of a plane jet towards landing gear noise reduction.     

PSP measurement of flow fields in a supersonic combustor with a backward-facing step

The mixing fields within a SCRAM-jet combustion chamber are visualized using pressuresensitive paint (PSP) as an oxygen sensor. The experiments are performed in a small supersonic wind tunnel at the National Aerospace Laboratory — Kakuda Research Center (NAL-KRC). The main stream Mach number is 2.4, and the dynamic pressure ratios between the injected gas and the main flow are 0.3, 0.7, 1.1 and 1.5. Three fuel injection nozzles are used; oxygen is injected from the central nozzle and air from the two nozzles at either side. The spread of the injected gas is measured to observe the effects of placing the nozzles in different positions. The results show that the jet has its own independent flow structure, and that little mixing of gases occurs between the flow structures created by each nozzle. When the injection dynamic pressure ratio is increased, the oxygen fraction rises in the recirculation zone and falls in the separation zone downstream of the injection.     

The Acoustic Performance of 3D Printed Multiple Jet Nozzles with Different Configurations

This work investigated multiple jet nozzles with various geometrical shape, number of exits, and material on reducing noise radiated from jet flows. Nozzles are categorized in two groups with few and many exit numbers, each with various exit shapes, slot and circular, and geometry. Firstly, nozzles are designed and then fabricated by a 3D printer, Form Labs, Form2USA, with polymeric resin. Also, the nozzle with the most noise reduction made of stainless steel. Noise and air thrust were measured at three air pressure gauges, 3, 5, 7 BAR and directions from nozzle apex, 30°, 90°, 135°. Nozzles with slot exit shape made of both plastic and stainless steel revealed the most noise reduction among all nozzles with few exit numbers, nearly 11–14 dB(A) and 11.5–15 dB(A), respectively. On average, slotted nozzle noise reduction was nearly 5–6 dB(A) more than finned nozzle. However, nozzles with more exit numbers, finned and finned-central exit, illustrated much more noise reduction than nozzles with few exit numbers, by almost 16–18 dB(A), they represented similar sound. All tested nozzles and open pipe demonstrated equal air thrust at each pressure gauges. The nozzles with slotted exit shape, either plastic or stainless steel, can provide reasonable noise reduction in comparison to other configuration with few exit numbers. In contrast, nozzles with more exit numbers demonstrated the most noise reduction.     

Comparative Study of the Propagation of Jet Noise in Static and Flow Environments

In order to analyze the effect of the background flow on the sound prediction of fine-scale turbulence noise, the sound spectra from static and flow environments are compared. It turns out that, the two methods can obtain similar predictions not only at 90 deg to the jet axis but also at mid- and high frequencies in other directions. The discrepancies of predictions from the two environments show that the effect of the jet flow on the sound propagation is related to low frequencies in the downstream and upstream directions. It is noted that there is an obvious advantage of computational efficiency for calculating in static environment, compared with that in flow environment. A good agreement is also observed to some extent between the predictions in static environment and measurements of subsonic to supersonic. It is believed that the predictions in static environment could be an effective method to study the propagation of the sound in jet flow and to predict the fine scale turbulence noise accurately in a way as well.     

Experimental investigation of acoustic self-oscillation influence on decay process for underexpanded supersonic jet in submerged space

Intensification of mixing between the gaseous working body ejected through a jet nozzle with ambient medium is an important scientific and technical problem. Effective mixing can increase the total efficiency of power and propulsion apparatuses. The promising approach, although poorly studied, is generation of acoustic self-oscillation inside the jet nozzle: this impact might enhance the decay of a supersonic jet and improve the mixing parameters. The paper presents peculiar properties of acoustic self-excitation in jet nozzle. The paper presents results of experimental study performed for a model injector with a set of plates placed into the flow channel, enabling the excitation of acoustic self-oscillations. The study reveals the regularity of under-expanded supersonic jet decay in submerged space for different flow modes. Experimental data support the efficiency of using the jet nozzle with acoustic self-oscillation in application to the systems of gas fuel supply. Experimental results can be used for designing new power apparatuses for aviation and space industry and for process plants.     

Application of RANS-based method to predict acoustic noise of chevron nozzles

Passive noise control devices for jet flows, such as chevron nozzles, have been studied for a long time due to their large application in turbofan engines. The main purpose of their usage is the reduction of low-frequency noise generation and thus decreasing the noise perceived at the far field. This work is a numerical study of acoustic noise generated by jet flow operating at Mach number 0.9 and Reynolds number 1.38 × 10⁶, considering two chevron nozzle geometries that differ from each other by the penetration angle into the flow. The main aim was to demonstrate that Reynolds averaged Navier Stokes (RANS)-based methods are reliable means to obtain acoustical noise predictions for the industry with a considerably low computational cost. In order to achieve this objective, computational fluid dynamics (CFD) RANS simulations were performed with a cubic k-ɛ model and the acoustic noise spectrum for different angles of radiation was obtained via the Lighthill ray-tracing (LRT) method. The numerical results for the acoustic and flow fields were seen to be in reasonable agreement with the experimental data, suggesting that this methodology can be used as a fast and useful option to predict acoustic noise of jet flows from chevron nozzles.     

等离子体激励器的静特性与高速特性仿真研究

A parametric study of high-frequency plasma jet actuator was carried out, using the experiment- tally measured energy distribution law of arc discharge as an ideal heat source. The influence of the exit angle of the actuator on the flow field was explored. The jet flow field characteristics of the spark discharge actuator under supersonic flow (Ma0=2.0) were investigated. The results show that the energy density of heat flux increases and the jet front and forward shock wave moves faster with the decrease of discharge region, and the smaller the exit angle of the jet is, the stronger the momentum injection ability of the actuator along the flow direction is. The rule still applies under high-speed air flow conditions. Compared to the static condition, the momentum injection capability of the jet is stronger and the influence domain is larger under supersonic flow conditions.     

超音速射流火焰的DNS研究:计算方法与涡结构

介绍了可压缩反应流的计算方法,然后计算了超音速射流火焰,对超音速射流火焰中的涡结构进行了分析。认为超音速射流火焰中的涡结构特征,与亚音速射流中有本质差异,且涡结构在燃料与组分的混合中起到了重要作用,进而直接影响火焰结构和燃烧效率。     

Acoustic properties of heated twin jets

A thorough experimental study of the noise characteristics of twin jets is presented in this paper. Twin round jets are investigated at typical jet engine conditions: that is, with heated high velocity flow. By varying the nozzle to nozzle spacing, it is possible to discriminate between the effects of turbulent mixing and acoustic shielding. As a result of this investigation, it was established that the turbulent mixing effects (both interaction noise generation and mixing suppression) occur for closely spaced nozzles. While acoustic shielding occurs at all nozzle spacings, it plays the dominant role at wide nozzle spacings. The levels of this acoustic shielding afforded by an adjacent jet can be sufficient to cause a nearly complete masking of the noise of the shielded jet. A significant discovery of this investigation was the importance of the layer of cooler, slower moving ambient air that exists between the twin jet plumes. This inter-jet layer causes acoustic refraction and reflection, and as the nozzle separation increases, the layer extends to shield more of the jet noise sources.     

Detonation simulations in supersonic flow under circumstances of injection and mixing

The unsteady, reactive Navier-Stokes equations with a detailed chemical mechanism of 11 species and 27 steps were employed to simulate the mixing, flame acceleration and deflagration-to-detonation transition (DDT) triggered by transverse jet obstacles. Results show that multiple transverse jet obstacles ejecting into the chamber can be used to activate DDT. But the occurrence of DDT is tremendously difficult in a non-uniform supersonic mixture so that it required several groups of transverse jets with increasing stagnation pressure. The jets introduce flow turbulence and produce oblique and bow shock waves even in an inhomogeneous supersonic mixture. The DDT is enhanced by multiple explosion points that are generated by the intense shock wave focusing of the leading flame front. It is found that the partial detonation front decouples into shock and flame, which is mainly caused by the fuel deficiency, nevertheless the decoupled shock wave is strong enough to reignite the mixture to detonation conditions. The resulting transverse wave leads to further mixing and burning of the downstream non-equilibrium chemical reaction, resulting in a high combustion temperature and intense flow instabilities. Additionally, the longitudinal and transverse gradients of the non-uniform supersonic mixture induce highly dynamic behaviors with sudden propagation speed increase and detonation front instabilities.     

In-flight simulation experiments on turbulent jet mixing noise

The effects of aircraft forward motion on pure turbulent mixing noise from unheated jets have been examined experimentally in the in-flight simulation mode. Both acoustic and flow characteristics were determined by testing model-scale nozzles in an anechoic free jet facility and a wind tunnel, respectively. Scaling laws were derived from each set of experiments and found to be complementary. The implications are discussed in detail. In particular, it is shown that the measured reduction in noise at 90° to the jet axis is a pure source alteration effect.