Experimental study on hysteresis phenomena of shock wave structure in an over-expanded axisymmetric jet

In recent studies on two-dimensional supersonic jets, it is reported that the hysteresis phenomenon for the reflection type of shock waves in the jet flow field is occurred under the quasi-steady flow condition and this phenomenon is affected by the transitional pressure ratio between the regular reflection and Mach reflection. However, so far, there are few researches on the hysteresis phenomenon for the transition of shock waves between regular and Mach reflection in over-expanded supersonic jets and the phenomenon has not been investigated satisfactorily. Therefore, the purpose of this study is to clarify the hysteresis phenomena for the reflection type of shock wave in the over-expanded axi-symmetric supersonic jet experimentally, and to discuss the relationship between hysteresis phenomenon and rate of the change of pressure ratio with time. Furthermore, the effect of Mach number at the nozzle exit on hysteresis loop was investigated for two kinds of nozzle.     

An experimental study of supersonic dual coaxial free jet

A supersonic dual coaxial jet has been employed popularly for various industrial purposes, such as gasdynamic laser, supersonic ejector, noise control and enhancement of mixing. Detailed characteristics of supersonic dual coaxial jets issuing from an inner supersonic nozzle and outer sonic nozzles with various ejection angles are experimentally investigated. Three important parameters, such as pressure ratios of the inner and outer nozzles, and outer nozzle ejection angle, are chosen for a better understanding of jet structures in the present study. The results obtained from the present experimental study show that the Mach disk diameter becomes smaller, and the Mach disk moves toward the nozzle exit, and the length of the first shock cell decreases with the pressure ratio of the outer nozzle. It was also found that the highly underexpanded outer jet produces a new oblique shock wave, which makes jet structure much more complicated. On the other hand the outer jet ejection angle affects the structure of the inner jet structure less than the pressure ratio of the outer nozzle, relatively.     

An experimental study of the nozzle lip thickness effect on supersonic jet screech tones

It is well known that screech tones of supersonic jet are generated by a feedback loop driven by the instability waves. Near the nozzle lip where the supersonic jet mixing layer is receptive to external excitation, acoustic disturbances impinging on this area excite the instability waves. This fact implies that the nozzle lip thickness can influence the screech tones of supersonic jet. The objective of the present study is to experimentally investigate the effect of nozzle-lip thickness on screech tones of supersonic jets issuing from a convergent-divergent nozzle. A baffle plate was installed at the nozzle exit to change the nozzle-lip thickness. Detailed acoustic measurement and flow visualization were made to specify the screech tones. The results obtained obviously show that nozzle-lip thickness significantly affects the screech tones of supersonic jet, strongly depending on whether the jet at the nozzle exit is over-expanded or under-expanded.     

An experimental study on the effect of square grooves on decay characteristics of a supersonic jet from a circular nozzle

In this experimental work, the effect of square grooves on the structure of a supersonic jet emanating from a circular nozzle has been investigated at three different nozzle inlet total pressures i.e 360 kPa, 550 kPa and 720 kPa. The nominal exit Mach number is 1.8. A new empirical relation for predicting the supersonic core length for grooved nozzle has been suggested. Further, a new parameter “groove effectiveness” has also been suggested to quantify the effect of the groove by using the total pressure data in the supersonic core length. Experimental results suggest that at higher nozzle inlet total pressure, the groove effectiveness plays a minor role. From the jet centreline total pressure data, supersonic core length, the locations at which 50 % and 90 % decay occurs have been obtained. It has been observed that higher groove effectiveness is associated with smaller values of supersonic core length, L_50% and L_90%. Schlieren images of the jet structure shows unsymmetrical shock pattern of jets emanating from a single grooved nozzle.     

二维超音速喷管型线设计仿真研究

采用计算软件FLUENT,对四种经典收缩段型线下的流场特性进行数值模拟,为选择超声速风洞收缩段的型线提供依据。基于特征线理论,利用解析法完成超音速喷管膨胀段型线设计,通过分析总压恢复系数及均匀度等流场参数,确定型线膨胀角角度及喷管长度。结果表明,收缩段型线选用双三次曲线,膨胀角度3.5°的情况下,超音速喷管出口达到了设计要求马赫数,并获得了较好的气流品质。     

Effect of boundary layer swirl on supersonic jet instabilities and thrust

This paper reports the effects of nozzle exit boundary layer swirl on the instability modes of underexpanded supersonic jets emerging from plane rectangular nozzles. The effects of boundary layer swirl at the nozzle exit on thrust and mixing of supersonic rectangular jets are also considered. The previous study was performed with a 30° boundary layer swirl (S=0.41) in a plane rectangular nozzle exit. At this study, a 45° boundary layer swirl (S=1.0) is applied in a plane rectangular nozzle exit. A three-dimensional unsteady compressible Reynolds-Averaged Navier-Stokes code with Baldwin-Lomax and Chien’sk-ε two-equation turbulence models was used for numerical simulation. A shock adaptive grid system was applied to enhance shock resolution. The nozzle aspect ratio used in this study was 5.0, and the fully-expanded jet Mach number was 1.526. The “flapping” and “pumping” oscillations were observed in the jet’s small dimension at frequencies of about 3,900Hz and 7,800Hz, respectively. In the jefs large dimension, “spanwise” oscillations at the same frequency as the small dimension’s “flapping“ oscillations were captured. As reported before with a 30° nozzle exit boundary layer swirl, the induction of 45° swirl to the nozzle exit boundary layer also strongly enhances jet mixing with the reduction of thrust by 10%.     

收缩喷嘴内部流道型线对射流流场的影响

为研究收缩喷嘴内部流道形成对射流流场的气体动特性参数的影响，根据可压缩流体轴对称N-S方程，采用非结构网格和二阶精度的有限体积法，对不同内部流道形线的喷嘴自由射流进行数值模拟。亚声速射流采用RNGk-ε湍流模型，超声速射流采用S-A湍流模型，计算结果与实验较吻合。在亚声速流动中，收缩喷嘴的收缩角大小会影响其对射流的阻滞效果，内部流道形线设计为维多辛斯基曲线可以获得更好的流场动特性参数，有利于提高喷嘴的工作效率。在超声速流动中，喷嘴流道型线对出口膨胀波的角度与强弱影响较大，要根据射流的有效作用区域选择合适的喷嘴，才能使能量的损失最小。若要获得较佳的外部流场参数，优化喷嘴内部流道设计十分重要。     

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.     

基本式和差动式旋转执行器研究 

基于磁控形状记忆合金（MSMA）直线执行器的研究,提出了基本式和差动式两种类型的MSMA旋转执行器。分别介绍了两种执行器的工作原理、结构和控制策略,推导了执行器的位移和转速公式,绘制出了MSMA元件的相对磁导率与磁通密度的关系曲线。在磁场有限元分析的基础上设计并研制了旋转执行器样机,比较了不同结构和不同控制策略下样机的实验速度曲线。样机的实验结果验证了MSMA旋转执行器工作机理的正确性和设计方法的可行性。     

Optimal design of supersonic nozzle contour for altitude test facility

This paper develops a robust and practical design for supersonic nozzles to be used in an altitude engine test facility. Although many studies have been conducted on nozzle design, none of these present a robust yet practical and simple method for designing supersonic nozzles. This research attempts to develop such design for supersonic nozzles by combining method of characteristics (MOC), optimization algorithm, and computational fluid dynamics analysis for design verification. Preliminary design optimal techniques were adopted to reduce nozzle length while keeping the exit area constant in the design. Optimization produced a smooth flow by generating a parallel and uniform flow at the exit. A two-dimensional model was initially used because of the axisymmetrical characteristic of the flow in this study. The optimal nozzle was designed for the operation of a test facility at Mach number 2.3 and altitude of 7 km. The optimal design produced a uniform and parallel flow at the given test condition.     

An experimental investigation of the near-field region of free turbulent round central and annular jets

The present study is an experimental investigation of two limiting cases of coaxial turbulent jet namely the round central jet and the annular jet, using Laser Doppler Anemometry. The results of the round jet are presented and discussed for two exit mean velocities. While the annular jet is tested for one exit velocity. The variations along the jets centerlines and the radial profiles for mean and fluctuating longitudinal velocities are presented. ln addition a comparison between the present results and the corresponding results of other investigators are also included. The obtained data revealed that the results of the turbulent central round jet are in a general agreement with those of the previous investigators. Meanwhile, the results of the annular jet followed the same trend as the results of the previous investigators with differences in magnitudes due to the high turbulence intensity at the exit of the present tested jet.     

Simultaneous internal/external flow calculation of a solid fuel ramjet projectile: a design analysis

SFRI has been regarded as a prime mode of secondary propulsion which enables the projectile to achieve a higher terminal velocity. Geometry of SFRJ is quite simple due to the lack of turbomachinery that conventional jet engines need. The complete internal/external flow field of an SFRJ-TGTR projectile at a cruising Mach number is calculated by an unstructured triangular compressible RANSE solver. To complete the analysis, single-component fuel combustion is added in the combustor flow calculation. A commercial CFD package is used with structured rectangular meshes for such computation. The results from the two computations are compared with existing computational and experimental data for validation. The design parameters like thrust, total pressure loss, inlet and nozzle efficiencies and drag coefficient are derived from the detailed flowfield data. Different values of cruising Mach number and nozzle throat area have been tried as a preliminary design procedure. Change in the nozzle throat area directly affects the inlet flow, because the internal flow is mostly subsonic and is linked directly to the nozzle flow.     

Acoustic properties associated with nozzle lip thickness in screeching jets

Imperfectly expanded supersonic jets generate discrete frequency sound known as screech tones. In the low supersonic Mach number, the screech phenomenon is axisymmetric. In this study the effect of nozzle lip thickness on the axisymmetric jet screech is investigated both by screech scattering computation and screeching jet simulation. The conservative form of the axisymmetric Euler equations in generalized coordinates are used to simulate a scattering problem and the Reynolds-Averaged Navier-Stokes equations with the modified Spalart-Allmaras turbulence model is employed for screeching jet simulation. Numerical results of scattered directivity pattern in scattering computation will be reported. Influence of the nozzle lip thickness on the momentum thickness, shock-cell spacing and screech tone wavelength is examined.     

A computational analysis of unsteady transonic/supersonic flows over backward facing step in air jet nozzle

A transonic/supersonic axisymmetric backward facing step nozzle flow in an air-jet loom has been analyzed numerically by using a time accurate characteristic based upwind flux difference splitting compressible Navier-Stokes method. The unsteady pressure and Mach number behavior along the center line of the main nozzle were analyzed by periodic inlet condition changes to simulate the intermittent flow inside main nozzle of an air-jet loom.     

新型钢板桩液力自旋式喷嘴设计及流场分析

针对钢板桩遇密实土层难以沉桩的问题,设计了一种新型液力自旋式喷嘴结构。基于流体力学理论,利用Fluent软件,通过改变斜射流口的偏心距、斜射流口与水平面的夹角等结构参数,得到了该喷嘴内腔的压力、流速及旋转体转矩的变化规律。结果表明:随着偏心距的增大,旋转体转矩增大;斜射流口的出口平均流速在小偏心距变化微小,在大偏心距逐渐减小;随着水平夹角的增大,旋转体转矩减小;斜射流口的出口平均流速随着水平夹角的增大先增大后减小,且随着偏心距的增大其峰值点逐渐后移。     

激光制造中载气式同轴送粉粉末流场的数值模拟

应用气固两相流理论对载气式同轴送粉系统中粉末流出送粉喷嘴后的粉末流场进行模拟计算,研究分析粉末流场浓度分布规律及粉末流参数(聚焦焦距和聚焦半径)的变化规律,计算结果与数字粒子图像测速的试验结果吻合较好.结果表明,载气式同轴送粉系统中,送粉喷嘴结构参数一定时,粉末流浓度和粉末流参数受送粉量及送粉气流量的影响.送粉气流量主要影响送粉喷嘴出粉口处粉末流的速度,对粉末流浓度分布和粉末流聚焦参数影响较小;送粉量增加,聚焦点处浓度值增大,粉流聚焦焦点略微下移,焦距处半径增大,聚焦焦深减小.建立粉末流简化物理模型,所建模型对同轴送粉喷嘴的设计和性能优化具有参考价值.     

A study on prediction of the base pressures for an axi-symmetric body

A flow modeling method has been developed to analyze the flow in the annular base (rear-facing surface) of a circular engine nacelle flying at subsonic speed but with a supersonic exhaust jet. Real values of exhaust gas properties and temperature at an altitude of 30,000 feet are employed. Potential flows of the air and gas streams are computed for the flow past a separated wake. Then a viscous jet mixing is superimposed on this inviscid solution. Conservation of mass, momentum and energy for the wake flow field is achieved by multiple iterations with modest computer requirements.     

Design and evaluation of high-pressure nozzle assembly for laser cutting of thick carbon steel

Laser cutting of carbon steel is extensively used across a range of industries, due to its advantage of high speed, low kerf and high quality. Currently, a 1-kW carbon dioxide (CO₂) laser with its subsonic nozzle assembly can be used only to cut steel plates up to around 10 mm. This paper aims to design and evaluate a high-pressure supersonic laser cutting nozzle assembly, which can enable a 1-kW CO₂ laser to cut steel of up to 50 mm thickness. Basic gas dynamic and compressible flow equations were used to design the supersonic nozzle assembly. The flow of the high-pressure gas jet inside the nozzle assembly was investigated using computational fluid dynamics (CFD), and the structural integrity of the high-pressure nozzle assembly was ensured using finite element analysis (FEA). The gas flow pattern at the exit of the nozzle assembly was computed and compared with the experimental observation made through a shadowgraph technique. Laser cutting experiments were performed with the developed supersonic nozzle assembly to demonstrate cutting of 50-mm-thick low carbon steel with 1-kW CO₂ laser.     

Supersonic jet noise control via trailing edge modifications

Various experimental data, including mixing areas, cross correlation factors, surface flow patterns on nozzle walls, and far field noise spectra, was used to draw a noise control mechanism in a supersonic jet. In the underexpanded case, mixing of the jet air with ambient air was significantly enhanced as presented before, and mixing noise was also dramatically reduced. Screech tones, in the overexpanded case, were effectively suppressed by trailing edge modifications, although mixing enhancement was not noticeable. From mixing and noise performance of nozzles with modified trailing edges, enhancing mixing through streamwise vortices seems an effective way to reduce mixing noise in the underexpanded flow regime. However, screech tones in the overexpanded flow regime is well controlled or suppressed by making shock cells and/or spanwise large scale structures irregular and/or less organized by a proper selection of trailing edges. The noise field in the overexpanded flow regime was greatly affected by the symmetricity of the nozzle exit geometry. In the underexpanded flow regime, the effects of the symmetricity of the nozzle exit on mixing were negligible.     

Analysis and modeling of a two-phase jet pump of a thermal management system for aerospace applications

Jet pumps are devices capable of pumping fluids to a higher pressure by inducing the motion of a secondary fluid employing a high-speed primary fluid. The main components of a jet pump are a primary nozzle, secondary fluid injectors, a mixing chamber, a throat, and a diffuser. The work described in this paper models the flow of a two-phase primary fluid inducing a secondary liquid (saturated or subcooled) injected into the jet pump mixing chamber. The model is capable of accounting for phase transformations due to compression, expansion, and mixing. The model is also capable of incorporating the effects of the temperature and pressure dependency in the analysis. The approach adopted utilizes an isentropic constant-pressure mixing in the mixing chamber and at times employs iterative techniques to determine the flow conditions in the different parts of the jet pump.