The effect of the secondary annular stream on supersonic jet

The present study addresses an experimental investigation of the near field flow structures of supersonic, dual, coaxial, free, jet, which is discharged from the coaxial annular nozzle. The secondary stream is made from the annular nozzle of a design Mach number of 1.0 and the primary inner stream from a convergent-divergent nozzle. The objective of the present study is to investigate the interactions between the secondary stream and inner supersonic jets. The resulting flow fields are quantified by pitot impact and static pressure measurements and are visualized by using a shadowgraph optical method. The pressure ratios of the primary jet are varied to obtain over-expanded flows and moderately under-expanded flows at the exit of the coaxial nozzle. The pressure ratio of the secondary annular stream is varied between 1.0 and 4.0. The results show that the secondary annular stream significantly changes the Mach disc diameter and location, and the impact pressure distributions. The effects of the secondary annular stream on the primary supersonic jet flow are strongly dependent on whether the primary jet is underexpanded or over-expanded at the exit of the coaxial nozzle.     

基于CFD的喷嘴结构对高压水射流反推特性的影响

喷嘴作为无人机新型射流反推弹射的重要元件,其结构参数对无人机弹射动态特性具有重要影响.以圆柱形、圆锥形和余弦形结构的喷嘴为研究对象,基于CFD建立不同结构喷嘴射流流动的数学模型,采用VOF多相流模型和k-ε湍流模型等对不同结构喷嘴射流流态进行数值模拟,获得不同结构喷嘴内外壁面的压力场、速度场分布,对比分析喷嘴结构特征和...     

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 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.     

The self-induced oscillations of the under expanded jets impinging upon a cylindrical body

The present study addresses the flow characteristics involved in the self-induced oscillations of the underexpanded jet impinging upon a cylindrical body. Both experiment and computational analysis are carried out to elucidate the shock motions of the self-induced oscillations and to find the associated major flow factors. The underexpanded sonic jet is made from a nozzle and a cylindrical body is placed downstream to simulate the impinging jet upon an obstacle. The computational analysis using TVD scheme is applied to solve the axisymmetric, unsteady, inviscid governing equations. A Schlieren system is employed to visualize the self-induced oscillations generated in flow field. The data of the shock motions are obtained from a high-speed video system. The detailed characteristics of the Mach disk oscillations and the resulting pressure variations are expatiated using the time dependent data of the Mach disk positions. The mechanisms of the self-induced oscillations are discussed in details based upon the experimental and computational results.     

Effect of the Pitot microtube diameter on pressure measurement in plane supersonic microjets

The article reports on results of estimating the Pitot tube diameter effect on the streamwise and lateral pressure distributions in plane supersonic air microjets. The study is aimed at obtaining reliable information on the microjet structure. The nozzles with the size of 22.3 × 2593 μm and 83.3 × 3823 μm are used in experiments. The supersonic flow from that nozzles is investigated by glass Pitot tubes in inner/outer diameters of 24/70 and 16/42 μm. Additional measurements are performed by a Pitot microtube 8 μm in outer diameter and ≈0.1 μm wall thickness. The value ranges for accuracy determination of supersonic jets main characteristics (shock cell sizes and supersonic core length) have been found.     

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%.     

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.     

Flow characteristics of pyramidal shaped small sonic nozzles

Four types of pyramidal sonic nozzles made of silicon crystal were studied experimentally. The throat sizes varied from 38 to 140 μm for type A and D nozzles and from 75 to 188 μm for type B and C nozzles. For each of the nozzle types, the results show that the discharge coefficient is proportional to the throat size, and the critical back pressure ratio for choking is insensitive to Reynolds’ number. In parallel, the flow field of a type B nozzle was investigated by numerical simulation. The effect of heat flux coming from the nozzle body was examined and the flow patterns obtained from Spalart-Allmaras and standard k−ω turbulence models were compared. The simulation results indicate the heat flux does not noticeably change the velocity field and discharge coefficient. Also, the flow downstream of the nozzle throat develops into an under-expanded supersonic jet in which expansion and oblique shock waves appear alternately.     

An experimental investigation of the near-field region of a free turbulent coaxial jet using LDA

The near field-region of the flow field of a coaxial jet configuration is investigated experimentally using Laser Doppler Anemometer. In order to obtain a better definition and, possibly, a deeper physical understanding of the flow field under consideration, the flow was simplified so that the jet is made to issue from concentric round profiled nozzles and to discharge freely into still ambient air. The velocity ratio between the inner and the outer streams of the jet, λ, is varied for a fixed upstream conditions. In addition a relatively wide interface between the two streams was introduced. This interface affects the structure of the shear layers between the two streams, thus affecting the velocity decay characteristics along the centerline. The obtained results show that the inner potential core length of the of coaxial jet strongly depends on the velocity ratio λ while the outer potential core for jets having velocity ratios greater than unity seems to be insensitive to the velocity ratio. The investigated coaxial jet flow fields did not show self-similarity up to x/D=25. Jets with velocity ratio less than unity were resulted to develop faster than those with λ greater than unity. In addition increasing the velocity ratio was seen to accelerate the jet decay along the centerline.     

喷嘴内壁压力分布的研究

喷嘴是能量转换部件,其效率直接影响航行器的总体性能,如续航能力、航速等.根据喷嘴内部等压面的特征,运用量守恒定理,建立喷嘴内壁面压力分布模型.对两种喷嘴(圆柱、圆锥)内壁面压力分布进行仿真与试验研究.比较分析模型、仿真和试验的结果.因结果可知:①内壁面压力分布在接近喷嘴时急剧下降,且随孔径的增加,壁面压力变化率减小;②...     

气溶性射流对钢板污垢清洗的应用研究

为了获得更好的清洗效果和节能环保,提出了一种新型的射流方式——气溶性射流。基于流体力学和气溶性射流的工作原理,根据气溶性射流的特性和喷嘴的几何特征,研究了气液相的流动特性方程,得到了气溶性射流的速度分布规律。利用自行设计的超音速喷嘴组装的带电气溶性射流实验装置对钢板进行清洗,经简易检测表明：气溶性射流喷嘴产生的超音速极大地提高了洗涤效率,与同流量的喷嘴相比,效率提高近10倍,具有广泛的应用前景。     

Transitional behavior of a supersonic flow in a two-dimensional diffuser

Two-dimensional blow-down type supersonic wind tunnel was designed and built to investigate the transient behavior of the startup of a supersonic flow from rest. The contour of the divergent part of the nozzle was determined by the MOC calculation. The converging part of the nozzle, upstream of the throat was contoured to make the flow profile uniform at the throat. The flow characteristics of the steady supersonic condition were visualized using the highspeed schlieren photography. The Mach number was evaluated from the oblique shock wave angle on a sharp wedge with half angle of 5 degree. The measured Mach number was 2.4 and was slightly less than the value predicted by the design calculation. The initial transient behavior of the nozzle was recorded by a high-speed digital video camera with schlieren technique. The measured transition time from standstill to a steady supersonic flow was estimated by analyzing the serial images. Typical transition time was approximately O.1sec.     

淹没环境下高压水射流喷嘴的结构优化

为提高高压水射流在辅助大型沉井沉降施工中的效率,对水射流关键元件喷嘴进行选型和结构优化,要求射流具备高能量、低衰减特性。从4类回转形喷嘴中优选速度衰减小的喷嘴,确定合理的速度衰减研究指标,再结合尺寸参数设计正交实验,利用回归分析建立预测模型,可以确定各个尺寸参数对速度指标的影响程度。由高压泵站输入压力和流量确定喷嘴的最大直径,再由预测模型确定最佳喷嘴是出口直径2.8 mm、入口直径5.26 mm、收缩角为12.3°的锥形喷嘴。     

超音速射流噪声突变现象的实验和数值模拟研究

在利用“放气法”测量喷管的超音速射流噪声的实验过程中,发现部分结构喷管产生的射流噪声大小不是随着上游压强的减小而减小,而是在特定压强点出现突增现象。为了在喷管的设计阶段就能预测其喷流噪声是否发生突变,对两种结构喷管的射流流场进行了实验和数值模拟研究。结果表明,射流流场的轴线总压和噪声声压级存在强相关性,可将流场总压的变化作为噪声是否突变的判据。因此,在设计喷管时对其射流流场进行仿真研究,就可预测其噪声突变现象的产生。     

Numerical investigation of transient side-loads in the start-up process of a rocket nozzle

A numerical investigation of transient side-loads in axisymmetric over-expanded thrust optimized contour nozzles is presented. These nozzles experience side-loads during start-up and shut-down operations because of flow separation at the nozzle walls. Two types of flow separations, FSS and RSS shock structures, also occur. A two-dimension numerical simulation was carried out over axisymmetric TOC nozzles to validate the present results and investigate oscillatory flow characteristics for start-up processes. Reynolds Averaged Navier-Stokes equations are numerically solved using a fully implicit finite volume scheme. Governing equations are solved via the coupled implicit scheme. The Reynolds Stress turbulence model is selected for this work. It was found that the present computed pressure at the nozzle walls closely matched the experimental data. The phenomenon of hysteresis was also observed between these two shock structures. The transition from an FSS to RSS pattern during the start-up process showed maximum nozzle wall pressure and fluctuations in shear stress values. Oscillatory pressure was observed on the nozzle walls with high pressure ratio. The present results show that the magnitude of nozzle wall pressure variation is high for the phenomenon of oscillation.     

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。     

缩扩型超音速喷管的设计与仿真

缩扩型的拉瓦尔喷管可以使通过其的气流获得超音速,出口速度的大小以及气流是否稳定是决定拉瓦尔喷管性能优劣的关键因素.通过Fluent软件对拉瓦尔喷管的相关流场进行了数值仿真,分析了入口压力、面积比、收缩段型面及扩张段的锥角对喷管出流速度的影响,可为喷嘴的设计提供理论指导.     

高压水除鳞喷嘴结构参数对喷嘴射流性能的影响

为了进一步研究高压水除鳞喷嘴的射流性能,优化喷嘴的结构参数,选取出口扩张角、锥孔深度、入口收缩角作为参考因素,以喷射角和射流流量作为评价指标,对喷嘴各参考因素对射流性能的影响进行了仿真分析。结合正交试验的方法,对喷嘴各结构参数及其交互作用对其性能影响的显著性及重要性次序进行分析,获得最优的参数组合并对其进行了实验验证。结果表明：出口扩张角的增大会使喷射角减小但会使射流流量增加,锥孔深度的增加可以较为全面地提升喷嘴的射流性能,收缩角的增大会使得喷嘴的射流性能减弱,喷嘴的除鳞效果变差;3个参考因素中锥孔深度是对射流性能影响最大的因素;当出口扩张角为80°、锥孔深度为8 mm、入口收缩角为65°时喷嘴的射流性能最优。通过将参数优化后的喷嘴的实验结果与原始喷嘴的实验结果对比发现,优化后喷嘴的喷射角由原始值35.7°提升至44.1°,射流流量的大小由原始值10.17 L/min提升至28.3 L/min。     

Supersonic gas jets for laser-plasma experiments

We present an in-depth analysis of De Laval nozzles, which are ideal for gas jet generation in a wide variety of experiments. Scaling behavior of parameters especially relevant to laser-plasma experiments as jet collimation, sharpness of the jet edges and Mach number of the resulting jet is studied and several scaling laws are given. Special attention is paid to the problem of the generation of microscopic supersonic jets with diameters as small as 150 μm. In this regime, boundary layers dominate the flow formation and have to be included in the analysis.