Increasing Jet Pressure Research Articles

Numerical investigation on direct water injection characteristics under different injection and ambient conditions within oxygen/argon atmosphere

Improving efficiency and reducing emissions are essential to achieve carbon neutrality in transportation sector. The hydrogen-fueled argon power cycle (H2-APC) is a novel power system with high efficiency and zero emission by utilizing argon-oxygen mixture as working fluids. However, knock suppression is a problem within H2-APC engines. Direct water injection (DWI) is an effective method to inhibit detonation. Spray morphology greatly influences the effectiveness of DWI, the investigation of spray characteristics in oxygen/argon atmosphere is critical. This paper established a computational model based on experimental data to investigate DWI characteristics under different injection and ambient conditions within oxygen/argon atmosphere. The results reveal increased jet temperature leads to stronger superheated jet spray collapse, while Sauter mean diameter (SMD) decreases and atomization improves. Increasing jet pressure effectively reduces spray SMD and improves atomization effect and evaporation rate. Higher ambient temperature reduces ambient gas density, spray SMD and penetration are decreased and increased, respectively. Moreover, spray evaporation rate and evaporated mass are dropped with higher ambient pressure for both superheated and subcooled sprays. As ambient density rises, the capacity for jet perturbation and fragmentation is enhanced, and spray SMD decreases. The research results can provide an effective guide for the DWI strategy of H2-APC engines.

Modeling and analysis of plastic abrasive jet flow characteristics

In this study, the particle velocity and spatial distribution of a plastic abrasive jet were modeled for the removal of surface coatings from a new generation of ship hulls by plastic abrasive jet machining. The particle velocity in the jet has been accurately tested through high-speed photography. It was found that the particle velocity of plastic abrasives with the same particle size was larger than conventional abrasives. Downstream from the nozzle exit axis, the divergence profile of the abrasive jet is approximately linear at distances up to 6.2dN; at distances higher than 6.2dN, the divergence profile line of the abrasive jet is approximately “flared”. The particle velocity of the plastic abrasive increased with increasing jet pressure, and no significant decrease of particle velocity was found in the limited measurement range. The instantaneous velocity distribution of the particles has been discussed in terms of axial and radial directions along the jet. For the radial velocity distribution of the abrasive jet, a relatively steady state is observed in the cross-section of the jet near the nozzle outlet. The results of the model calculations are in accordance with the experimental test data in terms of both tendency and margin of change.

Numerical study on drag and heat flux reduction induced by a counterflowing jet for rarefied hypersonic flow over a blunt body

This paper focuses on the drag and heat flux reduction induced by a counterflowing jet located on the leading edge of the blunt body head in rarefied hypersonic flows using the direct simulation Monte Carlo method. Flow structures in the flowfield, such as detached shock wave, Mach disk, contact surface, jet layer, and recompression shock wave, are all weakened gradually with the increase in the freestream altitude, and they eventually disappear at the altitude of 90 km. The increase in the jet pressure provides a great drag reduction by up to 53% when it increases from 800 to 1600 Pa, but the proportion of drag on the blunt body head to the total drag is only affected slightly by the jet pressure. A noteworthy finding is that further increasing jet pressure almost have no effect on heat flux variation when it is larger than 1200 Pa. On the whole, jet temperature has a quite weak influence on both flow structures and drag, while heat flux on the blunt body head is closely related to jet temperature. The results suggest that jet temperature should vary with that of blunt body surface, and moreover, the optimal jet temperature should be moderately lower than the wall surface temperature. In addition, increasing freestream altitude can provide excellent performance of drag reduction, but it causes non-monotonic variation of heat flux. In view of this, it is worth noting that heat flux on the blunt body head actually increases with altitude when the blunt body is in a severely rarefied atmospheric environment, such as the altitude H > 70 km.

Morphological study of cortical bone damage caused by high-pressure water jet erosion based on SPH-FEM coupling algorithm

Currently, traditional processing methods such as drilling, sawing, and milling are primarily utilized in orthopedic surgery, generating a significant amount of heat during the cutting process, leading to irreversible damage to bone and surrounding tissues. High-pressure water jet technology has demonstrated the capability to effectively reduce cutting temperatures and prevent thermal damage during the cutting process. It holds promising potential as an alternative approach to traditional surgical techniques in orthopedic surgery. This study selected the SPH-FEM (Smoothed particle hydrodynamics-Finite element method) coupling algorithm and LS-DYNA software to simulate the effects of high-pressure water jet erosion on cortical bone. The study analyzed the impact of jet pressure on damage morphology and the degree of cortical bone breakage. The simulation results indicate that the concave section of cortical bone damaged by high-pressure water jet is “U” shape. The maximum damage depth of cortical bone increases with the increase in jet pressure. Specifically, as jet pressure increases from 80 MPa to 245 MPa, the increase rate of the average cortical bone damage depth increased from about 0.01 mm/s to more than 0.45 mm/s. The beginning of the concave section of cortical bone damage presents a “funnel” shape, and the width of cortical bone damage reaches its maximum. When the jet pressure is below 180 MPa, there is a significant fluctuation in the width of cortical bone damage. However, with increasing jet pressure, the fluctuation lessens. Physical experiments were carried out under identical technological parameters to verify the numerical results. The experimental results are basically consistent with the numerical results, which can provide a theoretical basis for improving the surface quality of jet erosion of cortical bone.

Fracture propagation and induced strain response during supercritical CO2 jet fracturing

To investigate fracture generation and strain variation during SC-CO2 (supercritical carbon dioxide) jet fracturing, the model of induced strain is established and the experiments are comprehensively studied. The influence factors are comprehensively explored, such as jet pressure, ambient pressure, etc. With the increasing jet pressure, the fracture morphology changes from parallel cracks to oblique cracks. Both the mass loss of specimen and CO2 absorption increase significantly, and the growth rate and minimum value of strain also rise exponentially. Under a high ambient pressure of 8.0 MPa, the main fractures mostly propagated from the surface to the bottom surface of the specimen. The maximum strain and the stable duration under higher ambient pressure are 1.5 times and 10 times, respectively, of the case under the ambient pressure of 5.0 MPa. The comparison shows that the optimal jet distance is 5–7 times the nozzle diameter, resulting in massive mass loss, large CO2 absorption, and peak strain. Moreover, the nonlinear variation of strain curve during jet pressurization is related to the type of rock and ambient pressure. These studies clearly show the relationship between the fracture morphology and induced strain, which are crucial for SC-CO2 fracturing in shale gas reservoirs.

An experimental study of ultra-high pressure water jet-induced fracture mechanisms and pore size evolution in reservoir rocks

Ultra-high pressure water jets (UHP-WJ) rock breaking is a promising technology for developing unconventional natural gas reserves . In this study, rock-breaking experiments were conducted using typical reservoir rock samples such as sandstone , shale, and coal. The failure patterns in rocks after impact by UHP-WJ were analyzed. Moreover, the 3D damage field model of the rock was reconstructed based on CT scanning, and the internal crack distribution was visualized and examined. Furthermore, the fracture mechanisms of three types of reservoir rocks under the UHP-WJ impingement were studied. The results indicate that intergranular fractures produced due to shear stress are the dominant type of fracture in sandstones, tensile failure due to stress wave was the main type of fracture observed in shale, and coal exhibited the double breaking characteristics of stress wave effect and water wedge effect. The fragments size distribution of samples shows the largest fragments in shale, followed by coal, and the smallest fragments are observed in sandstone. Finally, the evolution of rock pore structure before and after the impact of UHP-WJ was monitored by nuclear magnetic resonance (NMR) methods. The results reveal that the micropores transform into mesopores and macropores in the sandstone, where the total porosity increased by 14.3%–24.8%. As the jet pressure exceeds 100 MPa, the porosity of shale increases significantly, such that the total porosity of the shale sample increased by 7.6%–37.1%. The mesopores transform into macropores and fractures with the influence of the water wedge effect in coal samples, resulting in the reduction of mesopores and the increase of macropores, where the total porosity of the coal sample increased by 14.6%–40%. The complexity and heterogeneity of pores were reduced under UHP-WJ with the increasing jet pressure. The findings provide a theoretical reference for optimizing the recovery from unconventional natural gas reservoirs .

Coal breakage features and fragment size distribution in water jet drilling for coalbed methane development

Water jet drilling (WJD) technology has been widely used to extract coalbed methane. Coal breakage features and fragment size in WJD determine the drilling efficiency. The smaller the coal fragments, the easier the fragments are discharged from hydraulic boreholes. This study discloses the coal breakage features and size distributions of fragments, field experiments under the original in-situ stress condition were conducted to obtain the coal fragments generated in WJD. The effects of the jet pressure and coal strength on fragment size distributions were investigated based on the Weibull distribution and fractal model. The results indicate that fine particles dominate the coal fragments and its proportion increases with the increase of jet pressure and the decrease of coal strength. The fragment size distributions accord with the Weibull distribution. The breakage degree index of fragments decreases in the logarithmic form as the jet pressure increases. The coal fragments have the fractal characteristic, and the fractal dimension increases with the increasing jet pressure and decreases with the increasing coal strength. Additionally, the fractal dimension increases with the decreases in breakage degree index. This work could provide a guide for research on the coal fragments transportation and the selections of hydraulic parameters in WJD.

Experimental study on the rock erosion performance of a pulsed abrasive supercritical CO2 jet

An experimental study on the erosion behaviors of pulsed abrasive supercritical CO2 jets (PASJ) was performed using sandstone as the target specimen. The erosion depth, erosion area, and mass loss of the crater caused by PASJ were discussed in detail. The optimal configuration of the Helmholtz nozzle was first determined, and then influences of erosion-related working parameters including the jet pressure, abrasive particle size, ambient fluid and jet impingement angle on the rock erosion performance were analyzed with this nozzle. The results indicate that increasing jet pressure helps improve the erosion depth and area, which is mainly the result of increased jet kinetic energy. The erosion capability of PASJ is highly dependent on the size of abrasive particles. Coarser particles shows greater effectiveness in producing deeper erosion craters and the jets are more likely to retain rock-erosion ability as the standoff distance increases. At a given standoff distance, the crater eroded in air is deeper than those eroded in gaseous CO2 and in supercritical CO2, because the particles are slowed much more slowly in air under decreased drag. Moreover, the area of the crater eroded by PASJ in air is also largest due to the highest radial spreading rate. Finally, it was found that mass loss and erosion depth first increase and then decrease with the increase of jet impingement angle (θ) and the maximum values are reached at θ = 70°, whilst the erosion area increases monotonically with decreasing jet impingement angle.

Experimental and numerical investigation of the abrasive waterjet machining of aluminum-7075-T6 for aerospace applications

The machining of hard-to-cut materials with a high degree of precision and high surface quality is one of the most critical considerations when fabricating various state-of-the-art engineered components. In this investigation, a comprehensive three-dimensional model was developed and numerically simulated to predict kerf profiles and material removal rates while drilling the aluminum-7075-T6 aerospace alloy. Kerf profile and material removal prediction involved three stages: jet dynamic flow modeling, abrasive particle tracking, and erosion rate prediction . Experimental investigations were conducted to validate the developed model. The results indicate that the jet dynamic characteristics and flow of abrasive particles alter the kerf profiles, where the top kerf diameter increases with increasing jet pressure and standoff distance. The kerf depth and hole aspect ratio increase with jet pressure, but decrease with standoff distance and machining time. Cross-sectional profiles were characterized by progressive edge rounding and parabolic shapes. Defects can be minimized by utilizing high jet pressure and small standoff distance. The material removal rate increases with increasing jet pressure, abrasive particle size, and exposure time, but decreases with increasing standoff distance.

Thermal protection performance of a low pressure short penetration mode in opposing jet and its application

An experiment about the flow mode of opposing jet is performed in a supersonic quiet tunnel, and a Nanoparticle-based plaser scattering method(NPLS) is used to acquire the shock wave information. A stable short penetration mode (SPM) in low jet pressure is observed in experiment aside from the traditional unstable long penetration mode (LPM) and high-pressure stable SPM. Hence, the flow mode of the opposing jet presents three kinds structures as the increasing jet pressure: low-pressure stable SPM, unstable LPM and high-pressure stable SPM. Then, numerical analysis about the flow-field and surface heat transfer on the opposing jet is conducted. The low-pressure SPM in hypersonic flow is also acquired in numerical results. Comparing to the unstable LPM, the low-pressure SPM has better cooling efficiency than the unstable LPM with smaller coolant gas. Nevertheless, the low-pressure SPM is not suitable for opposing jet TPS than the high-pressure SPM because its intensity is not strong enough to provide available thermal protection performance. The low-pressure SPM can be introduced to the combinational opposing jet and platelet transpiration TPS which the cooling efficiency in reattachment region is strengthened by transpiration coolant so the intensity of the opposing jet can be reduced to save coolant mass. The numerical results indicate that opposing jet and platelet transpiration combinational TPS with low-pressure SPM mode can realise better cooling effect and high efficiency than the traditional high-pressure opposing jet TPS.

Thermal and mechanical analysis of LN2 jet impinging on rock surface

The transient cooling of rock surface by liquid nitrogen (LN2) jet was experimentally studied in this article. Experimental conditions cover a range of 1.4–2.4 MPa for jet pressure and 3–5 cm for stand-off distance. The nozzle diameter was fixed at 1 mm and the rock specimen was a circular plate, with diameter of 16 cm and thickness of 2 cm. The temperature curves at different radial locations in rock were measured during LN2 jet impingement. Based on these data we mapped the complete heat flux across the rock surface and attained the three dimensional temperature field within the rock. Thermal stresses inside the rock were further computed according to the experimentally determined temperature field. It is shown that the partial wetting of rock surface by LN2 jet created sharp thermal gradients in both radial and vertical directions. The induced thermal stress was in tensile state and the spatial range of thermal stress is consistent with the wetted region by LN2. The propagation speed of wetting front increases with increasing jet pressure and decreasing stand-off distance. Under the present experimental conditions, the maximum heat flux of LN2 jet was around 2.4 × 105 W/m2. The maximum tensile stress on rock surface was calculated to be ~5 MPa, which could potentially damage the rock structure and deteriorate rock’s strength.

Pectin-microfibrillated cellulose microgel: Effects on survival of lactic acid bacteria in a simulated gastrointestinal tract

Using high pressure microfluidization, we prepared micro-fibrillated soybean cellulose (MFSC) and analyzed its morphology and structure. MFSC was then incorporated into low-methoxyl pectin (PC) to coat lactic acid bacteria (LAB) by ionotropic gelation, and the effects of PC-MFSC microgel on LAB survival in a simulated gastrointestinal tract were investigated. Particle size analysis showed that the MFSC particle size decreased significantly with increasing jet pressure. Transmission electron microscopy analysis indicated that many cellulosic microfibers appeared at 150 MPa. Infrared spectroscopy and X-ray diffraction analysis revealed that the crystal structure changed from β-cellulose I type to II type with increasing jet pressure, but excessive pressure (200 MPa) damaged the crystalline structure of MFSC. Scanning microscopy indicated that cellulosic microfibers not only promoted a compact pectin gel morphology but also adhered to and coated the LAB in the pectin gel. MFSC-150 stabilized the pectin gel network, preventing the weakening of the gel under low pH conditions. Compared with other PC-MFSCs, PC-MFSC-150 microgel significantly decreased LAB susceptibility to gastrointestinal juice and increased the viability of LAB.

Effect of shale mineralogy characteristics on the perforation performance and particle fragmentation of abrasive waterjet

Abrasive waterjet perforation was performed on siliceous, calcareous and carbonaceous shales to study the effect of mineralogy characteristics on shale failure behaviors, and size distribution and fractal dimension of broken abrasives were analyzed to determine fragmentation characteristics of particles. Results indicate that particle detachment is the primary failure mode for siliceous and calcareous shales, while particle scratching plays a more dominant role in carbonaceous shale. Due to weak cementation and high brittleness, siliceous shale has the highest rate of penetration (ROP), followed by calcareous and carbonaceous shales. Among the three shales, abrasive particles have the lowest median-diameter and the highest fractal dimension (D) in the carbonaceous shale perforation. Compared to garnet, silica-sand is lower in median-diameter but higher in D after perforation. Increasing jet pressure helps improve ROP and D, but intensify particle fragmentation simultaneously. There is a positive correlation between ROP and D, depending on mineralogy characteristics of shale.

An enhanced coalbed methane recovery technique based on CO2 phase transition jet coal-breaking behavior

In recent years, CO2 phase transition jet (CPTJ) coal-breaking technology has been reported to increase coal seam permeability. However, the structural evolution effect of coal subjected to CPTJ technology is unclear, which restricts widespread CPTJ technology application. In this study, a laboratory experimental system and field technical equipment were developed, and coal-breaking experiments under different CPTJ pressure conditions were conducted. We investigated pore structure changes by combining scanning electron microscopy (SEM) and mercury intrusion porosimetry (MIP) to better understand the coal pore structure evolution characteristics under different CPTJ pressures. Furthermore, an enhanced coalbed methane (ECBM) recovery experiment using CPTJ coal-breaking technology was conducted with the self-developed technical equipment. The results show that notable damage occurs in the coal body influenced by CPTJ, and the damage area increases with increasing jet pressure. The SEM results revealed that more pores and cracks are produced due to liquid CO2 CPTJ, and the porosity and crack size increase with increasing jet pressure. MIP analysis indicates that the pore structure of the breaking coal samples mainly includes macropores, and coal sample macroporosity increases significantly with increasing jet pressure. The field ECBM experiment showed that CPTJ technology can reduce the coalbed methane (CBM) drainage decay coefficient and increase the CBM extraction pure flow rate and recovery efficiency 8.3–10.4 and 20.4 times, respectively.

Numerical analysis on the flow field structure and deflection characteristics of water jets under nozzle moving conditions

High-pressure water jets under nozzle moving conditions are widely utilized for rock breakage in geotechnical engineering. The nozzle motion state significantly influences the jet flow field and flow characteristics, thus changing its rock-breaking performance. In this paper, a single-jet nozzle model with different traverse speeds was established to investigate the jet flow field features and deflection characteristics and analyze the effects of nozzle traverse speed and jet pressure through numerical simulations. The jet flow fields under nozzle fixed and moving conditions were compared in terms of the distributions of vorticity, velocity, pressure and turbulence kinetic energy, and deflection characterization. Jet flow patterns photographed by an ultra-high-speed camera indicated the deflection of the jet flow field under the nozzle moving condition. The simulation results show that the flow field structure of the moving jet contains the non-deflection zone and deflection zone. The jet time-averaged velocity and vorticity are distributed symmetrically in the non-deflection zone, which is similar to the flow patterns under the nozzle fixed condition. As the jet prolongs further, the vorticity value increases and the vorticity distribution is more concentrated and closer to the nozzle outlet in the moving direction owing to more intense turbulence, resulting in the non-self-similarity of jet velocity dimensionless distribution on cross-sections and jet flow deflection to the opposite direction. The faster the traverse speed and the lower the jet pressure, the greater the jet flow deflection degree. The jet centerline velocities decay exponentially with the standoff distance, and the attenuation degree increases as the traverse speed increases. As the standoff distance increases, the jet deflection distance increases in a quadratic parabola and the deflection angle and impact angle both increase linearly, and decrease exponentially with increasing jet pressure. There is a positive relationship between the jet deflection characterization and the traverse speed.

Key Experimental Investigations of cutting dimensionality by Abrasive Water Jet Machining on Basalt Fiber /Fly ash Reinforced Polymer Composite

Abstract Abrasive water jet machining is a versatile in function to cut the extremely hard and inhomogeneous structured materials. Thermo set polymer matrix designed with multiple layer of hard rock fiber and fly ash rendered a high-performance structural material. The aim of the work is to investigate the cutting dimensionality of Basalt fiber/fly ash reinforced vinyl ester composite on abrasive water jet machining. Composite laminates -are prepared by multi stacking of basalt fiber with variation in fly ash particle loading (5-20 wt. %) using a compression molding technique. Improved mechanical properties of composites are obtained on varying the composition of fly ash. The linear progression of mechanical proprieties was limited by the agglomeration of fly ash particles. Mechanical properties enhancement is attributed to the polymer chain termination with fly ash particles leading to cross link density. The fracture mechanism and mode failure observed to be matrix cracking, particles crushing and brittle failure of fiber. Analysis of kerf dimension and surface cut quality were done by varying the cutting parameters of Abrasive Water Jet Machining (AWJM). Kerf dimensional deviation reducing with fly ash loading however exceeding critical loading there is a more edge damage observed. The dimensional deviation was controlled by varying the process parameters and determined the acceptance level of cutting as found by fly ash loading up to 10%. It is observed that cutting dimension deviation more when exceeding the level of 10% fly ash. Increasing jet pressure intensity reduces the fiber edge pull out and damages. Consequently, cut surface smoothness was majorly affected by fly ash loading. Surface roughness of cut surface was varied from 2 to 5µm as increasing in fly ash loading.

Rock-Breaking Properties Under the Rotatory Impact of Water Jets in Water Jet Drilling

Water jet drilling is widely used to develop coalbed methane reservoirs. The water jet drill bit is the core component, and a self-rotating bit is an economical bit because of its high rock-breaking efficiency and low energy consumption. Because the important parameters concerning the rock-breaking efficiency of these drill bits are unclear, this study carried out rock-breaking experiments on water jet rotation under different conditions of drill bit rotation speed, jet pressure, and jet impact angle. How the rock was fractured and eroded under these different conditions was analyzed. The results show that the volume of rock broken under rotary jet erosion increases exponentially with increasing jet pressure. The rock-breaking depth is the most important factor that influences the volume of rock broken, whereas the diameter of the area broken is a secondary factor. There is an optimum water jet rotation speed for the most efficient rock breakage, and this rotation speed is positively correlated with jet pressure. There is also an optimum water jet impact angle for rock breaking, and, in our experiments, this angle was 10°. The rotary impact of the water jet causes the rock to be in a three-way tension state, and this reduces the water cushion effect and jet reflection. This study can be used as a reference and guide for optimizing the design of self-rotating water jet bits and the determination of reasonable drilling parameters.

Experimental and theoretical analysis of a supercritical carbon dioxide jet on wellbore temperature and pressure

Along with the fast development of society, the demand for energy continually increases. Shale gas, a type of unconventional natural gas trapped within shale formations, is considered to be one of the best clean alternative fuels with the most potential because of large reserves all over the world. However, hydraulic fracturing, a widely used technology in shale gas exploitation, may cause serious environment issues. Therefore, the application of supercritical carbon dioxide (SC-CO2) instead of water to fracture shale has been mentioned. This novel approach could enhance shale gas recovery with no harm to the environment. To investigate the use of a SC-CO2 jet on the wellbore pressure and temperature, a series of experiments based on impinging theory have been conducted in the present study. In the first experiment, a specially designed apparatus with four visual windows was used to intuitively observe the jet's structure and character with a high speed video system recording the entire experimental process. In the second experiment, a high precision sensor was assembled in the vessel to measure the impinging pressure. The result shows that: (1) a SC-CO2 jet has a similar structure as a water jet, while a SC-CO2 jet dissipates slowly due to its low viscosity; (2) the impinging pressure increases with the increasing jet pressure, but the increasing rate is different because dissipation ratio decreases; (3) the ambient pressure affects the jet energy transfer rate though flow resistance and the ambient density. Under present experimental conditions, the critical pressure is 14.5 MPa; (4) a predictive equation is listed to calculate the impinging pressure based on the jet pressure and ambient pressure. Considering the application, the minimum jet pressure should be chosen to be more than 1.5 times the ambient pressure, and a suitable target distance for a SC-CO2 jet is no more than 7.5 nozzle diameters. These results help optimize the parameters of the SC-CO2 jet and promote the application of SC-CO2 in shale gas exploitation.

Study on the Frequency Characteristic of Self-Excited Oscillation Pulsed Water Jet

The self-excited oscillation pulsed water jet (SEOP Jet ) has been used extensively in oil and gas well drilling, mineral mining, cutting and industrial cleaning. In order to improve the valid stand-off distance of water jet pressure and its range of action, the model of oscillation frequency of jet was given on the basis of hydroacoustics and fluid dynamics for better understanding the principle of increasing jet pressure amplitude. The SD150test system was used to analyze the effects of dynamic parameters on the frequency, the effects of pump pressure and the cavity length on the frequency were studied in detail. The test results showed that the peak value of the SEOP Jet is larger by about 20% than that of continuous jet. The oscillation frequency increased while increasing pump pressure and decreased while increasing cavity length. There was an optimum cavity length corresponding to the pressure peak value of the SEOP Jet. Experimental result is in good agreement with that of theoretical analysis.

Oscillation modes of underexpanded jets issuing from square and equilateral triangular nozzles

The oscillation modes of underexpanded jets exhausted from square and equilateral triangular nozzles were investigated experimentally. Two stages were observed for both jets. The oscillation modes corresponding to these stages were identified by using optical and acoustical methods. The experimental results show that oscillation modes for both jets are first axisymmetric and then sinuous with increasing jet pressure ratio. The helical oscillation mode, which appeared for the circular jet, was not observed in these jets. The hysteresis phenomenon found for the circular jet cannot be seen in these jets.