Jet Coherence Research Articles

A review of additives in abrasive water jet machining and their performance

Abrasive Water Jet Machining (AWJM) is an advanced machining process that has a variety of applications, including machining of hard-to-machine materials, drilling, milling, coating removal, polishing, etc. They are attractive because of their versatility, absence of heat-affected zones, low thermal-distortions, low cutting-forces, and environment-friendly nature. However, use of plain-water in AWJM poses some problems due to lack of jet coherence, poor suspension characteristics, and unsatisfactory performances in submerged-cutting and cutting of hollow surfaces. Countering these problems appropriately can lead to better cutting performances in terms of material removal rate, depth of cut, kerf-width, and kerf-taper. It can be achieved with addition of some additives in abrasive water-jet. These additives are mostly polymers of high molecular mass, which increases viscosity, suspension capability, and stability of jet, while reducing drag. In addition to AWJM, the effect of polymeric-additives in other water-based jets like Abrasive-Slurry-Jet (ASJ), High-pressure ASJ, etc. are also discussed. Therefore, this study aims to identify and list of different additives that are used in AWJM and other water-jet-based machining-processes over a past few decades, and to provide a run-through on their performances. It can provide the significant information to researchers working in fields related to waterjets.

Simplified prediction of near-field jet coherence using the cross-power spectral density acoustic analogy

The cross-power spectral density (CPSD) acoustic analogy is a generalization of Lighthill’s analogy that directly predicts near-field statistics at multiple spatial points. The methodology was previously applied and validated to predict noise from isotropic turbulence and off-design supersonic jet flows. We present a simplification of the CPSD model for near-field predictions of acoustic coherence from jet fine-scale mixing noise. Validation predictions are presented with a NASA Glenn Research Center experimental jet database. The simplified mathematical model is presented along with the physical significance of each term. The application of the model can be applied to predict loading and coherence on the fuselage of aircraft, rockets, and associated structures. This approach can help designers minimize sonic fatigue or failure in flight-vehicles and support structures.

High-coherence jets for focused fluid delivery in grinding

Jet coherence is of critical importance in grinding fluid application. Prior works to this end have largely delved into engineering contoured nozzles to promote coherence, with modest results. Inspired by decorative laminar fountains that display coherent jets, the present research focused instead on laminarizing the flow through a simple cylindrical nozzle. Augmented by the intriguing phenomenon of hydraulic flip, such jets possess extreme coherence and resemble smooth, transparent glass rods. Investigations indicate them to be valuable in applications like flute/groove grinding that necessitate precision fluid delivery and in instances that constrain the proximity of the nozzle to the grinding zone.

A Study of the detonation wave shape inside confined explosive with a metallic copper liner

The behaviour of shock wave inside a shaped charge is so important that the detonation wave nature characterizes (determines) the degree of jet coherency at the liner interface. The detonation wave shape inside the shaped charge depends on the casing thickness, material, the liner material, geometry and the applied wave shaper geometry. In this paper, a series of numerical simulation models were performed to study the incident and reflected waves and to characterize the resulting shock wave front to evaluate its effect on the produced (generated) jet characteristics. The used code was Autodyn 3D based on non-linear dynamic analysis with jetting analysis model. Inert wave shapers (Lead, Aluminium and steel) and active ones (TNT and PETN) behaviour were studied. The liner type used in this study was copper with in which the apex angle of the liner cone was 40°. It was concluded that the wave front shape has an obvious influence on the characteristics of the jet formation and its properties such as mass of jet and velocities of its tip. Also, the bulk sound speed of the liner itself in addition to the collapse characteristics of the liner was discussed in this paper successfully.

Ceramic sponge Abrasive Waterjet (AWJ) precision cutting through a temporary filling procedure

Ceramic sponge machining after firing is a great issue, requiring special tools and procedures because of the material peculiar macro-structure and its intrinsic brittleness. This study approaches the problem by exploiting the Abrasive Waterjet technology (AWJ) and showing its potential as a flexible tool. Nowadays, AWJ is coming up as an alternative to other ceramic manufacturing processes such as Grinding, Ultrasonic Machining and Laser Machining. The influence of a temporary pore filling agent, infiltrated in the already sintered sponge, is evaluated and its effect on the jet coherence is investigated through both modelling and experimental approaches. The most suitable process parameters are assessed in order to reduce the main AWJ defects in these conditions, setting the feed rate (vf) at 150mm/min on a 35mm thick 30 PPI (pores per inch) ceramic sponge on a conventional cutting equipment. The overall kerf divergence is therefore reduced down to less than 1° thanks to the filling procedure and then it is compensated by exploiting a 5-axis cutting centre. Defects are measured, using both conventional and ad hoc tools (e.g. CMM, grazing light surface inspection and digital image analysis). No thermal or chemical actions are applied by the AWJ cutting process and the negligible forces exerted on the struts preserve their integrity. A case study geometry is machined, fulfilling tight tolerances of 0.1mm on a Ø 10mm ceramic sponge cylinder over a 15mm thickness. A complex-shaped component is cut on 35mm thick sponge.

CFD aided design and experimental validation of an innovative Air Assisted Pure Water Jet cutting system

Water jet cutting has always been a promising technology because of its extreme simplicity and flexibility, even if it often suffers a lack of control on its process parameters, especially if compared to technologies such as laser cutting or electro-discharge machining. Recent studies have showed how the presence of water inside the orifice causes disturbances and instabilities which systematically affect the jet structure, both during the jet formation and the cutting process. These disturbances can be neglected in industrial applications, but they can play a relevant role in case of high-precision water jet machining. The aim of the research presented in this paper is to develop an innovative system able to modify the orifice flow field by means of a simple modification of the standard cutting head geometry; the system allows the controlled injection of air inside the primary orifice to prevent the jet instabilities and to adapt the level of jet coherence to the specific machining operation. The fluid dynamics aspects of the outflow process are investigated by means of a 3D numerical simulation with the Ansys Fluent CFD solver, while considerable experimental efforts are provided in order to validate the numerical model and finally evaluate the system performances on real case studies.

Jet coherence in QCD media: the antenna radiation spectrum

Abstract We study the radiation of a highly energetic partonic antenna in a colored state traversing a dense QCD medium. Resumming multiple scatterings of all involved constituents with the medium we derive the general gluon spectrum which encompasses both longitudinal color coherence between scattering centers in the medium, responsible for the well known Landau-Pomeranchuk-Migdal (LPM) effect, and transverse color coherence between partons inside a jet, leading, in vacuum, to angular ordering of the parton shower. We discuss shortly the onset of transverse decoherence which is reached in opaque media. In this regime, the spectrum consists of independent radiation off the antenna constituents.

On the Coherent Length of Fluid Nozzles in Grinding

The delivery of grinding fluid to the contact zone is generally achieved via a nozzle. The nozzle geometry influences the fluid velocity and flow pattern on exit from the nozzle orifice. It is important to the efficiency of the process and to the performance of the operation that the fluid is delivered in a manner that ensures the desired jet velocity has adequate coverage of the contact zone. Often, assumptions about adequate coverage are based on visual inspections of the jet coherence. This paper provides new insight into the internal nozzle flows and the coherent length of a wide range of nozzle designs. The work presents a new analytical model to predict coherent length which is shown to correlate well with measured data from experiment. Recommendations are given to guide a user to optimal design of nozzles to ensure adequate fluid supply to the contact zone.

The effect of nozzle geometry on the flow characteristics of small water jets

A wide variety of processes make use of plain orifice nozzles. Fuel injectors for internal combustion engines incorporate these nozzles to generate finely atomized sprays. Processes such as jet cutting, jet cleaning, and hydroentanglement, on the other hand, use similar nozzles, but require coherent jets. The spray or jet characteristics depend on the stability of the flow emerging from the orifice. This problem has been extensively researched for nozzles with diameters above 300 μm. Much less is known about the characteristics of jets produced by nozzles with smaller diameters, where viscous effects and small geometric variations due to manufacturing tolerances are likely to play an increasing role. Results are presented of a wide-ranging investigation of geometry effects on the flow parameters and jet characteristics of nozzles with diameters between 120 and 170 μm. Nozzles with circular cross-section and conical, cone-capillary and capillary axial designs were investigated. For conical and cone-capillary nozzles, the effect of cone angle and effects due to interactions between adjacent nozzles in the multi-hole cone-capillary nozzles were studied. For capillary nozzles, the effects of diameter variations and inlet edge roundness for capillary nozzles were considered. Furthermore, the effect of varying the aspect ratio (ratio of major and minor axes) of elliptical nozzles was studied. Flowrate and jet impact force measurements were carried out to determine the discharge coefficient Cd, velocity coefficient Cv, and contraction coefficient Cc of the nozzles for supply pressures between 3 and 12 MPa. Visualizations of the jet flow were carried out in the vicinity of the nozzle exit in order to identify near-nozzle flow regimes and to study jet coherence. The relationship between nozzle geometry, discharge characteristics, and jet coherence is examined.

PERFORMANCE OF A COHERENT JET COOLANT SYSTEM IN NON-CONTINUOUS DRESS CREEP-FEED GRINDING OF INCONEL 718

In this work, the effect of the degree of jet coherency on the occurrence of workpiece burn when creep-feed grinding Inconel 718 under non-continuous dress conditions is studied. Different degrees of jet coherency are compared on the basis of the maximum material removal rate achievable before workpiece burn occurs. The comparison is performed at different coolant flow rates as well as at different distances between nozzle and grinding zone. It is found that a more coherent jet results in considerably higher possible material removal rates before workpiece burn occurs. Furthermore, it is determined that nozzle distance from the grinding zone and nozzle orientation and position with respect to the grinding zone, using a coherent jet, does not affect cooling performance. It is also shown that an increase in flow rate will increase cooling performance but only up to a point.

Experiments with large diameter gravity driven impacting liquid jets

The phenomenon of a liquid jet released under gravity and falling through or impacting onto another liquid before colliding with an obstructing solid surface has been studied experimentally under isothermal conditions. Usually the jet diameter was sufficiently large to ensure jet coherency until collision. Direct flow visualization was used to study jets released into water pools with no air head space and jets impacting onto water pools after falling through an air head space. It is shown that distances predicting the onset of buoyancy and the entrainment of air using derivations from continuous plunging jets, are not applicable for impacting jets. The morphology of jet debris after collision with the solid surfaces correlates with the wetting properties of the jet liquid on the surface.