Author

Paradigm

References

Research

Abrasive Waterjet Research

1. Introduction

The aim of this Web page is to help researchers make an innovative, original and constructive contribution to abrasive waterjets and in particular the fluid dynamics of abrasive waterjet cutting heads.

Researchers and their supervisors need to understand the challenge they are taking on in investigating cutting head fluid dynamics. The following observations give a feel for the task:

• Fluid dynamics is a difficult science to work in because the equations for fluid flows have only been solved for a few simple flow situations
• Fluid dynamics is pre-eminently an experimental science, although computational fluid dynamics (CFD) is widely used for simulation of flows where the underlying flow behaviour is well understood as a result of experimental studies
• AWJ cutting head flows have the highest kinetic energy densities and velocities and involve the most complex three-phase flow phenomena encountered in industry
• Fluid dynamicists usually work with scale models and non-dimensional flow parameters but AWJ cutting heads involve so many flow variables that scale modelling is not an option
• The environment inside a cutting head is so aggressive and dynamic that it is not possible to directly measure events within cutting head flows
• Much of fluid dynamics is contra intuitive and so it is very easy to form a flawed mental model of a flow
• We are only likely to discover we have formed a flawed model if we measure the behaviour of a particular flow, or see a realistic simulation of the flow, or have a flow’s behaviour described in a clear and reasoned way.
• Researchers have mainly published information and data about cutting head flows that is not fluid dynamically correct. It is, therefore,necessary to be careful and selective in using published information and data.

Clearly, there is a need for guidance on carrying out studies on cutting heads. The Reference page provides information relevant to assessing the literature related to cutting heads.

There are also benefits to be gained from having a maintained source of desirable research studies, including recommendations as to the experimental procedures to be followed. A start on developing a library of research projects and specification for carrying out studies is reported in Section 5. Input on content and needs for research projects would be welcome.

2. Postgraduate Researchers

If you are a postgraduate research student, then you are a very valuable resource for your professor/supervisor. His/her status, job, departmental survival may depend on a steady flow of postgraduate students like you. You are devoting part of your career to a project so you deserve:

• To work on a project that will not only contribute to your development and future but gives you the opportunity to make an original and worthwhile contribution to the knowledge about abrasive waterjets. It will be your legacy so make it a good one
  
• To be provided with, or to be able to obtain, borrow or otherwise procure, the resources that give you the potential to make your project a success
  
• To be given adequate support by your supervisor. You should talk privately to the supervisor’s researchers, past and present, to establish that he/she will be concerned for your development and not just use you as a resource.

You should realise that:

• You are responsible for driving your project and how you invest your time
• That almost everything about abrasive waterjets is new to you but not necessarily to others. It is your responsibility to find out the state of the art in your intended research area. As the main centres of expertise are within manufacturers and user of AWJ systems you should do all you can to access individuals in these centres of expertise. However, you should do your homework first – people need to feel you know what you are talking about and that you know something about their business and needs
• If you are working on AWJs, probably the best experience you could gain for your project is to persuade an abrasive waterjet job shop to let you work as an operator and programmer for a short while. Better still to have a supervisor who has arranged for you to get operating experience, even if this is on a training course. Your supervisor should have sorted out the legal, health and safety and other issues for you, or point you in the right direction
• Modern measurement systems allow you to collect nonsensical data at an extraordinary rate. However, to obtain the first reliable measurement from a measurement system, such as a laser anemometer, may take 10 or more person years of effort. If you are part of an ongoing effort to develop a unique measuring capability, that is highly likely to be funded and driven to a successful conclusion, this can be worthwhile activity.

You should:

• Make sure your cutting head configuration matches or is better than the best commercial cutting heads. There are no data in the research literature from experiments at AWJ cutting jet energy densities achieved by commercially available cutting heads and this situation has gone on for over ten years
• Be aware that over 90% of references commonly quoted in research papers on cutting head flows are unreliable. Some totally erroneous papers are quoted many times in the literature in support of other erroneous research results. Your work will be suspect if you refer to papers that contain erroneous data and conclusions and you do not point this out
• Avoid application projects for established abrasive waterjet systems as the chances are one or more abrasive system manufacturers and/or end-users will have already done the work. A common comment at conferences, by attendees from industry, is “We did that work five, ten or fifteen years ago”
• After a short time you should know more about your specific topic than your supervisor. Your role then is to subtly educate and steer your supervisor so you can carry out research that will allow you to make the best contribution to the understanding of abrasive waterjets and whatever objectives you have set yourself.
• Think about the ultimate end user for the output from your project and not just satisfying academic requirements. Ensure that you present information and data in a form the end user will find useful
• Use equations sparingly, unless they are essential for others to use your work
• Think laterally, doing what others have already done is not innovative or original
• If you can see a business opportunity for yourself out of your research go for it
• Have fun, if not, move on.
  

2. Cutting Head Measurements

In the 1930s researchers at Munich University defined unambiguously how to collect and present data for single-phase flow through components. Forty years later the author was able to bring together data on internal flows from many sources (Miller 1990) because researchers had used the Munich University procedures for their experimental work.

It would be ideal if unambiguous data collection and presentation procedures could be developed for cutting heads. The reality is that this is highly unlikely because of the large number of flow, geometric and abrasive parameters involved and the desire of cutting head manufacturers to have “unique competitive features”. However, it is important that basic details about cutting heads and test conditions are recorded. As far as the author is aware, no published paper provides sufficient geometric details of a cutting head for other researchers to repeat or to check experimental work. This is not a professional approach to research.

The need for geometric similarity underpins experimental fluid dynamics. In the literature on AWJ cutting heads geometric similarity has largely been ignored. Disturbingly, experimenters do not interpret data against changes in non-dimensional geometric parameters – some would probably have drawn very different conclusions about flow processes in cutting heads if they had analysed their data in non-dimensional form.

The basic measurements for internal flows are flow rates and static pressures. The next level is point velocity measurements within a flow followed, in the case of multi-phase flows, with component velocities and concentrations. The aggressive environment within cutting heads precludes measuring velocities and component concentrations. It is also not possible, at present, to measure component distributions and velocities at the outlet from cutting nozzles. This means the only measurement options available are input flow rates - water, abrasive and air - and static pressure distributions through a cutting head.

A surprising observation is that after twenty years of research on AWJ cutting heads, detailed static pressure measurements through a cutting head have yet to be made. In particular the rapidly varying static pressures in the inlet and the initial section of cutting nozzle bores. A useful start in understanding air compression prior to cutting nozzle bores, and other flow phenomena, would be measurements of static pressure without abrasive flow through cutting heads.

Air plays a crucial role in all the flow processes within cutting heads but only a few researchers have bothered to measure airflows. Even fewer have carried out compressible flow calculations to try and understand the role air plays in accelerating particles in cutting nozzles and in nozzle wear processes.

A major contributory factor to the misunderstanding of cutting head flows is the difficulty of visually representing such flows. Outputs from CFD studies are usually very visual with velocities, concentration gradients, flow separation points and other detail clearly displayed through the use of colour, shading and directional arrows. It is desirable to have similar visual representations for cutting head flows but events in AWJ cutting heads are spread out over flow passages that are 400 or so waterjet diameters long. The elongated nature of cutting head flows makes it extremely difficult to represent flow behaviour in a visually meaningful way.

Published visual representations of flows in cutting heads foreshorten flow processes, such as waterjet break-up. As visual representations are powerful inputs when forming mental models it is understandable why new researches form erroneous conceptual model of cutting head flows. There is a need to see if state of the art computer simulation programs can be used to provide realistic representations of cutting head flows.

Having mentioned CFD programs, it is appropriate to point out that AWJ cutting head flows fall outside the capabilities of any CFD program. CFD comes into its own for simple to moderately complex flows taking place in or over complex geometries. Experimental data is available to calibrate CFD programs when used for such applications. In AWJ cutting heads very complex flows occur in simple flow geometries.

A feature that differentiates abrasive waterjets from other cutting beam technologies, such as lasers, is an abrasive waterjet is a multi-component beam whereas other cutting beams consist of a single component, such as photons (it could be argued that lasers and other cutting methods us oxygen and burning of the material being cut). Ninety percent or so of the kinetic energy of an abrasive waterjet is carried by the water that is not directly involved in the cutting process. It is impractical to determine cutting energy density because of the multi-component nature of an abrasive waterjet, and the wide spread of jet abrasive particle velocities and particle sizes.

A consequence of not being able to determine cutting energy density is the need for cutting trials to establish a jet’s cutting capabilities. Work is needed by the abrasive waterjet community to establish standard procedures for cutting trials. These include workpiece materials, non-dimensional workpiece thickness in terms of cutting jet diameter, profile shapes to be cut and characterisation of cut surfaces.

4. The Research Community

The research community is in a difficult position because it mainly has AWJ equipment but it cannot keep up with manufacturers in developing new capabilities, nor has it been innovative in researching new methods of abrasive waterjet generation. As a result the research base is not used by manufacturers for incremental improvements or for research that could lead to paradigm shifting products. In countries that were once leaders in abrasive waterjet research, funding organisations have virtually ceased supporting the technology because of a lack of innovation by the research community,

Organisations in the research community have the opportunity to actively contribute to changing the abrasive waterjet industry from a single product industry - AWJs for general machining. The industry needs to provide products that compete with and complement lasers for micro, fine and general machining. Research organisations can encourage entrepreneurial researchers to create start up companies in applications such as micro machining - Start up companies have probably been the most effective way of bringing about paradigm shifts in comparable industries to abrasive waterjets, such as lasers.

The abrasive waterjet research community is mainly composed of small groups distributed in universities around the world. Probably 80% or so of the projects these groups carry out could be described as “something to do projects” - project objectives, experimental resources and supervisors knowledge of fluid dynamics and industries needs being inadequate for useful outcomes. If supervisors and researchers had the option of getting informed comments on proposed projects more research would feed into industry. Even better would be a dynamic interaction between research groups and the wider abrasive waterjet community.

The idea of research groups interacting and collaborating may seem idealistic, however, the World Wide Web provides the communications vehicle for such collaboration; the problem is the driver/s for developing and maintaining collaboration between groups. The probable profile for the driver/s is individuals who see the opportunity to organise such collaboration as part of their contribution and career development in abrasive waterjets.

5. Research Projects

This part of this Web Page is still under development – the author would be pleased for suggestions of how this Section could become a Web Site in its own right that is owned by the abrasive waterjet research community.

5.1. Research Project Specifications

Two examples of research project specifications are provided for AWJ cutting heads. The aim is to use the examples to iterate towards a standard format for research project specifications.

Project. 1 - Static Pressure Distribution in AWJ Cutting Heads - Without Abrasive

Project 2 - Penetration of Abrasive Particles Upstream of Waterjet Orifices

5.2 Collaborative Projects

End user needs include:

• Faster cutting
• Reduced abrasive use
• Longer lives from cutting head orifices and focus tubes
• Improved reliability
• Better information on cutting head operating parameters to achieve particular machining objectives.

Projects aimed at meeting end user needs will involve cumulative cutting times running into thousands of hours. The direct hourly operating costs quoted by job shop operators (Waterjet Web Reference Discussion Group www.waterjets.org) is between 50 and 90 US dollars. This means that experimental data needs to be generated from day to day operations in job shops and production facilities to avoid cutting time cost that could exceed a hundred thousand Euros/dollars.

Setting up a successful project involving a number of end users is major project in itself. Committed participants have to be found and a pilot study carried out to establish what data needs to be collected, measurement and data collection procedures, data analysis methods and other details.

Additional costs to operators have to be minimized and perceived benefits maximised. This requires a good understanding of machine shop operations. Ideally the project manager should gain experience in a machine shop before finalizing what is required from machine operators.

A future extension to this Web page will include outlines for collaborative projects.

6. References

Miller, D. S. (1990) "Internal Flow Systems", 2nd Edition, BHR Group, Cranfield, UK.