Foil assisted multihull design

Discussion in 'Boat Design' started by groper, Sep 29, 2013.

  1. petereng
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    petereng Senior Member

    Hi Ad Hoc - I'm aware of the required variables and these have been considered. The panel shown is quite representative of a typical 600x600x5mm thick Eglass test panel using a typical epoxy resin. The avi is quite a bit more then a pretty video. Regards Peter s
     
  2. Ad Hoc
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    Ad Hoc Naval Architect

    Happy new year to you too Peter :)

    In the absence of any hard data stating what assumptions have been made for the simulation and whether the results have been verified (which can also be demonstrated) that is all it is, a pretty video I'm afraid.

    Any thing that is modelled in FEA/CFD that is beyond "very simple" requires considerable verification. The software itself is not "proof" of any kind...
     
  3. groper
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    groper Senior Member

    Experience trumps all IMHO... I can successfully predict most flow patterns based on what I've seen before and understanding how it works. All the infusions I do these days are a non-event... Problem areas are identified, and strategies are implemented to mitigate risks, much there same as any risk management process.

    Regardless of flow modelling, most infusion problems occur as a result of poor execution or mistakes made during the assembly of the preform. In other words, operator error. I don't see how this kind of of software prevents this- looks cool tho :)
     
  4. petereng
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    petereng Senior Member

    Hi Groper - yep I agree, but at least I now have a tool to do "what if" stuff. Always trying to decide what is the shortest runner system to do the job. I tend to use perimeter feeds these days and this will help predict where the "end" point is for the vacuum. Cheers Peter s
     
  5. Ad Hoc
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    Ad Hoc Naval Architect

    Well, call me a stick in the mud, I like quantitative independent results. I fail to see how a computer generated small 600x600mm over simplified unverified patch relates to a complex 3D dimensional vessel measured in many metres, not mm's. With variations in pressure, porosity, gravity/flow, shape and everything else that comes with such an under taking, to be considered as validation of a simulated "what if" scenario. Pretty pictures though...but that's the beauty of software, its big on presentation.
     
  6. rxcomposite
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    rxcomposite Senior Member

    Groper, I agree that a good infusion set up, once established is a no brainer, however to set it up, a lot of experience and intuitive knowledge comes with this. Infusionist carries this knowledge and setting up is almost second nature to them.

    A good software can establish the source/sink points along with the variables that goes with it. However, for it to be work, it must be in the real 3D world not just the X-Y plane. The hull has breadth, width, and depth. Same with the materials, it has varying permeability. The mold can even have high permeability areas if the sharp chines, deep stems, and transom corners are taken into account. Fibers have limited flexibility to form radiuses.

    Darcy’s law is flow of liquid along a path taking into account the permeability of the material and the viscosity of the resin. The law however works only on the assumption of a fixed permeability per material. When two or more materials are stacked, the flow pattern changes and should be mathematically defined in 3 axis. In its simplest form, the flow patterns are different individually for a WR, UNI, or CSM.

    A good software that predicts this pattern should be proven reliable and accurate. It is a tool that levels the playing field between the set-up, the materials, and the operator’s error. Once the software results are validated, product defect can be then isolated or reduced to material or operators defect. A good QA system can take care of this.
     
  7. petereng
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    petereng Senior Member

    Hello RX - Yes this is correct. The FE software I've used can include gravity, multiple materials, runners, the thru thickness etc. By building models using brick elements that are tesselated on top of each other or offset plate elements that are connected using conduction links then all of these variables can be accounted for. I have done physical tests with vertical surfaces up to 2m high and gravity in these cases has not changed the flow patter or time at all, probably due to the runner spacing at 600mm means gravity over that distrnace does not effect it much.. I have also done downhill filling with not much change either. Cheers Peter s
     
  8. rxcomposite
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    rxcomposite Senior Member

    Hello Petereng. If you have considered those variables, then your numerically derived software is competive. Perhaps you can post a link to the software you created so that viewers can see what are the variables you have included. If it is for your own personal use, maybe just a glimpse of what it can do.

    What is misleading in the simulation you posted is that it is seen as a simple 2 dimensional flow or flow over a homogenous fiber arrangement. Perhaps if you have included a different layer/color showing the differing flow pattern, then it would be interpreted more accurately. For example, the flow of resin on a CSM (or flow media) base stacked with a denser laminate such as a WR or Uni where the fluid flow speed changes from axial to vertical. The high permeability layer gets filled up first and diffuses verically into the upper denser ply.
     
  9. petereng
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    petereng Senior Member

    Hi RX - WR and CSM have a very similar fill pattern. UD has an assymetric pattern as the resin flows along the fibres better then across it (usually at about a 2:1 ratio). It would be unusual to model say a WR and a CSM as seperate materials . Usually we would lay the proposed laminate in a strip say 300mmx2m long with the proposed peel ply or perf and media on it. Then we fill the strip from one end and note how far the resin flows each minute. This is usully asymptotic in nature as the longer the distance of fill the greater the internal friction adds up. Most fills stop at 1m or 1.5m some can get to 2m (I have had one go to 2.5m!!). I then graph the time/distance and this allows me to calculate the permiability/porosity factor (using darcys Law). These get lumped together usually. Although the porosity is (1-fibre volume fraction) which is generally about 50% for WR,UD and CSM. So porosity is about 50% for all glass laminates at ~1bar infusion pressure. The front flow path (or top flow front) is usually about 25-50mm ahead of the bottom flow front (so we only concern ourselves about the top front). In some grooved core type infusions there can be 200mm difference in the flow fronts which can cause trouble at core gaps and corners (so would be worthwhile modelling this effect if interested in it). So in short unless you are doing this at a research level we usually figure the consolidated or lumped (average) hydraulic permiability values not the individual ones. This includes the media stack. Otherwise you would have to do a flow strip for all the components of the stack then build a model with all the individual layers. Happy to do this if someone pays for it!! I'm using Strand7 and using the thermal solver. All FE systems usually have a thermal solver. There is usually an example of a "seepage" calculation or just speak to the the FE support you are using and they will tell you how to do a porous flow problem analogy using the thermal solver. There's a few things to get your head around but if you do FE regularly it will be straightforward. Happy to explain anything in more detail if you ask a question. So to summarise. Temperature is Pressure, I have used 100degs to equal 100kPa so its simple to think about. Specific heat is internal resistance to flow or how "fast" each element fills up is one way of looking at it, conductivity is the combined permiability/porosity or how fast the flow front moves relative to "losing" heat to the elements. Density, thickness and geometry are the other variables that need to be kept close to reality but do need to be scaled (don't scale shape). I used the 600x600mm as over the last 15 years I've done or supervised 100's of these and know what time and shape to expect. 600x600 is used as 600mm is about the top flow for the type of laminates I use regularly and we need 600x600 to send to the uni for mechanical and other testing. If its a laminte or material that is new I set up a 2m strip usually.

    Once you have figured the variables, you nthen model the flow strip and adjust the density, Cp and k to acheive the right fill time. If you model a vertical fill you need to include gravity and this means you need to do a vertical strip as well as a flat strip. Once you have tuned the numbers so your model agrees with the actual you nthen have an accurate model.

    I've noticed over the years that when there are small gaps to fill near the end of fill they seem to not connect but fill to the vacuum port. If that makes sense. ie if the fill occurred regulary from the sides then the gap would close and we would need two vac ports not one. Here's a model to illustrate this effect. This is the type of thing that modelling will allow me to do. The runner in this case has very very low conductivity so gets around the perimeter nearly instantaneously (if you use 10mm spiral runners this is what I'd expect, it moves very, very fast) so you can't see the runner flow in this video. The video size is limited that I can upload so they are not as detailed as I see them. Usually I'm looking at 60Mb videos and I have to cut them down for here. The initial flow is modelled at 0.1secs intervals then I move to 20sec intervals a few seconds in.

    Boat structures can usually be broken into square or triangular sections that are relativley easy to visualise how it will fill. I'm involved with converting metal machine elements into composites and they are complicated. They have holes, islands... vary in thickness from 10mm to 45mm solid in the one part, are slender so have a tendency to close out when you use multiple resin ports. So I have to contend with a few things the boat builders don't. Visuallising these things is a bonus.
    Cheers Peter S
     

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  10. groper
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    groper Senior Member

    Peter, next job im going to infuse is a transverse cambered, elliptical section forebeam for my catamaran... im going to get the elliptical section with trampoline net insert included, hotwire cut from a block of PU foam and wrap my reinforcement around it. Im going to simply put a runner along the leading edge and pull the resin toward the trailing edge, the flow fronts should converge at the trailing edge vacline. Its just a small step towards infusing something like a mast or spar etc... remember our discussion on a solid carbon hydrofoil? 1 step at a time...
     
  11. petereng
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    petereng Senior Member

    Yes sounds good. Get some Tyvek (type of building sarking) and wrap around your vac line. Tyvek is like goretex, it allows gases to pass but not liquids. I've tried a couple of small scale tests and it looks promising. I'm still confused about cutting a PU with hotwire though. PU is a thermoset not a thermoplastic? Cheers

    Oh yes I have also placed the media on the foam when I have used foam and PVC tube moulds and left it in there, saves using peel ply and things then throughing the infusion stack away.

    Peter s
     
  12. groper
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    groper Senior Member

    Nice... Poor mans MTI hose hey? I was wondering how you got away with vac ports on the job without creating issues under the vac port... I guess some of this stuff solves it, good idea.

    I'm a fan of resin brakes to control the similar issues tho... Always stop the flow media 50mm short and it slows it right down and allows the front to even out both across the front and tool side lag...

    The PU was just PU foam... Serves as a light weight mandrel that remains inside the spar permanently...
     

  13. petereng
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    petereng Senior Member

    Resin breaks or brakes eg peel ply edges at least 50mm wide are fine if you filling linearly but many of the parts I'm involved with its best to fill from the perimeter. eg we are steadily moving towards machined matched moulds so an edge fill is the way to go for every part. I have a friend who uses MTI and its very impressive stuff. Not available retail in Oz, he brings it in from europe. Regards Peter s
     
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