[hpv-boats] CFD

[Bruno Sertier]

Hello Rick,

Thank you for your recommandations and advices. Actually my CFD quest is not 
only for the submarine drag reduction and estimation aims but for more 
general questions such as sizing of the tail and interaction with hull and 
transcient analysis when pitching, yawling and rolling.

Your point about efficiency with an arm swinging system for propulsion is 
interesting but looking at the video on your weblink it seems that you are 
pedaling (with high speed results!) or did I miss something? Could you be 
more specific about this system please?

Thank you.
Bruno
--------------------------------------------------
From: "Rick Willoughby" <rickwill at bigpond.net.au>
Sent: Sunday, February 27, 2011 6:23 AM
To: "HPV Boat" <hpv-boats at bikelist.org>
Subject: [hpv-boats] CFD

> Bruno
> re you question:
> Other question : I'm looking for a CFD software (such as Fluent +Gambit 
> from
> Ansys or Flow-3D) else than the opensource ones, free but not always  easy 
> to
> use. Has anyone a clue about how to get an efficient and pro class one
> without spending a fortune (or downloading cracked, uncomplete, and  of 
> high
> risk of malware infection)?
>
>
> I had a look at your sub.  I expect that you could get performance  gains 
> simply by working to minimise wetted surface and enclosed  volume.  You 
> can calculate the drag reasonably accurately for  operating in deep water 
> by using the ITTC57 skin friction line:
> Cf = 0.075/(log10 (Re#)-2)2
>
> The drag can then be approximated to:
>
> Rv=1/2*rho*U2*S*Cf
>
> Where U is speed and S is surface area both in compatible units with  rho.
>
> You need to do these calculation for the entire wetted surface  including 
> the fins and the prop protectors.  Each fin and protector  should be 
> treated as separate for determining its Re#. Remember to  allow for both 
> sides.
>
> Further comments:
> 1. This simple analysis excludes form factors, which will be low  unless 
> you have some abrupt transition in section.  The optimum  fineness ratio 
> for water is around 8 but the main aim is to keep the  volume and overall 
> surface area as small as possible while having the  room to operate. 
> Fineness of less than 4 would start to make form  more significant.
> 2. If you are near the surface then there will be waves.  There is  freely 
> available thin ship software called Michlet that gives  accurate wave data 
> fore fineness down to around 5.  Michlet will also  calculate the skin 
> friction based on the ITTC line.
> 3. The ITTC friction calculation assumes turbulent flow over the  surface. 
> There are so-called laminar flow hulls that give favourable  pressure 
> profiles to maintain laminar flow but I doubt whether they  can achieve 
> this in a hull disturb by pedalling.  Some info on that  here:
> http://www.iag.uni-stuttgart.de/luftfahrzeugaerodynamik/paper/ 
> melbourne_9_98_lutz.pdf
>
> One area where you might gain efficiencies is to consider less  turbulent 
> methods of power input than cycling. I have done  biomechanical modelling 
> of swing arm system in air compared with  cycling.  In the harmonic 
> regime, up to moderate power, it is more  efficient than cycling.  I 
> tested that system on the boat seen here:
> http://www.youtube.com/watch?v=PYoW3XjHRbw
> It is not suited to energetic power level and the harmonics are  probably 
> less beneficial when the legs are buoyant but it still  offers lower 
> velocity of the moving parts - predominantly legs.
>
> I expect you would get reasonable drag results for your existing sub 
> using this approach.
>
> You should find there are more efficient ways to adjust pitch and  roll 
> using moveable ballast (or air) at low speed and fins for higher  speed 
> rather than the twin side thrusters.
>
> Rick
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