[hpv-boats] CFD

[Larry H. Smith]

I assume mine is not the only dropped-jaw about the knowledge (and  
materials/technique/sources info) possessed and shared here by the HPB  
Wizard of Oz.

Speechless as usual, but still able to type. ;-)

Thanks Rick!




On Feb 26, 2011, at 9:23 PM, Rick Willoughby wrote:

> 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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