[hpv-boats] CFD

[Rick Willoughby]

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