As you may know, I've been worried about ROV hydrodynamics from my arrival to this site.From that beggining, as well, I'm impressed by the very good, nice and smart job that is being done around the project. Hence, please don't take what follows as an attack, but as a contribution from my side.
Always from the "Underwater Vehicle" and not from the Robotic point of view ...........
When a body moves through a fluid, it changes its state from steady to distorted. The more difference between both states, the more energy is obviously transferred from the body to the fluid.
In the case of a submersible of anykind, the best performance is wanted. Meaning that the less energy must be tranferred from the propulsive + hull system, to the fluid.
Have in mind, that all that wasted energy, in the form of turbulence, heat and noise, also comes from the batteries, shortening their life and the "sub" range.
First thing that jumps to the eye when having a look to the ROV, is its asymmetry with respect to an horizontal plane.
Even at the low speed of 1.08 m/s turbulences have a big effect.
Pressures are not evenly distributed. A body moving under such conditions will dive or go up. In the case of the ROV, it dives. The faster ahead, the bigger that diving condition.
We know it dives from two sources: Reports from users, and computation of pressures distributions.
The stagnation point of the ROV when going ahead, is located on a horizontal line on the E_Cylinder surface.
From this point, flow is broken into two main streams. One runs over the "smooth" upper chassis cover, while the other, has to turn around the cylinder, for finnally arriving to the complex internal ROV framing.
That structure is so complex from the fluid point of view, that stream twists again and again, leading to a low pressure area which "sucks" the ROV down.
Battery cylinders, which will be discussed later, help for this effect due to their plannar ends.
Now, we have two streams: A "clean" one running over the body and a "dirty" one which runs below and inside it.
The upper one, almost entirely passes by, not entering the propeller ducts. While the "dirty" one, is the only one feeding the propellers.
Propulsion has now to do three works.
1-) To attrack fluid that is not leading towards its rotation plane.
2-) To tidy flow up.
3-) To drive flow astern, allowing for propulsion.
Only number three is useful. At the same time, a big ammount of energy is extracted from propulsion for eddies formation inside the framing.
The pic below, froma CFD study, shows what is said above.
Note the blue area under the cylinder (low pressure-Coanda effect) and the heavy eddies area inside the ROV.
Well. Once problems stated, solutions can be found.
The question is: Holding main dimensions, price and simplicity, try to improve the "sub" performance.
Two main things have to be taken into account. Fluids don't "like" to be distorted.
Symmetry, where possible is a MUST.
Hence a new internal structure and a partially changed external chassis has been designed. It has been done by trying to approach the ideal shape, but being restricted by construction materials and building tools. It means, no curved surfaces and no complex parts.
Holding the size and the E_Cylinder with all its parts, allowed for a flow study and hence for finding the shape that better fits the fluid paths around the ROV.
At the same time, flow has been symmetrically directed to the propeller ducts in the more possible tidy state.
With both conditions, manouverability and propulsive performance would have to be improved.
Many attempts have been done (Surely you know that fluid dynamics is not an "exact" science), and after what seems to be a reasonable time spent, with a reasonable result, an improved ROV hull has been obtained.
The pic below, shows the new flow path.
Both convergent planes above an under the central girder are only two sheets from the same material alredy used for the ROV which have been sanded at the E_Cylinder edge.
Volume between both planes is opened at its after end. Its not an enclosed volume.
The tail section, from the same material, has also been sanded, for turbulence avoidance. The vertical plane shape is designed that way for decreasing surface dragg.
The pseudo-conical part ahead of the motors come from a model airplane catalogue. Are the parts used as propeller cones.
Similar cones have been fitted ahead and astern the battery cylinders. The dragg reduction has been quite big.
Both sides of the ROV are a lot opned in order to allow water flow to the props while decreasing the body surface as well.
All curves are elliptical or parabolloidal shaped.
A few drawings of this "hull":
Force(Y) Vertical dragg. Note it has been reduced almost ten times.
Force(Z) Horizontal dragg. Reduced in a 50%
Propeller flow rate: Water propelled though the ducts. Also improved.
Hope to have the building plans in two days.
Moderators ..... Where is the place for sharing those plans ?
Regards and ........... Happy 2014.