Reducing drag on v2.3


#1

Excelsior!!

I was thinking of trying to reduce drag on this version by drilling holes in various parts of the motor chassis, and outer hull (obviously not the e-chassis lol). Has anyone attempted this, and is there any reason anyone can think of making this a terrible idea..

Thanks

Marius


#2

Hi Excelsior:

May I have a more detailed explanation about your idea ? From only reading the post, I cannot get how those holes would reduce drag.

Regards

Ion


#3

See photo
1139-image.jpg (196 KB)


#4

Ouch ¡¡

What a solid wall. Didn't realize it.

Well, the ROV design is not an example of applied hydrodynamics. Reducing flat surfaces normal to the fllow stream will, of course reduce drag. You are right.

Anyway, for the working speed of this ROV, price and building simplicity, the design is quite good.

The only thing that could turn your idea in a horrible one, is that related to structural integrity.

Check how stress is applied to that ROV part. Does it work bending, twisting, stretching ? Is it vital holding distances-angles between that component and others ?

Are other parts diverting flow before reaching that flat plate ?

Here you have attached an estimation of the dragg force difference between both designs.

Fluid: Fresh water, "Normal" conditions.

Flow: Perpendicular to the plate surface. Speed 1.3 m/s -> 2.52 Knots

Arrangement: Isolated plate, no flow interference.


Estimated dragg reduction: Around 30%

Best regards

- ion -


#5

That's pretty good wouldn't you say? Thanks a bunch!


#6

Yeah, it is.

But note that the numbers above are related to an isolated plate. May be the arrangement around the ROV part, diverts the flow in such a way that dragg reduction results improved or negligible.

Main term of dragg force can be written as:

Fd= 1/2 x fluid density x Area x velocity(squared) x dragg coefficient.

Fd= 1/2 d A V² Cd

(remember that velocity is a vector)

(Area is measured projecting it on a plane normal to the flow)

Hence, any surface reduction will reduce dragg.

But, the term velocity is related to the "relative" velocity between the submerged body and fluid.

If the flow is distorted before arriving to the involved part, velocity vectors will change.

Two main things will happen:

Flow can not be taken as uniform anymore.(each flow portion will have different direction and speed)

Speed and incidence angles on the "plate", can be so diferent than those corresponding to the "isolated" conditions, that holes could have no practical effect.

From my point of view, the ROV would wellcome a complete hydrodynamic hull cover.

That cover does not have to be watertight, but only "flow friendly".

Regards

- Ion -


#7

Dear Marius:

I had a little computer free time and put a whole ROV model into the CFD.

The model has been built from the plans and documentation for the 2.4 version.

Results are:

Z, Longitudinal axis

Y, Vertical axis

X, Transversal axis


Speeds are choosen as "logical" speeds for this ROV

Hope this information can be useful for you.

Regards