Towed cable calculations 2

[engrnick]

Hi all,

I'm trying to work out an equation of motion for a project I'm working on. It's a towed system with a cable and mass at the end and I would like to be able to come up with an equation of motion for the system. I have the mass' drag values from some COMSOL simulations, but i'm feeling like my knowledge of steady state controls systems is lacking (I'm a structures guy) and I'm feeling kind of stupid.

Can someone direct me to a good example of how to go about developing this? A link? A textbook? something that explains how to set up the equation? Been out of school a long time so this feels really alien to me.

thanks!

 

[GregLocock]

I can't make much sense out of what you need that you haven't got. What does the system actually look like and what are you trying to predict?

Cheers

Greg Locock


New here? Try reading these, they might help FAQ731-376

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[engrnick]

Morning, you make an excellent point. sorry for the confusion.

I've got a towed system on the end of a cable pulled at 3 knots. I've got to find an equation of motion for the force needed to pull the system with the drag from the cable, the system and i've been told we need to be able to predict the motion using kinematic equations for the dynamics of the system as well.

I'm looking at papers published in ocean engineering to understand the concepts, and I see the math, but not sure how to relate this to my situation - mostly because I don't have an ocean engineering background.. it's like a baker looking at a fish - he knows the concept of how to cook, but this is totally different thing. I'm looking for a reference where I can learn how to set up this system logically and correctly without going back to school to get another degree, I guess is what i"m looking for.

Any help would be great, thanks!

 

[GregLocock]

Is the catenary in the cable important? is the towed body a paravane? are wave effects important? is this thing floating or submerged? is the compliance of the cable important?

Basically i could model it all sorts of level of detail, whereas equations might be more difficult.

Cheers

Greg Locock


New here? Try reading these, they might help FAQ731-376

http://eng-tips.com/market.cfm?

 

[engrnick]

Good questions - wasn't even aware that I should be asking this, and this is why I'm looking for a way to learn this.

catenary no
paravane a certain type
wave effects no
submerged
compliance not sure how to determine that.

I understand modeling it is possible, but i"m looking for how to learn how to do this. I imagine this was Ocean engineering 101, right?

Thanks for your help!

 

[GregLocock]

OK, so you have a massless, but possibly draggy, cable, and a paravane which i'm guessing can be considered to be neutrally buoyant, and for which you know the hydrodynamic coefficients.

I suspect that a draggy cable is a computational nightmare so i'll ignore that.

OK, that's enough random thoughts.

Draw the thing up in plan viwe and front view, work out your free body diagrams, using d'Alemebert forces, and you may have something usable.


Since I can't tell what level of fidelity you are after i can't say whetehr that is enough, but it is certainly an essential first step.

Cheers

Greg Locock


New here? Try reading these, they might help FAQ731-376

http://eng-tips.com/market.cfm?

 

[chicopee]

The best thing to do is to draw a free body diagram of the forces acting on the paravane. Knowing its features such as the center of gravity, the towing point, volume displacing water, wing configuration, you should be able to locate on the free body diagram the locations of the towing point, lift, drag, weight, buoyancy; at towing point, towing force T can be broken into Ty and Tx which can be equated from the other forces mentioned; then at center of gravity, you can sum the moments caused by the lift, drag and towing forces Tx and Ty in order to verify if the velocity of the object is correct to keep the paravane stable.