Drag coefficient of this contraption?

[beej67]

Civil engineer needing advice - figured here was a reasonable place to get it.

Consider this product:

It's basically a floating trash scooper you anchor into a stream or river and it collects litter. I can calculate the design flow rates and velocities easily, but I need to know the drag force to tell the structural engineer how to design the anchors.

I can get plans of the product, but there's no budget for testing, so I need to come up with a conservative drag coefficient and calculate the drag force on the contraption. I foresee two possible approaches. 1) I make some assumptions about simple geometry and pull Cds from a table, or 2) I jump down the rabbit hole of computational fluid dynamics and try to model the thing in Solid Works or similar. The latter avenue seems burdensome.

Any advice?


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[GregLocock]

Low speed drag is dominated by skin friction drag, so you need to look at your skin friction coefficients. Then double it.

Cheers

Greg Locock


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[beej67]

This will definitely be high speed drag. Velocities over 10 ft/sec, Reynolds numbers well into the fully turbulent flow regime. Remember, I'm a civil engineer, so I'm designing for peak floods, not daily flow. I don't have numbers from the manufacturer yet, but for the sake of argument lets presume Re > 4000.

My initial impression is that it will be almost entirely dominated by form drag, and my inclination is to just calculate the area facing the oncoming flow and treat the thing as a vertical plane, with a Cd around 2. Love to hear other opinions though.

Hydrology, Drainage Analysis, Flood Studies, and Complex Stormwater Litigation for Atlanta and the South East -

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[GregLocock]

Ah sorry I assumed that was a picture of typical conditions.

I'd have thought the mechanical overload drag when it collects a powerboat/tree or whatever would be the limit case.

If you really want to model it then look for a program called Michlet, which calculates the wave froming drag for symmetrical hull shapes.

However, that seems over the top. Cd=2 seems reasonable to me as well.





Cheers

Greg Locock


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[beej67]

Sorry for not being clearer in the beginning. Thanks for the recommendation. I'll give Michlet a look. Seems a little complicated, but probably no worse than HEC-1 or HEC-2.

Hydrology, Drainage Analysis, Flood Studies, and Complex Stormwater Litigation for Atlanta and the South East -

http://www.campbellcivil.com

 

[GregLocock]

It's a lovely program!

I did have another thought, presumably there are charts for calculating the resistance of rough river beds and the like, which given the amount of junk in that thing might not be a bad approximation.

Cheers

Greg Locock


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[samanaPE]

I don't think Michlet would provide reliable results in this particular case. The thin-ship theory implemented in Michlet really only applies to vessels of high length-to-beam and length-to-displacement ratios, and without the large submerged transom.

@GregLocock: I'd also think that the overload case would be the limit case. However much force is required to anchor this thing in a current, it's going to be much higher when some large object comes its way.