Vanity: Need to Calculate Drag on Sq Plate in 6 mph current

Email | 05-04-2006 | R J Cook

[HKMk23]

D=(0.5)(rho)(v^2)(A)(Cd)

Assume a common Cd of 1.17 for a regular, square, flat plate.

rho=density of water=62.427lb/cu.ft.
v=flow velocity=8.8ft/sec
A=area of plate=1sq.ft.

D=2830 lb-ft/sec^2

-- OR --

Assume a common Cd of 2.1 for a smooth brick.

rho=density of water=62.427lb/cu.ft.
v=flow velocity=8.8ft/sec
A=area of plate=1sq.ft.

D=5080 lb-ft/sec^2

Does that get into the ballpark?

[mission9]

On a related - there is enough energy in the Gulf Stream to meet the new electricity needs of the entire eastern seaboard for years to come. www.energy.gatech.edu/presentations/mhoover.pdf

[Ursus arctos horribilis]

Pressure times area = force.

[Malsua]

Well, I'd just hook a heavy duty scale, like those used to weigh Tarpon's to a cable attached to the plate at the four corners. Toss 'er in, record the result :)

[Robert A Cook PE]

8<)

[Robert A Cook PE]

Passed along your figures.

[RAY]

F = CdxAxV^2xdensity/g

Cd = 1.16 (Mech Engr Handbook square plate)

V 8.8 ft/sec

Density = 62.4 lbs/ft^3

g = 32.2 ft/sec^2

F = lbs

V^2= (ft/sec)^2

D = lbs/ft^3

g = ft/sec^2

A = ft^2

F = 174.081272 lbs

It will also depend upon the temperature and make up of the water, all of which influence viscosity - which influences the flow coefficient)

[processing please hold]

I read this thread just out of curiosity -- now I wish I hadn't. It didn't do anything for my self-esteem realizing that I couldn't understand half of what was written.

So did I. It all looks chinese to me. Must be rewarding to crunch numbers. I envy them. I must remember from here on out...curiosity killed the cat.

[spunkets]

I get ~600lbs(26771N) with C_d=2. Rectangular "pier". A square block is somewhat less, but HK's brick is 2.1. Mine came from shape tables. drag should come out in units of lbs(force).

[patton]

Yep -I hate fluid dynamics. My brain is full. Blow the dang thing up, and the problem goes away. Something about del/phi del/q...this is why one becomes a mechanical engineer. Kill the target.

[spunkets]

check 600lbs. drag = 1/2 C_d*density*A*v²

C_d= 2

density = 1000kg/m³

A= 4ft² = 0.372m²

v= 6mph = 2.68m/s

drag = 1000 * 0.375 * 2.68 *2.68

drag = 2693N = 606 lbs. (0.225 lb/N)

This is for the shape with a rectangular xsection with the face perp to the flow. If it's rotated to place an edge to the flow, the C_d drops to 1.6. For the triangular columns(piers), the C_d is slightly higher with the blunt face in the flow (C_d=2.2). If the 120^o angle apex faces the flow, the C_d drops to 1.7. I think you need to keep the C_d up at ~2.

[spunkets]

Forgot to ping you to the last one. The Reynolds number for your 100ft river 20ft deep is 20K, ~10K at 5ft, so those Cd numbers are good and in a flat range.

[Jeff Chandler]

You guys are giving me a headache.

[Robert A Cook PE]

Headache accepted.

It's an engineering thing, and comes with the territory of analysis, calculation, critique, and determined, focused, explicitly-detailed-point-of-view of seemingly nit-picking technical jargon.

On the other hand, if you don't do things that way, the building (or brdige pier) falls down on those people who don't examine things that way.

[Robert A Cook PE]

Concur with your comment about a Cd of 2.0 .

[Toby06]

Civil Engineers ping

I'm filling in for Fierce Allegiance, let me know if you would like on or off the list.

[Robert A Cook PE]

The help is appreciated.

[Toby06]

Interesting problem.

I do estimating, so I'd say $37,000 and i'll have your answer to 4 decimal places. Precise, not necessarily accurate, though.

[Robert A Cook PE]

!

Contestant: "Alex, I'll try "More Engineering Answers" for $1000.

Reply: And, the answer is, "More Dollars."

Contestant: "Now, what is the question?"

[grey_whiskers]

The answer is 3.14

Naaah, it's 42.

Cheers!