Force due to Velocity of Water

Question

Force due to Velocity of Water

AN1 (Structural)

(OP)

9 May 06 12:14

I have a question regarding the generally accepted practice of calculating forces on a stationary structural element due to water moving past it. Essentially, I have a cantilevered pipe affixed to a stationary block. The pipe will have instruments attached to it. I need to design the pipe and the connection of the pipe to the block.

Ussuri (Civil/Environmental) · Answer 1 · 2006-05-10 03:54:28

Very generally, wave forces on structures can be calculated using Morisons equation.  Morisons equation states that the total force is given by the sum of the drag forces and inertia forces.

There are two types of drag force, Form drag which is dependent on shape and skin friction drag.  Drag force is gernated as a result of the viscous action of the fluid.

The inertia force represents the reluctance of the water particle to be accelerated.  This covers the F=ma part and the 'added mass' force.

The 'added mass' is a concept used to describe the force required to displace the water.  To move an object it must be given kinetic energy but in addition to this the fulid displaced must be given kinnetic energy also.  So work done in moving the object is greater than that of moving the object alone.  It is as if the object has been given some 'added mass'

If you talking about offshore applications (and I assume you are because you posted in this forum) you will need to calculated the wave particle velocities and accelerations.  This is done using wave theory.  There are quite a few of them so it is important to use the correct one.  You may also have additional velocities from current or tidal action.

You could have a look at the links below for guidance.

http://exchange.dnv.com/ExchangeMenu/TaskManager.asp?WCI=MenuArea&WCE=PMA_6_812!816&uid=ID20060510348198915926

http://www.standard.no/imaker.exe?id=537

rconner (Civil/Environmental) · Answer 2 · 2006-05-10 10:59:29

The basic concepts of "fluid drag" are of course good, but in some applications I guess it should be remembered that if the "moving water" happens to contain a large solid object (e.g. a tree or even a runaway barge) that comes to bear on the pipe, the demands on the strength of the pipe material and its anchorage can go way up in the way of impact and increased "drag" on that object as well as the pipe!! For perhaps this reason and others I think designers/regulators etc. generally like to keep pipes out of the flow path when they have any other choice (I suspect however you probably need to have this pipe in some flow for some pretty good reasons!)

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Force due to Velocity of Water