# Measure Energy from Spring Motion (VIV)

1. Sep 22, 2011

### MechaMZ

Hi,

i hope someone can kindly give me some opinions for my idea :)

I've a project related to vortex induced vibration. The prototype has a cylinder submerging into moving fluid, and it generates a upward/ downward motion in cycles.

I want to measure the efficiency of my prototype, which energy can be extracted by my prototype from the moving fluid.

Due to prototype's limitation, it seems hard for me to install a small scale alternator to measure energy output. Thus, i've an idea to measure it by the spring movement. i.e. spring extension and compression.

the idea is:

https://www.physicsforums.com/attachment.php?attachmentid=39181&d=1316715663

there are 4 phases of the spring, i.e.
(2) extended by cylinder and its supports, the cylinder is under equilibrium by spring force, bouncy force and weight.
(3) cylinder is moving downward by exerted force due to vortex, spring extended.
(4) cylinder is moving upward by exerted force due to vortex, spring compressed.

the cylinder is moving up and down continuously. my method is:

(1) let the energy extracted from moving fluid = total energy stored in the spring
(2) total energy stored in the spring = 0.5k(XB)2 + 0.5k(XC)2 - 0.5k(XA)2

here i only look in to one cycle of motion. hope anyone could kindly enlighten me, thank you and appreciated!

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2. Sep 22, 2011

### AlephZero

I think this idea is fundamentally wrong, because on average the spring does not extract any energy from the system. The spring contains some strain energy when it is compresssed, but that energy is returned to the rest of the system (by exerting a force on the cylinder) when the spring returns to its original length.

This is not the same as taking mechanical energy out of the system completely, by comverrting it into electrical energy.

You might be able to use a mechanical damper (e.g. a dashpot) instead of the spring, but you will have to select the damping coefficient so that it dissipates the same amount of energy as your electrical generator would do.