Expert: Mark Janus - 12/21/2009

Question
Hi Mark,

I have a question about fluid mechanics but it's specifically relevant to costal engineering, so if you're not able to help then I completely understand. Anyhow here's my problem:

I've done an experiment in the labs where a wavemaker was used to produce waves of constant frequency in an enclosed channel. Dye was injected at a point. If you're familar with the concept of particles moving in circular orbits in waves, and stokes drift then you wouldn't be surprised that the dye 'drifts' in the motion of the wave propagation. But dye also travelled in the opposite direction of wave propagation (towards the wavemaker) which is really puzzling me.

Do you have any ideas what might be causing this?

Many thanks!

Answer
You are correct in that free surface fluids is not my specialty but I'll through some possibilities out there...   First, I take it that the propagation in the opposite direction is slow(er), also I'm picturing that the wavemaker produces planar waves traveling down the channel...  I would consider looking for any possible reflective surface (or instrument) in the channel... (there may be some low amplitude reflection traveling in the other direction)...

Also, being somewhat ignorant of the subject matter, does the wavemaker produce a pure sinusoidal wave (i.e. symmetric) or not...  does that play a role (i.e. is that key to Stokes drift theory)...

Also, depending on the "speed" of reverse propagation, it make simply be Brownian motion (due to molecular activity)...

Just some thoughts...

mj