Ripple pattern on stainless steel-lined sluice 2

[Lechtal]

Think of a large sluiceway through a dam, lined with stainless steel, and subject to flow-through velocities of 20 m/s and carrying large quantities of sand and gravel. The steel surface after time has taken on a ripple-like surface erosion not unlike a sand dune pattern. Thought to be due to erosion by sand in high-intensity turbulence in the boundary layer, but are there any other thoughts? The ripples are typically about 10 mm crest to crest and a fraction of a millimetre deep, and the erosion is almost like the steel has been "stretched".

 

[ishvaaag]

This means that the turbulence is not as chaotic as one might think. Surely circular movements akin to those present in waves have something to do in carving such features.

 

[ACtrafficengr]

It's probably a bit outdated now, but James Gleick's book "Chaos" has discussions of orderly results arising from turbulent behavior.

 

[ishvaaag]

Really the creation of a set turbulent rolls seems a energetically efficient way of dividing the central core of speedy water flush and the steel surface. The fact of the separations between crests being small seems to indicate to me that the big speeds in the flow affect the bulk of the section, and only tiny rolls commensurate to the distances between crests take (rolling) most of the gradient of velocity between the bulk and 0 at the interface. The rolls being sandy cause the erosion.

The erosive properties of the mix can be evaluated efficiently by taking a mold of the actual erosion and evaluating the steel loss.

 

[cvg]

It would be due to sand, or other erosive particles transported by the water. The only other mechanism would be cavitation, which normally causes pitting. The process is likely caused by the presence of dunes and anti-dunes. Anti-dunes allow erosion to occur between the dunes. This is a fairly well documented phenomena for sediment transport in rivers.

 

[Lechtal]

I can't figure out how to reply to cvg and isvaaag, so I am messaging here as if to myself!
Thanks isvaaag - the turbulent "rolls" idea is what I have conceived is happening, after some surface roughness of scraping or denting was initiated by sand or gravel impacting the surface. Thereafter the turbulent BL, containing 'rolls' within circulations which result from Reynolds stress variations along the surface of the steel.
Thanks cvg. I have discounted cavitation, but thanks for the thought. My experience with cavitation damage on stainless steel is not only microscale pitting but also irregular dents in very severe (high head) conditions. The analogy with antidunes is helpful too - that is just what might be happening with sand grains eddying around in violent motion.

 

[dik]

Could it be caused by cavitation and not wear due to water born particles?