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So the title of this post caught my interest, as I just graduated aerospace engineering and want to find *some* way to apply my shiny new degree :) Reading this post, I'm realizing how little I remember from the past four years.

I had a couple questions, though. Reading through the posts, I wasn't sure if there was the underlying assumption that either laminar or turbulent flow was better for the tank, or if that point itself was what was being discussed. If one or the other is better, could someone explain why that is? Is it in general better to have suspended particles, particles in the gravel, or do you want those particles filtered out? I assume that you would want the dissolved nutrients evenly distributed throughout the tank.

A couple posts caught my attention. One subtle distinction (not sure if it matters) is that a flow goes turbulent once a flow has achieved a high reynolds number (Re), but the turbulent flow itself doesn't have a high Re. There is no corresponding descriptor for turbulent flow, actually. Also, one thing that wasn't discussed (I don't think...sorry if it was) was the property of turbulent flow to "stick" to a surface better than a laminar flow. It's for this reason that airfoil designers sometimes actually want turbulent flow in some regions of an airfoil. This property might be relevant in considering the flow past leaves. The flow is much more likely to separate from the surface of a leaf if it is laminar. Another property to consider is that turbulent flow is an inherently chaotic process. This means that something like cigarette smoke is not (usually) a turbulent flow in the technical sense, though in the colloquial sense of "turbulent" it might be described as such. With regards to the coral vs plant discussion, if I understand correctly, coral is a colony of tiny polyps. Because they're a colony, they have no central means of distribution like a plant does. Thus, each polyp must have all its nutrients flow past it, whereas a plant uptakes many of its nutrients through its roots (but not CO2, I believe). That would extend the discussion to what would provide good flow through the gravel. Another distinction that just came to mind was turbulent flow in general versus in the boundary layer. The boundary layer (region close to a surface) of a laminar flow is turbulent due to the imperfections of the surface. The thickness of the boundary layer is dependent partially on Re.

Phew...sorry if that wasn't useful, but it felt good to try to remember what I know. I'm about to start my first planted tank after five years of lurking but never having enough time to start a tank. Maybe I'll get to experiment with some flow properties!
 

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@Ekrindul
That was a fascinating read! They focused on phosphate and ammonia - I wonder if their results hold true for trace nutrients.

@digital_gods
I think we're using two different definitions of boundary layer. If I understand it correctly, you're using it to describe the boundary between air and water, whereas I'm using it in the fluid dynamics sense - as the boundary between a fluid and a solid. With air, at least, when you have a flow past a solid surface, the boundary layer is referring to the thin layer of air between the main airstream and the solid surface. The properties of that layer of air can be radically different than the properties of the surrounding air.
 

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Unfortunately, much of the small amount of research there seems to be available on the subject isn't freely available.
Do you have names of articles you'd like to take a look at? I get access to a lot of databases through my university. Please, not too many though (I'm a busy grad student) :D But feel free to send me some article titles that you think would be useful and I'll see if I can post them.

The flow near the surface was of course a pretty turbulent but only a little past half way.
Just wondering, how did you determine that it was turbulent? Did you use food dye or just observe particles in the flow?
 

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I actually read (ok...skimmed) a few interesting articles after my interest was piqued.

Effect of boundary layer transport on the fixation of carbon by the giant kelp Macrocystis pyrifera
I couldn't find this one in the databases, but I'll try again tomorrow. The abstract contained this interesting statement, though:
"Experiments in water tunnels indicate that the boundary layer adjacent to the M. pyrifera blade may be turbulent in water speeds as low as 1 cm sec-1. Photosynthetic output of the blade can be increased by a factor of 300% by increasing water speeds over the blade surface from 0 to 4 cm sec-1. This is consistent with a decrease in the thickness of the boundary layer. Above 4 cm sec-1, the assimilation of carbon was limiting."
Velocity gradients and turbulence around macrophyte stands in streams
http://www.mediafire.com/?a9dvlukzg7czwbn
Turbulence was maintained in the attenuated flow inside the plant canopies, despite estimates of low Reynolds numbers, demonstrating that reliable evaluation of flow patterns requires direct measurements.
You can't trust your intuition as to if it's laminar or turbulent - you have to measure the flows. This article talked about how the plant beds alter the flow. The flows can become more OR less turbulent depending on plant bed characteristics.

Beyond light: physical, geological, and geochemical parameters as possible submersed aquatic vegetation habitat requirements
http://www.mediafire.com/?bzw1tng221vgjag
This had a TON of awesome information. For example:
Production of turbulence within the vegetation is dominated by the plant wake rather than by the bottom boundary shear, as in open channel flows (Nepf et al. 1997).
meaning that in streams and rivers, the turbulence is caused by the shear forces with the channel bed, whereas it was caused by the plants themselves in lakes (where the flow is due to wind driven waves). Also:
The optimal turbulence levels for SAV is yet unknown but it is interesting to note that most SAV beds tend to occur in areas where flow is characterized by the laminar-turbulent transition (Ackerman 1998)
meaning it might be best for the flow to start out laminar but transition to turbulent as it hits the plants! There's a lot more to dig out in this paper, but I don't have time to read the whole thing. Someone please post any more interesting bits if you have a chance to read more.
 
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