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Old 08/17/2007, 05:09 PM   #8
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aninjaatemyshoe's Avatar
Join Date: Mar 2006
Location: Dallas, Texas
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I think you might be confusing laminar flow with low flow. To put it simply, laminar flow is where all of the water is moving steadily at more or less the same rate and in the same direction. This is the general kind of flow that we want. It moves the water so as to assist respiration and waste removal, but it doesn't jet the corals and cause high shear stress that can damage tissue. The opposite end of the spectrum would be turbulent flow where the water is moving in many different directions at varying rates. Turbulent flow is far less energy efficient for our purposes and it typically comes as a result of jetstreams, which is how you would describe the flow coming out of a powerhead for instance. Now when you see corals being violently beaten back and forth by turbulent/jetstream flow, you would tend to think that they are in a tank with a lot more flow than one in which the corals are resting gently. This is not necessarily the case. Just because the flow is high and violent in certain areas does not mean it is high in all areas. Turbulent flow will almost always create dead zones where there is little to no water movement (places for detritus to accumulate and foul the water). As I see it, there are two main reasons why we tend to have tanks with turbulent flow patterns than ones with nice laminar flow. First, good laminar flow is difficult to produce given the equipment we have to work with. Certain things help, such as using prop-pumps instead of the traditional powerheads and using eductors on return outlets. These tend to generate flow that has much less shear stress and won't beat down sessile inverts so much. However, they don't really eliminate the dead zones unless you have a tank that is set up just right for uniform flow, which most tanks are not. Such tanks do exist; they employ what is called a gyre design ( ). This is not how most people want to set up their tanks. The second problem is that we desire random flow, which will help prevent accumulation of waste in specific areas. This is particularly important for SPS corals, which have difficulty eliminating wastes if all sides of them are not at least occasionally met with flow. Thing is, we see turbulent flow and think that it is random, which it is to an extent, but it still leads to dead zones. You really need some sort of switching mechanism that causes flow to go from one pattern to another. I think most of us tend to meet the problem left from turbulent flow by simply having more of it so that it can make up for its inefficiencies. This then leads to a greater likelihood that you'll put too much stress on a coral. At large public aquariums they use more suffisticated methods for generating flow than we have available to us (such as olloid wavemakers). We have to be creative and thoughtful to generate proper flow and not just a high-enough general flow rate.

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