Hey y'all,
Niko and I had a really stimulating talk the other day about flow regimes in planted aquaria and it got me thinking. His example of laminar flow in the San Marcos in a previous post is awesome and I thought I'd expand on it a little. After keeping planted tanks by rote for so long, having a deeper understanding of the science behind what's going on in my aquaria has helped me immesurably. I thought I'd talk a little bit about what I've learned in the hopes it'll increase your understanding of your aquaria and improve your hobby just as it has mine.
Reynold's number (Re)
http://en.wikipedia.org/wiki/Reynolds_number
The initial statement "...such as
laminar or
turbulent flow: laminar flow occurs at low Reynolds numbers, where viscous forces are dominant, and is characterized by smooth, constant fluid motion, while turbulent flow occurs at high Reynolds numbers and is dominated by inertial forces, which tend to produce chaotic
eddies,
vortices and other flow instabilities." contains the critical element' smooth, constand fluid motion vs. chaotic and other flow instabilities.
Going back to the San Marcos and other plant-filled streams many of us have seen; the flow in such streams is for the most part
laminar. It may not be purely unidirectional and constant throughout the water column; however overall the stream flow isn't
chaotic. Considering channel morphology; such streams are characterized by a general broad U shape. This is important! Although discharge may be high, the open nature of the channel allows for high flow with little impediment. Such flow regimes are beneficial to aquatic vegetation for multiple reasons:
1) Low shear forces- Although shear stress is present in all natural fluid flow situations; eddies and other turblent flow instabilities are areas of high localized shear stresses. We see this in our aquariums when using spray bars. The flow nearest the outlet is very fast relative to the area around it. This differential velocity creates eddies and vortexes in which shear forces can cause plants to lift out of the substrate or which can tear leaves off the stem; thereby damaging the plant.
2) Chaotic flow (high Re) likewise creates zones of increased and decreased water movement. We see this characterized by zones of deposition and resuspension of mulm in our tanks. The zones of high flow receive good input of nutrients and CO2; whereas the zones with low flow may be deficient in one or the other. Likewise, the deposition of mulm can create zones of localized water quality instability and become a source of harmful chemicals.
Consider the placement of filter outputs for a moment. The convention of placing the filter outflow on the side of the aquarium rather than the back aids in the creation of laminar flow.
Why? The increased length of flow helps to disperse the energy related to filter discharge. The farther the stream travels before hitting an impediment such as the side of the tank, the greater the impact viscosity plays in the flow dynamics. Conversely, if I put a spraybar or other outflow on the rear of the tank facing the front, the short distance between the initial outlet and impediment decreases the contribution of viscosity which causes an increase in energy when the flow hits the impediment. The higher the flow energy at point of impediment, the greater the Reynold's number, and the greater the turbulence of flow at the point of impact.
Using the below equation we see the following:
Given a constant viscosity, discharge velocity, and density of water leaving our filters; L, distance to impediment or width of stream flow is the characteristic which defines flow regime. If L is small, Re will be large. If L is large, Re will be small. Remember, we're multiplying L by two constants and dividing by another constant. The greater the pVL term the lower the overall ratio.
What the hell does this mean for my aquarium? In order to create an environment with the greatest potential for laminar flow we must set up a situation where either L is high or V (discharge velocity) is low. To use my aquarium as an example. I have spraybars placed along the substrate surface and tank sides which face the front of the tank (small L). In order to maximize laminar flow potential I have to decrease V. Assuming my pump is always discharging at a constant rate and the viscosity and density of the water leaving the spray bars is constant, I must either increase the size of the holes in the bar, increase the number of points of discharge, or both in order to reduce the Re.
Why would I care about that? Don't I want the highest velocity possible from my spraybar to suspend mulm so it can be picked up by the filter or transport it to a place where I can syphon it out? No!!!!!!! Remember, high V creates turbulent flow which, in turn, creates flow instabilities as discussed above. I want smooth, laminar flow to uniformly distribute water. This uniform flow distribution may be slower than turbulent distribution, but it is more effective in both nutrient distribution and removal of undesireable materials from the water column over the long term. Why?! Because laminar flow reduces differential zones of suspension and deposition. The greater the proportion of zones of suspension to deposition, the greater the volume of particles removed from the water column OVER TIME. This is the big secret and science behind the Lily Pipe and convention of placing the filter outlet on the side, rather than back, of the aquarium. Since stability is, by definition, a characteristic of change over time, one must consider efficacy over time rather than instantaneous or immediate efficacy. Turbulent flow can improve results in the short term, that's why we stir the water in planted areas at times when cleaning our aquaria. The instantaneous turbulence suspends the particles we want to remove. In general, however, this is not a desireable condition.
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