ReefTank Fluid Mechanics

[hesaias]

I have been really thinking about this subject since I started DIYing a Maxi-Stream and hot rodding my SCWD.

Water on a reef flows for the most part with the tides, right? If this is true, then current switchers that do more than this, or in more directions than this, would be unnatrural, correct?

Regarding the Rule of Thumb of 10X tank volume as a good starting point for flow, should we not look at flow rates in velocity, rather than GPM? The 2 are not equal.

All this stems from my looking at pump sizes, piping diameters, head pressures etc so I can get my Mag7 putting out top flow efficently through the SCWD. Turning over a lot of water may not be what we should be after, given that most aquarium pumps have a narrow outlet. I contend that this, as opposed to lower GPM, larger outlets that switch in 2 directions, might be a more natural approach, if the switch happens several hours apart, and there is a lull between the switch.

Anybody got any data on Hydrodynamics on a reef?

 

[hesaias]

I found two links that say that ammonium and phosphate uptake by corals is directly correlated to the velocity of the water!

Here and here

 

[hesaias]

Here is one directly related,

http://www.breedersregistry.org/Articles/v4_i3_riddle/riddle1.htm

 

[hesaias]

Okay
According to this AA article,
http://www.advancedaquarist.com/2006/8/aafeature/view
The best water velocity observed was between 10 abd 15 cm/s, or .32 and .49 ft/s for those of us who are metricly challenged.

The output of my Mag 7 is 700 GPH at 0 head. To achieve the flow rate above, my return line (water to the tank) should be between 2.5 and 3 inches. Since my return line is not that big, but infact 1/2", my water velocity is way to high at the return outlet.

Just as I suspected.

For an MJ 1200, the outlet size to achieve the proper flow should be 2 1/4", but its about 3/8 so the velocity at the outlet is closer to 30 ft/s at the outlet, or about 75 times what the animals need.

 

[hesaias]

Looking a bit deeper, to achieve the proper water velocity at the return outlet, I would need a pump that produced 50GPH with a 3/4" outlet.

All this is understanding that the flow drops off sharp the further away from the outlet. I am gonna address that in a sec.

FYI< I am using the formulas and nomographs in Crane Technical paper 410 for my calculations.

 

[hesaias]

Has anyone looked at setting up the water return parallell to the outlet of the pump? I was thinking if you had the outlets on one side of the tank, and the pump suction on the other side, you could generate a true current across the tank.

Getting it to switch will be the trick.

 

[Bryan89]

I think that you are simplifying things a bit in stating that 10-15 cm/s water velocity is the "best". The article that you reference above specifies very specific coral species in giving that velocity. Also, corals will adapt to their environment by changing shape and growth patterns based on flow and light patterns. Finally, specific species need more or less flow and wild caught colonies would tend to do better in conditions similar to their previous domain. Therefore, I would try to alternate flows to all of the corals rather than attempt one specific flow pattern for all corals. If you caught your own wild colonies, you could optimize their growth by replicating the exact conditions in which you found them. Just my 2 cents.

If you are trying to obtain a turbulent flow pattern, a pair of Tunze Streams with a Tunze Wavebox would give you a turbulent and shifting, random flow pattern.

HTH

Bryan

 

[hesaias]

Thats not really what I am shooting for. Im looking at setting up a current in the tank. This would use the shape of the rock etc in the tank to create the turbulence like it is on the reef in the wild.

The jist of the whole thing is that most of the time, our tanks have much different water dynamics than the reef, to the detriment of our animals. We spend tons of ttime obsessing about light (with good reason, I might add) and just kinda SWAG at flow. A powerhead is so not correct in providing flow in a tank, and I have yet to find a pump that will provide high head discharge pressure with around 50-100GPM, which, from looking at all the data I see, would be more natural than what most of us use.

My name is Scott, I am a pump geek.

 

[Bryan89]

Penductors would provide that tight, high velocity flow. I run four penductors, two at a time, off an actuated 3 way valve and a Blueline 70 pump. About 800 - 1000 gph off the pump with the 5x factor from the penductor means a directional, turbulent and random 5000 gph of flow. I have the penductors setup to oppose one another.

 

[hesaias]

More articles from AA

http://www.advancedaquarist.com/2006/9/aafeature2

Having a better knowledge of the environmental factors that make corals healthier can help aquarists grow corals faster and it might help us to understand the care requirements of still difficult species such as Goniopora and Dendronepthya. More importantly, a better understanding of the synergy between water flow speed and lighting intensity can help aquarists make better informed decisions about the equipment they should employ for their aquariums. Coming back to one of the first things I discussed in this article series, aquarists often put much more effort into considerations for lighting than water flow. As you can see from figure 7, if an aquarists provides very high light intensities but fails to deliver an equivalent water flow speed, instead of benefiting it is more likely that their corals will suffer from photoinhibition stress. The excessive lighting equipment will not yield the desired increase in growth rate but instead it will likely cause an increased heat input to the aquarium, increased algae growth and a considerable waste of money and energy. If you take nothing else from this article series just remember that:
The more light a coral receives, the more flow it will need
figure8.jpg

Figure 8.

Since P. damicornis occurs in a wide range of habitats, I am going to take some liberty to use the results from this research and apply it to what one might consider as an average coral’s basic flow requirements. Figure 8 shows some suggested minimum water flow speeds for optimum coral growth relative to the available lighting intensity. Loosely interpreted, the green line from 0-100 µmol photons·m²·s is roughly the average amount of light one could expect from a minimally lit aquarium with normal output fluorescent or just a few power compact or very high output fluorescent bulbs. The yellow line from 100-400 µmol photons·m²·s would represent an aquarium with moderate intensity lighting provided by mostly power compact or very high output fluorescent bulbs or a few low intensity metal halide bulbs. The red line would represent an aquarium that is really shallow or lit with banks of power compact or very high output fluorescent bulbs, very high intensity metal halides or a combination of all of these. As most of you are aware, lighting intensity within an aquarium can vary greatly depending on the age of the bulb, type of ballast reflector types and the depth of the tank. Therefore, since lighting intensity is highest closer to the light source, another way to interpret the graph would be to provide higher water flow speeds at the top of the tank, moderate flow speeds at the middle of the tank and lower flow speeds at the bottom of the tank. I would like to emphasize that these suggested water flow speed values are minimums which are only meant to be a relative guideline as to how aquarists should match up their aquarium’s lighting equipment with the delivery of appropriate water flow speeds. In next month’s article I will discuss why aquarium hobbyists should move away from gauging the water movement in their aquarium by using "X" amount of turnover, I will introduce some useful principles of fluid dynamics and I will also cover various techniques for producing mass water movement within our coral aquariums.

 

[hesaias]

Bryan
Maybe its me, but Im not sure we are talking about the same thing. "Tight, high velocity flow" is opposite of what I am trying to get. In the ocean, you do not have water velocities of 30ft/s like we use in out tanks. you have steady, low velocity flow. Think of what would happen to a diver on a reef if the water was moving at 30ft/s. Now imagine what would happen to a fish exposed to that.

In the ocean, the turbulence is created by the environment. The stuff in the water makes the flow change. I have some tiny micro bubbles in my tank right now. I can see dead zones, see the high velocity jets from my returns and see the fish avoide some areas of the water because of the velocity of the water.

FYI< right now I have 1 MJ1200 from L to r, 1 MJ 1200 on the Remora, my Ehiem Ecco running carbon and a MAG 7 through a SCWD with 2 returns. This gives me about 1300-1400 GPH in a 58 Gallon reef.

 

[Fredfish]

Water flow on a reef depends in part on the location of the reef. In some cases, flow will be dominated by tide. In other cases, it will be determined by predomiating currents in an area ie: flow only in one direction.

Either way, what you are looking for is mass flow that approximates the laminar flow found on a lot of reefs. Look at the Tunze stream pumps. They move a LOT of water using little power and the flow is more widely distributed. I recently added a tunze nanostream 625 (659gph) to my 40 gallon tank. Now ALL of the water in the tank moves at a reasonable velocity. I am getting good single direction flow across some gorgonians I have just added to the tank.

If you really want to have alternating current like that provided by tides, you can use multiple streams pointing in opposite directions on timers to reverse the current.

Fred

 

[Fredfish]

FWIW, when diving on the GBR I noticed different flows depending on how deep I was diving. On a dive on the lea side of Osprey reef, at 5 meters the flow was in one direction, but pulsed as water was pushed over the reef by waves.

At 12 meters, flow was all in one direction and constant. It didn't really feel like flow at all, but all the water was moving at a constant velocity.

Fred

 

[JGross4]

you must however realize that the flow you are measuring is the flow at the exit of the pump. If you take tunze nano stream 6025 for example (the pump i now use in my tank) is measured at about 600 gph at the exit.

Now one must consider that is the flow rate at the exit, but however the pump is designed to create an outward cone shapped flow pattern, if one were to measure the flow rate say 6 inchs away from the exit, and consider our systems ideal (which they are not) the flow rate should theoretically be the same, however the diameter of the flow will be greatly increased, and therefore the average overall velocity will be greatly reduced. further since our systems are not ideal, the volumetric flow rate would be less, and therefore the velocity would be even less. so as long as you dont have your corals really close to the exit your making great semi-linear flow.

The actual numerical values i do not have (im sure tunze dose though it you contact them), but i bet a short distance away from the exit of the pump it meets you flow criteria. If you were to purchase a tunze like a 6055 with a controller, the flow could be become pulsing, and mimic ocean surface conditions.

As for linear flow, i for one would be concerned dead spots in my tank, such as behind rock structures, i think it would create problems.

Anyway hope this helps, and you understand what i am saying, i have to go to class, ill try to get some data to show the distributions later today.

 

[BeanAnimal]

I think the the point that hesaias is trying to make the point that our fish tanks are like hot tubs and the natural reef is like a river.

Both may have Xgpm of flow over the same volume or area, but one is tubulant from many point sources interacting with each other, the other more laminar from a source that comes from one direction at a time.

Does it make a difference? That could be debated. How do you simulate the flow in a captive reef? You need to move a large volume of water in one direction for an extended period of time.

Several large closed loops operating in the same direction would give you just such a flow pattern. With some creative valving, the flow could be reversed on a tide schedule. With several pumps, the flow could also be varied in intensity according to tide level.

It is a fairly trivial project, but the question becomes one of benefit.

 

[hesaias]

Fred

I think you and I are on the same page. What you said you experienced about 12' down is what i am looking at replicating. Based on teh research that was done for the AA articles, this is what the test subject was exposed to, and what it responded best to.

JGross
Laminar flow cannot be achieved easily in a pipe <3", at least not at the rates we pump water at. A low head, high GPM pump is really opposite of what a tanl needs to achieve as close to laminar flo as possible. You would need say 75 GPH at 50psi with 1-1.5" outlets and returns opposite from each other. This would create a "current" in your tank. Since all the water is moving in the same direction, you would not experience the dead spots anywhere but directly under the outlets.

The turbulent flow around the corals is created by the shape of the corals themselves, not by the water flow. Closer to the surface, the water "pulses" due to the waves at the surface.

I really think that if laminar flow at a low, steady velocity was achieved, like there is on the reef itself, reefkeepers may see much better growth rates, and see a lot less problems like algae bloom etc because the stuff in the water would be kept in suspension until it was filtered out.

 

[MiddletonMark]

IMO, this partly brings up how some flow devices may be better suited than others [such as a MJ1200 vs. a wider stream-type pump].

 

[hesaias]

<a href=showthread.php?s=&postid=8927195#post8927195 target=_blank>Originally posted</a> by BeanAnimal
I think the the point that hesaias is trying to make the point that our fish tanks are like hot tubs and the natural reef is like a river.

Both may have Xgpm of flow over the same volume or area, but one is tubulant from many point sources interacting with each other, the other more laminar from a source that comes from one direction at a time.

Does it make a difference? That could be debated. How do you simulate the flow in a captive reef? You need to move a large volume of water in one direction for an extended period of time.

Several large closed loops operating in the same direction would give you just such a flow pattern. With some creative valving, the flow could be reversed on a tide schedule. With several pumps, the flow could also be varied in intensity according to tide level.

It is a fairly trivial project, but the question becomes one of benefit.

From the stufy done an documented in AA, the benefit is tremendous in terms of coral growth, respiration and nutrient uptake.

I also don't think that it would be that difficult to simulate, but getting a quiet pump that provided enough discharge pressure at a low enough flow (GPH) may be a challenge. The flow need not alternate either I don't think. If you wanted it to, you could do it with 1 pump still, using one inlet and one outlet.

 

[BeanAnimal]

I think you are making the pressure/velocity thing much to complicated. Pressure is not the concern, volume is.

With the proper volume we can diffuse the flow through a plate or many outputs that have a wide dispersion pattern. As long as the intake is on the opposite side of the vessel, then the overall flow volume will move in one direction.

However, you may want to consider using a "lazy river" type of setup where water on one side of the reef structure moves on the opposite direction of the water on the other side. You will not likely have dead spots in the structure due to "short circuiting" of some of the flow. We may know it is a small whirlpool we created, but the reef inhabitants do not. The fish may understand, but non motile creates would simply be stuck in laminar flow on their side of the reef. Those at the top would be caught in the turbulent eddy currents where both flow vectors meet... so you really get the best of both worlds, deep water simulation of directional currents and shallow breakwater zones where turbulence is abundant.

 

[BeanAnimal]

Another great idea would be the dump tank type of setup but on a different type of time interval.

Lets take a 125 gallon display. Above it place a 150 gallon holding tank with enough height to gain substantial head. Place the same size holding tank below.

One end of the 125 is fitted with several large diameter bulkheads and diffusers. The other with an eggrcrate or similar baffle that hides a large over under type of baffle and spillway overflow

Release that 150 gallons of water over 1 minute or so where it will sweep over the length of the tank and then find its way out the baffles and into the holding tank. The setup could trigger every few minutes.

Maybe not an efficient way to pump water, but it would offer a very easy way to produce high volume laminar flow. This is very similar to how a wave pool works.