Water flow

I reckon these topic's on water flow are an excellent read. Lot's of good info
http://www.advancedaquarist.com/2006...m=water%20flow
http://www.advancedaquarist.com/2006...m=water%20flow
http://www.advancedaquarist.com/2006...m=water%20flow
http://www.advancedaquarist.com/2006...m=water%20flow
And the most relevant IMO
http://www.advancedaquarist.com/2007...m=water%20flow

I'm working on summarising the above links, plus adding to the topic's with further research. I'm also adding to this with my own experiences. I've really gotten into this topic due to my old tank crashing as a result of water flow issues.

Water flow, why’s it so important to our system? Why do we even need water flow, and how can we best achieve this. In my experience, many people do not appreciate the need for water flow. A lot of reefers think lighting is more important, or that you need a big skimmer, but then skimp on the equipment that truly brings life to our system. I’ve often seen aquarist’s ask, What’s more important? Lighting, skimmer or flow? And the most common answer is they are equal. This is usually a question that is asked when someone is looking to upgrade their system, but sometimes when starting out.Water flow is almost always secondary to other decisions made when assembling a reef set up when in fact, water flow is paramount to the health and vitality of a reef system.

So let’s begin with the reasons why water flow is so important.

Water flow is important for many aspects of coral biology. Water flow determines how efficiently corals capture food, it helps corals rid themselves of metabolic waste and it also determines where corals occur by distributing their spawn and gametes. Most importantly, water flow is critical in driving the mechanisms of respiration and photosynthesis.

Which is more important, light or water flow? How many coral species do you know of that can live without light? How many corals do you know of that can live without flow? Well, there are hundreds of soft and stony coral species that live without light.Tubastrea, Dendrophylia and Dendronepthea . However, there are no corals that can live without water flow. Even in the case of a photosynthetic coral, how long can it live without light? All of us have had power outages, burned out bulbs and ballast failures, and most aquarists would agree that a coral can endure a week without light before its health becomes severely impacted. Even in illuminated aquaria there are reports of people forgetting frags in the sump for months at a time where they seemingly survive on the food that they can trap. But how long can a photosynthetic coral live without flow? The most catastrophic tank crashes are almost always due to a lack of circulation. In the case a photosynthetic coral, it not only has to breathe for itself but it must also support the respiration of the zooxanthellae living within it. Water flow, therefore, is more important to coral health than light, since corals will stress or die much more quickly when flow is inadequate.

Respiration and Photosynthesis

For physiological purposes, the accepted measures for how well a coral (or any other organism) “performs” are usually related to either how much energy the coral consumes or how much energy it produces. Energy is consumed during respiration and energy is produced during photosynthesis. Respiration (R) is the process of combining oxygen (O2) and sugar to produce energy with carbon dioxide (CO2) as a byproduct, whereas photosynthesis (P) is basically the same reaction in reverse: energy and carbon dioxide are combined to form sugars with oxygen as a byproduct. The rates of these processes are driven by the delivery of the input (CO2 for P or O2 for R) and the removal of the output (CO2 for R or O2 for P). Respiration and Photosynthesis can be measured by an increase or decrease of CO2 or O2. Although most people are aware that photosynthesis occurs only in the presence of light, it is important to note that respiration is constantly occurring in all organisms.

The most fundamental of biological processes are photosynthesis and respiration. The response of photosynthesis and respiration to environmental factors such as flow is easily monitored by measuring oxygen or carbon dioxide consumption or production. Additionally, the photosynthetic efficiency can be evaluated by measuring electron transport rate within the photosystems.

An experiment showed that corals maintained under stirred conditions show significantly higher rates of photosynthesis, respiration and calcification than corals maintained in unstirred conditions. Corals maintained under unstirred conditions photosynthesized and respired about 25% less than corals in stirred conditions. The calcification rates of corals in unstirred conditions were also lower than corals in stirred conditions but this reduction was not statistically significant.

Gas Exchange.

Gas exchange, is the delivery and removal of CO2 and O2 which is the sum of the rate of diffusion into and out of an organism. The total amount of gas exchange is dependent on the rates of diffusion and the rates of diffusion are dependent on the availability of moisture, surface area and concentration gradients. Since moisture isn’t a problem, we don’t need to worry about this.

Surface area is controlled mostly by the corals themselves; in the short term corals can change their polyp extension and colony expansion, and in the long term corals can modify their morphology. The only factor left for aquarists to control is the concentration gradient, which is affected by the degree and type of water motion.

Water Flow > Concentration Gradient > Diffusion (Gas Exchange) > rate of R and P

Corals have no specialized respiratory structures. Their external anatomy only features tentacles, a mouth, some tissue in between the polyps and, in the case of soft corals, they also have some pinnules along the sides of the tentacles. None of these are differentiated into specialized respiratory structures even though they have to rely on these anatomical features for gas exchange. If you had to breathe (respire) as a coral breathes, this would be the equivalent of holding your lungs outside of your body, inside out, and just hoping that the wind would blow hard and long enough for you to be able to breathe. This is how corals breathe in their environment and the scenario above illustrates the importance of water flow for adequate gas exchange in corals. This perspective might make you think twice about reducing your aquarium's flow at night.

Surface Area
As mentioned earlier, one of the only things corals can do to alter rates of gas exchange is to change their surface area either through polyp extension or morphology. Having a higher surface area increases the amount of gas exchange which can take place.

Corals can modify to change surface area is their morphology. A coral that is grown in higher water flow, would take twice as long to grow to the same occupied volume but it will have more than twice the mass of the lower flow specimen.
This means, that to look at the coral, it will have a more full and dense appearance, than a coral not in the same conditions. Where as the other coral will look like it has year’s of growing to reach the same result.

Concentration Gradients
Rates of diffusion are determined by concentration gradients which can be manipulated by water motion. A large difference in concentration yields a higher concentration gradient. The effect of concentration gradients on the rate of diffusion is analogous to the effect that the steepness of a slope will have on an object which is moving downhill: the greater the slope the faster the object will move. For the illustration, refer to part 1.

Imagine that there is a high concentration of a solute within the coral. The solute can be gas, nutrients, or minerals. On the left side there is a high concentration of solute in the water surrounding the coral. If the coral is trying to rid itself of a solute, this side has a low concentration gradient because there is not much of a difference in the concentration between the coral and the outside environment. In this scenario it will be more difficult for the coral to rid itself of a particular solute. On the right side, there is a low concentration of solute in the water column surrounding the coral. Since there is a high concentration within the coral, this scenario is an example of a high concentration gradient: there is a large difference between concentrations within and outside the coral. A low to moderate flow environment is where the available water motion is not sufficient to dilute the solute that the coral is trying to release. Whether a coral is absorbing or releasing solutes, greater water motion will always produce a concentration outside the coral which is favourable for creating a high concentration gradient. A high concentration gradient will lead to higher diffusion rates which in turn will support higher rates of respiration and photosynthesis.

Summary
1. Corals are dependent on diffusion for gas and nutrient exchange across their tissue layers.
2. The rate of diffusion is determined by concentration gradients
3. High gradients yield high diffusion rates and
4. The concentration gradient of solutes can be manipulated by water motion

Particle Capture

Depending on the type of corals you wish to keep, you will need good flow to assist in feeding the corals. At the lowest flow speeds, particle capture suffers from unfavourable hydrodynamics and a lack of particle “traffic”. So corals need decent flow not only for waste removal, but to fed. But be careful, some species of corals will not tolerate excessive flow rates, and the polyps will close up as a result. SPS benefit high flow, where softies, you want an intermediate amount of flow. Softies can be kept in higher flow’s if the flow is directed at the coral in question.

Growth Rate
The effect of water flow on growth rate is a multi pronged affair. Growth rate overall is a reflection of how well corals can photosynthesize, catch food, resist bleaching, avoid disease and predation. Growth rates are enhanced by flow due to decreased sedimentation, increased photosynthetic and respiration rates, greater abundance of food “traffic” and more efficient food particle capture. Calcification improves with greater water flow as well.

Photoinhibition, is a precursor to coral bleaching which occurs when corals receive more light than they can handle. When photoinhibition occurs, corals begin to stress and their photosynthesis rate decreases. Photoinhibition is reduced at higher flow speeds but they also showed that at high light intensities and low flow speeds, photodamage of the coral symbionts is amplified which in turn makes corals more sensitive to light and temperature extremes. This can lead to the bleaching of the coral. Higher flow and less stabile temperatures can help the coral build up a resistance to less than optimal conditions. In the event something does vary, the coral has a chance to adapt and not stress as much. So if you planning an upgrade of your lights, you will want to make sure the water flow is increased in your system.

Water flow continued,
The study of how objects are affected by moving fluid is called fluid dynamics. Fluid dynamics may also be used to describe the particles, nutrients and gases which are transported by moving fluids.

Viscosity and Inertia
A fluid can be described by the properties of density, pressure, buoyancy and viscosity. Of these, viscosity is the most significant property governing the behaviour of fluid motion. Viscosity is the resistance of a fluid to a change of shape or a resistance to flow and it can be thought of as fluid friction.

The other important factor governing hydrodynamics is inertia which is the resistance of a body to a change of motion. Whether an object or fluid is stationary or moving, both will tend to keep still or to keep moving. The effect of inertia is solely dependent on mass so if scale is increased then both size, mass and inertia will increase. Therefore, viscous forces are strongest at small scale and inertial forces are strongest at large scales. To a copepod, seawater is very sticky and it will stop moving immediately if it stops swimming. At this scale, viscosity is the dominant force. To a whale, seawater provides very little resistance and it will keep moving for quite a distance if it stops swimming. At this scale, inertia is the dominant force. Since the properties of viscosity and inertia dictate how a fluid motion behaves, the transition from viscosity dominated scale to inertia dominated scale also marks the transition from laminar to turbulent flow.

Turbulent and laminar flow
Turbulence has long been the characteristic which aquarists use to describe the desired water motion for reef aquariums. The trouble with that description is that when circulating seawater in volumes larger than a test tube it is very difficult to produce anything but turbulent flow. With that in mind, all water flow which aquarists produce in their aquaria would be described as directional turbulence or turbulent flow which is mostly laminar.

Turbulent flow is characterized by randomness and it is generally thought of as being rough and chaotic. Laminar or streamline flow is basically the opposite of turbulent flow and it is characterized by an evenness of direction of water movement even though parallel streamlines of flow can be moving in relation to each other. The smooth motion of laminar flow occurs because viscous forces cause parallel streamlines to stick to each other. In this situation the viscous forces of the fluid are dominant over inertial forces. If the velocity of the fluid is increased inertial forces will increase, the sticky effect of viscosity will be dampened and the evenness of the laminar flow will progress into chaotic and turbulent flow.
Likewise, as turbulent water flow approaches a solid surface, viscous forces transmit friction between the water flow and the surface. The friction causes a decrease in velocity so that the flow becomes more laminar as it approaches the surface. As water flow approaches a solid surface, viscous friction increases and the flow velocity continually decreases. The fine micro-layer of water which is in contact with the surface has zero velocity, it does not move and it exhibits a “no-slip” condition. The region above a surface where the characteristics of water flow change in type and speed is called the boundary


Boundary Layers
The boundary layer describes the actual region of interaction between a surface and a fluid. When the boundary layer is defined by the type or speed of the flow it is called a momentum boundary layer. The momentum boundary layer defined by flow type is the region between a surface and the point where flow changes from laminar to turbulent. The momentum boundary layer can also be defined by water flow speed as the region above a surface which ranges from 0% to 99% of mainstream flow. Stated simply it is the region where flow slows down. In some cases the boundary layer may be more turbulent than laminar so the definition of a boundary layer in terms of velocity is preferred.

The consequences of the boundary layer are obvious to anyone who has ever grown out a fragment of stony coral. When first attached, small fragments of coral tend to grow at a moderate rate but once they gain height above the substrate, growth rate tends to increase. We've all had that one fragment which grew at a crawling pace but once it reached a certain size, it projected out of the boundary layer and then grew at break neck speed. Once a coral attains a certain profile, it will experience more turbulent flow, increasing diffusion rates and maximizing the corals respiration and photosynthesis. This phenomenon is well known to the coral farmers who attach coral fragments to projecting bases or suspend them in the water column using string. Although they may not be aware of the boundary layer, these attachment techniques effectively minimize boundary layer effects and it maximizes growth by exposing coral fragments to optimal flow.

Viscosity and inertia are the most important properties of fluid motion. Viscosity is the resistance of liquids to flow and inertia is the resistance of a body to a change of motion. The ratio of inertial to viscous forces is called the Reynolds’s number (Re). At small scales (Re<1>1), the effect of inertia is more important. The motion of a moving fluid can be described as being turbulent or laminar. Turbulent flow is characterized as uneven in velocity and direction whereas laminar flow can be defined as being non-turbulent. Laminar flow is characterized as having even, parallel streamlines of motion. At lower velocities, water flow tends to be laminar and at higher velocities, water flow tends to become turbulent. Since fluid velocity is included in the calculation of the Reynolds’s number, the value of Re can predict the transition from laminar to turbulent flow. Fluid motion can break down into smaller cells of turbulent flow called eddies. If the formation of eddies occurs from the contact of fluid motion with an object, the eddies will produce a pulling force called drag. If laminar flow is redirected by a surface, the surface will experience a perpendicular force called lift.
Ok, so that’s getting a bit technical, but now we can move on to how to create the ideal flow’s to suit our system.

Describing the ideal flow
For lack of a better value, aquarists describe the amount of water motion in their reef aquariums in terms of turnover rate. If identical power heads are placed at opposite ends of an aquarium but one power head is facing the centre of the aquarium and the other power head is facing the aquarium glass, the power head which is directed towards the centre of the aquarium will undoubtedly produce faster flow speeds and more circulation throughout the aquarium. Although the power heads both have the same turnover rate their orientation to the main water mass has a great effect on the flow speeds they will produce. Since flow speed is the critical measure for determining the rate of gas exchange, turnover does little to convey how fast a coral will respire and photosynthesize

In the natural environment, the reef surface and the corals which live on it experience mostly random, chaotic flow in the form of oscillatory surge. In most cases, aquarists interpret “random, chaotic flow” to exclude laminar or unidirectional flow.

Although reef aquariums are an attempt at recreating a natural environment, trying to reproduce surge with the scale and energy of the natural environment would take tremendous effort and resources. Whereas the oscillatory surge of the natural environment entails movement of the entire water mass, the typical aquarium features small plumes of water movement which lose velocity and momentum with distance away from the source of water flow (Harker 1998). Water flow which is exiting a power head or other outlet begins as high speed, unidirectional flow. However, as the flow increases distance from its source, resistance from other flows and viscous friction cause the orderly flow to quickly lose momentum. At this point the flow loses velocity and it increasingly becomes multidirectional, turbulent flow.

It is true that turbulence leads to an increased rate of mixing, but fast laminar flow will become turbulent as soon as it encounters an irregular surface such as that of a coral. The faster the flow speed, the greater the amount of turbulence produced when the flow encounters a surface. Although turbulence is the desired end product of water movement, aquarists should be more focused on producing faster unidirectional flow.

The Flow Environment
The environment in which fluid movement occurs has a great effect on how the fluid will behave. The three things that will impact water movement in an aquarium are the dimensions of the aquarium, the relief of the live rock reef structure including the corals, and the force and duration of the water motion

When considering an aquarium for use as a reef tank, it is important to remember that the dimensions of the reef tank will have a great influence on what kind of reef it will be. The size and shape of the tank will determine the type of lighting to be used, how maintenance will be performed, the type of fish and corals it can hold and how water flow will behave within the glass.

In a larger aquarium, it will take more force to move the entire water mass and it will take longer for the entire volume to circulate. However, once the entire water mass of a larger aquarium is moving, it will have more inertia and it will be less impeded by the reef structure or corals which project into mainstream flow. It is easier to produce a variety of flow regimes in larger tanks. Since a larger tank will be governed more by kinetic than viscous forces it is more likely to feature a narrow band of faster flow at the surface and a broader band of slower flow at the bottom.

In the ocean, surface currents of water are driven mostly by wind blowing across the surface of the sea. The amount of water moving in those currents is proportional to the force of the wind and the duration for which it blows, which is called the fetch. Since aquarists do not use wind to move water, for our aquariums we can think of the fetch as the duration that a mass of water is pushed in a particular direction.


Commercially available “wave makers” are not designed or constructed on anything more than the status quo of the coral hobby which is that corals and reef aquariums need “random, turbulent flow.” Apart from their high price, the biggest complaint about these pump controlling devices is that their outlets are switched on and off with such short intervals that they do not allow for an optimized fetch of water flow. By turning off a pump before it has had a chance to reach its full water movement potential, a water pump in this scenario essentially sends out a plume of water movement which encounters a lot of resistance from the inertia of the water volume

By increasing the duration that a water pump is turned on, the moving parcel of water will gain size and momentum so that when the pump is turned off, the water volume should continue to move through the aquarium for a short time. The capacity for wave makers to produce mass water movement can be ameliorated by increasing the timing interval between pumps and designing pump circuits which work together to move the entire volume of the aquarium.

The final consideration for the flow environment is the placement of invertebrates in regions of the aquarium which combine suitable lighting intensities and water flow speeds. Since the upper region of the aquarium is often the preferred placement for high light corals, it is doubly advantageous to concentrate the fast water movement in the upper layers of the aquarium. Be mindful that when water flows around a shape, there is usually more turbulence and therefore more gas exchange on the downstream face of the shape. If you are looking at the upstream surface of a coral for indications of the coral’s behavioural response to water flow, you could be missing the more significant response on the downstream face of a coral. The bottom line is that you should do a colony wide inspection of your corals for indications of the suitability of the flow to which the coral is exposed.

Mass Water Movement

In order to maximize the output of water flow equipment, aquarists should design water movement systems so that all the components work together to minimize resistance and move the entire water mass of the aquarium. The best way to combine the energy of moving water to produce maximum water motion for an aquarium is to encourage the formation of a circular course of water movement called a gyre. Like the wheel, a gyre takes advantage of feedback mechanisms which preserve momentum by minimizing resistance. An aquarium gyre somewhat resembles a conveyor belt of water movement and it is characterized by mostly laminar, unidirectional flow. By alternating the rotation of the gyre from one side to the other, it is possible to evenly distribute turbulence on all sides of corals and therefore increase photosynthesis and respiration.

Gyres in Reef Aquariums
An aquarium does not necessarily need a divider to produce gyres of the water mass. Although the water movement will not be as complete and uniform as it is with a gyre tank, it is still advantageous to encourage water movement to follow a circuitous path. In a reef aquarium with live rock and coral on the bottom, the water surface of the aquarium provides the least resistance to moving water. Because of the lack of friction, moving water which is directed in this region will produce the most momentum of the water mass. If there is an even transport of the surface water from one side of the aquarium to the other, the entire water mass should begin to gain momentum as it is moved at both ends. At one end of the aquarium, the water will begin to “pile up” and then sink down. At the other end, water will rise up to replace the volume which is displaced by the water motion. Although it is easiest to create gyres which follow the top and bottom surfaces of the aquarium, this is not the only way to create gyres.

A Chauvet light timer can be used to alternate the flow between two circuits of power heads. Each circuit contains pumps which are diagonal to each other and in this fashion the force of both pumps were working together to move the entire water mass. The power heads then are run for a period of time, could be up to an hour or more, then the timer switches to the other power heads, which turn flow in the opposite direction.


Not only can mass water movement techniques help aquarists produce higher water flow speeds in the aquarium but it can also encourage more water movement through the interstices of live rock and corals with open growth forms. Normally an aquarist might target one or more plumes of water movement at corals which require fast water flow speeds. In this scenario, the turbulent water flow plume encounters a lot of friction on its way to the desired location of the reef aquarium: it will experience resistance from the still water around it, it will experience drag from the shape of the corals it encounters and the turbulent nature of the water flow plume will do little to preserve the momentum of the water movement.

In a scenario with the employment of mass water movement, the behavior of the fluid will be much different. The plume of water motion from the same source will encounter less resistance from the water around it since both parcels of water are moving in the same direction. The decreased resistance will straighten out the flow and preserve more momentum. Not only will the water be moving faster once it reaches a coral, since water is moving away from the coral on the downstream end, water will be forced through the normally stagnant water which is present at the interior of corals with open growth forms.

When using mass water movement techniques in cases of dense coral growth, water flow speed can actually accelerate as more water is pushed through spaces with a smaller area. Aquarists who wish to encourage additional water movement at the inside of dense coral colonies will see great benefits from using mass water movement techniques.

Great post Blacjack

I didn't read all of it yet, but great info. Thank you for good reading blacjack

I've read that a while ago, and I re-did my flow pattern in response to it.

very informative!

Well done

Interesting infomation for sure

Seen this on another forum as well

Thanks for the feed back. I've taken points of usefull info from various articles, but left out the really scientific stuff, so it's easy to read and understand.

Good to see mate It is a good read !

A lot of really good information there...thanks for sharing...maybe we could format this later for our wikipedia?!

not reefwiki?

something like that Paul

I just got around to reading this post. Very good information. Thank you blacjack.

Cheers mate.

Great read blacjack

Thanks for the info!!

No worries. I just like the idea of changing the old way's of thinking. The more I read, the more I'm learning we've done things based on best guesses instead of actual facts. I spend alot of time reading as much as I can about all aspects of our hobby to learn more. I need 2 lives to keep up

I agree BJ. Its funny how much stuff is passed on as fact when all it is is something someone said and has stuck. It just keeps getting repeated over and over again.