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Advanced flow principles and recreating the hydrodynamics of the reef environment By Simon@Reef-Eden

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#1 jason@jasonsaquatics


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Posted 06 November 2010 - 03:51 PM

Advanced flow principles and recreating the hydrodynamics of the reef environment

Over the last decade, there have been few aspects of modern reef-keeping ‘apart from chemistry’ that have received as much attention as ‘hydrodynamics’. Or more simply put, the action of water as it flows around the aquarium, and the various methods we employ to produce it within the confines of our systems. From the smallest Nano-reef to the largest public aquariums, we frequently hear recommendations about ‘X’ times flow per hour, wave simulation, tidal simulation, surge control. And a whole host of other fanciful terms to relay what we are trying to achieve. Frequently though, it is all too easy to get wrapped up in the various recommendations banded about, so much so, that we loose sight of what the original goal was in the first place. Which was to create as accurate a representation of wild conditions as we can feasibly apply, in our rather limited volumes of water?

When we look at coral reefs in the wild, we already accept that they are supremely diverse and dynamic environments. If we look closely, we may even consider that no two reefs or even ‘sections’ of a reef are even remotely the same, with each square meter seemingly infinitely different to its neighbour. But to understand reefs in their entirety, and to take on board both the major, and minor factors that determine what breaths life into a coral reef, we need to step back a little from that macroscopic view we become so accustomed to, whilst looking through the front panes of our relatively diminutive salty puddles, or at still images on the internet.

Although loosely related, we will steer clear of hard core chemistry issues over the course of this article and stick to the subject at hand. Suffice to say that as you will no doubt be aware already, circulation’ is essential for all marine life to one degree or another, and most definitely to sedentary organisms that rely on the prevailing currents to not only bring food, but also replenish, and remove respiratory gases, mucous secretions, and provide a constant stream of other chemicals essential for growth and reproduction. Additionally, these organisms rely almost totally on the prevailing currents to spread both sperm and eggs at times of reproduction, so that any given species may thrive and populate new areas.

So what is ‘current’, and what does the term ‘Hydrodynamics’ really mean. Well simply put. In aquatic terms, ‘current’ is the movement of water from one location to another. Hydrodynamics is the ‘nature’ of that movement, be it on a large scale, miniature, fast, slow, constant or pulsing in form.

One of the most common quotes in the marine hobby is that all too readily banded theory, that flow within a reef aquarium should be as random as possible. Remember our aim is always to replicate as feasibly as possible ‘natural’ conditions, so that our animals will both behave and grow as they would in the wild. As with most things in nature, there is usually some semblance of order underneath that chaotic surface layer we are so used to seeing. Therefore, our logical starting point is to look at natural coral reefs.

The major players.

First of all, let’s step back a little, no further, much further…right, that’s better. At this point, you should now be looking at the earth from quite a high altitude, if not from space. What do we see? well a lot of water for one, but look at the globe with different eyes and we see some surprising actions going on across the surface of its vast oceans. Satellite thermal imagery of our oceans combined with a worldwide network of persistently monitoring research buoys, shows us that whilst varying slightly in velocity dependant on the oceans vast weather systems that help drive them, predominant surface currents, actually follow pretty consistent and predictable paths across the globe 24hrs a day, seven days a week, 365 days a year, and have done so for millennia only occasionally shifting path in the event of a global climate change.

This consistency also includes tropical regions around the equatorial zone, that are home to the vast majority of the worlds coral formations. If we look at key areas, we can see that there are currents running across all these regions that flow consistently from a given direction all year round. The main factor in creating these surface currents is heat energy from the sun warming the upper water layers of all the main ocean basins called ‘gyres’, plus assistance from wind friction as it passes across the surface. Likewise, these winds are also generated by convectional forces born of our suns radiated heat. As an example of these perpetual currents. The Great Barrier Reef receives its main water flow from its northern end, travelling down its length to its southern reaches. The Maldives receives its water from the south, flowing upwards towards the north east and then away to the west in a huge arc. And Bali has two opposing currents, with water being pushed downwards between Borneo and Sumatra, then down through the Java Sea in a south easterly direction across its northern shores, whilst getting a somewhat opposing flow from the Timor Sea across its southern shores which runs east to west. In fact nearly all reefs have a ‘predominant’ and consistent flow that runs ‘along’ the length, rather than strait towards the reef front. This is largely to do with the fact that surface currents intercepting with land masses have little choice but to be deflected along the path of least resistance, i.e. along the shoreline and away from the trailing pressure front. This is much like when water from a powerhead hits the front pane of your tank, and then travels across it, pushed by more water from behind. If we were to talk about volumes, then we are talking not just millions, but ‘billions’ of gallons of water passing any given area of any chosen reef every hour of every day, of every year. Putting that into perspective, it’s like doing a 100% water change on a 6x2x2 reef tank, every 10-30 seconds. That’s a lot of water movement.

So what does this mean to you, the reef-keeper? Well, simply put, it means that contrary to what we may believe from observing wave action at the surface. Most coral reefs do indeed have an ambient ‘underlying’ flow direction that is largely unchanging in nature.

When we look at waves crashing against a shoreline we imagine there is nothing more than chaos at play. But much like any open body of water, what’s happening at the surface, may only bare a minimal resemblance to what’s happening a meter below. Despite the fact that winds may change direction and velocity from one day to the next causing varying wave patterns and forces at the surface, the underlying current direction caused by large scale oceanic convection, will remain pretty much the same, day in day out, year in, year out. This can easily be seen if snorkelling off the front of a reef crest in calm weather. Despite the lack of wave force, you will still get pulled steadily along in the underlying current, as it pushes its way along the front of the reef slope. Dependant on the surrounding geography which may include neighbouring islands or landmasses, this flow may be sedate, or compressed into moving faster. We give the term ‘laminar flow’ to this type of motion. In essence we are talking the smooth and consistent motion of fluid regardless of its velocity, in a constant direction and at a reasonably constant speed. Having little if any turbulence.

Less constant but equally impressive in force and scale, we then get to ‘tidal’ forces, i.e. those generated by the Luna cycles and gravitational pull, which causes the mass movement of water towards the side of the earth nearest the moon. And at the same time an equal shift to the apposing side of the planet. Depending on the phase of the moon, these forces and the subsequent effects of tidal shift can vary from place to place. Added to this is bathymetry (the contour of the land masses and surrounding ocean bed) which means that tidal depth change, or ‘surge’, varies from one location to another even within a fairly short distance of a few hundred miles. For example, a shoreline that has a long, gently sloping contour will have a more aggressive surge, than one that has a steeply angled contour such as an atoll. Even the ‘number’ of tidal shifts within a 24 hour period can vary according to location, with some having two high tides, whilst others only have one. In relation to reefs, this action has tremendous influence as massive volumes of water are shifted in and across the reef crest to the reef flat behind during the incoming tide, and visa versa as the tide turns and withdraws. Many a snorkelling reef keeper has noticed its far better to go snorkelling during the low tide phase on fringing reefs with a back lagoon, than it is during the incoming tide phase, because sediment gets stirred up with water rushing into the lagoon area causing reduced visibility. Once the tide has reduced to the degree the reef crest has formed a near surface wave barrier again, visibility returns to normal as the flow of water subsides, and sediment falls back out of solution in the more relaxed conditions between phases. Any suspended sediment that makes its way back out to the front of the reef with the outgoing tide, is quickly washed away and down the coast with the long shore currents, leaving the reef clear again. Unlike long shore oceanic currents, Tidal Surge is on the whole a ‘direct’ incoming volume of water that runs at 90deg to the reef front travelling towards, up and over to pass into the Lagoonal areas behind. The volumes of water moved by this force can be tremendous. With millions of gallons of sea water shifting through and across the reef over anything from 12 down to 6 hour intervals. From a corals point of view, this type of water motion has a similar effect to laminar flow. The only difference being that in this case, the flow periodically reverses on itself by 180deg to flow in the opposite direction as the tide turns with a short period of turbidity in between each cycle.

Our third and most complex force is wave action, which is a science in itself by rights. To understand waves, we must first break the myth in that a wave is a ‘moving’ body of water. It isn’t. It’s actually a moving pulse of stored ‘energy’ travelling through the upper layer of water. Its strength and effect are directly proportional to the forces that generate it, which in the oceans case, is predominantly wind. When air passes across the surface of water, the friction (drag) generated, transfers energy to the surface of the water causing water particles to move forward in compliment to the frictional force. As energy grows, these particles start to move in a circular motion as particles behind are pushed ‘up and over’ them. With increasing energy input (wind) the amount of friction generated, grows to a point where a wave is slowly formed. It is this rolling body of energy that we see travelling across the surface as a wave. Equally, the temperature of air passing across the- surface will have a greater or lesser effect. The cooler it is, the greater the effect as cold air is heavier than warm with a resulting rise in surface friction.

Diagram 1: shows how water particles move under influence of this energy wave, as it passes overhead.


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A particle motion in an ocean wave.

[Soft Break]A=In deep water.[Soft Break]B=In shallow water. The circular movement of a surface particle becomes elliptical with increasing forward momentum as the pressure wave is compressed.[Soft Break]1= Progression of wave[Soft Break]2= Crest[Soft Break]3=Trough

As the wave passes, water particles are moved in a circular motion on a vertical plane of axis. The amount of motion is directly proportional to the distance the particle is away from the wave itself. With greater depth, the influence decreases to the point no effect is felt. Equally, it should be understood that a particles motion at the surface, doesn’t follow the same wave. Its forward momentum is an accumulation of several circular motions as it is transferred through that orbit from one wave crest, down to the trough and back up to the next wave crest. In essence, the forward motion of any given particle is far less than that of the visible wave. It is only in shallow water (B) that we see an increase in the velocity and forward momentum of a given particle, as the pressure wave is compressed into an elliptical orbit against the rising substrate. At this point the trough speed and width is reduced by drag, whilst the crest carries on at the same speed, eventually the wave ‘breaks’ and forms its typical white crest as it falls into the leading trough.

This is an important factor for us to consider. Because unless you are keeping corals and animals that are normally found right at the top of the reef ‘on the crest’ then chances are, that water or ‘particle momentum’ created by wave action at greater depth, may be far, far less than we would at first assume when looking at the images of waves passing over the top of a reef. The areas we commonly identify as the reef-crest which shows the most turbulent wave action, can in fact be some distance behind the more tranquil areas where many of our reef inhabitants actually come from. I.e. the reef slope proper, which is commonly, inhabited by branching Acropora’s, plating corals such as Montipora’s and Millipora’s, and a few LPS. It’s in this 2-10 meter range that we commonly find our most regularly imported species, leaving the more robust encrusting and short branching corals such as A.Humillis and Porites fighting for existence in the high energy and violent environment of the upper reef crest.

When we look at most modern methods of creating wave action in reef aquaria. We are limited by the fact, that unlike the ocean, we only have a finite span of water available to generate a wave before it hits the other end of the aquarium and rebounds back, to offset the following wave. We also have an inherent problem with depth dissipation. What we commonly end up with, instead of the circular ‘rolling’ particle motion, is nothing more than a simple forward and backwards equal motion of the particles in a horizontal plane. This is much like the ‘in and out’ motion of tidal surge, but on a super condensed time scale of just a few seconds, rather than hours in either direction. If we were to tie together both wave action, and tidal surge, we end up with a pattern of steps that can easily be observed again by snorkeling in the ocean. When the tide is coming in, Wave action will give water particles their usual circular trajectory moving forwards bit by bit with each wave. But joining this motion is the incoming tide which adds a further push, forwards and across the reef.. In effect, we get two steps forward, one step backwards motion. As the tide turns, the effect is reversed somewhat to a two steps backwards, one step forward motion as the forward trajectory of the water particle is offset slightly by the receding tide. Although both of these ‘step’ systems are variable, dependant on the force and size of the wave action, they still serve as a good benchmark to set flow patterns by and demonstrate the inaccuracies of many modern wave simulation methodologies..

Our final type of current is ‘random’ or ‘turbulent’ flow, which is generated by a combination of the above 3 major forces, and its interaction with the irregular structure of the reef itself. As water is forced by either wave, tidal or oceanic current towards and along the reef, it will try to take the path of least resistance, that being either around, or over. The irregular contour of the reef structure ‘including the attached coral’s creates a drag zone ‘around’ and just over its surface, much like the dimples of a golf ball, but on a much larger scale. Following water flow is then pushed up and around this layer to pass by unhindered at its original rate. We call this layer the ‘bottom boundary layer’. Not to be confused with the boundary layer we commonly associate when talking at a microscopic level. This layer can be from just a few inches thick, to several feet deep in the case of a very convoluted and rough reef structure. It’s the reef structure and corals themselves in many cases, that cause turbulent flow patterns as water passes by, forming both pressure and velocity increases at the leading edge of the coral and its sides, and turbulent suction vortices behind, similar to the drag induced vortices behind a car in a wind tunnel. These vortices which move around the coral as the tide turns, are what help in gaseous exchange, slime removal, and the clearing of detrimental accumulations of loose sediment that would otherwise smoother the corals quite quickly. When it comes to replicating this scenario within the confines of an aquarium, life couldn’t be easier. The moment we move water around the tank, these areas of turbulent ‘deflected’ flow are naturally created for us by the corals themselves, dependant on the velocity of water as it passes across and around the corals. Diagram 2 shows this feature in action.

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1.Wave driven water particle trajectory.

2.Boundary layer limit

3.Turbulent zone / vortices caused by drag

In the event of a changing tide. The flow reverses, causing a shift in turbulence zones around, and to the opposite side of the corals as water passes in the opposite direction. Although wave action will still induce rotational particle motion towards the reef, the underlying flow will be determined by the volume of water exiting the area with the falling tide.

NB: Not accounted for in this image, is lateral long-shore oceanic drift.

When we combine these four principles of water movement, we have what forms the basis of understanding and ground rules for what we are trying to achieve within the confines of our reef aquaria. In the second part of this feature, we will be covering the methods and technology available to the reef aquarist, and how to put those principles into practice, avoiding that age old X times flow trap.


National Oceanic and Atmospheric Administration (NOAA). Dr. Stephen Monismith.

Naval Research Laboratory (NRL) Function and particle motion within a wave.

#2 jason@jasonsaquatics


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Posted 06 November 2010 - 04:26 PM

Article 2

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In the last installment, we ran through the causes, scope, and nature of flow patterns across wild reefs. Namely, an underlying and constant large scale flow in any given direction we call ‘oceanic current’ or ‘laminar flow’ We then had durational changes in direction, brought about by ‘tidal surge’ coming in either 6 or 12 hour cycles which oppose or combine with wave or oceanic current dependant on the geography. We then had ‘wave surge’ causing a horizontal rolling motion of the water particles as the wave pressure passes, with decreasing effect as we move deeper and deeper. We finally had ‘random flow’ generated by all the above factors interacting with the irregular physical structure of the reef itself, and the resident corals.

In this edition, we will run through some of the methods and technology employed to bring these patterns into the reef aquarium. But before we do, we need to cover one more factor that commonly causes a lot of confusion. That being ‘Flow rate’ or ‘velocity’, velocity being the distance any water particle moves within a given amount of time, in any single direction.

For many years now it has been standard practice to talk about flow within our aquariums by quoting the number of times the volume of the display aquarium, passes through circulation devices. We have commonly called this ‘X time’s turnover’. In its simplest form, this is little more than the combined liter per hour output of all circulation devices, divided by the display aquariums volume. We don’t include sumps etc, as we are only interested in the circulation within the actual tank where the corals and fish are kept.

Over recent years it has become an increasing trend, whereby extremely high turnover rates are sought as though there is some kind of competition to see who can get the highest flow rate, or that a higher figure will give better results in all cases. In essence many believe that 60 x turnover is better than 30 x turnover. Unfortunately, this couldn’t be farther from the truth for a great many situations, and more importantly, many of the organisms we keep.

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One factor that turnover figures don’t take into account is the layout of the rockwork, or the physical shape of the tank itself. To put that into perspective, a tank that has a central rockwork area, or ‘ridge’, will have a significantly faster flow of water between the glass sides of the tank and that rock, compared to the same tank with a similar amount of rock placed against the back glass, because flow compression causes water to speed up as it builds momentum traveling ‘around’ the central rock feature, compared to dispersing more evenly in the larger ‘open water’ volume of the rear rock wall display type display, Likewise, if there is a very varied and random assortment of rock breaking up the flow from circulation devices before it’s had a chance to build a fluid momentum, then we will lose ‘overall’ flow velocity due to the random frictional forces generated by diverting the flow over and around these larger obstacles. It is important to remember though that whilst a high flow rate may be advantageous in certain areas of the tank, it may actually ‘inhibit’ the growth and spread of some of the many benthic organisms we wish to encourage to grow in more secluded areas. Quite often these organisms can form a valuable and interesting addition to the biodiversity within the reef tank, and serve a key roll how the system controls free waste levels.

A way round this dilemma yet again, is to look at wild reefs and follow their lead. Studies of various reefs, from shallow ‘sloping’ types, to steeply walled atoll type reefs such as the Maldives, have shown surprisingly similar trends, with average water flow velocities in the regions of between 5-25cm/sec in the depth ranges 2-25m and within the ranges 0 to 3.5m of the effective bottom boundary layer which is the area that is most important to us as reef keepers. Although flow velocity above the bottom boundary layer may be significantly higher, it is of little consequence to the environment we are trying to create in the tank.

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In most instances, flow decreases with depth until we are only left with tidal drift and the change over zone between surface oceanic currents, and deep oceanic current. This would collaborate with the wave motion principle where the circular trajectory of particles is compressed into a more oval path with extended lateral travel per revolution in shallower depths which adds to the core water movement. Equally, the combined effect of tidal current and long shore oceanic current are being compressed onto the reef face and upwards over its top causing a further increases in velocity. The important fact here though, is that ‘range’ of velocity i.e. the 5-20cm/sec velocity range. Why is it so important? Well simply put, it’s because the vast majority of photosynthetic corals that we keep have become adapted over millions of years to live and thrive within these ranges. Whilst occasional storms or freak surges may cause large increases in the upper margins of this range, by and large, these are the accepted norms to which our corals and fish are adapted. Too much velocity constantly above 25cm/sec and it becomes not only harder for coral polyps to catch passing food. But it’s also more likely that damage will occur to the delicate tissues as they rub against the coral skeleton beneath, or polyps may refuse to expand properly at all. This is commonly seen if a powerhead is placed too near a coral, causing tissue recession or stripping and poor expansion in LPS corals such as Lobophyllia or Euphyllia. Conversely, too ‘slow’ flow will reduce the corals ability to exchange gases and nutrients into and from the water column effectively, as well as catch sufficient food.

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It may also cause the buildup of excessive amounts of settled detritus within the folds and branches of a coral, leading to bacterial infection and possible demise.

As with many things within our hobby though, more is not always necessarily better in all cases. There have been studies that confirm a higher degree of growth in certain SPS species under increasing flow velocities, however these studies also tied in with the previously and already agreed acceptable ranges of between 5–20cm/sec.

So how do we go about ensuring we are within the acceptable ranges? Well, to go through each of the coral groups in this article would be virtually impossible, so in this instance we will steer clear of coral ‘morphology’ till a later date and agree, that we should always research where our corals have originated from, and the type of conditions they are adapted to ‘prior’ to purchase, so that we can ensure their chances as much as possible once in the home reef. To find out whether our aquarium is within this 5-25cm/sec velocity range, we can perform a simple test whereby we release a small amount of food into the water column in various places to measure how far it travels over a 2 second period. Simply divide the distance by two, and you have your flow rate for that particular area. Remember though that randomization of flow around corals is perfectly normal and desirable to a certain degree so we are more interested in the flow in the main water column and over corals, rather than through, but we certainly don’t want the coral being battered from all directions as this isn’t very natural at all. We can also use this test, to check the minimum distances we should have our corals from any pump outlets or powerheads. A surprising fact that we will commonly encounter, is that flow rates within a relatively short distance of a powerhead or pump outlet can be greatly in excess of this range, so care should always be taken not only when placing powerheads or designing the outlets for closed loop type systems, but also in the placing of corals, and the areas into which they may grow. Fundamentally we should never place a coral directly in the path of a powerhead or outlet as serious damage may occur to the colony within a very short period of time.

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For many people, putting all that information into practice is one of the most interesting and taxing ‘from an engineering point of view’ aspects to designing a reef system, be it small or large in scale. The following sections will cover the various methods you can employ to replicate more natural flow patterns and velocities within the aquarium. In the examples shown, some systems have a higher degree of emphasis shown towards one pattern over another as the main method of water movement. Suffice to say that rarely is a single method employed as the sole means of circulation unless it is a dedicated biotope aquarium. Usually it is a combination of two or more principles for greater success and realism.

Wave action.

Fig 1:

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One method more commonly employed these days by larger system designers is the ‘closed loop’ which consists of one or more high powered pumps drawing water from the aquarium through hidden outlets protected by strainers, and then pumped strait back to the tank through one or more strategically placed nozzles.

Looking along the length of the aquarium, We see the effects of having 4 individual closed loop nozzles along the front bottom edge of the aquarium, which blow water through very wide bore outlets that disperse the flow into a wide blanket of motion, upwards towards the surface at 45 degrees causing a rolling motion up and over the rockwork in a very similar rolling motion to that encountered as waves pass over the reef. The strainers to feed the pumps in this instance are commonly hidden within the rock structure giving the added bonus of preventing detritus buildup underneath the rockwork as it is sucked up and blown back out again to be taken out by a skimmer after exiting via a surface weir. Another advantage to this method is that you are efficiently mixing bottom layer water with that at the top of the aquarium, aiding gas exchange throughout the entire water body and creating good surface agitation. Using variable speed pumps, you can feasibly create very realistic pulses of motion very akin to natural wave action.

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However, even without a quick pulsing facility, It has still proven to be a very successful method indeed of creating realistic wave simulation / water particle roll. There is no reason why this method can not be employed on a smaller scale with more moderate sized plumbing and hobby pumps, or strategically placed powerheads hidden within small rock structures near the bottom front of the tank. The need for pumps or outlets with a ‘wide bore’ cannot be stressed enough in this instance though. Corals don’t like being subjected to high velocity ‘jets’ of water at the best of times. Plus they should be positioned in locations that will give the maximum available dispersion distance before that flow comes into contact with a coral.

Fig 2:
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This method employs what is commonly known as a surge device or dump tank. Commonly set up in accordance with the design realized by Dr. Bruce Carlson, or by way of a flapper valve and float assembly. The system simply comprises a remote raised reservoir or header tank of given volume, which is gradually filled from either the main tank or sump to a point it suddenly causes a break in pressure differential and starts a fast siphoning effect that dumps its entire contents back to the main display aquarium in one large surge of flow though a wide bore pipe. The frequency of the surge is simply limited by the rate at which the header tank can be re-filled. Although supremely powerful, this system does have a few drawbacks, in that salt spray and creep from the inrushing water is a constant bug bare, as is the noise generated by the amount of air commonly drawn into the flushing system which causes a multitude of fine bubbles which may prove unsightly for some. The only other drawback is a fluctuating water level in the display tank which must have a Weir or overflows set at a height to allow for the extra volume being dumped.

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There are various offshoots to this idea that have been employed to varying degrees of success, from the reverse Carlson device, to flapper valve dump tanks.
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A more common way of simulating this effect without the salt creep and noise drawbacks, is with one or more of the many ‘stream’ type pumps on the market today. By setting a short pulse period we can simulate the repetitiveness of passing wave pulses to good effect. The only drawback with this method though, is that due to the twisting nature of the pumps outlet surge, much of the rolling particle motion associated with real wave action, is lost or disturbed. Plus this type of twisting flow may impact adversely on any near surface corals that are caught in the immediate path of the pump. In such cases it may be worth angling these pumps up towards the surface slightly, to level out that flow and minimize the twisting action.

Fig 3:
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The final and most recent method for creating wave motion is the ‘wave box’. This system comprises of a high output pump mounted within a box that sits inside the aquarium. The process is similar to that employed by the previous device i.e. the forced transfer of fluid from one container to another, however this time water is ‘forced’ out of the box by the pump rather than gravity, and into the main tank volume causing a waterless void behind it. As soon as the pump switches off, water rushes back into the box from a grill at the bottom to re-fill the void. Dependant on the interval the pump runs for, and the dimensions of the tank. A quite sizable surface wave can be encountered at each end of the aquarium. Unlike the two previous methods however that create the large scale ‘circular’ particle motion that is encountered in the wild, this method generally creates a more ‘to and fro’ rocking motion of water within the aquarium, with a moderate pulse of flow at the pump outlet. It is still very effective though, as many will pay testament to the amount of hidden detritus that is forced out of nooks and crannies on its first application, which can only be a good thing.

Oceanic current and tidal flow.
Fig 3:
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We can easily create both alternating tidal flow, and constant oceanic current by careful placement of powerheads or closed loop outlets at alternate ends of our aquarium.

By having pumps switching in alternate pairs, we can use the shape of the aquarium to push water around the tank in a circular or laminar motion and alternate its direction, dependant on our preferred cycle of tidal flow patterns be they 6 or 12 hour cycles. By using timers in the example above, we could run both pumps ‘A’ for 6 hours and then switch to pumps B for the next 6 hours. We can also advance on this principle by creating slack ‘between tide’ periods where only a little laminar flow is encountered. In this instance we may wish to run ‘both’ pumps A for 5.5 hours running anti clockwise, then switch to a single pump A for an inter-tidal period of say 30 minuets, before switching to one pump B in a clockwise direction for another 30 minuets before giving full alternate flow for 5.5 hours using both B pumps. This would give us a very realistic ‘build up’ of energy, peaking at mid tide times where flow rates are at a maximum.

As such we would be running as follows.

A+A=5.5 hours. to A=30min to B=30min to B+B=5.5 hours to A=30min.
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The above might sound a strange way of doing it. But remember, the ocean doesn’t have glass side walls. Unfortunately it’s impossible to flow water in a single direction within the confines of a glass box without it bouncing back somewhere. To push water one way, you must have an equal and opposing flow in another area to replace what is being moved. Luckily for us, the corals don’t actually recognize the back of the aquarium ‘as the back’ all they know is that they are receiving flow in one direction for a full tidal phase, and then it’s switching to the opposite direction. The corals at the front of the aquarium, are in the same boat, only they are sitting in the opposing tidal phase.

If we combine one of the methods of wave generation with the laminar flow method, and add in some degree of timer control, be it with individual timers or one of the many computerized aquarium controllers. We can effectively create very realistic flow patterns within the confines of a home aquarium, and because of the differing trajectories and natures of both flow patterns, they will combine around, and through corals to give us those all important random, localized flow patterns encountered within the bottom boundary layer that add that final element of realism for our corals and other animals to thrive in.

With the careful placement of individual corals and research into their natural conditions there is no reason why we can’t keep a mixture of species that require differing flow characteristics, within the same aquarium by taking advantage of these variations. Or alternately taking into account dedicated flow velocity scenarios, create realistic biotope systems from full on ‘reef crest’ wave surge systems, to more sedate laggoonal type protected systems that utilize ‘gentle’ laminar tidal flow as a main or sole feature to encourage a profusion of benthic filter feeding organisms mixed with more delicate LPS corals.

Hopefully this article has been of some use to you in putting into perspective what we are trying to simulate with all the technology currently available. The limits are boundless as to what we can achieve with a little imagination.

Simon Garratt.

References :

American Society of Limnology and Oceanography,

Matthew A. Reidenbach

Stephen G. Monismith

Jeffrey R. Koseff

Gitai Yahel3 and Amatzia Genin

#3 jason@jasonsaquatics


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Posted 06 November 2010 - 04:29 PM

Just like to say Thanks Simon ,,

In my and Lindsay View this is the best article around flow that can be found on the net .

Chris if you get a chance can you sticky this in the right place m8


#4 JayK


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Posted 06 November 2010 - 04:42 PM

Just like to say Thanks Simon ,,

In my and Lindsay View this is the best article around flow that can be found on the net .

Chris if you get a chance can you sticky this in the right place m8


Thanks for taking the time to post this up Jas - will be interesting read!

Keep it real - Ride Hard or Stay at Home!!!!

#5 dab



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Posted 06 November 2010 - 09:23 PM

an interesting read chaps

#6 Guest_lindsay_*

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Posted 07 November 2010 - 02:04 PM

As said above a very good read and its great to have it on here,good of Si to send it over to put it on here.




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Posted 07 November 2010 - 09:06 PM

As above very interesting read, thanks to si for taking the time to do this article and thanks to the boys for putting it up :good:

#8 ReefAnon



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Posted 09 November 2010 - 01:01 PM

Nice to see the "my tank has 100000000x turnover so everything must be great" myth knocked down as it's a meaningless comment that I see all too often.

Interesting to see all the strands of 'flow' brought together like this though there is perhaps a risk that some reading the first article only may walk away with the impression that 'laminar' flow is ok in our tanks - as Simon himself sums up in the start of the second article the general thrust of the first article is that in the wild there is "an underlying and constant large scale flow in any given direction we call ‘oceanic current’ or ‘laminar flow’". To me the big point about laminar flow in the wild is not so much that it's laminar (on a zoomed out view) but rather it's function as an auto feeder and water changer. That point is made in the article of course.

It's also clearer to to me now what Simon's fascination with a tidal tank was - is that still going?

I spent many happy(?) hours experimenting with my streams over the years in our tank and in the end settled for pulsing plus alternating flow. Over the longer term the benefits in terms of coral health and growth in our tank are obvious. I guess where it differs is that the change in direction happens every minute or so rather than every 6-12 hours. That's an option and I've sometimes wondered if it's one worth experimenting with but I kind of feel that the scale of our tanks is so infinitessimaly small compared to the real thing that this might be out of scale. And of course, growth and health has been good (as far as I can tell) for a number of years so the impetus to change it is not great.

Now, is that 20cm or 25cm per second - just kidding :good:

Thanks again Simon for another great read and contribution to the hobby and thanks to Jas for making it available on here.

Tank thread

#9 Guest_lindsay_*

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Posted 11 November 2010 - 07:57 PM

100000000xs,better check i got the right amount of 0s lol,on the other hand how many times have you read an article about a well grown out tank of sps that the guy doing the article adds,surprisingly the tanks flow turnover is 15 times, and often 20 times or less.

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