Published in: Das Schwimmbad und sein Personal, edition 07/2018
… or how we an attraction, that deserves its name, can create.
My old and therefore experienced chef, he is 80 year old, said in a very emotional situation to me: "When an architect knows nothing, he makes a circle (that's a rhyme in German)!" For a pool architects this is often synonymous: "If the pool just looks rectangular, I make a corner around!" And already the engineer and the builder has the problem, that the round corner should be called "lazy river" and mutates into an energy destroyer.
And because no one knows nothing and everyone afraid is of water, the money will be wasted instead of a few thoughts. Admittedly, at first I believed too, that one has to protect the environment of the lazy river over big waves.
But now back to the order, first a beautiful photo from today's example: May I introduce, Erlebnisbad Sumpfmühle, opening 1995:
Photo: Weners 14:03, 15. Jul. 2010, CC BY-SA 3.0, https://commons.wikimedia.org/w/index.php?curid = 23593140
For explanation, the water color in the following examples is not due to anything else, than that these pictures were taken out of season. Who does not believe, also the reference photo of the local site does not, finds the latest in the popular search engines for "Erlebnisbad Sumpfmühle Hetzdorf" a proof. A tip, also a virtual or real visit in the same named hotel is recommended.
Photo: http://www.sumpfmuehle.de
It should not be the aim of this article, to write about the usefulness of such a device "lazy river" or even to judge. Because that's what the architectural competitions are for
and the opinions of the local pool master. No, the goal is to be, some basic informations about the lazy river, to carry this "unknown hydraulic entity" on the edge of the German pools landscape and to get a feeling for the effects of the hydraulics.
Special attention will be devoted to the water surface, which is free from the will of the owners and architects, here behaves in a special way NOT horizontally and quiet. At the latest at the moment, in which the participants deliberately, that interferes here the centrifugal force, begin discussions on the protection of dry surfaces or non-chlorine-resistant planting against the spreading water masses.
As mentioned above, almost every architect has the special form of the circle very close to the heart as a last resort from a creative crisis . Therefore, the simplification of the lazy river on a closed circuit ring at this point was initially allowed.
The closed circuit
Let's assume, we have an annular river, filled with water. Suppose further, the ring has rotated a flat bottom and let this water.
At what speed? To the standards I know say, guidelines or brochures nothing. Well, that would have been the first point of contention, would be clarified in advance in competition. Or was even the water speed predetermined? So I once watched a few pools and see, in the majority of visits to structures arises an average flow speed of 1,2 m/s. So let's pretend, as if it would be the task, and go to the following observations from this target value.
How does behave the water surface? For now quite simplified, act on the water level 2 forces, the gravitational force and the centrifugal force. As so often in this physical world, arises in the steady state a force-equilibrium. So we expect the gravitational acceleration g, the average speed v, the outer radius of the channel Raussen etc..
Storm in the water bowl, Photo: aqua&pools
This balance of power leads to the first acceptance in a tilt of the water level. The tilt, usually expressed as tangent of an angle, is the ratio of gravitational force to centrifugal.
Various factors beginning of a fictitious example, for illustration immediately confirmed with values. The calculation in Hz instead of rad/sec may be permitted here, we do also operate a little math.
Us, and probably many before us, becomes terribly clear, more than 8cm is the water level after the current calculation outwardly higher than the inside. A tsunami is to be feared! A vicious circle, this lazy river! Every man for himself!
Or not?
Consistent volume
For all the joy of having exactly met the expected result, nevertheless, the initial condition must be remembered: A closed ring! At no point was the adoption, that water is added. So, if the water level rises outside, the additional volume must have its origin. Not hard to guess, what outside too much, must be too little inside! But where is the center? At which point the water level remains constant?
Let's get away with simplification and become a little bit more special. Suppose now, that this imaginary center would characterized, that is without considering the tilt off center just large volume, such as within the center.
The following is the calculation of the volumes and their identification. With simple mathematics, we obtain the radius, should specify, at which point the water level remains constant.
Is the tendency of this center set, then we find a water level at the outer edge. The fact, is that by the inclination of the volume of off-center slightly larger than within the center, We want to continue to be neglected at this point. The height of the supernatant thus falls even in the millimeter range. Before 82 mm, now 33mm! Hurraaaa, a mathematical success!
The parable
Now there may be resourceful people, the while looking into her tea glass during handling of the spoon with sharp
look recognize, that the water level curves. From here to the knowledge, that this should be the case with the lazy river, it is only a small step.
So we test! Correct, we know the inclination of each of the water level at any point of the imaginary diameter. But the tendency is simply a function of the distance to the center. Unless we stick to the simplification, that the water is to move at a constant angular velocity, we have a function of the radius available.
Luck, that early mathematicians have the "first derivative of a function" is recognized as the tilt (Differential calculation). We know the tilt and want to know the function, so let the recalculation missing, we can not do without integral calculus.
Behold, the dependency is square. We now know, however, which height of the water level relative to the center takes. But we also recognize, that in these large radii the math is exaggerated. Who cares, whether 23mm or 29mm overrun!
The open circle
Have we used a closed ring for our consideration in the first section, is of course to be observed, that this is a really rare case. How should swimmers get into the water? Is that in the meaning of an extra pool, with extra treatment, with extra control? Ultimately, there is almost always a link to the main pool. Let's take a closer look at this connection.
The connection to the main pool
Are we trying to squaring the circle? On the one hand we want with little effort, because in the end we are talking about kilowatt hours per swimmer, set a volume of water in motion, on the other hand we want, that the swimmer comes from the main pool to the lazy river. The two volumes are connected. It should be our endeavor, to separate the two systems hydraulically.
The water surface
It does not require much arithmetic, to prove, that between the main pool and the lazy river no step in the water surface will occur. Our unloved centrifugal force causes, that excess water volume is fed into the main pool, and the water level at the lazy river remains outside in a plane to the main pool.
Who does not believe, the following test procedure is advised:
- Pool without visitors in a steady state (Flow fixed) operation.
- fix water level in the balance tank.
- Approach flow channel.
- Identify additional water volume in the balance tank.
Admittedly, it is a few liters. Who does not have this range, should observe the water movement at the surface after the switching off of the lazy river. Water flows, not without reason in the direction of the lazy river!
What does that mean? If we have one connected to the pool lazy river, there are only small areas, where something more (not sea) Water in the gutter runs. Then you can adjust the channel and its processes. How it works? see brochure 03/2015, it was more detailed in those days. Meanwhile, you will find the article from 03/2015 linked www.aquaandpools.de downloadable too.
The dilemma, the chord of the circle
If the hydraulic separation between the lazy river and the pool is insufficiently successful, then we waste energy, to bring the main pool into an unwanted rotational movement. This suggestion will never be completely avoided, but let's give the water at least the chance!
Erlebnisbad Sumpfmühle, Hetzdorf, Sketch: aqua&pools
Often it is like, that the circuit is arranged at the pool on the drawing board, while the circular area is shifted on the pool area. See this example above. As a result, the virtual door, in the form of a chord in the circle.
But who convinced now the water, to move in the lazy river and not, following the centrifugal force, to give way to the main pool?
It will be clear, that the tangential arrangement of the door again chord is to prefer.
The tangent to the circle
The door to the main pool so should be on the tangent. A stratification of the water is then surely, but the water moves in a first approximation from the "pivot" in a straight line, but not to the imaginary line of the circumference.
Erlebnisbad Sumpfmühle, Hetzdorf, Sketch: aqua&pools
Known fact is, that a water jet can be directed on pressure side, but suction always a hemisphere emerged. considers the entire movement of water in the lazy river, is nothing more than a beam with amazing cross-section. The beam in the direction of movement should therefore "slip past" the main pool and form an undisturbed boundary layer as possible.
This is hardly possible in practice, but can be supported by skillful arrangement of the entrance to the lazy river. The entrance should be widened so, that the water, which has slowed down as it according to law of continuity, is recaptured in full surface. Here the symmetry of the circle and the idea of the architect would then be lost ...
Here's an example, in which that interpretation is demonstrably succeeded (Self-congratulation allowed, or?)
Erlebnisbad Saaleperle, Bernburg, Photo: Müller
The direction of flow does not go toward the opening of the main pool, the water speed is lowered in the region of the opening through the pool floor. The required capacity of the pumps is about. 50% to the above example and must be further reduced after starting for operation.
The drive beam
In the majority of the previously built lazy rivers, the drive of the volume is made of several nozzles in the outer wall of the river.
This jet hits the volume of water and injects its momentum. It accelerates the water. Nevertheless, a change affects the jet, because the secondary vortex convert the energy in a low distance. At the same time we can observe, that there is a layering of the water in the circumferential direction, and the beam is "applies to the wall". Common mistake is so, that the drive jet has too high a tangential part and does not sufficiently cross the water volume in the axial direction. In other words: Keep the angle to the wall always greater than 30°!
Erlebnisbad Sumpfmühle, Hetzdorf, Sketch: aqua&pools
In consequence, there is usually a partial increase in water speed at the outer wall of the lazy river. This area is usually no thicker than 20cm and so completely ineffective for the swimmer. The effect can at beam angles 0 be observed up to 30° relative to the tangent.
With a little attention can be seen on the upper Photo, that the water is in the small groove deeper than the lazy river. The sloshing quantities are therefore barely sufficient to fill this little gutter. The above assertion, the water level will not go on, will be supported is here and proved.
The optimum angle has been in various attempts the range 40 to 50° illustrated. Here usually the center of the water volume is achieved and correspondingly accelerated with less effort.
The arrangement of the drive jet with respect to the door to the main pool has a prominent importance. This last pulse on the water can and should move in the direction of the lazy river and keep the water, to evaporate with the water of the main pool. It is therefore advantageous, to arrange the nozzle or the nozzle group in the direction of flow BEFORE the connection with the main pool and "impose" the direction of movement. This does not work on "suction side of the lazy river".
The suction point
The operation of the nozzle is usually done with water from the pool. It is not irrelevant to the question of, where this water removed, so sucked, becomes. This is done in the main pool we create a flow, equal to the pump output, from the lazy river to the pool. A significant loss, because the entire volume flow must indeed from the nozzle to suction, so get out of the circle. Gone stupid!
The following sketch shows an unfavorable arrangement, in which all flows are formed in opposite directions. The red rectangle represents the position of the suction and explains, why the pumping capacity of about. 300m³/h, respectively 44kW as heat output in the pool end.
Erlebnisbad Sumpfmühle, Hetzdorf, Sketch: aqua&pools
Easier it is therefore, the suction in the lazy river itself, to arrange in the flow direction after the "door". The pump power would support the nozzles instead of producing unwanted side streams. Please the requirements of DIN EN 123451 concerning the suction openings not to ignore!
Cabriosol Pegnitz, Photo: aqua&pools
Here is another example with some strange proportions. In the outer ring, we see the 8 (!) suction boxes, which inclusive of the piping had to be placed on 130mm height. The interior 3 boxes are air geysers and make the center island to a second attraction.
Who above ground piping "without Tichelmann" notice, please be patient until the matching item. Always according to the slogan: "There is another way, if one calculates!“. Who is shocked only by the optics at the beginning, for whom is the following picture of the exit of the same lazy river after completion.
Cabriosol Pegnitz, Photo: aqua&pools
Summary
Thus, a lazy river is a complex issue. Its functions to summarized in a circle is, as so often in this world, very difficult. Even if not always the money is there, in a hydraulic model, the effect
to test, then the individual components should nevertheless be well selected and arranged.
In the end, all of us may have noticed, that it is not the centrifugal forces that are responsible, if the water flooded over an edge. With a little courage and consideration can be applied to this edge, which do the children (and the supervisory staff) not like, be completely dispensed with. But let's look at the picture from Bernburg and the hidden details again in terms of a recommendation to:
Erlebnisbad Saaleperle, Bernburg, Photo: Müller
- In the period of planning and I did not think about the water surface. Overseeing the area opposite is very difficult. Please make this detail differently!
- The area of the door is arranged a convex curved wall.
- Therefore, at the outlet of the flow channel no nozzles are needed, which impose the direction.
- The right visible "stop wall" of a 25-meter pool prevents the excitation of the main pool to a large rotation.
- The "door posts" are maximum rounded to minimize the risk of injury.
- In the area of the door is the pool floor on 1.35m water depth. At the same time the width is maximized. Correspond continuity law is the water speed there is reduced so that a simple "exit" is possible.
- In the area outside the door of the pool floor is build like a wave. therefore the water speed changes, what gives users a special pleasure.
- Not to mention there are users, who want to test their bounce to the center island.
Thank you for the interest! I am glad about every feedback.
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