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Newsletter Nr. 2/2019

A Word from the President Dear WFPhC-Members, Colleagues and Friends,

With this Newsletter we are publishing an article of Paul Pshenichka from Ukraine concerning some low-cost-experiments with a wide range of theoreti-cal backgrounds. Paul Pshenichka gave a speech and a workshop dealing with these phenomena during the WFPhC-Congress-2018 in Vienna.

Following his ideas, I used the third experiment, which is described in his article, during a seminar for physics teachers being involved in the training of the participants of our Austrian Physics Olympiad. It was interesting to watch all the discussions and questions how to vary these experiments in order to give pupils and students more detailed insights into the described phenomena. There were e.g. discussions about the following questions and experiments:

• A narrow rectangle starts to rotate when being dropped, whereas a broad one doesn´t. What´s the reason for that?

• Up to which size does it start to move this way?

We as Federation would like to be a platform for all being involved in competitions for second-ary school students all over the world and we cordially invite you to send us an article.

Also, we are proud to announce the next WFPhC-Congress that will take place in the charming city of Ohrid in North Macedonia from September 20-th to 24-th, 2020.

We cordially invite all of you to participate in this event. A flyer about it will be sent to all our members soon.

Enjoy the article of Paul Pshenichka and all your own work concerning Physics Competitions!

With best regards

(Helmuth Mayr – President of the WFPhC)

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Upcoming International Competitions ► 3. European Physics Olympiad (EuPhO) in Riga, Latvia May 31 to June 4, 2019

► 32. International Young Physicists´ Tournament (IYPT) in Warsaw, Poland July 6 to 13, 2019

► 50. International Physics Olympiad (IPhO) in Tel Aviv, Israel July 7 to 15, 2019

► 13. International Olympiad on Astronomy and Astrophysics (IOAA) in Keszthely & Heviz, Hungary August 2 to 10, 2019

► 7. International Young Naturalists´ Tournament (IYNT) in Minsk, Belarus August 18 to 24, 2019

► 24. International Astronomy Olympiad in Piatra Neamt, Romania October, 2019

► 16. International Junior Science Olympiad (IJSO) in Doha, Qatar December 3 to 12, 2019

(Stefan Petersen – WFPhC secretary)

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The Beauty of Fluid Dynamics

(Paul Pshenichka)

In the field of Fluid Dynamics it is possible to learn complex phenomena using very simple experi-mental tools. At the same time some of the experi-ments are very spectacular and compelling.

Let’s start with the Bernoulli Effect: there more the fluid speed – the less the pressure (Fig. 1). The red current streamlets are showing the laminar flow features.

Remark: The experiments are not explained in all details, for not to spoil the investigation pleasure of the reader.

First experiment. Place a sheet of paper (say an A4 printout paper) on the table surface and blow the air over and along the sheet. As the result, the air pressure over the paper decreases and the lift force raises the sheet. Simultaneously the viscosity friction force between the air flow and the paper surface moves the sheet forward (Fig. 2). If the test doesn’t succeed imme-diately, find out the cause.

Second experiment. All aforesaid is true if the air velocity does not exceed the Reynold’s Limit and the flow re-mains laminar. But if the air velocity exceeds the limit, the flow become turbulent and the paper starts fluttering (Fig. 3a-b). The effect caused the phenomena is named Kar-man’s Vortex Street. Due to the Bernoulli Effect the pres-sure near the vortexes decreases and uplifts the proper part of the paper.

Fig. 1. In the narrow part of the tube the air (or water) flow velocity increases and the pressure decreases.

Das Bild kann nicht angezeigt werden.

Fig. 2. The pressure difference lifts the paper and the friction force moves it to the left.

Fig. 3b. The pressure difference lifts the paper and the friction force moves it to the left.

Fig. 3a. A vortex street photo [2].

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In nature you can find many beautiful periodic patterns in the sky, snow or sand (Fig. 4).

Third experiment. Drop a narrow (say 2 cm x 10 cm) paper band (as you can see on Fig. 5) keeping it in the vertical plane. The falling band starts to rotate and glides like an airframe.

Here we should ask two questions. The first one is: why starts the band to rotate?

The flow separation at the lower edge of the band forms a vortex which lowered the pressure and caused a clockwise torsional moment [2]. The band carried along the air creating a rotating air-band cylinder (Fig. 6 and 7). The rotating cylinder wind a thin air layer which together with the incoming flow increases the air velocity on the upper part of the cylinder and decreases the velocity on the lower part. As the result, a lift force appeared (Magnus Effect).

It is also possible to calculate the Aerodynamic Quality of this airframe which is equal to the ratio Lift / Air Resistance – it is equal to the tangent of the angle between the gliding line and the horizon.

Fig. 5. Drop the band keeping it vertically.

Fig. 6. The flow separation on the band’s edge caused a vortex (scheme and photo [2]).

Fig. 4. The Karman’s Vortex Street causes very nice periodic structures in nature.

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Fourth experiment. Place a Ping-Pong ball in an upcoming stream of air or water. In Fig. 8 there are used a labor air blower but at home you can use a hairdryer. The ball take the equi-librium position in the stream and oscillates around it, describing a trajectory which reminds the number “8” (this problem was proposed at 26th IYPT-2013). The dynamic equilibrium is ensured by weight, flow resistance and pressure forces as well as Bernoulli and Magnus Ef-fects. Even if you decline the stream (Fig. 8) the ball still keeps its position in the flow.

If the ball begins to lose the equilibrium it starts to rotate in order to increase the restoring force due to the Magnus Effect. In the water stream the counterclockwise rotation of the ball be-comes visible (Fig. 9). A similar experiment is also possible to provide using a disk (Fig. 10).

Fifth experiment. If air is pumping into the bottom of a container filled with sand an interesting effect appeared – the sand starts to behave like water (Fig. 11).

Now the qualitative explanation is easy – the sand grains are freely “dancing” in the upcoming stream like balls. Hence, they are surrounded by air and separated from other grains. The static friction is near to zero and the whole mass of sand become “watery”.

References.

1. Landau L.D. & Lifschitz E.M. - Vol. 6, Fluid Mechanics (Course of Theoretical Physics), 2nd Edition.

2. Milton Van Dyke, - An Album of Fluid Motion, 14th Edition.

Fig. 7. The rotation of the cylinder caused a lift force.

Fig. 8. The ball remains Fig. 9 … or water stream. stable in the air …

Fig. 10. The disk rotates clockwise. Fig. 11. The “fish” toy swims in sand.