History    

The Forschungsanstalt Graf Zeppelin studied under direction of Georg Madelung the problems of diving torpedos thrown from airplane.

With the growing speed of airplanes it became more and more difficult to guarantee the steering of torpedos after water entry. In shallow water the torpedo often touched the ground, left its trajectory sideward or even jumped out of the water.

H.G. Snay build a test facility for modeling the water entry.

The plant consisted of a closed water basin with a pump for air evacuation.
External dimensions: 2.5m x 2m x 0.8m (see figure).

Front and back side of the basin consisted of glass plates.

The back side was illuminated by photoflood lamps.

The samples were accelerated by air compression.

The water entry was registered by a highfrequency camera.

In a presentation (November 1942) held by Snay in the ministry of aviation in Berlin concerning the water entry of torpedos under simulation tests he presented some pictures showing the different behavior of entering models.

Apparently these different behaviors occur, when a body plunges with high velocity in water and creates a cavity which is essentially larger then the dimensions of the body. In this case only the head of the body touches the water. Depending on the position of the center of gravity in the moment of water entry the body lands on the upper side of the cavity when the center of gravity lies above the trajectory or on the lower side when the center of gravity lies beneath the trajectory. Because of the existing forces and momentum the body can not leave the cavity wall and follows a circular path.

The problem of a straight line water entry was experimentally solved by a stretched cone supplied with a front plate. In a letter (August 1943) to H.Reichardt (KWI Göttingen) Snay transmitted a series of pictures showing a diving body. The pictures present 4 stages of a diving cone shaped body shot with a speed of circa 50 m/s from the right side of the basin under an angle of 15° to the horizontal. The first picture shows the long cavity which already narrows at the end. The following picture shows the body with adherent cavitation bubble followed by a series of detached bubbles.

These pictures are the oldest photographs of a "supercavitating" body I know assuming a supercavitating body is understood as a body which is nearly completely surrounded by a thin layer of gas and only with his front in touch with water.

Snay intensively exchanged experiences and experimental results with H.Reichardt from the Kaiser-Wilhelm-Institut für Strömungsforschung in Göttingen.

The Kaiser-Wilhelm-Institut für Strömungsforschung (KWI) under the guidance of Prof. Ludwig Prandl and his team of theoretical and experimental scientists was engaged on the problems of cavitation.

Since 1927 the KWI made experiments in a cavitation tunnel (see figure).

In the closed chanal they could modify the conditions of water flow by varying the jet velocity and the air pressure.

The controlling number for the measured section is the non-dimensional cavitation number σ, defined as
                                  with


                    
where   p₀   is the pressure of undisturbed flow,
  p_k    pressure on free surface,   q₀   dynamic pressure,
  ρ   density of water and   v    velocity of flow.

The test facility delivered good results down to cavitation numbers of    0.1.

For simulation the real behavior of diving bodies at higher velocities it was nessessary to experiment with smaller cavitation numbers.

In the KWI therefore a new cavitation tunnel was calculated and built (1943). The new facility had many advantages.

A description of the new plant and the arising problems during the development gives H.Reichardt (Lit.1: UM 6620, Feb. 1945)

Characteristics of the new tunnel:

  -    free jet cross section 15 cm x 20 cm,
  -    minimal cavitation number ca.   0.01,
  -    flow velocity    > 10 m/s,
  -    and a 3-component balance for
       precise measurements of forces.

The figure shows the measuring section with opened windows.



In this plant Reichardt made his experiments with rotational bodies and there adjacent cavitation bubbles. His investigations were finished in 1943.

The results were finaly published in his famous work: " Gesetzmäßigkeiten der Kavitationsblasen an umströmten Rotationskörpern".