Prandtl's flow visualization film C1 revisited

Kurzfassung

We would like to report on the techniques used around 1930 for the creation of a film consisting of sequences visualizing various forms of flow separation produced by L. Prandtl and his colleagues O. Tietjens and W. Müller in open-surface water channels. After first experiments with cinematography in the 1910s, Prandtl began to produce flow visualisation films in the 1920s, in order to better capture the dynamics of the unsteady flow phenomena. However, due to the short exposure times provided by the film camera the particles appeared as dots rather than the anticipated streaks, which were desired as representations of the streamlines. Therefore Prandtl and his colleagues first decided to work with a modified camera to provide longer exposure time and consequently streaked images. At this point, the real potential of animated film, for instance for the demonstration of flow separation, was realized and resulted in a flow visualization film, originally named "The Production of Vortices by Bodies Travelling in Water", that Prandtl showed 1927 in London and while travelling the globe from 1929 to 1930. The sequences of this film were found to be highly instructive from an educational point of view such that the film, now named C1, was made available in 1936 through the Reich Office for Teaching Films and in the 1950s by the Institute of Scientific Film. During the preparation of this paper, new movies recorded by Prandtl and his colleagues have been found in the archives. They are discussed in this paper in order to demonstrate the feasibility of such flow visualization movies for evaluation with modern PIV correlation and tracking analysis methods in a broader frame. While the film sequences were mainly intended to illustrate the temporal evolution leading to boundary layer separation and separated flow, numerous references confirm that Prandtl and his colleagues were fully aware of the quantitative nature of the acquired imagery. With much of the material preserved over the decades, it nowadays is possible to use advanced particle image velocimetry techniques such as PIV and Lagrangian particle tracking to retrieve this quantitative information and make a clear statement on the importance of proper data archival.