Visualisation des écoulements relatifs dans les machines tournantes rotoscope

The visual observation of relative flow in rotating machines. The rotoscope

Maître-Assistant à la chaire d'Electrotechnique, Faculté des Sciences de Paris.

Résumé

It is often useful to be able to establish the fluid stream lines or the paths of given particles in suspension in a rotary machine. The various methods whereby this can he achieved are reviewed in this article, with special emphasis on the use of the rotoscope, an instrument designed to 'freeze' the rotation al motion and only leave relative motion visible. Where stream lines are made visible by the addition of different substances to the fluid under investigation__especially gas bubbles__very careful Interpretation is necessary in analysing any results. Stroboscope technique can be improved with special lighting, for example where threads are used as an aid to visual observation, but such methods are unsuitable where there are gas bubbles in the flow. The rotoscope has been designed in an attempt to overcome these limitations. It is similar to a Poggendorf mirror and comprises an electrically-driven prism rotating at half the speed of the impeller under investigation (see Fig. 1). A pump impeller viewed through a rotoscope appears to be stationary. Relative particle paths and directions of motion can be shown up by taking a time exposure. By using stroboscopic lighting, they can be related to a time base and their velocity and acceleration defined. This is a difficult instrument to set up. The optical axis of the prism must coincide with the axis of rotation. Several adjustment methods are available. Image stability depends on adjustment accuracy. The speed of the pump tested varied from rest up to several thousand r. p. m. The rotoscope faithfully followed the impeller motion and constantly gave a fixed image providing a fresh view of the various effects investigated and making them easier to comprehend. The equipment can also be used to study leakage along blades or at blade tips, or cavitation. A large number of photographs of various techniques show such familiar effects as stream lines in various impeller types, swirl, bubble motion, and the flow pattern in a bladeless impeller. Used in conjonction with such visual observation aids as threads or corpuscules, the rotoscope becomes an essential instrument for researchers and test bed operators interested in relative flows in hydraulic and gas flow rotary machines.