On the Flow of Fluids Made Visible

Leonardo, Vol. 8, pp. 329-331. Pergamon Press 1975. Printed in Great Britain ON THE FLOW OF FLUIDS MADE VISIBLE Henri WerlC* 1 . Intrductim In 1883 Osborn Reynolds used a filament of dye solutionat the axisof a streamof water flowingthrough a transparent tube to show the occurrence of either laminar or turbulent flow [l]. Sincethen the technique of fluid flow visualization has played an important role in the development of the science of fluid mechanics. This Note concerns tests made at the Hydraulic Analogues Laboratory of the Office National #Etudes et de Recherches Akrospatiales (O.N.E.R.A.) at Chatillon, France. The aim of these tests was to observe and analyze the characteristics of airflows around aircraft and aircraft componentsby the indirect technique of hydraulic analogy. The tests yield data enablingcomparison with 'Office National $Etudes et de Recherches Akrospatiales (O.N.E.R.A.), 29 Av. de la Division Leclerc, 92320 Chatillon, France. (Based on a text in French.) (Received 21 December 1975). Fig. 1. Photograph showing 'traced' vortices on the upper surface of an inclined object having an ogive profile mounted in u uniform flow of water. (Photo: O.N.E.R.A., France.) theory and with data obtained by other means, for example in wind tunnels and in flight. The examples chosen have proved useful for aeronautical design and instruction, and the variety of the flow patterns in different colors have an artistic appeal [2]. A report by Douthat, Nagib and Fejer on the Fig. 2. Photograph showing the breaking up of a wing tip vortex. Model in a uniform waterflow. (Photo: O.N.E.R.A., France.) 329 330 Henri WerlP Fig. 4. Photograph, wing trailing edge view, of the upper surface vortices of the wings of a model of the Concorde supersonic airplane in a landing position. The model is in a water tunnel. (Photo: O.N.E.R.A., France.) artistic application of fluid flow patterns was published recently in Leonard0 [3]. 2. Experimental techniques The flow patterns shown in Figs. 1-5 (Fig. 3, cf. color plate) were obtained in a vertical water or hydrodynamic tunnel in which water flows downward under gravity through a test chamber of square crosssection [4], Models placed in the water flow are supported by various types of mountings attached to the walls of the test chamber. For some experiments, a dye solution, passing through passagesin the mountings and in the model, is injected into the flow from points on the surface of the model. Such passagescan also be used for the injection of a liquid to simulate a jet engine exhaust. Mounts have also been designed that permit rotation of a model in the water flow. The character of the flow around models mounted in the test chamber is made visible in the following two ways [5]: (1) A tracer liquid, a deeply colored solution with the same density and viscosity of water, is injected through tiny orifices drilled into the surface of a model. Tracer liquids of different color may be used to facilitate recognition of filaments originating fromthe orifices(Figs. 1-3). Theflow patternsdelineated by the tracer liquid may be observed directly or photographed in black and white or in color. (2) Air bubbles are introduced into the water flow upstream of the model. A ‘slice’of the flowfieldisilluminatedby a line source of light perpendicularto the line of sight of the observer. The flow field made visible by the illuminated moving bubbles (Figs. 4 and 5) may be observed directly or photographed. 3. Illustratedexamples As shown in Fig. 1, flow separation over a blunt but slender body, when placed at a high angle to a fluid flow,producesan organizedflowstr&urecharacterized by two symmetrical and steady vortices, which join together, leavingno vortex-free zone between them. It is the case for an ogive shaped airplane nose conethat is symmetricalabout its long axis. Fig. 2 shows the formation of a wing-tip vortex shed from a model of an airplane wing and its breakup downstream under the influence of adverse pressure gradients [6]. The complex flow pattern around a model of a complete airplane with a delta wing provided with the injection of a tracer liquid to simulate...