Flow Visualization in Wind Tunnels

Flow visualization is an experimental means of examining the flow patterns around a body or over its surface. The flow is “visualized” by introducing dye, smoke or pigment into the flow in the area under investigation. The primary advantage of such a method is its ability to provide a description of the flow over a model without a complicated data reduction and analysis. Smokeflow visualization (described by Bradshaw, 1970 and Rae and Pope 1984) involves the injection of streams of vapor into the flow. The vapor follows filament lines (lines made up of all the fluid particles passing through the injection points) in steady flow the filament lines are identical to stream lines. (Lines everywhere tangent to the velocity vector). Smokeflow visualization thus reveals the entire flow pattern around a body. It is difficult to exaggerate the value of flow visualization. The ability to see flow pattern on a model often gives insight into a solution to an aerodynamic problem. Flow visualization can be divided into two broad categories the first is surface flow visualization when the visualization media is applied to the surface such as tufts and oil flow etc. The second type is off surface such as smoke and streams. There are basically four methods of recording the flow visualization test. The first and the best but the least permanent method is for the scientist and the engineer to observe with his eyes. Because of the depth perception one can see a three dimensional picture. The other three common methods of recording the result of flow visualization are by film, either still or movie or television camera or video and magnetic tapes. It must be realized that all three of these methods are using a two dimensional medium to often record a three dimensional phenomena. This is especially fine when using a smoke or helium bubbles to trace flow stream lines pass the model. All three of these methods can be used either black and white or color. The photography methods while recording more time for developing and printing stills, when compared to video, yield higher resolution.