Flow visualization
The visualization of flow (also: flow visualization ) is a common experimental method in engineering to visualize flow processes. In nature, flow processes are sometimes also visible, such as the Kármán vortex street .
history
One of the best-known examples of processes made visible in fluid mechanics are the experiments of Osborne Reynolds , who postulated the Reynolds number as a universal connection, named after him, between flow velocity, a characteristic length and viscosity . With an experiment in which paint was introduced into a flow of water, Reynolds made the differences between laminar and turbulent flow visible and attributed this effect to the relationship between geometric dimensions, speed and viscosity of the water.
Experimental fluid visualization vs computational fluid mechanics
For some years now, classic flow visualization has been in strong competition with numerical methods in which flow processes are simulated using the finite element method (FEM). The results of FEM simulations can be represented graphically and can also be referred to as flow visualization.
The result, which can be recorded quickly and intuitively, can be seen as an advantage of experimental flow visualization compared to numerical ones. Changes to parameters such as model size or flow speed can usually be observed and interpreted without delay. However, the experimental visualization of the flow requires a relatively large amount of equipment, which usually consists of a wind tunnel , the model to be considered and the method used for visualization. The result of such a visualization of the flow must still be documented appropriately photographically. In the experimental visualization of flow processes, care must be taken that the selected method does not itself influence the flow or this influence must be chosen as small as possible.
The advantage of the numerical simulation of flow processes is the possibility of simulating very complex geometries, for example of complete aircraft. The results of a numerical flow simulation can be evaluated in a variety of ways; processes on the surface of a body in flow as well as downstream of it can be considered. In addition, the flow simulation can be combined with other numerical methods such as aeroacoustic or aeroelastic methods, which enables a variety of additional findings. The quality and costs of a numerical flow simulation depend on the method used and the resolution of the geometry into the individual grid elements as well as the modeling of the flow used.
Experimental procedure
There are different methods for making flow phenomena visible for the different fluids (water, air, other liquids or gases). The methods include the introduction of additional substances (paint / air bubbles / steam) as well as the application of paint or thin threads to the surface of an object surrounded by a flow, for example a wing .
Steam in air currents
The introduction of steam is a very suitable method for making flow processes visible. This method is limited to lower flow velocities - if the velocity increases too much, no defined areas with steam can stay in the flow and there is a mixing with the environment. Both steam and evaporated oil can be used as steam, with the latter the health hazard from the oil used must be taken into account. The periodic flow separation behind a circular cylinder with a flow around it can be represented well in a wind tunnel by introducing a suitable high-contrast fluid. The flow is generated in an Eiffel-style wind tunnel, to make it visible, evaporated oil is blown in upstream of the cylinder, which makes the periodic separation that occurs downstream visible due to the steam introduced. The same technology can also be used to represent more complex flow processes, for example in models of structures or means of transport such as ships, aircraft or motor vehicles.
Color in water currents
If a dye is added to a water flow, the spreading and mixing processes can be made visible. With this technique, it must be ensured that the entire volume of liquid is colored in a closed circuit. The water either has to be completely replaced or there is a possibility that the dye will decompose and so the visualization process can be repeated after a while.
Gas bubbles in water currents
Flow processes in water can be visualized well using the finest gas bubbles (hydrogen or oxygen). Using a fine wire, hydrogen or oxygen bubbles are electrolytically generated upstream of the object to be examined with a suitable voltage source . The gas bubbles must be so small that, despite the buoyancy they create due to their significantly lower density compared to water, there is no significant natural movement of the gas bubbles in the area of observation. Since with this method transparent objects like here the NACA airfoil model 0009 have to be used in order to make both sides of the flow around the model visible, the lighting conditions are complicated. The light section used sometimes leads to unintentional scattering by the test object.
Particles on the water surface
If the finest particles (aluminum powder or bear moss seeds) are scattered on the water surface , flow processes can also be made visible. The figure shows the current that is created by moving a cylinder rod through a static water surface.
Particle and dye in water
The mixture of suitable particles and dye can be used to visualize the vortex system, as three-dimensional phenomena can also be visualized in this way. The figure shows the phenomenon known as Taylor instability, which develops when a cylinder with a small gap width rotates in a cylindrical vessel. This creates a ring-shaped vortex system, a Couette flow , which occurs evenly distributed over the height of the cylinder.
Painting in air currents
The flow directly on the surface of bodies in flow is of great interest in experimental fluid mechanics. The objects examined can be aircraft models, parts of aircraft (such as the wings) or motor vehicles or ships. The painting technique can provide information about the flow forms laminar and turbulent, the flow direction on the object, secondary flows and detachments can be made visible. The illustration shows the flow visualization on the top of a wing model in a wind tunnel . The wing model (flow from bottom to top in the picture) has a black surface on which a coat of titanium dioxide oil paint that has been heavily diluted with petroleum has been applied. If the air blows against the wing, characteristic patterns are formed which can be interpreted with appropriate prior knowledge. In the picture below you can see a zigzag tape stuck on. A fine, strip-like structure is formed above the zigzag band - this is a turbulent overflow that is forced by the presence of the zigzag band. To the right and left of the zig-zag ribbon there is a less strongly striped structure that indicates a laminar flow. On the right in the picture, the relative profile depth is plotted on the wing model on the white stripe (0 corresponds to the leading edge, 100 corresponds to the trailing edge of the wing profile). At about 50% of the profile depth, the natural change from laminar to turbulent flow can be seen.
This visualization method must be carried out carefully, if too much paint or paint that is too viscous is applied it will affect the flow and the result will not reflect the actual flow conditions. The consistency of the color used and the application technique used must be determined in preliminary tests.
literature
- W. Merzkirch: Flow visualization . Academic Press, New York 1987, ISBN 0124913512 .
- M. Van Dyke: An album of fluid motion . Parabolic Press, Stanford, CA 1982, ISBN 0915760037 .
- Hans Lugt: Eddy currents in nature and technology . G. Braun, Karlsruhe 1979, ISBN 3765020281 .