Visualization Techniques in a Wind Tunnel

The needs of aircraft designers and aerodynamicists have begun to grow at a pace that requires changes in equipment and techniques used by those that produce aerodynamic data for the design of aircraft. New techniques of flow visualization have been developed, and as technology improves, new techniques will become more widely available in tunnels throughout the world.

Older flow visualization techniques are still, in large part, standards being used in almost every model test. These visualizations include:

Smoke:

This is a classic visualization that most are already familiar with.  This technique involves sending a stream of smoke down the wind tunnel, allowing researchers to see how airflow interacts with a model in real time. This method can be used with any type of model and in almost any type of wind tunnel.

Tufts:

This technique involves the use of a filament or string attached in either a few or many positions on the model. Tufts are similar to blades of grass moving in the wind, and many types of tuft installations will use a dye or luminescent coating that allows movement to be tracked in regular light.

Oil:

Oil is generally dyed and applied to the model prior to turning the wind on in the tunnel. This technique leaves a removable pattern of surface flow on the model after the test mode is completed. This solution also precludes the model from changing position during wind-on time, to ensure that the oil residue has stabilized over the area of interest. This technique has been used since the inception of wind tunnel testing, and clearly illustrates how wind flow interacts with an aircraft. Oil pools up or forms streams during low friction, such as flow separation on a wing, and areas that experience high friction (like that of a vortex) will be clear of oil.

Schlieren:

Schlieren flow visualization is based on the principle that light rays bend when they encounter changes in density. This technique stands to benefit from future technologies; however, historically, these images are 2D in nature, while the shockwaves that they are recording are 3D. This creates an interpretation problem for many engineers, and maybe one of the more difficult visualization methods to both understand and utilize.

Many older techniques are being improved with technology and better data collection processes to improve the output of wind tunnel tests. Although not every technique on this list is new, advances in capture and application technologies have improved their output to a degree not possible before.

Techniques on the cutting edge:

Pressure sensitive paint (PSP):

This flow visualization technique is a process of applying paint that is pressure and temperature sensitive, often by using O2 interaction with luminescent molecules within the paint itself. This technique allows researchers to understand highly complex flow patterns in relation to pressure and temperature. This also means that researchers can study more than just flow effects over a surface.

Laser Sheet:

Laser sheet visualization is accomplished by saturating the wind tunnel upstream with a particulate that will be illuminated by light. Lasers are generally used as a light source because the light quality is both uniform and measurable. Laser sheet visualization is primarily used to track airflow after it has made contact with the model, this helps understand vortices and the effect of air throughout a model’s motion.

Digital Particle Image Velocimetry (DPIV):

This technique is one of the largest beneficiaries of faster computers and the readily available processing power needed to analyze a complex series of images. The airflow surrounding the model is unaffected by DPIV and laser sheet imaging, which means that this process is becoming one of the more attractive options for gathering visual data. DPIV uses pulsed laser sheets to scatter light off of particulates (oil droplets, smoke particles, etc.) that are seeding into the flow field. The scatted light is captured by digital cameras to determine each particles positions with the two dimensional light sheet. By pulsing the laser and camera twice in rapid succession, it is possible to determine the movement of the particles between the two images. Using powerful computers, the correlation between the particles in the successive images is determined for the entire flow field and a two dimensional map of the flow field velocity is created. Utilizing the flow velocity data from multiple image pair, investigators, can create an understandable database of information used to validate a model or illustrate changes that must be made to the design.