Qualitative visualization methods are used to under-stand the behavior of the flow field and can often be of great help in establishing the cause of unexpected readings of forces or pressure distributions for example. Both large scale phenomena like trailing

vortices (smoke) and small scale phenomena (surface methods) like locally separated flow and point of laminar-to-turbulent transition can be studied. For supersonic speeds, Schlieren can be used for visualization of compressibility phenomena.

All our high speed wind tunnels are equipped with high quality windows on both sides of the test section in order to allow for Schlieren imaging. We perform both black and white and color Schlieren. A camera as well as a video can be used for recording.

 

Schlieren pictures are generally used to visualize boundary layers, shock waves, wakes and separated flow regions.

Smoke is entered into the wind tunnel, in front of the test object or the part of it that want to be studied, to make the air flow visible to the eye.

 

A smoke wand, hand held or attached to some traversing system, is generally used. Smoke can be emitted from only one point that can be arbitrary moved in the wind tunnel or from a "fork", giving a 2D or 3D pattern of smoke trails across the model.

 

Using a sheet laser together with the smoke, cross sections of the smoke trail can be illuminated to give a good picture of the structure of a vortex or even an estimate of the size of a separation bubble for example. 

 

The optical access in STARS low speed tunnels; LT1 and LT5 is excellent thanks to the large windows, which makes it easy to observe and record the visualization.

 

 

 

Various methods for visualization of the flow close to the surface of the test object can be used. Areas of separated flow, laminar-to-turbulent flow transition and general visualization of the direction of the flow across the surface of the model can be achieved.

 

 

Tufts are typically suitable for showing large to medium size flow field structures, depending on size of the tufts relative to the model. The easy installation also makes it a useful "quick-and-dirty" tool. Unlike the sublimation methods, tufts stay constantly active and is therefore suitable for wide slow sweeps, studying for example the conditions for stall.

 

Thanks to the easy installation, tufts can also be very illustrative on in-situ testing in moderate speeds. It should be pointed out that alike most of the qualitative visualization methods, the benefit of using tufts rely on whether the observer is capable of understanding the implications of the visualized flow field on the characteristics and performance of the test object.