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Measurement Techniques: Their Advantages and
limited temporal resolution is specifically a disadvantage when measuring sweeping jet actuators considering their frequency on the order of 1 kHz. Assuming the interaction of flows rapidly change near the surface, this technique won’t be able to catch all the relevant flow features.
Pressure Sensitive Paint (PSP): This technique is quite new and is still in continuous development.
While it has an extremely high spatial resolution (limited only by the camera’s pixel resolution) the temporal resolution is still limited. Additionally, the technique currently struggles to work efficiently and reliable at low freestream velocities of less than 50 m/s. Below this velocity the accuracy drops rather quickly and the quantitative measurements become qualitative measurements. This might still be helpful in certain situations. Also, because there is so much interest and development in this technique, the accuracy at low freestream velocities will most likely increase significantly over the next few years. Thus, it is definitely worthwhile to keep an eye on this technique.
Schlieren: Visualizing the flow via Schlieren can give a lot of insight into flow motion and its interactions. Seeing an otherwise invisible flow is essential as it can make complicated things easier to understand if key features can be distinguished. However, Schlieren is a qualitative technique and can not provide qualitative data in a simple manner. Nevertheless, it’s detailed visualization of the internal flow structures of a sweeping jet actuator helped a lot to understand its functionality (see Chapter 2). Unfortunately, Schlieren has a limited scalability. The observation volume is directly linked to the lens (or mirror) diameter due to the need for collimated light. Large diameter lenses (and mirrors) have an exponential growing cost to a point, where there are not affordable anymore except for a space telescope with billions of dollars of funding. This is one of the reasons why Schlieren is rarely used in a large wind tunnel setup. An additional problem is the need for a straight, unobstructed optical path through the test section, which sometimes is difficult to achieve without the use of mirrors inside the test section (which then obstruct the flow). Additionally, the windows through which the Schlieren observes the test section would need to be at optical quality as well to not negatively affect the image.
Digital Particle Image Velocimetry (DPIV): Nowadays, DPIV is the state of the art for quan- titative fluid measurements. Its temporal and spatial resolution depend mainly on the laser and camera setup. Highly temporal-resolved data can be achieved with a continuous laser source and
a high-speed camera. However, while this works well in water, doing these measurements in air is much more challenging. First, the particles required for seeding can’t be solid in air and are usually a liquid-gas-combination such as a helium or a soap bubble. These particles are much more likely to be destroyed during the experimental process than the solid particles used in water. As a conse- quence, a continuous supply of particles is required that needs to be tuned based on the experiment.
Second, due to the fact that the particles in air are normally much smaller than comparable ones in air, the laser power required is much higher. In most setups this prohibits the use of continuous lasers, and therefore limits the temporal resolution because a pulsed laser is necessary. Also DPIV is a two-dimensional technique and can’t reliably capture three-dimensional phenomena. However, compared to pressure taps and pressure sensitive paints it has the advantage that it is not bound to the wing surface and the interrogation plane can freely be chosen in space.
Defocusing Digital Particle Image Velocimetry (DDPIV): DDPIV is very similar to DPIV. But instead of being a two-dimensional technique, it uses additional cameras to extend into the third dimension preserving the high spatial resolution. In theory this is a simple opto-mathematical exercise, but in reality the problems of DPIV are intensified: the technique requires an illuminated volume instead of a mere sheet, asking for even more laser power, to a point where the cost of the laser is unaffordable. The large velocity differences between boundary layer, freestream, and jet velocity pose another problem that is more severe than in DPIV. Multiple laser shots are now needed to account for the differences, requiring not only a powerful, but also a multi-shot laser or multiple laser that can be synchronized. Cameras with multi-frame readout, such as high-speed cameras are scarce or expensive. Additionally, if the jet itself has no seeding particles, meaning it does not have its separate seeding supply, it will create “holes” in the flow that can’t be quantified during the analysis. This obviously applies to DPIV as well but to a much lesser extent. The seeding density of the flow is even more critical and needs to be adjusted with great care. The sum of these problems make it near impossible to do three-dimensional DPIV inside a large scale wind tunnel on an AFC wing model. However, the mentioned issues should resolve themselves within a few years with more powerful and cheaper lasers with multi-shot capabilities to come. The created need for appropriate cameras will make them available as well.
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