Crossflow instability

The interest in swept wings for high-speed applications is due to the fact that in supersonic regimes a bow shock-wave originates in front of "regular" wings, i.e. straight wings with rounded-nose airfoil, leading to an area of very high pressure just ahead of the leading edge. This dramatically increases the drag and hence the fuel consumption. If the wing is swept backwards, however, the pressure drop across the shock wave is reduced and so is the wave drag. When the sweep is large enough so that the leading edge is completely contained within the so-called Mach angle, the flow in front of the leading edge itself is subsonic and the shock wave cannot originate. This eliminates completely the wave drag. Besides wave-drag reduction/elimination, other advantages of swept wings are the reduction of sonic-boom effects in supersonic flight and the increase of critical Mach number in transonic flight.

If wave drag can be reduced or totally eliminated by sweeping the wing, a more difficult-to-control source of drag is skin-friction due to viscous effects, which take place inside the boundary layer. This type of drag is directly related to laminar-to-turbulent boundary-layer transition, which in high Mach-number regimes affects not only drag but also heat loads because of mixing enhancement, typical of turbulent flows. Allowing laminar flow over most of the wing of a modern airplane, therefore, would result in remarkable engineering benefits. This is the reason why transition to turbulence in boundary layers has received a considerable attention over the past decades.

From the point of view of practical applications, the understanding of basic transition mechanisms will allow us to employ flow control strategies in order to delay or avoid turbulence. Among these, passive ones would probably be preferable since they are more easily realizable and, in general, require less complications and less maintenance compared to active flow strategies.

Despite considerable efforts, the physics of transition in high-speed boundary layers are still poorly understood, in part due to the lack of experiments. By presenting some results in supersonic swept-wing flows, the present study aims at proposing infrared thermography as a technique to easier this type of investigations.

Thanks to the state-of-the-art infrared equipment available today, greater success and improved resolution of details of various flow phenomena have become possible in the last years. Since modern infrared cameras are able to measure temperature difference with an accuracy of nearly a tenth of a degree over a small area, infrared thermography (IRT) represents a very powerful tool to visualize flow phenomena that create measurable temperature changes on the model, such as shock waves, flow separation and laminar-to-turbulent boundary-layer transition.

The basic principle behind the use of IRT for transition-detection is the difference in the convective heat-transfer coefficients between laminar and turbulent flows. The laminar boundary layer allows very low heat exchanges between the model surface and the surrounding freestream flow. Laminar regions are thus characterized by a very low convective heat-transfer coefficient and behave as an insulator when compared with turbulent regions. In practice, this means that the surface in presence of laminar flow has the tendency to keep its initial temperature, which, in general, is different from the temperature of the external flow. On the contrary, the turbulent boundary layer features high mixing and therefore high convective heat exchange. As a direct consequence, a surface characterized by a turbulent boundary layer will reach the temperature of the surrounding, incoming flow faster than a laminar boundary layer.

IRT can offer several advantages compared to the other quantitative techniques previously presented. The set-up and the necessary acquisition system is very simple and compact. There is no need to paint or spray polymers, liquids or other substances on the model surface before each run and external light sources (like UV or others) are not necessary. Moreover, the user is not required to perform any calibration, being done and certified directly by the vendor. Like TSP, IRT can be used as a quantitative measurement technique or qualitative flow visualization tool. It provides a global flow-field image without the necessity of multiple local measurements (contrary to thermocouples) and if local information is needed, data can be retrieved from the whole image using the post-processing software that comes with the most common IR cameras used for scientific purposes. Besides all these advantages, IRT is perhaps the least intrusive technique because it does not require any kind of modification or interaction with the model or the flow, which could alter the phenomenon under investigation. This is preferable when transition is studied because of its extreme sensitivity to environmental disturbances via receptivity. In some cases, if a better image contrast is needed, the model can be heated or cooled with respect to its natural temperature, making IRT more intrusive. In general, however, IRT is a very poorly intrusive technique.

For more details, see the following related publications.

Zuccher, S. & Saric, W. 2008 Infrared Thermography Investigations in Transitional Supersonic Boundary Layers. Experiments in Fluids, 44, 1, 145-157.

Zuccher, S., Saric, W., Reed H. & McNeil, L. 2003 The Role of Infrared Thermography in the Study of Crossflow Instability at M=2.4. In Proceedings of the 7th Triennial International Symposium on Fluid Control, Measurement and Visualization. Sorrento, Italy.