>Measurement Techniques>Infrared Thermography (IRT)
Thursday, 23. March 2017

Infrared Thermography (IRT)

The Infrared Thermography (IRT) is based on the measurement of the infrared radiation from surfaces and allows a global determination and visualisation of the surface temperature distribution with high accuracy. In aerodynamic research (in wind tunnel and flight tests) the thermography is used for the investigations of the boundary layer. Due to the jump in the wall stress coefficient and therefore in the heat transfer coefficient at the laminar-turbulent transition it allows the detection and visualisation of the transition from laminar to turbulent flow as well as laminar separations and in some cases also vortices.

In most projects of the past the steady investigation of the boundary layer has been the point of interest. For example the determination of the laminar-turbulent transition has been done point for point as a function of the angle of attack. But in the mean time many requirements for an investigation of the unsteady behaviour of the boundary layer has come up. The enhanced infrared technique of today allows the measurement of the temperature distribution along a surface with very high precision (20 mK) and at very high sampling rates (up to some 100 Hz). But the response time of the former wind tunnel models and wings due to their high heat capacity are too low to take advantage of the features of the new infrared cameras. Therefore to achieve higher response times of the investigated surfaces a modification of the surface with the aim of a very low heat capacity and therefore a fast response is necessary. At former flight tests with a glider (see Figure 1 and Figure 2) and wind tunnel tests with a first approach of such a modified surface on the wing, respectively the model the principal feasibility of unsteady Infrared Thermography has been shown. But there remain some problems concerning the applicability of these modified surfaces. In addition the results could not be validated due to unavailable CFD calculations. Therefore further development of suited modification of the surfaces of models respectively wings and corresponding CFD unsteady calculations and wind tunnel as well as flight tests for the validation are necessary.

At wind tunnel and especially at flight tests the position of the infrared system and therefore the distance between camera and the region of interest on the investigated surfaces is more or less predetermined. In addition to this only a limited number of (expensive) lenses for infrared cameras are available. In summary the choice of a suitable lens is therefore always a more or less good compromise and only in a few cases the region of interest could be mapped to the infrared image with a sufficient number of involved pixels (sensor elements). A solution of this problem could be found in the usage of special designed infrared mirrors. The usage of these mirrors has the outstanding advantage of adjusting the best focal length via the radius of curvature and therefore to get an optimised mapping of the region of interest to the sensor chip with a maximum of useful pixels and therefore a maximum of spatial resolution.

Figure 1: DLR glider Janus with modified wing surface and infrared camera
Figure 2: Infrared image of left wing with transition and turbulent wedges

Compared to the notable costs of a flight test the costs for specially designed infrared mirrors are more or less low.

Another problem which arises especially at flight test is the very low angle of view when the infrared camera is mounted on the fuselage or behind a window in the cabin to measure the temperature of an outer part of the wing. The strongly varying distance between camera and surface causes a strong distortion and a high reduction of the spatial resolution in the outer part. In these cases a specially designed infrared mirror with corresponding variation of the focal length along the span wise direction should overcome this problem because the spatial resolution remains span wise constant and sufficient. There are no experiences with the usage of mirrors for IRT and only very little information can be found in the literature. Therefore, the feasibility of the usefulness of mirrors for flight tests will be investigated.

For efficient designing of the special infrared mirrors and the exact determination of the field of view or region of interest (previewing) the methods of so called ray tracing which are well known from computer graphics should be made available to the Infrared Thermography system.

At previous flight tests it was found that the post-processing of the infrared data (images) could be improved by the development of correction methods (i.e. specific algorithms). As an example, the viewing angle for the infrared system at flight tests is very low in most cases. This leads to distorted (warped) infrared images and to problems at the manual (visual) detection respectively the assignment of the transition line to the geometrical location at the real wing. In this context the visual determination of the reference points which have been attached to the surface for assignment of the temperature values to the real location on the wing post-processing of the infrared images can also be a problem. Additionally there is an angle dependence of the emission coefficient of the surface materials which leads to some small error in the infrared radiation and therefore in the calculated temperatures.

Within AIM² the IRT will be improved towards unsteady transition and shock measurements in-flight. Furthermore the application of mirror techniques for IRT will be evaluated. As the laminar-turbulent transition of the boundary layer has a significant influence on the aerodynamic forces of wings, rudders and the fuselage of an aircraft, the benefit of the development of unsteady IRT is the possibility of easy non-intrusive measurements of the boundary layer characteristics.

AIM² Advanced In-Flight Measurement Techniques, c/o German Aerospace Center (DLR), Bunsenstrasse 10, 37075 Goettingen, Germany, Tel: +49 551 709 2252, Fax: +49 551 709 2830, Email: