Background Oriented Schlieren Method (BOS)
The Background Oriented Schlieren Method is a very simple and robust technique to visualise density gradients. It makes use of the deviation of light rays due to refractive index changes. The relation between the refractive index and the density is described by the Gladstone Dale equation for gases. The visualised density gradients can be caused by a compressible flow regime, non-uniform temperature distribution or mixture processes of different gas species.
The BOS technique captivates by its simple setup shown in Figure 1 in particular compared to other Schlieren techniques. The optical system just consists of a camera focused on a background onto which a random pattern is applied. If an object with a density gradient is located between the camera and the background, the position of a dot on the background, which is imaged through the density object, is shifted compared to a reference without density gradient field in the line of sight of the camera. The local displacement of elements of the dot pattern is calculated by a cross correlation between the measurement recording and the undisturbed reference recording of the background pattern.
Knowing the location of the deviation (density object), the deviation angle of the light ray can be derived. Because the deviation angle results from the integration of the refractive index gradient along the light path, the refraction index field and furthermore the density gradient field can be derived for 2D or axis symmetrical cases with a single camera setup. For a quantitative evaluation of real 3D structures a tomographic approach using more camera views is necessary.
BOS has already been applied to investigate the compressible blade tip vortices (BTV) on the rotor blade of a helicopter. Measurements were performed with ground based systems as well as in flight. Further tests investigated the applicability of several natural backgrounds, such as grass, forests and the outskirts of the forest. Though the signal-to-noise ratio of the tested natural backgrounds is reduced compared to synthetic patterns, it is still sufficient for a BOS measurement. In conclusion the imaging of grass or small and dense leaves is preferable, leading to more homogeneous and reliable correlation data.
As part of an AIM measurement campaign, the jet wash of a business jet (Dassault Falcon 900) was visualised by a ground based BOS system where the camera system has been installed on the tower of the airport Braunschweig-Wolfsburg (Figure 2).
The measurements indicated that for ground based BOS systems, the flight path of the aircraft with respect to the camera and the used background must be well defined to ensure a successful measurement.
The simple and robust optical setup of BOS makes this technique interesting for in-flight applications. However, the special conditions induced by the large scale and the in-flight restrictions like the optical access require special solutions. The main task is to find an appropriate background and optimal camera position depending on the object to be investigated. There are two general approaches: referring to a natural background pattern or attaching a pattern on the aircraft surface.
Several natural backgrounds like grass and woods had already been investigated in the past. A test with a ground based BOS system within the AIM project showed, that the weather conditions can strongly influence the results. Taking the reference and measurement image at different times, the motion of the grass and woods caused by the wind can disturb the evaluation remarkably.
Using a ground fixed background pattern and an airborne BOS system, the reference and measurement image must be recorded simultaneously using a stereoscopic system or 2 consecutive images, underlying that the density structure is located in different areas and that the mean shift caused by the aircraft movement is small enough to identify corresponding areas in both pictures.
The approach of using the aircraft surface as a background has the advantage, that the pattern can be optimised for a particular application. Hence, the quality of the random pattern is much better than using a ground fixed pattern and the pattern is available for the complete flight envelope. However, this approach is often limited by the possibilities to mount the cameras in the right position. In addition, a separation between surface deformation and density effects must be possible. Using a fixed pattern on the wing surface of a fixed wing aircraft is of special interest for the application of BOS for shock detection over the wing. If the viewing conditions are optimal also the vortex trajectory over a flap or behind a propeller can be measured with BOS. Within AIM² BOS is further developed towards easy in-flight application. Different backgrounds are investigated and the combination of BOS with numerical calculations and a digital mock-up will be used to improve the technique for shock detection measurements. As the measurement setup can be similar to IPCT a combination of both techniques are evaluated to enable combined flow-structure coupling. The BOS technique can give basic information on the flow topology on the wing for high mach numbers and localize vortex trajectories with a very simple setup. Therefore it can be used to identify the potential for improvement of the aerodynamics of the wing and its high lift devices to reduce drag and identify flow-structure interactions causing vibrations in a very simple manner.