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Description
The aerodynamics and heat transfer of a vehicle is strongly impacted by the development of the boundary layer. In particular, the transition from laminar to turbulent leads to increased heat transfer and aerodynamic drag. In hypersonic regime, this phenomenon needs to be carefully evaluated to ensure vehicles reliability.
The use of computational techniques alone is not always reliable in predicting boundary layer state, thus experiments must be used alongside to understand boundary layer transition physics. Three type of testing can be identified for boundary layer studies: real scale flight tests, wind tunnel tests, and free-flight tests with ballistic range. Whilst real scale flight tests represent the ultimate golden standard, they are the most expensive and can incur in technical difficulties. Wind tunnel studies have historically provided an abundance of data, with the advantage of allowing ample instrumentation opportunities for the model, however the test section needs to be carefully designed to remove the influence of the internal walls. Finally, ballistic range facilities do not suffer from free stream turbulence like wind tunnels, and offer the opportunity to study the model free-flying at design speed through quiescent air. The downside of such facilities is that it is impossible or rather pointlessly expensive to instrument the model as such is destroyed during the test. Furthermore, with object travelling at Mach 5-7, test time is limited. Thus, optical measurement techniques are fundamental. Infrared thermography (IR) is a non-intrusive technique that offer interesting opportunities for boundary layer visualisation. The use of IR for this scope is still relatively uncommon, particularly in ballistic range facilities.
This article presents results from boundary layer visualisation experiments conducted in the hypervelocity ballistic range facility at the Japan Aerospace Exploration Agency (JAXA), Chofu, Japan. The facility is a two stage light gas gun which can fire models up to 4.5 km/s. The projectile used for this study has a cone shape, diameter of 18 mm and cone angle of 10°, was made from aluminium and was fired at a speed of 2 km/s (Mach 5.8). The detector used in the experiments was a InSb midwave infrared detector, with working wavelength between 1-5.4 μm, and an FPA of 640x512 pixels. An integration time of 0.69 μs was selected, which given the projectile speed of approximately 2 km/s leads to an image blur in the direction of motion of approximately 1.5 mm. After a description of the facility, results of boundary layer visuatisation through IR are provided
