ADVANCES Seeing real-world sonic 'booms'

09 March 2019

SIG: Imaging Science

All photos courtesy NASA

High above California's Mojave Desert, two T-38 supersonic jets pass just 600 metres beneath a slow-flying twin-propeller aircraft. But this was no random near-miss, this was a flight by NASA that utilises a revolutionary system to visualise the shock waves created by supersonic aircraft with the aim of designing the sonic 'boom' out of future high-speed aircraft.

While wind tunnels are incredibly useful in aerodynamics, there is nothing like gathering real-world data. The problem is being able to visualise shock waves generated by supersonic aircraft in a real world situation. Schlieren photography, a technique invented in 1864 by German physicist August Töpler, can be used to visualise changes in air density, but traditionally involves a collimated light source to produce parallel rays of light passing between two parabolic mirrors. Not something easy to arrange at 10,000 metres.

However, recent advances in image processing have enabled the development of a technique known as 'Background Oriented Schlieren', or BOS. In this, the image in a wind tunnel is taken against a speckled background target. Comparing images before and during the test allows image processing to determine any distortion in the image due to changes in air density, showing where shock waves or other aerodynamic effects take place.

Complex shock waves over a NASA T-38 jetNASA scientists have taken this one step further with AirBOS, in which the BOS system is carried on a Beechcraft B200 King Air twin-prop aircraft. The camera is placed to look straight down, the desert landscape below forming the speckled background required. First, a run is made to image the background. Then a run over exactly the same flight path is made, but with the test aircraft making a supersonic pass just 600 metres below and flying about 650 km/h faster. The camera system captures 3 seconds of video shot at 1400 frames per second. The processing software then compares the test images with the backgrounds shot previously, determines any distortions from shock waves in each image then stacks the resulting frames together to give a high-resolution image. The images produced are monochrome, with brightness directly related to air density, but may be digitally coloured to highlight structures in the shock waves.

The most recent tests have been complicated by flying two T-38 jets in formation. Flying at Mach 1.09 (1.09 times the speed of sound) the pilots fly as little as 10 metres apart while making the pass beneath the camera aircraft, something akin to threading a needle in the sky. The resulting images reveal much about the complex way in which shock waves interact and either reinforce or cancel out.

With more detailed data coming from these flight tests, NASA has refined the design of its X-59 Quiet SuperSonic Technology (QueSST) aircraft. The intention is to diffuse the shock waves, so that an overflying jet only creates a low rumble heard at ground level instead of the troublesome sonic 'boom'. This could open the way for supersonic airliners allowed to fly over populated areas, something the Anglo-French Concorde was banned from doing.