Felix Baumgartner's daring leap from the stratosphere on October 14, 2012 set a world record: It was the first time ever that a human being had broken the sound barrier while in free fall. Prof. Ulrich Walter, head of the TUM Chair for Astronautics, recognized Baumgartner's record leap as a unique opportunity to study how an irregularly shaped object falls: "In the past nobody knew how rough and irregularly formed surfaces like the folds of the protective suit and the backpack Baumgartner wore would impact fluid dynamics."
A jump raises plenty of questions
The first surprise came shortly after the landing, recalls Walter, who followed the jump live as scientific advisor to the Stratos team: "Our calculations, based on the fluid dynamics of a smooth body, indicated that Baumgartner would need to jump from an altitude of about 37 kilometers in order to break through the sound barrier, i.e. to fall faster than Mach 1 or about 1200 kilometers per hour. But in reality Baumgartner reached a much higher speed of Mach 1.25."
But how could an athlete equipped with a protective suit and a backpack fall faster than a symmetrically shaped object with a smooth surface? Using data collected for example on atmospheric pressure, temperature, Baumgartner's speed and his position in space at every point in time during the jump, for the first time it was possible to investigate the aerodynamics of irregularly shaped bodies at extreme speeds.
When the air stiffens
Calculating fluid dynamics in the transonic range close to the sound barrier is not all that easy, since a number of different physical phenomena overlap here: At speeds between Mach 0.7 and 1.3 the flow of air around a moving object is no longer elastic, but rather air reacts stiffly: Shock waves form, resulting in turbulence. In turn this turbulence absorbs energy, leading to an increase in aerodynamic drag at speeds close to the speed of sound. Conversely, under certain flow conditions surface irregularities can reduce aerodynamic drag: Just as a golf ball with small dimples on its surface flies better, a body in free-fall can be faster if it doesn't have a smooth surface.
Data from sensors and videos
In a theoretical analysis Walter first established the mathematical basis for calculating the flow resistance of arbitrarily shaped bodies directly from measured data. With this and from the measured values from Baumgartner's record jump the drag coefficient and the aerodynamics could be drived.
"We consolidated data from various sources in a variety of different formats – some of the data consisted of measured values, but we had to extract some of the information from videos," recalls Markus Gürster, who prepared the data and applied various analytic methods to it in his Bachelor's thesis.
"The results really surprised us," recalls the aviation and astronautics engineer, who is presently working on a doctorate at MIT after completing his Master's thesis there: "While the drag coefficient of a smooth cube increases continuously from Mach 0.6 to Mach 1.1, according to our results, the coefficient remained almost unchanged during Baumgartner's flight – that means the sound barrier hardly generated any additional drag at all."
Dents and bumps mean more speed
"The investigation shows that any variety of dents, wrinkles and irregularities on the surface significantly decrease aerodynamic drag at transonic speeds," Walter explains. Irregularly formed surfaces mean higher speed: Compared with a smooth objects, their drag coefficient and thus also their aerodynamic drag is cut almost in half.
Walter added that these calculations are still purely fundamental research, but adds that if for example the cruising speed of aircraft continues to rise, the results may be useful one day. He summarizes: "If you're trying to approach the speed of sound, dents and bumps can actually be very useful."
Markus Guerster, Ulrich Walter. Aerodyamics of a Highly Irregular Body at Transonic Speeds - Analysis of STRATOS Flight Data. PLOS One, December 7, 2017.
Prof. Ulrich Walter
Technical University of Munich
Chair for Astronautics
+49 (0) 89 289 160 03