How often do you wonder who that person standing or sitting next to you on a crowded bus or plane really is? How often do we walk away oblivious to an identity that would surprise and delight us? My frequency for almost mystical encounters seems to defy chance, and occasionally an encounter like the one I describe here leads me to suspect that the universe is messing with me.
In the summer of 2003 I had been in England for two months and according to plan was heading back to California for a few weeks of catch up work. I was lucky enough to capture an exit row seat, which is about the only way I can survive a 12 hour flight on American/Sardine-Can Airlines. In the normal seat, especially after their recent further reduction in seat pitch, my knees become fused to the seat in front of me after about five hours, and removing me from the seat requires a crow bar. I took an aisle seat overlooking the right hand wing and was soon joined by a young man who took the window. We greeted each other and said no more, each of us being totally absorbed into our own worlds.
Not long after leaving London Heathrow and achieving 39000 feet, I retrieved my briefcase from the overhead and pulled out a few papers to review for the journal, Measurement Science and Technology. I could not help but notice the man whose face was pressed against the window, studying the wing intensely. He seemed a bit frustrated as he fell back into his seat. A few minutes later, I could see that my reading material, and the article I was reviewing caught his eye. At that point he leaned over, examined the nametag that dangled from the handle on my briefcase and then looked at me.
And then after a brief moment of hesitation leaned over and spoke for the first time in the flight. “Excuse me. My name is Holger Babinsky and I teach aerodynamics at Cambridge. I thought that you looked familiar when I first sat down and now I realize after seeing your name that I have heard you lecture on flow visualization in aerodynamics. This is quite a coincidence to be sitting next to you because I have been observing something on the wing and I would love to hear your opinion about what I think I can see. Holgar continued by telling me that he had been lecturing on boundary layer theory, specifically on normal shock waves that form in the flow over a wing like that of the very aircraft in which we were sitting.
“I have analyzed, predicted, and taught about such shocks but I have never actually seen one. Now I am looking along this wing and think maybe I can see the shock wave. Then I realized that I had packed my camera and have no way to record it and am missing a great opportunity. Now I discover that I am sitting next to ‘a world expert’ on flow diagnostics. Would you mind taking a look at this and see if you can see anything or is it just my imagination?”
Suddenly, an otherwise uneventful flight had become interesting. I leaned over against the window and studied the wing. As serendipity would have it, the sun was immediately above us making a perfect light source for a flow visualization technique known as indirect shadowgraph. The method produces something like an xray of the flow and if everything is perfect, an expert can easily see the strong density gradients in the airflow, in a “shadow” of the flow. With this position of the sun, one would see a thin line or shadow caused by the so-called schlieren effect caused by the abrupt gas density change across the shock wave.
Sure enough, the line was there, barely visible in the shadow of the flow. One problem was that a 747 wing has a lot of joints and lines running along the same direction, so distinguishing these from a faint shadow caused by a shock wave is not trivial. It always helps to have a closer look, which, of course we could not do. Also, pictures with and without the shock wave to study later with some image enhancement would also help. Nevertheless, we convinced each other that what we were seeing was a shock wave. Holgar’s excitement was evident and his eyes lit up when I produced a digital camera from my briefcase.
This is not a Nobel prize winning discovery, of course, but nevertheless an amazing coincidence for two such people to be sitting together on a flight at the right altitude in the right seats and with the sun in exactly the right position when one of the two recognizes a name on a nametag and so on.
We took turns shooting pictures of the wing with different angles and amounts of zooming, and I promised him I would send him copies by email. “It would be great if I had pictures that I could combine with my lectures, and indeed it would be great to tell the story of how the pictures came about. He proposed to do whatever enhancements and analysis necessary to establish the viability of what we were seeing. By now our interests in each others knowledge led to a long and enriching conversation in which we exchanged ideas and questions about boundary layers and the associated aero optics of looking through them. Our knowledge bases made us exact complements of each other, and the time passed quickly.
The coincidence continued to grow. Holger was on his way to spend a summer sabbatical at Eglin Air Force Base to work with a friend of mine who headed a group with which I myself had worked on several occasions.
After arriving back in California, it was easy to forget about the pictures for a few weeks until one day as I was sorting pictures and happened upon them again. This time I was able to zoom in and study the shadowgraphs more carefully. A little image enhancement clearly showed the shock wave that Holger had discovered on the wing surface. See photographs below. I dug out the business card he had given me and emailed a set of the pictures to him back at Cambridge.
A year passed before we made contact again. Holger had expanded his lecture, predicted exactly where the shock should be, enhanced the photographs to show clearly that the shock was visible and had incorporated the whole thing into his lectures. The figure below describes the exact nature of the shock wave on the Boeing 777 wing. Since that time we have greeted each other at meetings on several occasions. By now we not only knew each other but also had mutual friends. All of this grew out of an extremely improbable chance encounter at 30000 feet.
Nowadays when flying I always check to see if the sun has will cooperate again the way it did when I sat next to Holger Babinsky. There was something very mystical about that combination that I don’t expect to experience again, but as life has shown me, an encounter that is just as mystical always lies around the next corner.