Fixing the Rack on the Scout and von Karman

We officially left Norman on Tuesday, discovering to our annoyance that a spare instrument rack, which had been attached to the roof of our vehicle, Scout 1, resonated very loudly at certain speeds. This sounds like a trivial thing, but it makes radio and cell communication pretty much impossible at peak noise, which is a safety issue when you’re coordinating with several other vehicles in the vicinity of a large supercell thunderstorm. The instrument technician, Sherman, was called upon to help us with the noise, and also Scout 2, which has a similar problem. The first solution was to plug all holes, but this didn’t work. Second, he added more weight to the structure, thinking to muffle the vibration. No dice. I should point out that these racks are typically mounted with all sorts of instruments, and they don’t resonate at any speed. Finally, Sherman decided to try adding a “J tube” to the appropriate spot on the rack, thinking this an easier thing to try than disassembling to rack for stowage. He had to visit the local hardware store to get PVC supplies. In the meantime, I sat around second-guessing these efforts, thinking that there was no way that this tube would alter the air flow around the rack so as to kill the resonance. Guess what? I was wrong! The J tube effectively stopped the resonance and made the Scout much more usable.

When I expressed my surprise to Sherman, he told me the story of Theodore von Karman, a famous fluid dynamicist from the middle of the 20th century. Sherman had read his biography, and he related a fascinating tale from the early days of aircraft construction, when planes were breaking up at high speeds. The engineers called in von Karman, and he was able to explain the problem as one of turbulence generated by the sharp corners where the wings met the fuselage. These corners generate errant vortices that were causing resonance in the structure, which was literally tearing the plane apart. He was able to look at the planes structure and visualize these vortices shedding off of the body of the plane. This amazing ability to visualize fluid flow is what makes him a legend to people like me, who are interested in modeling such flows, though for him, equations were secondary to the physical phenomenon. I think this matches up nicely to my last post about data. We can theorize all day long, but at the end of the day, nature does what it likes, whether we can model it correctly or not.