There we sat in class, hearing about how the layer of fluid that has become stagnant by collisions with a moving surface, grows in thickness as the flow travels. If, for example, this is the upper surface of a wing, the layer thickens, begins to display instability, and finally the action causes the airflow, up to now obediently following the surface, to separate from it.

I saw this at Eastern Creek, in 1993 the venue for the Australian GP. Ad photos show graceful streamlines parting to flow smoothly around the nose of a vehicle, then rejoining behind it. Lovely. This is the ideal—that every molecule disturbed by the moving vehicle is, after its passing, put back where it originally was, with its original direction and velocity

This is a “conservative flow” – the word conservative meaning that no energy is lost.



But in a misty morning practice I, all those tiny droplets gave me natural flow visualization. I could see that, rather than smooth flow lines rejoining behind the vehicle, I saw whirling random turbulence. Energy a-plenty was being given to the air, and that air was not giving any back. The energy being left behind in this way is a major component of aero drag.

On race bikes with a water temperature gage, I always asked the rider to tell me if temperature jumped up 5 degrees C. What this could mean was that the insulating effect of boundary layers on the piston crown and combustion chamber surfaces was being swept away, causing more energy to flow into engine coolant. What was doing the sweeping? Detonation—an abnormal form of combustion that produces shock-waves. Heavy deto blasts softened aluminum out of pistons, but light deto' just blows the boundary layer away, allowing coolant temp to rise. Without the protection of the boundary layer, the piston heats up, and its aluminum material loses strength.

But I wanted actual touchy-feely boundary layer experience. I realized I’d been feeling it for years. As the tub fills and steam rises to invite you to settle in, the hot water feels great all over.

But like so many of life’s pleasures, this doesn’t last. The water that felt so life-giving at first now feels…tepid. No problem – as we all know, giving the water a good stir restores the warmth. At least for a while.

What has happened is that the nice hot water next to your skin has cooled in giving its heat to you: a boundary layer of cooler water has formed, much to your discomfort.

It was to defeat this boundary layer formation and unwanted cooling that jet-baths were created. The water is continuously heated to the desired temperature, and a pump forces it through nozzles aimed at you (underwater) from many directions. Like light detonation, the water jets scour away the cooling boundary layer, allowing the feeling of just having settled into a hot bath to continue as long as you please.




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