Re: does water flowing from under a bridge whirl beacause of turburlent flow

Hi Nick,

The eddies that you see as water flows past the obstacles of a bridge aren't caused by turbulence, but they may be the beginnings of it.

Imagine a nice, uniformly flowing stream that encounters an obstacle, such as a bridge support. The obstacle forces the fluid to part as it flows around it. On the downstream side, the fluid streamlines will attempt to close inwards. This introduces vorticity (rotational motion) to the flow of the water. You can see some images that show this at www.raczynski.com/pn/fluids.htm

It may just stay as an eddy, in which case the flow is not actually turbulent. The signature of turbulence is that a large scale motion, such as the initial eddy, generates smaller motions, which generate smaller motions, and so on, until you have almost completely chaotic flow.

Whether or not the obstacle generates turbulence is determined by the Reynolds number, which multiplies the size of the flow (the width of the river, say) by the speed of the flow, and divides by the viscosity of the fluid. Viscosity is a measure of "thickness." Water, for example, has a much lower viscosity than molasses. The more viscous a fluid is, the more it tends to damp out random, turbulent motions. A high Reynolds number means that there is not much damping, and the fluid is susceptible to turbulence.

For gentle flows, you don't see turbulence. For rapid motions, however, you will start to see whitewater downstream of the obstacle, indicative of the onset of turbulence.

Turbulence is actually much more of a problem in aircraft design, where you are worried about the very rapid flow of a fluid (air) over an airfoil.