“When a fluid flows across a solid surface—any fluid, any speed, any surface—the speed of flow at the surface is zero. This is the so-called no-slip condition. Its existence (or nonexistence) occasioned hot debate back in the mid-nineteenth century, but it fits so perfectly with both theory and measurement that it has long outlived any controversy. So where does the real flow begin? Just beyond some molecularly thick layer on the surface, which is to say beyond a negligibly thin layer. But this real flow begins as very slow flow near the surface, gradually speeding up at greater distances until it eventually reaches (put strictly, it approaches) the full speed of stream, bloodstream, or wind—as in figure 4.3”