Laminar and Turbulent Fluid Flows
Experiments show that Equations 4.14 and 4.15 start to underestimate the drag force as the speed v, diameter d, or the ratio ρ/μ, of the fluid's density to viscosity increases. One reason for that breakdown in accuracy is that the analysis behind those equations specifies that the fluid flows smoothly around the sphere, similar to the sketch in Figure 4.27(a). Such a uniform type of fluid motion is called laminar, and it is associated with situations where the fluid is moving relatively slowly, the exact definition of "relative" being given shortly. As fluid flows faster around the sphere, the flow pattern eventually begins to break up and become random-looking, particularly on the sphere's trailing edge. The irregular flow pattern shown in Figure 4.27(b) is said to be turbulent.