When the speed of a flowing fluid exceeds a certain critical value, the flow is no longer laminar. Instead, the flow pattern becomes extremely irregular and complex, and it changes continuously with time; there is no steady-state pattern. This irregular, chaotic flow is called turbulence. Figure 12.20 shows the contrast between laminar and turbulent flow for smoke rising in air. Bernoulli’s equation is not applicable to regions where turbulence occurs because the flow is not steady.
Whether a flow is laminar or turbulent depends in part on the fluid’s viscosity. The greater the viscosity, the greater the tendency for the fluid to flow in sheets (laminae) and the more likely the flow is to be laminar. (When we discussed Bernoulli’s equation in Section 12.5, we assumed that the flow was laminar and that the fluid had zero viscosity. In fact, a little viscosity is needed to ensure that the flow is laminar.)
For a fluid of a given viscosity, flow speed is a determining factor for the onset of turbulence. A flow pattern that is stable at low speeds suddenly becomes unstable when a critical speed is reached. Irregularities in the flow pattern can be caused by roughness in the pipe wall, variations in the density of the fluid, and many other factors. At low flow speeds, these disturbances damp out; the flow pattern is stable and tends to maintain its laminar nature (Fig. 12.30a). When the critical speed is reached, however, the flow pattern becomes unstable. The disturbances no longer damp out but grow until they destroy the entire laminarflow pattern (Fig. 12.30b).