Figure 30-16 Laminar and turbulent flow. In a smooth-walled tube at low flow rates (i.e., small pressure gradients), the flow rate is laminar; that is, flow moves smoothly in concentric circles, with the centermost area having the greatest flow velocity and the area nearest the wall of the tube being virtually stationary. As the flow rate (and pressure gradient) increases, the flow transitions from laminar to turbulent. Instead of a neatly ordered flow, the velocities are more randomly distributed, energy is dissipated as heat, and the energy needed for a given flow rate increases. Many factors govern this transition, including size of the tube, viscosity of the fluid, flow rate, and pressure gradient. These factors are combined in determination of the Reynolds number (see Appendix 5 ).