Publisher Summary This chapter discusses selected topics in flow mechanics. For turlbulent flow in a smooth pipe, the Blasius formula gives a fair approximation. Whereas the Prandtl-Kármán formula is valid over the entire turbulent region but its implicit form makes it difficult to manipulate. For turbulent flow, no satisfactory result is to be expected save if the walls can be regarded as hydraulically smooth. Quite often the inner pipe is eccentrical within the outer pipe. According to Deyssler and Taylor, the friction factor decreases with increasing eccentricity. The density and flow velocity of a gas flowing in a pipe will significantly vary along the pipeline as a result of temperature and pressure changes. The energy equation valid for steady flow will thus hold for infinitesimal lengths of pipe dl only when the pressure differential between the two ends of the infinitesimal section dl is dp. The chapter also discusses the pressure drop of gas flow in low-pressure pipes, pressure drop of gas flow in high-pressure pipes, mean pressure in gas pipes, and flow of non-newtonian fluids in pipes. Other things related to flow mechanics are also discussed in the chapter.