ELEMENTARY FLUID MECHANICS REVJEW（基本的流体力学REVJEW）.pdf

ELEMENTARY FLUID MECHANICS REVJEW The subjectoffluid mechanicsis usuallydividedinto fluid staticsand fluid dynamics. In static fluids, the only force exertedbythe fluid is a pressureforce~nonnalto the surface. The basic equation is F ==f areapd~ and the pressure may be found by the fundamental equation of hydrostatics: dp = .:ydz. Fluid dynamicsproblemsaresolved using con!;ervationof mass,energy, and momentum. Elementaryfluid mechanicsproblemsusuallydealonly with incompressiblefluids in either the static or the steady,one-dimensionalflow condition. There are five general categoriesof problems: three types of forces (hydrostatic,dynamic,and viscous)andtwo types of flow (confinedand unconfined). Many complicated problemsinvolve several categories. Almost all equationsfound in fluid mechanicste~:tbooksand handbookswere derived using certain simplifying assumptions. Do not use an equation unlessyou are sure that in making these assumptionsyou arenotintroducinganintolerableerrorto the accuracyof your answer. Checkthe answerto insure the assumptionwasvalid. For example,if you use a laminar flow equationto find the velocity in a pipe, checkthe Reynolds number usingthat velocity to be sure that it is within the laminarrange. HYDROSTAllC FORCESAND PRESSURES A For incompressible fluids, the fundamental equation (dp = -ydz) simplifies to: p + yz = constant. The only force exerted by a static fluid is a pressure force normal to the surface: F = f areapdA = volume of pressureprism. Itthe fluid is incompressible,the f