Fluid Mechanics,Lectuer:Sun Gang,Introduction,1-2 Definition of a Fluid The solid object will no change inside the a closed container The liquid will change its shape to conform to that of the container and will take on the same boundaries as the container up to the maximum depth of the liquid,Fluid mechanics:the behavior of fluids at rest and in motion A fluid is a substance that deforms continuously under the application of a shear(tangential) stress no matter how small the shear stress may be A solid deforms when a shear stress is applied does not continue to increase with the timeDye maker to outline a fluid element,Introduction,The deformation of solid Experience a DeformationFinite(solid)Continuously increasing Shear stress is proportional To the rate of change of The deformation,At the atomic and molecular level: Solid:the molecular are packed so closely together that their nuclei and electrons form a rigid geometric structure,”glued”together by powerful intermolecular forces. Liquid:the space between molecular is large,the intermolecular forces allow enough movement of the molecules to give the liquid its “fluidity” Gas:the spacing between molecular is much larger, the influence of the intermolecular forces is much weaker,and the motion of the molecules occurs rather freely throughout the gas,Introduction,1-4 Basic Equation The ideal gas equation of state,The Basic laws governing the flow motion include: 1、The conservation of mass 2、Newtons second law of motion 3、The principle of angular momentum 4、The first law of theromdynamics 5、The second law of theromdynamics,Introduction,1-5 Methods of analysis,The system that you are attempting to analyze Basic mechanics : free-body diagramthermodynamics: closed system(terms: system and control volume),1-5.1 System and Control Volume,A system is defined as a fixed, identifiable quantity of mass; the system boundaries separate the system from the surroundings(fixed or movable), no mass crosses the system boundaries. A control volume is an arbitrary volume in space through which fluid flows. The geometric boundary of the comtrol volume is called the control surface.(include real or imaginary),1-5.2 Differential versus Integral Approach,The basic laws can be formulated in terms of infinitesimal or finite systems and control volumes. The first case the resulting equation are differential equation. The integral formulations of basic laws are easier to treat analytically, for deriving the control volume equation , we need the basic laws of mechanics and thermodynamics ,formulated in terms of finite systems,1-5.3 Methods of Description,Use of the basic equations applied to a fixed , identifiable quantity of mass, keep track of identifiable elements of mass(in particle mechanics: the Lagrangian method of description) Example: th eapplication of Newtons second law to a particle of fixed massConsider a fluid to be composed of a very large number of particle whose motion must be described With control volume analyses, the Eulerian on the properties of a flow at a given point in space as a function of time,1-6 Dimension and Unit,The physical quantities of engineering problems include units: length,time,mass, and temperature as dimension The primary quantities(length,time,mass, and temperature as dimension )and secondary quantities (dimensions are expressible in terms of the dimension of the primary quantities),1-6 Dimension and Unit,1-6.1 System of Dimension,a) mass(M), length(L), time(t), temperature(T)(MLtT) b) Force(F), length(L), time(t), temperature(T)(FLtT) c) Force(F),mass(M) ,length(L),time(t), temperature(T) (FMLtT),1-6.2 system of unit,a. SI, MLtT(primary)unit of mass(kg)length (meter)time (second) stemperature (kelvin)Absolute metric system of unit,Secondary dimensionForce(N)1N=1kg. m/s21dyne=1.g.cm/s2,FLtT, British Gravitational system: force(1bf); length(ft); time(second);temperature(degree Rankine),1 slug = 1lbf.s2/ft,FMLtT (English Engineering system) force(1bf) mass(lbm) length(foot) time (second) temperature (degree Rankine),gc=32.2 ft.lbm/(lbf. S2),4. State the three basic system of dimension 5. The typical units of physical quantities in the SI, British Gravitational, and English Engineering system of units,SI: 1N=1kg. m/s2,FLtT: 1bf Mass: 1slug = 1lbf.s2/ft,FMLtT: gc=32.2 ft.lbm/(lbf. S2),Chapter 2 Fundamental Concept,Mechanics and thermodynamics,2-1 Fluid as a continuum,The average or macroscopic effects of many molecules.continuum(classic fluid mechanics)(p, ,T,V is continuous function of position and time) the mean free path of the molecules is same order of magnitude as the smallest significant characteristic dimension of the problem(rarefied gas flow),For determine the density at a point , In fig.2.1 point C(x,y,z)s density is defined as mass per unit volume, the mean density within volume V would be given by = m/V, at point C,2-2 velocity field,2-2.1 one-,two-,and three-Dimensional flows,