流体力学与工程Chapter 2 Main Physical Properties of Fluid.ppt

Chapter 2Properties of Fluids 2.1.1 Characteristic Fluidity Definition of fluid: A Kind of Substance that deforms continuously when subjected to a shear stress. Difference of fluidity 0 greater 2.1.2 Continuous Medium Model: Fluid Parcel or Fluid Particle Definition of the model The space is full of molecules without any clearance. Purpose Such variables as volume, pressure, temperature, density, viscosity, velocity, acceleration and so on can be described by differential equations. Applied condition When the characteristic length of an object is far greater than the mean free distance of molecules. Mean statistic law of a lot of molecules. 2.2.1 Density 2.2.2 Relative Density 2.2.3 Specific Volume 2.2.4 Density of Mixed Gas 2.3.1 Compressibility Compression Coefficient Bulk Modulus of Elasticity 2.3.2 Dilatability When p=constant, and T=273+t 0C 2.3.3 Compressible Fluid and Incompressible Fluid Incompressible Fluid (Simplification) Liquid Gas with low speed and constant pressure (e.g. fan) Compressible Fluid Gas with high speed and variable pressure 2.4.1 Mass Force 2.4.2 Surface Force 2.5.1 Definition and Reason Definition of viscosity The property of showing interior friction between two adjacent flow layers for retarding their relative motions. Reasons Slowing down the fast flow layer Speeding up the slow flow layer 2.5.2 Newton’s Law of Viscosity τ----Tangent stress, N/m2 μ----Dynamic viscosity, Pa·s (μ/ρ)----Kinematic viscosity, m2/s Example 1 In the following figure, a lubricated shaft rotates inside a concentric sleeve bearing at 1200 rpm. The clearance δ is small with respect to the radius R so a linear velocity distribution in the lubricant may be assumed. What are the power requirements to rotate the shaft? R=2cm, L=6cm, δ=0.1mm, and μ=0.2 Pa.s. Solution The energy l