流体力学英文版part3.pptVIP

Fluid Flow Concepts Flow Classification : Ideal Fluid Flow Frictionless (zero viscosity) Incompressible Can be solved mathematically Real Fluid Flow Shear stress develops where there is velocity gradient Fluid in contact with wall has zero velocity Flow Classification Laminar Flow Occurs at low Reynolds number Fluid moves along smooth layers Motion governed by Newton’s law of viscosity Turbulent Flow Occurs at high Reynolds number Fluid moves along irregular, fluctuating and random paths Flow Classification Rotational (vortex) flow Fluid undergoes net rotation about some axis Irrotational flow Fluid has no net rotation, only linear translation All ideal fluid flows are irrotational flows Variation of fluid properties (velocity, pressure, density, temperature etc) Temporal Variation : Steady flow –Properties at a point not changing with flow Unsteady flow – changing with time Spatial Variation : Uniform flow – Properties at an instant do not change with space Non-uniform flow – changing from point to point Four possible flow types : Steady uniform flow Steady non-uniform flow Unsteady uniform flow Unsteady non-uniform flow Flow Classification One-dimensional flow Flow properties are function of time (t) and one space coordinate (e.g. x) Two-dimensional flow Flow properties are function of time (t) and two space coordinates (e.g. x, y) Three-dimensional flow Flow properties are function of time (t) and three space coordinates (x, y, z) Engineering Simplification Many engineering problems are simplified as one-dimensional problems Pipe Flow : Open Channel : Vm = Q/A Engineering Simplification Axi-Symmetric Flow through Circular Pipe Elemental discharge through elemental area : ?Q = v . ?A = v . 2?r?r Total Q through pipe section given by integration : Mean velocity Vm = Q/A = Q/(?R2) Fluid kinematics Under the ‘continuum’ hypothesis, a fluid body is considered to be made up of infinitesimal fluid ‘particles’ tightly packed