北京航空航天大学：《工程流体力学》课程PPT教学课件（双语版）Chapter 4 Differential Relations For Viscous Flow

4.3 Differential Equation of Mass conservation X inlet(mass flow) → mass flux per unit area 12+0(pu)b X outlet pudydz Ipu+cou dx]dya= z dz ax dx X flow out o(pu dxdydz Infinitesimal fixed cv In the like manner a(p Lxdvdz z Flow out off the Cv V. pl)dxdydz

6 X inlet (mass flow) udydz X outlet ( ) [ ] u u dx dydz x    +  dx y z x dz dy Infinitesimal fixed CV udydz ( ) [ ] u u dx dydz x    +  u mass flux per unit area → X flow out ( ) u dxdydz x    4.3 Differential Equation of Mass Conservation In the like manner ( ) v dxdydz y    ( ) w dxdydz z    Flow out off the CV  ( ) V dxdydz

Loss of mass in the cv op ydvdz Ot a(pu dxdydz+o(pv) dxdydz (p) dxdyd az at at a(pu) dxdydx+ a(pv dxdydz a(pr OX Bp, d(pu) a(pv)*20 ),a( +V·(p)=0 at ax at p dp a Av.=0 dp+pv.V=0 dt

7 ( ) ( ) ( ) u v w dxdydz dxdydz dxdydz dxdydz x y z t         + + = −     ( ) ( ) ( ) 0 u v w t x y z         + + + =     ( ) 0 V t    +   =  u v w V 0 t x y z          + + + +  =     + V = 0 dt d    Loss of mass in the CV dxdydz t  −  ( ) ( ) ( ) 0 u v w dxdydz dxdydz dxdydz dxdydz t x y z         + + =    

For steady flow V·()=0 For incompressible flow vv-0 Example 1 Under what conditions does the velocity field V=(a,x+b,y+Cz)i+(a,x+6,y+c22)j +(asx+bay+crzk represents an incompressible flow which conserves mass?( where ai,bi,c=const

8 For steady flow  = ( ) 0 V For incompressible flow V = 0  Example 1 Under what conditions does the velocity field 1 1 1 2 2 2 3 3 3 ( ) ( ) ( ) V a x b y c z i a x b y c z j a x b y c z k = + + + + + + + + represents an incompressible flow which conserves mass? ( where ) , , a b c const iii =

Solution u=ax+,y+Cz v=a2x+b,y+C22 w=aix+bay+Cz Continuity for incompressible flow × +c=0→a1+b2+c3=0 Example 2 An incompressible velocity field u=a(x2-y,w=b, a, b are const, what v=? Solution ouX 02ax+ 0 v=-2axy+((x, t) An arbitrary function of x, z, t

9 Solution 0 u v w x y z    + + =    1 1 1 u a x b y c z = + + 2 2 2 v a x b y c z = + + w a x b y c z = + + 3 3 3 1 2 3 a b c + + = 0 Continuity for incompressible flow Example 2 An incompressible velocity field: u=a(x2 -y 2 ),w=b, a,b are const,what v=? Solution 0 u v w x y z    + + =    2 0 v ax y  + =  2 v ax y  = −  v axy f x z t = − + 2 ( , , ) An arbitrary function of x,z,t

Assignment P264:P41(a)2P42,P4.4,P49(a)

10 Assignment: P264: P4.1(a), P4.2, P4.4 ,P4.9(a)