F.yp?F1.yp1?F2'.yp2'?F2''.yp2''?17640?1.5?23520?0.5?9800?0.333?41483.441483.441483.4yp???0.814(m)F50960
22. A diverging nozzle that discharges an 8-in-diameter water jet into the air is on the right end of a horizontal 6-in-diameter pipe. If the velocity in the pipe is 12fps, find the magnitude and direction of the resultant axial force the water exerts on the nozzle. Neglect fluid friction. (10分) Solution:
A162v2?v1?2?12?6.75(ft/s)A282 p1v12p2v2??z1???z2?
?2g?2g
?262.4 ?p1?(v2?v12)?(6.752?122)??95.38(lb/ft2)2g2?32.2
?Fx??Q(V2?V1)?P1A1?Fbx
?F??F?PA??Q(V?V)??95.38???(6)2?62.4?12???(6)2?(6.75?12)xbx112141232.2412
? 5 . 24 ( lb ) to the right
23. A horizontal 100-mm-diameter pipe (f=0.027) projects into a body of water (ke=0.8) 1m below the surface. Considering all losses, find the pressure at a point 5 m from the end of the pipe if the velocity is 4 m/s and the flow is (a) into the body of water; (b) out of the body of water. (10分)
Solution: (a) 2p1v12p2v2 ?z1???z2??hL?2g?2g
p1v12v12lv12
??z2?z1?f?(k') ?2gd2g2g p1542?1?0.027?
98000.12?9.8
p1?20.6(kN/m2)
2(b) p1v12p2v2?z1???z2??hL ?2g?2g
p1v12v12lv12??z2?z1?f?ke
?2gd2g2g
?p154242?1?0.027??1.8?98000.12?9.82?9.8p1??15.4(kN/m2)
24. A rectangular plate 5 ft by 4 ft is at an angle of 30°with the horizontal, and the 5-ft side is horizontal. Find the magnitude of the force on one side of the plate and the depth of its center of pressure when the top edge is (a) at the water surface; (b) 1 ft below the water surface. (10分)
Solution:
4(a)
F??hcA?62.4??sin30??(5?4)?1248(lb) 222
hp?h??4?sin30??1.333(ft) 334(b)
F??hcA?62.4?(1??sin30?)?(5?4)?2496(lb) 2131bl?5?43
Ic1412?(?)??4?12?4.333(ft) yp?yc?14ycAsin30?24?(4?5)(?)A
sin30?2
hp?ypsin30??4.333?0.5?2.17(ft)
1. 25. Water in a reservoir is discharged from a vertical pipe. If a=25 ft, b= 60 ft, c=40 ft, d= 2ft. All the losses of energy are to be ignored when the stream discharging into the air at E has a diameter of 4 in. What are pressure heads at B, C and D, if the diameter of the vertical pipe is 5 in? (10分)
22 pAvApEvE??zA???zE? ?2g?2g2 vE0?(25?60?40)?0?0?0? 2g
?vE?2?32.2?125?89.72(ft/s) AE42?vB?vC?vD?vE?2?89.72?57.42(ft/s) AB5 22pAvApBvB ??zA???zB??2g?2g
2 PBvB57.422??a??25???26.2(ft) ?2g2?32.2
22 pCvCpBvB??zB???zC? ?2g?2g
PP ?C?B?(zB?zC)??26.2?60?33.8(ft)?? 22pBvBpDvD ??zB???zD??2g?2g
PP ?D?B?(zB?zD)??26.2?60?40?2?71.8(ft)??
26. A nozzle that discharges a 60-mm-diamater water jet into the air is on the right end of a horizontal 120-mm-diameter pipe. In the pipe the water has a velocity of 4 m/s and a gage pressure of 400 kPa. Find the magnitude and direction of the resultant axial force the water exerts on the nozzle, and the head loss in the nozzle. (10分)
Solution:
A11202v2?v1??4?16(m/s)2A260 ?F??Q(V?V)?PA?Fx2111bx
??22 ?F??F?PA??Q(V?V)?400000??0.12?1000?4??0.12?(16?4)xbx112144
?4523.9?542.9?3981(N)
to the right
2 p1v12p2v2??z1???z2??hL
?2g?2g
2222pvv40000416112 ?hL???????28.57(m)?2g2g98002?9.82?9.8
?
27. A 450-ft-long pipeline runs between two reservoirs, both ends being under water, and the intake end is square-edged and nonprojecting. The difference between the water surface levels of the two reservoirs is 150ft. (a) What is the discharge(流量) if the pipe diameter is 12 in and f = 0.028? (b) When this same pipe is old, assume that the growth of tubercles has reduced the diameter to 11.25 in and that f=0.06. What will the rate of discharge be then? (10分)
Solution: (a) 22pvpv1 ?z1?1?2?z2?2?hL?2g?2g
2v2 lvz1?z2?f?(ke?k') d2g2g
450v2150?(0.028?0.5?1)
122?32.2
12
v?26.2(ft/s)
12 ??()212?20.6(cfs) Q?v.A?26.2?4
2(b) p1v12p2v2?z1???z2??hL ?2g?2g 2v2lv z1?z2?f?(ke?k')d2g2g
450v2
150?(0.06?0.5?1)11.25 2?32.2 12 v?17.8(ft/s) 11.252??() 12Q?v.A?17.8??12.32(cfs) 4
一、单项选择题(本大题共20小题，每小题1分，共20分)在每小题烈出的四个
选项中只有一个选项是符合题目要求的，请将正确选项前的字母填在题后的括号内。
1． 若流体的密度仅随___________变化而变化，则该流体称为正压性流体。（ ）
A． 质量 B．体积 C．温度 D．压
强
2．亚声速流动，是指马赫数__________时的流动。（ ）
A． 等于1 B．等于临界马赫数 C．大于1 D．小
于1
3．气体温度增加，气体粘度
A． 增加 B．减小 C．不变 D．增
加或减小
4．混合气体的密度可按照各种气体_____________的百分比数来计算。（ ）
A． 总体积 B．总质量 C．总比容 D．总
压强
5．某单位购买了一台提升汽车的油压升降机（如图一所示），原设计操纵方法是：从B秘进高压油，A管排油时平台上升（图一的左图）；从A管进高压油，B管排油时平台下降。在安装现场工人不了解原设计意图，将A、B两管联在一起成为C管（图一的右图）。请你判断单靠一个C管通入高压油或排油，能操纵油压机升降吗？你的判断( )
A． 可以 B．不能动作 C．能升不能降 D．能
降不能升
图一 图二
6．在一个储水箱的侧面上、下安装有两只水银U形管测压计（如图二），当箱顶部压强Po＝1个大气压时，两测压计水银柱高之差△h=h1-h2=760mm(Hg)，如果顶部再压入一部分空气，使Po＝2个大气压时。则△h应为（ ）
A． △h=－760mm(Hg) B．△h= 0mm(Hg) C．△h= 760mm(Hg) D．△h=1520mm(Hg) 7．流体流动时，流场各窨点的参数不随时间变化，仅随空间位置而变，这种流动称为（ ）
A．定常流 B．非定常流 C．非均匀流 D．均匀流
8．流体在流动时，根据流体微团________来判断流动是有旋流动还是无旋流动。（ ）
A． 运行轨迹是水平的 B．运行轨迹是曲线 C．运行轨迹是直线 D．是否绕自身轴旋转 9．在同一瞬时，流线上各个流体近质点的速度方向总是在该点与此线（ ）
A． 重合 B．相交 C．相切 D．平
行
10．图示三个油动机的油缸的内径D相等，油压p也相等，而三缸所配的活塞结构不同，三个油机动的出力F1，F2，F3的大小关系是（忽略活塞重量）（ ）