流体力学双语3-动力学.ppt

1 注：压力表测得数据为相对压强，工程上称之为表压。又表盘刻度常用 表示，应掌握其与 的关系。 3—4 如图所示一弯管喷嘴，管径 D =75 mm ，喷嘴出口直径 d =25 mm ，压力表读数 ，求法点上、中、下螺栓的受力情况。四个螺栓对角中心距为 150 mm ，弯管及水重 100 N ，作用位置如图示。 解：此题是以动量方程为主，连续性方程、能量方程、动量矩方程综合运用的问题。 （1）以弯管入口和出口截面以管壁面包围的流体为控制体，列 x 轴和 y 轴方向的动量方程。 题 3—4 附 图 m/s 255 . 1 , m/s 29 . 11 ) ( 2 2 2 2 1 2 2 1 2 2 1 1 1 2 1 2 2 2 2 2 2 2 1 1 1 = = = + = - T + + = + + V V d d V V p z g V V g V p z g V p z and known data into it , obtain Substitue List the energy equation of sections (take nozzle axex as norm) ） （ g g g N 6 . 83 4 N 43 . 334 , N 43 . 334 3 = ￠ = - = - = ￠ = x x x x R F R R R the orientation is leftwards and pull the bolts The pull on each bolt is. ）substitute them into (1), （ N 53 . 158 93 . 74 6 . 83 N 6 . 83 N 67 . 8 93 . 74 6 . 83 = + = ￠ - = - = ￠ + F F f F f F F f ＝ ＝ ＝ ＝ the stresses on each bolt are respectively 下 中 上 N 93 . 74 15 . 0 24 . 11 , m) N ( 24 . 11 3 . 0 100 3 . 0 025 . 0 4 29 . 11 1000 4 2 2 2 2 2 2 2 2 2 2 2 2 2 2 - = - = = ￠ = ￠ × - = - ′ × = - = = = + l M F M l F Gx y A V M y A V y QV Gx M so the outside momentum is clockwise. The influence of syphon and weight of water do momentum to origin of coordinate. The anticlockwise orientation is positive. ） （ p r r r × 现在我们用动量方程来确定这种作用力。 同理，对于 y 轴方向有 从以上公式可求出 与 ，从而可以计算R。 于是按式（3—40），有 沿 x 轴方向的动量变化为 沿 x 轴方向的作用力总和为 我们用两个分量来分析： (2) The counter pressure of jet（reverse thrust） In the container of Figure 3—5 on the position of depth equals h below the liquid surface there is a outflow hole whose area is A and much smaller than the area of liquid surface. On the precondition of the outflow hole is very small we assume that the outflow process can be regarded as approximate stationary flow in a very short time . At this time the outf