问题 单项选择题

关于流线的特征的描述,错误的是()。

A.一般情况下流线不能相交

B.恒定流中,流线的形状与位置不随时间发生变化

C.流线充满整个流体运动的空间

D.流线就是流体质点运动的轨迹线

答案

参考答案:D

解析:

流线是某一时刻在流场中画出的一条空间曲线,在该时刻,曲线上所有质点的流速矢量均与这条曲线相切。而流体质点运动的轨迹线是迹线。流线与迹线是两个不同的概念,在恒定流动中,流线与迹线可以重合,在非恒定流动中,二者不一定重合。

[点评] 本题考察的是对流线的概念及特征的理解,同时要区分流线与迹线二者在本质上的区别。

阅读理解

阅读理解。

      A lot of boys and girls in western countries are wearing the same clothes, and many boys have long

hair, so it is difficult to tell whether they are boys or not.

      One day an old man went for a walk in a park in Washington and when he was tired, he sat down

on a chair by a swimming pool. A young person was standing on the other side of the pool.

     "Oh!" the old man said to the person sitting next to him, "Do you see the person with the loose

clothes and long hair? Is it a boy or a girl?"

      "A girl," said the person, "She is my daughter. "

      "Oh!" the old man said quickly, "Please forgive me. I don't know that you are her mother. "

      "I'm not," said the person, "I'm her father. "

1. What did the old man see on the other side of the pool?

A. A girl. 

B. A boy. 

C. An old woman. 

D. A man. 

2. The old man thought the person sitting next to him was ____. 

A. a woman

B. a man

C. a girl

D. a boy

3. "Please forgive me." in the passage means "____" .

A. I'm not sure. 

B. I'm surprised.  

C. I'm sorry. 

D. I'm not living here. 

4. Which is right according to the passage?

A. Most people in western countries wear the same clothes. 

B. The old man thought the person next to him was a man. 

C. It's difficult to tell boys from girls because they wear the same clothes. 

D. Sometimes men also have long hairs as women in western countries. 

5. Which can be the best title for the passage?

A. Are they wearing the same clothes?

B. Can you tell boys from girls?

C. Are they boys or girls?

D. What's wrong with the boys?

填空题