Fluid mechanics test paper and answer one
1. True or False Questions
1. According to Newton's law of internal friction, when the fluid flows, the internal friction force inside the fluid is directly proportional to the flow velocity there.
2. The hydrostatic pressure at the centroid of a plane wall in contact with liquid is exactly equal to the average value of the hydrostatic pressure at all points on the entire pressurized wall.
3. When the fluid flows, the momentum changes only when the flow velocity changes.
4. Under the same conditions, the nozzle flow coefficient is greater than the orifice flow coefficient.
5. A steady (steady) flow must be a slowly changing flow.
6. The root cause of water hammer is that the liquid is viscous.
7. The long pipe refers to the flow pipe whose flow rate and head cannot be omitted during calculation.
8. The so-called hydraulic smooth pipe refers to a pipe with a small inner wall surface roughness.
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2. The relationship between kinematic viscosity and dynamic viscosity is , and its international unit is .
3. The basic principle of dimensional analysis is: ; There are two specific calculation methods.
4. The difference between the cross-sectional average flow velocity V and the actual flow velocity u is.
5. The expression of the Bernoulli equation for the total fluid flow is,
The applicable conditions are.
6. The head H of the pump refers to.
7. The expression of momentum equation of steady flow is:
8. The formula for calculating head loss is and.
9. The expression of Newton's law of internal friction, its scope of application is.
10. The pressure center refers to.
3. Short answer questions
1. Stable flow and unstable flow.
2. Causes of flow resistance.
3. Hydraulic characteristics of series pipelines.
4. How to distinguish between hydraulic smooth pipe and hydraulic rough pipe, are they fixed?
5. Two characteristics of static pressure.
6. The content of the continuum assumption.
7. The expression of the Bernoulli equation of the actual total fluid flow and its applicable conditions.
8. The basic principle of dimensional analysis method.
9. The definition formula and physical meaning of Euler number.
10. Definition of pressure pipeline.
11. The first type of problems in long pipe calculations.
12. Definition of acting head.
13. The working principle of jet pump.
14. The scalar form of the momentum equation.
15. Characteristics of isobaric surfaces.
16. Space continuity differential equation and its physical meaning.
17. Analyze the causes of local head loss.
18. The definitions and physical meanings of Reynolds number, Froude number and Euler number three similar quasi-numbers.
19. Streamline characteristics.
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reference answer
1. True or False Questions
×√×√× ×××√×
2. Fill in the blanks
1、 3 L/s
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3. Short answer questions
1. Stable flow and unstable flow. ---In the flow field, when the fluid particle passes through the space point, all the moving elements do not change with time, this flow is called a steady flow; on the contrary, all or part of the moving elements of the fluid particle passing through the space point change with time, This flow is called unsteady flow.
2. Causes of flow resistance. ---External factors: the size of the hydraulic radius; the size of the pipeline length; the size of the pipe wall roughness. Internal cause: There is always friction and impact of particles in fluid flow. The viscosity shown by particle friction and the inertia of movement speed change caused by particle collision are the fundamental reasons for flow resistance.
3. Hydraulic characteristics of series pipelines. ---When there is no midway diversion and confluence in the series pipeline, the flow is equal and the resistance is superimposed. The total head loss of the series pipeline is equal to the sum of the head loss of each pipe section in series, and the flow rate of the latter pipe section is equal to the flow rate of the previous pipe section minus the discharge flow at the end of the former pipe section.
4. How to distinguish between hydraulic smooth pipe and hydraulic rough pipe, are they fixed? --- Not fixed. The laminar side layer thickness is compared with the pipe wall roughness value.
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5. Two characteristics of static pressure. ---1. The direction of static pressure is vertical to the pressure surface and pointing to the pressure surface. 2. The magnitude of the static pressure at any point has nothing to do with the direction of the pressure surface, or the static pressure at any point in all directions is equal.
6. The content of the continuum assumption. --- That is to say, real fluids and solids can be approximated as a continuous medium that fills the entire space, and the macroscopic properties of matter are still dominated by Newtonian mechanics . This assumption ignores the specific microstructure of matter, and uses a set of partial differential equations to express macroscopic physical quantities (such as mass, density, pressure, etc.). These equations include equations describing the properties of the medium and basic physical laws, such as the law of conservation of mass , the law of conservation of momentum , etc.
7. The expression of the Bernoulli equation of the actual total fluid flow is (
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), the applicable condition is steady flow, incompressible fluid, the mass force acting on the fluid is only gravity, and the section taken is slowly changing flow.
8. The basic principle of dimensional analysis method. ---It is the principle of dimension harmony. According to the fact that the dimensions of each item in the physical equation must be the same, various physical quantities describing complex physical phenomena are combined to form a dimensionless group π, thereby reducing variables.
9. The definition formula and physical meaning of Euler number. ---
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, whose physical meaning is the ratio of pressure to inertial force.
10. Definition of pressure pipeline. ---Any pipeline in which the liquid flow fills the whole pipe and flows under a certain pressure difference is called a pressure pipeline.
11. The first type of problems in long pipe calculations. ——Knowing the pipe diameter, pipe length and terrain, when a certain flow of liquid passes through, determine the pressure drop in the pipeline, or determine the pressure head required for the starting point, or calculate the hydraulic gradient.
12. Definition of acting head. ---- The energy per unit weight of water at any section is equal to the specific energy divided by the acceleration due to gravity . Including position head , pressure head and velocity head. The unit is m.
13. The working principle of jet pump. --- The jet pump is mainly composed of a nozzle, a suction chamber and a diffusion chamber. The working fluid enters the nozzle through the tube under pressure, and is ejected from the nozzle outlet at a high speed. Due to the extremely high speed of the ejected working fluid, the liquid (or gas) near the nozzle is taken away. At this time, a vacuum is formed in the suction chamber at the rear of the nozzle outlet, so the suction chamber can suck fluid from the suction pipe and mix it with the working fluid, and enter the discharge pipe through the diffuser pipe. If the working fluid is continuously sprayed, the liquid (or gas) can be delivered continuously.
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1. The characteristics of the streamline. --- (1) In the case of steady flow, because the velocity of each fluid particle in the flow field does not change with time, the shape of the streamline passing through the same point remains unchanged , so the streamline and the trajectory coincide. In unsteady flow, generally speaking, streamlines change with time, so streamlines and traces do not coincide. (2) Only one streamline can pass through a certain space point at a given moment, and generally streamlines cannot intersect or branch. Otherwise, the fluid particle will have several different flow directions at the same space point. Streamlines can intersect only at those points in the flow field where the velocity is zero or infinite, because at these points there is no problem of different flow directions at the same point. The point at which the velocity is zero is called a stationary point, and the point at which the velocity is infinite is called a singularity. (3) The streamline cannot be turned suddenly, it is a smooth continuous curve. (4) Where the streamlines are dense, it means that the flow velocity in the flow field is relatively large, and where the streamlines are sparse, it means that the flow velocity is relatively small.
4. Calculation questions
1. See textbook p.77
2. Refer to textbook p.63/p.70
3. See textbook p.111
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