Fluid Physique Points:
1.The fluid mass point is wireless scale, and only the translational movement is done.
2.The fluid point does not do the immediate thermal movement, but only the macroscopic movement under the action of external force;
3.The average value of the fluid-related properties in the range of the surrounding frontage volume centered on the fluid mass point is used as the physical property of the fluid mass point
Fluid Element:There is a line-scale fluid unit, which is called the fluid "plasma", referred to as the fluid element. Fluid elements can be seen as tiny units composed of a large number of fluid mass points.
Continuum Assumptions:Suppose a fluid is a medium composed of fluid particles that are continuously distributed.
Contents of the Continuity Media Model:According to the concept of fluid pointing and the continuum model, each fluid particle has a definite macroscopic physical quantity, when the fluid fluid point is located at a certain spatial point, if the physical quantity of the fluid mass point, the physical spatial continuous distribution function can be established, according to the basic laws of physics, the differential equation that satisfies the physical quantity can be established, and the mathematical continuous function theory can be used to solve these equations, and the continuous change law of the physical quantity with space position and time can be obtained.
Cohesion of molecules:When the two layers of liquid do relative motion, the average distance between the molecules of the two layers of liquid increases, and the force between the molecules is realized into the force of attraction, which is the cohesion of the molecule.
The liquid fast flow layer drives the slow flow layer through molecular cohesion, and the diffuse flow layer blocks the movement of the fast flow layer through the cohesion of molecules, which is manifested as internal friction. γ
Non-slip condition of the fluid on the surface of the solid: The cohesive force between the molecules adheres the fluid to the surface of the solid, moving with the solid or coming to rest.
Newtonian fluids:Fluids with constant dynamic viscosity are called Newtonian fluids.
Newton's law of viscosity states:The viscous shear stress of Newtonian fluid is directly proportional to the shear rate of the fluid and also shows that for a given fluid, the viscous shear stress acting on the fluid is determined by the velocity gradient between two adjacent layers of fluids, not by velocity:
Effect of temperature on viscosity:Temperature has a great influence on the viscosity of a fluid. The viscosity of a liquid decreases with increasing temperature, while the viscosity of a gas increases with increasing temperature.
Effect of pressure on viscosity:Changes in pressure have little effect on viscosity, and only when a change in pressure of a few hundred atmospheres occurs, the viscosity changes significantly, and the viscosity of gases and liquids increases at high pressure.
Capillary phenomenon:The phenomenon that the liquid level in the glass tube rises or decreases under the action of surface tension is called capillary phenomenon;
There are two ways to describe fluid motion.
Lagrange Method:The Lagrangian method is also known as the follow-up method. It focuses on the fluid points, moves with the flow points, records the changes of various physical quantities with the position and time during the movement of the fluid points, and understands the whole picture of the entire fluid movement by following all the points.
Eulerfa:Euler's method is also known as local law. It looks at points in space and expresses the physical quantities of fluids as a function of position and time in space. The physical quantity of a point in space refers to the amount that occupies a point in space at a certain time.
The physical quantity of the fluid mass point, the fluid mass point occupying the space point at different times is different.
Velocity Field:The velocity field is a field composed of velocity vectors at each coordinate point in fluid space. The velocity field describes not only the spatial distribution of velocity vectors, but also the variation of this distribution over time.
Steady Flow:Flow in which the flow parameters do not change over time. Conversely, the flow of fluid parameters with varying time is called indefinite length flow.
Trace:The trajectory of the fluid point movement. Marking a particle in the flow field and connecting its location points at different times into lines is the trace of the fluid point.
Streamline:A streamline is an imaginary curve that indicates the direction of the velocity vector at each point in the flow field at a certain time.
Flow Surface:A polygon that passes through all the streamlines at each point on a non-streamlined curve.
For a stationary flow field, the streamlines are also traces.
Pulse:A vein is a line that connects cities through a fixed point of fluidity one after another.
Fluid Lines:The connection of a series of fluid points connected from end to end marked at a certain time in the flow field is called the fluid line at that time. Because this series of fluid texture points is marked by the same time, each particle reaches the same fluid line position at the next time, so it is also called a timeline.
Flow Tube:The tubular surface in the flow field that is bounded by each point of the streamline passing through an arbitrary non-streamline closed curve is called a flow tube.
Flow Beam:The fluid inside the flow tube is called the flow beam.
Total Flow:In engineering, the fluid enclosed by the pipeline and the pipe wall is also regarded as the sum of countless micro-element flow beams, which is called the total flow.
Constant Flow:In terms of time, if the flow parameters (velocity, pressure, density, etc.) at each spatial point do not change with time, such a flow is a constant flow, and vice versa.
Uniform flow:If the migration acceleration of the particle is zero, the flow is uniform, and vice versa.
Inflow:The viscous flow that is confined between solids and avoidance is called influx.
Outflow:Outflow usually refers to the external circumference of a fluid to an object, and the effect of a solid wall on the flow is usually limited to a limited extent, and the flow field can be infinite.
According to whether the vorticity in the flow field is zero or not, it can be said that the fluid can be divided into rotationless flow and spiral flow.
System:It refers to a group of definite fluid points, and the shape, volume, and surface area of the system can be constantly changed during movement, but these definite fluid points should always be included. The sum of the physical quantities of all fluid mass points is called the physical quantity of the system, and more accurately it should be called the extension quantity of the system. The rate of change of the extension of a system over time is called the system derivative.
Control Body:Artificially selected areas of spatial geometry in the flow field. Its boundary faces are called control planes.
The principle of fluid continuity: According to Lagrangian, the fluid substance (mass) contained in a fluid system remains constant during the flow processAccording to Euler, if the density of the fluid does not change (incompressible fluid), the substance flowing into the control body (mass) should be equal to the substance flowing out of the control body (mass). The latter is often referred to as the principle of continuity.
Conditions for the application of the Bo Effort Equation:1.No viscous fluids. 2.Incompressible fluids. 3.Fixed-length flow. 4.Along the streamline;
The conditions for the application of the equation along the total flow of Bo effort: 1No viscous fluids. 2.Incompressible fluids. 3.Steady Flow:4.Along the flow beam, and the calculated cross-section meets the conditions for a slow variable flow.
Non-uniform flow:It is divided into gradual flow and rapid flow flow, and the flow with a small migration acceleration of the flow point or the flow with a streamline close to a parallel straight line is defined as a gradual flow, and vice versa is a rapid flow.
The reasons why the properties of uniform flow are approximately valid for gradient flow are: 1. The overflow section of gradient flow is close to the plane, and the velocity direction of each point on the surface is nearly parallel2. The distribution law of dynamic pressure and static pressure on the flow section of gradient flow is the same.
Turbulence:Turbulent motion is a flow formed by the superposition of vortices of various sizes and different vorticities, and then coexists with reverse motion in turbulent motion.
Sticky Affected Area:By the condition of non-slipping of the wall, an area of velocity gradient from zero to the outer flow velocity is formed around the object.
Cavitation:The process of formation, development and collapse of vapor or gas vacuoles in the liquid or at the interface between liquid and solid when the local pressure in the liquid is reduced to the saturated vapor pressure of the liquid.
Cavitation:When the cavitation generated in the low-pressure area of the flow field moves to the high-pressure area, or when the local flow field periodically changes from low-pressure to high-pressure, the cavitation will collapseThe deformation and material erosion phenomenon on the surface of a moving object in a liquid after being impacted by cavitation is also known as denudation or cavitation.
Two forms of failure of cavitation erosion:
1.When the cavitation is close to the wall, the cavitation collapses by forming a microjet that continuously hits the wall, causing direct damage
2.The cavitation collapse forms a shock wave and impacts the wall at the same time, and the continuous impact caused by the collapse of countless cavitations will cause fatigue damage of the wall material
Boundary Layer:When reγ1, the viscous affected area is reduced to the narrow area of the wall area, which is called the boundary layer.
Boundary Layer Features:1.The thickness is very small;2.The thickness of the boundary layer grows as the flow along the flat plate progresses;
Boundary layer separation:Boundary layer separation, also known as flow separation, refers to the phenomenon that the boundary layer that flows close to the wall surface is separated from the wall.
Speed of sound:Sound velocity is a general term for the propagation velocity of a weak disturbance in an elastic medium. The speed of propagation is only related to the ratio of elasticity to mass of the ring.
Shock:Both theoretical analysis and experiments show that when a strong compressive disturbance propagates in the supersonic flow field, it descends under certain conditions to form a strong pressure wave front, which is called a shock wave.
Fanno Line:1.When MA=1 and DS=0, the velocity of the Fannot line at the point of maximum entropy reaches the speed of sound;
2.When MA <, DS and DT have different names, indicating that when the temperature drops, the specific entropy increases, and the state proceeds in a clockwise direction along the half-branch of the Fanno line. The subsonic flow accelerates in the adiabatic friction tube, but reaches the speed of sound at most (ma=1), and the temperature, pressure, and density are all reduced, and the total pressure is also reduced. γ
3.When MA>1, DS and DT have the same sign, indicating that when the temperature rises, the specific entropy also increases, and the state proceeds in a counterclockwise direction along the lower branch of the Fannot line. The supersonic velocity decelerates in the adiabatic friction tube, but reaches the speed of sound at most (ma = 1), and the temperature, pressure, and density in the flow all increase, and the total pressure decreases.
Yong congestion phenomenon in adiabatic friction tubes
The actual tube length l>lmax will occur when the congestion phenomenon occurs.
1.For subsonic flows, the pressure disturbance caused by the Yong plug can propagate upstream to the inlet, causing the inlet to overflow until the outlet section is exactly a critical section.
2.For supersonic flow, the Yong generates a shock wave in the tube, which then becomes a subsonic flow, moving the critical section to the outlet section. The location of the shock wave depends on the severe condition of the Yong, especially when the location of the shock wave occurs even before the outlet section, forming an overflow, which is a decrease in flow.
Rayleigh Line:1.For subsonic flow (ma<1), heating (dq>0) will cause the flow to accelerate (dv>0), but at most to ma=1 (halfway along the Rayleigh line in a clockwise direction);2.For supersonic flow (ma>1), cooling (dq<0) will cause the flow to accelerate further (dv>0) (halfway along the Rayleigh line in a clockwise direction) and vice versa.
Heating-induced congestion:
1.For subsonic flow, the pressure disturbance is transmitted in the reverse direction to the inlet section, resulting in overflow and reducing the flow rate
2.For supersonic flow, the plug generates a shock wave in the tube, when the total pressure loss is greater, and the shock wave moves upstream, and the process does not stop until the inlet is crossed. The supersonic gas flow is first converted into a subsonic flow through a shock wave, and then an overflow is created, and the flow is reduced before it can pass through the pipe.
Doppler effect:Due to the different density of the sound waves, observers from different positions will hear different frequencies, a phenomenon known as the Doppler effect.
Mach cone:When the fluid flows at supersonic speed, the Mach wave no longer remains flat at this time, but the expanding cone surface in the direction of the flow field velocity with O as the apex, and the wave front of the spherical pressure emitted from the point sound source is tangent to the cone, the cone is called the Mach cone, the bus is called the Mach line, and the half cone angle of the cone is called the Mach angle
The basic characteristics of the supersonic flow field: the propagation of weak perturbation waves in the supersonic flow field is bounded;
Composition of head loss:
1.Loss along the way is the frictional loss caused by the shear stress of the pipe wall when flowing along the pipe of equal section;
2.Local losses are made up of 1Redistribution of velocity due to cross-section changes;2.Fluid elements collide with each other and increase friction;3.Secondary Flow;4. Losses caused by flow separation, formation of vortices, etc.
Physical Meaning of Acceleration Formula:
Acceleration at point b = rate of change of velocity at point b with time (local rate of change of b) + rate of change due to difference in spatial position of b (rate of change of migration along each axis of point b).
Physical Significance of the N-S Equation:
Mass Acceleration (inertial force) = volumetric force + differential pressure force (pressure gradient) + viscous force (divergence of viscous shear stress).
Physical Significance of Birch Effort Equation:
Velocity head + position head + pressure head = total head.
Position head + pressure head = pressure head.
Helmholtz's law of velocity decomposition significance:
The velocity of another point m in the field of the m0 point = the velocity of the m0 point + the relative velocity due to (fluid rotation + linear strain rate + angular deformation rate).
Commonly used flow analysis methods:
1.The fundamental laws of physics (the law of conservation of mass, Newton's laws of motion (the law of conservation of momentum and momentum moment), the first law of thermodynamics (the law of conservation of energy)).
2.System and Control Body Analysis;
3.Methods of differentiation and integration;
4.Dimensional analysis;