流体简介
The fluid is a matter that tends to deform continuously under Shear Stress. It consists of both liquid and gases. All the matters on earth can be classified based on their behavior into two categories, i.e., Solid and Fluid. The substance that tends to flow under certain given conditions is called fluid. It becomes essential to understand different types of Fluid and the difference between fluid and solid for better understanding and visualization of types of fluid flow. In this article, we will be discussing types of fluid and the difference between solid and fluid in brief and then move onto types of Fluid flow.
种类Fluids
Fluids are classified based on their behavior under Shear stress. The Behaviour is analyzed with the help of Quantity calledViscosityand density of Fluid. Viscosity is like frictional force present in Solid Particles, which resist the Motion of Solid. It resists the fluid flow and induces a relative motion between the subsequent fluid layers.
要对分类进行分类并有更好的理解，请考虑以下方程：
注意：上方方程表示沿一个方向流动的流体（x方向），并且在所有其他方向上流动为零。
We can Classify Fluid into 6 Different types, which are discussed as follows:
1.理想的液体：
在这种类型的流体中，粘度被认为为零，并且密度到处都是恒定的。这意味着流体流中流体层之间没有相对运动，并且所有层都以相同的速度移动。理想的液体是一个假设，它们在现实中不存在。这些假设是为了分析给定条件下某些流体的行为。
In Short, we can say that,
2. Real Fluid:
In this type of Fluid, Viscosity is not zero, and Density varies everywhere in the Fluid. It means that there is relative motion between the layers of fluid in fluid flow. Real Fluid is the behavior possessed by Fluid in reality but is often ignored to make the analysis simpler. In Real Fluid, we don’t have any fixed formula for the variation of Density and fixed value for Fluid’s Viscosity. All the Fluids are real Fluid in Nature.
In Short, we can say that,
3.牛顿液：
In real fluid, we don’t have the exact formula for calculating Density, and we don’t know the Viscosity of fluid.牛顿流体是粘度定义值的流体，指数（n）的值为1。所有牛顿流体的方程式可以写如下：
牛顿液can have Constant and variable Density, but the Variation of Density
with respect to time and space will be known to us.
简而言之，我们可以说，
4.非牛顿液：
In this type of Fluid, Viscosity is not zero and is defined accurately. Density can vary or remain constant with respect to Time and Space. The main difference arises with the value of Exponent ‘n’, which is not equal to 1 and depends on the type of Non-Newtonian fluid. The Equation for all Non-Newtonian Fluid can be written as follows:
5.可压缩液：
The fluid is said to be compressible Fluid if density varies with time and space. We can’t say about the Viscosity in this case as it can either be Zero or non-Zero.
简而言之，我们可以说，
6.不可压缩的液体：
如果流体的密度随时间和空间的变化而变化，则流体是不可压缩的。我们不能说在这种情况下的粘度，因为它可以为零或非零。
简而言之，我们可以说，
下面的分类列出了不同流体的粘度和密度的特性：
| S.no. | Type of Fluid | Viscosity | Density |
| 1 | Ideal Fluid | Zero | 持续的 |
| 2 | Real Fluid | 非零 | 多变的 |
| 3 | 牛顿液 | 非零and have definite formula | Can be either Constant or variable |
| 4 | 非牛顿液 | 非零and formula depends on type of Non-Newtonian fluid | Can be either constant or Variable |
| 5 | Compressible Fluid | 零/非零 | 多变的 |
| 6 | 不可压缩的液体 | 零/非零 | 持续的 |
Solid vs Fluid: Differences Between Them
固体和流体的行为不同，并且不会以相同的方式遵守物理规则。由于它们在性质方面的差异，我们拥有用于流体的固体和流体力学的固体力学。当我们在各种条件下密切观察它们的行为时，我们可以注意到它们之间的许多差异。为了将其行为分为更直接的方式，我们使用称为剪切应力的属性。它定义了流体的流量及其行为与固体不同。在剪切应力的作用下，固体倾向于弯曲和变形。因此，它们的剪切应力随弯曲或变形而线性变化。在剪切应力的作用下，液体倾向于连续变形，并且剪切应力的变化与变形无线性。这是固体和流体之间的主要区别。
Fig:Shear Stress Behaviour
流体流动的驱动力
Flow Characteristics of Fluid lead to various Phenomena such as Cyclone, Change in Weathers, Cooling of Internal Combustion Engine, and many more things. The question arises, what is the reason for the flow of Fluid? And can we predict the Fluid flow behavior at every condition? Fluid Flows from One Point to another because of the Pressure difference between the two Points. The Natural Flow of Fluid Occurs from High-pressure to Low-Pressure to equalize the pressure difference at these two points. Flow from Low-Pressure to High-pressure region can be achieved with External Driving force, such as Pump, Etc. Fluid flow Patterns can be classified but can’t be accurately predicted at an Instant of time. All the Prediction for the Fluid flow behavior is made with the use of Softwares that use Numerical Techniques to Approximate the behavior of flow at a given instant.
To Start Our Study for Fluid Flow, let us first learn about different Parameters in fluid flow. These Parameters are Timelines, Pathlines, Streaklines, and Streamlines.
时间线：
如果我们在给定的时间瞬间标记相邻的流体颗粒的流动，则形成时间表。例如，为了在恒定剪切应力的作用下证明流体颗粒的行为，引入时间轴以在每个时间瞬间给出流体的变形。因此，在时间轴中，在给定时间瞬间，每个流体的粒子都会追溯到。
图4：时间轴
Pathline:
如果we trace the path of a fluid particle for some time, it forms Pathline. For Example, take dye and smoke, and take a long exposure photograph of its subsequent motion. The path traced by the Particle is Pathline. Here, we consider a source Particle of fluid and Observe its Path for a given Time. The Path traced by the Particles during that Time is the Pathline of the Particle.
图：途径
Streakline:
如果we mark the Fluid particle Path at a given location for some time, it forms Streakline. During Wind tunnel aerodynamic testing of Automobile, smoke is released towards the car for aerodynamic force and Drag evaluation. The Path traced by the smoke over the car is Streakline. Here, we consider the flow of subsequent layers, observe their Position at a given Instant of time and trace the Position to form Streakline.
无花果：条纹线
Streamline:
它是流体颗粒的路径，因此与之切线给出了该点流体颗粒速度的方向。由于它们是流动的切线，因此可以沿着流线路流动。它们用于计算机仿真进行流动可视化，其中绘制流线以表示流体颗粒所追踪的速度场。
Fig.:Streamline
流体流
流体流可以分为以下类型：
- 统一和非均匀流动
- 稳定而不稳定的流动
- 旋转和无关流
- Compressible and Incompressible Flow
- 粘性和无粘性流
- External and Internal Flow
- Laminar and Turbulent flow
- 1D, 2D, and 3D Flow
Let us Study about them one by one:
1.均匀和非均匀流动
Fluid flow is said to be Uniform if the Velocity of fluid does not change with Space. Hence, in this types of fluid flow, Velocity is only dependent on Time and not on X, Y, Z Coordinates of fluid Particle.
如果流体的速度随空间的变化，则说流体流动不均匀。因此，在这种类型的流动中，速度是时间的功能，x，y，z z坐标的流体颗粒的坐标。例如，如图所示，当横截面区域不变时，流量的速度是恒定的，但是随着横截面区域的变化，随着流体移入截面的变化，速度会变化。流动本质上是非燃料的。
流体通过具有均匀横截面的管道流动称为均匀的流动，如果流体流通过没有均匀（或锥形）横截面的管道，称为非均匀流动
2. Steady and Unsteady flow
Fluid Flow is said to be Steady if Fluid Properties such as velocity and Pressure don’t vary with time. Hence, in this flow, Fluid Properties are only dependent on the X, Y, Z Coordinates of a fluid particle. For this type of flow, streaklines, streamlines, and pathlines are identical.
Fluid flow is Unsteady if fluid properties such as velocity and Pressure vary with time. Hence, in this flow, Fluid properties are dependent on time and X, Y, Z Coordinates of a fluid particle. For this type of flow, streaklines, streamlines, and pathlines are not identical.
在给定的图中,第一个表达式翅ts Steady flow, while the second expression is for Unsteady flow. A constant Discharge through Pipe will be a steady flow, while Variable discharge through the pipe will be Unsteady flow.
3.旋转和无关流动
如果the Fluid particles rotate about their axis while moving in the streamline, it is called Rotational flow.
如果the fluid particles move in a streamline and don’t rotate about their axis, it is called Irrotational flow.
We can identify this type of flow by calculating the flow’s vorticity, which depends on the velocity of flow. If Vorticity is Zero, fluid flow is Irrotational; otherwise, it is rotational flow.
4. Compressible and Incompressible Flow
在可压缩流中，流体的密度随时间和空间而变化。而在不可压缩的流动中，流体的密度保持恒定。该流程在制动液中找到了其应用。在制动系统中，制动液会将脚产生的压力转移到车轮中以折断。如果流体不可压缩，它将将脚施加的精确压力转移到车轮上以进行有效制动。如果流体可压缩，则将转移到车轮的压力将小于施加的压力。它甚至可能为零。因此，制动液本质上应该不可压缩。
We can quickly identify these types of fluid flow with the help of Mach Number. It is defined as:
Ma= V/Vs
- Vs= Velocity of Sound in fluid
- V= Velocity of fluid
| 0 <= ma <0.33 | Incompressible flow |
| Ma>0.33 | 可压缩流 |
5.粘性和非粘性流：
在粘性流中，流体颗粒在随后的层之间经历粘度，因此相对运动发生在流体颗粒层之间。在非粘性流中，流体颗粒在随后的层之间没有任何粘度，因此，流体颗粒之间没有相对运动。
6. External and Internal Flow:
Internal Fluid Flow
壁的存在决定了这种类型的流动模式。完全由固体界定的流动称为内流或管道流动。
如果任何实体不绑定流动，则称为外流。例如，在汽车上流过的流动称为外流，在软件和风洞测试的帮助下可视化。圆管内的流动是内部流动的，可以在软件和简单的实验室实验的帮助下轻松可视化。
7.层流和湍流
在层流中，流体颗粒在不同的层移动，并且不会宏观混合。在这种类型的流体流中，我们可以预测给定时间瞬间的流动模式。所有后续层在流中彼此平行。
在湍流中，流体颗粒Intermix和流动变得随机。在这种类型的流动中，在给定时间的瞬间无法准确预测流的模式。发生涡形的形成，导致大量能源损失。
Reynolds numberis used to predict Flow, i.e., Whether it is turbulent and Laminar. The formula is given by:
re = v*l/µ
Where,
- Re = Reynolds number
- V= Velocity of fluid
- L= Characteristics length of Object where flow is taking place
- µ =粘度系数
For Internal Flow,
| 0 <= re <= 2000 | 层流 |
| 2000 |
从层流过渡到动荡 |
| 4000 |
Turbulent Flow |
对于外流，
| 0<=Re<=100000 | 层流 |
| 100000 |
从层流过渡到动荡 |
| 500000 |
Turbulent Flow |
8. 1-D, 2-D, and 3-D Fluid flow:
在1-D类型的流体流中，流体参数（例如速度）是时间的函数，仅是一个空间坐标。
In the 2-D types of Fluid flow, Fluid parameters such as velocity are the function of time and two spatial Coordinates.
In the 3-D Fluid flow, Fluid parameters such as velocity are the function of time and all three Spatial Coordinates.
| 1-D fluid Flow | u = f（x，t），v = 0，w = 0 |
| 2-D流体流 | u = f（x，y，t），v = g（x，y，t），w = 0 |
| 3-D流体流 | u = f（x，y，z，t），v = g（x，y，z，t），w = h（x，y，z，t） |
FAQs
Q.1。可压缩流体和可压缩流量有什么区别？
Answer-Compressible Fluid talks about Fluid and its density variation, while compressible flow only talks about Fluid in Motion and the density variation of Fluid in Motion. Fluid can have a constant density during static conditions and variable density during dynamic condition. Mach Number determines whether the flow is compressible or not. It does not decide the fluid characteristics.
Q.2。在过渡方案中考虑了哪种类型的流体流量？
Answer-It depends upon the Situation and on your Computer Condition. If you have a value near to turbulent in transition regime and have an excellent Computer for Simulation, go for turbulent. If not, then go for Laminar.
Q.3. Is Flow of air passed through the car is External and Internal Flow
Answer-如果我们分析汽车的空气动力学阻力，则是外流。汽车内部有一些空气。由于它受固体的界限，因此可以将其视为内部流动。
Q.4。在制动液应用中应用的法律名称是什么？
Answer-法律的名称是Pascals Law。它指出，对于不可压缩的流体，压力在所有方向上平均转移。
Q.5。为什么在稳定且不稳定的流程中不提及时间表？
Answer-There is no concept of timeline for Steady flow, since fluid parameters doesn’t depend on time.
Q.6。什么是泵？
Answer-Pump is an External agent that is used in movement of fluid, against their natural direction of flow. For Example, Pump is used in Steam Power plant for taking the water from Condenser to the Boiler at a height.
Q.7. What is Shear stress?
Answer-Shear stress is the stress developed by the force applied tangentially to the Object, on which the force is applied.
