Turbulent flow: concepts, properties and applications

Turbulent flow is one of the two fundamental types of fluid motion and the opposite of laminar flow. Due to its complexity and irregularity, this type of flow is of great importance in engineering, physics, and environmental science. In turbulent flow, fluid particles move irregularly and turbulently, and intensive mixing of the fluid layers occurs. In this article, we review the basic concepts, properties, defining equations, and applications of turbulent flow.

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1.     Basic concepts of turbulent flow

Turbulent flow is the irregular, turbulent motion of a fluid in which the   fluid particles   move randomly and in different directions. This type of flow typically occurs at high speeds and in fluids with low viscosity. The Reynolds number is used to distinguish between turbulent and laminar flow. The Reynolds number is a dimensionless parameter that indicates the ratio of inertial forces to viscous forces. In turbulent flow, the Reynolds number is typically greater than 4000.

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2.     Properties of turbulent flow

Turbulent flows have unique properties that distinguish them from laminar flows. These characteristics include:

a) Irregular and turbulent movement

In turbulent flow, the particles of a fluid move chaotically and in different directions. This irregular movement creates vortices and intensive mixing between the fluid layers.

b) Rapid changes in speed and pressure

In turbulent flow, the fluid’s velocity and pressure change continuously and rapidly. These changes cause strong fluctuations in the flow rate.

c) Greater heat and mass transfer

Due to the intensive mixing of the fluid layers, the intensity of heat and mass transfer in turbulent flow is significantly higher than in laminar flow. This property is important for applications such as heat exchangers and cooling systems.

d) greater pressure drop

Turbulent flow results in a greater pressure drop than laminar flow due to turbulent motion and higher internal friction. This must be taken into account when designing fluid transport systems.

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3.     Basic equations of turbulent flow

Turbulent flows are described by partial differential equations that govern fluid dynamics. These equations include the continuity equation, the Navier-Stokes equations, and the energy equation. However, due to the complexity of turbulent flows, the analytical solution of these equations is extremely difficult, which is why numerical methods and computer models are typically used.

a) Continuity equation

The continuity equation states that mass   is conserved in a closed system. For an incompressible flow, the equation is:

$$\nabla \cdot v$$

where     $velocity\; vector$   of the fluid.

b) Navier-Stokes equation

The Navier-Stokes equation describes the motion of a fluid. For turbulent incompressible flow, the equation is:

ρ(∂v∂t+v⋅∇v)=−∇p+μ∇2v+f    ρ    (    ∂    t    ∂    v    ​+    v    ⋅    ∇    v    )    =    −    ∇    p    +    μ    ∇    2    v    +    f

Where      ρ   $is$     the density of the liquid,     p    $is\; the$    pressure    ,     μ    $is\; the$     dynamic viscosity and     f    $is\; the$     external force.

c) Energy equation

The energy equation describes heat transfer in a fluid. For turbulent flow, the equation is:

ρcp(∂T∂t+v⋅∇T)=k∇2T+Φ    ρ    c    p    ​(   ∂    t    ∂    T    ​+    v    ⋅    ∇    T    )    =     k    ∇    2    T    +    Φ

where     $is\; the$     temperature,     cp is    the heat capacity    ,     $the$     thermal conductivity and     Φ    $is$ the$viscosity\; loss$     .

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4.     Applications for turbulent flows

Turbulent flows are widely used in industry and science due to their unique properties. Here are some of them:

a) Liquid transport systems

In pipelines and channels used to transport fluids , turbulent flow is sometimes preferable due to the intensive mixing and more intensive heat and mass transfer. This is important for the design of heating and cooling systems, oil and gas pipelines, and air conditioning systems.

b) Heat exchanger

In heat exchangers, turbulent flow increases the heat transfer rate between fluids. This is used in the chemical industry, power plants, and cooling systems.

c) Airspace

In aircraft and rocket construction, turbulent airflow around the wings and fuselage creates aerodynamic   forces. Understanding these flows is crucial for improving efficiency and reducing fuel consumption.

d) Environment

In the study of ocean and air currents, turbulent flows play an important role in the transport of heat, mass, and energy. This is important for weather forecasting and climate change studies.

d) Food and pharmaceutical industries

In mixing and conveying processes in the food and pharmaceutical industries, turbulent flow increases efficiency and improves product quality.

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5.     Advantages and disadvantages of turbulent flow

Turbulent flows have advantages and disadvantages that must be taken into account when designing different systems.

Advantages:

• Higher heat and mass transfer rates
• Better mixing of materials
• Improving the efficiency of some processes such as heat exchangers

Defects:

• further pressure drop
• Complexity of analysis and control
• It requires more energy to create and maintain turbulent flow.

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6.     Conclusion

Turbulent flow plays a vital role in industry and science as one of the most important types of fluid motion. Due to the irregular and turbulent nature of fluid motion, this type of flow is widely used in fluid transport systems, heat exchangers, aerospace engineering, and environmental protection. However, complexity and high pressure drops also pose serious problems in turbulent flows. With the development of modeling methods and experimental precision, it has become possible to analyze and optimize turbulent flows in various applications, leading to greater efficiency and lower costs.