Introduction to RANS Turbulence Models: Fundamentals and Applications

Engineering has always been a field of progress and development, continually pushing the boundaries of human innovation and discovery. The Reynolds-Averaged Navier-Stokes Equations (RANS) are a major part of this advancement as they form the basis for Computational Fluid Dynamics (CFD) and enable us to simulate complex turbulent flows. 

In this article, we explore the fundamentals of RANS and its applications, giving you a comprehensive overview of the RANS model and the basics of its usage in engineering. So, let's dive in and learn the details of the RANS models!

Overview of the Reynolds-Averaged Navier-Stokes Equations

Reynolds-Averaged Navier-Stokes (RANS) equations are a set of simplified equations derived from the Navier-Stokes equations and used to describe the average behavior of fluid flows. It provides a mathematical description of the behavior of fluids in a steady state, turbulent, compressible, or noncompressible. The equations are based on the conservation of mass, momentum, and energy principles. 

They are expressed as a system of non-linear partial differential equations, where the unknown variables are the dependent variables that depend on the state and properties of the fluid. The RANS equations model turbulent flows in computational fluid dynamics (CFD). These equations can predict flow characteristics such as pressure drop, drag, and heat transfer in a fluid system.

Characteristics of the RANS Model

The RANS (Reynolds-Averaged Navier-Stokes) model is widely used to simulate internal and external energy production and transfer that occurs in fluids and gases. It is based on the Navier-Stokes equations, a set of fundamental equations that govern the motion of fluids and gases within a given system. The RANS model is utilized for various applications, including industrial and automotive solutions, aerospace and aeronautical engineering, marine solutions, and more. 

The RANS model relies on several underlying assumptions to generate a highly accurate representation of the state of a fluid or a gas. These assumptions are referred to as "the sub-models" of the RANS model, and each provides a different level of detail for the simulation. 

The four most commonly used sub-models are: 

1. Turbulence: This sub-model considers the effects of small-scale motions on the overall flow of the fluid. Turbulence is used to model how energy is added to the system and ultimately dissipates.
2. Diffusion: It considers the effect of diffusion on gas or fluid properties. This includes the diffusion of heat, momentum, and mass. This sub-model can significantly affect the resolution of the simulation results. 
3. Advection: The model takes into account the motion of the gas or fluid caused by the presence of a pressure gradient or an external force field. It simulates how energy is moved about in a given system, such as for convection currents.
4. Viscosity: It examines the shear stresses that can arise in fluids and can attempt to account for various forms of shear stresses that can be present, such as those due to turbulent motions. Additionally, a viscosity sub-model can help to stabilize the flow at any boundaries. 

Overall, the RANS model is a widely used simulation method utilised in various industries and applications. It relies on a set of assumptions known as sub-models which provide varying levels of detail for the simulation. These four sub-models (turbulence, diffusion, advection, and viscosity) can all influence the accuracy of the RANS model as they each account for different effects in the system.

Advantages and Disadvantages of a RANS Model

A Random Sampling (RANS) Model is a statistical model used to assess the effects of various variables on random samples. This model type is commonly used in investment banking, marketing, and economics. It can be used to determine the probability of certain outcomes or to forecast future behavior. 

Advantages of the RANS Model 

• Simpler than other models: The RANS Model is much simpler to use than other statistical models. It involves applying one formula to the data to achieve the desired results. This makes it easier to understand and interpret, even for those with limited statistical knowledge.
• Cost-effective: The RANS Model is much more cost-effective than other statistical models. As a result, it is suitable for individuals on a small budget. 
• Flexible: The RANS model is also highly-flexible, allowing users to apply different formulas to the same data set. This ensures that the results obtained are accurate, reliable, and valid. 
• Easy to implement: The RANS model is straightforward to implement. This makes it ideally suited for companies with limited resources. 

Disadvantages of the RANS Model 

• Difficulty interpreting results: The main disadvantage of the RANS Model is its difficulty interpreting the results. For those unfamiliar with statistics, it may be difficult to interpret the results obtained from a RANS Model. 
• The simplicity of results: The simplicity of the RANS Model yields results that may not accurately reflect the complexities of the data. This could lead to incorrect predictions or analysis. 
• The limited number of variables: The RANS model is limited by the number of variables that can be included in the formula. This makes it unsuitable for larger data sets or those involving multiple variables. 
• Ignoring underlying relationships: Another disadvantage of the RANS Model is its propensity for ignoring underlying relationships. By basing its analysis on only one formula, it fails to account for the possible interactions among different variables.

Real-World Applications of the RANS Model

Since the last few decades, RANS models have been extensively used in the aerospace, automotive, offshore, naval, and energy sectors, among others. They are often employed to simulate the behavior of flows encountered in aerodynamics, combustion, and multiphase phenomena. 

Real-world applications of RANS models include the analysis of lid-driven cavity flows in a closed channel, the flow in a boiler, or the study of the effect of cavitations on a moving vehicle. RANS models have also been used to design and develop aircraft engines and wings, study aerofoil performance, and analyze complex industrial burner mixing and chemical reaction behaviors. 

In addition, RANS models are often used to predict the hydrodynamic behavior of ships, submarines, and offshore platforms under different sea states. These models can provide insight into ship handling, maneuvering performance, and hydrodynamic coefficients. 

The automobile sector uses RANS turbulence models to optimize vehicle designs and investigate fluid-structure interactions, including engine components, aerodynamic drag, ground clearance, slipstream effects, and wind tunnel testing. They've also been used to research turbochargers, exhaust systems, and drive trains.

In the energy sector, RANS models are used to predict fluid flow behavior in nuclear reactors and optimize thermal power plants, including the analysis of combustion chambers and steam turbine components.

Overall, RANS models can be used to accurately predict and simulate various physical phenomena associated with the behavior of fluids. These models have significantly contributed to developing and improving industrial and commercial products in many different industries.

Conclusion

From learning the fundamentals of RANS to understanding its applications in research, industry, and engineering, this article has been an insightful guide for every learner. While the Reynolds-Averaged Navier-Stokes equations provide a set of simplified equations capable of modeling turbulent flow, it is important to understand the basics and continue to study the principles and methods for turbulent flow. 

To learn more about Turbulence and RANS Modelling, enroll in Skill-Lync’s course. Talk to our experts to know more about our curriculum and projects you will work on during the course.