Great post to share: Written for beginners just starting with Fluent

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Table of Contents 1 Getting Started 2 Basic Concepts and Terms Related to Fluids and Flow in CFD Calculations 2.1 Ideal Fluids and Viscous Fluids 2.2 Newtonian Fluids and Non-Newtonian Fluids 2.3 Compressible Fluids and Incompressible Fluids 2.4 Laminar Flow and Turbulent Flow 2.5 Steady Flow and Unsteady Flow 2.6 Subsonic Flow and Supersonic Flow 2.7 Heat Transfer and Diffusion 3 What is the purpose of discretization in numerical simulations? How to discretize the computational domain? What grids are commonly used for discretization? How to discretize the control equations? What are the common methods for discretization? What’s the difference between them? 3.1 Purpose of discretization 3.2 Discretization of the computational domain and commonly used grids 3.3 Discretization of governing equations and its methods 3.4 Differences among various discretization methods 4 Comparison of the performance of common discretization schemes (stability, accuracy, and efficiency) 5 What is the purpose of numerical computation of flow fields? What are the main methods? What is its basic idea? What is their respective scope of application? 6 What are the respective characteristics of numerical solution methods for compressible flow and incompressible flow? Why is solving incompressible flow actually more difficult than solving compressible flow? 6.1 Numerical solutions to compressible Euler and Navier-Stokes equations 6.2 Solving incompressible Navier-Stokes equations 7 What are boundary conditions? What is its physical meaning? What is its relationship with the initial conditions? 8 In numerical computation, what are the differences between hyperbolic, elliptic, and parabolic partial differential equations? 9 In grid generation techniques, what is a conformal coordinate system? What is a grid-independent solution? 10 In GAMBIT, what are the main methods used to determine the quality of a grid as indicated by the “check”? And what details does it generally pay attention to when creating grids? 11 If the grid spacing differs at the junction of two surfaces, that is, when the two grids are not continuous, how can this situation be overcome? 12 Several points to note when setting the GAMBIT boundary layer type: a. How should undefined boundary lines be handled? b. How are the internal boundaries within the calculation domain handled (2D)? 13 Why is it necessary to specify boundary types and region types after dividing the grid? What are the common boundary types and region types? 14 20 What are fluid zones and solid zones? Why use the concept of regions? How does FLUENT use regions? 15 21 How to monitor FLUENT’s calculation results? How to determine whether a calculation converges? How is the convergence criterion defined in FLUENT? Analyze the control parameters that determine the convergence of calculations, and explain how to select and set these parameters? What are the common methods for solving the problem of non-convergence? 16 22 What is the relaxation factor? What impact does the relaxation factor have on the calculation results? What impact does it have on the convergence of the computation? 17 23 During FLUENT simulations, it is common for the “turbulence viscous rate” to exceed its limit value; how should this be addressed? And what value does the limit value here refer to? What is the impact of these corrections on the calculation results? Why does “reversed flow” sometimes occur when running calculations in FLUENT? 18 24 What is its specific meaning? Is there any way to avoid it? If this continues to be the case, what impact will it have on the final calculation results? How to get started learning *any software – everyone goes through a period of getting started. It is necessary to study diligently and earnestly. As for what the best learning method is, I cannot make any definitive conclusions. Here, I would like to share my experiences from when I first started using FLUENT three years ago, in the hope that it can provide some help to those who are new to FLUENT. At that time, I needed to learn FLUENT for my graduation project, so my teacher gave me a book: Han Zhanzhong’s \"FLUENT Fluid Engineering Simulation Examples and Applications.\" Of course, there were two prerequisites for studying this book: first, a foundation in fluid mechanics; second, access to the FLUENT software. Then, follow the two-dimensional calculation examples in the book, learning one example and one step at a time*, after that move on to three-dimensional calculations, and then perform specific calculations for the projects you encounter. One should not rush things; by learning step by step, from the preprocessor GAMBIT, to simulation using FLUENT, and then to post-processing with tools like TECPLOT, persistence will yield very significant results. If there is a teacher who knows FLUENT around you, consulting that teacher when facing problems is the most effective approach; for issues you don’t understand, you can also search the Internet or look up relevant books to find answers. Additionally, I also have Wang Fujun’s book “Computational Fluid Dynamics Analysis”. Studying both books together yields better results. Basic concepts and terms related to fluids and flow in CFD calculations: ideal fluids and viscous fluids ； Newtonian fluids and non-Newtonian fluids ； Compressible fluids and incompressible fluids ； Laminar and turbulent flow ； Steady flow and unsteady flow ； Subsonic and supersonic flow ； Heat conduction and diffusion, etc. Ideal fluids and viscous fluids: Although fluids at rest cannot withstand shear stress, when in motion they resist the relative movement between adjacent layers of fluid, that is, the relative sliding velocity; this resistance is known as viscous stress. This property of a fluid to resist the relative sliding velocity between two layers of fluid, or more generally, to resist deformation, is called viscosity. The degree of viscosity depends on the properties of the fluid and changes significantly with temperature. Experiments show that the magnitude of viscous stress is proportional to viscosity and relative velocity. When the viscosity of the fluid is low (in fact, the viscosity of the most important fluids such as air and water is very low), and the relative velocity of motion is also low, the viscous stress generated is negligible compared to other types of forces such as inertial forces. At this point, we can approximately regard the fluid as inviscid; such a fluid is called an ideal fluid. It is quite obvious that an ideal fluid has no resistance whatsoever to tangential deformation. In terms of viscosity, we can divide fluids into two main categories: ideal fluids and viscous fluids. It should be emphasized that a true ideal fluid does not exist in reality; it is merely an approximate model of real fluids under certain conditions. Newtonian fluids and non-Newtonian fluids are the types of fluids that are most commonly encountered in daily life and engineering practice. Those whose shear stress is in a linear relationship with the shear deformation rate according to the following formula are known as Newtonian fluids. Those in which shear stress is not linearly related to the rate of deformation are called non-Newtonian fluids. Figure 2-1(a) shows the relationship curve between shear stress and deformation rate. Those that satisfy the linear relationship in the above formula are Newtonian fluids. The others are non-Newtonian fluids. Among non-Newtonian fluids, they are further classified into dilatant fluids, pseudoplastic fluids, ideal Bingham fluids with a yield stress, and plastic fluids, based on the relationship between their shear stress and deformation rate. Generally, oils, paints, milk, toothpaste, blood, mud, and the like are all non-Newtonian fluids. The study of non-Newtonian fluids has wide applications in the fiber industry, plastics, petroleum, the chemical industry, food processing, and many light industries. Figure 2-1(b) also shows that for some non-Newtonian fluids, their viscosity properties exhibit a time effect; that is, the shear stress depends not only on the rate of deformation but also on the duration of application. When the deformation rate remains constant, the shear stress increases over time; such a non-Newtonian fluid is known as a rheopectic fluid. A non-Newtonian fluid in which the deformation rate remains constant while the shear stress decreases over time is called a thixotropic fluid. In the flow of compressible fluids and incompressible fluids, due to changes in factors such as pressure and temperature, the volume (or density, since the mass of the fluid particles remains constant) of these particles changes to some extent. The property whereby the volume or density of a fluid particle can change under certain pressure differences or temperature differences is known as compressibility. All real fluids are compressible. Its degree of compression depends on the properties of the fluid and the external conditions. For example, at 100 atmospheres of pressure, the volume of water decreases by 0.5%; when the temperature changes from 20° to 100°, the volume decreases by 4%. Therefore, under normal circumstances, liquids can be approximated as incompressible. However, in certain special cases, such as explosions in water or water hammer, it is necessary to treat the liquid as compressible. The compressibility of gases is much greater than that of liquids; therefore, under normal circumstances they should be treated as compressible fluids. However, if the pressure difference is small, the rate of motion is low, and there is no significant temperature difference, then the resulting volume change of the gas is actually quite minor. At this point, the gas can also be approximated as incompressible. The density appears in the continuity equation for compressible fluids; therefore, it can be treated as an independent variable in the continuity equation for solution. Subsequently, the pressure can be determined using the equation of state for gases. The pressure field of an incompressible fluid is indirectly specified by the continuity equation. Since there is no equation for directly solving for pressure, solving the flow equations for incompressible fluids presents special difficulties. Experiments on laminar flow and turbulent flow show that viscous fluid motion exists in two forms: laminar flow and turbulent flow. The nature of these two forms is completely different. Laminar flow is a regular pattern of fluid motion in which different layers of the fluid flow separately without mixing. The trajectories of fluid particles are smooth, and the flow remains stable. The characteristics of turbulence are completely opposite: the fluid motion is highly irregular, its various parts mix vigorously, the trajectories of particles are chaotic, and the flow field is extremely unstable. These two distinctly different forms of motion can transform into one another under certain conditions. Steady flow and unsteady flow are classified based on time; fluid flow is divided into these two categories according to whether physical quantities such as velocity, pressure, and temperature change over time. When the physical quantities of the flow do not change over time, it is a steady-flow ； The opposite is called unsteady flow. Steady flow is also known as constant flow, or steady-state flow ； Unsteady flow is also known as non-constant flow or unstationary flow. The fluid flow in many fluid machines during startup or shutdown is generally unsteady, while it can be regarded as steady during normal operation. Subsonic flow and supersonic flow: When the flow velocity is high or there are significant pressure changes in the flow field, the fluid is affected by compressibility. The Mach number is defined as the ratio of local velocity to local sound speed. When the Mach number is less than 1, the flow is subsonic ； When the Mach number is much less than 1 (such as M