# What Everybody Ought to Know About CFD

**CFD or Computational Fluid Dynamics **is one of the key analysis methods used in engineering applications. CFD’s origin lies in the human’s efforts to better understand the power of natural elements like wind, storms, floods, or sea waves.

**What do we Know about Flows?**

Historical evolution of sciences started to classify the natural power and associated reaction of air, water or gases in the physical discipline named Fluid Dynamics. This is offering a systematic structure that embraces empirical laws derived from flow measurement used to solve practical problems. A typical Fluid Dynamics problem involves basic fluid properties like flow velocity, pressure, density, and temperature, in relation with time and space.

In everyday life, we meet fluid flows in: meteorology (rain, wind, floods, hurricanes), heating, ventilation and air conditioning, aerodynamic design, engines combustion, industrial processes, or human body – blood flow, and so on. Fluid Dynamics has a wide range of applications, including calculating forces on aircraft, determining the mass flow rate of petroleum through pipelines, predicting weather patterns.

**What is CFD?**

Gas and liquid flows behavior are quantified by partial differential equations representing conservation laws for the mass, momentum, and energy. Computational Fluid Dynamics is a branch of Fluid Mechanics that uses numerical analysis and algorithms to solve fluid flows situations. High-performing computers are used to perform the calculations required to simulate the interaction of liquids and gases with surfaces defined by boundary conditions. [1]

CFD is based on the **Navier-Stokes equations**. Arising from applying Newton’s second law to fluid motion, together with the assumption that the stress in the fluid is the sum of a diffusing viscous term and a pressure term; these equations describe how the velocity, pressure, temperature, and density of a moving fluid are correlated. [2]

The development of CFD has been closely associated with the evolution of high-speed computers.

**Brief CFD History**

1922 – Basis of modern CFD and numerical meteorology made by Lewis Fry Richardson in a weather forecasting scheme using differential equations and finite differences [3];

1933 – Earliest numerical solution for flow past a cylinder developed by A. Thom [4]

1950 – First 24 hours weather forecast performed by the ENIAC modern computer [5]

1955 – Particle-in-cell simulation method for transient 2D fluid flow developed by Los Alamos National Lab [6]

1963 – Vorticity-stream-function method for 2D, transient, incompressible flow [7]

1965 – Marker-and-cell method for time dependent viscous flow developed by Los Alamos National Lab [8]

1966 – Fluid-in-cell method developed for unsteady of compressible flow problems [9]

1967 – First 3D model based on panels discretization published by Douglas Aircraft [10]

1968 – First lifting Panel Code (A230) described by Boeing Aircraft [11]

1970 – First description of Full Potential equations published by Boeing [12]

1981 – 3D FLO57 code based on Euler equations for transonic flows [13]

After 1981 – many fundamental researches contributed to CFD 2D and 3D methods focused on airfoil design and analysis. NASA researches dedicated to the Navier–Stokes equations developed 2D codes ARC2D and 3D codes like ARC3D, OVERFLOW, and CFL3D, being main sources for modern commercial CFD packages.

**Why is it Important to Use CFD?**

With a **CFD analysis** we can understand the flow and heat transfer throughout a design process. The basic methodology for any engineering CFD analysis is developed on few procedures:

• Understanding flow model – Flow separations, transient effect, physical interactions;

• Proving assumed model – Experimental results validation, parametric studies, structural simulations;

• Model optimizing – Reducing pressure drops, flow homogenization, improving laminar and turbulent mixing.

Without numerical simulations of fluid flow it is very difficult to imagine how:

• meteorologists can forecast the weather and warn of natural disasters;

• vehicle designers improve the aerodynamic characteristics;

• architects can design energy saving and safe living environments;

• oil and gas engineers can design and maintain optimal pipes networks;

• doctors can prevent and cure arterial diseases by computational hemodynamic.

**Performing Sophisticated CFD Simulations with SimScale**

Being considered as main mechanical fluid simulation, Fluid Dynamics is one of the key analyses methods used by the SimScale platform together with Structural Mechanics, Thermodynamics, Acoustics and Particles Analyses. As main modelling included by CFD simulation, SimScale is offering:

• Multiple laminar and turbulence models – based on the Reynolds number of the fluid flow;

• Steady-state applications and transient solvers set up;

• Mass transport within fluid flows;

• Access to multiple incompressible and compressible fluids solvers;

• Single- and multiphase flows simulation;

• Advanced modelling concepts as porous media or rigid body movement of fluid domains

**CFD Models Diversity in SimScale Public Projects**

For better illustrating the multitude of applications of CFD analyses, let’s take a look at some relevant examples available in the SimScale Public Projects library:

Many projects had as main goal the aerodynamic analysis of different mobile vehicles like Formula 1 cars or motorbike turbulence airflow simulation based on a steady-state, turbulent, incompressible modelling strategy using a k-omega-SST turbulence model.

Many times during a football or basketball game the ball takes very strange trajectories. This project shows how a flow analysis can be used to investigate the aerodynamic behavior of a football.

During his session of SimScale free Formula 1 aerodynamics, Nic Perrin gave a lot of great insights into race car fascinating aerodynamics, such as the interaction between the front wing and the wheels or how the vortices help to improve the downforce.

An interesting multiphase flow analysis can be found in the free surface simulation of a waterfall.

A large applicability of CFD simulation is in industrial areas. Below we have a water purification process simulation example. A maze model is used for multiple analysis of the mean flow field and the change in contamination distribution for the purification process.

Very interesting are flow simulations for valves, with large applicability in the industries. Here is a simple flow simulation in a globe valve where the results can be analyzed and visualized in the integrated post-processing environment.

Highly important are projects related to theoretical models research, founding assumptions for real cases studies.

Here is an example of a non-newtonian flow analysis through a sudden expansion. For detailed study please go to **“Non-Newtonian flow through an expansion channel” documentation. **This project validates the flow velocity profile for a non-newtonian fluid via the steady-state Reynolds-Averaged Navier–Stokes (RANS) approach and Power-Law model for non-newtonian fluids.

All the projects presented in this article can be imported into your own workspace and used as templates. Feel free to browse the SimScale Public Projects for other interesting simulations.

References:[1] – Wikipedia, Computational fluid dynamics [2] – Wikipedia, Navier-Stokes Equations [3] - Richardson L.F. -“Weather Prediction by Numerical Process”, Cambridge University Press, 1922, Reprinted by Dover Publications, New York, 1965 [4] – Thom A. –“The Flow Past Circular Cylinders at Low Speeds”, Royal Society, 651-666, London, 1933 [5] – Lynch P. -"The origins of computer weather prediction and climate modelling", Journal of Computational Physics, University of Miami, 2008 [6] – Harlow F.H. - "A Machine Calculation Method for Hydrodynamic Problems", Los Alamos Scientific Laboratory, 1956 [7] - Fromm, J. E., Harlow, F. H. -"Numerical solution of the problem of vortex street development", Physics of Fluids 6: 975, 1963 [8] - Harlow, F. H.; Welch J. E.,"Numerical calculation of time-dependent viscous incompressible flow of fluid with a free surface", Physics of Fluids 8: 2182–2189, 1965 [9] - Gentry, R. A., Martin, R. E., Daly, J. B. -"An Eulerian differencing method for unsteady compressible flow problems". Journal of Computational Physics 1: 87–118, 1966 [10] - Hess, J.L.; Smith A.M.O., -"Calculation of Potential Flow about Arbitrary Bodies".Progress in Aerospace Sciences 8: 1–138, 1967 [11] - Rubbert, P., Saaris, G. -"Review and Evaluation of a Three-Dimensional Lifting Potential Flow Analysis Method for Arbitrary Configurations,"AIAA paper, 72-188, 1968 [12] – Murman, E., Cole, J. -"Calculation of Plane Steady Transonic Flow," AIAA paper, 1970 [13] – Jameson, A., Schmidt, W. and Turkel, E. -"Numerical Solution of the Euler Equations by Finite Volume Methods Using Runge-Kutta Time-Stepping Schemes,"AIAA paper 81-1259, 1981