Glossary of Terms in Computational Fluid Dynamics

Acronym for 6 Degrees Of Freedom.

6-DOFⁱ is an advanced simulation capability that allows solid objects to move responding to forces of gravity and fluid pressures. Each degree of freedom represents a form of movement consisting of translations in 3 principal directions (x,y & z) and rotations (roll, pitch & yaw) about about axes aligned with the same 3 principal directions.

6-DOFⁱ is especially critical to modeling floating platforms and sea vessels. However, it can also be used to assess the stability of an aircraft.

Numbers that describe the aerodynamic performance of a vehicle. Drag and lift coefficients are typical "aero coefficients" that most vehicle designers are interested in optimizing.

A typical drag coefficient for a sedan is 0.32, while for a truck it can be around 0.7. The more streamlined a vehicle is, the smaller is its drag coefficient. However in Formula 1, designers are more interested in increasing the down force as opposed to reducing the drag. Hence as streamlined as Formula vehicles look, their drag coefficients can be as high as 0.8 and the (negative) lift coefficients as large as -1.6.

Aeroelasticity studies the interaction of inertial, elastic, and aerodynamic forces on a solid structure. In essence, under aero/hydrodynamic loads a solid structure like an airplane wing or a yacht sail can flex and vibrate. This behavior is the aeroelastic response of a structure to fluid dynamic forces. A CFDⁱ solver coupled with a structural FEAⁱ solver can be utilized to study aeroelastic properties of a structure.

Acronym for the American Society of Heating, Refrigerating and Air-Conditioning Engineers.

Boundary layer is an imaginary layer of moving fluid in the immediate vicinity of a bounding surface. Due to viscosity the fluid tends to slow down from its free stream speed to zero at the surface. The zone through which this change happens is called the boundary layer. Boundary layers can be as thin as a few millimeters on the surface of a wing or as thick as a few hundred meters on the surface of earth.

Acronym for Computer-Aided Design.

Software solutions for authoring product design.

Acronym for Computer-Aided Engineering.
Software solutions for simulating the physical behavior of a future product.

Acronym for Computational Fluidⁱ Dynamics.

CFDⁱ is a computer-based mathematical modeling tool that incorporates the solution of the fundamental equations of fluid flow, the Navier-Stokes equations, and other allied equations. CFDⁱ incorporates empirical models for modeling turbulence based on experimentation, as well as the solution of heat, mass and other transport and field equations. There are three phases to CFDⁱ: pre-processing, simulation and post-processing.

Acronym for Computational Heat Transfer.

CHTⁱ is similar to the finite element analysis used for structural analysis, except that the solved equations model the heat exchanges through, to and from solids. A CFDⁱ solver can be coupled with a CHTⁱ solver to model cooling due to convection.

If the density of the fluid changes with variations in pressure, then the problem can be classified as compressible flow. In fluid dynamics, if flow speed reaches values larger than 1/3 of the speed of sound, then it is in the compressible regime.

An airplane flying at 200-300 mph is in the compressible regime. For a high speed fan, the tips of the fan blades can reach the compressible regime. A Formula 1 vehicle can easily reach speeds in the compressible regime...

Typically, CFDⁱ solvers are different for compressible and incompressible flows. Compressible flows are more expensive to compute. At CAEbridge, we specialize on incompressible flows which constitute a larger class of problems.

A multi-section tower or storage unit that can accommodate several computers as typically seen in data centers and HPCⁱ facilities.

A specialized CFDⁱ software tool for the modeling the thermal environment in Data Centers and to reduce their carbon signature. CoolSimⁱ is a product of Applied Math Modeling Inc.

Acronym for Computer Room Air Conditioner.

In mathematics, discretization is the process of transferring continuous models and equations into discrete counterparts. The grid generation process is closely related to discretization, as it involves the process of breaking a continuous surface or volume into discrete (finite) elements over which the discrete form of the equations are solved.

Acronym for Direct Numerical Simulationⁱ.
This is a CFDⁱ method by which the complete turbulent flow field is solved directly numerically without any form of time or length averaging in the domain, i.e. both the mean flow and all turbulent eddies (fluctuations) are simulated. DNSⁱ is very computationally intensive and currently only practicable for simple flows at low Reynolds Numbers.

Acronym for Finite Element Analysis.

Performing an analysis based on the FEMⁱ approach.

Acronym for Finite Element Method.

This is a numerical technique for solving boundary-value problems. In a fluid flow application, the flow domain can be represented by finite elements, i.e. a geometrically similar model consists of several simplified discrete regions, which are linked. Flow governing equations (mass and momentum conservation, constitutive equations) are applied to each element, and a system of equations is constructed in terms of nodal unknowns. Techniques of linear algebra, combined with non-linear numerical schemes are subsequently invoked for solving the system.

When flow stream lines stop following the surface of a wing, a car or a ship hull, we refer to the phenomena as "Flow Separationⁱ".

When flow separates or detaches from the surface, it usually becomes chaotic (turbulent) and creates adverse effects like increasing drag and/or reducing lift. Aerodynamic design can involve the prevention of flow separations by morphing (or "streamlining") the solid surfaces.

A fluid is defined as a substance that continually deforms (flows) under an applied shear stress. Fluids are a subset of the phases of matter and include liquids, gases, plasmas and to some extent plastic solids. What differentiates these fluids from each other are their characteristic viscosity and density values. As long as these values are properly defined, CFDⁱ can model any fluid and its dynamics.

Acronym for Fluidⁱ-Structure Interaction.

This is a model that allows for the deformation of solid boundaries to be taken into account when there is physical interaction with a flow field. FSIⁱ encompasses those physical phenomena for which aerodynamic (hydrodynamic), elastic (structural), and inertial (dynamic) forces influence each other and interact in a significant way. Aeroelasticity (Hydroelasticity) is an important subset of fluid-structure interaction. Modeling FSIⁱ can usually be achieved by coupling a CFDⁱ code with a structural analysis code.

Acronym for Finite Volume Method.
This is a method for representing and evaluating partial differential equations (PDEⁱ) as algebraic equations. The finite elements in the shape of cubes, pyramids or tetrahedra represent the finite volumes over which PDEⁱ's are solved and assembled to represent the big picture.

The finest element size in a CFDⁱ or FEAⁱ grid.

Typically, the finest resolution for the aerodynamic simulation of a car of 5m length would go down to 10-20mm selectively on the surface. For a ship of 200m, the finest cell size may be as high as 0.5m to keep simulation costs down. The coarser the grid is, the less accuracy one can expect from a simulation.

Acronym for Graphical User Interface.

This is a computer screen interface that presents information in a user-friendly way using pictures, graphics and icons.

Acronym for High Performance Computing.

HPCⁱ uses supercomputers and computer clusters to solve advanced computation problems. Today, computer systems approaching the teraflops-region are counted as HPCⁱ-computers. For HPCⁱ computer clusters constitute an affordable and expandable solution, as they can be built by connecting several regular desktop computers. Latest multi-core CPU chip architectures allow HPCⁱ to be performed on a single PC or a small cluster of PCs.

Acronym for Initial Graphics Exchange Specification.

This is an agreed international neutral file format used to translate geometrical information between different CADⁱ, CAEⁱ and analysis software packages. Visit www.nist.gov/iges/ for details.

Antonym for Compressible Flowⁱ

Laminar flow, sometimes known as streamlined flow, occurs when a fluid flows in parallel layers, with no disruption between the layers. It is the opposite of turbulent flow. In nonscientific terms laminar flow is "smooth," while turbulent flow is "rough."

Acronym for Large Eddy Simulationⁱ

This is a compromise between DNSⁱ and RANSⁱ. In LESⁱ, the unsteady flow equations are solved for the mean flow and large eddies of a flow field and a “sub grid scale” model is used to simulate the effects of the smaller eddies. Since it is the largest eddies which contain the most energy and interact most with the mean flow, the LESⁱ approach results in a good model of the main effects of turbulence. Since the grid size no longer has to be small enough to allow for the smallest turbulent eddies, this method is much less computationally expensive as DNSⁱ and may be applied to a wider range of flows.

Acronym for Multiple Reference Frame.

This is a steady state CFDⁱ method that simplifies the flow effects of a transient moving body with a fixed shape in a computer simulation. It is very commonly used for modeling impellers in mixing tanks and rotor/stator turbomachinery interactions for instance.

Fluidⁱ dynamic flow problems that involve the coexistence of multiple phases like gas and liquid.

Combustion processes which involve the liquid fuel and output gasses constitute a typical multiphase problem. Marine applications that involve both air and water can be considered multiphase flow problems.

The set of PDEⁱ's that describe fluid dynamics. Their solution using CFDⁱ output velocity and pressure values of the fluid anywhere inside the simulation volume.

Acronym for Open Field Operation and Manipulation.

OpenFOAMⁱ Toolbox can simulate anything from complex fluid flows involving chemical reactions, turbulence and heat transfer, to solid dynamics, electromagnetics and the pricing of financial options. OpenFOAMⁱ is produced by OpenCFD Ltd and is freely available and open source, licensed under the GNU General Public Licence.

Acronym for Partial Differential Equations.

PDEs are used to formulate, and thus aid the solution of, problems involving functions of several variables; such as the propagation of sound or heat, electrostatics, electrodynamics, fluid flow, and elasticity.

Last phase of the 3-step CFDⁱ process.

Post-processingⁱ involves the reduction of the simulation output numerical data
into human interpretable plots, tables, figures, movies and Powerpoint style presentations or reports. Usually an appropriate visualization software is used interactively to generate the desired output. Post-processingⁱ is traditionally a time consuming manual process. For standard problems with repetitive analysis, the process can be automated with appropriate scripting techniques.

1st phase of the 3-step CFDⁱ process.

Pre-processingⁱ consists of:

a. the creation a geometry usually done in a CADⁱ tool
b. the generation of a suitable grid of the computational domain to solve the flow equations on

Acronym for Reynolds Averaged Navier-Stokes.

This is a form of the Navier-Stokes equations for describing fluid flow in which additional terms (known as Reynolds Stresses) are included in the solver to account for the time-averaged effects of turbulence.

In fluid mechanics, the Reynolds number Re is a dimensionless number that gives a measure of the ratio of inertial forces to viscous forces. So Reynolds number consequently quantifies the relative importance of these two types of forces for given flow conditions.

Reynolds numbers are used to characterize different flow regimes, such as laminar or turbulent flow: laminar flow occurs at low Reynolds numbers, while turbulent flow occurs at high Reynolds numbers.

2nd phase of the 3-step CFDⁱ process.

Simulationⁱ is a usually fully automated process in which the HPCⁱ resources are used to solve PDEⁱ's that describe flow motion.

Acronym for STereoLithography.

STLⁱ mesh file format is the most standard 3D surface geometry representation used across several industries. Typically a client will supply either and IGESⁱ CADⁱ file or an STLⁱ mesh to transfer existing design information to the CFDⁱ analyst.

Turbulenceⁱ is that state of fluid motion which is characterized by apparently random and chaotic three-dimensional fluctuations. When turbulence is present, it usually dominates all other flow phenomena and results in increased energy dissipation, mixing, heat transfer, and drag. Most flows in engineering are turbulent and require the use of an appropriate turbulence model in CFDⁱ simulations.

Empirical models used in CFDⁱ to describe the effects of turbulent fluctuations. There is no turbulence model that is universally applicable to all classes of flow problems. The CFDⁱ analyst chooses an appropriate turbulence model from a set of 10-20 based on published recommendations and his own experience.

The study of how a vehicle like an airplane or ship responds to external forces and moments.

External forces can include gravity, buoyancy, aerodynamic lift, drag and side forces. The moments are in a sense "rotational forces" and would rotate the vehicle about its center of gravity. External forces create moments if their application centers are not aligned with the center of gravity of the vehicle.

Viscosityⁱ is a measure of the resistance of a fluid which is being deformed by either shear stress or extensional stress. In everyday terms (and for fluids only), viscosity is "thickness." Thus, water is "thin," having a lower viscosity, while honey is "thick" having a higher viscosity. Viscosityⁱ describes a fluid's internal resistance to flow and may be thought of as a measure of fluid friction.

Acronym for Volume of Fluidⁱ

The VOFⁱ method is used mainly for multiphase flows, and in particular free surface (or wave) flows. Most typical application of the VOFⁱ method is marine problems that involve both air and water media in the same problem.