CFD – What Is Computational fluid dynamics -Fluid Mechanics

Computational fluid dynamics (CFD) is the use of applied mathematics, physics and computational software to visualize how a gas or liquid flows — as well as how the gas or liquid affects objects as it flows past. Computational fluid dynamics is based on the Navier-Stokes equations. These equations describe how the velocity, pressure, temperature, and density of a moving fluid are related.

Definition Of  CFD

“Computational fluid dynamics (CFD) is a branch of fluid mechanics that uses numerical analysis and data structures to solve and analyze problems that involve fluid flows. ”

Mathematical modelling of a continuum problem leads to a set of differential, integral or integro-differential equations. Exact analytical solution of such equations is limited to problems in simple geometries. Hence, for most of the problems of practical interest, an approximate numerical solution is sought. In the context of mechanics, the science and practice of obtaining approximate numerical solution using digital computers is termed Computational Mechanics. For thermo-fluid problems, this approach is popularly known as Computational Fluid Dynamics (CFD)

Computers are used to perform the calculations required to simulate the interaction of liquids and gases with surfaces defined by boundary conditions. With high-speed supercomputers, better solutions can be achieved. Ongoing research yields software that improves the accuracy and speed of complex simulation scenarios such as transonic or turbulent flows. Initial experimental validation of such software is performed using a wind tunnel with the final validation coming in full-scale testing, e.g. flight tests.

The fundamental basis of almost all CFD problems is the Navier–Stokes equations, which define many single-phase (gas or liquid, but not both) fluid flows. These equations can be simplified by removing terms describing viscous actions to yield the Euler equations. Further simplification, by removing terms describing vorticity yields the full potential equations. Finally, for small perturbations in subsonic and supersonic flows (not transonic or hypersonic) these equations can be linearized to yield the linearized potential equations.

CFD deals with approximate numerical solution of governing equations based on the fundamental conservation laws of physics, namely mass, momentum and energy conservation.

The CFD solution involves

• Conversion of the governing equations for a continuum medium into a set of discrete algebraic equations using a process called discretization.
• Solution of the discrete equations can using a high speed digital computer to obtain the numerical solution to desired level of accuracy.

CFD Methodology:

• Physical bounds of the problem defined
• Volume defined by the bounds divided into cells or meshes
• Physical modeling defined: the equations of motion, radiation, enthalpy and species conservation
• Boundary conditions defined
• Simulation is started
• Data analysis and visualization performed

The main components of a CFD design cycle are the following:

• Analyst – states the problem to be solved
• Model and methods – expressed mathematically
• Software – embodies knowledge and provides algorithms
• Computer hardware – for actual calculations, and an analyst must inspect and interpret simulation results

APPLICATIONS OF CFD

CFD is being used for fundamental research as well as industrial R&D. CFD analysis forms an integral part of design cycle in most of the industries: from aerospace, chemical and transportation to bio-medical engineering. The length scales range from planetary boundary layers to micro-channels in electronic equipments. Following is a short-list of some of more prominent applications of CFD :

• Meteorology: weather forecasting
• Aerospace: design of wings to complete aircraft aerodynamic design
• Turbomachines: design of hydraulic, steam, gas, and wind turbines; design of pumps, compressors, blower, fans, diffusers, nozzles.
• Engines: combustion modelling in internal combustion engines
• Electronics: cooling of micro-circuits
• Chemical process engineering
• Energy systems: analysis of thermal and nuclear power plants, modelling of accident situations for nuclear reactors.
• Hydraulics and hydrology: flow in rivers, channels, ground aquifers, sediment transport.
• HVAC: Design of ducts, placement of heating/cooling ducts for optimum comfort in a building
• Surface transport: aerodynamic design of vehicles
• Marine: hydrodynamic design of ships, loads on off-shore structures
• Biomedical: simulation of blood flow through arteries and veins, fluid flow in renal and ocular systems.
• Fundamental flow physics: dynamics of laminar, transitional and turbulent flows.

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