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Ventilation Design Handbook on Animal Research Facilities Using Static Microisolators 
Motion (F = ma), the time rate of change of velocity, or acceleration, is equal to the force/unit 
mass. If the quantities depend on both time and space, the equations are written to take this into 
account and they are known as partial differential equations, or PDE's. In most general 
formulations, the governing equations for physical phenomena are written in terms of rates of 
change with respect to time and space, or as partial differential equations. 
5.1.5 Flow Variables 
The flow variables contain information about the fluid state at a point in space. Enough 
information must be maintained in order to specify a valid fluid state; i.e., two thermodynamic 
variables, such as pressure and temperature, and one kinematic variable, such as velocity. A 
velocity will usually have more than one component, i.e., in three dimensions it will have three 
components. 
In this research, the variables under consideration are the three components of velocity, pressure, 
temperature, concentration, and two variables characterizing turbulent levels: turbulent kinetic 
energy and its rate of dissipation. 
Over the past 25 years, CFD techniques have been used extensively and successfully in the 
mainly high-end sectors, such as the nuclear and the aerospace industries. In its raw and general 
form, CFD has always been the forte of fluids experts. The recent concept of tailoring CFD 
software, combined with the expertise in heating and ventilation in buildings, has made it 
possible to apply these powerful methods to provide fast and accurate results to designers under 
severe time and budgetary constraints. In fact, this project would not have been practical without 
these new elements in place. 
5.1.6 How Does it Work? 
In order to generate a CFD solution, two processes must be accomplished, namely; 
• geometry definition and grid generation 
• numerical simulation 
In broad terms, grid generation is the act of specifying the physical configuration to be simulated 
and dividing it up into a three-dimensional grid containing a sufficient number of small regions 
known as control volume cells so that the Navier-Stokes partial differential equations can be 
solved iteratively. Numerical simulation is the process of applying a mathematical model to that 
configuration and then computing a solution. These two stages are sequential. The grid 
generation is performed before any numerical simulation work can be done. 
