Biomedical Engineering Reference
In-Depth Information
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Figure 24 shows the position of the ball as a function of time.
In this paper, we describe our work to more accurately model the flow
inside the barrel and to verify the results experimentally.
Computational Analysis
Describing the flow along the barrel during firing is challenging because it
is dominated by features that are not considered in order to simplify its
mathematical analysis. The flow is transient and compressible. The geometric
boundary conditions change rapidly with time. The annular gap between the
ball and the barrel is small enough that viscosity cannot be ignored. Clearly, a
precise description of the flow requires a computational fluid dynamic (CFD)
analysis.
FLUENT
TM
CFD software was used to analyze the flow inside the gun.
Modelling the flow around a fixed ball is relatively simple, but the moving
boundary condition created by the ball cannot be ignored. For the analysis, the
ball moves under the influence of the calculated aerodynamic forces. The
software supports this capability, though it is not a standard feature of the
software. Rather, a user-defined function had to be written to account for
motion of the ball.
Computational Fluid Models
The purpose of CFD software is to generate approximate solutions to the
Navier-Stokes' (NS) equations. The NS equations describe fluid flow exactly,
but are impossible to solve for all but the most basic geometries. For any
realistic problem, only approximate solutions are possible. There are generally
two levels of approximation, as shown in Figure 25. The first level of
approximation eliminates or simplifies terms in the NS equations to produce a
more tractable set of differential equations. The second level of approximation
discretizes the model geometry so that the differential equations can be
replaced by a set of coupled algebraic equations.
CFD is a computational technology that enables one to study the dynamics
of particles that flow. Using CFD, it is possible to build a computational model
that represents a system or device that one wants to study. A virtual prototype
of the device is built and the software will output a prediction of the fluid
dynamics and related physical phenomena. Using CFD software a ‗virtual
