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Concept
Boundary conditions in fluid dynamics
Summary
Boundary conditions in fluid dynamics are the set of constraints to boundary value problems in computational fluid dynamics. These boundary conditions include inlet boundary conditions, outlet boundary conditions, wall boundary conditions, constant pressure boundary conditions, axisymmetric boundary conditions, symmetric boundary conditions, and periodic or cyclic boundary conditions.
Transient problems require one more thing i.e., initial conditions where initial values of flow variables are specified at nodes in the flow domain. Various types of boundary conditions are used in CFD for different conditions and purposes and are discussed as follows.
In inlet boundary conditions, the distribution of all flow variables needs to be specified at inlet boundaries mainly flow velocity. This type of boundary conditions are common and specified mostly where inlet flow velocity is known.
In outlet boundary conditions, the distribution of all flow variables needs to be specified, mainly flow velocity. This can be thought as a conjunction to inlet boundary condition. This type of boundary conditions is common and specified mostly where outlet velocity is known.
The flow attains a fully developed state where no change occurs in the flow direction when the outlet is selected far away from the geometrical disturbances. In such region, an outlet could be outlined and the gradient of all variables could be equated to zero in the flow direction except pressure.
The most common boundary that comes upon in confined fluid flow problems is the wall of the conduit. The appropriate requirement is called the no-slip boundary condition, wherein the normal component of velocity is fixed at zero, and the tangential component is set equal to the velocity of the wall. It may run counter to intuition, but the no-slip condition has been firmly established in both experiment and theory, though only after decades of controversy and debate.
Heat transfer through the wall can be specified or if the walls are considered adiabatic, then heat transfer across the wall is set to zero.
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