06-bound - Lecture 6 Boundary Conditions Applied...

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1 Lecture 6 - Boundary Conditions Applied Computational Fluid Dynamics Instructor: André Bakker © André Bakker (2002-2006) © Fluent Inc. (2002)
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2 Outline Overview. Inlet and outlet boundaries. Velocity. Pressure boundaries and others. Wall, symmetry, periodic and axis boundaries. Internal cell zones. Fluid, porous media, moving cell zones. Solid. Internal face boundaries. Material properties. Proper specification.
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3 Boundary conditions When solving the Navier-Stokes equation and continuity equation, appropriate initial conditions and boundary conditions need to be applied. In the example here, a no-slip boundary condition is applied at the solid wall. Boundary conditions will be treated in more detail in this lecture.
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4 Example: face and cell zones associated with pipe flow through orifice plate inlet outlet wall orifice (interior) orifice_plate and orifice_plate-shadow fluid Overview Boundary conditions are a required component of the mathematical model. Boundaries direct motion of flow. Specify fluxes into the computational domain, e.g. mass, momentum, and energy. Fluid and solid regions are represented by cell zones. Material and source terms are assigned to cell zones. Boundaries and internal surfaces are represented by face zones. Boundary data are assigned to face zones.
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5 Neumann and Dirichlet boundary conditions When using a Dirichlet boundary condition, one prescribes the value of a variable at the boundary, e.g. u(x) = constant . When using a Neumann boundary condition, one prescribes the gradient normal to the boundary of a variable at the boundary, e.g. n u(x) = constant . When using a mixed boundary condition a function of the form au(x)+b n u(x) = constant is applied. Note that at a given boundary, different types of boundary conditions can be used for different variables.
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6 Flow inlets and outlets A wide range of boundary conditions types permit the flow to enter and exit the solution domain: General: pressure inlet, pressure outlet. Incompressible flow: velocity inlet, outflow. Compressible flows: mass flow inlet, pressure far-field. Special: inlet vent, outlet vent, intake fan, exhaust fan. Boundary data required depends on physical models selected. General guidelines: Select boundary location and shape such that flow either goes in or out. Not mandatory, but will typically result in better convergence. Should not observe large gradients in direction normal to boundary near inlets and outlets. This indicates an incorrect problem specification. Minimize grid skewness near boundary.
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7 Pressure boundary conditions require static gauge pressure inputs: The operating pressure input is set separately.
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