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Flux conditions specify how the surroundings affect and interact with the model at the boundary, often expressed as an applied force, flux, or current. This type of boundary condition is also called a Neumann boundary condition.
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Constraints specify the result of the interaction between the model and its surroundings, expressed as prescribed values of the dependent variables. This type of boundary condition is also called a Dirichlet boundary condition.
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A Boundary Load in a solid model, which prescribes the stress acting on the boundary.
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Heat Flux in a heat transfer model, which prescribes the heat per unit area flowing into (or out of) the model across the boundary.
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A Normal Current Density in an AC/DC model, which prescribes the electrical current per unit area entering (or exiting) the model at the boundary.
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In COMSOL Multiphysics, by convention, the force acting on the model or the flux into the model is specified. That is, specify how the surroundings affect the model and not how the model affects its surroundings. You can change the direction by reversing the sign before the numerical value of the force or flux.
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A Prescribed Displacement of the boundary of a solid object.
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That the velocity is zero on a Wall boundary in a CFD model.
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The Temperature at the boundary of a heated solid.
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The Electric Potential on an electrode in an AC/DC model.
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In most physics features, the default boundary condition is of a flux type and does not fix a reference level for the dependent variable. Therefore, when solving certain study types (notably stationary studies), you must manually add at least one boundary condition of constraint type (or a point constraint) for the model to be well-defined.
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