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[FAQ]

In a 3D calculation, the absolute pressure at a die exit does not seem to match the assigned boundary condition. Is there a way to find a gauge/absolute pressure at the exit of a channel? For 3D flow calculation, the reported feature on the pressure is mainly a graphical artefact and may actually originate from a “conflict” between the (discontinuous) interpolation used in the calculation for the pressure and its (continuous) graphical representation in FLPOST or FieldView. In 3D, the interpolation for the pressure is usually constant i.e. one value is calculated at the centre of each element and it is thus discontinuous across the element border. It is “legal” from the point of view of finite element calculation, since the part integration applied to the momentum equation allows to get rid of the derivative. However… however … Graphical post-processor usually handle continuous quantities which are actually known only at vertices. Hence, from the value of the pressure at the centre of each element, a pressure is “evaluated” at each vertex by averaging the values of the pressure in the elements surrounding that vertex. When calculating a simple 3D Poiseuille flow with vanishing normal force at the exit, the pressure does not vanish at the centre of the last layer of elements (at the exit). Therefore, averaging these central pressure values at the vertices on the exit section cannot produce a vanishing value. In other words, what the user sees on the screen is a bit different from what has been calculated. Yet, by extrapolating to the exit the values obtained at the centre of the elements, the exit boundary condition should be retrieved.