Matched Outlet - Boundary Condition

Matched Outlet is a boundary condition used to dynamically adjust the velocity distribution at an outlet so that the net flow rate (mass or volumetric) across that boundary patch matches a user-specified inlet. It iteratively corrects the boundary velocity profile so that the integrated flow rate at the outlet aligns with the desired inlet.

In many engineering and CFD simulations, it is common to specify an exact outflow rate rather than a fixed velocity at an outlet. For instance, the net flow leaving the domain might be known, but not a velocity profile in advance. Traditional boundary conditions like Fixed Value or Zero Gradient cannot directly guarantee a specific total flow rate. Matched Outlet is designed to solve this problem.

Mathematically, the boundary condition can be expressed as:

Matched Outlet is a sophisticated boundary condition that bridges the gap between Fixed Value or Zero Gradient conditions and the practical necessity of controlling outflow rates in CFD simulations. It provides a feedback loop that adjusts the outlet velocity distribution to achieve a predefined mass or volumetric flow rate, leading to more realistic and controlled simulation outcomes.

Matched Outlet is similar to Flow Rate Outlet Velocity. However, unlike that condition, which sets a fixed target flow rate, Matched Outlet automatically adjusts based on the specified inlet. In other words, if the inlet flow changes, Matched Outlet dynamically updates the outlet conditions accordingly.

Matched Outlet is applied at the exit of the domain to prescribe the flow rate, based on the inlet mass or volumetric flow. In a real-world scenario, if the amount of fluid entering a domain changes, the outflow typically adjusts naturally to maintain mass conservation. The Matched Outlet boundary condition mimics this behavior numerically.

The definition of boundary conditions in SimFlow is both simple and intuitive. To specify the Matched Outlet boundary condition, the user must navigate to the Boundary Conditions panel, select the appropriate boundary, and choose the correct option from the drop-down menu.

Matched Outlet requires specifying an inlet boundary that will serve as the reference point for calculating the flux - Figure 1.

Flux From - specifies the inlet patch from which the flow rate is taken to match at this outlet patch.

In this section, we propose boundary conditions that are alternative to Matched Outlet. While they may fulfill similar purposes, they might be better suited for a specific application and provide a better approximation of physical world conditions.