Embedded bursting membrane or flap

Understand how the flow solver mitigates the destabilizing effects of pressure wave propagation caused by switching between closed and open modes, using a screen relaxation method with variable head loss over multiple iterations.

Switching between closed and open modes leads to a sudden change from an impermeable surface to a fully permeable interface in a flow direction. This cause a numerical pressure wave propagation destabilizing the convergence of the solver.

To alleviate this issue, the flow solver uses a screen relaxation method, where switching between the embedded flow surface and the zero head loss screen is carried out by imposing variable head loss for the screen through many iterations:

• In a steady state model, the relaxation is performed through steady state iterations.
• In a transient model, the relaxation is performed in one time step and through many non-linear iterations.

Once switching the condition is initiated, the flow solver calculates automatically the variable screen head loss coefficient with the dynamic pressure-based screen formulation, according to:

where

• h is the head loss coefficient.
• is the area averaged density.
• is the average velocity in the upsteam fluid domain.

At each iteration, the head loss coefficient decreases towards zero.

For a flap, the switching process from the open state to the closed state starts with increasing the head loss coefficient, h, at each iteration from zero to its most up-to-date value at previous closed state. The number of iterations to complete the switching process depends on the maximum number of iterations in the simulation and the simulation type: transient or steady state.