{"id":462093,"date":"2026-08-02T09:54:11","date_gmt":"2026-08-02T09:54:11","guid":{"rendered":"https:\/\/innovationspace.ansys.com\/forum\/forums\/topic\/help-in-deciding-boundary-conditions-for-my-computational-domain\/"},"modified":"2026-08-02T09:54:11","modified_gmt":"2026-08-02T09:54:11","slug":"help-in-deciding-boundary-conditions-for-my-computational-domain","status":"publish","type":"topic","link":"https:\/\/innovationspace.ansys.com\/forum\/forums\/topic\/help-in-deciding-boundary-conditions-for-my-computational-domain\/","title":{"rendered":"Help in deciding boundary conditions for my computational domain."},"content":{"rendered":"

<p>I am simulating <strong data-start=”341″ data-end=”466″>subsonic compressible flow through a pipe and a distortion screen, followed by discharge into a still ambient environment<\/strong>. The computational fluid regions are shown in the attached images; the gaps between the internal fluid blocks represent the solid portions of the distortion screen. The flow passes through the pipe and screen and then expands into the larger cylindrical fluid domain. Experimentally, I know the <strong data-start=”762″ data-end=”786″>inlet total pressure<\/strong> and the <strong data-start=”795″ data-end=”822″>ambient static pressure<\/strong>.<\/p><p><\/p><p><\/p><p>My main uncertainty is how to define the boundaries of this larger cylindrical domain. I have labelled them as <strong data-start=”936″ data-end=”1013″>1 – front annular face, 2 – cylindrical curved surface, and 3 – rear face<\/strong>. I tried the following three approaches.<\/p><p><strong data-start=”1056″ data-end=”1067″>Case 1:<\/strong> I specified faces <strong data-start=”1086″ data-end=”1140″>1, 2 and 3 as pressure outlets at ambient pressure<\/strong>. During the calculation, Fluent reported reversed flow over portions of the pressure-outlet boundaries. I understand that this can represent ambient fluid entering the domain due to entrainment. However, the solution eventually became highly unstable and diverged, with very large mass imbalance and pressure\/temperature limiting.<\/p><p><strong data-start=”1615″ data-end=”1626″>Case 2:<\/strong> I changed faces <strong data-start=”1643″ data-end=”1663″>1 and 2 to walls<\/strong> and kept only face <strong data-start=”1683″ data-end=”1729″>3 as a pressure outlet at ambient pressure<\/strong>. This case initially behaved much better and the solution appeared to converge for several hundred iterations. However, reversed flow eventually developed at the rear pressure outlet and increased significantly. My interpretation is that the jet naturally requires entrainment from the surrounding fluid, but faces 1 and 2 prevent any normal flow because they are walls. Therefore, the only open boundary available for the solver is the rear pressure outlet<\/p><p><strong data-start=”2242″ data-end=”2253″>Case 3:<\/strong> I then specified faces <strong data-start=”2277″ data-end=”2327″>1 and 2 as pressure inlets at ambient pressure<\/strong> and face <strong data-start=”2337″ data-end=”2383″>3 as a pressure outlet at ambient pressure<\/strong>. My intention was to allow ambient fluid to enter naturally through faces 1 and 2 while the main flow exits through face 3. However, this case was even more unstable. Fluent reported significant reversed flow at both the pressure-inlet and pressure-outlet boundaries, and the solution diverged.<\/p><p>I suspect that an important issue is that the <strong data-start=”2726″ data-end=”2801″>outer cylindrical boundary is relatively close to the jet\/screen region<\/strong>, so entrainment has a strong influence on these boundaries. I am therefore unsure whether changing only the boundary-condition type can solve the problem, or whether the ambient computational domain itself needs to be enlarged. Unfortunately, I am not sure whether I can change in my computational domain or not as of now. I have not used a pressure-far-field boundary because the surrounding experimental environment is  still air, whereas the pressure-far-field condition requires a freestream Mach number.<\/p><p>Could you please advise what would be the <strong data-start=”3268″ data-end=”3383″>physically appropriate boundary condition for faces 1 and 2 for this type of jet-into-quiescent-ambient problem<\/strong>? Should these boundaries permit entrainment or walls, and if so, which Fluent boundary condition would be most appropriate? Alternatively, should the cylindrical ambient domain first be extended farther away from the jet before applying an ambient-pressure boundary condition?<\/p><p>I have run these cases on my institute HPC system, and I can also attach the Fluent console messages\/residual behaviour from all three cases if they would help diagnose the issue. Please answer taking into account of these attempts I have made. Thanks in advance.<\/p><p><\/p><p>The above one is for first case.<\/p><p><\/p><p>This is for 2nd case.<\/p><p><\/p><p>This is for 3rd case, residual value is even more higher. Highly unstable.<\/p><\/p>\n","protected":false},"template":"","class_list":["post-462093","topic","type-topic","status-publish","hentry"],"aioseo_notices":[],"aioseo_head":"\n\t\t\n\t\n\t\n\t\n\t\n\t\t\n\t\t\n\t\t\n\t\t\n\t\t\n\t\t\n\t\t\n\t\t\n\t\t\n\t\t\n\t\t\n\t\t