I am working with a uds convection-diffusion equation. I have already solved the background flow which is steady, turbulent, in a rectangular cavity (1 inlet, 1 outlet, and mass sinks on 1 side wall). All equations are turned off except the uds equation for a transient simulation. The cavity is initialized with a uds concentration of 0 and a copncentration of 1 is specified at the inlet. <\/p>\n
I was experiencing small areas of non-physical negative concentrations early on in the simulation (around 1s), as the simulation progressed the minimum concentration continued to increase in magnatude to a considerable value. The simulation could progress for a while before the negative concentrations cause convergence or scalar mass imbalance issues but eventually it would run away enough that it did. <\/p>\n
I determined the cause of the negative concentration to be the small diffusion coeffecient (roughly 1e-12). If I set a large diffusion coefficient (greater than 1e-4) the diffusion spread out and eliminated any significant negative concentration. If I set the diffusion coefficient to 0 there was no negative concentration. In all cases the uds equation was well converged an the backround flow was well converged with good mass conservation. <\/p>\n
I came across an old forum post by another reporting a similar issue but unfortunately there is no solution presented. https:\/\/forum.ansys.com\/forums\/topic\/anisotropic-diffusion-and-negative-concentration\/ It sounds as though Fluent is having issues calculating gradients for diffusion terms on the highly non-orthogonal mesh. I switched to Green-Gauss node based but that had little improvement. <\/p>\n
I have a polyhedral mesh with very fine prism layers near the wall. I have used this uds model on similar meshes for different flows (convction dominant) without any issues of negative concentration. The flow in this particular cavity where I am having issues is stratified and the lower portion is nearly stagnant. This is where the negative concentrations seem to arise first, where the convection term is not dominant.<\/p>\n
As a work around I have created a modified diffusion coefficient that uses my desired value (turbulent + brownian diffusion) when the value is above a threshold and switches to 0 when below the threshold. This works to prevent negative concentration and my simulation is numerically stable however i am forced to neglect secondary dioffusion effects especially near the walls. The concentration builds up too large positive values faster. <\/p>\n
Obviously switching to a coarser orthogonal cartesian mesh would aleviate the negative concentration issues but I am already invested in this current mesh and need the near wall refinement for specific models in the background flow. I’d greatly appreaciate any suggestions to improve things numerically speaking on my current mesh. <\/p>\n","protected":false},"template":"","class_list":["post-370136","topic","type-topic","status-closed","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