{"id":429946,"date":"2025-04-24T14:40:19","date_gmt":"2025-04-24T14:40:19","guid":{"rendered":"https:\/\/innovationspace.ansys.com\/forum\/forums\/topic\/dpm-source-terms-udfs-units\/"},"modified":"2025-04-24T14:40:19","modified_gmt":"2025-04-24T14:40:19","slug":"dpm-source-terms-udfs-units","status":"publish","type":"topic","link":"https:\/\/innovationspace.ansys.com\/forum\/forums\/topic\/dpm-source-terms-udfs-units\/","title":{"rendered":"DPM Source Terms UDFs Units"},"content":{"rendered":"

<p>Hey all.<br><br>I’m currently using Fluent’s DPM to simulate the release of 10,000 water vapor bubbles into a tall cubic domain filled with water, with a hot and cold lower and upper plates, respectively. I have a few custom forces to implement. Those forces are supposed to intervene in the tracking of the bubbles, and in a momentum source term to the carrier phase. If I understood correctly, the forces added in the DEFINE_DPM_BODY_FORCE UDF only impact the trajectory calculations. Thus, I also add those forces in the momentum source of the carrier phase using the DEFINE_DPM_SOURCE UDF, by writing: S->momentum_s[i] += F[i], with F being my forces array, and i = {0, 1, 2} the cartesian component. So, F[i] is in Newton. In addition to this, I also implement a very simple heat exchange mechanism between the bubbles and the carrier phase, which is nearly identical to Fluent’s Inert Heating, with the only change being how I define the heat transfer coefficient h. Since Fluent doesn’t provide a way to customize it (as it does with the drag coefficient), I compute it in the DEFINE_DPM_SOURCE UDF, and use it in the usual formula: Q = pi*d*d*(T_b – T_f)*h, with h = k*Nu\/d. In those, d is the bubble diameter, T_b the bubble T° (constant), T_f the fluid T° at the bubble center, k the conduction coefficient, and Nu the bubble Nusselt number. I thus write: S->energy += Q, where Q is in Watt. I don’t make use of the “strength” parameter which, since I enforce 1 bubble per parcel, is equal to the inverse of my fluid timestep, which is constant at 0.01s. I of course disable Fluent’s laws in the GUI, so no laws are active. Bubbles’ radius is constant, as is their density. Finally, I use node-based averaging of the source terms to distribute those terms to the neighbouring cells (with a Gaussian kernel). Simulation is obv unsteady, with unsteady particle tracking etc.<br><br>Here’s my problem: results weren’t great, so I wanted to compare Fluent’s Inert Heating law to mine. It turns out that Fluent’s Nusselt number is often very close to mine (1-10% away), which would indicate that Fluent’s Inert Heating and my law yield the same results. But, when I compute the volume-averaged enthalpy source during the simulation (enthalpy source = carrier phase energy source in this case), mine’s magnitude is around 1e-8 to 1e-10, whereas Fluent’s is around 1e-15. What am I missing here? Should I make use of the “strength” parameter, or maybe my units are not right? In the documentation’s exemple of the DEFINE_DPM_SOURCE UDF, the variable put into S->energy is in Watt too.*I can provide more information on the case if needed. Thanks!<\/p><\/p>\n","protected":false},"template":"","class_list":["post-429946","topic","type-topic","status-publish","hentry","topic-tag-multiphaseudfevaporationdpmspeciestransportdefinesourcesourcetermeulerianlagrangian-1","topic-tag-bubbles-1"],"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