UDFs to implement pressure-drop vs massflow rate data (in a given direction) for a fluid zone
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This solution describes a UDF that can be used for easy setup of 1D resistance inside a fluid zone given the pressure-drop vs mdot data (for a direction) in tabular format. One approach to do this in Fluent is using the porous-media model, which requires transforming the discrete data into an equation that can be used by either the inertial or viscous resistance form specified for the porous media model. This pressure drop characteristics (as function of massflow rate) is typically non-linear. In this approach, the one dimensionality of the given resistance data is still assumed. An alternative approach has been developed using a UDF to directly implement the user specified 1D resistance as momentum sink terms. This approach avoids the potentially time-consuming effort to curve fit the tabular data and then transform it to the standard form used by either the viscous and/or inertial resistance in the porous media model. What do the UDFs do ? ----------------------------------------- - Read the table of pressure-drop vs massflow rate data - Convert this table to pressure-drop-per-unit-length vs normal velocity (which is velocity in direction-1) - Apply resistance to each cell (using momentum source terms) according to the local velocity and use the table to get the pressure-drop-per-unit-length This defines the resistance in the axial direction (direction-1) Note that the resistance values inside the porous panel (of that fluid zone) must be set to zero. The porous model must still be activated for that fluid zone and the directions be correctly specified. How to use the UDFs ? ------------------------------------- Assume that a fluid zone has been designated as a porous zone. 1.a. Prepare the input file for the pressure drop vs massflow rate b. The file must be named as delP_vs_mdot.txt c. The first line is the number of data d. The left column is massflow rate (SI) e. The right column is pressure drop (Pa) 2.a. Determine the normal/axial direction vector of the porous zone This will be the direction-1 b. Determine direction-2 --> These two directions will be inputs inside the porous zone fluid panel 3.a. Find the thickness (Lp) of the porous zone b. Find the cross-sectional area of the porous zone c. Find the zone ID of the porous zone --> Edit porous.c and use these three values appropriately d. Compile porous.c 4.a. Load the case file, scale, and do the rest of the setup b. Load the library c. Execute the on-demand UDF "alloc_dyn_mem" Define -> User-Defined -> Execute On Demand This UDF needs to be executed only once per Fluent session d. Execute the on-demand UDF "read_table" Define -> User-Defined -> Execute On Demand e. Hook the "exec_end" Define -> User-Defined -> Function Hooks... Hook to the Execute At End Function f. Open the boundary condition panel of the porous zone and setup the direction-1 and direction-2 vectors Make sure that all resistances are zero e. In the same boundary condition panel, enable the source terms and hook the UDFs "porous_x", "porous_y", and "porous_z" to the x-, y-, and z-momentum respectively 5.a. Initialize the flowfield b. Start iterating How to restart the case ? ---------------------------------------- 1. Read the case file ONLY The UDFs will be automatically loaded 2. Execute the "read_table" Define -> User-Defined -> Execute On Demand 3. Read the data file 4. Execute the "init_vn" Define -> User-Defined -> Execute On Demand Verification: ------------------ A sample case of channel flow with porous zone in the middle. The input file delP_vs_mdot.txt is such that a pressure drop of 2000Pa will be observed for the prescribed boundary conditions. - sample-1-serial.cas.gz : channel is alligned with the x-axis and sample-1-serial.dat.gz flow is initialized with 7m/s - sample-2-serial.cas.gz : same as above except that the channel is sample-2-serial.dat.gz rotated 30deg wrt to the z-axis with 42m/s the flow is also initialized differently - sample-3-parallel.cas.gz : based on sample-2 except that another sample-3-serial.dat.gz rotation (about y-axis) is added The UDFs have also been tested for a thicker porous zone and the same pressure drop is observed. The UDF and the corresponding sample cases have been uploaded to the web. <a target=_blank href="http://www.fluentusers.com/support/solutions/1085/porous.tar.gz">http://www.fluentusers.com/support/solutions/1085/porous.tar.gz</a>http://www.fluentusers.com/support/solutions/1085/porous.tar.gz |
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