writing an expression involving concentration gradient

- October 24, 2023 at 1:30 pm
  
  Ramesh Chandra Mishra
  Subscriber
  
  Hi
  I am trying to evaluate the following equation using custom field functions
  but getting stuck because of concentration gradient. Does anyone know how to evaluate concentration gradient using expression or custom field functions so that above equation can be calculated.
- October 24, 2023 at 3:13 pm
  
  Rob
  Forum Moderator
  
  Some of the derivatives are available in solver, otherwise you may need to look at UDM.
- October 24, 2023 at 3:24 pm
  
  Ramesh Chandra Mishra
  Subscriber
  
  So there is no way I can solve this without using UDF?
- October 24, 2023 at 3:45 pm
  
  Rob
  Forum Moderator
  
  Possibly not, check the Derivatives and if you can patch a solver result (concentration) to UDM and if that has an available derivative. Otherwise, you may need a UDF.
- October 24, 2023 at 3:46 pm
  
  Ramesh Chandra Mishra
  Subscriber
  
  There is no option for concentration in the derivatives. I was wondering if I can do it using gradient option in expressions?
- October 24, 2023 at 4:26 pm
  
  Rob
  Forum Moderator
  
  I'm not sure there's an option for the gradients in the Expression system. It's rapidly evolving feature so that may change with 2024Rx, otherwise the gradients are available for UDM as UDF macros.
- October 24, 2023 at 4:48 pm
  
  Ramesh Chandra Mishra
  Subscriber
  
  This is what I am talking about. But I have a bit problem with this . It's syntax is Gradient but how to know if it is gradient wrt to x, y or z.
  Secondly, can you guide me a bit towards UDF. As far as i know, concentration can be obtained by multiplying mass fraction with density. Not so sure about concentration gradient though. Can you help me proceed in this regard?
- October 24, 2023 at 5:01 pm
  
  Rob
  Forum Moderator
  
  Concentration is equivalent to volume fraction, and mass fraction is available so you may need to do some maths to convert.
- October 24, 2023 at 5:36 pm
  
  Ramesh Chandra Mishra
  Subscriber
  
  That part is fine by me but suppose I get the concentration, how do I convert it to gradient is my question. I can see macros for temperature and pressure gradient in the manual but could not find one for concentration gradient.
- October 25, 2023 at 7:02 am
  
  Ramesh Chandra Mishra
  Subscriber
  
  Is it possible to get the gradient of a variable created in the udf?
- October 25, 2023 at 8:05 am
  
  Rob
  Forum Moderator
  
  Check the User Defined Memory (UDM) macros - https://ansyshelp.ansys.com/account/Secured?returnurl=/Views/Secured/corp/v232/en/flu_udf/flu_udf_DataAccessMacros.html%23flu_udf_example_udm_uds_usage