# What is UDS and When/Why use it?

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 August 15, 2017, 11:29 What is UDS and When/Why use it? #1 New Member   RJC Join Date: Jun 2015 Posts: 18 Rep Power: 10 Hello, I am asking a very basic question. What is User Defined Scalar (UDS)? When/Why use UDS? Can someone explain it in simple terms? I was unable to get a clear understanding of UDS after reading the FLuent user guide. So I am hoping that someone can explain in simple terms.

 August 15, 2017, 20:22 #2 Senior Member   Lucky Join Date: Apr 2011 Location: Orlando, FL USA Posts: 5,238 Rep Power: 63 At risk of circular definition, a user defined scalar is quite simply a transported scalar that is defined by the user. It allows you to make up and solve transport equations of make-believe (but can be real) quantities so that you can solve for how stuff would flow. Without user-defined scalars, you would be stuck with only continuity (mass transport), momentum (momentum transport), and sometimes energy (internal energy transport). A popular use of scalar transport equations is for mixing problems. You can seed different parts of the flow with your scalar and see how this scalar advects and mixes with different parts of the flow. Anything that flows might possibly be studied using a UDS, so it's uses are many. You can even use a UDS to calculate flow residence time.

 August 16, 2017, 10:53 #3 New Member   RJC Join Date: Jun 2015 Posts: 18 Rep Power: 10 Thanks a lot for your reply.

 Tags fluent - udf, uds