用户自定义函数，或UDF，是用户自编的程序，它可以动态的连接到Fluent求解器上来提高求解器性能。用户自定义函数用C语言编写。使用DEFINE宏来定义。(User defined function or UDF is a program written by user. It can connect to Fluent solver dynamically and improve the performance of solver. The user-defined function is w