上面这段代码就是设置x轴方向的速度为30米每秒（UDF默认是SI单位制）。vel[0]表示x、vel[1]表示y、vel[2]表示z。细心读者肯定会有疑问，这里并没有指定是哪条边界移动，别急，这需要到后面，在Fluent里设置。-When one of these general multiphase models is enabled, storage must be set aside for the mixture as well as the individual phases. This functionality is manifested in the code through the use of additional thread and domain data structures. Consequently, some predefined macros have been added that allow access to data contained within mixture-level and phase-level domain and thread structures. See Section　 3.11 for details on writing UDF for multiphase applications.