Let's say there are 5 applications and the transaction log getting filled. How to identify which application is the main culprit out of the running 5!?
You can display the application statistics with db2 get snapshot for applications on <dbname> (this will display a lot of statistics for each application). By default the UOW statistics are not collected but they could show the log space used and the last commit time. You can let DB2 collect those statistics with db2 update monitor switches using uow on but I'm not sure if this will show all statistics for currently running processes. You can probably find the log filling application by checking the number of rows updated/deleted etc.
If you want to collect the UOW statistics (or other staitics) always you can set the database manager parm dft_mon_uow to ON, but this might cause the CPU usage the grow a little bit. If your system is already heavy loaded be carefull with it.