What is the location of alert.log file in Oracle Database 11g?

SQL> select value from v$parameter where name='background_dump_dest';

VALUE
--------------------------------------------------------------------------------
/u0/app/oracle/diag/rdbms/orcl/orcl/trace

$ cd /u0/app/oracle/diag/rdbms/orcl/orcl/trace
$ ls 
alert_orcl.log   orcl_j001_25686.trm  orcl_ora_20641.trc

or in SQL *PLUS

SQL> show parameter background_dump_dest;

SQL> select value from v$parameter where name='diagnostic_dest';

VALUE
--------------------------------------------------------------------------------
/u0/app/oracle

To know the current ORACLE_SID, you can use following select:

SQL> select instance_name from v$instance;

INSTANCE_NAME
----------------
ORCL

Alert log file consist alert_.log