How to clear alert.log without ADRCI? I know there is ADRCI purge command since 11g, but how can I delete part of the alertlog without ADRCI for a specified time period, I want delete everything older than 15 days.
I would rotatelogs command to periodically copy the logs into a new one, and then delete the old log after a point in time. You can put it into a cron job.

However, during troubleshooting it is VERY beneficial to have the entire alert logs - and in one file. Be sure to keep it somewhere close, when trouble comes by.