SQL Server – Clean Buffers
Use DBCC DROPCLEANBUFFERS to test queries with a cold buffer cache without shutting down and restarting the server. To drop clean buffers from the buffer pool, first use CHECKPOINT to produce a cold buffer cache. This forces all dirty pages for the current database to be written to disk and cleans the buffers. After you do this, you can issue DBCC DROPCLEANBUFFERS command to remove all buffers from the buffer pool. 
CHECKPOINT; GO DBCC DROPCLEANBUFFERS; GO
It is recommended these commands should not be executed in a production environment where the SQL Server caching helps in gaining performance. Running these commands could adversely impact the server performance.
1. DBCC DROPCLEANBUFFERS (Transact-SQL)