Arunachalam and Tiru-annamalai (or Tiruvannamalai) are the same. People would have very well seen the description "Arunachala's abode". Tiruvannamalai is a combination of words. 'Thiru' in Tamil means 'SHRI' as in Sanskrit 'Malai' in Tamil means 'Mountain' in English or 'GIRI' as in Sanskrit
The whole word TIRUVANNAMALAI translates to SHRI ARUNACHALA'S HILL. The Hill which is the abode of Lord Arunachala.
And hence TIRUVANNAMALAI and ARUNACHALAM, though may sound unrelated, are the same.
