LONDON, April 20 (Reuters) - British emerging markets-focused hedge fund Onslow Capital Management has closed after a long period of low volatility hit returns and assets fell below a sustainable level, it said in a letter to investors. The move by founder Nicolas Galperin to close after 11 years comes despite forecasts for markets to be more turbulent in the coming months as central banks shift their easy monetary policies, and follows the closure of Harmonic Capital Partners in February for a similar reason.