A roof 201 has an apex 200. A wind turbine 202 is positioned above the apex 200. Air flows through the gap 204 between the apex 200 and the turbine 202. A flow optimizer 203 diverts a section of airflow close to the apex 200 into the region 205, downstream of the apex. This section of diverted airflow is turned away from the local airflow direction towards the surface of the roof 201, downstream of the apex. This reduces the boundary layer separation and turbulence downstream of the apex 200, which reduces energy loss associated with sub-optimal static pressure recovery downstream of the apex 200, and thus increases the discharge coefficient of the system.