Evolution of angular velocity of central black holes of accretion disks

The evolution characters of angular velocity Omega(h) of central black holes of accretion disks (including thin and thick disks) are investigated by considering both accretion and the Blandford-Znajek (BZ) process. It is shown that the rate of change of Omega(h) with respect to time turns from the positive value to the negative, as the dimensionless angular momentum a, evolves to a critical value. Evolving in a non-monotonous way, Omega(h) increases at first, and then decreases in a prograde accretion process. Therefore there exists a maximum of Omega(h). It is pointed out that dimensionless angular momentum a, is more appropriate to describe the spinning state of a central black hole of an accretion disk than angular momentum J and angular velocity Omega(h).