  Low and intermediate frequency quasi-periodic oscillations (QPOs) in black
hole candidates are believed to be due to oscillations of the Comptonizing
regions in an accretion flow. Assuming that the general structure of an
accretion disk is a Two Component Advective Flow (TCAF), we numerically
simulate the light curves emitted from an accretion disk for different
accretion rates and find how the QPO frequencies vary. We use a standard
Keplerian disk residing at the equatorial plane as a source of soft photons.
These soft photons, after suffering multiple scattering with the hot electrons
of the low angular momentum, sub-Keplerian, flow emerge out as hard radiation.
The hydrodynamic and thermal properties of the electron cloud is simulated
using a Total Variation Diminishing (TVD) code. The TVD code is then coupled
with a radiative transfer code which simulates the energy exchange between the
electron and radiation using Monte Carlo technique. The resulting localized
heating and cooling are included also. We find that the QPO frequency increases
and the spectrum becomes softer as we increase the Keplerian disk rate.
However, the spectrum becomes harder if we increase the sub-Keplerian accretion
rate. We find that an earlier prediction that QPOs occur when the infall time
scale roughly matches with the cooling time scale, originally obtained using a
power-law cooling, remains valid even for Compton cooling. Our findings agree
with the general observations of low frequency QPOs in black hole candidates.
