  In outburst sources, quasi-periodic oscillation (QPO) frequency is known to
evolve in a certain way: in the rising phase, it monotonically goes up till a
soft intermediate state is achieved. In the propagating oscillatory shock
model, oscillation of the Compton cloud is thought to cause QPOs. Thus, in
order to increase QPO frequency, Compton cloud must collapse steadily in the
rising phase. In decline phases, exactly opposite should be true. We
investigate cause of this evolution of the Compton cloud. The same viscosity
parameter which increases the Keplerian disk rate, also moves the inner edge of
the Keplerian component, thereby reducing the size of the Compton cloud and
reducing the cooling time scale. We show that cooling of the Compton cloud by
inverse Comptonization is enough for it to collapse sufficiently so as to
explain the QPO evolution. In the Two Component Advective Flow (TCAF)
configuration of Chakrabarti-Titarchuk, centrifugal force induced shock
represents boundary of the Compton cloud. We take the rising phase of 2010
outburst of Galactic black hole candidate H~1743-322 and find an estimation of
variation of $\alpha$ parameter of the sub-Keplerian flow to be monotonically
rising from $0.0001$ to $0.02$, well within the range suggested by
magneto-rotational instability. We also estimate the inward velocity of the
Compton cloud to be a few meters/second which is comparable to what is found in
several earlier studies of our group by empirically fitting the shock locations
with the time of observations.
