  We examine the effects of magnetic field on low angular momentum flows with
standing shock around black holes in two dimensions. The magnetic field brings
change in behavior and location of the shock which results in regularly or
chaotically oscillating phenomena of the flow. Adopting fiducial parameters
like specific angular momentum, specific energy and magnetic field strength for
the flow around Sgr A*, we find that the shock moves back and forth in the
range 60--170R_g, irregularly recurring with a time-scale of ~5 days with an
accompanying more rapid small modulation with a period of 25 hrs without
fading, where R_g is the Schwarzschild radius. The time variability associated
with two different periods is attributed to the oscillating outer strong shock,
together with another rapidly oscillating inner weak shock. As a consequence of
the variable shock location, the luminosities vary roughly by more than a
factor of 3. The time-dependent behaviors of the flow are well compatible with
luminous flares with a frequency of ~one per day and bright flares occurring
every ~5 -- 10 days in the latest observations by Chandra, Swift and XMM-Newton
monitoring of Sgr A*.
