  We develop a passive control strategy for suppressing thermoacoustic
instability in a bluff-body stabilized premixed turbulent combustor. When the
equivalence ratio is varied, there is a transition from combustion noise to
thermoacoustic instability via intermittency in the combustor. We perform
simultaneous acoustic pressure, 2D-PIV, and CH$^*$ chemiluminescence
measurements to capture the pressure fluctuations, the velocity field, and the
heat release rate (HRR) field during the transition. We measure the spatial
distribution of the amplitude of turbulent velocity at the acoustic frequency,
time-averaged vorticity, time-averaged HRR, and Rayleigh index and identify
various regions of significance. We implement a passive control strategy by
targeting these regions with a steady injection of secondary micro-jet of air
to optimize the injection location and determine the critical region. Targeting
the critical region with secondary air leads to greater than 20 dB suppression
of the dominant thermoacoustic mode. We observe that the coherent structure
forming from the shear layer following the dump plane gets suppressed, leading
to an incoherent spatial distribution of HRR fluctuations. We find that the
turbulent velocity amplitude correctly identifies the critical region for
optimized passive control during thermoacoustic instability. In contrast, the
Rayleigh index identifies the region of the most significant acoustic driving;
however, it does not identify the region most sensitive to control. Finally, we
extend our analysis by determining the spatial distribution of the Hurst
exponent measured from the turbulent velocity field. We show that the Hurst
exponent identifies the critical region during thermoacoustic instability and
intermittency, unlike the other physical measures.
