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The present study numerically investigates the effect of a partially covered cylindrical shield on the flow/shock oscillation characteristics of a Hartmann whistle when the pulsating jet exits through the two small openings, (a) close to the cavity inlet, and (b) away from the cavity inlet, of the cylindrical shield. The relevant parameters that modify the flow/shock oscillations of the Hartmann whistle are the stand-off distance, nozzle pressure ratio, cavity length, cavity shield, jet diameter, etc. The pulsating nature of flow in a partially shielded Hartmann whistle is investigated for various stand-off distances to understand its effect in achieving effective flow control. The velocity vectors indicate that the partly shielded Hartmann whistle operates in the jet regurgitant mode with different regurgitant phases. It also shows that some amount of the jet near the cavity inlet gets diverted towards the shield and gets attached to it, whereas some exits out through the two shield openings which can be injected into the flow to be controlled. The Mach number contours indicate the flow deceleration/reacceleration zones, shock-cell structures as well as fluid column oscillations in shock-cells/cavity regions. The present study reveals that the stand-off distance and the jet diameter are the crucial parameters, which control the oscillation mechanisms in a partially covered Hartmann whistle for achieving effective flow control. Thus, this paper sufficiently demonstrates the role of stand-off distances, openings in the shield as well as jet diameter in modifying the flow/shock oscillation characteristics of a partially shielded Hartmann whistle in achieving the finest flow control.

The present study experimentally and numerically investigates the flow past flat plates and NACA0012 airfoils with the different trailing edge (i.e., blunt, sharp, and rounded) and both end-rounded configurations on the far-field acoustic emissions and complex flow phenomena to determine the best edge geometry for low noise. The studies are conducted for various jet speeds of 20, 30, and 40 m/s (chordwise Reynolds numbers of 1.812 × 105, 2.72 ×105, and 3.62 ×105). It is observed that all the edge-modified flat plates show the highest directivity at an emission angle of 60° for all jet velocities studied. The comparison of power directivities between realistic and flat plate airfoils reveals that the realistic airfoils radiate lower acoustic emissions as compared to flat plate airfoils, albeit they show a common feature of downstream directivity. In general, the far-field acoustic emissions of both end-rounded plates are observed to be lower as compared to blunt, sharp, and rounded trailing edge geometries. The leading edge modification is also observed to reduce airfoil self-noise. The likely reason for the lower far-field acoustic emissions provided by both end-rounded trailing edged foils is due to the modification in the boundary layer characteristics owing to the presence of smooth flow around the rounded leading edge as well as reduced vortex strength as compared to the other foil geometries. Thus, the present study demonstrates that passive modifications of leading edge profiles are essential along with trailing edge for achieving lower acoustic emissions and higher noise reductions.