Numerical Simulation of Partially Covered Hartmann Whistle in a Sonic-Underexpanded Jet

The current study provides numerical investigation into the use of “Hartmann whistle” as an effective passive flow control device by covering the major area between the nozzle exit and cavity inlet using a cylindrical shield. The passive control is accomplished by allowing the pulsating jet to exit through two small openings in the shield so that it can be utilized for various flow control applications such as mixing enhancement, drag reduction, noise mitigation. The current study numerically investigates the effect of partially covered cylindrical shield on the shock as well as regurgitant oscillation characteristics of a Hartmann whistle when the pulsating jet exits through the two small openings of the cylindrical shield. The relevant parameters that modify the flow/shock oscillations of the Hartmann whistle are the cavity standoff distance, nozzle pressure ratio, cavity length, cavity shield, etc. The studies were performed for various standoff distances values of 10, 20, and 30 mm to demonstrate the role of standoff distance in effective flow control. The modifications in the shock as well as regurgitant oscillation features of partially covered Hartmann whistles are systematically compared using transient velocity vectors, Mach number contours, etc. for various standoff distances. The velocity vectors indicate flow diversion features near the cavity mouth as well as inflow and outflow jet regurgitant phases. The Mach contours of partially shielded Hartmann whistles indicate shock structures, zones of flow deceleration and re-acceleration. It also clearly demonstrates that the resonant oscillations are primarily driven by jet regurgitance at smaller standoff distances, but at higher standoff distances they are primarily driven by the fluid column oscillations in the shock-cells, shield as well as in the cavity zones. Thus, the current study reveals that the standoff distance is a crucial parameter that controls the strength of shock, regurgitant as well as fluid column oscillations in a partially covered Hartmann whistle in order to achieve an effectual flow control.