On the Stability of Subsonic Impinging Jets

Impinging jets are relevant in many applications, especially those involving heating and cooling processes. In such context, several techniques have been proposed to enhance the heat transfer at the wall: in particular, experimental investigations demonstrated that the use of a pulsating inlet with an appropriate pulsation frequency may result in a 40% enhancement of heat transfer compared to a non-pulsating configuration. Such a result is related to the enlargement of the generated toroidal vortices which cause higher wall shear stresses. The mechanism underlying the generation of these larger vortex rings at a specific frequency is still unclear and the explanation of the effects of pulsation on heat transfer is still an open question. In order to shed light on such process, in this work, we present a modal analysis of a subsonic impinging jet confined between two horizontal walls placed at a distance of 5D, being D the diameter of the orifice in the uppermost wall from which the jet issues. Initially, a direct numerical simulation (DNS) of a round jet with a Mach number of 0.8 and a Reynolds number of 3300 is performed and the main flow characteristics (including dominant frequencies and spatial features) are retrieved through a dynamic mode decomposition (DMD) analysis. Subsequently, a global stability analysis on the same physical configuration is carried out and the spatial structures and frequencies of the resulting leading unstable modes are discussed and compared with the DMD and DNS data.