Evidence of flow vortex signatures on wall fluctuating temperature using unsteady infrared thermography for an acoustically forced impinging jet

Infrared thermography is used to investigate unsteady wall temperature in the configuration of a round jet impinging orthogonally on a uniform flux heated plate. The Reynolds number is 28,000 and the orifice to plate distance is H/D=3. The flow can be forced using a loudspeaker located on the top of the injection module. Two forcing Strouhal numbers are then studied, StLS=0.26 and 0.79. The unprocessed results reveal a first propagation mode of the fluctuating temperatures along the impingement plate. It does not depend on the forcing and is characterized by very low frequencies and corresponds to the propagation of warm and cold bubbles in privileged azimuthal directions. Their amplitudes reach 0.5K. Using processing tools such as phase averaging or high-pass filtering, another propagation mode is revealed: the radial spread of cold and warm fronts, linked to the convection of vortices along the impingement plate. Indeed, their characteristic frequencies and convection velocities correspond to those of the vortices along the plate and the amplitudes reach 0.1K.