Aeroelastic instabilities of a rigid flat plate impacted by an impinging air jet

Impinging air jets are widely used in steel production for the purpose of cooling. On thin steel sheet galvanizing lines, numerous observations reported that the impinging jets impact force can trigger flutter or divergence instabilities. This leads to significant oscillations of the sheet between the nozzles, resulting in detrimental effects on the thermal and mechanical properties of the product. Unfortunately, the stability criterion for this phenomenon remains poorly defined.

We conducted a simplified experimental model with a horizontal rigid flat plate, connected to springs, allowing it to oscillate vertically. This system acts as a damped oscillator with a single degree of freedom. An air jet blows orthogonally from a rectangular nozzle to the plate. The stability of the system depends on various parameters such as the shape of the nozzle or the jet velocity, but mainly the nozzle-plate distance. Below a critical distance, the plate is unstable and starts to flutter. Experimental results lead us to develop a theoretical model for the jet added damping and stiffness term.