Impinging planar air jet has essential role to control final coating thickness in many industrial processes such as photographic film manufacturing, painting, and especially steel strip coating. It is well known that the wall pressure and the wall shear stress profiles at the strip are important jet characteristics that directly impact on the coating quality. One of the causes of the coating defect is attributed to the oscillation of the wall pressure and the wall shear stress. Hence, intensive investigation of these characteristic behaviors is necessary to better control the coating thickness and enhance the quality of the coating surface.
When the supplied air exhausts from the jet exit, it immediately begins to entrain the still ambient air. The two mixing layers appear and develop from each side of the jet exit. As the jet flow propagates, more ambient fluid is entrained and the mixing layers become thicker. The two mixing layers evolve and converge toward the jet centerline whilst simultaneously making the jet flow wider. The centerline velocity remains unchanged as the exit velocity on a certain distance until the two mixing layers meet each other at the centerline. The zone bounded by the two mixing regions where no mean velocity gradients exist is named as the potential core region1). It was demonstrated that vortex structures start soon near the jet exit at the two mixing layers that border the potential core. Due to the tangential velocity gradient that exists between the high speed jet flow and the surrounding relatively stationary environment, momentum is transferred from the faster layers to slower layers. One important consequence of this interaction is the generation of vortices by the shear layers separating the jet flow and the ambient air. These vortex structures convect downstream and combine together into larger structures. Subsequently, they traverse into the impinging region at the strip resulting in the fluctuation of the wall pressure and the wall shear stress. The frequency components of the jet flow can alter significantly as a consequence of downstream joining of vortices2).
There is evidence that both symmetric and anti-symmetric fluctuations coexist in the initial region of the jet. From study of the transition of a two-dimensional jet there are identified two types of velocity fluctuations in the region near the jet exit and, additionally, the fluctuations at higher frequencies are found to be symmetric with respect to the jet centerline, whereas low-frequency fluctuations are anti-symmetric3). Velocity fluctuation measurements indicate the presence of symmetric and anti-symmetric modes in the region prior to the end of the jet potential core where the symmetric mode is noted to be dominant with higher frequencies than those of the anti-symmetric mode4). Furthermore, it has been demonstrated that the symmetric mode is dominant in the near-field region or potential core region of the jet flow and the anti-symmetric mode dominates in the far-field region leading to the flapping motion of the jet flow5-7).
The presence of the strip which barricades the free jet flow is assumed to cause more disturbances to the jet flow through a feedback loop mechanism8-12). The loop firstly starts at the jet nozzle and is derived by the instabilities in the mixing layers. These structures grow in size becoming larger-scale coherent structures and convect downstream toward the strip. Acoustic waves are produced as the coherent structures impinging upon the strip. These waves are reflected and travel back upstream as acoustic waves and then they advance the mixing layer instabilities when arriving at the jet nozzle.
This study aims to inspect the relations of the mean flow characteristics on the strip and H/d ratio, Reynolds number by means of steady CFD simulation. Then, unsteady impinging jet flow is examined by means of Larger Eddy Simulation (LES). The migration of stagnation points is monitored to explore the flapping behavior along the strip of the wall pressure and wall shear stress distribution.