Influence of splitter plate thickness on flow structure, turbulence, and mixing in high-aspect-ratio plane parallel jets

The influence of the middle plate thickness (t) on the flow structure of a free, parallel-jet rectangular nozzle was experimentally investigated. In this work, four middle plate thicknesses were employed, with the remaining geometry unchanged, to assess their effect on the flow structure. Mean and turbulent characteristics of the jet were measured along the streamwise and lateral directions using a two-dimensional Constant Temperature Hot-Wire Anemometry. Also, Particle Image Velocimetry (PIV) and Proper Orthogonal Decomposition (POD) were employed to investigate the flow characteristics. The findings indicated that increasing the middle plate thickness fundamentally changes the near-field flow structure and the interaction of the inner shear layers. A linear bending trend in the lateral velocity toward the symmetry line was observed as plate thickness increased; however, its magnitude showed a nonlinear trend. Furthermore, changing the plate thickness results in a different flow structure: from a stream of small vortices to a flapping motion to a separated flow. The most effective middle plate thickness (among the four tested) enhances velocity fluctuations and increases turbulent transport of normal and shear stresses via lateral velocity fluctuations, thereby improving mixing and increasing lateral spreading. PIV and POD demonstrated vortex interaction in the inner and outer shear layers, vortex behavior within the recirculation zone, and a coherent structure with significant variation. The Strouhal number (Sth) was found to increase as the middle plate thickness decreased. This experiment demonstrates that altering a single parameter can influence flow control, with implications for a broad range of industrial free-turbulent-jet applications.