Turbulent Flows Structures Crossing Conical Diffusers: Angle Effect Analysis using PIV Technique and POD for Post-Processing

In this work, we investigated experimentally the hydrodynamics of flows crossing conical diffusers. On our previous work (Aloui et al., 2011), CFD turbulent models were validated for flows crossing the critical angle (2=16°). Indeed, the PIV data base constructed was exploited to validate a variant of SST-RLC model. Taking into account the conical diffuser angle effect, the apparition and the development of vortices were observed and studied. The dynamics of the recirculation zones which may be observed at the lower and higher parts of the singularity, has not formed the subject of numerous studies. There were no studies that characterize the vortices at the conical diffusers in terms of size, centre positions, and vortex intensity. Consequently, two conical diffusers were studied using the Particle Image Velocimetry technique (PIV). The results illustrate effects of “opening angle” (2=16°) and (2=30°) on the flow structures developed in such type of diffusers. From such opening angle of conical diffusers, the progressive angle increasing generates a detachment of the boundary layer of the conical diffuser depending on the turbulence level. This detachment may lead to a coherent flow structures. We applied the coherent structures criterion 2 to the recorded velocity fields to detect and characterize the vortices at the conical diffusers. We used the Proper Orthogonal Decomposition (POD) to filter the PIV data base constructed and to extract the most energetic modes. The results illustrate that the turbulent flow structures can be constituted using a limited number of energetic modes.