Unsteady characteristics of three-dimensional turbulent flow with separation in an asymmetric diffuser

  • Author / Creator
    Elyasi, Mohammad Mahdi
  • A great number of studies have been undertaken to understand separated turbulent boundary layers (TBLs) due to its importance in the performance, design and control of many engineering systems (Hal 2000). Flow separation has a complicated three-dimensional nature in many flow configurations including wings, flow intakes and swept edges emanating from an apex (Simpson 1981). The three-dimensional nature of flow separation adds to its more complexity (Simpson et al. 1977), and its complicated nature remains unknown (Tobak and Peake 1982; Délery et al. 2001). In this study, a TBL is subjected to adverse pressure gradient (APG) by an increase in the cross section area of the flow in a diffuser-like section, although the upstream flow is not fully developed. The free-stream velocity drops from 0.45 m/s upstream of the diffuser section to 0.36 m/s at the downstream region. The TBL separates from the diverging wall of the diffuser. Advanced particle image velocimetry (PIV) techniques are applied to provide new insight into statistical understanding and turbulent structures involved in turbulent flow separation. The velocity fields captured by multiple planar PIV cameras are stitched together to provide a large field of view (FOV) with high dynamic rang. Tomographic PIV (Tomo-PIV) provides three-dimensional velocity fields at the location of flow detachment in a small volume near the wall to reveal the three-dimensional characteristics of the flow. The three-dimensional average flow field shows a vortex structure with the rotation axis of the wall-normal direction. All Reynolds stresses show a similar pattern in the FOV of planar PIV, and they are almost consistent with the distributions provided in the previous research literature on two-dimensional flow separation. The presence of attached or separated flow is characterized by monitoring the direction of streamwise velocity close to the wall. The structure of the instantaneous flow fields that contain a coherent recirculation region is characterized using conditional averaging. For these flow fields, a region with strong forward streamwise fluctuation appears upstream of the detachment line. Quadrant analysis also shows the frequent occurrence of strong positive streamwise fluctuations upstream of flow detachments. The turbulent statistics of the conditionally selected flow fields reveal a low turbulence region appeared at the detachment point and a strong turbulence at the upstream of flow detachments. Proper orthogonal decomposition (POD) technique was used to identify the flow motions with large turbulent kinetic energy. The energetic POD modes represent the movement of the shear layer (breathing motion) and the strength of velocity gradient of the shear layer (the strength of vortex shedding) as the motions with large kinetic energy in the flow. The three-dimensional POD analysis reveals also other high energy three-dimensional structures, including vortices with wall-normal axis of rotation (similar to the mean flow structure) and saddle-point structures. Similarities between POD modes and flow structures captured by conditionally averaging analysis are found, which associates these dominant flow motions with the structure of flow detachment. Reconstructed flow fields using the first four POD modes visualize the flow development at the instant of single-detachment flow fields, which illustrates the movement of the center of a large vortex with wall-normal axis from the downstream to the upstream of the FOV. This dislocation of the vortex supplies strong forward and backward motion before and after the instant of flow separation.

  • Subjects / Keywords
  • Graduation date
    Fall 2017
  • Type of Item
  • Degree
    Master of Science
  • DOI
  • License
    This thesis is made available by the University of Alberta Libraries with permission of the copyright owner solely for non-commercial purposes. This thesis, or any portion thereof, may not otherwise be copied or reproduced without the written consent of the copyright owner, except to the extent permitted by Canadian copyright law.