Large-scale clustering of coherent fine-scale eddies in a turbulent mixing layer

• Published in: The International journal of heat and fluid flow Vol. 72; pp. 100 - 108
• Main Authors: Itoh, Toshitaka, Naka, Yoshitsugu, Minamoto, Yuki, Shimura, Masayasu, Tanahashi, Mamoru
• Format: Journal Article
• Language: English
• Published: Elsevier Inc 01.08.2018
• Subjects: Direct numerical simulation; Inter-scale energy transfer; Turbulence structure; Turbulent mixing layer; Turbulent mixing layer; Direct numerical simulation; Inter-scale energy transfer; Turbulence structure
• ISSN: 0142-727X; 1879-2278
• DOI: 10.1016/j.ijheatfluidflow.2018.05.007

•Clustering is measured by the number density of coherent fine-scale eddies.•The large-scale enstrophy increases with the number density.•In the core of the cluster, the large-scale strain rate and vorticity are less aligned.•The large-scale strain rate is active in the periphery of the cluster. Clustering of coherent fine-scale eddies in a turbulent mixing layer has been analyzed by using direct numerical simulation (DNS) data at Reλ ≃ 250. The coherent fine-scale eddies are defined based on the second invariant of the velocity gradient tensor and the vorticity vector. The clustering is evaluated by the number density of coherent fine-scale eddies, and the large-scale structures are extracted by low-pass filtered velocity fields. Conditional averaging shows that the large-scale enstrophy increases with the number density, whereas the large-scale strain rate stays around the average in the high number density region. The alignments of the vorticity vector and the eigenvectors of the large-scale strain rate tensor are conditioned by the number density or the strain rate magnitude. The eigenvectors and the vorticity vector indicate strong preferential alignments under the intense large-scale strain rate condition. On the other hand, those alignments become weak in the high number density regions. The inter-scale energy transfer between grid and subgrid scales is significantly correlated with the magnitude of the large-scale strain rate while there is no apparent correlation with the number density.