Comprehending and describing turbulent structures play a crucial role in a wide range of engineering applications. Additionally, they contribute to enhancing our understanding of turbulent flows, which is vital for advancing research in fluid dynamics. Hairpins are one kind of structure observed in turbulent flows. A wide range of solutions from numerical methods including the Lagrangian viewpoint to experimental techniques has been used to understand hairpin evolution. The significance of the Lagrangian viewpoint in the study of hairpins lies in its ability to capture the underlying physics at a fundamental level.
A Lagrangian particle tracking code was developed in MATLAB. The validation of the tracer was conducted using a trusted arbitrary velocity field and the numerical schemes were examined in terms of accuracy. Adams Bashforth 4th order and Lagrangian interpolation of fifth-degree were selected as the Ordinary Differential Equation solver and interpolation of the velocity field, respectively.
The evolution and generation of hairpins are studied in a Lagrangian perspective for plane turbulent channel flow. Direct numerical simulation database of turbulent channel flow at friction Reynolds numbers Re_τ=590, Re_τ=390, and Re_τ=180 is employed to perform tracking and interpolation algorithms of fluid particles on hairpins. These particles are carefully selected using a vortex line identification technique developed to extract the hairpin’s core line. Four methods of vortex identification were examined in which the λ^2 criterion together with Liutex were used for visualization purposes.
Interesting results are observed regarding the Reynolds number impact on the evolution and regeneration of hairpins. As the Reynolds number increases, the head and the vortical tongues are compressed and aligned in the main flow direction and the hairpin structures tend to orient more horizontally. The deformation shape of hairpins using the strain state parameter, which is correlated with the dissipation rate, is analyzed using the probability density function. Most phenomena related to the generation of secondary hairpins, including the existence of vortical tongues, occur in the range of 35
كليدواژه هاي فارسي
اثر رينولدز , توسعه لاگرانژي , تغيير شكل هرپين , رهگيري ذره سيال , توربولانس , كانال
كليدواژه هاي لاتين
Reynolds number effect , Lagrangian evolution , hairpin deformation , fluid particle tracking , turbulent channel flow
Author
Behnam Kazemi Majd
SuperVisor
Dr. Zeinab Pouransari
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