@Article{IJNAM-5-17,
author = {G. Chen, Z. Lin, D. Morse, S. Snider, S. Apte, J. Liburdy and E. Zhang},
title = {Multiscale Feature Detection in Unsteady Separated Flows},
journal = {International Journal of Numerical Analysis and Modeling},
year = {2018},
volume = {5},
number = {5},
pages = {17--35},
abstract = {<p style="text-align: justify;">Very complex flow structures occur during separation that
appear in a wide variety of applications involving flow over a bluff
body. This study examines the ability to detect the dynamic interactions
of vortical structures generated from a Helmholtz instability caused by
separation over bluff bodies at large Reynolds number of approximately $10^4$ based on cross stream characteristic length of the geometry.
Accordingly, two configurations, a thin airfoil with flow at an angle of
attack of $20^0$ and a square cylinder with normally incident flow are
examined. A time-resolved, three-component PIV data set is collected in
a symmetry plane for the airfoil, whereas direct numerical simulations
are used to obtain flow over the square cylinder. The experimental data
consists of the velocity field, whereas simulations provide both
velocity and pressure-gradient fields. Two different approaches
analyzing vector field and tensor field topologies are considered to
identify vortical structures and local, swirl regions. The vector field
topology uses (1) the $\Gamma$ function that maps the degree of rotation
rate (or pressure-gradients) to identify local swirl regions, and (2)
Entity Connection Graph (ECG) that combines the Conley theory and Morse
decomposition to identify vector field topology consisting of fixed
points (sources, sinks, saddles) and periodic orbits, together with
separatrices (links connecting them). The tensor field feature uses (1)
the $\lambda_2$ method that examines the gradient fields of velocity or
pressure-gradient to identify local regions of pressure minima, and (2)
tensor field feature that decomposes the velocity-gradient or pressure
Hessian tensor into isotropic scaling, rotation, and anisotropic
stretching parts to identify regions of high swirl. The vector-field
topology requires spatial integration of the velocity or
pressure-gradient fields and represents a global descriptor of vortical
structures. The tensor field feature, on the other hand, is based on
gradients of the velocity of pressure-gradient vectors and represents a
local descriptor. A detailed comparison of these techniques is performed
by applying them to velocity or pressure-based data and using spatial
filtered data sets to identify the multiscale features of the flow. It
is shown that various techniques provide useful information about the
flow field at different scales that can be used for further analysis of
many fluid engineering problems of practical interest.</p>},
issn = {2617-8710},
doi = {https://doi.org/},
url = {http://global-sci.org/intro/article_detail/ijnam/837.html}
}