 
Summary: Detection and Analysis of Separated Flow Induced Vortical
Structures
Stephen Snider, Daniel Morse, Guoning Chen,
Sourabh V. Apte, James A. Liburdy§, and Eugene Zhang¶
Oregon State University, Corvallis, OR, 97331, USA
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
104 based on a cross stream characteristic length of the geometry. Accordingly, two configurations, a square
cylinder with normally incident flow and a thin airfoil with flow at an angle of attack of 200 are examined.
Direct numerical simulation is used to obtain flow over the square cylinder. A timeresolved, threecomponent
PIV data set is collected in a symmetry plane for the airfoil. Different approaches analyzing vector field
and tensor field topologies are considered to identify vortical structures and local, swirl regions: (i) the
function that maps the degree of rotation rate (or pressuregradients) to identify local swirl regions, (ii) 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), and (iii) the 2 method that examines the gradient fields of velocity to identify local
regions of pressure minima. Both velocity and pressuregradient fields are analyzed for the DNS data, whereas
only velocity field is used for the experimental data set. The vectorfield topology requires spatial integration
of the velocity or pressuregradient fields. The tensor field topology, on the other hand, is based on gradients of
the velocity of pressuregradient vectors. A detailed comparison of these techniques is performed by applying
