 
Summary: Correlation of Surface Pressure and Vortical Flow Structures in
an Unsteady Separating Flow
Stephen Snider, Daniel Morse,
Sourabh V. Apte, and James A. Liburdy,
Oregon State University, Corvallis, OR, 97331, USA
Controlling flow in an unsteady separating flow requires accurate information about the surface forces and
the effect that vortical structures in the flow field have on those surface forces. A correlation between vortex
passage and surface pressure is defined to determine appropriate locations for sensors on the surface of a
square cylinder for control feedback. Data are generated using a large eddy simulation of flow over a square
cylinder at Reynolds number of 21,000. The flow is decomposed using proper orthogonal decomposition (POD)
based on the frequency and energy content of the velocity and pressure fields around the square cylinder.
Two data sets are reconstructed from the POD: (i) the first two modes containing approximately 75% of the
energy of the flow and representing the large scale wake oscillation, and (ii) higher order modes containing the
remainder of the energy of the flow representing smaller scale features in the flow, such as those shed from the
leading edge. Vortex detection using the function is performed on both data sets, where the function is
an areaaveraged measure of the swirl around each point in the flow. Vortices in the data set composed of the
first two modes tend to be large scale features and oscillate with the same frequency as the bluff body wake
oscillation. The vortices detected in the higher order modes are smaller and tend to be the vortical structures
shed from the leading edge of the square cylinder and convected downstream. In both data sets, the correlation
values increase from the leading edge to the trailing edge, indicating that the downstream half of the square
