Spatio-temporal changes to coherent structures in a TBL caused by unsteady pressure gradients

A temporally-strengthening favorable-adverse pressure gradient sequence was imposed on a flat plate turbulent boundary layer (TBL). A series of steady pressure gradients that instantaneously matched the time-varying pressure gradients were also independently imposed. The TBL response was captured in the adverse pressure gradient region using time-resolved particle image velocimetry. Energetically-dominant structures of the flow and their spatio-temporal variations were extracted using space-time proper orthogonal decomposition (ST-POD) for the unsteady case and spectral proper orthogonal decomposition (SPOD) for the corresponding steady cases. Differences in modal changes in the TBL between the unsteady and steady pressure gradient impositions were quantitatively tracked as a function of time, accounting for differences in size, angle of inclination, height from the wall, etc. In this talk, we discuss the structural differences, make connections to differences in turbulent statistics previously identified by the authors, and likewise study the effect of timescale of pressure gradient imposition.