  Most of the studies on pressure fluctuations in wall-bounded turbulent flows
aim at obtaining statistics as power spectra and scaling laws, especially at
the walls. In the present study we study energetic coherent pressure structures
of turbulent channel flows, aiming at a characterization of dominant coherent
structures throughout the channel. Coherent structures are detected using
spectral proper orthogonal decomposition (SPOD) and modeled using resolvent
analysis, similar to related works dealing with velocity fluctuations, but
using pressure fluctuations as the output of interest. The resolvent operator
was considered with and without the Cess eddy viscosity model. Direct numerical
simulations (DNSs) of incompressible turbulent channel flows at friction
Reynolds numbers of approximately 180 and 550 were employed as databases. Three
representative dominant structures emerged from a preliminary spectral
analysis: near-wall, large-scale and spanwise-coherent structures. For
frequency-wavenumber combinations corresponding to these three representative
structures, SPOD results show a strong dominance of the leading mode,
highlighting low-rank behavior of pressure fluctuations. The leading resolvent
mode closely agrees with the first SPOD mode, providing support to studies that
showed better performance of resolvent-based estimators when predicting
pressure fluctuations compared to velocity fluctuations. The dominant
mechanisms of the analyzed modes are seen to be the generation of
quasi-streamwise vortices with pressure fluctuations appearing close to vortex
centers. A study on the individual contributions of the nonlinear terms
(treated as forcing in resolvent analysis) to the pressure output reveals that
each forcing component plays a constructive role to the input-output
formulation, which also helps understanding the weaker role of forcing color in
driving pressure fluctuations.
