  Pressure-gradient-induced separation of swept and unswept turbulent boundary
layers, based on the DNS studies of Coleman et al. (J. Fluid Mech. 2018 &
2019), have been analyzed for various nonequilibrium effects. The goal is to
isolate physical processes critical to near-wall flow modeling. The
decomposition of skin friction into contributing physical terms, proposed by
Renard and Deck (J. Fluid Mech. 2016) (short: RD decomposition), affords
several key insights into the near-wall physics of these flows. In the unswept
case, spatial growth term (encapsulating nonequilibrium effects) and TKE
production appear to be the dominant contributing terms in the RD decomposition
in the separated and pressure-gradient zones, but a closer inspection reveals
that only the spatial growth term dominates in the inner layer close to the
separation bubble, implying a strong need for incorporating nonequilibrium
terms in the wall modeling of this case. The comparison of streamwise RD
decomposition of swept and unswept cases shows that a larger accumulated
Clauser-pressure-gradient parameter history in the latter energizes the outer
dynamics in the APG, leading to diminished separation bubble size in the
unswept case. The spanwise RD decomposition in the swept case indicates that
the downstream spanwise flow largely retains the upstream ZPG characteristics.
This seems to ease the near-wall modeling challenge in the separated region,
especially for basic models with an inherent log-law assumption. Wall-modeled
LES of the swept and unswept cases are then performed using three wall models,
validating many of the modeling implications from the DNS. In particular, the
extension of RD decomposition to wall models underpins the criticality of
spatial growth term close to the separation bubble, and the corresponding
superior predictions by the PDE wall model due to its accurate capturing of
this term.
