Direct Numerical Simulation of a Turbulent Boundary Layer over Acoustic Liners

The nacelle of aircraft engines is coated with acoustic liners to reduce engine noise emissions. An undesirable side effect of acoustic liners is that they increase aerodynamic drag. For the first time, the authors study this drag penalty through pore-resolved direct numerical simulation (DNS) of a flat-plate zero pressure gradient turbulent boundary layer at friction Reynolds number [Formula: see text], which is high enough to be representative of liners in operating conditions. In the configuration under scrutiny, the turbulent boundary layer experiences a step change in surface topography passing from a smooth wall to an acoustic liner array, allowing one to study the streamwise adaptation length of the boundary layer. It is found that the mean velocity profile adjusts to the new surface condition in a nearly negligible distance (less than 10 local boundary-layer thicknesses), whereas turbulent fluctuations take much longer. DNS is also performed with external acoustic noise in the form of planar monocromatic waves grazing the boundary layer with an amplitude of 150 dB. In agreement with some earlier studies, it is found that sound waves do not affect aerodynamic drag at these flow conditions.