Turbulence and added drag over acoustic liners

TL;DR

This study uses detailed numerical simulations to analyze how acoustic liners on aircraft engines influence turbulence and drag, revealing they behave as porous surfaces with turbulence-driven flow inside the orifices.

Contribution

The paper develops a robust theory for estimating added drag from acoustic liners based on pore-resolved DNS across various parameters and flow conditions.

Findings

Acoustic liners act as porous surfaces with wall-normal permeability.

Flow inside orifices is fully turbulent with significant velocity fluctuations.

Acoustic liners exhibit a fully rough regime with pressure drag comparable to sand-grain roughness.

Abstract

We present pore-resolved direction numerical simulations (DNS) of turbulent flows grazing over perforated plates, that closely resemble the acoustic liners found on aircraft engines. Our DNS explore a large parameter space including the effects of porosity, thickness, and viscous-scaled diameter of the perforated plates, at friction Reynolds numbers $Re_\tau = 500$-$2000$, which allows us to develop a robust theory for estimating the added drag induced by acoustic liners. We find that acoustic liners can be regarded as porous surfaces with a wall-normal permeability and that the relevant length scale characterizing their added drag is the inverse of the wall-normal Forchheimer coefficient. Unlike other types of porous surfaces featuring Darcian velocities inside the pores, the flow inside the orifices of acoustic liners is fully turbulent, with a magnitude of the wall-normal velocity fluctuations comparable to the peak in the near wall cycle. We provide clear evidence a fully rough regime for acoustic liners, also confirmed by the increasing relevance of pressure drag. Once the fully rough asymptote is reached, canonical acoustic liners provide an added drag comparable to sand-grain roughness with viscous-scaled height matching the inverse of the viscous-scaled Forchheimer permeability of the plate.