Acoustic liners have been extensively studied from a purely acoustic perspective. Recently, the interest in characterising their aerodynamic performance has increased, as they are known to provide an increase in drag, compared to their respective smooth counterpart, up to 70%. The goal of this work is to experimentally test various acoustic liner samples to study this drag contribution. The hypothesis is that it is possible to decrease the added drag without having to compromise the acoustic performance.
A variety of orifice cross-section configurations are investigated. Samples are tested in different experimental campaigns, combining the use of two different wind tunnels (the Delft University- Boundary
Layer Facility in Delft and the Flow Duct Facility at NLR, Marknesse) and two different experimental measurement setups, both to investigate the setup effects and to provide a deep understanding of liner performances in different operating conditions. All sample porous plates are 3D-printed by means of Stereolitography (SLA). Direct drag measurements are performed to obtain the aerodynamic response. Insertion Loss measurements are carried out to compare the acoustic attenuation of the different samples.
The direct drag measurements show an increase in added drag for all samples in all tested conditions,
with 0% < ∆CD < 37%. Compared to the conventional cylindrical orifice shape, two configurations show a lower added drag: the tapered and the inverse chamfered. Due to shape simplicity, which makes it easier to manufacture with different techniques, the tapered is taken as reference ’improved’
configuration. The relative drag reduction caused by the tapered configuration ranges from 20% to 40%, with consistent results between different campaigns. The insertion loss measurements, on the other hand, show no significant difference between different samples. All samples have a low acoustic
attenuation in the no-flow case, reaching a maximum of 2.5dB. With increasing M, the attenuation increases in the whole frequency range, probably due to the testing configuration and wind tunnel effects.
The measurements show important novel insights on the drag increase caused by acoustic liners. Data from different campaigns, facilities and measurement systems compare positively and a possible model is proposed, which relates the drag increase to the product of the Open Area Ratio σ and the inner-scaled pore diameter at the top of the liner porous facesheet, d+. It allows to estimate the drag increase,
given the geometric parameters of the liner and the flow conditions (in terms of a viscous parameter, e.g. uτ). This model does not take into account the effect of the material surface roughness, which has
a considerable impact, due to the contribution of the ’pure skin-friction drag’ on the total drag. From the acoustic measurements, no significant difference is found between the acoustic attenuation of all
liner samples, showing that a lower drag increase can be achieved without sacrificing the acoustic performance. All in all, further development in acoustic liners is considered crucial to achieve optimal noise attenuation as well as aerodynamic performance.
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