PECULIARITY OF EXPERIMENTAL RESEARCH OF SOUND-ABSORBING LINER SPECIMENS PRODUCED BY 3D PRINTING
Fan noise propagating in ducts of bypass aircraft engine is suppressed with resonant acoustic liners. To design effective liner, in particular, a new resonator geometry, numerical simulation can be used [1, 2]. However, sound-absorption efficiency is verified by experiments carried out in grazing flow facility  or impedance tube with normal incident waves . To determine acoustic properties of the liner on the first facility it is necessary to process experimental data by special numerical technique . The transfer function method  used in impedance tube with normal incidence waves allows finding acoustic properties at once. The carrying out experiments encounters with a problem of specimen production, because technology for liners with a new-designed resonator geometry is not yet provided. Thus, 3D printing is a rational decision.
In this case, the production cycle includes designing specimen geometry in 3D modeling program, data translation into the format supported with 3D printer and finally 3D print of specimen. However, there are both strengths and weaknesses of 3D printing from ABS plastic. Its technology consists in layer-by-layer extrusion of ABS plastic fibers. There are two types of material used in 3D printer. The base material is ABSplus thermoplastic. The second material is to support base material by production of special layers at the necessary places. The support material is removed from the open areas by mechanical means, such as a screwdriver or stationery knife. From the hard-to-reach areas the support material is removed by dissolving with a special chemical composition.
Cell of the resonant liner is closed cavity with one or more holes with diameter of 1 mm. If cell is printed entirely with perforated plate and hard bottom plate, then all internal cavity is filled with support material. Because access to the interior of the cavity can be realized only through one hole, within a cell there is a stagnation zone, which complicates washing out the support material with liquid solvent. It causes too long time for dissolution of the support material, and in some cases it is absolutely impossible to remove completely support material. For the reasons outlined, it was decided to print specimens by parts with their following alignment in impedance tube. This approach allows variation of porosity of perforated plates and dimensions of the honeycomb cells.
Another limitation of this production method of liner specimens is minimum possible thickness of the wall. As each layer is formed of continuous filament, the 3D printer puts the filament in one direction, then turns around in the end of the segment and puts the filament in the opposite direction in order to move on to the next part geometry. As a result, the minimum wall thickness of the honeycomb cells is not less than 1 mm.
To carry out experimental studies one-layer specimens were produced by 3D printing from ABS plastic. The inner volume of the specimen resonators is 5.2355·10-6 and 1.5751·10-6 m3, and porosity of perforated plates is 5, 7 and 11 %. Comparison of 3D printed specimens was made with the honeycombs from standard materials. 3D printed and standard honeycomb cells were covered by either 3D perforated plate or plate from composite material. The thickness of the perforated plates and arrangement of the holes are the same.
As is known, the efficiency of the liner is described by acoustical impedance, which has to be correctly matched to the modal structure of the sound field, propagating in aircraft engine ducts. However, at the initial stage of the search of effective resonator geometry the estimation can be performed by sound absorption coefficient α. Its values were determined by measurements in an impedance tube with normal incident waves . Experiments were carried out in the frequency range 500-6400 Hz at sound pressure level 140 dB.
Obtained data have shown that sound absorption coefficient of the liner specimens produced by 3D printing from ABS plastic, can be differ by 0.2 from those for standard material liners, which is considerably. These results can be explained by different thickness of the specimen walls (it is 1 mm for 3D printing which exceeds wall thickness of honeycomb cell made of standard material) causing different wall stiffness and internal volume of the resonators.
Thus, one can conclude that production of promising liners by 3D printing can be used to experimental research of acoustic properties of the new resonator geometry, but consideration must be given to the fact, that properties of the produced liner specimens would be different from those produced by standard technology and with other materials. It is necessary to carefully select the 3D printer to produce liner specimens.
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