13:20

conference time (CEST, Berlin)

conference time (CEST, Berlin)

From Micro-structure to Component Behavior: Acoustic Properties of Polymer Foams

27/10/2021 13:20 conference time (CEST, Berlin)

Room: J

C. Moser, C. Schuecker (Montanuniversitaet Leoben, AUT); K. Echallier, M. H. Luxner (Luxner Engineering ZT GmbH, AUT)

In the aviation industry, passenger comfort is one of the highest priorities. The motivation for this project is to reduce the noise the passengers perceive (e.g. from the turbine). As an acoustic quantity, the Sound Transmission Loss (STL) of polymer foams is investigated in this work.
The approach is to predict the STL response of foams based on their micro-structure. To do so, a tool is developed which generates arbitrary geometries. Those geometries are considered to be the periodically arranged unit cells of a foam. Using the Multi-scale Asymptotic Method (MAM), the parameters for the acoustic Johnson-Champoux-Allard-Lafarge (JCAL) material model are computed. As a final step, the poro-elastic Biot-Johnson material model implemented in ABAQUS (since version 2019) is used to predict the acoustic response based on the cell structure. This poro-elastic material model does not only consider the path along which the air is flowing through the structure of the foam but also accounts for the interaction between the air and the polymer frame. Furthermore, this material model is a homogenized material model which allows to model the acoustic response of complex components efficiently without modelling the micro-structural details for the finite element simulation.
As the goal is to provide an ideal micro-structure to improve the STL response of polymer foams, the effects of one step to the next one is investigated. Firstly, the effect of geometric features onto the JCAL parameters is studied. These parameters then define the STL response, so the knowledge of the effects of each parameter is of interest. Finding an improved micro-structure is then reversed engineering. Knowing the effect on the STL, a set of target JCAL parameters can be defined. This set of parameters is then to be reached by altering the micro-structure leading to an optimized micro-structure for a polymer foam in terms of the acoustic response.

micro-structure, polymer, foams, acoustic analysis, Sound Transmission Loss, JCAL, MAM, poro-elastic, material modelling

13:40

conference time (CEST, Berlin)

conference time (CEST, Berlin)

Simulation of Alpha Cabin Reverberent Room to Estimate Absorption Coefficient under Diffuse Sound Field

27/10/2021 13:40 conference time (CEST, Berlin)

Room: J

X. Robin, E. Richard, M. Raskin, T. Poulos (Hexagon, BEL)

The acoustic efficiency of components used in the automotive industry for noise insulation can be characterized by their absorption coefficient. It can be measured either based on the normal acoustic incidence or a diffuse field. The diffuse field approach is more realistic for the car interior since the sound field inside the car is diffuse. Automotive OEMs and suppliers are usually applying experimental approaches to determine the absorption coefficient of a given porous material utilizing setups that act as small reverberant rooms called “alpha cabins”.
This paper discusses a methodology based on a frequency-domain Finite Element (FE) method to simulate an alpha cabin in the range of 400-10000 Hz and determine the absorption coefficient in a computationally efficient way.
To overcome computational challenges emanating from the size of the cabin, an innovative three-step approach is proposed. Below the Schroeder frequency (~1500 Hz), where the alpha cabin is not large enough to be considered as reverberant, the absorption coefficient is determined in the same way as in the experimental approach. This involves the reconstruction of the time-domain signal from the response spectrum on a series of microphones for estimating the sound-decay values inside the cabin (RT60). For the mid-range frequencies, between 1500 and 5000 Hz, energetical quantities calculated via the finite element method in the fluid and porous parts of the setup are used to determine the RT60 values without the need of time-domain reconstruction. Finally, at high frequencies, between 5000 and 10000 Hz, the model is scaled so as to reduce its size while still keeping the cabin sufficiently diffuse, and the same post-processing method as in the mid-frequency range is used.
The results from this methodology are compared with previously published results in order to evaluate the viability of the method, which provides the possibility to reduce expensive experimental work.

alpha cabin, trim, porous material, absorption coefficient, diffuse field, finite element

14:00

conference time (CEST, Berlin)

conference time (CEST, Berlin)

Analytical Prediction of Whoosh Noise & Blade Pass Frequency Noise at AIS Orifice

27/10/2021 14:00 conference time (CEST, Berlin)

Room: J

S. Mishra, A. Karim (Ford Motor Company, USA)

During acceleration of a vehicle the turbocharger operates very close to the surge line. At this operating regime of the turbocharger, the pressure ratio increases substantially in comparison to the increase in airflow. This leads to a NVH error state generically known as whoosh noise which is a broadband noise between 4000 Hz to 11000 Hz. A test for whoosh noise is usually performed late on the design stages and usually performed on the vehicle. This late testing in the design phase, increases risk of late changes if whoosh is detected. This also inhibits any design changes that could have mitigated the issue and requiring to take only palliative measures such as silencers or damping pads. These palliative measures to contain whoosh noise can be significantly expensive. Therefore, an analytical method becomes more desirable that could potentially save cost if whoosh noise is predicted early in the design process. These in-vehicle tests are performed with a microphone at the air induction system orifice location. So, the analytical model would not only need to predict the source accurately, but also it must predict the propagation of acoustic energy through the AIS without numerical loss to predict sound pressure level at the orifice location accurately.
This CFD investigation aims to achieve the above; asses sound pressure level at different location in the air induction system including at the orifice. The results are then validated with actual gas turbine lab test results. The CFD uses transient aeroacoustics (CAA) along with rigid body motion to assess sound pressure level. The simulation demands a high level of spatial and temporal accuracy because the speed of rotation of turbocharger could be more than 100,000 RPM and to minimize numerical loss. The current investigation takes 6 weeks of CPU time on 384 Processors. The software StarCCM+ is used for CFD analyses & visual post-processing and LMS Testlab is used for NVH post-processing.

CFD, LES, Whoosh, blade pass frequency, nvh, aeroacoustic, turbocharger, caa