Aeroaccoustic Perfomance of Air Diffusors and Their Psychoactousic Evaluation

Methods

The transient flow simulations are performed using the commercial software STARCCM+ 18.04.008. In earlier stages of the project earlier versions of the software were also used. The airflow is modeled as an incompressible fluid. The flow model consists of the main model of the diffuser to which a periodic duct segment is attached. The periodic duct segment is connected to the main flow model with a data mapper that supplies the flow field as inlet boundary condition. The computation is split into several stages, where a steady RANS approach is used to initialize the flow model. To reduce the required to be computed time period, the transient is initialized with a combination of a steady and a transient solution. The final stages are computed following a LES-WALE approach. To evaluate the aeroacoustic processes, broadband source models are applied to the steady RANS results. In the case of LES computation a hybrid coupled approach is used, where an implementation of the perturbed convective wave equation is utilized. Here, the acoustic pressure is sampled at several locations and on cut sections. From the acoustic pressure signals the power spectral density is computed at the specified locations and also on the cut sections. The computational grids of both diffusers feature more than 100 × 10⁶ cells with local grid resolutions down to 0.25 mm. The time step is set to 1 × 10⁻⁵ s and a total time period of up to 0.45 s is computed. Due to their large plenum volumes both diffusers feature strong eigenmodes that develop inside the plenums and produce noise in different frequency ranges. To quantify the influence of the plenums the plenum walls are treated with non-reflecting behavior regarding the acoustic waves. Computing both the non-reflecting and the reflecting variant of one diffuser flow model requires more than 0.4 × 10⁶ core-h on the CLAIX-23 HPC system.

Results

The computed results of both diffusers were validated with various experimental investigations. In both cases the aeroacoustic emissions and the main flow properties such as velocity distribution and pressure loss were in close agreement to the measured results. When evaluating the results from the non-reflecting variants of the diffusers’ flow models, the main noise source regions are identifiable. The swirl diffuser features its main noise sources in the outflow, mainly driven by the interaction of the individual jet flows with the surrounding environment. The slot diffuser features its main noise sources inside the slots where a strong deflection of the flow interacts with a recirculation flow pattern. By treating this area with a modified geometry, the noise emissions are reduced in both the non-reflecting and the reflecting variant of the flow model. A subsequent experimental investigated of the manufactured prototype of the modified geometry confirms the reduction of the sound emissions.

Discussion

By performing just two detailed simulations, a non-reflecting and a reflecting variant of the diffuser, a deep insight in the physical processes is obtained. The aeroacoustic emissions and the most important noise sources are identifiable and quantifiable from the rather short computed time period. Although the computational effort is substantial, the solution process profits from the utilization of GPGPU computation. Therefore, it is expected that at least in the academic field such computations are becoming more feasible in the near future, when GPUs are more widely available. Even in industrial applications such computation should become feasible, considering what information can be gained from a few simulations.

Additional Project Information

DFG classification: 402-04 Acoustics
Software: Star-CCM+
Cluster: CLAIX

Publications

Phillip Ostmann et al.,
“Numerische Untersuchung des aeroakustischen Verhaltens eines
generischen Schlitzauslasses”,
https://doi.org/10.18154/RWTH-2022-08418

Philipp Ostmann, Martin Kremer, and Dirk Müller,
“Evaluation of the Aeroacoustic Sources of a Swirl Diffusor”,
https://doi.org/10.3397/IN_2023_0038

Philipp Ostmann, Lara Stürenburg, and Dirk Müller,
“Aeroakustische Performance von Luftdurchlässen”,
https://doi.org/10.37544/1436-5103-2025-01-02-42

Philipp Ostmann, Martin Kremer, and Dirk Müller,
“Identification of the Aeroacoustic Emission Source Regions Within a Ceiling Swirl Diffuser”,
https://doi.org/10.3390/acoustics7010009

Philipp Ostmann, Martin Kremer, and Dirk Müller,
“Identifying and optimizing the aeroacoustic source regions of a slot air diffuser”,
https://doi.org/10.1016/j.apacoust.2025.111002

Thesis:
Christian Bruchhaus. “Numerische Studie des aeroakustischen Verhaltens von Luftdurchlässen in HVAC-Systemen”,
Masterarbeit. Aachen: RWTH Aachen University, Apr. 2022

Lisa Krüger. “Numerische Untersuchung des Einflusses geometrischer Merkmale auf die aeroakustischen Eigenschaften eines Schlitzdurchlasses”,
Masterarbeit. Aachen: RWTH Aachen University, Apr. 2023

Deborah Sonnenberg. “Detektierung und Bewertung schallverursachender geometrischer Merkmale eines Schlitzdurchlasses auf Basis numerischer Strömungssimulationen”,
Masterarbeit. Aachen: RWTH Aachen University, Feb. 2024