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Numerical and experimental analysis of vibroacoustic field of external gear pumps
Despite the increasing demand for the hydraulic pump noise reduction, there is yet to be an established straightforward solution to reduce noise emissions. This is primarily due to a lack of understanding of the complete mechanism underlying noise generation and propagation, which involves complex interactions between three domains. Study of the physical phenomena of the hydraulic pump noise is typically separated into three categories, namely fluid-borne noise (FBN), structure-borne noise (SBN), and air-borne noise (ABN). In this light, this study examines the noise generation and propagation of hydraulic pumps in all three domains numerically and experimentally, taking external gear pumps (EGPs) as a reference.
In conventional pump noise studies, the outlet pressure ripple in the fluid domain, which typically refers to has been the key focus to minimize, and FBN typically refers to the outlet pressure ripple. Fortunately, attempts to minimize ripples have resulted in some promising solutions that are now on the market (e.g., dual-flank gear pumps). However, since the noise generated by gear pumps involves several other significant and coherent noise sources, this approach has some limitations. In view of this, the current study describes FBN in a wider context to include all potential noise sources in the fluid domain, and their mutual effects on noise are investigated.
Another aspect of the vibration and noise of the pump that is not often investigated is its “field” behaviors. Many significant works in vibroacoustic analysis or noise solutions rely on the simple measurements of acceleration or sound pressure at a single or few local points. Since vibration and noise are functions of not only time but also "space", this practice has also served as one of the obstacles to a comprehensive understanding of noise generation. Therefore, this study contributes to topic of the vibroacoustic field behaviors.
Furthermore, when prototyping or designing new pumps, inefficient trial-and-error methods are often used, and it demonstrates the necessity of the acoustic model of the pumps for virtual prototyping. The major limiting factor towards the development of this type of models is high computational costs. Another technical challenge is that most of vibroacoustic analysis commercial software usually requires the user’s manual works for the simulation setup. In this regard, another aim of this study is to develop a computationally inexpensive and automated acoustic model that does not need manual inputs of users, so that the model can be used as a virtual prototyping tool with various design parameters.
To sum up, the primary goal of this research is to numerically and experimentally investigate the vibroacoustic field behaviors and formulate the acoustic model to be used as a virtual prototyping tool with the experimental validation. To achieve these objectives, this research employs the well-established computational and experimental methods of vibro-acoustic analysis.
The analysis of FBN makes use of the HYGESim tool, which has been developed to study EGMs at Maha Fluid Power Research Center. This tool solves the main flow based on the lumped parameter approach in conjunction with different solution schemes for lubricating interfaces and body dynamics. From the HYGESim results, all potential noise sources within the working fluid, such as inlet and outlet pressure ripple and dynamic pressure at the tooth space volumes, hydrodynamic journal bearings, and the lateral lubricating interface, are properly mapped to the structure using appropriate simplifications.
When it comes to SBN, the modal superposition approach is exploited for the fast prediction of vibration fields. Therefore, considerable efforts are expended both numerically and experimentally to obtain accurate modal information. Particular attention is paid to the modeling of the mechanical connections between components and modeling of constraints in numerical modal analysis using the finite element method (FEM). Moreover, the vibration mode shapes are categorized according to the dominant motions that the pump body exhibits. Then, two different approaches, namely the full numerical model and the hybrid model, are introduced for the estimation of the vibration field during the operation; for the modal expansion, the former uses numerical modal information, while the latter uses experimentally determined modal information. Finally, the numerical model results are compared to the operational deflection shape (ODS) measured during pump operation, and a good agreement is observed.
For the ABN prediction, the boundary element method (BEM) is used by taking the predicted vibration information as an input. The BEM solver development is elaborated to numerically replicate the acoustic environments where the noise measurement is conducted. With the developed BEM solver, two units that have the different gear and groove designs that fit into the same casing are tested, and as the key outcome, their sound power level, sound power spectrum, sound pressure distributions are presented. For model validation, the noise measurements are performed according to the ISO standard in the semi-anechoic chamber at Maha using a custom-designed robot arm. These validations demonstrate the ability of the developed model to predict the overall sound power levels with an averaged error of 1.87 dB and capture the general trends of measured sound power spectrum and sound pressure level distribution under various operating conditions. Furthermore, the developed model provides the reasonably fast computation time.
Finally, using the developed acoustic model, a parametric study is performed with the backflow groove as a design variable. It is discussed how the volumetric efficiency and noise performance vary with the design changes, which demonstrates the model potential as a virtual prototyping tool.