Coupling Sound Absorbing Materials With an Air Cavity Using Frequency Dependent Bulk Material Properties

This paper presents the acoustic finite element method and the modal solution method for coupling sound absorbing materials with an air cavity to predict the sound pressure frequency response. The sound absorbing materials are represented with complex, frequency-dependent, effective mass-density and bulk-modulus properties obtained from the acoustic impedance of material samples. To couple the sound absorber cavity and air cavity, the boundary conditions at the interface between the cavities requires equality of pressure and equality of acoustic volume flow. Two modal solution methods are developed to compute the frequency response of the coupled system with frequency dependent material properties: the component mode method and the coupled mode method. The finite element and modal solution methodology is developed in a form readily adaptable for implementation in commercially available codes. The accuracy of the modal solution methodology is assessed for modeling a one-dimensional air tube terminated with absorbent material and the seats in an automobile passenger compartment.

Development and Validation of an In-Situ Utility Pole Simulation Model for Virtual Modal Testing

Modal testing is being investigated as a non-destructive test (NDT) method for wood poles. Modal properties of the pole must be extracted from sensor data containing frequency content associated with the interaction of the pole with its conductors. A dynamic model of a utility pole with attached conductors has been developed and validated through experimentation. The model will allow controlled, repeatable simulations of modal hammer hits for preliminary verification of pole property identification methods. The cable is modeled as a series of point masses connected by translational springs. The pole is represented by a modal expansion based on separation of variables. To facilitate creating and connecting the pole and cable models, scaling the model to represent longer pole lines, and introducing modal hammer inputs; the bond graph formalism was employed. To validate the model, an instrumented reduced-scale pole and cable system was built and tested in the laboratory. Time series measurements of cable tension and transverse motion, along with frequency-domain accelerometer data, show that the simulation model has sufficient fidelity to predict the effect of conductors on a pole’s response spectrum over the frequency range of interest.

Design and Prototype of a Two-Axis Acoustic Levitator

The purpose of this paper is to document the process required to design and prototype a two-axis acoustic levitator and to show that the two-axis levitator improves the stability of a particle in an acoustic levitation field. The levitator design consists of the following subsystems: the transducer assemblies, which are responsible for generating the acoustic pressure field needed for levitation; the electrical system, which is responsible for providing the transducer assemblies with adequate power to maintain levitation; and the frame structure, which is responsible for locating and rigidly supporting the transducer assemblies. The two-axis levitator is designed to have four transducers that operate at 27.2 kHz, and simulated results show that the system satisfies nearly all the design criteria and objectives. A transducer test stand and prototype were constructed to verify the design. The test stand was used to characterize all four transducers, and once the assembly was constructed the prototype operating frequency was determined to be 27.5 kHz. The prototype was used to successfully levitate Styrofoam pellets, a plastic pellet, and water droplets of various sizes. The displacement of a water droplet of approximately 1 mm in diameter was measured when levitated with both one-axis (vertical) and two-axis (vertical and horizontal) levitation. Using one-axis levitation, the water droplet displaced a maximum of 1.1 mm in the horizontal direction and 0.17 mm in the vertical direction. Using two-axis levitation, the horizontal displacement was 0.07 mm and the vertical displacement was 0.05 mm. Therefore, the two-axis acoustic levitator provides significant improvements in levitated particle stability.

Compressible 2D Flow Field Interaction of Two Contra-Rotating Blades

The blades of coaxial, contra-rotating rotor systems cross each other in close proximity and at high relative speeds. This crossing event is a potential source of noise and severe blade loads. Effects of compressibility can aggravate the interaction and significantly alter the pressure field signature and phase relationships. A 2-D analysis of this phenomenon is performed by simulating two airfoils passing each other at specified speeds and vertical separation distances. Several test cases spanning a relevant range of Reynolds numbers, angles of attack, and relative Mach number are considered. The Mach number is varied to simulate the radial variation of velocity from the root to tip of a rotor blade to capture the pressure signature, lift, and drag of the airfoils. The velocity and pressure distributions on the airfoils, and in the space between the airfoils are computed before, at, and after airfoil crossing. The variations of lift and drag coefficients through the interaction are captured. The upper airfoil experiences an increase in lift followed by a very sharp drop in lift during the interaction. When relative Mach numbers are transonic, the region of interaction is greatly extended, with shock interactions occurring. The results show the complex nature of the aerodynamic and fluid dynamic impulses generated by blade-blade interactions, with implications to aeroelastic loads and aeroacoustic sources.

Mesh-Independent Design of Phononic Crystals Using an Advanced Finite Element Formulation

The numerical modeling of phononic crystals using the finite element method requires a mesh that accurately describes the geometric features. In an optimization setting, involving shape and/or topological changes, this implies that a new matching mesh needs to be generated in every design iteration. In this paper a mesh-independent description for both the interior and exterior boundaries of the periodic unit cell is proposed. A method is developed to apply Bloch-Floquet periodic boundary conditions to edges that are non-matching to the mesh. The proposed method is applied to a one-dimensional phononic crystal and is demonstrated to exhibit improved performance over the commonly used interface material averaging. We show that this method provides an accurate mesh-independent model.

An Investigation Into Acoustic Dipole Source Calibration Methods for Turbomachinery Sound Radiation in Reverberant Fields

In-situ calibration methods using a single spherical-shaped transmitting hydrophone (idealized as a monopole acoustic source) have traditionally been used for radiated sound measurements of turbomachinery performed in the Garfield Thomas 1.22-m diameter water tunnel located at The Pennsylvania State University’s Applied Research Laboratory (ARL Penn State). In this reverberant field, the monopole source containing known transmitting characteristics was used to calibrate acoustic sensors that were either near or far from the source. This method typically works well when the type of source is monopole in nature; however, many acoustics sources can be dipole or quadrupole in nature. In this study we investigated the applicability of using dipole sources in a space such as a well-characterized reverberant tank, and we found through a virtual dipole method that the radiation still appears monopole in the reverberant field. The method was extended for the vibration of a panel (a known dipole source) and once again the monopole assumption for the in-situ calibration for a near-field hydrophone and conventional reverberant hydrophones remained consistent.

Effect of Vibrations on the Performance of a Centrifugal Pump-Impeller When Using a Variable Frequency Drive

This paper deals with turbomachinery, such as pumps or turbines, which are very sensitive to changes in fluid speed over the contours of the blades when the volumetric flow is varied. These changes modify the fluid incidence angle, causing a rapid decline in pump performance. Our research focuses on an analysis of the performance or efficiency of a centrifugal pump with a variable frequency drive, where losses in efficiency are caused by turbulence generating harmful vibrations in the installation. The methodology consists of measuring the magnitude of the vibrations. The data obtained are compared to the performance reached when the change in velocity has been produced with the regulation of the volumetric flow to a partial load of the pump. This suggests an analysis to attempt to resolve the issue of density variation that occurs when pumping liquefied petroleum gas (LPG) under regular operating conditions.

Minimizing the Acoustic Signature of a Cooling Fan Using the Mesh Morpher Optimizer

Noise reduction is considered as a challenging task in the engineering field. The main objective of this study is focused on providing an optimal new design of a cooling fan with better performance by minimizing the acoustic signature using the surface dipole acoustic power as function. The process of designing a new cooling fan with optimal performance and reduced acoustic signature can be fairly lengthy and expensive. With the use of CFD and specific tools like mesh morphing, in conjunction with state-of-the-art optimization techniques such as Simple model, a given baseline design can be optimized for performance and acoustics. The present study focuses on minimizing the acoustic signature of a given cooling fan using the surface dipole acoustic power as the objective function. The Mesh Morpher Optimizer (MMO) in ANSYS Fluent is used in conjunction with a Simplex model of the broadband acoustic modeling. The broadband model estimated the acoustic power of the surface dipole sources on the surface of the blade without the need for expensive unsteady simulations. It has been shown in the previous work that such a model can provide reliable design guidance. The new promising approach has shown a reduced dipole surface intensity of around 46% of the original value. Other acoustics sources (quadropole noise) are ignored due to the relatively low fan speed considered in this study. Considering this as first attempt study, it is believed that advanced additional studies may improve the model in changing the mesh and objective function.

Effects of Kurtosis Value on the Response of Vibrating Beam With Different Boundary Conditions

Effect of signal kurtosis value on the response of the beam with different boundary conditions has been studied. The signals with various Kurtosis have been generated in a Signal generator code- combined with an algorithm for random data generation for various kurtosis values. Subsequently, a simple Finite Element model of a rectangular Euler-bernoulli beam is introduced and the PSD1s are applied to the model in the form of low frequency base excitation acceleration input. The results show that, generally, the signals with higher kurtosis tend to create more stress and deformation in the structure due to the more frequent existence of peaks with higher amplitudes. However, at lower kurtosis values near normal distribution, the response behavior is different. Also, the stress induced in the beam end is the most critical in clamped-clamped conditions, and where both ends are excited.