Research on the Farfield Tonal Fan Noise at Small Mach Number

The calculation model of the farfield tonal noise of a subsonic fan was studied in this paper in order to find the main sources of tonal noise. A full model and a simplified model, which neglects the acceleration noise, were brought out. Experiment was done to verify the full model and good fitness were found between the calculated result and the measured result. Numerical experiments were then done to compare among those two models and the Gutin formula, which calculates the acceleration noise of steady force. Results reveal that at small Mach number the simplified model, which takes only the fluctuating force into account, approximates exactly with the full model. Because the noise calculated by Gutin formula is very small, it is concluded that the tonal noise of a subsonic fan with the Mach number less than 0.2 is caused mainly by the fluctuating force. It gives a guideline to cut fan noise down.

CFD Simulation of Flow Tones From Grazing Flow Past a Deep Cavity

Locked-in flow tones due to shear flow over a deep cavity are investigated using Large Eddy Simulation (LES). An isentropic from of the compressible Navier-Stokes equations (pseudo-compressibility) is used to couple the vertical flow over the cavity mouth with the deep cavity resonances (1). Comparisons to published experimental data (2) show that the pseudo-compressible LES formulation is capable of predicting the feedforward excitation of the deep cavity resonator, as well as the feedback process from the resonator to the flow source. By systematically increasing the resonator damping level, it is shown that strong lock-in results in a more organized shear layer than is observed for the locked-out flow state. By comparison, weak interactions (non-locked-in) produce no change in the shear layer characteristics. This supports the 40 dB definition of lock-in defined in the experiment.

Full Band-Gap Silicon Phononic Crystals for Surface Acoustic Waves

Periodic elastic structures, called phononic crystals, show interesting frequency domain characteristics that can greatly influence the performance of acoustic and ultrasonic devices for several applications. Phononic crystals are acoustic counterparts of the extensively-investigated photonic crystals that are made by varying material properties periodically. Here we demonstrate the existence of phononic band-gaps for surface acoustic waves (SAWs) in a half-space of two dimensional phononic crystals consisting of hexagonal (honeycomb) arrangement of air cylinders in a crystalline Silicon background with low filling fraction. A theoretical calculation of band structure for bulk wave using finite element method is also achieved and shows that there is no complete phononic band gap in the case of the low filling fraction. Fabrication of the holes in Silicon is done by optical lithography and deep Silicon dry etching. In the experimental characterization, we have used slanted finger interdigitated transducers deposited on a thin layer of Zinc oxide (sputtered on top of the phononic crystal structure to excite elastic surface waves in Silicon) to cover a wide range of frequencies. We believe this to be the first reported demonstration of phononic band-gap for SAWs in a hexagonal lattice phononic crystal at such a high frequency.

Absolute Band Gaps in Two-Dimensional Phononic Crystal Plates

The elastic band structures of two-dimensional phononic crystal plates are computed with the help of a super-cell plane wave expansion (PWE) method. These band structures strongly differ from the infinite 2D phononic crystal dispersion curves. In particular, these band structures exhibit surface modes and guided modes. The influence of the constituent materials, of the plate thickness and of the geometry of the array on the band structure is investigated. We focus more specifically on determining the thicknesses of the plate for which absolute forbidden bands exist. Namely, we show that absolute forbidden bands occur in the band structure if the thickness of the plate is of the same order of magnitude as the periodicity of the array of inclusions.

Band Gap Engineering in N-Dimensional Phononic Crystals

Periodic binary elastic/acoustic composites can give rise to genuine band gaps in the band structure. The term genuine refers to the complete gaps, which persist independently of the polarization of the wave and of its direction of propagation. Within these complete gaps sound and vibrations are forbidden, the "acoustic crystals" stand still, and the total silence reigns. Thus a vibrator (or defect) introduced into a periodic elastic composite would be unable to generate sound or vibrations within the gap. The existence of complete gaps in the band structure is closely associated with the (classical) Anderson localization of sound and vibrations. The search for phononic band-gap materials is of comparable interest to the pursuit of photonic band-gap materials. Thus the phononic crystals are to acoustics as photonic crystals are to optics. In comparison to the photonic crystals, there are additional parameters (the mass densities and two velocities - longitudinal and transverse) involved in the phononic crystals, which make the physics richer and leaves us with more options in the quest of creating full stop bands in the system. As regards the applications, the phononic crystals are envisioned to find ways in the acoustic waveguides, improvements in designing the transducers, elastic/acoustic filters, noise control, ultrasonics, and medical imaging, to name a few. Since the interesting phenomena emerging from the phononic crystals are all consequences of the existence of the gap(s), a major part of the research efforts has focused on the search for phononic band-gap crystals. As such, we report and emphasize on the spectral gaps in the band structure for cleverly synthesized N-dimensional (N = 1, 2, 3) phononic crystals. PACS numbers:

Flow/Acoustic Characteristics for Flow Over Two Tandem Cylinders

Flow over two tandem cylinders is simulated using an immersed-boundary method. Cases with different distances between the cylinders are investigated. The flow field is then used to provide information to analyze the pressure fluctuations both in the cylinders (assumed rigid and hollow) and at the far field. The results show that the downstream cylinder pertains higher levels of pressure fluctuations both inside the cylinder and at far field than the upstream cylinder. When the distance between the cylinders increases from the quasi-reattachment flow pattern to the critical regime, the pressure fluctuation levels from the downstream cylinder increase significantly. The spectral analysis reveals only one significant peak at the frequency of twice of the corresponding Strouhal number for the far-field acoustics, but two peaks at the frequencies of both the Strouhal number itself and twice of it for pressure fluctuations inside the cylinders.

Platform Protection Through Acoustic Signature Management

A naval ship's acoustic signature is known after a ranging but changes the longer it is in-service away from a range. The Ship Signatures Management System (SSMS) provides an organic real-time capability to predict their own signature and enough information to mitigate signature issues. SSMS provides the Commanding Officer with a tool to determine the ship's acoustic signature in order to evaluate the impact of his/her proposed actions on the ship's counter-detection range and sensor performance. In this manner, the ship's protection is enhanced through insightful and timely signature management. DRDC has upgraded the SSMS hardware to state-of-the-art components to increase the number of sensors, the fidelity of the logged data, the dynamic range, and the processing power. This paper discusses some of the advanced SSMS features developed like tonal detection and tracking, tonal association, and the diagnostics used to determine the cause of features in the acoustic signature.

Objective Evaluation of Diesel Combustion Noise in Vehicle Passenger Compartment

The combustion noise of diesel engine during run up and idle condition makes unpleasant interior noise in passenger car whose market share is increasing nowadays. There were few studies to quantify the unpleasantness of combustion noise. We tried to introduce an objective metrics which matches well with subjective ratings of combustion noise. To get the objective metrics, the combustion noise signal was analyzed by many different methods based on the human hearing sense. In the end, we found out that the modulation amplitudes of each frequency band match well with the unpleasant characteristics of combustion noise. Then, an objective index was developed from the combination of modulation amplitudes on the basis of the human hearing sense. The index was shown to be in good correlations with subjective ratings in general.

Substructuring Developments in the Energy Finite Element Formulation

In Naval applications of the Energy Finite Element Analysis (EFEA) there is an increasing need for developing comprehensive models with a large number of elements which include both structural and interior fluid elements, while certain parts of the structure are considered to be exposed to an external heavy fluid loading. In order to accommodate efficient computations when using simulation models with a large number of elements, joints, and domains, a substructuring computational capability has been developed. The new algorithm is based on dividing the EFEA model into substructures with internal and interface degrees of freedom. The system of equations for each substructure is assembled and solved separately and the information is condensed to the interface degrees of freedom. The condensed systems of equations from each substructure are assembled in a reduced global system of equations. Once the global system of equations has been solved the solution for each substructure is pursued. Important issues which have been considered in the new development originate from the necessity to define substructure interfaces along joint locations. The discontinuity of the energy density variables and the proper formulation of the joints across substructure interfaces have been considered in the new algorithm. In order to demonstrate the validity of the developments and the computational savings a set of previous applications where simulation results were compared to test data is repeated using the substructuring algorithm.

Design Space Exploration of Multi-Phase Layered Phononic Materials via Natural Evolution

A phononic material is commonly characterized by its dispersive frequency spectrum. With appropriate spatial distribution of the constituent material phases, spectral stop bands could be generated. Moreover, it is possible to control the number, the width, and the location of these bands within a frequency range of interest. This study aims at exploring the relationship between the unit cell configuration and its frequency spectrum characteristics. Focusing on 1D layered phononic materials, and longitudinal wave propagation in the direction normal to the layering, the unit cell features of interest are the number of layers and the material phase and relative thickness of each layer. An evolutionary search for multi-phase cell designs exhibiting a wide stop band, or a series of wide stop bands, is conducted using a specially formulated representation and set of operators that break the symmetries in the problem. An array of optimal designs for a range of ratios in Young's modulus and density are obtained and the corresponding objective values are plotted as a function of the ratios of the phase properties. Structures composed of the designed phononic materials are excellent candidates for use in a wide range of applications including vibration and sound isolation.