Search Results

Over 30 years ago, A. H. Vincent of Westland Helicopters demonstrated that if a structure is excited harmonically, the response at another position (at a fixed frequency) will trace a circle in the complex plane as a result of a dynamic stiffness modification between two points. As either the real or imaginary part of an introduced dynamic stiffness is varied from minus infinity to plus infinity, the structural or acoustic response on any position will map a circle in the complex plane. This paper reviews the basis for this little known principle for vibro-acoustics problems and illustrates the viability for a cantilevered plate example. The applicability of the method is then considered for strictly acoustic systems like intake and exhaust systems. Specifically, it is shown that the response traces a circle in the complex plane if either the real or imaginary parts of the source or termination impedance are varied from minus to plus infinity.

It was demonstrated that a bypass duct similar to a Herschel- Quincke tube could be used to increase the transmission loss of mufflers at selected frequencies. In many cases, the duct can be short and thought of as a leak. It was shown that the optimal length and cross-sectional area could be determined by using a simple optimization technique known as the Vincent Circle. Most importantly, it was demonstrated that the attenuation at low frequencies could be improved by as much as 15 dB. To prove the concept, a muffler was designed and optimized using transfer matrix theory. Then, the optimized muffler was constructed and the transmission loss measured using the two-load method. The measured results compared well with prediction from transfer matrix theory. Boundary element simulation was then used to further study the attenuation mechanism.

Transmission loss (TL) is a good performance measure of mufflers since it represents the muffler's inherent capability of sound attenuation. There are several existing numerical methods, which have been widely used to calculate the TL from numerical simulation results, such as the four-pole and three-point methods. In this paper, a new approach is proposed to evaluate the transmission loss based on the reciprocal work identity. The proposed method does not assume plane wave propagation in the inlet and outlet ducts, and more importantly, does not explicitly apply the anechoic termination impedance at the outlet. As a result, it has the potential of extending TL computation above the plane wave cut-off frequency.

The two-load method is used to measure the transmission loss of thin panels in two different sized impedance tubes (3.49 cm and 10.16 cm). Samples were initially tested with a clamped boundary condition. This was followed by tests with an elastomer inserted between the tube and tested sample to adjust the boundary condition at the periphery. In all tests performed, the influence of the sample holding method could not be removed from the test. The measured transmission loss was compared to finite element simulation with good agreement for both impedance tubes. Additionally, the effect of a compliant boundary condition along the periphery of the sample was also validated via simulation.

Transmissibility is the most common metric used for isolator characterization. However, engineers are becoming increasingly concerned about energy transmission through an isolator at high frequencies and how the compliance of the machine and foundation factor into the performance. In this paper, the transfer matrix approach for isolator characterization is first reviewed. Two methods are detailed for determining the transfer matrix of an isolator using finite element simulation. This is accomplished by determining either the mobility or impedance matrix for the isolator and then converting to a transfer matrix. It is shown that results are similar using either approach. In both cases, the isolator is first pre-loaded before the transfer matrix is determined. The approach to find isolator insertion loss is demonstrated for an isolator between two plates, and the effect of making changes to the structural impedance on the machine side of the isolator by adding ribs is examined.

A recently developed superposition approach for determining the insertion loss of a two-inlet muffler is reviewed. To validate the approach, calculated and measured insertion losses are compared for a small engine muffler with two inlets and one outlet. After which, the phasing between the two inputs is varied and the insertion loss is evaluated. Results show that the insertion loss is strongly affected by the phasing between sources at low frequencies while phasing between sources has a lesser impact at high frequencies. At the conclusion of the paper, the theory for applying the superposition approach to transmission loss is reviewed.

Piezoelectric materials are smart materials that can undergo mechanical deformation when electrically or thermally activated. An electric voltage is generated on the surfaces when a piezoelectric material is subjected to a mechanical stress. This is referred to as the ‘direct effect’ and finds application as sensors. The external geometric form of this material changes when it is subjected to an applied voltage, known as ‘converse effect’ and has been employed in the actuator technology. Such piezoelectric actuators generate enormous forces and make highly precise movements that are extremely rapid, usually in the micrometer range. The current work is focused towards the realization and hence application of the actuator technology based on piezoelectric actuation. Finite element simulations are performed on different types of piezoelectric actuations to understand the working principle of various actuators.

Microperforated panel (MPP) absorbers are rugged, non-combustible, and do not deteriorate over time. That being the case, they are especially suitable for long term use in harsh environments. However, the acoustic performance is modified when contaminated by dust, dirt, or fluids (i.e. oil, water). This paper examines that effect experimentally and correlates the absorption performance with Maa's theory for micro-perforated panels. Transfer impedance and absorption coefficient are measured for different levels of aluminum oxide and carbon dust accumulation. The amount of dust contamination is quantified by measuring the luminance difference between clean and dirty panels with a light meter. The porosity and hole diameter in Maa's equation are modified to account for dust obstruction. The effect of coating the MPP with oil, water, and other appropriate viscous fluids was also measured. This effect was simulated by modifying the viscous factor in Maa's equation.

A process for predicting the transmission and insertion losses of multi-component exhaust systems is detailed in this paper. A two-tiered process incorporating boundary element analysis to evaluate multi-component systems is implemented. At the component level, the boundary element method is used to predict the transfer matrix for larger components where plane wave behavior is not expected within the component. The transfer matrix approach is then used to predict insertion loss for built-up systems with interconnecting duct or pipe work. This approach assumes plane wave behavior at the inlet and outlet of each component so it is limited to the low frequency regime. Results are compared with experimental results for HVAC systems.

This paper documents an inverse visible element Rayleigh integral (VERI) approach. The VERI is a fast though approximate method for predicting sound radiation that can be used in the place of the boundary element method. This paper extends the method by applying it to the inverse problem where the VERI is used to generate the acoustic transfer matrix relating the velocity on the surface to measurement points. Given measured pressures, the inverse VERI can be used to reconstruct the vibration of a radiating surface. Results from an engine cover and diesel engine indicate that the method can be used to reliably quantify the sound power and also approximate directivity.

The paper presents work on development, testing and vehicle integration of inflatable wings for small UAVs. Recent advances in the design of inflatable lifting surfaces have removed previous deterrents to their use and multiple wing designs have been successfully flight tested on UAVs. Primary benefits of inflatable wings include stowability (deploy upon command) and robustness (highly resistant to damage). The inflatable planforms can be either full- or partial-span designs allowing a large design space and mission adaptability. The wings can be stowed when not in use and inflated prior to or during flight. Since inflatable designs have improved survivability over rigid wings, this has the prospect of increasing vehicle robustness and combat survivability. Damage resistance of inflatable wings is shown from results of laboratory and flight tests.

This paper considers the use of partial enclosures and absorbing materials inside those enclosures to dissipate energy. Several experiments were conducted where various parameters of an enclosure were altered and the effect on the noise radiating through the opening was measured. From these results, the parameters that play the most important role in sound radiation through the opening of an enclosure were determined. The two-point method and decomposition theory were used to calculate the transmission loss, which was used as the primary variable to analyze the enclosure's performance; the transmission loss is shown to be a better variable than sound pressure or output sound power for this purpose. Numerical simulations were conducted using the indirect boundary element method, and the results were compared with experimental results.

This paper considers two questions: how does one know when a boundary element mesh is reliable, and what are the advantages and potential pitfalls of various methods for sound radiation prediction. To answer the first question, a mesh checking method is used. With this method velocity boundary conditions are calculated on the nodes of the mesh using a point source or sources placed inside the mesh. A boundary element program is then used to calculate the sound power due to these boundary conditions. The result is compared to the known sound power of the point source or sources. This method has been used to determine the maximum frequency of a mesh, how many CHIEF points to use, etc. The second question is answered by comparing the results of several numerical methods to experimental results for a running diesel engine. The methods examined include the direct and indirect boundary element methods and the Rayleigh integral.

The most common approach for measuring the transmission loss of a muffler is to determine the incident power by decomposition theory and the transmitted power by the plane wave approximation assuming an anechoic termination. Unfortunately, it is difficult to construct a fully anechoic termination. Thus, two alternative measurement approaches are considered, which do not require an anechoic termination: the two load method and the two-source method. Both methods are demonstrated on two muffler types: (1) a simple expansion chamber and (2) a double expansion chamber with an internal connecting tube. For both cases, the measured transmission losses were compared to those obtained from the boundary element method. The measured transmission losses compared well for both cases demonstrating that transmission losses can be determined reliably without an anechoic termination. It should be noted that the two-load method is the easier to employ for measuring transmission loss.

This paper explores the use of inverse numerical acoustics to reconstruct the surface vibration of a noise source. Inverse numerical acoustics is mainly used for source identification. This approach uses the measured sound pressure at a set of field points and the Helmholtz integral equation to reconstruct the normal surface velocity. The number of sound pressure measurements is considerably less than the number of surface vibration nodes. A brief guideline on choosing the number and location of the field points to provide an acceptable reproduction of the surface vibration is presented. The effect of adding a few measured velocities to improve the accuracy will also be discussed. Other practical considerations such as the shape of the field point mesh and effect of experimental errors on reconstruction accuracy will be presented. Examples will include a diesel engine and a transmission housing.

Insertion loss in one-third or octave bands is widely used in industry to assess the performance of large silencers and mufflers. However, there is no standard procedure for determining the transmission loss in one-third or octave bands using measured data or simulation. In this paper, assuming that the source is broadband, three different approaches to convert the narrowband transmission loss data into one-third and octave bands are investigated. Each method is described in detail. To validate the three different approaches, narrowband transmission loss data of a simple expansion chamber and a large bar silencer is converted into one-third and octave bands, and results obtained from the three approaches are demonstrated to agree well with one another.

The principle of vibro-acoustic reciprocity is reviewed and applied to model sound radiation from a shaker excited structure. Transfer functions between sound pressure at a point in the far field and the velocity of a patch were determined reciprocally both for the to-scale structure and also for a half-scale model. A point monopole source was developed and utilized for the reciprocal measurements. In order to reduce the measurement effort, the boundary element method (BEM) was used to determine the reciprocal transfer functions as an alternative to measurement. Acceleration and sound intensity were measured on patches of the vibrating structure. Reciprocally measured or BEM generated transfer functions were then used to predict the sound pressure in the far field from the vibrating structure. The predicted sound pressure compared favorably with that measured.

This paper presents a stability analysis of the free-piston Stirling engine and linear alternator power generation system. Since such a system operates under sustained mechanical oscillations, stability of the system is important for proper operation, and as a criterion in selecting the tuning capacitor. The stability criterion of the system is that the rate of change in power dissipation and electric power output is always faster than the rate of the power generated by the engine. The dynamic equations and model of the system are developed in this paper. Frequency domain analysis and Bode plot techniques are utilized in the study. The stable operating frequency region corresponding to different levels of power output are then determined.

This paper presents the criteria in selecting the size of the tuning capacitor, and the cost tradeoff between magnet volume and tuning capacitor in a free piston Stirling engine power generation system. The permissible range of capacitor size corresponding to different magnet volume, in order to prevent magnet demagnetization and stabilize the operation of the system, is determined. Within the permissible range suitable capacitor size may be selected to compensate the inductive load of the system to improve the overall power factor. If the capacitor size is not in the permissible range, there would exists a danger of losing magnet strength, or unstable operation of the engine that would destroy the engine due to unbounded amplitude of piston oscillations. The theory developed is then applied to a practical system, and the cost tradeoff between magnet volume and capacitor is studied.

The high temperature and high neutron flux environment of a thermionic space power reactor presents a challenge in the design of the sheath insulator within a thermionic fuel element. The present alumina insulator design is suspect to degradation due to the neutron flux. The alumina insulator also requires a barrier coating to isolate it from the liquid alkali metal coolant. Although the alumina sheath development is progressing, the alumina insulator remains a potential point of significant performance loss in the thermionic fuel element. The recent successes in depositing polycrystalline diamond film onto cylindrical refractory metal substrates has led to the consideration of diamond as a potentially ideal sheath insulator. Investigations have been conducted into the durability of diamond thin film under exposure to simulated thermionic reactor conditions.