Search Results

The two-load method is commonly applied to determine the transmission loss for a muffler especially if an impedance tube rig is used. Although one procedure and algorithm is detailed in ASTM E2611, the quality of the transmission loss curve is dependent on several factors that are not discussed in detail in the standard. In this paper, several practical concerns are investigated including (1) the number of channels used in the measurement, (2) the selection of the reference channel, and (3) the choice of data processing algorithm (transfer or scattering matrix). Results are compared for a simple expansion chamber first, then for mufflers of other types. Recommendations are made for obtaining smoother transmission loss curves for various measurement methods.

The insertion loss of louvered terminations positioned at the end of a rectangular duct is determined using acoustic finite element analysis. Insertion loss was determined by taking the difference between the sound power with and without the louvers at the termination. Analyses were conducted in the plane wave regime and the acoustic source was anechoic eliminating any reflections from the source. The effect of different louver configurations on insertion loss was examined. Parameters investigated included louver length, angle, and spacing between louvers. Based on the analyses, equations were developed for the insertion loss of unlined louvers.

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.

An energy source simulation method (ESSM) has been developed to determine sound energy density. Using this approach, a specified intensity boundary condition on the surface of a vibrating body is approximated by superimposing energy density sources placed inside the body. The unknown strengths for these sources are then found by minimizing the error on the boundary, using a least squares technique. The superposition of these energy density sources should then approximate the sound radiating from the body. The approach was evaluated in two-dimensions for a circle, square, and a more general geometry. The ESSM proved an excellent tool for predicting the energy density provided that power radiated uniformly in all directions. However, the ESSM could not accurately predict the directional characteristics of the energy density field if the power radiated significantly higher from one side of an object than other sides.

3D printing is a revolutionary manufacturing method that allows the productions of engineering parts almost directly from modeling software on a computer. With 3D printing technology, future manufacturing could become vastly efficient. However, it has been reported that the 3D printed parts exhibit anisotropic behaviors in microstructure and mechanical properties, that is, depending on the positions (infill orientations) that the parts are placed on a printer platform, the properties of resultant parts can vary greatly. So far, studies on anisotropic behaviors of 3D printed parts have been mostly limited to the static properties (modulus of elasticity, failure strength, etc.); there is a lack on the understanding of mechanical responses of 3D printed parts under dynamic conditions. In the present study, the anisotropic behaviors of 3D printed parts are investigated from the dynamic aspect.

Traditional manufacturing processes such as injection or compression molding are often enclosed and pressurized systems that produce homogenous products. In contrast, 3D printing is exposed to the environment at ambient (or reduced) temperature and atmospheric pressure. Furthermore, the printing process itself is mostly “layered manufacturing”, i.e., it forms a three-dimensional part by laying down successive layers of materials. Those characteristics inevitably lead to an inconsistent microstructure of 3D printed products and thus cause anisotropic mechanical properties. In this paper, the anisotropic behaviors of 3D printed parts were investigated by using both laboratory coupon specimens (bending specimens) and complex engineering structures (A-pillar). Results show that the orientation of the infills of 3D printed parts can significantly influence their mechanical properties.

Literature has shown that 3D printed composites may have highly anisotropic mechanical properties due to variation in microstructure as a result of filament deposition process. Laminate composite theory, which is already used for composite products, has been proposed as an effective method for quantifying these mechanical characteristics. Continuous fiber composites traditionally have the best mechanical properties but can difficult or costly to manufacture, especially when attempting to use additive manufacturing methods. Traditionally, continuous fiber composites used specialized equipment such as vacuum enclaves or labor heavy hand layering techniques. An attractive alternative to these costly techniques is modifying discontinuous fiber additive manufacturing methods into utilizing continuous fibers. Currently there exist commercial systems that utilize finite-deposition (FD) techniques that insert a continuous fiber braid into certain layers of the composite product.

The current Si/polymeric medical diagnostic sensors that are on the market only feature a one-point calibration system [1]. Such a measurement results in less accurate sensing and more in-factory sensor rejection. The two-point calibration fluidic method introduced here will alleviate some of the shortcomings of such current miniature analytical systems. Our fluidic platform is a disposable, multi-purpose micro analytical laboratory on a compact disc (CD) [2, 3]. This system is based on the centrifugal force, in which fluidic flow can be controlled by the spinning rate of the CD and thus a whole range of fluidic functions including valving, mixing, metering, splitting, and separation can be implemented. Furthermore, optical detection such as absorption and fluorescence can be incorporated into the CD control unit to obtain signals from pre-specified positions on the disc.

Transfer path analysis is commonly used to determine input forces indirectly utilizing measured responses and transfer functions. Though it is recommended that the source should be detached from the vibrating structure when measuring transfer functions, engineers and technicians frequently have a difficult time in doing so in practice. Recently, a substitute for inverse force determination via transfer path analysis has been suggested. The indirectly determined forces are termed blocked forces and are usable so long as the source and machine are not detached from one another. Blocked forces have the added advantage of being valid even if the machine structure is modified. In this research, a typical automotive engine cover is considered as a receiver structure and is bolted to a plastic source plate excited by an electromagnetic shaker.

Finite element modeling has been used extensively nowadays for predicting the noise and vibration performance of whole engines or subsystems. However, the elastomeric components on the engines or subsystems are often omitted in the FE models due to some known difficulties. One of these is the lack of the material properties at higher frequencies. The elastomer is known to have frequency-dependent viscoelasticity, i.e., the dynamic modulus increases monotonically with frequency and the damping exhibits a peak. These properties can be easily measured using conventional dynamic mechanical experiments but only in the lower range of frequencies. The present paper describes a method for characterizing the viscoelastic properties at higher frequencies using fractional calculus. The viscoelastic constitutive equations based on fractional derivatives are discussed. The method is then used to predict the high frequency properties of an elastomer.

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.

Helmholtz resonators are often used in the design of vehicle mufflers to target tonal noise at a few specific low frequencies generated by the engine. Due to the uncertainty of temperature variations and different engine speeds, multiple resonators may have to be built in series to cover a narrow band of frequencies. Double-tuned Helmholtz resonators (DTHR) normally consist of two chambers connected in series. Openings or necks are created by punching small slots into a thin-walled tube which provide a natural neck passage to the enclosing volume of the Helmholtz resonator. In this paper, numerical analyses using both the boundary element (BEM) and the finite element (FEM) methods are performed and simulation results are compared against one another. A typical real-world muffler configuration commonly used in passenger vehicles is used in a case study.

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.

Microperforated panel absorbers are best considered as the combination of the perforate and the backing cavity. They are sometimes likened to Helmholtz resonators. This analogy is true in the sense that they are most effective at the resonant frequencies of the panel-cavity combination when the particle velocity is high in the perforations. However, unlike traditional Helmholtz resonators, microperforated absorbers are broader band and the attenuation mechanism is dissipative rather than reactive. It is well known that the cavity depth governs the frequency bands of high absorption. The work presented here focuses on the development, modeling and testing of novel configurations of backing constructions and materials. These configurations are aimed at both dialing in the absorption properties at specific frequencies of interest and creating broadband sound absorbers. In this work, several backing cavity strategies are considered and evaluated.

Additive manufacturing is slowly changing how components are developed and manufactured. As the technology develops over time, it is anticipated that industry will 3D print sound absorbers in production. Configurations may be considered that would be difficult to manufacture in another way. For exploratory purposes, several designs were 3D printed and positioned in an impedance tube for testing. Though the absorbers developed are based on well-established strategies, the absorbers considered are either difficult to manufacture by another means or take advantage of the unique features of 3D printed parts. The samples measured include long perforations, lightweight panels, and Helmholtz resonators with spiral wound necks. Selected results are compared with acoustic finite element analysis.

Micro-perforated (MPP) panels are acoustic absorbers that are non-combustible, acoustically tunable, lightweight, and environmentally friendly. In most cases, they are spaced from a wall, and that spacing determines the frequency range where the absorber performs well. The absorption is maximized when the particle velocity in the perforations is high. Accordingly, the absorber performs best when positioned approximately a quarter acoustic wavelength from the wall, and larger cavity depths improve the low frequency absorption. At multiples of one half acoustic wavelength, the absorption is minimal. Additionally, the absorption is minimal at low frequencies due to the limited cavity depth behind the MPP. By partitioning the backing cavity, the cavity depth can be strategically increased and varied. This will improve the absorption at low frequencies and can provide absorption over a wide frequency range.

Typical automotive muffler designs contain complex internal components such as extended inlet/outlet tubes, thin baffles with eccentric holes, internal connecting tubes, perforated tubes, perforated baffles, flow plugs and sound-absorbing materials. An accurate performance prediction for highly complicated muffler designs would greatly reduce the effort in fabricating and testing of multiple design iterations for engineers. This paper discusses the use of a component-based computer simulation tool for design and analysis of exhaust mufflers. A comprehensive computer program based on the Direct Mixed-Body Boundary Element Method was developed to predict the transmission loss characteristics of muffler systems. The transmission loss is calculated by an improved four-pole method that does not require solving the boundary element matrix twice at each frequency, and hence, it is a significantly faster approach when compared to the conventional four-pole method.

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.

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.

Experiments were performed to investigate the convective heat transfer from a two-dimensional slot jet of the dielectric liquid PAO to a smooth 15.2 mm by 9.5 mm film resistor surface. The effects of nozzle width, nozzle-to-plate distance, impinging velocity, and liquid properties have been examined. Heat transfer correlations and a discussion of relative parametric effects are provided.