Leslie Robert Koval

On sound transmission into a thin cylindrical shell under “flight conditions”☆

Abstract In the context of the transmission of airborne noise into an aircraft fuselage, a mathematical model for sound transmission into a thin cylindrical shell is used to study sound transmission under “flight conditions”: i.e., under conditions of external air flow past a pressurized cylinder at flight altitude. Numerical results for different incidence angles are presented for a typical narrow-bodied jet in cruising flight at 10 660 m (35 000 ft) with interior pressure at 2440 m (8000 ft). A comparison is made between no-flow sound transmission at standard conditions on the ground to sound transmission under flight conditions. It is shown that at M = 0, the cylinder transmission loss has dips at fR (cylinder ring frequency) and fc (critical frequency for a flat panel of same material and thickness as shell). Below fR cylinder resonances affect TL. Between fR and fc, cylinder TL follows a masslaw behavior. Flow provides a modest increase in TL in the mass-law region, and strongly interacts with the cylinder resonances below fR. For normally-incident waves, TL is unaffected by flow.

Sound transmission into a laminated composite cylindrical shell

Abstract In the context of the transmission of airborne noise into an aircraft fuselage, a mathematical model is presented for the transmission of an oblique plane sound wave into a laminated composite circular cylindrical shell. Numerical results are obtained for geometry typical of a narrow-bodied jet transport. Results indicate that from the viewpoint of noise attenuation a laminated composite shell does not appear to offer any significant advantage over an aluminum shell. However, the transmission loss of a laminated composite shell is sensitive to the orientation of the fibers and this suggests the possibility of using a laminated composite shell to tailor the noise attenuation characteristics to meet a specific need.

On sound transmission into an orthotropic shell

Abstract A mathematical model is presented for the transmission of airborne noise through the walls of an orthotropic cylindrical shell. Parameters were varied to see how orthotropicity affected noise transmission. When compared to that for an isotropic shell, the cylinder transmission loss was found to be quite sensitive to the ratio of circumferential to axial modulus of elasticity. A modulus ratio greater than unity appears to enhance transmission loss in the mass-controlled region, while a ratio less than unity degrades it. Below the ring frequency, the trends appear to be reversed. The cylinder transmission loss appears to be relatively insensitive to changes in the shear modulus.

Effects of cavity resonances on sound transmission into a thin cylindrical shell

Abstract In the context of the transmission of airborne noise into an aircraft fuselage, a mathematical model is presented for the effects of internal cavity resonances on sound transmission into a thin cylindrical shell. The “noise reduction” of the cylinder is defined and computed, with and without including the effects of internal cavity resonances. As would be expected, the noise reduction in the absence of cavity resonances follows the same qualitative pattern as does transmission loss. Numerical results show that cavity resonances lead to wide fluctuations and a general decrease of noise reduction, especially at cavity resonances. Modest internal absorption is shown to greatly reduce the effect of cavity resonances. The effects of external airflow, internal cabin pressurization, and different acoustical properties inside and outside the cylinder are also included and briefly examined.

On Sound Transmission into a Stiffened Cylindrical Shell with Rings and Stringers Treated as Discrete Elements

Abstract In the context of the transmission of airborne noise into an aircaft fuselage, a mathematical model is presented for the transmission of an oblique plane sound wave into a finite cylindrical shell stiffened by stringers and ring frames. The rings and stringers are modeled as discrete structural elements. The numerical case studies was typical of a narrow-bodied jet transport fuselage. The numerical results show that the ring-frequency dip in the transmission loss curve that is present for a monocoque shell is still present in the case of a stiffened shell. The ring frequency effect is a result of the cylindrical geometry of the shell. Below the ring frequency, stiffening does not appear to have any significant effect on transmission loss, but above the ring frequency, stiffeners can enhance the transmission loss of a cylindrical shell.

Structural health monitoring using wavelet transforms

A new method of damage detection using wavelet transforms and curvature mode shapes is proposed in this paper. A damage in the structure results in changing its dynamic characteristics such as natural frequencies, damping, and mode shapes. A number of researchers have investigated structural health monitoring techniques for identifying, locating, and quantifying the damage using the changes in the dynamic response of a damaged structure. Curvature mode shape and wavelet maps are two such methods that have already been used to locate damages. These methods have some limitations in determining the exact location of the damages. We have developed a technique by combining these two methods for enhancing the sensitivity and accuracy in damage location. The mode shapes are double differentiated using the central difference approximation to obtain the curvature mode shape. Then a wavelet map is constructed for the curvature mode shape. It is shown that this method can be used to determine the location of the damages. The proposed method is applied to detect damage in an experimental lattice structure and a cantilever beam with multiple damages. The mode shapes are obtained analytically using finite element analysis and also experimentally using laser vibrometer. The experimental results obtained are satisfactory.

A FINITE ELEMENT MODEL FOR SOUND TRANSMISSION THROUGH LAMINATED COMPOSITE PLATES

The finite element method is used to model the noise transmission through unstiffened and stiffened laminated composite panels of finite size into a closed cavity. Plate and acoustic finite elements are coupled and the frequencies of the coupled modes are determined. The model is then used to calculate the noise reduction of the panel. Results are compared to experimental values obtained at the NASA Langley Research Center. The purpose of this paper is to demonstrate the use of finite elements to model, for noise transmission calculations, complex structures, such as a stiffened composite panel or a composite panel with windows.

Modeling and robust control of smart structures

The design and implementation of control strategies for large, flexible smart structures presents challenging problems. One of the difficulties arises in the approximation of high- order finite element models with low order models. Another difficulty in controller design arises from the presence of unmodeled dynamics and incorrect knowledge of the structural parameters. In this paper, the balance-truncation reduced-order models are employed in deriving lower-order models for complex smart structures. These methods do not introduce any spill-over problems in the closed-loop response of the system. The simplified analytical models are compared with models developed by structural identification techniques based on vibration test data. To minimize the effects of uncertainties on the closed-loop system performance of smart structures, robust control methodologies have been employed in the design of controllers. The reduced order models are employed in the design of robust controllers. To demonstrate the capabilities of shape-memory-alloy actuators, we have designed and fabricated a three-mass test article with multiple shape-memory-alloy (NiTiNOL) actuators. Generally, the non-collocation of actuators and sensors presents difficulties in the design of controllers. Controllers for a test article with non-collocated sensors and actuators are designed, implemented and tests. The closed-loop system response of the test article with two actuators and sensors has been experimentally determined and presented in the paper.

On sound transmission into a heavily-damped cylinder

A mathematical model for the transmission of sound into a thin monocoque cylindrical shell is discussed. The model is used to evaluate an oblique plane wave incident upon a flexible thin cylindrical shell. The solution is applicable to the transmission of sound under actual flight conditions. The model is then used to determine curves of cylinder-transmission loss for heavily damped cylinders. Numerical results are found for several plane-wave incidence angles for a narrow-bodied jet fuselage made of aluminum. It is noted that damping (i.e., the loss factor) increases, dips because of reduced cylinder resonances, and eventually disappears when the loss factor of the shell is large enough.

Reduced-Order Modeling of Flexible Structures

An alternate procedure for deriving a reduced-order model is presented. The Routh expansion method is used and preserves the original system impulse response energy. This procedure does not acquire knowledge of the system eigenvalues/eigenvectors and guarantees a stable reduced-order model if the original system is stable. The method is illustrated on a simply supported beam subjected to moment excitation. A comparison is made with reduced-order models obtained using other methods.