R. Benjamin Davis

Cylindrical Shell Submerged in Bounded Acoustic Media: A Modal Approach

The dynamics of a simply-supported cylindrical shell submerged in liquid hydrogen (LH 2 ) and liquid oxygen (LOX) are considered. The shell itself is bounded by a rigid outer cylinder with closed rigid ends. This conOguration gives rise to two ∞uid-Olled cavities{ an inner cylindrical cavity and an outer annular cavity. Such geometries are common in rocket engine design. This study computes the natural frequencies and modes of the ∞uid-structure system by combining the rigid wall acoustic cavity modes and the in vacuo structural modes into a system of coupled ordinary diAEerential equations. Eigenvalue veering is observed near the intersections of the curves representing natural frequencies of the rigid wall acoustic and the in vacuo structural modes. In the case of a shell submerged in LH 2 , system frequencies near these intersections are as much as 30% lower than the corresponding in vacuo structural frequencies. Due to its high density, the frequency reductions in the presence of LOX are even more dramatic. The forced response of the ∞uid-loaded shell subject to a harmonic point excitation is also presented. The forced response in the presence of ∞uid is diAEerent from the response of the structure in vacuo in a variety of ways. The frequency shifts that arise from consideration of the ∞uid alter the order of the resonant response peaks. In some cases, modes that are well separated in the in vacuo case are within close proximity in the ∞uid-loaded case (and vice-versa). The ∞uid-loaded structural responses also contain relatively small resonant peaks corresponding to system modes that are dominated by contributions from the ∞uid.

Validation of Measured Damping Trends for Flight-Like Vehicle Panel/Equipment including a Range of Cable Harness Assemblies

This validation study examines the effect on vibroacoustic response resulting from the installation of cable bundles on a curved orthogrid panel. Of interest is the level of damping provided by the installation of the cable bundles and whether this damping could be potentially leveraged in launch vehicle design. The results of this test are compared with baseline acoustic response tests without cables. Damping estimates from the measured response data are made using a new software tool that leverages a finite element model of the panel in conjunction with advanced optimization techniques. While the full test series is not yet complete, the first configuration of cable bundles that was assessed effectively increased the viscous critical damping fraction of the system by as much as 0.02 in certain frequency ranges.

The Response of a Mechanical Oscillator Due to Swept and Dithered Excitation

A single degree-of-freedom oscillator subject to linearly swept and/or dithered excitation is considered. Dither refers to the variation of an excitation frequency about a given nominal, or primary, frequency. Dither in rocket engine turbopump shaft speeds can be an important consideration when analyzing the dynamic response of turbomachinery components such as turbine blades. Results indicate that the incorporation of dithered excitation into a fatigue analysis may extend the predicted fatigue life of the structure by a factor of two or more.

An Efficient Modal Analysis Method for Structures Coupled to Fluid-Filled Cavities

This work discusses the implementation of a component mode synthesis technique for use with geometrically complex acoustic-structure systems. The feasibility of conceptually similar techniques was Orst demonstrated over 30 years ago. Since that time there have been remarkable advancements in computational methods. It is therefore reasonable to question the extent to which component mode synthesis remains a computationally advantageous approach for acoustic-structure systems of practical interest. This study demonstrates that relative to the most recent release of the popular Onite element software package, ANSYS, component mode synthesis techniques have a signiOcant computational advantage when the forced response of an acoustic-structure system is of interest. However, recent improvements to the unsymmetric eigensolver available in ANSYS have rendered component mode synthesis a less e±cient option when calculating system frequencies and modes. The e±ciency and accuracy of the proposed technique is assessed in the context of systems that are relevant to the design of liquid rocket engines.

Implementation of Speed Variation in the Structural Dynamic Assessment of Turbomachinery Flow-Path Components

During the design of turbomachinery flow path components, the assessment of possible structural resonant conditions is critical. Higher frequency modes of these structures are frequently found to be subject to resonance, and in these cases, design criteria require a forced response analysis of the structure with the assumption that the excitation speed exactly equals the resonant frequency. The design becomes problematic if the response analysis shows a violation of the HCF criteria. One possible solution is to perform “finite-life” analysis, where Miner’s rule is used to calculate the actual life in seconds in comparison to the required life. In this situation, it is beneficial to incorporate the fact that, for a variety of turbomachinery control reasons, the speed of the rotor does not actually dwell at a single value but instead dithers about a nominal mean speed and during the time that the excitation frequency is not equal to the resonant frequency, the damage accumulated by the structure is diminished significantly. Building on previous investigations into this process, we show that a steady-state assumption of the response is extremely accurate for this typical case, resulting in the ability to quickly account for speed variation in the finite-life analysis of a component which has previously had its peak dynamic stress at resonance calculated. A technique using Monte Carlo simulation is also presented which can be used when specific speed time histories are not available. The implementation of these techniques can prove critical for successful turbopump design, as the improvement in life when speed variation is considered is shown to be greater than a factor of two.

A simplified method for approximating the natural frequencies of acoustic‐structure systems

It is well known that the natural frequency of a flexible structure in contact with a fluid‐filled cavity can be approximated with a closed‐form expression that considers the coupling of only two component modes (i.e., a rigid‐wall acoustic mode and an in vacuo structural mode) [F. Fahy, Sound and Structural Vibration (Academic, London, 1985), Chap. 6, pp. 249–256]. However, this expression requires volume and area integrations over the component mode shapes. For many practical configurations, the effort to determine the component mode shapes and compute the necessary integrals counteracts much of the time and computational savings afforded by the closed‐form expression. Here, the closed‐form expression is recast as a function of the nondimensional frequency separation between the component modes and a new nondimensional coupling parameter, Ψ. Design curves representing the value of Ψ for common geometries and boundary conditions are presented. With the use of these design curves and a knowledge of the co...