A9947879 ~ AI&i 99-1918

INTELLIGENT SYSTEM FOR VIBROACOUSTIC AND SHOCK

ENVIRONMENT PREDICTIONS’

Sally A. M?nemy, Brandon Dixon, Allen Parrish The University of Alabama

Tuscaloosa, AL

Sheldon S. Rubm Rubin Engineering Company

Sherman Oaks, CA

Don Wong, Carole Tanner The Aerospace Corporation

El Segundo, CA

ABSTRACT

Prediction of vibroacoustic environments for Department of Defense (DOD) and commercial launch vehicles, as well as spacecraft, has become a costly, labor-intensive process. Yet such analysis is essential to assure reliability of vehicle structures and airborne equipment. The objective of this research and development program is to develop a sof3ware package that uses a combination of theoretical and empirical methods in an automated sequence to provide predictions of vibroacoustic and shock environments. This intelligent design tool will enable more responsive, cost effective, and accurate analyses than are currently possible. The software to be developed will incorporate a graphical user interface and multiple, modular analysis tools. The program will utilize standard input / output data formats, so that the results f%om other analytical prediction tools may be imported for further processing. This intelligent design tool will be developed in three phases: Phase I - Top Level Requirements Specification and the Development Roadmap; Phase II - Software Prototype; Phase III - FulI Scale Program Development. This paper describes the work completed under Phase I and currently underway under Phase II.

INTRODUCTION

Prediction of vibroacoustic and shock environments for NASA, DOD and commercial launch vehicles, as well as spacecraft, is currently a costly and labor- intensive process. However, the analyses used to predict these environments are essential in order to ensure reliability of vehicle structures and airborne equipment. Levels predicted in prehminary analyses are usefhl both in spacecraft design and also in the development of specifications for qualification tests, which ensure that components and systems are robust enough to withstand transportation, lift-off, aero- dynamic and separation loads.

The Aerospace Corporation has initiated a multi- year program to develop a next generation design and analysis tool that will advance the state-of-theart for vibroacoustic and shock predictions. The objective of this program is to develop a software package that uses a combination of theoretical and empirical methods to provide, in an automated sequence, rapid and accurate estimates of vibroacoustic and shock environments. The Vibroacoustic Intelligent System for Prediction of Environments Reliability and Specifications (VESPERS) will incorpomte:

’ Copyright 0 1999 The American Institute of Aeronautics and Astronautics Inc., All rights reserved.

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The best available analytical tools for vibroacoustic and shock enviromneIlt predictions; Data scaling tools / methods; An extensive graphical user interface; A comprehensive centralized database of launch vehicle and spacecraft flight and test data; Help utilities that include brief tutorials and references for further information on the prediction tools.

This paper provides an overview of the VISPERS sofhvare package and its development plan.

Consider the process of predicting lift-off vibration levels for a component mounted on an acousticaLly receptive satellite inside a launch vehicle fairing. Assuming the primary excitation source to be lift-off acoustic loads, the first step is the prediction of the external acoustic levels during launch.’ There is no single accepted method of predicting lift-off acoustic levels and there appear to be no available computer tools that can perform this type of calculation. Two currently accepted procedures, both of which entail calculating estimates by hand, are outlined in Eldred’s 1971 NASA Technical Report and in Gruner’s 1964 paper.2B3 Because the generation of lift-off acoustic loads and the parameters upon which they depend are not fully understood, both the Eldred and Gnmer procedures are empirical methods. Since there is an inherent level of uncertainty in the predictions obtained using either method, it is valuable to be able to compare the predicted levels with levels measured on a similar launch vehicle / pad. However, what constitutes similar is not always clear, nor is the required historical data always readily accessible (if available at ah). Correct interpretation of these methods and their application to new launch vehicle configurations requires an understanding of aeroacoustics (near- and far-field), turbulence, compressible shear layers, and shocks.

Once an estimate of the external acoustic loads is obtained, the next step is to predict sound transmission through the fairing. Typically, a structural dynamics group will estimate the fairing vibration response at low frequencies using finite element analysis (FEA). In this frequency range, the effect of the internal acoustic volume is likely to be considered negligible and the low frequency, long wavelength sound levels inside the fhiring are not predicted using the FEA results. The internal sound levels at high frequencies can be estimated using statistical energy analysis (SEA) and

there are several computer programs available to do this: VAPEPS, which is available from NASA, requires command line input; SEAM@*, which is a program developed and marketed by Cambridge Collaborative; and AutoSEA, a program developed and marketed by Vibro-Acoustic Sciences that utilizes a graphical user interfkce.66 These programs provide estimates that are valid at high modal densities and their correct application requires a solid understanding of structural vibrations (both wave and modal behavior). In or&r to understand the limitations of SEA, one must have a background in random vibrations, probability and statistics.

Given the uncertainties in SEA prediction procedures, it is again helpful if calculated estimates of the internal acoustic levels can be compared to those measured in similar vehicles. If this type of historical data is available, it must be corrected for differences in fill effects and internal fairing absorptive treatments.’ Finally, once the internal acoustic levels have been predicted, the satellite vibration levels can be estimated. If an SEA program is used, the high frequency satellite vibration response will be obtained f?om the overall system analysis that generated the estimates of internal acoustic levels.

Each organization compensates for uncertainties in the overall prediction process with what they consider an appropriate safety factor. Consistent, reliable estimates require an experienced analyst with a strong technical background in aeroacoustics, structural vibrations and statistics. Such technically proficient analysts are few, yet most organizations, seeing that the prediction process appears to be unsophisticated, do not feel the need for a centralized vibroacoustics group to estimate vibration and acoustic environments. Instead, each program (launch vehicle or satellite) may have one or two individuals who generate such estimates, on an infrequent basis, as part of a broad set of job responsibilities.

The need for an infrastructure that provides for ongoing refinement and improvement of the vibroacoustic prediction process is clear. This iIlhStl-UCture should include an iwlwe4 unambiguous three-dimensional geometry display, a database of appropriate physical properties, rapid computerized predictions based on sound principles, and an ability to readily compare and refine estimates based on comparison with historical data (appropriately scaled). VISPERS will provide this critically important i l l&lStl-UCaUe.

SEAM is a registered trademark of Cambridge Collaborative, Inc.

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&‘ROJECT DESCRIPTION

Phase I - Program Definition and DeveloDment Plan

The VISPERS project began with a Phase I effort to defme both the program capabilities and a development phi. Table I summarizes the essential VISPERS capabilities. Figure 1 is a flow chart for a typical vibroacoustics prediction process using VISPERS. This flow chart is representative of the majority of the environments predictions encountered in practice, but not the prediction of mid-field and far-field acoustic levels during launch. Fig. 1 can be modified to include estimation of the response of a satellite to internal acoustics by adding an arrow out of the Sound Transmission Prediction Tools box and back into the Structural Response Prediction Tools box.

Given the technical breadth of the vibroacoustics estimation process, it is important that VISPERS provide the user with access to explanations of the prediction procedures. This is especially important for engineers new to the vibroacoustic environments field. VESPERS will incorporate an extensive help / tutorial facility. By clicking on keywords, users will be able to view explanations of the procedures and a list of relevant references. Synopses of key references, written by the experts in the field, will also be available.

VISPERS will make extensive use of graphical user interfaces (GUYIs) and multiple, modular analysis tools. This modularity will allow program capabilities to be added incrementally, as time and funding permits. The breakdown of program components in Table II reflects this modularity. To the extent possible, development will leverage the capabilities of existing tools in the areas of GUI, geometric display, and interactive modeling. The software will use standard input I output data formats, so that results can be imported from or exported to other programs.

Standard programming languages, middleware, commercial off the shelf (COTS) products and component-based development techniques will be used to develop the VISPERS software. The package will be deployable as either a single-platform or client-server system. As such we expect to employ the following generic software architecture:

l A separable GUI component deployable as a thin client, developed using a combination of visual and scripting languages;

l An ODBC-compliant database (e.g., Oracle 8) for the historical data, along with appropriate ODBC drivers and libraries to support the interface between the GUI and database;

e Industry standard middleware (e.g., COM, DCOM and/or CORBA) to support the interface between GUI and prediction tools.

Phase II - Prototvoe

A VISPERS software prototype is currently in development. The prototype will have only limited prediction capabilities. The intent of the prototype is two-fold. First, it will define the software architecture. Second, the prototype will be used to demonstrate the VISPERS concept to potential funding agencies.

The prediction capabilities of the prototype will be limited to external lift-off acoustics (using the Gnmer method), sound transmission loss (using an SEA cylinder model developed by Don Wong and available on EnviroNET), absorption corrections for a typical blanket treatment, and fill factor corrections per Hughes et a.l.**“s The database structure will be developed for external and internal lift-off acoustics data. The database will be populated with launch data and scaling methods developed for internal and external launch acoustic data. The prototype soflware will include GUIs for all of these modules.

Phase III - Full Scale DeveloDment

The goals for VISPERS, as represented in Tables I and II, are clearly ambitious. Expertise from many technical disciplines and multiple organizations is required to meet these goals. The prototype software will be completed in fiscal year 1999 under existing funding. Limited funding is available for fiscal year 2000. Participation in this effort by NASA, DOD, and private sector organizations, both in terms of technical expertise and funding, is sought.

CONCLUSION

A multi-year program to develop a next generation design and analysis tool that will advance the state-of- the-art for vibroacoustic and shock predictions has been described. The envisioned software package, VISPERS, will use a combination of theoretical and empirical methods to provide rapid and accurate estimates of vibroacoustic and shock environments. VISPERS will include a database and data scaling methods, so that predictions may be compared to data measured on similar launch vehicles. VISPERS will provide an infi-astructure that provides for ongoing refinement and improvement of the vibroacoustic prediction process.

ACKNOWLEDGMENT

This work was supported by The Aerospace Corporation.

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TABLE I - VISPERS TOP LEVEL SPECIFICATION

USEROBIECTPJES _ . PREDICT

FAR-FIELDLEVELSDURINGLAUNCH MID-FIELDLE~ELS DURINGLAUNCH EXTERNALACOUSTICSONLAUNCHSTAND EXTERNALACOUSTICLOADSON~EHICLESKIN AEROACOUSTICL~ADSDURINGFLIGHT VIBRATIONLEVELSOFACOMPONENISONSRIN INTERIORFAIRINGACOUSTICLEWLS VIBRATIONLE'+'ELSOFSPACECRAFT/CORECOMPONENTS VIBRATIONLEVELSFORENGINEMOUNTEDCOMPONENTS cOMPONENTSHOCKLEvELs

. GENERATETESTSPECIFICATIONDOCUMENIS l EVALUATETHENEEDFORRE-QUALIFICA-IION . DETERMINE IFSHOCKTESTEIWIRONMENTISCOVEREDBYR4.NDOMWBRATIONTEST . PREDICTRhlAININGLIFEOFACOMPONENTTHATHASUNDERGONETESTING . HIGHCYCLEFATIGUE/LIFEPREDICTION

METHODS . /%NALYTICALPREDICTIONToOL-S l EMPIRICALPREDICTIONT~OLS . APPLYSCALINGRULESTODATA(CASEBASEDREASONING) . EsTIMATEsBASEDONONEORMOREOFTHESEMETHODS

LE~ELSOFACCURACY . INITIALBALLPARKESTIMATES l PRELIMINARYDESIGNESTIMATES . EsTIMATEsBASEDONFINALDESIGN

INPUTSFROMEXTERNALPROGRAMS . IMPORTRESULTSFROMOTHERANALYSIS'IWXS . l [MPORTANDREFORMATDATAFROMDATABASE(S)

OUTPUT . PREDICTEDMEANANDSTATWI'ICALESTIMATES . ' I-ESTSPECIFICATIONDOCUMENT

ENVIRONMENTALTESTLE~ELS DURATIONOFENVIRONMENTALTEST ~HOCKRESPONSESPECTRUM

. &MAINlNGCOMPONENTLIFE

. ELECTRONICANDPRINTEDRECORDOFINP~&OUTPUT PARAMETERSSUPPLIEDBYUSER PREDICTIONMETHOD(S)UHLI~ED SOURCEOFRESULTSIMPORTEDFROMOTHERPROGRAMS SOURCEOFSCALEDDATA

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Excitation

Tools

1 Tools 1 -

Transmission

/

t, - OR I

Figure 1. Typical VISPERS Prediction.

Table II - Breakdown of VISPERS Program Components

Excitation Source Prediction Tools Lift-Off Acoustic Loads Aerodynamic Loads (Max Q / Transomc).

Sound Transmission Prediction Tools Mid-frequency transmission loss High-frequency SEA Predictions

Internal Acoustic Adjustments Absorption Fill Factor

Random Vibration Predictions Mid-frequency Predictions High-frequency, SEA Predictions

Random Vibration Adjustments Mass Loading Force Limiting

Excitation Source Scaling Methods Lift-Off Acoustic Loads Aerodynamic Loads (Max Q / Transonic)

Response Scaling Methods Internal Acoustics Random Structural Vibrations Transient Shock Response

Generation of Test Specifications Reverberant Acoustic Tests Random Vibration Testing (based on equivalent

fatigue)

Environmental Mitigation Guidelines

Guidelines for Test Requirements

Data Processing Guidelines

Interface / Conversion Tools CAD/CAM Program Interface(s) FEA Program Interface(s) External SEA Program Interface(s)

Graphical User Interfaces / Database Links Lift-Off Acoustics Prediction

Lit&Off Acoustics Scaling m Database Sound Transmission Prediction

Internal Acoustics Scaling ti Database Random Vibrations Prediction

Random Vibration Scaling a Database Shock Prediction

Shock Response Scaling e Database Aerodynamic Loads Prediction

Aerodynamic Loads Scaling ti Database ’ Test Guidelines

Test Guidelines m Test Instrumentation and Facilities Database

Environments Database External acoustic data Aerodynamic pressure data Launch Vibration data Internal acoustic data Shock data

Material Databases Typical aerospace structures (e.g. honeycomb

panels) Damping treatments Absorptive treatments /blankets

Test Facilities Database

Test Instrumentation Database

REFERENCES

1. Himelblau, H., Kern, D., Manning, J., Piersol, A. Rubin, S., “Handbook for Dynamic Environmental Criteria,” Final Draft, January 1999. Requests for copies should be sent to NASA Engineering Standards, EL02, MSFC, AL, 35812.

2. Eldred, K. M., “Acoustic Loads Generated By The Propulsion System,” NASA SP-8072, June, 197 1.

3. Gruner, W. J., and Johnston, G. D., “An Engineering Approach to Prediction of Space Vehicle Acoustic Environments,” presented to the 67th Meeting of the Acoustical society of America, May 6-9, 1964, New York, New York.

4.

5.

6.

7.

8.

“Vibroacoustic Payload Environment Prediction System (VAPEPS),” NASA JPL, Pasadena, CA. SEAM@‘, Cambridge Collaborative, Inc., Cambridge, MA. “AutoSEA Users Guide”, Vibro-Acoustic Sciences, Inc., Version 1.4, 1995. Hughes, W.O., McNelis, M.E., and Manning, J.E., “NASA LeRc’s Acoustic Fill Effect Test Program and Results,” NASA TM 106688, prepared for the 65& Shock and Vibration Symposium, San Diego, California, 31 Oct. - 3 Nov. 3, 1994. Messore, P. J., and Chissotimos, C., “EnviroNET,” http://envnet.gsfc.nasa.gov/, Dec. 1998.

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