Numerical methods for load and response prediction for use in acoustic fatigue
(2016) Abstract (Swedish)
 Acoustic fatigue can occur in structural elements of an aircraft exposed to very high sound pressures. To deal with acoustic fatigue, mainly empirical methods have been applied and often late in the design phase. Current design guidelines have three main limitations. First, they do not say anything about the load intensities. The load levels can be determined either experimentally or numerically. Experimental testing tends to be expensive and time consuming. It is also desired to deal with acoustic fatigue early in the design phase. Therefore, it is desired to turn to numerical methods to determine the load levels. Second, the design guidelines assume that the spatial distribution of the load is uniform. In other words, the load is assumed... (More)
 Acoustic fatigue can occur in structural elements of an aircraft exposed to very high sound pressures. To deal with acoustic fatigue, mainly empirical methods have been applied and often late in the design phase. Current design guidelines have three main limitations. First, they do not say anything about the load intensities. The load levels can be determined either experimentally or numerically. Experimental testing tends to be expensive and time consuming. It is also desired to deal with acoustic fatigue early in the design phase. Therefore, it is desired to turn to numerical methods to determine the load levels. Second, the design guidelines assume that the spatial distribution of the load is uniform. In other words, the load is assumed to be perfectly in phase over the entire structural element. This assumption limits the accuracy of the response prediction and by extension the fatigue prediction. Third, the design guidelines are limited to a simple, single surface panel with linear response.
In this thesis, both the load and response prediction are performed by numerical methods. The load is determined using Computational Fluid Dynamics (CFD). From the CFD simulations, both the load intensities and the spatial distributions are extracted. This solves the first and second mentioned limitations. The extracted load is used as force input to a Finite Element (FE) simulation of the exposed panel structure. Since complex structures and nonlinearities can be handled using the FEmethod, it avoids the third mentioned limitation.
Two cases of separated flow are used as model problems for acoustic fatigue in this thesis. In both model problems, the simulations are compared to existing measurements. In Paper A, a ramped backwardfacing step is used. The flow over the step induces a load on an aluminium sheet fitted downstream of the step. With the exception of the cutoff, or shedding mode, frequency being overpredicted, the spectral qualities of the load and the load intensities are well captured. The panel response prediction compares reasonably well with the existing measurements. In Paper B, a reduction in a range of low frequencies of the downstream load is observed when the ramped backwardfacing step is lined with chevrons or serrations.
The model problem used in Papers CE is flow over an inclined fence at transonic Mach number and realistic Reynolds number for aircraft operation. A segment with cyclic boundary conditions of the flow setup is simulated in Paper C. This result in well predicted crossspectra, but an energy concentration in the autospectra is not properly resolved. In Paper D, a full threedimensional simulation of the entire setup is performed and it is concluded that the missing energy concentration in the autospectra is properly captured. In Paper E, the response of a realistic aircraft panel structure is simulated using FE random response analysis with the CFDsimulated load as input. The response is found to be sensitive to the crossspectra of the input load. The strain predictions vary with strain gauge location. However, only one strain gauge is off by more than a factor of two, which appears to be the best one can hope for when using the design guidelines in favourable conditions and with a measured load. Therefore, the main conclusion of this thesis is that the method of using CFD to calculate the load which is to be used as input to an FE response simulation can produce useful results for acoustic fatigue. (Less)  Abstract
 Acoustic fatigue can occur in structural elements of an aircraft exposed to very high sound pressures. To deal with acoustic fatigue, mainly empirical methods have been applied and often late in the design phase. Current design guidelines have three main limitations. First, they do not say anything about the load intensities. The load levels can be determined either experimentally or numerically. Experimental testing tends to be expensive and time consuming. It is also desired to deal with acoustic fatigue early in the design phase. Therefore, it is desired to turn to numerical methods to determine the load levels. Second, the design guidelines assume that the spatial distribution of the load is uniform. In other words, the load is assumed... (More)
 Acoustic fatigue can occur in structural elements of an aircraft exposed to very high sound pressures. To deal with acoustic fatigue, mainly empirical methods have been applied and often late in the design phase. Current design guidelines have three main limitations. First, they do not say anything about the load intensities. The load levels can be determined either experimentally or numerically. Experimental testing tends to be expensive and time consuming. It is also desired to deal with acoustic fatigue early in the design phase. Therefore, it is desired to turn to numerical methods to determine the load levels. Second, the design guidelines assume that the spatial distribution of the load is uniform. In other words, the load is assumed to be perfectly in phase over the entire structural element. This assumption limits the accuracy of the response prediction and by extension the fatigue prediction. Third, the design guidelines are limited to a simple, single surface panel with linear response.
In this thesis, both the load and response prediction are performed by numerical methods. The load is determined using Computational Fluid Dynamics (CFD). From the CFD simulations, both the load intensities and the spatial distributions are extracted. This solves the first and second mentioned limitations. The extracted load is used as force input to a Finite Element (FE) simulation of the exposed panel structure. Since complex structures and nonlinearities can be handled using the FEmethod, it avoids the third mentioned limitation.
Two cases of separated flow are used as model problems for acoustic fatigue in this thesis. In both model problems, the simulations are compared to existing measurements. In Paper A, a ramped backwardfacing step is used. The flow over the step induces a load on an aluminium sheet fitted downstream of the step. With the exception of the cutoff, or shedding mode, frequency being overpredicted, the spectral qualities of the load and the load intensities are well captured. The panel response prediction compares reasonably well with the existing measurements. In Paper B, a reduction in a range of low frequencies of the downstream load is observed when the ramped backwardfacing step is lined with chevrons or serrations.
The model problem used in Papers CE is flow over an inclined fence at transonic Mach number and realistic Reynolds number for aircraft operation. A segment with cyclic boundary conditions of the flow setup is simulated in Paper C. This result in well predicted crossspectra, but an energy concentration in the autospectra is not properly resolved. In Paper D, a full threedimensional simulation of the entire setup is performed and it is concluded that the missing energy concentration in the autospectra is properly captured. In Paper E, the response of a realistic aircraft panel structure is simulated using FE random response analysis with the CFDsimulated load as input. The response is found to be sensitive to the crossspectra of the input load. The strain predictions vary with strain gauge location. However, only one strain gauge is off by more than a factor of two, which appears to be the best one can hope for when using the design guidelines in favourable conditions and with a measured load. Therefore, the main conclusion of this thesis is that the method of using CFD to calculate the load which is to be used as input to an FE response simulation can produce useful results for acoustic fatigue. (Less)
Please use this url to cite or link to this publication:
http://lup.lub.lu.se/record/278b86ef6b4e4f32a7bf32a282a0f56a
 author
 Nilsson, Johan ^{LU}
 supervisor

 Per Erik Austrell ^{LU}
 organization
 publishing date
 20160518
 type
 Thesis
 publication status
 published
 subject
 keywords
 Acoustic fatigue, Sonic fatigue, Computational Fluid Dynamics, Large Eddy Simulation, Finite Element Method, Random response analysis, Separated flow, High Re, Backwardfacing step, Fence flow, Aircraft, Proper Orthogonal Decomposition, Acoustic fatigue, Sonic fatigue, Computational Fluid Dynamics, Large Eddy Simulation, Finite Element Method, Random response analysis, Separated flow, High Re, Backwardfacing step, Fence flow, Aircraft, Proper Orthogonal Decomposition
 publisher
 Structural Mechanics, Lund University
 ISBN
 9789176237861
 language
 English
 LU publication?
 yes
 id
 278b86ef6b4e4f32a7bf32a282a0f56a
 date added to LUP
 20160517 16:12:31
 date last changed
 20160919 08:45:20
@phdthesis{278b86ef6b4e4f32a7bf32a282a0f56a, abstract = {Acoustic fatigue can occur in structural elements of an aircraft exposed to very high sound pressures. To deal with acoustic fatigue, mainly empirical methods have been applied and often late in the design phase. Current design guidelines have three main limitations. First, they do not say anything about the load intensities. The load levels can be determined either experimentally or numerically. Experimental testing tends to be expensive and time consuming. It is also desired to deal with acoustic fatigue early in the design phase. Therefore, it is desired to turn to numerical methods to determine the load levels. Second, the design guidelines assume that the spatial distribution of the load is uniform. In other words, the load is assumed to be perfectly in phase over the entire structural element. This assumption limits the accuracy of the response prediction and by extension the fatigue prediction. Third, the design guidelines are limited to a simple, single surface panel with linear response.<br> <br> In this thesis, both the load and response prediction are performed by numerical methods. The load is determined using Computational Fluid Dynamics (CFD). From the CFD simulations, both the load intensities and the spatial distributions are extracted. This solves the first and second mentioned limitations. The extracted load is used as force input to a Finite Element (FE) simulation of the exposed panel structure. Since complex structures and nonlinearities can be handled using the FEmethod, it avoids the third mentioned limitation.<br> <br> Two cases of separated flow are used as model problems for acoustic fatigue in this thesis. In both model problems, the simulations are compared to existing measurements. In Paper A, a ramped backwardfacing step is used. The flow over the step induces a load on an aluminium sheet fitted downstream of the step. With the exception of the cutoff, or shedding mode, frequency being overpredicted, the spectral qualities of the load and the load intensities are well captured. The panel response prediction compares reasonably well with the existing measurements. In Paper B, a reduction in a range of low frequencies of the downstream load is observed when the ramped backwardfacing step is lined with chevrons or serrations. <br> <br> <br> The model problem used in Papers CE is flow over an inclined fence at transonic Mach number and realistic Reynolds number for aircraft operation. A segment with cyclic boundary conditions of the flow setup is simulated in Paper C. This result in well predicted crossspectra, but an energy concentration in the autospectra is not properly resolved. In Paper D, a full threedimensional simulation of the entire setup is performed and it is concluded that the missing energy concentration in the autospectra is properly captured. In Paper E, the response of a realistic aircraft panel structure is simulated using FE random response analysis with the CFDsimulated load as input. The response is found to be sensitive to the crossspectra of the input load. The strain predictions vary with strain gauge location. However, only one strain gauge is off by more than a factor of two, which appears to be the best one can hope for when using the design guidelines in favourable conditions and with a measured load. Therefore, the main conclusion of this thesis is that the method of using CFD to calculate the load which is to be used as input to an FE response simulation can produce useful results for acoustic fatigue.}, author = {Nilsson, Johan}, isbn = {9789176237861}, keyword = {Acoustic fatigue,Sonic fatigue,Computational Fluid Dynamics,Large Eddy Simulation,Finite Element Method,Random response analysis,Separated flow,High Re,Backwardfacing step,Fence flow,Aircraft,Proper Orthogonal Decomposition,Acoustic fatigue,Sonic fatigue,Computational Fluid Dynamics,Large Eddy Simulation,Finite Element Method,Random response analysis,Separated flow,High Re,Backwardfacing step,Fence flow,Aircraft,Proper Orthogonal Decomposition}, language = {eng}, month = {05}, publisher = {Structural Mechanics, Lund University}, school = {Lund University}, title = {Numerical methods for load and response prediction for use in acoustic fatigue}, year = {2016}, }