Methods to Predict Structural Response due to Random Sound Pressure Fields
Independent thesis Advanced level (degree of Master (Two Years)), 20 credits / 30 HE creditsStudent thesis
To predict structural responses due to random sound pressure fields are of great interest within many fields of aircraft development, particularly within acoustic fatigue problems and definition of vibration requirements. Today there exist some methods to quantify sound pressure fields affecting the air-fighters. Some of them are considered to be expensive, time consuming or with high computational cost. Examples of this would be to measure a real flight, produce data from wind tunnels, use Computational Fluid Dynamics (CFD) or obtain data from an engineering database. Once the sound pressure levels are known they can be applied as loads to structural models and this is the area studied in this work. To study these problems a new working tool is made using MATLAB. The tool’s main purpose is to give an opportunity to study structural responses caused by random sound pressure fields with different correlation methods. Because of the complexity of both the sound pressure and different structures of the aircraft a few limitations are considered. The plate is used since this makes is easy to produce different mode shape functions. The mode shape function is an important part in this work as it can be used to create all possible frequency response functions in a structure. Then, to determine a structure response, different methods to produce pressure fields are used. The methods are called correlation-models and five different models are considered: uncorrelated, fully correlated and moving correlated load (MCL) and two empirical models due to the similarity to real sound pressure fields called Turbulent Boundary Layer (TBL) and a diffuse excitation model. To prove the accuracy of the created working tool, an independent FE-solver is used called Abaqus. Abaqus is used to validate the mode shape- and the frequency response-fucntions. Another advantage with Abaqus is that the solver already includes three of the correlation models which therefore simplify the verification of the new tool. Finally, a simulation study is carried out in order to validate the MATLAB functions and test the sensitivity to different correlation models. In order to do this, the sound pressure field is to be reasonable approximated and therefore data from the database ESDU (acronym of Engineering Sciences Data Unit) is used that predicts sound pressure fields for different flight envelopes. In the simulation study all correlation models are compared to TBL due to its sound pressure and here it can be seen that fully correlated loads fails to predict response due to certain modes. On the other hand, the MCL model increases this accuracy for low Mach numbers and even more for high Mach numbers due to its velocity dependence. The diffuse model, which is supposed to imitate a real pressure chamber load, is often believed to be conservative but in this study it can be seen that this is not always the case.
Place, publisher, year, edition, pages
2015. , 33 p.
IdentifiersURN: urn:nbn:se:liu:diva-121632ISRN: LIU-IEI-TEK-A--15/02367--SEOAI: oai:DiVA.org:liu-121632DiVA: diva2:857430
Subject / course
Aronsson, Carl-Gustaf, Universitetslektor
Schmidt, Peter, Universitetslektor
ProjectsRandom Vibration, Power Spectral Density, Cross Correlation, Turbulent Boundary Layer, Random Response