Sstructural-acoustic optimization of structures excited by turbulent boundary layer flow

Open Access
Author:
Shepherd, Micah R
Graduate Program:
Acoustics
Degree:
Doctor of Philosophy
Document Type:
Dissertation
Date of Defense:
February 25, 2014
Committee Members:
  • Stephen A Hambric, Dissertation Advisor
  • Victor Ward Sparrow, Committee Member
  • John Brian Fahnline, Committee Member
  • Ashok D Belegundu, Committee Member
Keywords:
  • structrual acoustics
  • optimization
  • turbulent boundary layer flow
Abstract:
In order to reduce noise radiation of aircraft or marine panels, a general structural-acoustic optimization technique is presented. To compute the structural-acoustic response, a modal approach based on finite element / boundary element analysis is used which can easily incorporate fluid loading, added structures and static pre-loads. Simple deterministic or complex random forcing functions are included in the analysis by transforming their cross-spectral density matrices to modal space. Particular emphasis is placed in this dissertation on structures excited by the fluctuating pressures due to turbulent boundary layer (TBL) flow. An efficient frequency-spacing is also used to minimize evaluation time but ensure accuracy. The response from the structural-acoustic analysis is coupled to an evolutionary strategy with covariance matrix adaptation (CMA-ES) to find the best design for low noise and weight. CMA-ES, a stochastic optimizer with robust search properties, samples candidate solutions from a multi-variate normal distribution and adapts the covariance matrix to favor good solutions. The optimization procedure is validated by minimizing the sound radiated by a point-driven ribbed panel and comparing the optimization results to an exhaustive search of the design space. Structural-acoustic optimization is then performed on a curved marine panel with heavy fluid loading excited by slow TBL flow. A weighted combination of noise radiation and mass are minimized by changing the thickness of strips and patches of elements. An uncorrelated pressure approximation is used to estimate the modal force due to TBL flow thus reducing the evaluation time required to compute the objective function. The results show that the best noise reduction is achieved by minimizing the modal acceptance of energy by the panel. This is equivalent to pushing the structural modes away from the peak frequency range of the forcing function. Additionally, the Pareto trade-off curve between total sound power and panel mass is estimated to show the best designs which will simultaneously reduce both noise and weight. As a final case, the sound power radiated is minimized for a ribbed aircraft panel excited by TBL flow at typical cruise conditions. A static pressure load is applied to the panel to simulate cabin pressurization during flight and the rib locations and cross-sectional area are used as the design variables during optimization. Nearly 10 dB of reduction is achieved by pushing the ribs to the edge of the panel, thus lowering the modal amplitudes excited by the forcing function. The optimal configuration is also found for a higher speed and a larger downstream distance. The design variables are then separated, and the optimization is repeated to determine the coupling between the design variables. Finally, a static constraint is included in the procedure using a very low-frequency dynamic calculation to approximate a static response. The constraint limits the amount of reduction that can be achieved by the optimizer. Guidance for designing quiet aircraft panels is then presented.