A numerical investigation on effects of structural flexibility on aerodynamic far field sound Ashish Purohit, Ashish K. Darpe ⇑ , S.P. Singh Vibration Research Laboratory, Department of Mechanical Engineering, Indian Institute of Technology Delhi, New Delhi 110016, India article info Article history: Received 14 June 2013 Received in revised form 1 October 2013 Accepted 29 October 2013 Available online 11 November 2013 Keywords: Aerodynamic sound Fluid structure interaction Structural flexibility Laminar flow Hybrid method abstract A comparative study of aerodynamic sound from flexible and rigid bodies under the interaction of vortex wake is presented in this work. A bluff body with a trailing splitter plate in a low Reynolds number and low Mach number flow is considered for generation of sound. The vortices are generated by the upstream bluff body, which in turn excites the trailing plate. The objective of the study is to investigate the effect of structural flexibility on the aerodynamic sound. The trailing plate is initially considered to be rigid and then flexibility is introduced to study the influence of flexibility of the plate on the far field acoustics. Three different test cases are considered: a square block with rigid splitter plate, square block with flex- ible splitter plate and square block without splitter plate. A two-step computational aeroacoustic (CAA) hybrid method is used for computing aerodynamic sound. To account for a bi-directional interaction between the flexible plate and flow field, a two-way fluid structure interaction model is used for the flow simulation. The acoustic propagation is simulated by surface source method based on the Euler equa- tions, wherein the surface source term is determined by transient compressible flow simulation. It is observed that incorporation of flexibility produces significant influence on vortex field and has subse- quent effect on far field aerodynamic sound. A higher sound pressure and a changed directivity pattern are observed for the case of flexible plate compared to the case of rigid plate. The results of the study reveal the importance of the flexibility of structure in the character of the far field sound. Ó 2013 Elsevier Ltd. All rights reserved. 1. Introduction With increased computational power and precise computa- tional fluid dynamics (CFD) techniques, focus of aeroacoustic re- search has also shifted from simple problems like flow over rigid bodies to more complex cases of aerodynamic sound from flexible structures. In the past few years, rapid growth of computational aeroacoustics (CAA) has made it possible to apply CAA to a variety of problems in industries e.g. automobile, aerospace, air condition- ing, wind power, etc. Flexible parts of a vehicle like doors, panels, antennas, pantographs of high speed trains, flexible ducts of build- ing air-conditioning systems, etc. are major sources of undesirable aerodynamic noise. Therefore, the need of controlling this undesir- able sound is increasing day by day. Investigation of aerodynamic sound from vibrating bodies has become a key topic in aerody- namic research. Extensive research in the area of aeroacoustics mainly started in 1953, largely through a landmark paper by Lighthill [1] in the field of aerodynamic sound modeling. He proposed an acoustic analogy for solving jet flow problems. Later Curle [2] and Williams and Hawkings [3] introduced a surface source term in the acoustic analogy. The acoustic analogy is an integral based modeling, where in-homogeneities of mean flow are not accounted. After the devel- opment of acoustic analogy, many techniques have been devel- oped. Some of those are based on direct numerical simulation (DNS) [4,5]. Hardin and Pope [6] developed a hybrid method that splits the complete process into a non-linear sound generation and a linear acoustic propagation. Past literature shows that efforts have been made to explore the area of acoustics generated from vibrating structures. In 1969, Da- vies [7] worked on excitation of flexible panels by turbulent boundary layers. Graham [8] studied acoustics from an elastic plate under boundary layer excitation. To include the effects of plate motion on the external pressure field, he assumed a weak coupling between solid and fluid. Wu and Maestrello [9] estimated the acoustic response of a baffled plate excited by a turbulent bound- ary layer. Excitation of elastic body under the action of boundary layer has been studied extensively [10–12]. In most of these stud- ies, excitation force on the elastic body has been modeled by a combination of fluctuating pressure due to turbulent layer and acoustic perturbations over the body. Thus, a unidirectional inter- action has been considered. In a more comprehensive study by Schafer et al. [13], aerodynamic sound from a thin flexible plate of fixed–fixed boundary condition, using a bi-directional fluid 0045-7930/$ - see front matter Ó 2013 Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.compfluid.2013.10.042 ⇑ Corresponding author. Tel.: +91 1126596095. E-mail addresses: ashish.purohit@mech.iitd.ac.in (A. Purohit), akdarpe@me- ch.iitd.ac.in (A.K. Darpe), singhsp@mech.iitd.ac.in (S.P. Singh). Computers & Fluids 89 (2014) 143–152 Contents lists available at ScienceDirect Computers & Fluids journal homepage: www.elsevier.com/locate/compfluid