Final Report Summary - AM10 (The Aeroacoustics of Elastic Structures)
The work carried out in this project combines knowledge in two major disciplines in aeronautical sciences: aeroacoustics and aeroelasticity. While each of the disciplines has been the subject of extensive studies for more than half a century, the combination of the two, particularly in the context of thin-structure aeromechanics, supplies a new set of challenging problems that need to be addressed.
The project has developed a theoretical scheme for studying the combined effects of flow unsteadiness (incident gust or localized turbulence) and external forcing (boundary actuation) on the aeroelastic and aeroacoustic behaviors of elastic structures.
The formulation of the aeroacoustic problem was based on a compact-body acoustic analogy, thus avoiding the traditional difficulty in obtaining the weak acoustic far field from direct simulations.
The results obtained have assisted in shedding light on the mechanisms coupling between the motion and sound of thin elastic structures, and in suggesting novel elasto-acoustic noise-control methodologies.
These are highly desirable in various applications, including the development of ``green" noise-control systems for the reduction of trailing edge noise, the monitoring of the acoustic signature of flapping-wing micro-unmanned-aerial-vehicles (MUAVs), and the analysis of natural phenomena such as insect-flight sound.
The objectives set for the project were in line with ongoing efforts in the European Community to develop new techniques for noise reduction in both civil and military applications.
Having defined the effect of structure elasticity on system acoustic signature to be the topic of the research, the work carried out has focused on non-linear investigations of the near- and far-field responses of flexible single- and multi-airfoil configurations to external hydrodynamic and mechanical actuations.
The main purpose of the set of works published during this project was to gain further insight into the added impact of elasticity on system acoustic signature, through the mechanism of fluid-structure interactions.
This contribution is novel in the sense that acoustic radiation resulting from fluid-structure interactions has focused mainly on analyses of single rigid (even if vibrating) structures. In addition, past studies have not investigated the combined effects of incoming-flow unsteadiness and mechanical actuation, and settled with studying ``passive" radiation resulting from the former factor only.
In each specific problem studied, the project breaks up naturally into two parts.
In the first part, the dynamical near-field response of the system is analyzed. In the second part, the outcome acoustic signature is studied. Generally, the near-field calculation is based on incompressible high-Reynolds number potential flow theory for the fluid, coupled with an equation of motion to describe structure dynamics. Coupling between the two descriptions is made through a pressure-jump term, occurring between upper and lower surfaces of the structure, caused by asymmetric motion of the body.
Incoming flow unsteadiness is represented through a given distribution of upstream fluid vorticity; mechanical structure actuation is modeled through prescribed leading-edge excitations, to imitate flapping-flight conditions. Being forced by a given input signal, it is found that the system normally amplifies actuations containing one of its natural frequencies. This clearly demonstrates the importance of combining structure elastic degrees of freedom into the system description.
The resonance mechanism is found later on to have a major effect on the radiation of far-filed sound.
Once the near-field dynamical description is obtained, it serves as an effective source term in calculation of the outcome acoustic radiation. To calculate the far-field pressure actuation efficiently, we make use of an acoustic analogy, where the full fluid equations are rewritten in the form of a linear wave operator acting on the pressure, balanced by a near-field source term. Assuming the source is acoustically compact (meaning that the body size is much smaller than emitted sound wavelength, a valid assumption in the present low-Mach setup), we make use of the Powell-Howe acoustic analogy, where the sources of sound include fluid vorticity and structure motion. Our analyses compare the relative importance of each source term, showing that, in many cases, fluid vorticity overcomes structure motion as the dominant source of sound. Structure dynamics is nevertheless an indirect cause of radiation, as vorticity is generated owing to the motion of the body.
In the process of working on the main project theme, the PI has initiated a separate funded project on a related topic, in collaboration with local industry. This project has initiated during the first part of the project, and has been extended since.
The purpose of this ongoing project is to analyses the vibroacoustic response of a thin-shell cylinder to external acoustic excitation.
The project combines experimental and analytic examination of the vibratory response of a cylinder set in a reverberation chamber and actuated by broadband noise. It is mainly of interest to capture the essential structural behavior using a low-order model. Here, as well, a phenomenon of self-resonance excitation was found, which is of practical significance. The project serves as an example showing the outer impact of the reintegration grant in broadening the scope of research of the PI.
The project has developed a theoretical scheme for studying the combined effects of flow unsteadiness (incident gust or localized turbulence) and external forcing (boundary actuation) on the aeroelastic and aeroacoustic behaviors of elastic structures.
The formulation of the aeroacoustic problem was based on a compact-body acoustic analogy, thus avoiding the traditional difficulty in obtaining the weak acoustic far field from direct simulations.
The results obtained have assisted in shedding light on the mechanisms coupling between the motion and sound of thin elastic structures, and in suggesting novel elasto-acoustic noise-control methodologies.
These are highly desirable in various applications, including the development of ``green" noise-control systems for the reduction of trailing edge noise, the monitoring of the acoustic signature of flapping-wing micro-unmanned-aerial-vehicles (MUAVs), and the analysis of natural phenomena such as insect-flight sound.
The objectives set for the project were in line with ongoing efforts in the European Community to develop new techniques for noise reduction in both civil and military applications.
Having defined the effect of structure elasticity on system acoustic signature to be the topic of the research, the work carried out has focused on non-linear investigations of the near- and far-field responses of flexible single- and multi-airfoil configurations to external hydrodynamic and mechanical actuations.
The main purpose of the set of works published during this project was to gain further insight into the added impact of elasticity on system acoustic signature, through the mechanism of fluid-structure interactions.
This contribution is novel in the sense that acoustic radiation resulting from fluid-structure interactions has focused mainly on analyses of single rigid (even if vibrating) structures. In addition, past studies have not investigated the combined effects of incoming-flow unsteadiness and mechanical actuation, and settled with studying ``passive" radiation resulting from the former factor only.
In each specific problem studied, the project breaks up naturally into two parts.
In the first part, the dynamical near-field response of the system is analyzed. In the second part, the outcome acoustic signature is studied. Generally, the near-field calculation is based on incompressible high-Reynolds number potential flow theory for the fluid, coupled with an equation of motion to describe structure dynamics. Coupling between the two descriptions is made through a pressure-jump term, occurring between upper and lower surfaces of the structure, caused by asymmetric motion of the body.
Incoming flow unsteadiness is represented through a given distribution of upstream fluid vorticity; mechanical structure actuation is modeled through prescribed leading-edge excitations, to imitate flapping-flight conditions. Being forced by a given input signal, it is found that the system normally amplifies actuations containing one of its natural frequencies. This clearly demonstrates the importance of combining structure elastic degrees of freedom into the system description.
The resonance mechanism is found later on to have a major effect on the radiation of far-filed sound.
Once the near-field dynamical description is obtained, it serves as an effective source term in calculation of the outcome acoustic radiation. To calculate the far-field pressure actuation efficiently, we make use of an acoustic analogy, where the full fluid equations are rewritten in the form of a linear wave operator acting on the pressure, balanced by a near-field source term. Assuming the source is acoustically compact (meaning that the body size is much smaller than emitted sound wavelength, a valid assumption in the present low-Mach setup), we make use of the Powell-Howe acoustic analogy, where the sources of sound include fluid vorticity and structure motion. Our analyses compare the relative importance of each source term, showing that, in many cases, fluid vorticity overcomes structure motion as the dominant source of sound. Structure dynamics is nevertheless an indirect cause of radiation, as vorticity is generated owing to the motion of the body.
In the process of working on the main project theme, the PI has initiated a separate funded project on a related topic, in collaboration with local industry. This project has initiated during the first part of the project, and has been extended since.
The purpose of this ongoing project is to analyses the vibroacoustic response of a thin-shell cylinder to external acoustic excitation.
The project combines experimental and analytic examination of the vibratory response of a cylinder set in a reverberation chamber and actuated by broadband noise. It is mainly of interest to capture the essential structural behavior using a low-order model. Here, as well, a phenomenon of self-resonance excitation was found, which is of practical significance. The project serves as an example showing the outer impact of the reintegration grant in broadening the scope of research of the PI.