Aeroacoustics of Electrically Propelled Air Vehicles

A transformational era in flight is upon us with the increased ubiquity of Unmanned Aerial Systems (UAS), in particular, the burgeoning interest in Unmanned Aerial Mobility (UAM) for mobilizing cargo and passengers. While the precise application and market for these vehicles is still developing, there will definitely be more of these UAM vehicles flying in and around populated areas. One of the key enabling factors for realizing the quantity and frequency of flights that are envisioned will be the noise levels produced by UAM as they operate in populated areas. In order to develop low noise UAM, air vehicle designers need data, design tools, and technology to enable the development of quiet vehicles from conceptual design stage all of the way through detailed design. Currently, there is a lack of data and capability for predicting the noise produced by UAM until millions of dollars have been spent to design, fabricate, and fly the vehicle. This is why acoustics needs to be factored into the earliest stages of conceptual design in order to develop a vehicle that achieve the quietest design possible.

In order to execute this multidisciplinary research program, we need to develop capability for advanced measurement techniques to further the understanding of the noise source mechanisms, fluid dynamics involved with noise generation, the impact of propulsor-airframe integration, and to generate the data needed to build low-order acoustic prediction tools. Research projects may include:

1. Design, purchase, and assemble a mobile acoustics data acquisition platform for anechoic facility and outdoor measurements. This platform will include microphones, measurement arrays, data acquisition equipment, and software for acquiring and processing acoustic data
2. Develop a test stand for operating electric motor and rotor configurations in our anechoic facility
3. Develop an experimental setup for measuring the velocity field (velocity and turbulence) of various propulsor configurations. This task will utilize a high-speed Particle Image Velocimetry (PIV) system
4. Develop data processing software to synthesize acoustics and flowfield data to provide data-driven insights into the dominant noise mechanisms 

Optical measurements of aerodynamic flows provides a means for engineers to visualize the invisible. The advent of high-speed cameras, faster and more powerful lasers, and advances in computing power and data storage has resulted in imaging diagnostics that are capable of very high spatial (space) and temporal (time) measurements. Fluid dynamics researchers utilize these imaging diagnostics to study the physics of high-speed and high-temperature flows to advance aerospace designs for greater efficiency and performance. Advanced aircraft concepts are moving towards further integrating the airframe and propulsion system to achieve greater aerodynamic efficiency, reduced fuel burn, and reduced aircraft noise. This trend toward close integration of the propulsion system with the airframe is resulting in new challenges as the complex aerodynamic flows interact with the airframe surface.

This project will involve utilizing imaging diagnostics including Particle Image Velocimetry (PIV) to measure gas velocity and Infrared Imaging Spectroscopy to measure surface temperatures for a high-velocity, high-temperature gas flow interacting with a surface. These measurements will use high-speed cameras capable of kilohertz acquisition rates, meaning thousands of frames per second. This will allow us temporally resolve these unsteady processes due to turbulence fluctuations in the high-speed flows. The unique aspect of this project will be to develop new methods for acquiring simultaneous velocity and temperature from these measurement techniques. The ability to understand these short timescale processes will have major implications in the design of future aerospace vehicle systems with closely integrated propulsion systems.

Active flow control is a field of study focusing on using actuators and sensors to impart energy to a fluid flow in order to control or modify the fluid behavior. An example of this is injecting a fluid into a low momentum boundary layer (friction layer between a fluid and a surface) in order to delay separation of the boundary layer. This can have many desirable effects such as reducing aerodynamic drag or providing a desirable change in the flow properties on a surface. Current modern aircraft are geometrically optimized for efficiency by using high-fidelity computational tools and building on 50 years of design modifications to the basic “tube and wing” aircraft design. In order to achieve further performance improvements and capabilities beyond what passive geometry can provide, active flow control can be harnessed. The applications of active flow control can be used to improve wing performance at high-angles of attack, improve engine efficiency, control undesirable fluid behaviors, improve aircraft maneuverability, and to reduce control surface sizes or eliminate them entirely.

This project will involve designing, building, and testing actuators for active flow control. The focus will be on a piezoelectric actuator that is driven by an electronic controller to achieve air flow pulsations. Actuators of different designs and sizes will be tested to characterize the output flow velocity and frequencies. The goal of the project will be to develop an understanding of the actuator capability and limitations in regards to the size and power output. The project will entail design (CAD), fabrication, and assembly of the actuators along with development of a data acquisition system (hardware and software) for controlling the actuators and recording data. This project will eventually result in testing of the actuators in a wind tunnel with an aerodynamic model.

Aeroacoustics is the study of noise generated from aerodynamic phenomena. A common source of noise, aerodynamic turbulence, is found everywhere from nature to aircraft systems. This project will involve measuring the turbulent flow over a human head and human ear across a range of conditions in a low-speed wind tunnel. This project will provide detailed measurements of the turbulent flow, realistic acoustics in the human ear, and study flow control to reduce turbulence noise on the human ear to improve audibility in windy environments.

The project will involve the scoping and design of an experimental test campaign and application of advanced fluid dynamic measurement techniques to quantify the flow field and acoustic sources. This interdisciplinary research spans engineering, biological systems, and acoustics.