Research

Our research addresses areas across physical acoustics, computational modeling of physical systems, stochastic modeling, data analytics and engineering education. We explore problems through an interdisciplinary lens with researchers from physical sciences, social-science, education and business management. Some of these topics are described here. Two recently funded NSF projects address Graduate Education in Cyber-Physical Systems Engineering and the Design of AI-assisted Augmented Reality based Education and Training Systems.

Acoustic Streaming in the Inner Ear

The cochlea of the inner ear is a small fluid-filled chamber responsible for converting acoustically induced mechanical vibrations into the neural signals that we use to interpret sound. The internal sensory structures within the cochlea can be excited by air and bone conduction. The mechanical process of bone conduction hearing is shown in Fig. 1. The arrows in the figure denote relative velocity directions of the tympanic membrane (TM), oval window (O), and round window (R).

The mechanical features of bone conduction are shown in the Figure. In contrast to air conduction, sound induces vibration of the otic capsule, the bony outer wall of the inner ear that includes the cochlea. Due to the incompressible behavior of the enclosed fluid in the cochlea, the sum of the volume velocities at the oval and the round windows must equal the time-rate of change of the volume of fluid in the cochlea. In this research acoustic streaming resulting from the time-harmonic compression of the cochlear capsule is examined.

Passive Radio Frequency Excited Acoustic Transduction

In this work, the physical processes that govern the operation of a passive acoustic transducer, shown if Fig 1, are analyzed and modeled. The wireless battery-free transducer derives its power from an externally applied electromagnetic field generated by a radio transmitter. The audio signal is encoded in the backscattered electromagnetic field. Electro-mechano-acoustical analogies are developed and presented. Power generation, sound transduction, and radio frequency backscatter transmission of the audio signal are analyzed.

Introduction to the special issue on the theory and applications of acoustofluidics

J. Friend, C. Thompson, K. Chitala and M. Denis. The Journal of the Acoustical Society of America 150, 4558 (2021);

Acoustofluidics is a burgeoning field that applies ultrasound to micro-scale to nano-scale fluidic systems. The discovery of the ability to effectively manipulate fluids and particles at small scales has yielded results that are superior to other approaches and has been built into a diverse range of research. Recasting the fundamentals of acoustics from the past to include new phenomena observed in recent years has allowed acoustical systems to impact new areas, such as drug delivery, diagnostics, and enhanced chemical processes. The contributions in this special issue address a diverse range of research topics in acoustofluidics. Topics include acoustic streaming, flows induced by bubbles, manipulation of particles using acoustic radiation forces, fluid and structural interactions, and contributions suggesting a natural limit to the particle velocity, the ability to deliver molecules to human immune T cells, and microdroplet generation via nozzle-based acoustic atomization.

Padé approximants and their application to scattering from fluid media

In this work, a numerical method for modeling the scattered acoustic pressure from fluid occlusions is described. The method is based on the asymptotic series expansion of the pressure expressed in terms of sound speed contrast between the host medium and entrained fluid occlusions. Padé approximants are used to extend the applicability of the result for larger values of sound speed contrast. For scattering from a circular cylinder, an improvement in convergence between the exact and numerical solutions is demonstrated. In the case of scattering from an inhomogeneous medium, a numerical solution with reduced order of Padé approximants is presented.

Chaotic Motion in Oscillatory Flows

Over the last 150 years, the problem of oscillatory motion of fluids has been examined by numerous investigators. However, only recently have some of the causes for instability and chaotic motion in such flows been uncovered. Chaotic oscillations in wall-bounded oscillatory flows have been linked to the nonlinear growth of vortical disturbances introduced into the viscous boundary layer. The temporal evolution of these disturbances gives rise to the basic features evident in the resulting chaotic fluid motion. Disturbances are generated by the environment and consequently are comprised of numerous modes having a varied spatial wave number and amplitude. Once the fluid motion becomes unstable, according to linear theory, the least linearly stable mode will prevail and chaotic oscillation will ensue at a threshold value of the controlling parameter. Nonlinear amplitude growth can yield alternative solutions, which display chaotic oscillations at lower threshold values. To determine the least stable disturbance, the sensitivity of the fluid motion to the wave number composition and amplitude of an imposed disturbance must be understood. The figure depicts the state space trajectory of the kinetic energy of a chaotic three-dimensional disturbance in an oscillatory boundary layer. The orbits demonstrate an exponential sensitivity to initial conditions. The feature indicates the presence of chaotic oscillations.

Network Traffic and Channel Characterization

Internet traffic resulting from world-wide-web access is increasing at exponential rates and resulting in congested networks. At the same time, Internet voice and video applications are being seen as a cost-effective means of communication for both residential and corporate users. The end- to-end delay, delay variation and packet losses are traffic impairments that influence the audio and visual quality of packet voice and video applications. The characterization of Internet traffic and traffic impairments is therefore an important problem for the testing and implementation of protocols that support real-time applications. This research is focused on developing probabilistic models that can predict the delay and loss experienced by packet streams arising from real-time applications. These models are based on the analysis of measurements of delay and packet loss experienced by probe packets sent out at periodic intervals over the Internet. These measurements provide the seasonal time-of-day patterns of delay and delay variation. The figure depicts the time-of-day variation of the average round-trip delay experienced by a fifteen-minute constant bit rate packet stream when traversing a sequence of fourteen hops between the source and destination on the Internet. The maximum delay typically occur during peak occurs of 10-12 am and 2-5 PM. Predictive models of such patterns comprising of a deterministic cyclic behavior superimposed by stochastic disturbances are being investigated.

Spectrum Sharing

This work examines the blocking performance of narrow-band (NB) and wideband (WB) sources that access a shared pool of wireless channels. The NB sources utilize a single channel, whereas WB sources access simultaneously a larger group of channels. This system is analyzed in the context of maximizing the utilization of spectrum that is allocated to the WB system by allowing NB sources random access to the same spectrum. The performance measure to be controlled is the blocking experienced by WB sources. A queueing model of this system is proposed that allows evaluation of the impact of NB sources on WB performance. Assuming Poisson arrival process models and negative exponential distributions for the channel holding times, the dependence of the blocking probability of the WB source is derived as a function of the NB utilization factor. In an uncontrolled NB access scheme, the WB blocking is lower bounded by the blocking probability experienced by NB sources. The performance improvement afforded by adding extra NB channels and by controlling the access rates of the NB source for finite population WB sources is presented. The analysis presented here provides an assessment of some of the emerging paradigms for adaptive spectrum allocation. Of particular interest are the criteria under which spectrum agile, cognitive and software radios functioning in an opportunistic fashion can access unused spectrum allocated to a primary system without adversely affecting the performance of the primary system.

Engaging graduate students as co-creators of educational modules on an interdisciplinary topic [to appear ASEE 2022]

Co-creation in higher education is the process where students collaborate with instructors in designing the curriculum and associated educational material. This can take place in different scenarios, such as integrating co-creation into an ongoing course, modifying a previously taken course, or while creating a new course. In this work, we investigate training and formative assessment models for preparing graduate students in engineering to participate as co-creators of educational material on an interdisciplinary topic. The topic of cyber-physical systems engineering and product lifecycle management with application to structural health monitoring is considered in this co-creation project. This entails not only topics from different disciplines of Civil, Computer, Electrical and Environmental engineering, Business and Information Sciences, but also humanistic issues of sustainability, environment, ethical and legal concerns in data-driven decision-making that support the control of cyber-physical systems.

Aside from the objective of creating modules accessible to students with different levels of disciplinary knowledge, the goal of this research is to investigate if the co-creation process and the resulting modules also promote interest and engagement in interdisciplinary research. A literature survey of effective training approaches for co-creation and associated educational theories is summarized. For students, essential training components include providing (i) opportunities to align their interests, knowledge, skills and values with the topic presented; (ii) experiential learning on the topic to help develop and enhance critical thinking and question posing skills, and (iii) safe spaces to reflect, voice their opinions, concerns and suggestions. In this research we investigate the adaption of project-based learning (PjBL) strategies and practices to support (i) and (ii) and focus groups for participatory action research (PAR) as safe spaces for reflection, feedback and action in item (iii). The co-creation process will be assessed through qualitative analysis of data collected through the PjBL activities and PAR focus groups and other qualitative data (i.e., focus group transcripts, interview transcripts, project materials, fieldnotes, etc.). The outcome of the co-creation process is an on-line course module that is designed to be integrated in existing engineering graduate and undergraduate courses at four different institutions, that includes two state universities and two that are historically black colleges and universities.

Engaging Women Engineering Undergraduates as Peer Facilitators in Participatory Action Research Focus Group [ASEE 2021]

This study is part of a longitudinal research project examining the design of summer bridge and subsequent undergraduate engineering programs in the College of Engineering at University of Massachusetts Lowell, with the long-term view of how these programs can create more supportive, inclusive environments for women to become engaged as leaders in their educational pathways and future careers. A summer bridge program prepares first-year women engineering students for the academic and cultural opportunities and challenges they may face. Through an immersion in focus groups constructed for participatory action research (PAR), students learn to use their voices for change and also to lead in taking action to improve their experiences in the program. This summer experience is leveraged in the academic year by additional training for a small group of women participants from the summer program to facilitate focus groups open to all undergraduate students in the college of engineering. Using PAR and an asset-based approach emphasizing listening and learning from student voices, this study suggests how engaging women as peer facilitators in PAR focus groups builds leadership and communication skills, as well as increases understanding of student perceptions and experiences in their engineering majors. Further, using the results of a survey administered to all engineering students in their Junior and Senior years on their experiences in their majors, the responses from participants in the summer program are compared with those from students who did not have this opportunity. Women engineering students in the summer program were statistically more likely to see themselves as having stronger critical thinking and communication skills than women who did not participate in this program, but differences between these two groups in perceptions of sense of belonging and leadership skills were not statistically significant.

Participatory Action Research (PAR) as Formative Assessment of a STEM Summer Bridge Program (ASEE 2021 )

Research, Academics and Mentoring Pathways (RAMP) is a six-week summer bridge program offered to incoming female undergraduate engineering students. Initiated in 2018, the goal of this program is to increase the enrollment, retention, and success of female engineering students as they enter the College of Engineering, continue with their studies, and graduate into the workforce. The objectives are to encourage research participation, improve student content knowledge in gateway courses such as Calculus, and increase their sense of belonging, preparedness, and self-efficacy. To understand student perspectives and experiences, we utilized Participatory Action Research (PAR) to construct a series of formative assessments prioritizing the views and participation of the RAMP students themselves. PAR was selected as a research and assessment strategy due to its emphasis on student participation and empowerment linked with action for positive change. Online surveys and four focus groups involved the students in topics geared towards developing a psychologically safe space for sharing experiences, providing feedback on program activities, and reflecting on personal goals, values, and aspirations.

Analysis of the assessment data from these activities and surveys in 2018 revealed concrete suggestions that were used to improve the RAMP program in 2019. Changes made included using peer facilitators for the focus groups, providing workshops in a variety of locations, including more interactive research-based projects, using undergraduates for project support, and retaining several aspects of the 2018 program that were highly rated, such as industry visits, kayaking trips, and financial aid workshops. This iterative process of listening to student feedback and using their suggestions to make program changes was repeated in RAMP 2019, and will be continued in RAMP 2020. Based on our findings, we will identify the insights learned from using PAR as a formative assessment and explain why this approach may be especially helpful in creating more supportive and beneficial environments for women in engineering education.