NSF Graduate Research Fellowship

The present reporting period spans the last part of my second year through my third year as an Aerospace Engineering graduate student at University of Illinois, Urbana-Champaign (UIUC) under the advice of Prof. Theresa Saxton-Fox, PhD. My work throughout this last year presents a focus on exploring underlying mechanisms of organization and interactions between coherent structures in turbulent boundary layers (TBLs) and working towards sharing and publishing those findings. Additionally, I was able to apply what I learned about experimental fluid dynamics and data analysis to collaborate on other projects. Overall, I further created connections between my work and the skills acquired in the past years to current and upcoming projects as well as planned and collaborated in outreach efforts.

Intellectual Merit. Previously, I conducted a series of wind tunnel experiments where a small cylinder was inserted in a turbulent boundary layer developed over a flat plate at different wall-normal distances to study the effects of the interaction between an injected small-scale structure, which are the vortices shed from the cylinder, and the coherent structures present in the TBL, which span lengths in the order of the boundary layer thickness. With the objective of understanding the interactions between structures of different scales, Particle Image Velocimetry (PIV) was used to obtain the velocity field right behind the cylinder for each case. Through the current reporting period, I started exploring different data analysis techniques. Having sets for all the cases I studied be time-resolved and non-time-resolved, there were different approaches possible. Through the non-time resolved data I obtained statistics for mean streamwise velocity, streamwise and wall-normal fluctuations, and Reynolds shear stresses. Interesting distributions were observed as the flow evolved and compared with a TBL without a cylinder immersed at the same freestream velocity. From asymmetry at the cylinder location varying in behavior with wall-normal location to decreases in streamwise fluctuations near the wall happening as the flow evolved, some spatial trends were observed and led to interesting insights. After this, I performed modal analysis techniques such as Spectral Proper Orthogonal Decomposition (SPOD) and Dynamic Mode Decomposition (DMD) to explore connections between structures energetically, which gave some insights but did not fully connect what statistical trends were showing. Later I implemented a technique known as Conditional Projection Averaging (CPA)1, where I average different flow quantities based on a global projection coefficient and a model for a large-scale structure of interest. This work is in preparation to be submitted this Spring to the Physical Review Fluids journal for publication. I also started doing vortex tracking for the vortices shed by the cylinder to observe the change in organization of the flow. With the knowledge on PIV, vortex tracking techniques, and data analysis, I contributed to a project in collaboration with other students in my group and another research group. This work consisted of studying the interaction of the wake of a propeller with a structure, which in this case was an airfoil. The objective was to compare the performance of a propeller to mitigate blade vortex interaction.

Broader Impacts. After doing further analysis on the experimental data I collected, I shared my data at different forums. On May 2022, my advisor presented my work at the 1st Direct In-person Colloquium on Vortex Dominated Flows (DisCoVor) in Switzerland and preliminary results involving Conditional Projection Averaging and flow statistics to the 75th Annual Meeting of the American Physical Society, Division of Fluid Dynamics (APS-DFD) held in Indianapolis, IN from November 20-22, 2023. I am also currently finishing preparation for publication on a paper related to my findings and analysis. Additionally, I contributed to my first conference paper. I wrote about phase-averaged flow statistics for an experimental study on propeller design and interactions with an airfoil2, which was presented at AIAA SCITECH Forum 2023 in National Harbor, MD. Apart of my research efforts, I also did some outreach activities. On December 2023, I planned and presented a virtual panel of women graduate students and introduction to graduate school virtually hosted by the Association of Females in Mechanical Engineering at University of Puerto Rico at Mayagüez. On April 2023, I volunteered for the Women in Aerospace (WIA) student association at UIUC’s Engineering Open House doing wind tunnel demonstrations to the public.

References: 1. Saxton-Fox, T., Lozano-Durán, A., & McKeon, B. J. (2022). Amplitude and wall-normal distance variation of small scales in turbulent boundary layers. Physical Review Fluids, 7(1), 014606. 2. Kopperstad, T. E., Borra, A., Beusse, A., Kim, C., Torres De Jesus, E., Saxton-Fox, T., & Ansell, P. J. (2023). Design of Propellers with Passive Mitigation of Coherent Tip Vortex Roll-up. In AIAA SCITECH 2023 Forum (p. 2614).

The present reporting period spans the last part of my first year through my second year as an Aerospace Engineering graduate student at University of Illinois, Urbana-Champaign (UIUC) with Prof. Theresa Saxton-Fox, PhD as my research advisor. My research aims to fundamentally improve the understanding of the underlying mechanisms behind the organization and interaction of coherent structures in the outer region of a turbulent boundary layer through wind tunnel experiments and novel analysis techniques. The insights my project could give in understanding wall-bounded turbulent flows could mean an important contribution to improving predictive models used for applications such as aircraft design and performance evaluation. These improvements in predictive models have the potential of aiding in the enhancement of aircraft efficiency, representing savings for the industry, while reducing CO2 emissions by optimizing fuel consumption.

Intellectual Merit.

As part of my fellowship application, I proposed an experiment where vorticity would be inserted artificially into a turbulent boundary layer over a flat plate through the placement of a small cylinder that would form a Von Kármán vortex street, a well-known and understood phenomenon. This was proposed to study how the coherent structures organize and interact energetically in the flow, which means observing the patterns in velocity and how these patterns change physically and what that means for the energy of the flow at present. At the same flow conditions, different cases varying the cylinder’s distance from the plate were considered. During this reporting period, I designed and built the experimental setup and learned how to measure the velocity field through two-dimensional, two-component Particle Image Velocimetry (2D2C-PIV), a non-invasive method that uses optical elements and high-speed imaging to measure the velocity on the test section. Through Fall 2021, I conducted the experiments and processed the PIV data obtained for all the cases. For each case, time-resolved and non-time-resolved PIV measurements were done, which broadens the options on methods of analyzing the results. I performed preliminary statistics such as conditional averaging and two-point correlations and compared with measurements in the same test section with the same flow conditions but without the cylinder. The analysis is currently in progress through Spring 2022. Preliminarily, it has been observed that as the flow goes downstream, the vortices shed by the cylinder have a prevalent effect in the fluctuation of the streamwise component of the velocity. This prevalence in the velocity fluctuation has been observed to be less prevalent for the cases closest to the wall, which may imply the effect of the vortices is less strong there. Currently, I aim to do other types of analysis to correlate with the statistics such as modal decomposition methods, to understand the energetic aspect of the flow, and vortex tracking for the structure organization aspect.

Broader Impacts.

After conducting my experiments and obtaining preliminary results from the different cases, it is a priority for me to start disseminating results to practice my scientific communication skills and create collaborations with other researchers. I presented my preliminary work in the 74th Annual Meeting of the American Physical Society, Division of Fluid Dynamics (APS-DFD) held in Phoenix, AZ from November 21-23, 2021. Also, I collaborated with other graduate students in my research group in submissions to national and international conferences. I am second author in two submissions in this reporting period with them: a paper submission that was accepted at the 12th International Symposium on Turbulence and Shear Flow Phenomena (TSFP) and a presentation at the 2022 AIAA Aviation Forum, both events to be held this Summer 2022. Additionally, my advisor will be presenting on findings of my research project in the 1st Direct In-person Colloquium on Vortex Dominated Flows (DisCoVor).

Another priority to me is to support undergraduate and graduate students, especially from underrepresented groups. In Summer 2021, I participated as a panelist on three instances for the Summer Predoctoral Institute, a program which gives incoming graduate students of all disciplines at UIUC opportunity to work on their research and learn about resources on campus. During Fall 2021, I gave individual support to two students from my Alma mater institution, University of Puerto Rico-Mayagüez (UPRM), with the application process for the NSF-GRFP and one of them was awarded this year. Additionally, I was invited by a former teacher, Prof. Sylvia Rodríguez-Abudo, PhD to be a panelist in a webinar titled “Writing a Competitive Graduate Fellowship Application: Finding your Research Idea and Selling Point” targeted to UPRM undergraduate and graduate students applying for various national fellowships.