Mitul Luhar

Interactions between internal solitary waves and surface canopies of varying length and porosity are examined via laboratory experiments and complementary simulations for a miscible, two-layer system. In both approaches, internal solitary waves of varying amplitudes are generated by a jet-array mechanism that is driven by the nonlinear extended Korteweg–de Vries solution. Pycnocline displacements, phase speeds and velocity fields are obtained using synchronised planar laser-induced fluorescence and particle imaging velocimetry systems in the experiment. In the simulations, the canopy is represented as a porous zone with prescribed porosity and hydraulic conductivity determined by the Kozeny–Carman model, which is validated by comparing simulated and measured horizontal velocity profiles. The higher-porosity (transitional) canopy produces a nearly monotonic, albeit minor, amplitude reduction and negligible wave energy dissipation after the interaction. However, the shear layer developed at the bottom edge of the lower-porosity (dense) canopy grows to a strength comparable to the shear sustained by the internal solitary wave profile at the pycnocline. The vortex pair generated by this shear accelerates the upper-layer fluid beneath the canopy, leading to complex nonlinear amplitude modulation and significant wave transformation. With an extended canopy length, the internal solitary waves settle to a quasi-steady state with a significant phase speed reduction. Upon the wave exiting the canopy, flow separation at the downstream edge of the canopy again pairs with the shear at the pycnocline, inducing an intensified jet. This complex interaction leads to energy transfer between kinetic and potential energy under the dense canopy.

Do you remember the moment you decided to become a scientist, engineer, or health professional? Maybe it was while gazing at the stars with your grandparents on a summer night. Maybe it was while building a volcano in grade school, volunteering at a nature center, or seeing a loved one's life transformed by a new treatment. Maybe it was not a single moment, but rather a gradual pathway with a series of experiences. However it happened, one thing is likely true: your decision was grounded in a deeply human experience. You chose science because it interested you and touched life, yours or someone else's. There's a beautiful harmony in that, because science is the study of life and the world around us. It's woven into the human experience, not separate from it. Many of us in science understand this, but we are not always effective at conveying this perspective to our families, friends, neighbors, and the public. That's a loss, not just for science, but for all of us. Because science exists to serve: to make our world better, our communities safer, and our families healthier, thereby enriching our lives with greater access to art, music, and the many other joys of the human experience. At a time when access to critical research and patient care funding is changing, when institutions are being restructured, and when trust in science is wavering, the gap between what we know and what others perceive carries real consequences (1–3). There are likely many ways to help bridge this divide. But here's one small, meaningful place to start: let's simply show up as people. As neighbors, family, and community members, each of us has a story that brought us to science and a hope that science can continue to enrich everyone's lives. Let's be ambassadors and storytellers of scientific discoveries in our fields—not just through facts and findings, but through shared human values and experiences. This strategy reflects insights from the learning sciences, which emphasize the value of a person-centered approach to science, technology, engineering, and medicine (STEM) outreach. Encouraging individuals to connect with STEM through relatable personal stories and real-world industry examples has been shown to boost K-12 student engagement and foster motivation to pursue STEM careers (4). Across age groups, narratives capture attention by engaging emotions and imagination, making information easier to process and more meaningful (5). Because people often gravitate toward content that aligns with their existing beliefs and because the perceived truth of a topic does not always shape reactions, storytelling in science is considered a powerful tool for overcoming these biases (6). By creating emotional and cognitive involvement, narratives can achieve the level of audience connection that traditional science communication frequently struggles to deliver (7). Still, while recognizing the unique strength of narratives, it helps to understand why audiences may respond to the same scientific message in such different ways. Many of the reactions that shape whether information feels convincing, confusing, or dismissible grow out of well-documented cognitive patterns that influence how people process new information. Motivated reasoning research shows that individuals often rely on cognitive processes that favor the conclusions they already prefer, selectively accessing beliefs and strategies that allow those preferred conclusions to feel justified (8). Selective exposure research similarly demonstrates that people tend to seek information that reinforces their existing attitudes, especially when digital environments give them more control over what they see, even though they do not always avoid opposing views outright (9). Work on cultural cognition offers an additional lens, showing that individuals interpret scientific information through the values and identities of the groups they belong to, which helps explain why facts alone often fail to change minds (10). At the same time, trust building can be especially challenging in misinformation-rich environments where false claims continue to shape judgments even after correction, where factual backfire effects are rarer than commonly assumed, and where inoculation-style interventions have shown promise in boosting resilience against misinformation (11–13). The question then becomes how we can use stories in our everyday interactions to break through these barriers, counter misinformation, and help people make sense of the science that shapes their lives. We offer the advice below as scientists, engineers, and health professionals who use science every day and continue to be in awe of new scientific discoveries. As a scientist, your passion and excitement are boundless; you can inspire younger people to pursue a STEM career and help anyone, of any age, connect science to everyday life. In your neighborhood school and broader community, the next generation of innovators is watching the moon, climbing trees, falling down, and asking every question possible. Why can I sometimes see the moon during the day? What makes certain trees grow better than others? What happens to my skin after I scrape it? Freely engage with the curiosities wherever you meet them: in your day-to-day life or in a volunteer organization that relies on scientists to teach and mentor. Be patient, joyful, and silly. When your niece brings you slime to play with, you can mention to her that it's a polymer and it's viscoelastic (or “sticky stretchy”) and then have the patience to explain what those words mean. When you’re helping your friend's child bake a cake, tell them about the chemical reaction between the lemon and the baking soda that produces a gas that makes the cake fluffy. Trust that your excitement and knowledge, once picked up by them, will continue to ripple through their friends, family, and broader communities. Those ripples of curiosity we help create are often carried forward through the stories people remember and share. Research in science communication reinforces this observation. Narratives are one of the ways people naturally make sense of the world, and studies show why they work so well. They can increase interest, engagement, and comprehension, helping people understand and remember scientific ideas more easily than traditional logical explanations (14). A meta-analysis of narrative studies also found consistent persuasive effects, with stories shaping beliefs, attitudes, intentions, and even behaviors in meaningful ways (15). When science is shared through stories that feel connected to real people and real situations, it becomes easier for those ideas to take root and continue spreading. Research indicates that authentic, hands-on activities are also effective in sparking interest in science, particularly among K-12 students (16). For adults, who often learn through free-choice experiences, such as watching documentaries, visiting museums, reading magazines, or exploring online resources, similar principles apply (17). Evidence suggests that while formal education predicts self-reported science and technology knowledge, it is not as strong of a predictor as participation in free-choice learning experiences (18). Therefore, instead of relying solely on formal conversations with the people in our lives, scientists can encourage family and friends to engage in these informal learning opportunities and even join them in activities, such as museum visits or documentary viewings. Equally important is fostering bidirectional conversations, rather than one-way lectures, about these experiences, similar to how people talk about sports or current events. This practice helps integrate science into everyday life and positions it as a natural and engaging part of the social fabric (19). As an engineer, you are constantly contemplating how to build a better world. You recognize that your unique skill set allows you to define and solve problems that, while initially daunting, have the potential to enhance human life. Good engineers are good listeners. Work to engage users and stakeholders in the design process; don’t just build for them, but build solutions “with” them. Understand the limitations of your own knowledge, biases, and assumptions, and work with experts outside your own field who can augment and complement your skill set. Ensure that you are working with communities in mutually beneficial ways and that you are centering your user throughout the design process. Take the time to ask: Why are some steps currently done in a certain way? What constraints are involved? Who should be involved in the design process? Engaging the broader community in the design and development of the products, services, and systems that drive our world will create buy-in and trust in engineered solutions. This advice aligns with ongoing efforts to integrate participatory design and co-design practices into engineering curriculum and practice. Participatory design and co-design methods are both human-centered design approaches that actively involve the communities and individuals who are affected by or will use a product, system, or service in the design process itself (20). Instead of treating users as passive recipients of technology, participatory design emphasizes engagement with marginalized and/or underserved populations, helping to dismantle potential power dynamics between the “designer” and the “designed for” (21, 22). Emerging work suggests that both participatory and co-design methods not only build early stakeholder buy-in but can also result in innovative and technologically advanced solutions, such as buildings with enhanced functional complexity and transparent AI systems (23–25). These ideas also reflect what long-standing community-based participatory research efforts have taught us about working with communities. At the heart of this approach is a clear recognition that power shows up in many forms, including who makes decisions and whose knowledge is valued. Partners in this work emphasize that knowledge itself is a form of power and that naming expertise wherever it exists, sharing information openly, and acknowledging historical inequities all matter (26). They also note that building trust requires more than good intentions. It means working together to create operating norms that support open communication, mutual respect, and shared decision-making, because these practices help address the lack of trust and unequal power relationships that often stand in the way of progress (27, 28). If engineers bring this same mindset to their design partnerships, they can likely strengthen the process and create solutions that reflect the lived experience, priorities, and strengths of the people most affected. As a health professional, you may feel hesitant to bring broader conversations about science into your patient interactions. But the truth is, those conversations are already happening. Patients often arrive having done their own research, bringing questions about new therapies they’ve read or heard about. These moments offer a valuable opportunity not just to clarify facts but to engage in thoughtful dialogue. Sharing insights about scientific studies, including their strengths, limitations, and the importance of ongoing research, can help build trust and reinforce the idea that medicine is constantly evolving in response to new evidence. Rather than limiting a conversation to something like, “We’ll plan to treat your high blood pressure with a medication called amlodipine,” you can more explicitly highlight the role of science by adding that: “Based on research involving many people, this medication is expected to work for you. Science is not always one-size-fits-all, but it helps guide our decisions. I’ll walk you through what that means for you and our plan for monitoring your blood pressure.” By weaving these narratives into everyday care, we not only inform but also empower patients to see themselves as partners in a shared scientific journey. More specifically, narrative communication has been shown to influence health behavior and decision-making through several well-studied mechanisms. These include reducing counterarguing, increasing engagement, and helping health information feel more personal and memorable. Narratives also support observational learning and shape perceptions of what is normal or expected in a given situation (29). In clinical settings, these same mechanisms help explain why patient-centered stories and the ways clinicians bring scientific evidence to life through narrative can meaningfully enhance understanding, motivation, and trust (30). Narrative perspectives also align with the well-documented literature on shared decision-making (SDM), which highlights the importance of clear communication, partnership, and patient involvement. SDM is defined as a collaborative process in which clinicians and patients engage in joint deliberation to make informed healthcare decisions (31). This approach integrates a discussion of the best available clinical evidence and scientific data with the patient's unique values, preferences, and contextual factors. In clinical settings, SDM is increasingly recognized as a critical component of patient-centered care, with empirical evidence suggesting its positive impact on the development of trust within the physician–patient relationship (32). Furthermore, the impact of SDM extends well beyond the traditional patient–physician dyad. Increasingly, SDM is being designed and implemented through interprofessional healthcare teams that include providers across multiple disciplines and levels of care (33). This collaborative model helps promote better integration of services, break down professional silos, and strengthen continuity of care (34). Moreover, SDM is not confined to the patient alone; families can play an active role even when the patient is neither incapacitated nor a minor (35, 36). Recognizing and incorporating family perspectives ensures that decisions reflect the broader context of patient values and support systems, ultimately fostering more holistic and sustainable care outcomes (37). As scientists, engineers, and health professionals, we often see ourselves as contributors to society through our work in advancing knowledge, solving problems, and improving lives. But we are also members of that same society, shaped by its systems and challenges. Our relationship with science is not fixed; it evolves with our experiences, both professional and personal. Consider the story of a parent who spent her adult life in STEM academia. When her first child was diagnosed with a rare, complex genetic condition, she encountered aspects of healthcare and education systems that she had never needed to navigate before. Through her interactions with other caregivers, she gained new perspectives on how people engage with science-informed systems in their everyday lives. These conversations were shaped by shared hopes, concerns, and a deep desire to advocate for their children. Her commitment to science remained strong, but her understanding of its role in people's lives became more nuanced. She recognized that the values often emphasized in professional spaces, such as precision, innovation, and achievement, must be complemented by compassion, inclusion, and a recognition of diverse lived experiences. Similar eye-opening experiences happen when doctors or other healthcare workers become patients themselves (38). Going through illness from the patient's perspective often uncovers gaps in communication, empathy, and system design that were not seen before. These experiences can increase appreciation for patient-centered care and stress the importance of SDM and trust (39). They remind us that knowledge does not protect us from vulnerability. Humility and listening are important in every interaction. As our lives unfold, we all encounter moments that challenge our assumptions and expand our understanding. These moments are not separate from our scientific and clinical work. They are part of it. They remind us that science is not only something we practice in our professions but also something we experience in our lives. By embracing this, we can help ensure that science remains relevant, trusted, and responsive to the needs of all people. As STEM professionals, we have the opportunity and the responsibility to advance knowledge in ways that serve both the research community and society. It's helpful to revisit that purpose from time to time and reflect on what it means for us to meet society's current needs—recognizing that the path forward may not always be straightforward and that communicating our role is part of the challenge. In the 1800s, James Clerk Maxwell formulated the theoretical equations that predicted the existence of electromagnetic waves, which were experimentally confirmed later that century by Heinrich Hertz (40). Neither was driven by commercial application. Instead, both were compelled by a desire to understand the natural world. Yet, their discoveries laid the foundation for one of the most transformative technologies of the 20th century: Guglielmo Marconi's wireless telegraph, the precursor to modern radio communication (41). History is full of such stories of foundational scientific breakthroughs that quietly seeded world-changing innovations like GPS, AI, the internet, and life-saving vaccines (42,43) . Unfortunately, most of society remains unaware of these origin stories, and thus, many people fail to assign the same value to basic science that scientists instinctively do. They overlook what Abraham Flexner so powerfully called “the usefulness of useless knowledge.” In today's rapidly evolving world, it is not enough for scientists to make discoveries. As we work with other disciplines and sectors, we must also be good listeners and become storytellers. We must embrace our humanity and own personal vulnerabilities as we share the journey of scientific exploration, the wonder of curiosity-driven research, and the limitless possibilities it unlocks. By leading with our humanity and sharing stories, we enable society to see not only what science accomplishes but also why it truly matters. The authors thank Alison Boland-Reeves for her editorial input and assistance with the editing of the manuscript. The authors declare no conflicts of interest. All authors are members of the New Voices Program at the National Academies of Sciences, Engineering, and Medicine. The content of the manuscript reflects the authors’ views and not the views of their institutions or the National Academies of Sciences, Engineering, and Medicine.

This work models flight mission segments for battery-powered electric aircraft. Battery specific energy (BSE) requirements at aircraft level are identified to make electric vertical take-off and landing (eVTOL) and electric conventional take-off and landing (eCTOL) commuter aircraft feasible under future technology scenarios. For eVTOLs, the analysis pays particular attention to the critical transition segment from vertical takeoff to horizontal fixed-wing flight to calculate power and energy requirements. For a 5-person vehicle on a 37.5-nmi (69.5-km) mission similar to that of the Joby S4, the transition segment requires an instantaneous power of 980 kW, largely due to the induced drag. Due to this high power, the climb has an energy consumption similar to that of the cruise, despite being shorter in duration. The eCTOL aircraft is sized by converting a fuel-burning commuter aircraft (the Cessna 208B Grand Caravan) and quantifying changes in the range and total weight as a function of the BSE and empty weight fraction (EWF). Both aircraft can fly longer-range missions as BSE increases, and as EWF and hence, total aircraft weight decrease. For eVTOL and eCTOL aircraft to be feasible, the required effective BSE is estimated to be 400 Wh/kg and 300 Wh/kg respectively. A dimensionless metric -- the energy consumed per payload weight-range (EPPR) -- is used to compare the performance of existing and future aircraft. For eVTOL cases, the EPPR shows asymptotic behavior at higher range and BSE and plateaus at 0.8. The eCTOL cases follow a similar trend, but the EPPR plateaus at 0.28 regardless of the EWF values. Results show that the EPPR for the modeled eCTOL aircraft shows energy efficiency gains over the conventional Cessna 208B for ranges higher than 200 km and an effective BSE of 300 Wh/kg.

Linear-stability modelling suggests that all sufficiently large riblets promote maximally growing spanwise rollers (García-Mayoral & Jiménez 2011 J. Fluid Mech. vol. 678, 317–347), yet direct numerical simulations (DNS) have shown that this is not the case (Endrikat et al. 2021 J. Fluid Mech. vol. 913, A37) some riblet shapes do not form spanwise rollers at all. Thus, the drag-reduction breakdown across all riblet shapes cannot be solely attributed to maximally growing spanwise rollers, prompting a reappraisal of the modelling. In this paper, comparing DNS data with riblet-resolving linear-stability predictions shows that the spanwise rollers are actually marginal modes, not maximally growing instabilities. This riblet-resolved linear analysis also predicts that not all riblet shapes promote spanwise rollers, in agreement with DNS, and unlike earlier linear-stability modelling, which relied on a one-dimensional (1-D) mean flow and on an over-simplified effective wall-admittance boundary condition. These riblet-resolved calculations further inform how to capture the effect of the riblet shape in a 1D model. Once captured, predictions with an effective boundary condition match riblet-resolved results, but still do not indicate what features of the riblet geometry promote the roller instability. Thus, the wall admittance is measured near the riblet crests, in both the riblet-resolved linear analysis and DNS, to show that the in-groove dynamics is dominated by a balance between the overlying pressure and unsteady inertia, and not viscous diffusion, as previously assumed. This pressure–unsteady-inertia balance sets the linear scaling of the wall admittance with riblet size, as observed in DNS, and is a key factor in setting the streamwise wavelength of the spanwise rollers. Furthermore, modelling this pressure–unsteady-inertia balance in the wall admittance reveals the role of riblet slenderness in promoting spanwise rollers, which provides the missing link in previous correlations between the riblet geometry and the presence or lack of rollers.

Internal solitary waves (ISWs) consist of a non-periodic single-crest profile resulting from the balance between non-linearity and dispersion. They can be a significant source of momentum transport in any stratified systems, such as oceans and estuaries. Previous experiments have primarily utilized lock- release mechanisms to generate internal solitary waves in two-layer systems. This provides limited control over wave properties and limits its studies with barotropic wave interactions. The present effort attempts to validate the performance of a new wave generation method, termed the Jet Array Wave Maker (JAW). Experiments show that the JAW system reliably generates ISWs with interfacial displacements and velocity fields in close agreement with theoretical predictions. Future experimental work will target surface solitary waves (SSWs) interaction with ISWs.

Abstract High‐resolution measurements of nutrients in rivers are vital to assess water quality and catchment material balances. Yet, such measurements are often cost‐prohibitive. To improve sampling efficiency, data‐driven sparse sensing (DSS) is proposed to recover high‐resolution nutrient time‐series from sparse flow and concentration measurements. DSS leverages dimension‐reduction to identify basis functions that optimally represent available data, and analyzes these basis functions to identify optimal times and locations for future measurements. A model trained on high‐resolution flow and concentration measurements from few locations accurately reconstructed nutrient concentration time‐series and annual loads at target sites spanning the Midwest region of the US. Optimal sampling times occurred in spring, while sampling locations were distributed across catchment area and flow. Sparse measurements (20–80 per year) at optimal sampling times and locations were sufficient to accurately estimate nutrient concentrations and loads (error <±2% for NOx; <±9% for total phosphorus). DSS promises to enable cost‐effective water quality monitoring.

Abstract This paper evaluates the experimental generation of internal solitary waves (ISWs) in a miscible two-layer system with a free surface using a jet-array wavemaker (JAW). Unlike traditional gate-release experiments, the JAW system generates ISWs by forcing a prescribed vertical distribution of mass flux. Experiments examine three different layer-depth ratios, with ISW amplitudes up to the maximum allowed by the extended Korteweg-de Vries (eKdV) solution. Phase speeds and wave profiles are captured via planar laser-induced fluorescence and the velocity field is measured synchronously using particle imaging velocimetry. Measured properties are directly compared with the eKdV predictions. As expected, small- and intermediate-amplitude waves match well with the corresponding eKdV solutions, with errors in amplitude and phase speed below 10%. For large waves with amplitudes approaching the maximum allowed by the eKdV solution, the phase speed and the velocity profiles resemble the eKdV solution while the wave profiles are distorted following the trough. This can potentially be attributed to Kelvin-Helmholtz instabilities forming at the pycnocline. Larger errors are generally observed when the local Richardson number at the JAW inlet exceeds the threshold for instability.

This paper evaluates the experimental generation of internal solitary waves (ISWs) in a miscible two-layer system with a free surface using a jet-array wavemaker (JAW). Unlike traditional gate-release experiments, the JAW system generates ISWs by forcing a prescribed vertical distribution of mass flux. Experiments examine three different layer-depth ratios, with ISW amplitudes up to the maximum allowed by the extended Korteweg-de Vries (eKdV) solution. Phase speeds and wave profiles are captured via planar laser-induced fluorescence and the velocity field is measured synchronously using particle imaging velocimetry. Measured properties are directly compared with the eKdV predictions. As expected, small- and intermediate-amplitude waves match well with the corresponding eKdV solutions, with errors in amplitude and phase speed below 10%. For large waves with amplitudes approaching the maximum allowed by the eKdV solution, the phase speed and the velocity profiles resemble the eKdV solution while the wave profiles are distorted following the trough. This can potentially be attributed to Kelvin-Helmholtz instabilities forming at the pycnocline. Larger errors are generally observed when the local Richardson number at the JAW inlet exceeds the threshold for instability.

We consider the relationship between Eulerian modal decompositions and Lagrangian coherent structures (LCSs). The model sensitivity framework developed by Kaszás and Haller (2020) is used to express data-driven modal representations of fluid flow in a Lagrangian space. The method, based on the computation of the finite-time Lyapunov exponent, computes the amplitude perturbations experienced by fluid particles due to specific modal components of the flow. Demonstrations of the method are presented for both periodic and turbulent flows, including experimental data from the wake past an oscillating foil, numerical data of the classical cylinder wake flow, and a direct numerical simulation (DNS) of a turbulent channel flow. This method provides a way to understand how Eulerian mode structures interact dynamically with features of the Lagrangian coherent structure across scales, offering additional physical insight into modal decompositions.

The global challenges of climate change and water scarcity demand increased energy efficiency of desalination technologies, especially the dominant seawater reverse osmosis (SWRO). While many studies have assessed individual improvements to SWRO like energy recovery devices, their combined potential for maximizing system energy efficiency has not been systematically explored. Therefore, this study combines a comprehensive data collection from 64 facilities, with detailed modeling of conventional, state-of-the-art, and practical minimum energy use configurations. For each of these three ef-ficiency levels, we synthesized performances benchmarks for pump efficiency, membrane permeability, membrane spacer mass transfer coefficient, and pre-and post treatment. We use these parameters to benchmark potential ef-ficiency gains of 39 of the facilities, examine performance across different recovery ratio and feed salinity, and prioritize the most promising innovations. The methodology encompasses detailed process-and component-level modeling, and includes the derivation of a maximum membrane mass transfer coefficient from the Chilton-Colburn J-factor analogy. We found that the combined state-of-the-art methods, including semi-batch reverse osmo-∗

Abstract Previous theoretical and simulation results indicate that anisotropic porous materials have the potential to reduce turbulent skin friction in wall-bounded flows. This study experimentally investigates the influence of anisotropy on the drag response of porous substrates. A family of anisotropic periodic lattices was manufactured using 3D printing. Rod spacing in different directions was varied systematically to achieve different ratios of streamwise, wall-normal, and spanwise bulk permeabilities ( $$\kappa _{xx}$$ <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML"> <mml:msub> <mml:mi>κ</mml:mi> <mml:mrow> <mml:mi>xx</mml:mi> </mml:mrow> </mml:msub> </mml:math> , $$\kappa _{yy}$$ <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML"> <mml:msub> <mml:mi>κ</mml:mi> <mml:mrow> <mml:mi>yy</mml:mi> </mml:mrow> </mml:msub> </mml:math> , and $$\kappa _{zz}$$ <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML"> <mml:msub> <mml:mi>κ</mml:mi> <mml:mrow> <mml:mi>zz</mml:mi> </mml:mrow> </mml:msub> </mml:math> ). The 3D printed materials were flush-mounted in a benchtop water channel. Pressure drop measurements were taken in the fully developed region of the flow to systematically characterize drag for materials with anisotropy ratios $$\frac{\kappa _{xx}}{\kappa _{yy}} \in [0.035,28.6]$$ <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML"> <mml:mrow> <mml:mfrac> <mml:msub> <mml:mi>κ</mml:mi> <mml:mrow> <mml:mi>xx</mml:mi> </mml:mrow> </mml:msub> <mml:msub> <mml:mi>κ</mml:mi> <mml:mrow> <mml:mi>yy</mml:mi> </mml:mrow> </mml:msub> </mml:mfrac> <mml:mo>∈</mml:mo> <mml:mrow> <mml:mo>[</mml:mo> <mml:mn>0.035</mml:mn> <mml:mo>,</mml:mo> <mml:mn>28.6</mml:mn> <mml:mo>]</mml:mo> </mml:mrow> </mml:mrow> </mml:math> . Results show that all materials lead to an increase in drag compared to the reference smooth wall case over the range of bulk Reynolds numbers tested ( $$\hbox {Re}_b \in [500,4000]$$ <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML"> <mml:mrow> <mml:msub> <mml:mtext>Re</mml:mtext> <mml:mi>b</mml:mi> </mml:msub> <mml:mo>∈</mml:mo> <mml:mrow> <mml:mo>[</mml:mo> <mml:mn>500</mml:mn> <mml:mo>,</mml:mo> <mml:mn>4000</mml:mn> <mml:mo>]</mml:mo> </mml:mrow> </mml:mrow> </mml:math> ). However, the relative increase in drag is lower for streamwise-preferential materials. We estimate that the wall-normal permeability for all tested cases exceeded the threshold identified in previous literature ( $$\sqrt{\kappa _{yy}}^+&gt; 0.4$$ <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML"> <mml:mrow> <mml:msup> <mml:msqrt> <mml:msub> <mml:mi>κ</mml:mi> <mml:mrow> <mml:mi>yy</mml:mi> </mml:mrow> </mml:msub> </mml:msqrt> <mml:mo>+</mml:mo> </mml:msup> <mml:mo>&gt;</mml:mo> <mml:mn>0.4</mml:mn> </mml:mrow> </mml:math> ) for the emergence of energetic spanwise rollers similar to Kelvin–Helmholtz vortices, which can increase drag. The results also indicate that porous walls exhibit a departure from laminar behavior at different values for bulk Reynolds numbers depending on the geometry. Graphical abstract

We present experiments on oscillating hydrofoils undergoing combined heaving and pitching motions, paying particular attention to connections between propulsive efficiency and coherent wake features extracted using modal analysis. Time-averaged forces and particle image velocimetry measurements of the flow field downstream of the foil are presented for a Reynolds number of $Re=11\times 10^3$ and Strouhal numbers in the range $St=0.16\unicode{x2013}0.35$ . These conditions produce 2S and 2P wake patterns, as well as a near-momentumless wake structure. A triple decomposition using the optimized dynamic mode decomposition method is employed to identify dominant modal components (or coherent structures) in the wake. These structures can be connected to wake instabilities predicted using spatial stability analyses. Examining the modal components of the wake provides insightful explanations into the transition from drag to thrust production, and conditions that lead to peak propulsive efficiency. In particular, we find modes that correspond to the primary vortex development in the wakes. Other modal components capture elements of bluff body shedding at Strouhal numbers below the optimum for peak propulsive efficiency and characteristics of separation for Strouhal numbers higher than the optimum.

Turbulent flows over porous lattices consisting of rectangular cuboid pores are investigated using scale-resolving direct numerical simulations. Beyond a certain threshold which is primarily determined by the wall-normal Darcy permeability, ${{\mathsf{K}}_y}$ , near-wall turbulence transitions from its canonical regime, marked by the presence of streak-like structures, to another marked by the presence of Kelvin–Helmholtz-like (K–H-like) spanwise-coherent structures. The threshold agrees well with that previously established in studies where permeable-wall boundary conditions had been used as surrogates for a porous substrate (Gómez-de Segura &amp; García-Mayoral, J. Fluid Mech. , vol. 875, 2019, pp. 124–172). In the smooth-wall-like regime, none of the investigated substrates demonstrate any reduction in drag relative to a smooth-wall flow. At the permeable surface, a notable component of the flow is that which adheres to the pore geometry and undergoes modulation by the turbulent scales of motions due to the interaction mechanism described by Abderrahaman-Elena et al. ( J. Fluid Mech. , vol. 865, 2019, pp. 1042–1071). Its resulting effect can be quantified in terms of an amplitude modulation (AM) using the approach of Mathis et al. ( J. Fluid Mech. , vol. 628, 2009, pp. 311–337). This pore-coherent flow component persists throughout the porous substrate, highlighting the importance of a given substrate's microstructure in the presence of an overlying turbulent flow. This geometry-related aspect of the flow is not accounted for when continuum-based models for a porous medium or effective representations of them, such as wall boundary conditions, are used. The intensity of the AM effect is enhanced in the K–H-like regime and becomes strengthened with larger permeability. As a result, structured porous materials may be designed to exploit or mitigate these flow features depending upon the intended application.

Abstract Characterizing the mechanisms that contribute to the onset of drag increase over micro-grooves (riblets) as the spacing increases is critical to design strategies for riblet-based drag reduction. This study decomposes the roughness function to investigate different mechanisms associated with the breakdown of drag reduction as riblet spacing is increased. We obtain the roughness function through direct numerical simulations (DNS) in a minimal channel and restricted nonlinear (RNL) models. Both the traditional RNL decomposition and an augmented RNL (ARNL) model that includes additional nonlinear interactions are employed as computationally tractable, reduced order representations of the flow field. RNL and ARNL results are compared to those of DNS in minimal channels to investigate the role of the different scale-dependent nonlinear interactions contributing to the roughness function. A comparison of the co-spectra arising from the minimal channel DNS with that from RNL and ARNL simulations indicates that general trends are captured by both reduced order models. However, the additional nonlinearity introduced in the ARNL model produces closer correspondence in the observed structural features of the DNS results. In particular, the ARNL better captures the signatures of the dispersive flow and the texture-coherent fluctuations. There is also a noticeable improvement observed in the profiles of the added stress contributions obtained with the ARNL model versus the RNL model.

Abstract The global challenges of climate change and water scarcity demand increased energy efficiency of desalination technologies, especially the dominant seawater reverse osmosis (SWRO). While many studies have assessed individual improvements to SWRO like energy recovery devices, their combined potential for maximizing system energy efficiency has not been systematically explored. Therefore, this study combines a comprehensive data collection from 64 facilities, with detailed modeling of conventional, state-of-the-art, and practical minimum energy use configurations. For each of these three efficiency levels, we synthesized performances benchmarks for pump efficiency, membrane permeability, membrane spacer mass transfer coefficient, and pre- and post treatment. We use these parameters to benchmark potential efficiency gains of 39 of the facilities, examine performance across different recovery ratio and feed salinity, and prioritize the most promising innovations. The methodology encompasses detailed process- and component-level modeling, and includes the derivation of a maximum membrane mass trans- fer coefficient from the Chilton-Colburn J-factor analogy. We found that the combined state-of-the-art methods, including semi-batch reverse osmosis, could save 69% of the energy excess beyond the thermodynamic minimum, while the practical minimum, including batch RO, could save 82% of excess energy. The compiled data also show that modern facilities achieve a lower specific energy consumption (SEC) when using isobaric–energy recovery devices (ERD) compared to older devices like Pelton turbines. Factors affecting SEC and energy savings (i.e., capacity, equipment efficiency, and year of installation) are also analyzed for 39 of the SWRO facilities. Further substantial savings can be obtained by improving membrane spacers and shifting to more batch efficient configurations

We present experiments on oscillating hydrofoils undergoing combined heaving and pitching motions, paying particular attention to connections between propulsive efficiency and coherent wake features extracted using modal analysis. Time-averaged forces and particle image velocimetry (PIV) measurements of the flow field downstream of the foil are presented for a Reynolds number of Re=11$\times$10$^3$ and Strouhal numbers in the range St=0.16-0.35. These conditions produce 2S and 2P wake patterns, as well as a near-momentumless wake structure. A triple decomposition using the optimized dynamic mode decomposition (opt-DMD) method is employed to identify dominant modal components (or coherent structures) in the wake. These structures can be connected to wake instabilities predicted using spatial stability analyses. Examining the modal components of the wake provides insightful explanations into the transition from drag to thrust production, and conditions that lead to peak propulsive efficiency. In particular, we find modes that correspond to the primary vortex development in the wakes. Other modal components capture elements of bluff body shedding at Strouhal numbers below the optimum for peak propulsive efficiency and characteristics of separation for Strouhal numbers higher than the optimum.

View Video Presentation: https://doi.org/10.2514/6.2023-3446.vid This paper applies structured input-output (I/O) methods to analyze turbulent channel flows over riblets. Structured I/O provides a reduced-complexity framework for accounting for the nonlinear terms in the Navier-Stokes equations, thus providing a greater degree of fidelity in the analysis of nonlinear flows than the related linear I/O and resolvent-based analysis techniques. We show that structured I/O is capable of predicting I/O gains relevant to the nonlinear flow physics as well as the associated modes that reveal the underlying instability mechanisms driving turbulent flows over riblets. Particularly, we show that the structured I/O analysis highlights dominant instabilities associated with the lift-up mechanism, Tollmien-Schlichting waves, and Kelvin-Helmholtz-type vortices for the turbulent channel flow model. The associated wavenumbers identified for these instabilities are physically consistent with the results found in prior DNS studies. Furthermore, we show that the rectangular and triangular riblets analyzed in this study minimize the I/O gains of the near-wall cycle, leading to the emergence of new smaller-scale structures that have been highlighted in previous DNS studies. Results and findings are reported and discussed for turbulent channel flows at Re = 180 with smooth-walls and both rectangular and triangular riblet patterning.

View Video Presentation: https://doi.org/10.2514/6.2023-3448.vid Characteristics of flow through anisotropic porous materials (APMs) with periodic lattice microstructures are investigated. Periodic cubic lattice samples are designed with various projected area porosities along the principal axes of the sample. Isotropic porous lattices of comparable porosities are designed as well, as a mechanism for further validating APM data and use of the data analysis model used here. One-inch cubic samples are fabricated using stereolithography printing and installed at the end of a long square duct capable of producing fully developed laminar flow up to Re = 2000 based on the hydraulic diameter and bulk velocity of the duct. Experiments are also performed without the duct, with the specimen exposed to approximately uniform flow at the flow conditioner exit. The Darcy-Forchheimer model is used as the basis for data analyses with consideration to nonlinear inertial effects at high Re. The exact point when inertial effects are larger than viscous effects depends on the porosity of the design. Experimental data are analyzed for various designs with high (∼ 0.85), medium (∼ 0.50), and low (∼ 0.15) projected area porosities and compared with select Direct Numerical Simulation (DNS) data. Non-dimensional data are analyzed using two methods, 1) a single projected zone and 2) a zonal weighting approach based on the various flow blockage regions. The data are used to investigate scaling laws that may be useful for APM designs in future passive flow control applications.

Abstract This study seeks to provide physical insight into the friction-driven crawling locomotion of systems with radially symmetric bodies. Laboratory experiments with a tripedal robot show that both translation and rotation can be achieved with just three independently actuated rigid limbs, i.e., with 3 degrees-of-freedom. These observations are rationalized using a simple mathematical model, which assumes that the friction at each limb is linearly proportional to the normal force at the contact point, and opposes the direction of motion. This dynamic model reproduces experimental observations across an extensive parametric sweep involving sinusoidal rotation of the limbs with varying amplitudes and phase shifts. Model predictions highlight the role played by time-varying normal forces at the contact points. These predictions are confirmed using embedded force transducers in the limbs. We present a further simplified analysis explaining that a geometric nonlinearity is induced in the dynamics from the radial symmetry and that this nonlinearity is essential to the generation of pure translation. We also show that this nonlinearity can be amplified by a cyclic time-varying limb length variation. These results provide a framework for further study of radially symmetric movers.

Abstract Many rivers and streams are ungauged or poorly gauged and predicting streamflow in such watersheds is challenging. Although streamflow signals result from processes with different frequencies, they can be “sparse” or have a “lower‐dimensional” representation in a transformed feature space. In such cases, if this appropriate feature space can be identified from streamflow data in gauged watersheds by dimensionality reduction, streamflow in poorly gauged watersheds can be predicted with a few measurements taken. This study utilized this framework, named data‐driven sparse sensing (DSS), to predict daily‐scale streamflow in 543 watersheds across the contiguous United States. A tailored library of features was extracted from streamflow training data in watersheds within the same climatic region, and this feature space was used to reconstruct streamflow in poorly gauged watersheds and identify the optimal timings for measurement. Among different regions, streamflow in snowmelt‐dominated and baseflow‐dominated watersheds (e.g., Rocky Mountains) was more effectively predicted with fewer streamflow measurements taken. The prediction efficiency in some rainfall‐dominated regions, for example, New England and the Pacific coast, increased significantly with an increasing number of measurements. The spatial variability of prediction efficiency can be attributed to the process‐driven mechanisms and the dimensionality of watershed dynamics. Storage‐dominated systems are lower‐dimensional and more predictable than rainfall‐dominated systems. Measurements taken during periods with large streamflow magnitudes and/or variances are more informative and lead to better predictions. This study demonstrates that DSS can be an especially useful technique to integrate ground‐based measurements with remotely sensed data for streamflow prediction, sensor placement, and watershed classification.