Waterloo Research

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Now showing 1 - 20 of 2517

Antarctic basal environment shaped by high-pressure flow through a subglacial river system
(Springer Nature, 2022-10-27) Dow, Christine F.; Ross, Neil; Jeofry, Muhammad Hafeez; Siu, Kevin; Siegert, Martin J.
The stability of ice sheets and their contributions to sea level are modulated by high-pressure water that lubricates the base of the ice, facilitating rapid flow into the ocean. In Antarctica, subglacial processes are poorly characterized, limiting understanding of ice-sheet flow and its sensitivity to climate forcing. Here, using numerical modelling and geophysical data, we provide evidence of extensive, up to 460 km long, dendritically organized subglacial hydrological systems that stretch from the ice-sheet interior to the grounded margin. We show that these channels transport large fluxes (~24 m3 s−1) of freshwater at high pressure, potentially facilitating enhanced ice flow above. The water exits the ice sheet at specific locations, appearing to drive ice-shelf melting in these areas critical for ice-sheet stability. Changes in subglacial channel size can affect the water depth and pressure of the surrounding drainage system up to 100 km either side of the primary channel. Our results demonstrate the importance of incorporating catchment-scale basal hydrology in calculations of ice-sheet flow and in assessments of ice-shelf melt at grounding zones. Thus, understanding how marginal regions of Antarctica operate, and may change in the future, requires knowledge of processes acting within, and initiating from, the ice-sheet interior.
Totten Glacier subglacial hydrology determined from geophysics and modeling
(Elsevier, 2020-02-01) Dow, Christine F.; McCormack, Felicity S.; Young, Duncan A.; Greenbaum, Jamin S.; Roberts, Jason L.; Blankenship, Donald D.
Aurora Subglacial Basin (ASB), which feeds Totten Glacier, is a marine basin lying below sea level and contains up to 3.5 m of global sea level equivalent. Rates of future sea level rise from this area are primarily dependent on the stability of Totten Ice Shelf and the controls on ice flow dynamics upstream of the grounding line, both of which may be influenced by subglacial hydrology. We apply the GlaDS subglacial hydrology model to ASB to examine whether the spatial patterns of distributed and efficient drainage systems impact the dynamics of Totten Glacier. We determine the most appropriate model configuration from our series of sensitivity tests by comparing the modeled basal water pressure and water depth results with specularity content data. Those data are derived from ICECAP radar surveys over the same region and represent regions of basal water accumulation. The best match between simulated basal hydrology properties and specularity content shows a strong correspondence in regions of distributed water in the ASB troughs for both water depth and water pressure, but weak correspondence between water depth and specularity content near the grounding line. This may be due to the presence of several large channels draining over the grounding line into the head of Totten Ice Shelf, which are likely not as well represented in the specularity content data as distributed systems. These channels may have a significant impact on melt, and therefore the stability, of Totten Ice Shelf. Within ASB, regions of high water pressure and greater water accumulation correspond well with regions of faster ice flow, suggesting some control of basal hydrology on ice dynamics in this region.
Induced subgraphs and tree decompositions XIV. Non-adjacent neighbours in a hole.
(Elsevier, 2025-02) Chudnovsky, Maria; Hajebi, Sepehr; Spirkl, Sophie
A clock is a graph consisting of an induced cycle C and a vertex not in C with at least two non-adjacent neighbours in C. We show that every clock-free graph of large treewidth contains a "basic obstruction" of large treewidth as an induced subgraph: a complete graph, a subdivision of a wall, or the line graph of a subdivision of a wall.
Polynomial Bounds for Chromatic Number. IV: A Near-polynomial Bound for Excluding the Five-vertex Path.
(Springer, 2023-09-15) Scott, Alex; Seymour, Paul; Spirkl, Sophie
A graph G is H -free if it has no induced subgraph isomorphic to H. We prove that a P5-free graph with clique number ω ≥ 3 has chromatic number at most ωlog2(ω). The best previous result was an exponential upper bound (5/27)3ω, due to Esperet, Lemoine, Maffray, and Morel. A polynomial bound would imply that the celebrated Erd˝os-Hajnal conjecture holds for P5, which is the smallest open case. Thus, there is great interest in whether there is a polynomial bound for P5-free graphs, and our result is an attempt to approach that.
Pure Pairs. VIII. Excluding a Sparse Graph.
(Springer, 2024-08-05) Scott, Alex; Seymour, Paul; Spirkl, Sophie
A pure pair of size t in a graph G is a pair A, B of disjoint subsets of V(G), each of cardinality at least t, such that A is either complete or anticomplete to B. It is known that, for every forest H, every graph on n ≥ 2 vertices that does not contain H or its complement as an induced subgraph has a pure pair of size (n); furthermore, this only holds when H or its complement is a forest. In this paper, we look at pure pairs of size n1−c, where 0 < c < 1. Let H be a graph: does every graph on n ≥ 2 vertices that does not contain H or its complement as an induced subgraph have a pure pair of size (|G| 1−c)? The answer is related to the congestion of H, the maximum of 1 − (|J | − 1)/|E(J )| over all subgraphs J of H with an edge. (Congestion is nonnegative, and equals zero exactly when H is a forest.) Let d be the smaller of the congestions of H and H. We show that the answer to the question above is “yes” if d ≤ c/(9 + 15c), and “no” if d > c.
Induced subgraphs and tree decompositions VI. Graphs with 2-cutsets
(Elsevier, 2025-01) Abrishami, Tara; Chudnovsky, Maria; Hajebi, Sepehr; Spirkl, Sophie
This paper continues a series of papers investigating the following question: which hereditary graph classes have bounded treewidth? We call a graph t-clean if it does not contain as an induced subgraph the complete graph Kt, the complete bipartite graph Kt,t, subdivisions of a (t x t)-wall, and line graphs of subdivisions of a (t x t)-wall. It is known that graphs with bounded treewidth must be t-clean for some t; however, it is not true that every t-clean graph has bounded treewidth. In this paper, we show that three types of cutsets, namely clique cutsets, 2-cutsets, and 1-joins, interact well with treewidth and with each other, so graphs that are decomposable by these cutsets into basic classes of bounded treewidth have bounded treewidth. We apply this result to two hereditary graph classes, the class of (ISK4, well)-free graphs and the class of graphs with no cycle with a unique chord. These classes were previously studied and decomposition theorems were obtained for both classes. Our main results are that t-clean (ISK4, wheel)-free graphs have bounded treewidth and that t-clean graphs with no cycle with a unique chord have bounded treewidth.
Current Research on Gaseous Ammonia Detecting and Capture Technologies
(Elsevier, 2023-10-27) Akhand, Aamer; Wu, Xiao-Yu
Ammonia is currently widely used for fertilizers, and it continues to grow as a potential clean fuel source or hydrogen carrier, which may result in growing trace amounts in the atmosphere. Increasing anthropogenic ammonia emissions raises concerns such as disrupting the global nitrogen cycle and negating the greenhouse gas reduction. This review article outlines and compiles recent studies on gaseous ammonia sensing and absorption/adsorption, which could alert or reduce rogue emissions, i.e., unintended anthropogenic releases of ammonia. Different sensors and their characteristics are outlined with a focus on the more popular chemoresistive technologies. Several absorption/adsorption methods from metal-organic frameworks (MOFs) and deep eutectic solvents (DESs) are described and highlighted.
Validated Dynamic Stall Simulation of Pitching Low Reynolds Number Airfoils
(American Institute of Aeronautics and Astronautics, 2024-08) Zilstra, Alison; Johnson, David A.
Deep dynamic stall is one of several complex behaviors that result in extreme variation of the aerodynamic loads on small wind turbine (SWT) blades during unsteady wind conditions. In this study, unsteady Reynolds-averaged Navier–Stokes simulations are performed for two low Reynolds number (Re) airfoils where sinusoidal pitching is applied to replicate the dynamic stall that occurs on rotating SWT blades. The SD 7037 airfoil is simulated at Re=4.1x10⁴ and a pitching reduced frequency of k=0.08, and the S833 airfoil is at Re=1.7x10⁵ and k=0.06. The simulated lift coefficient and dynamic stall timing agree with experimental data, which is attributed to the wall-normal resolution of the mesh and is an advancement from the early prediction of stall seen consistently in previous numerical studies. The accurate prediction of dynamic stall is found to be dependent on the correct simulation of the bursting of the laminar separation bubble (LSB), which initiates the complete separation of the boundary layer and the formation of a leading-edge vortex. The γ-Reθ,t k-⍵ model combined with the use of a fine mesh at the airfoil leading edge results in an accurate simulation of the bursting LSB and the correct prediction of the deep dynamic stall.
The current state of integrating equity, diversity and inclusion into knowledge mobilization: a systematic literature review
(Emerald Publishing, 2024-08-15) Cornelius-Hernandez, Tracelyn; Clarke, Amelia
Purpose – The purpose of this paper is to systematically review and analyze the academic literature on integrating equity, diversity, and inclusion (EDI) into knowledge mobilization (KMb). Design/methodology/approach – This systematic literature review of the body of scholarly literature published on integrating EDI with KMb follows established methods and protocols proposed by Popay et al. (2006) and Page et al. (2021). Using a relevant keyword string, a search was conducted in ProQuest and SCOPUS to find peer-reviewed articles, which were then screened using predetermined inclusion and exclusion criteria. Finally, inductive and deductive analyses were conducted on the selected articles. Findings – The findings suggest that most of the authors are based in the Global North, the majority of literature was published in the last two years, and that it is conceptual. This synthesis highlights five solution-oriented themes: acknowledging inherent bias, centering marginalized groups, promoting responsible knowledge mobilization, establishing partnerships, and advocating for transformational and systemic change. This study also identifies four broad barriers: inherent, unconscious, and implicit biases, a lack of evidence-based best practices, siloing of research and information, and a lack of institutional support and resources. Findings also highlight the value of further research into barriers, gaps and opportunities. Originality/value – By studying the intersection of EDI and KMb, this contemporary synthesis of the state of the field presents opportunities for future research into gaps, barriers and potential solutions.
Never Skip Leg Day Again: Training the Lower Body with Vertical Jumps in a Virtual Reality Exergame
(Association for Computing Machinery, 2023-04-23) Cmentowski, Sebastian; Karaosmanoglu, Sukran; Nacke, Lennart E.; Steinicke, Frank; Krüger, Jens
Virtual Reality (VR) exergames can increase engagement in and motivation for physical activities. Most VR exergames focus on the upper body because many VR setups only track the users’ heads and hands. To become a serious alternative to existing exercise programs, VR exergames must provide a balanced workout and train the lower limbs, too. To address this issue, we built a VR exergame focused on vertical jump training to explore full-body exercise applications. To create a safe and effective training, nine domain experts participated in our prototype design. Our mixed-methods study confirms that the jump-centered exercises provided a worthy challenge and positive player experience, indicating long-term retention. Based on our findings, we present five design implications to guide future work: avoid an unintended forward drift, consider technical constraints, address safety concerns in full-body VR exergames, incorporate rhythmic elements with fluent movement patterns, adapt difficulty to players’ fitness progression status.
Born to Run, Programmed to Play: Mapping the Extended Reality Exergames Landscape
(Association for Computing Machinery, 2024-05-11) Karaosmanoglu, Sukran; Cmentowski, Sebastian; Nacke, Lennart E.; Steinicke, Frank
Many people struggle to exercise regularly, raising the risk of serious health-related issues. Extended reality (XR) exergames address these hurdles by combining physical exercises with enjoyable, immersive gameplay. While a growing body of research explores XR exergames, no previous review has structured this rapidly expanding research landscape. We conducted a scoping review of the current state of XR exergame research to (i) provide a structured overview, (ii) highlight trends, and (iii) uncover knowledge gaps. After identifying 1318 papers in human-computer interaction and medical databases, we ultimately included 186 papers in our analysis. We provide a quantitative and qualitative summary of XR exergame research, showing current trends and potential future considerations. Finally, we provide a taxonomy of XR exergames to help future design and methodological investigation and reporting.
Robust-stochastic models for profit maximizing hub location problems
(INFORMS, 2021) Taherkhani, Gita; Alumur, Sibel A.; Hosseini, Mojtaba
This paper introduces robust-stochastic models for profit maximizing capacitated hub location problems in which two different types of uncertainty including stochastic demand and uncertain revenue are simultaneously incorporated into the problem. First, a two-stage stochastic program is presented where demand and revenue are jointly stochastic. Next, robust-stochastic models are developed to better model uncertainty in the revenue while keeping the demand stochastic. Two particular cases are studied based on the dependency between demand and revenue. In the first case, a robust-stochastic model with a min-max regret objective is developed assuming a finite set of scenarios that describe uncertainty associated with the revenue under a revenue-elastic demand setting. For the case when demand and revenue are independent, robust-stochastic models with a max-min criterion and a min-max regret objective are formulated considering both interval uncertainty and discrete scenarios, respectively. It is proved that the robust-stochastic version with max-min criterion can be viewed as a special case of the min-max regret stochastic model. Exact algorithms based on Benders decomposition coupled with sample average approximation scheme are proposed. Exploiting the repetitive nature of sample average approximation, generic acceleration methodologies are developed to enhance the performance of the algorithms enabling them to solve large-scale intractable instances. Extensive computational experiments are performed to consider the efficiency of the proposed algorithms and also to analyze the effects of uncertainty under different settings. The qualities of the solutions obtained from different modeling approaches are compared under various parameter settings. Computational results justify the need to solve robust-stochastic models to embed uncertainty in decision making to design resilient hub networks.
An alternative approach to address uncertainty in hub location
(Springer, 2023) Janschekowitz, Marc; Taherkhani, Gita; Alumur, Sibel A.; Nickel, Stefan
In this paper, optimization and simulation techniques are integrated to address single and multiple allocation hub network design problems under uncertainty. Using a scenario-based iterative optimization-simulation approach four sources of uncertainty are considered: the demand to be transported within the network, the associated transportation costs as well as the fixed costs for both opening hub facilities and establishing the connections between them. Additionally, flow-dependent economies of scale on all network connections are incorporated in the simulation phase. A value of simulation measure is introduced to evaluate the performance of the methodology. The computational tests conducted on the well-known CAB data set with varying levels of uncertainty show that the approach can result in better solutions with up to 6.6% lower cost compared to its deterministic counterpart.
Observations of submesoscale eddy-driven heat transport at an ice shelf calving front
(Springer Nature, 2022-06-22) Friedrichs, Drew M.; McInerney, Jasmin B. T.; Oldroyd, Holly J.; Lee, Won Sang; Yun, Sukyoung; Yoon, Seung-Tae; Stevens, Craig L.; Zappa, Christopher J.; Dow, Christine F.; Mueller, Derek; Steiner, Oscar Sepulveda; Forrest, Alexander L.
Antarctica’s ice shelves buttress the continent’s terrestrial ice, helping slow the loss of grounded ice into the ocean and limiting sea level rise. Ice-ocean interaction plays a critical role in ice shelf stability by driving basal melt rates. Consequently, improved prediction of the future state of ice shelves lies in understanding the coastal ocean mechanics that deliver heat to their cavities. Here, we present autonomous glider-based observations of a coherent structure at the calving front of a cold-water cavity ice shelf (Nansen Ice Shelf, East Antarctica). This ~10 km-wide eddy dominated the local ocean circulation in the austral summer of 2018/2019, promoting an upwelling of cold ice shelf water and a deepening of warm surface water. Microstructure turbulence measurements show a resulting maximum vertical heat transport of 10 W m−2 at depths equivalent to the ice shelf draft. Similar eddy-driven heat transport further into the ice shelf cavity would support enhanced summertime melt in regions of shallower ice draft.
Modeling the Deformation Regime of Thwaites Glacier, West Antarctica, Using a Simple Flow Reaction for Ice Anisotropy (ESTAR)
(Wiley; American Geophysical Union (AGU), 2022-02-21) McCormack, Felicity S.; Warner, Roland C.; Seroussi, Helene; Dow, Christine F.; Roberts, Jason L.; Treverrow, Adam
ce deformation dominates the evolution of ice shelf flow and the slow-moving regions in the interior of ice sheets. However, deformation may be poorly represented in large-scale ice sheet models that use the Glen flow relation, due to its questionable applicability to the steady-state flow of anisotropic ice that prevails in ice sheets, having been derived from secondary creep rates of isotropic ice. We assess the deformation regimes of Thwaites Glacier, West Antarctica, using the Glen and “Empirical Scalar Tertiary Anisotropy Regime”, (ESTAR) flow relations, the latter being derived from steady-state deformation rates of anisotropic ice. For grounded ice, the character of the flow relation determines the contribution of deformation to overall flow, with ESTAR producing greater bed-parallel shear deformation than the standard Glen flow relation. The ESTAR experiments show larger basal shear stress maxima than the standard Glen experiment because ESTAR treats the responses to simple shear stresses and compression stresses differently, reducing the role of lateral and longitudinal stresses in momentum balance. On the Thwaites Glacier Tongue, ESTAR provides the best match to observed speeds by accounting for the differing effects of stresses on ice flow. Our results highlight the importance of the numerical description of anisotropy, particularly: In regions of transition from deformation-dominated to sliding-dominated flow; in the approach to the grounding line, and across ice shelves. Given the importance of these locations in determining mass flux into the ocean, our results have implications for projections of sea level change from Antarctic ice loss.
Basal channels drive active surface hydrology and transverse ice shelf fracture
(Science, 2018-06-13) Dow, Christine F.; Lee, Won Sang; Greenbaum, Jamin S.; Greene, Chad A.; Blankenship, Donald D.; Poinar, Kristin; Forrest, Alexander L.; Young, Duncan A.; Zappa, Christopher J.
Ice shelves control sea-level rise through frictional resistance, which slows the seaward flow of grounded glacial ice. Evidence from around Antarctica indicates that ice shelves are thinning and weakening, primarily driven by warm ocean water entering into the shelf cavities. We have identified a mechanism for ice shelf destabilization where basal channels underneath the shelves cause ice thinning that drives fracture perpendicular to flow. These channels also result in ice surface deformation, which diverts supraglacial rivers into the transverse fractures. We report direct evidence that a major 2016 calving event at Nansen Ice Shelf in the Ross Sea was the result of fracture driven by such channelized thinning and demonstrate that similar basal channel-driven transverse fractures occur elsewhere in Greenland and Antarctica. In the event of increased basal and surface melt resulting from rising ocean and air temperatures, ice shelves will become increasingly vulnerable to these tandem effects of basal channel destabilitization.
Seismic evidence for complex sedimentary control of Greenland Ice Sheet flow
(Science, 2021-08-16) Kulessa, Bernd; Hubbard, Alun L.; Booth, Adam D.; Bougamont, Marion; Dow, Christine F.; Doyle, Samuel H.; Christoffersen, Poul; Lindback, Katrin; Pettersson, Rickard; Fitzpatrick, Andrew A. W.; Jones, Glenn A.
The land-terminating margin of the Greenland Ice Sheet has slowed down in recent decades, although the causes and implications for future ice flow are unclear. Explained originally by a self-regulating mechanism where basal slip reduces as drainage evolves from low to high efficiency, recent numerical modeling invokes a sedimentary control of ice sheet flow as an alternative hypothesis. Although both hypotheses can explain the recent slowdown, their respective forecasts of a long-term deceleration versus an acceleration of ice flow are contradictory. We present amplitude-versus-angle seismic data as the first observational test of the alternative hypothesis. We document transient modifications of basal sediment strengths by rapid subglacial drainages of supraglacial lakes, the primary current control on summer ice sheet flow according to our numerical model. Our observations agree with simulations of initial postdrainage sediment weakening and ice flow accelerations, and subsequent sediment restrengthening and ice flow decelerations, and thus confirm the alternative hypothesis. Although simulated melt season acceleration of ice flow due to weakening of subglacial sediments does not currently outweigh winter slowdown forced by self-regulation, they could dominate over the longer term. Subglacial sediments beneath the Greenland Ice Sheet must therefore be mapped and characterized, and a sedimentary control of ice flow must be evaluated against competing self-regulation mechanisms.
Characterizing subglacial hydrology within the Amery Ice Shelf catchment using numerical modeling and satellite altimetry
(Wiley, 2024-04-20) Wearing, Martin G.; Dow, Christine F.; Goldberg, Daniel N.; Gourmelen, Noel; Hogg, Anna E.; Jakob, Livia
Meltwater forms at the base of the Antarctic Ice Sheet due to geothermal heat flux (GHF) and basal frictional dissipation. Despite the relatively small volume, this water has a profound effect on ice-sheet dynamics. However, subglacial melting and hydrology in Antarctica remain highly uncertain, limiting our ability to assess their impact on ice-sheet dynamics. Here we examine subglacial hydrology within the Amery Ice Shelf catchment, East Antarctica, using the subglacial hydrology model GlaDS. We calculate subglacial melt rates using a higher-order ice-flow model and two GHF estimates. We find a catchment-wide melt rate of 7.03 Gt year−1 (standard deviation = 1.94 Gt year−1), which is ≥50% greater than previous estimates. The contribution from basal dissipation is approximately 40% of that from GHF. However, beneath fast-flowing ice streams, basal dissipation is an order of magnitude larger than GHF, leading to a significant increase in channelized subglacial flux upstream of the grounding line. We validate GlaDS using high-resolution interferometric-swath radar altimetry, with which we detect active subglacial lakes and fine-scale ice-shelf basal melting. We find a network of subglacial channels that connects areas of deep subglacial water coincident with active subglacial lakes, and channelized discharge at the grounding line coinciding with enhanced ice-shelf basal melting. The concentrated discharge of meltwater provides 36% of the freshwater released into the ice-shelf cavity, in addition to ice-shelf basal melting. This suggests that ice-shelf basal melting is strongly influenced by subglacial hydrology and could be affected by future changes in subglacial discharge, such as lake drainage or channel rerouting.
Subglacial discharge accelerates future retreat of Denman and Scott Glaciers, East Antarctica
(American Association for the Advancement of Science, 2023-10-27) Pelle, Tyler; Greenbaum, Jamin S.; Dow, Christine F.; Jenkins, Adrian; Morlighem, Mathieu
Ice shelf basal melting is the primary mechanism driving mass loss from the Antarctic Ice Sheet, yet it is unknown how the localized melt enhancement from subglacial discharge will affect future Antarctic glacial retreat. We develop a parameterization of ice shelf basal melt that accounts for both ocean and subglacial discharge forcing and apply it in future projections of Denman and Scott Glaciers, East Antarctica, through 2300. In forward simulations, subglacial discharge accelerates the onset of retreat of these systems into the deepest continental trench on Earth by 25 years. During this retreat, Denman Glacier alone contributes 0.33 millimeters per year to global sea level rise, comparable to half of the contemporary sea level contribution of the entire Antarctic Ice Sheet. Our results stress the importance of resolving complex interactions between the ice, ocean, and subglacial environments in future Antarctic Ice Sheet projections.
Radarsat constellation mission derived winter glacier velocities for the St. Elias Icefield, Yukon/Alaska: 2022 and 2023
(Taylor & Francis, 2023-10-10) Van Wychen, W.; Bayer, Courtney; Copland, Luke; Brummel, Erika; Dow, Christine
Here we use high resolution (5 m) Radarsat Constellation Mission (RCM) imagery acquired in winters 2022 and 2023 to determine motion across glaciers of the St. Elias Icefield in Yukon/Alaska. Our regional velocity mapping largely conforms with previous studies, with faster motion (>600 m/yr) for the glaciers originating in the Yukon that drain southward and westward to the coast of Alaska and relatively slower motion (100–400 m/yr) for the land terminating glaciers that drain eastward and northeastward and stay within the Yukon. We also identify two new glacier surges within the icefields: the surge of Nàłùdäy (Lowell) Glacier in Winter 2022, and Chitina Glacier in Winter 2023, and track the progression of each surge from January to March utilizing ∼4-day repeat RCM imagery. To evaluate the quality of RCM-derived velocities, we compare our results with 50 simultaneous measurements at three on-ice dGPS stations located on two Yukon glaciers and find the average absolute difference between measurements to be 6.6 m/yr. Our results demonstrate the utility of RCM data to determine glacier motion across large regions with complex topography, to support process-based studies of fast flowing and surge-type glaciers and continue the legacy of velocity products derived from the Radarsat-2 mission.

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