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PNAS Nexus · 2026-04-30

Flight Mission Modeling of eVTOL and eCTOL Commuter Aircraft

2026-01-08

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.

Stream Nutrient Load and Concentration Estimation From Minimal Measurements

Geophysical Research Letters · 2025-04-22 · 3 citations

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.

Why do only some riblets promote spanwise rollers?

Journal of Fluid Mechanics · 2025-11-03 · 1 citations

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.

EXPERIMENTAL STUDY OF INTERNAL SOLITARY WAVES INTERACTION WITH SURFACE SOLITARY WAVES

Coastal Engineering Proceedings · 2025-05-29

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.

Internal solitary wave generation using a jet-array wavemaker

Experiments in Fluids · 2025-02-20 · 1 citations

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.

Internal Solitary Wave Generation Using A Jet-Array Wavemaker

Research Square · 2024-11-25

Connections between propulsive efficiency and wake structure via modal decomposition

Journal of Fluid Mechanics · 2024-06-04 · 8 citations

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.

Internal Solitary Wave Generation Using A Jet-Array Wavemaker

arXiv (Cornell University) · 2024-11-07

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.

The role of nonlinear interactions in the onset of drag increase in flow over riblets

Journal of Physics Conference Series · 2024-04-01 · 1 citations

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.