arXiv subjects

Owen Williams

Publications and source records attributed to Owen Williams.

11 recordsLinked to original sources

arXiv2026

Unsteadiness in turbulent separated flow over a three-dimensional Gaussian bump

The unsteady separated flow over the three-dimensional Boeing Gaussian Bump is investigated at a Reynolds number based on bump height $Re_H = 2.26\times10^5$ using unsteady wall-pressure measurements and planar particle image velocimetry (PIV). Four major unsteady broadband phenomena spanning more than two decades in frequency are identified: (1) a very-low-frequency (VLF) spanwise motion centered at a Strouhal number of $St_H\sim10^{-3}$ (1 Hz) based on bump height, (2) a low-frequency breathing motion of the separation zone centered at $St_{L_{\rm{sep}}}=0.068$ (13.5 Hz) where $L_{\rm{sep}}$ is the mean separation length, (3) a 20 Hz frequency that appears to be associated with vortex shedding from the lateral shear layers, and (4) a centreline shear-layer vortex shedding at $St_{L_{\rm{sep}}}=0.68-1.01$ (135-200 Hz). Interestingly, while the VLF mode has a characteristic frequency of the same order to that often reported for other rectilinear bodies and hills that exhibit bistable asymmetric wake-switching, it is found that the VLF mode for this geometry exhibits a continuous spanwise meandering motion. Joint symmetric-antisymmetric proper orthogonal decomposition modal statistics from top-down PIV data further show that the spanwise meandering and streamwise stretching of the wake -- likely associated with the breathing motion -- are dynamically coupled, with the separation zone reaching its greatest streamwise extent when in a symmetric state. In this paper, the observed hierarchy of spectral features is comparable with those observed for a wide range of geometries, suggesting connections between geometric lengthscales and the low-frequency dynamics.

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arXiv2025

Impacts of Blade Camber on Cross-Flow Turbine Performance and Loading

Cross-flow turbines show promise for renewable energy generation from wind and tidal sources. The rotating reference frame of cross-flow turbine blades results in virtual camber and incidence due to streamline curvature, altering the lift, drag and pitching moment of the blades. Adding geometric camber is therefore likely to alter performance and loading, however there is little consensus regarding the direction of camber that might be most favorable. This study compares 2% concave-in and concave-out cambered blades (NACA 2418) with symmetrical NACA 0018 foils for a turbine with a 0.49 chord-to-radius ratio. Experimental performance measurements are compared across a range of tip-speeds, and particle image velocimetry is used to explore the in-rotor flow evolution through the cycle. Concave-out blades, which enhance virtual camber and lift in the power stroke are found to exhibit sub-optimal performance. In contrast, concave-in cambered blades slightly improved symmetrical blade performance by enhancing downstream flow reattachment, more than compensating for reduced peak power generation. The difference between each cambered foil is seen to grow with increasing tip-speed ratio. Moreover, these concave-in blades reduce peak loading by 13%, which may prove critical in future designs, especially at high tip-speed ratios. Exploration of the near-blade flow fields suggest that the influence of geometric camber is non-linear, and that use of a simplistic summation of both geometric and virtual camber to account for camber effects may be overly simplistic. Despite this, corresponding validated simulations suggest that a small but positive total camber (geometric plus virtual) is optimal for this turbine.

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arXiv2025

Optimizing the Interplay Between the Chord-to-Radius Ratio, Camber and Pitch of Cross-Flow Turbine Blades

This study examines the combined impact of the chord-to-radius ratio (c/R), blade camber, and preset pitch on cross-flow turbine performance. While prior research has examined individual effects of c/R and pitch, this work focuses on their interaction with camber and the resulting aerodynamic behaviors. Central to the analysis is the concept of virtual camber and virtual incidence, which arise from the curved trajectory of the blades, and are quantified using a conformal transformation. Using two experimental datasets -- one varying preset pitch, c/R, blade count, and Reynolds number, and the other examining geometric blade camber and pitch under identical conditions -- this work demonstrates links between blade pitch and camber influences. We propose a potential pathway for reducing the design space by combining geometric variables and coupled virtual effects into two effective parameters: net camber and net incidence. These composite variables provide a more unified and descriptive framework for understanding turbine performance and design behavior, offering a pathway for more consistent optimization of turbine geometry.

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arXiv2025

Dynamics of intracycle angular velocity control applied to cross-flow turbines

Understanding the intricate dynamics of cross-flow turbines (CFT) is critical to the improvement of performance and optimal control strategies. The current study numerically investigates intracycle control by modulating the angular velocity as a function of blade position for a 2-bladed NACA0018 turbine at a lab-scale chord-based Reynolds number of 45,000. Previous work has implemented intracycle control in attempts to improve turbine efficiency at the best performing tip-speed ratio (TSR). However, intracycle modulation of angular velocity simultaneously changes the time-averaged TSR, making it difficult to understand if the effects on performance are due to changes in mean TSR or imposed by the intracycle dynamics. Thus, this work explores a wider region of TSR across which intracycle control is applied, and assesses turbine performance with respect to time-averaged TSR. The effect of intracycle amplitude and phase shift of the velocity modulation is reported in terms of power generation and blade-level forces, and time-resolved flow fields reveal mechanisms behind changes in efficiency. For the 2-bladed turbine explored, the peak performance at constant angular velocity occurs at approximately TSR = 2. Intracycle control is found to be most beneficial at TSR < 2 where power is increased up to 71% over its constant speed baseline and 12% over the peak performance without control. This is accomplished through boundary layer reattachment through the acceleration portion of the stroke. In contrast, applying control when TSR $\ge 2$ is not beneficial due to degraded performance in the downstream portion of the stroke.

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arXiv2025

Experimental identification of blade-level forces, torque, and pitching moment for cross-flow turbines

Cross-flow turbine power is a net sum of power generation from rotating blades and power loss from rotating support structures. While the aggregate forces and torques at the turbine level are important for end use, these can inhibit a deeper understanding of fluid-structure interactions. Identification of blade-level forces and torques allows for specific investigations into how the fluid forcing on the blade drives rotation and can aid blade structural design. Here, we present a physics-based methodology for extracting blade-level forces and torques from experimental measurements at the axis of rotation of a cross-flow turbine, and demonstrate strong agreement with equivalent blade-only simulations. In doing so, we highlight the often-overlooked pitching moment, which offsets continuous increases in power generation from the tangential force and leads to net-zero power generation at freewheel.

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arXiv2024

Experimental validation of a linear momentum and bluff-body model for high-blockage cross-flow turbine arrays

In a confined flow, the performance of a turbine and its near-wake fluid dynamics depend on the blockage ratio, defined as the ratio of the turbine projected area to the channel cross-sectional area. While blockage is understood to increase the power coefficient for turbine "fences" spanning a channel, most investigations at the upper range of practically-achievable blockage ratios have been theoretical or numerical in nature. Furthermore, while linear momentum actuator disk theory is frequently used to model turbines in confined flows, as confinement increases, the ability of this idealized model to describe performance and flow fields has not been established. In this work, the performance and near-wake flow field of a pair of cross-flow turbines are experimentally evaluated at blockage ratios from 30% to 55%. The fluid velocity measured in the bypass region is found to be well-predicted by the open-channel linear momentum model developed by Houlsby et al. (2008), while the wake velocity is not. Additionally, self-similar power and thrust coefficients are identified across this range of blockage ratios when array performance is scaled by the modeled bypass velocity following Whelan et al.'s (2009) adaptation of the bluff-body theory of Maskell (1963). This result demonstrates that, despite multiple non-idealities, relatively simple models can quantitatively describe highly confined turbines. From this, an analytical method for predicting array performance as a function of blockage is presented. Overall, this work illustrates turbine performance at relatively high confinement and demonstrates the suitability of analytical models for predicting and interpreting their hydrodynamics.

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arXiv2024

Influence of the downstream blade sweep on cross-flow turbine performance

Cross-flow turbine (known as vertical-axis wind turbines or ``VAWTs'' in wind) blades encounter a relatively undisturbed inflow for the first half of each rotational cycle (``upstream sweep'') and then pass through their own wake for the latter half (``downstream sweep''). While most research on cross-flow turbine optimization focuses on the power-generating upstream sweep, we use single-bladed turbine experiments to show that the downstream sweep strongly affects time-averaged performance. We find that power generation from the upstream sweep continues to increase beyond the optimal tip-speed ratio. In contrast, the downstream sweep consumes power beyond the optimal tip-speed ratio due to unfavorable lift and drag directions relative to rotation and a potentially detrimental pitching moment arising from rotation-induced virtual camber. Downstream power degradation increases faster than upstream power generation, such that downstream sweep performance determines the optimal tip-speed ratio. In addition to performance measurements, particle image velocimetry data is obtained inside the turbine swept area at three tip-speed ratios. This illuminates the mechanisms underpinning the observed performance degradation in the downstream sweep and motivates an analytical model for a limiting case with high induction. Performance results are shown to be consistent across 55 unique combinations of chord-to-radius ratio, preset pitch angle, and Reynolds number, underscoring the general significance of the downstream sweep.

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arXiv2023

Cycle-to-cycle variations in cross-flow turbine performance and flow fields

Cross-flow turbine performance and flow fields exhibit cycle-to-cycle variations, though this is often implicitly neglected through time- and phase-averaging. This variability could potentially arise from a variety of mechanisms -- inflow fluctuations, the stochastic nature of dynamic stall, and cycle-to-cycle hysteresis -- each of which have different implications for our understanding of cross-flow turbine dynamics. In this work, the extent and sources of cycle-to-cycle variability for both the flow fields and performance are explored experimentally under two, contrasting operational conditions. Flow fields, obtained through two-dimensional planar particle image velocimetry inside the turbine swept area, are examined in concert with simultaneously measured performance. Correlations between flow-field and performance variability are established by an unsupervised hierarchical flow-field clustering pipeline. This features a principal component analysis (PCA) pre-processor that allows for clustering based on all the dynamics present in the high-dimensional flow-field data in an interpretable, low-dimensional subspace that is weighted by contribution to overall velocity variance. We find that the flow-field clusters and their associated performance are correlated primarily with inflow fluctuations, despite relatively low turbulence intensity, that drive variations in the timing of the dynamic stall process. Further, we find no evidence of substantial cycle-to-cycle hysteresis. Clustering reveals persistent correlations between performance and flow-field variability during the upstream portion of the turbine rotation. The approach employed here provides a more comprehensive picture of cross-flow turbine flow fields and performance than aggregate, statistical representations.

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arXiv2023

Compressible Boundary Layer Velocity Transformation Based on a Generalized Form of the Total Stress

The effects of density and viscosity fluctuations on the total stress balance are identified and used to create a new mean velocity transformation for compressible boundary layers. This work is enabled by an extensive database of direct numerical simulations that incorporate wall-cooling, semi-local Reynolds numbers ranging from 800 to 34000, and Mach numbers up to 12. The role, significance and physical mechanisms connecting density and viscosity fluctuations to the momentum balance and to the viscous, turbulent and total stresses are presented,allowing the creation of generalized formulations. We identify the significant properties that thus-far have been neglected in the derivation of velocity transformations: (1) the Machinvariance of the near-wall momentum balance for the generalized total stress, and (2) the Mach-invariance of the relative contributions from the generalized viscous and Reynolds stresses to the total stress. The proposed velocity transformation integrates both properties into a single transformation equation and successfully demonstrates a collapsing of all currently considered compressible cases onto the incompressible law of the wall, within the bounds of reported slope and intercept for incompressible data. Based on the physics embedded in the two scaling properties, the success of the newly proposed transformation is attributed to considering the effects of the viscous stress and turbulent stresses as well as mean and fluctuating density viscosity in a single transformation form.

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arXiv2020

Simulations of Intracycle Angular Velocity Control for a Cross-Flow Turbine

Straight-bladed cross-flow turbines are computationally explored for harvesting energy in wind and water currents. One challenge for cross-flow turbines is the transient occurrence of high apparent angles of attack on the blades that reduces efficiency due to flow separation. This paper explores kinematic manipulation of the apparent angle of attack through intracycle control of the angular velocity. Using an unsteady Reynolds-averaged Navier-Stokes (URANS) model at moderate Reynolds numbers, the kinematics and associated flow physics are explored for confined and unconfined configurations. The computations demonstrate an increase in turbine efficiency up to 54%, very closely matching the benefits shown by previous intracycle control experiments. Simulations display the time-evolution of angle of attack and flow velocity relative to the blade, which are modified with sinusoidal angular velocity such that the peak torque generation aligns with the peak angular velocity. With optimal kinematics in a confined flow there is minimal flow separation during peak power generation, however there is a large trailing edge vortex (TEV) shed as the torque decreases. The unconfined configuration has more prominent flow separation and is more susceptible to Reynolds number, resulting in a 41% increase in power generation under the same kinematic conditions as the confined flow.

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arXiv2019

Robust Principal Component Analysis for Modal Decomposition of Corrupt Fluid Flows

Modal analysis techniques are used to identify patterns and develop reduced-order models in a variety of fluid applications. However, experimentally acquired flow fields may be corrupted with incorrect and missing entries, which may degrade modal decomposition. Here we use robust principal component analysis (RPCA) to improve the quality of flow field data by leveraging global coherent structures to identify and replace spurious data points. RPCA is a robust variant of principal component analysis (PCA), also known as proper orthogonal decomposition (POD) in fluids, that decomposes a data matrix into the sum of a low-rank matrix containing coherent structures and a sparse matrix of outliers and corrupt entries. We apply RPCA filtering to a range of fluid simulations and experiments of varying complexities and assess the accuracy of low-rank structure recovery. First, we analyze direct numerical simulations of flow past a circular cylinder at Reynolds number 100 with artificial outliers, alongside similar PIV measurements at Reynolds number 413. Next, we apply RPCA filtering to a turbulent channel flow simulation from the Johns Hopkins Turbulence database, demonstrating that dominant coherent structures are preserved in the low-rank matrix. Finally, we investigate PIV measurements behind a two-bladed cross-flow turbine that exhibits both broadband and coherent phenomena. In all cases, we find that RPCA filtering extracts dominant coherent structures and identifies and fills in incorrect or missing measurements. The performance is particularly striking when flow fields are analyzed using dynamic mode decomposition, which is sensitive to noise and outliers.

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