About

Nathaniel J. Wei, Ph.D., is an Assistant Professor in the Department of Mechanical Engineering and Applied Mechanics at the University of Pennsylvania. He is also a Faculty Fellow with the Environmental Innovations Initiative and holds secondary faculty status in the General Robotics, Automation, Sensing, & Perception (GRASP) Lab. Additionally, he is affiliated with the Vagelos Institute for Energy Science and Technology. Dr. Wei earned his Ph.D. in Aeronautics from the California Institute of Technology in 2023, following an M.S. in Mechanical Engineering from Stanford University in 2020 and a B.S.E. in Mechanical and Aerospace Engineering from Princeton University in 2017.

Research topics

• Mechanics
• Computer Science
• Physics
• Mathematics
• Geometry
• Geology
• Acoustics
• Environmental science
• Astronomy
• Telecommunications
• Meteorology

Selected publications

• Time-varying wind-turbine wakes at high Reynolds numbers

  ArXiv.org · 2025-05-28

  preprintOpen access1st authorCorresponding

  Wind turbines operating in the atmospheric boundary layer are constantly exposed to time-varying flow conditions. These disturbances often occur on similar time scales to wind-turbine controllers, which may interfere with wind-farm control strategies that operate under steady-flow assumptions. This study aims to investigate the significance of such time variations on wind-turbine wake dynamics, focusing on slow time scales representative of quasi-steady processes in large wind farms. Experiments are conducted at near utility-scale Reynolds numbers ($Re_D=4\times10^6$) in a pressurized-air wind tunnel, with a wind turbine forced in periodic rotation-rate oscillations by means of a time-varying generator torque at low Strouhal numbers ($St=0.04$). Flow measurements in the wake of the turbine demonstrate that disturbances propagate through the wake as traveling waves, which are advected nonlinearly at the velocity of the wake rather than that of the free stream. The wake behavior can be described in a quasi-steady manner, but only after wake advection is accounted for by a Lagrangian transformation. Even in the quasi-steady regime, the spatiotemporal evolution of the wake can be controlled by independently varying the turbine thrust and tip-speed ratio. The results suggest that wake advection is important to consider for wind-farm modeling and control, and that time-varying control may allow wind-turbine wake interactions to be tuned even in nominally quasi-steady conditions for optimal wind-farm performance.

  PublisherOA PDFDOI

• Phase-averaged analysis of jet dynamics in a scaled-up vocal fold model with asymmetric motions

  Physics of Fluids · 2025-09-01 · 1 citations

  article

  This study focuses on the effects of glottal jet dynamics on phonation when one of the vocal folds does not move as much as the other. This can be a pathological condition, such as vocal fold paresis, in which a vocal fold is completely or partially paralyzed. Experiments were conducted using a 10× scaled-up model in a free-surface water tunnel. Two-dimensional vocal fold models with semi-circular ends were computer-driven inside a square duct with constant opening and closing speeds. Four cases were studied in which one vocal fold moved 0%, 50%, 75%, and 100% of the other; the last case being, of course, the nominally “healthy,” symmetric case. Time-resolved Digital Particle Image Velocimetry and pressure measurements along the duct centerline were made at a Reynolds number of 7200 and reduced frequency of 0.0261, corresponding to an equivalent life frequency of 97.5 Hz. Phase-averaged analysis of key contributors to sound production was conducted using terms in the streamwise integral momentum equation. The ultimate goal is understanding how asymmetric gap opening affects the dynamics, specifically vocal fold drag, and therefore, sound production. Results of this experiment show that this nominally simple flow encompasses multiple effects due to varying maximum gap opening, asymmetry due to partial motion of one vocal fold, blockage by the wall boundary layers, and pseudo-frequency effects arising when the two vocal folds move at different speeds. There is indeed a dependence of vocal fold drag on gap opening.

  PublisherDOI

• Flow Conditioner Development for Turbulence Reduction in a Fan Array Wind Tunnel

  ScholarlyCommons (University of Pennsylvania) · 2025-09-15

  otherOpen accessSenior author

  Wind in a closed, controlled environment is best suited for testing when in the form of a streamlined, consistent flow. The AWARE Lab’s new Windshaper, which is a Fan-Array Wind Tunnel, is a 1.5 m x 1.5 m open section wind tunnel with 648 individually controllable computer fans. This design causes the raw flow to be quite turbulent and spatially variable; when testing projects that require idealized air flow, a flow conditioner is necessary. This study shows the design and creation of a turbulence-reducing flow conditioner as well as the characterization, analysis, and optimization of the prototype. Through this iteration, the working flow conditioner was roughly 10% of the cost of the prefabricated one. Initial results indicated that there must be a compromise between mean wind speed vs. turbulence intensity to make a streamlined, testable flow. The final design is a plastic honeycomb with a 1⁄4” hole diameter, 2” thick + 0.013” wire diameter fiberglass screen, each spaced 1 ft apart; this allowed for a turbulence reduction by 33% at 50% duty cycle as well as the flow made more consistent across positions. Further work for this study includes adding a more rigid screen and sharing the design as a low-cost option for other open-jet wind tunnel facilities.

  Publisher

• Wake dynamics of wind turbines in unsteady streamwise flow conditions

  arXiv (Cornell University) · 2024-06-17

  preprintOpen access1st authorCorresponding

  The unsteady flow physics of wind-turbine wakes under dynamic forcing conditions are critical to the modeling and control of wind farms for optimal power density. Unsteady forcing in the streamwise direction may be generated by unsteady inflow conditions in the atmospheric boundary layer, dynamic induction control of the turbine, or streamwise surge motions of a floating offshore wind turbine due to floating-platform oscillations. This study seeks to identify the dominant flow mechanisms in unsteady wakes forced by a periodic upstream inflow condition. A theoretical framework for the problem is derived, which describes traveling-wave undulations in the wake radius and streamwise velocity. These dynamics encourage the aggregation of tip vortices into large structures that are advected along in the wake. Flow measurements in the wake of a periodically surging turbine were obtained in an optically accessible towing-tank facility, with an average diameter-based Reynolds number of 300,000 and with surge-velocity amplitudes of up to 40% of the mean inflow velocity. Qualitative agreement between trends in the measurements and model predictions is observed, supporting the validity of the theoretical analyses. The experiments also demonstrate large enhancements in the recovery of the wake relative to the steady-flow case, with wake-length reductions of up to 46.5% and improvements in the available power at 10 diameters downstream of up to 15.7%. These results provide fundamental insights into the dynamics of unsteady wakes and serve as additional evidence that unsteady fluid mechanics can be leveraged to increase the power density of wind farms.

  PublisherOA PDFDOI

• Comment on wes-2023-164

  2024-04-30

  peer-reviewOpen access1st authorCorresponding

  <strong class="journal-contentHeaderColor">Abstract.</strong> Many traditional methods for wind turbine design and analysis assume quasi-steady aerodynamics, but atmospheric flows are inherently unsteady and modern turbine blades are susceptible to aeroelastic deformations. This study therefore evaluates the effectiveness of simple analytical models for capturing the effects of such unsteady conditions on wind-turbine blades. We consider a pitching and plunging airfoil in a periodic transverse gust as an idealization of unsteady loading scenarios on a blade section. A potential-flow model derived from a linear combination of canonical problems is proposed to predict the unsteady lift on a two-dimensional airfoil in the small-perturbation limit. We then perform high-fidelity two-dimensional numerical simulations of a NACA-0012 airfoil over a range of periodic pitch, plunge, and gust disturbances, and quantify the amplitude and phase of the unsteady lift response. Good agreement with the model predictions is found for low to moderate forcing amplitudes and frequencies, while deviations are observed when the angle-of-attack amplitudes approach the static flow-separation limit of the airfoil. Potential explanations are given for the cases in which the ideal-flow theory proves insufficient. This theoretical framework and numerical evaluation motivate the inclusion of unsteady flow models in design and simulation tools in order to increase the robustness and operational lifespans of wind turbine blades in real flow conditions.

  PublisherDOI

• Comment on wes-2023-164

  2024-05-03

  peer-reviewOpen access1st authorCorresponding

  <strong class="journal-contentHeaderColor">Abstract.</strong> Many traditional methods for wind turbine design and analysis assume quasi-steady aerodynamics, but atmospheric flows are inherently unsteady and modern turbine blades are susceptible to aeroelastic deformations. This study therefore evaluates the effectiveness of simple analytical models for capturing the effects of such unsteady conditions on wind-turbine blades. We consider a pitching and plunging airfoil in a periodic transverse gust as an idealization of unsteady loading scenarios on a blade section. A potential-flow model derived from a linear combination of canonical problems is proposed to predict the unsteady lift on a two-dimensional airfoil in the small-perturbation limit. We then perform high-fidelity two-dimensional numerical simulations of a NACA-0012 airfoil over a range of periodic pitch, plunge, and gust disturbances, and quantify the amplitude and phase of the unsteady lift response. Good agreement with the model predictions is found for low to moderate forcing amplitudes and frequencies, while deviations are observed when the angle-of-attack amplitudes approach the static flow-separation limit of the airfoil. Potential explanations are given for the cases in which the ideal-flow theory proves insufficient. This theoretical framework and numerical evaluation motivate the inclusion of unsteady flow models in design and simulation tools in order to increase the robustness and operational lifespans of wind turbine blades in real flow conditions.

  PublisherDOI

• Wake dynamics of wind turbines in unsteady streamwise flow conditions

  Journal of Fluid Mechanics · 2024-11-28 · 10 citations

  articleOpen access1st authorCorresponding

  The unsteady flow physics of wind-turbine wakes under dynamic forcing conditions are critical to the modelling and control of wind farms for optimal power density. Unsteady forcing in the streamwise direction may be generated by unsteady inflow conditions in the atmospheric boundary layer, dynamic induction control of the turbine or streamwise surge motions of a floating offshore wind turbine due to floating-platform oscillations. This study seeks to identify the dominant flow mechanisms in unsteady wakes forced by a periodic upstream inflow condition. A theoretical framework for the problem is derived, which describes travelling-wave undulations in the wake radius and streamwise velocity. These dynamics encourage the aggregation of tip vortices into large structures that are advected along in the wake. Flow measurements in the wake of a periodically surging turbine were obtained in an optically accessible towing-tank facility, with an average diameter-based Reynolds number of 300 000 and with surge-velocity amplitudes of up to 40 % of the mean inflow velocity. Qualitative agreement between trends in the measurements and model predictions is observed, supporting the validity of the theoretical analyses. The experiments also demonstrate large enhancements in the recovery of the wake relative to the steady-flow case, with wake-length reductions of up to 46.5 % and improvements in the available power at 10 diameters downstream of up to 15.7 %. These results provide fundamental insights into the dynamics of unsteady wakes and serve as additional evidence that unsteady fluid mechanics can be leveraged to increase the power density of wind farms.

  PublisherOA PDFDOI

• Lagrangian particle tracking in the atmospheric surface layer

  Measurement Science and Technology · 2024-06-11 · 2 citations

  articleOpen access

  Abstract Field measurements in the atmospheric surface layer (ASL) are key to understanding turbulent exchanges in the atmosphere, such as fluxes of mass, water vapor, and momentum. However, current field measurement techniques are limited to single-point time series or large-scale flow field scans. Extending image-based laboratory measurement techniques to field-relevant scales is a promising route to more detailed atmospheric flow measurements, but this requires significant increases in the attainable measurement volume while keeping the spatiotemporal resolution high. Here, we present an adaptable particle tracking system using helium-filled soap bubbles, mirrorless cameras, and high-power LEDs enabling volumetric ASL field measurements. We conduct analyses pertinent to image-based field measurement systems and develop general guidelines for their design. We validate the particle tracking system in a field experiment. Single-point Eulerian velocity statistics are presented and compared to data from concurrently operated sonic anemometers. Lagrangian displacement statistics are also presented with a comparison to Taylor’s theory of dispersion. The system improves the state-of-the-art in field measurements in the lower atmosphere and enables unprecedented insights into flow in the ASL.

  PublisherDOI

• Power-generation enhancements and upstream flow properties of turbines in unsteady inflow conditions

  Journal of Fluid Mechanics · 2023 · 19 citations

  1st authorCorresponding
  • Computer Science
  • Mechanics
  • Environmental science

  Energy-harvesting systems in complex flow environments, such as floating offshore wind turbines, tidal turbines and ground-fixed turbines in axial gusts, encounter unsteady streamwise flow conditions that affect their power generation and structural loads. In some cases, enhancements in time-averaged power generation above the steady-flow operating point are observed. To characterize these dynamics, a nonlinear dynamical model for the rotation rate and power extraction of a periodically surging turbine is derived and connected to two potential-flow representations of the induction zone upstream of the turbine. The model predictions for the time-averaged power extraction of the turbine and the upstream flow velocity and pressure are compared against data from experiments conducted with a surging-turbine apparatus in an open-circuit wind tunnel at a diameter-based Reynolds number $Re_D = 6.3\times 10^5$ and surge-velocity amplitudes up to 24 % of the wind speed. The combined modelling approach captures trends in both the time-averaged power extraction and the fluctuations in upstream flow quantities, while relying only on data from steady-flow measurements. The sensitivity of the observed increases in time-averaged power to steady-flow turbine characteristics is established, thus clarifying the conditions under which these enhancements are possible. Finally, the influence of unsteady fluid mechanics on time-averaged power extraction is explored analytically. The theoretical framework and experimental validation provide a cohesive modelling approach that can drive the design, control and optimization of turbines in unsteady flow conditions, as well as inform the development of novel energy-harvesting systems that can leverage unsteady flows for large increases in power-generation capacities.

  PublisherDOI

• Large-eddy simulations of turbulent flows in arrays of helical- and straight-bladed vertical-axis wind turbines

  Journal of Renewable and Sustainable Energy · 2023-11-01 · 2 citations

  articleOpen access

  Effects of helical-shaped blades on the flow characteristics and power production of finite-length wind farms composed of vertical-axis wind turbines (VAWTs) are studied numerically using large-eddy simulation (LES). Two helical-bladed VAWTs (with opposite blade twist angles) are studied against one straight-bladed VAWT in different array configurations with coarse, intermediate, and tight spacings. Statistical analysis of the LES data shows that the helical-bladed VAWTs can improve the mean power production in the fully developed region of the array by about 4.94%–7.33% compared with the corresponding straight-bladed VAWT cases. The helical-bladed VAWTs also cover the azimuth angle more smoothly during the rotation, resulting in about 47.6%–60.1% reduction in the temporal fluctuation of the VAWT power output. Using the helical-bladed VAWTs also reduces the fatigue load on the structure by significantly reducing the spanwise bending moment (relative to the bottom base), which may improve the longevity of the VAWT system to reduce the long-term maintenance cost.

  PublisherOA PDFDOI

Frequent coauthors

• John O. Dabiri

  13 shared

• Timothy Wei

  Saratoga Hospital

  9 shared

• Michael Krane

  Pennsylvania State University

  9 shared

• Hunter Ringenberg

  University of Colorado Boulder

  8 shared

• Dylan Rogers

  University of Nebraska–Lincoln

  8 shared

• David E. Rival

  7 shared

• JiaCheng Hu

  Anhui University

  4 shared

• Frieder Kaiser

  Queen's University

  4 shared

Labs

• AWARE Lab @ PennPI

Education

• Ph.D., Aerospace Engineering

  University of California, Berkeley

  2003

• M.S., Aerospace Engineering

  University of California, Berkeley

  1999

• B.S., Aerospace Engineering

  University of California, Berkeley

  1997