About

James Brasseur is a Research Professor in the Ann and H.J. Smead Aerospace Engineering Sciences at the University of Colorado Boulder, with a focus on fluid dynamics, turbulence, aerodynamics, geophysical flows, and biomechanics. His educational background includes a Ph.D. in Aeronautical and Astronautical Science from Stanford University, along with a master's degree in Aeronautical and Astronautical Engineering from Stanford and a bachelor's degree in Aerospace Engineering from the University of Maryland. He has held numerous academic and professional positions, including guest professorships at Uppsala University in Sweden, affiliate scientist roles at the National Center for Atmospheric Research (NCAR), and visiting scientist positions at institutions such as the University of Rome Ter Vergata, Imperial College London, and the University of Cambridge. Brasseur's research interests encompass turbulence physics, turbulent flows, and turbulent combustion, as well as geophysical turbulence, wind energy, biofluid dynamics, and pharmaceutical transport in the gastrointestinal tract. His work involves large-eddy simulation, direct numerical simulation, and scientific visualization, contributing to both fundamental fluid mechanics and applied biomedical engineering.

Research topics

• Mechanics
• Physics
• Gastroenterology
• Classical mechanics
• Thermodynamics
• Chemistry
• Internal medicine
• Materials science
• Physical chemistry
• Meteorology
• Medicine
• Atmospheric sciences
• Environmental science
• Organic chemistry
• Pharmacology

Selected publications

• Mechanisms underlying the generation and generalisation of the surface layer

  Journal of Fluid Mechanics · 2025-04-25 · 2 citations

  articleOpen accessCorresponding

  We define ‘surface layer’ (SL) as an inertia-dominated turbulence region outside a viscous or roughness surface-adjacent sub-layer (SAS) that is characterised by linear scaling of specific coherence length scales on wall-normal distance, $z$ . We generalise the mechanisms that underlie the formation of the classical inertial SL in the shear-dominated turbulent boundary layer (TBL) to wall-bounded turbulent flows with zero mean shear. Using particle image velocimetry data from two wind tunnel facilities, we contrast the classical TBL SL with a non-classical shear-free SL generated within grid turbulence advected over an impermeable plate using two grids with different turbulence length scales. Integral-scale variations with $z$ and other statistics are quantified. In both shear-dominated and shear-free SLs we observe well-defined linear increases in $z$ of the streamwise integral scale of vertical velocity fluctuations. In grid turbulence the shear-free SL initiates just above the SAS that confines friction-generated motions. By contrast, the TBL SL forms with non-zero mean shear rate that extends streamwise coherence lengths of streamwise fluctuations. In both flow classes only the integral scales of vertical fluctuating velocity increase linearly with $z$ , indicating that the SL is generated by the blockage of vertical fluctuations in the vertical. Whereas the SAS in the TBL is much thinner than in the grid-turbulence flows, the generation of a shear-free SL by the interaction of turbulence eddies and a surface depends on the relative thinness of the SAS. We conclude that the common generalisable SL mechanism is direct blockage of vertical fluctuations by the impermeable surface.

  PublisherOA PDFDOI

• Mechanisms underlying the generation and generalisation of the surface layer – ERRATUM

  Journal of Fluid Mechanics · 2025-07-10

  erratumOpen access
  PublisherOA PDFDOI

• Nonsteady Load Responses of Wind Turbines to Atmospheric and Mountain-Generated Turbulence Eddies, With Impacts on the Main Bearing: A Validation Study

  2024-08-01 · 1 citations

  reportOpen access1st authorCorresponding

  Previous computational and field experiments identify three characteristic time scales in the aerodynamic responses of utility-scale wind turbine loads to atmospheric boundary layer (ABL) turbulence: a 30-90 second time scale for the passage of high/low speed "streaks" through the rotor plane, the blade and rotor rotation time scales (- 1-5 seconds), and a sub-second time scale created by blade rotation through gradients within eddy coherent structure. In the current study we compare aerodynamic load responses from daytime ABL turbulence quantified with large-eddy simulation and a actuator line model of the NREL 5 MW wind turbine with analysis of field data from the NREL/GE 1.5 MW wind turbine 5 kilometers east of the Rocky Mountain Front Range in Colorado. In addition, we contrast the responses to the passage of the mountain-generated eddies embedded within the westerly winds with the ABL eddies embedded within northerly/southerly winds. These analyses are in context with the nonsteady forcing of the main bearing by the aerodynamic generation of nontorque bending moments on the main shaft. Potentially relevant to main bearing failure mechanisms, both computational and field data show that the magnitudes of turbulence-generated nontorque bending moments, that we show generate nonsteady force on the main bearing, are of order, and often larger than, torque (which underlies power). However, the temporal variations in these two responses are uncorrelated, implying that the aerodynamic mechanisms that drive power and main bearing response are fundamentally different. We find this to be the case in the field with both mountain-generated eddies (westerly winds) and ABL-generated eddies (northerly/southerly winds). Whereas the time and length scales are comparable, the mountain eddies were somewhat more energetic than the northerly/southerly ABL eddies. Interestingly, however, the fluctuations in nontorque bending moment that force the main bearing were found to be stronger when forced by the ABL eddies than the mountain eddies. The field studies validate the key results from the computational study and show even stronger response in the nontorque bending moment than in the computer simulations. In all cases, the torque and nontorque bending moments are temporally uncorrelated, torque and power are driven by time variations in rotor-averaged horizontal wind velocity and nontorque bending moments are driven by time changes in the degree of nonuniformity in the distribution of velocity over the rotor plane. Thus the results generalize the mechanisms underlying nonsteady aerodynamic forcing to classes of turbulence eddy types with strength of order or stronger than ABL eddies with transverse scale of order the wind turbine rotor. These include atmospheric turbulence eddies, topography-generated turbulence eddies and, by extension, impacts of turbine-wake-scale turbulence eddies on downstream wind turbine rotors.

  PublisherOA PDFDOI

• Main Bearing Replacement and Damage - A Field Data Study on 15 Gigawatts of Wind Energy Capacity

  2023-07-01 · 11 citations

  reportOpen access

  This study seeks to establish a comprehensive baseline of knowledge for the replacement and damage of main bearings in wind turbines. The purpose of this report is to provide a high-level summary of the data set, methodology, and results of this work. Full technical details and an extended analysis will be made available in a future publication. We collected data on main bearing replacements and reported damage from industrial partners based in Europe and the United States. In total, we obtained data for 167 wind power plants, with a combined capacity of 15.3 gigawatts (GW). Most of the data set was comprised of land-based, three-point mount, spherical roller bearings. Within this data set were 689 instances of main bearing replacement. Analysis was undertaken in two parts: first, a statistical analysis of the main bearing time-to-replacement data using survival analysis techniques; second, quantitative and qualitative analyses of the obtained damage information. Our results showed that 10% of a fixed main bearing population would be expected to have been replaced by 10.5 years. This is close to half of the 20-year design value. Fitted parametric distributions then indicated that by year 20, between 22% and 25% of main bearings are expected to have been replaced. Analysis of the damage reports revealed spalling to be the main type of damage listed. The additional presence of surface damage in the collected data indicates that at least part of the spalling cases are likely due to surface-initiated rolling contact fatigue. At this stage is not clear what proportion of spalling cases result from "wear induced", surface-initiated and subsurface-initiated rolling contact fatigue. While this work provides important insights into the current state of main bearing replacements and damage, many questions remain. An ongoing and expanding data collection and analysis effort focused on wind turbine main bearings is therefore recommended.

  PublisherOA PDFDOI

• Wind turbine main bearing rating lives as determined by IEC 61400‐1 and ISO 281: A critical review and exploratory case study

  Wind Energy · 2023-11-20 · 23 citations

  reviewOpen access

  Abstract This paper studies the rating lives of wind turbine main bearings, as determined by the IEC 61400‐1 and ISO 281 standards. A critical review of relevant bearing life theory and turbine design requirements is provided, including discussion on possible shortcomings such as the existence (or not) of the bearing fatigue load limit and the validity of assuming linear damage accumulation. A detailed exploratory case study is then undertaken to determine rating lives for two models of main bearing in a 1.5 MW wind turbine. Rating life assessment is carried out under different conditions, including various combinations of main bearing temperature, wind field characteristics, lubricant viscosity, and contamination levels. Rating lives are found to be sufficiently above the desired 20‐year design life for both bearing models under expected operating conditions. For the larger bearing, operational loads are shown to be below or close to the bearing fatigue load limit a vast majority of the time. Key sensitivities for rating life values are temperature and contamination. Overall, the results of this study suggest that an ISO 281 rating life assessment does not account for reported rates of main bearing failures in 1 to 3 MW wind turbines. It is recommended that a similar analysis be undertaken for ISO/TS 16281 rating lives, along with further efforts to identify principal root causes of main bearing failures in future work, possibly leading to a new application standard specific to this component. It is also recommended that the impacts of partial wake impingement on main bearing rating lives are investigated.

  PublisherOA PDFDOI

• Nonsteady Load Responses to Mountain-Generated Turbulence Eddies on the DOE 1.5 MW Wind Turbine at the National Wind Technology Center

  OSTI OAI (U.S. Department of Energy Office of Scientific and Technical Information) · 2023-06-19

  paratextOpen access1st authorCorresponding

  Field data collected from the NREL/GE 1.5MW wind turbine and met tower at the NREL Wind Technology Center near Boulder, Colorado from June-October 2018 were analyzed to quantify the impacts of turbulence eddies on the load responses measured from sensors on the main shaft, blade and tower. The passage of individual mountain-generated eddies from the met tower to the wind turbine were critically determined by correlating the optimal time shifts in signal between met tower and nacelle anemometers with mean advection time. Loading responses from mountain eddy passage were compared with atmospheric eddies from the north/south, unimpeded by the mountains, and found to be similar. Whereas time variations in torque were highly correlated with time changes in horizontal eddy velocity, the out-of-plane bending moments on the main shaft (directly forcing the main bearing) were uncorrelated with horizontal eddy velocity. This result is consistent with a previous LES study indicating that the main bearing is forced by asymmetrical interactions between the rotor and turbulence eddies, while power fluctuations respond primarily to advective eddy velocity. Surprisingly, the nacelle anemometer produced statistics very similar to the met tower.

  Publisher

• A foundational step towards understanding and improving the transition between URANS and LES in hybrid URANS-LES methodology

  Computers & Fluids · 2023-04-20 · 3 citations

  articleOpen access

  Appropriate transition between URANS and LES plays an integral role in the performance of hybrid URANS-LES techniques. However, dynamics of the transition region (also known as the Gray Area) have not yet been well understood, despite several efforts to mitigate negative impacts of inappropriate modeling of this region such as deviation from law-of-the-wall (also referred to as log-layer mismatch (LLM)) in turbulent attached boundary layers. The current paper centers on Scale Adaptive Simulation (SAS) hybrid URANS-LES methods and aims to address some shortcomings of the SAS methodology while providing new insights into the dynamics of the Gray Area. More specifically, necessary requirements, including the length scale, grid design and underlying RANS model to appropriately model essential dynamics of the Gray Area and, thus, mitigating the log-layer mismatch, are presented, which could be used for broader framework of hybrid URANS/LES and probably wall-modeled LES approaches . SAS methods traditionally have difficulty transitioning from URANS to LES without a sufficiently unstable base flow. Here we introduce an improved triggering mechanism based on departure-from-equilibrium dynamics formulated for the SAS method, primarily to enable the model to transition from URANS to scale resolving mode in attached/mildly separated flows , a well-known deficiency of the classical SAS formulations. The improved triggering mechanism considers deviations from equilibrium through a vortex stretching term in the dynamical equation for turbulent dissipation rate . The present study demonstrates that the length scale obtained for the gray area using straightforward blending of URANS and LES regions results in inaccurate transition dynamics. Therefore, we introduce a transport equation for the length scale of energy-containing eddies that allows an appropriate transition of length scale from URANS to LES mode. The modified framework uses the k − ɛ − ζ − f model as the underlying RANS model and is shown to trigger a transition from URANS to LES in stable attached stationary turbulent channel flow and mitigate the log-layer mismatch problem to a great extent. When applied to flows that feature boundary layer separation (flow over a periodic hill and flow over the wall-mounted hump), the current hybrid framework delivers results in agreement with existing experimental data and comparable to high fidelity LES calculations. • Improvements to hybrid URANS-LES Scale Adaptive-Simulation methods are developed. • A dynamical length scale prediction adjusts eddy viscosity in the transition region. • The length scale prediction and other improvements mitigate the “Log-Layer Mismatch”

  PublisherDOI

• Author response for "Wind turbine main bearing rating lives as determined by IEC 61400‐1 and ISO 281: A critical review and exploratory case study"

  2023-09-19

  review
  PublisherDOI

• Impacts of wind field characteristics and non-steady deterministic wind events on time varying main-bearing loads

  2022-01-18 · 3 citations

  preprintOpen access

  Abstract. This work considers the characteristics and drivers of the loads experienced by wind turbine main-bearings. Simplified load response models of two different hub and main-bearing configurations are presented, representative of both inverting direct-drive and four-point mounted geared drivetrains. The influences of deterministic wind field characteristics, such as wind speed, shear, yaw offset and veer, on the bearing load patterns are then investigated for similarity scaled 5, 7.5 and 10 MW reference wind turbine models. Main-bearing load response in cases of deterministic gusts and extreme changes in wind direction are also considered for the 5 MW model. Perhaps surprisingly, veer is identified as an important driver of main-bearing load fluctuations. Upscaling results indicate that similar behaviour holds as turbines become larger, but with mean loads and load fluctuation levels increasing at least cubically with the turbine rotor radius. Strong links between turbine control and main-bearing load response are also observed.

  PublisherDOI

• Impacts of wind field characteristics and non-steady deterministic wind events on time-varying main-bearing loads

  Wind energy science · 2022-06-08 · 12 citations

  articleOpen access

  Abstract. This work considers the characteristics and drivers of the loads experienced by wind turbine main bearings. Simplified load response models of two different hub and main-bearing configurations are presented, representative of both inverting direct-drive and four-point-mounted geared drivetrains. The influences of deterministic wind field characteristics, such as wind speed, shear, yaw offset, and veer, on the bearing load patterns are then investigated for similarity scaled 5, 7.5, and 10 MW reference wind turbine models. Main-bearing load response in cases of deterministic gusts and extreme changes in wind direction are also considered for the 5 MW model. Perhaps surprisingly, veer is identified as an important driver of main-bearing load fluctuations. Upscaling results indicate that similar behaviour holds as turbines become larger, but with mean loads and load fluctuation levels increasing at least cubically with the turbine rotor radius. Strong links between turbine control and main-bearing load response are also observed.

  PublisherDOI

Recent grants

• NIH Grant R01DK041436

  NIH · $808k · 1997

• Space-time Loadings on Wind Turbine Blades driven by Atmospheric Boundary Layer Turbulence: Coupling LES and DES

  NSF · $337k · 2009–2013

• Micro-scale Transport as a Critical Link between Molecular-scale Absorption and Macro-scale Mixing in Gut Physiology and Function

  NSF · $720k · 2005–2010

• NIH Grant R41DK052058

  NIH · $100k · 1997

Frequent coauthors

• Anupam Pal

  Physical Research Laboratory

  57 shared

• Werner Schwizer

  University of Zurich

  51 shared

• K. Indireshkumar

  Princeton Plasma Physics Laboratory

  41 shared

• Geoff Hebbard

  The Royal Melbourne Hospital

  41 shared

• Henryk Faas

  University of Nottingham

  40 shared

• Patrik Kunz

  Maastricht University

  40 shared

• Larry S. Miller

  Feinstein Institute for Medical Research

  38 shared

• Michael D. Fried

  University of California, Irvine

  36 shared

Education

• B.S.

  University of Maryland

  1973

• M.S.

  Stanford University

  1976

• Ph.D., Aeronautical and Astronautical Science (minor, Physics)

  Stanford University

  1979

Awards & honors

• Elected Inaugural Chair of the APS Topical Group on the Phys…
• Elected Fellow of the American Physical Society (via the Div…
• Elected to the Executive Committee of the American Physical…
• Elected to the Permanent Scientific Committee of the World O…
• Inducted into The Johns Hopkins University Society of Schola…