About

Marcus Hultmark is a Professor in the Department of Mechanical and Aerospace Engineering at Princeton University. His research interests include a variety of problems related to fluid mechanics, with a focus on turbulence, heat and mass transfer, drag reduction, and wind energy. He has been recognized for his contributions with awards such as the 2016 Air Force Young Investigator Award, the 2017 NSF CAREER award, and the 2017 Nobuhide Kasagi Award. Hultmark received his M.Sc. degree from Chalmers University in Sweden and completed his Ph.D. at Princeton University in 2011.

Research topics

• Physics
• Mechanics
• Materials science
• Composite material

Selected publications

• Drag on a hollow sphere can increase with porosity

  Journal of Fluid Mechanics · 2026-01-23

  articleOpen accessSenior authorCorresponding

  Previous literature has shown that the introduction of homogeneous perforation on plates and cylinders decreases aerodynamic drag. Here, it is shown that the opposite is true for a sphere; drag can increase with porosity. Hollow porous spheres exposed to a uniform free stream are studied experimentally using force and flow field measurements. The parameter space encompasses moderate to high Reynolds numbers ( $5 \times 10^4 \leq \textit{Re} \leq 4 \times 10^5$ ) and porosities ranging from $0\,\%$ to $80\,\%$ . The main conclusion is that drag increases with porosity, at super-critical Reynolds numbers, for all studied porosities. At low porosities (less than $9\,\%$ ), the effect of porosity on drag can be explained by shifts in the separation point. At higher porosities the drag increase cannot be explained by separation shifts, and instead is explained by two competing forms of kinetic energy dissipation: (i) shear on the macro-scale of the body, and (ii) hole losses from flow through the pores. The former generally decreases with porosity, as bleeding flow passing through the body decreases the characteristic velocity difference in the body-scale wake. In a sphere, hole losses increase with porosity sufficiently fast to overcome decreasing body-scale shear losses, in contrast to plates and cylinders where this is not the case. Relatively weak wake vortex structures, and associated low drag coefficient at zero porosity, for a sphere reduce the impact of wake bleeding. Moreover, fluid entering the fore of a sphere can exit perpendicular to the free stream, further reducing wake bleeding while still contributing to hole losses.

  PublisherOA PDFDOI

• Geometric sensitivity of the NSTAP

  Experiments in Fluids · 2025-02-11 · 1 citations

  article
  PublisherDOI

• Aerodynamic testing of rotor sails: A scaling challenge

  2025-01-01

  articleOpen accessSenior author
  PublisherOA PDFDOI

• Understanding the effects of rotation on the wake of a wind turbine at high Reynolds number

  Flow · 2025-01-01

  articleOpen accessSenior author

  Abstract The wake of a horizontal-axis wind turbine was studied at a Reynolds number of $Re_D=4\times 10^6$ with the aim of revealing the effects of the tip speed ratio, $\lambda$ , on the wake. Tip speed ratios of $4\lt \lambda \lt 7$ were investigated and measurements were acquired up to 6.5 diameters downstream of the turbine. Through an investigation of the turbulent statistics, it is shown that the wake recovery was accelerated due to the higher turbulence levels associated with lower tip speed ratios. The energy spectra indicate that larger broadband turbulence levels at lower tip speed ratios contributes to a more rapidly recovering wake. Wake meandering and a coherent core structure were also studied, and it is shown that these flow features are tip speed ratio invariant, when considering their Strouhal numbers. This finding contradicts some previous studies regarding the core structure, indicating that the structure was formed by a bulk rotor geometric feature, rather than by the rotating blades. Finally, the core structure was shown to persist farther into the near wake with decreasing tip speed ratio. The structure’s lifetime is hypothesised to be related to its strength relative to the turbulence in the core, which decreases with increasing tip speed ratio.

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• Spanwise wall forcing can reduce turbulent heat transfer more than drag

  Journal of Fluid Mechanics · 2025-05-10

  articleOpen access

  Direct numerical simulations are performed for turbulent forced convection in a half-channel flow with a wall oscillating either as a spanwise plane oscillation or to generate a streamwise travelling wave. The friction Reynolds number is fixed at $Re_{\tau _0} = 590$ , but the Prandtl number $Pr$ is varied from 0.71 to 20. For $Pr\gt 1$ , the heat transfer is reduced by more than the drag, 40 % compared with 30 % at $Pr=7.5$ . This outcome is related to the different responses of the velocity and thermal fields to the Stokes layer. It is shown that the Stokes layer near the wall attenuates the large-scale energy of the turbulent heat flux and the turbulent shear stress, but amplifies their small-scale energy. At higher Prandtl numbers, the thinning of the conductive sublayer means that the energetic scales of the turbulent heat flux move closer to the wall, where they are exposed to a stronger Stokes layer production, increasing the contribution of the small-scale energy amplification. A predictive model is derived for the Reynolds and Prandtl number dependence of the heat-transfer reduction based on the scaling of the thermal statistics. The model agrees well with the computations for Prandtl numbers up to 20.

  PublisherDOI

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

  ArXiv.org · 2025-05-28

  preprintOpen accessSenior author

  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.

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• Kirigami-inspired wind steering for natural ventilation

  Journal of Wind Engineering and Industrial Aerodynamics · 2024-02-13 · 13 citations

  article
  PublisherDOI

• A non-intrusive volumetric camera calibration system

  Measurement Science and Technology · 2024-06-12 · 2 citations

  articleOpen access

  Abstract When acquiring quantitative data using cameras, calibration is required to establish the mapping relation between the image space and physical space. Calibration targets with known dimensions are often used, with the most popular being physical targets. In setups where physical access is a challenge, using physical targets may not be possible, and so we develop an adaptive non-intrusive calibration target capable of conducting volumetric calibrations in free space. The calibration target is formed by two intersecting laser beams traversed in the test domain. A novel algorithm is presented for accurately finding the beam intersections, even at small crossing angles. The error sources are assessed along with their scaling behavior with respect to key parameters. The performance of the calibration method is evaluated by using it to examine a test object with known dimensions.

  PublisherDOI

• Lagrangian particle tracking in the atmospheric surface layer

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

  articleOpen accessSenior author

  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

• Contact-angle hysteresis provides resistance to drainage of liquid-infused surfaces in turbulent flows

  arXiv (Cornell University) · 2024-01-10

  preprintOpen access

  Lubricated textured surfaces immersed in liquid flows offer tremendous potential for reducing fluid drag, enhancing heat and mass transfer, and preventing fouling. According to current design rules, the lubricant must chemically match the surface to remain robustly trapped within the texture. However, achieving such chemical compatibility poses a significant challenge for large-scale flow systems, as it demands advanced surface treatments or severely limits the range of viable lubricants. In addition, chemically tuned surfaces often degrade over time in harsh environments. Here, we demonstrate that a lubricant-infused surface (LIS) can resist drainage in the presence of external shear flow without requiring chemical compatibility. Surfaces featuring longitudinal grooves can retain up to 50% of partially wetting lubricants in fully developed turbulent flows. The retention relies on contact-angle hysteresis, where triple-phase contact lines are pinned to substrate heterogeneities, creating capillary resistance that prevents lubricant depletion. We develop an analytical model to predict the maximum length of pinned lubricant droplets in microgrooves. This model, validated through a combination of experiments and numerical simulations, can be used to design chemistry-free LISs for applications where the external environment is continuously flowing. Our findings open up new possibilities for using functional surfaces to control transport processes in large systems.

  PublisherOA PDFDOI

Recent grants

• UNS: Physical Mechanisms of Wall-Bounded Turbulence and Turbulent Mixing at Extreme Reynolds

  NSF · $319k · 2015–2018

• Collaborative Research: Parameterization of the Land-Surface Thermal and Moisture Heterogeneities

  NSF · $300k · 2017–2021

• CAREER: Revealing the characteristics of high Reynolds number wakes with rotation

  NSF · $550k · 2017–2022

Frequent coauthors

• Alexander J. Smits

  Princeton University

  73 shared

• Sean Bailey

  University of Kentucky

  29 shared

• Matthew Fu

  28 shared

• Janik Kiefer

  Technical University of Denmark

  22 shared

• Margit Vallikivi

  Princeton University

  21 shared

• Claudia Brunner

  Princeton University

  19 shared

• Alexander Piqué

  19 shared

• Liuyang Ding

  Guangdong Pharmaceutical University

  16 shared

Labs

• Fluid Mechanics Transport Phenomena GroupPI

  Fundamental and Applied Studies in Turbulence

Education

• M.Sc., Mechanical. Eng.

  Chalmers University of Technology

• M.A., Mech. and Aero. Eng.

  Princeton University

• Ph. D, Mech. and Aero. Eng.

  Princeton University

Awards & honors

• 2016 Air Force Young Investigator Award
• 2017 NSF CAREER award
• 2017 Nobuhide Kasagi Award