Marcus Hultmark
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View article: Spanwise wall forcing can reduce turbulent heat transfer more than drag
Spanwise wall forcing can reduce turbulent heat transfer more than drag Open
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 f…
View article: Understanding the effects of rotation on the wake of a wind turbine at high Reynolds number
Understanding the effects of rotation on the wake of a wind turbine at high Reynolds number Open
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 investi…
View article: Acceleration is the key to drag reduction in turbulent flow
Acceleration is the key to drag reduction in turbulent flow Open
A turbulent pipe flow experiment was conducted where the surface of the pipe was oscillated azimuthally over a wide range of frequencies, amplitudes, and Reynolds numbers. The drag was reduced by as much as 35%. Past work has suggested tha…
View article: Understanding the effects of rotation on the wake of a wind turbine at high Reynolds number
Understanding the effects of rotation on the wake of a wind turbine at high Reynolds number Open
The wake of a horizontal-axis wind turbine was studied at $Re_D=4\times10^6$ with the aim of revealing the effects of the tip speed ratio, $λ$, on the wake. Tip speed ratios of $4<λ<7$ were investigated and measurements were acquired up to…
View article: Spanwise wall forcing can reduce turbulent heat transfer more than drag
Spanwise wall forcing can reduce turbulent heat transfer more than drag Open
Direct numerical simulations are performed of turbulent forced convection in a half channel flow with wall oscillating either as a spanwise plane oscillation or to generate a streamwise travelling wave. The friction Reynolds number is fixe…
View article: A non-intrusive volumetric camera calibration system
A non-intrusive volumetric camera calibration system Open
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 phy…
View article: Lagrangian particle tracking in the atmospheric surface layer
Lagrangian particle tracking in the atmospheric surface layer Open
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 singl…
View article: Contact-angle hysteresis provides resistance to drainage of liquid-infused surfaces in turbulent flows
Contact-angle hysteresis provides resistance to drainage of liquid-infused surfaces in turbulent flows Open
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 …
View article: Contact-angle hysteresis provides resistance to drainage of liquid-infused surfaces in turbulent flows
Contact-angle hysteresis provides resistance to drainage of liquid-infused surfaces in turbulent flows Open
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 …
View article: Acceleration is the Key to Drag Reduction in Turbulent Flow
Acceleration is the Key to Drag Reduction in Turbulent Flow Open
A turbulent pipe flow experiment was conducted where the surface of the pipe was oscillated azimuthally over a wide range of frequencies, amplitudes and Reynolds number. The drag was reduced by as much as 30\%. Past work has suggested that…
View article: Kirigami-inspired wind steering for natural ventilation
Kirigami-inspired wind steering for natural ventilation Open
Ensuring adequate ventilation of exterior and interior urban spaces is essential for the safety and comfort of inhabitants. Here, we examine how angled features can steer wind into areas with stagnant air, promoting natural ventilation. Us…
View article: Similarity of length scales in high-Reynolds-number wall-bounded flows
Similarity of length scales in high-Reynolds-number wall-bounded flows Open
The wall dependence of length scales used to describe large- and small-scale structures of turbulence is examined using highly resolved experiments in zero-pressure-gradient turbulent boundary layers and pipe flows spanning the range $2000…
View article: Evaluation of Elastic Filament Velocimetry (EFV) Sensor in Ventilation Systems: An Experimental Study
Evaluation of Elastic Filament Velocimetry (EFV) Sensor in Ventilation Systems: An Experimental Study Open
Determination of airflow rates is an inevitable part of the energy-efficient control of ventilation systems. To achieve efficient control, the flowmeters used must be suitably accurate and create minimum disturbance to the airflow. In this…
View article: FlowDrone: Wind Estimation and Gust Rejection on UAVs Using Fast-Response Hot-Wire Flow Sensors
FlowDrone: Wind Estimation and Gust Rejection on UAVs Using Fast-Response Hot-Wire Flow Sensors Open
Unmanned aerial vehicles (UAVs) are finding use in applications that place increasing emphasis on robustness to external disturbances including extreme wind. However, traditional multirotor UAV platforms do not directly sense wind; convent…
View article: Fast-response hot-wire flow sensors for wind and gust estimation on UAVs
Fast-response hot-wire flow sensors for wind and gust estimation on UAVs Open
Due to limitations in available sensor technology, unmanned aerial vehicles (UAVs) lack an active sensing capability to measure turbulence, gusts, or other unsteady aerodynamic phenomena. Conventional in situ anemometry techniques fail to …
View article: Comparison of dynamic stall on an airfoil undergoing sinusoidal and VAWT-shaped pitch motions
Comparison of dynamic stall on an airfoil undergoing sinusoidal and VAWT-shaped pitch motions Open
The aerodynamics of vertical axis wind turbines (VAWTs) are inherently unsteady because the blades experience large angle of attack variations throughout a full turbine revolution. At low tip speed ratios, this can lead to a phenomenon kno…
View article: Heat transfer increase by convection in liquid-infused surfaces for laminar and turbulent flows
Heat transfer increase by convection in liquid-infused surfaces for laminar and turbulent flows Open
Liquid-infused surfaces can reduce friction drag in both laminar and turbulent flows. However, the heat transfer properties of such multi-phase surfaces have still not been investigated to a large extent. We use numerical simulations to st…
View article: A stereoscopic PIV system for the Princeton Superpipe
A stereoscopic PIV system for the Princeton Superpipe Open
Herein, we describe the design and testing of a stereoscopic PIV system uniquely adapted for the high pressure environment of the Princeton Superpipe. The Superpipe is a recirculating pipe facility that utilizes compressed air as the worki…
View article: Reynolds stress scaling in the near-wall region of wall-bounded flows
Reynolds stress scaling in the near-wall region of wall-bounded flows Open
A new scaling is derived that yields a Reynolds number independent profile for all components of the Reynolds stress in the near-wall region of wall bounded flows, including channel, pipe and boundary layer flows. The scaling demonstrates …
View article: Reynolds stress scaling in the near-wall region
Reynolds stress scaling in the near-wall region Open
A new scaling is derived that yields a Reynolds number independent profile for all components of the Reynolds stress in the near-wall region of wall bounded flows. The scaling demonstrates the important role played by the wall shear stress…
View article: Scaling turbulence in the near-wall region
Scaling turbulence in the near-wall region Open
A new velocity scale is derived that yields a Reynolds number independent profile for the streamwise turbulent fluctuations in the near-wall region of wall bounded flows for $y^+<25$. The scaling demonstrates the important role played by t…
View article: Solidity effects on the performance of vertical-axis wind turbines
Solidity effects on the performance of vertical-axis wind turbines Open
The variety of configurations for vertical-axis wind turbines (VAWTs) make the development of universal scaling relationships for even basic performance parameters difficult. Rotor geometry changes can be characterized using the concept of…
View article: Characterization of the wake behind a horizontal-axis wind turbine (HAWT) at very high Reynolds numbers
Characterization of the wake behind a horizontal-axis wind turbine (HAWT) at very high Reynolds numbers Open
The wake of a three-bladed horizontal axis wind turbine was studied at aerodynamic conditions similar to what is experienced by commercially available turbines. Field relevant Reynolds numbers and tip speed ratios were obtained through the…
View article: Unsteady effects on a pitching airfoil at conditions relevant for large vertical axis wind turbines
Unsteady effects on a pitching airfoil at conditions relevant for large vertical axis wind turbines Open
Interest in offshore vertical axis wind turbines (VAWTs) has created a need to study VAWTs at much higher Reynolds numbers than they have previously been studied at. VAWTs are characterised by unsteady aerodynamics, and high Reynolds numbe…
View article: Dynamic stall at high Reynolds numbers due to variant types of airfoil motion
Dynamic stall at high Reynolds numbers due to variant types of airfoil motion Open
Unsteady airfoil experiments were conducted in a high-pressure wind tunnel at chord Reynolds numbers of Re c = 3.0 × 10 6 . A moderately thick NACA0021 airfoil was pitched from rest beyond the static stall angle in six individual ramp test…
View article: A Soft Material Flow Sensor for Micro Air Vehicles
A Soft Material Flow Sensor for Micro Air Vehicles Open
To control and navigate micro air vehicles (MAVs) efficiently, there is a need for small, lightweight, durable, sensitive, fast, and low-power airspeed sensors. When designing sensors to meet these requirements, soft materials are promisin…