Aliza Abraham

University of Minnesota Twin Cities | UMN · Department of Mechanical Engineering

Wakes behind rotors such as wind turbines, propellers, and helicopters can have detrimental effects on downstream structures including increased structural loading. These wakes are characterized by helical vortices shed from the blade tips, which are subject to instabilities that cause the vortices to break down, reducing the strength of the wake....

Helical vortex systems, such as those found in the wakes of wind turbines, helicopter rotors and propellers, are subject to instabilities that lead to pairing between adjacent vortex loops. Certain modes of these instabilities can be triggered by an asymmetry in the rotor generating the vortices. In three-vortex systems, like those formed by many i...

Wakes behind rotors such as wind turbines, propellers, and helicopters can have detrimental effects on downstream structures including increased structural loading. These wakes are characterized by helical vortices shed from the blade tips, which are subject to instabilities that cause the vortices to break down, reducing the strength of the wake....

Atmospheric turbulent velocity fluctuations are known to increase wind turbine structural loading and accelerate wake recovery, but the impact of vortical coherent structures in the atmosphere on wind turbines has not yet been evaluated. The current study uses flow imaging with natural snowfall with a field of view spanning the inflow and near wake...

We present a field study of snow settling dynamics based on simultaneous measurements of the atmospheric flow field and snow particle trajectories. Specifically, a super-large-scale particle image velocimetry (SLPIV) system using natural snow particles as tracers is deployed to quantify the velocity field and identify vortex structures in a 22 m $\...

The current study uses large eddy simulations to investigate the transient response of a utility-scale wind turbine wake to dynamic changes in atmospheric and operational conditions, as observed in previous field-scale measurements. Most wind turbine wake investigations assume quasi-steady conditions, but real wind turbines operate in a highly stoc...

Atmospheric turbulent velocity fluctuations are known to increase wind turbine structural loading and accelerate wake recovery, but the impact of vortical coherent structures in the atmosphere on wind turbines has not yet been evaluated. The current study uses flow imaging with natural snowfall with a field of view spanning the inflow and near wake...

With the rapid growth of wind turbine installation in recent decades, fundamental physical understanding of the flow around wind turbines and farms is becoming increasingly critical for further efficiency increases. However, the effort to develop this understanding is hindered by the significant challenges involved in modelling such a complex dynam...

We present a field study of snow settling dynamics based on simultaneous measurements of the atmospheric flow field and snow particle trajectories. Specifically, a super-large-scale particle image velocimetry (SLPIV) system using natural snow particles as tracers is deployed to quantify the velocity field and identify vortex structures in a 22 m $\...

As wind energy continues to expand, increased interaction between wind farms and their surroundings can be expected. Using natural snowfall to visualize the air flow in the wake of a utility-scale wind turbine at unprecedented spatio-temporal resolution, intermittent periods of strong interaction between the wake and the ground surface are observed...

The effect of turbulence on snow precipitation is not incorporated into present weather forecasting models. Here we show evidence that turbulence is in fact a key influence on both fall speed and spatial distribution of settling snow. We consider three snowfall events under vastly different levels of atmospheric turbulence. We characterize the size...

In collaboration with 16 musicians from the Minnesota Orchestra, we assess the airflow and particle concentration emitted from ten wind instruments under realistic performance conditions. Anemometer and schlieren measurement techniques are used to quantify the air flow, and aerodynamic particle sizer, laser sheet, and digital inline holography tech...

The simulation (validated with experimental measurements) showing the distinct patterns of aerosol emission from seven different wind instruments.

Taking advantage of the thermal plume that drives the aerosols moving upwards, the filter placed on top of the trumpet outlet showing a significantly higher efficacy of removing the aerosols (>90%) generated during trumpet play than the filter placed on the ground in front of the instrument (<5%).

Simulations show that the enhancement of thermal plume by lowering the ambient temperature can increase the efficiency of filters placed on top of the instrument to remove aerosols generated during trumpet play.

Schlieren imaging and aerosol measurements showing the changing flow patterns and aerosol emission during clarinet play

Schlieren imaging and aerosol measurements showing the changing flow patterns and aerosol emission during trumpet play

There has been growing concern about high risk of airborne infection during wind instrument performance as the COVID-19 pandemic evolves. In collaboration with 16 musicians from the Minnesota Orchestra, we employ multiple experimental and numerical techniques to quantify the airflow and aerosol concentration emitted from ten wind instruments under...

High-resolution field-scale experiments using flow visualization with natural snowfall and high-fidelity large eddy simulations are combined to investigate the effect of dynamic turbine operation and atmospheric conditions on wind turbine wake mixing and recovery in the wake of a 2.5 MW wind turbine. Instantaneous near-wake expansion and deflection...

The current study uses large eddy simulations and field-scale measurements to investigate the transient response of a utility-scale wind turbine wake to dynamic changes in atmospheric and operational conditions. Most wind turbine wake investigations assume quasi-steady conditions, but real wind turbines operate in a highly stochastic atmosphere, an...

The effect of turbulence on snow precipitation is not incorporated into present weather forecasting models. Here we show evidence that turbulence is in fact a key influence on both fall speed and spatial distribution of settling snow. We consider three snowfall events under vastly different levels of atmospheric turbulence. We characterize the size...

As wind energy continues to expand, increased interaction between wind farms and their surroundings can be expected. Using natural snowfall to visualize the air flow in the wake of a utility-scale wind turbine at unprecedented spatio-temporal resolution, we observe intermittent periods of strong interaction between the wake and the ground surface a...

A better understanding of the intense interaction between the turbulent inflow and rotating wind turbine components is critical to accelerating the path towards larger wind turbines. The present investigation provides the first field characterization of the influence of turbulent inflow on the blade structural response of a utility-scale wind turbi...

This paper provides a review of the general experimental methodology of snow-powered flow visualization and super-large-scale particle image velocimetry (SLPIV), the corresponding field deployments and major scientific findings from our work on a 2.5 MW utility-scale wind turbine at the Eolos field station. The field measurements were conducted to...

We present incoming flow characterization of a 2.5 MW turbine at high spatio-temporal resolution, using super-large-scale particle image velocimetry (SLPIV). The datasets have a field of view of 85 m (vertical) × 40 m (streamwise) centered at 0.2 rotor diameter upstream. The mean field shows the presence of the induction zone and a distinct region...

The present investigation provides the first field characterization of the influence of turbulent inflow on the blade structural response of a utility-scale wind turbine (2.5MW), using the unique facility available at the Eolos Wind Energy Research Station of the University of Minnesota. A representative one-hour dataset under a stable atmosphere i...

Understanding wind turbine wake mixing and recovery is critical for improving the power generation and structural stability of downwind turbines in a wind farm. In the field, where incoming flow and turbine operation are constantly changing, the rate of wake recovery can be significantly influenced by dynamic wake modulation, which has not yet been...

Super-large-scale particle image velocimetry and flow visualization with natural snowfall is used to collect and analyse multiple datasets in the near wake of a 2.5 MW wind turbine. Each dataset captures the full vertical span of the wake from a different perspective. Together, these datasets compose a three-dimensional picture of the near-wake flo...

Let it snow: combining super-large-scale snow visualization and large eddy simulation to reveal interesting near wake phenomena of a utility-scale (2.5 MW) wind turbine.

This paper provides a review of the general experimental methodology of snow-powered flow visualization and super-large-scale particle imaging velocimetry (SLPIV), the corresponding field deployments and major scientific findings from our work on a 2.5 MW utility-scale wind turbine at the Eolos field station. The field measurements were conducted t...

Super-large-scale particle image velocimetry and flow visualization with natural snowfall is used to collect and analyze multiple datasets in the near wake of a 2.5 MW wind turbine. Each dataset captures the full vertical span of the wake from a different perspective. Together, these datasets compose a three-dimensional picture of the near-wake flo...

The atmospheric incoming flow of a wind turbine is intimately connected to its power production as well as its structural stability. Here we present an incoming flow measurement of a utility-scale turbine at the high spatio-temporal resolution, using super-large-scale particle image velocimetry (SLPIV) with natural snowflakes. The datasets include...

Understanding wind turbine wake mixing and recovery is critical for improving the power generation and structural stability of downwind turbines in a wind farm. In the field, where incoming flow and turbine operation are constantly changing, wake recovery can be significantly influenced by dynamic wake modulation, which has not yet been explored. H...

Super-large-scale particle image velocimetry (SLPIV) using natural snowfall is used to investigate the influence of nacelle and tower generated flow structures on the near-wake of a 2.5 MW wind turbine at the EOLOS field station. The analysis is based on the data collected in a field campaign on March 12th, 2017, with a sample area of 125 m (vertic...

This thesis describes the use of a combination of various visual techniques to characterize the flow-structure interaction of a breaking wave impacting a flexible vertically mounted plate. Several experiments were conducted on a simulated dam break in which water was rapidly released from a reservoir to generate a wave, which impinged on a cantilev...

A mechanistic understanding of goal directed behavior in vertebrates is hindered by the relative inaccessibility and size of their nervous systems. Here, we have studied the kinematics of prey capture behavior in a highly accessible vertebrate model organism, the transparent larval zebrafish (Danio rerio), to assess whether they use visual cues to...