Research Projects

Wind turbines are already among the most massive structures in the world and continue to increase in size each year. The large size of wind turbines leads to aerodynamics problems at extreme Reynolds numbers. At these high Reynolds numbers, there is a lack of experimental data for airfoil geometries relevant to wind turbines due to the difficulty and cost of replicating these flows in a traditional wind tunnel.

This work utilizes the High Reynolds Number Testing Facility (HRTF), a high pressure wind tunnel, to achieve the highest Reynolds number airfoil experiments in the world. The pressure distribution on the surface of the airfoil is measured to calculate key aerodynamic characteristics such as lift and drag. With these measurements, the effects of Reynolds number, surface roughness, airfoil geometry, and dynamic pitching are characterized.

Wind turbine blades are becoming longer and more slender to decrease the cost of energy at the expense of making them more susceptible to vibrations and flutter. Unsteady aerodynamic loading and vibrations have resulted in occasional structural damage to parked turbines over a large range of rotor sizes and inflow conditions, but existing engineering models of this behavior yield limited accuracy for circumstances when the blade is parked, or not rotating. A leading hypothesis for these uncharacterized vibrations is Stall-Induced Vibrations (SIVs) from transverse flow over the rotor.

At Princeton, researchers are using the High Reynolds Test Facility (HRTF), a variable-density wind tunnel, to approach dynamic similitude in their experiments. Increasing the Reynolds number with density rather than flow velocity allows for measurable vibrational response times when characteristic length scales are shortened. To emulate the various inflow conditions a parked turbine may experience (sheared and veered), a range of inflow directions is tested using predetermined mounting angles and motorized rotation. Custom accelerometer instrumentation, base-mounted load cells, and hotwire traverses enable simultaneous wake and structural measurements. These 3-dimensional aerodynamics experiments are among the first of their kind, and a key to better understanding the loading on parked turbines.

This work is being completed in collaboration with MIT X GE Vernova Energy and Climate Alliance.

Around the globe, hurricanes, tornadoes, and flash floods cause significant loss of life and cost billions of dollars in damage annually. Large structures, like bridges and skyscrapers, have been subject to unexpected failure due to a lack of understanding of the fluid-structure interactions at High Reynolds numbers. Flow-induced flutter, buffeting, and wake oscillations degrade structural integrity over time, cause deformations and vibrations unfit for users, and can result in complete failure. In terms of the ratio between inertial and viscous forces, the forcing on a submerged bridge in a flash flooding event is similar to wind loading from hurricanes on similar-sized structures.

Structures are intentionally being designed with openings (porosity) to reduce wind loading. These openings, which allow wind to pass through the side of a bridge or building, are a breeding ground for turbulent boundary layers and flow separation to occur. At such high Reynolds numbers, 𝒪(10⁶), current modeling software is unable to resolve all the scales of the problem, so the extent of the interaction between these highly turbulent flows is unknown. To better design structures to survive extreme flows, sample structures are tested in the High Reynolds Test Facility (HRTF), a variable-density wind tunnel at Princeton University. The facility leverages the fluid properties of compressed gas (up to 200 Bar) to replicate the loads on large objects within a smaller working area. This novel method of structural design promises more physically relevant results than small-scale atmospheric wind tunnels, and reduced model construction costs compared to massive-scale atmospheric wind tunnels.

This work is being completed in collaboration with Damario Berry and Professor Maria Garlock in the Creative & Resilient Urban Engineering (CRUE) Lab.

Wall-bounded flows in the environment are often modulated by buoyancy. For example, vertical temperature gradients in the atmospheric boundary layer introduce forces that modify turbulent transport. When density decreases with height, the fluid is stably stratified. Turbulent eddies attempting to lift dense fluid parcels away from the surface have their kinetic energy reduced and converted to potential energy. At the same time, shear due to the no-slip condition drives turbulence production. The ratio of buoyancy to shear effects is characterized by the Richardson number.

Emulating these flows in the laboratory is extremely difficult because Reynolds numbers are generally very large. Achieving high Reynolds numbers by increasing freestream velocity drives down the Richardson number. A different mechanism is therefore required to increase the Reynolds number.

The SuperTank is an unconventional closed-circuit wind tunnel designed to simultaneously achieve high Reynolds numbers and geophysically relevant Richardson numbers. Experiments are conducted within a 12.5 m-long cylindrical pressure vessel with a 3 m diameter. Compressed dry air, pressurized up to 88 bar, flows through a square test section measuring 0.88 m in width and 7.5 m in length. Freestream velocities range from 0.5 to 10 m/s. To create bulk temperature differences of up to 80 °C, a plate with 18 individually controlled heating zones is mounted on the ceiling of the test section. Reynolds numbers based on freestream velocity and boundary-layer height range from 220,000 to 2,500,000. Bulk Richardson numbers up to 1 can be achieved.

The zero-pressure gradient flat plate turbulent boundary layer is one of the most well studied fundamental problems in turbulence, however, there remain many open questions. This canonical flow has a wide range of engineering applications and acts as a baseline for more complex flows. This work aims to develop a better understanding of how fluid structures affect the pressure and shear stress forces acting on the wall, as well as evaluate new methods for measuring the pressure field.

Experiments are performed in the Smoke Tunnel, a traditional wind tunnel with a test section of 1 m by 0.5 m and flow speeds up to 5 m/s. Flow field measurements are made using four cameras oriented at different angles to allow for 3D particle tracking velocimetry. Additionally, simultaneous measurements of the wall pressure and shear stress are made using a microphone and a direct shear stress sensor.

As modern wind turbines continue to grow to diameters approaching 200 m, it becomes increasingly difficult to perform computer simulations or laboratory experiments that match all governing parameters simultaneously. Prior work has been limited by tradeoffs among three important nondimensional numbers for wind-turbine aerodynamics: Reynolds number, tip-speed ratio, and Mach number. In traditional small-scale wind tunnels, these parameters cannot all be matched to full-scale values.

Researchers in the Hultmark Lab use a high-pressure wind tunnel in which density can be varied, allowing the Reynolds number to be adjusted independently of the tip-speed ratio. The facility enables controlled laboratory studies that match key full-scale wind-turbine flow physics. It is instrumented with a hot-wire traverse for detailed measurements of turbine wakes. Topics include the dynamics and wake structure of yaw-misaligned turbines, differences between actuator-disc and turbine-wake profiles, and the effects of dynamic induction control on wake recovery.