The John Field Laboratory
Measurements in fluid mechanics are crucial for several reasons, including:
- Validation of theoretical models and simulations: Experimental data is necessary to confirm the accuracy of theoretical models and computational simulations.
- Design and optimization of engineering systems: Measurements assist in the design and optimization of engineering systems such as pumps, turbines, and pipelines.
- Understanding of flow phenomena: Detailed measurements are important for understanding complex flow phenomena such as turbulence and vortex flows.
- Driving force for research and innovation: Accurate measurements enable the exploration of new concepts and the validation of new theories, driving current research and innovation forward.
Many technological developments, such as airplanes and water turbines, which emerged during the 20th century, were driven by experimental research and development.
Our equipment
Below is a brief summary of some of our equipment. These primarily consist of optical systems, which are used to measure flow velocity in various applications non-intrusively.
PIV is a laser-based measurement method for flow velocity, where the area or volume under study is illuminated with a light sheet or a light volume, depending on the nature of the measurement. The light is typically generated by a laser or an LED. Measurements can be conducted of two velocity components in two dimensions, known as 2D2C PIV, of three velocity components in two dimensions (2D3C), or of three velocity components in three dimensions (3D3C). Flow velocity is measured indirectly by adding particles of size 1-200µm to the flow. A pulsed light source illuminates the particles, which are then photographed by a set of cameras. Advanced computer algorithms estimate the displacement of the particles between the laser and the images, which ultimately can be related to the flow velocity.
Tomographic PIV
Fluid mechanics has access to a FlowMaster Tomographic (Tomo) PIV system from LaVision. This is a so-called 3D3C PIV, which can simultaneously measure all three velocity components in three dimensions. The system's flexibility also allows for stereoscopic or planar measurements if the flow phenomenon being studied does not require three-dimensional measurements.
The PIV system has mainly been used to study the flow through ordered porous materials.
Stereoscopic PIV System
In 2023, Fluid Mechanics acquired a stereoscopic PIV system from Dantec Dynamics. Like the tomographic system, Dantec's system can be used for two-component measurements in a plane.
The system has been used to study time-dependent flow in jets, as well as the flow through a model of a water turbine in Vattenfall's laboratory in Älvkarleby.
Micro-PIV
Micro-PIV is an extension of regular PIV for the study of small flow fields; typically, flow fields of the order of micro- to millimeters. To enable studies on such a small scale, a microscope is used, connected to the camera that photographs the flow. Fluid Mechanics has a micro-PIV system from Dantec Dynamics. The system was upgraded in 2024 with an LED as the light source. At the John Field Laboratory, we typically study the flow of lubricants.
Fluid Mechanics' tomographic PTV system, FlowMaster 4D-PTV / Shake-the-Box from LaVision, enables tomographic measurements of velocity. Through these velocity measurements, time-resolved estimates of pressure can also be extracted in the same volume where the velocity is measured. Typically, pressure can only be measured at single points on the edges of a flow geometry, so this estimation of pressure provides a unique insight into the flow. This system illuminates the studied volume using an LED. PTV, which stands for particle tracking velocimetry, differs from PIV. In PIV, flow velocity is estimated based on the movement of a large number of particles, while in PTV, velocity is estimated based on the movement of individual particles. Advantages of LaVision's PTV system include high accuracy for velocity and acceleration, as well as shorter data evaluation times.
The system has been used in studies of flow around skiers in the climate wind tunnel at Mid Sweden University in Östersund. In addition to the measurements in the climate wind tunnel, the system has been used to study ecohydraulic flows with free water surfaces, as well as after pipe bends related to hydrogen transport in pipelines.
LDV is a technique used to measure the velocity of liquids or gases using laser light. Two laser beams intersect at a point in the flow, creating an interference pattern. Particles moving through this pattern scatter light, resulting in a Doppler shift in the frequency of the scattered light proportional to their velocity (similar to the phenomenon that occurs with the sound of an ambulance as it passes by). This frequency shift is detected and analyzed to determine the particle's velocity. LDV is non-intrusive and allows for accurate measurements without disturbing the flow. LDV can be used in a variety of applications within fluid mechanics. The technique provides high spatial and temporal resolution, making it ideal for detailed flow analysis.
Fluid Mechanics has an LDV system from TSI and has previously had a system from Dantec Dynamics. These systems have been used to measure flow velocity in time-dependent pipe flow at LTU, secondary flows in a simplified setup of a rotating kiln, and in Vattenfall's laboratory in Älvkarleby.
CTA is used to measure velocity or wall shear stress in turbulent flows. The sensor used in CTA, which consists of either a thin wire or a thin film, acts as a resistor in a Wheatstone bridge. When fluid flows around a heated object, which in this case is the sensor, heat is transferred from the object to the flowing medium. A servo amplifier ensures that the bridge remains balanced by adjusting the electrical current through the sensor to keep the temperature (and therefore the resistance) constant, regardless of the flow speed over the sensor. The resulting electrical voltage across the sensor can thus be related to the flow velocity or wall shear stress through calibration.
Fluid Mechanics has used CTA to study wall shear stress in time-dependent pipe flow. These studies have included both transient and pulsating flow cases. Measurements have been conducted in a 100 mm pipe at LTU at relatively low Reynolds numbers (Re<50,000), as well as in a larger setup at NTNU in Norway (Re>1,500,000).
Our set-ups
In the John Field Laboratory, we have the following setups, among others. These are used in doctoral projects, undergraduate education, and by senior researchers. The experimental measurement equipment described above is utilized in these setups. Many of our setups are connected to the hydropower industry. Studies are conducted on both the flow through water turbines and environmental issues, such as those related to fish migration. Fluid Mechanics also has flow cells of ordered porous materials and a setup for tomographic measurement of the flow after a pipe bend.
A simplified model of a hydrogen pipeline is available for studying the flow after geometric alterations such as pipe bends, contractions, and valves. Specifically, the phenomenon of "swirl-switching" is studied, which is an interesting phenomenon that occurs in turbulent pipe flow after a bend.
So far, studies have been conducted in a mixture of water and glycerol. This enables the use of the tomographic PTV system described above.
In the long term, studies with hydrogen as the flowing medium are also planned. The aim is for these to be conducted in the H2-Labs, which is a large-scale hydrolysis facility that is planned outside of Piteå.
Flow cells representing ordered porous materials have been used in several studies to conduct tomographic PIV measurements. The porous beds have been in the form of staggered and cubic structures. The studied Reynolds numbers have ranged from 10 < Re < 1000, covering the laminar, transitional, and turbulent regimes of the flow.
A 1:15.5 scaled-down model of the Kaplan turbine Porjus U9 is available in the Fluid Mechanics laboratory. Research projects conducted with this setup have mainly focused on the so-called vortex rope, a phenomenon that occurs when water turbines operate at relatively low power output. To optimize the use of hydropower as a source of regulating power, it is important to find methods to combat the harmful phenomena resulting from the vortex. In this setup, stereoscopic and planar PIV measurements, as well as LDV measurements, have been conducted to characterize the vortex rope. Various methods to combat the vortex have been studied, with both pressure and LDV measurements conducted to examine the impact of these methods on the vortex.
Since the turbine is a scaled-down model of Porjus U9, there is a unique opportunity to study scale effects, both against the prototype and the 1:3.875 model of Porjus U9 available in Vattenfall's laboratory in Älvkarleby.
The setup is also used in undergraduate education in the course Water Turbine - F7017T.
In our open channel, secondary flows generated by various geometric shapes that mimic natural obstacles in our rivers are studied. The goal is to map the flow mechanical structures generated by these geometric shapes, ultimately to understand which flow structures attract fish during their upstream migration for spawning.
In this channel, the optical measurement methods PTV and LDV are primarily used.
The pressure-time method is a primary method for determining the flow in water turbines. The flow is determined by measuring the pressure between two cross-sections in a circular pipe. According to the international standard IEC60041, certain requirements are set for the measurement section to ensure that the results are sufficiently accurate.
In the Fluid Mechanics laboratory, there is a setup to study the pressure-time method under conditions that do not meet the requirements of IEC60041. Swedish hydropower plants often have a low head, which means they rarely meet the requirements of the IEC standard. Therefore, to make the method more useful under conditions prevalent in Swedish hydropower plants, the method needs to be developed. In the setup, water flows between two tanks that are displaced 3.6 meters in the vertical direction. The water is continuously pumped back from the lower to the upper tank, and weirs ensure that the water levels are always at the same height in both tanks. The maximum flow in the system is 15 l/s. Among other things, the possibility of developing the pressure-time method to apply when the flow passes through a pipe bend or a contraction is being investigated.
Field Studies and External Measurement Campaigns
Fluid Mechanics conducts research in close collaboration with industrial partners and other universities. Below is a selection of studies that have been conducted in full-scale setups and in external laboratories.
Mitigation of Vortical structures in Full-Scale Hydropower Plants
The so-called vortex rope that can occur when water turbines operate at a relatively low power output limits hydropower's ability to provide regulating power. Within the project AFC4Hydro, which concluded in 2023, tests were conducted to mitigate the harmful vortex rope in two full-scale water turbines: the 10 MW Kaplan turbine Porjus U9 and a 230 MW Francis turbine in Oksla, Norway.
Image-based river surface velocimetry downstream Boden hydropower plant
Imaging techniques have been used to measure river velocity, specifically emphasizing natural surface floating patterns. By employing a multi-camera system, 3D measurements on river surfaces, including surface velocity and water surface reconstruction were performed using Lagrangian Pattern Tracking Velocimetry and Large-scale Particle Image Velocimetry. The discharge during these large-scale tests reached almost 1000 cubic meters per second.
The flow around cross-country skiers has been studied at Mid Sweden University's climate wind tunnel in Östersund using the tomographic PTV system. Aerodynamics plays a crucial role in sports because air resistance is one of the forces that counteract movement in many disciplines, such as cross-country skiing, cycling, and speed skating.
The study in the climate wind tunnel focused on the wake flow formed behind the skier during skiing. In all contexts where air resistance is significant, the wake flow is particularly interesting. The properties of the wake affect the total amount of generated air resistance, and detailed measurements of these properties provide a unique insight into the air resistance generated by skiers.
Fluid Mechanics has been conducting research at Vattenfall's laboratory in Älvkarleby for more than 15 years.
Turbine Model
Several studies have been conducted on a model of the research turbine Porjus U9, located 50 km north of Jokkmokk. Often, laser-based methods such as PIV and LDV have been used, but high-speed photography and pressure measurements have also been carried out to improve the understanding of the flow in different parts of the turbine.
Hydraulic Engineering
Studies have been conducted in channels with rough surfaces to simulate the flow in blasted rock tunnels. Optical methods such as PIV have been used for these studies, but the acoustic method Acoustic Doppler Velocimetry (ADV) has also been utilized. Additionally, the flow in spillways has been studied using ADV.
Investments
Fluid Mechanics constantly strives to have access to the latest technology for advanced measurements of fluid mechanical phenomena. Our lab is one of the most modern and best-equipped in Sweden for fluid mechanics studies, mainly through optical measurement methods. In recent years, we have invested in:
- Stereoscopic PIV system from Dantec Dynamics (2023).
- Tomographic PTV system from LaVision (2021).
- Tomographic PIV system from LaVision (2018).
Funding
The funding for these investments comes from the Kempe Foundations and LTU's lab fund, as well as the following financiers:
- Swedish Research Council
- European Union
- Swedish Hydropower Centre
- Vinnova
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