Session L4: Industrial Application: Power Generation

Simulations of Oscillating Hydrofoils in Array Configurations

The vortex and wake interactions of multiple oscillating foils are investigated computationally for energy harvesting applications. Oscillating with high pitch and heave amplitudes to maximize power production, the elliptical-shaped foils generate large coherent vortices at the leading and trailing edge, which are shed downstream to create a large highly structured wake of vortices with alternating sign. Downstream foils oscillate within the large organized wake at a relative phase angle to the lead foil such that power efficiency is optimized. When placed directly downstream of one another, the optimal phase of a second foil is to avoid interactions with the first foil’s wake, generating less than half of the total power of the first foil. However, when placed in a staggered configuration the downstream foil has an increase in efficiency through constructive vortex-foil interactions.   

Effect of Elevated Free Stream Turbulence on the Hydrodynamic Performance of a Tidal Turbine Blade Section

The effects of elevated freestream turbulence (FST) on the performance of a tidal turbine blade is studied using laboratory experiments. Of interest for the current investigation is elevated levels of FST in the range of 6-24{\%} that is prevalent in deployment sites of tidal turbines. A constant chord, no twist blade section (SG6043) is tested at an operating Reynolds number of 1.5x10$^{\mathrm{5}}$ and at angles of attack ranging from -90$^{\mathrm{o}}$ to $+$90$^{\mathrm{o}}$. The parameter space encompasses the entire operational range of a tidal turbine that includes flow reversal. Multiple levels of controlled FST are achieved using an active grid type turbulence generator placed at the entrance to the water tunnel test section. The hydrodynamic loads experienced by the blade section are measured using a 3-axis load cell; a Stereo-PIV technique is used to analyze the flow field around the blade. The results indicate that elevated levels of FST cause a delay in flow separation when compared to the case of a laminar freestream. Furthermore, the lift to drag ratio of the blade is considerably altered depending on the level of FST and angle of attack tested.   

The effect of blade pitch in the rotor hydrodynamics of a cross-flow turbine.

In this work we will show how the hydrodynamics of the rotor of a straight-bladed Cross-Flow Turbine (CFT) are affected by the Tip Speed Ratio (TSR), and the blade pitch angle imposed to the rotor. The CFT model used in experiments consists of a three-bladed (NACA-0015) vertical axis turbine with a chord (c) to rotor diameter (D) ratio of 0.16. Planar Digital Particle Image Velocimetry (DPIV) was used, with the laser sheet aiming at the mid-span of the blades, illuminating the inner part of the rotor and the near wake of the turbine. Tests were made by forcing the rotation of the turbine with a DC motor, which provided precise control of the TSR, while being towed in a still-water tank at a constant Reynolds number of 61000. A range of TSRs from 0.7 to 2.3 were covered for different blade pitches, ranging from 8$^{\circ}$ toe-in to 16$^{\circ}$ toe-out. The interaction between the blades in the rotor will be discussed by examining dimensionless phase-averaged vorticity fields in the inner part of the rotor and mean velocity fields in the near wake of the turbine.   

Shape Optimization of A Turbine-99 Draft Tube Using Design-by-Morphing

We have found the “optimal” shape of a turbine-99 draft tube that maximizes its pressure recovery factor using a new design method called design-by- morphing. In design-by- morphing, new draft tubes are created by morphing multiple baseline draft tubes with different weights. The surfaces of baseline draft tubes are approximated by a summation of spectral coefficients multiplied by spectral basis functions. Then, a morphed draft tube is produced by computing a new set of spectral coefficients which are a weighted average of the spectral coefficients of the baseline draft tubes. The “optimal” draft tube is obtained by finding the weights such that the mean pressure recovery factor is maximized. After optimization is carried out using design-by- morphing, the high static pressure region is significantly reduced, and the flow is smoother and more uniform than it was in any of the baseline turbine-99 draft tubes. The optimal draft tube shows a 10.9\% improvement over the turbine-99 draft tube. We have applied this method to trains and to aircrafts, and have reduced the drag and the drag-to-lift ratio by 13.2\% and 23.1\%, respectively. We believe that this optimization method is applicable to many engineering applications in which the performance of an object depends on its shape.   

Effects of free surface on flow energy harvesting system based on flapping foils

Here, we consider a flapping foil based energy harvester, which is modelled by a 2D NACA0015 foil performing coupled motions of pitching and heaving. Volume of fraction(VOF) method is employed to capture the free surface. We fix the Reynolds number at $Re=900$, and the Froude number at $Fr=0.32$. We fix the non-dimensional flapping frequency at $f=0.16$, the pitching amplitude at $\theta_0=75^\circ$, and the heaving amplitude at $h_0=1c$, where $c$ is the chord length. With these parameters, the harvester has been proved to reach its highest efficiency of $\eta=0.34$ in a single phase flow. By varying the submergence $d$, which is defined as the distance between the calm free surface and the highest position of the pitching pivot of the flapping foil, we find that the free surface affects pronouncedly the energy harvesting efficiency $\eta$. As $d$ decreases from $24c$ to $0.5c$, $\eta$ increases from $0.34$ to $0.41$, getting a $20\%$ promotion of the efficiency. To reveal the underlying physical mechanism of the effects of free surface, we examine the time histories of hydrodynamic forces on the foil. We find that due to the existence of the the free surface, the lift force and pitching moment experience asymmetric time histories during the upstroke and downstroke of the foil.   

Vortex wake interactions and energy harvesting from tandem pitching and heaving hydrofoils

Measurements of flow structure and power extraction by tandem pitching and heaving hydrofoils are conducted in a flume. The leading and trailing hydrofoils are synchronized and aligned parallel to the oncoming flow. Force measurements and time-resolved PIV are used to characterize the system. The system efficiency of tandem foils with the same kinematics is quantified as a function of the phase difference between the foils and there exist favorable and unfavorable phase angles and that system efficiencies can be as large as 0.45. For unfavorable phase angles, PIV indicates that the leading edge vortex generated by the trailing foil, which is critical to good energy harvesting, is weakened by the oncoming wake from the leading foil. Conversely, at a favorable phase, the vortex shed from the leading foil enhances the performance of the trailing foil, compensating for the otherwise negative aspects of operating in the wake. A model, combining frequency, separation distance and a characteristic convection velocity, is introduced to predict the optimal phase region and is validated over a range of parameters. By changing the pitching amplitude and phase angle in trailing foil we show that relatively larger pitching amplitudes can further improve the system efficiency.   

The Effects of Inducer Inlet Diffusion on Backflow

High suction performance inducers are used as a first stage in turbopumps to hinder cavitation and promote stable flow. Despite the distinct advantages of inducer use, an undesirable region of backflow and cavitation can form near the tips of the inducer blades. This flow phenomenon has long been attributed to ``tip leakage flow'', or the flow induced by the pressure differential between pressure and suction sides of an inducer blade at the tip. We examine the backflow of inducer geometries with a tip clearance of 0.4 mm to allow tip leakage flow and a tip clearance of 0 mm to remove tip leakage flow at varying flow coefficients under both single phase and cavitating conditions. Despite removal of the tip leakage flow, backflow persists, and upstream propagation is essentially unaffected. We have observed backflow penetrating 1.1 tip diameters upstream of the leading edge in the inducer with tip clearance, and 0.95 tip diameters in the inducer without tip clearance under the same flow coefficient for single phase conditions. A comprehensive analysis of these simulations suggests that blade inlet diffusion, not tip leakage flow, is the driving force for the formation of tip backflow.   

Comparison of atomization characteristics of drop-in and conventional jet fuels

Surge in energy demand and stringent emission norms have been driving the interest on alternative drop-in fuels in aviation industry. The gas-to-liquid (GTL), synthetic paraffinic kerosene fuel derived from natural gas, has drawn significant attention as drop-in fuel due to its cleaner combustion characteristics when compared to other alternative fuels derived from various feedstocks. The fuel specifications such as chemical and physical properties of drop-in fuels are different from those of the conventional jet fuels, which can affect their atomization characteristics and in turn the combustion performance. The near nozzle liquid sheet dynamics of the drop-in fuel, GTL, is studied at different nozzle operating conditions and compared with that of the conventional Jet A-1 fuel. The statistical analysis of the near nozzle sheet dynamics shows that the drop-in fuel atomization characteristics are comparable to those of the conventional fuel. Furthermore, the microscopic spray characteristics measured using phase Doppler anemometry at downstream locations are slightly different between the fuels.   

Design of an anemometer to characterize the flow in the ducts of a hydrogenerator rotor rim

Due to its complex geometry, the airflow within hydrogenerators is difficult to characterize. And although CFD can be a reliable engineering tool, its application to the field of hydrogenerators is very recent and has certain inherent limitations, which are due in part to geometrical and flow complexities, including the coexistence of moving (rotor) and stationary (stator) components. For this reason, experimental measurements are required to validate the CFD simulations of such complex flows.~ To this end, a 1:4 scale model of a hydrogenerator was constructed at the IREQ (Hydro-Qu\'{e}bec Research Institute) to better understand the flow dynamics in the rotor and stator components, and to help benchmark its CFD simulations.~ However, new flow sensors must be developed to quantify the flow in the confined and harsh regions of hydrogenerators. Of particular interest is the flow within the rotor rim ducts, since it is directly responsible for cooling one of the most critical components, the poles. This rather complex task required the design of an anemometer that had to be accurate, durable, cost-effective, easy to install, and able to withstand the extreme conditions (temperatures of 50\textdegree C, centrifugal forces of 300g, etc.) found in hydrogenerators. This paper presents two preliminary designs of such sensors and a series of tests that were performed to calibrate and test them.~