The wake of a bluff body is a representative issue in vortex dynamics that plays a central role in civil engineering, ocean engineering and thermal engineering. In this work, a flowing soap film was used to investigate the wakes of multiple stationary circular cylinders and of a single oscillating cylinder. Corresponding computer simulations were also conducted. Vortex formation of a stationary circular cylinder was analyzed by proper orthogonal decomposition (POD). The POD analysis was used to define an unsteady vortex formation length, which suggests a relationship between the vortex formation length of a single cylinder and the critical spacing of two cylinders in a tandem arrangement. A systematic parametric study of the wake structure was conducted for a controlled transversely oscillating cylinder. Neural network and support vector machine codes assisted the wake classification procedure and the identification of boundaries between different wake regimes. The phase map of the vortex shedding regimes for the (quasi) two-dimensional experiment qualitatively agrees with previous three-dimensional experiments. The critical spacings of two identical tandem circular cylinders in a flowing soap film system were determined using visual inspections of the wake patterns and calculations of the Strouhal frequencies. The dimensionless spacing was both increased and decreased quasi-statically. Hysteresis was observed in the flow patterns and Strouhal numbers. This study appears to provide the first experimental evidence of critical spacing values that agree with published computational results. The wake interaction between a stationary upstream circular disk and a free downstream circular disk was also investigated. With the ability to tie together the wake structure and the object motion, the relationship between energy generation and flow structure in the simplified reduced order model system was studied. The research results find the optimal efficiency of the energy harvesting system by a parametric study. / PHD / The wake of a bluff body is a classic issue in vortex dynamics that has been the subject of much research in civil engineering, ocean engineering and thermal engineering. Bluff bodies, especially circular cylinders, can be found extensively in heat exchangers, cooling systems and offshore structures. Flow-induced vibration of a bluff body due to the formation of a wake is an important problem in many fields of engineering. Flow-induced vibration determines the oscillation of flexible pipes that transfer oil from the seabed to the surface of the ocean, for example [71]. In civil engineering, flow-induced vibration affects the design of bluff structures in wind such as bridges, chimneys and buildings [62]. Flow-induced vibration caused by vortices being shed from a bluff body is also a promising way to extract energy from geophysical flows [10]. FIV energy harvesting systems are especially suitable for slow flow speeds in the range 0.5-1.5m/s which cannot be efficiently harvested by traditional hydroelectric power technologies. When a pair of tandem cylinders is immersed in a flow, the downstream cylinder can be excited into wake-induced vibrations (WIV) due to the interaction with vortices coming from the upstream cylinder. In this work, a flowing soap film was used to investigate the flow-induced vibration of the downstream cylinder of a tandem pair. With the ability to tie together the wake structure and the object motion, we investigate the relationship between energy generation and flow structure in the reduced order model system. The research results find the optimal efficiency of the energy harvesting system by a parametric study. To get deep physical understanding of the flow-induced vibration, wake structures of a circular cylinder undergoing controlled motion and the critical spacing of two identical tandem circular cylinders were also investigated in this research. These research results can help not only the optimization of energy harvesting systems based on flow-induced vibration of the circular-cylinder system, but also will benefit the understanding of wake interactions between multiple bluff bodies such as schooling fish, natural draft cooling towers and wind turbine farms.

The status of marine current tidal energy technology is currently in the research and development phase, with a few deployments and tests of prototypes under-way in some countries. There is a huge pressure for tidal farms to be of Gigawatt scale in order to have a real, economically viable impact on renewable energy utilization targets outlined for 2020. A route to achieving this is the large scale energy farm philosophy, similar to wind farms, based on very large numbers of unit current tidal stream devices. However, this emerging technology development raises different research questions which lead to further problems in the practical implementation of tidal stream devices. Thus, the aim of this study was: (i) to develop a new, computationally cheap computational fluid dynamics (CFD) based model of the Momentum-Reversal-Lift (MRL) tidal turbine, and (ii) to perform a detailed calculations of the flow field of single and multiple turbines using the developed model to investigate the flow features such as, downstream wake structures, dynamics of the free surface, wake recovery, and the influence of wake interactions on the performance of individual devices. A new CFD based Immersed Body Force (IBF) model has been developed to represent the MRL turbine. The IBF model was developed based on the concept of actuator disc methodology by incorporating additional geometric features that induce energy absorption from the flow which also lead to a downstream wake structure intended to reflect more closely those of the real turbines than simple momentum sink zone models. This turbine model was thoroughly used to investigate the performance of the MRL turbine and the associated flow characteristics and proved its capability in analysing several issues relating to this design of tidal turbines. Several calculations have been carried out and a full range of operating points of the MRL turbine was formulated. A maximum power coefficient of, Cp = 0.665, was obtained with a blockage ratio of, B = 0.016. However, the performance of the turbine was improved at a higher blockage ratio both in a single and tidal stream farm investigations. The power coefficient of a single turbine was improved by about 3% when simulated with a blockage ratio of, B = 0.029, and even a higher value was obtained in a tidal stream farm containing three turbines configured in the spanwise direction which reached up to, CP = 0.761, with a global blockage ratio of, B = 0.027. These power coefficients are higher than the Lanchester-Betz limit of CP = 0.593 obtained at B = 0, which is mainly due to the tidal turbine operating in a constrained environment, high blockage ratio, that increases the thrust force on the device. The power coefficient of the IBF model showed consistently higher values compared to experiments and a detailed CFD model results. This indicates that the power coefficient calculated using the IBF model includes some other losses within the turbine region, such as losses due to viscous, shear etc. Investigations on the influence of closely packed clusters of turbines in a tidal streamfarm showed that a laterally close configuration of turbines improved the performance of individual turbines due to the blockage effect, which is created by the array of turbines in the span-wise direction. In contrast, a small longitudinal spacing between turbines inflicted a massive energy shadowing that affects the performance of downstream turbines. However, a tidal stream farm with a staggered turbine layout can reduce the longitudinal spacing by about 50% with a minimum of 6D lateral spacing compared with a regular turbine layout due to the advantage of using an accelerated bypass flows.

Over the last few decades, wind power has shown a continuous and significant developmentin the energy market globally. Having reached a certain level in both technologyand in dimensions, the role of optimizing wind turbines as well as wind farms hasbecome an additional aspect to future development and research. Since turbine wakescan cause significant power deficits within a farm, research in this area has the potentialfor large improvements in wind farm design. A wake is described as the downstream flow behind the rotor of an operating windturbine. The two main characteristics of wakes are a velocity (momentum) deficit and anincreased turbulence level. The velocity deficit behind the upwind turbine results in apower loss of the downstream turbines, whereas the higher turbulence causes additionalloads on the downstream turbines’ structures resulting in fatigue problems. The study of wakes is a complex topic, they are influenced by an interconnection of anumber of parameters like ambient wind speed and turbulence, atmospheric stabilityconditions (stable, unstable, and neutral), the turbines’ operational characteristics, andthe terrain properties. In order to assess the power deficits affected by wake interaction between turbines,an analysis can be realized by processing SCADA data of turbines in a wind farm. The collected data is treated by a comprehensive filtration process, excluding events of icing, curtailment, faults, etc. and by grouping into different atmospheric conditions, windspeed intervals and wind speed sectors. Finally, power deficit values, as a function ofwind direction, are calculated and quantified, and thereafter analyzed to assess the wakebehavior at different conditions for different cases.In this thesis, the wake-induced power deficit has been investigated in a specificstudy case for three pairs of two neighboring turbines in a forested moderately complexterrain using SCADA data. The production losses amounted between the range of 32% to 67% for the specific site with turbine spacing around 4D. The obtained results werepartially unsatisfactory, caused by the reasons of inaccurate wind direction values due toyaw misalignment issues and challenging separation into different stability conditions. Moreover, the power deficits showed a clear reduction of losses with increasing windspeed. A conclusion regarding the differences between stable and near neutral conditionscould not be determined from the data.

During the last decades the exploitation of energy from the wind has become one of the most promising renewable energy technologies. The main strive in today’s development of wind turbines is to increase the efficiency of the turbine and to build bigger rotors that are able to extract more power out of the wind. When it comes to the planning and designing of a wind park, also the aerodynamic interactions between the single turbines must be taken into account. The flow in the wake of the first row turbines is characterized by a significant deficit in wind velocity and by increased levels of turbulence. Consequently, the downstream turbines in a wind farm cannot extract as much power from the wind anymore. Furthermore, the additional turbulence in the wake could be a reason for increased material fatigue through flow-induced vibrations at the downstream rotor. The main focus of this experimental study is to investigate the local velocity deficit and the turbulence intensities in the wake behind an array of two model wind turbines. For two different turbine separation distances, the wake is scanned at three different downstream positions. The experiments are performed at the wind tunnel (1.9m x 2.7m cross section) at NTNU Trondheim using two model wind turbines with a rotor diameter of 0.9m. A hot wire probe is used to scan the wake behind the model turbines in defined positions. Moving axially downstream the velocity deficit in the wake gradually recovers and the turbulence intensity levels slowly decrease. Furthermore, a gentle expansion of the wake can be observed. The wake profiles measured in close distances behind the rotor are characterized by evident asymmetries. Further downstream in the wake turbulent diffusion mechanisms cause a more uniform and more symmetrical flow field. Moreover, the turbulence intensity behind the second wind turbine is found to be significantly higher than behind one unobstructed turbine. Also, considerably higher velocity deficits are found in the near wake behind the second turbine compared to the wake behind one unobstructed turbine. However, the velocity profile at five rotor diameters downstream in the wake behind the second turbine is already very similar to the velocity distribution behind the first turbine. Furthermore, the velocity field and turbulence intensity distribution in the wake behind the second turbine is more symmetrical and more uniform than behind the first turbine.

The unsteady behavior of a tip leakage flow downstream of a simulated axial compressor rotor has been studied. The Virginia Tech low speed linear cascade wind tunnel was adapted to model the unsteady tip leakage flow produced by a rotor operating in the vortical wakes of a set of stator vanes. The cascade, consisting of 8 GE rotor B blades, has adjustable tip gap, inlet angle of 65.1 degrees, turning angle of 11.8 degrees and solidity of 1.076. The cascade Reynolds number, based on blade chord, was 393,000. A moving end wall was used to simulate the relative motion between rotor and casing, and vortex generators attached to the moving end wall were used to produce an idealized periodic unsteady vortical inflow similar to that shed by the junction of a row of inlet guide vanes. Measurements of the vortical inflow to the cascade produced by the generators and of the mean blade loading at the mid span are presented. The periodic and aperiodic behavior of the tip leakage flow downstream of the cascade, produced by this vortical disturbance, is also presented using phase and time averaged 3-component turbulence and pressure fluctuation measurements. These measurements are made for tip gap from 0.83% to 3.3% chord and streamwise locations from 0.772% to 1.117% blade spacing axially downstream of the cascade. The phase averaged inflow measurements reveal that the inflow produced by the vortex generators consists of a pair asymmetric counter-rotating vortices embedded in a thin (4.6% chord) endwall boundary layer. The vortices extend some 7.4% chord from the end wall. Their strength is about two orders smaller than the typical circulation of the tip leakage vortices produced by the cascade. Phase averaged single point three component hot-wire measurements downstream of the cascade reveal that the vortical inflow is, however, capable of producing significant large scale fluctuations in the size, strength, structure and position of the tip leakage vortex. These effects increase in magnitude with increase of tip gap. For small tip gaps these effects appear to be due to simple superposition between the inflow vortices and the tip leakage vortex. However for larger tip gaps these effects appear primarily a consequence of the inflow vortices interfering with the shedding of circulation from the blade tip. The fact that the circulation fluctuation is consistent with the inviscid unsteady loading prediction suggests that the inviscid response may be a major mechanism for generating the tip leakage unsteadiness. Although there is large periodic fluctuation in the tip leakage flow disturbed by the inflow, there is a larger aperiodic component. Two point correlation measurements and linear stochastic estimation are used to reveal the structure of this aperiodic part for a tip gap of 3.3% chord. The aperiodic fluctuation, containing most of the turbulence energy, is found appearing to be organized structures in large scale, and making the estimated instantaneous velocity field significantly different from the phase averaged periodic velocity field. Phase averaged pressure fluctuation measurements made using a microphone in the tip leakage vortex downstream of the cascade reveal that there are significant periodic fluctuating pressure waves and intense mean square fluctuation of the aperiodic fluctuating pressure. They are consistent with the measured periodic flow and aperiodic flow field respectively. These microphone measurements are validated using fluctuating pressure gradient estimates determined from the hot-wire measurements. / Ph. D.

Detailed surface pressure measurements have been made on a NACA 0015 immersed in two grid generated homogenous flows at Re = 1.17 x 10⁶ for a = 0°, 4°, 8°, 12°, 16°, and 20°. The goal of this measurement was to reveal and highlight mean loading and turbulence scale effects on surface pressure fluctuations resulting from turbulence/airfoil interaction. Also, measurements are compared with the theory of Amiet (1976a,b). The surface pressure response shows a dependance on angle of attack, the nature of which is related to the relative chord/turbulence scale. The dependance on turbulence scale appears to be non-monotonic at low reduced frequencies, wr = Pi*f*c/U with both an increase and decrease in unsteady pressure magnitude occurring with increasing mean load. A reduced frequency overlap region exists at wr > 10 where the two different scale flows begin to produce similar effects on the surface pressure with increasing angle of attack manifesting as a rise in unsteady surface pressure magnitude. Also, the interaction of the full 3-dimensional wavenumber spectrum affects the distance over which pressure fluctuations correlate and the extent of correlation is affected by angle of attack as demonstrated in the chordwise and spanwise pressure correlation. Amiet's theory is shown to agree favorably with measurements in the leading edge region although demonstrates insufficiencies in predicting unsteady pressure phasing. / Master of Science

Wind energy is on an upswing due to climate concerns and increasing energy demands on conventional sources. Wind energy is attractive and has the potential to dramatically reduce the dependency on non-renewable energy resources. With the increase in wind farms there is a need to improve the efficiency in power allocation and power generation among wind turbines. Wake interferences among wind turbines can lower the overall efficiency considerably, while offshore conditions pose increased loading on wind turbines. In wind farms, wind turbines* wake affects each other depending on their positions and operation modes. Therefore it becomes essential to optimize the wind farm power production as a whole than to just focus on individual wind turbines. The work presented here develops a hierarchical power optimization algorithm for wind farms. The algorithm includes a cooperative level (or higher level) and an individual level (or lower level) for power coordination and planning in a wind farm. The higher level scheme formulates and solves a quadratic constrained programming problem to allocate power to wind turbines in the farm while considering the aerodynamic effect of the wake interaction among the turbines and the power generation capabilities of the wind turbines. In the lower level, optimization algorithm is based on a leader-follower structure driven by the local pursuit strategy. The local pursuit strategy connects the cooperative level power allocation and the individual level power generation in a leader-follower arrangement. The leader, could be a virtual entity and dictates the overall objective, while the followers are real wind turbines considering realistic constraints, such as tower deflection limits. A nonlinear wind turbine dynamics model is adopted for the low level study with loading and other constraints considered in the optimization. The stability of the algorithm in the low level is analyzed for the wind turbine angular velocity. Simulations are used to show the advantages of the method such as the ability to handle non-square input matrix, non-homogenous dynamics, and scalability in computational cost with rise in the number of wind turbines in the wind farm.

Simultaneous two-point three-component four-sensor hot-wire velocity measurements taken in three flows of aeroacoustic interest are here analyzed. The analyses provide information on the turbulence structure of the flows as it would be encountered by hypothetical noise producing blades passing through the flows. Two-point measurements taken in the first flow, a lifting wake from a rectangular NACA 0012 half wing, are used to calculate space-time correlation functions and 'pointwise' wave number frequency spectra. Two upwash spectra, calculated for locations in the region of the wake that is roughly homogenous in the spanwise direction, are direct estimates of the full wave number frequency spectra at their locations. As such, they are used to perform aeroacoustic calculations, and the results are compared to results achieved using the von Kármán isotropic spectrum. Amiet's approximation, where the wave number frequency spectra can be represented by the correlation length scales is found to hold reasonably well for the measured spectra. The two-point measurements in the second flow, a vortex/blade-tip interaction, are analyzed to provide information useful to researchers of blade-wake interaction noise produced by helicopter rotors. Space-time correlation functions and wave number frequency spectra are calculated for five cuts through the region of interaction. The correlation functions provide information concerning the turbulence length scales found in the interaction region. The spectra are compared to the von Kármán isotropic spectrum and found to be greatly different. However, the spectra do bear some resemblance to spectra calculated in the spanwise homogenous region of the lifting wake. The two-point measurements taken in the third flow, the wake from a fan cascade, are analyzed to provide information of use to modelers of broadband noise produced through rotor wake/stator interactions. In particular, space-time correlation functions are calculated for a grid of two-point measurements, which allows the estimation of the turbulence structure as seen by a passing stator blade. Space-time correlation functions and wave number frequency spectra are calculated for various stator configurations. The implications of engine operating speed and stator configuration for broadband noise production are discussed. <i> [Vita removed March 2, 2012. GMc]</i> / Master of Science

The effects of serrated trailing edge designs, designed for noise reduction, on the flow-field downstream of an idealized aircraft engine fan blade row were investigated in detail. The measurements were performed in the Virginia Tech low speed linear cascade tunnel on one set of baseline GE-Rotor-B blades and four sets of GE-Rotor-B blades with serrated trailing edges. The four serrated blade sets consisted of two different serration sizes (1.27 cm and 2.54 cm) and for each different serration size a second set of blades with added trailing edge camber. The cascade row consisted of 8 GE-Rotor-B blades and 7-passages with adjustable tip gap settings. It had an inlet angle of 65.1º, stagger angle of 56.9º and a turning angle of 11.8º. The tunnel was operated with a tip gap setting of 1.65% chord, with a Reynolds number based on the chord of 390,000. Blade loading measurements performed on each set of blades showed that it was slightly dependent on the serration shape. As the serration size was increased the blade loading decreased, but adding droop increased the blade loading. The Pitot-static cross-sections showed that flow-fields near the upper and lower endwalls cascade tunnel were similar with the baseline or the serrated blade downstream of the blade row. In the wake region, the individual trailing edge serrations tips and valleys could be seen. As the wake convected downstream, the individual tips and valleys became less visible and the wake was more uniform in profile. The tip leakage vortex was only minimally affected by the trailing edge serrations. This conclusion was further reinforced by the three-component hot-wire cross-sectional measurements that were performed from the lower endwall to the mid-span of the blade. These showed that the mean streamwise velocity, turbulence kinetic energy and turbulence kinetic energy production in the tip leakage region were nearly the same for all four serrated blades as well as the baseline. The vorticity in this region was a more dependent on the serration shape and as a result increased with serration size compared to the baseline. Mid-span measurements performed with the three-component hot-wire showed the spreading rate of the wake and the decay rate of the wake centerline velocity deficit increased with serration size compared to the baseline case. Drooping of the trailing edge only minimally improved the spreading and decay rates. This improvement in these rates was predicted to reduce the tonal noise at the leading edge of the downstream stator vane because the periodic fluctuation associated with the sweeping of the rotor blade wakes across it, was due to the pitchwise variation in the mean streamwise velocity. The wakes were further compared to the mean velocity and turbulence profiles of plane wakes, which the baseline and the smallest serration size agreed the best. As the serration size was increased and drooping was added, the wakes became less like plane wakes. Spectral plots at the wake centerline in all three velocity directions showed some evidence of coherent motion in the wake as a result of vortex shedding. / Master of Science

Understanding the flow around wind turbines is a highly relevant research question due to the increased interest in harvesting energy from renewable sources. This thesis approaches the topic by means of numerical simulations using the actuator line method and the incompressible Navier–Stokes equations in the spectral element code Nek5000. The aim is to gain enhanced understanding of the wind turbine wake structure and wind turbine wake interaction. A verification study of the method and implementation is performed against the finite volume solver EllipSys3D using two types of turbines, an idealized constant circulation turbine and the Tjæreborg turbine. It is shown that Nek5000 requires significantly lower resolution to accurately compute the wake development, however, at the cost of a smaller time step.The constant circulation turbine is investigated further with the goal of establishing guidelines for the use of the actuator line method in spectral element codes, where the mesh is inherently non-equidistant and currently used guidelines of force distribution based on Gaussian kernels are difficult to apply. It is shown that Nek5000 requires a larger kernel width in the fixed frame of reference to remove numerical instabilities. Further, the impact of different Gaussian widths on the wake development is investigated in the rotating frame of reference, showing that the convection velocity and the breakdown of the spiral tip and root vortices are dependent on the Gaussian width. In the second part, the flow around single and multiple wind-turbine setups at different operating conditions is investigated and compared with experimental results. The focus is placed on comparing the power and thrust coefficients and the wake development based on the time-averaged streamwise velocity and turbulent stresses. Further the influence of the tower model is investigated both upstream and downstream of the turbine. The results show that the wake is captured accurately in most cases. The loading exhibits a significant dependence on the Reynolds number at which the airfoil data is extracted. When the helical tip vortices are stable the turbulent stresses at the tip vortices are underestimated in the numerical simulations. This is due to the finite resolution and the projection of the actuator line forces in the numerical domain using a prescribed Gaussian width, which leads to lower induced velocities in the helical vortices. / <p>QC 20170523</p>