Rayleigh–Taylor and Richtmyer-Meshkov instability induced flow, turbulence, and mixing. II

Mechanical model development of rolling bearing-rotor systems: A review

Results from large eddy simulations (LES) and direct numerical simulations (DNS) of a two-dimensional, spatially developing, compressible planar free jet undergoing an idealized, exothermic, chemical reaction of the type F+rOx→(1+r)P are presented in order to assess several subgrid-scale (SGS) combustion models. Both a priori and a posteriori assessments are conducted. The SGS turbulence model used is the dynamic Smagorinsky model (DSM). Two classes of SGS combustion models are employed in this study. These include the conserved scalar approach and the direct closure approach. Specifically, the SGS combustion models involve several forms of direct filtered reaction rate closures, including a scale similarity filtered reaction rate model (SSFRRM), and a mixing controlled strained laminar flamelet model (SLFM) in the form of thermochemical state relationships, obtained from the DNS, and two assumed forms for the subgrid mixture fraction filtered density function (FDF). In general, LES results are in reasonable agreement with DNS results and highlight the performance of the various SGS combustion models. In particular, in the context of the present study, it is found that: (1) the SLFM cases overpredict product formation due to their inability to capture finite-rate chemistry effects; (2) due to the relatively low values of the SGS mixture fraction variance in the flow under study, the SLFM results are not sensitive to the form of the assumed FDF; and (3) in comparison to the other models investigated, the SSFRRM combustion model provides the best agreement with the DNS for product formation.

Large eddy simulation of a nonpremixed reacting jet: Application and assessment of subgrid-scale combustion models

The power generated by a wind turbine largely depends on the wind speed. During time periods with identical hub-height wind speeds but different shapes to the wind profile, a turbine will produce different amounts of power. This variability may be induced by atmospheric stability, which affects profiles of mean wind speed, direction and turbulence across the rotor disk. Our letter examines turbine power generation data, segregated by atmospheric stability, in order to investigate power performance dependences at a West Coast North American wind farm. The dependence of power on stability is clear, regardless of whether time periods are segregated by three-dimensional turbulence, turbulence intensity or wind shear. The power generated at a given wind speed is higher under stable conditions and lower under strongly convective conditions: average power output differences approach 15%. Wind energy resource assessment and day ahead power forecasting could benefit from increased accuracy if atmospheric stability impacts were measured and appropriately incorporated in power forecasts, e.g., through the generation of power curves based on a range of turbulence regimes.

/pdf/atmospheric-stability-affects-wind-turbine-power-collection-5g4jrjnn30.pdf

Atmospheric Stability Affects Wind Turbine Power Collection

https://www.brown.edu/research/projects/scientific-computing/sites/brown.edu.research.projects.scientific-computing/files/uploads/High%20order%20WENO%20and%20DG%20methods%20for%20time-dependent%20convection-dominated%20PDEs.pdf

High order WENO and DG methods for time-dependent convection-dominated PDEs

A six-degree-of-freedom model was developed and used to simulate the motion of all elements in a cylindrical roller bearing. Cage instability has been studied as a function of the roller-race and roller-cage pocket clearances for light-load and high-speed conditions. The effects of variation in inner race speed, misalignment, cage asymmetry, and varying size of one of the rollers have been investigated. In addition, three different roller profiles have been used to study their impact on cage dynamics. The results indicate that the cage exhibits stable motion for small values of roller-race and roller-cage pocket clearances. A rise in instability leads to discrete cage-race collisions with high force magnitudes. Race misalignment leads to a rise in instability for small roller-cage pocket clearances since skew control is provided by the sides of the cage pocket. One roller of larger size than the others causes inner race whirl and leads to stable cage motion for small roller-race clearances without any variation in roller-cage pocket clearance. Cage asymmetry and different roller profiles have a negligible impact on cage motion.

Cage Instabilities in Cylindrical Roller Bearings

In this study, we address the benefits of a vertically staggered (VS) wind farm, in which vertical-axis and horizontal-axis wind turbines are collocated in a large wind farm. The case study consists of 20 small vertical-axis turbines added around each large horizontal-axis turbine. Large-eddy simulation is used to compare power extraction and flow properties of the VS wind farm versus a traditional wind farm with only large turbines. The VS wind farm produces up to 32% more power than the traditional one, and the power extracted by the large turbines alone is increased by 10%, caused by faster wake recovery from enhanced turbulence due to the presence of the small turbines. A theoretical analysis based on a top-down model is performed and compared with the large-eddy simulation. The analysis suggests a nonlinear increase of total power extraction with increase of the loading of smaller turbines, with weak sensitivity to various parameters, such as size, and type aspect ratio, and thrust coefficient of the vertical-axis turbines. We conclude that vertical staggering can be an effective way to increase energy production in existing wind farms. Copyright © 2016 John Wiley & Sons, Ltd.

Benefits of collocating vertical-axis and horizontal-axis wind turbines in large wind farms

Large-eddy simulation (LES) has been used previously to study the effect of either configuration or atmospheric stability on the power generated by large wind farms. This is the first study to consider both stability and wind farm configuration simultaneously and methodically with LES. Two prevailing wind directions, two layouts (turbines aligned versus staggered with respect to the wind) and three stabilities (neutral and moderately unstable and stable) were evaluated. Compared with neutral conditions, unstable conditions led to reduced wake losses in one configuration, to enhanced wake losses in two and to unchanged wake losses in one configuration. Conversely, stable conditions led to increased wake losses in one, decreased wake losses in two and unchanged wake losses in one configuration. Three competing effects, namely, rates of wake recovery due to vertical mixing, horizontal spread of wakes and localized regions of acceleration caused by multiple upstream wakes, were identified as being responsible for the observed trends in wake losses. The detailed flow features responsible for these non-linear interactions could only be resolved by the LES. Existing analytical models ignore stability and non-linear configuration effects, which therefore need to be incorporated. Copyright © 2017 John Wiley & Sons, Ltd.

Evaluation of layout and atmospheric stability effects in wind farms using large‐eddy simulation

Layout studies are critical in designing large wind farms, since wake effects can lead to significant reductions in power generation. Optimizing wind farm layout using computational fluid dynamics is practically unfeasible today because of their enormous computational requirements. Simple statistical models, based on geometric quantities associated with the wind farm layout, are therefore attractive because they are less demanding computationally. Results of large-eddy simulations of the Lillgrund (Sweden) offshore wind farm are used here to calibrate such geometry-based models. Several geometric quantities (e.g., blockage ratio, defined as the fraction of the swept area of a wind turbine that is blocked by upstream turbines) and their linear combinations are found to correlate very well (correlation coefficient of ~0.95) with the power generated by the turbines. Linear models based on these geometric quantities are accurate at predicting the farm-averaged power and are therefore used here to stud...

https://journals.ametsoc.org/doi/pdf/10.1175/JTECH-D-14-00199.1

Geometry-Based Models for Studying the Effects of Wind Farm Layout

A novel wind turbine configuration comprising of four identical rotors mounted on a single tower is studied using large-eddy simulation (LES) with turbine forces modeled using an actuator drag-disk model. The characteristics of the wakes of the multi-rotor turbine are compared to those of the wake of a conventional turbine comprised of a single rotor per tower. The single-rotor turbine has twice the diameter and the same thrust coefficient as the rotors of the multi-rotor turbine. Several multi-rotor configurations, with varying horizontal and vertical spacings between the four rotors, are evaluated. The multi-rotor turbine wakes are found to recover faster, while the turbulence intensity in the wake is smaller, compared to the wake of the conventional turbine. The mean velocity profiles obtained from the LES results are predicted accurately by a semi-analytical model assuming Gaussian radial profiles of the velocity deficits, and either linear or quadratic superposition of multiple wakes. The classical Jensen wake model, assuming top-hat radial profiles, with quadratic superposition of wakes is also found to accurately predict the decay of the axial mean velocity in the streamwise direction. The interaction between multiple multi-rotor turbines is contrasted with that between multiple single-rotor turbines by considering wind farms with five turbine units aligned perfectly with each other and the wind direction and separated by four single-rotor diameters. The wake losses are found to be significantly smaller in such a wind farm comprising of multi-rotor turbines as compared to single-rotor turbines. The top-hat analytical model is employed to quantify the benefits of the multi-rotor configuration over the conventional single-rotor configuration for a wide range of thrust coefficients and axial spacings. These results suggest that a larger planform energy flux can be achieved without significantly increased fatigue loads by using multi-rotor turbines instead of conventional, single-rotor turbines.

https://iopscience.iop.org/article/10.1088/1742-6596/1037/7/072021/pdf

Niranjan S. Ghaisas

Papers

Cage Instabilities in Cylindrical Roller Bearings

Benefits of collocating vertical-axis and horizontal-axis wind turbines in large wind farms

Evaluation of layout and atmospheric stability effects in wind farms using large‐eddy simulation

Geometry-Based Models for Studying the Effects of Wind Farm Layout

Large-eddy simulation study of multi-rotor wind turbines