Flow Kinematics in Variable-Height Rotating Cylinder Arrays

Experimental data are presented for large arrays of rotating, variable-height cylinders in order to study the dependence of the three-dimensional mean flows on the height heterogeneity of the array. Elements in the examined arrays were spatially arranged in the same staggered paired configuration, and the heights of each element pair varied up to ±37.5% from the mean height (kept constant across all arrays), such that the arrays were vertically structured. Four vertical structuring configurations were examined at a nominal Reynolds number (based on freestream velocity and cylinder diameter) of 600 and nominal tip-speed ratios of 0, 2, and 4. It was found that the vertical structuring of the array could significantly alter the mean flow patterns. Most notably, a net vertical flow into the array from above was observed, which was augmented by the arrays' vertical structuring, showing a 75% increase from the lowest to highest vertical flows (as evaluated at the maximum element height, at a single rotation rate). This vertical flow into the arrays is of particular interest as it represents an additional mechanism by which high streamwise momentum can be transported from above the array down into the array. An evaluation of the streamwise momentum resource within the array indicates up to a 56% increase in the incoming streamwise velocity to the elements (from the lowest to highest ranking arrays, at a single rotation rate). These arrays of rotating cylinders may provide insight into the flow kinematics of arrays of vertical axis wind turbines (VAWTs). In a physical VAWT array, an increase in incoming streamwise flow velocity to a turbine corresponds to a (cubic) increase in the power output of the turbine. Thus, these results suggest a promising approach to increasing the power output of a VAWT array

Low order physical models of vertical axis wind turbines

In order to examine the ability of low-order physical models of vertical axis wind turbines to accurately reproduce key flow characteristics, experimental data are presented for the mean flow patterns and turbulence spectra associated with pairs of rotating turbines, rotating solid cylinders, and stationary porous flat plates (of both uniform and non-uniform porosities). The experiments were conducted at a nominal model-diameter Reynolds number of 600 and rotation tip speed ratios between 0 and 6. By comparing the induced flow fields of the different models both qualitatively and quantitatively, it was concluded that the two dimensional horizontal mean flow fields induced by the porous flat plates were quantitatively similar to those induced by slowly rotating turbine models. However, over the range of the experimental parameters examined, the porous flat plates were unable to produce vertical flows similar to those associated with the slowly rotating turbine models. Conversely, the moderately rotating cylinders induced three dimensional mean flow fields quantitatively similar to those induced by rapidly rotating turbine models. These findings have implications for both laboratory experiments and numerical simulations, which have previously used analogous low order models in order to reduce experimental/computational costs. Specifically, over the range of parameters examined, the comparison between induced flow fields of the different model fidelities allows identification of the lowest cost model for which the specific goals of a study can be obtained, to within the desired accuracy. And if a lower fidelity model is used, it is possible to incorporate into the analysis of the collected data an understanding of how the results would be expected to vary from a higher fidelity case

Fish schooling as a basis for vertical axis wind turbine farm design

Most wind farms consist of horizontal axis wind turbines (HAWTs) due to the high power coefficient (mechanical power output divided by the power of the free-stream air through the turbine cross-sectional area) of an isolated turbine. However when in close proximity to neighbouring turbines, HAWTs suffer from a reduced power coefficient. In contrast, previous research on vertical axis wind turbines (VAWTs) suggests that closely-spaced VAWTs may experience only small decreases (or even increases) in an individual turbine's power coefficient when placed in close proximity to neighbours, thus yielding much higher power outputs for a given area of land. A potential flow model of inter-VAWT interactions is developed to investigate the effect of changes in VAWT spatial arrangement on the array performance coefficient, which compares the expected average power coefficient of turbines in an array to a spatially-isolated turbine. A geometric arrangement based on the configuration of shed vortices in the wake of schooling fish is shown to significantly increase the array performance coefficient based upon an array of 16x16 wind turbines. Results suggest increases in power output of over one order of magnitude for a given area of land as compared to HAWTs.Comment: Submitted for publication in BioInspiration and Biomimetics. Note: The technology described in this paper is protected under both US and international pending patents filed by the California Institute of Technolog

A new approach to wind energy: Opportunities and challenges

Despite common characterizations of modern wind energy technology as mature, there remains a persistent disconnect between the vast global wind energy resource—which is 20 times greater than total global power consumption—and the limited penetration of existing wind energy technologies as a means for electricity generation worldwide. We describe an approach to wind energy harvesting that has the potential to resolve this disconnect by geographically distributing wind power generators in a manner that more closely mirrors the physical resource itself. To this end, technology development is focused on large arrays of small wind turbines that can harvest wind energy at low altitudes by using new concepts of biology-inspired engineering. This approach dramatically extends the reach of wind energy, as smaller wind turbines can be installed in many places that larger systems cannot, especially in built environments. Moreover, they have lower visual, acoustic, and radar signatures, and they may pose significantly less risk to birds and bats. These features can be leveraged to attain cultural acceptance and rapid adoption of this new technology, thereby enabling significantly faster achievement of state and national renewable energy targets than with existing technology alone. Favorable economics stem from an orders-of-magnitude reduction in the number of components in a new generation of simple, mass-manufacturable (even 3D-printable), vertical-axis wind turbines. However, this vision can only be achieved by overcoming significant scientific challenges that have limited progress over the past three decades. The following essay summarizes our approach as well as the opportunities and challenges associated with it, with the aim of motivating a concerted effort in basic and applied research in this area

Thoracic fat volume is independently associated with coronary vasomotion.

Thoracic fat has been associated with an increased risk of coronary artery disease (CAD). As endothelium-dependent vasoreactivity is a surrogate of cardiovascular events and is impaired early in atherosclerosis, we aimed at assessing the possible relationship between thoracic fat volume (TFV) and endothelium-dependent coronary vasomotion. Fifty healthy volunteers without known CAD or major cardiovascular risk factors (CRFs) prospectively underwent a (82)Rb cardiac PET/CT to quantify myocardial blood flow (MBF) at rest, and MBF response to cold pressor testing (CPT-MBF) and adenosine (i.e., stress-MBF). TFV was measured by a 2D volumetric CT method and common laboratory blood tests (glucose and insulin levels, HOMA-IR, cholesterol, triglyceride, hsCRP) were performed. Relationships between CPT-MBF, TFV and other CRFs were assessed using non-parametric Spearman rank correlation testing and multivariate linear regression analysis. All of the 50 participants (58 ± 10y) had normal stress-MBF (2.7 ± 0.6 mL/min/g; 95 % CI: 2.6-2.9) and myocardial flow reserve (2.8 ± 0.8; 95 % CI: 2.6-3.0) excluding underlying CAD. Univariate analysis revealed a significant inverse relation between absolute CPT-MBF and sex (ρ = -0.47, p = 0.0006), triglyceride (ρ = -0.32, p = 0.024) and insulin levels (ρ = -0.43, p = 0.0024), HOMA-IR (ρ = -0.39, p = 0.007), BMI (ρ = -0.51, p = 0.0002) and TFV (ρ = -0.52, p = 0.0001). MBF response to adenosine was also correlated with TFV (ρ = -0.32, p = 0.026). On multivariate analysis, TFV emerged as the only significant predictor of MBF response to CPT (p = 0.014). TFV is significantly correlated with endothelium-dependent and -independent coronary vasomotion. High TF burden might negatively influence MBF response to CPT and to adenosine stress, even in persons without CAD, suggesting a link between thoracic fat and future cardiovascular events

A Kinematic Description of the Key Flow Characteristics in an Array of Finite-Height Rotating Cylinders

Experimental data are presented for large arrays of rotating, finite-height cylinders to study the dependence of the three-dimensional (3D) mean flows on the geometric and rotational configurations of the array. Two geometric configurations, each with two rotational configurations, were examined at a nominal Reynolds number of 600 and nominal tip-speed ratios of 0, 2, and 4. It was found that the rotation of the cylinders drives the formation of streamwise and transverse flow patterns between cylinders and that net time–space averaged transverse and vertical flows exist within the developed flow region of the array. This net vertical mean flow provides an additional mechanism for the exchange of momentum between the flow within the array and the flow above it, independent from the turbulent exchange mechanisms which are also observed to increase by almost a factor of three in a rotating array. As an array of rotating cylinders may provide insight into the flow kinematics of an array of vertical axis wind turbines (VAWTs), this planform momentum flux (both mean and turbulent) is of particular interest, as it has the potential to increase the energy resource available to turbines far downstream of the leading edge of the array. In the present study, the streamwise momentum flux into the array could be increased for the rotating-element arrays by up to a factor of 5.7 compared to the stationary-element arrays, while the streamwise flow frontally averaged over the elements could be increased by up to a factor of four in the rotating-element arrays compared to stationary-element arrays

Flow Kinematics in Variable-Height Rotating Cylinder Arrays

Experimental data are presented for large arrays of rotating, variable-height cylinders in order to study the dependence of the three-dimensional mean flows on the height heterogeneity of the array. Elements in the examined arrays were spatially arranged in the same staggered paired configuration, and the heights of each element pair varied up to ±37.5% from the mean height (kept constant across all arrays), such that the arrays were vertically structured. Four vertical structuring configurations were examined at a nominal Reynolds number (based on freestream velocity and cylinder diameter) of 600 and nominal tip-speed ratios of 0, 2, and 4. It was found that the vertical structuring of the array could significantly alter the mean flow patterns. Most notably, a net vertical flow into the array from above was observed, which was augmented by the arrays' vertical structuring, showing a 75% increase from the lowest to highest vertical flows (as evaluated at the maximum element height, at a single rotation rate). This vertical flow into the arrays is of particular interest as it represents an additional mechanism by which high streamwise momentum can be transported from above the array down into the array. An evaluation of the streamwise momentum resource within the array indicates up to a 56% increase in the incoming streamwise velocity to the elements (from the lowest to highest ranking arrays, at a single rotation rate). These arrays of rotating cylinders may provide insight into the flow kinematics of arrays of vertical axis wind turbines (VAWTs). In a physical VAWT array, an increase in incoming streamwise flow velocity to a turbine corresponds to a (cubic) increase in the power output of the turbine. Thus, these results suggest a promising approach to increasing the power output of a VAWT array

Low order physical models of vertical axis wind turbines

In order to examine the ability of low-order physical models of vertical axis wind turbines to accurately reproduce key flow characteristics, experimental data are presented for the mean flow patterns and turbulence spectra associated with pairs of rotating turbines, rotating solid cylinders, and stationary porous flat plates (of both uniform and non-uniform porosities). The experiments were conducted at a nominal model-diameter Reynolds number of 600 and rotation tip speed ratios between 0 and 6. By comparing the induced flow fields of the different models both qualitatively and quantitatively, it was concluded that the two dimensional horizontal mean flow fields induced by the porous flat plates were quantitatively similar to those induced by slowly rotating turbine models. However, over the range of the experimental parameters examined, the porous flat plates were unable to produce vertical flows similar to those associated with the slowly rotating turbine models. Conversely, the moderately rotating cylinders induced three dimensional mean flow fields quantitatively similar to those induced by rapidly rotating turbine models. These findings have implications for both laboratory experiments and numerical simulations, which have previously used analogous low order models in order to reduce experimental/computational costs. Specifically, over the range of parameters examined, the comparison between induced flow fields of the different model fidelities allows identification of the lowest cost model for which the specific goals of a study can be obtained, to within the desired accuracy. And if a lower fidelity model is used, it is possible to incorporate into the analysis of the collected data an understanding of how the results would be expected to vary from a higher fidelity case

The role of suction thrust in the metachronal paddles of swimming invertebrates

© The Author(s), 2020. This article is distributed under the terms of the Creative Commons Attribution License. The definitive version was published in Colin, S. P., Costello, J. H., Sutherland, K. R., Gemmell, B. J., Dabiri, J. O., & Du Clos, K. T. The role of suction thrust in the metachronal paddles of swimming invertebrates. Scientific Reports, 10(1), (2020): 17790, doi:10.1038/s41598-020-74745-y.An abundance of swimming animals have converged upon a common swimming strategy using multiple propulsors coordinated as metachronal waves. The shared kinematics suggest that even morphologically and systematically diverse animals use similar fluid dynamic relationships to generate swimming thrust. We quantified the kinematics and hydrodynamics of a diverse group of small swimming animals who use multiple propulsors, e.g. limbs or ctenes, which move with antiplectic metachronal waves to generate thrust. Here we show that even at these relatively small scales the bending movements of limbs and ctenes conform to the patterns observed for much larger swimming animals. We show that, like other swimming animals, the propulsors of these metachronal swimmers rely on generating negative pressure along their surfaces to generate forward thrust (i.e., suction thrust). Relying on negative pressure, as opposed to high pushing pressure, facilitates metachronal waves and enables these swimmers to exploit readily produced hydrodynamic structures. Understanding the role of negative pressure fields in metachronal swimmers may provide clues about the hydrodynamic traits shared by swimming and flying animals.This work was funded by National Science Foundation (NSF OCE 1829913 to SPC), the Alfred P. Sloan Foundation (to BJG) and the Gordon and Betty Moore Foundation (8835 to KRS). The work was also supported by the Roger Williams Foundation to Promote Scholarship and Teaching