CFD modelling of wind turbine airfoil aerodynamics

This paper reports the first findings of an ongoing research programme on wind turbine computational aerodynamics at the University of Glasgow. Several modeling aspects of wind turbine airfoil aerodynamics based on the solution of the Reynoldsaveraged Navier-Stokes (RANS) equations are addressed. One of these is the effect of an a priori method for structured grid adaptation aimed at improving the wake resolution. Presented results emphasize that the proposed adaptation strategy greatly improves the wake resolution in the far-field, whereas the wake is completely diffused by the non-adapted grid with the same number and distribution of grid nodes. A grid refinement analysis carried out with the adapted grid shows that the improvements of flow resolution thus achieved are of a smaller magnitude with respect to those accomplished by adapting the grid keeping constant the number of nodes. The proposed adaptation approach can be easily included in the structured generation process of both commercial and in-house structured mesh generators systems. The study also aims at quantifying the solution inaccuracy arising from not modeling the laminar-to-turbulent transition. It is found that the drag forces obtained by considering the flow as transitional or fully turbulent may differ by 50 %. The impact of various turbulence models on the predicted aerodynamic forces is also analyzed. All these issues are investigated using a special-purpose hyperbolic grid generator and a multi-block structured finitevolume RANS code. The numerical experiments consider the flow field past a wind turbine airfoil for which an exhaustive campaign of steady and unsteady experimental measurements was conducted. The predictive capabilities of the CFD solver are validated by comparing experimental data and numerical predictions for selected flow regimes. The incompressible analysis and design code XFOIL is also used to support the findings of the comparative analysis of numerical RANS-based results and experimental data

The role of fundamental solution in Potential and Regularity Theory for subelliptic PDE

In this survey we consider a general Hormander type operator, represented as a sum of squares of vector fields plus a drift and we outline the central role of the fundamental solution in developing Potential and Regularity Theory for solutions of related PDEs. After recalling the Gaussian behavior at infinity of the kernel, we show some mean value formulas on the level sets of the fundamental solution, which are the starting point to obtain a comprehensive parallel of the classical Potential Theory. Then we show that a precise knowledge of the fundamental solution leads to global regularity results, namely estimates at the boundary or on the whole space. Finally in the problem of regularity of non linear differential equations we need an ad hoc modification of the parametrix method, based on the properties of the fundamental solution of an approximating problem

Wake interaction in offshore wind farms with mesoscale derived inflow condition and sea waves

Numerical simulation is an indispensable tool for the design and optimization of wind farms layout and control strategies for energy loss reduction. Achieving consistent simulation results is strongly related to the definition of reliable weather and sea conditions, as well as the use of accurate computational fluid dynamics (CFD) models for the simulation of the wind turbines and wakes. Thus, we present a case study aiming to evaluate the wake-rotor interaction between offshore multi-MW wind turbines modelled using the Actuator Line Model (ALM) and realistic wind inflow conditions. In particular, the interaction between two DTU10 wind turbines is studied for two orientations of the upstream turbine rotor, simulating the use of a yaw-based wake control strategy. Realistic wind inflow conditions are obtained using a multi-scale approach, where the wind field is firstly computed using mesoscale numerical weather prediction (NWP). Then, the mesoscale vertical wind profile is used to define the wind velocity and turbulence boundary conditions for the high-fidelity CFD simulations. Sea waves motion is also imposed using a dynamic mesh approach to investigate the interaction between sea waves, surface boundary layer, and wind turbine wakes and loads

Clinical application of cardiac scintigraphy with bone tracers: controversies and pitfalls in cardiac amyloidosis

Cardiac amyloidosis (CA) is a life-threatening disease caused by extracellular deposition of amyloidogenic proteins in the heart tissue; it could be associated with a poor prognosis and remains underdiagnosed and underestimated. During the last years, bone scintigraphy has been widely used to facilitate the diagnosis of CA, avoid endomyocardial biopsy, and differentiate amyloid light-chain amyloidosis from transthyretin amyloidosis. Technetium-99m pyrophosphate (99mTc-PYP) is the most used tracer in the United States, but a standardized and shared acquisition protocol is still lacking; technetium-99m 3,3-diphosphono-1,2-propanodicarboxylic acid (99mTc-DPD) is widely used in Europe and can count on a more grounded data than 99mTc-PYP. Both tracers suffer from some diagnostic limitations (due to their biochemical characteristics) and pitfalls that can lead to a misdiagnosis of CA. We aim to briefly describe the main differences between 99mTc-PYP and 99mTc-DPD, analyzing the data available in the literature and highlighting the most frequent causes of misdiagnosis and pitfalls. Both 99mTc-DPD and 99mTc-PYP show good accuracy for the diagnosis of CA with high specificity and sensibility. Nevertheless, to achieve this accuracy, the correct acquisition protocols must be followed for each tracer, as suggested in the latest recommendation. Proper diagnosis of CA has a crucial role in patient management; therefore, it is important for nuclear physicians to have the most specific approaches in acquiring and interpreting bone scintigraphy for transthyretin cardiac amyloidosis

Uniform Gaussian estimates for the fundamental solutions for heat operators on Carnot groups

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"Is this case of a very rare disease work-related?" A review of reported cases of Pacinian neuroma.

Objective In response to a request for a medicolegal opinion from an orthopaedic technician with pacinian neuroma of the hand, we conducted a systematic review of reported cases designed to assess whether this rare disease could be considered to be work-related.Methods We used Medline, Embase, Google Scholar, Google, and a manual search to identify reports of histologically confirmed pacinian neuroma of the hand manifesting after 15 years of age. Cases with available information (from article/authors) on personal history were considered eligible. We tabulated information on age/gender; localization and histopathologic features; signs/symptoms; history of local trauma; occupation, sports/hobbies or other physical exposures; right- or left-handedness; and outcome.Results We found 44 eligible cases (including the present referral). Of these, 21 (48%) followed a trauma [<6 months from onset/presentation (N=7); 0.5\u20132 years from onset (N=6); 652 years from onset (N=7); and timing unknown (N=1)] involving the same anatomic site. Three of these 21 traumas definitely occurred at work, and a further 2 cases occurred at the site of definite work-related repetitive microtrauma.Conclusions The results reinforce the concept that pacinian neuroma of the hand can follow local trauma or repetitive microtrauma. Implicated traumas were either recent or remote, and they sometimes occurred while performing manual tasks at work. We concluded that the referred case could plausibly be considered work-related

Low-speed preconditioning for strongly coupled integration of Reynolds-averaged Navier–Stokes equations and two-equation turbulence models

Computational fluid dynamics codes using the density-based compressible flow formulation of the Navier–Stokes equations have proven to be very successful for the analysis of high-speed flows. However, solution accuracy degradation and, for explicit solvers, reduction of the residual convergence rates occur as the local Mach number decreases below the threshold of 0.1. This performance impairment worsens remarkably in the presence of flow reversals at wall boundaries and unbounded high-vorticity flow regions. These issues can be resolved using low-speed preconditioning, but there exists an outstanding problem regarding the use of this technology in the strongly coupled integration of the Reynolds-averaged Navier–Stokes equations and two-equation turbulence models, such as the k − ω shear stress transport model. It is not possible to precondition only the RANS equations without altering parts of the governing equations, and there did not exist an approach for preconditioning both the RANS and the SST equations. This study solves this problem by introducing a turbulent low-speed preconditioner of the RANS and SST equations that does not require any alteration of the governing equations. The approach has recently been shown to significantly improve convergence rates in the case of a one-equation turbulence model. The study focuses on the explicit multigrid integration of the governing equations, but most algorithms are applicable also to implicit integration methods. The paper provides all algorithms required for implementing the presented turbulent preconditioner in other computational fluid dynamics codes. The new method is applicable to all low- and mixed-speed aeronautical and propulsion flow problems, and is demonstrated by analyzing the flow field of a Darrieus wind turbine rotor section at two operating conditions, one of which is characterized by significant blade/vortex interaction. Verification and further validation of the new method is also based on the comparison of the results obtained with the developed density-based code and those obtained with a commercial pressure-based code

Potential theory results for a class of PDOs admitting a global fundamental solution

We outline several results of Potential Theory for a class of linear par-tial differential operators L of the second order in divergence form. Under essentially the sole assumption of hypoellipticity, we present a non-invariant homogeneous Harnack inequality for L; under different geometrical assumptions on L (mainly, under global doubling/Poincar\ue9 assumptions), it is described how to obtainan invariant, non-homogeneous Harnack inequality. When L is equipped with a global fundamental solution \u393, further Potential Theory results are available (such as the Strong Maximum Principle). We present some assumptions on L ensuring that such a \u393 exists

On the Hausdorff volume in sub-Riemannian geometry

For a regular sub-Riemannian manifold we study the Radon-Nikodym derivative of the spherical Hausdorff measure with respect to a smooth volume. We prove that this is the volume of the unit ball in the nilpotent approximation and it is always a continuous function. We then prove that up to dimension 4 it is smooth, while starting from dimension 5, in corank 1 case, it is C^3 (and C^4 on every smooth curve) but in general not C^5. These results answer to a question addressed by Montgomery about the relation between two intrinsic volumes that can be defined in a sub-Riemannian manifold, namely the Popp and the Hausdorff volume. If the nilpotent approximation depends on the point (that may happen starting from dimension 5), then they are not proportional, in general.Comment: Accepted on Calculus and Variations and PD

Harnack inequality for fractional sub-Laplacians in Carnot groups

In this paper we prove an invariant Harnack inequality on Carnot-Carath\'eodory balls for fractional powers of sub-Laplacians in Carnot groups. The proof relies on an "abstract" formulation of a technique recently introduced by Caffarelli and Silvestre. In addition, we write explicitly the Poisson kernel for a class of degenerate subelliptic equations in product-type Carnot groups