Fluid dynamic turbulence is one of the most challenging computational physics problems because of the extremely wide range of time and space scales involved, the strong nonlinearity of the governing equations, and the many practical and important applications. While most linear fluid instabilities are well understood, the nonlinear interactions among them makes even the relatively simple limit of homogeneous isotropic turbulence difficult to treat physically, mathematically, and computationally. Turbulence is modeled computationally by a two-stage bootstrap process. The first stage, direct numerical simulation, attempts to resolve the relevant physical time and space scales but its application is limited to diffusive flows with a relatively small Reynolds number (Re). Using direct numerical simulation to provide a database, in turn, allows calibration of phenomenological turbulence models for engineering applications. Large eddy simulation incorporates a form of turbulence modeling applicable when the large-scale flows of interest are intrinsically time dependent, thus throwing common statistical models into question. A promising approach to large eddy simulation involves the use of high-resolution monotone computational fluid dynamics algorithms such as flux-corrected transport or the piecewise parabolic method which have intrinsic subgrid turbulence models coupled naturally to the resolved scales in the computed flow. The physical considerations underlying and evidence supporting this monotone integrated large eddy simulation approach are discussed.

New insights into large eddy simulation

An efficient ghost-cell immersed boundary method (GCIBM) for simulating turbulent flows in complex geometries is presented. A boundary condition is enforced through a ghost cell method. The reconstruction procedure allows systematic development of numerical schemes for treating the immersed boundary while preserving the overall second-order accuracy of the base solver. Both Dirichlet and Neumann boundary conditions can be treated. The current ghost cell treatment is both suitable for staggered and non-staggered Cartesian grids. The accuracy of the current method is validated using flow past a circular cylinder and large eddy simulation of turbulent flow over a wavy surface. Numerical results are compared with experimental data and boundary-fitted grid results. The method is further extended to an existing ocean model (MITGCM) to simulate geophysical flow over a three-dimensional bump. The method is easily implemented as evidenced by our use of several existing codes.

A Ghost-Cell Immersed Boundary Method for Flow in Complex Geometry

Wind energy is the fastest growing alternate source of energy in the world since its purely economic potential is complemented by its great positive environmental impact. The wind turbine, whether it may be a Horizontal-Axis Wind Turbine (HAWT) or a Vertical-Axis Wind Turbine (VAWT), offers a practical way to convert the wind energy into electrical or mechanical energy. Although this book focuses on the aerodynamic design and performance of VAWTs based on the Darrieus concept, it also discusses the comparison between HAWTs and VAWTs, future trends in design and the inherent socio-economic and environmental friendly aspects of wind energy as an alternate source of energy.

Wind turbine design with emphasis on Darrieus concept Ion Paraschivoiu

Wind energy, together with other renewable energy sources, are expected to grow substantially in the coming decades and play a key role in mitigating climate change and achieving energy sustainability. One of the main challenges in optimizing the design, operation, control, and grid integration of wind farms is the prediction of their performance, owing to the complex multiscale two-way interactions between wind farms and the turbulent atmospheric boundary layer (ABL). From a fluid mechanical perspective, these interactions are complicated by the high Reynolds number of the ABL flow, its inherent unsteadiness due to the diurnal cycle and synoptic-forcing variability, the ubiquitous nature of thermal effects, and the heterogeneity of the terrain. Particularly important is the effect of ABL turbulence on wind-turbine wake flows and their superposition, as they are responsible for considerable turbine power losses and fatigue loads in wind farms. These flow interactions affect, in turn, the structure of the ABL and the turbulent fluxes of momentum and scalars. This review summarizes recent experimental, computational, and theoretical research efforts that have contributed to improving our understanding and ability to predict the interactions of ABL flow with wind turbines and wind farms.

https://link.springer.com/content/pdf/10.1007/s10546-019-00473-0.pdf

Wind-Turbine and Wind-Farm Flows: A Review

https://biblio.ugent.be/publication/533365/file/533369.pdf

Performance of a new partitioned procedure versus a monolithic procedure in fluid-structure interaction

In fluid-structure interaction simulations the meshes at the fluid-structure interface usually do not match, because of the different mesh requirements for the flow and structure. The exchange of data over the discrete interface becomes then far from trivial. In this paper we investigate the difference in accuracy and efficiency between a conservative and a consistent coupling approach. This is done for an analytical test problem as well as a quasi-1D FSI problem, for different coupling methods found in literature. It is found that when the coupling method is based on a weak formulation of the coupling conditions the conservative approach is the best choice. For other coupling methods the consistent approach provides the best accuracy and efficiency, because the conservative approach results in unphysical oscillations in the pressure received by the structure and is therefore not consistent.

/pdf/comparison-of-the-conservative-and-a-consistent-approach-for-1qstakmo8a.pdf

Comparison of the conservative and a consistent approach for the coupling of non-matching meshes

This study addresses the quantification of the aerodynamic performance of the ‘Delfly Micro’, which is currently the smallest ornithopter in the ‘Delfly’ family with an X-Wing configuration. Aerodynamic forces were measured on a full-scale ‘Delfly Micro’ by a sophisticated balance system. In view of the influence of the wing flexibility on the performance, wings with varying membrane thicknesses and stiffener orientations were examined in hovering case. To characterize the forward flight regime, aerodynamic forces were also measured with varying free stream velocities and angles of attack in wind tunnel. The results indicate that wing flexibility indeed influences the aerodynamic performance, where a more rigid wing can produce more force, whereas with a lower force to power ratio. According to the wind tunnel data, the ‘Delfly Micro’ can attain a flight speed up to 3 m/s, at a flapping frequency over 30 Hz and angle of attact of 30 degrees.

/pdf/experimental-investigation-of-the-flapping-performance-on-2u4ekj0h97.pdf

Experimental Investigation of the Flapping Performance on 'Delfly Micro'

Due to the continuously increasing computer capacity simulations of multi-physics systems belong to one of the options. However, computations of multi-physics systems are still time consuming and therefore acceleration techniques are often used. Solution based mesh adaptation is a commonly used technique to optimize steady and unsteady numerical computations. The question is if solution based mesh adaptation can also be used to increase the efficiency of unsteady fluid structure interaction (FSI) computations. The current research focuses on the application of solution based mesh adaptation to FSI computations. The FSI solver used in this work is a partitioned solver that consists of a Reynolds Averaged Navier Stokes (RANS) solver for the flow and a rigid body dynamics representation for the structure. The FSI problem concerns a NACA0012 airfoil that is allowed to perform in-plane translations. The performed mesh adaptations consist of refinements that are based on the solution for one or more periods. First results show potential by means of a more detailed representation of flow features in comparison with the solution obtained by using non-adapted grids. Therefore more effort is put in the further development of the preliminary version of the unsteady solution based mesh adaptation, whereby adaptation after each time step and implementation of coarsening are the key-issues. The adaptations are based on the solution of the previous time step(s). With this preliminary version, the unsteady adaptation cannot be applied in combination with FSI yet, however the structure of the code is such, that with few modifications unsteady adaptation with FSI is possible. In this paper, two issues are discussed: firstly the preliminary results for an FSI case with mesh refinements based on the solution of a period and secondly the preliminary results of mesh adaptation applied to an unsteady flow without FSI.

Solution Based Mesh Adaptation Applied to Fluid Structure Interaction Computations

The flow around a beveled flat plate model with an asymmetric 25 degrees trailing edge with three rounding radii is analyzed using a Navier-Stokes based open source software package OpenFOAM in order to predict the aeroacoustic properties of the models. A Large Eddy Simulation with a dynamic Smagorinsky and implicit model are used as closure model for the flow solver, and are compared regarding their aeroacoustic performance. Velocity coherence and pressure correlation is determined in spanwise direction. The acoustic far field spectrum is obtained by solving Curle’s analogy in frequency domain as a post-processing step.

/pdf/on-the-aeroacoustic-properties-of-a-beveled-plate-564li80dy7.pdf

On the aeroacoustic properties of a beveled plate

Accurate numerical prediction of aeroelastic instabilities requires correct representation of the transfer of energy between the fluid and the structure, particularly when determining the neutral-stability point In the numerical model, energy transfer depends on the discretization methods used for the fluid and the structure as well as on their coupling In this paper we consider a space-time Galerkin least-squares formulation of the fluid equations and use shape functions which are discontinuous in time For the time integration of the structure we investigate two essentially different methods, namely the trapezoidal method and a time-discontinuous Galerkin method In particular, we assess their ability to conserve energy in the presence of a forcing term Further, we show that these methods are different in their ability to conserve momentum and energy at the interface when coupled to the fluid discretization, We compare monolithic and partitioned fluid-structure coupling Partitioned schemes are typically unstable Structural prediction can improve their accuracy and stability, but the admissible time step still remains restricted Although monolithic schemes do not necessarily imply energy conservation at the interface, they appear to have better stability properties than partitioned schemes allowing for larger time steps Moreover, we show that the efficiency of the iterative solver employed in a monolithic scheme can be improved by prediction techniques We illustrate our results by numerical experiments for a one-dimensional model problem of a piston interacting with a fluid

H. Bijl

Papers

Comparison of the conservative and a consistent approach for the coupling of non-matching meshes

Experimental Investigation of the Flapping Performance on 'Delfly Micro'

Solution Based Mesh Adaptation Applied to Fluid Structure Interaction Computations

On the aeroacoustic properties of a beveled plate

Space-time discretization for Fluid-Structure Interaction