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

High fidelity analysis of fluid-structure interaction systems is often too timeconsuming when a large number of model evaluations are required. The choice for a solution procedure depends often on the efficiency of the method and the possibility of reusing existing field solvers. Aggressive Space-Mapping, a technique originally developed for multi-fidelity optimization, is applied to accelerate the partitioned solution procedure of a high fidelity fluid-structure interaction model. The method supports software modularity. Aggressive Space-Mapping (ASM) is applied to an academic testcase and the results are compared with the corresponding Incremental Quasi-Newton (IQN) method. An efficiency metric is defined to facilitate the comparison. The ASM method is found to be more efficient than the corresponding IQN method for the testcases considered. The efficiency of space-mapping increases with increasing fluid-to-structure mass ratio, indicating that the method is especially useful for strongly coupled problems.

/pdf/a-multi-model-incremental-adaptive-strategy-to-accelerate-75s5psst62.pdf

A multi-model incremental adaptive strategy to accelerate partitioned fluid-structure algorithms using space-mapping

In aerospace engineering Computational Fluid Dynamics (CFD) is often ap- plied to obtain values for quantities of interest which are global functionals of the solution of the CFD computation. For instance the lift, drag and control- and stability derivatives necessary in ight simulation models for ight simulators. In the application for ight simulation models it would require years of performing CFD computations to generate such a model. One way of reducing the computational time is to apply a mathemati- cal uid ow model which is suciently sophisticated to compute the quantity of interest with the required accuracy. The ultimate goal is to apply a model adaptive strategy which adapts the 'coarse' mathematical model (in parts of the computational domain) to a more sophisticated model when the modelling error in the quantity of interest is too large. This approach requires the application of adjoint techniques to couple the local modelling errors to the global quantity of interest. In this paper we study global modelling error estimation in a quantity of interest by a dual weighted residual method, as described in (2), to a simple linear, scalar model problem of which the analytical solutions are known exactly.

/pdf/model-error-estimation-in-global-functionals-based-on-39w41skpco.pdf

Model error estimation in global functionals based on adjoint formulation

A three-dimensional simulation of hovering flapping wings was performed using an immersed boundary method. This was done to investigate the effects of chordwise wing deformation on three important unsteady aerodynamic mechanisms found in flapping flight, namely Leading Edge Vortex (LEV) shedding, wake capture and clap and fling. A wing was modeled as a flat plate, flapping close to a symmetry plane. Three different deforming chords were defined, a rigid case, a case with maximum deformation at the trailing edge and increased angle of attack (AoA) near the leading edge, and a case with the maximum deformation in the center of the chord and decreased AoA near the leading edge. All cases had zero deformation at the wing root and maximal deformation at the wing tip. A higher AoA near the leading edge resulted in faster LEV buildup and faster buildup of lift. No shedding of the LEV was observed in any of the cases even when deformation caused a high AoA near the leading edge. A distinct dip in lift buildup was observed and shown to be caused by the interaction between the previously shed vortex and the newly developing LEV. This interaction occurred faster when the AoA at the leading edge was increased, and slower when the angle of attack was decreased. Moving the wing closer to the symmetry plane had a positive effect on the cycle average value of CL. This positive effect was reduced however by the earlier interaction between the LEV and the previously shed vortex.

/pdf/effect-of-chordwise-deformation-on-unsteady-aerodynamic-1hnvq4jnt5.pdf

Effect of chordwise deformation on unsteady aerodynamic mechanisms in hovering flapping flight

In this paper, the influence of wing flexibility and the 'clap-and-peel' motion (one versus two wings) on the upward (lifting) force in hover is studied. CFD simulations based on the ALE Navier-Stokes equations together with radial basis function mesh interpolation are used. For the 'clap-and-peel' motion an immersed symmetry plane is introduced into the computational domain. The flexible shapes of the wing are obtained from experiments. From the preliminary results it can be concluded that flexibility enhances the production of the upward force.

Aerodynamic analysis of the wing flexibility and the clap-and-peel motion of the hovering DelFly II

Noting that time-accurate computations of the unsteady incompressible Navier-Stokes (INS) equations can be computationally expensive, a family of higher order implicit multi-stage time integration schemes (namely ESDIRK) is used for advancing the solution to the unsteady INS in time. The higher order time integration schemes have the potential to decrease the computational cost of obtaining engineering levels of accuracy relative to the traditionally used 2nd order implicit schemes. The finite volume method is used for spatial discretization, and co-located arrangement of the primitive variables is considered. Furthermore, an iterated PISO algorithm is used to solve the incompressible Navier-Stokes equations. By using a temporally consistent Rhie-Chow interpolation, higher order temporal accuracy in solving the INS equations on co-located grids is achieved. For a two-dimensional lid driven cavity test case, the temporal convergence of the solution is investigated, with the third and fourth order ESDIRK schemes for time integration. The results demonstrate the temporal consistency and temporal order preservation of the algorithm.

/pdf/higher-order-implicit-time-integration-schemes-to-solve-46bb2j20zh.pdf

H. Bijl

Papers

A multi-model incremental adaptive strategy to accelerate partitioned fluid-structure algorithms using space-mapping

Model error estimation in global functionals based on adjoint formulation

Effect of chordwise deformation on unsteady aerodynamic mechanisms in hovering flapping flight

Aerodynamic analysis of the wing flexibility and the clap-and-peel motion of the hovering DelFly II

Higher order implicit time integration schemes to solve incompressible navier-stokes on co-located grids using consistent unsteady rhie-chow