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A hybrid filament-mesh vortex method is proposed and validated to predict the aerodynamic performance of wind turbine rotors and to simulate the resulting wake. Its novelty consists of using a hybrid method to accurately simulate the wake downstream of the wind turbine while reducing the computational time used by the method. The proposed method uses a hybrid approach, where the near wake is resolved by using vortex filaments, which carry the vorticity shed by the trailing edge of the blades. The interaction of the vortex filaments in the near vicinity of the wind turbine is evaluated using a direct calculation, whereas the contribution from the large downstream wake is calculated using a mesh-based method. The hybrid method is first validated in detail against the well-known MEXICO experiment, using the direct filament method as a comparison. The second part of the validation includes a study of the influence of the time-integration scheme used for evolving the wake in time, aeroelastic simulations of the National Renewable Energy Laboratory 5 MW wind turbine and an analysis of the central processing unit time showing the gains of using the hybrid filament-mesh method. Copyright © 2017 John Wiley & Sons, Ltd. [ABSTRACT FROM AUTHOR]

We present a new class of C∞-smooth finite element spaces on Cartesian grids, based on a partition of unity approach. We use these spaces to construct smooth approximations of particle fields, i.e., finite sums of weighted Dirac deltas. In order to use the spaces on general domains, we propose a fictitious domain formulation, together with a new high-order accurate stabilization. Stability, convergence, and conservation properties of the scheme are established. Numerical experiments confirm the analysis and show that the Cartesian grid-size σ should be taken proportional to the square-root of the particle spacing h, resulting in significant speed-ups in vortex methods. [ABSTRACT FROM AUTHOR]

A high-accuracy numerical scheme is proposed for vortex methods of flow simulation around airfoils of arbitrary shape including airfoils with sharp edges, because it does not require the solution continuity on the airfoil. [ABSTRACT FROM AUTHOR]

We present a Brinkman penalization method for three-dimensional (3D) flows using particle vortex methods, improving the existing technique by means of an iterative process. We perform simulations to study the impulsively started flow past a sphere at Re = 1000 and normal to a circular disc at Re = 500 . The simulation results obtained for the flow past a sphere are found in qualitative good agreement with previously published results obtained using respectively a 3D vortex penalization method and a 3D vortex method combined with an accurate boundary element method. From the results obtained for the flow normal to a circular disc it is found that the iterative method enables the use of a time step that is one order of magnitude larger than required by the standard non-iterative Brinkman penalization method. [ABSTRACT FROM AUTHOR]

In the present work, an in-house UnSteady Double Wake Model (USDWM) is developed for simulating general flow problems behind bodies. The model is presented and used to simulate flows past a circular cylinder at subcritical, supercritical, and transcritical flows. The flow model is a two-dimensional panel method which uses the unsteady double wake technique to model flow separation and its dynamics. In the present work the separation location is obtained from experimental data and fixed in time. The highly unsteady flow field behind the cylinder is analyzed in detail. The results are compared with experiments and Unsteady Reynolds-Averaged Navier Stokes (URANS) simulations and show good agreement in terms of the vortex shedding characteristics, drag, and pressure coefficients for the different flow regimes. [ABSTRACT FROM AUTHOR]

A synthetic jet is generated by an oscillating flow that a time-averaged mass flow is zero, unlike continuous jets. Synthetic jets have been used for various applications such as mixing enhancement and active controls. The present study investigated flow characteristics of a synthetic jet by using the vortex method. Continuous and synthetic jets were simulated by distributed sources on a slot and vortex pairs discharged from slot lips and their images. The wall boundary was simulated by the images of vortices. The onset condition of the synthetic jet by the vortex method agrees well with the Holman's result. The mean velocity distribution on the x axis by the vortex method reasonably agrees with the experimental one. For the dimensionless stroke lengths larger than 3.14, vortices remain in the flow field after the period of the suction. The induced velocity of the vortex remaining in the flow field generates the synthetic jet. During the suction, the largest velocity is located near a pair of the vortex groups. The 2nd local maximum in the mean velocity distribution corresponds to that in the frequency distribution of the maximum velocity. The 2nd local maximum is caused by a pair of the vortex groups flowing between x=15 and x=25 slowly.

An air jet, which remains laminar and axisymmetric in the single-phase flow condition, is simulated numerically in the particle-laden condition. The vortex method for particle-laden gas jet proposed by the authors is employed for the simulation. An air issues with velocity U0 from a round nozzle into the air co-flowing with velocity Ua. The Reynolds number based on U0 and the nozzle diameter is 1333, the velocity ratio Ua/U0 is 0.4. Spherical glass particles with diameter 65μm are loaded at the mass loading ratio 0.025. The particle velocity at the nozzle exit is 0.68U0. The particles impose disturbances on the air and induce the three-dimensional flow, resulting in the transition from the axisymmetric flow to the non-axisymmetric one. As the particles make the air velocity fluctuation increase, the air momentum diffuses more in the radial direction, and accordingly the spread of the jet becomes larger. The abovementioned results agree well with the trend of the existing experiments. The proposed vortex method can successfully capture the flow transition caused by the particles laden on an axisymmetric air jet.

The present paper investigates the performance of various inflow turbulence generation techniques (ITGT) for large eddy simulation (LES) of flow and dispersion around a model building in a turbulent atmospheric boundary layer. Four different ITGT comprising 1 – no fluctuations, 2 – spectral method, 3 – vortex method and 4 – internal mapping, based on two basic methodologies (i.e. precursor and synthetic turbulence methods), are employed. These techniques are evaluated by considering their prediction accuracy, computational costs, complexity of implementation, inflow information required to operate and impacts on the flow downstream of the inlet, particularly in the wake region of the model building. Results indicate that the accuracy of LES predictions is greatly reliant on ITGT. It is shown that ITGT not only have significant effects on flow field vortical structures, but also influence frequency contents of velocity fluctuations, recirculation regions and plume shapes in the wake region. [ABSTRACT FROM PUBLISHER]

The Diffused Vortex Hydrodynamics (DVH) is a recent numerical model born as an evolution of the classical vortex particle methods, where an improvement of the solution quality has been achieved through a regularization of the particles spatial distribution during the vorticity diffusion process. The DVH method is a pure meshless method which adopts a body-fitted approach to enforce no-slip boundary condition on solid surfaces. In the present work it is exploited to perform an accurate analysis of the vorticity field generated by the incompressible flow around bodies with geometrical singularities. In common academic or technological applications, such singularities are quite frequent ( e.g. trailing edges of wing profiles or of propeller blades) and the classical Euler mesh-based methods can suffer in modeling these geometries. In the recent years a wide literature was produced in order to describe enhanced numerical methods that could overcome such challenging problems. The potentialities of the DVH approach to problems where bodies with geometric singularities are involved are deeply discussed and some examples are finally offered. [ABSTRACT FROM AUTHOR]

A nonlinear aerodynamic modeling based on conformal mapping is presented to obtain semi-analytical formulas for the unsteady aerodynamic force and pitching moment on a flat-plate airfoil in arbitrary motion. The aerodynamic model accounts for large amplitudes and non-planar wake and is used to study the aeroelastic behavior of a flat-plate airfoil elastically connected to a support. The fluid is assumed to be inviscid and incompressible, while the flow is assumed to be attached, planar, and potential. Within these hypotheses, conformal mapping and a complex-potential representation of unsteady aerodynamics are used to simplify the theoretical formulation. The vorticity shed at the trailing edge is discretized in desingularized point vortices in order to allow free-wake dynamics. The unsteady aerodynamic model is validated with classical linearized formulations based on the assumption of small disturbances, and with experimental data and theoretical predictions for a large-amplitude pitch-up, hold, pitch-down maneuver. The aeroelastic model is then used to simulate the response of a flat-plate airfoil to sudden starts and body–vortex interactions. Numerical results show that the proposed approach can be an effective tool to model the aeroelastic behavior of an arbitrarily-moving wing section in a time-dependent potential stream of incompressible fluid. [ABSTRACT FROM AUTHOR]