Vortex Particle Velocity Diffusion Method Using Dynamic Turbulence Control Based on Enstrophy

The rotor interaction phenomenon occurs frequently in unmanned aerial vehicles (UAVs) with multi-rotor configuration. In this study, we investigated the rotor interaction effects of a small-size quadcopter UAV on the aerodynamic performance and acoustic characteristics. The numerical simulation of DJI Phantom 2 model in the hover flight condition was conducted using the nonlinear vortex lattice method (NVLM) with the vortex particle method (VPM) and acoustic analogy based on the Farassat-1A formula. NVLM can consider the nonlinear aerodynamic characteristics that are mainly associated with viscous and low Reynolds number effects by incorporating airfoil look-up table and vortex strength correction. VPM is well suitable for simulating complex wake structures of quadcopter UAV because maintaining the connectivity between adjacent vortex particles is not required. Calculations for an isolated rotor were compared against measurements to validate the predicted results, including thrust force and tonal noise at 1st blade passing frequency (BPF). The calculations for a quadcopter configuration show that the average thrust coefficient decreases and thrust coefficient begins to fluctuate dramatically due to severe rotor interaction. In addition, the sound pressure level of the quadcopter is much higher than that of the isolated rotor and a significant difference in noise directivity between the quadcopter and isolated rotor is clearly captured. It is observed that unsteady loading introduced by rotor interaction leads to a considerable increase in noise. This study is a preliminary study to the aeroacoustic problem in multicopter UAV. The discussion in this work gives scope for further research on noise reductions of multicopter UAV by controlling a separation distance between the rotors.

In this paper, we present a hybrid method, which integrates PIC/FLIP and vortex particle methods into a unified framework, to efficiently simulate vortex shedding that happens when fluids flow around internal obstacles. To improve efficiency and reduce the numerical dissipations, we first solve the governing equations on a coarse grid using PIC/FLIP, and then interpolate the intermediate results to a finer grid to obtain the base flow. When the regular particles in PIC/FLIP enter the boundary layer, if the specified conditions are satisfied to cause vortex shedding, they are selected as vortex particles by assigning additional vorticity related attributes. The vortex particle dynamics are controlled by the vorticity form of NS equations, and several efficient methods are proposed to solve them on the finer grid. Finally, the obtained turbulence flow is added to the base flow. As a result, we are able to simulate turbulent water with rich wake details around the internal obstacles.

Vortex Particle Methods are one of the most convenient ways of tracking the vorticity evolution. In the article we presented numerical recreation of the real life experiment concerning head-on collision of two vortex rings. In the experiment the evolution and reconnection of the vortex structures is tracked with passive markers (paint particles) which in viscous fluid does not follow the evolution of vorticity field. In numerical computations we showed the difference between vorticity evolution and movement of passive markers. The agreement with the experiment was very good. Due to problems with very long time of computations on a single processor the Vortex-in-Cell method was implemented on the multicore architecture of the graphics cards (GPUs). Vortex Particle Methods are very well suited for parallel computations. As there are myriads of particles in the flow and for each of them the same equations of motion have to be solved the SIMD architecture used in GPUs seems to be perfect. The main disadvantage in this case is the small amount of the RAM memory. To overcome this problem we created a multiGPU implementation of the VIC method. Some remarks on parallel computing are given in the article.

The modelling of diffusive terms in particle methods is a delicate matter and several models were proposed in the literature to take such terms into account. The diffusion velocity method (DVM), originally designed for the diffusion of passive scalars, turns diffusive terms into convective ones by expressing them as a divergence involving a so-called diffusion velocity. In this paper, DVM is extended to the diffusion of vectorial quantities in the three-dimensional Navier–Stokes equations, in their incompressible, velocity–vorticity formulation. The integration of a large eddy simulation (LES) turbulence model is investigated and a DVM general formulation is proposed. Either with or without LES, a novel expression of the diffusion velocity is derived, which makes it easier to approximate and which highlights the analogy with the original formulation for scalar transport. From this statement, DVM is then analysed in one dimension, both analytically and numerically on test cases to point out its good behaviour.

This Paper presents a coupled vortex particle method–computational fluid dynamics solver. The vortex particle method is used to prevent dissipation of the vortex structure on coarse computational fluid dynamics meshes. Implementation of the approach uses the split velocity method that specifies the fluid velocity as the sum of the induced vortex particle velocity and a remaining velocity. Dissipation of the vortex velocities on coarse meshes is removed, and the computational fluid dynamics equations solved for the remaining velocity have a form identical to those for a moving mesh, but with additional source terms. The coupled solver is demonstrated on a selection of two-dimensional test cases, and the results are compared to the solutions of the computational fluid dynamics solver on its own using a coarse mesh and a fine mesh. It is shown that the coupled solver preserves the vortices on a coarse mesh and is computationally more efficient than using the fine mesh.

This research is concerned with the two-dimensional vortex method (VM) solvers. We develop and investigate the performance of the Vortex-In-Cell (VIC) and Vortex Particle Method (VPM) which are well known as the VM’s family members. The advantage of these both methods are that we can accelerate velocity computation procedure, an N-body problem in numerical methods, by using Fast Fourier Transform (FFT) and Fast Multipole Method (FMM), respectively. In addition, the viscous calculation process in VPM can be accelerated by using a scheme of Nearest Neighbor Particle Searching (NNPS) algorithms. Moreover, the no-through boundary condition treatment issue can be easily handled by using an immersed boundary condition for both methods. The accuracy and numerical cost of both numerical methods will be examined by simulating flow over an Impulsively Started Circular Cylinder and comparisons

Rotor wake causes unsteady aerodynamics of rotor blade. So, accurate prediction of wake is very important and vortex method is good solution for this problem. Aerodynamic force of the rotor blade is calculated by potential panel method and the rotor wake is simulated by vortex particle method. The vortex particle method is easier to treat wake-body interaction and has better performance to expect the effect of ground and fuselage interaction. Rotor in hovering and forward flight condition is simulated through these methods. Thrust and surface pressure of rotor are compared with experiment data.

Review for "Estimation of power performances and flow characteristics for a Savonius rotor by vortex particle method"

El presente trabajo de investigación se ocupa del estudio de las vibraciones verticales inducidas por vórtices (VIV) en aquellos puentes que, por sus características geométricas y propiedades dinámicas, muestran cierta sensibilidad este tipo de fenómeno aeroelástico. El objeto principal es el análisis del mecanismo de interacción viento-estructura sobre secciones no fuseladas de geometría simple, con objeto de realizar una adecuada caracterización del problema y poder abordar posteriormente el análisis de otras secciones de geometría más compleja, representativas de los principales elementos estructurales de los puentes, como arcos, tableros, torres y pilas. Este aspecto es fundamental durante la fase de diseño del puente, donde deberían tenerse en cuenta también una serie de detalles que pueden influir significativamente en su sensibilidad ante problemas aerodinámicos, como la morfología y dimensiones principales de la sección transversal del tablero, la disposición de barreras de seguridad y barreras cortaviento, o las riostras que unen diferentes elementos estructurales. La configuración de dos elementos en tándem o la construcción de un puente en las inmediaciones de otro existente son otros aspectos a considerar respecto a la sensibilidad frente a efectos aeroelásticos. El estudio se ha llevado a cabo principalmente mediante la implementación de simulaciones numéricas que reproducen la interacción entre la corriente de aire y secciones representativas de modelos estructurales, a partir de un código CFD basado en el método de las partículas de vórtices (VPM), siguiendo por tanto un esquema Lagrangiano. Los resultados han sido validados con datos experimentales existentes, valores procedentes de ensayos en túnel de viento y registros reales a partir de diferentes casos de estudio: Alconétar (2006), Niterói (1980), Trans-Tokyo Bay (1995) y Volgogrado (2010). Finalmente, se propone un modelo semi-empírico para la estimación del rango de velocidades críticas y amplitudes de oscilación basado en la utilización de las derivadas de flameo de Scanlan y la densidad espectral de las fuerzas aerodinámicas en el dominio de la frecuencia. The present research work concerns the study of vertical vortex-induced vibrations (VIV) in bridges which show certain sensitivity to this type of aeroelastic phenomenon. It focuses on the analysis of the wind-structure interaction mechanism on bluff sections, with the objective of making a good characterisation of the problem and subsequently addressing the analysis of sections with a complex geometry, which are representative of the bridge structural elements, such as arches, decks, towers and piers. This issue is of relative importance during the bridge design phase, since minor details of the aforementioned elements can significantly influence its sensitivity to aerodynamic problems. The shape and main dimensions of the deck cross section, the addition of safety barriers and windshields, the presence of braces to enhance the structure mechanical properties, the utilisation of cross sections in tandem arrangement, or the erection of a new bridge in the vicinity of another existing one are some of the aspects to be considered regarding the sensitivity to the aeroelastic effects. The study has been carried out mainly through the implementation of numerical simulations that reproduce the interaction between the airflow and the representative cross section of a structural bridge model, by the use of a CFD code based on the vortex particle method (VPM), thus following a Lagrangian scheme. The results have been validated with existing experimental data, values from wind tunnel tests and full scale observations from the different case studies: Alconétar (2006), Niterói (1980), Trans-Tokyo Bay (1995) and Volgograd (2010). Finally, a new semi-empirical model is proposed for the estimation of the critical wind velocity ranges and oscillation amplitudes based on the use of the Scanlan’s flutter derivatives and the power spectral density of aerodynamic force time history in the frequency domain.

The algorithm of the Viscous Vortex Domain method implemented in the author’s code VM2D is investigated. This code allows simulating 2D viscous incompressible flows and solving fluid-structure interaction problems. The verification of the algorithm, namely, the correctness of taking into account the viscosity effect, is performed on well-studied test problems of the flow simulation around a circular cylinder for Reynolds numbers in the range 20 . 200: the problem of the impulse start of the circular cylinder is considered, and the dependency of the flow separation angle on the Reynolds number is investigated. The comparison shows that for low Reynolds numbers (up to 140), the flow separation angle obtained in numerical simulations by using the VM2D code is in good agreement with the results of other studies. For higher Reynolds numbers, the estimated separation angle is slightly overestimated.

The possibility to represent a given input velocity field using vortex particles is investigated in this chapter. This operation is referred to as velocity reconstruction. A direct and simple approach is chosen to perform such operation. The errors inherent to the method, but also linked to these approximations, are discussed. These errors include: the divergence error of the vortex particles vorticity field, the numerically finite support of the vorticity domain (absence of Neumann-to-Dirichlet map for the external flow), the differentiation error, the regularization error and the discretization error. A method to investigate the discretization error using sinusoidal signals is used to study the approximate cut-off of the method. The method is applied to a turbulent velocity field in an unbounded domain. The results were presented in the publication titled “Impact of a wind turbine on turbulence: un-freezing the turbulence by means of a simple vortex particle approach” (Branlard, Gaunaa, 2016, [4]). Applications of the method are given in Chaps. 29 and 30.

The algebraic turbulent eddy viscosity model of Baldwin and Lomax has been critically examined for the case of two-dimension al (2-D) supersonic compression corner interactions. The flowfields are computed using the Navier-Stokes equations together with three different versions of the Baldwin-Lomax model, including the incorporation of a relaxation technique. The turbulence models are evaluated by a detailed comparison with available experimental data for compression ramp flows over a range of corner angle and Reynolds number. The Baldwin-Lomax outer formulation is found to be unsuitable for separated 2-D supersonic interactions due to the unphysical streamwise variation of the computed length scale in the vicinity of separation. Minor modifications are proposed to partially remedy this difficulty. The use of relaxation provides significant improvement in the flowfield prediction upstream of the corner. However, the relaxation length required is one-tenth of that employed in a previous computational study. AH of the turbulence models tested here fail to simulate the rapid recovery of the boundary layer downstream of reattachment.

The existence of universal power laws at low wavenumbers (K) in the energy spectrum (Eu) of the turbulent longitudinal velocity (u) is examined theoretically and experimentally for the near-neutral atmospheric surface layer. Newly derived power-law solutions to Tchen's approximate integral spectral budget equation are tested for strong- and weak-interaction cases between the mean flow and turbulent vorticity fields. To verify whether these solutions reproduce the measured Eu at low wavenumbers, velocity measurements were collected in the dynamic sublayer of the atmosphere at three sites and in the inner region of a laboratory open channel. The atmospheric surface layer measurements were carried out using triaxial sonic anemometers over tall corn, short grass, and smooth desert-like sandy soil. The open channel measurements were performed using a two-dimensional boundary-layer probe above a smooth stainless steel bed. Comparisons between the proposed analytical solution for Eu, the dimensional analysis by Kader and Yaglom, and the measured Haar wavelet Eu spectra are presented. It is shown that when strong interaction between the mean flow and turbulent vorticity field occurs, wavelet spectra measurements, predictions by the analytical solution, and predictions by the dimensional analysis of Kader-Yaglom (KY) are all in good agreement and confirm the existence of a -1 power law in Eu(= Cuuu2* K-1, where Cuu is a constant and u* is the friction velocity). The normalized upper wavenumber limit of the -1 power law (Kz = 1, where z is the height above the zero-plane displacement) is estimated using two separate approaches and compared to the open channel and atmospheric surface-layer measurements. It is demonstrated that the measured upper wavenumber limit is consistent with Tchen's budget but not with the KY assumptions. The constraints as to whether the mean flow and turbulent vorticity strongly interact are considered using a proposed analysis by Panchev. It is demonstrated that the arguments by Panchev cannot be consistent with surface-layer turbulence. Using dimensional analysis and Heisenberg's turbulent viscosity model, new constraints are proposed. The new constraints agree with the open channel and atmospheric surface-layer measurements, Townsend's ‘inactive eddy motion hypothesis’, and the Perry et al. analysis.

A first-order turbulence model based on a turbulent viscosity formulation is derived from a second-order turbulence closure developed for bubbly flows. The interpretation of the second-order turbulence closure in terms of the turbulent viscosity model leads to an analytical analysis of homogeneous bubbly flows. Based on this analysis, a kε turbulence model is then developed and applied to two homogeneous bubbly flows (uniform and with a constant shear). The simulations show that this kε turbulence model that involves a supplementary scale related to the bubble effects succeeds in the representation of the interfacial effects on the turbulence structure in the two homogeneous bubbly flows.

In this paper we have basic explanation of vortex and deeply explain the various vortex particle methods of water, smoke, and explosions. So from we knew that in water,smoke and explosion field we have getdifferent vortex particle effects which are useful for this topic. We have also explain Helmholtz’s theorems as basic principle of vortex for achieving better flow mixing and we have describe various experimental results of interaction of different vortices. And from all we are trying to achieve a good flow mixing using vortex enhancement. Keywords: Vortex, Helmholtz’s Theorem, vortex methods, fluids, smoke, water, explosion Cite this Article Maitri Dave, Vinal Virani. Vortex Enhanced Flow Mixing: A Review. Journal of Thermal Engineering and Application . 2015; 2(3): 8–13p.