High-Resolution Simulation of Parallel Blade-Vortex Interactions

Numerical Simulation of Two-Dimensional Parallel Blade-Vortex Interactions Using Large Eddy Simulation

Helicopter Blade-Vortex Interaction (BVI) generally occurs under certain conditions of powered descent or during extreme manoeuvring. The vibration and acoustic problems associated with the interaction of rotor tip vortices and the following blades is a major aerodynamic concern for the helicopter community. Numerous experimental and computational studies have been performed over the last two decades in order to gain a better understanding of the physical mechanisms involved in BVI. The most severe interaction, in term of generated noise, happens when the vortex fllament is parallel to the blade, thus afiecting a great portion of it. The majority of the previous numerical studies of parallel BVI fall within a potential ∞ow framework therefore precluding vortex-induced ∞ow separation. Some Navier-Stokes approaches using dissipative numerical methods in conjunction with RANS-type turbulence models have also been attempted, but with limited success. In this work, the situation is improved by increasing the fldelity of both the numerical method and the turbulence model. A kineticenergy conserving flnite-volume scheme using a collocated-mesh arrangement, specially designed for simulation of turbulence in complex geometries, was implemented. For the turbulence model, a costefiective zonal hybrid RANS/LES technique is used. The concentrated tip vortex is not attenuated as it is convected downstream and over a NACA-0012 airfoil. The lift, drag, moment coe‐cients induced by the passage of the vortex are monitored in time and compared with available experimental data.

PurposeThis paper aims to investigate the mechanisms lying behind the cycloidal rotor under hovering status.Design/methodology/approachExperiments were conducted to validate the numerical simulation results. The simulations were based on unsteady Reynolds-averaged Navier–Stokes (URANS) equations solver and the sliding mesh technique was used to model the blade motion. 2D and 2.5D simulations were made to investigate the 3D effects of turbulence. The effects of pressure and viscosity were compared to study the significance of the blade motion on force generation.FindingsThe 2.5D numerical simulation cannot produce more accurate results than the 2D counterpart. The pitching motion of the blade results in dynamic stall. The dynamic stall vortices induce parallel blade vortex interaction (BVI) upon downstream blades. The interactions between the blades delay the stall of the blade which is beneficial to the thrust generation. The blade pitching motion is the dominant contributor to the force generation and the turbulence is the secondary. Strong downwash in the rotor cage varied the inflow velocity as well as the effective angle of attack (AOA) of the blade.Practical implicationsCycloidal rotor is a propulsion device that can provide omni-directional vectored thrust with high efficiency and low noise. To understand the mechanisms lying behind the cycloidal rotor helps the authors to design efficient cycloidal rotors for aircraft.Originality/valueThe authors discovered that the blade pitching motion plays primary role in force generation. The effects of the dynamic stall and BVI were studied. The reason why cycloidal rotor can be more efficient was discussed.

Experimental study of thermal performance and flow behaviour with winglet vortex generators in a circular tube

The Vortex Generators located over the airfoils are generally small in size. Due to its small size and shape, the mesh requirements are high and mesh generation becomes complex. In this study, the source term modelling approach termed as BAY model developed by Bender et al. is used to simulate the effect of Vortex Generators. One of the advantages of BAY model is its simplicity eliminating the complex grid requirements around the Vortex Generator for the Computational Fluid Dynamic simulations. Comparing with the BAY model approach, mesh resolved Vortex Generator approach will need more number of cells in the domain. Hence using the B AY model is advantageous in computational cost also. The solver EllipSys3D, which is an incompressible structured multi-block finite volume based RANS (Reynolds Averaged Navier-Stokes) solver, is used for the studies. Parametric studies using BAY model are carried out for different heights, lengths, chord wise locations and spacing of Vortex Generators on a wind turbine airfoil. The qualitative results predicted using BAY model are in lined with experimental results from literature showing that it is capable to mimic the effect of Vortex Generators. So overall due to its simplicity source term modelling through BAY model can be used for quick parametric studies.

In the present work the Local Discontinuous Galerkin (LDG) method with polynomial adaptivity is applied to the Large Eddy Simulation (LES) of the parallel blade-vortex interaction (BVI). The BVI phenomenon occurs on helicopter and drone rotors in manoeuvring conditions and it produces impulsive changes in the pressure distributions, vibrations and noise. To deeply understand the mechanism of load generation related to the pressure field and three dimensional perturbations growth, to focus on the interaction between the vortex and the three dimensional structures in boundary layer and wake, accurate 3D unsteady numerical simulations of turbulent flows are necessary. For this reason, it is very important the use of a numerical code based on high order schemes such as LDG. Moreover, in the LDG approach, the numerical resolution can be varied on each element and in time, adapting to the requirement of the simulated flow and saving a large amount of computing resources. In the used numerical code the criterion for variation of the polynomial order is based on a refinement indicator especially suited for LES and based on the structure function. The local polynomial representation directly provides a means to separate large from small scale modes, thus providing the starting point for the definition of the subgrid scale models. In the present simulations, the subgrid scales contribution is represented with a sophisticated dynamic anisotropic subgrid model, suitable and well tested for wall resolved LES and complex separated turbulent flows. The BVI is simulated highlighting the effect of the vortex on the pressure distributions, on the boundary layer separation and on the resulting forces.

A novel CFD approach for the prediction of ride control system response on wave-piercing catamaran in calm water

A comprehensive wind tunnel campaign was performed to assess the results of a computational framework aimed to study the optimal layout and position of vortex generators for helicopter fuselage drag reduction. The most promising vortex generators configurations indicated by the numerical study were tested on a heavy-class helicopter fuselage model. The experimental activity, including loads, pressure measurements and stereo particle image velocimetry surveys, confirmed the main trends predicted by numerical simulations. In particular, the wind tunnel tests results showed that the best array of vortex generators positioned on the model back-ramp downstream the fuselage upsweep produces a maximum drag reduction of about 5% with respect to the measured clean fuselage drag. Moreover, velocity fields and pressure distributions evaluated around the backdoor/tail-boom junction enabled to investigate the flow physics related to the vortex generators functioning for fuselage drag reduction.

For gas turbine combustors using liquid fuel injectors, introducing the spray into crossflowing air to achieve superior fuel-air mixing characteristics can improve performance and reduce emissions. Robust numerical design tools are needed to accelerate the development of low-emissions technologies. CFD models are used to assess the design, performance, and emissions of novel combustion systems. Atomization modelling is often facilitated by using the Lagrangian approach with breakup models rather than the more complex and computationally expensive higher-fidelity Volume-of-Fluid (VOF) approach. However, breakup models used in state-of-the-art CFD rely on empirical constants that have not yet been fully calibrated for jets in crossflow or for representative gas turbine conditions. Coupling the Lagrangian and VOF approaches could deliver a better compromise between accuracy and computational cost. Among the few studies conducted on the VOF-Lagrangian approach for jets in crossflow, validation was often limited to the near-field spray trajectory under low Weber numbers using either non-turbulent jets or water at ambient pressure. The predictive capabilities in the far-field region were only validated using deterministic breakup models and for non-turbulent jets under low pressure conditions. This study proposes and validates a novel numerical methodology for coupling the Lagrangian and VOF approaches using a Stochastic Secondary Droplet (SSD) breakup model with Adaptive Mesh Refinements (AMR) for turbulent liquid fuel jets in high-speed crossflow at more representative conditions. The predictions within both near and far-field breakup regions were validated using experimental datasets at high pressures (1–8 bar), Weber numbers (720–1172) and momentum flux ratios (6–33). The predictive capabilities and computational cost were also compared to the Lagrangian approach used with LES and used with an unsteady RANS numerical methodology previously developed and validated by the authors. The effects of different VOF-Lagrangian transition criteria on the computational cost were also assessed and recommendations have been provided for further improvements. The overall predictive capabilities of LES were significantly improved by the novel hybrid methodology proposed by the authors. Although it tends to underpredict the spray trajectory and Sauter Mean Diameter (SMD), it better captures the SMD in the wake region of the jet and the overall droplet velocities compared to the URANS methodology.

The design of rigid vortex generators (RVG) influences the thermal performance of various technologies. We employed Discrete Adjoint-Based Optimization to show the optimal development of vortex generators. Under turbulent flow conditions, different bi-objective functions on the RVG design were examined. Specifically, we aimed at an optimal RVG shape that minimizes the pressure drop and maximizes the local heat transfer in a rectangular channel. We show that an optimal design of an RVG can be obtained using computational fluid dynamics in conjunction with the Pareto Front at a computational cost of the order ~O(10^{-1}). We obtained three essential vortex generator shapes based on the RVG morphing technique. Compared to the baseline geometry of a delta winglet pair DWP, the first morphed design reduced the pressure drop by 39%, however, at the expense of a 21% reduction in the Nusselt number. The second vortex generator design enhanced the heat transfer by 18%, however, at the cost of a significant increase in pressure drop of about 40%. The final morphed design achieved the highest thermal performance factor of 1.28, representing a heat transfer enhancement of 6% with a moderate increase in pressure drop of about 13% compared to DWP vortex generators. Furthermore, we investigated the effect of introducing different size holes on the mass reduction of vortex generators and their thermal performances. The mass of vortex generators can be reduced by 9% and with an increase of 7% in thermal performance factor concerning the DWP baseline. The findings of this study will lead to highly efficient lightweight heat exchangers.

This paper analyzes the effect of wind tunnel wall interference on the airfoil aerodynamic performance, based on the results of experiment and CFD numerical simulation. The experimental measurement was made in IET using the low-speed and closed wind tunnel. The airfoil was DU91-W2-250. The numerical simulation with tunnel wall (wall), without tunnel wall (no wall) and without side tunnel wall (no side wall) were performed. Through the comparison the results between Exp and CFD, it is feasible to use CFD numerical simulation in this paper. And we get the following conclusions: the upper and lower tunnel walls make the flowing area decrease and the velocity increase due to the thickness of boundary layer on the walls, and they also casuse that C l and C d increase. The side tunnel walls cause span-wise flowing and delay the flow separation on the surface of airfoil, meanwhile, they cause that the velocity decreases, C l decreases and C d increases. At small angles of attack, the effect of side tunnel walls on the flowing of airfoil surface can be neglected. However, this effect is very significant at large angles of attack. The classical method can not work well at large angle of attacks for the correction of wall interference in wind tunnel because it doesn’t consider the effect of tunnel wall boundary layer. The C l should be increased through the correction, meanwhile, it is found that the flowing is not two-dimensional flow at large angles of attack in the free-air condition, so we suggest that the pressure measurement with more cross sections or using balance method should be used to obtain the aerodynamic performance of airfoil.

This manuscript contains the lecture notes for a course taught from July 7th through July 11th at the 1997 Numerical Analysis Summer School sponsored by C.E.A., I.N.R.I.A., and E.D.F. The subject area was chosen to support the general theme of that year`s school which is ``Multiscale Methods and Wavelets in Numerical Simulation.`` The first topic covered in these notes is a description of the problem domain. This coverage is limited to classical PDEs with a heavier emphasis on hyperbolic systems and constrained hyperbolic systems. The next topic is difference schemes. These schemes are the foundation for the adaptive methods. After the background material is covered, attention is focused on a simple patched based adaptive algorithm and its associated data structures for square grids and hyperbolic conservation laws. Embellishments include curvilinear meshes, embedded boundary and overset meshes. Next, several strategies for parallel implementations are examined. The remainder of the notes contains descriptions of elliptic solutions on the mesh hierarchy, elliptically constrained flow solution methods and elliptically constrained flow solution methods with diffusion.

A novel technique regarding the reduction of helicopter blade-vortex interaction noise is proposed. The proposed technique is based on the idea of injecting air at the leading edge of the blade to alter the vortex characteristics (strength and core size). The numerical investigations are performed using the large-eddy simulation (LES) approach. The simulations were performed for a Reynolds number, Re = 1.3 x 10 6 , based on the NACA0012 airfoil chord and free-stream velocity. The present study shows that by injecting air at the leading edge of the blade, the influence of blade-vortex interaction on the aerodynamic coefficients and aeroacoustic noise is significantly reduced.

The paper is concerned with the prediction and analysis of dynamic stall of flow past a pitching NACA0012 airfoil at 1 million Reynolds number based on the chord length of the airfoil and at reduced frequency of 0.25 in a three dimensional flow field. The turbulence in the flow field is resolved using large eddy simulations with the dynamic Smagorinsky model at the sub grid scale. The development of dynamic stall vortex, shedding and reattachment as predicted by the present study are discussed in detail. This study has shown that the downstroke phase of the pitching motion is strongly three dimensional and is highly complex, whereas the flow is practically two dimensional during the upstroke. The lift coefficient agrees well with the measurements during the upstroke. However, there are differences during the downstroke. The computed lift coefficient undergoes a sharp drop during the start of the downstroke as the convected leading edge vortex moves away from the airfoil surface. This is followed by a recovery of the lift coefficient with the formation of a secondary trailing edge vortex. While these dynamics are clearly reflected in the predicted lift coefficient, the experimental evolution of lift during the downstroke maintains a fairly smooth and monotonic decrease in the lift coefficient with no lift recovery. The simulations also show that the reattachment process of the stalled airfoil is completed before the start of the upstroke in the subsequent cycle due to the high reduced frequency of the pitching cycle.

The shape optimal design of the plate-fin type heat sink with vortex generator is performed to minimize the pressure loss subjected to the desired maximum temperature numerically. Evaluation of the performance function, in general, is required much computational cost in fluid/thermal systems. Thus, global approximate optimization techniques have been introduced into the optimization of fluid/thermal systems. In this study, Kriging method is used to obtain the optimal solutions associated with the computational fluid dynamics (CFD). The results show that when the temperature rise is less than 40 K, the optimal design variables are B₁ = 2.44 mm, B₂ = 2.09 mm,, t = 7.58 mm. Kriging method can dramatically reduce computational time by 1/6 times compared to SQP method so that the efficiency of Kriging method can be validated.