Moving Reference Frame Method Research Articles

Numerical investigation of the effect of winglet configurations with multiple cant angles on the aerodynamic performance of wind turbine blade

ABSTRACT A numerical investigation of the effects of winglets with multiple cant angles on the power output of the blade of a horizontal axis wind turbine (HAWT) is conducted. Several winglet configurations with single and multiple cant angles are designed and their performance is evaluated. Numerical solutions obtained by RANS equations and moving reference frame method were compared with the experiments and demonstrated that they concur well with the experimental data. In the torque and thrust comparison, the ones with multiple cant angles present better power output enhancement than those with a single cant angle at a relatively high wind speed. The configurations of multiple cant angles are effective in reducing the influence of wing tip vortices, resulting in a higher torque while maintaining the same winglet length and height.

Effects of wing loading on the aerodynamic characteristics of autorotating maple seeds

Natural maple seeds can fall into stable autorotation when descending. Their excellent aerodynamic performance can be applied to biomimetic aircraft design. Wing loading plays an important role in flight performance. To make clear the effects of wing loading on the aerodynamic characteristics of autorotating maple seeds, experiments integrated with numerical simulation were performed. In the experiment, the free fall test and the wind tunnel test were conducted on maple seeds with variable wing loadings. During the free fall process, four typical stages can be divided for the maple seed according to different kinematic characteristics. In the numerical simulation, the Moving Reference Frame method was used to simulate the fluid of autorotating maple seeds. Both experiment and simulation results show that descending velocities and spinning rates rise almost linearly with the increase in wing loadings, and descending velocity is closely related to spinning rate. Obvious leading-edge vortexes were observed on seed wing, which are the flow mechanism of high lift. The pressure differences between the top and lower surfaces of seeds grow with increasing wing loadings. Larger wing loadings lead to more serious separation of leading-edge vortex. The parameter influence analysis demonstrates that smaller conning angles or larger spinning rates are beneficial for high-lift flight. The spinning rate has a more significant effect on the autorotating maple seeds. However, when the spinning rate is larger than 200 rad/s, the lift does not increase anymore. The spinning rate should be controlled smaller than 200 rad/s when applied to biomimetic aircraft.

Critical issues in the moving reference frame CFD simulation of small horizontal axis wind turbines

Steady computational fluid dynamics with the Moving Reference Frame method has shown promise as a tool for assessing the aerodynamic performance of small horizontal-axis wind turbines, offering sufficient accuracy with low computational costs, thus supporting the development of these machines and facilitating their vital role in the growing distributed energy systems. However, accurately simulating the intricate low Reynolds number flow around the blades of these turbines remains a significant challenge, with certain critical issues yet to be fully resolved. Through a comparative literature review, this study firstly uncovered a lack of standardized simulation settings for small horizontal axis wind turbines, resulting in inconsistencies across different studies. Addressing this gap, the research delves into the critical issues identified, specifically the computational domain cross-section shape and size, discretization scheme accuracy, meshing criteria, freestream turbulence intensity, and turbulence modelling. To rectify these challenges, the study conducts a detailed sensitivity analysis of the aforementioned parameters applied on a well-documented turbine in the literature as a case study. For validation purposes, an empirical method is employed to correct the tunnel wall blockage effect on the case study turbine experimental data. This method is verified, and the correction of the closed test section tunnel data, notably, shows a significant blockage effect, contradicting the notion of insignificance for a blockage ratio of 11.8% in the literature. The analysis revealed how incorrect simulation settings can yield misleading results that initially may align with experimental data but fail to guarantee a reliable numerical setup for extending the study. Ultimately, this research furnishes best practice guidelines for establishing accurate simulation setups.

A Numerical Study of a Pico Hydro Turbine

Pico hydro turbines are suitable for low head applications in power plants since their efficiency is more stable than other turbine types. In some situations, computational fluid dynamics (CFD) has also been utilized as well as experimental studies for the performance prediction of water turbines at a pico scale. Also, CFD methods are getting much closer to real conditions in terms of steady-state with moving references and transient domains with rotor movements. For this purpose, electricity production related to the flow in a PHT was investigated numerically. This study presents six degrees of freedom (6-DOF) and moving reference frame (MRF) methods to predict the maximum conditions of a pico scale two-dimensional turbine by comparing the torque and angular velocities on the runner based on the turbine output power of 1 W determined by an experimental study. Besides, the effect of the torque, angular velocity, tip speed ratio, and turbine body profile was investigated comprehensively. In this regard, MRF and 6-DOF methods were performed to validate and compare the numerical model with the experimental results. Also, the results obtained from 6-DOF and MRF methods were compared to experimental study. It is concluded that PHT is generating 0.3 W power under 6.47 rad/s angular velocity with 6-DOF method, however; this value corresponds to 31.4 rad/s angular velocity against 1 W with the MRF method. Also, the maximum velocity of the turbine was 6.1 m/s according to the simulation result. It is accepted that the turbine maximum velocity inlet was 0.53 m/s based on the experimental study. As a conclusion, numerical results for the pico hydro turbine were reasonable taking the experimental study into account. It is also concluded that there is a tip speed ratio of 2.36 with the MRF method and 0.48 with the 6-DOF method between water tangential velocity and runner velocity for the turbine model.

Numerical investigation of the flow past two transversely forced oscillating cylinders in a tandem arrangement

A numerical investigation is conducted of the flow past two transversely oscillating cylinders in a tandem arrangement for Reynolds number 100, gap L = 2.0D to 7.0D, and oscillation amplitude A = 0.1D to 0.7D, where D is the width of each cylinder. The moving reference frame method combined with the spectral/hp element method is employed to simulate the flow in the lock-in region. The results reveal that the fluid forces and flow structures are sensitive to the oscillation amplitude and gap. The competition of the lift forces was observed: the lift forces of the up-cylinder are higher than the down-cylinder at small gaps (L/D = 2–3) while they gradually show inverse relations at wide gaps (L/D = 6–7). Moreover, all the flow structures behind the down-cylinder show weak 2S or 2S mode for the stationary cases, whereas the type of vortex mode changed into 2S (L/D = 2.0), 2P (L/D = 5.0), and subsequently to C(2S) (L/D = 6.0) with oscillating amplitude increased to A/D = 0.7. The relations between the fluid forces and statistical flow structures as well as the vortex modes transition were discussed in detail for various oscillation amplitudes and gaps. This research is expected to enhance the fundamental understanding of flow-induced forces and the interaction between a flow and tandem square cylinders.

Dynamic mode decomposition analysis of the two-dimensional flow past two transversely in-phase oscillating cylinders in a tandem arrangement

The flow through tandem square cylinders was investigated at a Reynolds number of 100 for oscillation amplitudes A = 0.1D to 0.7D and gaps L = 2.0D, 5.0D, and 6.0D, where D is the width of the cylinders. A moving reference frame method combined with the spectral/hp element method was employed to simulate the two-dimensional flow in the lock-in regime. Fluid forces, vorticity fields, power spectrum density, and pressure distribution were first investigated. Since surface pressure is directly connected with fluid forces, pressure and velocity field were synchronously analyzed by employing optimal dynamic mode decomposition. An underlying link between fluid forces and coherence modes was then uncovered. The results reveal that the move-induced forces and flow structures strongly depend on gaps and amplitudes in the lock-in regime. With respect to the dynamic mode decomposition analysis, odd-order modes contribute to lift forces, while even-order modes result in drag forces. The flow structures are dominated by at most three modes; as the amplitude increases, the high-order mode energy increases, coinciding with corresponding power spectrum density results of forces. Typical 2S, 2P, and C(2S) wakes were observed for various gaps and two representative amplitudes (A/D = 0 and 0.7), and their dominant modes show distinctive differences that lead to different local pressure shapes on the cylinders. It is the combined effects of local mode shape and global mode energy that account for the change in fluid forces for various gaps and two oscillating amplitudes.

A multiscale methodology for small-scale bubble dynamics in turbulence

We formulate in this paper a multiscale numerical framework that handles small-scale bubble dynamics in turbulence. Our framework involves bubbles with arbitrary density ratios with the carrier phase. We use a Moving Reference Frame method that follows a bubble to deal with a fast rising of bubbles present at high density ratios between the phases. We use a Proportional Integral Derivative controller to handle an additional acceleration term in the governing equations that stems from the change of a coordinate system from a fixed to a non-inertial one. Our framework accounts for the fact that the dynamics of bubbles are significantly influenced by the unsteadiness of the small-scale turbulent liquid fluctuations that modify the bubble shapes and alter their motion. In addition, we improve and speed up, with at least two orders of magnitude in computational time, the numerical framework recently proposed by Milan et al. (2020). The developed numerical framework can capture processes occurring at time scales even smaller than the Kolmogorov times. It can be applied to droplets, bubbles or particle systems in both laminar and turbulent flows, using any general DNS technique that handles two-phase flows.

Numerical Investigation of the Effect of Winglets on Wind Turbine Blades

Numerical investigation of the effect of winglets on NREL Phase VI wind turbine blade is performed. In this research, two different winglet configurations upstream winglet and downstream winglet are compared to see which configuration has the benefit to improve the blade performance. The thickness of winglet tip with the blade shape varied from 100% to 85% and 70% of tip blade thickness to see the effect on blade performance. Moving reference frame method was chosen to perform the blade rotating motion. Computational results were compared with experimental data and it was found that upstream winglet able to increase torque production compared to normal blade up to 1.6%. While downstream winglet decreases. Reducing winglet thickness tends to increase the torque.

Leakage and rotordynamic characteristics of labyrinth seal and hole-pattern damping seal with special-shaped 3D cavity

Purpose The purpose of this paper is to enhance sealing and rotordynamic performance of hole-pattern damping seal (HPDS) and labyrinth seal (LS) by structural innovation and geometrical optimization of special-shaped hole or annular-groove cavity. Design/methodology/approach The unsteady flow was transformed into steady one using moving reference frame method. The full period numerical models of LS and HPDS were established. The influence of special-shaped hole or annular-groove cavity at axial inclined angle on leakage rate and rotordynamic coefficient of these two seals at different whirl angular speed were investigated. Findings The results show that dynamic characteristics of straight-tooth LS are better than that of slanted-tooth LS. Compared to typical straight-hole damping seal, HPDS with windward oblique-hole when axial inclined angle ranges from 50 to 60° has superiority in both leakage and rotordynamic characteristics by considering smaller cross-coupled stiffness coefficient and whirl frequency ratio, larger direct damping coefficient and effective damping coefficient. Originality/value A novel HPDS with special-shaped three-dimensional hole cavity was proposed to enhance leakage and rotordynamic performance. The optimized geometrical structures of HPDS for excellent sealing and rotordynamic characteristics were obtained. Peer review The peer review history for this article is available at: https://publons.com/publon/10.1108/ILT-07-2020-0262/

A Numerical Investigation of Air/Mist Cooling Through a Conjugate, Rotating 3D Gas Turbine Blade With Internal, External, and Tip Cooling

Abstract This paper describes a numerical investigation to study the effect of injecting mist (tiny water droplets) into the cooling air used to cool down rotating gas turbine blades. In this study, the conjugate heat transfer method is used which consists of the simulation of the air/mist fluid flow inside and outside the blades as well as the heat conduction through the blade body. The complete 3D blade with internal cooling passages and external film cooling holes on the surface and blade tip is simulated in a rotating, periodic sector of the blade. The discrete phase model (DPM) is used to simulate and track the evaporation and movement of the tiny water droplets. The rotation effect of the turbine blade is included in the computational fluid dynamics (CFD) simulation by using the moving reference frame method. The effects of different parameters such as the mist/air ratio (10–20%) and the mist droplets size (20–40 µm) on mist cooling enhancement are investigated. The results show that the mist cooling enhancements are about 10–25% on the outer surface of the blade and reach 50% in some locations inside the blade on the internal cooling passages walls. Most of the liquid droplets completely evaporate inside the internal cooling passages; only a limited amount of mist is able to escape from the film cooling holes to enhance the blade outer surface and blade tip cooling. The effect of 10% mist on enhanced cooling can be converted to an equivalent of a 30% reduction in cooling air flow.

Development of an icing simulation code for rotating wind turbines

This study aims to develop a three-dimensional icing simulation code named WISE (Wind turbine Icing Simulation code with performance Evaluation) integrated into OpenFOAM®. The freely available source code can contribute to icing simulations that require parallel computations. The rotational motion is explained by a Moving Reference Frame (MRF) in both aerodynamic and droplet fields. The thin water film theory is applied in the thermodynamic module. To verify WISE, ice accretion shapes on NREL Phase VI under rime and glaze icing conditions were considered. The ice accretion shapes obtained by WISE were compared against FENSAP-ICE and another numerical simulation without the MRF method for the droplet field. For the rime condition, the icing limits, maximum thickness, and its location are well predicted by WISE compared with FENSAP-ICE while the simulation without the MRF method overestimates the icing limits and maximum thickness. For the glaze condition, only WISE and FENSAP-ICE results are compared where the icing limits are slightly different. On the suction side, WISE accurately predicts the maximum thickness, ice growth direction, and icing limits. However, the thickness of ice on the pressure side is underestimated. It might be necessary to have a turbulence model that can predict the flow transition.

Transient boundary condition approach for simulating mechanical mixing in large wine tanks

Since wine is not continuously stirred during or after fermentation, it is common to use movable mixers on a variety of tank sizes or to install small, fast-rotating propeller mixers in large tanks. Many practically relevant optimization problems relate to processes occurring directly after the start of the propeller (flow field initialization period), which require 3D instationary models if CFD simulations are used. Based on traditional instationary models, these simulations are very time-consuming even on fast computers and limit the part of the flow field initialization period that can be computed. In this work, a new method was developed to speed up the transient simulation of mechanical mixing in large wine tanks. The method uses the traditional CFD techniques of moving reference frame (MRF) and several ensuing propeller rotations using sliding mesh (SM), to calculate a steady-state flow field and to compute the flow behavior in the propeller region. Subsequently, the flow’s key figures are extracted for the propeller region at multiple time-points during one rotation and continuously mapped to a new simulation starting from a flow field with zero velocity. This new approach using a transient boundary condition (BC), was validated with tank mixing experiments and literature data. In contrast to the MRF method, it allows e.g. for an estimation of minimum mixing times similar to the SM method, while computation times are reduced by a factor of up to 20 for tank-mixer scenarios evaluated in this work. For a mixing simulation in a wine tank, the new method takes only 3 days compared to more than 60 days using the traditional SM approach.

CFD simulation of floating particles suspension in a stirred tank

Computational fluid dynamics (CFD) simulations were performed to predict the floating particles suspension in a baffled tank stirred by a standard Rushton turbine. An Eulerian multiphase model and a standard k-e turbulence model with mixture properties were used in the CFD simulation. The impeller rotation was solved using a moving reference frame method. Flow pattern, power number and solid holdup distribution were obtained and compared with the results in literature. The effects of operating condition on floating particles suspension characteristics were studied. It indicated that the influences of impeller speed and solid loading on particle suspension varied with particle sizes. For small particles, the impeller speed and solid loading have no obvious effects on solid holdup distribution and suspension quality. For large particles, particle suspension quality becomes better first, and then keeps almost unchanged with enhancing of the impeller speed. Suspension quality is better for higher solid loading of large particles. Within the scope of the present study, solid loading has no great effect on suspension quality. Suspension quality becomes worse with increasing of the particle size. Large particles are easy to accumulate in the centres of the liquid free surface and the upper circular loop, and the vicinity of the shaft.

Design and analysis of flow velocity distribution inside a raceway pond using computational fluid dynamics.

Open raceway ponds are widely adopted for cultivating microalgae on a large scale. Working depth of the raceway pond is the major component to be analysed for increasing the volume to surface area ratio. The working depth is limited up to 5-15cm in conventional ponds but in this analysis working depth of raceway pond is considered as 25cm. In this work, positioning of the paddle wheel is analysed and corresponding Vertical Mixing Index are calculated using CFD. Flow pattern along the length of the raceway pond, at three different paddle wheel speeds are analysed for L/W ratio of 6, 8 and 10, respectively. Effect of clearance (C) between rotor blade tip and bottom surface is also analysed by taking four clearance conditions i.e. C=2, 5, 10 and 15. Moving reference frame method of Fluent is used for the modeling of six blade paddle wheel and realizable k-ε model is used for capturing turbulence characteristics. Overall objective of this work is to analyse the required geometry for maintaining a minimum flow velocity to avoid settling of algae corresponding to 25cm working depth. Geometry given in [13] is designed using ANSYS Design modular and CFD results are generated using ANSYS FLUENT for the purpose of validation. Good agreement of results is observed between CFD and experimental Particle image velocimetry results with the deviation of 7.23%.

동유체력 계산을 이용한 수중운동체의 횡동요 계수 변화 예측

For Underwater vehicles, Unwanted roll excursions are inevitable as they are caused by induced propeller torque, disturbances, and banking motion during turns. To estimate the manoeuvring performance of underwater vehicle, it is necessary to obtain the roll coefficient of body. This paper was covered estimation of roll coefficient of underwater vehicle using STAR-CCM+, commercial CFD(Computational Fluid Dynamics) code. The RANS equations for incompressible fluid flows was solved numerically by using a finite volume method. An MRF(Moving Reference Frame) Method was Also adopted for rotations of body. For the validation, the flow around a DARPA SUBOFF bare hull model was simulated and good agreement with experiments was obtained. And Pure roll coefficients were calculated and campared with the experimental data which were presented by Seoul National University. Finally, an underwater vehicle model with propeller was simulated and analyzed for estimation of roll coefficient variation caused by induced propeller torque.

Spurious Current Suppression in VOF-CSF Simulation of Slug Flow through Small Channels

A numerical treatment is proposed to minimize the creation of unphysical, spurious currents in modeling liquid–gas slug flow using the volume of fluid-continuum surface force (VOF-CSF) method. An elongated gas slug drawn into a small circular channel initially filled with liquid is considered. To suppress spurious currents formed by numerical errors in calculation of the surface tension force at small capillary numbers (Ca < 0.01), an artificial relative reference frame is specified with motion in a direction opposite to the flow. An increase in the local relative velocity magnitude near the interface is demonstrated to be the key mechanism for spurious current suppression. A comparison of simulations performed with and without this treatment shows that spurious currents are eliminated at Ca = 0.0029; liquid film thickness, gas slug velocity, and liquid-phase circulation near the leading slug interface are preserved and the computed values agree with the literature. This demonstrates that the proposed moving reference frame method does not influence the computed physical phenomena of interest while suppressing unphysical spurious velocities.

Numerical Simulation of a Three Bladed Marine Propeller in Steady and Unsteady State

In this paper the numerical analysis for an unstructured polyhedral 3bladed un-skewed propeller in steady and transient analysis is discussed and the results are compared with the experimental data. A moving reference frame method is adopted by creating an interface between the propeller and the domain for the rotation of the propeller. The hydrodynamic coefficients such as Thrust coefficient (Kt), Torque coefficient (Kq), circumferentially averaged axial, radial and tangential velocity are compared with the experimental data. The computational time for each simulation is compared and concluded that marine propellers consume less computational time in the steady analysis when compared with the unsteady analysis.