Overset Grid Research Articles

ExaWind: Open‐source CFD for hybrid‐RANS/LES geometry‐resolved wind turbine simulations in atmospheric flows

AbstractPredictive high‐fidelity modeling of wind turbines with computational fluid dynamics, wherein turbine geometry is resolved in an atmospheric boundary layer, is important to understanding complex flow accounting for design strategies and operational phenomena such as blade erosion, pitch‐control, stall/vortex‐induced vibrations, and aftermarket add‐ons. The biggest challenge with high‐fidelity modeling is the realization of numerical algorithms that can capture the relevant physics in detail through effective use of high‐performance computing. For modern supercomputers, that means relying on GPUs for acceleration. In this paper, we present ExaWind, a GPU‐enabled open‐source incompressible‐flow hybrid‐computational fluid dynamics framework, comprising the near‐body unstructured grid solver Nalu‐Wind, and the off‐body block‐structured‐grid solver AMR‐Wind, which are coupled using the Topology Independent Overset Grid Assembler. Turbine simulations employ either a pure Reynolds‐averaged Navier–Stokes turbulence model or hybrid turbulence modeling wherein Reynolds‐averaged Navier–Stokes is used for near‐body flow and large eddy simulation is used for off‐body flow. Being two‐way coupled through overset grids, the two solvers enable simulation of flows across a huge range of length scales, for example, 10 orders of magnitude going from O(μm) boundary layers along the blades to O(10 km) across a wind farm. In this paper, we describe the numerical algorithms for geometry‐resolved turbine simulations in atmospheric boundary layers using ExaWind. We present verification studies using canonical flow problems. Validation studies are presented using megawatt‐scale turbines established in literature. Additionally presented are demonstration simulations of a small wind farm under atmospheric inflow with different stability states.

Aerodynamic performance analysis of a floating wind turbine with coupled blade rotation and surge motion

The unsteady aerodynamic characteristics and interference effects of a floating wind turbine are significantly influenced by the six-degree-of-freedom (6DoF) motions. In this paper, the computational fluid dynamics (CFD) method using the overset grid technique and the improved delayed detached eddy simulation (IDDES) model was adopted to investigate the effects of harmonic enforced surge motions on the aerodynamic performance and wake characteristics of wind turbines. The aerodynamic load coefficients including the constant term, damping coefficients (surge velocity-induced), and additional mass coefficients (surge acceleration-induced), were fitted by using the least squares method at different surge frequencies and surge amplitudes. The blade pressure distributions and wake characteristics were analysed in detail. The Ω ~ R of the third-generation vortex identification method was applied to visualize vortex evolution. The numerical results are evaluated against the data obtained by wind tunnel experiments, indicating an acceptable relative error of 3.45 % for the power coefficient, C p , at the rated tip speed ratio (TSR). The higher frequency and greater amplitude surge motions lead to more dramatic fluctuations for the power coefficient than the axial load coefficient. The neglect of the additional mass coefficient only results in an acceptable error. Furthermore, the mean damping coefficient is on average approximately 3.5 times the mean constant term coefficient for C p , approximately 1.5 times for C z . In terms of the blade pressure distributions and wake characteristics, a higher comprehensive induced velocity, V c , is induced to expand the wider high-pressure area and shrink the low-pressure area at the pressure surface. Meanwhile, toroidal vortex ropes were induced by surge motion. Besides, the surge motion at high frequency and greater amplitude positively impacts enhancing wake deficit recovery.

Experimental and numerical investigation on the unsteady interaction in longitudinal stage separation for parallel-staged two-body configuration

Hypersonic stage separation is a significant process, probably involving complex aerodynamic interaction, which determines the survival of two-stage-to-orbit (TSTO) vehicles. The longitudinal stage separation (LSS) scheme is proposed to improve the safety of the parallel-staged TSTO vehicle, where a small interstage gap may result in weak interference. Therefore, an experimental and numerical study of LSS for the parallel-staged TSTO vehicle at Mach 7 with different angles of attack (AoA) is carried out. The dynamic interaction, including variations in the shock structure, wall pressure distribution, and unsteady aerodynamics, is investigated by testing and numerical simulation. The LSS experiments for the TSTO vehicle were performed using a high-speed pneumatic ejection launch system in the JF-12 shock tunnel, and the method was developed using high-speed visualization and image processing techniques to capture the separating trajectory. The numerical simulations were carried out using the overset grid method and solving the Navier–Stokes equations coupled with the rigid body dynamics equations to obtain the laminar flows over the TSTO vehicle during LSS. The qualitative and quantitative comparison of the test and numerical results showed good agreement in terms of aerodynamic performance, flowfield pattern, wall pressure, and separation trajectory. They show that the small interstage gap of the LSS leads to weak type I and VI shock–shock interactions, with short-duration weak shock reflection at a higher AoA. Furthermore, no shock reflection or interstage gap is observed at lower AoA. Moreover, no stage recontact is observed, and the safety and feasibility of LSS for parallel-staged TSTO vehicles are demonstrated.

Research of Unsteady Aerodynamic Characteristics of Electrically Controlled Rotor Airfoils with Trailing-Edge Flaps

An electrically controlled rotor (ECR), also known as a swashplateless rotor, eliminates the swashplate system to implement the primary control via the trailing-edge flaps (TEFs), which can result in enhancements in rotor performance, as well as substantial reductions in weight, drag, and cost. In this paper, the unsteady aerodynamic characteristics of the airfoil with TEF of a sample ECR under unsteady freestream condition are investigated. The CFD results are obtained with sliding and overset grid techniques that simulate the airfoil pitching and flap deflection. Comparative analysis of the aerodynamic characteristics under steady and unsteady freestream conditions at different advance ratios is conducted. At various advance ratios, the lift and drag coefficients are higher at a small angle of attack under unsteady freestream condition; however, it is the opposite at a large angle of attack. The peak values of the lift and drag coefficients show an increased trend with the increase in the advance ratio. On the contrary, the pitch moment and flap hinge moment coefficients demonstrate minor variation under unsteady freestream condition. Furthermore, the aerodynamic characteristics of airfoils become more unsteady with variation in the freestream. Therefore, the lift and drag coefficients of the ECR airfoil with TEF show significant differences between steady and unsteady freestream conditions; however, the pitch moment and the flap hinge moment coefficients show little difference.

Experimental and CFD modeling of a progressive cavity pump (PCP) using overset unstructured mesh part 1: Single-phase flow

Progressive Cavity Pumps (PCP) are integral in industries for transporting high-viscosity fluids due to their efficacy. Nevertheless, their computational modeling via Computational Fluid Dynamics (CFD) poses challenges, primarily attributed to meshing algorithms and computational demands. This research underscores that structured meshes are not imperative for assessing PCP efficiency, given that operational parameters reflect a 10% accuracy when juxtaposed with experimental data. An overset mass conservation model was harnessed to rectify mass conservation errors and the computational burden from interpolation. The overset grid methodology emerged as a favorable technique for PCP modeling, with its applicability critically analyzed. Experimental data comparison revealed that variables like volumetric flow rate, power consumption, and efficiency were accurately depicted with a marginal error of less than 10%. Interestingly, while high-viscosity oils resonated with the Hagen-Poiseuille flow equation in clearance zones, water showcased elevated velocities, inducing a non-linear pressure profile. Analysis of slip components indicated that high-viscosity oils offered superior efficiencies due to augmented slip resistance. Thus, CFD modeling of PCPs across different viscosity spectrums is feasible without mandating operations to enhance mesh quality. This study pioneers in offering insights into PCP dynamics, emphasizing the accuracy and optimization potential of the overset unstructured mesh approach.

CFD investigation into the effect of GM variations on ship manoeuvring characteristics

The change in a ship's loading condition may induce a displacement of its vertical centre of gravity, subsequently impacting the metacentric height (GM) of the vessel. Such alterations can have unforeseen effects on the ship's manoeuvring behaviour. This study investigates these effects by developing a free running manoeuvre model for the KRISO Containership (KCS) based on the Unsteady Reynolds-Averaged Navier-Stokes (URANS) method. A multi-level dynamic overset grid and the body-force propeller method were employed to simulate the ship motion involving rudder manipulations. The numerical calculations of zigzag and turning manoeuvre revealed that the variations in GM produce significant alternations in the roll motion during manoeuvres, consequently affecting the zigzag and turning trajectories of the vessel.

Numerical Study of Self-propulsion Resistance of KCS Based on CFD

In this study, we used computational fluid dynamics (CFD) techniques to conduct self-propulsion tests to evaluate vessel resistance. We conducted self-propulsion calculations on the Korea Research Institute of Vessels and Ocean Engineering (KRISO) container ship (KCS) at a model scale, directly discretizing the propeller and rudder. We used the overset grid method, which allows the propeller to rotate arbitrarily during the calculation process, and used the volume of fluid (VOF) method for multiphase flow to handle the free surface effect. The accuracy of the computational results was verified by comparison with existing experimental data.

Efficient Upwind Finite-Difference Schemes for Wave Equations on Overset Grids

Efficient Upwind Finite-Difference Schemes for Wave Equations on Overset Grids

Dynamics and “falling deep” mechanism of submerged floating body under internal solitary waves

Internal solitary waves (ISWs) can result in notable alterations to the hydrodynamic characteristics of submerged bodies, affecting their motion and even leading to serious “falling deep” accidents. This study utilizes a combination of Computational Fluid Dynamics (CFD) and the modified Korteweg-de Vries (mKdV) theory. By combining techniques such as the Volume of Fluid (VOF) model and overset grid, a numerical method for simulating the interaction between ISWs and submerged bodies is established. The dynamics of submerged floating bodies under internal solitary waves are systematically investigated. The results demonstrate that the primary cause of changes in motion and force is the rotating velocity field induced by ISWs. Additionally, the influences of different locations and internal solitary wave amplitudes on the hydrodynamic characteristics of the submerged body are examined. The submerged body is more likely to experience “falling deep” due to the influence of ISWs when it is situated in the lower layer of the fluid.

Application of a Modification of the Grid-Characteristic Method using Overset Grids for Explicit Interface Description to Modelling the Relief of the Ocean Shelf

Introduction. The problem of modelling the propagation of elastic waves is of great practical importance when conducting seismic exploration. Based on it, a model of the environment under study is being built. At the same time, the quality of the constructed model is determined by the accuracy of solving the modelling problem, which ensures constantly increasing requirements for modelling accuracy. For accurate modelling, it is important to correctly describe and take into account the boundaries of the media. At the same time, the quality of the constructed model is determined by the accuracy of solving the modelling problem, which ensures constantly increasing requirements for modelling accuracy.Materials and Methods. We have studied a modification of the grid-characteristic method on rectangular grids using overset grids to describe the interface of media of complex shape. This approach has previously been used to describe the earth’s surface when conducting simulations on land. This paper describes its application in modelling the relief of the ocean shelf.Results. The use of the overset grid reduces the modelling error, the number of parasitic waves and artifacts and makes it possible to get a more visual picture.Discussion and Conclusions. Overset grids can be used to describe the interface of media in modelling seismic exploration of the ocean shelf. Their use makes it possible to increase the accuracy of modelling and reduce the number of artifacts compared to using only one grid.

Fish injury analysis and flip-blade type optimization design of an undershot waterwheel

In this study, the performance of the ultra-low head undershot fixed-blade and flip-blade waterwheels are studied via numerical simulations to evaluate their efficiency and stability. On the other hand, the accuracy of the numerical simulation of the fixed-blade waterwheel is verified by field tests before the flip-blade waterwheel is designed on its basis. Moreover, the likelihood of fish injury in the fixed-blade waterwheel is predicted to evaluate the fish-friendliness of the waterwheel. Applying the overset grid algorithm, the results indicate that the fish is most vulnerable to damage in the rotor area and that the fish suffer some impact damage but almost no pressure and shear stress damage, and a minimal probability of mortality present, confirming that the waterwheel is fish friendly. Furthermore, the power coefficient of the waterwheel with the rise of tip speed ratio tends to increase first and then decrease, while maintaining a positive correlation with the immersion depth within a certain range. Additionally, flip-blade owns higher efficiency than fixed-blade waterwheel under the same working conditions but is slightly less stable. We expect that these results will help establish design guidelines for more efficient flip-blade waterwheels to break the impression that traditional undershot waterwheels are less efficient.

CFD simulations targeting the performance of the NTNU BT1 wind turbine using overset grids

CFD simulations targeting the performance of the NTNU BT1 wind turbine using overset grids

Numerical simulation of flow-induced noise generation and propagation of a floating offshore wind turbine with prescribed pitch motion

It is important to evaluate flow-induced noise emitted due to operations of floating offshore wind turbines since noise pollution can damage the environments of seabirds and aquatic species. This paper studies the aerodynamic and aeroacoustic performance of a laboratory three-blade wind turbine model with an airfoil profile of NREL S826. The wind turbine blade rotation motion is coupled with the prescribed pitch motion of the floating offshore wind power platform. The influence of prescribed pitch motions on the noise generated by wind turbine blades and noise propagation was discussed. Adopting the computational fluid dynamics (CFD) method, the aerodynamic noise field on a blade is simulated by the improved delayed detached eddy simulation (IDDES) model and Ffowcs Williams–Hawkings (FW-H) acoustic analogy. The overset grid technique is adopted to simulate the six degrees of freedom (DoFs) motions of the floating offshore wind turbine model. The sound source distributions on blades for noise generation are obtained, and the flow-induced noise propagation characteristics are analyzed. It is found that the influence of the pitch amplitude and frequency on the flow-induced noise field as well as the wake vortex characteristics should not be neglected.

Numerical simulation of planing hull motions in calm water and waves with overset grid

This paper presents simulations of planing hull motions in calm water and waves with the unsteady RANS solver and the overset grid in Star-CCM+. The calm-water simulations were performed for the Fridsma planing hull model at Froude numbers from 0.59 to 1.78. Convergence studies were first carried out on dimensions of overset domain, grid resolution, time step and turbulence model, including k–ɛ, Realizable k–ɛ, k–ω, and SST k–ω models. Uncertainties in numerical solutions due to spatial discretization were examined. The best-practice settings were summarized and used for simulations of the Fridsma planing hull with calm water and regular waves, as well as the investigations of scale effect. For the calm-water simulations, resistance, sinkage and trim of the Fridsma planing hull and LRI-II hull were compared with experimental data and empirical results based on the Savitsky method. Additionally, the recommended settings were also applied to two full-scale planing hulls, LRI-II and OTH-Vp, in irregular waves. Convergence studies were also carried out on wave generation. Predicted motions and vertical accelerations were compared with sea trial data. Validation studies show that the predictions are in good agreement with experimental data.

Numerical simulation of near-surface turn maneuver of a generic submarine

Aiming at the actual demand of improving the numerical simulation of the unsteady motion of submarines in maneuvering conditions, the overset grid is used to model six degrees of freedom coupling motions of the submarine and the independent motion of each control plane, and the sliding grid is used to simulate the rotation of the propeller, to carry out the research on the numerical simulation approach of a generic submarine in near-surface maneuvering condition. The VOF method is utilized to capture the free surface. By comparing the submerged turn maneuver numerical simulation results of typical dynamics parameters with the experiment's results, the practicability of the approach for engineering prediction is verified. Then the simulations of near-surface maneuvers under different submergence depths are conducted, and the influence of submergence depth on kinematics and dynamics parameters of Joubert BB2 in maneuvering conditions are analyzed. Results demonstrate that the control performance of the stern planes will be sharply degraded when maneuvering too close to the free surface, resulting in a significant increase in the tactical diameter of the turn maneuver. The research can guide the prediction and evaluation of submarine maneuverability and seaworthiness. It is beneficial for the improvement of free running submarine model tests.

Characteristics of cavity dynamics, forces, and trajectories on vertical water entries with two spheres side-by-side

In the present work, a three-dimensional numerical model with six-degrees-of-freedom is adopted to carry out the investigation of twin water entries with two spheres side-by-side. The shear-stress transport k–ω model is used to detail the turbulence structure, and the air–water interface is tracked through the volume of fluid method. The motion of consecutive water entry with spheres is simulated using an independent overset grid technology. Numerical results reveal the variation of different kinds of lateral forces experienced by the trailing sphere following water entry, while simultaneously clarifying the governing law regarding the effects of time interval, lateral distance, and entry velocity on the trajectory behaviors. The repulsive and attractive forces are found upon the trailing sphere (sphere II) entering water, inducing different deviation directions, which are affected by the time interval and lateral distance. In addition, increasing water-entry velocity results in an increase in both attractive and repulsive force acted on sphere II; notwithstanding, the effect of repulsive force on trajectory characteristics is more noticeable.

Blockage effect and ground effect on oscillating hydrofoil

Oscillating hydrofoils located in a channel or limited zone by two side walls will be affected by both the blockage effect and ground effect. To study the contribution of these two effects on the energy-extraction performance of hydrofoils, a two-dimensional numerical model was established based on the overset grid technology. The differences in the energy extraction of hydrofoil with different wall settings were compared, and the blockage effect and ground effect in the double-wall case were mainly studied. The results show that compared with the open flow field, the single-wall can only improve the hydrofoil energy extraction by 1.75%, while the double-wall with the same foil-wall spacing can greatly improve the oscillating hydrofoil efficiency by 19.34%. By linearly fitting the results at low blockage ratios and performing the subtraction process, the influence of blockage effect and ground effect can be separated preliminarily. These two effects have the same order of magnitude in improving the energy extraction of hydrofoils when the blockage ratio is high. In addition, the present study proposes to add the correction of the drag coefficient to remedy the shortcoming of the correction method proposed by Barnsley and Wellicome that cannot deal with the high blockage ratio cases, which means the influence of the ground effect is also corrected. Then the blockage effect and the ground effect can also be separated by the subtraction process. Based on the above analysis, it is found that the ground effect has little influence when the hydrofoil oscillates at high frequencies, while the influence of the blockage effect is mainly related to the blockage ratio rather than the oscillating frequency.

Aerodynamic characteristics of fully enclosed sound barrier induced by the passing trains with 400 km/h

The fully enclosed sound barrier (FESB) has excellent sound reduction effect, but the relatively closed space will make train-induced wind effect more obvious. This study aims to quantify the aerodynamic characteristics on FESB during the trains passing through using a computational fluid dynamic (CFD) method, considering the effects of opening spacing ΔH, train length, and trains passing each other. An overset grid approach is applied to stimulate the relative motion between the train and the FESB, which is verified by correlated test data. The results demonstrate that compression and expansion waves induced by passing trains are the primary sources of pressure fluctuations within the FESB. Implementing a pressure relief scheme with a top opening, which modifies the propagation law of pressure fluctuations inside the FESB, can effectively mitigate the train-induced aerodynamic effects on the structure. Moreover, the head-wave effect on the FESB is less affected by the train length in such schemes. When the opening spacing ΔH value exceeds 1.5 m, the peak-to-peak pressure ΔPH induced by a train passing by gradually reaches a stable state, and there exists a perfectly negative exponential relationship between the aerodynamic force per unit length of FESB and the opening spacing ΔH. However, the train-induced aerodynamic effect on the FESB is amplified when two trains pass each other. In pressure relief scheme with ΔH of 1.5 m, compared to the ΔF observed on the near and far line sides during a single train passing by, those obtained during trains meeting operation are found to be 0.66 times and 2.64 times higher.

Hydrodynamic Performance of Soft Robotic Fish Actuated by Ionic Polymer-Metal Composite Pectoral Caudal Fin

Soft robotic fish has a lot of benefits. This paper aims to create a robotic fish with a soft pectoral caudal fin which was powered by IPMC material. The soft robotic fish’s hydrodynamic performance during straight swimming is numerically computed using the overset grid approach. Compared with the soft tail fin driven with a driving frequency of 0.05 Hz and those driven together with three soft pectoral fins with different driving frequencies and amplitude, the hydrodynamic coefficients and pressure of the bionic robotic fish are analyzed. We can find the regions of distinct positive and negative pressure during the IPMC soft caudal fin flapping. And the forward pressure component provides forward propulsion for the bionic fish. The greater the flapping frequency of the IPMC soft pectoral fin is, the smaller the amplitude is, and the greater the drag force receives. It is concluded that while the driving frequency of soft pectoral fins is 0.05 Hz, the amplitude is 5.64 mm, and the soft pectoral fin driving effect is good. Further research into how soft pectoral caudal fin frequency and amplitude affect the hydrodynamic capabilities of bionic fish was also made.

Numerical Evaluation of Hydrodynamic Coefficients in Roll Motion

The determination of ship motions in a seaway heavily depends on hydrodynamic coefficients. However, the coefficients related to roll-motion exhibit nonlinear behavior due to viscous effects and relatively small wave damping, which pose significant challenges. To overcome these challenges, several studies have employed computational fluid dynamics (CFD) to analyze hydrodynamic performance, leveraging the advancements in computational performance. Nevertheless, limited research has investigated the error and uncertainty of these numerical solutions. In this study, we conducted a forced oscillation test on a ship-like section that induces lateral vortex shedding using the OpenFOAM with overset grid method. We validated the numerical results of the roll hydrodynamic coefficients with experiments and the results of potential theory. We used the procedure proposed by the ITTC to perform uncertainty analysis on the grid size and time step size, which presented the overall uncertainty of the numerical analysis results. Our findings indicate that the grid uncertainty of the added moment of inertia was significant in the low frequency region, while the time step uncertainty was significant in the high frequency region.