Sliding Mesh Interface Research Articles

Qualitative Investigation of Wake Composition in Offshore Wind Turbines: A Combined Computational and Statistical Analysis of Inner and Outer Blade Sections

High-fidelity numerical simulations are used to thoroughly analyze the evolution of the wake behind a megawatt-scale offshore wind turbine. The wake features are classified in terms of wake dynamics composition and the associated turbulence characteristics originating from the inner and outer sections of the blades. Understanding the wake is essential for developing compact layouts for future wind farms. We employed a transient Sliding Mesh Interface (SMI) technique to analyze the fully dynamic wake evolution of the offshore NREL 5MW full turbine. Our high-fidelity results have been validated against previously published results in the literature. We thoroughly investigated the dominant structures of the wake using Proper Orthogonal Decomposition (POD) techniques, which we applied to transient simulations of fully developed flows after five wind turbine revolutions over the snapshot data. Our findings show that the inner section of the blades, which is composed of airfoils with larger cross-sections, is responsible for the dominant components of the wake, while the contribution of the wake from the outer section of the blade is significantly lower. Therefore, designing more aerodynamic sections for the blade’s inner section can help reduce the dominant wake components and thus decrease the inter-turbine distance in future wind farms.

A sliding-mesh interface method for three dimensional high order spectral difference solver

In this study a sliding-mesh interface method for three-dimensional unstructured mesh is developed and implemented to the high order spectral difference (SD) solver. The mortar approach is utilized for data exchange on the sliding interface for non-uniform unstructured mesh. The factors affecting the accuracy of the mortar method are analyzed, including mortar number, polynomial degree of mortar, non-uniformity of mortars, and mortar omission in polygon clipping. The method is validated with both inviscid and viscous problems with rotating flows. The numerical results show that the present method is able to retain the high order accuracy of the SD method. The influence of mortar omission in polygon clipping for poor quality complex mesh is discussed and evaluated with several validation problems. The errors with and without mortar omission are compared. The results show that the developed method is reliable and accurate even with mortar omission occurred occasionally in the simulation if the maximum omitted area is less than about 15% of its connected element on the sliding interface.

An efficient sliding mesh interface method for high-order discontinuous Galerkin schemes

Sliding meshes are a powerful method to treat deformed domains in computational fluid dynamics, where different parts of the domain are in relative motion. In this paper, we present an efficient implementation of a sliding mesh method into a discontinuous Galerkin compressible Navier-Stokes solver and its application to a large eddy simulation of a 1-1/2 stage turbine. The method is based on the mortar method and is high-order accurate. It can handle three-dimensional sliding mesh interfaces with various interface shapes. For plane interfaces, which are the most common case, conservativity and free-stream preservation are ensured. We put an emphasis on efficient parallel implementation. Our implementation generates little computational and storage overhead. Inter-node communication via MPI in a dynamically changing mesh topology is reduced to a bare minimum by ensuring a priori information about communication partners and data sorting. We provide performance and scaling results showing the capability of the implementation strategy. Apart from analytical validation computations and convergence results, we present a wall-resolved implicit LES of the 1-1/2 stage Aachen turbine test case as a large scale practical application example.

Computational Analysis and Design Improvement of an Industrial Centrifugal Pump with Experimental Validation

Centrifugal pumps are one of the work horses in any process industries. Over the past few years, efforts to increase the performance of the centrifugal pump by various means have been ongoing. The performance of centrifugal pump is significantly affected by the path the fluid traverses along the geometry of blade and casing of the pump. Visualization of the fluid flow is difficult to comprehend. In the present work, commercial computational fluid dynamics (CFD) code, ANSYS CFX package is used to numerically simulate the 3D flow of water inside an industrial centrifugal pump. The effect of changing the volute geometry at regions of interest is studied. Steady-state analysis is performed using shear stress transport (SST) turbulence model using mixing plane interface between moving and stationary zones of the pump. Further, transient simulation using sliding mesh interface was carried out using results obtained from steady analysis as initial conditions. From the above results, recirculation was observed in the casing and minor losses near the neck region of the pump. Based on the results of transient simulation, the casing area was reduced at 2 regions by 4%, 8% and 12% to understand its effect on recirculation and minor losses. The results of transient analysis on the casing geometry showed that 8% reduction in area improved the pump efficiency by 4.85% at the design flow rate of 111.11 kg/s. The efficiency improvement of the pump is attributed to the reduction in recirculation and minor losses. The improvements in the efficiency were validated by experimental results. The experimental results showed an improvement of efficiency by 4.62%.

Validation of the numerical simulations of flow around a scaled-down turbine using experimental data from wind tunnel

Aerodynamic characteristic of a small scale wind turbine under the influence of an incoming uniform wind field is studied using k-w Shear Stress Transport turbulence model. Firstly, the lift and drag characteristics of the blade section consisting of S826 airfoil is studied using 2D simulations at a Reynolds number of 1X 10. After that, the full turbine including the rotational effects of the blade is simulated using Multiple Reference Frames (MRF) and Sliding Mesh Interface (SMI) numerical techniques. The differences between the two techniques are quantified. It is then followed by a detailed comparison of the turbine\'s power/thrust output and the associated wake development at three tip speeds ratios ( = 3, 6, 10). The phenomenon of blockage effect and spatial features of the flow are explained and linked to the turbines power output. Validation of wake profiles patterns at multiple locations downstream is also performed at each . The present work aims to evaluate the potential of the numerical methods in reproducing wind tunnel experimental results such that the method can be applied to full-scale turbines operating under realistic conditions in which observation data is scarce or lacking.

High Fidelity Computational Fluid Dynamics Assessment of Wind Tunnel Turbine Test

We present, what to our best knowledge, is the most accurate numerical investigation of the wind tunnel tests carried out over a model wind turbine (known as NTNU Blind Test) at the Norwegian University of Sciences and Technology. We show numerical benchmarking of wake measurements against experimental data and similar investigations performed previously by researchers using Computational Fluid Dynamics (CFD) simulations. We have made a full 3D model of the wind turbine and used Sliding Mesh Interface (SMI) approach to handling the rotation of the rotor. The simulations are done with the use of OpenFoam and the k — ω Shear Stress Transport model to resolve turbulence using the Reynolds Average Navier-Strokes (RANS) technique. We present the numerically simulated spatial distribution of the flow field across the wake at zero angles of yaw for horizontal lines downstream of the rotor plane as that was the focus of the NTNU Blind Test presented in [1]

Aeroelastic simulation of the first 1.5-stage aeroengine fan at rotating stall

A massive parallel aeroelastic simulation platform has been built to investigate the first 1.5-stage fan of an aeroengine at rotating stall. The Computational Fluid Dynamics (CFD) solver and Computational Structural Dynamics (CSD) solver are coupled directly by non-matching mesh interfaces. The unsteady rotor/stator interaction is solved by the Sliding Mesh Interface method. The original rotor blades are shrouded by the midspan shrouds. An unshrouded fan is also created to investigate the effects of the midspan shrouds. Both the shrouded fan and unshrouded fan have stable aeroelasticity at the designed state. At rotating stall, the stalled region rotates at 30% of the rotor speed on the absolute reference frame. The energy spectrum of the rotating stalled flow is measured quantitatively. It shows that the first two order excitations are much stronger than the higher order excitations. In the flow of rotating stall, the fifth backward travelling wave mode of shrouded fan is resonated by the fifth excitation of the rotational stalled flow because the rotational speed of the stalled region coincides with the modal rotational speed, while for the unshrouded fan, the first bending mode is resonated by the second excitation of the rotational stalled flow, forming two waves in the circumference of the annulus blades. At rotating stall, the vibration of the shrouded blades is still under control but the vibration of the unshrouded blades is diverged and out of control. A novel tool, i.e., resonance map, is proposed to predict the resonance. It provides a perspective to explain the effects of midspan shrouds at a theoretical level, and it would also be helpful in the structural design of blades.

Industrial scale turbine and associated wake development -comparison of RANS based Actuator Line Vs Sliding Mesh Interface Vs Multiple Reference Frame method.

This current work compares the three methodologies (Actuator Line model (ALM), Sliding Mesh Interface (SMI) and Multiple Reference Frame (MRF)) in modeling an industrial scale reference turbine at different tip speed ratios (TSR). The comparison shows that all the 3 models qualitatively predict the expected trend of power coefficient (Cp) vs TSR curve with an optima around TSR ≈ 7.5. But the quantitative values of the predicted Cp, the wake deficits and the flow patterns differ from model to model. Between ALM and MRF, the former predicts a relatively milder variation of Cp with TSR. A deeper analysis of the flow pattern and wake deficit behind the turbine helps in understanding the behavioral characteristics of these models. MRF shows variations in flow pattern with TSRs, like the associated stall conditions at TSR = 6 and an optimum angle of attack condition at TSR = 7.5. ALM shows only a slight variation in the flow pattern near the hub region (DU40 location) at different TSRs. This is because the blades are not resolved in ALM. Perhaps, such differences in flow-pattern predictions result in the differences in Cp vs TSR trends predicted by the MRF and ALM models. SMI, on the other hand, captures the complex 3D flow structures. The wake deficit comparison shows that both ALM and MRF model captures qualitatively higher wake deficit at TSR = 7.5 in the core wake region 0.8 > z/R > 0.2 as compared to TSR = 9 and TSR = 6. This behavior too is related to the observed flow pattern as captured by these models. Future studies may involve using LES in ALM to see if it improves the RANS predictions.

Quasi-Static & Dynamic Numerical Modeling of Full Scale NREL 5MW Wind Turbine

Simulations of the National Renewable Energy Laboratory (NREL) 5MW wind turbine under quasi-static Multiple Reference Frame (MRF) and dynamic Sliding Mesh Interface (SMI) methodologies are presented. Two reference zone approach is considered, inertial and moving reference frame. The former contains nacelle and tower, while the later constitutes of the rotor assembly. Predictive capabilities of both simulation techniques are exploited, and verification is performed against the Blade Element Momentum (BEM), and Large Eddy Simulation (LES) results in literature [1], [2]. The simulations are parametrized at variable tip-speed ratios (6, 6.5, 7, 7.5, 8, 8.5,9) and a uniform incoming velocity of 9m/s using unsteady Reynolds-Averaged Navier-Stokes (RANS). The MRF simulation techniques accuracy and robustness are exploited, hereafter key features at various operating conditions inside flow field are identified up to three radii (3R) distance. Aerodynamic torque in the dynamic SMI simulations is observed to oscillate and vary between 2,550 kN m and 2,650 kN m over a revolution. The wake evolution adjacent to the turbine is found to characterized by three massive vortices along with a central vortex which determined the dynamics of the wake. The three blade vortices interact with the central vortex and get dissipated at the 3R distance from the turbine. Immediately behind the tower, increased turbulent intensity levels are reported which gradually reduce after ≈1.5R distance both in the vertical and horizontal direction.

Numerical and experimental investigations of the propeller characteristics of an electrically powered ultralight aircraft

The performance and efficiency of a propeller is crucial for electrically powered propulsion systems. Since the energy of the batteries is limited, it is important to develop propellers with high efficiency. Therefore, numerical and experimental investigations of the propeller characteristics are performed. The wind tunnel experiments are performed on a fuselage–propeller configuration. The electrical motor, batteries, and control units are designed to be integrated in the fuselage. Furthermore, force measurements are conducted to provide a data base for the validation of the numerical results. Two different numerical approaches are presented. First, the propeller is fully resolved by applying a rotational domain and a sliding mesh interface. Second, an actuator disk approach including blade element theory with a panel method one-way coupled with a boundary layer integration method is presented. The latter shall be used to reduce computational and mesh generation costs. The thrust, efficiency as well as pressure distribution and the flow field downstream of the propeller are analyzed. The obtained numerical results show a good agreement with the experimental data for the integral values over a wide operating range. Moreover, the results of the inter-method comparison of the two numerical approaches are in a good accordance regarding the local effects for the two highlighted operating points.

Near wake region of an industrial scale wind turbine: comparing LES-ALM with LES-SMI simulations using data mining (POD)

Accurate prediction of power generation capability needs proper assessment of blade loading and wake behavior. In this regard, the Sliding Mesh Interface (SMI) approach and the Actuator Line Model (ALM) are two diverse computational fluid dynamics (CFD) based approaches of simulating the turbine behavior, each having its own merits and demerits. The SMI technique simulates the unsteady flow by explicitly modeling the blades and their rotation using a dynamic mesh, while in Actuator Line Model, the blades are not modeled explicitly but each blade is resolved as a rotating line (made of N actuator segments), over which the forces are computed. The current work focuses on simulating an industrial scale reference turbine and in differentiating the near wake dynamics predicted by these two approaches using Large Eddy Simulation (LES) and Proper Orthogonal Decomposition (POD) technique (a data mining tool). Initially, the ALM is compared with FAST model for the prediction of variation of power coefficient with the Tip Speed Ratio (TSR). The ALM is able to capture the varying trend and it predicts a similar optimum tip speed ratio as the FAST model. At this optimum TSR condition, the ALM is compared with the SMI method for a study limited to the near wake region. Comparisons between SMI and ALM shows that : (a) The SMI is predicting more complex 3D nature of the flow, and (b) the POD shows that ALM captures the shear regions of wake but it does not capture the vast compendium of length and time scales of eddies as SMI does. However, despite these limitations, the ALM has been able to capture the qualitative trend in wake deficit and the power coefficient variation with tip speed.

Numerical Analysis of NREL 5MW Wind Turbine: A Study Towards a Better Understanding of Wake Characteristic and Torque Generation Mechanism

With the increased feasibility of harvesting offshore wind energy, scale of wind turbines is growing rapidly and there is a trend towards clustering together higher number of turbines in order to harvest maximum yield and to leave a smaller footprint on the environment. This causes complex flow configurations inside the farms, the study of which is essential to making offshore wind energy a success. The present study focuses on NREL 5MW wind turbine with the following objectives (a)To compare Sliding Mesh Interface and Multiple Reference Frame modeling approaches and their predictive capabilities in reproducing the characteristics of flow around the full scale wind turbine. (b)To get a better insight into wake dynamics behind the turbine in near and far wake regions operating under different tip-speed-ratio and incoming turbulence intensities.

Aerodynamical phenomena in a large top covered wind mill with vertical axis wind turbine

Purpose– The purpose of this paper is to examine the aerodynamical and air mass flow phenomena taking place in the channel of a modified version of one of the well-known Sistan wind mills, in order to improve its aerodynamic performance.Design/methodology/approach– The simulations are done by means of the finite volume method associated to the realizable k-ε turbulence model. The computational domain consists in a rotating sub domain including the wind turbine equipped with nine blades and a fixed sub domain including the rest of the computational domain. Both are connected by means of a sliding mesh interface. Calculations are done for 8×105-4×106 Reynolds number range, corresponding to inlet velocities varying from 2 to 10 m s−1.Findings– The velocity fields are presented for the stopped and operating turbine (static and dynamic conditions). A careful examination of the aerodynamic phenomena is performed to detect potential vortices that could develop in the central cavity of the active assembly, and then influence the wind turbine’s operation.Originality/value– The modification proposed in this survey is easy to realize, consisting in covering the top of the entire original assembly that avoids the extraction of a large part of the air mass flow occurring through the open top of the original version. The aerodynamic phenomena occurring across the channel of this large vertical axis wind turbine are substantially different from those of the original version.

Numerical modelling of rotor–stator interaction in rim driven thrusters

An electric rim driven thruster is a relatively new marine propulsion device and the associated fluid dynamics have not been fully investigated. This work develops a robust CFD method and investigates both frozen rotor and unsteady simulations of rotor–stator interaction. Two solvers from OpenFOAM were used. Steady state simulations were performed using MRFSimpleFoam with a frozen rotor treatment of the interface between static and rotational reference frames. The solver for unsteady simulations was pimpleDyMFoam, utilising a sliding mesh interface to handle the dynamic meshing. Both methods are thoroughly verified and validated against experimental data. The k–omega SST turbulence model is found to be robust down to low advance ratios. For the rim driven thruster, analytical models are used to estimate friction forces in the rim gap and their contribution to torque losses. The frozen rotor and unsteady treatments of rotor–stator interaction are compared and found to have similar trends in the variation of thrust produced. However, the frozen rotor method does not predict the same variation of instantaneous torque and does not capture the rotor–stator interaction fully. Analysis of the unsteady rotor–stator interaction shows an oscillating flow over the stators and thus inflow to the blades.

A simple, efficient, and high-order accurate curved sliding-mesh interface approach to spectral difference method on coupled rotating and stationary domains

This paper presents a simple, efficient, and high-order accurate sliding-mesh interface approach to the spectral difference (SD) method. We demonstrate the approach by solving the two-dimensional compressible Navier–Stokes equations on quadrilateral grids. This approach is an extension of the straight mortar method originally designed for stationary domains [7,8]. Our sliding method creates curved dynamic mortars on sliding-mesh interfaces to couple rotating and stationary domains. On the nonconforming sliding-mesh interfaces, the related variables are first projected from cell faces to mortars to compute common fluxes, and then the common fluxes are projected back from the mortars to the cell faces to ensure conservation. To verify the spatial order of accuracy of the sliding-mesh spectral difference (SSD) method, both inviscid and viscous flow cases are tested. It is shown that the SSD method preserves the high-order accuracy of the SD method. Meanwhile, the SSD method is found to be very efficient in terms of computational cost. This novel sliding-mesh interface method is very suitable for parallel processing with domain decomposition. It can be applied to a wide range of problems, such as the hydrodynamics of marine propellers, the aerodynamics of rotorcraft, wind turbines, and oscillating wing power generators, etc.

Computational analysis of flow physics of a combined three bladed Darrieus Savonius wind rotor

A 3D unsteady computational Fluid Dynamics (CFD) model is utilized to investigate the aerodynamics of a combined three-bladed Darrieus Savonius wind rotor. Unsteady Reynolds-averaged Navier–Stokes (RANS) equations of the CFD software are modelled to obtain the fluid flow distributions. In this, initial unsteady effects of the flow interactions are covered to reach to the steady state condition at which the flow physics is analysed. The fluid zone has a large stationary domain and rotating sub-domain connected by a sliding mesh interface. The 3D wake structure of the combined rotor is analysed to understand the physical processes responsible for its power production in low wind speed regime. Steep Coanda like vortices are observed in the rotor wake that migrates from the downstream of Savonius rotor on to the downstream of Darrieus rotor. This effect results into compression of vortices on the downstream of Darrieus rotor thereby pushing the rotor in its rotation, hence augment power production. Presence of attached stall vortices on the Darrieus blades and increased concentration of vortices on the downstream of Darrieus rotor justifies the design of Darrieus rotor on top of Savonius rotor, which could be reflected in high value of power coefficient of the rotor. The outcome of this research would improve the viability of such combined rotor for its use in the built environment.

Unsteady aerodynamics of a twist bladed H-Darrieus rotor in low Reynolds number flow

In this paper, an attempt is made to investigate the unsteady aerodynamics of a two-bladed H-Darrieus rotor having blade twist of 30° at the trailing edge when placed in low Reynolds number flow. The effect of low Re flow on the unsteady blade-fluid interactions has been studied in some details to understand the high power coefficient of such twist bladed rotor in low wind speed condition. Two-dimensional unsteady Reynolds-averaged Navier–Stokes equations are solved using Fluent 6.0 CFD software. The computational domain comprises of a large static domain and a rotating sub-domain, both are connected by a sliding mesh interface. The blade-fluid interactions have been investigated at different rotor azimuthal positions. The effect of low Reynolds number flow from dynamic stall is shown in the velocity profiles, pressure and torque-coefficients distributions. The combination of interactions of vortices on the leading edge and interactions of the same on the trailing edge of the returning blade increases torque coefficient of the rotor during its power stroke compared to untwisted rotor. The present numerical model could be built into a 3D CFD model to investigate the effect of different blade aspect ratios on the stall vortex interactions at trailing edge so as to improve the unsteady performance of such twist bladed rotor in low Re flow.

Computational Studies of Flow through Cross Flow Fan: Effect of Casing Parameters

This present paper describes three dimensional computational analysis of complex internal flow in a cross flow fan, (CFF) with a special emphasis on the performance improvement by varying impeller and casing geometries, inlet and tongue clearance gaps. RNG k- two-equation turbulence model is used to simulate the model with unstructured mesh. Sliding mesh interface is used at the interface between the rotating and stationary domains to capture the unsteady interactions. Three impeller geometries, seven casing configurations and three inlet and tongue clearances are varied in the present study. The efficiency of the fan has improved from 59 % to 71 % and that of casing from 37 % to 61 %. Maximum energy transfer through the impeller takes place in the region where the flow follows the blade curvature.

Numerical investigation of a jet-and-vortex-actuator without and with cross-flow boundary layer

In this study we consider an active flow control actuator which was first investigated experimentally by Lachowicz et al. (1999a,b) and called “Jet and Vortex Actuator” (JaVA). The Fluent® CFD software package has been used because of its ability to employ user-defined functions for specification of the unsteady boundary conditions and a sliding mesh interface for discretization of the moving parts of the JaVA. Three cases are presented, one without cross flow and two with a cross-flow boundary layer. The difference in the latter two is that the actuator arrangement with respect to the cross flow is reversed from one case to the other. The results for the first case prove that the available experimental results can be reproduced in a computational study and that the numerical simulation provides additional information because it covers the unsteady details as well. The other two cases show that the actuator increases the wall-normal velocity gradient by a redistribution of momentum within the boundary layer, especially when the wide gap of the actuator is behind the small gap and the moving actuator plate.

An Investigation of Turbine Wheelspace Cooling Flow Interactions With a Transonic Hot Gas Path—Part II: CFD Simulations

A computational model has been developed to study the mechanisms responsible for hot gas ingestion into the wheel-space cavity of a stationary high pressure turbine (HPT) cascade rig. Simulations were undertaken for the stationary rig described by Bunker et al. (2009, “An Investigation of Turbine Wheelspace Cooling Flow Interactions With a Transonic Hot Gas Path—Part I: Experimental Measurements,” ASME Paper No. GT2009-59237) in a companion paper. The rig consists of five vanes, a wheel-space cavity, and five cylinders that represent the blockage due to the leading edge of the rotor airfoils. The experimental program investigated two cylinder diameters and three clocking positions for a nominal coolant flow rate. Comparisons are made between the computed and measured flow-fields for the smaller of the two cylinders. It is demonstrated that the circumferential variation of pressure established by the vane wake and leading edge bow wave results in an unstable shear layer over the rim seal axial gap (trench) that causes hot gases to ingest for a nominal coolant flow. Steady-state computational fluid dynamics (CFD) simulations did not capture this effect and it was determined that an unsteady analysis was required in order to match the experimental data. Favorable agreement is noted between the time-averaged computed and measured pressure distributions in the circumferential direction both upstream and downstream of the trench, as well as within the trench itself. Furthermore, it is noted that time-averaged buffer cavity effectiveness agrees to within 5% of the experimental data for the cases studied. The validated CFD model is then used to simulate the effect of rotation by rotating the cylinders and disk at rotational rate that scales with a typical engine. A sliding mesh interface is utilized to communicate data between the stator and rotor domains. The stationary cases tend to ingest past the first angel-wing for a nominal coolant flow condition, whereas the effect of rotation helps pressurize the cavity and is responsible for preventing hot gas from entering the buffer cavity.