Influence of turbulence on the performance of a laboratory scale HAWT

The oncoming wind to horizontal axis wind turbines (HAWT) may change its speed and direction stochastically in time. Hence, turbine blades are exposed to flows both with fluctuating angle of attack and fluctuating yaw angles. The modern wind turbines are reacting to those changes by pitch angle and torque control not only to exploit as much power as possible but also stabilize energy production and prevent any damage of the turbine. However, time scales of wind fluctuations and sudden changes of wind properties can be very short and with very high in amplitude. In the present study we focus on the influence of turbulence on the performance of a HAWT. Main motivation of the investigations is to figure out best strategies for the aerodynamic design of the blades operating under turbulent conditions. A laboratory scale HAWT and a performance measurement set-up are employed to measure the influence of the oncoming wind. The tests are conducted in the closed loop wind tunnel of our institute. The test section of the tunnel is 1.87 m in width, 1.4 m in height and 2 m in length. The rotor blades are specially designed and optimized for this wind tunnel and the generator used. The turbulence is generated by two static squared mesh grids; fine and coarse one. Hence, two mainly different turbulence scales are obtained. In addition, the distance between the wind-turbine and the grid is adjusted to have additional sub-turbulence scales for each grid. The turbulence is nearly isotropic and decays in the flow direction. The developments of Taylor's micro scale (λ_g) and integral scale (L_g) of the turbulence in the flow direction at various incoming wind velocities (8−16 m/s) are measured. Hence, the facility allows to expose the wind-turbine to turbulence with various energy and length scale content. Those measurements are conducted with hot-wire anemometry in the absence of the wind-turbine. Upstream and downstream turbulence intensities (TI) distributions are measured to give insight on the surrounding free stream and turbine wake interaction and how can different turbulence eddies scales contribute in the influence of the performance of the turbine. Performance measurements are conducted with and without turbulence and the results are compared. The study shows that the higher the turbulence, the more the power extracted by the turbine. This is due to the higher interaction of large eddies with the turbine wake and with the boundary layer, which helps to keeping it attached. Furthermore, higher TI's help in suppressing the tip vortex, thus, reduce turbine tip losses. (© 2014 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim)     

Two Way Coupled Micro/Meso Scale Method for Wind Farms

Wind turbines extract energy from the approaching flow field resulting in reduced wind speeds, increased turbulence and a wake downstream of the wind turbine. The wake has a multitude of negative effects on downstream wind turbines. This includes reduced efficiency and increased unsteadiness resulting in vibrations and potentially in material fatigue. Moreover, the maintenance can increase compared to non-interfering wind turbines. The simulation of these effects is challenging. Computational fluid dynamics (CFD) simulations of these large and complex geometries requires exceedingly large computational resources. With present Reynolds Averaged Navier-Stokes (RANS) or Large Eddy Simulation (LES) based CFD methods it is virtually impossible to perform such simulations of the interaction between individual wind turbines in a complete wind turbine farm. Coupling to the mesoscale accounting for local weather situations becomes yet more challenging. This is due to the wide range of length and time scales that have to be considered for these simulations and therefore the tremendous computational power needed to perform such simulations. To investigate these effects we propose to combine ideas from existing methods, the Coarse-Grid-CFD (CGCFD) ( [1]) developed at the KIT and the meso-/ micro scale method developed at the University of Thessaloniki ( [2]). Goal of the proposed methodology is to provide a numerical method that allows to implement a wind farm in a meso-scale weather simulation which includes two-way coupling. Thus both the micro and the meso scale wind and energy production of wind farms can be addressed. This proposed multi scale coupling strategy can also be applied in two hierarchies reducing the numerical effort of the global approach yet more. (© 2014 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim)     

RANS closures for non-neutral microscale CFD simulations sustained with inflow conditions acquired from mesoscale simulations

This study focuses on bridging the gap between the turbulence modelling methodologies of meterological and engineering codes by proposing a novel methodology to define the closure coefficients of Reynolds-Averaged Navier-Stokes turbulence models consistently with the physics of the atmospheric boundary layer. In this framework, different turbulence closures have been developed and tested on different full-scale test cases corresponding to different atmospheric stability conditions by performing microscale simulations with the inflow conditions provided by a numerical weather prediction (NWP) code. Developed turbulence models have been implemented into the open source computational fluid dynamics (CFD) toolbox, OpenFOAM and the inflow conditions have been acquired with another open source code, the Weather Research and Forecasting (WRF) model.     

Investigation of the effect of the wall roughness and free stream turbulence on the boundary layer development

Development of a flat plate boundary layer (grad P = 0) was investigated in the closed type wind tunnel (0.5 × 0.9 × 2.7 m³). The plate was either aerodynamically smooth or covered with a thin plate (10 mm thick) with the surface made from the sand paper (grits 80). FST is generated by square mesh plane grids/screens set across the flow as to produce homogeneous, close to isotropy turbulence with various scales of velocity and length. The surface roughness was of transient type. The comparison is made of the effect of various grouping of the wall roughness and FST on the boundary layer development. (© 2008 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)     

On the Turbulence Modelling of 3D-Atmospheric Boundary Layer Flow

The paper presents a mathematical and numerical investigation of the 3D–flow in the atmospheric boundary layer (ABL) over complex relief. The two–equation k - ε model is applied to account for the turbulence. The flow is also supposed to be viscous, incompressible and stationary. The boundary conditions are realized through the wall-functions. (© 2005 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)     

Numerical Solution of Transonic Flow in SE1050 Cascade

The SE1050 cascade is an open test case (QNET network) of plane turbine cascade measured at the IT ASCR wind tunnel. The two regimes with subsonic and supersonic outletMach number were selected for numerical simulation. Several numerical methods have been developed and also several turbulence models have been implemented. Comparison of computed results and experimental data gives us opportunity to discuss main features of transonic flow field in well designed turbine cascade, possibilities of its numerical capturing (grid quality, numerical viscosity, turbulence model, boundary layer transition) and its influence on prediction of energy losses. (© 2005 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)     

Turbulent Boundary Layer Under the Influence of Adverse Pressure Gradient

The paper deals with the experimental analysis of turbulent boundary layer at the flat plate for large value of Reynolds number equal Re_θ ≈︁ 3000. The adverse pressure gradient generated by curvature of the upper wall corresponded to the case of pressure variation in axial compressor. The fully developed structure of turbulence was achieved by proper triggering of the boundary layer. The mean and turbulent flow-fields were investigated with the use of hot-wire technique while mean and instantaneous pressure fields were examined with piezoelectric transducers. The scaling and turbulence structure of fully developed turbulent boundary layer under the influence of adverse pressure gradient revealed the more pronounced contribution of outer region to the downstream development of turbulent boundary layer. (© 2009 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim)     

Mathematical Modelling of Atmospheric Flow over a Shelter-belt

The paper presents a mathematical and numerical study of the flow with pollution dispersion in the atmospheric boundary layer (ABL) over a simplified topography with a shelter-belt placed before the pollutant (coal dust) source. The flow is supposed to be viscous, incompressible, turbulent and stationary. A two different numerical models are briefly mentioned. The shelter-belt is supposed to be a solid and impermeable obstacle significantly changing the flow-field and hence it gives the possibility of influencing the level of the pollutant concentrations in the downstream region. (© 2006 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)     

Numerical Simulation of Pollution Dispersion over Real Terrain

The paper presents a mathematical and numerical investigation of the atmospheric boundary layer (ABL) flow over coal depot. Two mathematical models have been mentioned based upon: 1) the RANS equations in the conservative form and 2) the Boussinesq approximation of RANS equations in the non–conservative form, both formulated for an incompressible flow with a simple algebraic turbulence closure and given stationary boundary conditions. Also pollution dispersion of passive pollutants has been considered.     

Variation of drag coefficient with wind wave development

Turbulent air flows over developing wind waves in the air-sea boundary layer are numerically simulated without considering wave breaking. Influences of wind waves on air flows are considered using a model of significant wave and surface roughness, with a formula proposed for calculating the surface roughness. κ-ε model is adopted to simulate turbulent flows. The results of the drag coefficient and turbulence characteristics agree well with the observations. Project supported by the National Natural Science Foundation of China (Grant No. 19332010).     

Two-equation eddy viscosity models based on the Monin–Obukhov similarity theory

The present study is devoted for the development of two equation Reynolds-Averaged Navier–Stokes (RANS) closures for computational fluid dynamics (CFD) modelling of the atmospheric boundary layer. By using the inflow conditions based on the Monin–Obukhov similarity theory, the closure coefficients of the proposed models are derived from the analytical solutions of simplified turbulent transport equations. Modifications are conducted for three different turbulence models, which are standard $k-ϵ,$$k-\omega$ and Re-Normalisation Group (RNG) $k-ϵ$. Numerical experiments are performed for the homogeneous atmospheric boundary layer and the results are compared with the theoretical values in comparison to the standard versions of modified turbulence models. Developed models are implemented to open source CFD toolbox OpenFOAM.     

Aircraft wake vortices – prediction and mitigation

The current abstract presents selected topics investigated within the wake-vortex research program of DLR. Two approaches are addressed that both aim at increasing airport capacity without compromising safety. One approach is to directly alleviate wake vortex strength and stability by constructive measures at the aircraft wings. The other approach utilizes the dominant influence of meteorological parameters like turbulence, wind shear, and temperature stratification on wake vortex fate. (© 2008 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)     

Direct numerical simulation of turbulent non-Newtonian flow using OpenFOAM

Understanding transition and turbulence in the flow of shear-thinning non-Newtonian fluids remains substantially unresolved and additional research is required to develop better computational methods for wall-bounded turbulent flows of these fluids. Previous DNS studies of shear-thinning fluids mainly use purpose-built codes and simple geometries such as pipes and channels. However in practical application, the geometry of mixing vessels, pumps and other process equipment is far more complex, and more flexible computational methods are required. In this paper a general-purpose DNS approach for shear-thinning fluids is undertaken using the OpenFOAM CFD library. DNS of turbulent Newtonian and non-Newtonian flow in a pipe flow are conducted and the accuracy and efficiency of OpenFOAM are assessed against a validated high-order spectral element-Fourier DNS code – Semtex. The results show that OpenFOAM predicts the flow of shear-thinning fluids to be a little more transitional than the predictions from Semtex, with lower radial and azimuthal turbulence intensities and higher axial intensity. Despite this, the first and second order turbulence statistics differ by at most 16%, and usually much less. An assessment of the parallel scaling of OpenFOAM indicates that OpenFOAM scales very well for the CPUs from 8 to 512, but the intranode scalability is poor for less than 8CPUs. The present work shows that OpenFOAM can be used for DNS of shear-thinning fluids in the simple case of pipe flow, and suggests that more complex flows, where flow separation is often important, are likely to be simulated with accuracies that are acceptably good for engineering application.     

Turbulent Entrainment in the Atmospheric Boundary Layer

Turbulent entrainment – the process by which turbulence inside of the atmospheric boundary layer entrains air from the free troposphere above it – has been investigated using direct numerical simulations in two configurations, one without a cloud and one with a cloud. With the first configuration, we have learned that the entrainment zone in a convective boundary layer growing into a linearly stratified troposphere is better described in terms of a two-layer structure, with different characteristic scales associated with each of the two sub-layers. With the second configuration, we have explained how wind shear across the entrainment zone capping a stratocumulus cloud can render evaporative cooling as important as radiative cooling in driving convective motions. (© 2014 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim)     

Mathematical and Numerical Study of the 3D Atmospheric Boundary Layer Flows Including Pollution Dispersion

The paper presents a mathematical and numerical investigation of the atmospheric boundary layer (ABL) flow over 3D complex terrain part of which is represented by the real topography of the Krkonose mountains located in the Czech Republic. The flow is supposed to be turbulent, non‐stratified, viscous, incompressible and stationary. Two mathematical models have been formulated. The first model is based upon the RANS equations in the conservative form and the second one uses the Boussinesq approximation of RANS equations and takes the non‐conservative form. Also pollution dispersion over the complex 3D terrain has been considered in both models. The problem closure is achieved by an algebraic turbulence model and given boundary conditions.     

Simulation of round jets with nozzle-dependent inflow conditions using the k−ε turbulence model

In view of the “round jet initial condition anomaly”, discussed in literature, we investigate the effect of inflow conditions resulting from the use of different nozzle geometries to form the jet. RANS simulations in the framework of OpenFOAM using the k − ε turbulence model are performed. As the standard model coefficient C_ε1 = 1.44 is known to overpredict spreading rates for round jets, a value of C_ε1 = 1.6 was recommended for this case already in the 1970's. While this works well for jets issuing from long pipes, it does not give satisfactory results for other nozzle geometries. To overcome this deficiency while keeping the k − ε model, we suggest modified coefficients C_ε1 based on profiles of mean flow and turbulence at the nozzle exit. We determine optimal values of C_ε1 for three different nozzle geometries, and test them at various Reynolds numbers. Good agreement with experimental data is obtained. (© 2014 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim)     

Inflow Treatment for the Simulation of Subsonic Jets with Turbulent Nozzle Boundary Layers

The state of the boundary layer at the nozzle exit of a circular nozzle-jet configuration has an important influence on the development of the shear layer and the emitted sound. Of special interest is the acoustic near-field obtained when the nozzle exit boundary layer is fully turbulent. The turbulent inflow generation and the inflow boundary treatment are important issues to be addressed. We use the Synthetic Eddy Method (SEM) to generate a turbulent inflow which reproduces mean flow and Reynolds stress profiles of specified reference data. The spatially and temporally varying synthetic fluctuations are imposed in the simulation by a forcing term added to the governing equations which is active in a small region downstream of the inflow boundary. This forcing in combination with characteristic boundary conditions allows for passing of upstream-propagating acoustic waves and avoids an uncontrolled drift of mean-flow quantities. We employ this inflow boundary treatment for a subsonic nozzle-jet flow simulation at a Reynolds number of ∼ 9500 and Mach number of 0.9. (© 2011 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim)     

Numerical simulation of vortex induced vibrations and its control by suction and blowing

The vortex formation and shedding behind bluff structures is influenced by fluid flow parameters such as, Reynolds number, surface roughness, turbulence level, etc. and structural parameters such as, mass ratio, frequency ratio, damping ratio, etc. When a structure is flexibly mounted, the Kármán vortex street formed behind the structure gives rise to vortex induced oscillations. The control of these flow induced vibrations is of paramount practical importance for a wide range of designs. An analysis of flow patterns behind these structures would enable better understanding of wake properties and their control. In the present study, flow past a smooth circular cylinder is numerically simulated by coupling the mass, momentum conservation equations along with a dynamical evolution equation for the structure. An active flow control strategy based on zero net mass injection is designed and implemented to assess its efficacy. A three actuator system in the form of suction and blowing slots are positioned on the cylinder surface. A single blowing slot is located on the leeward side of the cylinder, while two suction slots are positioned at an angle α = 100°. This system is found to effectively annihilate the vortex induced oscillations, when the quantum of actuations is about three times the free stream velocity. The dynamic adaptability of the proposed control strategy and its ability to suppress vortex induced oscillations is verified. The exact quantum of actuation involved in wake control is achieved by integrating a control equation to decide the actuator response in the form of a closed loop feed back system. Simulations are extended to high Reynolds number flows by employing eddy viscosity based turbulence models. The three actuator system is found to effectively suppress vortex induced oscillations.     

On Results' Accuracy at Two‐Dimensional Transonic Wind Tunnel Testing

The principal factors which influence the accuracy of two‐dimensional wind tunnel test results are analyzed. The influences of Reynolds number, Mach number and wall interference with reference to solid and flow blockage (blockage of wake) as well as the influence of side‐wall boundary layer control are analyzed.