Anshul Mittal

Investigation of Two Analytical Wake Models Using Data From Wind Farms

A code ‘Wind Farm Optimization using a Genetic Algorithm’ (referred as WFOG) was developed for optimizing the placement of wind turbines in large wind farms. It utilizes an analytical wake model (by Jensen et al.) to minimize the cost per unit power for the wind farm. In this study, a new wake model by Ishihara et al. is tested in WFOG. The wake model takes into account the effect of atmospheric turbulence and rotor generated turbulence on the wake recovery. Results of the two wake models are compared with data from Horns Rev and Nysted wind farm. The maximum error (Horns Rev wind farm) for Ishihara’s wake model was 7% as compared to 15% for Jensen’s wake model. The optimal results obtained in earlier studies (using Jensen’s wake model) are compared to wind farm configurations obtained for Ishihara’s wake model. The optimization is carried out for the simplest wind regime: Constant wind speed and fixed wind direction.Copyright

High-Fidelity Computational Simulation of the Interaction between Tandem Wind Turbines

Simulations of a two wind turbines operating in tandem at various tip-speed-ratios are carried out using Tenasi, a node-centered, finite volume unstructured flow solver. The model wind turbines were designed using the NREL S826 airfoils as cross sections and detailed experimental data is available for a variety of flow conditions. The simulations included the tunnel walls as the blockage (based on tower area and the swept area of the rotor) was 12%. The results presented here are for tip-speed-ratios of 2.5, 4, and 7 for the rear turbine while the front turbine was always operated at a tip speed ratio of 6, with 6 being the design point. All simulations were carried out at a freestream velocity of 10 m/s and the wind turbine RPM was varied to achieve the desired tip-speed-ratios. Results were obtained using both oneand two-equation turbulence models. Turbine performance as well as wake data at various locations is compared to experiment. The overall agreement between the computations and experiment is very good.

High-Fidelity Computational Simulation of the Wake Characteristics of a Model Wind Turbine

Simulations of a model wind turbine at various tip-speed-ratios are carried out using Tenasi, a node-centered, finite volume unstructured flow solver. The model wind turbine was designed using the NREL S826 airfoils as cross sections and detailed experimental data is available for a variety of flow conditions. The simulations included the tunnel walls as the blockage (based on tower area and the swept area of the rotor) was 12%. The results presented here are for tip-speed-ratios of 3, 6 and 10, with 6 being the design point. All simulations were carried out at a freestream velocity of 10 m/s and the wind turbine RPM was varied to achieve the desired tip-speed-ratio. Results are presented for various oneand two-equation turbulence models. Turbine performance as well as wake data at various locations is compared to experiment. The overall agreement between the computation and experiment is good.

A Parabolic Method without Pressure Approximations for Wind Turbines

A Parabolized Navier-Stokes (PNS) approximation for the incompressible equations is developed to simulate flows through a wind turbine. The PNS formulation differs from commonly used parabolic formulations in that the pressure field is calculated as a dependent variable without any approximation for the streamwise pressure gradient. The turbine influence is modeled by incorporating time-averaged aerodynamic forces predicted by an Actuator Line model (FAST developed at NREL). These forces are time averaged and incorporated as localized source terms near the turbine. Using this coupled time-averaged spatial-marching method, the calculation can begin well upstream of the turbine and continue through the turbine to predict the entire wake region. Laminar and turbulent flatplate boundary-layer cases are used here for basic validation of the PNS Algorithm. Computed solutions for the NREL offshore 5-MW baseline wind turbine are compared with the blade-resolved Navier-Stokes solutions for verification.

Improvements to the Actuator Line Modeling for Wind Turbines

The Actuator Line method of modeling wind turbine rotors has become popular over the last several years. There are various issues pertaining to the use of this method from a practitioner’s stand point including grid dependent solutions and strong effect of projection width on the predicted power. Some improvements to the existing actuator line method are investigated in this paper and discussed. The strategies include utilizing two projection widths based on the physical attributes of the blade (chord and width of the actuator element) and reducing the sensitivity of the AL model to the input velocities by averaging them.

A Parabolic Method for Accurate and Efficient Wind Farm Simulation

A Parabolized Navier-Stokes (PNS) approximation for the incompressible equations is developed and utilized to simulate flow through a wind farm of five wind turbines. The PNS formulation differs from commonly used parabolic formulations in that the pressure field is calculated as a dependent variable without any approximation for the streamwise pressure gradient. The wind turbine is modeled by incorporating time-averaged aerodynamic forces predicted by an Actuator Line model (FAST developed at NREL). Using this coupled timeaveraged spatial-marching method, the calculation can begin well upstream of the wind farm and continue through the turbines to predict the entire flow field including the wakes. The model was verified for wind turbines by comparing the computed solutions for the NREL offshore 5-MW baseline wind turbine with the blade-resolved Navier-Stokes solutions. Very good agreement was obtained and the runtime on a single desktop computer was less than 80 minutes. Results for the wind farm are presented. The simulation on a desktop computer took 271 minutes.

Extension of a Parabolic Method without Pressure Approximations for Wind Turbines in ABL Flows

A Parabolized Navier-Stokes (PNS) approximation for the incompressible equations is developed to simulate flows through a wind turbine embedded in an atmospheric boundary layer (ABL). The PNS formulation differs from commonly used parabolic formulations in that the pressure field is calculated as a dependent variable without any approximation for the streamwise pressure gradient. The turbine influence is modeled by incorporating timeaveraged aerodynamic forces predicted by an actuator-line model (FAST developed at NREL). Using this coupled time-averaged spatial-marching method, the calculation can begin well upstream of the turbine and continue through the turbine to predict the entire wake region. In this study, the original model is extended to simulate neutral atmospheric boundary layers (ABL) and study the wake development of a single wind turbine. The PNS model has been validated for laminar and turbulent flat-plate boundary-layer cases and verified for a wind turbine (uniform inflow conditions) by comparing the computed solutions with the blade-resolved Navier-Stokes solutions for the NREL offshore 5-MW baseline wind turbine.

Investigation of Rotor Models for Wind Turbine Simulations

In this paper, air flow around a model wind turbine and the NREL offshore 5-MW baseline wind turbine is simulated using different rotor modeling techniques/methodologies. The actuator line (AL) method of modeling the rotor is compared with the fully resolved rotor simulation. The aerodynamic forces for the AL method are computed using an open source code, FAST, from NREL. An in-house code, Tenasi, is used to carry out the CFD simulations. Validation of the computational methodology is carried out by comparing the simulation results with the experimental data for the model wind turbine. Since the NREL offshore 5-MW baseline wind turbine is notional, experimental data is not available for validation. The simulation results are verified using the results from other studies.

A parabolic velocity-decomposition method for wind turbines

An economical parabolized Navier-Stokes approximation for steady incompressible flow is combined with a compatible wind turbine model to simulate wind turbine flows, both upstream of the turbine and in downstream wake regions. The inviscid parabolizing approximation is based on a Helmholtz decomposition of the secondary velocity vector and physical order-of-magnitude estimates, rather than an axial pressure gradient approximation. The wind turbine is modeled by distributed source-term forces incorporating time-averaged aerodynamic forces generated by a blade-element momentum turbine model. A solution algorithm is given whose dependent variables are streamwise velocity, streamwise vorticity, and pressure, with secondary velocity determined by two-dimensional scalar and vector potentials. In addition to laminar and turbulent boundary-layer test cases, solutions for a streamwise vortex-convection test problem are assessed by mesh refinement and comparison with Navier-Stokes solutions using the same grid. Computed results for a single turbine and a three-turbine array are presented using the NREL offshore 5-MW baseline wind turbine. These are also compared with an unsteady Reynolds-averaged Navier-Stokes solution computed with full rotor resolution. On balance, the agreement in turbine wake predictions for these test cases is very encouraging given the substantial differences in physical modeling fidelity and computer resources required.

Exploration of Modal Decomposition Techniques for Wind Turbines

The objective of this study is to investigate the modal decomposition techniques namely, Proper Orthogonal Decomposition (POD) and Dynamic Mode Decomposition (DMD) for application in wind energy. An unsteady dataset is decomposed into modes and associated coefficients which are then used to reconstruct solution(s). The sensitivity of the number of modes on the accuracy of the reconstructed solution is evaluated. Test cases comprise of linear and nonlinear equation in one dimension and flow around a NACA 0012 airfoil apart from the CFD data of the NREL offshore 5-MW baseline wind turbine. The motivation for this work comes from the fact that reconstruction of a solution using the modes is several order of magnitudes more efficient (in terms of the computational cost) as compared to Navier-Stokes simulations.