SimCenter National Center for Computational Engineering

Chattanooga, TN, United States

SimCenter National Center for Computational Engineering

Chattanooga, TN, United States

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Taylor L.K.,University of Tennessee at Chattanooga | Taylor L.K.,SimCenter National Center for Computational Engineering | Sreenivas K.,University of Tennessee at Chattanooga | Sreenivas K.,SimCenter National Center for Computational Engineering | And 3 more authors.
44th AIAA Thermophysics Conference | Year: 2013

An implicit algorithm is presented for the unsteady three-dimensional incompressible Navier-Stokes equations including the thermal energy equation, formulated in a strongly coupled manner. Thermal buoyancy is addressed using the Boussinesq approximation. The method of artificial compressibility is introduced to cast the resulting equations into a time marching form. A finite volume discretization is applied for general unstructured grids with nonsimplical elements. Numerical flux formulations are presented for both Roe's approximate Riemann solver and an HLLC approach. A backward Euler scheme is utilized for temporal discretization. The resulting implicit nonlinear equation is solved at each time step using an approximate Newton iteration algorithm. All boundary conditions are applied implicitly. The resulting algorithm has been implemented in Tenasi, an in-house developed flow solver, and validated for canonical test cases from the three regimes (forced, natural, and mixed) of laminar heat transfer and one regime (mixed) of turbulent heat transfer. A final application of the current algorithm is the prediction of forced turbulent heat transfer in a two-pass ribbed turbulator. All computed solutions are in close agreement with analytical solutions, other benchmark simulations, or experimental data.


Sreenivas K.,University of Tennessee at Chattanooga | Sreenivas K.,SimCenter National Center for Computational Engineering | Mittal A.,University of Tennessee at Chattanooga | Mittal A.,SimCenter National Center for Computational Engineering | And 4 more authors.
33rd Wind Energy Symposium | Year: 2015

Simulations of two wind turbines operating in tandem (with an offset in the transverse direction) 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 3.5, 4.75, and 8 for the rear turbine while the front turbine was always operated at a tip speed ratio of 6, with 6 being the design point. Two different sets of test cases are presented here, one corresponding to low freestream turbulence intensity and the other corresponding to high freestream turbulence intensity. 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 one-and 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. © 2015, American Institute of Aeronautics and Astronautics Inc. All rights reserved.


Mittal A.,University of Tennessee at Chattanooga | Mittal A.,SimCenter National Center for Computational Engineering | Sreenivas K.,University of Tennessee at Chattanooga | Sreenivas K.,SimCenter National Center for Computational Engineering | And 4 more authors.
33rd Wind Energy Symposium | Year: 2015

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. © 2015, American Institute of Aeronautics and Astronautics Inc. All rights reserved.


Sreenivas K.,University of Tennessee at Chattanooga | Sreenivas K.,SimCenter National Center for Computational Engineering | Mittal A.,University of Tennessee at Chattanooga | Mittal A.,SimCenter National Center for Computational Engineering | And 4 more authors.
34th Wind Energy Symposium | Year: 2016

A higher order reconstruction approach (up to 7th order) based on WENO methodology is applied to grids typically found in wind turbine applications. The presence of an essentially structured grid to resolve the wake is exploited in this implementation. The advantage of this approach over traditional finite-volume schemes applied to unstructured meshes is that it fits within the context of existing flow solvers and can be implemented with minimal modifications. Results presented here for the BT1 configuration show overall improvements in thrust and power coefficient predictions, while significant improvement can be observed in the velocity fluctuation predictions. The current implementation requires about 35% more computational time compared to traditional quadratic reconstruction. © 2016, American Institute of Aeronautics and Astronautics Inc, AIAA. All rights reserved.


Mittal A.,SimCenter National Center for Computational Engineering | Mittal A.,University of Tennessee at Chattanooga | Sreenivas K.,SimCenter National Center for Computational Engineering | Sreenivas K.,University of Tennessee at Chattanooga | And 4 more authors.
34th Wind Energy Symposium | Year: 2016

A Parabolized Navier-Stokes (PNS) approximation for the incompressible equations is utilized to simulate flow through a wind farm. The PNS formulation differs from commonly used parabolic formulations in that the pressure field is calculated as a dependent variable. The wind turbine is modeled by incorporating time-averaged aerodynamic forces predicted by an Actuator Line model (FAST developed at NREL). Using this coupled time-averaged spatial-marching method, the computation begins well upstream of the wind farm and continues through the turbines to predict the entire flow field including the wakes. The model has been 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 runtimes on a single core of a desktop computer range from an hour for a single turbine to several hours for a wind farm. The PNS model is used to obtain sensitivity derivatives in order to maximize the power produced by a wind turbine array by optimizing the location of turbines. Excellent results obtained for the test cases show great promise for the PNS model to be utilized for optimization purposes. © 2016, American Institute of Aeronautics and Astronautics Inc, AIAA. All rights reserved.


Druyor Jr. C.,SimCenter National Center for Computational Engineering | Karman Jr S.,SimCenter National Center for Computational Engineering
50th AIAA Aerospace Sciences Meeting Including the New Horizons Forum and Aerospace Exposition | Year: 2012

An adaptive hybrid mesh generation method is described to automatically provide spa-tial discretizations suitable for computational uid dynamics or other 2D solver applica-tions. This method employs a hierarchical grid generation technique to create a background mesh, an extrusion-type method for inserting boundary layers, and an unstructured tri-angulation to stitch between the boundary layers and background mesh. This method provides appropriate mesh resolution based on geometry segments from a file, and has the capability of adapting the background mesh based on a spacing field generated from solu-tion data or some other arbitrary source. By combining multiple approaches to the grid generation process, this method seeks to benefit from the strengths of each, while avoiding the weaknesses of each. Copyright © 2012 by University of Tennessee Chattanooga, SimCenter: National Center for Computational Engineering. Published by the American Institute of Aeronautics and Astronautics.


Anderson W.K.,SimCenter National Center for Computational Engineering | Wang L.,SimCenter National Center for Computational Engineering | Kapadia S.,SimCenter National Center for Computational Engineering | Tanis C.,SimCenter National Center for Computational Engineering | Hilbert B.,SimCenter National Center for Computational Engineering
Journal of Computational Physics | Year: 2011

Finite-element discretizations for Maxwell's first-order curl equations in both the time domain and frequency domain are developed. Petrov-Galerkin and discontinuous-Galerkin formulations are compared using higher-order basis functions. Verification cases are run to examine the accuracy of the algorithms on problems with exact solutions. Comparisons with other, well accepted, methodologies are also considered for problems for which exact solutions do not exist. Effects of several parameters, including spatial and temporal refinement, are also examined and the relative efficiency of each scheme is discussed. By considering test cases previously considered by other researchers, it is also demonstrated that the algorithms do not exhibit spurious solutions. Finally, three-dimensional results are compared with test results for a rectangular waveguide for which experimental data has been obtained with the explicit purpose of code-validation. The ability to predict changes in scattering parameters caused by variations in geometric and material properties are examined and it is demonstrated that the algorithms predict these changes with good accuracy. © 2011 Elsevier Inc.


Ji L.,SimCenter National Center for Computational Engineering | Wilson R.,SimCenter National Center for Computational Engineering | Sreenivas K.,SimCenter National Center for Computational Engineering | Hyams D.,SimCenter National Center for Computational Engineering
28th AIAA Applied Aerodynamics Conference | Year: 2010

Many approaches for moving and deforming mesh have been developed, but the approach adopted often depends on both the meshing scheme and the proposed application. Approaches based on a spring analogy with linear torsional springs or solution of partial differential equations have been used, but are generally very expensive to solve at each time step and are not trivial to parallelize. Here, a universal approach to grid motion known as the algebraic interpolation method (AIM) is followed to manage deforming surfaces. This method is universal and applicable to any grid type. Also, it is perfectly suitable to a parallel platform and can be implemented efficiently. The original scheme has some difficulty handling two-node bending mesh deformation involved in various fluid-structure interaction problems and other cases in which mesh deformation is driven solely by the surface motion. Several modifications have been made for these applications. It is determined that the grid quality can be improved significantly by adding a smoothing algorithm. Extra connectivities can also help improve the grid quality. The current scheme is applied to several well known synthetic jet applications from a NASA Langley Workshop for validation. Results are presented for the mesh deformation of NACA0012 airfoil and Suboff body. Free surface evolution of S175 container ship is also included along with its two-node bending mesh deformation. © 2010 by the American Institute of Aeronautics and Astronautics, Inc.


Pankajakshan R.,SimCenter National Center for Computational Engineering | Mitchell B.J.,SAIC | Taylor L.K.,SimCenter National Center for Computational Engineering
Computers and Fluids | Year: 2011

The present study details the implementation of a time accurate method for the tracking of particles being acted upon by a continuous gas phase and gravity. The Lagrangian particle tracking approach was implemented within the framework of a parallel, incompressible, unstructured, node-centered finite-volume flow solver. The paper gives a method for selecting time steps for individual particles such that interactions with the continuum phase are updated at particle locations nearest the continuum-phase nodes while constraining the particle from passing beyond boundaries of the relevant adjacent cell. An implementation of this technique for three-dimensional nonuniform multi-element unstructured grids is given in the context of domain decomposition for implementation on distributed-memory parallel computers. Results of simulations with and without particle-particle collisions compare favorably with experimental validation results. © 2010 Elsevier Ltd.


Stephen D.S.,SimCenter National Center for Computational Engineering
41st AIAA Fluid Dynamics Conference and Exhibit | Year: 2011

Although modern turbulence models have been greatly improved in recent years by advances in sub-scale techniques, little has been done to address the interaction between turbulence modeling and aerodynamic shocks. Consequently, modern turbulence models are hampered by inadequate source terms and eddy viscosities that work well on a case by case basis. The objective of this study is to demonstrate straightforward modifications to the blended kεkω turbulence model that result in vastly improved computations for aerodynamic shocks. A flapped nozzle is chosen as the primary test case due to the complexities of the three dimensional flow field over a wide range of flow conditions. © 2011 by D. Stephen Nichols, SimCenter.

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