Mina Zaki

Assessment of Rotor-Stator Interface Boundary Condition Techniques for Modeling Axial Flow Turbines

An existing 3-D Navier-Stokes analysis for modeling single stage compressor and turbine rotors has been modified to model multi-stage axial compressors and turbines. As part of this effort, several rotor-stator interface boundary conditions have been systematically evaluated. The first stage of the two stage fuel turbine on the space shuttle main engine (SSME) has been used to determine which of these boundary conditions conserve global properties such as mass, momentum, and energy across the interface, while yielding good performance predictions. All the methods gave satisfactory results and a characteristic based approach was found to work best.

A normal ray refinement technique for Cartesian-grid based Navier–Stokes solvers

In this work, a novel technique called normal ray refinement (NRR) is developed, implemented and investigated. Normal ray refinement is designed to allow for the viscous fluid flow simulations using an unstructured Cartesian grid framework in a computationally efficient manner. A key benefit of using a Cartesian grid method is that the grid can be automatically generated, thereby saving a vast amount of time and effort for complex geometries. The main drawback of using the Cartesian grid method is the large number of cells required to resolve viscous boundary layers, and it is this problem that the NRR approach addresses. The NRR approach relies on the use of refined normal rays of cells emanating from the body surface and spanning the boundary layer. Separating these rays along the body surface are relatively large cells too coarse to accurately capture viscous gradients. The heart of the NRR approach lies in the inter-ray communication strategies used between the normal rays that allow the accurate simulation of boundary layers even though the cells separating the rays are large. This yields a large reduction in the number of cells in the grid, which reduces the computational cost of simulation. This paper provides a background on different viscous Cartesian grid-based methods, followed by an explanation of the NRR approach, then some initial 2D results obtained using NRR for Reynolds numbers up to 1 million. It is shown that NRR can yield substantial reduction in computational cost relative to the standard Cartesian approach.

Hybrid Reynolds-Averaged Navier-Stokes/Kinetic-Eddy Simulation of Stall Inception in Axial Compressors

A hybrid Reynolds-averaged Navier-Stokes/kinetic-eddy simulation turbulence model is used for the stall predictions in a transonic axial compressor stage. This hybrid Reynolds-averaged Navier-Stokes/kinetic-eddy simulation model solves Menters k-ω-shear-stress-transport model near walls and switches to the kinetic-eddy simulation model away from walls. The kinetic-eddy simulation model solves directly for local turbulent kinetic energy and local turbulent length scales, thus alleviating the grid spacing dependency found in other detached-eddy simulation and hybrid Reynolds-averaged Navier-Stokes/large-eddy simulation models. The current methodology is used in the prediction of the performance map and the stall inception for the NASA stage 35 compressor configuration as a representative of a modern compressor stage. The present approach is found to satisfactorily predict the onset of stall. It is found that the rotor blade-tip leakage vortex and its interaction with the shock wave is the main reason behind the stall inception in this compressor stage.

A Hybrid RANS/KES Model for External and Internal Flow Applications

A hybrid RANS/KES (HRKES) turbulence model is proposed for external and internal flow applications. This HRKES model solves Menter’s k-ω-SST model near walls and switches to Kinetic Eddy Simulation (KES) model away from walls. The KES model solves directly for local turbulent kinetic energy and local turbulent length scales, thus alleviating the grid spacing dependency found in other DES and HRLES models. This closure is considered RANS near walls and VLES-LES away from walls. Within the HRKES model, four different options (a combination of two different blending functions and the use of realizability constraints to bound the KES model parameters) have been evaluated and studied for flows over a turbine vane configuration, airfoils (RAE2822, NACA0036), and a compressor configuration. While all four options showed good correlation with the test data, blending k-ω-SST with KES using the k-ω-SST F 2 function showed better predictions in separated flow regions than the baseline k-ω-SST model.

Evaluation of Viscous Flow Solvers with Adaptive Cartesian Meshes for Hypersonic Flows

The present work seeks to advance and document the ability of Cartesian grid based formulations to model hypersonic viscous flows. This capability is investigated in the adaptive Cartesian mesh solvers, NASCART-GT and UFS. The effectiveness of the immersed boundary ghost cell method in these solvers to model viscous effects in hypersonic nonreacting flow is investigated. In addition, the viscous, chemically reacting flow capability is developed, installed, and tested into the Cartesian framework in the NASCART-GT code. This thermochemical nonequilibrium methodology accounts for temperatures associated with the equilibrated translational-rotational modes and the vibrational-electronic modes. Viscous diffusion terms are added to the species and energy conservation equations, and collision cross-section based transport coefficients are implemented. Initial comparisons of skin-friction coefficients and surface heat transfer predictions in a non-reacting, viscous environment are conducted. In addition, comparisons of off-surface flow features in a reacting, viscous environment are performed.

Hybrid Reynolds-Averaged Navier-Stokes and Kinetic Eddy Simulation of External and Internal Flows

thus alleviating the grid-spacing dependency found in other detached eddy simulation and hybrid Reynoldsaveraged Navier–Stokes and large eddy simulation models. Within the hybrid Reynolds-averaged Navier–Stokes andkinetic eddy simulation model, fourdifferent options (a combination of two different blending functions andthe use of realizability constraints to bound the kinetic eddy simulation model parameters) have been evaluated and studied for flows over airfoils (RAE2822, NACA0015) and a turbine vane configuration. Although all four options showed good correlation with the test data, blendingk-!-SST with kinetic eddy simulation using Menter’sk-!-SST F2 function showed better predictions in separated flows than using the baseline k-!-SST model. OST current computational fluid dynamics (CFD) methods used in the simulation and analysis of external and internal flows are based on Reynolds-averaged Navier–Stokes (RANS) approaches, which depend on turbulence-closure models to provide flowfield turbulent variables. Although RANS approaches yield good predictions for attached flowfields, they fail to accurately predict flow structures in separated-flow regions because they resolve only a portion of the turbulence scales of interest. Of course, the ideal approach would be direct numerical simulation (DNS), because the entire range of spatial and temporal scales of turbulence is resolved, but the computational cost of DNS is prohibitive. Another intermediate technique between DNS and RANS has been proposedtoreplaceRANSinsuchcases;thisapproachiscalledlarge eddy simulation (LES). In LES, the contribution of large energy-containing structures and all scales larger than the grid resolution to momentum and energy transfer is computed, and the effect of subgrid unresolved small

CONSERVATION AND GRID ADAPTATION ENHANCMENTS TO A NORMAL RAY REFINEMENT TECHNIQUE FOR CARTESIAN-GRID BASED NAVIER-STOKES SOLVERS

The Normal Ray Refinement (NRR) technique allows for viscous fluid flow simulations using an unstructured Cartesian grid framework in a more computationally efficient manner. The NRR approach relies on the use of refined normal rays of cells emanating from the body surface and spanning the boundary layer. Separating these rays along the body surface are relatively large cells too coarse to accurately capture viscous gradients. Accuracy is maintained between the normal rays through use of an appropriate inter-ray communication technique. NRR yields a large reduction in the number of cells in the grid, thereby reducing the computational cost of simulation. Previous studies showed that the NRR has the capability to simulate viscous flows with boundary layers efficiently with far fewer cells than in a uniform Cartesian grid. In this paper, the NRR methodology is further developed and investigated. For improved efficiency, variable length NRR and adaptive length NRR capabilities have been implemented and validated for external flows. For improved accuracy, a conservation formulation have been added to the NRR technique and tested for internal and external flows.

Numerical Simulations of the Aerodynamics of Circulation Control Wind Turbines Under Yawed Flow Conditions

The aerodynamic performance of a wind turbine rotor equipped with circulation control technology is investigated using a three-dimensional unsteady viscous flow analysis. The National Renewable Energy Laboratory (NREL) Phase VI horizontal axis wind turbine (HAWT) is chosen as the baseline configuration. Experimental data for the baseline case is used to validate the flow solver, prior to its use in exploring these concepts. Steady and pulsed Coanda jet calculations have been performed for axial and yawed flows at several wind conditions. Results presented include radial distribution of the normal and tangential forces at selected radial locations, shaft torque, and root flap bending moments. At low wind speeds where the flow is fully attached, it is found that steady and pulsed Coanda jets at the trailing edge are both effective at increasing circulation resulting in an increase of lift and the chordwise thrust force. This leads to an increased amount of net power compared to the baseline configuration for moderate blowing coefficients. Preliminary calculations are also shown to demonstrate how Coanda jets may be used as jet spoilers to alleviate structural loads under extreme wind conditions.Copyright

Numerical Stall Inception Predictions for Axial Compressors Using a Hybrid RANS/KES Turbulence Model

A hybrid RANS/KES (HRKES) turbulence model is used for the stall predictions in a transonic axial compressor stage. This HRKES model solves Menter’s k-ω-SST model near walls and switches to Kinetic Eddy Simulation (KES) model away from walls. The KES model solves directly for local turbulent kinetic energy and local turbulent length scales, thus alleviating the grid spacing dependency found in other Detached Eddy Simulation (DES) and Hybrid RANS/LES (HRLES) models. The current methodology is used in the prediction of the performance map and the stall inception for the NASA Stage 35 compressor configuration as a representative of a modern compressor stage. The present approach is found to satisfactory predict the onset of stall. It is found that the rotor blade tip leakage vortex and its interaction with the shock wave is the main reason behind the stall inception in this compressor stage.