Development of an Unsteady Aeroelastic Solver for the Analysis of Modern Turbomachinery Designs

Developers of aircraft gas turbine engines continually strive for greater efficiency and higher thrust-to-weight ratio designs. To meet these goals, advanced designs generally feature thin, low aspect airfoils, which offer increased performance but are highly susceptible to flow-induced vibrations. As a result, High Cycle Fatigue (HCF) has become a universal problem throughout the gas turbine industry and unsteady aeroelastic computational models are needed to predict and prevent these problems in modern turbomachinery designs. This research presents the development of a 3D unsteady aeroelastic solver for turbomachinery applications. To accomplish this, a well established turbomachinery Computational Fluid Dynamics (CFD) code called Corsair is loosely coupled to the commercial Computational Structural Solver (CSD) Ansys® through the use of a Fluid Structure Interaction (FSI) module. Significant modifications are made to Corsair to handle the integration of the FSI module and improve overall performance. To properly account for fluid grid deformations dictated by the FSI module, temporal based coordinate transformation metrics are incorporated into Corsair. Wall functions with user specified surface roughness are also added to reduce fluid grid density requirements near solid surfaces. To increase overall performance and ease of future modifications to the source code, Corsair is rewritten in Fortran 90 with an emphasis on reducing memory usage and improving source code readability and structure. As part of this effort, the shared memory data structure of Corsair is replaced with a distributed model. Domain decomposition of individual grids in the radial direction is also incorporated into Corsair for additional parallelization, along with a utility to automate this process in an optimal manner based on user input. This additional parallelization helps offset the inability to use the fine grain mp-threads parallelization in the original code on non-distributed memory architectures such as the PC Beowulf cluster used for this research. Conversion routines and utilities are created to handle differences in grid formats between Corsair and the FSI module. The resulting aeroelastic solver is tested using two simplified configurations. First, the well understood case of a flexible cylinder in cross flow is studied with the natural frequency of the cylinder set to the shedding frequency of the Von Karman streets. The cylinder is self excited and thus demonstrates the correct exchange of energy between the fluid and structural models. The second test case is based on the fourth standard configuration and demonstrates the ability of the solver to predict the dominant vibrational modes of an aeroelastic turbomachinery blade. For this case, a single blade from the fourth standard configuration is subjected to a step function from zero loading to the converged flow solution loading in order to excite the structural modes of the blade. These modes are then compared to those obtained from an in vacuo Ansys® analysis with good agreement between the two

Investigation of Advanced Counterrotation Blade Configuration Concepts for High Speed Turboprop Systems. Task 8: Cooling Flow/heat Transfer Analysis User's Manual

The focus of this task was to validate the ADPAC code for heat transfer calculations. To accomplish this goal, the ADPAC code was modified to allow for a Cartesian coordinate system capability and to add boundary conditions to handle spanwise periodicity and transpiration boundaries. This user's manual describes how to use the ADPAC code as developed in Task 5, NAS3-25270, including the modifications made to date in Tasks 7 and 8, NAS3-25270

A Time-linearized Navier-Stokes Solver for Annular Gas Seal Rotordynamic Analysis

A time-linearized CFD solver for analyzing rotordynamics of gas seals is presented offering an improvement over existing linearized CFD solvers. Previous linearized solvers required structured grids and axisymmetric domains, limiting the complexity of the geometries of the seals that could be analyzed. A preexisting, full-order, in-house CFD solver was available which operated on fully 3D and unstructured grids and was well suited for complex seal geometries. A linearized version of the in-house code is developed as a companion to the full-order solver, retaining its unstructured and fully 3D features. Furthermore, boundary conditions are developed for the linearized solver allowing it to take advantage of the geometric symmetries that were required by earlier linearized solvers without necessarily being limited to them. Additionally, a linearization procedure is presented which is general enough to be used for the many various features of the full-order solver. As the in-house code continues to be developed and new features are included, the same linearization procedure can be used to keep the companion code up to date. The full-order, in-house solver and the time-linearized companion code combine to become a powerful CFD-perturbation solver accessible to all complexities of seal geometries. This dissertation also presents an analytical formula that describes features of cavity flow as it pertains to annular gas seals in order to progress the fundamental understanding of the flow physics of roughened seals. An existing semi-empirical analytical formula, developed to describe the cavity flow of aircraft bomb bays, is modified using the full-order, in-house CFD solver. The numeric model is validated against experimental measurements and used to adjust empirical parameters of the formula to match cavity flow conditions unique to annular seals. The modified analytical formula is able to predict features of cavity flows found in annular gas seals better than existing formulae. Finally, the companion, time-linearized CFD solver is verified using two simple cases and the combined full-order and time-linearized CFD-perturbation solver is used to predict rotordynamic properties for two gas seal geometries. The first case used to verify the linearized solver is a channel flow with an oscillating back-pressure and the second is a stationary flow with an oscillating wall. The first gas seal case the combined CFD-perturbation solver is used for is a straight seal based on the High Pressure Oxidizer Turbopump (HPOTP) of the Space Shuttle Main Engine (SSME). The second is a stepped labyrinth seal. The rotordynamic predictions are compared with established bulk-flow models of the two cases and conclusions are presented

Comparative assessment of the harmonic balance Navier Stokes technology for horizontal and vertical axis wind turbine aerodynamics

Several important wind turbine unsteady flow regimes, such as those associated with the yawed wind condition of horizontal axis machines, and most operating conditions of all vertical axis machines, are predominantly periodic. The harmonic balance Reynolds-averaged Navier-Stokes technology for the rapid calculation of nonlinear periodic flow fields has been successfully used to greatly reduce runtimes of turbomachinery periodic flow analyses in the past fifteen years. This paper presents an objective comparative study of the performance and solution accuracy of this technology for aerodynamic analysis and design applications of horizontal and vertical axis wind turbines. The considered use cases are the periodic flow past the blade section of a utility-scale horizontal axis wind turbine rotor in yawed wind, and the periodic flow of a H-Darrieus rotor section working at a tip-speed ratio close to that of maximum power. The aforementioned comparative assessment is based on thorough parametric time-domain and harmonic balance analyses of both use cases. The paper also reports the main mathematical and numerical features of a new turbulent harmonic balance Navier-Stokes solver using Menter’s shear stress transport model for the turbulence closure. Presented results indicate that a) typical multi-megawatt horizontal axis wind turbine periodic flows can be computed by the harmonic balance solver about ten times more rapidly than by the conventional time-domain analysis, achieving the same temporal accuracy of the latter method, and b) the harmonic balance acceleration for Darrieus rotor unsteady flow analysis is lower than for horizontal axis machines, and the harmonic balance solutions feature undesired oscillations caused by the wide harmonic content and the high-level of stall predisposition of this flow field type