Laith Zori

Unsteady CFD Methods in a Commercial Solver for Turbomachinery Applications

Modern CFD flow solvers can be readily used to obtain time-averaged results on industrial size turbomachinery flow problem at low computational cost and overall effort. On the other hand, time-accurate computations are still expensive and require substantial resources in CPU and computer memory. However, numerical techniques such as phase shift and time inclining method can be used to reduce overall computational cost and memory requirements. The unsteady effects of moving wakes, tip vortices and upstream propagation of shock waves in the front stages of multi-stage compressors are crucial to determine the stability and efficiency of gas turbines at part-load conditions. Accurate predictions of efficiency and aerodynamic stability of turbomachinery stages with strong blade row interaction based on transient CFD simulations are therefore of increasing importance today. The T106D turbine profile is under investigation as well as the transonic compressor test rig at Purdue. The main objective of this paper is to contribute to the understanding of unsteady flow phenomena that can lead to the next generation design of turbomachinery blading. Transient results obtained from simulations utilizing shape correction (phase shift) and time inclining methods in an implicit pressure-based solver, are compared with those of a full transient model in terms of computational cost and accuracy. NOMENCLATURE Acronyms DP

Investigation of Transient CFD Methods Applied to a Transonic Compressor Stage

Understanding unsteady flow phenomena in compressor stages often requires the use of time-accurate CFD simulations. Due to the inherent differences in blade pitch between adjacent blade rows, the flow conditions at any given instant in adjacent blade rows differ. Simplified computation of the stage represented by a single blade in each row and simple periodic boundary conditions is therefore not possible. Depending on the blade counts, it may be necessary to model the entire annulus of the stage; however, this requires considerable computational time and memory resources. Several methods for modeling the transient flow in turbomachinery stages which require a minimal number of blade passages per row, and therefore reduced computational demands, have been presented in the literature. Recently, some of these methods have become available in commercial CFD solvers. This paper provides a brief description of the methods used, and how they are applied to a transonic compressor stage. The methods are evaluated and compared in terms of computational efficiency and storage requirements, and comparison is made to steady stage simulations. Comparisons to overall performance data and two-dimensional LDV measurements are used to assess the predictive capabilities of the methods. Computed flow features are examined, and compared with reported measurements.© 2011 ASME

A Comparison of Advanced Numerical Techniques to Model Transient Flow in Turbomachinery Blade Rows

Computational predictions of the transient flow in multiple blade row turbomachinery configurations are considered. For cases with unequal numbers of blades/vanes in adjacent rows (“unequal pitch”) a computation over multiple passages is required to ensure that simple periodic boundary conditions can be applied. For typical geometries, a time accurate solution requires computation over a significant portion of the wheel. A number of methods are now available that address the issue of unequal pitch while significantly reducing the required computation time. Considered here are a family of related methods (“Transformation Methods”) which transform the equations, the solution or the boundary conditions in a manner that appropriately recognizes the periodicity of the flow, yet do not require solution of all or a large number of the blades in a given row. This paper will concentrate on comparing and contrasting these numerical treatments. The first method, known as “Profile Transformation”, overcomes the unequal pitch problem by simply scaling the flow profile that is communicated between neighboring blade rows, yet maintains the correct blade geometry and pitch ratio. The next method, known as the “Fourier Transformation” method applies phase shifted boundary conditions. To avoid storing the time history on the periodic boundary, a Fourier series method is used to store information at the blade passing frequency (BPF) and its harmonics. In the final method, a pitch-wise time transformation is performed that ensures that the boundary is truly periodic in the transformed space. This method is referred to as “Time Transformation”. The three methods have recently been added to a commercially-available CFD solver which is pressure based and implicit in formulation. The results are compared and contrasted on two turbine cases of engineering significance: a high pressure power turbine stage and a low pressure aircraft engine turbine stage. The relative convergence rates and solution times are examined together with the effect of non blade passing frequencies in the flow field. Transient solution times are compared with more conventional steady stage analyses, and in addition detailed flow physics such as boundary layer transition location are examined and reported.Copyright

Efficient Time Resolved Multistage CFD Analysis Applied to Axial Compressors

Unsteady computations are necessary if blade row interactions effects are relevant, for example for detailed optimization of a compressor at off-design conditions towards the aerodynamic stability limit, or for structural mechanical tuning of the blades. Modeling time accurate transient multistage flow is expensive both in terms of computer time and memory. Recently the implicit time-resolved Time Transformation method (based on Giles’ time inclining) has been shown to be computationally efficient and a good alternative for modeling transient flow in a single stage (one pitch ratio) turbomachinery configuration. A further advantage of this time resolved method is its ability to capture not only blade passing frequencies but also self-excited frequencies such as in wakes and tip vortex shedding.In this work, an extension of the Time Transformation method (TT) to multistage modeling has been employed to assess the method’s ability in predicting modern multistage compressor performance speedline curve, as well as its ability in capturing dominant machine frequencies. The multistage TT method is verified on a two and a half stage modified Hannover compressor, followed by an industrial validation on a Siemens Energy half scale six stage axial compressor based on the last stages of the Siemens Platform Compressor (PCO). Reference transient solutions on reduced portions of the compressor and/or modified blade count solutions are obtained and compared directly to single passage multistage Time Transformation predictions for the Hannover compressor. The method is then applied directly to the full six stage Siemens compressor employing the true blade counts for all six stages.The first goal of this work is to investigate the ability and accuracy of the multistage TT method to capture all relevant blades passing frequencies, including the impact of different degrees of pitch change between components. The second goal of this work is to explore how best to apply the method for the prediction of a compressor map, up to the surge line. Solutions are compared to experimental test rig data. Physical explanations of the key flow features observed in the experiment, as well as of the differences between the predictions and experimental data, are given.Copyright

Efficient Computation of Large Pitch Ratio Transonic Flow in a Fan With Inlet Distortion

Modeling the unsteady flow of a fan subject to an inlet distortion is computationally expensive due to the need to model the full-annulus. Using the Fourier Transformation (FT) method in ANSYS CFX, which recognizes phase-shifted periodic boundary conditions, the fan inlet distortion simulation can be achieved efficiently by solving just two passages. The FT method can handle very large inlet distortion to blade passage pitch ratios such as the case of the problem simulated in this work. The analysis considers transonic flow through a fan with high bypass ratio subjected to an inlet total pressure distortion. The inlet disturbance traverses the inlet once per revolution and is intended to simulate the inlet flow distortion seen by an aircraft engine fan during take-off conditions. The pressure ratio across the fan is chosen so that the fan moves from a started to un-started condition as the disturbance moves past the inlet. This condition will provide a rigorous test of the FT method. The FT method is validated by comparing to the equivalent full-annulus unsteady solution. The FT unsteady solution compares remarkably well with the reference solution and is able to reproduce the detailed dynamics of the shock movement. Moreover, the solution from the FT method is also able to reproduce the efficiency, viscous effects and blade loading from the full-annulus case. The FT solution is obtained with a 5X reduction in CPU time and a 10X reduction in memory requirement.

Investigation of Time/Frequency Domain CFD Methods to Predict Turbomachinery Blade Aerodynamic Damping

Accurate and efficient prediction of blade aerodynamic damping is critical for the design of turbomachines such as gas and steam turbines. Traditional unsteady time-marching CFD methods used in aerodynamic damping calculations are expensive because they require simulation of many or all blade passages in a given blade-row. In order to reduce computational cost, one can use a pitch-change method and reduce the problem to a small sector of the geometry (one or two blades). Even still, the time-marching method is expensive as many vibration cycles must be simulated to reach a quasi-steady periodic state. To further reduce computational cost, a time/frequency solution method is required.This paper uses an implicit pressure-based time/frequency solution method in combination with a Fourier transformation (FT) pitch-change method and validates its implementation in ANSYS CFX solver. Three cases are investigated, including Standard Configuration 11 (subsonic and transonic), and NASA Rotor 67 transonic fan. Predictions of unsteady pressure coefficient are compared against the experimental data and reference full wheel simulations, over a range of nodal diameters. Computational resources (CPU time) required by the time/frequency method are compared to time transient simulations and discussed in detail.© 2016 ASME

Time Transformation Simulation of 1.5 Stage Transonic Compressor

The accurate prediction of the aerodynamic and aeromechanical performance in a modern transonic compressor often exceeds the capability of traditional steady state mixing plane simulation methods. Time accurate transient blade row simulation approaches are required when there is a close coupling of the flow between the blade rows, and for fundamentally transient flow phenomena such as aeromechanical analysis including blade flutter and forced response, aerothermodynamic analysis and aero-acoustic analysis.Transient blade row simulations can be computationally impractical when all of the blade passages must be modeled to account for the unequal pitch between the blade rows. Most turbomachines consist of multiple stages, further exacerbating the computational challenge. In order to reduce the computational cost, time accurate pitch-change methods are utilized so that only a sector of the turbomachine (one or few passages per row) is modeled. The extension of the time-transformation pitch-change method to multistage machines has recently shown good promise in predicting both aerodynamic performance and resolving dominant blade passing frequencies for a subsonic compressor, while keeping the computational cost affordable.In this work, a modified one and a half stage Purdue transonic compressor (modified for unequal pitch for all three blade rows) is examined. The goal is to assess the ability of the multistage time-transformation method to accurately predict the aerodynamic performance and transient flow details in the presence of transonic blade row interactions. The results from the multistage time-transformation simulation are compared in detail with a transient full-wheel simulation, a profile transformation simulation, as well as to a steady-state mixing-plane model. Flow details are examined including an FFT analysis of select signals, and the onset of stall is compared between all methods. The relative computational effort is compared between all of the analysis methods.Copyright