Interaction In Turbomachinery Research Articles

Unsteady Flows and Component Interaction in Turbomachinery

Unsteady component interaction represents a crucial topic in turbomachinery design and analysis. Combustor/turbine interaction is one of the most widely studied topics both using experimental and numerical methods due to the risk of failure of high-pressure turbine blades by unexpected deviation of hot flow trajectory and local heat transfer characteristics. Compressor/combustor interaction is also of interest since it has been demonstrated that, under certain conditions, a non-uniform flow field feeds the primary zone of the combustor where the high-pressure compressor blade passing frequency can be clearly individuated. At the integral scale, the relative motion between vanes and blades in compressor and turbine stages governs the aerothermal performance of the gas turbine, especially in the presence of shocks. At the inertial scale, high turbulence levels generated in the combustion chamber govern wall heat transfer in the high-pressure turbine stage, and wakes generated by low-pressure turbine vanes interact with separation bubbles at low-Reynolds conditions by suppressing them. The necessity to correctly analyze these phenomena obliges the scientific community, the industry, and public funding bodies to cooperate and continuously build new test rigs equipped with highly accurate instrumentation to account for real machine effects. In computational fluid dynamics, researchers developed fast and reliable methods to analyze unsteady blade-row interaction in the case of uneven blade count conditions as well as component interaction by using different closures for turbulence in each domain using high-performance computing. This research effort results in countless publications that contribute to unveiling the actual behavior of turbomachinery flow. However, the great number of publications also results in fragmented information that risks being useless in a practical situation. Therefore, it is useful to collect the most relevant outcomes and derive general conclusions that may help the design of next-gen turbomachines. In fact, the necessity to meet the emission limits defined by the Paris agreement in 2015 obliges the turbomachinery community to consider revolutionary cycles in which component interaction plays a crucial role. In the present paper, the authors try to summarize almost 40 years of experimental and numerical research in the component interaction field, aiming at both providing a comprehensive overview and defining the most relevant conclusions obtained in this demanding research field.

Discrete Optimization on Unsteady Pressure Fluctuation of a Centrifugal Pump Using ANN and Modified GA

Pressure fluctuation due to rotor-stator interaction in turbomachinery is unavoidable, inducing strong vibration in the equipment and shortening its lifecycle. The investigation of optimization methods for an industrial centrifugal pump was carried out to reduce the intensity of pressure fluctuation to extend the lifecycle of these devices. Considering the time-consuming transient simulation of unsteady pressure, a novel optimization strategy was proposed by discretizing design variables and genetic algorithm. Four highly related design parameters were chosen, and 40 transient sample cases were generated and simulated using an automatic program. 70% of them were used for training the surrogate model, and the others were for verifying the accuracy of the surrogate model. Furthermore, a modified discrete genetic algorithm (MDGA) was proposed to reduce the optimization cost owing to transient numerical simulation. For the benchmark test, the proposed MDGA showed a great advantage over the original genetic algorithm regarding searching speed and effectively dealt with the discrete variables by dramatically increasing the convergence rate. After optimization, the performance and stability of the inline pump were improved. The efficiency increased by more than 2.2%, and the pressure fluctuation intensity decreased by more than 20% under design condition. This research proposed an optimization method for reducing discrete transient characteristics in centrifugal pumps.

The noise-generating mechanism of rod-airfoil configuration and the effect of spanwise length on noise prediction

The wake-blade interaction in turbo-engines is the primary source of broadband noise. In this work, the noise mechanism of a benchmark rod-airfoil configuration is exhaustively analyzed using compressible Large Eddy Simulation (LES) and Ffowcs Williams-Hawkings (FW-H) methods. The baseline model with a spanwise length shortened by 7d (d is the rod diameter) is simulated under periodic boundary conditions, and the effect of spanwise lengths on noise prediction and correlation is investigated in combination with two other models (3d and 1d). In terms of the baseline model, the noise predicted by different FW-H surfaces is compared first, and the noise radiated from different chordwise and spanwise regions of the airfoil surface reveal the generating mechanism of the main dipolar noise and the effect of periodic boundary conditions on noise prediction, respectively. Additionally, the fluctuating surface pressure characterized at four frequency bands confirms that the wake-airfoil interaction dominates the leading-edge noise at low frequencies. The characteristic frequencies of the noise and transient velocity fields reveal that the primary broadband noise source is the interaction of large-scale vortices with the airfoil surface at the leading-edge region. Finally, the effect of spanwise length on noise prediction and correlation is studied for three models. The differences in the flow structures lead to different features of their coherence coefficients. The results presented in this work provide a comprehensive understanding of the noise-generating mechanism of wake-airfoil interaction in turbomachinery and the practical application of simplified models in the future numerical simulations.

Simulation of Indexing and Clocking with Harmonic Balance

The aim of this paper is to demonstrate how the harmonic balance method can be used to predict rotor–rotor and stator–stator interactions in turbomachinery. These interactions occur in the form of clocking and indexing. Whereas clocking refers to the dependency of the performance on the relative circumferential positioning of the rotors or stators, the term indexing is used when different blade (or vane) counts lead to an aperiodic time-averaged flow. The approach developed here is closely related to the one presented by He, Chen, Wells, Li, and Ning, who generalised the Nonlinear Harmonic method to zero-frequency disturbances. In particular, configurations with only one passage per blade row are used for the simulations. We validate the methods by means of the simulation of a fan stage configuration with rotationally asymmetric inlet conditions. It is demonstrated that the harmonic balance solver is able to accurately predict the inhomogeneity of the time-averaged flow field in the stator row. Moreover, the results show that the approach offers a considerable gain in computational efficiency.

Blade dynamical response based on aeroelastic analysis of fluid structure interaction in turbomachinery

The flow around airfoils being an energy tank; instabilities are then possible, leading often to dangerous vibratory levels. In order to predict and control aeroelastic instabilities, this investigation aims to provide a detailed understanding of the flow interaction within the complex geometry of moving blades cascade. Among the working parameters, we consider the effect of the pressure rate on the blades aeroelastic instabilities under an unsteady compressible flow. We investigate the consequences of the airfoil motion onto the aerodynamic performances, the energy transfer between the flow and the structure and the aerodynamic stability. The nonlinear aeroelastic model is based on a dynamic fluid code, a structure code and a weak coupling algorithm. Numerical simulations are conducted in three airfoil situations: fixed, pitching and free motions. The simulation results show that the pressure rate has an effect on the blade aeroelastic instabilities, their amplitude and their type: flutter and limit cycle oscillations (LCOs). The oscillation amplitude and the energy transferred to the airfoil increase more rapidly as the pressure rate increases. These instabilities have significant effects on the aerodynamic loads. Analysing the energy exchange and the aerodynamic work, we note that the aerodynamic stability is very sensible to the pressure ratio.

Interactions of Separation Bubble With Oncoming Wakes by Large-Eddy Simulation

The unsteady flow physics and heat transfer characteristics due to interactions of periodic passing wakes with a separated boundary layer are studied using large-eddy simulation (LES). A series of airfoils of constant thickness with rounded leading edge are employed to obtain the separated boundary layer. Wake data extracted from precursor LES of flow past a cylinder are used to replicate a moving bar that generates wakes in front of a cascade (in this case, an infinite row of the model airfoils). This setup is a simplified representation of the rotor–stator interaction in turbomachinery. With a uniform inlet, the laminar boundary layer separates near the leading edge, undergoes transition due to amplification of disturbances, becomes turbulent, and finally reattaches forming a separation bubble. In the presence of oncoming wakes, the characteristics of the separated boundary layer have changed and the impinging wakes are found to be the mechanism affecting the reattachment. Phase-averaged results illustrate the periodic behavior of both flow and heat transfer. Large undulations in the phase-averaged skin friction and Nusselt number distributions can be attributed to the excitation of the boundary layer by convective wakes forming coherent vortices, which are being shed and convect downstream. Further, the transition of the separated boundary layer during the wake-induced path is governed by a mechanism that involves the convection of these vortices followed by increased fluctuations, where viscous effect is substantial.

The Influence of Structural Parameters on the Stability of Blade-Casing Interactions in Turbomachinery

Blade-casing interactions are common in modern turbomachinery. They occur due to small tip clearances between the blades and the casing, which are designed to be minimal for efficiency reasons. Rubbing between the two components may cause severe vibrations, ultimately resulting in structural failure. Recently an analogy between this problem and the machine tool chatter has been proposed. It results in a representation where the dynamics are governed by linear time invariant delay-differential equations. The stability analysis of such systems is complex and clearly of critical importance. Stability characteristics are causally related to the system parameters. Following some earlier results, the present article reveals the relation between these parameters and the stability characteristics of the system. The results are demonstrated on a case study.

Detection of Rotor Forced Response Vibrations Using Stationary Pressure Transducers in a Multistage Axial Compressor

Blade row interactions in turbomachinery can lead to blade vibrations and even high cycle fatigue. Forced response conditions occur when a forcing function (such as impingement of stator wakes) occurs at a frequency that matches the natural frequency of a blade. The objective of this research is to develop the data processing techniques needed to detect rotor blade vibration in a forced response condition from stationary fast-response pressure transducers to allow for detection of rotor vibration from transient data and lead to techniques for vibration monitoring in gas turbines. This paper marks the first time in the open literature that engine-order resonant response of an embedded bladed disk in a 3-stage intermediate-speed axial compressor was detected using stationary pressure transducers. Experiments were performed in a stage axial research compressor focusing on the embedded rotor of blisk construction. Fourier waterfall graphs from a laser tip timing system were used to detect the vibrations after applying signal processing methods to uncover these pressure waves associated with blade vibration. Individual blade response was investigated using cross covariance to compare blade passage pressure signatures through resonance. Both methods agree with NSMS data that provide a measure of the exact compressor speeds at which individual blades enter resonance.

Fluid Structure Interaction for Hydraulic Problems

Fluid Structure interaction plays an important role in engineering applications. Physical phenomena such as flow induced vibration in nuclear industry, fuel sloshing tank in automotive industry or rotor stator interaction in turbo machinery, can lead to structure deformation and sometimes to failure.In order to solve fluid structure interaction problems, the majority of numerical tests consists in using two different codes to separately solve pressure of the fluid and structural displacements. In this paper, a unique code with an ALE formulation approach is used to implicitly calculate the pressure of an incompressible fluid applied to the structure. The development of the ALE method as well as the coupling in a computational structural dynamic code, allows to solve more large industrial problems related to fluid structure coupling.

Numerical Prediction of Sound Generated from Flows with a Low Mach Number

Prediction and reduction of sound generated from a flow are becoming increasingly important in various fields of engineering. This is mainly due to the fact that the magnitude of such sound rapidly increases with increasing flow velocity. In fact it grows with 5th to 8th power of the flow velocity.This paper describes methods to predict sound that is generated from a relatively low-speed flow often times encountered in mechanical engineering, such as flows around propeller fans, high-speed trains and automobiles, and demonstrates their capability at present. It is essentially the unsteady motions of vortices in flow that generate sound. Prediction of sound is thus possible if source fluctuations in flow are adequately computed. Furthermore, because the feedback effects of sound onto the source flow field are in general negligible for such sound that is generated from low-speed flows, it is possible to predict sound by separate computations of the source flow field and resulting acousitical field. Source fluctuations in the flow are computed by a large-eddy simulation (LES) with Dynamic Smagorinsky Model (DSM) and they are fed to the following acoustical computation as input data. Applications of this method will be presented for various classes of flow-induced sound, including bluff body sound, sound resulting from rotor-stator interactions in turbomachinery and sound generated from a transitional/turbulent boundary layer on an aerofoil. Finally, direct Navier-Stokes computations of sound will be briefly described and their possible applications to the prediction of sound generated from low-speed flows will be discussed with some examples.

Unsteady Fluid Structure Interaction in Turbomachinery

Unsteady Fluid Structure Interaction in Turbomachinery

Coupled simulation of flow-structure interaction in turbomachinery

The aeroelastic behaviour of vibrating blade assemblies is usually investigated in the frequency domain where the determination of aeroelastic stability boundaries is separated from the computation of linearized unsteady aerodynamic forces. However, nonlinear fluid-structure interaction caused by oscillating shocks or strong flow separation may significantly influence the aerodynamic damping and hence effect a shift of stability boundaries. In order to investigate such aeroelastic phenomena, the governing equations of structural and fluid motion have to be simultaneously integrated in time. In this paper a technique is presented which analyses the flutter behaviour of turbomachinery bladings in the time domain. The structural part of the governing aeroelastic equations is time-integrated according to the algorithm of Newmark, while the unsteady airloads are computed at every time step by a Navier–Stokes code. The link between the two time integrations is an automatic grid generation in which the used mesh is dynamically deformed so that it conforms with the deflected blades at every time step. The computed time series of the aeroelastic simulation of an assembly of highly loaded compressor blades vibrating freely in transonic flow are presented. The energy transfer between fluid and structure is here dominated by vibrating shocks and shock-boundary layer interaction. It is investigated if the predicted aeroelastic stability boundaries differ from those of a linearised method.

Characteristic Regimes of Transitional Separation Bubbles in Unsteady Flow

This experimental investigation deals with transition phenomena of a separated boundary layer under unsteady inlet flow conditions. The main purpose of this investigation is to understand the influence of the rotor-stator interaction in turbomachinery on the subsequent, highly loaded boundary layer. The research project is divided into two phases. In the first phase, which has been completed recently, only the variation of mean velocity caused by upstream blades was simulated in the experiments while the free-stream turbulence intensity was retained at a constant low level. The experiments are carried out in an Eifel-type wind tunnel to investigate the laminar separated boundary layer of a flat plate under oscillating inlet conditions. The adverse pressure gradient, similar to that of turbomachines, is generated by the contoured upper wall. The unsteadiness is produced by a rotating flap located downstream of the test section. The reduced frequency, the amplitude and the mean Reynolds number are varied to simulate the conditions prevailing in turbomachines. In addition to the Kelvin–Helmholtz instability of the separated shear layer, a lower frequency instability was observed. This is frequently referred to as `free shear layer flapping' and results in two distinctly different ways of re-attachment, depending primarily on the Reynolds number. For low momentum thickness Reynolds numbers at the separation point, large-scale vortices locked to the frequency of the unsteady main flow are identified. They originate nearly at the top of the separation bubble and are ejected downstream. A fully turbulent boundary layer develops after these vortices mix out. For higher Reynolds numbers, transition is completed within a short length of the free shear layer and there-attachment region. The characteristic momentum thickness Reynolds number separating these two regimes in unsteady flow is about 125. The Strouhal number (reduced frequency) does not appear to have any significant effect. Based on the experimental results, this behaviour is discussed in some detail.

Three-Dimensional Disturbance Vortex Method for Simulating Rotor/Stator Interaction in Turbomachinery

References 1Schetz, J. A., Billig, F. S., and Favin, S., “Analysis of External Burning on Inclined Surfaces in SupersonicFlow,” Journalof Propulsionand Power, Vol. 10, No. 5, 1994, pp. 602–608. 2Kahaner, D., Moler, C., and Nash, S.,Numerical Methods and Software, Prentice–Hall, Englewood Cliffs, NJ, 1988, Chap. 8. 3Colasurdo,G., and Pastrone, D., “Indirect Optimization Method for Impulsive Transfers,” AIAA/AAS Astrodynamic Conference, AIAA, Washington, DC, 1994, pp. 441–448; also AIAA Paper 94-3762, 1994.

Vortex Simulation of Rotor/Stator Interaction in Turbomachinery

A new numerical method is presented in this paper to simulate rotor/stator interaction in turbomachinery by means of a vortex method based on a Lagrangian frame. The algorithm takes the result from steady solution as input, which gives an initial description of the unsteady disturbance flow field. To calculate the unsteady response to these disturbances, the Lagrangian vortex method is used to capture the convective process, and the deterministic vortex scheme to approximate the viscous diffusion process. The application of Baldwin–Lomax turbulence model in wakes is developed, to overcome difficulties such as the much higher calculated viscosity in the outer region than that in the boundary regions, and the difficulty in continuously tracing moving wake centerlines encountered when other numerical methods are used. Generally the agreement between the computational and experimental results is good. The sweeping characteristic of wakes, the influence of unsteadiness on incidence, and the decaying features of unsteady velocities and pressure are studied in the paper.

Numerical Prediction of Unsteady Transitional Boundary Layer Caused by Rotor-Stator Interaction

The objective of the research reported in this paper was to develop computational techniques for the prediction of transitional flows associated with the rotor-stator interaction in turbomachinery. Three lowReynolds-number turbulence models were incorporated in the Navier- Stokes code and evaluated for accuracy in predicting the onset and end of unsteady transitional patches caused by wake passing. The best model was then used for the modification and improvement of the leading-edge effect. A comparison with the experimental data indicated that the quality of prediction was as good as those derived from an unsteady boundary-layer code. A comparison of the instantaneous shape factor, the skin friction coefficient, and the momentum thickness indicated that the Navier-Stokes predictions were reasonably good and should aid the designers of turbomachiner y in allowing for the unsteady transitional flows caused by rotor-stator interaction. The simulation results were interpreted to derive new insight on the flow physics associated with transitional flow including turbulence properties.

Computational Study of Stall Flutter in Linear Cascades

Aeroelastic interaction in turbomachinery is of prime interest to opertors, designers, and aeroelasticans. Operation at off-design conditions may promote blade stall; eventually the stall pattern will propagate around the blade annulus. The unsteady periodic nature of propagating stall will force blade vibration and blade flutter may occur if the stall propagation frequency is entrained by the blade natural frequency. In this work a computational scheme based on the vortex method is used to simulate the flow over a linear cascade of airfoils. The viscous effect is confined to a thin layer, which determines the separation points on the airfoil surfaces. The preliminary structural model is a two-dimensional characteristic section with a single degree of freedom in either bending or torsion. A study of the relationship between the stall propagation frequency and the blade natural frequency has been conducted. The study shows that entrainment, or frequency synchronization, occurs, resulting in pure torsional flutter over a certain interval of reduced frequency. A severe blade torsional amplitude (of order 20 deg) has been computed in the entrainment region, reaching its largest value in the center of the interval. However, in practice, compressor blades will not sustain this vibration and blade failure may occur before reaching such a large amplitude. Outside the entrainment interval the stall propagation is shown to be independent of the blade natural frequency. In addition, computational results show that there is no entrainment in the pure bending mode. Rather, “de-entrainment” occurs with similar flow conditions and similar stall frequencies, resulting in blade buffeting in pure bending.

A moving-grid finite element algorithm for supersonic flow interaction between moving bodies

The present study describes a finite element scheme which provides for the computer simulation of the motion of one finite element mesh relative to another along a smooth interface through which flow occurs. The scheme is designed to simulate transient flow interaction phenomena encountered in situations in which two or more solid bodies move relative to one another through an arbitrary flow field. Particular applications include simulations of rotor-stator flow interactions in turbomachinery. Although the method is applicable to any type of flow interaction problem, the applications considered here focus on the modeling of supersonic flow interactions characterized by the time-dependent Euler equations, and on problems of pure advection of a scalar field, all in two-dimensional flow domains. Several numerical examples are given to demonstrate the robustness and the performance of the numerical scheme.

The Unsteady Wake Interaction in Turbomachinery and Its Effect on Cavitation

This paper attempts first to predict theoretically the influence on the nonsteady pressure perturbations on a blade row as a result of periodic wakes shed from an upstream blade row, and then to compare such theoretical predictions with a series of carefully conducted cavitation experiments at the Garfield Thomas Water Tunnel of the Ordnance Research Laboratory. The results indicate that the experimental values of pressure perturbations are somewhat lower than theoretically predicted values. The general trend, e.g., the influence of wake width and wake spacing, is in substantial agreement between theory and experiments.

Closure to “Discussion of ‘The Unsteady Wake Interaction in Turbomachinery and Its Effect on Cavitation’” (1959, ASME J. Basic Eng., 81, p. 189)

Closure to “Discussion of ‘The Unsteady Wake Interaction in Turbomachinery and Its Effect on Cavitation’” (1959, ASME J. Basic Eng., 81, p. 189)