Downstream Blade Row Research Articles

Numerical investigation of secondary flows in a high-lift low pressure turbine

In turbomachines, secondary flows (or endwall flows) typically originate at the junction between endwalls and the blade surface. Within the blade passage, the strength of the secondary flows is amplified by the crossflow from the pressure to the suction surface of the blade. The enhanced mixing due to secondary flows induce additional losses into the system. This decreases the overall work output and also changes the flow incidence onto the downstream blade rows. Using a series of high-fidelity eddy resolving simulations, the current study attempts to provide an improved understanding for the complex flow physics over the endwalls of a high-lift Low Pressure Turbine (LPT) blade. The effect of three different inflow conditions has been studied. These include a laminar boundary layer (LBL), a turbulent boundary layer (TBL) and wakes with secondary flow (W&S) from an upstream blade row. For the simulations with TBL and W&S, precursor eddy resolving simulations were used to prescribe realistic inflows. The loss generation mechanisms were subsequently studied both at the endwall and the midspan, which includes evaluating the mass-averaged total pressure loss coefficient (Yp) and the loss generation rate.When compared to LBL, additional disturbances from an incoming TBL and wakes with secondary flows enhanced the mixing within the blade passage resulting in a substantial increase in the total pressure loss. Prior to flow transition, incoming wakes with secondary flows increased the local loss generation rate at both the endwall and the midspan in the front portion of the blade passage (x/Cx < 0.84). In contrast, in the aft portion of the passage (x/Cx > 0.8), the incoming wakes effectively suppressed the separation bubble at the midspan thereby decreasing the local loss generation rate. It is also demonstrated that the wakes shed from the trailing edge at the mid-span mix out rapidly when compared to the passage vortex at the endwall.

Multiple-Frequency Phase-Lagged Unsteady Simulations of Experimental Axial Compressor

This paper presents an evaluation of the multiple-frequency phase-lagged approach, which enables the performance of unsteady Reynolds-averaged Navier–Stokes simulations on multistage turbomachinery configurations using a time-marching method. The major advantage of this approach is to reduce the computational domain to one single blade passage per row. The first part of the paper presents the method and discusses the associated assumptions and limitations. The method is then evaluated on the axial compressor configuration “Compresseur de Recherche pour l’Etude des effets Aérodynamiques et TEchnologiques” investigated experimentally at Laboratory of Fluid Mechanics and Acoustics. The computational fluid dynamics results are analyzed and compared both with experimental data and with a reference multipassage computation based on a sliding mesh approach. These comparisons enable the highlighting of the interests of this approach but also the underlining of its limits. The multiple-frequency phase-lagged approach enables the simulation of unsteady effects on a multistage turbomachinery and access to unsteady information that would not be available with a mixing-plane approach. However, if the method is capable of capturing unsteady effects linked to the adjacent upstream and downstream blade rows passing frequency, it fails modeling clocking effects, i.e., the relative influence between rows and .

Aerodynamic Analysis of Steam Turbine Feed-Heating Steam Extractions

Feed-heating in steam turbines, the use of steam extracted from the turbine to heat the feed-water, is known to raise the plant efficiency and so is included in most steam turbine power plant designs. The steam is extracted through an extraction slot that runs around the casing downstream of a rotor blade row. The slot is connected to a plenum, which runs around the outside of the turbine annulus. Steam flows to the feed-heaters through a pipe connected usually to the bottom of the plenum. The steam extraction is driven by a circumferentially nonuniform pressure gradient in the plenum. This causes the mass flow rate of steam extracted to vary circumferentially, which affects the main passage flow downstream of the extraction point. The flow in the extraction plenum and the influence of the steam extraction on the mainstream aerodynamics is analyzed numerically in this paper. A complete annulus with the extraction slot and plenum together with the downstream stator and rotor blade rows is modeled in this study. The results reveal a highly nonuniform steam extraction around the annulus with the highest extraction rates from the bottom nearest the extraction pipe and the lowest at the top of the annulus. This difference in extraction rates modifies the flow angle and loss circumferential distribution downstream of the stator blade row. This study finds out that the distribution of steam extraction around the annulus and its influence on the main passage flow could be greatly improved by changing the shape and increasing the volume of the extraction slot and plenum.

Numerical investigation of the superimposed effects on stator wake oscillation in an axial-radial combined compressor

The superimposed influences of the blade rows in a multistage compressor are important because different matches of upstream and downstream blades can result in significant differences in the stator wake oscillation. Numerical investigation of the axial stator wake oscillation, which is affected upstream by the axial rotor and downstream by the radial rotor, was performed in an axial-radial combined compressor. Many configurations with different blade numbers and locations, which influence axial stator wake oscillation were investigated. When rotors have equal blade numbers, the axial stator wake oscillates periodically versus time within time T (moving blade passing 1/3 revolution). In contrast, stator wake oscillates irregularly within T when rotors have different blade numbers. A model-split subtraction method is presented in order to separate the influences of the individual blade rows on the wake oscillation of the axial stator. Analysis from the rotor-stator configuration showed that the unsteady flow angle fluctuation response is caused by the upstream rotor. For the rotor-stator-rotor configuration, the unsteady flow angle fluctuations are influenced by up- and downstream blade rows. With the model-split subtraction method, the up- and downstream influences on the flow angle fluctuation could be clearly separated and quantified. Low amplitudes could be observed when the influences from up- and downstream moving rows were superimposed with the “positive peak-negative peak” type wave. Clocking investigations were carried out to change the relative superimposed phase of influences from the surrounding blade rows in order to modulate the amplitudes of the axial stator wake oscillation. However, the amplitudes did not reach the maximum when they were superimposed with “positive peak-positive peak” type wave due to the impact of the interaction between the two moving blade rows.

20603 曲がりダクトによる軸流タービン翼列内の二次流れと損失生成機構の解明 : 後縁ウェークの影響

Various secondary flows are generated in an axial turbine cascade, and the most dominant one is the passage vortex. Additionally, the trailing edge wake discharging from an upstream blade row affects the internal flow of a downstream blade row in an axial turbine. In this study, the curvature of a curved duct, which is considered to be basic model to generate the passage vortex, is related to the passage of an axial turbine cascade. And the flow in the curved duct was analyzed numerically in order to reveal the influence of the strength and the pitch-wise location of wake at the inlet section on the secondary flow structure and the associated loss generation. The computed results showed that the wake distributed on the suction side weakens the passage vortex, on the other hand, that distributed on the pressure side splits it into two vortices in the pitch-wise direction.

Tip Flow Unsteadiness and Blade Row Interactions for a Low-Speed Compressor Rotor

A numerical investigation has been conducted to determine the effects of self-excited tip-leakage flow unsteadiness, upstream stator wakes, and downstream blade row interactions on the performance prediction of a low-speed research compressor rotor. Calculations included a single blade row rotor configuration as well as two multiblade row configurations: one in which the rotor was modeled with an upstream stator and a second in which the rotor was modeled with a downstream stator. Steady-state and time accurate calculations were performed using a Reynolds averaged Navier–Stokes solver, and the results were compared with detailed experimental data at operating conditions corresponding to the compressor design point, near stall, and a midpoint between the two. Differences in the performance predictions between the three configurations were then used to determine the effect of the upstream stator wakes and the downstream blade row interactions. Results obtained show that for this compressor with rather large tip clearance time accurate calculations are a vast improvement over steady-state calculations, but the upstream stator wakes and the downstream blade row interactions have only a small effect on the numerical solution. In this paper, changes in the rotor flowfield due to the upstream and downstream stators are investigated and discussed to explain the limited impact of the stator wakes and downstream blade row interactions on this tip dominated flow. In addition, the unsteady nature of the rotor tip vortex at near stall conditions is discussed to explain the improvement of time accurate analysis over steady-state computations.

Aero‐design of transonic LH2 and LOX contra rotating turbopumps in an expander rocket engine

PurposeContra‐rotating turbines offer reduced size, weight, and cooling requirements, compared to conventional co‐rotating machinery. In spite of the associated mechanical complexity, their aero‐thermal performance is superior to conventional turbines, not only due to the elimination of stator blade rows, but also because lower turning airfoils can be implemented as a result. The purpose of this paper is to present a methodology to determine the optimum velocity triangles of the turbine, together with a two‐dimensional design and optimization tool to minimize the blade unsteady force using radial basis function network, coupled to a genetic algorithm. The proposed design methodology is illustrated with the aerodynamic design of a contra‐rotating two‐axis turbine, which is able to deliver the power necessary to drive the LOX and LH2 pumps of an improved expander rocket engine.Design/methodology/approachThis paper presents a methodology to determine the optimum velocity triangles of the turbine, together with a two dimensional design and optimization tool to minimize the blade unsteady force using radial basis function network, coupled to a genetic algorithm. The proposed design methodology is illustrated with the aerodynamic design of a contra‐rotating two‐axis turbine, which is able to deliver the power necessary to drive the LOX and LH2 pumps of an expander rocket engine, namely the Japanese LE‐5B.FindingsThe airfoil optimizer allows reductions in the downstream pressure distortion of 40 per cent. Consequently, the unsteady forces in the downstream blade row are minimized.Originality/valueThis paper presents to turbomachinery designers in liquid propulsion a novel tool to enhance the aerodynamic performance while reducing the unsteady forces on the blades.

Compressor Efficiency Variation With Rotor Tip Gap From Vanishing to Large Clearance

Compressor efficiency variation with rotor tip gap is assessed using numerical simulations on an embedded stage representative of that in a large industrial gas turbine with Reynolds number ∼ 2 × 106 to 7 × 106. The results reveal three distinct behaviors of efficiency variation with tip gap. For relatively small tip gap (less than 0.8% span), the change in efficiency with tip gap is nonmonotonic with an optimum tip gap for maximum efficiency. The optimum tip gap is set by two competing flow processes: decreasing tip leakage mixing loss and increasing viscous shear loss at the casing with decreasing tip gap. An optimum tip gap scaling is established and shown to satisfactorily quantify the optimal gap value. For medium tip gap (0.8%–3.4% span), the efficiency decreases approximately on a linear basis with increasing tip clearance. However, for tip gap beyond a threshold value (3.4% span for this rotor), the efficiency becomes less sensitive to tip gap as the blade tip becomes more aft-loaded thus reducing tip flow mixing loss in the rotor passage. The threshold value is set by the competing effects between increasing tip leakage flow and decreasing tip flow induced mixing loss with increasing tip gap. Thus, to desensitize compressor performance variation with blade gap, rotor should be tip aft-loaded and hub fore-loaded while stator should be tip fore-loaded and hub aft-loaded as much as feasible. This reduces the opportunity for clearance flow mixing loss and maximizes the benefits of reversible work from unsteady effects in attenuating the clearance flow through the downstream blade-row. The net effect can be an overall compressor performance enhancement in terms of efficiency, pressure rise capability, robustness to end gap variation, and potentially useful operable range broadening.

Filtered chorochronic interface as a capability for 3-D unsteady throughflow analysis of multistage turbomachinery

This note reviews the widely used phased-lagged [Erdos, J. L., E. Alzner, and W. McNally. 1977. AIAA Journal 15: 1559–68.] approach and corresponding chorochronic interface relations [Gerolymos G. A., G. J. Michon, and J. Neubauer. 2002. Journal of Propulsion and Power 18: 1139–52.] and explores its potential extension to the approximate unsteady throughflow analysis of multistage turbomachinery. The basic relations pertaining to the binary blade-row interaction case, for which chorochronic periodicity is exact in a phase-averaged rans framework, are briefly formulated, and selected computational examples illustrate the application of the method. Then, the filtered chorochronic interface is defined as the unsteady counterpart of the well-known mixing-plane concept. This interface takes into account only those tθ-waves which are compatible with the interaction of the immediately upstream and downstream blade-rows. The concept, which is similar to the decomposition-and-superposition method [Li, H. D., and L. He. 2005. ASME J ournal of Turbomachinery 127: 589–98.], is illustrated by 3-D computations of a ½-stage transonic compressor.

J055043 航空エンジン用低圧タービン翼の高効率化に関する研究 : Wake流入条件下における二次流れ損失の調査(〔J055-04〕流体機械の研究開発におけるEFD/CFD(4))

This paper details experimental studies on the secondary flow losses in low-pressure turbine (LPT) cascades. A large portion of overall loss in LPT is due to secondary flow loss. Hence it is important to understand the mechanisms of secondary flow loss production. Moreover, strong wake generated by the upstream blade rows exists in a turbine. This causes aerodynamic interaction of the secondary flow and boundary layer separation in the downstream blade rows, resulting in a complex flow field in the secondary flow region. The objective of this study is to investigate the detail of secondary flow losses under the effect of wake-passing.

Heat Transfer for the Film-Cooled Vane of a 1-1/2 Stage High-Pressure Transonic Turbine—Part I: Experimental Configuration and Data Review With Inlet Temperature Profile Effects

This paper investigates the vane airfoil and inner endwall heat transfer for a full-scale turbine stage operating at design corrected conditions under the influence of different vane inlet temperature profiles and vane cooling flow rates. The turbine stage is a modern 3D design consisting of a cooled high-pressure vane, an un-cooled high-pressure rotor, and a low-pressure vane. Inlet temperature profiles (uniform, radial, and hot streaks) are created by a passive heat exchanger and can be made circumferentially uniform to within ±5% of the bulk average inlet temperature when desired. The high-pressure vane has full cooling coverage on both the airfoil surface and the inner and outer endwalls. Two circuits supply coolant to the vane, and a third circuit supplies coolant to the rotor purge cavity. All of the cooling circuits are independently controlled. Measurements are performed using double-sided heat-flux gauges located at four spans of the vane airfoil surface and throughout the inner endwall region. Analysis of the heat transfer measured for the uncooled downstream blade row has been reported previously. Part I of this paper describes the operating conditions and data reduction techniques utilized in this analysis, including a novel application of a traditional statistical method to assign confidence limits to measurements in the absence of repeat runs. The impact of Stanton number definition is discussed while analyzing inlet temperature profile shape effects. Comparison of the present data (Build 2) to the data obtained for an uncooled vane (Build 1) clearly illustrates the impact of the cooling flow and its relative effects on both the endwall and airfoils. Measurements obtained for the cooled hardware without cooling applied agree well with the solid airfoil for the airfoil pressure surface but not for the suction surface. Differences on the suction surface are due to flow being ingested on the pressure surface and reinjected on the suction surface when coolant is not supplied for Build 2. Part II of the paper continues this discussion by describing the influence of overall cooling level variation and the influence of the vane trailing edge cooling on the vane heat transfer measurements.

A Physical Interpretation of Stagnation Pressure and Enthalpy Changes in Unsteady Flow

This paper provides a physical interpretation of the mechanism of stagnation enthalpy and stagnation pressure changes in turbomachines due to unsteady flow, the agency for all work transfer between a turbomachine and an inviscid fluid. Examples are first given to illustrate the direct link between the time variation of static pressure seen by a given fluid particle and the rate of change of stagnation enthalpy for that particle. These include absolute stagnation temperature rises in turbine rotor tip leakage flow, wake transport through downstream blade rows, and effects of wake phasing on compressor work input. Fluid dynamic situations are then constructed to explain the effect of unsteadiness, including a physical interpretation of how stagnation pressure variations are created by temporal variations in static pressure; in this it is shown that the unsteady static pressure plays the role of a time-dependent body force potential. It is further shown that when the unsteadiness is due to a spatial nonuniformity translating at constant speed, as in a turbomachine, the unsteady pressure variation can be viewed as a local power input per unit mass from this body force to the fluid particle instantaneously at that point.

Efficiency enhancement mechanisms of a new type of full reversible axial-flow fan

A row of airfoil-shaped support struts has been proposed to be installed on both sides of the rotor, so as to remedy the configuration asymmetry of a conventional fully reversible fan in its forward and reverse flow modes. In the forward mode, the upstream support struts act as inlet guide vanes (GVs), and the downstream ones as outlet GVs; whereas in the reverse mode, the roles played by the two support-strut rows switch over. Among the five representative airfoils, symmetric airfoil NACA0012-64 is found to be the most suitable airfoil used as the section of the two rows of the airfoil-shaped support struts. Detailed numerical investigations on four such new fans, whose design points cover a wide range, reveal that the installation of the two rows of blades improves the performance of the new type fan, when compared with the one-rotor fully reversible axial-flow fan without such support-struts. The upstream blade row has little influence on the direction of incoming flow and causes little aerodynamic losses. By contrast, the downstream blade row plays an important role in efficiency improvements: (1) the downstream blade row, which acts as outlet GVs, significantly reduces the rotor-exit tangential velocity and recovers part of the swirl energy in the flow downstream of the rotor, although sometimes the aerodynamic losses caused by such support-strut-like outlet GVs may be high at a large incidence; (2) due to the obviously reduced tangential components of the flow velocities downstream of the outlet GV, the air travels a much shorter distance toward the exit of the annular diffuser, compared with that in the fan without outlet GVs. This leads to a markedly reduced friction loss. In addition, the larger the equivalent divergence angle of the annular diffuser is, the more effectively does the installation of the two rows of the support-strut-like blade improve the performance of the new type fans.

Integrated Motor/Propulsor Duct Optimization for Increased Vehicle and Propulsor Performance

This paper presents a computational study to better understand the underlying fluid dynamics associated with various duct shapes and the resultant impact on both total vehicle drag and propulsor efficiency. A post-swirl propulsor configuration (downstream stator blade row) was selected with rotor and stator blade number kept constant. A generic undersea vehicle hull shape was chosen and the maximum shroud radius was required to lie within this body radius. A cylindrical rim-driven electric motor capable of generating a specific horsepower to achieve the design operational velocity required a set volume that established a design constraint limiting the shape of the duct. Individual duct shapes were designed to produce constant flow acceleration from upstream of the rotor blade row to downstream of the stator blade row. Ducts producing accelerating and decelerating flow were systematically examined. The axisymmetric Reynolds Averaged Navier–Stokes (RANS) version of fluent® was used to study the fluid dynamics associated with a range of accelerated and decelerated duct flow cases as well as provide the base total vehicle drag. For each given duct shape, the propeller blade design code, PBD 14.3, was used to generate an optimized rotor and stator. To provide fair comparisons, the maximum rotor radius was held constant with similar circulation distributions intended to generate equivalent amounts of thrust. Computations predicted that minimum vehicle drag was produced with a duct that produced zero mean flow acceleration. Ducted designs generating accelerating or decelerating flow increased drag. However, propulsive efficiency based exclusively on blade thrust and torque was significantly increased for accelerating flow through the duct and reduced for decelerating flow cases. Full 3D RANS flow simulations were then conducted for select test cases to quantify the specific blade, hull, and shroud forces and highlight the increased component drag produced by an operational propulsor, which reduced overall propulsive efficiency. From these results, a final optimized design was proposed.

Numerical Investigation on a New Type of Fully Reversible Axial Fan with Two Blade Rows on Both Sides of the Rotor

The fully reversible axial flow fans,which can rotate reversibly to ventilate in the opposite direction with the same performance curves,are widely demanded in subways,tunnels,underground warehouses and many other fields. A new configuration method,which can obviously improve the efficiency of reversible axial flow fans,is proposed to meet this special requirement. The new approach is to install two rows of blade with reverse symmetrical airfoil section at stagger angle of 90° on both sides of the rotor of reversible axial fans. The same computational domain is used for reversible flow fans with and without the two-row guide vanes,and the detailed numerical studies reveal that the upstream blade row has little influence on the direction of the incoming flow while the downstream blade row working as a straightener reduces the circumferential velocity of the rotor-exit air remarkably and makes the outlet air flow in nearly axial direction in the diffuser. This leads to a greatly decreased friction loss,and the greater the rate of divergence in the diffuser is,the more obvious the effect will be. In addition,part of the swirl dynamic energy of the rotor-exit air is recovered and changed into static pressure rise. The above two mechanisms contribute to the evident increase in both total pressure rise and efficiency of the new type of reversible axial fans with additional blade rows than those without any stators. Furthermore,the installation of the blade rows on both sides of the fully reversible axial fan guarantees a symmetric configuration and consequently guarantees the fully reversible performance also.

Unsteady Aerodynamic Forces Caused by Oscillation of Blades in Multistage Bladerows

The unsteady lifting surface theory for a model composed of five bladerows is developed to predict unsteady aerodynamic forces on oscillating blades of aerodynamically coupled multistage annular bladerows. The theory is formulated to calculate generalized aerodynamic forces which appear in Lagrange’s equations of blade motions, so that it can be used as the aerodynamic tool for the inter-row coupling flutter analysis of multistage bladerows. Numerical studies were also conducted to investigate the mutual aerodynamic influences among bladerows. The simultaneous effects of co-rotating upstream and downstream bladerows located at next to the next position significantly modify the aerodynamic response of oscillating blades. The aerodynamic force induced by the oscillating bladerow is not very small even on the bladerows remoter than the next-to-the-next bladerow.

Shorten the Intermediate Turbine Duct Length by Applying an Integrated Concept

The demand of further increased bypass ratio of aero engines will lead to low pressure turbines with larger diameters, which rotate at lower speed. Therefore, it is necessary to guide the flow leaving the high pressure turbine to the low pressure turbine at larger diameters minimizing the losses and providing an adequate flow at the low pressure (LP)-turbine inlet. Due to costs and weight, this intermediate turbine duct has to be as short as possible. This would lead to an aggressive (high diffusion) s-shaped duct geometry. It is possible to shorten the duct simply by reducing the length but the risk of separation is rising and losses increase. Another approach to shorten the duct and thus the engine length is to apply a so called integrated concept. These are novel concepts where the struts, mounted in the transition duct, replace the usually following LP-vane row. This configuration should replace the first LP-vane row from a front bearing engine architecture where the vane needs a big area to hold bearing services. That means the rotor is located directly downstream of the strut. This means that the struts have to provide the downstream blade row with undisturbed inflow with suitable flow angle and Mach number. Therefore, the (lifting) strut has a distinct three-dimensional design in the more downstream part, while in the more upstream part, it has to be cylindrical to be able to lead through supply lines. In spite of the longer chord compared with the base design, this struts have a thickness to chord ratio of 18%. To apply this concept, a compromise must be found between the number of struts (weight), vibration, noise, and occurring flow disturbances due to the secondary flows and losses. The struts and the outer duct wall have been designed by Industria de Turbopropulsores. The inner duct was kept the same as for the base line configuration (designed by Motoren und Turbinen Union). The aim of the design was to have similar duct outflow conditions (exit flow angle and radial mass flow distribution) as the base design with which it is compared in this paper. This base design consists of a single transonic high pressure (HP)-turbine stage, an aggressive s-shaped intermediate turbine duct, and a LP-vane row. Both designs used the same HP-turbine and were run in the continuously operating Transonic Test Turbine Facility at Graz University of Technology under the same engine representative inlet conditions. The flow field upstream and downstream the LP-vane and the strut, respectively, has been investigated by means of five hole probes. A rough estimation of the overall duct loss is given as well as the upper and lower weight reduction limit for the integrated concept.

CFD MODELLING OF WATER INJECTION FOR TURBINE BLADE CLEANING

CFD modeling of water injection for turbine blade cleaning has been carried out to predict the water coverage on a stationary blade row, which will enable a better understanding on the interactions between hot-air flow and cold-water droplets. A generic configuration was used in a priori in-house experiment, which provides test data for CFD validation. The two-phase flow CFD model adopts the Eulerian-Lagrangian approach, in which the air-flow was treated as the continuous phase and water droplets as the dispersed phase. CFD predictions are found in fairly good agreement with test results, particularly the water coverage on the downstream blade row. Moreover, CFD modeling provides further details, including the trajectory of water droplets, which are difficult to be obtained by experiments, and yet extremely useful for understanding the flow physics.

Control of Shroud Leakage Loss by Reducing Circumferential Mixing

Shroud leakage flow undergoes little change in the tangential velocity as it passes over the shroud. Mixing due to the difference in tangential velocity between the main stream flow and the leakage flow creates a significant proportion of the total loss associated with shroud leakage flow. The unturned leakage flow also causes negative incidence and intensifies the secondary flows in the downstream blade row. This paper describes the experimental results of a concept to turn the rotor shroud leakage flow in the direction of the main blade passage flow in order to reduce the aerodynamic mixing losses. A three-stage air model turbine with low aspect ratio blading was used in this study. A series of different stationary turning vane geometries placed into the rotor shroud exit cavity downstream of each rotor blade row was tested. A significant improvement in flow angle and loss in the downstream stator blade rows was measured together with an increase in turbine brake efficiency of 0.4 %.

Multibladerow Forced Response Modeling in Axial-Flow Core Compressors

This paper describes the formulation and application of an advanced numerical model for the simulation of blade-passing and low-engine order forced response in turbomachinery core compressors. The Reynolds averaged Navier–Stokes equations are used to represent the flow in a nonlinear time-accurate fashion on unstructured meshes of mixed elements. The structural model is based on a standard finite-element representation. The fluid mesh is moved at each time step according to the structural motion so that changes in blade aerodynamic damping and flow unsteadiness can be accommodated automatically. A whole-annulus 5-bladerow forced response calculation, where three upstream and one downstream bladerows were considered in addition to the rotor bladerow of interest, was undertaken using over 20 million grid points. The results showed not only some potential shortcomings of equivalent 2-bladerow computations for the determination of the main blade-passing forced response, but also revealed the potential importance of low engine-order harmonics. Such harmonics, due to stator blade number differences, or arising from common symmetric sectors, can only be taken into account by including all stator bladerows of interest. The low engine-order excitation that could arise from a blocked passage was investigated next. It was shown that high vibration response could arise in such cases.