Downstream Airfoil Research Articles

Airfoil response to periodic vertical and longitudinal gusts

Gust response has consistently been a concern in engineering. Critical theories have been proposed in the past to predict the unsteady lift response of an airfoil experiencing vertical gusts by Atassi, and longitudinal gusts by Greenberg. However, their applicability for an airfoil with non-zero angles of attack still needs clarification. Thus, force measurements are conducted to examine these theories’ validity and quasi-steady corrections are applied to compensate potential disparities between the idealised and real flow conditions. Velocity measurements are performed to scrutinise the effect of gusts on the flow around the airfoil, and subsequently to reveal the underlying mechanism governing the airfoil's response to gust-induced perturbations. In the study, two pitching vanes are arranged upstream to generate periodic vertical and longitudinal gusts, whereas a downstream airfoil with angles of attack of 0–12° is subjected to two gust types. It is found that Greenberg's theory demonstrates superior predictive capability in pre-stall regimes, with the potential for its effectiveness to be expanded to post-stall regimes through theoretical refinements. In contrast, Atassi's theory exhibits significant deviations from experimental outcomes across the measured angles of attack. Nevertheless, a modified version of the theory aligns better with experimental results at small angles of attack, whereas substantial discrepancies persist as the angle of attack increases. In the pre-stall regime, the aerodynamic response of the airfoil to vertical gusts displays a linear correlation with the flow angle near the leading edge. In the post-stall regime, the vertical gust induces dynamic stall of the airfoil. The flow angle has an essential effect on the lift coefficient but it alone is inadequate to dictate the trend of the lift coefficient. The vorticity statistics show that negative vortex circulation strongly correlates with the lift coefficient. Thus, further correction of the theory or a new vortex model can be expected to predict the lift variation.

Suppression of airfoil-leading-edge turbulence interaction noise using detached micro-perforated fairings

This study investigates the use of detached micro-perforated fairings for reducing turbulence interaction noise generated by the wake impingement of a solid/perforated rod onto the leading edge (LE) of a downstream NACA0012 airfoil in an anechoic wind tunnel. The fairings, which have the same profile as the airfoil LE, are positioned upstream of the airfoil and in close proximity to the LE. The efficiency of noise control is evaluated by varying the porosity σ, windward width A, and gap distance G between the fairing and airfoil LE, and assessed using near- and far-field microphone arrays. The acoustic measurements indicated that micro-perforated fairings can significantly reduce airfoil LE turbulence interaction noise with a reduction of overΔLp_LE = 12 dB when using a fairing with σ = 28.4% and A/tmax = 1 at the gap distance G/c = 0.133 (where tmax and c represent the maximum thickness and chord length of the airfoil, respectively). Additionally, it was observed that changes in porosity σ had less sensitivity in noise control compared to A and G parameters, with nearly similar noise reductions under the range of porosity tested (16.9% ∼ 31.9%). Observations made using wall-pressure sensors on the LE surface and particle image velocimetry (PIV) demonstrated that the applied noise reduction mechanism prevented turbulence from impinging upon the airfoil LE area in the chord range of 0 ∼ 0.04.

Flow control with synthetic jets on two tandem airfoils using machine learning

Active flow control was applied to a tandem configuration of two SD7003 airfoils. The tandem configuration consisted of an upstream airfoil (forefoil) with a pitching motion at a fixed frequency and a downstream airfoil (hindfoil) that was not moving. Synthetic jet actuators (SJAs) were applied on both airfoils to control the flow fields at the low Reynolds number of 30 000. The flow physics inherently involved three different frequencies: frequency of the pitching forefoil and two actuation frequencies of the two of SJAs. In this study, we kept all three frequencies fixed at 5 Hz. However, we allowed for phase differences between them. An optimization study was conducted in order to improve total aerodynamic performance defined as the combined total time-averaged value of lift-to-drag ratio of both airfoils (L/D)tot. Injection angle of the two SJAs, phase differences between each SJA frequency, and frequency of the pitching motion in addition to vertical spacing between the airfoils were considered as design variables of the optimization study. Optimization algorithm was coupled with a machine learning method to reduce computational cost. We found that lift coefficients were enhanced, and drag coefficients were reduced for the optimum controlled case in comparison with the uncontrolled case, which led to an aerodynamic performance improvement of 304%. However, drag force was the dominant parameter in determining final performance value. For all design variables, drag force determined the final optimum values.

Neural network flow optimization using an oscillating cylinder

Flow behaviors of a downstream object can be affected significantly by an upstream object in close proximity. Combined with the neural network algorithms, this concept is used for flow control in this study to optimize the aerodynamic performance of a downstream object. Flow with an oscillating cylinder placed upstream is systematically studied because there are multiple control parameters that influence the flow dynamics around the downstream object. These control parameters are used as the input factors of a back-propagation neural network, and then a revised genetic algorithm is applied to find the optimal set of control parameters. In the current study, we use an airfoil in a low-Reynolds-number flow as an example to investigate the proposed neural network flow optimization concept. The datasets used to train the neural network are from the computational simulation with a previously validated immerse-boundary method to accommodate the motion of the cylinder. The results show that by optimally placing an upstream moving cylinder, it is possible to enhance the aerodynamic performance of the downstream object. Compared to the reference case, the optimized lift/drag ratio of the downstream airfoil can be achieved 2.4 times of its reference value, while maintaining a relatively high lift coefficient.

Reduction of noise generated by cylinder-airfoil interaction using grooved structures on the upstream cylinder

The cylinder-airfoil interaction noise can be reduced by changing the shape of the leading edge of the downstream airfoil. Generally, this way not only can reduce the interaction noise at middle and high frequency, but also can change the peak noise at the low frequency. This study attempts to affect the cylinder-airfoil interaction noise from the perspective of reducing the intensity of the upstream wake shedding vortex. In order to achieve this target, the equally spaced grooves were cut into the upstream cylinder, and the acoustic wind tunnel tests at various incoming velocities (20–60m·s−1) were conducted to compare the interaction noise of cylinder-airfoil (NACA0012) models. It is found that the grooved structure can effectively reduce the peak noise at characteristic frequencies bellow 1000 Hz and the broadband noise in the mid-frequency ranging from 1000 Hz to 3000 Hz, especially for the higher incoming velocity. Thereinto, the peak noise and overall sound pressure level (OASPL) with the grooved cylinder are reduced by 13 dB and 7.2 dB, respectively at the incoming velocity of 60 m·s−1. The numerical simulations based on the large eddy simulation (LES) and Ffowcs Williams–Hawkings (FW-H) acoustic analogy were performed to further reveal the mechanisms of noise reduction when the velocity is 60 m·s−1. The results show that the vortex shedding from cylinder wake is suppressed by the grooved cylinder and the vortex structure at the leading edge of the airfoil is also cut into the small-scale vortex structures by the grooved structure. The pressure fluctuation amplitude and the peak value turbulent kinetic energy in the wake of the grooved cylinder are significantly reduced. In addition, the further spectrum analysis reveals that the weak correlation of the vortex shedding on the grooved cylinder could lead to the suppression of the pressure fluctuation in the cylinder wake, and thereby the interaction noise is significantly reduced.

Influence of lengthways spacing and phase difference on traveling wave energy absorption characteristics of flexible airfoils in a diamond array

Under certain motion parameters, traveling wave motion can absorb energy from incoming flow. In addition to the geometric and motion parameters of each unit, the arrangement also has a great influence on the energy absorption efficiency of traveling wave bodies. Using NACA0012 airfoil as a two-dimensional simplified model, the influence of lengthways spacing and phase difference on energy absorption efficiency of airfoils in the diamond array is studied by numerical calculation in this paper. The results show that with the increase of the lengthways spacing between the midstream airfoils and the downstream airfoil, the transverse interference between the airfoils decreases gradually until disappear. Specifically, with the increase of lengthways spacing, the energy absorption efficiency of the midstream airfoils decreases first and then stays constant, while the downstream airfoil shows a simple harmonic fluctuation with decreasing amplitude. When there exists transverse interference in the array, the energy absorption efficiency of the midstream and downstream airfoils shows a simple harmonic fluctuation concerning the phase difference between the upstream airfoil and the downstream airfoil. When there is no transverse interference in the array, the energy absorption efficiency of the downstream airfoil is a simple harmonic fluctuation concerning the phase difference between the upstream airfoil and the downstream airfoil. Under the optimal arrangement, the energy absorption efficiency of the diamond array is 35% higher than that of a single traveling wave airfoil.

Numerical Simulation of Ground Effect on Circulation Control Airfoil

Circulation control (CC) is used extensively to control the attitude of rudderless aircraft experiencing ground effect in take-off and landing phases. The investigation of ground effect on airfoil CC is necessary to improve flight performance and quality in proximity to the ground. The aerodynamics and flow field of a modified NACA0012 airfoil with CC in ground effect are investigated with numerical simulations. The compressible Reynolds-averaged Navier–Stokes equations with the shear-stress transport k − ω turbulence model equations are solved using the Finite Volume Method (FVM). Simulation results show that the ground effect changes the lift increment per unit jet momentum coefficient, and CC can reverse the polarity ground effect. The effective angle of attack α E and the downwash space downstream airfoil are altered by the ground effect resulting in variations of airfoil surface pressure and lift. Unlike the unbounded flow field, the jet attachment distance is not only determined by the jet momentum coefficient but it can change with the ride height, which is the distance from the ground to the center of the semicircular Coanda surface, for the same jet momentum coefficient.

Rod–Airfoil Interaction Noise Reduction Using Gradient Distributed Porous Leading Edges

Rod–airfoil interaction noise is a major concern in several practical industrial and aeronautical applications. In this study, we constructed bio-inspired gradient distributed porous leading edges to reduce rod–airfoil interaction noise. Noise radiations by NACA 0012 airfoils with nonporous aluminum and porous leading edges were experimentally compared in an anechoic wind tunnel by changing the streamwise gap between the upstream rod and the downstream airfoil, as well as the angle of attack of the airfoil. The results of detailed acoustic tests showed that the proposed gradient distributed porous leading edges can significantly reduce noise radiation around and above the peak frequency of the baseline rod–airfoil interaction. Parametric studies on the piecewise porous characteristics showed that rod–airfoil interaction noise reduction is sensitive to the coverage percentage, position, and arrangement order of the porous materials. Porous leading edges with lower pores per inch, larger coverage, and gradually sparse distributed pores better reduced noise. Moreover, the position of the porous material affected the frequency band of noise reduction, and the noise reduction performance was better when it was located in the downstream strips of the porous leading edge.

Effect of phase difference on energy absorption performance of undulating airfoils in a triangular array

Undulatory flexible body can absorb energy from the flowing fluid under certain motion parameters. The NACA0012 airfoil is used as the two-dimensional model of fishlike body in this paper. Through numerical simulation, the effect of phase difference on the energy absorption performance of each undulating airfoil in the triangular array is studied at Re=1.95×105. For the convenience of analysis, the energy absorption efficiency (EAE) has been calculated separately for each airfoil. The results show that: with the change of phase difference between the upstream airfoil and downstream airfoils, the EAE of each airfoil in the triangular array appears a simple harmonic fluctuation. Under the optimal phase difference, the EAE of the upstream airfoil has decreased by 9.6% while the EAE of downstream airfoils has increased by 13% compared with a single undulating airfoil. When the two downstream airfoils undulate in-phase, their EAE changes synchronously with the phase difference between the upstream airfoil and downstream airfoils. When the two downstream airfoils undulate anti-phase, their EAE shows an antisymmetric form with the period of 360∘. The best arrangement of a triangular array is that the upstream airfoil and downstream airfoils undulate anti-phase, and the two downstream airfoils undulate in-phase.

Configuration optimization of two tandem airfoils at low Reynolds numbers

Optimization was used to find the best configuration of two airfoils in tandem placed into an incoming flow. Upstream airfoil (forefoil) was pitching at a fixed frequency, while the downstream airfoil (hindfoil) was kept at a fixed angle of attack. Study was performed at a low Reynolds number of 30,000 based on chord length. Selig-Donovan 7003 (SD7003) was used for both airfoils, which is a high-performance airfoil specially designed for low Reynolds number flows. The optimization studies were conducted using a genetic algorithm (GA) to maximize aerodynamic performance. The design variables in this study were: horizontal and vertical spacing between the airfoils and hindfoil's angle of attack. Since the optimization process is time-consuming, machine learning was used to train four artificial neural networks (ANNs) to be coupled with genetic algorithm to reduce the computational cost. Two separate optimization cases were considered at two different orders of magnitude in pitching amplitudes of the forefoil, while the pitching frequency was kept at constant value. We found that in both cases, optimum tandem configurations had a smaller combined drag coefficient in comparison with the addition of two separate airfoils. The case with high pitching amplitude produced higher magnitude of lift, while the low amplitude case resulted in a significant improvement in aerodynamic performance.

Large-Eddy Simulation of Low-Pressure Turbine Cascade with Unsteady Wakes

To better understand the wake effects at low Reynolds numbers, large-eddy simulations of a 50% reaction low-pressure turbine stage and a linear cascade with two different bar wake generators were carried out for a chord Reynolds number of 50,000. For the chosen front-loaded high-lift airfoil, the endwall structures are stronger than for more traditional mid-loaded moderate-lift airfoils. By comparing the 50% reaction stage results with the bar wake generator results, insight is gained into the effect of the three-dimensional wake components on the downstream flow field.For the cases with bar wake generator, the endwall boundary layer is growing faster because of the relative motion of the endwall with respect to the freestream. The half-width of the wake is approximately matched for the larger one of the two considered bar wake generators. To improve the quality of the phase-averaged flow fields, the proper orthogonal decomposition was employed as a filter to remove the low-energy unsteady flow field content. Both the mean flow and filtered phase-averaged flow fields were analyzed in detail. Visualizations of the phase-averaged flow field reveal a periodic suppression of the laminar suction side separation from the downstream airfoil even for the smaller bar wake generator. The passage vortex is entirely suppressed for the 50% reaction stage and for the larger bar wake generator. Furthermore, the phase-averaged data for the 50% reaction stage reveal a new longitudinal flow structure that is traced back to near-wall wake vorticity. This flow structure is missing for the bar wake generator cases.

Three-cylinder rotating system flows and their effects on a downstream dimpled airfoil

Effects from turbulence levels created by a three-cylinder rotating system on the flow over a dimpled surface of an airfoil are reported. The three-cylinder rotating system effect is tested on the flow over a NACA0015 with leading-edge dimples at various angles of attack at Reynolds numbers of 0.5 × 105 and 1.0 × 105 in a low-speed wind tunnel. To the effect of mimicking free-stream flow perturbations, the three-cylinder system had small dimensional characteristics compared to those of the airfoil. Two-dimensional (2D) Particle Image Velocimetry (PIV) was used to characterize the flow with illumination from a dual-head Nd:YAG laser and imaging from two high-resolution PIV cameras setup to capture details of the three-cylinder system and the airfoil flow simultaneously. Results show wake characteristics that create complex unsteady and turbulent flows of various intensities as they rotate at various speeds in the free-stream. These free-stream flow disturbances establish the unsteady and turbulent flows that interact with the airfoil flow. The study finds substantial effects occurring for certain cylinder-system settings, airfoil angle-of-attack, and Re number, notably detecting a diminishing separation region and a decrease in its turbulence levels when compared to those for the flow without turbulence generated in the free stream.

Parallel vortex body interaction enabled by active flow control

An experimental investigation has been conducted to demonstrate the utility of active flow control as a disturbance generator for vortex body interaction studies. The technique is used to explore the flow physics of parallel vortex body interaction between two NACA 0012 airfoils in series. Experiments were carried out at a chord-based Reynolds number of 740,000 relative to the first airfoil. Active flow control in the form of nanosecond pulse-driven dielectric barrier discharge plasma actuation, originating close to the leading edge, was used to produce vortex shedding from the upstream (disturbance) airfoil at various frequencies ($$0.038 \le F^+ \le 0.762$$). These vortices were characterized, showing reduced circulation and diameter with increasing frequency, before examining the downstream wake-airfoil interactions. Time-resolved pressure and phase-locked PIV measurements were taken on the downstream (target) airfoil for multiple angles of attack. For $$F^+ \le 0.5$$, the target airfoil is subject to strong oscillations from the wake of the disturbance airfoil that lead to large fluctuations in lift and pitching moment. However, a further increase in $$F^+$$ reattaches the flow over the disturbance airfoil and no major vortex body interactions are observed on the target. Governing parameters for this type of vortex body interaction are explored, and differences between isolated and non-isolated encounters as well as the presence of a viscous response are examined. Finally, means to alleviate loads caused by the incident vortex are explored.

Investigation on rod-airfoil noise with high-order cell-centered finite difference method and acoustic analogy

This paper focuses on the radiation noise of a rod-airfoil configuration based on a high-order cell-centered finite difference method (CCFDM) and Ffowcs Williams-Hawkings (FW-H) acoustic analogy. To reduce numerical dissipation and dispersion, a class of optimized compact schemes is firstly proposed and then validated with benchmark cases from computational aeroacoustic workshops. For turbulence, the delayed detached eddy simulation (DDES) with Spalart-Allmaras model is adopted and is validated through a canonical periodic hill problem. For the far-field radiated acoustics, the FW-H acoustic analogy is utilized with validation of laminar flow past NACA0012. Finally, a rod-airfoil configuration is investigated. The results show reasonable agreement with the experimental data in terms of both near-field aerodynamics and far-field acoustics. The interaction between the upstream turbulence wake from the rod and the downstream airfoil is evidenced to be the dominant contributor to the far-field noise.

Flow/noise control of a rod–airfoil configuration using “natural rod-base blowing”: Numerical experiments

When the vortices shedding from an upstream rod impinge upon the surface of a downstream airfoil, the resultant vortex–body interaction noise can be significant. The use of “natural rod-base blowing” to reduce this interaction noise is investigated numerically using a 3D hybrid computational aeroacoustics approach. The natural blowing is generated through an internal slot that interconnects the stagnation and base region of the rod. Numerical simulation is performed for a straight blowing at blowing rates (BRs) between 6.4% and 16.3% and an oblique blowing with slot–incidence angles (θ) between 0° and 15°, respectively. Far-field noise evaluation demonstrates that the natural rod-base blowing under tested BRs can reduce significantly the noise emission, whereas loses its efficiencies gradually with the increase of angle θ. The most effective case is the straight blowing (θ=0°) at BR≥13.6% where the far-field tonal noise associated with the steady-periodic von Karman vortex shedding is annihilated. The changes in transient flow structures indicate that the natural blowing gradually attenuates the von Karman vortex shedding when BR increases, resulting in a mitigation of vortex–body​ interaction on the airfoil leading surface and hence a reduction of the unsteady lift. At BR=13.6%, as the angle θ reaches 15° from below, a new von Karman vortex street reforms in the flow field, leading to another tonal noise. Linear stability analysis based on a local concept of absolute/convective instability suggests that the absolutely unstable region in the near-wake of the rod elongates first and then shrinks with the increase of BR, while the local absolute growth rate decreases monotonously.

Erratum to: Experimental Investigation on a Single NACA Airfoil’s Nonlinear Aeroelasticity in Wake Induced Vibrations

In order to meet the economic and environmental requirements, turbomachine blades and aircraft wings are becoming more light and flexible, and bearing more mechanical and aerodynamic loads. However aerodynamic excitation would bring more variables into the structural vibration, and becoming an aeroelasticity problem. Unlike mechanical resonance vibration, the structure would interact with the aerodynamic excitation, and the aerodynamic excitation frequency would lock into structural natural frequency even the frequency margin is more than 10%. This phenomenon extends the high amplitude response range and should be noticed in safety design in order to deal with the margin in specific resonance conditions. In this paper, the aerodynamic excitation induced forced response is investigated with experimental setup including upstream cylinder and a downstream single NACA airfoil in wind tunnel. The upstream cylinder generates the vortices imposed on the NACA airfoil, brings periodic excitation on the flexible blade. Flow velocity is measured with hot wire anemometer (HWA) at upstream and downstream of the blade synchronously. Numerical simulation is conducted based experimental condition and verified by the measurements. Proper Orthogonal Decomposition (POD) is applied to obtain the major flow structure at one typical flow condition. The structural properties of the airfoil including natural frequency and damping are evaluated through finite element analysis and hammer test. Based on the fluid and structure properties, coupled test and analysis can be conducted. The vibration characteristics of NACA airfoil at 1st and 2nd order modes are explored by altering the freestream velocity and cylinder diameter. The forced vibration of 1st order mode has the lock-in phenomenon, and the maximum amplitude point is not at the resonance point. But 2nd order mode shows typical resonance behavior.

Hybrid RANS/LES Turbulence Model Applied to a Transitional Unsteady Boundary Layer on Wind Turbine Airfoil

A hybrid Reynolds-averaged Navier Stokes/large-eddy simulation (RANS/LES) turbulence model integrated with a transition formulation is developed and tested on a surrogate model problem through a joint experimental and computational fluid dynamic approach. The model problem consists of a circular cylinder for generating coherent unsteadiness and a downstream airfoil in the cylinder wake. The cylinder flow is subcritical, with a Reynolds number of 64,000 based upon the cylinder diameter. The quantitative dynamics of vortex shedding and Reynolds stresses in the cylinder near wake are well captured, owing to the turbulence-resolving large eddy simulation mode that was activated in the wake. The hybrid model switched between RANS and LES modes outside the boundary layers, as expected. According to the experimental and simulation results, the airfoil encountered local flow angle variations up to ±50°. Further analysis through a phase-averaging technique found phase lags in the airfoil boundary layer along the chordwise locations, and both the phase-averaged and mean velocity profiles collapsed into the Law-of-the-wall in the range of 0 < y + < 50 . The features of high blade-loading fluctuations due to unsteadiness and transitional boundary layers are of interest in the aerodynamic studies of full-scale wind turbine blades, making the current model problem a comprehensive benchmark case for future model development and validation.

Experimental Investigation on a Single NACA Airfoil’s Nonlinear Aeroelasticity in Wake Induced Vibrations

In order to meet the economic and environmental requirements, turbomachine blades and aircraft wings are becoming more light and flexible, and bearing more mechanical and aerodynamic loads. However aerodynamic excitation would bring more variables into the structural vibration, and becoming an aeroelasticity problem. Unlike mechanical resonance vibration, the structure would interact with the aerodynamic excitation, and the aerodynamic excitation frequency would lock into structural natural frequency even the frequency margin is more than 10%. This phenomenon extends the high amplitude response range and should be noticed in safety design in order to deal with the margin in specific resonance conditions. In this paper, the aerodynamic excitation induced forced response is investigated with experimental setup including upstream cylinder and a downstream single NACA airfoil in wind tunnel. The upstream cylinder generates the vortices imposed on the NACA airfoil, brings periodic excitation on the flexible blade. Flow velocity is measured with hot wire anemometer (HWA) at upstream and downstream of the blade synchronously. Numerical simulation is conducted based experimental condition and verified by the measurements. Proper Orthogonal Decomposition (POD) is applied to obtain the major flow structure at one typical flow condition. The structural properties of the airfoil including natural frequency and damping are evaluated through finite element analysis and hammer test. Based on the fluid and structure properties, coupled test and analysis can be conducted. The vibration characteristics of NACA airfoil at 1st and 2nd order modes are explored by altering the freestream velocity and cylinder diameter. The forced vibration of 1st order mode has the lock-in phenomenon, and the maximum amplitude point is not at the resonance point. But 2nd order mode shows typical resonance behavior.

Vortical Gusts: Experimental Generation and Interaction with Wing

We describe the experimental generation of isolated vortical gusts and the interaction between these gusts and a downstream airfoil at a Reynolds number of 20,000. A standard method of generating a vortical gust has been to rapidly pitch an airfoil. A different approach is presented here: heaving a plate across a tunnel and changing direction rapidly to release a vortex. This method is motivated by the desire to limit a test article’s exposure to the wake of the gust generator by moving it to the side of the tunnel. Two suites of experiments were performed to characterize the performance of the gust generators and to measure the forces on and flow around the downstream airfoil. The novel mechanism allowed for measurement of the resumption of vortex shedding from the downstream airfoil, which was impossible with the pitching generator.

Analysis of the Effect of Multirow and Multipassage Aerodynamic Interaction on the Forced Response Variation in a Compressor Configuration—Part I: Aerodynamic Excitation

The aeroelastic prediction of blade forcing is still a very important topic in turbomachinery design. Usually, the wake from an upstream airfoil and the potential field from a downstream airfoil are considered as the main disturbances. In recent years, it became evident that in addition to those two mechanisms, Tyler–Sofrin modes, also called scattered or spinning modes, may have a significant impact on blade forcing. It was recently shown in literature that in multirow configurations, not only the next but also the next but one blade row is very important as it may create a large circumferential forcing variation, which is fixed in the rotating frame of reference. In the present paper, a study of these effects is performed on the basis of a quasi three-dimensional (3D) multirow and multipassage compressor configuration. For the analysis, a harmonic balancing code, which was developed by DLR Cologne, is used for various setups and the results are compared to full-annulus unsteady calculations. It is shown that the effect of the circumferentially different blade excitation is mainly contributed by the Tyler–Sofrin modes and not to blade-to-blade variation in the steady flow field. The influence of various clocking positions, coupling schemes and number of harmonics onto the forcing is investigated. It is also shown that along a speed-line in the compressor map, the blade-to-blade forcing variation may change significantly. In addition, multirow flutter calculations are performed, showing the influence of the upstream and downstream blade row onto aerodynamic damping. The effect of these forcing variations onto random mistuning effects is investigated in the second part of the paper.