Blade Wake Interaction Research Articles

Flow field studies on a micro-air-vehicle-scale cycloidal rotor in forward flight

This paper examines the flow physics and principles of force production on a cycloidal rotor (cyclorotor) in forward flight. The cyclorotor considered here consists of two blades rotating about a horizontal axis, with cyclic pitch angle variation about the blade quarter-chord. The flow field at the rotor mid-span is analyzed using smoke flow visualization and particle image velocimeV are compared with flow fields predicted using 2D CFD and time-averaged force measurements acquired in an open-jet wind tunnel at three advance ratios. It is shown that the experimental flow field is nearly two dimensional at μ = 0.73 allowing for qualitative comparisons to be made with CFD. The incoming flow velocity decreases in magnitude as the flow passes through the retreating (upper) half of the rotor and is attributed to power extraction by the blades. A significant increase in flow velocity is observed across the advancing (lower) half of the rotor. The aerodynamic analysis demonstrates that the blades accelerate the flow through the lower aft region of the rotor, where they operate in a high dynamic pressure environment. This is consistent with CFD-predicted values of instantaneous aerodynamic forces which reveal that the aft section of the rotor is the primary region of force production. Phase-averaged flow field measurements showed two blade wakes in the flow, formed by each of the two blades. Analysis of the blades at several azimuthal positions revealed two significant blade-wake interactions. The locations of these blade-wake interactions are correlated with force peaks in the CFD-predicted instantaneous blade forces and highlight their importance to the generation of lift and propulsive force of the cyclorotor.

Relevance of aerodynamic modelling for load reduction control strategies of two-bladed wind turbines

A new load reduction concept is being developed for the two-bladed prototype of the Skywind 3.5MW wind turbine. Due to transport and installation advantages both offshore and in complex terrain two-bladed turbine designs are potentially more cost-effective than comparable three-bladed configurations. A disadvantage of two-bladed wind turbines is the increased fatigue loading, which is a result of asymmetrically distributed rotor forces. The innovative load reduction concept of the Skywind prototype consists of a combination of cyclic pitch control and tumbling rotor kinematics to mitigate periodic structural loading. Aerodynamic design tools must be able to model correctly the advanced dynamics of the rotor. In this paper the impact of the aerodynamic modelling approach is investigated for critical operational modes of a two-bladed wind turbine. Using a lifting line free wake vortex code (FVM) the physical limitations of the classical blade element momentum theory (BEM) can be evaluated. During regular operation vertical shear and yawed inflow are the main contributors to periodic blade load asymmetry. It is shown that the near wake interaction of the blades under such conditions is not fully captured by the correction models of BEM approach. The differing prediction of local induction causes a high fatigue load uncertainty especially for two-bladed turbines. The implementation of both cyclic pitch control and a tumbling rotor can mitigate the fatigue loading by increasing the aerodynamic and structural damping. The influence of the time and space variant vorticity distribution in the near wake is evaluated in detail for different cyclic pitch control functions and tumble dynamics respectively. It is demonstrated that dynamic inflow as well as wake blade interaction have a significant impact on the calculated blade forces and need to be accounted for by the aerodynamic modelling approach.Aeroelastic simulations are carried out using the high fidelity multi body simulation software SIMPACK. The aerodynamic loads are calculated using ECN's AeroModule and NREL's BEM code Aerodynl3.

A Review of Active Blade Twist Technology: Part I – Actuation Concepts

Marques P. (2014). A review of active blade twist technology: Part I – Actuation concepts. International Journal of Unmanned Systems Engineering. 2(S2): 16-25. Helicopter rotors operate in a complex unsteady aerodynamic environment. Variations in aerodynamic loads on the rotor caused by phenomena such as shock waves, dynamic stall, stall flutter, blade-wake interaction, and blade vortex interaction generate vibration and noise. Suppression of rotor vibration and noise has several advantages, particularly enhanced forward flight performance and less environmental impact. Active blade twist attenuates noise and vibration, and improves helicopter performance through a better spanwise distribution of loads. Rotor-based active control systems that attenuate vibrations and noise at their source, at the rotor blade, are considered superior. This paper reviews actuation concepts for the active twist of the rotor blade and provides a valuable reference in the design of rotors for advanced rotor-UAVs. The simplest method to modify the rotor blade pitch is to apply harmonic frequencies above the rotor rotational frequency using Higher Harmonic Control (HHC). Individual blade control (IBC) is superior to HHC, however IBC necessitates high hydraulic power. The Smart Spring concept provides IBC by actively altering the blade dynamic stiffness, rather than the aerodynamic characteristics, and overcomes the limitations of piezoceramic actuators. The SMART system integrates piezoelectric actuator materials into the rotor blade. There are different means to obtain active twist, including relatively-simple discrete flaps, Smart Tabs, and the Active Trailing Edge which twists the blade aeroelastically. © Marques Engineering Ltd.

Wake-blade interaction in steam turbine stages

Abstract The article discusses the phenomenon of stator Wake/Rotor cascade (W/R) interaction in a steam turbine stage, and the ability to capture it in turbine stage design calculations making use of standard numerical codes. Firstly, the W/R interaction is analysed by comparing its real, experimentally recorded course with the numerical results obtained using vortex theory models and methods. This part of the analysis ends with formulating a conclusion about stochastic nature of the W/R interaction and indicating its reason, which is the vortex structure of the stator wake. Next, a question is discussed whether and how this stochastic nature of the examined phenomenon can be taken into account in calculations of Reynolds Averaged Navier-Stokes (RANS) equations. Differences are indicated between the uniform pattern of the stator wake obtained using a RANS code and the vortex structure of the real wake. It is concluded, however, that despite these differences the RANS results correctly reflect the time-averaged course of the real W/R interaction, and the process of averaging the flow parameters on the sliding plane between stator and rotor calculation areas can be treated as sort of “numerical averaging” of different realisations of the W/R interaction.

The Thermodynamics of Wake Blade Interaction in Axial Flow Turbines: Combined Experimental and Computational Study

This paper reports on insights into the detailed thermodynamics of axial turbine nozzle guide vane (NGV) wakes as they interact with the rotor blades. The evidence presented is both computational and experimental. Unsteady Reynolds-averaged Navier–Stokes (RANS) simulations are used to compare the experimental observations with theoretical predictions. Output processing with both Eulerian and Lagrangian approaches is used to track the property variation of the fluid particles. The wake is found to be hot and loses heat to the surrounding fluid. The Lagrangian output processing shows that the entropy of the wake will fall due to heat loss as it passes through the rotor and this is corroborated experimentally. The experimental vehicle is a 1.5-stage shroudless turbine with modest Mach numbers of 0.5 and high response instrumentation. The entropy reduction of the wake is determined to be about four times the average entropy rise of the whole flow across the rotor. The results show that the work done by the wake fluid on the rotor is approximately 24% lower than that of the free-stream. The apparent experimental efficiency of the wake fluid is 114% but the overall efficiency of the turbine at midheight is around 95%. It is concluded that intrafluid heat transfer has a strong impact on the loss distribution even in a nominally adiabatic turbine with moderate row exit Mach numbers of 0.5.

Aerodynamics of a Low-Pressure Turbine Airfoil at Low Reynolds Numbers—Part II: Blade-Wake Interaction

Part II of this two-part paper presents the aerodynamic behavior of a low-pressure high-lift turbine airfoil, PakB, under the influence of incoming wakes. The periodic unsteady effects of wakes from a single upstream blade-row were measured in a low-speed linear cascade facility at Reynolds numbers of 25,000, 50,000 and 100,000 and at two freestream turbulence intensity levels of 0.4% and 4%. In addition, eight reduced frequencies between 0.53 and 3.2, at three flow coefficients of 0.5, 0.7 and 1.0 were examined. The complex wake-induced transition, flow separation and reattachment on the suction surface boundary layer were determined from an array of closely-spaced surface hot-film sensors. The wake-induced transition caused the separated boundary layer to reattach to the suction surface at all conditions examined. The time-varying profile losses, measured downstream of the cascade, increased with decreasing Reynolds number. The influence of increased freestream turbulence intensity was only evident in between wake-passing events at low reduced frequencies. At higher values of reduced frequency, the losses increased slightly and, for the cases examined here, losses were slightly larger at lower flow coefficients. An optimum wake-passing frequency was observed at which the profile losses were a minimum.

A vortex panel model for the simulation of the wake flow past a vertical axis wind turbine in dynamic stall

ABSTRACTA 2D vortex panel model with a viscous boundary layer formulation has been developed for the numerical simulation of a vertical axis wind turbine (VAWT), including the operation in dynamic stall. The model uses the ‘double wake’ concept to reproduce the main features of the unsteady separated flow, including the formation and shedding of strong vortical structures and the wake–blade interaction. The potential flow equations are solved together with the integral boundary layer equations by using a semi‐inverse iterative algorithm. A new criterion for the reattachment of the boundary layer during the downstroke of a dynamically stalled aerofoil is implemented. The model has been validated against experimental data of steady aerofoils and pitching aerofoils in dynamic stall at high and low Reynolds numbers (Re = 1.5 × 106 and Re = 5 × 104). For the low Reynolds number case, time‐resolved 2D particle image velocimetry (PIV) measurements have been performed on a pitching NACA 0012 aerofoil in dynamic stall. The PIV vorticity fields past the oscillating aerofoil are used to test the model capability of capturing the formation, growth and release of the strong leading edge vortex that characterizes the dynamic stall. Furthermore, the forces extracted from the PIV velocity fields are compared with the predicted ones for a quantitative validation of the model. Finally, the model is applied to the computation of the wake flow past a VAWT in dynamic stall; the predicted vorticity fields and forces are in good agreement with phase‐locked PIV data and CFD‐DES available in the literature. Copyright © 2012 John Wiley & Sons, Ltd.

Analysis of the Helishape Descent Database with Regards to Blade-Wake Interaction Noise

Helicopter rotor Blade Wake Interaction (BWI) noise is known to be significant during take-off and level flight. Less attention has been given to descent flight conditions, where Blade Vortex Interaction (BVI) noise is dominant. The present study investigates BWI noise in descent flight conditions by analyzing the HELISHAPE descent case database. Through signal analysis of both acoustic waveforms and spectra, the rotor azimuthal region responsible for BWI noise is localized and the dominance of BVI noise in the BWI frequency region is shown. Coherence analysis of the blade pressure data indicate significant chordwise coherence in the 3 to 4 Strouhal number range and absence of acoustic dipoles in the BWI frequency range. Comparison with analysis of a swept-back blade-tip database showed that blade-tip shape does not affect BWI noise. The findings of this study support BWI prediction models based on Amiet's theory and suggest that BWI noise can be ignored for predictions of rotor noise in descent flight conditions.

Multiblade Reduced-Order Aerodynamics for State-Space Aeroelastic Modeling of Rotors

Reduced-order aerodynamic models are tools that may be conveniently applied in a wide range of research and design applications in the aeronautical and mechanical fields. This paper presents a methodology for the identification of a reduced-order model (ROM) describing the linearized unsteady aerodynamics of helicopter rotors in arbitrary steady flight, which is particularly suited for the derivation of the state-space perturbation aeroelastic operators and is hence useful for stability analysis and aeroservoelastic applications. It is defined in terms of multiblade coordinates and yields a (finite state) constant-coefficient, linear, differential form relating them to the corresponding multiblade aerodynamic loads. This approach requires the prediction of a set of harmonic perturbation responses by an aerodynamic solver. The accuracy of the identified ROM in describing unsteady aerodynamics phenomena is strictly connected to that of the aerodynamic solver. Complex aerodynamic effects (like wake roll-up and wake―blade interactions) are included in the ROM if they are taken into account in evaluating the harmonic responses. Numerical results concerning a flap-lag helicopter rotor in forward flight are presented. These examine the accuracy the aerodynamic ROM introduced both in terms of aerodynamic loads predictions and in terms of aeroelastic stability analysis. An aeroservoelastic application is also included in order to demonstrate the suitability of the ROM proposed for the design of controllers.

Numerical Simulation of Blade-Wake Interaction of Vertical Axis Tidal Turbine

To study the blade-wake interaction of vertical axis tidal turbine (VATT)，particles were placed in the flow field to trace blade wake during numerical simulation. Numerical simulations were conducted utilizing Euler-Lagrange model. In the simulations, the continuous phase was solved by Reynolds-averaged Navier-Stocks(RANS) equation combined with SST turbulence model and the particle trajectories of the dispersed phase were determined by momentum equation. Numerical results of predicting instantaneous blade forces and blade wakes showed good agreement with the test data. The model was also compared with previous classic free vortex model (V-DART), vortex method combined with finite element analysis (FEVDTM) and 2-D vortex panel model (VPM2D). It showed that the present model was much better than the former.

Internal Flow Measurement with LDV at Design Point of Contra-Rotating Axial Flow Pump

The adoption of the contra-rotating rotor has been proposed in response to a demand for the axial flow pump with higher specific speed. The internal flow field and blade rows interactions between front and rear rotors should be considered in the design for higher performance and more stable operation, but has not been clarified yet. The flow field in contra-rotating axial flow pump was measured at various flow rates with the LDV and wall pressure measurements. In the present paper, the experimental and numerical results in the flow field between front and rear rotors are shown at the design flow rate and the flow behaviors are discussed, related on blade loading, blade tip leakage flow, wake and blade rows interaction.

Internal Flow Measurement with LDV at Design Point of Contra-Rotating Axial Flow Pump

The adoption of the contra-rotating rotor has been proposed in response to a demand for the axial flow pump with higher specific speed. The internal flow field and blade rows interactions between the front and rear rotors should be considered in the design for higher performance and more stable operation, but has not been clarified yet. The flow field in contra-rotating axial flow pump was measured at various flow rates with the LDV and wall pressure measurements. In the present paper, the experimental and numerical results in the flow field between front and rear rotors are shown at the design flow rate and the flow behaviors are discussed, related on blade loading, blade tip leakage flow, wake and blade rows interaction.

Nonlinear Analysis of Unsteady Flows in Multistage Turbomachines Using Harmonic Balance

A harmonic balance technique for the analysis of two-dimensional linear (small-disturbance) and nonlinear unsteady flows in multistage turbomachines is presented. The present method uses a mixed time-domain/frequency-domain approach that allows one to compute the unsteady aerodynamic response of multistage machines to both blade vibration (the flutter problem) and wake interaction (the forced response problem). In general, the flowfield may have multiple excitation frequencies that are not integer multiples of each other, so that the unsteady flow is (sometimes) aperiodic in time. Using our approach, we model each blade row using a computational grid spanning a single blade passage. In each blade row, we store several subtime level solutions. For flows that are periodic in time, these subtime levels span a single time period. For aperiodic flows, the temporal period spanned by these subtime level solutions is sufficiently long to sample the relevant discrete frequencies contained in the aperiodic flow. In both cases, these subtime level solutions are related to each other through the time-derivative terms in the Euler or Navier-Stokes equations and boundary conditions; complex periodicity conditions connect the subtime levels within a blade passage, and interrow boundary conditions connect the solutions among blade rows. The resulting discretized equations, which are mathematically steady because time derivatives have been replaced by a pseudospectral operator in which the excitation frequencies appear as parameters, can be solved very efficiently using multigrid acceleration techniques. In this paper, we apply the technique to both flutter and wake-interaction problems and illustrate the influence of neighboring blade rows on the unsteady aerodynamic response of a blade row.

Large Eddy Simulation of Acoustical Sources in a Low Pressure Axial-Flow Fan Encountering Highly Turbulent Inflow

A large eddy simulation (LES) was applied to predict the unsteady flow in a low-speed axial-flow fan assembly subjected to a highly “turbulent” inflow that is generated by a turbulence grid placed upstream of the impeller. The dynamic Smagorinsky model (DSM) was used as the subgrid scale (SGS) model. A streamwise-upwind finite element method (FEM) with second-order accuracy in both time and space was applied as the discretization method together with a multi-frame of reference dynamic overset grid in order to take into account the effects of the blade-wake interactions. Based on a simple algebraic acoustical model for axial flow fans, the radiated sound power was also predicted by using the computed fluctuations in the blade force. The predicted turbulence intensity and its length scale downstream of the turbulence grid quantitatively agree with the experimental data measured by a hot-wire anemometry. The response of the blade to the inflow turbulence is also well predicted by the present LES in terms of the surface pressure fluctuations near the leading edge of the blade and the resulting sound power level. However, as soon as the effects of the turbulent boundary layer on the blades become important, the prediction tends to become inaccurate.

Comprehensive validation of an intermittency transport model for transitional low-pressure turbine flows

AbstractA transport equation for the intermittency factor is employed to predict transitional flows under the effects of pressure gradients, freestream turbulence intensities, Reynolds number variations, flow separation and reattachment, and unsteady wake-blade interactions representing diverse operating conditions encountered in low-pressure turbines. The intermittent behaviour of the transitional flows is taken into account and incorporated into computations by modifying the eddy viscosity, μτ with the intermittency factor, γ. Turbulent quantities are predicted by using Menter’s two-equation turbulence model (SST). The onset location of transition is obtained from correlations based on boundary-layer momentum thickness, accelaration parameter, and turbulence intensity. The intermittency factor is obtained from a transport model which can produce both the experimentally observed streamwise variation of intermittency and a realistic profile in the cross stream direction.The intermittency transport model is tested and validated against several well documented low pressure turbine experiments ranging from flat plate cases to unsteady wake-blade interaction experiments. Overall, good agreement between the experimental data and computational results is obtained illustrating the predicting capabilities of the model and the current intermittency transport modelling approach for transitional flow simulations.

353 航空エンジン用高負荷低圧タービン動翼の研究 : CFDによる後流干渉効果の検証(G05-6 流体機械(1),G05 流体工学)

353 航空エンジン用高負荷低圧タービン動翼の研究 : CFDによる後流干渉効果の検証(G05-6 流体機械(1),G05 流体工学)

Unsteady RANS modelling of wake–blade interaction: computational requirements and limitations

An investigation is presented of the ability of the unsteady RANS approach to represent, from a numerical point of view, the characteristics and evolution of moving wakes as they interact with the flow around a turbine blade and in a related variable-passage configuration. The context of the study is wake-induced transition in unsteady turbomachine flows. Its emphasis is on numerical and computational issues, with physical aspects of predicting unsteady transition in the boundary layer of a turbine blade being covered in a companion paper. Issues addressed include: resolution and time-step requirements––especially in relation to the convective transport of scalar-wake properties, the sensitivity of the solution to the numerical scheme, and the relative merits and limitations of either including the wake-generating moving devices––in the present case an array of circular bars––as part of the computational process, or of prescribing the moving wakes as part of the flow-inlet conditions upstream of the blade passage. In relation to the latter option, the viability of extracting the wake conditions from a precursor RANS computation for an isolated wake-generating body is examined. In total, results for 21 test conditions are reported in an effort to derive some general conclusions and recommendations. The investigation demonstrates that (i) extremely high resolution is needed to ensure that numerical issues do not obscure physical phenomena and turbulence-model capabilities; (ii) the inclusion of the wake-generating device in the computation poses serious problems associated with vortex shedding; and (iii) the results of precursor calculations for the wake can be seriously flawed because of inherent limitations in the RANS approach.

Simulation of unsteady motion of a propeller in a fluid including free wake modeling

The problem of flow around a marine propeller performing a general 3D unsteady motion in an infinitely extended fluid is formulated and solved using a boundary element method. Hydrodynamic modeling of the freely moving—unsteady—trailing vortex sheet emanating from each blade is achieved, using vortex filaments and a time stepping method. Thus vortex wake–blade interaction can be taken correctly into consideration. The method uses bilinear panels for the representation of blade and free wake geometry and constant panels for the representation of the unknown dipole intensity. The proposed method produces very stable rollup wake patterns for both steady and unsteady problems for a broad range of discretization parameters. Application of the method to a number of specific cases of steady and unsteady propeller motion has shown the very good numerical performance of the method and that good predictions for forces and pressure distributions can be obtained.

Blade-Wake Interaction Noise for Turbines With Downwind Rotors

The interaction between the rotor and the tower wake is an important source of noise for wind turbines with downwind rotors. The tower wake modifies the dynamic pressure and the local flow incidence angle as seen by the blades and, hence, modifies the aerodynamic loading of the blade during blade passage. The resulting n per revolution fluctuation in the blade loading (where n is the number of blades) is the source of low frequency but potentially high amplitude sound levels. The Wind Turbine Company (WTC) Proof of Concept 250 kW (POC) wind turbine has been observed by field personnel to produce low-frequency emissions at the National Wind Technology Center (NWTC) site during specific atmospheric conditions. Consequently, WTC is conducting a three-phase program to characterize the low frequency emissions of its two-bladed wind turbines and to develop noise mitigation techniques if needed. This paper summarizes the first phase of this program including recent low-frequency noise measurements conducted on the WTC POC250 kW wind turbine, a review of the wake characteristics of circular towers as they pertain to the blade-wake interaction problem, and techniques to attenuate the sound pressure levels caused by the blade-wake interaction.

Experimental Investigation of Unsteady Flow Field Within a Two-Stage Axial Turbomachine Using Particle Image Velocimetry

Detailed measurements of the flow field within the entire 2nd stage of a two-stage axial turbomachine are performed using particle image velocimetry. The experiments are performed in a facility that allows unobstructed view on the entire flow field, facilitated using transparent rotor and stator and a fluid that has the same optical index of refraction as the blades. The entire flow field is composed of a “lattice of wakes,” and the resulting wake-wake and wake-blade interactions cause major flow and turbulence nonuniformities. The paper presents data on the phase averaged velocity and turbulent kinetic energy distributions, as well as the average-passage velocity and deterministic stresses. The phase-dependent turbulence parameters are determined from the difference between instantaneous and the phase-averaged data. The distributions of average passage flow field over the entire stage in both the stator and rotor frames of reference are calculated by averaging the phase-averaged data. The deterministic stresses are calculated from the difference between the phase-averaged and average-passage velocity distributions. Clearly, wake-wake and wake-blade interactions are the dominant contributors to generation of high deterministic stresses and tangential nonuniformities, in the rotor-stator gap, near the blades and in the wakes behind them. The turbulent kinetic energy levels are generally higher than the deterministic kinetic energy levels, whereas the shear stress levels are comparable, both in the rotor and stator frames of references. At certain locations the deterministic shear stresses are substantially higher than the turbulent shear stresses, such as close to the stator blade in the rotor frame of reference. The nonuniformities in the lateral velocity component due to the interaction of the rotor blade with the 1st-stage rotor-stator wakes, result in 13 percent variations in the specific work input of the rotor. Thus, in spite of the relatively large blade row spacings in the present turbomachine, the nonuniformities in flow structure have significant effects on the overall performance of the system.