Blade Boundary Layer Research Articles

Wind tunnel research, dynamics, and scaling for wind energy

The interaction of wind turbines with turbulent atmospheric boundary layer (ABL) flows represents a complex multi-scale problem that spans several orders of magnitudes of spatial and temporal scales. These scales range from the interactions of large wind farms with the ABL (on the order of tens of kilometers) to the small length scale of the wind turbine blade boundary layer (order of a millimeter). Detailed studies of multi-scale wind energy aerodynamics are timely and vital to maximize the efficiency of current and future wind energy projects, be they onshore, bottom-fixed offshore, or floating offshore. Among different research modalities, wind tunnel experiments have been at the forefront of research efforts in the wind energy community over the last few decades. They provide valuable insight about the aerodynamics of wind turbines and wind farms, which are important in relation to optimized performance of these machines. The major advantage of wind tunnel research is that wind turbines can be experimentally studied under fully controlled and repeatable conditions allowing for systematic research on the wind turbine interactions that extract energy from the incoming atmospheric flow. Detailed experimental data collected in the wind tunnel are also invaluable for validating and calibrating numerical models.

High-Fidelity Simulation Study of the Unsteady Flow Effects on High-Pressure Turbine Blade Performance

Abstract Unsteadiness, in the form of both broadband background disturbances and discrete coherent wakes, can have a strong effect on the performance of turbomachinery blades. The influence of the incoming flow has received much interest as it inevitably affects the blade boundary layers and develops as it passes through the machine. In the present work, we investigate the effect of unsteady flow on high-pressure turbines (HPTs), using high-fidelity datasets produced by wall resolved large-eddy simulation of an HPT stage. The effects of incident wakes from an upstream stator, compounded by the presence of freestream turbulence, on the downstream rotor are investigated. Based on analyzing cases with different turbulence intensities and length scales prescribed at the inlet, we show that changing the freestream turbulence characteristics has a direct effect on the unsteady behavior of the stator wakes. As a result, the performance of the rotor is also significantly affected. By detailing the influence of the wake–turbulence interaction, we aim to distinguish driving forces on rotor performance, be it changes in the incident wakes or direct influence from the freestream turbulence. Furthermore, the aerothermal behaviors of the rotor blades have been extensively investigated, showing that the blade boundary layers on the suction and pressure sides respond differently to external disturbances. The insights gained can provide designers with guidelines in understanding the unsteady flow effects of a given flow state, and how the unsteadiness present, either broadband or deterministic, will affect the performance of downstream blades.

Multi-scale Navier-Stokes analysis of geometrically resolved erosion of wind turbine blade leading edges

A multi-scale computational fluid dynamics analysis of wind turbine blade leading edge erosion is presented. The test case is a large set of eroded blade sections. These are obtained by fitting the resolved eroded leading edge geometry of the outboard part of a multi-megawatt offshore wind turbine to the NACA633-618 airfoil. The erosion geometry measured by a blade laser scan is geometrically resolved in the aerodynamic simulations, whereas the aerodynamic effects of unresolved lower-amplitude scales are accounted for by using distributed surface roughness models. The simulations also account for the laminar-to-turbulent transition of the blade boundary layers with and without distributed roughness. An existing semi-empirical model and simulations of the nominal airfoil enable one to estimate the roughness level needed to trip leading edge boundary layer transition at the considered Reynolds number of 9 million. It is found that a) the mean roughness heights of the observed geometry perturbations are well above the critical roughness height, and b) consideration of either large or small erosion scales in isolation results in underestimating the airfoil performance impairment.

On the receptivity of low-pressure turbine blades to external disturbances

In the present work, the laminar–turbulent transition of the flow evolving around a low-pressure turbine blade has been investigated. Direct numerical simulations have been carried out for two different free stream turbulence intensity (FSTI) levels to investigate the role of free stream oscillations on the evolution of the blade boundary layer. Emphasis is placed on identifying the mechanisms driving the formation and breakup of coherent structures in the high FSTI case and how these processes are affected by the leading-edge receptivity and/or by the continuous forcing in the blade passage. Proper orthogonal decomposition (POD) has been adopted to provide a clear statistical representation of the shape of the structures. Extended POD projections provided temporal and spanwise correlations that allowed us to identify dominant temporal structures and spanwise wavelengths in the transition process. The extended POD analysis shows that the structures on the pressure side are not related to what happens at the leading edge. The results on the suction side show that the modes defining the leading edge and the passage bases correlate with coherent structures responsible for the transition. The most energetic mode of the passage basis is strongly related to the most amplified wavelength in the boundary layer and breakup events leading to transition. Modes with a smaller spanwise wavelength belong to the band predicted by optimal disturbance theory, they amplify with a smaller gain in the rear suction side, and they show the highest degree of correlation between the passage region and the rear suction side.

Trapped Acoustic Modes in an Axial Multi-Stage Compressor Leading to Non-Synchronous Blade Vibrations

Non-synchronous blade vibrations have been observed in an experimental multi-stage high-speed compressor setup at part-speed conditions. A detailed numerical study has been carried out to understand the observed phenomenon by performing unsteady full-annulus Reynolds-Averaged Navier–Stokes (RANS) simulations of the whole setup using the solver elsA. Several operating conditions have been simulated to observe this kind of phenomena along a speedline of interest. Based on the simulation results, the physical source of the non-synchronous blade vibration is identified: An aerodynamic disturbance appears in a highly loaded downstream rotor and excites a spinning acoustic mode. A “lock-in” phenomenon occurs between the blade boundary layer oscillations and the spinning acoustic mode. The establishment of axially propagating acoustic waves can lead to a complex coupling mechanism and this phenomenon is highly relevant in understanding the multi-physical interactions appearing in modern compressors. It is shown that aerodynamic disturbances occurring downstream can lead to critical excitation of rotor blades in upstream stages due to an axially propagating acoustic wave. The paper includes the analysis of a relevant transient test and a detailed analysis of the numerical results. The study shows the capability and necessity of a full-annulus multistage simulation to understand the phenomenon.

Assessment of Machine-Learned Turbulence Models Trained for Improved Wake-Mixing in Low-Pressure Turbine Flows

This paper presents an assessment of machine-learned turbulence closures, trained for improving wake-mixing prediction, in the context of LPT flows. To this end, a three-dimensional cascade of industrial relevance, representative of modern LPT bladings, was analyzed, using a state-of-the-art RANS approach, over a wide range of Reynolds numbers. To ensure that the wake originates from correctly reproduced blade boundary-layers, preliminary analyses were carried out to check for the impact of transition closures, and the best-performing numerical setup was identified. Two different machine-learned closures were considered. They were applied in a prescribed region downstream of the blade trailing edge, excluding the endwall boundary layers. A sensitivity analysis to the distance from the trailing edge at which they are activated is presented in order to assess their applicability to the whole wake affected portion of the computational domain and outside the training region. It is shown how the best-performing closure can provide results in very good agreement with the experimental data in terms of wake loss profiles, with substantial improvements relative to traditional turbulence models. The discussed analysis also provides guidelines for defining an automated zonal application of turbulence closures trained for wake-mixing predictions.

Super-large-scale flow visualization using natural snowfall for the study of utility-scale wind turbine flows

With the rapid growth of wind turbine installation in recent decades, fundamental physical understanding of the flow around wind turbines and farms is becoming increasingly critical for further efficiency increases. However, the effort to develop this understanding is hindered by the significant challenges involved in modelling such a complex dynamic system with a wide range of relevant scales (blade boundary layer thickness at ∼ 1 mm to atmospheric scales at ∼ 1 km). Additionally, conventional methods used to measure air flow around wind turbines in the field (e.g., lidar) are limited by low spatio-temporal resolutions.

Effects of Surface Waviness on Fan Blade Boundary Layer Transition and Profile Loss—Part II: Experimental Assessments and Applications

AbstractPart II describes the experimental assessment and the application of the ideas in Part I concerning the mechanisms that determine the role of blade surface waviness on laminar-turbulent transition and their consequent effect on civil aircraft fan performance. A natural transition wind tunnel was designed and constructed to characterize the impact of surface waviness on transition, using both hotwire anemometry and infrared thermography. The experimental results support the new hypothesis presented in Part I, concerning the way in which blade surface waviness affects fan performance through motion of the transition onset location due to interaction between surface waviness and Tollmien–Schlichting (TS) boundary layer instability. In particular, the theoretical amplification of the TS waves, and the corresponding transition onset location movement due to surface waviness, was borne out over a range of variations in Reynolds number, nondimensional surface wavelength, nondimensional surface wave height, and location of surface wave initiation, relevant to composite fan blade parameters. Further, the increase of receptivity coefficient, and thus, the initial amplitude of disturbances due to geometric resonance between surface wavelength and TS wavelength was also confirmed by the experiments. Surface waviness was estimated, in some cases, to result in a nearly 1% decrease in fan efficiency compared to a nonwavy blade. Suggestions are given for mitigation of the effects of waviness, including the idea of blade curvature rescheduling as a method to delay transition and thus decrease loss.

Effects of Surface Waviness on Fan Blade Boundary Layer Transition and Profile Loss—Part I: Methodology and Computational Results

AbstractThis two-part paper describes a new approach to determine the effect of surface waviness, arising from manufacture of composite fan blades, on transition onset location movement and hence fan profile losses. The approach includes analysis and computations of unsteady disturbances in boundary layers over a wavy surface, assessed and supported by wind tunnel measurements of these disturbances and the transition location. An integrated framework is developed for analysis of surface waviness effects on natural transition. The framework, referred to as the extended eN method, traces the evolution of disturbance energy transfer in flow over a wavy surface, from external acoustic noise through exponential growth of Tollmien–Schlichting (TS) waves, to the start and end of the transition process. The computational results show that surface waviness affects the transition onset location due to the interaction between the surface waviness and the TS boundary layer instability and that the interaction is strongest when the geometric and TS wavelengths match. The condition at which this occurs, and the initial amplitude of the boundary layer disturbances that grow to create the transition onset is maximized, is called receptivity amplification. The results provide first-of-a-kind descriptions of the mechanism for the changes in transition onset location as well as quantitative calculations for the effects of surface waviness on fan performance due to changes in surface wavelength, surface wave amplitude, and the location at which the waviness is initiated on the fan blade.

Time-Resolved Measurements of the Unsteady Boundary Layer in an Annular Low-Pressure Turbine Configuration With Perturbed Inlet

AbstractIn this study, the unsteady behavior of the boundary layers developing on a low-pressure turbine (LPT) stator profile and their effect on secondary flow patterns in a 1.5-stage turbine configuration are investigated under the influence of periodic inflow perturbations. The experimental setup previously employed to analyze the unsteady secondary flow in the stator wake has been enhanced by hot-film sensor arrays placed on the stator profiles at different blade height positions to provide time-resolved data from within the passage. The stator inflow is perturbed by a rotating wake generator, and a modified T106 profile has been considered for the blading. The modified profile, labeled as T106RUB, was developed for matching the transition and separation characteristics of the popular original T106 profile at low flow speeds, thus facilitating measurements to be taken in a large-scale test rig with its improved accessibility. The transition phenomena occurring in the profile boundary layers are investigated under both unperturbed and periodically perturbed inflow by means of spectral analysis, the semi-quantitative characterization of the wall-stress system, and an evaluation of the statistic quantities. In particular, the periodic changes of the suction-side boundary layer flow region toward the trailing edge are studied in detail. Furthermore, time-resolved hot-film measurements at different blade height positions facilitate a detailed comparison of the quasi two-dimensional mid-span profile flow and the near end-wall profile flow, which is subject to the influence of secondary flow structures. These information are employed to assess to which extent the additional turbulence originating from the wakes affects the blade boundary layers and thus the secondary flow structures. Furthermore, the role of the perturbation frequency on the coupled system of boundary layers and secondary flow structures is evaluated.

Прогнозирование объема каверн на вращающихся лопастях и масштабные эффекты

Object and purpose of research. Pressure pulsations induced by cavitating blades substantially contribute to flowinduced loads and amplify structural vibration. These pulsations depend on oscillation of the volume of cavities over blades. Prediction of them usually involves model tests and there are three kinds of scale effects influencing the cavity volumes. The first one is associated with the non-uniform inflows. The second one is associated with the combined influence of the blade boundary layer and surface tension on the cavity surface. The third one is associated with the cavity buoyancy. Materials and methods. Because of complexity of blade flows, a qualitative analysis of similar unsteady non-uniform flows around 3D hydrofoils is useful. This paper presents such an analysis for a hydrofoil with the sections copied from a marine propeller blade. The inflows correspond to the wakes of a ship and of her model. Computations carried out using an analysis of viscous-inviscid interaction. Main results. The qualitative explanation of observed trends and scale effects is obtained due to this analysis. In particular, the role of pressure side cavitation in full scale conditions is pointed out. Conclusion. The difference of model and ship wakes results in the substantial difference in blade section angles of attack at the same blade loading. Therefore, in model tests the suction side cavitation is more extensive, whereas the pressure side cavitation may not appear, though it exists on full-scale ship propeller blade. This substantial scale effect has been usually out of previous considerations.

Flow Coefficient and Reduced Frequency Effects on Wake-Boundary Layer Interaction in Highly Accelerated LPT Cascade

The paper presents a detailed analysis of particle image velocimetry (PIV) measurements performed in a turbine cascade representative of highly accelerated low-pressure turbine (LPT) blades. Two cameras have been simultaneously used to observe a great portion of the suction side boundary layer with the highest possible spatial resolution, thus allowing us to solve the interaction process between impinging upstream wakes and the blade boundary layer. Four unsteady inflow conditions, characterized by different incoming wake reduced frequencies and flow coefficients, have been examined at fixed Reynolds number. The highly resolved flow fields have been processed to explore reduced frequency and flow coefficient effects on the boundary layer unsteady transition process and, consequently, on loss production. For a deep physical insight on the mechanisms responsible for loss generation, proper orthogonal decomposition (POD) has been applied at different phases of the wake passing period. This has provided the dominant structures affecting the cascade aerodynamics during the wake period. Moreover, the examination of POD modes has allowed us to show the effects induced by the parameter variation on the turbulent kinetic energy production and thus to the unsteady loss production mechanisms.

Acoustic Resonance in an Axial Multistage Compressor Leading to Non-Synchronous Blade Vibration

Abstract Significant non-synchronous blade vibrations (NSVs) have been observed in an experimental three-stage high-speed compressor at part-speed conditions. High-amplitude acoustic modes, propagating around the circumference and originating in the highly loaded stage-3, have been observed in coherence with the structural vibration mode. In order to understand the occurring phenomena, a detailed numerical study has been carried out to reproduce the mechanism. Unsteady full-annulus Reynolds-averaged Navier–Stokes simulations of the whole setup have been performed using the solver elsA. The results revealed the development of propagating acoustic modes which are partially trapped in the annulus and are in resonance with an aerodynamic disturbance in rotor-3. The aerodynamic disturbance is identified as an unsteady separation of the blade boundary layer in rotor-3. The results indicate that the frequency and phase of the separation adapt to match those of the acoustic wave and are therefore governed by acoustic propagation conditions. Furthermore, the simulations clearly show the modulation of the propagating wave with the rotor blades, leading to a change of circumferential wave numbers while passing the blade row. To analyze if the effect is self-induced by the blade vibration, a non-coherent structural mode has been imposed in the simulations. Even at high vibration amplitude, the formerly observed acoustic mode did not change its circumferential wave number. This phenomenon is highly relevant to modern compressor designs, since the appearance of the axially propagating acoustic waves can excite blade vibrations if they coincide with a structural eigenmode, as observed in the presented experiments.

High-Fidelity Simulations of a High-Pressure Turbine Vane Subject to Large Disturbances: Effect of Exit Mach Number on Losses

Abstract We report on a series of highly resolved large-eddy simulations of the LS89 high-pressure turbine (HPT) vane, varying the exit Mach number between Ma = 0.7 and 1.1. In order to accurately resolve the blade boundary layers and enforce pitchwise periodicity, we for the first time use an overset mesh method, which consists of an O-type grid around the blade overlapping with a background H-type grid. The simulations were conducted either with a synthetic inlet turbulence condition or including upstream bars. A quantitative comparison shows that the computationally more efficient synthetic method is able to reproduce the turbulence characteristics of the upstream bars. We further perform a detailed analysis of the flow fields, showing that the varying exit Mach number significantly changes the turbine efficiency by affecting the suction-side transition, blade boundary layer profiles, and wake mixing. In particular, the Ma = 1.1 case includes a strong shock that interacts with the trailing edge, causing an increased complexity of the flow field. We use our recently developed entropy loss analysis (Zhao and Sandberg, 2019, “Using a New Entropy Loss Analysis to Assess the Accuracy of RANS Predictions of an HPT Vane,” ASME Turbo Expo 2019: Turbomachinery Technical Conference and Exposition, Paper No. GT2019-90126) to decompose the overall loss into different source terms and identify the regions that dominate the loss generation. Comparing the different Ma cases, we conclude that the main mechanism for the extra loss generation in the Ma = 1.1 case is the shock-related strong pressure gradient interacting with the turbulent boundary layer and the wake, resulting in significant turbulence production and extensive viscous dissipation.

The effect of the wake on the separated boundary layer in a two-stage compressor

This experimental study provides striking examples of the separated boundary layer development resulting from blade–wake interaction in a multistage turbomachine. Particle image velocimetry measurements are performed within the second-stage rotors of a two-stage compressor. Phase-lock results confirm that wake impingement greatly changes the passage flow, as well as affecting the boundary layer flow. The high turbulence level and the negative jet behavior of the wake dominate the interaction between the unsteady wake and the separated boundary layer on the suction surface. By correlating the flow state of the boundary layer with the spatial position of the wake, the influence of the wake on the blade boundary layer flow is revealed, and the mechanism restraining boundary layer separation on the suction surface is studied. It is found that the wake itself does not inhibit separation, and instead, the boundary layer of the region swept by the wake is thickened and separation is strengthened. However, the wake impingement produces a turbulent spot, and the calmed region behind this spot inhibits separation, as well as making the boundary layer thinner. As a consequence, the periodic sweeping of the wake makes the boundary layer exhibit a clear periodicity.

Numerical investigation of tip clearance effects on rotating instability of a low-speed compressor

To investigate the effects of tip clearance on the rotating instability of a compressor, full-annulus numerical simulations, with and without clearance, were conducted for a low-speed compressor rotor. The analyses were focused on three spanwise positions: 99%, 50% and 10% span of the blade. The frequency, circumferential mode characteristics, and instantaneous flow structures were obtained from unsteady simulations. In the tip region, the transverse vortex in each passage and the shedding vortex at the rear of the blade suction side every two passages are associated with rotating instability (RI) when clearance exists. Without clearance, comb-like frequency distributions appear from upstream to downstream in the stationary frame at all the three radial positions, and the mode order of the RI changes from 31 to 10. The flow separations occur earlier, and the wake vortices shed periodically, similar to a Kármán vortex street every two passages. According to the cross-correlation analysis of the unsteady pressure signals at different spanwise and circumferential positions, an alternative hypothesis for the RI mechanism is proposed: the instability waves generated by the blade boundary layer separation on the suction side will diffuse throughout the upstream and downstream, and a feedback mechanism will be triggered when the aerodynamic loading exceeds certain thresholds. According to this hypothesis, changing the surface feature of the suction side could have influence on the RI behaviours as well as the corresponding noise and vibration problems.

Direct numerical simulation and stability analysis of the transitional boundary layer on a marine propeller blade

It is generally assumed that the cross-flow instability exists on propeller blade boundary layers due to their similarity with the rotating disk flow. However, to the best of our knowledge, there is no experimental or numerical evidence that directly shows the existence of the cross-flow instability on propellers. In this paper, we present a direct numerical simulation of the boundary layer flow around a marine propeller blade. For the investigated case, the cross-flow velocity inside the attached laminar boundary layer is smaller than the rotating disk case, but it is large enough to lead to the cross-flow instability. The wave-numbers and wave-direction of the cross-flow vortices observed agree well with the prediction of linear stability theory. Linearized Navier–Stokes simulations show that the boundary layer is absolutely unstable in the radial direction but convectively unstable in the streamwise direction. A significant flow direction deviation between laminar and turbulent regions is also observed on the blade surface. In the separation bubble, there is an unusually large radial velocity (around 50% of the inviscid streamwise velocity). Both the flow direction deviation and the large radial velocity in the separation bubble can be explained by the relationship between Coriolis forces and circumferential velocities.

STUDI NUMERIK SEPARASI ALIRAN 3D AKIBAT PENAMBAHAN FFST PADA BIDANG TUMPU AIRFOIL ASIMETRI

3D flow separation is a form of flow loss that cannot be avoided on turbo engines. In the axial compressor, 3D flow separation is due to the interaction between the blade boundary layer and the casing boundary layer or the hub boundary layer. The result of the secondary flow causes blockage of the flow which causes the pressure on the compressor to decrease. Efforts to reduce secondary flow are carried out by adding a FFST to endwall. This research was conducted in a numerical simulation using FLUENT 6.3.26 software. The parameters used in the free stream flow Re = 1.64 x 105 and Turbulence Intensity Tu = 0.3% to assess the comparison of the flow characteristics on the endwall of the British 9C7 / 22.5C50 asymmetric airfoil due to the addition of a FFST and without FFST with variations angle of attack (α) of 00, 80, 120, 140, 160 .The results show that the addition of FFST can increase the turbulent intensity in the area near the wall which turns into momentum, so that it has an impact on the ability of the flow to overcome the adverse pressure in the trailing edge area and further backward (delayed) separation which results in smaller wake. With the addition of the angel of attack, the saddle point position is more directed to the lower side and the attachment line is not induced by the horseshoe vortex, so that the flow is more able to follow the contours of the body, as a result the curling flow is weaker and the wake is narrower and the blockage (energy loss) can be reduced. The most effective energy reduction due to secondary flow through FFST occurs at α = 8 ° at 7.36%.

Numerical simulation of the Mexico wind turbine using the actuator disk model along with the 3D correction of aerodynamic coefficients in OpenFOAM

A foremost element in aerodynamic behavior analysis of wind turbines is numerical simulation. Actuator disk model is a method which enables us to simulate and analyze the flow fields and wakes behind the rotor by reducing the fluid computation around the rotor and solving the Navier-Stokes equations without considering the blade boundary layer. In addition of the need for constructing a realistic geometrical model, one also must run a simulation with very small grid spacing and time steps to resolve the blade boundary layer dynamics leading to the aerodynamic forces, which is also extremely expensive. In this paper actuator disk model along with a 3D corrected aerodynamic coefficients was implemented in the OpenFOAM software to simulate the MEXICO wind turbine rotor. The simulations were performed under three conditions: turbulent, design and stall conditions. The results of simulation including an estimation of the blade forces and the wakes velocity field were compared with the experimental results. It was found that the 3D corrected aerodynamic coefficients of airfoils led to an improved agreement between the simulation and experimental results, compared to a model implementing original aerodynamic coefficients.

Numerical Simulation of a Counter-Rotative Open Rotor Using Phase-Lagged Conditions: Initial Validation on a Single Rotor Case

Abstract This paper presents the large Eddy simulation (LES) of a propeller representative of the first rotor of a counter rotative open rotor (CROR) configuration based on a multiple frequency phase-lagged approach in conjunction with a proper orthogonal decomposition (POD) data storage. This method enables to perform unsteady simulations on multistage turbomachinery configurations including multiple frequency flows with a reduction of the computational domain composed of one single blade passage for each row. This approach is advantageous when no circumferential periodicity occurs in the blade rows of the configuration and a full 360 deg simulation would be required. The data storage method is based on a POD decomposition replacing the traditional Fourier series decomposition (FSD). The inherent limitation of phase-shifted periodicity assumption remains with POD data storage but this compression method alleviates some issues associated with the Fourier transform, especially spectrum issues. The paper is first dedicated to compare the flow field obtained with the LES with phase-lagged condition against full-matching URANS, LES simulations, and experimental data available around the blade and in the wake of the rotor. The study shows a close agreement of the phase-lagged LES simulation with other simulations performed and a thicker wake compared with the experiments with lower turbulent activity. The analysis of the losses generated in the configuration, based on an entropy formulation and a splitting between boundary layer and secondary flow structures, shows the strong contribution of the blade boundary layer in the losses generated.