Blade Wake Research Articles

Some Aspects of the Transonic Compressor Tandem Design

For the development of the latest generation of axial compressors, it is necessary to enlarge the design space by using advanced aerodynamic processes. This enables a further increase in efficiency and performance. The use of a tandem blade configuration in a transonic compressor row provides the possibility to enlarge the design space. It is necessary to address the design aspects a bit more in detail in order to efficiently apply this blading concept to turbomachinery. Therefore, in the current study, the known design aspects of tandem blading in compressors will be summed up under the consideration of the aerodynamic effects and construction characteristics of a transonic compressor tandem. Based on this knowledge, a new transonic compressor tandem cascade (DLR TTC) with an inflow Mach number of 0.9 is designed using modern numerical methods and a multi-objective optimization process. Three objective functions as well as three operating points are used in the optimization. Furthermore, both tandem blades and their arrangement are parameterized. From the resulting database of 1246 members, a final best member is chosen as the state-of-the-art design for further detailed investigation. The aim of the ensuing experimental and numerical investigation is to answer the question, whether the tandem cascade resulting from the modern design process fulfills the described design aspects and delivers the requested performance and efficiency criteria. The numerical simulations within the study are carried out with the DLR flow solver TRACE. The experiments are performed at the transonic cascade wind tunnel of DLR in Cologne. The inflow Mach number during the tests is 0.9, and the AVDR is adjusted to 1.3 (design value). Wake measurements with a three-hole probe are carried out in order to determine the cascade performance. The experimental results show an increase in losses and a reduction of the cascade deflection by about 2 deg compared to the design concept. Nevertheless, the experimental and numerical results allow a good understanding of the aerodynamic effects. In addition, planar PIV was applied in a single S1 plane located at midspan to capture the velocity field in the wake of blade 1 in order to analyze the wake flow in detail and describe its influence on the cascade deflection and loss behavior. Finally, an outlook will be given on what future tandem compressor research should be focused.

Entropy production analysis for the clocking effect between inducer and impeller in a high-speed centrifugal pump

The clocking effect between the inducer and the impeller has a certain impact on the performance of the high-speed centrifugal pump, which however, is often ignored by designers. In the present study, three-dimensional numerical simulation based on Reynolds-averaged Navier–Stokes method is adopted to evaluate the influence of this clocking effect on the performance of a full-scale liquid rocket engine oxygen turbopump. A novel entropy production method with the correction of wall effects was introduced to evaluate the energy loss generated in the pump and to clarify the formation mechanism of this clocking effect from the perspective of the second law of thermodynamics. Results show that the best performance is captured when the relative circumferential angle between the inducer blade trailing edge and the impeller blade leading edge was set as 0° and the maximum difference in pump efficiency is approximately 1.5% at different clocking positions. The entropy production analysis of each component of the pump reveals that the clocking effect on the pump performance mainly originates from the turbulent dissipation in the impeller and the diffuser. The study of the local entropy production rate and the streamline distributions shows that the formation of this clocking effect is owing to the different extent of the separation vortices in the impeller passage near the shroud and the impeller blade wake in the diffuser inlet as well as the backflow vortices in the diffuser blade passage near the volute tongue.

A strongly-coupled model for flexible rotors

A fluid–structure model describing the equilibrium state of a flexible blade rotor with its own wake is derived for various external axial flow conditions. The model is based on three building blocks. The two-dimensional lifting-line theory is first used to compute the local aerodynamic loads and the blade circulation profile. The blade deformation is then obtained by solving the nonlinear equations for bending and twisting angles deduced from a one-dimensional beam model. Finally, the wake is obtained using a Joukowski model. In this wake model, the wake of each blade is modeled by two small-core-size counter-rotating vortices emitted from the rotor axis and blade tip. The velocity field induced by these vortices is computed using the Biot–Savart law. We show that, in the rotor frame, we can obtain a stationary vortex structure for almost any vertical flight regimes. This wake solution can then be used to compute the induced velocity in the rotor plane and apply the two-dimensional lifting-line theory again. By iterating a few times this loop, we converge toward a nonlinear solution of the problem for which the aerodynamics loads, blade deformation and wake structure are compatible.As illustration, this newly-developed model is applied to two rotors. We analyze the effects of the external wind conditions, geometry and material properties of the blades on the blade deformation and wake characteristics. We show that we can describe slow descending regimes for which the classical momentum theory does not apply.

Steady wind performance of a 5 kW three-bladed H-rotor Darrieus Vertical Axis Wind Turbine (VAWT) with cambered tubercle leading edge (TLE) blades

Abstract The performance of a 5 kW three-bladed H-rotor Darrieus Vertical Axis Wind Turbine with cambered Tubercle Leading Edge NACA 0025 blades was established using computational fluid dynamics. 3D models and simulations of the VAWT were realized using CAD and CAE software, after which post-processing analyses provided understanding of the VAWT and blade flow physics. Results showed the TLE to be detrimental to flow and performance of a cambered VAWT. Using torque, lift and drag data, the cambered TLE VAWT was shown to deviate significantly against the cambered VAWT over one complete converged rotation. Cambered TLE VAWT blades encounter reduced lift forces and increased drag forces leading to lowered and eventual reversal of torque values. The z-vorticity and Q-criterion visualizations further supported the numerical results with the post-processed images showing vortices the size of the blade chord generated at blade wakes of the cambered TLE. The vortices emanate from the flow separation at the blade crests creeping in at the blade trailing edge at 75° azimuth and reaching halfway through the blade chord length at 106° azimuth. Spanwise separation was observed to be restricted at blade crests. The resulting flow degradation translated to the poor performance of the wind turbine.

A spline ROM of blade aerodynamic force to upstream wake

All related research nowadays are devoted to build ROM of aerodynamic forces induced by vibration of blades or wings. There is no discussion of ROMs of the aerodynamic forces of the blades corresponding to upstream wakes, which is more important to turbine machines' safety. In this paper, a spline Volterra ROM method is proposed to predict the blades' aerodynamic forces induced by the upstream wakes to fasten the blade–wake interaction analysis in turbine machine design. Traveling wave method is applied to simplify the inputs representing the upstream wakes. The kernels of the Volterra ROM are expressed in spline to reduce the number of parameters to be identified. The coefficients of the spline kernels are identified by minimizing the difference between ROM results and CFD data. The ROM method is used to predict a blade's aerodynamic forces under an actual wake. The results show that the ROM can capture most information of the aerodynamic forces due to the wake.

The effect of blade pitch on the flow dynamics inside the rotor of a three-straight-bladed cross-flow turbine

This study analyses the variations in flow dynamics inside the rotor of a three-straight-bladed cross-flow turbine for different blade pitches. A water towing tank facility has been used with a turbine model based on symmetric NACA-0015 profiles with a chord-to-diameter ratio of 0.16. The set-up was especially designed to measure the flow field inside and around the rotor, using planar digital particle image velocimetry. The experiments were made at a constant turbine diameter Reynolds number, [Formula: see text], of 6.1 × 104. The forced rotation of the cross-flow turbine using a direct current motor allowed changes in the ratio of the blade tangential speed to the freestream velocity (tip speed ratio), covering the range of 0.7–2.3. Toe-in and excessive toe-out angles have been associated in the past to low performances in this type of turbines. Pitch angle cases with expected low performances have been chosen, namely, [Formula: see text] of 8° toe-in and 16° toe-out, as well as a case with a high expected efficiency of 8° toe-out and another with [Formula: see text] as the reference case. The the goal is to better understand the flow dynamics and the blade–wake interactions inside the rotor of this type of machines, by studying cases with very different expected performances.

Advanced Medium-Order Modelling of a Wind Turbine Wake with a Vortex Particle Method Integrated within a Multilevel Code

The current paper describes an aerodynamic model for treatment of wind turbine wakes. For accurate treatment of the wake evolution for the near wake, along with interaction with local flow features, a model with low numerical diffusion has been chosen, a vortex particle method, which inherently allows treatment also of shearing effects and viscous diffusion. Treatment of blade loading is facilitated with the use of a lifting-line model. Details of correct specification of distributed and shed vortical elements in the blade wake are provided. Reduction of the computation cost has been achieved by implementing the model within a multilevel framework. In addition the model has been highly parallelised, so that relatively quick simulations at high fidelity can be achieved on the order of seconds. The ability of the model to produce results of comparable accuracy to CFD is demonstrated by comparison to the MEXICO test rotor.

A panel method for floating offshore wind turbine simulations with fully integrated aero- and hydrodynamic modelling in time domain

ABSTRACTThe further development of the first-order panel method panMARE for simulating floating offshore wind turbines (FOWTs) in time domain is presented in this work. Based on a surface discretisation of platform, tower and rotor with its wake, the three-dimensional aerodynamic and hydrodynamic flow fields are calculated. A free wake deformation model is integrated to capture blade–wake interaction. Hydrodynamic, aerodynamic and mooring loads are cumulated by a six-degrees-of-freedom solver to compute the motion of the FOWT. The presented method is able to simulate unsteady and aperiodic motions and to predict the wake deformations and their influence on the rotor loads due to the platform motion. In order to verify the method, simulation results of the DeepCWind floater with the NREL 5-MW baseline turbine are compared with those obtained in the OC4 project. The ability to capture highly unsteady situations is studied as well by simulating a gust with varying length.

The Influence of Different Wake Profiles on Losses in a Low Pressure Turbine Cascade

Large eddy simulations were carried out in order to investigate the influence of unsteady incoming wakes with different profiles on the loss mechanisms of the high lift T106Alinear low-pressure turbine (LPT) cascade. Bars placed upstream of the LPT blade were set into rotation around their axis, thus generating circulation, as well as asymmetrical wake profiles. Three different rotation rates were simulated, yielding different wake parameters that were then compared to an actual turbine blade wake profile. Whereas the commonly-used non-rotating bars generated wakes with turbulent kinetic energy levels several times higher than that of an actual blade wake, the case with counter-clockwise rotation led to more rapid wake mixing. All three wakes were able to trigger boundary layer transition and thus intermittently prevent separation on the suction surface. However, the weaker the wakes, the larger and longer lasting the separation bubbles became, and an increase in profile losses could be observed. Interestingly, the configuration with the weakest wake and the largest separation bubble resulted in a reduction of the overall LPT loss.

Prediction of the Open-Water Performance of Ducted Propellers with a Panel Method

In the present work, a comparison between the results obtained by a panel code with a Reynolds-averaged Navier-Stokes (RANS) code is made to obtain a better insight on the viscous effects of the ducted propeller and on the limitations of the inviscid flow model, especially near bollard pull conditions or low advance ratios, which are important in the design stage. The analysis is carried out for propeller Ka4-70 operating inside duct 19A. From the comparison, several modelling aspects are studied for improvement of the inviscid (potential) flow solution. Finally, the experimental open-water data is compared with the panel method and RANS solutions. A strong influence of the blade wake pitch, especially near the blade tip, on the ducted propeller force predictions is seen. A reduction of the pitch of the gap strip is proposed for improvement of the performance prediction at low advance ratios.

Similarity of the Output Characteristics of the Small Vertical Axis Wind Turbine

Several models have been proposed to predict the shaft output characteristics of the vertical axis wind turbine from the aerodynamic characteristics of the blade. However, it has not predict the output characteristics of the small wind turbine even the multiple stream tube model included effect of the momentum reduction. Therefore, the purpose of this study was to find out a similarity parameter of the shaft output characteristics based on blade wake interference to establish a prediction model for the small wind turbine. From the measurement of the blade wake using the turbine model with single blade, the scale of the blade wake depends on the ratio of the rotational speed to the free stream velocity, and the advection velocity of it depends on the free stream velocity. The shaft output characteristics of the small wind turbine indicate good similarity for the parameter, λ3σn/D , based on interference of blade wake.

Numerical Investigation of the Impact of the Compressor Operation Mode on Working Process of the Combustion Chamber

The method of integrated compressor/combustor simulation was used to investigate the impact of flow distortion, appeared due to compressor blades, during the combustion chamber workflow. The method was improved in terms of generating a common grid and of principles of the boundary conditions settings. The geometric model includes four geometric volume bodies: guide vanes of the penultimate stage of high-pressure compressor, the impeller and guide vanes of the last stage and the flow path of combustion chamber. The calculation was carried out for some operation mode of the engine (nominal, 0.7 of nominal and 0.5 of nominal regimes) with and without compressor. The results were compared with the results of combustion chamber simulation without the compressor. Simulations showed that blade wakes extend up to the flame tube head. These wakes influence on the flame tongue, pressure field, temperature and velocity in the recirculation-mixing zone. It can influence on combustion efficiency, ecological performance and on temperature field at the combustor outlet. Thus, the simulations, which take into account combustion chamber and compressor, are more fully represent the characteristics of the working process of the combustion chamber and increase the efficiency of the design of new products.

Identification of flow phenomena over NACA 4412 wind turbine airfoil at low Reynolds numbers and role of laminar separation bubble on flow evolution

Objective of this study is to elucidate relationship with aerodynamic properties and vortex shedding from suction surface and wake of wind turbine blade at low Reynolds numbers. Force measurement of NACA 4412 airfoil was conducted at various angles of attack and Reynolds numbers. Furthermore, smoke-wire experiment was performed to clearly visualize flow patterns such as flow separation or laminar separation bubble (LSB) over the airfoil. Also, velocity measurements at near wake region were done to highlight coactions among LSB and trailing edge vortices. Hot-film experiment was performed to obtain more information with regards to progress and formation of LSB. Experiments indicated that location and formation of LSB were affected by variety of both Reynolds number and angle of attack. Besides, mean and rms velocities showed that flow characteristics near wake were influenced by LSB and trailing-edge separation. At lower angles of attack, short bubble occurred, and frequency of vortex shedding due to short bubble were high whereas shedding frequency of long bubble at moderate angles of attack were low. The bubbles can cause vibration and noise at the wind turbine blade, hence they should be better known and eliminated, which aerodynamic performance and energy efficiency of turbine can be increased.

DNS investigation of the dynamical behaviour of trailing vortices in unbaffled stirred vessels at transitional Reynolds numbers

Flow in an unbaffled stirred vessel agitated by a four-bladed radial impeller is investigated by using direct numerical simulations at Re = 320 and 1600. We observe fluctuations in the power consumption with a peak frequency at ca. three times the impeller rotational speed for both Reynolds numbers. It is discovered that these fluctuations are associated with a periodic event in the wake of the blades, which involves alternating growth and decay of the upper and lower cores of the trailing vortex pair as well as up-and-down swinging motion of the radial jet. Moreover, the phase relation between the wakes of the different blades is examined in detail. Further studies using fractal-shaped blades show that the exact blade shape does not have a strong influence on this phenomenon. However, the wake interaction between the blades, hence the number of blades, has a direct influence on the unsteadiness of trailing vortices.

Near wake region of an industrial scale wind turbine: comparing LES-ALM with LES-SMI simulations using data mining (POD)

Accurate prediction of power generation capability needs proper assessment of blade loading and wake behavior. In this regard, the Sliding Mesh Interface (SMI) approach and the Actuator Line Model (ALM) are two diverse computational fluid dynamics (CFD) based approaches of simulating the turbine behavior, each having its own merits and demerits. The SMI technique simulates the unsteady flow by explicitly modeling the blades and their rotation using a dynamic mesh, while in Actuator Line Model, the blades are not modeled explicitly but each blade is resolved as a rotating line (made of N actuator segments), over which the forces are computed. The current work focuses on simulating an industrial scale reference turbine and in differentiating the near wake dynamics predicted by these two approaches using Large Eddy Simulation (LES) and Proper Orthogonal Decomposition (POD) technique (a data mining tool). Initially, the ALM is compared with FAST model for the prediction of variation of power coefficient with the Tip Speed Ratio (TSR). The ALM is able to capture the varying trend and it predicts a similar optimum tip speed ratio as the FAST model. At this optimum TSR condition, the ALM is compared with the SMI method for a study limited to the near wake region. Comparisons between SMI and ALM shows that : (a) The SMI is predicting more complex 3D nature of the flow, and (b) the POD shows that ALM captures the shear regions of wake but it does not capture the vast compendium of length and time scales of eddies as SMI does. However, despite these limitations, the ALM has been able to capture the qualitative trend in wake deficit and the power coefficient variation with tip speed.

Direct Numerical Simulation Based Analysis of RANS Predictions of a Low-Pressure Turbine Cascade

The state-of-the-art design of turbomachinery components is based on Reynolds-averaged Navier–Stokes (RANS) solutions. RANS solvers model the effects of turbulence and boundary layer transition and therefore allow for a rapid prediction of the aerodynamic behavior. The only drawback is that modeling errors are introduced to the solution. Researchers and computational fluid dynamics developers are working on reducing these errors by improved model calibrations which are based on experimental data. These experiments do not typically, however, offer detailed insight into three-dimensional flow fields and the evolution of model quantities in an actual machine. This can be achieved through a direct step-by-step comparison of model quantities between RANS and direct numerical simulation (DNS). In the present work, the experimentally obtained model correlations are recomputed based on DNS of the same turbine profile simulated by RANS. The actual local values are compared to the modeled RANS results, providing information about the source of model deficits. The focus is on the transition process on the blade suction side (SS) and on evaluating the development of turbulent flow structures in the blade's wake. It is shown that the source of disagreement between RANS and DNS can be traced back to three major deficiencies that should be the focus of further model improvements.

Wind Tunnel Validation of Double Multiple Streamtube Model for Vertical Axis Wind Turbine

The performance and annual energy output have to be predicted to maximize the economic benefits from a wind turbine. Mathematically predicting the performance of Darrieus type lift based turbines are challenging due to the inconsistent angle of attack, blade wake interaction and local induced velocities giving rise to complex flow physics. A reliable and validated mathematical model is therefore essential to optimize the various design parameters prior to manufacture. The objective of the current study is to evaluate widely employed aerodynamic models based on their prediction accuracy, limitations, and computational requirements. Double multiple stream tube models have been discussed in detail and the predictions are experimentally validated through the wind tunnel test of three-bladed H-Darrieus rotor in terms of torque and power coefficient. The possible sources for the deviation between the predicted and measured values have been discussed and concluded with potential solutions.

Influence of Solidity and Wind Shear on the Performance of VAWT using a Free Vortex Model

Performance analysis of a VAWT and HAWT is highly complex due to fluid structure interactions and blade vortex interactions.However, there are simplified methods such as momentum theory to most expensive CFD modelsavailable forperformance prediction. CACTUS is neither as simple as the momentum theory nor as complex as the CFD models, it is an open source code that uses the free vortex method to predict the performance of a wind turbine.In this paper, the effect of solidity and wind shear on the performance of an H-type Darrius VAWT is studied using CACTUS. Variation of solidity was achieved by changing the chord length ( c/R 0.04-0.07) and number of blades ( N 2,3). It has been observed that at lower tip speed ratio (TSR 3) shorter c/R was more efficient due to relatively low wake blade interaction. Effect of wind shear due to the tower height from the ground using the power law equation with values of power law coefficient ranging from 0.1 to 0.3has also been studied.Itis observedthat the power coefficients of the VAWT under a turbulent boundary layer are in congruence with the experimental results.

An improved prediction model of vortex shedding noise from blades of fans

The main source of the noise of an axial flow fan is the fluctuating pressure field on blade surfaces caused by the shedding of vortices at the trailing edge of blades. An analytical model to predict the vortex shedding noise generated at the trailing edge of blades of axial flow fans was proposed by Lee in 1993. In this model, for mathematical convenience, an idealized vortex street is considered. However, the agreement between the analytical results and the experimental data needs to be improved because of the simplification about the Karman vortex street in the wake of blade. In the present study, a modified model is proposed based on the prediction model by Lee. The boundary layer theory is used to analyze and calculate the boundary layer development on both the pressure and the suction sides of blades. Considering the effect of boundary layer separation on the location of noise source, the predicted overall sound pressure level compares favorably with the experimental data of an axial fan. In the calculation of A-weighted sound pressure level (LA), considering the effect of static pressure on radiate energy, the predicted broadband noise with the modified model compares favorably with the experimental data of a multiblade centrifugal fan.

Synthesis of Active Twist Controller for Rotor Blade–Vortex Interaction Noise Alleviation

This paper deals with an efficient computational process for the synthesis of a low-frequency controller aimed at alleviating helicopter rotor blade–vortex interaction noise. It is based on an optimal, multicyclic, control approach that exploits distributed torque loads actuated by smart materials to twist rotor blades at the 2/rev frequency (active twist rotor concept). Aeroacoustic rotor simulations are based on aerodynamic and aeroelastic tools capable of accurately predicting wake–blade miss distance, which plays a crucial role in blade–vortex interaction noise emission. Its modification is the main objective of the proposed controller action. The control law is identified through a numerically efficient aeroacoustic solver based on analytical–numerical sectional aerodynamics modeling. Numerical investigations first examine noise emission sensitivity to active twist actuation, then identify control and output variables suitable for the closed-loop controller. Finally effectiveness of the proposed low-frequency feedback control law for blade–vortex interaction noise alleviation is assessed by application to helicopter rotor in descent flight.