Axial Flow Conditions Research Articles

A Simplified Model for Propeller Thrust in Oblique Flow

New aircraft architectures are being proposed for unmanned aerial vehicles and air taxis, which include tilt-able motor and propellers. These propulsive units operate with a propeller axis at an angle oblique to the flight direction, and thus it is important to understand and model how thrust is produced by a propeller operating under these conditions. Propellers in oblique flow have been modeled using Blade Element Momentum Theory coupled to an inflow model, and the Vortex Lattice Method. In the present work, we develop a much simpler approach that neglects the crossflow component of the incoming air velocity. An advance ratio is developed based on the parallel inflow component, and is coupled to existing propeller data collected in axial flow conditions. The proposed model is evaluated using existing experimental data collected under oblique flow conditions, and is predicts thrust to within \(5\,\%\) of experimental values for most conditions. The greatest discrepancy between the model and experiments occurs in the pure crossflow case, which is of lesser importance in the application to unmanned aerial vehicles and air taxis.

Aerodynamic Characterization of the IEA 15 MW Reference Wind Turbine by Code-to-Code Comparison

The consistency of different aerodynamic formulations applied to the analysis of a modern multi-megawatt horizontal axis wind turbine rotor is investigated. The proposed code-to-code comparison involves specific implementations of a hierarchy of solvers based on Blade Element Momentum Theory (AEOLIAN), Actuator Line Modelling (OpenFOAM), free-wake Panel Method (FUNAERO) and blade-resolved Computational Fluid Dynamics (OpenFOAM ). The analysis addresses local and integral aeroloads and flow physical quantities concerning the state-of-the-art IEA 15 MW reference wind turbine in axial uniform flow conditions. The proposed solvers predict consistent rotor performance and blade aeroloads (also in line with data from of IEA Task 47). However, differences emerge close to blade root, where blade-resolved CFD reveals a significant flow separation on the suction side. Furthermore, scattering of induction factors computations is observed, especially in the axial direction. Different methodologies and numerical setup used in blade-resolved simulations allow achieving physically-consistent induction values, especially at blade tip. Finally, flow-field predictions by Computational Fluid Dynamics (CFD) and Panel Method are consistent upstream and close to the disk downstream (except where significant flow separation occurs), whilst a more detailed study on the effect of extending wake refinement zone in CFD simulation is advisable.

Hydrodynamic design of the Separate Effect test facility for Flow-Accelerated Corrosion and Erosion (SEFACE) studies in liquid lead

Flow-accelerated corrosion and erosion (FACE) phenomena can be crucial for performance of structural elements in heavy liquid metal (HLM) cooled reactor systems. Existing experimental observations indicate that turbulent flow characteristic can affect FACE, but there is no quantitative data that can be used for model development and validation. Main recirculation pump impellers, which operate at high relative velocities and rotational flow conditions can be especially vulnerable to FACE. For comparison, the core internals operate at lower velocities and in axial flow conditions, but at higher temperatures and neutron fluence. Hence, systematic experimental data is needed to improve our knowledge on FACE phenomena. The Separate Effect Test Facility for Flow-Accelerated Corrosion and Erosion (SEFACE) is designed to obtain such experimental data including high relative velocities (up 20 ms−1) and high temperatures (400 to 550 °C) of liquid lead. This article focuses on the hydrodynamic design of SEFACE. The aim of the design is to achieve well defined flow conditions for experiments and ensure safe operation of the facility. First, we examine three design concepts (i.e., forced convection loop, rotating cylinder, and rotating disk) and motivate the choice of the rotating disk approach for SEFACE. Second, we discuss different design options, i.e., a confined rotor–stator test chamber and the unconfined rotating disk configuration. We used Reynolds-Averaged Navier Stokes (RANS) calculations to identify and solve the issues stemming from the high rotational speed. These include, for instance, lead free surface deformation, radial pressure buildup, and axial bending forces due to asymmetric test chamber. The CFD-derived torque and power predictions in rotor–stator and rotating disk systems are verified with selected empirical turbulent friction factor correlations or/and DNS calculations. We demonstrate that the developed hydrodynamic design of SEFACE solves identified issues and enables obtaining experimental data under well-defined flow conditions. The findings are deemed to also be applicable to the design of rotating disk-type FACE installations for other liquid mediums.

Flow Confinement Effects on sUAS Rotor Noise

The goal of this paper is to investigate the effects of flow confinement on the noise generated by a small unmanned aerial system rotor measured and simulated in a partially closed test room. More specifically, the influence of the flow recirculation in the test room and the consequent blade turbulence interaction on both the tonal unsteady loading noise and the broadband laminar separation noise components are scrutinized. The study also aims at improving our prediction capabilities of drone rotor noise, and deepening our fundamental knowledge of rotor aeroacoustics in transitional boundary layer regimes. Numerical simulations are conducted by means of a scale-resolving wall-modelled lattice Boltzmann solver to calculate the unsteady flow and the noise generated by the same reference configuration used in our previous works, which consists in a rotor operated at a chord-based Reynolds number of about 70000 at 75% of the tip radius, both in hover and axial flow conditions. Results for a rotor in pristine flow conditions are compared to results obtained by simulating the rotor in a confined environment that reproduces the A-Tunnel test chamber of Delft University of Technology. One one hand, our study reveals that previously observed discrepancies between measured and predicted noise spectra in hover conditions are not due to the influence of the flow confinement on the laminar separation mechanisms, as hypothesized in our previous study, rather to a lack of numerical resolution. On the other hand, the new results reveal that, in hover conditions, the dominant effect of the flow recirculation is to increase the unsteady loading tonal noise components, whereas, in axial flow conditions, no significant effect due to the test environment is observed. Some of our conclusive observations are supported by results obtained using a source identification technique herein presented for the first time.

Evaluation of the influence of the viscous sublayer on the mechanical stability of fuel plates under axial flow conditions

The current work aims to investigate the influence of the viscous sublayer on the mechanical stability of fuel element plates under axial flow conditions by means of two-way Fluid Structure Interaction (FSI) numerical simulations. The methodology adopted is that proposed by (Mantecón, 2019; Mantecón and Mattar Neto, 2018), who observed a transition from linear to non-linear behavior between the maximum deflection of the plates in their leading edge with the square of the velocity of the cooling fluid in the channel. The speed at which the transition is identified is the critical speed (Vc). In order to verify the influence of the viscous effects, the CFD domain was discretized from its viscous sublayer. As this approach greatly increases the computational cost, where the characteristics of the flow allowed, symmetry boundary conditions were used. In addition to this approach, it was decided to investigate the ability to solve the FSI problem in steady state. The obtained results confirmed that the boundary layer modeling is sufficient to determine the critical velocity. Furthermore, they also suggest that the steady-state approach and the application of symmetry boundary conditions, where possible, can be used in the design of new fuel elements, supporting traditional methods.

Progress in the validation of rotor aerodynamic codes using field data

Abstract. Within the framework of the fourth phase of the International Energy Agency (IEA) Wind Task 29, a large comparison exercise between measurements and aeroelastic simulations has been carried out featuring three simulation cases in axial, sheared and yawed inflow conditions. Results were obtained from more than 19 simulation tools originating from 12 institutes, ranging in fidelity from blade element momentum (BEM) to computational fluid dynamics (CFDs) and compared to state-of-the-art field measurements from the 2 MW DanAero turbine. More than 15 different variable types ranging from lifting-line variables to blade surface pressures, loads and velocities have been compared for the different conditions, resulting in over 250 comparison plots. The result is a unique insight into the current status and accuracy of rotor aerodynamic modeling. For axial flow conditions, a good agreement was found between the various code types, where a dedicated grid sensitivity study was necessary for the CFD simulations. However, compared to wind tunnel experiments on rotors featuring controlled conditions, it remains a challenge to achieve good agreement of absolute levels between simulations and measurements in the field. For sheared inflow conditions, uncertainties due to rotational and unsteady effects on airfoil data result in the CFD predictions standing out above the codes that need input of sectional airfoil data. However, it was demonstrated that using CFD-synthesized airfoil data is an effective means to bypass this shortcoming. For yawed flow conditions, it was observed that modeling of the skewed wake effect is still problematic for BEM codes where CFD and free vortex wake codes inherently model the underlying physics correctly. The next step is a comparison in turbulent inflow conditions, which is featured in IEA Wind Task 47. Doing this analysis in cooperation under the auspices of the IEA Wind Technology Collaboration Program (TCP) has led to many mutual benefits for the participants. The large size of the consortium brought ample manpower for the analysis where the learning process by combining several complementary experiences and modeling techniques gave valuable insights that could not be found when the analysis is carried out individually.

Numerical investigation of hydraulic instability of circular arc shell-type fuel elements under axial flow conditions

Shell-type fuel elements in nuclear assemblies can collapse under high-speed coolant flows and impede channels, which is known as hydraulic instability. Stiffer than flat plates, curved shells are commonly in involute shape and circular arc shape. In this study, two parallel circular arc shells with three channels are numerically simulated for hydraulic instability prediction. Fluid-structure interaction between shells and fluid is achieved, and results show that the two arc shells are asymmetrically deformed in opposite directions. A “valley morph” pops near the leading edge at a channel velocity and becomes increasingly visible as the flow rate rises. The maximum deflection occurs at the leading edge, which is linearly related to the squared channel velocity below a value but nonlinearly steeply ascends otherwise. The two characteristic velocities are congruent and thus can be deemed as the “critical velocity” at which the response of fuel elements changes although shells remain elastic all through. The derived critical velocity is about 80% of Miller’s theoretical estimate.

Experimental studies on combustion performance of beeswax-paraffin blended solid fuels in a hybrid rocket

The study intends to investigate the physical, chemical and thermal characteristics of paraffin blended fuels to determine their suitability as a solid fuel in a hybrid rocket. Wax fuels are a viable and efficient alternative to conventional rocket fuels, having excellent structural strength and thermal and mechanical properties. By utilizing both axial and swirl injection technique, the combustion performance of paraffin – beeswax blended fuels have been tested with a fabricated cylindrical grain in a laboratory-scale rocket setting along with oxygen. The test outcomes revealed solid fuel compositions of more beeswax content in paraffin wax on an oxygenated gaseous environment with a swirl-flow injection method has the highest average regression rate of 1.649 mm/sec at 181 kg/m2s mass flux. Axially injected oxygen with pure paraffin wax has the lowest value of 0.85 mm/sec at 96 kg/m2s. The regression rate comparisons revealed that oxygen injection by a swirl injector increased the regression rates by 40% for mass fluxes greater than 80 kg/m2s. Compared to other studies, the combustion efficiencies have been obtained in this study are good. Blended fuels can manage and increase combustion efficiencies for axial and swirl flow conditions. Swirl injectors outperform axial injectors for oxygen injection and allow for a higher proportion of Beeswax combined with paraffin. This study exclusively designed and manufactured an axial injector and swirl injector, according to the required dimensions of a lab-scale hybrid rocket's combustion chamber, injector, and exhaust nozzle, and their performances have been evaluated.

Development of a fluid structure interaction tool based on an actuator line model

In the present paper the methodology and the procedures for the implicit coupling of an Actuator Line (AL) aerodynamic code with a beam like structural code for the analysis of wind turbine rotors are detailed. Results from benchmark aeroelastic simulations of canonical inflow conditions, comparing the newly developed AL model against a standard Blade Element Momentum (BEM) model are presented in the paper. The two models provide very similar results in simple, uniform inflow, axisymmetric flow cases. The advantages of this newly developed tool emerge when more complex inflow conditions are addressed. In the present paper, besides axial flow conditions, operation under high yaw misalignment is also considered. BEM model accounts for the effect of the wake skewness through the application of an a posteriori engineering correction. Therefore, in this particular non symmetric flow case, deviations between AL and BEM are expected to be higher, especially as yaw misalignment angles increase. In the paper the above differences are assessed and interpreted.

Application of multi-axis force/torque sensor system for experimental study of small unmanned aerial vehicles propulsion systems – Preliminary results

The experimental evaluation of the performance of small propellers plays important role in the process of multicopter UAV design perfection. This paper presents a small UAV propeller test stand, based on a multi-axis Force/Torque sensor system. The system makes possible to measure the thrust, torque and the pitching moment or lateral force (when in oblique flow) of the tested propeller. The data processing of the experimental results is discussed. Results for static, axial flow and oblique flow conditions are presented. Some conclusions about the measurement methodology are drawn. The results verify that the test bench has good accuracy and is easy for operation.

A Review of Passive Micromixers with a Comparative Analysis.

A wide range of existing passive micromixers are reviewed, and quantitative analyses of ten typical passive micromixers were performed to compare their mixing indices, pressure drops, and mixing costs under the same axial length and flow conditions across a wide Reynolds number range of 0.01–120. The tested micromixers were selected from five types of micromixer designs. The analyses of flow and mixing were performed using continuity, Navier-Stokes and convection-diffusion equations. The results of the comparative analysis were presented for three different Reynolds number ranges: low-Re (Re ≤ 1), intermediate-Re (1 < Re ≤ 40), and high-Re (Re > 40) ranges, where the mixing mechanisms are different. The results show a two-dimensional micromixer of Tesla structure is recommended in the intermediate- and high-Re ranges, while two three-dimensional micromixers with two layers are recommended in the low-Re range due to their excellent mixing performance.

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.

Numerical Simulation of Horizontal Axis Wind Turbines with Vortex Generators

In the present study, a simulation about the effects of vortex generators on horizontal axis wind turbine rotor blade was numerically conducted using a static coupled CFD–CSD method. A Navier–Stokes CFD flow solver based on unstructured meshes was used to obtain the blade aerodynamic loads. A FEM-based CSD solver employing a nonlinear coupled flap-lag-torsion beam theory was utilized to calculate the blade elastic deformation. The coupling of the CFD and CSD solvers was accomplished in a loosely coupled manner by exchanging the information between the two solvers at infrequent intervals. The static coupled CFD–CSD method was applied to the NREL 5 MW reference wind turbine rotor under steady axial flow conditions. Triangular counter-rotating vortex generators were adopted to control flow separation and radial flow in the inboard section of the NREL 5 MW reference rotor blades. They were installed on the inboard part of the blade from 0.2 to 0.4 R. As a result of the flow analysis considering the counter-rotating vortex generators, strong vortices were generated by counter-rotating vortex generators. It can be seen that the regions where flow separation and radial flow occur in the inboard sections were reduced compared to the baseline wind turbine. For this reason, the maximum power improvement due to counter-rotating vortex generators was 1.04% at the rated wind speed.

An improved model for the tangential velocity distribution in strong free-surface vortices: an experimental and theoretical study

ABSTRACTA study of the tangential velocity field in a strong free-surface vortex is presented. Experiments were conducted on an open channel scroll type vortex chamber by investigating the tangential velocity for three approach flow conditions. The tangential velocity and circulation fields were ascertained at various sub-surface depths using particle tracking techniques. The results revealed that the experimental tangential velocity distribution is dependent on the initial circulation but is largely independent of the vertical axis. The theoretical tangential velocity profile was found to diverge from the experimental data in the near-field close to the core. It was concluded that the tangential velocity field in this region is strongly dependent on the axial flow conditions. Consequently, an alternative tangential velocity model was developed which models the near-field axial flow effects using the inverse circulation number and an empirical coefficient. Application of a nonlinear least squares multiple regression analysis identified values for the empirical coefficient to obtain a good solution.

Experimental Analysis of a Small-Scale Rotor at Various Inflow Angles

The performance characteristics of a rotor that is typically used for small unmanned aircraft were analyzed in a series of wind-tunnel experiments. Wind-tunnel measurements were conducted with the rotor at various inflow angles in order to investigate the effects on the rotor performance of partially or fully edgewise flow as they are typically encountered with small multirotor vehicles. Rotor tests were also performed under static and fully axial flow conditions in order to investigate the aerodynamic performance during hover as well as vertical climb and descent. The wind-tunnel data were corrected to account for the interference of wind-tunnel walls with the rotor wake and the blockage due to the presence of the rotor test stand in the wind-tunnel test section. The results are presented in terms of thrust, power, and roll moment coefficients under different rotor rotational speeds for a T-motor 18x6.1. Additionally, the measured thrust and power coefficients of Master Airscrew Electric 11x7 are compared with available propeller data under static and axial flow conditions for verification purposes. It is shown that the rotor performance characteristics are strongly affected by the freestream advance ratio and the freestream inflow angles. For example, at inflow angles that are typical for multirotor vehicles between about 15° and 0° with respect to the rotor disc, thrust coefficients stay constant or grow with increasing advance ratio, whereas power coefficients remain relatively constant with changing advance ratio.

Numerical methodology for fluid-structure interaction analysis of nuclear fuel plates under axial flow conditions

Shell-type fuel elements are widely used in nuclear research reactors. The nuclear fuel is contained in parallel shells, flat or curved, that are separated by narrow channels through which the fluid flows to remove the heat generated by fission reactions. A major problem of this fuel assembly design is the hydraulic instability of the shells caused by the high flow velocities. The objective of the study presented here is the development of a fluid-structure interaction methodology to investigate numerically the onset of hydroelastic instability of flat-shell-type fuel elements, also known as plate-type fuel assemblies, under axial flow conditions. The system analyzed consists of two nuclear fuel plates bounded by three-equal coolant channels. It is developed using the commercial codes ANSYS CFX for modeling the fluid flow and ANSYS Mechanical to model the plates. The fluid-structure interaction methodology predicts a behavior consistent with other theoretical and experimental works. Particularly, the maximum deflection of the plates is detected at the leading edge and it is a linear function of the square of the fluid velocity up to the Miller’s theoretical value. For velocities above this value, a nonlinear relationship is observed. This relationship indicates that structural changes are taking place in the plates. Furthermore, for fluid velocities greater than the Miller’s velocity, an extra deflection peak is observed near the trailing edge of the plates. Thus, structural alterations also happen along the length of the flat-shells.

3D-printed rotating spinnerets create membranes with a twist

Round hollow fiber membranes have been well established in applications such as gas separation, ultrafiltration and blood dialysis. Yet, it is well known that geometrical topologies can introduce secondary flow patterns counteracting mass transport limitations, stemming from diffusion resistances and fouling. We present a new systematic methodology to fabricate novel membrane architectures. We use the freedom of design by 3D-printing spinnerets, having multiple bore channels of any geometry. First, such spinnerets are stationary to fabricate straight bore channels inside a monolithic membrane. Second, in an even more complex design, a new mechanical system enables rotating the spinneret. Such rotating multibore spinnerets enable (A) the preparation of twisted channels inside a porous monolithic membrane as well as (B) a helical twist of the outside geometry. The spun material systems comprise classical polymer solutions as well as metal-polymer slurries resulting in solid porous metallic monolithic membrane after thermal post-processing. It is known that twisted spiral-type bore channel geometries are potentially superior to straight channels with respect to mass and heat polarization phenomena, however their fabrication was cumbersome in the past. Now, the described methodology enables membrane fabrication to tailor the membrane geometry to the needs of the membrane process. To showcase the delicate interplay between the geometry and radial and axial flow conditions, we report fluid mechanical simulations and flow magnetic resonance imaging measurements for a twisted tri-bore membrane during permeation.

A new stall delay algorithm for predicting the aerodynamics loads on wind turbine blades for axial and yawed conditions

AbstractStall delay is a complicated phenomenon that has gained for many years the attention of industry and academics in the fields of helicopter and wind turbine aerodynamics. Since most of the potential flow theories still rely on the use of 2D aerofoil data for simulating loads on a rotating blade, less degree of accuracy is expected because of 3D rotational effects. In this work, a new model for correcting the 2D steady aerodynamic data for 3D effects is presented. The model can reduce the uncertainty in the blade design process and, subsequently, make wind turbines more cost‐effective. This model combines the stall delay model of Corrigan and Schillings, a modified version of an inviscid stall delay model, a new modification factor to account for the effect of the angle of attack changes and a new tip loss factor. Furthermore, the model applies the use of the separation factor of Du and Selig to evaluate the area on the rotor disc where stall delay is most prominent. The new stall delay model was embedded in a free‐wake vortex model to estimate the aerodynamic loads on the National Renewable Energy Laboratory Phase VI rotor blades consisting of the S809 aerofoil sections. The results in this study confirm the validity of the 3D corrections by the proposed new model under both axial and yawed flow conditions. Copyright © 2017 John Wiley &amp; Sons, Ltd.

Generalized Aerodynamic Modeling of Dynamic Wake Curvature for Open Rotors With Slender Blades

This paper elaborates on the theoretical development of an analytical approach, capable of modeling the effect of dynamic wake curvature on the aeroelastic response of open rotors with slender blades. The classical solution of incompressible, potential flow derived for a curved vortex tube of uniform vorticity strength is employed. The previously developed curved vortex tube analysis is mathematically generalized to account for arbitrary radial and circumferential variations of circulatory disk loading. An orthogonality analysis is carried out to obtain a finite set of inflow perturbation coefficients that describe the aerodynamic effect of wake curvature in a generalized manner. The end result is a set of integral expressions that provide the interharmonic coupling between the inflow perturbations on the rotor disk due to a curved trailing wake and the corresponding variations of disk loading. The obtained perturbation coefficients are subsequently superimposed upon an existing finite-state induced flow model that assumes a skewed, noncurved cylindrical wake. The developed mathematical approach for fluid mechanics is coupled with an unsteady blade element aerodynamics model, a rotor blade structural mechanics model, and a nonlinear rotor dynamics model. The combined formulation is implemented in an existing helicopter flight mechanics code. The overall method is initially employed to assess the effect of wake curvature on the dynamic response of a small-scale articulated rotor with a flap frequency ratio equal to unity. Subsequently, the integrated model is deployed to investigate the influence of wake curvature and inflow modeling fidelity on the predicted oscillatory blade loads and transient control response of a full-scale helicopter rotor. Comparisons are carried out with flight test measurements as well as with complex free-wake analysis methods. It is shown that including the effect of wake curvature is essential for predicting the transient control response of the investigated rotor. Good agreement is demonstrated between the proposed analytical model and nonlinear predictions carried out by resolving the complex wake geometry. The developed fluid mechanics formulation is a time-accurate method derived from first-principles and is applicable to both axial and nonaxial flow conditions.

Angle of attack distribution on wind turbines in yawed flow

This paper presents a new approach to model the induction for wind turbines in yawed flow to find the effective angle of attack distribution along the blades. There is an inherent difficulty in determining the angle of attack required by aeroelastic codes as input to tabulated airfoil data. Unlike two-dimensional (2D) conditions where the angle of attack is the angle between the unperturbed flow and the airfoil chord, for the rotating blade, the incoming flow is bended because of rotation and affected by the induction of the blade circulation and the tip and root vortices, making the definition of the angle of attack uncertain. In this work, the analogy with 2D potential flows suggests that a coupling term between the unperturbed geometric angle of attack and the unitary velocity in the radial direction should be included to calculate the effective angle of attack seen by the blade strips. The model introduces two radially dependent interferences on the geometric angle, obtained from axial flow conditions that give the effective angle of attack under arbitrarily yawed flow conditions. The model gives a uniform treatment to the radial flows associated with yawed conditions or coned rotors and can be applied directly to calculate the effective angle of attack distribution on coned rotors in yawed flow. The accuracy of the method compares with results obtained from conventional or higher degree of complexity methods. The model can be incorporated into blade element momentum theory codes as an option to calculate angle of attack distributions. Copyright © 2015 John Wiley & Sons, Ltd.