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Guest Editorial

J. Fluids Eng. 2017;140(2):020301-020301-1. doi:10.1115/1.4037993.
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Although nearly 5000 years have passed since the wheel was invented, and even if modern pumps and turbines have been mostly developed during the last two centuries, major challenges and innovation are still present in the area of rotating flows and rotating machinery. This is certainly related to the variety of applications where rotating fluids are now involved: rocket engine inducers, hydraulic or gas turbines, marine propellers, compressors, turbochargers, ventilators and blowers, marine or wind turbines, and pumps for various hydraulic systems are just some examples of industrial areas where new concepts, new designs, and optimization are needed.

Commentary by Dr. Valentin Fuster

Research Papers: Flows in Complex Systems

J. Fluids Eng. 2017;140(2):021101-021101-6. doi:10.1115/1.4037972.
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This paper shows the results of an experimental activity developed in cooperation between Ansaldo Energia and the Department of Engineering and Applied Science of Bergamo University with the aim of assessing the impact of newly designed holes on the thermal protection of a rotor blade platform. The original rotor blade platform featured ten cylindrical holes located along the blade pressure side (PS). Moreover, the channel front side was cooled exploiting the seal purge flow exiting the stator to rotor interface gap. The front midchannel, and particularly the region around the interplatform gap, remained uncooled. To protect this region, two sets of cylindrical holes were designed and manufactured on a seven blade cascade model for experimental verification. Aerodynamic and thermal tests were carried out at low Mach number. To evaluate the interaction of injected flow with secondary flows a five hole probe was traversed downstream of the trailing edge plane. The thermal behavior was analyzed by using thermochromic liquid crystals technique, so to obtain film cooling effectiveness distributions. The seven-hole configuration coupled with a low blowing ratio of about 1.0 provided the best thermal protection without any impact on the aerodynamic performance.

Commentary by Dr. Valentin Fuster
J. Fluids Eng. 2017;140(2):021102-021102-9. doi:10.1115/1.4037975.

A method for evaluating the transient performance of a turbocharger (TC) is so-called load step tests. In these tests, the load of the engine is increased at constant engine speed and the time measured from the start to the end of the load step is measured. Usually, these tests can be run relatively late in the development process, since hardware needs to be already available. In order to judge the transient TC performance at an earlier stage, engine process simulations are run using maps of compressor and turbine. For the turbine, these maps usually need to be extrapolated, since only a certain range of each speed line can be measured on a standard gas stand. Furthermore, because of the exhaust gas pulsation of the engine, it is known that the turbine performance differs from the steady-state case which the maps rely on. This has to be respected by additional models. Using computational fluid dynamics (CFD) simulations, the transient performance of the turbine can be analyzed independent from steady-state maps. So far, these investigations have been usually performed with a constant turbine speed. In this paper, a method is presented which includes the speed fluctuations of the TC caused by the exhaust pulsations as well as the change in mean speed during the load step by including compressor and engine in the CFD analysis with User-Fortran models. Results for a load step from 21,000 rpm to 196,400 rpm are discussed.

Commentary by Dr. Valentin Fuster
J. Fluids Eng. 2017;140(2):021103-021103-8. doi:10.1115/1.4037976.

Large-scale radial blowers are widely used in factories and are one of the main sources of noise. The present study aims at identifying the noise generation mechanisms in such a radial blower in order to suggest simple modifications that could be made in order to reduce the noise. The flow in a representative large-scale radial blower is investigated thanks to unsteady Reynolds-averaged Navier–Stokes (URANS) numerical simulations. The radiated noise is calculated, thanks to an in-house propagation code based on the Ffowcs Williams Hawkings' (FWH) analogy, SherFWH. The results highlight the main noise generation mechanisms, in particular the interaction between the rotating blades and the tongue, and the interaction between the rotating blades and the trapdoors located on the volute sidewall. Some modifications of the geometry are suggested.

Commentary by Dr. Valentin Fuster
J. Fluids Eng. 2017;140(2):021104-021104-10. doi:10.1115/1.4037977.

The fluid-induced rotordynamic forces acting on a whirling space turbopump composed by an inducer and a radial impeller have been compared to the same forces measured on each single component of the turbomachine (i.e., on the inducer and on the radial impeller). The experimental campaign has been carried out in cold water at design and off-design conditions (80%, 100%, and 120% of the design flow rate) both in noncavitating (NC) and cavitating regimes. The paper illustrates the different trends of the rotordynamic forces on the axial and radial pumps and highlights their contributions on the overall turbomachine. At positive whirl ratios, the behavior of the inducer is dominant while, at negative ones, both the pumps show the same trends in such a way that the overall behavior is roughly the sum of each single component.

Commentary by Dr. Valentin Fuster
J. Fluids Eng. 2017;140(2):021105-021105-7. doi:10.1115/1.4037973.

Unsteady flow phenomena like rotating stall frequently occur in centrifugal pumps under off-design conditions. Rotating stall could lead to flow instabilities and pressure pulsation, which affect the normal operation of pumps. The mechanism of rotating stall has not been sufficiently understood in previous researches. In this study, the impact of rotating stall in the impeller on centrifugal pump stability and pressure pulsation is numerically investigated. This paper aims to detect the unsteady flow characteristics inside the centrifugal pump by computational fluid dynamics technology, to analyze pressure pulsations caused by rotating stall and to explore the propagation mechanism of rotating stall. Unsteady numerical simulations are performed by ANSYS 16.0 to model the unsteady flow within the entire flow passage of a centrifugal pump under 0.4QBEP and 0.6QBEP working conditions. Through flow characteristics research, the generation and propagation of rotating stall are discovered. Flow separation appears near the leading edge of the pressure side and transforms into vortices, which move along the passage. Meanwhile, the stall cells rotate circumferentially in the impeller. Additionally, frequencies and amplitudes of pressure pulsations related to rotating stall are investigated by spectrum analysis. The results detect a possible characteristic frequency of rotating stall and show that the interaction between stall cells and the volute tongue could have an influence on rotor–stator interaction (RSI).

Commentary by Dr. Valentin Fuster
J. Fluids Eng. 2017;140(2):021106-021106-9. doi:10.1115/1.4037974.

An analysis of the vortex dynamics in the wake of three different freestream turbine concepts is conducted to gain a better understanding of the main processes affecting the energy recovery in their wakes. The turbine technologies considered are the axial-flow turbine (AFT), the crossflow turbine (CFT), also known as the H-Darrieus turbine, and the oscillating-foil turbine (OFT). The analysis is performed on single turbines facing a uniform oncoming flow and operating near their optimal efficiency conditions at a Reynolds number of 107. Three-dimensional (3D) delayed detached-eddy simulations (DDES) are carried out using a commercial finite volume Navier–Stokes solver. It is found that the wake dynamics of the AFT is significantly affected by the triggering of an instability, while that of the CFT and the OFT are mainly governed by the mean flow field stemming from the tip vortices' induction.

Commentary by Dr. Valentin Fuster
J. Fluids Eng. 2017;140(2):021107-021107-6. doi:10.1115/1.4037978.

Recently, a new interest in vertical axis wind turbine (VAWT) technology is fueled by research on floating support structures for large-scale offshore wind energy application. For the application on floating structures at multimegawatt size, the VAWT concept may offer distinct advantages over the conventional horizontal axis wind turbine (HAWT) design. As an example, VAWT turbines are better suited for upscaling, and at multimegawatt size, the problem of periodic fatigue cycles reduces significantly due to a very low rotational speed. Additionally, the possibility to store the transmission and electricity generation system at the bottom, compared to the tower top as in a HAWT, can lead to a considerable reduction of material logistics costs. However, as most VAWT research stalled in the mid 1990s, no sophisticated and established tools to investigate this concept further exist today. Due to the complex interaction between unsteady aerodynamics and movement of the floating structure, fully coupled simulation tools modeling both aero and structural dynamics are needed. A nonlinear lifting line free vortex wake (LLFVW) code was recently integrated into the open source wind turbine simulation suite qblade. This paper describes some of the necessary adaptions of the algorithm, which differentiates it from the usual application in HAWT simulations. A focus is set on achieving a high robustness and computational efficiency. A short validation study compares LLFVW results with those of a two-dimensional (2D) unsteady Reynolds-averaged Navier–Stokes (URANS) simulation.

Commentary by Dr. Valentin Fuster
J. Fluids Eng. 2017;140(2):021108-021108-7. doi:10.1115/1.4037980.

Leading-edge protuberances on airfoils or wings have been considered as a viable passive control method for flow separation. In this paper, the aerodynamic performance of a modified airfoil with a single leading-edge protuberance was investigated and compared with the baseline NACA 634-021 airfoil. Spalart–Allmaras turbulence model was applied for the numerical simulation. Compared to the sharp decline of baseline lift coefficient, the stall angle of the modified foil decreased and the decline of the lift coefficient became mild. The poststall performance of the modified airfoil was improved, while the prestall performance was declined. Asymmetric flows along the spanwise direction were observed on the modified airfoil, and the local region around one shoulder of the protuberance suffered from leading-edge separation at prestall angles of attack, which may be responsible for the performance decline. At poststall angles of attack, the attached flows along the peak of the protuberance with a sideward velocity component would help improving the total performance of the airfoil. Experimental visualization methods, including surface tuft and smoke flow, were performed, and the asymmetric flow pattern past the protuberance was successfully captured. This specific phenomenon may be largely related to the formation of the biperiodic condition and other complicated flow patterns induced by multiple leading-edge protuberances. The formation mechanism and suppression method of the symmetry breaking phenomenon should be investigated more deeply in the future to guide the practical application of this passive control method.

Commentary by Dr. Valentin Fuster
J. Fluids Eng. 2017;140(2):021109-021109-7. doi:10.1115/1.4037982.

Mathematical model of oil flow in fluid film bearing in field of centrifugal forces is developed. Centrifugal forces for planet wheel bearing sliding surfaces and oil gap are formulated. This model is based on modification of two-dimensional Reynolds equation taking into account inertia centrifugal forces for oil film. Required modification of Reynolds equation is received from Navier–Stokes and continuity equations taking into account centrifugal forces acting on planet wheel bearing. Modified two-dimensional Reynolds equation is solved numerically using finite element discretization. Developed mathematical model, based on modified Reynolds equation, is verificated at comparison with solution of full Navier–Stokes equations system obtained in commercial software package. Results for pressure distribution in bearing with fixed axis and in planet wheel bearing are received and compared. The sufficient influence of centrifugal inertia forces in oil layer of planet wheel bearing on pressure distribution, bearing carrying force, and attitude angle is shown for specific shaft journal eccentricity ratio, eccentricity direction, and rotation velocity.

Commentary by Dr. Valentin Fuster
J. Fluids Eng. 2017;140(2):021110-021110-10. doi:10.1115/1.4037983.

This paper presents a numerical study of the impact of tip gap uncertainties in a multistage turbine. It is well known that the rotor gap can change the gas turbine efficiency, but the impact of the random variation of the clearance height has not been investigated before. In this paper, the radial seals clearance of a datum shroud geometry, representative of steam turbine industrial practice, was systematically varied and numerically tested by means of unsteady computational fluid dynamics (CFD). By using a nonintrusive uncertainty quantification (UQ) simulation based on a sparse arbitrary moment-based approach, it is possible to predict the radial distribution of uncertainty in stagnation pressure and yaw angle at the exit of the turbine blades. This work shows that the impact of gap uncertainties propagates radially from the tip toward the hub of the turbine, and the complete span is affected by a variation of the rotor tip gap. This amplification of the uncertainty is mainly due to the low-aspect ratio of the turbine, and a similar behavior is expected in high pressure (HP) turbines.

Commentary by Dr. Valentin Fuster
J. Fluids Eng. 2017;140(2):021111-021111-11. doi:10.1115/1.4037984.

This paper presents the capability of a numerical code, isis-cfd, based on the solution of the Navier–Stokes equations, for the investigation on the hydrodynamic characteristics of a marine propeller in open water. Two propellers are investigated: the Istituto Nazionale per Studi ed Esperienze di Architectura Navale (INSEAN) E779A model in straight-ahead flow and the Potsdam Propeller Test Case (PPTC) model in oblique flow. The objectives of this study are to establish capabilities of various turbulent closures to predict the wake propeller and to predict the instability processes in the wake if it exists. Two Reynolds-averaged Navier–Stokes (RANS) models are used: the k–ω shear stress transport (SST) of Menter and an anisotropic two-equation explicit algebraic Reynolds stress model (EARSM). A hybrid RANS–large eddy simulation (LES) model is also used. Computational results for global flow quantities are discussed and compared with experimental data. These quantities are in good agreement with the measured data. The hybrid RANS–LES model allows to capture the evolution of the tip vortices. For the INSEAN E779A model, the instability of the wake is only predicted with a hybrid RANS–LES model, and the position of these instabilities is in good agreement with the experimental visualizations.

Commentary by Dr. Valentin Fuster
J. Fluids Eng. 2017;140(2):021112-021112-6. doi:10.1115/1.4037986.

Rocket turbopumps sometimes experience self-excited shaft vibration due to rotordynamic forces. To prevent this vibration, a key step is to establish a method to measure and evaluate the rotordynamic forces that act on turbopump components. In this study, we measured rotordynamic forces acting on a two-stage inducer using a rotordynamic test stand developed in 2012 at Kakuda Space Center. In noncavitating conditions, we did not observe strong nonlinearities in rotordynamic forces in the inducer at low flow rate conditions. The results of the pressure fluctuation on the inducer showed that rotordynamic forces were mainly excited in the second stage of the inducer. In cavitating conditions, we found that there is no strong nonlinearity between cavitating rotordynamic forces and the whirling frequency ratio in the inducer. These results show the robustness of the rotordynamic forces acting on the inducer against the flow rate and cavitation.

Commentary by Dr. Valentin Fuster
J. Fluids Eng. 2017;140(2):021113-021113-8. doi:10.1115/1.4037987.

Rotating cavitation is an important problem, which makes it difficult to design reliable rotating machines. In this study, a simple analysis method that tried to evaluate the cavitation instabilities of a rotating machinery by using one-dimensional (1D) system analysis software was attempted. In this method, cavitation compliance and mass flow gain factor are distributed in each flow path of the inducer. Analysis results show that cavitation instabilities, including rotating phenomena, exist. With the evolved analysis model, effects of various parameters on the eigenvalues of the system were investigated. Analysis results agreed with inducer test results qualitatively. Furthermore, by the analysis considered whirl motion of the rotor, effects of it on cavitation instabilities were investigated.

Commentary by Dr. Valentin Fuster
J. Fluids Eng. 2017;140(2):021114-021114-9. doi:10.1115/1.4037988.

This paper presents the simulation of the dynamic behavior of variable speed pump–turbine. A power reduction scenario at constant wicket gate opening was numerically analyzed from 100% to 93% rpm corresponding to a power reduction from full load to about 70% with a ramp rate of 1.5% per second. The flow field analysis led to the onset and development of unsteady phenomena progressively evolving in an organized rotating partial stall during the pump power reduction. These phenomena were characterized by frequency and time–frequency analyses of several numerical signals (pressure, blade torque, and flow rate in blade passages). The unsteady pattern in return channel strengthened emphasizing its characteristic frequency with the rotational velocity decreasing reaching a maximum and then disappearing. At lower rotational speed, the flow field into the wickets gates channel starts to manifest a full three-dimensional (3D) flow structure. This disturbance was related to the boundary layer separation and stall, and it was noticed by a specific frequency.

Commentary by Dr. Valentin Fuster
J. Fluids Eng. 2017;140(2):021115-021115-8. doi:10.1115/1.4037985.

This contribution is concerned with the development and the implementation of a foil air journal bearing model. For this purpose, the numerical procedure solving the Reynolds equation for compressible fluids has to be coupled to a compliant foil model. The presented beam-based approach is supposed to reproduce most of the experimentally known particularities in the mechanical behavior of the foil structure, while being at least as runtime-efficient as the commonly used simple elastic foundation model. The developed modeling approach will be validated by comparing simulation results to data found with a more complex reference model. In the analysis part, most notably, the top foil compliance is shown to deteriorate the load-carrying capacity of air bearings. Moreover, the influence of the top foil compliance on the dynamics of a rigid rotor supported by two foil air journal bearings will be discussed.

Commentary by Dr. Valentin Fuster
J. Fluids Eng. 2017;140(2):021116-021116-9. doi:10.1115/1.4037990.

This paper presents an original experimental study concerning the structural response of a flexible lightweight hydrofoil undergoing various flow conditions including partial cavitating flow. It is based on the analysis of the static deformation, the vibrations, the strains, and the stresses of a polyacetal NACA0015 cantilevered hydrofoil in a hydrodynamic tunnel, at Reynolds numbers ranging from 3 × 105 to 6 × 105. A specific distance measurement laser device was developed to measure the static deformation of the hydrofoil. The vibration response was measured by means of two laser vibrometers in order to identify the structural modal response. The strains and stresses were obtained from integrated strain gauges embedded in the foil close to the root section. A high-speed camera was used in order to analyze unsteady features of the cavitating flow. This paper presents the experimental setup and several results in both noncavitating and cavitating flow that should be very useful for numerical developments of fluid structure interaction (FSI) in heavy fluid. Several observations are reported in the paper showing the strong coupling between the fluid and the structure. Particularly, a frequency lock-in of the cavity frequency to the first bending mode is clearly observed for a narrow band of cavitation numbers.

Commentary by Dr. Valentin Fuster

Research Papers: Fundamental Issues and Canonical Flows

J. Fluids Eng. 2017;140(2):021201-021201-8. doi:10.1115/1.4037981.

A major source of contra-rotating open rotor (CROR) tonal noise is caused by the interaction of the front-rotor (FR) wakes with the aft-rotor blades. Inspired by chevron nozzles, which increase the mixing process in jet shear layers, serrations are implemented at the FR trailing-edge in order to increase the wake mixing and thus reduce the tones. The depth and width of the serrations are optimized with a multi-objective, metamodel-assisted evolutionary algorithm. For each member, a steady-state Reynolds-averaged Navier–Stokes (RANS) simulation is performed, which is coupled with an analytical noise prediction method in order to evaluate the noise reduction due to the serrations. The results confirm that tonal interaction noise can be reduced by means of trailing-edge serrations. It is found that the major noise reduction mechanism for wake interaction is attributed to increased destructive interferences occurring in spanwise direction. The tonal noise generated through the interaction of the rear rotor (RR) potential field with the FR trailing edge is also slightly reduced because of the circumferential and axial shift of the serrated trailing edge. Furthermore, the present study demonstrates the feasibility of performing an acoustic optimization with a hybrid approach that predicts the noise analytically and extracts the aerodynamic input data from a steady-state RANS flow solution.

Commentary by Dr. Valentin Fuster

Research Papers: Multiphase Flows

J. Fluids Eng. 2017;140(2):021301-021301-13. doi:10.1115/1.4037979.

This paper investigates the performance of moment-based methods and a monodispersed model (Mono) in predicting the droplet size distribution and behavior of wet-steam flows. The studied moment-based methods are a conventional method of moments (MOM) along with its enhanced version using Gaussian quadrature, namely the quadrature method of moments (QMOM). The comparisons of models are based on the results of an Eulerian–Lagrangian (E–L) method, as the benchmark calculations, providing the full spectrum of droplet size. In contrast, for the MOM, QMOM, and Mono an Eulerian reference frame is chosen to cast all the equations governing the phase transition and fluid motion. This choice of reference frame is essential to draw a meaningful comparison regarding complex flows in wet-steam turbines as the most important advantage of the moment-based methods is that the moment-transport equations can be conveniently solved in an Eulerian frame. Thus, the moment-based method can avoid the burdensome challenges in working with a Lagrangian framework for complicated flows. The main focus is on the accuracy of the QMOM and MOM in representing the water droplet size distribution. The comparisons between models are made for two supersonic low-pressure nozzle experiments reported in the literature. Results show that the QMOM, particularly inside the nucleation zone, predicts moments closer to those of the E–L method. Therefore, for the test case in which the nucleation is significant over a large proportion of the domain, the QMOM provides results in clearly better agreements with the E–L method in comparison with the MOM.

Commentary by Dr. Valentin Fuster
J. Fluids Eng. 2017;140(2):021302-021302-7. doi:10.1115/1.4037989.

In this study, numerical analysis is carried out around the cyclic flat-plate cascade with symmetric and asymmetric slit, so as to examine the suppressing or controlling effect of the slit on cavitation instabilities such as cavitation oscillation (CO) which resembles cavitation surge, and rotating cavitation. These instabilities cause various problems for the turbomachinery, for example, rotating cavitation causes an asynchronous shaft vibration, and CO causes an oscillation of column of working fluid as a result of the resonance phenomenon of the system. In liquid propellant rocket engine, suppression device for these instabilities bring increase in cost of the launch. Therefore, it is thought that to develop effective suppression technique is important for turbopumps. Especially, in this paper, two types of the flat-plate three blades cascade which have symmetric slit on each blade and three types of the cascade which have asymmetric slit were analyzed, and the results are compared with those of cascade without slit. As a result, the CO is perfectly suppressed in both of two types cascade with asymmetric slit. Also, other examined cascades have suppression effect of CO These results indicate the possibility of suppressing cavitation instabilities in actual inducers or controlling the type of the cavitation instabilities by the arrangement of the slit. Moreover, the head performance is equal or slightly increased by arranging slit.

Commentary by Dr. Valentin Fuster

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