bluuebunny/crossref_metadata_2025_split huggingface dataset:
Two-Dimensional Numerical Analysis of Non-Darcy Flow Using the Lattice Boltzmann Method: Pore-Scale Heterogeneous Effects
Authors: Takeuchi Yuto , Takeuchi Junichiro , Izumi Tomoki , Fujihara Masayuki 
Published: January 2021

Abstract
This study simulates pore-scale two-dimensional flows through porous media composed of circular grains with varied pore-scale heterogeneity to analyze non-Darcy flow effects on different types of porous media using the lattice Boltzmann method. The magnitude of non-Darcy coefficients and the critical Reynolds number of non-Darcy flow were computed from the simulation results using the Forchheimer equation. Although the simulated porous materials have similar porosity and representative grain diameters, larger non-Darcy coefficients and an earlier onset of non-Darcy flow were observed for more heterogeneous porous media. The simulation results were compared with existing correlations to predict non-Darcy coefficients, and the large sensitivity of non-Darcy coefficients to pore-scale heterogeneity was identified. The pore-scale heterogeneity and resulting flow fields were evaluated using the participation number. From the computed participation numbers and visualized flow fields, a significant channeling effect for heterogeneous media in the Darcy flow regime was confirmed compared with that for homogeneous media. However, when non-Darcy flow occurs, this channeling effect was alleviated. This study characterizes non-Darcy effect with alleviation of the channeling effect quantified with an increase in participation number. Our findings indicate a strong sensitivity of magnitude and onset of non-Darcy effect to pore-scale heterogeneity and imply the possibility of evaluating non-Darcy effect through numerical analysis of the channeling effect.
The Current-Carrying Tribological Properties of Cu/Graphene Composites
Authors: Zhao Jing , Peng Yitian , Zhou Qianguang , Zou Kun 
Published: January 2021

Abstract
Excellent current-carrying tribological properties including the low-friction, high anti-wear, high current-carrying efficiency, and stability are important for the current-carrying application in transmitting electrical signals and power. Here, the Cu/graphene composites with graphene uniformly distributed in Cu matrix were successfully prepared by combining the electroless plating process and powder metallurgy process. The current-carrying tribological properties including friction, wear, and electrical stability of the Cu/graphene composites with brass pairs were investigated by varying normal applied load and sliding speed under multiple applied voltages. The friction reduction and anti-wear properties of Cu/graphene composites were enhanced by the introduction of graphene. The friction coefficient of the Cu/graphene composites keeps stable under current-carrying and non-current-carrying conditions due to the benefit of the graphene enhancement to Cu. The graphene on wear surface reduces friction force and wear. The current-carrying efficiency and stability increased with the increase of applied load but decreased with increasing sliding speed. The contact stability increased with applied load, while high sliding speed caused the drastic vibration of sliding contact. The studies can provide a beneficial guideline for the current-carrying applications of Cu/graphene composites to reduce the friction and wear.
A Novel Structural Configuration of Modular Floating Wind Farms With Self-Adaptive Property
Authors: Zhang Yuming , Liu Haixiao 
Published: January 2021

Abstract
The present work proposes a novel structural configuration for floating wind farms after an investigation into the structural configurations of current floating wind farms. The uniqueness of the structural configuration lies in the ability to adjust its heading to wind inflow directions, i.e., the self-adaptive property. To verify the existence of the self-adaptive property, a structural design is performed, and a quasi-static framework to analyze the responses is established. To simplify the loads on wind turbines under yawed inflow conditions, empirical expressions for wind loads are proposed, based on which the quasi-static analysis is performed under the yawed inflow direction of 90 deg from the structural heading. The analytical result indicates that under yawed inflow conditions, the structural heading will eventually turn to align with the wind inflow direction on account of the structural configuration and the wind turbine control. Then, the effects of inflow wind velocity, initial offset, number of wind turbines, complex environmental conditions, and hydroelastic properties on the responses of the structure are investigated.
Numerical Study of a Solar-Powered Stirling Engine With a Transparent Top Cover
Authors: Sanmiguel-Rojas E. , Gutierrez-Castillo P. , Auñón-Hidalgo J. A. 
Published: January 2021

Abstract
The present work is focused on the numerical study of a solar-powered Stirling engine, with the particularity that the solar radiation is injected through a transparent top cover. Thus, the working fluid absorbs the heat across a porous layer of a steel woven wire screen placed alongside the inner side of the transparent wall. The engine output net power and efficiency are studied as a function of the porosity, engine speed, temperature of the expansion chamber, and wire diameter of the screen. It is found that the engine efficiency remains practically constant for porosity values over 0.7, but there is a relevant increase of the engine output net power compared to the same working conditions without the absorbing layer. For a given porosity value, the most significant increase of net power due to introducing the porous layer was reached when doubling the engine speed resulting in an increment close to 40%.
A New Approach to Modeling Slip and Work Input for Centrifugal Compressors
Authors: Harrison Herbert M. , Key Nicole L. 
Published: January 2021

Abstract
A new method of modeling slip factor and work input for centrifugal compressor impellers is presented. Rather than using geometry to predict the behavior of the flow at the impeller exit, the new method leverages governing relationships to predict the work input delivered by the impeller with dimensionless design parameters. The approach incorporates both impeller geometry and flow conditions and, therefore, is inherently able to predict the slip factor both at design and off-design conditions. Five impeller cases are used to demonstrate the efficacy of the method, four of which are well documented in the open literature. Multiple implementations of the model are introduced to enable users to customize the model to specific applications. Significant improvement in the accuracy of the prediction of slip factor and work input is obtained at both design and off-design conditions relative to Wiesner’s slip model. While Wiesner’s model predicts the slip factor of 52% of the data within ±0.05 absolute error, the most accurate implementation of the new model predicts 99% of the data within the same error band. The effects of external losses on the model are considered, and the new model is fairly insensitive to the effects of external losses. Finally, detailed procedures to incorporate the new model into a meanline analysis tool are provided in the appendices.
Experimental Investigation of the Effects of Squeeze Film Damper Design on Highspeed Rotor System
Authors: Iacobellis Vincent , Behdinan Kamran , Chan Dennis , Beamish Dave 
Published: January 2021

Abstract
Squeeze film dampers (SFDs) are used in highspeed rotordynamics systems to mitigate vibrations while traversing critical speeds. SFDs are critical in dissipating large amplitude motions and dynamic loading transferred from the rotor to the bearing supports during highspeed operation. Little testing on the effect of SFDs on rotor shafts under highspeed operating conditions is available in the literature. Thus, a SFD-rotor test rig has been designed and built to study the effect of SFD oil supply pressure, oil temperature, oil inlet feed number/orientation, unbalance, and seals on the response of a Jeffcott rotor. In this paper, the test rig is introduced along with its calibration through static and dynamic testing. It was found that the rotor displacement results were improved through the addition of trial masses to provide a runout correction and that using rubber supports for the support structure generated more symmetric results in the vertical and horizontal planes compared to steel supports. Initial results for the test rig are also presented for different unbalances with and without oil supply and with and without SFD end seals. In these cases, increased unbalance produced higher amplitude motion with a corresponding increase in critical speed and decrease in damping.
An Analysis of the Secondary Flow Around a Tandem Blade Under the Presence of a Tip Gap in a High-Speed Linear Compressor Cascade
Authors: Konrath Liesbeth , Peitsch Dieter , Heinrich Alexander 
Published: January 2021

Abstract
Tandem blades have often been under investigation, experimentally as well as numerically, but most studies have been about tandem blade stators without tip gap. This work analyzes the influence of a tip gap on the flow field of a tandem blade for engine core compressors. Experiments have been conducted in a high-speed linear compressor cascade on a tandem and a reference geometry. The flow is analyzed using five-hole probe measurements in the wake of the blades and oil flow visualization to show the near surface stream lines. First, the results for design conditions (tandem and conventional blade) are compared to measurements on corresponding blades without tip gap. Similarities and differences in the flow topology due to the tip clearance are analyzed, showing that the introduction of the tip clearance has a similar influence on the loss and turning development for the tandem and the conventional blade. The tandem blade features two tip clearance vortices with a complex flow interaction and the possible formation of a third counter-rotating vortex between them. An incidence variation from 0° to 5° for both blades indicate at first a similar behavior. After a separation of the flow field into gap and non-gap half it becomes apparent that the tandem blade shows higher losses on the gap side, while featuring a close-to-constant behavior on the non-gap side. Further investigation of the flow on the gap side shows indicators of the front blade exhibiting tip clearance vortex break down, while the rear blade seems unaffected.
Design Guidelines for Axial Turbines Operating With Non-Ideal Compressible Flows
Authors: Giuffre’ Andrea , Pini Matteo 
Published: January 2021

Abstract
The impact of non-ideal compressible flows on the fluid-dynamic design of axial turbine stages is examined. First, the classical similarity equation is revised and extended to account for the effect of flow non-ideality and compressibility. Then, the influence of the most relevant design parameters is investigated through the application of a dimensionless turbine stage model embedding a first-principles loss model. The results show that the selection of optimal duty coefficients is scarcely affected by the molecular complexity of the working fluid, whereas compressibility effects produce an offset in the efficiency trends and in the optimal flow coefficient. Furthermore, flow non-ideality can lead to either an increase or a decrease of stage efficiency of the order of 2–3% relative to turbines designed to operate in dilute gas state. This effect can be predicted at preliminary design phase through the evaluation of the isentropic pressure-volume exponent. 3D RANS simulations of selected test cases corroborate the trends predicted with the reduced-order turbine stage model.
Large Eddy Simulation of Laminar and Turbulent Shock/Boundary Layer Interactions in a Transonic Passage
Authors: Priebe Stephan , Wilkin Daniel , Breeze-Stringfellow Andy , Jothiprasad Giridhar , Cheung Lawrence C. 
Published: January 2021

Abstract
Shock/boundary layer interactions (SBLI) are a fundamental fluid mechanics problem relevant in a wide range of applications including transonic rotors in turbomachinery. This paper uses wall-resolved large eddy simulation (LES) to examine the interaction of normal shocks with laminar and turbulent inflow boundary layers in transonic flow. The calculations were performed using GENESIS, a high-order, unstructured LES solver. The geometry created for this study is a transonic passage with a convergent-divergent nozzle that expands the flow to the desired Mach number upstream of the shock and then introduces constant radius curvature to simulate local airfoil camber. The Mach numbers in the divergent section of the transonic passage simulate single stage commercial fan blades. The results predicted with the LES calculations show significant differences between laminar and turbulent SBLI in terms of shock structure, boundary layer separation and transition, and aerodynamic losses. For laminar flow into the shock, significant flow separation and low-frequency unsteadiness occur, while for turbulent flow into the shock, both the boundary layer loss and the low-frequency unsteadiness are reduced.
Effect of Backward Sweep on Aerodynamic Performance of a 1.5-Stage Highly Loaded Axial Compressor
Authors: Huang Song , Zhou Chuangxin , Yang Chengwu , Zhao Shengfeng , Wang Mingyang , Lu Xingen 
Published: January 2021

Abstract
As a degree of freedom in the three-dimensional blade design of axial compressors, the sweep technique significantly affects the aerodynamic performance of axial compressors. In this paper, the effects of backward sweep rotor configurations on the aerodynamic performance of a 1.5-stage highly loaded axial compressor at different rotational design speeds are studied by numerical simulation. The aim of this work is to improve understanding of the flow mechanism of backward sweep on the aerodynamic performance of a highly loaded axial compressor. A commercial CFD package is employed for flow simulations and analysis. The study found that at the design rotational speed, compared with baseline, backward sweep rotor configurations reduce the blade loading near the leading edge but slightly increases the blade loading near the trailing edge in the hub region. As the degree of backward sweep increases, the stall margin of the 1.5-stage axial compressor increase first and then decrease. Among different backward sweep rotor configurations, the 10% backward sweep rotor configuration has the highest stall margin, which is about 2.5% higher than that of baseline. This is due to the change of downstream stator incidence, which improves flow capacity near the hub region. At 80% rotational design speed, backward sweep rotor configurations improve stall margin and total pressure ratio of the compressor. It’s mainly due to the decreases of the rotor incidence near the middle span, which results in the decreases of separation on the suction surface. At 60% rotational design speed, detached shock disappears. Backward sweep rotor configurations deteriorate stall margin of the compressor, but increase total pressure ratio and adiabatic efficiency when the flow rate is lower than that at peak efficiency condition. Therefore, it’s necessary to consider the flow field structure of axial compressors at whole operating conditions in the design process and use the design freedom of sweep to improve the aerodynamic performance.
Nonlinear Modal Analysis of Frictional Ring Damper for Compressor Blisk
Authors: Sun Yekai , Yuan Jie , Denimal Enora , Salles Loïc 
Published: January 2021

Abstract
The use of integrally blisk is becoming popular because of the advantages in aerodynamic efficiency and mass reduction. However, in an integrally blisk, the lack of the contact interface leads to a low structural damping compared to an assembled bladed-disk. One emerging damping technique for the integrally blisk is based on the use of friction ring damper which exploits the contact interfaces at the underneath of the disk. In this paper, three different geometries of the ring dampers are investigated for damping enhancement of a blisk. A full-scale compressor blisk is considered as a case study where a node to node contact model is used to compute the contact forces. The dynamic behaviour of the blisk with the ring damper is investigated by using nonlinear modal analysis which allows a direct estimation of the damping generated by the friction interface. The damping performance for the different ring dampers are evaluated and compared. It appears that the damping efficiency as well as the shift in the resonant frequency for the different geometries are highly related to the nodal diameter and contact pressure/gap distributed within contact interface. The geometry of the ring damper has significant impact on the damping performance.
Measurements of Density and Sound Speed in Mixtures Relevant to sCO2 Power Cycles
Authors: Park Suhyeon , Urso Justin , Manikantachari K. R. V. (Raghu) , Hosangadi Ashvin , Zambon Andrea , Vasu Subith S. 
Published: January 2021

Abstract
The objective of this research is to validate properties of mixtures relevant to supercritical carbon dioxide (sCO2) power cycles. Direct fired sCO2 cycles are promising technology for the future power generation systems. The working fluid of sCO2 cycles will be near and above critical point of CO2. One of the challenges is that the simulation of mixtures should consider real gas behavior. Expected operating conditions of Allam cycles reach up to 300 bar and 1000 °C. Characterizing the mixtures at the extreme conditions is an important issue in current researches and industrial applications. Thermophysical properties of mixtures may be beyond the valid range of the widely used database such as NIST REFPROP. Experimental data of mixtures properties in the literature is limited which is necessary to develop high-fidelity design tools for sCO2 power cycles.
We measured density and sound speed of several multi-component mixtures. A temperature-controlled high-pressure test cell was used for the density measurements. Sound speed was measured by resonant frequency detection using an external speaker and a piezoelectric pressure sensor. Mixtures studied in this work includes carbon dioxide, methane, oxygen and water vapor. Properties of pure CO2 were measured to show the validity of our technique. Compositions were selected to be close to frozen mixtures at the inlet, mid-progress and exhaust conditions of a model sCO2 combustor in the previous numerical simulation work. Corresponding reaction progress variables (RPV) were RPV = 0, 0.5, and 1. Temperature and pressure conditions of experiments are 310–450 K, and 0–150 bar. In our study, density and sound speed from NIST REFPROP database agree with experimental measurements within the range of our measurement uncertainties.
Non-Axisymmetric Endwall Contouring in a Linear Compressor Cascade
Authors: Ma Yuchen , Teng Jinfang , Zhu Mingmin , Qiang Xiaoqing 
Published: January 2021

Abstract
Modern axial compressors are designed to be highly loaded in terms of aerodynamics, which can lead to challenges of increasing the compressor efficiency. Losses associated with secondary flow effects are well known to be the major limiting factor of improving the compressor performance. In this study, non-axisymmetric endwall contouring in a linear compressor cascade was generated through the optimization process. Combined with numerical simulation, wind tunnel tests on linear cascades with flat and contoured endwall were performed with various measurement techniques at the design and off-design conditions.
The simulation results show that optimal endwall design can provide 3.08% reduction of the total pressure loss at the design condition. The reduction of pressure loss obtained is mainly below 24%span with the size of the high loss region being effectively reduced. At off-design condition, the numerical benefit of the endwall contouring is found less pronounced. The discrepancy is spotted between simulation and experiments. The experimental pressure loss reduction is mainly below 18% at ADP. And the pressure loss for the CEW increases greatly at offdesign condition in experiments.
Flow patterns revealed by numerical simulations show that the separation on the blade surface is mitigated with focus point disappearing, and reverse flow on the endwall near the suction side corner is moved away from the blade surface. CFD analysis indicates that the altered pressure distribution on the endwall accelerates the flow at the suction side corner and moves the reverse flow core further downstream. The weakened interaction between the corner vortex and tornado-like vortex from the endwall near the suction side corner is the main control mechanism of the CEW. The performance improvement in the linear compressor is mainly gained from it.
Higher-Order Conceptual Model for Labyrinth Seal Flutter
Authors: Corral Roque , Greco Michele , Vega Almudena 
Published: January 2021

Abstract
A simple non-dimensional model to describe the flutter onset of two-fin straight labyrinth seals [1] is extended to account for non-isentropic flow perturbations. The isentropic relationship is replaced by the more general integral energy equation of the inter-fin cavity. A new expression for the Corral & Vega stability criterion is derived which is very consistent with the previous model in the whole design space of the seal but for torsion centers located in the high-pressure side close to the seal. The new model formally depends on more dimensionless parameters since the existing parameter grouping of the previous model does not hold anymore, but this dependency is weak in relative terms. The model blends the limit where the discharge time of the inter-fin cavity is much longer than the vibration period, and the flow is nearly isentropic, and the opposite limit, where the perturbations are isothermic, gracefully. A few numerical examples obtained using a three-dimensional linearized frequency domain solver are included to support the model and show that the trends are correct but the body of the numerical work will be presented in a separated article. The matching between the work-per-cycle obtained with the model and frequency domain solver is good. It is shown that some weird trends obtained using linearized unsteady simulations are qualitatively consistent with the current model but not with the previous one [1]. The largest differences between the new and the previous model are seen when the seal is supported at the high-pressure side.
“I-Spots” and Emmons’ Spot Production Rate
Authors: Mayle R. E. , Stripf M. 
Published: January 2021

Abstract
A new intermittency function flagging the turbulent, becalmed, and non-becalmed regions of transitional flow is introduced. In addition, a method is developed to detect embedded incipient turbulent spots within the non-becalmed regions of the flow, which are then documented, counted, and related to Emmons’ spot production rate. This in turn is used to calculate an intermittency distribution which when compared with that measured displays an excellent agreement. Finally, it is shown that correlations based on Narasimha’s intermittency equation are highly questionable.