Reynolds-averaged Navier-Stokes Analysis Research Articles

Aerodynamic Analysis of Low-Pressure Axial Fans Installed in Parallel

Abstract Ducted rotor-only low-pressure axial fans play an integral role in automotive thermal management. The tightly packed under-hood region and down-stream heat-exchanger shape limit the fan diameter. In order to circumvent this limitation, multiple cooling fans of small diameters are tightly packaged and placed in parallel. Currently, there is limited scientific work, that study the aerodynamics of low-pressure axial fans when installed in parallel. This work aims to quantify the aerodynamic performance and the flow-field as a result of installing low-pressure axial fans in parallel through computational fluid dynamics (CFD). Publicly available experimental data from Friedrich-Alexander University is used for the validation of the numerical setup. Three-dimensional, full-annulus, unsteady Reynolds-averaged Navier-Stokes (URANS) analysis has been performed for both a single-fan and two-fans installed in parallel and their respective aerodynamic performance has been compared for the operation condition identified as the best efficiency point in experiments. Only small differences are observed in the overall aerodynamic performance of the two-fans in parallel compared to a single-fan. A circumferential nonuniformity in the form of a local high-pressure zone at the inlet of the fan is observed when the two-fans are placed in parallel. The resulting circumferential nonuniformity is quantified, both in space and time. A strong correlation is found between the pressure fields of the two-fans installed in parallel.

Design Optimization of a Dual-Bleeding Recirculation Channel to Enhance Operating Stability of a Transonic Axial Compressor

The present work performed a comprehensive investigation to find the effects of a dual-bleeding port recirculation channel on the aerodynamic performance of a single-stage transonic axial compressor, NASA Stage 37, and optimized the channel’s configuration to enhance the operating stability of the compressor. The compressor’s performance was examined using three parameters: The stall margin, adiabatic efficiency, and pressure ratio. Steady-state three-dimensional Reynolds-averaged Navier–Stokes analyses were performed to find the flow field and aerodynamic performance. The results showed that the addition of a bleeding channel increased the recirculation channel’s stabilizing effect compared to the single-bleeding channel. Three design variables were selected for optimization through a parametric study, which was carried out to examine the influences of six geometric parameters on the channel’s effectiveness. Surrogate-based design optimization was performed using the particle swarm optimization algorithm coupled with a surrogate model based on the radial basis neural network. The optimal design was found to increase the stall margin by 51.36% compared to the case without the recirculation channel with only 0.55% and 0.28% reductions in the peak adiabatic efficiency and maximum pressure ratio, respectively.

휴대용 수력 터빈 이노마드 우노의 성능향상을 위한 최적설계

In this study, an optimization design was performed to improve the hydrodynamic performances of an ultra-small turbine based on a 3-D Reynolds-averaged Navier-Stokes (RANS) analysis. The runner of the turbine was selected as the computational domain and optimization object. The locations of the inlet, outlet and external interface of the computational domain were set to 3 times, 6 times and 3 times the diameter of the runner, respectively. For the turbulent closure problem in the RANS analysis, the shear stress transport (SST) turbulence model was used, and a grid dependency test was performed by applying the grid convergence index (GCI) based on the Richardson extrapolation method considering the grid discretization error. Four design variables related to the geometry of the runner blade were selected to maximize the power coefficient. The radial basis neural network (RBNN) was used as the surrogate model and trained to improve prediction accuracy. The optimization results obtained by using machine learning method showed highly accurate prediction values, and compared with the reference design, the optimum design provided improved hydraulic performances.

Effect of Air Injection on the Internal Flow Characteristics in the Draft Tube of a Francis Turbine Model

Under low flow-rate conditions, a Francis turbine exhibits precession of a vortex rope with pressure fluctuations in the draft tube. These undesirable flow phenomena can lead to deterioration of the turbine performance as manifested by torque and power output fluctuations. In order to suppress the rope with precession and a swirl component in the tube, the use of anti-swirl fins was investigated in a previous study. However, vortex rope generation still occurred near the cone of the tube. In this study, unsteady-state Reynolds-averaged Navier–Stokes analyses were conducted with a scale-adaptive simulation shear stress transport turbulence model. This model was used to observe the effects of the injection in the draft tube on the unsteady internal flow and pressure phenomena considering both active and passive suppression methods. The air injection affected the generation and suppression of the vortex rope and swirl component depending on the flow rate of the air. In addition, an injection level of 0.5%Q led to a reduction in the maximum unsteady pressure characteristics.

공기 주입에 따른 프란시스 수차 모델의 흡출관 내 유동 특성

Undesirable pressure fluctuations in Francis turbines are caused by precession of the vortex rope in the draft tube under conditions of low flow rate. These pressure fluctuations can cause a resonance that seriously undermines the stability of the turbine system. In the previous study, the effects of anti-swirl fins in the draft tube were investigated to suppress the vortex rope. However, despite the installation of anti-swirl fins, the precession of vortex rope is occurred near the draft tube cone, and an additional suppression factor for vortex rope is required. Therefore, this preliminary study investigates the effect of air-injection to select flow rate of the air-injection for suppressing the vortex rope in the draft tube cone. Three-dimensional steady-state Reynolds-averaged Navier-Stokes analyses were conducted with a shear stress transport turbulence model to investigate the performance and internal flow characteristics according to the flow rate of air-injection. The efficiencies were decreased by the air-injection into the draft tube, however, the effect of vortex rope suppression was exhibited when the flow rate of air-injection was above 0.5%Q.

Effect of the leakage flow in runner on flow characteristics of a Francis turbine model

In the Francis turbine, there are gaps between the runner and the stator structure as the labyrinth seals. The flows inside gaps have the complicated flow phenomena; these complicated flows can affect the hydraulic performance and the downstream of the Francis turbine as the leakage flow. Therefore, it is important to investigate the flow characteristics induced by the runner gaps. However, normally the runner gap that has very complicated flow structure is often disregarded by considering the time and cost of the numerical analysis. But, it is necessary to apply all flow structures correctly to confirm the more accurate and reliable flow phenomena. In this study, the flow characteristics of gaps at the hub and shroud of the Francis turbine model runner were investigated with adding the leakage flow in the runner cone. In addition, the internal flow characteristics were compared by considering with and without the runner gaps application. For observing the influence of the leakage flow on the hydraulic performance and flow characteristics, the three-dimensional steady Reynolds-averaged Navier-Stokes analyses were performed using the shear stress transport turbulence model. The efficiency was decreased by applying the runner gaps; and the complicated flow phenomena were captured in the runner gaps.

Stability Enhancement of a Single-Stage Transonic Axial Compressor Using Inclined Oblique Slots

A casing treatment using inclined oblique slots (INOS) is proposed to improve the stability of the single-stage transonic axial compressor, NASA Stage 37, during operation. The slots are installed on the casing of the rotor blades. The aerodynamic performance was estimated using three-dimensional steady Reynolds-Averaged Navier-Stokes analysis. The results showed that the slots effectively increased the stall margin of the compressor with slight reductions in the pressure ratio and adiabatic efficiency. Three geometric parameters were tested in a parametric study. A single-objective optimization to maximize the stall margin was carried out using a Genetic Algorithm coupled with a surrogate model created by a radial basis neural network. The optimized design increased the stall margin by 37.1% compared to that of the smooth casing with little impacts on the efficiency and pressure ratio.

Relationship Between Flow Instability and Performance of a Centrifugal Pump With a Volute

Abstract Flow instability and its correlations with performance characteristics were investigated for a centrifugal pump with a volute. Unsteady three-dimensional Reynolds-averaged Navier–Stokes analysis was performed to analyze the flow and performance characteristics using the shear stress transport (SST) turbulence model. The grid dependence and temporal resolution were tested to evaluate the numerical uncertainties, and the numerical solutions were validated using experimental data. The total-to-static head coefficient, the impeller's total-to-static head coefficient, and the volute static pressure recovery coefficient were selected as performance parameters. To identify the flow instability, pressure fluctuations were monitored upstream of the impeller, at the volute inlet, and on the shroud wall of the impeller. Three different types of flow instability were detected in partial-load conditions: inside the volute, upstream of the impeller, and at the interface between the impeller and volute. The time-dependent flow structures were investigated to obtain insight into the onset of the flow instability. The correlation of the onset of the flow instability with the performance curves was discussed.

Analysis of Interaction Between Oscillating Jet Issuing from a Fluidic Oscillator and a Crossflow

In the present study, numerical simulation was conducted to study both the internal and external flows of a fluidic oscillator in a quiescent environment and with a crossflow. Unsteady Reynolds-averaged Navier–Stokes analysis was used to evaluate the flow field. The blowing ratio was varied to understand the interaction between the free stream and the oscillating jet. Results for the time-averaged and the time-resolved flow fields were presented. The outlet velocity from the oscillator was not affected by the freestream flow for the tested three blowing ratios. Q criterion was used to evaluate the interaction of turbulent vortical structures from the fluidic oscillator with the crossflow. It was observed that two counter-rotating vortices and ring-like vortices were the important flow features of the external flow. As blowing ratio increases, the counter-rotating vortices become more asymmetric, move further from the wall, and are more spread.

Effect of Fins on the Internal Flow Characteristics in the Draft Tube of a Francis Turbine Model

Undesirable flow phenomena in Francis turbines are caused by pressure fluctuations induced under conditions of low flow rate; the resulting vortex ropes with precession in the draft tube (DT) can degrade performance and increase the instability of turbine operations. To suppress these DT flow instabilities, flow deflectors, grooves, or other structures are often added to the DT into which air or water is injected. This preliminary study investigates the effects of anti-cavity fins on the suppression of vortex ropes in DTs without air injection. Unsteady-state Reynolds-averaged Navier–Stokes analyses were conducted using a scale-adaptive simulation shear stress transport turbulence model to observe the unsteady internal flow and pressure characteristics by applying anti-cavity fins in the DT of a Francis turbine model. A vortex rope with precession was observed in the DT under conditions of low flow rate, and the anti-cavity fins were confirmed to affect the mitigation of the vortex rope. Moreover, at the low flow rate conditions under which the vortex rope developed, the application of anti-cavity fins was confirmed to reduce the maximum unsteady pressure.

Effects of the Cross-Sectional Area of a Volute on Suction Recirculation and Cavitation in a Centrifugal Pump

Abstract The effects of the cross-sectional area of a volute on suction recirculation and cavitation in a centrifugal pump were investigated. The pump performance and fluid flow were analyzed using both steady and unsteady three-dimensional Reynolds-averaged Navier–Stokes analyses. The shear stress transport (SST) model was adopted as a turbulence closure model, and a simplified Rayleigh–Plesset cavitation model and a homogeneous two-phase mixture model were used to simulate the cavitating flow inside the pump. A constant to determine the designed circumferential velocity of the volute was selected as the geometric parameter for a parametric study. The hydraulic efficiency, head coefficient, blockage in front of the impeller, and critical cavitation number for a head-drop of 3% were selected as the performance parameters to evaluate the hydraulic performance. The results show that unlike the blockage, the hydraulic and suction performances were affected significantly by the volute shape. Both steady and unsteady flow analyses showed that the onset and development of suction recirculation were relatively unaffected by the volute geometry and the best efficiency point of the pump.

A Novel Algebraic Stress Model with Machine-Learning-Assisted Parameterization

Reynolds-stress closure modeling is critical to Reynolds-averaged Navier-Stokes (RANS) analysis, and it remains a challenging issue in reducing both structural and parametric inaccuracies. This study first proposes a novel algebraic stress model named as tensorial quadratic eddy-viscosity model (TQEVM), in which nonlinear terms improve previous model-form failure due to neglection of nonlocal effects. Then a data-driven regression model based on a fully-connected deep neural network is designed to determine the TQEVM coefficients. The well-trained data-driven model using high-fidelity direct numerical simulation (DNS) data successfully learned the underlying input-output relationships, further obtaining spatial-dependent optimal values of these coefficients. Finally, detailed validations are made in wall-bounded flows where nonlocal effects are expected to be significant. Comparative results indicate that TQEVM provides improvements both for the stress-strain misalignment and stress anisotropy, which are clear advantages over linear and quadratic eddy-viscosity models. TQEVM extends to the scope of resolution to the wall distance y + ≈ 9 as well as provides a realizable solution. RANS simulations with TQEVM are also carried out and the obtained mean-flow quantities of interest agree well with DNS. This work, therefore, results in a high-fidelity representation of Reynolds stresses and contributes to further understanding of machine-learning-assisted turbulence modeling and regression analysis.

A Simplified Numerical Model for the Prediction of Wake Interaction in Multiple Wind Turbines

In the design of wind energy farms, the loss of power should be seriously considered for the second wind turbine located inside the wake region of the first one. The rotation of the first wind-front rotor generates a high-vorticity wake with turbulence, and a suitable model is required in computational fluid dynamics (CFD) to predict the deficit of energy of the second turbine for the given configuration. A simplified numerical model based on the classical momentum theory is proposed in this study for multiple wind turbines, which is proposed with a couple of tuning parameters applied to Reynolds-averaged Navier-Stokes (RANS) analysis, resulting in a remarkable reduction of computational load compared with advanced methods, such as large eddy simulation (LES) where two parameters reflect on axial and rotational wake motion, simply tuned with the wind-tunnel test and its corresponding LES result. As a lumped parameter for the figure of merit, we regard the normalized efficiency on the kinetic power output of computational domain, which should be directed to maximize for the optimization of wind farms. The parameter surface is plotted in a dimensionless form versus intervals between turbines, and a simple correlation is obtained for a given hub height of 70% diameter and a fixed rotational speed tuned from the experimental data in a wide range.

Design and Verification of a Single-Channel Pump Model based on a Hybrid Optimization Technique

This paper handles a hybrid multiple optimization technique to concurrently enhance hydraulic efficiency and decrease unsteady radial forces resulting from fluid-induced vibration of a single-channel pump for wastewater treatment. A single-channel impeller and volute was optimized systematically by using a hybrid particle swarm optimization and genetic algorithm coupled with surrogate modeling. Steady and unsteady Reynolds-averaged Navier–Stokes analyses were conducted to optimize the internal flow path in the single-channel pump. Design variables for controlling the internal flow cross-sectional area of the single-channel impeller and volute in the single-channel pump were chosen to concurrently optimize objective functions with hydraulic efficiency and the unsteady radial forces resulting from impeller–volute interaction. The optimization results clearly showed that the arbitrary cluster optimum design considerably enhanced hydraulic efficiency and decreased the unsteady radial forces concurrently, compared to the reference design. Finally, the hydraulic performance of the optimized prototype model was verified experimentally. Then, it was proved that the proposed technique is a practical tool for designing a single-channel pump.

Generalized Treatment of Surface Deformation for High-Speed Computational Fluid Dynamic Surrogates

A new surrogate modeling approach is developed that augments the loads from an analysis over an undeformed surface in order to account for the effects of static surface deformations in high-speed flows. The approach relies on pointwise data-driven surrogates to approximate the nonlinear flow mechanics and theoretical expressions to identify the surrogate input parameters. The pointwise nature of the model enables efficient construction with minimal assumptions on the surface deformation. The accuracy of the modeling strategy is evaluated relative to steady Reynolds-averaged Navier–Stokes analyses for prescribed deformations and quasi-static aerothermoelastic response prediction. Basic fluid loads models and conventional kriging interpolation models, where deformation is parameterized using global characteristic shapes, are also considered. The new surrogate approach yields normalized root mean square errors of less than 5% relative to steady Reynolds-averaged Navier–Stokes predictions of pressure and heat flux over prescribed deformations. Results for aerothermoelastic simulations highlight that the approach outperforms all other models in terms of predicting the thermo-structural response and fluid loads. In regard to computational requirements, the new approach affords a 93% reduction in the offline costs compared with conventional kriging models and is several orders of magnitude faster than Reynolds-averaged Navier–Stokes simulations for online predictions.

Optimization of Supercritical Airfoil Considering the Ice-Accretion Effects

Ice formation on a wing poses threats to aircraft safety because it changes the effective shape of the airfoil and deteriorates its lift and moment performances dramatically. The present study develops a supercritical airfoil optimization design method that takes into account the ice-accretion effects. A multipoint/multiobjective aerodynamic optimization tool based on a response surface-enhanced evolutionary algorithm and a Reynolds-averaged Navier–Stokes analysis is constructed. A modified shear stress transport turbulence model is developed for predicting the stall behavior of an airfoil with a non-streamline leading-edge caused by horn-shaped ice. The aerodynamic coefficients of the design points at high-speed/low-speed/ice-accretion situations are set as the collaborative objectives. Pressure distribution constraints are applied to assure favorable pressure distribution, and they better coordinate these objectives. The results show that the present optimization improves the maximum lift coefficient of an iced airfoil by 16.5% while maintaining the cruise aerodynamic efficiency as well as its robustness. The selection of design objectives is discussed to achieve better design quality and efficiency. The optimized airfoil is analyzed with different ice shapes as well as icing conditions, and it is applied on a wing, which validates the improvement of aerodynamic performance.

Optimization of the Coanda bump to improve the film cooling effectiveness of an inclined slot

The present study attempted to improve film cooling effectiveness by installing a Coanda effect-inducing protrusion, called a Coanda bump, at the exit of the 45-degree inclined slot. The inclined slot used in this study was rectangular in shape with dimensions of 4 mm by 50 mm. For optimization, three geometric Coanda bump parameters – the width, height, and streamwise position of the peak point – were selected. The Box-Behnken design (BBD) method was employed to calculate the cases, and the response surface method (RSM) was applied to optimize the Coanda bump. The objective function (i.e., the area averaged film cooling effectiveness) was numerically investigated via a Reynolds-averaged Navier-Stokes (RANS) analysis. The results showed that when the optimized Coanda bump was applied to the slot exit, secondary flows such as the kidney vortex decreased and the coolant lift-off diminished compared to the reference design point without the Coanda bump. The area averaged film cooling effectiveness of the optimized design points improved by about 1.3, 2.4, and 7.6% at blowing ratios of 1.0, 1.5, and 2.0, respectively, compared to that of the reference design point.

Shape optimization of a bended film-cooling hole to enhance cooling effectiveness

To improve cooling effectiveness of film-cooling hole, bending of hole was investigated and optimized using surrogate modelling based on three-dimensional Reynolds-averaged Navier-Stokes analysis. The turbulence was modeled using a shear stress transport model. The spatially-averaged film-cooling effectiveness was employed as the objective function, and the injection angles of the lower and upper cylindrical parts and height of the bending point were selected as design variables for optimization. The Kriging model was used to approximate the objective function. 27 design points were selected by the Latin hypercube sampling, where objective function values were calculated to construct the surrogate models. The results at a blowing ratio of 0.5 showed that the optimum bended film-cooling hole improved the spatially-averaged film-cooling effectiveness by 39.9 % and 78.0 % compared to a reference bended hole and a cylindrical hole, respectively.

Comparative analysis of flow in a fluidic oscillator using large eddy simulation and unsteady Reynolds-averaged Navier–Stokes analysis

A fluidic oscillator with two feedback channels was numerically investigated using large eddy simulation (LES) and unsteady Reynolds-averaged Navier–Stokes (RANS) analysis. The shear stress transport model was used for turbulence closure in the RANS analysis, and the WALE model was used for the LES as a sub-grid model. The computational mesh size was determined based on the turbulent energy length scale inside the fluidic oscillator. The numerical results were validated through qualitative and quantitative comparisons with available experimental results for the flow field and frequency of the jet oscillation. The RANS analysis showed better overall agreement with the experimental data than the LES. The LES slightly under-predicted the reverse flow in the feedback channels and separation region in the mixing chamber. The results showed that the mass flow rate through a feedback channel is nearly independent of the total mass flow rate through the oscillator. The LES was able to resolve the turbulent mixing layer originating from the inlet wedge of the mixing chamber and two large streamwise vortices originating from the throat of the oscillator.

Fluid Structure Interaction Simulations of the NREL 5 MW Wind Turbine—Part I: Aerodynamics and Blockage Effect

Fluid structure interaction (FSI) simulations of the NREL 5 MW wind turbine are performed using a combination of two separate computational codes: abaqus for the finite element analysis (FEA) of turbine structures and STAR-CCM+ for the unsteady Reynolds-averaged Navier–Stokes analysis of flow around the turbine. The main aim of this study is to demonstrate the feasibility of using two-way coupled FSI simulations to predict the oscillation of the tower, as well as the rotor blades, of a full-scale wind turbine. Although the magnitude of the oscillation of the tower is much smaller than that of the blades, this oscillation is crucial for the assessment of the fatigue life of the tower. In this first part of the paper, the aerodynamic characteristics of the turbine predicted by the two-way coupled FSI simulations are discussed in comparison with those predicted by Reynolds-averaged Navier–Stokes simulations of a rigid turbine. Also, two different computational domains with a cross-sectional size of 2D × 2D and 4D × 4D (where D is the rotor diameter) are employed to investigate the blockage effect. The fatigue life assessment of the turbine is planned to be reported in the second part of the paper in the near future.