Reynolds-averaged Navier-Stokes Simulations Research Articles

Mitigation of rotor thrust fluctuations through passive pitch

A rotor blade operating in an unsteady or a sheared stream experiences force fluctuations, which increase the structural requirements for both the blades and the rotor system as a whole. In this paper, we investigate whether force fluctuations can be passively mitigated without compromising the mean load. We consider a tidal turbine rotor in a shear current at a diameter-based Reynolds number of 2×107. The blades are rigid and can passively pitch. A mass–spring system acts on the spanwise axis of the blade governing the pitch kinematics. The effectiveness of this system is demonstrated with three methodologies: an analytical model based on blade element momentum theory and Theodorsen’s theory, and two sets of Reynolds-averaged Navier–Stokes simulations performed with two independent codes. The analytical and the numerical models are validated against experiments and simulations in the literature. All methodologies predict a reduction of more than 80% of the thrust fluctuations. Furthermore, because of the more uniform thrust force exerted on the current, the wake behind a passive pitch blade does not diffuse the onset shear flow. This results in a more sheared wake and faster wake recovery. The present results demonstrate the potential benefits of passive pitch and may underpin future applications of this concept for different types of turbines and compressors, including wind turbines, propellers, helicopter rotors, etc.

A virtual laboratory for conducting “hands-on” experiments on water wave mechanics

Water wave mechanics are an important topic for scientist and practitioners of civil engineering due to the key role in coastal structure design and the understanding of beach erosion and coastal flooding. Different water wave theories have been proposed over the past two centuries to describe wave transformation, being the so-called linear water wave theory; the most fundamental but still widely used despite its limitations. The use of laboratory facilities in water wave courses, at both undergraduate and graduate levels, is fundamental to provide hands on experience and training on the experimental design and data analysis. Furthermore, it allows to evaluate the validity and limitations of linear wave theory. Unfortunately, such facilities are scarce and their use was further complicated owing to social distancing measures during the COVID-19 outbreak. On the other hand, the advent of computer technology and Computation Fluid Dynamics codes have allowed the development of virtual laboratories for science, technology, and engineering. Here, a widely validated numerical model, which solves the Reynolds Averaged Navier-Stokes (RANS) equations, is employed to develop a virtual wave flume for educational purposes. The virtual laboratory consists of three components: 1) An educational component consisting of lab assignments including background theory, mainly based on the water wave mechanics book by Dean and Dalrymple (1991), and the description of the experiment to be conducted in the virtual wave flume; 2) Synthetic data, corresponding to different RANS simulations, including the spatial-temporal variability of different hydrodynamic variables corresponding to realistic laboratory experiments; and 3) The Graphical User Interface (GUI) allowing the user to select the experiment and download the lab assignment instructions. Within each experiment the user may select different wave conditions and deploy the virtual “sensors” interactively. The experiment and time series corresponding to the virtual sensors can be visualized in real-time. The data acquired with the virtual sensors can be exported as a binary file for further analysis and comparison with theory. The virtual laboratory has been employed as a supporting tool in the Water Wave Mechanics and Nearshore Hydrodynamics graduate courses taught at the National Autonomous University of Mexico. A preliminary assessment of learning outcomes revealed that students agree on the usefulness of the virtual wave flume to the understanding of wave transformation processes and the enjoyment of the courses. The documentation, pre-run numerical simulations, and GUI are integrated in a self-application program developed in Matlab® available for download from a public repository.

Prediction of respiratory droplets evolution for safer academic facilities planning amid COVID-19 and future pandemics: A numerical approach

Airborne dispersion of the novel SARS-CoV-2 through the droplets produced during expiratory activities is one of the main transmission mechanisms of this virus from one person to another. Understanding how these droplets spread when infected humans with COVID-19 or other airborne infectious diseases breathe, cough or sneeze is essential for improving prevention strategies in academic facilities. This work aims to assess the transport and fate of droplets in indoor environments using Computational Fluid Dynamics (CFD). This study employs unsteady Reynolds-Averaged Navier-Stokes (URANS) simulations with the Euler-Lagrange approach to visualize the location of thousands of droplets released in a respiratory event and their size evolution. Furthermore, we assess the dispersion of coughing, sneezing, and breathing saliva droplets from an infected source in a classroom with air conditioning and multiple occupants. The results indicate that the suggested social distancing protocol is not enough to avoid the transmission of COVID-19 since small saliva droplets ( ≤ 12 μm) can travel in the streamwise direction up to 4 m when an infected person coughs and more than 7 m when sneezes. These droplets can reach those distances even when there is no airflow from the wind or ventilation systems. The number of airborne droplets in locations close to the respiratory system of a healthy person increases when the relative humidity of the indoor environment is low. This work sets an accurate, rapid, and validated numerical framework reproducible for various indoor environments integrating qualitative and quantitative data analysis of the droplet size evolution of respiratory events for a safer design of physical distancing standards and air cleaning technologies.

Uncertainty quantification (UQ) for CFD simulation of OECD-NEA cold leg mixing benchmark

An uncertainty quantification (UQ) analysis was conducted for the unsteady Reynolds Average Navier Stokes Simulation (URANS) of the OECD-NEA cold leg mixing benchmark. The methodology is based on the American Society of Mechanical Engineers Verification and Validation (ASME V&V) standard for UQ in Computational Fluid Dynamics (CFD) applications. The numerical, input and experimental uncertainties were calculated to determine the validation uncertainty for the open test (T1). Furthermore, based on validation uncertainty, model uncertainty was calculated. Finally, on the basis of open test (T1) the symmetric and asymmetric uncertainty band was achieved for the blind test (T2). The validation uncertainty closely covers the PIV data in the cold leg. Besides, the simulation results of the blind test, together with the uncertainty band, closely bound the PIV data of the mean velocity in the cold leg. However, a large simulation uncertainty band was obtained in the downcomer due to a large validation error.

Combustion characteristics of a reverse-cross-flow combustor

The effect of fuel-to-air jet momentum flux ratios (varied from 0.1 to 5.1) in a 6.25 kW Reverse-Cross-Flow (RCF) combustor is investigated at a maximum thermal intensity of 40 MW/m3-atm using methane fuel. Reaction zone is characterised by imaging OH radicals using planar laser-induced fluorescence (OH-PLIF) technique. Reynolds-averaged Navier-Stokes (RANS) simulations are performed at an equivalence ratio of 0.8 using Eddy Dissipation Concept (EDC) model for turbulence-chemistry interactions with detailed reaction mechanism (GRI-Mech 3.0). Numerical results show a qualitative match with the experimental results for averaged OH intensity distribution and NOx and CO emissions. Air jet is observed to deflect more with increase in fuel-to-air jet momentum flux ratio. Flame-front is observed to be located at the periphery of the air jet due to mixing of reactants and hot burned products in the shear layer. Instantaneous OH-PLIF data suggest that the flame-front location changes considerably with time. Both numerical and experimental results show increase in CO emissions and reduction in NOx emissions with increase in fuel-to-air jet momentum flux ratio. This may be linked to unfavorable residence time distribution, despite better mixing at high fuel injection momentum.

Coaxial-Injector Surrogate Modeling Based on Reynolds-Averaged Navier–Stokes Simulations Using Deep Learning

Facing the need to increase the accuracy of rocket engine design tools, the present work introduces an innovative methodology for the design and optimization of rocket engine combustion chambers using numerical simulations and deep learning. An experimental test case of a single coaxial injector is taken as a reference point, and a design of experiments is generated by varying nine parameters (geometrical and operative conditions). Reynolds-averaged Navier–Stokes simulations are carried out to generate the data set. The data are used to train surrogate models of different fidelities, from low-dimensional outputs (zero-dimensional and one-dimensional) toward the two-dimensional temperature field. Attention is given on the selection of the proper machine learning technique. For low-dimensional outputs, results show that deep neural networks outperform other standard machine learning tools, namely, radial basis function and kriging. Regarding high-dimensional outputs, convolutional neural networks with gradient-based loss functions are found effective to capture the large and smooth temperature variations, as well as the thin and sharp temperature gradients at the flame front. Eventually, the models are used in the framework of an optimization problem. Results highlight the benefits of new design and optimization tools based on deep learning, capable of real-time predictions of complex flowfields.

A dual-mesh hybrid Reynolds-averaged Navier-Stokes/Large eddy simulation study of the buoyant flow between coaxial cylinders

This study is concerned with the investigation of the suitability of the dual-mesh method for the buoyancy driven flow inside a cylindrical annulus with one internal cylinder. This flow situation can occur, for instance, in gas cooled nuclear reactors and is characterized by complex features, including a buoyant plume and a negatively buoyant wall-jet. The buoyancy forces in this flow are generated by the temperature difference between the inner and outer cylinders. The dual-mesh approach is a hybrid RANS-LES method in which an unsteady Reynolds-averaged Navier-Stokes (RANS) simulation and a coarse Large eddy simulation (i.e. an LES that is under-resolved near walls) are run simultaneously on two different grids. In this approach, a criterion is used to determine the locations at which each simulation is expected to perform better than the other. Consequently, at every location the less accurate simulation is forced and corrected towards the more accurate one. This correction is done using source terms that are included in the equations of the flow and thermal fields of the two simultaneous simulations. It is observed that the buoyancy-extended lengthscale resolution criterion behaves satisfactorily in defining the regions where the LES is corrected towards the RANS and vice versa. Moreover, both quantitative and qualitative analyses of the results are conducted using quasi direct numerical simulation results from the literature. It is shown that the dual-mesh method has the potential to yield results that are better than the results of the pure RANS and the pure coarse LES simulations.

Accelerating reactive-flow simulations using vectorized chemistry integration

The high cost of chemistry integration is a significant computational bottleneck for realistic reactive-flow simulations using operator splitting. Here we present a methodology to accelerate the solution of the chemical kinetic ordinary differential equations using single-instruction, multiple-data vector processing on CPUs using the OpenCL framework. First, we compared several vectorized integration algorithms using chemical kinetic source terms and analytical Jacobians from the pyJac software against a widely used integration code, CVODEs. Next, we extended the OpenFOAM computational fluid dynamics library to incorporate the vectorized solvers, and we compared the accuracy of a fourth-order linearly implicit integrator–both in vectorized form and a corresponding method native to OpenFOAM—with the community standard chemical kinetics library Cantera. We then applied our methodology to a variety of chemical kinetic models, turbulent intensities, and simulation scales to examine a range of engineering and scientific scale problems, including (pseudo) steady-state as well as time-dependent Reynolds-averaged Navier–Stokes simulations of the Sandia flame D and the Volvo Flygmotor bluff-body stabilized, premixed flame. Subsequently, we compared the performance of the vectorized and native OpenFOAM integrators over the studied models and simulations and found that our vectorized approach performs up to 33–35× faster than the native OpenFOAM solver with high accuracy.

Energy analysis of a detonation combustor with a bladeless turbine, a propulsion unit for subsonic to hypersonic flight

This paper details an energy conversion unit for the combined power extraction and thrust increase from high subsonic to hypersonic flows. The energy transfer is achieved through a wavy hub surface that promotes shocks and expansion fans. First, a design procedure and the non-dimensional steady-state performance characteristics (the reduced torque, non-dimensional mass flow, and non-dimensional speed) are presented (including an exergetic analysis) at on– and off-design conditions through time-averaged Reynolds Averaged Navier Stokes simulations. A cycle analysis was performed on a Mach 3 vehicle to demonstrate the feasibility of this technology. Second, energy extraction is achieved from the highly unsteady exhaust of rotating detonation combustors without needing an additional intermediate diffuser or nozzle. The rotating boundary conditions were computed from a reactive unsteady simulation. The influence of three shock parameters (pressure ratio across the shock, shock wave Mach number, and the number of waves) is investigated through a sensitivity study to assess the robustness of the new device. Furthermore, depending on the sense of rotation of the bladeless turbine relative to the rotation of the incoming shock and the helix angle, we can define two different modes of operation in which power extraction was achieved, namely, the shock mode and reversed mode.

On the combined use of Vortex Generators and Gurney Flaps for turbine airfoils

While Vortex Generators (VGs) and Gurney Flaps (GFs) are commonly used for airfoil flow control, studies of a combination of the two devices are rare. The present investigation aims at examining the combined effect of VGs and GFs on a 20% thick airfoil. To this end, a wind tunnel investigation coupled with a computational study was performed. The present paper presents force, pressure and Stereo Particle Image Velocimetry measurements, along with Unsteady Reynolds Averaged Navier Stokes simulations.

Blockage effects in a single row of wind turbines

Reynolds-averaged Navier-Stokes simulations are used to assess the magnitude and sign of turbine interaction loss in single-row wind farms with flow perpendicular to the row. Consistent with previous studies, the simulations show that lateral interactions between turbines can lead to a production gain when assuming idealised neutral conditions. These gains are amplified by using over-constraining boundary conditions. When more realistic conditions are simulated, the gains turn to losses, suggesting that losses are more likely to occur in actual single-row wind farms. The study results also indicate that commonly used assumptions in engineering models for blockage effects risk the underprediction of turbine interaction loss.

A data-informed analytic model for turbine power prediction with anisotropic local blockage effects

This paper presents a new analytic approach for estimating the local power coefficient of a turbine experiencing anisotropic local blockage effects. Data-driven methods are employed first to approximately obtain a known analytic expression for isotropic local blockage effects, and then deployed to find candidate expressions for anisotropic local blockage effects. The dataset for the analysis of anisotropic local blockage is collected from 3D Reynolds-averaged Navier-Stokes (RANS) simulations of an infinitely wide array of actuator disks for nearly 2,000 different blockage configurations. This study builds upon previous work in array optimisation of both tidal and wind turbines, where the local power coefficient may increase substantially for optimal array configuration. A brief discussion on the relationship between the local blockage and wind farm blockage is also provided. Other theoretical approaches and possible refinements to the presented analytic model are also discussed.

Numerical study of heat transfer, flow fields, turbulent length scales, and anisotropy in corrugated heat exchanger channels

In this study, we report on large eddy simulation (LES) of convectively dominated heat transfer in a corrugated heat exchanger channel using the computational fluid dynamics toolbox, OpenFOAM. A chevron pattern domain with 63 contact points is used to represent the conditions in a real plate heat exchanger (PHE). The unsteady nature of the flow is elucidated using visualization techniques based on volume rendering of temperature and iso surfaces of vorticity defined using the λ2-criterion and contours of wall shear stress and wall heat flux to illustrate the heat transfer process. Global surface averaged temperature and pressure drop are extracted from the LES on successively finer grids, approaching direct numerical simulation resolution, to increase the understanding of grid resolution requirements for LES in PHEs. Industry standard Reynolds-averaged Navier–Stokes simulations are compared to the LES results along selected profiles to demonstrate similarities and differences between the two techniques. The differences detected are further investigated using anisotropy invariant mapping, energy spectra, and turbulence length scale distributions. Significant differences between the model classes are detected and detailed. Moreover, the LES resolution requirements for the flow and the heat transfer processes are found to be different with the latter being more severe.

Faster wind farm AEP calculations with CFD using a generalized wind turbine model

Wind farm Annual Energy Production (AEP) calculations are required to design energy efficient wind farm layouts. We investigate methods that can reduce the computational effort of AEP calculations using Reynolds-averaged Navier-Stokes simulations of an idealized atmospheric wind farm setup. In addition, we introduce a generalized wind turbine model that compares well with wind turbine aerodynamic data covering a large range of wind turbine sizes. We apply the general wind turbine model to reduce the computational effort of the AEP calculations by decreasing the number of independent wind speed flow cases. Furthermore, we apply Reynolds-number similarity to compute the wind speed flow cases faster and we show how wind farm layout mirror- and rotational-symmetry can reduce the number of independent wind direction flow cases.

Lagrangian Investigation of Convective Mass Transfer of Dissolved Elements at Specimen Boundaries in a Flowing Molten Lead Loop

Mass transfer is the dominant mode of structural material corrosion in energy systems employing heavy liquid metal coolant such as lead-cooled reactors. Modeling efforts in the literature have focused on materials science aspects, such as diffusive transport of alloying elements in structural materials and oxide layers, oxide layer growth and erosion, and species dissolution at the interface, but they have overlooked convective transport which is often represented by simplified one-dimensional models with no transverse convection. Here, within a Lagrangian framework, we particularly study the convective transport of dissolved elements at specimen boundaries in a flowing molten lead loop. Three-dimensional transient Reynolds-averaged Navier-Stokes simulations coupled with particle transport are carried out to compare convective transport in lead and other coolants, such as lead-bismuth eutectic, pressurized water, and sodium. Transverse convection in the narrow test section is observed to occur at a timescale comparable to longitudinal (downstream) transport and removal of particles from the test section, which highlights the need for three-dimensional modeling in the present setup. The effects of temperature, surface roughness, and mean flow velocity on convective transport in lead are investigated. While mean flow velocity is the dominant variable affecting convective mass transfer, increased surface roughness and reduced temperature are also shown herein to moderately enhance convective transfer.

Rheological and flow characteristics of fresh concrete in the rotational rheometer

The coaxial cylinder rheometer is widely used to measure the torque and rotational speed (T-N) of fresh concrete. T-N can be transformed into the corresponding rheological parameters via the solution of the ‘Couette inverse problem’. However, the solution of the ‘Couette inverse problem’ is a theoretical solution, which may produce errors due to the complex geometric shape of the coaxial cylindrical rheometer and the possible plug or secondary flow. The purpose of this study is to investigate the rheological and flow behaviors of fresh concrete in the rotational rheometer by coupling CFD (Computational Fluid Dynamics) simulations and experiments. The efficient high-fidelity 3-D CFD simulations of fresh concrete flowing in the ConTec Viscometer 5 are conducted using the Reynolds Average Navier–Stokes (RANS) model with the Multi-Reference Frame (MRF). Both Bingham and Herschel–Bulkley (H–B) models are adopted, and the numerical simulations are validated against by experiments, which enhanced the confidence that RANS simulation with MRF could supplement the experiments and elucidate the rheological and flow behaviors of fresh concrete with the whole-flow domain information.

Experiments and CFD simulations of the LBE loop in HYST: A new concept for irradiation experiments in a fast-reactor-like environment

The current work investigates the thermal-hydraulics of a HYbrid fast/thermal core configuration Subcritical Testbed (HYST), a new concept proposed for irradiation experiments in a fast-reactor-like environment. Both the experimental studies and CFD simulations are performed to assist the development of HYST system. The experiments characterize heat transfer and pressure drop in the fast core as well as the heat transfer to thermal core from the fast core, while the CFD simulations only focus on the fast core to reduce the computational cost. The Reynolds-Averaged Navier Stokes (RANS) simulations using ANSYS Fluent are benchmarked through a two-step strategy first by existing data in literature and then by the newly collected data in HYST. By comparing with the existing data, confidence is gained in mesh generation, wire-rod contact modeling, and selected turbulence and Prt models. The mesh independence study in simulating HYST using Fluent indicates that the mesh convergence can be reached. Investigated parameters include the LBE temperature and velocity magnitude in all subchannels, heater rod surface temperature, outlet LBE temperature, and pressure drop between inlet and outlet. The effects of different Prt models and non-uniform power distribution on HYST simulation results are also investigated. Finally, these RANS simulations of HYST are benchmarked by preliminary experimental data in both adiabatic and heated tests. The experimental data of LBE outlet temperature for heated tests compare well with the simulations; however, the pressure drops are underestimated by the correlations and CFD simulations. In the next development phase, more detailed experimental studies and CFD simulations will be performed to assist the design of HYST system.

Evaluation of the global-blockage effect on power performance through simulations and measurements

Abstract. Blockage effects due to the interaction of five wind turbines in a row are investigated through both Reynolds-averaged Navier–Stokes simulations and site measurements. Since power performance tests are often carried out at sites consisting of several turbines in a row, the objective of this study is to evaluate whether the power performance of the five turbines differs from that of an isolated turbine. A number of simulations are performed, in which we vary the turbine inter-spacing (1.8, 2 and 3 rotor diameters) and the inflow angle between the incoming wind and the orthogonal line to the row (from 0 to 45∘). Different values of the free-stream velocity are considered to cover a broad wind speed range of the power curve. Numerical results show consistent power deviations for all five turbines when compared to the isolated case. The amplitude of these deviations depends on the location of the turbine within the row, the inflow angle, the inter-spacing and the power curve region of operation. We show that the power variations do not cancel out when averaging over a large inflow sector (from −45 to +45∘) and find an increase in the power output of up to +1 % when compared to the isolated case under idealised conditions (neutral atmospheric conditions, no vertical wind shear or ground effects). We simulate power performance “measurements” with both a virtual mast and nacelle-mounted lidar and find a combination of power output increase and upstream velocity reduction, which causes an increase of +4 % in the power coefficient under idealised conditions. We also use measurements from a real site consisting of a row of five wind turbines to validate the numerical results. From the analysis of the measurements, we also show that the power performance is impacted by the neighbouring turbines. Compared to when the inflow is perpendicular to the row, the power output varies by +1.8 % and −1.8 % when the turbine is the most downwind and upwind of the line, respectively.

Numerical modeling for local flame structure and pollutant formation in biodiesel and n-Dodecane spray jet flames

The present work performs unsteady Reynolds-averaged Navier-Stokes simulation to study the combustion characteristics of soybean methyl esters biodiesel and n-dodecane spray jet flames. To realistically account for the turbulence-chemistry interaction in computationally efficient manner, the Multi-Environment PDF model coupled with unsteady flamelet library are considered in the present work. The present comprehensive spray combustion model is validated against experimental data in terms of liquid and vapor penetration, ignition delay time and lift-off length. It was found that the biodiesel spray flame creates hot reaction zone at the relatively narrow central region with small flame volume. On the other hand, n-dodecane spray flame forms the hot reaction zone at the spreading flame field with large flame volume. These distinctive differences in local flame structures yield different NO distribution of the two spray flames due to its strong dependence on temperature. The biodiesel spray flame produces significantly less C2H2 at the fuel-rich flame zone than the n-dodecane spray flame. The present work clearly shows that the MEPDF model has capability to predict central partially premixed flame of biodiesel, diffusion flame of n-dodecane, and reduction of soot when oxygenate fuel is used in Diesel engine-like conditions.

Effect of Airfoil Dimple on Low-Reynolds-Number Differing Laminar Separation Behavior via Multi-Objective Optimization

Micro air vehicles and vertical-takeoff-and-landing drones usually operate at relatively low values of the Reynolds number (). In the present study, three airfoils from different categories (AG14, BE50, and SD7032) are studied at . How the behavior of the laminar separation bubble changes with the Reynolds number is investigated and is found to give rise to differing aerodynamic forces that can severely degrade the aerodynamic performance of low-Reynolds-number airfoils. To obtain airfoils with satisfactory aerodynamic performance over a wide range of low Reynolds number, a multi-objective optimization approach is proposed and implemented with different design considerations. The airfoils optimized at these Reynolds numbers have a geometric dimple, as confirmed by unsteady Reynolds-averaged Navier–Stokes simulations with a transition model, and by particle image velocimetry wind-tunnel testing. The dimple is effective in restricting the location and size of the laminar separation bubble, leading to robust and significantly improved aerodynamic performance. Also studied is how the number of nonuniform rational B-spline control points affects the optimum airfoil results, and the dimple always appears in the optimum geometry for .