Unsteady Reynolds Averaged Navier-Stokes Approach Research Articles

Efficient control of the fully passive oscillating foil in 2D confined flows with adjustment of the heave damping

A foil that is free to pitch and heave in an upstream flow can oscillate very regularly and with large amplitudes given that its inertial properties and support stiffness in pitch and heave are well adapted to the flow velocity. Useful energy can be extracted from these oscillations via an appropriate damping in heave that models the presence of an electric generator. In recent years, the structural parameters of such a fully passive oscillating-foil turbine (OFT) have been optimized, yielding a maximum energy extraction efficiency of 51.0% under the assumptions of 2D and unconfined flow. However, the turbine is normally deployed in channels with finite cross-sectional area, thus impacting the flow rate passing through the turbine via the blockage effect. In this work, we extend the applicability of the 2D optimized structural parameters to 2D confined scenarios with a simple tuning of the viscous heave damping coefficient. Performance is determined via a fluid-structure interaction solver based on an unsteady Reynolds-averaged Navier–Stokes approach. As expected, confining the turbine increases the heave amplitude and the power, up to a point where the motions become chaotic, and thus require an increase in the heave damping coefficient. This study shows that in all confined 2D scenarios, reasonably good performances of the fully passive OFT can be maintained when using its optimal structural parameters obtained in 2D unconfined conditions, given that the generator is adjusted accordingly.

Assessment of URANS-Type Turbulent Flow Modeling of a Single Port Submerged Entry Nozzle (SEN) for Thin Slab Continuous Casting (TSC) Process

The numerical methods based on the unsteady Reynolds-averaged Navier–Stokes (URANS) equations are robust tools to model the turbulent flow for the industrial processes. They allow an acceptable grid resolution along with reasonable calculation time. Herein, the URANS approach is validated against a water model experiment for the special single port submerged entry nozzle (SEN) design used in the thin slab casting (TSC) process. A 1-to-2 under-scaled water model was constructed, including the SEN, mold, and strand Plexiglas segments. Paddle-type sensors were instrumented to measure the submeniscus velocity supported by videorecording of the dye injections to provide both qualitative and quantitative verification of the SEN flow simulations. Two advanced URANS-type models (realizable k–ε and shear stress transport k–ω) were applied to calculate velocity pattern on meshes with various resolutions. An oscillating single jet flow was detected in the experiment, which the URANS simulations initially struggled to reflect. The dimensionless analysis of the mesh properties and corresponding adjustment of the boundary layers inside the SEN allowed to resolve the flow pattern. The performed fast Fourier transform (FFT) verified a good numerical prediction of the flow frequency spectrum. The corresponding simulation strategy is proposed for the industrial CC process using the URANS approach.

Experimental and numerical analysis of the physical characteristics of natural and ventilated supercavitating flows

In this experimental and numerical study, we investigate the physical characteristics of a supercavitating flow generated behind a disk-shaped cavitator under both natural and ventilated conditions, an area of research that has not been thoroughly examined. Initially, the experiment is conducted within a cavitation tunnel employing a forward-facing model, complemented by high-speed visualization techniques. Subsequently, an unsteady Reynolds-averaged Navier–Stokes approach is adopted to conduct numerical simulations along with the k–ε turbulent model and Ffowcs Williams–Hawkings (FW–H) methods. The outcomes of the study demonstrate that when considering fixed cavitation numbers, the profiles of natural and ventilated cavities are consistent. Under constant flow conditions, the introduction of ventilating air leads to a discernible reduction in hydroacoustic characteristics in the high-frequency spectrum and has the potential to improve flow stability behind the cavitator. The numerical results offer insight into the behaviors of the water, vapor, and ventilation air. In the foamy cavity stage, all the considered phases (water, vapor, and ventilation air) coexist inside the cavity. Upon the formation of a transparent supercavity, the ventilation air primarily gathers around the ventilation holes and the surrounding gas-leakage region. Meanwhile, the vaporous gas is dominant and is concentrated predominantly in the central region of the supercavity. The findings extracted from this study represent a significant advancement in our understanding of the intricacies of supercavities under ventilated and vaporous conditions. These insights hold the potential to drive groundbreaking innovations in the design and control of supercavitating vehicles.

Free fall drag estimation of small-scale multirotor unmanned aircraft systems using computational fluid dynamics and wind tunnel experiments

New European Union (EU) regulations for UAS operations require an operational risk analysis, which includes an estimation of the potential danger of the UAS crashing. A key parameter for the potential ground risk is the kinetic impact energy of the UAS. The kinetic energy depends on the impact velocity of the UAS and, therefore, on the aerodynamic drag and the weight during free fall. Hence, estimating the impact energy of a UAS requires an accurate drag estimation of the UAS in that state. The paper at hand presents the aerodynamic drag estimation of small-scale multirotor UAS. Multirotor UAS of various sizes and configurations were analysed with a fully unsteady Reynolds-averaged Navier–Stokes approach. These simulations included different velocities and various fuselage pitch angles of the UAS. The results were compared against force measurements performed in a subsonic wind tunnel and provided good consistency. Furthermore, the influence of the UAS`s fuselage pitch angle as well as the influence of fixed and free spinning propellers on the aerodynamic drag was analysed. Free spinning propellers may increase the drag by up to 110%, depending on the fuselage pitch angle. Increasing the fuselage pitch angle of the UAS lowers the drag by 40% up to 85%, depending on the UAS. The data presented in this paper allow for increased accuracy of ground risk assessments.

Numerical Investigation into Single and Double Gurney Flaps for Improving Airfoil Performance

The impact of single and double Gurney flaps of heights within the range of is determined for a typical symmetrical airfoil (NACA 0015) by numerical simulations using an unsteady Reynolds-averaged Navier–Stokes approach. The optimal scaling of the Gurney flap to maximize the lift-to-drag ratio of the airfoil is revisited and confirmed to depend on the boundary-layer thickness in the region where it is deployed. Appropriate scaling rules for both single and double Gurney flap are proposed. Studying the Gurney flap’s pressure distribution at a finely resolved level highlights its effect on the overall foil’s pressure distribution. Finally, the unsteady simulations carried out are harnessed to determine the shedding frequencies generated by the flaps and the flow structures associated with them, hinting at the physical mechanisms by which Gurney flaps may increase lift at a low drag cost, thus improving the lift-to-drag ratio of the airfoil.

Comparative Analysis of Patient-Specific Aortic Dissections through Computational Fluid Dynamics Suggests Increased Likelihood of Degeneration in Partially Thrombosed False Lumen.

Aortic dissection is a life-threatening vascular disease associated with high rates of morbidity and mortality, especially in medically underserved communities. Understanding patients' blood flow patterns is pivotal for informing evidence-based treatment as they greatly influence the disease outcome. The present study investigates the flow patterns in the false lumen of three aorta dissections (fully perfused, partially thrombosed, and fully thrombosed) in the chronic phase, and compares them to a healthy aorta. Three-dimensional geometries of aortic true and false lumens (TLs and FLs) are reconstructed through an ad hoc developed and minimally supervised image analysis procedure. Computational fluid dynamics (CFD) is performed through a finite volume unsteady Reynolds-averaged Navier-Stokes approach assuming rigid wall aortas, Newtonian and homogeneous fluid, and incompressible flow. In addition to flow kinematics, we focus on time-averaged wall shear stress and oscillatory shear index that are recognized risk factors for aneurysmal degeneration. Our analysis shows that partially thrombosed dissection is the most prone to false lumen degeneration. In all dissections, the arteries connected to the false lumen are generally poorly perfused. Further, both true and false lumens present higher turbulence levels than the healthy aorta, and critical stagnation points. Mesh sensitivity and a thorough comparison against literature data together support the reliability of the CFD methodology. Image-based CFD simulations are efficient tools to assess the possibility of aortic dissection to lead to aneurysmal degeneration, and provide new knowledge on the hemodynamic characteristics of dissected versus healthy aortas. Similar analyses should be routinely included in patient-specific hemodynamics investigations, to plan and design tailored therapeutic strategies, and to timely assess their effectiveness.

Aerodynamics and Power Balance of a Distributed Aft-Fuselage Boundary Layer Ingesting Aircraft

This paper presents a first investigation into the aerodynamics and performance breakdown of a distributed aft-fuselage boundary layer ingesting (BLI) tube-and-wing aircraft using fully coupled Unsteady Reynolds-Averaged Navier-Stokes (URANS) calculations that resolve the complete fan and installation geometries. Through the URANS simulations, the interaction between the turbulence from the fuselage boundary layer (BL) and the BLI propulsor is identified as an area that warrants further research. Using the URANS approach, the ingested turbulence leads to a 4.5% reduction in the propulsor stage total–total isentropic efficiency. A mechanical power balance has been drawn up to compare the power sources and sinks throughout the installation and propulsor for two test cases with different thicknesses of ingested BL. The test case with a thinner BL was found to generate significantly more dissipation in the BL development upstream of the propulsor, the flow separation over the outer cowl, and the interaction between the reversed flow over the cowl and the propulsor exhaust jet. Due to this increase in dissipation, the case with thinner ingested BL consumes 7% more power relative to a baseline case with thicker BL, representative of the upstream fuselage at cruise. This demonstrates the importance of matching the installation with the incoming fuselage BL.

Influence of Aerodynamic Interaction on Performance of Contrarotating Propeller/Wing System

This paper gives a quantitative account of the influence of slipstream on the aerodynamic performance of a contrarotating propeller (CRP)/wing system, and compares it with the CRP and clean wing. To accurately evaluate the complex aerodynamic interaction, the unsteady Reynolds-averaged Navier–Stokes approach using the sliding mesh method is performed at a typical freestream velocity of 30 m/s. Four different critical parameters, including the freestream angle of attack (AoA), axial spacing between the front propeller (FP) and rear propeller (RP), number of blades, and rotational speed, are considered in the present work. The results show that the thrust coefficient, power coefficient, and propulsion efficiency of the CRP/wing system change sharply and the difference in amplitude between adjacent waves is large. In particular, the propeller slipstream has a significant impact on the lift–drag performance of the wing in the case of a nonzero AoA. The presence of a wing also increases the efficiency of propulsion due to the recovery of vortices. In the case of a small axial spacing, the thrust coefficient value of the FP is significantly smaller than that of the RP. However, when the axial spacing exceeds a certain value, the opposite relationship is obtained. When the rotational speed increases from 3695 RPM to 8867 RPM, the lift coefficient and drag coefficient of the wing gradually increase.

Numerical simulation of bubbly jets in crossflow using OpenFOAM

This paper conducted a computational fluid dynamics study of bubbly jets (not bubble plumes due to pure gas injection) in crossflow to explore the hydrodynamics that are still unknown. A three-dimensional model was developed, calibrated, and validated by coupling the Euler–Euler two-fluid model with unsteady Reynolds-averaged Navier–Stokes approach in OpenFOAM. The results showed that the modeled gas void fraction, bubble velocity, water jet centerline trajectory, and jet expansion agree well with the experimental data. The vertical distribution of turbulent kinetic energy evolves from mono-peak to dual-peaks as the jet penetrates farther for the bubbly jet due to the interactions between bubbles and ambient water flow. Water velocity distribution was examined at cross sections of both the air- and water-phases of bubbly jets in crossflow, and counter-rotating vortex pairs can be clearly observed for both phases. Generally, the center-plane maximum concentration decreases in the crossflow direction. Compared to pure water jets, bubbly jets are stretched wider in the vertical direction due to the lift of bubbles, and thus, dilution is larger. Interestingly, the vorticity at water jet cross sections of bubbly jets evolves from two vertical “kidney-shapes” to two axisymmetric “thumb-up-shapes.” Moreover, effects of ambient crossflow on bubbly jet behaviors were systematically examined. As the crossflow velocity increases, the locations of maximum concentration, maximum velocity magnitude, maximum vorticity magnitude, as well as water jet centerline, all tend to be lower for bubbly jets.

Application of an overset grid method for the performance analysis of flapping airfoils

With the growing importance of renewable energy, the interest in energy-harvesting technologies based on flow-induced vibrations of airfoils has been rekindled in the past few years. Compared to conventional turbines, these devices are centrifugal stress-free and hence they are structurally more robust. They are also environmentally friendly due to the relatively low speeds, thus reducing the impact on flying animals. In order to numerically investigate these types of devices, mesh motion techniques must be used so that the computational grid can adapt to the time-varying shape of the domain and boundaries, thus preserving its robustness and quality. During the last years, many different dynamic mesh approaches have been developed to better predict the behaviour of a flow interacting with solid moving bodies. The aim of the present work is to apply and validate the overset grid method offered by the OpenFOAM software. In this type of mesh, one or more grid blocks are allowed to overlap with other sets of cells to solve the fluid domain with moving bodies, thus showing many potential advantages compared to the other existing dynamic mesh approaches. In the first part of this work, a numerical investigation of the flow over a stationary SD 7003 airfoil at low Reynolds number has been performed using an unsteady RANS approach. The computed results have then been compared to experimental and high-fidelity computational data available from the literature in order to validate the presented model in terms of numerical domain configuration, mesh refinement, and turbulence modelling. Subsequently, unsteady RANS simulations have been performed on the SD 7003 and NACA 0012 airfoils for two different amplitudes of flapping motion, i.e., 30° and 45°. Finally, the results obtained with the OpenFOAM overset grid solver have been compared with the numerical and experimental benchmarks presented by Kurtulus, showing an excellent agreement with both computed and measured data.

Hybrid VOF–Lagrangian CFD Modeling of Droplet Aerobreakup

A hybrid VOF–Lagrangian method for simulating the aerodynamic breakup of liquid droplets induced by a traveling shock wave is proposed and tested. The droplet deformation and fragmentation, together with the subsequent mist development, are predicted by using a fully three-dimensional computational fluid dynamics model following the unsteady Reynolds-averaged Navier–Stokes approach. The main characteristics of the aerobreakup process under the shear-induced entrainment regime are effectively reproduced by employing the scale-adaptive simulation method for unsteady turbulent flows. The hybrid two-phase method combines the volume-of-fluid technique for tracking the transient gas–liquid interface on the finite volume grid and the discrete phase model for following the dynamics of the smallest liquid fragments. The proposed computational approach for fluids engineering applications is demonstrated by making a comparison with reference experiments and high-fidelity numerical simulations, achieving acceptably accurate results without being computationally expensive.

Investigation of flame structure and precessing vortex core instability of a gas turbine model combustor with different swirler configurations

Numerical simulation of a dual-swirl gas turbine model combustor is performed under cold and reacting flow conditions using a three-dimensional unsteady Reynolds-averaged Navier–Stokes approach. A multi-species chemical mechanism is used in this study for the analysis of the numerous radicals participating in the ignition process and the flame structure. The other objective of this study is to investigate the flow field under different injector configurations, including both co-rotating and counter-rotating swirler arrangements, different swirl intensities, and vane areas. A comparison of the results with experimental data shows that the predicted velocity and temperature profiles follow the experimental data reasonably. In these studies, a precessing vortex core is found in the shear layer of the inner recirculation zone for all injector arrangements considered, and a co-rotating vortex exists in the outer shear layer for some of these arrangements. OH mass fraction field shows that the reactions take place mostly near the vortex core. Furthermore, it is shown that the build-up process of H2O2 and CH2O inside the cold jet has an important effect on combustion initiation. In addition, the formation and consumption of the H atoms in the recirculation zones and the balance between OH and H2O2 are shown to have important roles in the flame formation process. Finally, the precession frequency of the PVC is found to scale almost linearly with the spatial gradient of swirl velocity in the inner swirler and almost independent from the inclusion of the combustion reactions.

Study on Geese Array Effect and Optimal Layout of Herringbone PV array

Layout parameters play a significant role in wind loads of PV array. In view of this, wind loads of the herringbone PV array composed of 9 panels under five array angles (30°,40°,45°,50°,60°), five installation angles (30°,40°,45°,50°,60°) and five array distances (1.5m,1.75m,2m,2.25m,2.5m), are investigated by using the SST k-ω turbulence model and unsteady RANS approach. Then laws of wind pressure subjected to parameters are evaluated by drawing the contour and boxplot. Results show that: in the construction of herringbone photovoltaic panels, array angle is preferably not greater than 45°, installation inclination angle is not greater than 50°, and optimal array distance is between 1.75m and 2.5m. It is expected to provide theoretical basis and scientific guidance for layout of the herringbone array and sand control in desert areas.

Computational Evaluation of Shock Wave Interaction with a Liquid Droplet

This article represents the natural continuation of the work by Rossano and De Stefano (2021), dealing with the computational fluid dynamics analysis of a shock wave interaction with a liquid droplet. Differently from our previous work, where a two-dimensional approach was followed, fully three-dimensional computations are performed to predict the aerodynamic breakup of a spherical water body due to the impact of a traveling shock wave. The present engineering analysis focuses on capturing the early stages of the breakup process under the shear-induced entrainment regime. The unsteady Reynolds-averaged Navier–Stokes approach is used to simulate the mean turbulent flow field in a virtual shock tube device with circular cross section. The compressible-flow-governing equations are numerically solved by means of a finite volume method, where the volume of fluid technique is employed to track the air–water interface. The proposed computational modeling approach for industrial gas dynamics applications is verified by making a comparison with reference numerical data and experimental findings, achieving acceptably accurate predictions of deformation and drift of the water body without being computationally cumbersome.

Numerical characterization of seiche waves energy potential in river bank lateral embayments

Lateral embayments or cavities built in the river banks are used to accomplish a two-fold mission: channel stabilization and enhancement of morphological diversity and habitat suitability. The presence of these lateral embayments in the shallow flow of rivers may trigger standing resonant waves in the cross-flow direction. These water surface oscillations, typical of enclosed water domains, are called seiches. Amplitude and frequency of seiches are jointly controlled by the geometry of the embayments and by the flow rate. The study of seiches from an energy harvesting perspective is herein considered. In this work, an in-house numerical simulation model is used to assess the seiche wave energy potential in a channel with multiple lateral cavities. The model, which is based on a high-order discretization of the shallow water equations, allows to reproduce the resonant phenomena using an Unsteady Reynolds Averaged Navier-Stokes approach. The numerical results indicate that there is a range of hydraulic conditions under which energy is concentrated in a dominant cross-flow resonant seiche wave. The optimal hydraulic conditions to operate a seiche wave energy recovery are shown and discussed. High flow rates alone do not control the energy recovery efficiency, actually, a tipping point in the embayment efficiency ratio is identified. The co-existence of multi-modal seiche waves challenges the dominance of the standing wave period of interest for energy harnessing.

Analysis and Numerical Simulation of no Reacting Swirling Flow by Using URANS Approach

In this paper, we investigate the no-reacting swirling flow by using the numerical simulation based to the unsteady Reynolds-averaged Navier-Stokes approach. The numerical simulation was realized by using a computational fluid dynamics CFD code. The governing equations are solved by using the finite volume method with two classical models of turbulence K-epsilon and Shear Stress K-ω. The objective of this paper is therefore to evaluate the performance of the two models in predicting the recirculation zones in a swirled turbulent flow. The current models are validated by comparing the numerical results of the axial, radial and tangential velocities to the experimental data from literature.

Local and gross parameters of air distribution in a room with a sidewall jet: CFD validation based on benchmark test

The mixing ventilation system with diffusers placed on the side walls, above the occupied zone, is the most popular indoor air distribution system. Computational fluid dynamics (CFD) can support the design and optimization of this system, however the uncertainty of CFD results should be estimated. The CFD benchmark test of a room with a sidewall jet was proposed in 2015, and in the present work, this benchmark test was used to evaluate the quality of CFD predictions that used two turbulence modeling techniques: vortex-resolving large eddy simulation (LES) and unsteady Reynolds-averaged Navier–Stokes (URANS). Local data were used to determine the gross and integral parameters characterizing the velocity distribution in the jet at a certain distance from the inlet, like: maximum mean velocity, jet width, volume flux and momentum flux. The velocity distributions in the quasi free jet region were approximated with the model of a jet from the point source of momentum, its parameters were also established. The URANS approach did not improve the simulation results relative to the previous steady-state RANS modeling procedure. In terms of the gross and integral parameters the results of LES with refined mesh were more consistent with the measured results. In the jet zone and occupied zone, the gross parameters obtained from LES calculations differed by less than 15% from the measured values. The accuracy requirements for CFD predictions for the conceptual design of ventilation systems were therefore satisfied.

Aeroacoustic simulation of a cross-flow fan using lattice Boltzmann method with a RANS model

The present study developed an unsteady RANS approach based on the lattice Boltzmann method (LBM), which can perform direct aeroacoustic simulations of low-speed fans at lower computational cost compared with the conventional LBM-LES approach. In this method, the k-ω turbulence model is incorporated into the LBM flow solver, where the transport equations of k and ω are also computed by the lattice Boltzmann method, similar to the Navier-Stokes equations. In addition, moving boundaries such as fan rotors are considered by a direct-forcing immersed boundary method. This numerical method was validated in a two-dimensional simulation of a cross-flow fan. As a result, the simulation was able to capture an eccentric vortex structure in the rotor, and the pressure rise by the work of the rotor can be reproduced. Also, the peak sound of the blade passing frequency can be successfully predicted by the present method. Furthermore, the simulation results showed that the peak sound is generated by the interaction between the rotor blade and the flow around the tongue part of the casing.

Full configuration drag estimation of short-to-medium range fixed-wing UAVs and its impact on initial sizing optimization

The paper presents the derivation of a new equivalent skin friction coefficient for estimating the parasitic drag of short-to-medium range fixed-wing unmanned aircraft. The new coefficient is derived from an aerodynamic analysis of ten different unmanned aircraft used for surveillance, reconnaissance, and search and rescue missions. The aircraft is simulated using a validated unsteady Reynolds-averaged Navier Stokes approach. The UAV’s parasitic drag is significantly influenced by the presence of miscellaneous components like fixed landing gears or electro-optical sensor turrets. These components are responsible for almost half of an unmanned aircraft’s total parasitic drag. The new equivalent skin friction coefficient accounts for these effects and is significantly higher compared to other aircraft categories. It is used to initially size an unmanned aircraft for a typical reconnaissance mission. The improved parasitic drag estimation yields a much heavier unmanned aircraft when compared to the sizing results using available drag data of manned aircraft.

Computational Evaluation of Shock Wave Interaction with a Cylindrical Water Column

Computational fluid dynamics was employed to predict the early stages of the aerodynamic breakup of a cylindrical water column, due to the impact of a traveling plane shock wave. The unsteady Reynolds-averaged Navier–Stokes approach was used to simulate the mean turbulent flow in a virtual shock tube device. The compressible flow governing equations were solved by means of a finite volume-based numerical method, where the volume of fluid technique was employed to track the air–water interface on the fixed numerical mesh. The present computational modeling approach for industrial gas dynamics applications was verified by making a comparison with reference experimental and numerical results for the same flow configuration. The engineering analysis of the shock–column interaction was performed in the shear-stripping regime, where an acceptably accurate prediction of the interface deformation was achieved. Both column flattening and sheet shearing at the column equator were correctly reproduced, along with the water body drift.