Unsteady RANS Research Articles

Modeling of street-scale pollutant dispersion by coupled simulation of chemical reaction, aerosol dynamics, and CFD

Abstract. In the urban environment, gas and particles impose adverse impacts on the health of pedestrians. The conventional computational fluid dynamics (CFD) methods that regard pollutants as passive scalars cannot reproduce the formation of secondary pollutants and lead to uncertain prediction. In this study, SSH-aerosol, a modular box model that simulates the evolution of gas, primary and secondary aerosols, is coupled with the CFD software, OpenFOAM and Code_Saturne. The transient dispersion of pollutants emitted from traffic in a street canyon is simulated using the unsteady Reynolds-averaged Navier–Stokes equations (RANS) model. The simulated concentrations of NO2, PM10, and black carbon (BC) are compared with field measurements on a street of Greater Paris. The simulated NO2 and PM10 concentrations based on the coupled model achieved better agreement with measurement data than the conventional CFD simulation. Meanwhile, the black carbon concentration is underestimated, probably partly because of the underestimation of non-exhaust emissions (tire and road wear). Aerosol dynamics lead to a large increase of ammonium nitrate and anthropogenic organic compounds from precursor gas emitted in the street canyon.

Propeller Rotation Effects on Pitching Moment for Transport Aircraft with Conventional Tail

The effects of the propeller rotation direction on the pitching moment characteristics for a typical twin-engine turboprop aircraft with a conventional tail at low and moderate angles of attack under high-power conditions are investigated through unsteady Reynolds-averaged Navier–Stokes simulations. The numerical method is validated using wind-tunnel data for a twin-engine T-tail turboprop aircraft. The computational results of the conventional tail configurations indicate that the corotating propellers significantly degrade the longitudinal stability at low angles of attack in the takeoff configuration. By comparison with corotating propellers, counter-rotating inboard-up propellers not only increase the lift but also lead to a notable improvement in longitudinal stability, especially at low angles of attack. In contrast, the counter-rotating outboard-up propellers further reduce the negative pitching moment curve slope of the airframe. Analysis of the flowfield demonstrates that the effects of the propeller rotation direction significantly alter the downwash gradient at the horizontal tailplane by affecting the vertical location of the high-downwash region. The effectiveness of the horizontal tailplane and the longitudinal stability of the aircraft are therefore changed.

Multi-Objective Airfoil Optimization Under Unsteady-Freestream Dynamic Stall Conditions

Aimed at alleviating dynamic stall, an optimization method for rotor airfoils is proposed considering the unsteady-freestream circumstance and pitching motion. The method is based on the unsteady Reynolds-averaged Navier-Stokes equations, kriging model, and non-dominated sorting genetic algorithm II (NSGA-II). Three objective functions are built for minimizing the area of lift hysteresis loop, value of peak drag, and peak pitching moment. An optimized airfoil with larger maximum thickness, maximum camber, and upper leading-edge radius is obtained based on the baseline SC1095 airfoil. Besides, the optimized airfoil shows smaller local thickness and camber near the trailing edge. The results show that the flow separation and dynamic stall are suppressed or alleviated under both design and off-design conditions, attributed to a significant reduction of negative pressure peak and adverse pressure gradient. The dynamic stall is also alleviated for the rotor configured with the optimized airfoil in forward flight. This indicates that the optimized airfoil performs well in a rotor environment.

Deep Learning-Based Prediction of Unsteady Reynolds-Averaged Navier-Stokes Solutions for Vertical-Axis Turbines

The following study investigates the effectiveness of a deep learning-based method for predicting the flow field and flow-driven rotation of a vertical-axis hydrokinetic turbine operating in previously unseen free-stream velocities. A Convolutional Neural Network (CNN) is trained and tested using the solutions of five two-dimensional (2-D), foil-resolved Unsteady Reynolds-Averaged Navier-Stokes (URANS) simulations, with free-stream velocities of 1.0, 1.5, 2.0, 2.5, and 3.0 m/s. Based on the boundary conditions of free-stream velocity and rotor position, the flow fields of x-velocity, y-velocity, pressure, and turbulent viscosity are inferred, in addition to the angular velocity of the rotor. Three trained CNN models are developed to evaluate the effects of (1) the dimensions of the training data, and (2) the number of simulations used as training cases. Reducing data dimensions was found to diminish mean relative error in predictions of velocity and turbulent viscosity, while increasing it in predictions of pressure and angular velocity. Increasing the number of training cases from two to three was found to reduce relative error for all predicted unknowns. With the best achieved CNN model, the variables of x-velocity, y-velocity, pressure, turbulent viscosity, and angular velocity were inferred with mean relative errors of 6.93%, 9.82%, 10.7%, 7.48%, and 0.817%, respectively.

Nonlinear URANS model for path-following control problem towards autonomous marine navigation under wave conditions

An accurate prediction of the performance of maritime autonomous surface ships (MASS) to follow a predefined path in waves is of critical importance for ensuring safe autonomous marine navigation. Traditional methods for the study of path-following problems are based on simplified mathematical ship models and are incapable of precisely resolving the complicated interactions between the hull, propeller, rudder, and incident waves under path-following control. In the present study, a CFD-based dynamic model for path-following problems is developed for autonomous marine navigation by means of a fully nonlinear unsteady Reynolds-Averaged Navier-Stokes (RANS) solver combined with the Line-of-Sight (LOS) guidance law. Fortunately, an unsteady RANS solver is capable of incorporating viscous and turbulent effects and the free surface resolution critical to path-following problems, allowing a better prediction of a ship's path-following performance in waves. To obtain practical insight into the performance of the ship executing path-following control for autonomous navigation in waves, a comprehensive analysis of path-following tasks in waves covering the whole range of important wave directions and wave heights was carried out in this study. The numerical results demonstrated that when the ship performed the path-following manoeuvre under the beam and quartering seas, the greatest oscillatory deviation from the predefined path was observed due to the asymmetric pressure distribution on the ship hull caused by the incident waves. It was also confirmed that wave heights strongly affected the trajectories experienced by the ship performing the path-following task in the beam and oblique seas; the deviation from the predefined path was found to increase with the increase in the wave height. As high-performance computational resources become increasingly available, the proposed CFD method will provide an accurate and efficient way to predict the performance of autonomous surface ships performing path-following tasks in waves, providing a valuable contribution to enhancing the safety of autonomous marine navigation.

Predicting unsteady heat-fluid interaction features and nonlinear acoustic behaviors in standing-wave thermoacoustic engines using unsteady RANS, LES and hybrid URANS/LES methods

The study presented in this paper aims to simulate a standing-wave thermoacoustic engine (SWTAE) system by capturing its nonlinear thermoacoustic features with three-dimensional (3D) unsteady Reynolds-Averaged Navier-Stokes (URANS) and hybrid URANS/LES (Large Eddy Simulation) models such as Detached-Eddy Simulations (DES) and Stress-blended Eddy Simulation (SBES) models. For comparison, LES is also conducted as a reference case to examine the turbulence model effect on capturing the nonlinear features of heat-driven acoustics in the SWTAE system. The 3D model is validated against numerical results available in the literature, along with mesh- and time-independence studies. The comparison studies show that the computation time required for modeling SWTAE using SBES and DES is 28.4% and 36.5% less than LES, respectively. It is also shown that the prediction of the acoustic power of SWTAE using URANS is about 21.0% lower than that using LES, while the results from SBES and DES are in relatively good agreement with LES. Comparative studies of nonlinear hydrodynamics in the flow fields show that the results from SBES are closer to LES than DES, which can be attributed to the SBES model providing a faster transition to an explicit LES model outside the wall boundary layer. Furthermore, the heat transfer characteristics are compared by analyzing heat leaks and transversal heat flux, and it is found that the URANS model overestimates the heat transfer characteristics, while the results of the other three models are smaller than those obtained by URANS. In conclusion, the SBES model has great potential to be applied in simulating thermoacoustic nonlinear and flow behaviors of SWTAEs. It has the attractive features of delivering relatively accurate predictions and a significant reduction in computational times and costs compared to LES.

Numerical Evaluation of the Wave-Making Resistance of a Zero-Emission Fast Passenger Ferry Operating in Shallow Water by Using the Double-Body Approach

The consideration of shallow water effects has gained in importance regarding inland operations. The interaction between the keel and the riverbed affects the hydrodynamic characteristics of marine vessels. The highly complex nature of the interference phenomenon in catamarans makes the shallow water problem more complicated as compared to monohulls. Hence, catamarans are very sensitive to speed changes, as well as to other parameters, such as the shallow water effects. This makes the design of catamarans more challenging than their monohull equivalents. At lower Froude numbers, the higher importance of the frictional resistance makes the greater wetted surface of the catamaran a disadvantage. However, at higher speeds, there is the potential to turn their twin hulls into an advantage. This study aims to investigate the wave-making resistance of a zero-carbon fast passenger ferry operating in shallow water. The URANS (unsteady Reynolds-averaged Navier–Stokes) method was employed for resistance simulations. Then, the double-body approach was followed to decompose the residual resistance into viscous pressure and wave-making resistance with the help of the form factors of the vessel calculated at each speed. The characteristics of the separated wave-making resistance components were obtained, covering low, medium, and high speeds. Significant findings have been reported that contribute to the field by providing insight into the resistance components of a fast catamaran operating in shallow waters.

Load Characteristics and Extreme Response of Straight-Bladed Floating VAWT Using a Fully Coupled Model

Operating Offshore Floating Vertical Axis Wind Turbines (OF-VAWT) have the potential to perform well in the deep-sea area. Some researchers gave performance prediction by developing simplified computing models. However, these models have imperfections in considering load and motion nonlinearity, especially in extreme environments. In this work, a numerical model is developed composed of Computational Fluid Dynamics (CFD) and Dynamic Fluid Body Interaction (DFBI) to acquire the aero-hydrodynamic load and performance of OF-VAWT in general and extreme environments. Unsteady Reynolds-Averaged Navier-Stokes (URANS), SST k-ω and Eulerian Multi-Phase (EMP) models are combined to generate a gas-liquid two-phase flow field; the Volume of Fluid (VOF) model is employed to capture free-surface and make numerical wind-wave. DFBI superposition motion technology is proposed for local motion definition and motion solution, and overset with sliding meshes is introduced to achieve the grid motion. The numerical approach is verified by the tunnel and tank experimental data from the available literature. Simulation results of general cases, such as variable wind speed, wave height and wave length, are compared to discuss the effect of environmental parameters on load and performance. Comparison shows that this straight-bladed OF-VAWT is more susceptible to wind speed. Furthermore, the aerodynamic load generated by the shut-down rotor is still significant in extreme environment, which has implications for the development of OF-VAWT controller.

Investigating roughness effects on ship resistance in shallow waters

Shallow waters influence the behaviour and performance of a vessel by modifying the local pressure distribution, wave making, and boundary layer thickness. The boundary layer thickness is also influenced by surface roughness. No previous studies have investigated the combined effects of shallow water contributions and roughness on ship resistance. This study aims to fill this knowledge gap in the literature by using Unsteady Reynolds Averaged Navier-Stokes modelling. Results show that the total resistance coefficient increases between approximately 22% and 36% in the presence of roughness depending on the speed and depth-to-draft ratio. The numerical model used shows that pressure resistance grows at a faster pace than frictional resistance and increases its relative contribution to the total when roughness is applied, contrary to deep water cases.

Investigation on the flow around a submarine under the rudder deflection condition by using URANS and DDES methods

Considering the complex flow features of a submarine under the maneuvering condition and their close relationship with the maneuverability, a numerical study is performed for the flow around the submarine under the rudder deflection condition. Taking the submarine model DARPA SUBOFF as the study object, the rudder force test with a wide range of rudder angles is simulated based on the open source CFD platform OpenFOAM. Two numerical methods of Unsteady Reynolds-Averaged Navier-Stokes (URANS) method and Delayed Detached Eddy Simulation (DDES) method are employed. A systematic convergence study is performed to clarify the effect of grid spacing and time step on the numerical solution. The computed hydrodynamic forces are compared with the available experimental data, and the effectiveness of the adopted numerical methods is validated. Besides, the obtained rudder forces and rudder parameters are compared and discussed. Based on the obtained flow field details of vortex structures, velocity fields, streamline distribution, turbulence levels, and pressure distribution, the flow mechanisms of the deflected rudders are analyzed. Compared with URANS method, DDES method presents a better capability of capturing the flow field details around the submarine under the tight maneuvering condition, where remarkable flow separation is involved.

Numerical course-keeping tests of ONR tumblehome in waves with different rudder control strategies

This study aims to present numerical analysis on the course-keeping performance of an Office of Naval Research (ONR) tumblehome ship model in waves using an enhanced open-source library. The unsteady Reynolds-averaged Navier–Stokes (URANS) model coupled with a hierarchical rigid body motion solver is adopted to simulate free-running ships. An enhanced body-force propeller model for nonuniform inflow is implemented, and an optimized overset grid technology with motion decomposition is used for grid transformation. A rudder control module with a new derived gain-scheduling method for wave cases is introduced for heading control purposes. Five benchmark tests with different wave directions are analyzed in terms of linear and nonlinear motions. The results show good agreement with benchmark data and possible reasons for numerical discrepancies are also discussed. As the P controller of the rudder in oblique waves is insufficient for maintaining a zero-heading, the behavior of P-, PD-, and PID controllers are studied in oblique waves. The optimal control strategy depends on the environmental conditions in which weak rudder control is preferred in rough seas. This work demonstrates that the developed solver is feasible for CFD applications to ship hydrodynamics in waves.

Hydrodynamic and radial force characteristics with design of a single-channel pump for wastewater treatment based on the similarity law

Single-channel pumps are widely employed for wastewater treatment in Korea. To render possible the application of this type of pump in other countries using different power frequencies, the present study applies the similarity law to redesign the impeller geometry of the single-channel pump optimized in a previous study. To this end, the numerical simulations are conducted using steady and unsteady Reynolds-averaged Navier–Stokes equations and a shear stress transport reattachment modification turbulence model. Numerical simulations are confirmed by an experiment based on the 11th API610 standard. At design flow rate condition, the novel pump model meets the design requirement with the minimum total head coefficient of 3.078. The distance between the center of the radial force and the origin of the new model is notably decreased in comparison with the reference model, especially at the design point with a reduction of 3.418 times. Moreover, the results of the unsteady radial force are consistent with those of the steady radial force. The pressure oscillations in the new model are likewise reduced compared to the reference model, particularly at several measuring points under design flow rate with pressure oscillations dropping from 8.5% and 10.1–5.3% and 1.2%, respectively. The non-uniform high-pressure region caused by the rotor-stator interaction at the outlet of the impeller is evidently suppressed in the new model. The new single-channel pump is feasible for new power frequency and performs better with low vibration and noise levels during the operation.

Influence of subway train fire locations on the characteristics of smoke movement in a curved tunnel.

Scenario models of a moving subway train can help investigate the influence of different fire locations on smoke propagation characteristics in curved tunnels. To this end, this study adopts the three-dimensional Unsteady Reynolds Average Navier-Stokes equations method and the renormalization group k-ε two-equation turbulence model with buoyancy correction for numerical analysis. The motion of the train is replicated using the slip grid technique. The results indicate that when a fire breaks out on a moving train in tunnels, the piston wind leads the longitudinal movement of the smoke. If a fire erupts in the head or middle car of a moving train, the time of smoke backflow is delayed by 30 s or 17 s, respectively, compared to that for the tail car. The obtained results provide a theoretical basis for reasonably controlling the smoke flow in subway tunnels and reducing casualties in fire accidents.

Small-width wall-attached Coandǎ jets for flow control

The flow dynamics of small-width wall-attached jets generated by a Coand-effect nozzle is investigated by unsteady Reynolds-averaged Navier–Stokes simulations. The data are validated by comparison with hot-wire velocity measurements performed on the same flow configurations. The jets exhibit a complex topology strongly influenced not only by the spanwise vorticity (as usually observed in wall jets) but also by a vorticity component normal to the wall and induced by the shear layer developing on the jet sides. This results in an original U-shaped jet whose characteristics are studied in detail for three different mass flow rates. The robustness of the flow topology on a larger range of injected mass flow rates is finally presented and discussed in terms of the injected momentum near the wall. The resulting flow profiles point out that our injector is expected to be a promising candidate for active flow control in gas-turbine compressors for aeronautical and energy applications.

CFD study to predict the effect of a passing ship on moored ships in a confined waterway

This study aims to present a practical practice to obtain a reliable numerical solution based on an Unsteady Reynolds Averaged Navier-Stokes (URANS) Computational Fluid Dynamics (CFD) to estimate the forces acting on moored ships during a passage of a ship in a confined waterway. The motion of the passing ship is implemented using an overset-grid method. The convergence of the numerical solutions depending on the spatial and temporal discretization are systematically analyzed, and the computational results are validated with a recent benchmark test data. In particular, the effect of the initial state of the passing ship in the unsteady simulation is dealt with in-depth. The simulation results when the passing ship departs at a constant speed and when it accelerates from a standstill are compared, and the influence of undesirable wave radiation generated with the departure of the passing ship at a constant speed is discussed.

Scale-resolving simulations of the flow in a nuclear fuel bundle with a channel spacer grid using partially averaged Navier–Stokes and large-eddy simulation

The objective of this study is to evaluate the capabilities of the Partially Averaged Navier–Stokes (PANS) method in simulations of the flow through a rod bundle representative of a nuclear fuel assembly with a channel-type spacer grid. In the PANS bridging turbulence model, filters can be applied to obtain any level of resolution from Reynolds Averaged Navier–Stokes (RANS) to Direct Numerical Simulation (DNS). The closure model is sensitive to the length-scale cutoff by means of unresolved to total kinetic energy ratio (fk) and unresolved to total dissipation ratio (fε). Simulations are conducted to study the effect of the cutoff of length scale on the results obtained for flow statistics at selected locations in the domain. The results obtained with different PANS filters are compared with Unsteady RANS (URANS), Large Eddy Simulation (LES), and experiments. The mean and fluctuating flow components are computed at a representative plane located at the inter-channel between rods. Other flow quantities analyzed include the pressure drop upstream and downstream of the spacer grid, the turbulent kinetic energy, and the unresolved eddy viscosity. Additionally, we use the Lumley triangle to study turbulence anisotropy and to compare the nature of the energy content captured with LES and one of the PANS models. It is shown that the PANS model with fk=0.4, which resolves 60% of the turbulent kinetic energy content, captures the most relevant flow physics and is a suitable modeling approach for this application.

Data-driven turbulence model for unsteady cavitating flow

Unsteady Reynolds-averaged Navier–Stokes (URANS) equations have been widely used in engineering fields to investigate cavitating flow owing to their low computational cost and excellent robustness. However, it is challenging to accurately obtain the unsteady characteristics of flow owing to cavitation-induced phase transitions. In this study, we propose an implicit data-driven URANS (DD-URANS) framework to analyze the unsteady characteristics of cavitating flow. In the DD-URANS framework, a basic computational model is developed by introducing a cavitation-induced phase transition into the equations of Reynolds stress. To improve the computational accuracy and generalization performance of the basic model, the linear and nonlinear parts of the anisotropic Reynolds stress are predicted through implicit and explicit methods, respectively. A data fusion approach, allowing the input and output of characterized parameters at multiple time points, is presented to obtain the unsteady characteristics of the cavitating flow. The DD-URANS model is trained using the numerical results obtained via large-eddy simulation. The training data consist of two parts: (i) the results obtained at cavitation numbers of 2.0, 2.2, and 2.7 for a Venturi flow, and (ii) those obtained at cavitation numbers of 0.8 and 1.5 for a National Advisory Committee for Aeronautics (NACA) 66 hydrofoil. The DD-URANS model is used to predict the cavitating flow at cavitation numbers of 2.5 for a Venturi flow and 0.8 for a Clark-Y hydrofoil. It is found that the DD-URANS model is superior to the baseline URANS model in predicting the instantaneous periodic shedding of a cavity and the mean flow fields.

Modeling of Sandia Flame D with the non-adiabatic chemistry tabulation approach: the effects of different laminar flames on NO X prediction.

The effects of different one-dimensional laminar flames on the prediction of nitrogen oxide (NO X ) emission in Sandia Flame D are numerically investigated using a flamelet method in the present work. In addition to the basic control variables of the mixture fraction and the reaction progress variable for chemistry tabulation in this combustion model, two additional variables of mixture fraction variance and enthalpy loss are added to the control variable space to improve prediction accuracy of the NO X pollutant. The former variable of mixture fraction variance is used for the presumed probability density function integration, and the latter takes into account the non-adiabatic effect. Two flamelet libraries are generated based on the one-dimensional unstretched premixed flame and the one-dimensional counterflow diffusion flame, respectively. An additional transport equation for the mass fraction of nitrogen oxide (NO) is solved for improving prediction accuracy. The unsteady Reynolds-averaged Navier-Stokes (URANS) simulation results are compared and analyzed with experimental data. The simulation results show dependence on the type of laminar flame. In the four-dimensional control variable space, the results with steady unstretched premixed flame indicate great agreement on the predictions of temperature and NO field. The computational method proposed in the present work sheds light on the high-precision combustion numerical simulation of NO X emission.

VALIDATION OF A STOCHASTIC BREAKUP MODEL FOR TURBULENT JETS IN HIGH-SPEED CROSSFLOW: ASSESSMENT OF TURBULENT INTERACTIONS AND SENSITIVITY TO BOUNDARY CONDITIONS

Improving the mixing of fuel and air by injecting a turbulent liquid fuel jet into a high-speed cross-flowing gas can reduce the emissions of gas turbine applications. To facilitate and hasten the development of such low-emissions technologies, accurate predictions of the spray characteristics are needed. The objective of the present study is to validate the predictive capabilities of a stochastic breakup model for turbulent transverse jets over a wide range of representative pressures and atomization characteristics. The effect of turbulence modeling is also assessed to provide accurate and computationally less expensive Eulerian-Lagrangian transient approaches. To do so, the predictions made with the large eddy simulation (LES) approach for different subgrid-scale (SGS) models and with the synthetic eddy method (SEM) are compared to the ones made using the unsteady Reynolds-averaged Navier-Stokes (RANS) approach with and without a turbulent dispersion model. The sensitivity of the numerical methodology to the upstream velocity profile, pressure, and momentum flux ratio were also assessed. Properly accounting for the upstream gas velocity profile was found to be critical to ensure accurate predictions of the spray characteristics. The unsteady RANS (URANS) turbulent approach coupled with the turbulent dispersion model showed good agreement with experimental data, but the LES approach tends to overpredict the spray penetration and underpredict the Sauter mean diameter (SMD). This could be due to the lower turbulent interactions it predicts, which may lead to lower momentum transfer between the phases.

Влияние структуры течения газа в осесимметричном канале на формирование неоднородного температурного поля в наполнителе из твердого легкоплавкого материала

This paper presents a study of the interaction between high-speed airflow and the surface of a solid low-melting material in a flowing channel of a model body. Both numerical and experimental approaches are used to solve the problem, which allows one to perform a comprehensive analysis of the processes under study. Numerical simulation conditions correspond to aerodynamic tests in the experimental facility. The unsteady Reynolds-averaged Navier-Stokes (URANS) equations are used to describe a gas flow. When solving the problem, the coupled heat-transfer and turbulence are taken into account. The low-temperature gas-dynamic processes are considered, while the chemical reactions and phase transition are neglected. As a result of numerical simulations, the flow structure and regime in a flowing channel of the model are determined, as well as the pressure and temperature distributions in the near-wall region of a solid combustible material. The gas flow regime corresponds to an underexpanded jet flow with the separation of the boundary layer and the formation of the intense heat-transfer regions at the initial section of the flowing channel. According to the numerical simulation results, in aerodynamic tests with a Mach number of 6, the melting point is attained in the near-wall region of the solid combustible material (polyethylene, polyoxymethylene, and wax). Aerodynamic tests are carried out to validate the obtained results. Experimental results show that the variation in the flowing channel diameter in the thick-wall cylinder made of polyethylene and polyoxymethylene is induced by thermal expansion. In aerodynamic tests with a wax cylinder, the mass reduction and the fusion of the solid-gas interface are revealed.