Reynolds-averaged Navier-Stokes Method Research Articles

Computational Fluid Dynamics Investigation of Hydrodynamic Forces and Moments Acting on Stern Rudder Plane Configurations of a Submarine

This study presents the predicted hydrodynamic characteristics of different rudder plane configurations on the stern of a full-scale submarine in deep water, which are obtained using the Reynolds-Averaged Navier–Stokes method in Ansys Fluent Solver. First, the results obtained for the X-rudder plane configuration are verified according to previous numerical and experimental results in order to assess the accuracy of the simulation procedure. The X-rudder plane, Y-rudder plane, and Cross-rudder plane configurations in deep water with deflection angles ranging from −21 degrees to +21 degrees are then simulated. Next, the hydrodynamic forces and moments of the Cross-plane, X-plane, and Y-plane rudder configurations obtained through simulation are analyzed using Taylor’s expansion to estimate the hydrodynamic coefficients. The obtained results demonstrate that the X-force of the X-plane rudder configuration is larger than the corresponding forces acting on the Cross-plane rudder and Y-plane rudder configurations. Meanwhile, the Y-force and Z-force of the X-plane rudder configuration are significantly greater than the corresponding forces of the left configurations. The same tendency can be seen in the moment of the X-plane rudder about the y- and z-axes. However, the roll moment induced by the Y-plane and Cross-plane rudder configurations is significantly larger than that under the X-plane rudder configuration.

Simulation and validation of instability transient process in multistage high-speed axial compressor based on the body-force model

Simulation and validation of instability transient process in multistage high-speed axial compressor based on the body-force model

Reducing the contaminant dispersion and infection risks in the train cabins by adjusting the inlet turbulence intensity: A study based on turbulence simulation

Existing studies on ventilation in closed spaces mainly considered the average inlet velocity and ignored the influence of inlet turbulent fluctuation. However, the variation in inlet turbulence intensity (TI) is considerable and significantly affects the dispersion of contaminants. This study conducts numerical simulations verified by experiments to investigate the effect of the inlet TI on train contaminants dispersion and analyze infection probability variation. Firstly, the unsteady Reynolds-averaged Navier-Stokes (URANS) method and improved delayed detached eddy simulation (IDDES) method are compared in simulating the internal airflow characteristics based on the on-site measurement. The results indicate that the latter dominates in capturing airflow pulsations more than the former, although the mean airflow results obtained from both methods agree well with experimental results. Furthermore, the IDDES method is employed to investigate the effect of the inlet TI on contaminant dispersion, and the infection risks are also assessed using the improved probability model. The results show that, with the increase of TI from 5 % to 30 %, the contaminant removal grows considerably, with the removal index rising from 0.23 to 1.86. The increased TI leads to the overall and local infection risks of occupants descending significantly, wherein the former decreases from 1.53 % to 0.88 % with a reduction rate of 42 %, and the latter drops from 3.30 % to 2.16 % with a mitigation rate of 35 %. The findings can serve as solid guidelines for numerical method selection in accurately capturing the indoor dynamic airflow distribution and for the ventilation parameters design regarding TI inside trains to mitigate the airborne infection risk.

CFD simulation of pumping ventilation with two asymmetrically arranged openings on the rear wall in a three-story isolated building

Pumping ventilation, driven by periodic vortex shedding, can effectively improve the ventilation rate of single-sided ventilation. Previous studies on pumping ventilation mainly considered symmetric opening conditions, while the asymmetric opening distribution has not been well studied. Therefore, this study investigates the pumping ventilation of an isolated three-story building with asymmetric opening conditions using computational fluid dynamics (CFD) simulations. Unsteady Reynolds-averaged Navier-Stokes (URANS) method with SST k-ω turbulent model is adopted to conduct the CFD simulations. The effects of horizontal translation distances, internal partitions, and inter-floor interference on time-averaged and fluctuating ventilation rates are investigated. The simulation results show that the mean ventilation rate is larger than the fluctuating ventilation rate. The mean ventilation rate increases while the fluctuating ventilation rate decreases with the increase of the horizontal translation distance. In addition, the internal partition decreases the pumping ventilation rate primarily by reducing the fluctuating ventilation rate rather than the mean ventilation rate. Furthermore, the ventilation rates of the 2nd floor are promoted by simultaneous pumping ventilation of the 1st and 3rd floors compared to the situation that only the 2nd floor is ventilated. The ventilation rates of the 1st floor are the largest, and of the 3rd floor are the lowest. The findings of this study benefit the deeper understanding of pumping ventilation and provide theoretical guidance on the opening designs of multi-story buildings for better ventilation efficiency.

Numerical study of film cooling in single-element injector gaseous CH4/O2 rocket engine with coupled wall function

The investigation of film cooling in CH4/O2 rocket engines holds paramount importance in the advancement of rocket propulsion. However, the wall heat flux is always overestimated in numerical simulation processes. Hence, this article proposes a numerical framework that employs the Reynolds averaged Navier–Stokes method to simulate the single-element gaseous CH4/gaseous O2 combustion chamber, which is the basis for rocket engine simulations. The coupled wall function that considers chemical reaction effects is introduced to enhance the accuracy of wall heat flux prediction. The impact of utilizing a coupled wall treatment on the prediction of wall heat flux and its fundamental parameters are examined. In addition, a single-element combustion chamber experiment is performed to validate the simulation. The results demonstrate that the implementation of the coupled wall function hardly influences the main flow characteristics, whereas the wall heat flux calculated with general wall functions shows an overestimation, which can be reduced by the coupled wall function. Finally, the case with a coupled wall function can improve the cooling efficiency with greater accuracy and the cooling systems with optimized design. The proposed numerical framework and the findings of the study provide profound insights that can improve the design and optimization of rocket engines.

Numerical investigations of the effects of spanwise grooves on the suppression of vortex-induced vibration of a flexible cylinder

Numerical investigations of Vortex-induced Vibration (VIV) suppression of a flexible cylinder with spanwise grooves in the uniform flow have been conducted by viv3D-FOAM-SJTU solver, which imports the thick strip model into OpenFOAM. The Reynolds-averaged Navier-Stokes (RANS) method is adopted to calculate hydrodynamic forces at each fluid strip. The vibrations of cylinder are solved through the Euler-Bernoulli bending beam hypothesis and the Finite Element Method (FEM) in two directions. During simulations, four spanwise grooves are arranged equally around the cylinder with an interval of 90° at two configurations. The spanwise groove keeps a constant width of w = 0.2D and a variable groove depth among 0.08D, 0.12D and 0.16D. Specific comparisons on Root Mean Square (RMS) vibration displacements, vibration modal responses, vibration frequency responses and vortex structures are conducted to analyze suppression effects of the spanwise grooves. Numerical studies indicate that the appropriate choose of groove geometry and arrangement can effectively reduce both crossflow and inline VIV responses.

Numerical study of ignition process of ternary hydrogenated catalytic biodiesel/methanol/n-octanol blends under high-temperature and high-pressure conditions

This article presents a numerical simulation study of the spray combustion process of methanol/n-octanol/hydrogenated catalytic biodiesel (HCB) blended fuel in a high-temperature and high-pressure constant volume combustion chamber environment, using the Reynolds-averaged Navier-Stokes (RANS) method. This study focuses on the effects of the fuel properties on two-stage ignition processes. The blended fuel, denoted as M15, is composed of methanol, octanol, and HCB in volume ratios of 15 %:17 %:68 %. The pure HCB (M0) was also calculated under the same operating condition as references. The results indicate that M15 exhibits significant delays in both high-temperature and low-temperature ignition compared to M0. At low ambient temperatures, methanol demonstrates a strong inhibitory effect on spray ignition in part due to its competition with alkanes for OH radicals. Additionally, high-temperature ignition of M0 primarily occurs at the periphery of the spray, whereas the ignition of M15 transitions toward the fuel-rich region near the nozzle exit. This transition is attributed to the ignition delay, which provides better-mixing characteristics in the M15 spray, along with the oxygen content introduced by methanol.

Study on the Interface Instability of a Shock Wave–Sub-Millimeter Liquid Droplet Interface and a Numerical Investigation of Its Breakup

This study investigated the influence of instability on the interaction between sub-millimeter liquid droplets and shock waves. Experiments were conducted using 0.42 mm diameter droplets with varying shock wave Mach numbers. The investigation quantified the effects of Weber numbers and initial diameters on the development of Rayleigh–Taylor and Kelvin–Helmholtz instabilities at the shock wave–sub-millimeter liquid droplet interface. Three-dimensional numerical simulations were performed to investigate the deformation and breakup behaviors of sub-millimeter liquid droplets under the impact of a shock wave with a Mach number of 2.12. The post-shock gas flow environment in this condition was in a supersonic state. The simulations utilized the volume-of-fluid method to model the gas–liquid interface, employed unsteady Reynolds-averaged Navier–Stokes methods to simulate turbulence, and incorporated grid gradient adaptive technology to enhance computational efficiency. The results revealed that by increasing the Weber number or decreasing the initial diameter, both the growth rate and the wavenumber extremum of the Rayleigh–Taylor and Kelvin–Helmholtz instability waves increased. The variation in the K–H instability’s growth rate extremum increasing Weber number surpassed that of the R–T’s instability. This indicated that both the R–T and K–H waves on sub-millimeter liquid droplets tended to exhibit increased growth rates and reduced scales. Moreover, as the Weber number increased, the K–H instability became dominant in the aerodynamic fragmentation. The numerical simulations showed good qualitative agreement with the experimental data, affirming the viability of numerical methods for addressing such challenges. The evolution of the sub-millimeter liquid droplets was marked by two primary stages, flattening and shear stripping, signifying that the K–H instability-driven SIE mechanism governed the aerodynamic breakup in the supersonic post-shock airflow.

CFD investigation into the effect of GM variations on ship manoeuvring characteristics

The change in a ship's loading condition may induce a displacement of its vertical centre of gravity, subsequently impacting the metacentric height (GM) of the vessel. Such alterations can have unforeseen effects on the ship's manoeuvring behaviour. This study investigates these effects by developing a free running manoeuvre model for the KRISO Containership (KCS) based on the Unsteady Reynolds-Averaged Navier-Stokes (URANS) method. A multi-level dynamic overset grid and the body-force propeller method were employed to simulate the ship motion involving rudder manipulations. The numerical calculations of zigzag and turning manoeuvre revealed that the variations in GM produce significant alternations in the roll motion during manoeuvres, consequently affecting the zigzag and turning trajectories of the vessel.

Influence of jet flow on hydrodynamic performance of a ducted propeller

This study introduces a concept that jet technology in the aeronautical field is used for active flow control to improve the hydrodynamic performance of a ducted propeller. Jet flow is added in front of the ducted propeller, and it produces a circumferential velocity that is opposite to the rotation direction of the rotor. An international standard ducted propeller was adopted to demonstrate this concept. The unsteady Reynolds-averaged Navier–Stokes method and the shear stress transport k−ω turbulence model were employed for the simulations. The open-source platform OpenFOAM was utilized. The overall efficiency η0 of the ducted propeller first increases and then decreases with increasing the jet flow velocity Rjf from 1 to 3 and the distance L to the rotation center from 0.2D to 0.4D. When the jet flow is at the optimal condition of Rjf=2 and L=0.3D, the maximum efficiency improvement of 3.1% is achieved for the ducted propeller. The reason is that the jet flow contributes to a pressure increase in the flow through the rotor. This effect is related to tip and hub vortices, which are disrupted by the jet flow and have relatively low vorticity magnitudes compared to the reference case without jet. The findings in this study have the potential to advance the development of active flow control technology for ships.

Numerical Approach Based on Solving 3D Navier–Stokes Equations for Simulation of the Marine Propeller Flow Problems

The report presents the approach implemented in the Russian LOGOS software package for the numerical simulation of the marine propeller flow problems using unstructured computational meshes automatically generated by the mesh generator. This approach includes a computational model based on the Navier–Stokes equation system and written with respect to the physical process: the turbulent nature of flow with transient points is accounted using the Reynolds Averaged Navier–Stokes method and the k–ω SST model of turbulence by Menter along with the γ–Reθ (Gamma Re Theta) laminar-turbulent transition model; the Volume of Fluid method supplemented with the Schnerr–Sauer cavitation model is used to simulate the cavitation processes; a rotating propeller is simulated by a moving computational mesh and the GGI method to provide conformity of the solutions on adjacent boundaries of arbitrarily-shaped unstructured meshes of the two domains. The specific features of the numerical algorithms in use are described. The method validation results are given; they were obtained because of the problems of finding the performance curves of model-scale propellers in open water, namely the problems of finding the performance of propellers KP505 and IB without consideration of cavitation and the performance of propellers VP1304 and C5 under cavitation conditions. The paper demonstrates that the numerical simulation method presented allows for obtaining sufficiently accurate results to predict the main hydrodynamic characteristics for most modes of operation of the propellers.

Acoustic Field Radiation Prediction and Verification of Underwater Vehicles under a Free Surface

This study aimed to examine the acoustic field radiated by propellers and underwater vehicles. For the verification of sound radiation in underwater vehicles, numerical methods are widely used in addition to experiments and propeller blade frequencies for calculation and validation. Numerical convergence and accuracy are more important for near-field and far-field problems. This paper uses the boundary element method (BEM) to assess the convergence of the finite volume method (FVM). In this study, the FVM, including the Reynolds-averaged Navier–Stokes method and the Ffowcs Williams–Hawkings (FW-H) acoustic model, is used to investigate the influence of various geometric inflows on the hydrodynamic and noise performance of the propeller. Then, the sound radiation of the FVM is compared with the BEM at the far field to determine the number of meshed elements. Furthermore, spectral analysis is being conducted to examine the noise generated by the underwater vehicle and propeller. The objective is to investigate the influence of the free surface on propeller efficiency. After verifying the numerical simulation, the results indicate that a relationship can be established between water pressure and propeller thrust under specific conditions. This relationship can be used to estimate the magnitude of propeller thrust at different water depths. The simulated results of propeller thrust, torque coefficient, propulsion efficiency, and sound radiation in this study are consistent with experimental values. This demonstrates the accuracy and practicality of the findings of numerical procedures in engineering applications.

Study on the influence of shaft brackets on the scale effect of the four-screw ship wake field

In the four-screw ship navigation, the load difference between the inner and outer propeller is found to be more than twice of that in the model test, indicating a serious scale effect. The stern attachment of the four-screw ship has a significant impact on the inner and outer propeller wake which directly affecting the load distribution. Thus, to investigate the impact of the brackets on the scale effect of the four-screw ship flow field, this paper conducted multi-scale simulations of a four-screw ship with and without bracket using the Reynolds-averaged Navier-Stokes (RANS) method. The feasibility and accuracy of the numerical method was verified by comparing with PIV test results. The result shows that the scale effect is more pronounced in the outer propeller compared to the inner propeller. The presence of bracket reduces the wake fraction difference between the inner and outer propellers, but significantly affects the inner radius region of the outer propeller disk, more than doubling the wake fraction in this region at full-scale. When the bracket Reynolds number falls below its critical Reynolds number, the inner radius region of the outer propeller flow field deviates notably from the full-scale results and loses its practical significance.

Effects of ammonia combustion on skin friction characteristics for supersonic flow

Boundary layer combustion is an effective method to reduce friction in scramjet combustors. In existing research on drag-reducing fuels, hydrocarbons pose challenges related to carbon deposition and emissions. Meanwhile, hydrogen presents significant difficulties in storage and transportation. For the first time, this paper explores ammonia as a drag-reducing alternative. We employed the Reynolds-averaged Navier-Stokes (RANS) method to investigate the characteristics of drag reduction by ammonia combustion in supersonic flow. Numerical results indicate that the local drag reduction by ammonia combustion is better than hydrocarbon and worse than hydrogen compared to existing research. Ammonia exhibits superior potential for drag reduction. However, the overall drag reduction at room ammonia injection temperature is limited due to the significant distance between the ammonia self-ignition point and the injection port. To address this challenge, we proposed two optimization strategies: increasing the ammonia injection temperature and increasing the inlet ammonia decomposition ratio. As the ammonia injection temperature increases, the self-ignition position moves towards the inlet, and the combustion drag reduction region expands upstream. Interestingly, a higher ammonia inlet decomposition ratio does not necessarily lead to a better drag reduction effect. In fact, an excessively high decomposition ratio can inhibit the combustion drag reduction effect. The most effective drag reduction occurs when the ammonia inlet mass fraction is 50%, resulting in a 42.8% reduction in integrated drag compared to the case without NH3 injection. Overall, this paper offers critical insights into the utilization of ammonia combustion in scramjets to reduce drag.

Wake physics of two-dimensional flapping-hydrofoil turbines

We present a numerical study on the wake of two-dimensional flapping-hydrofoil turbines using Reynolds averaged Navier–Stokes method with shear stress transport k–ω model. The adaptive mesh refinement was applied for vortex simulations. The pitching amplitude ranges from 50° to 90°, and the reduced frequency ranges from 0.10 to 0.20. By varying the reduced frequency and pitching amplitude, we visualized three different types of wakes, and they are the von Karman wake, the mixed wake, and the chaotic wake. We found that there is a critical value of the reduced frequency to determine whether the wake will eventually develop into a standard von Karman wake. When the vortices leave the hydrofoil, they first form a classical staggered arrangement and then develop into a stable double-row configuration. The regular motions of vortices along specific trajectories are explained by analyzing the resultant velocity using velocity polygon, taking into account the effects of vortex interactions and environmental factors. The main component of vortex induced velocities at specific locations is always opposite to the freestream velocity, which is the cause of velocity attenuation in the wake. With the increase in the reduced frequency and pitching amplitude, the velocity attenuation is getting worse. The maximum velocity attenuation usually occurs farther downstream from the hydrofoil for large reduced frequencies and large pitching amplitudes. The wake of a flapping-hydrofoil turbine is divided into four feature zones by studying the time-varying characteristics of velocities, which deepens the understanding of the wake and can provide important references in selecting the optimal location for downstream turbines.

Numerical Study on the Flow Field Characteristics and Efficiency of Different Underwater Gun Muzzle Brakes

The submerged gun’s mathematical and physical models were established, and an investigation was conducted on the development of the flow field and the recoiling efficiency of three representative muzzle brakes. The study employed the Schnerr-Sauer cavitation model and the k − ω transport turbulence model, both based on the Reynolds-averaged Navier-Stokes method. Numerical simulation of the underwater firing of a 12.7 mm gun’s muzzle flow field was accomplished using a combination of user-defined function (UDF) and overlapping mesh techniques. Detailed analysis was performed on the flow field characteristics encompassing density, velocity, and phase state fields, along with an in-depth examination of the efficiency of the three muzzle brakes. The simulation results demonstrated an enhanced cavitation effect when a brake was incorporated, with the impact muzzle brake exhibiting the highest efficiency, followed by the impact-reaction type, while the reaction type exhibited the lowest performance.

Ship energy-saving device: Influence of the scale effect and hull roughness on the resistance reduction effect of an interceptor

Saving energy of ships are key research topics in the shipping industry, and the scale effect of energy-saving devices is a critical issue. Based on the Reynolds-averaged Navier–Stokes method, free-surface and double-body simulations of model- and full-scale ships were performed in this study. The resistance reduction effect and interceptor mechanism were analyzed. In addition, the influence of the scale and roughness effects on the resistance reduction performance of the interceptor was studied. The results showed that the stern flow field was significantly improved after the interceptor was installed, which was mainly responsible for the ship resistance reduction. When the Froude number ≥0.334, the average resistance reduction rates of the model- and full-scale interceptors were 5.67% and 8.71%, respectively. Some differences were observed between the extrapolated results of the resistance reduction effect of the interceptor and the results of the full-scale simulation; therefore, the change in the form factor and the scale effect of the wave-making resistance were considered in the extrapolated method. In addition, with increasing hull roughness, the frictional, viscous pressure, and total resistances of the ship increased, which decreased the resistance reduction rate of the interceptor by 1%–3% and shortened the resistance reduction speed range of the interceptor.

Numerical study of the unsteady blade root aerodynamics of a 2 MW wind turbine equipped with vortex generators

Abstract. In order to design vortex generators for modern multi-megawatt wind turbines accurately, the 3D behaviour of the boundary layer has to be considered. Due to the rotation of the blade, the lift-enhancing rotational augmentation has a considerable impact, especially in the inner blade sections. To investigate the interaction of vortex generators and rotational augmentation, high-fidelity computational flow simulations by means of unsteady Reynolds-averaged Navier–Stokes methods are presented for a rotating blade of a generic 2 MW horizontal-axis wind turbine. The inner blade section is analysed with and without vortex generators for two different pitch settings, including one causing largely separated flow. Two ways of placing the vortex generators on the blade with different radial starting positions are investigated in order to find out if the coexistence of the two lift-enhancement methods (i.e. rotational augmentation and vortex generators) is beneficial. All simulations are performed with the flow solver FLOWer, and the vortex generators are modelled by the introduction of source terms into the computational domain through a so-called BAY (Bender–Anderson–Yagle)-type model. For the case without vortex generators, it is found that the strength of rotational augmentation largely depends on the effective angles of attack (i.e. the pitch setting). For the case with lower effective angles of attack, rotational augmentation is a cyclic phenomenon, whereas for the case with higher effective angles of attack, it generates large loads in the inner root section due to a constant centrifugal pumping mechanism in time. The results from the cases with vortex generators display a rather destructive interaction of vortex generators and rotational augmentation on the torque. For low effective angles of attack and thus attached flow conditions, vortex generators exhibit slight losses compared to the case without vortex generators, as they inhibit spanwise flow through rotational augmentation. For high effective angles of attack, the vortex generators placed over 30 % of the blade produce an increase of 3.28 % in torque compared to the case without vortex generators and high centrifugal pumping.

Impact of cavity and ramp configuration on the combustion performance of a strut-based scramjet combustor

Abstract The flow performance of a dual wall-mounted cavity in a strut-injector scramjet combustor in steady reacting flow conditions is computationally analyzed. A baseline configuration corresponding to DLR experiments and two proposed configurations with varying bottom wall cavity depth and fixed top wall ramp is considered. Steady-flow computations are performed using the 2-D Reynolds Averaged Navier–Stokes method with k-ω SST turbulence closure coupled and single-step reaction chemistry. The calculated flow patterns, density, pressure, and temperature fields are compared with shadowgraph and wall pressure measurements from DLR experiments. The cavity and strut are mounted downstream of the strut to analyze the shock patterns and their interference with the shear layer mixing features. The estimated flow patterns, density, pressure, and temperature fields are compared with shadowgraph and wall pressure measurements from DLR experiments. Incorporating cavity and ramp configuration provides earlier complete combustion compared to the baseline model, with a marginal rise in the total pressure caused by additional shock wave formation that emanates from the corners of the cavity and ramp. The combustion zone widens in the lateral direction as the cavity shifts the shock train downstream of the strut injector owing to intense shock shear layer interactions.

Hydrodynamics and Wake Flow Analysis of a Floating Twin-Rotor Horizontal Axis Tidal Current Turbine in Roll Motion

The study of hydrodynamic characteristics of floating double-rotor horizontal axis tidal current turbines (FDHATTs) is of great significance for the development of tidal current energy. In this paper, the effect of roll motion on a FDHATT is investigated using the Computational Fluid Dynamics (CFD) method. The analysis was conducted in the CFD software STAR-CCM+ using the Reynolds-averaged Navier–Stokes method. The effects of different roll periods and tip speed ratios on the power coefficient and thrust coefficient of FDHATT were studied, and then the changes in the vorticity field and velocity field of the turbine wake were analyzed by two-dimensional cross-section and Q criterion. The study indicates that roll motion results in a maximum decrease of 30.76% in the average power coefficient and introduces fluctuations in the instantaneous load. Furthermore, roll motion significantly accelerates the recovery of wake velocity. Different combinations of roll periods and tip speed ratios lead to varying degrees of wake velocity recovery. In the optimal combination, at a position 12 times the rotor diameter downstream, roll motion can recover the wake velocity to 92% of the incoming flow velocity. This represents a 23% improvement compared to the case with no roll motion.