Reynolds-averaged Naiver-Stokes Research Articles

Research on turbulence model parameters for the prediction of waterjet propulsion pump hydrodynamic performance

Abstract The hydrodynamic performance of waterjet propulsion pumps has an important impact on improving the speed of waterjet propulsion ships. High-precision prediction of the hydrodynamic performance of water-jet propulsion pumps based on CFD is of great significance to improving the pump design quality. Based on the Reynolds-averaged Naiver-Stokes equation, this paper conducts a grid-independence verification on a water jet propulsion pump based on hydrodynamic test data, uses the Latin hypercube sampling method to generate SST k-ω turbulence model parameter space calculation samples, performs steady-state calculation on all samples, analyzes the sensitivity of changes in the parameters of the turbulence model to the prediction of the external characteristics of the waterjet propulsion pump, and finally obtains the influence of changes in the parameters of the turbulence model on the prediction of the external characteristics of the pump. Among them, the sensitivities of the three parameters β *, β1 and a1 are higher. The results show that the prediction accuracy of the external characteristics of the water jet propulsion pump can be effectively improved by correcting these three highly sensitive parameters, and the correction values of the three key parameters are obtained. The prediction error of the external characteristics of the water jet propulsion pump is reduced from 4.3% to less than 2.1%. A feedforward neural network is used to establish a mapping model between the three turbulence model parameters and the head coefficient and power coefficient of the waterjet propulsion pump in order to achieve rapid prediction. The genetic algorithm is applied to optimize the weights, thresholds and other coefficients in the neural network, thereby improving the prediction accuracy. Based on the optimized neural network, the genetic algorithm is used to optimize the three turbulence model parameters, and the optimized coefficients are obtained. The optimized turbulence model parameters are put into the hydrodynamic model for RANS simulation to verify the degree of optimization of the model coefficients by the genetic algorithm.

Impact of ventilation efficiency on restraining hazardous toilet plume post-flushing in indoor environments

This study analyzed the hybrid ventilation efficiency of indoor toilet plumes generated post-flushing with various designs. Single-sided natural ventilation and hybrid ventilation were numerically investigated to determine the impact of vent locations commonly found in multistorey buildings. An in-situ experiment was performed to identify the size distribution of the plume with time series. A computational fluid dynamics model based on the Reynolds-averaged naiver-Stokes equations and baseline k-ω turbulence equations was used to predict aerosol pollutant transmission. The aerosol-laden particles receded gradually impacted by the convection of the ambient source. Besides flush volume, temperature and ventilation scenarios were also related to the generation and droplet transmission. The results indicated that the ventilation rates were generally increased with decreased residence time if the turbulence was not suppressed. This study suggested that appropriate ventilation design could significantly reduce the average particle residence time, in which case it could also reduce the rate of virus infection.

Effect of Different Flame Deflectors Plate Geometry on Supersonic Jet Impingement Flow and Heat Characteristics from Rocket Exhaust

The numerical investigation has been carried out on an under-expanded supersonic jet impingement on the perpendicular and inclined deflector plates of various shapes, such as flat, sinusoidal, and wedge types. The numerical study considers flat plates at Mach number of 2.2, and pressure ratio of 1.2, and results of flow and heat transfers are obtained using the Reynolds averaged Naiver Stokes (RANS) two-equation turbulence models. Various two-equation models (especially Standard k-ε, Reynolds Normalization Group k-ε, Realizable k-ε, Shear Stress Transport k-ω, and Baseline k-ω) were employed to predict the deflector’s surface temperature and pressure ratios. The pressure ratios are plotted with various two-equation turbulence models, and it is observed that among all turbulence models, the Shear Stress Transport k-ω shows close agreement with experimental results. Moreover, among the three deflectors, the wedge-shaped deflector shows improved performance in deflecting the rocket exhaust plume away from the surface. However, the maximum temperature on the deflector surface for a wedge-shaped deflector is 8.19% and 26% higher than the flat and sinusoidal-shaped deflectors, respectively.

3D NUMERICAL STUDY OF HYDROYNAMICS AROUND A MONOPILE FOUNDATION UNDER COMBINED WAVE CURRENT EFFECT

A numerical investigation of hydrodynamics around a monopile foundation is presented in the paper. An open-source CFD program REEF3D is used for the present study. The REEF3D model solves the Reynolds-averaged Naiver-Stokes equation with k-w turbulence closure to obtain the flow hydrodynamics. The code utilizes a staggered cartesian grid system. Also, Message Passing Interface (MPI) based parallelization is used for domain decomposition. In order to ensure the precision of the flow field, the model is first validated for four different combined wave-current scenarios, where the currents and waves are following each other. The results obtained from the validation studies exhibit good agreement with the already reported experimental results. Initially, a grid convergence study has been performed to ensure that the Numerical wave tank is independent of the mesh size. Finally, numerical experiments corresponding to four different combined wave-current cases for different wave steepness are simulated using the code.

Numerical investigations on the unsteady leakage flow and heat transfer characteristics of the turbine blade squealer tip

The unsteady leakage flow and heat transfer characteristics of the turbine blade squealer tip was numerically investigated using three-dimensional Unsteady Reynolds-Averaged Naiver-Stokes (URANS) and standard k-ω turbulence model. The numerical simulated heat transfer coefficients of the turbine blade tip were well agreement with the experimental data. The accuracy of the numerical method was validated. The steady and unsteady leakage flow and heat transfer coefficient of the turbine blade squealer tip with and without film cooling was conducted. The unsteady leakage flow and heat transfer characteristics of the turbine blade squealer tip with film cooling design at three different blow ratios was numerically investigated. The obtained results show that the corner vortex and scraping vortex near the pressure side shows periodic fluctuation along the separation line of the leakage flow under the function of the pressure difference between the suction and pressure side of the blade tip, as well as the upstream vane unsteady wake flow. This unsteady flow behavior results in the fluctuation of the high heat transfer coefficients at the blade tip. The film cooling design along the camber line can effectively reduce the thermal load of the turbine blade squealer tip by comparison to without film cooling case. The time-averaged heat transfer coefficients of the turbine blade squealer tip reduce 9.0%, 12.4% and 13.2% at blowing ratio of .0.5, 1.0 and 1.5 comparison to without film cooling design in respective. The present work can provide the reference of the film cooling design for the turbine blade squealer tip.

Quantification of Reynolds-averaged-Navier–Stokes model-form uncertainty in transitional boundary layer and airfoil flows

It is well known that the Boussinesq turbulent-viscosity hypothesis can introduce uncertainty in predictions for complex flow features such as separation, reattachment, and laminar-turbulent transition. This study adopts a recent physics-based uncertainty quantification (UQ) approach to address such model-form uncertainty in Reynolds-averaged Naiver–Stokes (RANS) simulations. Thus far, almost all UQ studies have focused on quantifying the model-form uncertainty in turbulent flow scenarios. The focus of the study is to advance our understanding of the performance of the UQ approach on two different transitional flow scenarios: a flat plate and a SD7003 airfoil, to close this gap. For the T3A (flat-plate) flow, most of the model-form uncertainty is concentrated in the laminar-turbulent transition region. For the SD7003 airfoil flow, the eigenvalue perturbations reveal a decrease as well as an increase in the length of the separation bubble. As a consequence, the uncertainty bounds successfully encompass the reattachment point. Likewise, the region of reverse flow that appears in the separation bubble is either suppressed or bolstered by the eigenvalue perturbations. This is the first successful RANS UQ study for transitional flows.

Development of turbulent heat flux model for unsteady forced convective heat transfer of small-to-medium Prandtl-number fluids based on deep learning

Turbulent heat flux (THF) models are used for the closure of the THF term when solving the steady/unsteady Reynolds-averaged scalar transport equation to simulate the turbulent heat transfer in industry. It is known that the simple gradient diffusion hypothesis (SGDH) has deficiencies under complex conditions with flow separations. To develop a more general THF model, this paper firstly establishes a high-fidelity database of forced convective heat transfer passing a circular cylinder under different Prandtl number (Pr) conditions at the Reynolds number (Re) of 500 via the direct numerical simulations. Proper orthogonal decomposition method is then employed for the triple decomposition on unsteady turbulent flow with the first two orders of eigenmodes reconstructing the large-scale field and the remaining reconstructing the turbulent field. Then, architectures using different neural network structures based on tensor basis neural network (TBNN), including MLP-TBNN-THF which adopts the multilayer perceptron (MLP) and ResNet-TBNN-THF that uses the residual network (ResNet), are constructed to predict normalized THF from large-scale flow features and Pr. Posterior tests are carried out on different Prs and three different Res: 500, 5000 and 16900 using the well-trained TBNN-THF models to evaluate their performance and generalization capability. Most models proposed in this paper predict the heat transfer more accurately than the SGDH model, even when extended to conditions out of the range trained. The failure of the isotropic assumption of SGDH model is observed in most regions. In that case, it is vitally necessary to use a model like proposed in this paper to simulate the THF term more accurately. In conclusion, the THF model, which is appropriate for complex working conditions and fluids with small-to-medium range of Pr, obtained by deep learning method in this paper, is helpful to improve the prediction accuracy of temperature field or concentration field by steady or unsteady Reynolds-averaged Naiver-Stokes approach in engineering. Besides, the current framework can be generalized to scalar-flux modeling under other conditions with the supplement of more databases in the future.

A high-fidelity CFD-based model for the prediction of ship manoeuvrability in currents

The manoeuvring behaviour of a vessel in currents differs remarkably from its behaviour in water without a current, stemming from hydrodynamic effects caused by the presence of the current. Given that vessels operating in open seas and coastal waters are mostly exposed to ocean currents, it is important to have an understanding of the influence of currents on ship manoeuvrability. In the present paper, by means of an unsteady Reynolds-Averaged Naiver-Stokes solver, a numerical study of ship manoeuvrability in different currents was performed. Firstly, a model-scale container ship (the KRISO Container Ship) was used to develop the Computational Fluid Dynamics (CFD) model capable of performing a self-propelled free manoeuvre. Then, a validation study was carried out to assess the validity of the CFD model by comparison with the available experimental results from a free-running test. Following this, a series of manoeuvring simulations (i.e., standard turning manoeuvres) in deep waters with current speed to ship speed ratios varying between −0.552 and −0.138/+0.138 and + 0.552 were conducted using the present CFD model. The numerical results demonstrated that the inclusion of the current has a remarkable influence on the turning performance of the ship, leading to significant changes in the ship trajectory and its turning parameters when compared to the inherent ship manoeuvrability in deep water without a current.

Effect of inflow conditions on transonic turbine airfoil limit loading

To better understand airfoil limit loading and the effect inflow conditions have on local efficiency, a computational fluid dynamic investigation was performed for four different transonic turbine airfoils under sub-critical, critical, and supercritical conditions. A computational baseline was established using data previously collected at the Pratt and Whitney Canada High-Speed Wind Tunnel at Carleton University near design conditions using the Reynolds-Averaged Naiver-Stokes shear stress transport k − ω turbulence model with γ transition. The effects of inflow conditions on aerodynamic performance were examined by varying incidence by ± 20°, mainstream turbulence intensity from 5 to 20% and mainstream turbulent length scale from 1 to 100% of the airfoil pitch. Quantitative data of mass-flow averaged Mach numbers, mass-flow averaged flow angles, surface isentropic Mach number distributions and mass-flow averaged total pressure loss coefficients were collected and are presented alongside flow visualizations of numerical Schlieren images to allow for a detailed description of the entire flow domain. Similar to previous experimental work the limit loading pressure ratio and the mass-flow averaged outlet flow angle were strongly correlated with the airfoil outlet metal angle. The influence of inflow conditions was minimal on the exit flow profile with the exception of the mass-flow averaged total pressure loss coefficients. Results show incidence variation to change the total pressure loss coefficient depending on the airfoil, whereas, turbulence intensity and turbulent length scale predicted a drastic increase in loss with increased turbulence level for all airfoils considered.

SHIP MANOEUVRING PREDICTION BASED ON NUMERICAL TOWING TANK TECHNIQUE

A practical procedure is proposed in this paper to predict ship manoeuvrability. A three degrees of freedom MMG (Japanese Manoeuvring Mathematical Modelling Group)-type model is established to simulate rudder manoeuver. Propeller thrust and rudder loads are calculated by empirical formulas, whereas the hull forces as well as moment are determined with hydrodynamic derivatives which are derived from CFD (Computational Fluid Dynamics) computations. An own developed RANS (Reynolds-Averaged Naiver-Stokes) solver on the base of OpenFOAM is applied to simulate a range of PMM (Planar Motion Mechanism) tests and Fourier analyses of the computed results are carried out to obtain the required derivatives. In order to demonstrate the effectivity of the whole procedure and the RANS computations, the US (United States) combatant DTMB 5415 is taken as a sample for an application. Forced motions of surge, sway, yaw and yaw with drift for the bare hull with bilge keels are simulated. Thereafter, simulations of standard rudder manoeuvers, i.e. turning and zigzag, are performed by applying the computed derivatives. The results are compared with available measured data. It has been shown that the present procedure together with the RANS method can be used to evaluate the manoeuvrability of a ship since general good agreements between the simulated results and measured data are achieved.

MODEL- AND FULL-SCALE RANS SIMULATIONS FOR A DRIFT TANKER

The flow around a full-scale (FS) ship can be simulated by means of Reynolds-Averaged Naiver-Stokes (RANS) method, which provides a way to obtain more knowledge about scale effects on ship hydrodynamics. In this work, the viscous flow around a static drift tanker in full scale is simulated by using the RANS solver based on the open source platform OpenFOAM. The k - w SST model is employed to approximate the eddy viscosity. To reduce computational time, wall function approach is applied for the FS simulation. The flow around the ship in model scale is simulated as well, but without using any wall function, i.e., using Low-Reynolds number mode. In order to verify the computations, de- tailed studies on the computational grid including investigation of the sensitivity of computed forces to y+ (dimension- less distance of first grid point to wall) and grid dependency study are carried out. The computed forces are compared with available measured data. The scale effects are analysed and discussed by comparisons.

Numerical and experimental study of scaling criteria accuracy for film cooling using CO2 coolant

To have a better understand of film cooling technique, many experiments to investigate parameters affecting the film cooling effectiveness and heat transfer have been conducted. In this kind of experiments, different sorts of coolant (CO2, N2 and etc.) are often applied to match coolant-to-mainstream properties or use some measuring techniques. However, the application of those coolant probably leads to a similarity problem, as the properties of them are different from air coolant in real engine conditions.This study investigates the accuracy of six criteria for the film cooling effectiveness scaling which changes the coolant from air to CO2 based on both experimental and numerical method, to promote the similarity to air coolant of film cooling experiments using CO2 as coolant. The study conducted on the pressure side of 1st stage stator from E3 aero engine. The mainstream inlet pressure and temperature of the experiment was 112kPa and 312K, respectively. Numerical simulations were conducted by solving the Reynolds Averaged Naiver-Stokes (RANS) equations and Re-Normalization Group (RNG) k-ε turbulence model. The effect of CO2 coolant on pressure side film cooling effectiveness was initially investigated, followed by the analysis of the accuracy for six scaling criteria. Finally, reasons leading to the deviation of scaling was studied.Both numerical and experimental results indicate that the application of CO2 coolant leads to an obvious reduction of ηAPS at relatively low coolant mass flow rate. That effect almost disappears at high coolant mass flow rate. In the experiment, η with the variation in VR has the highest Pearson correlation coefficient (Rp) between air and CO2 coolant at almost all measuring points on pressure side, illustrating VR criterion has the highest accuracy in the scaling which the coolant is changed from air to CO2. The cooling effectiveness deviation of VR criterion is lower than 3% at all coolant CO2 percentage(obtained from CFD results). The low specific heat capacity of CO2 will lead to a relatively faster decay of injection temperature compared with air, producing an adverse influence on cooling effectiveness. The high density of CO2 can increase attachment ability of injections(especially on mid-span of the blade) and form a similar cooling film state to the air coolant at higher coolant mass flow rate. The results of this study can be used to promote the accuracy of the experiment using CO2 as coolant at relatively low temperature conditions in terms of film cooling effectiveness and injection state(attachment or separation).

Experimental and modelling analysis of efficiency enhancement in a liquid piston gas compressor using metal plate inserts for compressed air energy storage application

In this work, experimental and modelling analyses are performed in order to improve the compression efficiency of a Liquid Piston Gas Compressor (LPGC), which utilizes a column of water for air compression. Due to low cost and easy manufacturing, aluminium parallel plates are used as the heat exchanger inside the LPGC to absorb the thermal energy from the compressed air and hence increase the efficiency of the compressor. A comprehensive set of experimental and numerical analyses are performed to gain deep insight into the flow and thermal characteristics of the air and water in the LPGC with plate inserts. A LPGC prototype including a steel cylinder with a height of 1.1 m and a diameter of 0.08 m is developed and experimental data is collected for air compression from 8 bar to 40 bar. Experiments are performed for three plate inserts with different heights of 0.2 m, 0.35 m and 0.5 m. Experimental data is acquired for 3 different water flow rates of 0.0005 m3s−1, 0.0007 m3s−1 and 0.0008 m3s−1 equivalent to compression times of 1.7 s, 2.6 s and 3.5 s, respectively. To gain further understanding of the flow and heat transfer characteristics inside the LPGC, three-dimensional modelling is performed by solving unsteady Reynolds-Averaged Naiver Stokes (RANS) equations and deploying the Volume of Fraction (VOF) approach for tracking the water-air interface in the LPGC cylinder. Experimental results show that in comparison to the no-insert LPGC case, the air temperature at the end of the compression can be reduced by about 50 K, 75 K, and 82 K with the inclusion of plate inserts with the height of 0.20 m, 0.35 m and 0.5 m, respectively. This leads to an increase in the LPGC compression efficiency by about 3%, 4% and 8%, respectively. The modelling tool validated against the experimental data, revealed that for a fixed number of plates in the LPGC, increasing the thickness of the plates, increases the compression efficiency. Additionally, the parametric study performed by the modelling tool showed that for a fixed plate thickness, increasing the plate height up to an optimum value, compression efficiency increases. After, further increase in the plate height, decreases the compression efficiency. For the LPGC geometrical property and compression conditions studied here, the optimum plate height, which maximizes the efficiency, is found to be 0.9 m for all plate thicknesses studied.

Backflow effects on mass flow gain factor in a centrifugal pump.

Previous researches has shown that inlet backflow may occur in a centrifugal pump when running at low-flow-rate conditions and have nonnegligible effects on cavitation behaviors (e.g. mass flow gain factor) and cavitation stability (e.g. cavitation surge). To analyze the influences of backflow in impeller inlet, comparative studies of cavitating flows are carried out for two typical centrifugal pumps. A series of computational fluid dynamics (CFD) simulations were carried out for the cavitating flows in two pumps, based on the RANS (Reynolds-Averaged Naiver-Stokes) solver with the turbulence model of k-ω shear stress transport and homogeneous multiphase model. The cavity volume in Pump A (with less reversed flow in impeller inlet) decreases with the decreasing of flow rate, while the cavity volume in Pump B (with obvious inlet backflow) reach the minimum values at δ = 0.1285 and then increase as the flow rate decreases. For Pump A, the mass flow gain factors are negative and the absolute values increase with the decrease of cavitation number for all calculation conditions. For Pump B, the mass flow gain factors are negative for most conditions but positive for some conditions with low flow rate coefficients and low cavitation numbers, reaching the minimum value at condition of σ = 0.151 for most cases. The development of backflow in impeller inlet is found to be the essential reason for the great differences. For Pump B, the strong shearing between backflow and main flow lead to the cavitation in inlet tube. The cavity volume in the impeller decreases while that in the inlet tube increases with the decreasing of flow rate, which make the total cavity volume reaches the minimum value at δ = 0.1285 and then the mass flow gain factor become positive. Through the transient calculations for cavitating flows in two pumps, low-frequency fluctuations of pressure and flow rate are found in Pump B at some off-designed conditions (e.g. δ = 0.107, σ = 0.195). The relations among inlet pressure, inlet flow rate, cavity volume, and backflow are analyzed in detail to understand the periodic evolution of low-frequency fluctuations. Backflow is found to be the main reason which cause the positive value of mass flow gain factor at low-flow-rate conditions. Through the transient simulations of cavitating flow, backflow is considered as an important aspect closely related to the hydraulic stability of cavitating pumping system.

High-resolution thermal analysis of nuclear thermal propulsion fuel element using OpenFOAM

This paper conducts high-resolution thermal hydraulic analysis of a Nuclear Thermal Propulsion (NTP) Fuel Element (FE) using a finite volume approach. Recent NTP design efforts have focused on using low-enriched uranium (LEU) fuel as an alternative to the historical highly-enriched uranium (HEU) fuel. In addition to changing materials’ properties, variations in dimensions and flow characteristics are also considered. This makes the applicability of legacy experimental data questionable for the new LEU designs, as the data was generated for a specific range of design characteristics. Nowadays, there is limited accessibility to experimental capability under realistic hot hydrogen conditions. However, some experimental setups could be replaced or complemented by higher-order numerical analysis, which is the end objective of the computational framework developed here. This study implements a 3D conjugate heat transfer (CHT) numerical solver between solid and fluid regions using OpenFOAM Computational Fluid Dynamic (CFD) toolbox. The hydrogen flow in the fluid region is simulated using Reynolds-averaged Naiver-Stokes (RANS) model, while the solid region is modeled using a conduction heat transfer solver. The 3D simulation results are validated against experimental data obtained from the Nuclear Engine for Rocket Vehicle Application (NERVA) Nuclear Rocket Experimental (NRX) A6 program. The latter shares similar design parameters with modern NTP systems, such as dimensions and flow conditions. In addition, legacy heat transfer correlations were implanted in a reduced-order 1.5D semi-analytic solution, and the results were compared against the OpenFOAM solution. The results presented in this paper conclude that OpenFOAM can serve as a high-resolution thermal hydraulic code both for reproducing legacy experiments and educating reduced-order models.

Numerical Study on the Unsteady Characteristics of the Propeller Cavitation in Uniform and Nonuniform Wake Flows

Propeller cavitation is a problematic issue because of its negative effects, such as performances losses, noise, vibration and erosion. Numerical methodology is an effective and efficient technical tool for the study of propeller cavitation, however, it is hard to capture tip-vortex cavitation in the previous work by using common turbulence models based on turbulent-viscosity hypothesis. In this work, the Reynolds-Averaged Naiver-Stokes (RANS) approach, adopting the Reynolds stress turbulence model (RSM), is taken to study the unsteady characteristics of the cavitation on the four-bladed INSEAN E779A model propeller. The numerical simulation was carried out using the commercial CFD software ANSYS Fluent 14.0. One kind of uniform wake flow and two kinds of nonuniform wake flows are considered here. The results in the uniform flow show a good agreement with previous experimental results on both the sheet cavitation and the tip vortex cavitation and prove the ability of the RSM on capturing the tip vortex cavitation. Two kinds of nonuniform wake flows are designed based on the previous experimental researches and the unsteady characteristics of the propeller cavitation are analyzed by comparing the results in the uniform and two nonuniform wake flows together.

Computational Study of Pressurized Oxy-fuel Combustion of a Multi-phase Medium comprised of Methane (Fluid) and Pulverized Coal (Solid)

Staged pressurized oxy-fuel combustion is an advanced technology to be employed for the combustion and carbon capturing processes. This technology is promising because of relatively low costs, low emissions and high-efficient power generation with exhausting pressurized carbon dioxide. Our computational combustion group at West Virginia University performs a numerical study of a lab-scale SPOC reactor by using the ANSYS Fluent software for steady and unsteady Reynolds-averaged Naiver-Stokes simulations as well as large-eddy simulations aiming to support the experiments on SPOC ongoing at Washington University in St. Louis,. The simulations consider the total power to be 100 kW, varying the inputs from coal and methane, with CO2 being the coal carrier. The ultimate computational goal is to continuously reduce the portion of CH4 and eventually perform the LES of pure coal combustion generating 100 kW without no assistants of CH4. In present work, the numerical analysis involves a two-phase flow, turbulence, heat transfer as well as the flame and particle dynamics. The species transport model with the finite rate/eddy dissipation turbulence-chemistry interaction is used for coal combustion, along with a non-premixed combustion model for coal-CH4 burning. The discrete ordinates model for radiation is employed. Being successful with the RANS for pure coal, we are currently on the transition to the LES of pure coal.

Modelling and validation of fluid flow inside a dissipative ladle shroud and a continuous casting tundish

Fluid flow plays a significant role in the continuous casting of molten steel. In this paper, two Reynolds Averaged Naiver-Stokes (RANS) models and a Large Eddy Simulation (LES) model were comparatively employed to characterize the fluid flow inside a dissipative ladle shroud and a tundish. LES model was proved to be powerful to characterize the turbulence structure inside the ladle shroud. The effect of meshing density on the computational accuracy was considered using the LES model. The fine-mesh model can capture multiscale vortices inside the ladle shroud; while, the coarse-mesh model disables the LES to obtain the detailed flow information. The experiment of particle image velocimetry (PIV) was used to verify the flow field obtained by the LES model inside the tundish. The PIV and the LES results agree well in terms of flow pattern and velocity vector.

Numerical simulation of unsteady propeller force for a submarine in straight ahead sailing and steady diving maneuver

In order to provide a complementary perspective to the effects of the maneuvering motions on the unsteady propeller performance, the numerical simulation of the flow field of the hull-rudder- propeller system is performed by Unsteady Reynolds-averaged Naiver-Stokes (URANS) method. Firstly, the flow fields around the submarine model without the presence of propeller in straight ahead motion and the steady diving maneuvers with submergence rudder deflections of 4°, 8° and 12° are predicted numerically. The non-uniformity characteristic of the nominal wake field is exacerbated with the increase of submergence rudder angle. Then the flow field around the SUBOFF-G submarine fitted with the 4381 propeller is simulated. The axial, transverse and vertical unsteady propeller forces in different maneuvering conditions are compared. In general, as the submarine maneuvers more violently, the harmonic amplitudes of the unsteady force at the 2BPF and 3BPF increased more significantly than that at BPF.

Prediction of sand erosion in CFD with dynamically deforming pipe geometry and implementing proper treatment of turbulence dispersion in particle tracking

Predicting more accurate sand erosion rate in oil and gas production is important for maintaining safe and reliable operations while maximizing output efficiency. Computational fluid dynamic (CFD) is a powerful tool for erosion prediction and provides detailed erosion pattern in complex geometry. To improve accuracy of erosion prediction, first part of this paper proposes an algorithm to include the effect of dynamically changing pipe geometry in CFD based erosion modelling and second part of this paper provides guidelines for proper treatment of turbulence dispersion in CFD based erosion modelling.Sand erosion is a time-dependent phenomenon where most severe erosion scars are localized and occurs under disturbed flow conditions. These localized high erosion rates cause substantial change in pipe geometry, creating cavities which modify the flow field, the particle trajectories and eventually the erosion pattern. This work has illustrated that the erosion prediction can be influenced by including the effect of dynamically changing pipe geometry in CFD modelling. An algorithm was developed to couple erosion rate calculations with a Moving-Deforming-Mesh, which dynamically deforms the solid wall surface geometry and computational mesh based on the local erosion rate at regular time intervals. This effect will be significant for geometries involving sharp corners and/or regions of recirculation zones.Sand erosion largely depends on particle impact location, impact angle and impact velocity - which is the commutative effect of particle trajectory leading to the bounding surface. The particle trajectory is determined based on mean and the turbulence of the flow field. In steady state CFD simulations with Reynolds Averaged Naiver Stokes (RANS) turbulence models, turbulent dispersion is obtained from mean turbulence quantities using a stochastic random walk model, which estimates the instantaneous characteristics of a turbulent eddy. However, stochastic random walk model parameters will have significant effects on turbulence dispersion of the particles and hence the erosion prediction. Specific guidelines for these model parameters can improve the accuracy and consistency in CFD based erosion prediction.