Modern CFD Research Articles

Assessment of implicit adaptive mesh‐free CFD modelling

SummaryThis work presents details and assesses implicit and adaptive mesh‐free CFD modelling approaches, to alleviate laborious mesh generation in modern CFD processes. A weighted‐least‐squares‐based, mesh‐free, discretisation scheme was first derived for the compressible RANS equations, and the implicit dual‐time stepping was adopted for improved stability and convergence. A novel weight balancing concept was introduced to improve the mesh‐free modelling on highly irregular point clouds. Automatic point cloud generations based on strand and level‐set points were also discussed. A novel, polar selection approach, was also introduced to establish high‐quality point collocations. The spatial accuracy and convergence properties were validated using 2D and 3D benchmark cases. The impact of irregular point clouds and various point collocation search methods were evaluated in detail. The proposed weight balancing and the polar selection approaches were found capable of improving the mesh‐free modelling on highly irregular point clouds. The mesh‐free flexibility was then exploited for adaptive modelling. Various adaptation strategies were assessed using simulations of an isentropic vortex, combining different point refinement mechanisms and collocation search methods. The mesh‐free modelling was then successfully applied to transonic aerofoil simulations with automated point generation. A weighted pressure gradient metric prioritising high gradient regions with large point sizes was introduced to drive the adaptation. The mesh‐free adaptation was found to effectively improve the shock resolution. The results highlight the potential of mesh‐free methods in alleviating the meshing bottleneck in modern CFD.

Effect of Erosion on Surface Roughness and Hydromechanical Characteristics of Abrasive-Jet Machining

The article contains the fundamental results of the experimental and numerical investigations for pneumo-abrasive unit nozzles with different geometries. The research was purposed by the pressing need to develop an inexpensive and effective working nozzle design of the air-abrasive unit which can be applied for surface processing before some technological processes are performed, as well as for surface coating, descaling after thermal treatment, processing of hollow holes of the crankshafts, smoothing of the inner surfaces of the narrow channels between the impeller blades after electric discharge machining for ultrahigh-pressure combination compressors. Several designs were considered, ranging from the simplest to those with a complicated inner channel geometry. The impact of the nozzle material and challenging inner surface application on its characteristics has also been studied. The research was done using the application of modern CFD complexes for numerical modeling of the air-abrasive mixture discharge from the working nozzle of the pneumo-abrasive unit. In addition, physical experimentation was provided. The methods applied in the research allow for profound, systematic research of spraying units operating on the air-abrasive mixture within a wide range of geometrical and mode parameters. The novelty of the gained results lies in the development of the mathematical model of the pneumo-abrasive nozzle operating process, the working out of a cheaper nozzle design, getting information about air-abrasive mixture distribution along the nozzle length, giving practical recommendations for calculation and designing a working nozzle for the jet-abrasive unit.

CFD design of a novel device for temperature profile measurement in Waste-to-Energy plants

Monitoring the flue gas temperature is a crucial issue for Waste-to-Energy (WtE) plants companies, since it is essential to preserve the materials in the post-combustion chamber, the energy efficiency of the plants as well as to control the emissions of pollutants. As of today, the temperature of flue gases in such plants is commonly monitored by means of thermocouples, infrared pyrometers or aspirated thermocouples. However, all these instruments show limitations in terms of accuracy and reliability inside post-combustion chambers. In this paper, the authors present the thermo-fluid dynamics design of a novel device aimed at mitigating the issues of existing technologies, realised by using the modern CFD techniques. Numerical analyses, performed with the open-source OpenFOAM code, allowed to find a suitable shape for the device and to give a first estimate of the measuring errors, which are of the order of 2% (∼26 K at the typical working temperature of WtE plants’ post-combustion chambers).

MODERN SOFTWARE FOR THE NUMERICAL STUDY OF FLOW IN HYDRAULIC MACHINES

In the past few decades, the field of developing computer software systems has been actively developing, which in turn leads to competition in the software market. Qualified engineers working in the hydroturbine industry must be able to use a computer not only at the user level, but also at the programmer level in order to program modules for their own needs based on existing software systems. Recently, numerical simulation has become applicable to an ever wider class of flows, replacing experimental research methods. Certain numerical models are characterized by different areas of applicability and expenditures of computational resources. The paper provides an analytical review of modern CFD software systems. The advantages and disadvantages of these programs are analyzed in terms of building a three-dimensional model of the object of study, creating a computational grid, setting boundary conditions and visualizing the calculation results. The analysis and comparison of existing mathematical models that used to calculate the spatial flow in the flow path of hydraulic machines has been carried out. There are many different programs for solving hydrodynamic problems, some of the advanced commercial software systems are Ansys, SolidWorks Flow Simulation, Autodesk CFD. There are also open source software products. These automatic design systems make it possible not only to perform high-quality modeling of systems of various physical nature, but also to study the response of these systems to external influences in the form of distributions of pressures, temperatures, and velocities. The calculation algorithms in the programs are similar; the distinctive features of the programs can be evaluated according to the following criteria: grid generation, accuracy, reliability (convergence), calculation speed, model physics, system flexibility. The use of modern software packages for studying the hydrodynamic characteristics of the flow in hydraulic machines significantly reduces the time and material resources in comparison with physical modeling.

Determination of steam leaks through faulty labyrinth seals of a steam turbine

BACKGROUND: When calculating the axial force acting on the thrust bearing of a steam turbine, when calculating the dimensions of the unloading piston, when calculating the efficiency of the turbine stages, it is necessary to determine the amount of steam leakage through its diaphragm and end seals. Existing methods make it possible to calculate leaks only through serviceable, undamaged labyrinth seals of several standard designs. However, during the operation of steam turbines, due to off-design axial and radial displacements of the rotor, labyrinth seals are often damaged - deformed, crushed or broken.
 AIMS: The purpose of the study is to develop a method for calculating leaks using direct CFD modeling in serviceable and damaged seals with typical failures, verify the simulation results by comparison with known methods and experimental data, and determine critical steam leaks through faulty labyrinth turbine seals.
 MATERIALS AND METHODS: A method for calculating the steam flow rate through serviceable and faulty labyrinth seals of a steam turbine using the capabilities of modern CFD methods is proposed and verified. The computational domain of modeling the front-end seal of the turbine, the features of setting the boundary conditions, the adaptive computational grid, and the numerical mathematical model used are described.
 RESULTS: The results of a numerical study of steam leakage through serviceable and damaged labyrinth end seals of the turbine are presented: with bent ridges in the seals, with partial or complete absence of ridges. The operation of the front-end seal of the turbine was simulated with several typical failures. It is shown that partial damage to the ridges of the front-end seal of the turbine, which is often encountered during operation, leads to a significant increase in steam leakage. It has been established that with significant damage to the ridges, an increase in steam leakage can lead to the exhaustion of the capacity of the steam pressure regulator in the seal, which leads to a malfunction of the turbine thrust bearing unloading system.
 CONCLUSIONS: The proposed technique and the results obtained can be used to calculate steam leakage through serviceable and faulty diaphragm and end labyrinth seals of turbines, when calculating the value of the axial force acting on the turbine thrust bearing in variable operating modes, the technique is useful in assessing the efficiency of the thrust bearing unloading system.

Aerodynamic Analysis of a Supersonic Transport Aircraft at Low and High Speed Flow Conditions

The recent improvement of technology readiness level in aeronautics and the renewed demand for faster transportation are driving the rebirth of supersonic flight for commercial aviation. However, the design of a future supersonic aircraft is still very challenging due to the complexity of several problems, such as static stability performance during the acceleration phase from subsonic speeds to supersonic speeds. Additionally, the interest of scientific community in open source numerical platform as a valid tool for a reliable and affordable aerodynamic design is considerably growing. In this framework, the present work addresses the aerodynamic performance of a Concorde-like aeroshape developed within the preliminary design of a high-speed civil transportation aircraft. Several flight conditions, ranging from subsonic to supersonic speeds, were investigated in detail by using Computational Fluid Dynamics. The aerodynamic force and moment coefficients are computed with fully three-dimensional and steady state Reynolds Average Navier-Stokes simulations, carried out in turbulent flow conditions. The effect of the Mach number variation on the shift of the aircraft aerodynamic center is detailed, by focusing on the aircraft pitching static stability. Flowfield numerical simulations are performed with both commercial (Ansys-Fluent) tool and open-source (SU2) code, which is also used extensively in multidisciplinary design procedures, for further comparisons. Particular attention is focused on the shift of the aeroshape aerodynamic center to verify that the provided wing design allows the aircraft static margin to be within 5% of the reference length, both at low-speed and high-speed flight conditions. The computed positions of the aerodynamic center are in agreement with the aeroshape surface pressure distributions and confirmed the literature results available for the Concorde aircraft. Therefore, in the view of future simulation campaigns for supersonic transportation aircraft, the present work aims to bridge the gap between previous aerodynamic design experiences, for instance matured on Concorde, and those carried out with modern CFD tools on full-scale aircraft, and on time-scales compatible with conceptual design practice. Finally, as the difference between the computed aerodynamic coefficients reflected mainly on drag computation performed with SU2, a special focus on numerical diffusion effect of the solver is also given and compared with a commercial certified CFD tool. This adds a unique further contribution to the SU2 community for aeronautics application.

NUMERICAL SIMULATION OF FLOW CHARACTERISTICS OF WATER-JET ASSEMBLY UNDER REVERSE STATE

Although the water-jet assembly has a lot of shipping applications after its development and has a good reputation, it is not known whether this creation 40 years ago can withstand the verification of modern CFD technology. Through the CFD calculation of water-jet assembly under forward state and reverse state, the internal flow of water-jet assembly during forward state and reverse state is studied. The results show that the flow characteristics of the water-jet assembly in the reverse state are different from the original design intention. There are reverse jets on both sides of the gap between the nozzle and the built-up rudder, and the non-twisted blades of the guide vane cannot completely eliminate the circumferential velocity of the fluid after the work of the impeller. Therefore, the built-up rudder and guide vane of waterjet assembly may need to be further optimized.

High-resolution ILW outflow boundary treatment for the Navier–Stokes equations

WENO schemes have high-resolution and great capability of capturing unsteady and non-linear phenomena. These phenomena occur in elaborated flows, such as cascade and external aerodynamic flows. Thus, high-resolution methods play an important role in modern CFD. Easiness and effectiveness are reached when implementing these methods in rectangular meshes. However, one has the non-boundary-conforming issue. To overcome it the ILW procedure can be employed. We showed that the ILW has a similar behavior to the far-field boundary treatment. Then, we proposed and tested a new ILW outflow boundary treatment and an WENO-type extrapolation. We employed the finite difference method with the positivity-preserving Lax–Friedrichs splitting, high-resolution viscous terms discretization, multi-resolution WENO scheme, and third-order strong stability preserving Runge–Kutta time discretization. We tested the proposed methods in a smooth test case, supersonic flow past a cylinder, NACA 0012 nitrogen flows with 0 and 12° angle of attack, and a NACA 9520 cascade airflow. The proposed methods have high order and resolution, allowed domain size reduction, and captured unsteady and non-linear phenomena. Although preliminary, the NACA 0012 and 9520 flows showed the methods are promising. Common issues with other methods are also reported and further improvements are needed. For example, local mesh refinement, high-order wall boundary treatment, and turbulence modeling.

Inka Hydraulic Engineering at the Tipon Royal Compound (Peru)

The Inka site of Tipon had many unique hydraulic engineering features that have modern hydraulic theory counterparts. For example, the Tipon channel system providing water to the Principal Fountain had a channel contraction inducing critical flow as determined by CFD analysis- this feature designed to induce flow stability and preserve the aesthetic display of the downstream Waterfall. The Main Aqueduct channel sourced by the Pukara River had a given flow rate to limit channel overbank spillage induced by a hydraulic jump at the steep-mild slope transition channel location as determined by use of modern CFD methods- this flow rate corresponds to the duplication of the actual flow rate used in the modern restoration using flow blockage plates placed in the channel to limit over-bank spillage. Additional hydraulic features governing the water supply to agricultural terraces for specialty crops constitute further sophisticated water management control systems discussed in detail in the text.

CFD Design of Waste Heat Boiler Burner in the Education System of Masters Heat Engineering Specialties

Modern CFD methods for calculating combustion processes make it possible to take into account changes in temperatures, heat loads, rates of coolants, as well as further changes in fuel quality. To develop the skills of CFD design and understanding of combustion processes among future specialists in thermophysical specialties, work was carried out to simulate the burner device of a waste heat boiler. For the study, the design of the gas burner of the waste heat boiler RB-70-4.0-440, which operates as a part of the power unit at the LLC “Rubezhansky Cardboard and Container Plant” in the city of Rubezhnoe, was selected. When constructing a geometric model, the hydraulic resistance to the flow of the supply and distribution manifolds was taken into account. To simplify the calculations, the problem was carried out in a two-dimensional, axisymmetric formulation. Analyzing the computational combustion models, the Non-Premixe Combustion model was chosen, which made it possible to take into account the entry of fuel and oxidizer into the reaction zone by two different flows, as well as turbulent diffusion flame propagation. Six variants of models were investigated: the first three variants with a flame tube with a solid disc with diameters of 32, 48, 56 mm, the next three variants, had a burner with a discontinuous disk 32 mm in diameter at a distance of 6, 16, 32 mm from the flame tube. As a result of the research, the optimal shape of the burner was chosen, which corresponds to model 4, and provides a high-quality combustion process, as evidenced by the high temperature of the torch and the lowest temperature at the disk. The conducted research gives future masters the skills of modeling combustion processes in power equipment.

Development of secondary vortex structures in rotor wakes

The blade tip vortex system is a crucial feature in the wake of helicopter rotors, and its correct prediction represents a major challenge in the numerical simulation of rotor flows. A common phenomenon in modern high-fidelity CFD simulations is the breakdown of the primary vortex system in hover due to secondary vortex braids. Since they are strongly influenced by the numerical settings, the degree to which these secondary vortex structures actually physically occur is still discussed and needs experimental validation. In the current work, the development of secondary vortex structures in the wake of a two-bladed rotor in hover conditions was investigated by combining stereoscopic particle image velocimetry measurements in different measurement planes and high-fidelity simulations. Secondary vortex structures were detected and quantified at different axial locations in the wake by applying an identical scheme to the measured and simulated velocity data. In agreement, it was found that the number of secondary vortices is maximum at a distance of 0.8,R below the rotor. The more intense secondary vortex structures were quantitatively well captured in the simulation, whereas in the experiment a larger number of weaker vortices were detected. No distinct preferential direction of rotation was found for the secondary vortices, but they tended to develop in vortex pairs with alternating sense of rotation. A clustered occurrence of secondary vortices was observed close to the primary tip vortices, where the rolled-up blade shear layer breaks down into coherent vortex structures.Graphical abstract

Comparative design and CFD analysis of 3-D printed acrylonitrile butadiene styrene nozzle aerator for discharge reduction

The flow nozzle aerator, which is the part of the water tap made up of acrylonitrile butadiene styrene, can be modified entirely with a new design. The curved and cone-shaped slopes are used to improve the smooth flow at uniform velocity. Simultaneously, discharge is optimum by modified interior design. The smooth laminar delivery of water with optimum pressure, the liquid element at the aerator end becomes smooth. The assembled nozzle aerator solid model has been generated before experimentation. This can promote through the prediction by the modern tool ANSIFLUENT and CFD for finding the flow behavior and its outlet characters. The solid model can be fabricated to prototype for accurate dimensions by using 3-D printing technology. Comparing fluid motion with the time consumption of filling water has been done over these different kinds of aerator and nozzle models, which are fabricated by 3-D printing.

Моделирование системы распределения воздуха пассажирского самолета

The paper considers an air distribution system for a pressurized cabin in a civil aircraft. We propose a method for simulating this system that involves developing a one-dimensional model based on hydraulic computation results for the three-dimensional geometry of the distribution system structure. This approach makes it possible to recompute air distribution in large branching systems relatively quickly and with sufficient accuracy when making changes to their design. The advantages of this method include lower computing costs and lower labour intensity compared to using a modern CFD software package to simulate a full three-dimensional flow in the distribution system structure. The paper describes the operation of the air distribution system prototype and presents the schematic diagram and the functional flow block diagram of the system. We show how to use three-dimensional simulation to determine the discharge characteristics of system pipelines. We present the numerical model constructed for the system and display the capabilities of this model in static and dynamic simulation modes. We analysed the simulation results and confirmed the possibility of using the method proposed for engineering computations. We validated the numerical model using experimental investigation results obtained by means of an empirical test bench simulating the air distribution system prototype.

Optimization of Return Channels of High Flow Rate Centrifugal Compressor Stages Using CFD Methods

Calculations performed with modern CFD programs aid in optimizing flow paths of centrifugal compressors. Characteristics of stator elements of flow paths, calculated via CFD methods, are considered quite accurate. We present optimized return channels (RCh) of three model industrial compressor stages with vaneless diffusers. A parameterized model was created for optimization. The MOGA (Multi-Objective Genetic Algorithm) optimization method was applied in the Direct Optimization program of the ANSYS (Analysis System) software package. Optimization objects were return channels of the stages with high flow rate 0.15. The stages have three different loading factors 0.45, 0.60, 0.70. The optimization goal was to achieve the minimum loss coefficient at the design point. During the optimization process, we varied the following: the number of vanes, the inlet angle of the vanes, the height of the vanes at the inlet, the outer and inner radii of curvature of the U-bend. The outlet angle of the vanes was selected to minimize outlet circumferential velocity. In comparison with preliminary design, the optimized RCh are more efficient across the entire range of flow rates. The optimization reduced the loss coefficient by 20% at the design flow rate.

Discontinuous Galerkin solution of the RANS and [formula omitted] equations for natural and bypass transition

Transition modelling represents a key ingredient for improving the performance predictions of many industrial applications. Among transition models, local formulations seem to guarantee better robustness, accuracy and easiness of implementation in modern CFD solvers. These models have been proposed in the finite volume context to predict the laminar-turbulent transition, but only few attempts have been made in the high-order framework. In this paper a new phenomenological transition model based on the concept of laminar kinetic energy has been proposed and implemented in a high-order accurate Discontinuous Galerkin code, named MIGALE. The transition model is validated and assessed by computing the transitional flow around flat plates with zero/adverse pressure gradients and through different turbine nozzles (T106A and LS89) for different values of Reynolds number and turbulence intensity. The computed results have been compared with experimental data and reference numerical solutions.

Stator elements optimization of centrifugal compressor intermediate type stage by CFD methods

Optimal gas-dynamic design is a complex and time-consuming process. Modern CFD methods help in solving optimization problems and reliably calculating characteristics of stator elements of centrifugal compressor stages. To carry out such calculations, it is necessary to create a parametrized model, which facilitates automation of the process of changing the flow path geometry, rebuilding its dimensions and the computational grid. Using the Direct Optimization program of the ANSYS software package, we have optimized the flow path of the stator elements of a centrifugal compressor intermediate type stage consisting of a vaneless diffuser and a return channel. In this paper, the MOGA (Multi-Objective Genetic Algorithm) optimization method was used. The object of the study was stator elements of one of the model stages designed by the Problem Laboratory of Compressor Engineering, SPbPU. The goal was to achieve the minimum value of the loss coefficient of stator elements when changing 5 geometric parameters: the number of vanes, the inlet vane angle, the height of the vane at the inlet to the return channel vane cascade, the radius of curvature of the leading edge and the thickness of the vane profile. For the best variants based on the results of optimization, the characteristics of the loss coefficient depending on the flow rate coefficient were calculated, their characteristics were compared with the initial variant of the stator elements. The best variant in the design mode has a loss coefficient 4.4% lower than the reference model. With a flow rate coefficient of 1.63 times greater than the calculated one, the optimized variant’s loss coefficient is 33% less.

СТАБІЛІЗАЦІЯ ТЕМПЕРАТУРНОГО РЕЖИМУ ПРИМІЩЕННЯ ЛАБОРАТОРІЇ ВИСОКОПРОДУКТИВНИХ ОБЧИСЛЕНЬ ДОННТУ

Purpose: to analyze the thermal load of the server cluster of the high-performance computing laboratory, stabilize the temperature regime of the room throughout the year, taking into account seasonal changes in the ambient temperature. Methodology: performing experimental studies using modern devices and analyzing experimental data to assess the heat load of server clusters at various degrees of their workload with calculations. Results: a thermal analysis of the work of the server cluster of a high-performance computing laboratory was carried out at various degrees of server load with calculations. Temperature measurements were carried out for various zones and elements of server racks, temperature changes were established when the servers were loaded with calculations. In the modern CFD system, the air flows of the server cooling system and the temperature regime of the server cluster and the server room are simulated, taking into account the location of the equipment and information about the measurements of temperatures and air flow rates. As a result, temperature distributions were obtained for different server loads. A comparison of the supply and exhaust ventilation options was carried out; equipment was selected to ensure the effectiveness of temperature stabilization in the server room at the required level. ISSN 2308-5312 Наукові праці ДонНТУ. Серія: «Машинобудування і машинознавство». No1(14)’2020 20 Scientific novelty: the assessment of heat dissipation during the operation of the server cluster was carried out, the temperature regime of the operation of the server room objects was simulated and the temperature distributions were obtained for different server loads. Practical significance: the design of the room with the location of equipment for supply and exhaust ventilation is proposed; the main parameters of equipment for effective ventilation to reduce the heat load of server racks are determined, recommendations for the use of heat laboratories of high-performance computing for partial heating of the educational building of the state higher educational institution

An augmented reality platform for interactive aerodynamic design and analysis

SummaryWhile modern CFD tools are able to provide the user with reliable and accurate simulations, there is a strong need for interactive design and analysis tools. State‐of‐the‐art CFD software employs massive resources in terms of CPU time, user interaction, and also GPU time for rendering and analysis. In this work, we develop an innovative tool able to provide a seamless bridge between artistic design and engineering analysis. This platform has three main ingredients: computer vision to avoid long user interaction at the pre‐processing stage, machine learning to avoid costly CFD simulations, and augmented reality for an agile and interactive post‐processing of the results.

CFD Simulation of Oil Jets for Piston Cooling Applications Comparing the Level Set and the Volume of Fluid Method

<div class="section abstract"><div class="htmlview paragraph">A new CFD simulation model and methodology for oil jet piston cooling has been developed using the modern level set approach. In contrast to the widely used volume of fluid (VOF) method, the level set approach explicitly tracks the interface surface between oil and air, using an additional field equation. The method has been extensively tested on two- and three-dimensional examples using results from literature for comparison. Furthermore, several applications of oil jet piston cooling on Ford engines have been investigated and demonstrated. For example, three-dimensional simulations of piston cooling nozzle jets on a diesel engine have been calculated and compared to test-rig measurements. Laminar jets, as well as jets with droplets and fully atomized jets, have been simulated using realistic material properties, surface tension, and gravity. Simulations of cooling jets on the undercrown of a gasoline piston and on a moving piston with a cooling gallery have been investigated and compared to test-rig measurements. Results of a VOF model with the CFD software STAR-CCM+ used in the Ford CAE workflow have been compared to the new level set method. Despite using different computational approaches (level set versus volume of fluid method), the results are similar for laminar jets. Differences occur for semi-turbulent and atomized oil jets with many droplets, which need highly resolved meshes. All in all, the modern CFD tools are a powerful way for investigating active cooling strategies for pistons in order to improve the efficiency of internal combustion engines and to reduce emissions.</div></div>

Implicit boundary equations for conservative Navier–Stokes equations

Co-existence of the physical and numerical boundary conditions makes implicit boundary treatment a particularly difficult problem in modern CFD simulations. Previous studies adopted space–time extrapolation or specially designed partial differential equations on the boundaries that are different from those of interior points. They are often formulated in terms of primitive variables, and are very challenging for complicated boundary types to be converted to, and implemented in, the conservative variables that are preferred in numerical simulations. More importantly, different boundary equations or different extrapolation techniques may compromise the stability, accuracy, or convergence rate of the A-stable schemes that are developed for interior points. A new methodology for implicit boundary treatment is proposed in this study. By introducing a simple correction matrix T, a set of generalized equations ∂Q/∂t=(I+T)R are developed in terms of conservative variables. It is applicable for both the interior domain (T=0) and the boundaries (T≠0). It is in a partial differential equation form, satisfies the boundary conditions accurately, independent of the time and spatial discretizations. Any one-sided schemes can be used on the boundaries but still maintain the upwind property. Implicit solution techniques are made significantly easy to implement using, for example, the data-parallel lower–upper relation method and the Newton method (combined with the GMRES method for subsidiary iterations). Numerical experiments show that the proposed methodology produces stable simulations for very large CFL numbers and preserve the imposed boundary values accurately.