CFD Modelling of Multiphase Flows: An Overview

The Computational Fluid Dynamics (CFD) is now increasingly being used for modeling industrial flows, i.e. flows which are multiphase and turbulent. Numerical modeling of flows where momentum, heat and mass transfer occurs at the interface presents various difficulties due to the wide range of mechanisms and flow scenarios present. This paper attempts to provide a summary of available mathematical models and techniques for two-phase flows. Some comments are also made on the models available in the commercially available codes.

Calculation of Flow Induced Vibration Amplitudes of Radial Sewage Water Pumps

The extreme vibrations of sewage water pumps with single-blade impellers are induced mainly by interaction of the flow in the impeller and the casing. The resulting periodically unsteady forces affect the impeller and produce radial deflections of the pump shaft. These oscillations of the rotor are transferred to the pump casing and attached pipes. They can be recognized as vibrations at the bearing blocks or at the pump casing. The present contribution describes the investigation of the transient flow in a sewage water pump. The three-dimensional, viscous, unsteady flow in a pump with a single blade impeller is determined by numerical simulation. After that the hydrodynamic stimulation forces are calculated from the so known transient flow field. The forces can be classified into pressure and friction forces. The pressure forces usually exceed the friction forces on several orders of magnitude. A separate view on the fluid-wetted impeller surfaces shows that the pressure forces acting on the blade are clearly larger than the forces at the hub and at the shroud. So they are decisive for the vibration amplitudes of single-blade sewage water pumps. By a following dynamic analysis of the pump rotor using a commercial Finite-Element-Method (FEM) the resulting vibration amplitudes are determined for several operating points. With the known pressure field and the calculated vibration amplitudes the vibration behavior of sewage water pumps can be influenced during the design by changing the relevant construction parameters.

Dynamic Fracture Parameters and Calculations for 3-D Cracks

Welded structures such as armor fighting vehicles, shipboard structures or munitions systems are required to sustain intense and rapidly applied dynamic loading due to gun firings, impact of enemy munitions and extreme loading from accident scenarios. Flaws are normally found in various extents in welds depending on quality control of the welding process. It is important to determine critical flaw sizes of three-dimensional cracks in a welded joint under dynamic loading introduced by the above scenarios. Calculation of dynamic fracture parameters of the three-dimensional cracks of various geometrical factors at different locations is important for use the crack growth evaluation, flacture and structural integrity assessment. This paper summarizes the methodologies and results of the dynamic fracture parameter calculations for stationary three-dimensional cracks in cracked structures subjected to both static and dynamic loads.

Basic Study for Destruction of Cryogenic Pressure Vessel By Shock Wave

In recent years, a use for cryogenic fluids as a coolant has been increasing because of the industrial development of low temperature technology. Therefore, the use of the cryogenic pressure vessel becomes general. LNG-tank, LN2, O2-tank at food factory and medical facilities are given for example. The shock tolerance of the various material under cryogenic temperature region that consist cryogenic pressure vessel is necessary with considering about them safety. The fact that material becomes fragile when it becomes a low temperature has already been known. Property of material in the temperature area of −200°C and under aren’t known very much. Therefore, we carried out experiments, which test piece are exposed to shock wave under cryogenic temperature region, and investigated destruction of test piece. The breaking surface of test piece was observed by using of the SEM. Structure of material was observed by an optical microscope.

Mixed Mode Loading of an Interface Crack Subjected to Creep Conditions

Interface cracks are seldom subjected to pure Mode I or pure Mode II conditions. Stationary interface cracks between two distinct, bonded elastic-creep materials subjected to remotely applied mixed mode loading are simulated. The finite element method (FEM) is used to examine crack tip fields and candidate driving force parameters for crack growth. Plane strain conditions are assumed. In most cases a functionally graded transition layer is included between the two materials. Examples of such systems include weld metal (WM) and base metal (BM) interfaces in welded or repaired boiler components subjected to elevated temperatures. Numerical solutions based on the asymptotic fields of the homogeneous and heterogeneous Arcan-type specimens are presented. Creep ductility-based damage models are used to predict the initial crack propagation trajectory. The incorporation of functionally graded transition layer regions affects the evolution of time-dependent stress components in the vicinity of the crack tip. The magnitude and direction of crack tip propagation can then be optimized with respect to interface properties.

The Design of Refinement Criteria for Dynamic Grid Adaptation Applied to Airfoil Flow Modeling

At present a large number of fluid dynamics applications are found in aerospace, civil and automotive engineering, as well in medical related fields. In many applications the flow field is turbulent and the computational modelling of such flows remains a difficult task. To resolve all turbulent flow phenomena for flow problems where turbulence is of key interest is a priori not feasible in a Computational Fluid Dynamics (CFD) investigation with a conventional mesh. The use of a Dynamic Grid Adaptation (DGA) algorithm in a turbulent unsteady flow field is an appealing technique which can reduce the computational costs of a CFD investigation. A refinement of the numerical domain with a DGA algorithm requires reliable criteria for mesh refinement which reflect the complex flow processes. At present not much work has been done to obtain reliable refinement criteria for turbulent unsteady flow. The purpose of the work is to implement a new refinement technique for the boundary layer in the vicinity of the wall. It is aimed to model the flow around an airfoil with a LES turbulence model and a new DGA algorithm. In addition to that several simulations have been carried out for parametric studies. In these studies the incompressible solver in REACFLOW has been used. This Computational Fluid Dynamics (CFD) code REACFLOW was developed in collaboration with the joint Research Centre (JRC) in Italy. The following aims are aspired: • A new mesh refinement criteria method suitable for boundary layers; • To carry out LES simulations to establish the performance of the refinement criteria. The new criteria which are created in this work are for the near wall region. This criteria uses the wall shear stresses for the refinement technique. For the main flow stream the refinement criteria proposed by de With et al [6] will be used.

Computational Fluid Dynamic Modelling of High Aspect Ratio Cross-Sectional Jets I: The Effect of Orifice Shape on Jet Behaviour

High aspect ratio cross-sectional jets (HAR jets) are significant for many industrial applications including offshore hydrocarbon production safety, manufacturing processes, aeronautics and others. Little interest has been paid to such jets as the common belief was that within an acceptable distance from the jet orifice the behaviour emulates that of an axisymmetric jet. Previous experimental and preliminary numerical work [1–4] has shown that this is not necessarily correct. Work has been done to investigate numerically the effect the orifice shape has on the behaviour of the jets. This will be in terms of the curvature of the orifice in comparison to the same aspect ratio with a straight rectangular shape. Simulations have been carried out relating to experimental work [1] as comparison and verification. The spreading of the jet will be assessed as it can have significance in terms of safety, performance and effectiveness. This work enhances previous work [3] and allows an assessment of whether such a curvature in the inlet significantly effects the jet behaviour for two pipe pressures. The choice of turbulence model will also be assessed in terms of the standard two-equation k-ε model and it’s variants the RNG and Realisable models. Later work will investigate the use of Large Eddy Simulation within the context of the geometry used here. This important information will allow for greater understanding for the modelling of such jets in a CFD simulation within a complex industrial problem such as gas dispersion with a hydrocarbon production area. It is realized that the fluid does not emerge as a single velocity from the pipe into the flange and hence to form the inlet for the jet. Therefore the effect of the flow within the pipe and how this effects the emerging jet behaviour is investigated in part II of this paper [5].

The Germano Large Eddy Simulation Model Used for the Simulation of Separated Flow

A computational study of unsteady, separated fluid flow was made using the Large Eddy Simulation (LES). As flow problem the turbulent flow past a circular cylinder at a Reynolds number of Re = 3900 was chosen. The objective of this work was to study the numerical and modelling aspects of the dynamic Germano-LES turbulence model. Before LES can be used for applications of practical relevance, such as the flow around a complete aircraft or automobile, extensive tests must be carried out on simpler configurations to understand the quality of LES. Also, the influence of different grid resolutions was examined. Due to the fact of a low Reynolds number, no-slip boundary conditions were used at solid walls. Two different subgrid scale models were applied. In recent years several simulations were carried out using the Smagorinsky-LES model but there is still a lack of experience using the dynamic Germano-LES model, which takes the local flow parameters into account. Several simulations with different parameters and grid-models were carried out both with the Germano-LES model and the Smagorinsky-LES model. Comparisons were made between these two models as well as with several experimental data taken from literature.

Circular Cylinder Exposed to Cross Flow Fluid Forces – Parameters of Influence – Limits of Numerical Models

In many technical fields, e.g. heat exchangers, circular cylinders are involved in Fluid Structure Interaction (FSI) problems. Therefore correct frequency and magnitude of fluid forces, respectively Strouhal number, drag and lift coefficient are needed. If fluid forces are evaluated with Computational Fluid Dynamics (CFD), mostly flow around a rigid cylinder is used to verify model and numerical methods. Unfortunately experimental as well as numerical results show great variation, making verification and testing of models difficult. Reynolds number is regarded as main influencing parameter for a rigid cylinder in cross flow. Most of experimental deviations can be related to other parameters, which differ from experiment to experiment. In this paper such parameters are specified and it is shown, that a closer look is needed, if one really wants to verify a model. Besides experimental results, which can be found in literature, some parameters are investigated by numerical simulation. Like experiments CFD (Computational Fluid Dynamics) simulations show a huge bandwidth of results, even when the same turbulence model is used. Flow around cylinders separates over a wide range of Reynolds numbers. It will be demonstrated that, using CFD, large deviations in fluid forces can often be related to miscalculation of the point of separation.

Comparison of Fracture Response of Preflawed Tubes Under Internal Static and Detonation Loading

Fracture experiments were performed on thin-walled and preflawed aluminum 6061-T6 tubes. Flaws were machined as external axial surface notches. The tubes were 1) statically loaded with oil, 2) statically loaded with nitrogen, and 3) dynamically loaded with gaseous detonations traveling at 2.4 km/s. The experiments were controlled so that comparisons could be made on sets of specimens with the same material, tube and flaw geometry, nominal loading amplitude, and flange supports, with the only difference being the dynamics of the loading. It was found that there is a significant difference in crack propagation behavior for the three types of loading. In this paper, fracture behavior will be discussed along with the fluid dynamics involved. The tubes were also instrumented with pressure transducers, crack detection gages, and strain gages so that data on loading, crack propagating speeds, and strain history can be compared.