A DIRECT NUMERICAL SIMULATION OF CHANNEL FLOW TRANSITION UNDER AN EXTERNAL BODY FORCE

This research seeks to increase our understanding on the laminar-turbulent transition under an external body force. Direct numerical simulation by the spectral method with a weak formulation is used to solve the transient 3-D Navier-Stokes equations. Initial disturbances consist of the finite-amplitude 2-D Tollmien-Schlichting wave and two 3-D oblique waves. Competitions among different modes were computed during transition for different Richardson numbers.

It is found that the body force can modify the transition mechanism of flows between two vertical plates. The body force was found to hasten the formation of three-dimensional flow. A non-laminar flow induced by the body force may present when the background flow is still laminar.     

Assessment of assembly shape and geometric effects on amplification of shear waves using discrete element method

The discrete element method (DEM) is a capable tool used to simulate shear wave propagation in granular assemblies for many years. Researchers have studied assembly shapes such as rectangles (in 2D simulations) or cylinders and cubes (in 3D simulations). This paper aimed to qualify the effect of assembly shape on the shear wave propagation and maximum amplification in the vertical plane (horizontal and vertical directions) caused by this propagation. To this end, shear wave propagations in different assembly shapes such as rectangle, trapezium, and triangle with rigid boundary conditions were simulated. A sine wave pulse was applied with a point source by moving a particle as the transmitter particle. To evaluate the shear wave velocity of the assemblies, the transmitter and receiver particles were simulated. All the simulations were performed with 2D DEM which is a useful tool to determine the amount and location of the maximum amplification factor of the assembly in both horizontal and vertical directions. An advantage of this study was assessing the effect of parameters such as input wave frequency, assembly height, shape, and aspect ratios on the amplification of the input waves.     

Three-dimensional numerical simulations of cross-flow around four cylinders in an in-line square configuration

This paper presents a numerical study of three-dimensional (3-D) laminar flow around four circular cylinders in an in-line square configuration. The investigation focuses on effects of spacing ratio (L/D) and aspect ratio (H/D) on 3-D flow characteristics, and the force and pressure coefficients of the cylinders. Extensive 3-D numerical simulations were performed at Reynolds number of 200 for L/D from 1.6 to 5.0 at H/D=16 and H/D from 6 to 20 at L/D=3.5. The results show that the 3-D numerical simulations have remedied the inadequacy of 2-D simulations and the results are in excellent agreement with the experimental results. The relation between 3-D flow patterns and pressure characteristics around the four cylinders is examined and discussed. The critical spacing ratio for flow pattern transformation was found to be L/D=3.5 for H/D=16, while a bistable wake pattern was observed at L/D=1.6 for the same aspect ratio. Moreover, a transformation of flow pattern from a stable shielding flow pattern to a vortex shedding flow pattern near the middle spanwise positions of the cylinders was observed and was found to be dependent on the aspect ratio, spacing ratio, and end wall conditions. Due to the highly 3-D nature of the flows, different flow patterns coexist over different spanwise positions of the cylinders even for the same aspect ratio. It is concluded that spacing ratio, aspect ratio, and the no-slip end wall condition have important combined effects on free shear layer development of the cylinders and hence have significant effects on the pressure field and force characteristics of the four cylinders with different spacing ratios and aspect ratios.     

Computational study of shock wave control by pulse energy deposition

We have developed a computational code based on the axisymmetric Navier–Stokes equations with thermochemical kinetics for assessing wave drag reduction and other effects in pulse-energy deposition ahead of a bow shock by means of full simulations from generation of a laser-induced blast wave to interaction with the bow shock. Thermochemical nonequilibrium computations can reproduce the process of blast wave formation with laser ray tracing, and the computed low-density core inside the blast wave has a teardrop-like shape, depending on the laser input condition. The flowfield interacting with a bow shock formed in Mach 5 flow was computed. The result suggests that the shape of the low-density core affects the resultant wave drag, and parameters of an incident laser beam should be taken into account in exploring the optimal condition of the proposed wave-drag scheme.     

Dependence of the force action of a water wave on the shape of its leading front

The results of experiments in which bore-type waves with various leading front shapes propagated first above an even horizontal bottom, then above a rising bottom, then again above a horizontal bottom segment, and were finally reflected from a vertical wall are presented. It is shown that the impact force is greatest if the leading front breaks directly on approaching the wall. This force is much greater than the impact force of smooth waves or waves that break before they reach the wall.     

Comparison of results from DNS of bubbly flows with a two-fluid model for two-dimensional laminar flows

Results from direct numerical simulations of laminar bubbly flow in a vertical channel are compared with predictions of a two-fluid model for steady-state flow. The simulations are done assuming a two-dimensional system and the model coefficients are adjusted slightly to match the data for upflow. The model is then tested by comparisons with different values of flow rate and gravity, as well as downflow. In all cases the results agree reasonably well, even though the simulated void fraction is considerably higher than what is assumed in the derivation of the model. The results do, however, suggest a need to understand the lift and the wall repulsion force on bubbles better, particularly in dense flows.     

Steam condensing flow modeling in turbine channels

The numerical method for modeling of the transonic steam flows with homogeneous and/or heterogeneous condensation has been presented. The experiments carried out for the Laval nozzles, for 2-D turbine cascades and for a 3-D flow in real turbine were selected to validate an in-house CFD code adjusted to the calculations of the steam condensing flows in complicated geometries. The sensitivity of the condensation model and difficulties in the validation process of the CFD code have been discussed. These difficulties limit the possibilities of verification and improvement of the condensation theory based on the existing experimental data.     

Re-entry body drag: shock tunnel experiments and computational fluid dynamics calculations compared

The best approach for conducting the research necessary for developing hypersonic flight vehicles is a close coupling between experiments that employ rapid measurement techniques and computational fluid dynamics (CFD) that appropriately accounts for the freestream nonuniformities, as well as for hypervelocity flow phenomena. This approach has been employed here, where stress wave force measurements and CFD calculations have been combined in an investigation of the axial drag on a generic re-entry body. Experiments were performed in argon and nitrogen, with test flows ranging in total enthalpy between 3 MJ/kg and 12 MJ/kg and Mach numbers varying from 6 to 13. The associated measured drag forces ranged from 300 to 360 N. For Mach 12 argon flows, the CFD overpredicted the drag by 8%, while for two hypervelocity nitrogen flows the CFD overpredicted the drag by at most 5%. Considering uncertainties in the force measurements and the CFD boundary conditions, the agreement is good, and the work highlights both the ability of the force measurement technique to respond to rapid changes in flow conditions and the importance of carefully accounting for flow gradients in the CFD boundary conditions. This paper was based on work that was presented at the 3rd International Symposium on Interdisciplinary Shock Wave Research, Canberra, Australia, March 1–3, 2006.     

Computational fluid dynamics applied to internal gas-turbine blade cooling: a review

This paper reviews current capabilities for predicting flow in the cooling passages and cavities of jet engines. Partly because of the need to enhance heat transfer coefficients, these flow domains entail complicated passage shapes where the flow is turbulent, strongly three-dimensional (3-D) and where flow separation and impingement, complicated by strong effects of rotation, pose severe challenges for the modeler. This flow complexity means that more elaborate models of turbulent transport are needed than in other areas of turbine flow analysis. The paper attempts to show that progress is being made, particularly in respect to the flow in serpentine blade-cooling passages. The first essential in modeling such flows is to adopt a low Reynolds number model for the sublayer region. The usual industrial practice of using wall functions cannot give a better than qualitative account of effects of rotation and curvature. It is shown that Rayleigh number effects can modify heat transfer coefficients in the cooling passages by at least 50%. The use of second-moment closure in the modeling is shown to be bringing marked improvements in the quality of predictions. Areas where, at present, more computational fluid dynamics (CFD) applications are encouraged are impingement cooling and pin-fin studies.     

Developed cavitation behind a disk in a vertical tube

The theoretical and experimental investigation of developed cavities in a vertical flow shows [1–3] that the size and shape of the cavities depend in this case to a greater extent on the Froude number than for horizontal flow. Experiments in a longitudinal gravity field are usually made under conditions of a confined flow, which requires analysis of the influence of this circumstance on their results. Such an analysis has been made for horizontal flow confined by walls in both an approximate [4–6] and an exact [7–10] formulation; the problems have been solved for both two- and three-dimensional flows. In the present paper we solve in a nonlinear formulation the axisymmetric cavitation problem of determining the influence of the restriction of flow in a vertical circular tube on a developed cavity. The obtained results describe well the cavities realized under experimental conditions even when there is an appreciable deviation of the shape of the tube section from a circle.     

Multi-dimensional simulation of thermal non-equilibrium channel flow

The Homogenous Relaxation Model (HRM) is used to study thermal non-equilibrium, two-phase flows with flash-boiling and condensation. Typically, such non-equilibrium phase-change models have been studied in one-dimensional flow, but the goal of the present work is to create and utilize a multi-dimensional CFD implementation. The simulations are able to handle general polyhedral meshes, an important convenience for irregular channel or nozzle shapes. The model is applied to flash-boiling flow in short channels and validated against experimental measurements. The simulations predict the multi-dimensional features that have been observed in the past in experiments. Nozzle choking is also observed in the calculations.     

Heterogeneous nucleation in CFD simulation of flashing flows in converging–diverging nozzles

Flashing flow is an important phenomenon in many industrial contexts; however simulation of these flows remains difficult. CFD simulations are able to describe the distribution and evolution of 3D structures in the flow but are dependent on good closure relations for interphase transfer. Nucleation during flashing flow is often neglected in CFD simulation where a minimum starting vapour fraction and a constant bubble number density are given. Models that include nucleation have used wall nucleation terms from 1D system code models, averaged over the domain. In this work, three models for wall nucleation are tested and compared with experimental data from a converging–diverging nozzle. Nucleation is applied at the walls of the domain, and various models are investigated. Good agreement with the critical flow rate and axial profiles are found, but agreement with the radial void fraction data is not satisfactory. Methods of addressing this are explored, and it is found that including a small bulk heterogeneous nucleation term gives the best agreement with the radial profiles, with negligible impact on the axial average properties.     

防风网透流风空气动力学特性大涡数值模拟研究

基于有限体积法建立不可压缩粘性流体运动的大涡模拟模型,采用Smagorinsky-Lilly亚格子模型,并引入浸入边界法(IBM)实现无滑移固壁边界条件,对雷诺数30~30000之间防风网透流风进行模拟研究。基于模拟结果,提出蝶型防风网透流风存在4个典型分区结构,流场中存在由蝶型形态引起的大尺度分层剪切流动,加强流体动能耗散。透流风在雷诺数300时发生层流至湍流的转捩,而在雷诺数增长至3000以上时,湍流充分发展,纵向流速脉动强度可达70%。防风网整体空气阻力远大于单个孔口射流阻力的线性叠加,射流间的相互作用以及大尺度的分层剪切结构大大增加流体阻力损失,这为通过优化孔口布置和网板形态来节省材料提供了科学依据。     

Investigations of shock wave interaction with nanosecond surface discharge

Non-stationary plane shock wave interaction with localized nanosecond-lasting surface discharges was investigated. Pulse discharge plasma glow evolution was recorded with a CCD camera and gas flow evolution was recorded with laser shadowgraphy. CFD simulations of pulse energy deposition near the horizontal wall surface in front of the shock wave were carried out. Non-stationary 2D symmetrical flow dynamics were studied and analysis of the instantaneous surface discharge energy rate through CFD and shadow images matching was carried out.     

Direct simulation of MHD flows in dual-coolant liquid metal fusion blanket using a consistent and conservative scheme

Direct simulation of 3-D MHD (magnetohydrodynamics) flows in liquid metal fusion blanket with flow channel insert (FCI) has been conducted. Two kinds of pressure equilibrium slot (PES) in FCI, which are used to balance the pressure difference between the inside and outside of FCI, are considered with a slot in Hartmann wall or a slot in side wall, respectively. The velocity and pressure distribution of FCI made of SiC/SiC_f are numerically studied to illustrate the 3-D MHD flow effects, which clearly show that the flows in fusion blanket with FCI are typical three-dimensional issues and the assumption of 2-D fully developed flows is not the real physical problem of the MHD flows in dual-coolant liquid metal fusion blanket. The optimum opening location of PES has been analyzed based on the 3-D pressure and velocity distributions.     

Three-dimensional hydrodynamic modelling of tidal flows interacting with aquaculture fish cages

Three-dimensional, non-linear, non-hydrostatic simulations of rotating tidal flows interacting with aquaculture cages (represented as drag elements) at the geophysical scale are performed using an adaptive, finite volume fluid code “Gerris”. Exploiting the Gerris grid structure, sub-metre scale resolution can be obtained even for the farm scale experiments, enabling examination of the impact of the cage on the imposed tidal flows. Passive tracers are used to try to quantify these cage impacts, representing either feed or faecal matter (with specified fall speeds), or other biogeochemical markers such as dissolved oxygen. Using a relatively simple drag formulation, we show that the model is able to reproduce laboratory observations. The farm scale simulations can also be “tuned” in a similar fashion, for example by comparison with observations of total drag force on such structures, or with field measurements of flow retardation by cages. Single and multi-scale cage experiments are then examined to explore the potential impacts of perturbed horizontal and vertical flows on material redistribution through and within the cages. Even with the relatively smooth forcing and drag formulation the experiments reveal a surprising level of complexity in terms of the perturbed flows and their impact on transporting and diffusing passive material.     

Mechanical action of a traveling hydraulic jump on a vertical cylinder

The technique and the results of an experimental investigation of the longitudinal force component and the vertical coordinate of the point of its application when bore-type waves impinge on a vertical cylinder are presented. The bore was generated by removing a vertical gate providing a difference of free surface levels in a channel with a smooth horizontal bottom. It was found that the presence of a free surface in the incident flow has a considerable effect on the force and its moment about the channel bottom.     

Near-wall imaging using toluene-based planar laser-induced fluorescence in shock tube flow

A quantitative thermometry technique, based on planar laser-induced fluorescence (PLIF), was applied to image temperature fields immediately next to walls in shock tube flows. Two types of near-wall flows were considered: the side wall thermal boundary layer behind an incident shock wave, and the end wall thermal layer behind a reflected shock wave. These thin layers are imaged with high spatial resolution (15μm/pixel) in conjunction with fused silica walls and near-UV bandpass filters to accurately measure fluorescence signal levels with minimal interferences from scatter and reflection at the wall surface. Nitrogen, hydrogen or argon gas were premixed with 1–12% toluene, the LIF tracer, and tested under various shock flow conditions. The measured pressures and temperatures ranged between 0.01 and 0.8 bar and 293 and 600 K, respectively. Temperature field measurements were found to be in good agreement with theoretical values calculated using 2-D laminar boundary layer and 1-D heat diffusion equations, respectively. In addition, PLIF images were taken at various time delays behind incident and reflected shock waves to observe the development of the side wall and end wall layers, respectively. The demonstrated diagnostic strategy can be used to accurately measure temperature to about 60 μm from the wall.     

Effects of a transverse flows with variable viscosity in micropolar fluids

A regular perturbation analysis is presented for three laminar natural convection flows in micropolar fluids in liquids with temperature dependent viscosity: a freely-rising plane plume, the flow above a horizontal line source on an adiabatic surface (a plane wall plume) and the flow adjacent to a vertical uniform flux surface. While these flows have well-known power-low similarity solutions when the fluid viscosity is taken to be constant, they are non-similar when the viscosity is considered to a function of temperature. A single similar flow, that adjacent to a vertical isothermal surface, is also analysed for comparison in order to estimate the extent of validity of perturbation analysis. The formulation used here provides a unified treatment of variable viscosity effects on those four flows. Computed first-order perturbation quantities are presented for all four flows. Numerical results for velocity, angular velocity and thermal functions has been shown graphically or tabulated for different values of micropolar parameters. Received on 20 October 1997     

Numerical investigation of drag and heat reduction in hypersonic spiked blunt bodies

In this investigation, the effects of spike as retractable drag and aerodynamic heating reduction into the reentry Earth’s atmosphere for hemispherical body flying at hypersonic flow have been numerically studied. This numerical solution has been carried out for different length, shapes and nose configuration of spike. Additional modifications to the tip of the spike are investigated in order to obtain different bow shocks, including no spike, conical, flat and hemispherical aerodisk mounted. Unsteady compressible 3-D Navier–Stokes equations are solved with k ? ω (SST) turbulence model for a flow over a forward facing spike attached to a heat shield for a free stream Mach number of 6. The obtained numerical results are compared with the experimental ones, and the results shows acceptable verification. This analysis shows that the aerodisk is more effective than aerospike. The designs produced 60 and 15 % reduction in drag and wall temperature responses, respectively.