Changes In Flow Field Research Articles

Thermal behavior of an isolator with mode transition inducing back-pressure of a dual-mode scramjet

Combustion mode transition is a valuable and challenging research area in dual-mode scramjet engines. The thermal behavior of an isolator with mode transition inducing back-pressure is investigated by direct-connect dual-mode scramjet experiments and theoretical analysis. Combustion experiments are conducted under the incoming airflow conditions of total temperature 1270K and Mach 2. A small increment of the fuel equivalence ratio is scheduled to trigger mode transition. Correspondingly, the variation of the coolant flow rate is very small. Based on the measured wall pressures, the heat-transfer model can quantify the thermal state variation of the engine with active cooling. Compared with the combustor, mode transition has a greater effect on the isolator thermal behavior, and it significantly changes the isolator heat-flux and wall temperature. To further study the isolator thermal behavior from flight Mach 4 to Mach 7, a theoretical analysis is carried out. Around the critical point of combustion mode transition, sudden changes of the isolator flowfield and thermal state are discussed.

The past, present, and future viscous heat dissipation available for Greenland subglacial conduit formation

Abstract. Basal hydrology of the Greenland Ice Sheet (GIS) influences its dynamics and mass balance through basal lubrication and ice–bed decoupling or efficient water removal and ice–bed coupling. Variations in subglacial water pressure through the seasonal evolution of the subglacial hydrological system help control ice velocity. Near the ice sheet margin, large basal conduits are melted by the viscous heat dissipation (VHD) from surface runoff routed to the bed. These conduits may lead to efficient drainage systems that lower subglacial water pressure, increase basal effective stress, and reduce ice velocity. In this study we quantify the energy available for VHD historically at present and under future climate scenarios. At present, 345 km3 of annual runoff delivers 66 GW to the base of the ice sheet per year. These values are already ∼ 50 % more than the historical 1960–1999 value of 46 GW. By 2100 under IPCC AR5 RCP8.5 (RCP4.5) scenarios, 1278 (524) km3 of runoff may deliver 310 (110) GW to the ice sheet base. Hence, the ice sheet may experience a 5-to-7-fold increase in VHD in the near future which will enhance opening of subglacial conduits near the margin and will warm basal ice in the interior. The other significant basal heat source is geothermal heat flux (GHF), which has an estimated value of 36 GW within the present-day VHD area. With increasing surface meltwater penetration to the bed the basal heat budget in the active basal hydrology zone of the GIS will be increasingly dominated by VHD and relatively less sensitive to GHF, which may result in spatial changes in the ice flow field and in its seasonal variability.

Computational fluid dynamics study on the drag and flow field differences between the single and coupled bends

Owing to the limited installation space and duct size, coupled fittings are common in the duct systems of buildings. The coupling effect leads to changes in drag and fan energy consumption. This study investigates duct drag and flow field characteristics under coupling conditions. Experiments and numerical simulations with the Reynolds stress model are conducted. Flow field changes, flow field deformation, and drag changes in the duct are analyzed. Regardless of the coupling form, the velocity near the inner arc is fast, whereas that near the outer arc is slow. Under three different coupling connection conditions (S-shaped, L-shaped, and U-shaped), the outlet velocity gradient of the U-shaped coupling connection is the least obvious. After the fluid flows through the bend, a significant centerline velocity reduction can be observed, even greater than that in the bend. The lowest centerline velocity lies within the range of 2.5 D to 4.5 D after the bend. Coupling connection has an insignificant effect on upstream duct resistance. The resistance of single bend is less than that of the downstream bend for the coupled bend and greater than that of the upstream bend under coupling conditions. Practical application: Coupling effect is common in practical application of ventilation engineering. This effect leads to the change of fluid resistance loss of ducts and pipes. However, few researchers focus on this effect. This study finds that regardless of the coupling form, the velocity near the inner arc is fast, whereas that near the outer arc is slow. It means the guide vane should be set near inner arc. L-shaped coupling connection has the largest downstream piping resistance. The resistance of the downstream piping under S-shaped coupling is the least, thus L-shaped coupling connection should be avoided as far as possible in practical application.

A dynamic heat transfer coefficient between fractured rock and flowing fluid

Estimation of heat production remains a major challenge for geothermal industry. In continuum mechanics two main approaches need to be separated to model heat transfer between fluid and rock: local thermal equilibrium (LTE) and local thermal non-equilibrium (LTNE). While LTNE does not require the strong assumption of instantaneous local thermal equilibrium, the parameters for explicit heat transfer between rock and fluid are only loosely defined. This work focuses on the heat transfer coefficient between rock walls and flowing fluid. Based on an experimental setup with simple geometry and a steady state scenario, we derive a dynamic heat transfer coefficient dependent on fracture aperture, flow velocity and thermal parameters. We compare our model to experimental data and achieve a good agreement for most temperatures. In comparison to a static heat transfer coefficient, a dynamic coefficient changes the fluid and rock temperature distribution in the fractured system. We then show possible extensions of our dynamic approach with a simulation on reservoir scale. In opposite to existing models and empiric approaches our model intrinsically adjusts to spatial heterogeneity and temporal changes in flow and temperature field. The model is based on well-defined physical parameters which can be easily obtained from standard laboratory tests and dependent on characteristic variables like velocity and rock temperature. Our model can be extended by including more constitutive relationships linking permeability, fracture aperture, fluid pressure and heat transfer.

Effects of wall conditions on flow field in a circular micro-channel

Effects of a gas-surface interaction model, the Cercignani-Lampise-Lord model, on primary fluid properties in a circular micro-channel are investigated in this paper. The tangential momentum accommodation coefficient (TMAC) has a significant influence on the flow field in the micro-channel. However, the variation of the flow field with the normal momentum accommodation coefficient is not as significant as that with the TMAC. The fundamental mechanism behind the change of flow field is revealed. Moreover, it is found that the effect of the wall roughness on the flow field is much greater than that of the wall material. When the wall roughness varies, the mass flux of the ring section region between the half radius and the radius changes a lot while the rest remains unchanged. However, the heat flux over the cross section remains almost unchanged with the variation of the wall material when the wall is rough.

Passive control of thermoacoustic instabilities in swirl-stabilized combustion at elevated pressures

In this study, a porous insert is placed at the dump plane of a swirl-stabilized lean premixed combustor to passively suppress thermoacoustic instabilities. The diffuser-shaped annular ring of porous inert material influences the turbulent flow field directly, including recirculation zones and vortical and/or shear layer structures to passively control the acoustic performance of the combustor. The porous inert material is made of silicon carbide–hafnium carbide coated, high-strength, high-temperature-resistant open-cell foam materials. In this study, the porous insert concept is investigated at above-ambient operating pressures to demonstrate its suitability for practical combustion applications. Experiments are conducted in quartz and metal combustors, without and with the porous insert while varying operating pressure, equivalence ratio, and reactant flow rate. Measurements show that the porous insert, and consequent changes in the combustor flow field, decrease the sound pressure levels at the frequency of combustion instability at all operating conditions investigated in this study. The porous insert also decreases the broadband combustion noise, i.e. the measured sound pressure levels over a wide frequency range.

Effects of rising angle on upstream blades and intermediate turbine duct

With the improvement of requirement, design and manufacture technology, aero-engines for the future are characterized by further reduction in fuel consumption, cost, but increment in propulsion efficiency, which leads to ultra-high bypass ratio. The intermediate turbine duct (ITD), which connects the high pressure turbine (HPT) with the low pressure turbine (LPT), has a critical impact on the overall performances of such future engines. Therefore, it becomes more and more urgent to master the design technique of aggressive, even super-aggressive ITDs. Over the last years, a lot of research works about the flow mechanism in the diffuser ducts were carried out. Many achievements were reported, but further investigation should be performed. With the aid of numerical method, this paper focuses on the change of performance and flow field of ITD, as well as nearby turbines, brought by rising angle (RA). Eight ITDs with the same area ratio and length, but different RAs ranges from 8 degrees to 45 degrees, are compared. According to the investigation, flow field, especially outlet Ma of swirl blade is influenced by RA under potential effect, which is advisable for designers to modify HPT rotor blades after changing ITD. In addition to that, low velocity area moves towards upstream until the first bend as RA increases, while pressure loss distribution at S2 stream surface shows that hub boundary layer is more sensitive to RA, and casing layer keeps almost constant. On the other hand, the overall total pressure loss could keep nearly equivalent among different RA cases, which implies the importance of optimization.

Oscillatory Characteristics of Shallow Open Cavities in Supersonic Flow

Numerical simulations and experiments were carried out to investigate the unsteady flow in a two-dimensional rectangular open cavity with emphasis on the effects of length-to-depth ratio on the shear-layer development and cavity drag. The cavity was exposed to a supersonic flow of Mach 2. The results of the simulations were compared and analyzed with experimental data. Detailed predictions of flow features and frequencies showed a good agreement with the experiments. Particularly, the results indicated how the cavity flowfield changes when the length-to-depth ratio of the cavity is changed. Below a certain value of length-to-depth ratio, self-sustained periodic oscillations are strong and coherent, and the cavity drag is low. Beyond this value, the oscillations are weak, frequencies are mostly broadband, and the drag is high. This transition was found to occur for a cavity of length-to-depth ratio of 5. The investigation has also explored features of cavity shear-layer development, hitherto unexplained, and these are elucidated.

Processing condition induced structural evolution in the alternating multi-layer structure during high speed thin-wall injection molding

Abstract Polymeric materials with alternating multi-layer structure have gained much attention in the field of biomimic, where many methods were used to prepare materials with such structure for various functionalities. A simple method based on high speed thin-wall injection molding (HSTWIM) has been proposed in our previous studies for the easy fabrication of multi-layer functional polymeric materials. Herein, the effect of various injection processing parameters: injection distance, injection speed, mold temperature and mold thickness on the phase morphology and molecular orientation is studied. The processing condition induced changes in flow field, temperature field, viscosity and shear time are thought to largely influence the confinement of melt during processing, thus, their final phase morphology. It is observed that moderate injection distance, moderate injection speed, moderate mold temperature and thin mold favors the formation of such alternating multi-layer structure. Such study could provide guidelines for the fabrication of functional multi-layered structure through HSTWIM as well as control of phase morphology through confined flow.

Evaluation of the successive approximations method for acoustic streaming numerical simulations.

This work evaluates the successive approximations method commonly used to predict acoustic streaming by comparing it with a direct method. The successive approximations method solves both the acoustic wave propagation and acoustic streaming by solving the first and second order Navier-Stokes equations, ignoring the first order convective effects. This method was applied to acoustic streaming in a 2D domain and the results were compared with results from the direct simulation of the Navier-Stokes equations. The velocity results showed qualitative agreement between both methods, which indicates that the successive approximations method can describe the formation of flows with recirculation. However, a large quantitative deviation was observed between the two methods. Further analysis showed that the successive approximation method solution is sensitive to the initial flow field. The direct method showed that the instantaneous flow field changes significantly due to reflections and wave interference. It was also found that convective effects contribute significantly to the wave propagation pattern. These effects must be taken into account when solving the acoustic streaming problems, since it affects the global flow. By adequately calculating the initial condition for first order step, the acoustic streaming prediction by the successive approximations method can be improved significantly.

Numerical analysis of hemodynamics in spastic middle cerebral arteries

Cerebral vasospasm (CVS) is the most common serious complication of subarachnoid hemorrhage. Among the many factors that are associated with the pathogenesis of CVS, hemodynamics plays an important role in the initiation and development of CVS. Numerical simulation was carried out to obtain the flow patterns and wall shear stress (WSS) distribution in spastic middle cerebral arteries. The blood was assumed to be incompressible, laminar, homogenous, Newtonian, and steady. Our simulations reveal that flow velocity and WSS level increase at the stenosis segment of the spastic vessels, but further downstream of stenosis, the WSS significantly decreases along the inner wall, and flow circulation and stagnation are observed. The hydrodynamic resistance increases with the increase of vessel spasm. Moreover, the change of flow field and hydrodynamic forces are not linearly proportional to the spasm level, and the rapid change of hemodynamic parameters is observed as the spasm is more than 50%. Accordingly, in the view of hemodynamic physiology, vessels with less than 30% stenosis are capable of self-restoration towards normal conditions. However, vessels with more than 50% stenosis may eventually lose their capacity to adapt to differing physiologic conditions due to the extreme non-physilogic hemodynamic environment, and the immediate expansion of the vessel lumen might be needed to minimize serious and non-reversible effects.

Effects of non-equilibrium condensation on deviation angle and efficiency in a steam turbine stage

In this paper, the AUSM-van Leer hybrid scheme is extended to solve the governing equations of two-phase transonic flow in a steam turbine stage. The dominant solver of the computational domain is the non-diffusive AUSM scheme (1993), while a smooth transition from AUSM in regions with large gradients to the diffusive scheme by van Leer (1979) guarantees a robust hybrid scheme throughout the domain. The steam is assumed to obey non-equilibrium thermodynamic model. The effects of condensation on different specifications of the flow field are studied at subsonic/supersonic flow regimes. It is observed that as a result of condensation, the aerothermodymics of the flow field changes. For example in supersonic wet case (Pb = 14.55 kPa), pressure loss coefficient of rotor and total entropy generation are, respectively, 77% and 29% more than those in dry conditions. Also the value of rotor deviation angle reaches 6.27° in wet case and Pb = 14.55 kPa.

Computational study of transient flow around Darrieus type cross flow water turbines

This study presents full transient numerical simulations of a cross-flow vertical-axis marine current turbine (straight-bladed Darrieus type) with particular emphasis on the analysis of hydrodynamic characteristics. Turbine design and performance are studied using a time-accurate Reynolds-averaged Navier–Stokes commercial solver. A physical transient rotor-stator model with a sliding mesh technique is used to capture changes in flow field at a particular time step. A shear stress transport k-ω turbulence model was initially employed to model turbulent features of the flow. Two dimensional simulations are used to parametrically study the influence of selected geometrical parameters of the airfoil (camber, thickness, and symmetry-asymmetry) on the performance prediction (torque and force coefficients) of the turbine. As a result, torque increases with blade thickness-to-chord ratio up to 15% and camber reduces the average load in the turbine shaft. Additionally, the influence of blockage ratio, profile trailing edge geometry, and selected turbulence models on the turbine performance prediction is investigated.

Effects of Straight and Serpentine Flow Field Designs on Temperature Distribution in Proton Exchange Membrane (PEM) Fuel Cell

Proton exchange membrane fuel cells or sometimes called as polymer electrolyte membrane (PEM) fuel cells is a device for energy transformation in a changing process from one form of energy to another form of energy. It became as an alternative especially for future use in stationary and vehicular applications. PEM fuel cells provide high efficiency and power density with null emission, low operating temperature, quickly start and long life. One of the aspects that are crucial in optimizing the PEM fuel cells performance is a flow field geometry. In this paper, a simulation case of PEM fuel cells was simulated to determine effects of a straight and serpentine flow field on temperature distribution in PEM fuel cells. ANSYS Fluent software was used to simulate 3-dimensional models of single PEM fuel cells in order to determine the effects of changes in the geometry flow field on temperature distributions. Results showed that the serpentine flow field design produces a better temperature distribution along the membrane. The simulation result shows a good agreement with the experiment, thus boost a higher confidence in the results to determine the effectiveness of the flow field design in PEM fuel cells.

プラズマ気流制御による水平軸風車周りの流れ場の変化

プラズマ気流制御による水平軸風車周りの流れ場の変化

Considerations for Measuring Compressor Aerodynamic Excitations Including Rotor Wakes and Tip Leakage Flows

The unsteady flow field generated by the rotor provides unsteady aerodynamic excitations to the downstream stator, which can result in vibrations such as forced response. In this paper, measurements of the rotor wake and rotor tip leakage flow from an embedded rotor in a multistage axial compressor are presented. A unique feature of this work is the pitchwise traverse of the flow field used to highlight the changes in the rotor exit flow field with respect to the position of the surrounding vane rows. Results acquired at midspan focus on characterizing an average rotor wake, including the effects on the frequency spectrum, from a forced response perspective. While many analyses use an average rotor wake to characterize the aerodynamic forcing function to the downstream stator, this study explores the factors that influence changes in the rotor wake shape and the resulting impact on the spectrum. Additionally, this paper investigates the flow near the endwall where the tip leakage vortex is an important contributor to the aerodynamic excitations for the downstream vane. For the first time, experimental data are presented at the rotor exit, which show the modulation in size and radial penetration of the tip leakage vortex as the rotor passes through the upstream vane wake. As computational models become more advanced, the ability to incorporate these aerodynamic excitation effects should be considered to provide better predictions for vane vibratory response.

Numerical Study of Flow Field and Aerodynamic Performance of Straight Bladed VAWT at Variable Tip Speed Ratios

This paper studies the relationship between unsteady flow features and instantaneous torque and power performance of straight bladed vertical axis wind turbine at variable tip speed ratios. The rotor unsteady flow field simulation was carried out by using computational fluid dynamics method. The flow physics and the principle of changing flow field acting on torque performance and power performance have been analyzed where the rotating rotor was the major concern. The results show that the flow feature alters from periodical blade dynamic stall vortexes generation, development and shedding at low tip speed ratio to cyclical formation, evolution and diffusion of blade wake flow with the rising tip speed ratio. Both vortex shedding around the blade and interaction of blade wakes degrade the rotor aerodynamic performance. It is suggested that, to absorb maximum wind energy, delay the blade vortex shedding and reduce the range of blade wake, evolution and diffusion should be included in the rotor aerodynamic design.

GW26-e3569 Flow field changes of right ventricle in diastole pre- and post-operation in patients with atrial septal defect by vector flow mapping

GW26-e3569 Flow field changes of right ventricle in diastole pre- and post-operation in patients with atrial septal defect by vector flow mapping

GW26-e2957 Age-Related Variation in Flow Blood Fields of The Aortic Arch by Vector Flow Mapping

To investigate age-related changes in blood flow fields through the aortic arch using velocity distribution, vortex formation, energy loss at different cardiac cycles. One hundred and seven healthy subjects (49males) were divided into four groups according to age : <18 years (group A, n=22), 18-39

Effect of blade vortex interaction on performance of Darrieus-type cross flow marine current turbine

In this work, in-house computational fluid dynamics (CFD) code was utilized to simulate a cross-flow vertical-axis marine current turbine (straight-bladed Darrieus type). Particular emphasis was placed on the influence of interaction between vortices and blades on hydrodynamic performance. A physical transient-rotor–stator model with a sliding mesh technique was used to capture changes in flow field at a particular time step. The Spalart–Allmaras turbulence model was adopted for the turbulence. For a Darrieus-type marine current turbine, hydrodynamic characteristics such as power coefficient and flow behavior were then numerically investigated. The results suggest that vortices shed from previous blade passages and the close encounter of a rotor blade with these vortices can cause a variation in performance for this type of turbine during operation at different tip speed ratios.