Outer Flow Field Research Articles

Analytical model of small- and large-amplitude drop oscillation dynamics

The mechanical energy balance over a bulk of fluid that oscillates between prolate and oblate shapes in another immiscible fluid is solved using a general spheroidal coordinate system. The drop shape is described by a unique parameter that may continuously vary over the time, making the implementation of the model rather simple. Potential flow is assumed, and inertial and viscous effects are accounted for in both the inner and outer flow fields. The characteristics of drop oscillation for small and large amplitudes are studied, and the results are compared with theoretical, experimental, and numerical data from the open literature. The rather satisfactory validation of the model over a large variety of operating conditions allows its extension to include other physical phenomenon, like evaporation and its effect on drop oscillation.

Study on the influence of shaft brackets on the scale effect of the four-screw ship wake field

In the four-screw ship navigation, the load difference between the inner and outer propeller is found to be more than twice of that in the model test, indicating a serious scale effect. The stern attachment of the four-screw ship has a significant impact on the inner and outer propeller wake which directly affecting the load distribution. Thus, to investigate the impact of the brackets on the scale effect of the four-screw ship flow field, this paper conducted multi-scale simulations of a four-screw ship with and without bracket using the Reynolds-averaged Navier-Stokes (RANS) method. The feasibility and accuracy of the numerical method was verified by comparing with PIV test results. The result shows that the scale effect is more pronounced in the outer propeller compared to the inner propeller. The presence of bracket reduces the wake fraction difference between the inner and outer propellers, but significantly affects the inner radius region of the outer propeller disk, more than doubling the wake fraction in this region at full-scale. When the bracket Reynolds number falls below its critical Reynolds number, the inner radius region of the outer propeller flow field deviates notably from the full-scale results and loses its practical significance.

Numerical Analysis of Pressure Load on Hypersonic Wing

The flight vibration environment of hypersonic vehicles has always been a hot topic in the field of aerospace at home and abroad. In this paper, the simple wing model and the outer flow field grid of the wing are established by using fluent software. A series of environmental parameter conditions are set in the hypersonic flow field, and the steady and unsteady flow fields are analyzed. The distribution characteristics of the pressure load on the wing surface in the hypersonic flow field are obtained. The results show that the pressure load measured at the head position of the wing model is larger, and the pressure load at the fuselage position is smaller and changes more gently. With the increase of Mach number, the pressure load on the wing model will also increase. Then MatLab software is used to convert the time domain curve of wing surface pressure load into a power spectral density curve, which provides load input for subsequent random vibration response simulation on finite element software.

Visual Comparative Analysis for the Oil-air Two-Phase Flow of An Oil-Jet Lubricated Roller-Sliding Bearing

To discuss the lubrication characteristics of roller-sliding bearing under oil-injected lubrication, the CFD method was introduced into the fluid-structure interaction model to realize the transient simulation of oil-air two-phase flow (OATPF). The volume of fluid (VOF) method is applied to capture the oil-air interface. A sliding mesh is established between the inner flow field and outer flow field. Moreover, the two design schemes of roller-sliding bearing are compared. A reasonable design scheme is obtained. The bearing rotation speed, oil velocity, oil viscosity, oil ratio, and oil temperature have a significant impact on the lubrication performance of the roller-sliding bearing. The results show that the oil distribution inside the bearing is uneven. The lowest oil volume fraction exists in the basin, which is near the upstream of the nozzle, and gradually increases from the inner raceway to outer raceway. The oil storing rate increases with the increase of oil viscosity. A novel method of oil volume prediction is proposed in a deep manner. It also provides some reference for the design of roller-sliding bearing and other bearings.

Infrared simulation of aircraft rear fuselage based on a coupled CFD method

Abstract As an inner heat source, the propulsion system of aircraft obviously influences the rear fuselage thermal status and thus produced infrared signature. In this paper, a coupled flow-heat simulation method is established to calculate the inner and outer flow fields of a realistic aircraft exhaust system. Considering multi heat transfer modes and fluid-solid thermal interactions, the method is examined by available experimental data. Different nozzle layouts are then provided to analyse the influence of inner hot parts including combustion gas and solid walls on the outer fuselage temperature changes. Thermal radiation is individually researched and analysed since radiation flux heats the outer fuselage skin significantly, especially in the cold convection channel. The simulation results indicate that the appearance of intermediate shield device effectively suppresses the outer fuselage temperature-rise and therefore resulting infrared signatures. In comparison, the exhaust system equipped with intermediate shield could decrease outer fuselage infrared signatures by 69 and 40% for 3–5 μm band and 8–12 μm band, respectively.

Nozzle resonance mechanism and cooperative optimization of self-excited oscillating pulse cavitation jet

The peak value and pulsation amplitude of the self-excited oscillating pulse cavitation jet nozzle are essential indices to evaluate the jet performance. We established a simulation model of the jet process of the nozzle to investigate the evolution mechanism of the inner and outer flow fields. We used the chamber fillet, chamber diameter, chamber length, and outlet-tube diameter as the design variables, and the peak value of the striking force and the amplitude of the pulsation of the striking force as the target variables. The collaborative optimization design method of the nozzle was determined by combining the orthogonal test method, the back propagation neural network, and the nondominated sorting genetic algorithm. As indicated by the results, when the inlet pressure was 3 MPa, the factors ranked as follows in terms of their effects on the jet performance of the nozzle: the chamber fillet, the outlet-tube diameter, chamber diameter, and the chamber length. To verify the feasibility of the collaborative optimization method, the nozzle was fabricated via 3D printing, and the simulation model was verified by testing. This study provides support to the development of design theory for self-oscillating pulsed cavitation jet nozzles.

Experimental investigation of internal and external flow fields of jetting nozzles with different structures

Jetting has been widely used in the petroleum fields, the nozzle has been investigated for a long time and the structure has been optimized for different applications as the key part of jetting. However, most of these researches are focused on the outer flow with simulations, the real flow characteristics are not been revealed clearly. A visual experimental platform was built with Particle Image Velocimetry (PIV) and a high-pressure pump. 40 nozzles of different profiles were machined and measured. The internal and outer flow fields were captured by PIV under low velocity, and the pressure drop and flow displacement were measured under high-pressure conditions in this paper. The jetting performance such as discharge coefficient, flow core length, jet angle, and velocity distributions were analyzed. The internal flow characteristics including the flow field near the nozzle wall were captured by PIV. The converging angle and the throat section length play an important role in the jetting performance and are suggested to be set to 15° and 3 times the nozzle diameter when designed. The throat length has little effect on the flow field of the streamlined nozzle compared to the cone-straight nozzle, especially the concave curve nozzle. The vortex generated at the corner and the entrance of the throat section can indicate the boundary layer separation and transition, which agrees with our previous simulation work. All work will provide theoretical support for the design of the nozzle on-site application.

Computational Modeling of Tubular Flow Fields for PEM Fuel Cells

The bipolar plates of polymer electrolyte membrane fuel cells (PEMFCs) are of focus for improvement since they are responsible for most of the fuel cell stack weight and volume. Traditionally these flow fields are machined or stamped out of a graphitic or metallic material. Recent research has shown that additively manufactured metal plates show comparable performance to those made with conventional method. Due to the layer-by-layer nature, there is a great deal of design flexibility for additively manufactured parts. This allows for far more freedom in flow field designs, such as a tubular flow field with internal channels.In recent years there have been developments of tubular PEMFCs. Numerical simulation reported in literature have shown that tubular PEMFCs provide higher power density [1], more uniform reactant gas velocity distribution [2], and a smaller pressure gradient [3].The goal of the tubular model in our study was to replicate in size a standard AAA battery , so the total dimensions of the fuel cell were limited at a height of 40.5 mm and a diameter of 10.5 mm, with a cell active area of 987 mm2. The tubular channel length was approximately 400 mm. For reference, a conventional (planar) single serpentine fuel cell with a comparable 982 mm2 active area was used, replicating a scaled down commercially available design from Fuel Cell store. This planar design had square channels with a depth of 1.016 mm and a channel width of 0.7874 mm. The length of the channel was approximately 550 mm. For both designs, the land widths were 0.7874 mm to maintain a 1:1 channel-land width ratio. The gas diffusion layer (GDL), microporous layer (MPL), catalyst layer, and membrane had respective thicknesses of 360 μm, 50 μm, 10 μm, and 50 μm. The channel geometry for the tubular fuel cell was changed from rectangular to a semicircle, as the additive manufacturing process is not adept at building overhangs with supports. To maintain an accurate comparison, the cross-sectional area of the planar and tubular channels was kept constant at 0.79 mm2. This flow field would be incorporated into the outer anode flow field of the final tubular design, so hydrogen at 80°C was used as the fluid. A volumetric flow of 0.24 L/min was used as the inlet boundary condition.Initially, basic fluid flow simulations were performed on several tubular serpentine flow field designs (Figure 1). The double corkscrew Tubular v4 design had the greatest pressure drop, which was due to the fact the cross-sectional area was divided into two smaller sections. Tubular v2 and v3 had similar results being the most alike geometrically. Tubular v1, which split off into two symmetrical flow channels, exhibited the lowest pressure drop as the half channels cut down on the distance the hydrogen would have to travel inlet to outlet.With the chosen design Tubular v1, this geometry was used for the PEMFC module. Various changes were made to the tubular model to simulate the new design. Our proprietary membrane electrode assembly (MEA) design had no GDL present in the cathode layer, with oxygen being pumped directly through the cathode catalyst layer. This design assured that the supplied O2 concentration is much higher since its transport is by convection instead of diffusion. The ANSYS PEMFC model will not run if there is no defined GDL between the gas channel and MPL, so this was circumvented by directly changing the properties of the catalyst layer. To model this, the mass fraction of the oxygen in the cathode catalyst layer was increased from 0.2 to 0.3. It is also expected for the flowing of oxygen to flush away any water formation, so the water formation was limited. Due to the water mass balance taking place in the model, limiting liquid water formation increased the dissolved water inside of the membrane. The electrochemical surface area was increased to account for the higher surface area of the Pt catalyst in our proprietary MEA design.These modifications showed an increase in performance for the tubular cell when compared with the standard planar design as seen in Figure 2. Limiting the liquid water formation and selecting a geometry with a more uniform pressure distribution resulted in a cell with higher power density. The removal of the GDL and increase of air flow allowed for a higher oxygen concentration at the cathode catalyst interface. With oxygen no longer being a limiting reactant, the cell was able to perform at a higher power output. Figure 1

Non-Invasive Measurement of Humidity Distribution in Polymer Electrolyte Fuel Cells (PEFCs): Part I. In Situ Proof of Concept

A non-invasive method to determine the local relative humidity (RH) and local high frequency resistance (HFR) distribution from inlet to outlet of polymer electrolyte fuel cells (PEFCs) is presented. The method uses a plurality of electrodes attached to the outer flow field surface to inject alternating currents and measure the resulting voltage amplitudes at the flow fields’ surface. Since the measured voltage amplitudes, are highly dependent on the actual electrode positioning on the flow field boundaries, they are interpreted by interpolation between reference measurements at homogeneous relative humidity conditions and known overall cell impedance. The method is implemented experimentally using alternating current injections at a frequency of 1 kHz at well-defined asymmetric in situ N2/N2 conditions on a 200-cm2 cell in a 6-cell short-stack. The comparison between the RH distribution and the outlet RH sensor data yields a very good match. The same can be said when comparing the HFR distribution and the global cell HFR. Finally, the sensitivity of the method is tested by varying the mass flow rate with asymmetrical RH conditions. The results are highly reproducible in all the tested conditions.

Acoustic streaming resulting from compression of the cochlear bony capsule.

Acoustic streaming resulting from the time-harmonic compression of the cochlear capsule is examined in this paper. The cochlear pressure is expressed as an integral equation in the cochlear partition velocity. Rapid spatial variation in the velocity of the cochlear partition requires one to treat high-order fluid modes within the cochlear fluid. Hence, evanescent pressure modes are needed in the analysis. Asymmetry in the oval and the round window velocity is shown to give rise to a pressure gradient across the cochlear partition. The time-average fluid motion is obtained using the method of matched asymptotic expansions in conjunction with numerical evaluation of the outer flow field.

Non-Stokes drag coefficient in single-particle electrophoresis: New insights on a classical problem

We measured the intrinsic electrophoretic drag coefficient of a single charged particle by optically trapping the particle and applying an AC electric field, and found it to be markedly different from that of the Stokes drag. The drag coefficient, along with the measured electrical force, yield a mobility-zeta potential relation that agrees with the literature. By using the measured mobility as input, numerical calculations based on the Poisson–Nernst–Planck equations, coupled to the Navier–Stokes equation, reveal an intriguing microscopic electroosmotic flow near the particle surface, with a well-defined transition between an inner flow field and an outer flow field in the vicinity of electric double layer’s outer boundary. This distinctive interface delineates the surface that gives the correct drag coefficient and the effective electric charge. The consistency between experiments and theoretical predictions provides new insights into the classic electrophoresis problem, and can shed light on new applications of electrophoresis to investigate biological nanoparticles.

The unusual fluid dynamics of particle electrophoresis

The classical problem of the electrophoretic motion of a spherical particle has been treated theoretically by Overbeek in his 1941 PhD thesis and almost 40 years later by O’Brien & White. Although both approaches used identical assumptions, the details are quite different. Overbeek solved for the pressure, velocity fields as well as the electrostatic potential, whereas O’Brien & White obtained the electrophoretic mobility without the need to consider the pressure and velocity explicitly. In this paper, we establish the equivalence of these two approaches that allow us to show that the tangential component of the fluid velocity has a maximum near the surface of the particle and outside the double layer, the velocity decays as 1/r3, where r is the distance from the sphere, instead of 1/r in normal Stokes flow. Associated with this behavior is that of an irrotational outer flow field. This is consistent with the fact that a sphere moving with a constant electrophoretic velocity experiences zero net force. A study of the forces on the particle also provides a physical explanation of the independence of the electrophoretic mobility on the electrostatic boundary conditions or dielectric permittivity of the particle. These results are important in situations where inter-particle interaction is considered, for instance, in electrokinetic deposition.

Direct numerical simulations of creeping to early turbulent flow in unbaffled and baffled stirred tanks

It has been known for a long time that the fluid flow and several global quantities, such as the power and pumping numbers, are about the same in baffled and unbaffled mechanically stirred vessels at low Reynolds numbers, but bifurcate at some intermediate Re and take drastically different values in fully turbulent flow. However, several details are not yet completely understood, notably concerning the relation of this bifurcation with the flow features and the transition to turbulence. In order to shed light on these issues, computational fluid dynamics was employed to predict the flow field in two vessels stirred by a six-bladed Rushton turbine at Reynolds numbers from 0.2 to 600 (covering the range from creeping flow to early turbulent flow). The two vessels differed only for the presence or absence of peripheral baffles. All simulations were conducted by a finite volume method in time-dependent mode, and a sliding-mesh technique was used in the baffled case to deal with the relative motion of baffles and impeller blades. A sensitivity analysis proved that a grid of about 5 million finite volumes was adequate to yield grid-independent results. The study proved that the bifurcation between quantities related to baffled and unbaffled tanks occurs when the inner (near-impeller) and outer (near-wall/baffles) flow fields interact significantly. It also elucidated the mechanisms of transition to turbulence in baffled and unbaffled tanks, notably showing in this latter case the existence (in the rotating reference frame of the impeller) of a periodic flow regime which involves a travelling wave instability.

Mixing and combustion enhancement of Turbocharged Solid Propellant Ramjet

Turbocharged Solid Propellant Ramjet is a new concept engine that combines the advantages of both solid rocket ramjet and Air Turbo Rocket, with a wide operation envelope and high performance. There are three streams of the air, turbine-driving gas and augment gas to mix and combust in the afterburner, and the coaxial intake mode of the afterburner is disadvantageous to the mixing and combustion. Therefore, it is necessary to carry out mixing and combustion enhancement research. In this study, the numerical model of Turbocharged Solid Propellant Ramjet three-dimensional combustion flow field is established, and the numerical simulation of the mixing and combustion enhancement scheme is conducted from the aspects of head region intake mode to injection method in afterburner. The results show that by driving the compressed air to deflect inward and the turbine-driving gas to maintain strong rotation, radial and tangential momentum exchange of the two streams can be enhanced, thereby improving the efficiency of mixing and combustion in the afterburner. The method of injecting augment gas in the transverse direction and making sure the injection location is as close as possible to the head region is beneficial to improve the combustion efficiency. The outer combustion flow field of the afterburner is an oxidizer-rich environment, while the inner is a fuel-rich environment. To improve the efficiency of mixing and combustion, it is necessary to control the injection velocity of the augment gas to keep it in the oxygen-rich zone of the outer region. The numerical simulation for different flight conditions shows that the optimal mixing and combustion enhancement scheme can obtain high combustion efficiency and have excellent applicability in a wide working range.

A research into the flow and vortex structures around vehicles during overtaking maneuver with lift force included

A simulation model of overtaking maneuvers is established to research into the flow and vortex structures around two identical Ahmed vehicles. The simulation demonstrates how the flow and vortex structures generate and change, which influences the aerodynamical coefficients of the vehicles most. In this study, a transient solver is used on the platform of ANSYS/Fluent, and the sliding mesh method is adopted to simulate the motion of the overtaking vehicle, which realizes a more actual recovery of the road conditions. Based on the fact that the primary factor that influences the aerodynamical coefficients of a vehicle is the change in the outer flow around it, a research into the variation in the outer flow field is conducive to in-depth analysis of the aerodynamical characteristics of the vehicle. Most previous studies focused only on the lateral aerodynamical coefficients during overtaking maneuvers. This article establishes a comprehensive model that takes the lift force into consideration to more accur...

MacCormack’s technique-based pressure reconstruction approach for PIV data in compressible flows with shocks

This paper proposes an improved approach for extraction of pressure fields from velocity data, such as obtained by particle image velocimetry (PIV), especially for steady compressible flows with strong shocks. The principle of this approach is derived from Navier–Stokes equations, assuming adiabatic condition and neglecting viscosity of flow field boundaries measured by PIV. The computing method is based on MacCormack’s technique in computational fluid dynamics. Thus, this approach is called the MacCormack method. Moreover, the MacCormack method is compared with several approaches proposed in previous literature, including the isentropic method, the spatial integration and the Poisson method. The effects of velocity error level and PIV spatial resolution on these approaches are also quantified by using artificial velocity data containing shock waves. The results demonstrate that the MacCormack method has higher reconstruction accuracy than other approaches, and its advantages become more remarkable with shock strengthening. Furthermore, the performance of the MacCormack method is also validated by using synthetic PIV images with an oblique shock wave, confirming the feasibility and advantage of this approach in real PIV experiments. This work is highly significant for the studies on aerospace engineering, especially the outer flow fields of supersonic aircraft and the internal flow fields of ramjets.

Integration of Research for an Exhaust Thermoelectric Generator and the Outer Flow Field of a Car

The exhaust thermoelectric generator (TEG) can generate electric power from a car engine's waste heat. It is important to maintain a sufficient temperature difference across the thermoelectric modules. The radiator is connected to the cooling units of the thermoelectric modules and used to take away the heat from the TEG system. This paper focuses on the research for the integration of a TEG radiator and the flow field of the car chassis, aiming to cool the radiator by the high speed flow around the chassis. What is more, the TEG radiator is designed as a spoiler to optimize the flow field around the car chassis and even reduce the aerodynamic drag. Concentrating on the flow pressure of the radiator and the aerodynamic drag force, a sedan model with eight different schemes of radiator configurations are studied by computational fluid dynamics simulation. Finally, the simulation results indicate that a reasonable radiator configuration can not only generate high flow pressure to improve the cooling performance, which provides a better support for the TEG system, but also acts as a spoiler to reduce the aerodynamic drag force.

Coupling a Wall Diffusion Model with an Immersed Boundary Technique

Immersed boundary methods showed a good compliance for a wide range of complex inviscid and viscous flows. Moreover, the high-Reynolds-number regime still remains an open issue. The paper describes a wall-layer approach to answer the need of a model that goes beyond the actual capability of classical wall functions. A near-wall region is established that gets information from the outer flowfield and returns back the wall stress. The latter is obtained by integrating simplified thin-boundary-layer equations along a normal to the wall subgrid. In the outer zone, the compressible Reynolds-averaged Navier–Stokes equations are solved by means of a finite volume method on two- or three-dimensional Cartesian meshes. The classic validation test dealing with a high-Reynolds-number flow over a flat plate is carried out. A benchmark two-dimensional complex flow is numerically investigated to assess the performance of the wall model in case of a pressure-induced separation. The results are compared with both experiments and body-conforming numerical solutions.

A solution of two-parameter asymptotic expansions for a two-dimensional unsteady boundary layer

A solution procedure based on two-parameter asymptotic expansions, in terms of a Blasius parameter and a dimensionless time, is presented for a two-dimensional, unsteady boundary layer over a flat surface. The Blasius parameter is used to scale the stretching of the boundary-layer length scale, and the dimensionless time represents the unsteadiness caused by the outer flow field. The matching conditions between the outer solutions and inner solutions are obtained according to the matching procedure from which the streamfunction, velocity and pressure are matched all at the same time. Closed-form solutions are obtained until the second-order expansions of the solution. Applications of the solution to example problems are given with comparisons to the results in the literature to show the validity and versatility of the current solution to accommodate a variety of outer flows. The solution is even valid for predicting the time and location when the flow separation first occurs in some applications.

Stokes Flow over Composite Sphere: Liquid Core with Permeable Shell

This paper presents an analytical study of an infinite expanse of uniform flow of steady axisymmetric Stokes flow of an incompressible Newtonian fluid around the spherical drop of Reiner-Rivlin liquid coated with the permeable layer with the assumption that the liquid located outside the capsule penetrates into the permeable layer, but it is not mingled with the liquid located in the internal concave of capsule. The flow inside the permeable layer is described by the Brinkman equation. The viscosity of the permeable medium is assumed to be same as pure liquid. The stream function solution for the outer flow field is obtained in terms of modified Bessel functions and Gegenbauer functions, and for the inner flow field, the stream function solution is obtained by expanding the stream function in terms of S. The flow fields are determined explicitly by matching the boundary conditions at the pure liquid-porous interface, porous-Reiner-Rivlin liquid interface, and uniform velocity at infinity. The drag force experienced by the capsule is evaluated, and its variation with regard to permeability parameter a, dimensionless parameter S, ratio of viscosities l2, and thickness of permeable layer d is studied and graphs plotted against these parameters. Several cases of interest are deduced from the present analysis. It is observed that the cross-viscosity increases the drag force, whereas the thickness d decreases the drag on capsule. It is also observed that the drag force is increasing or decreasing function of permeability parameter for l2 < 1.