Simplified Flow Field Research Articles

Transverse Jet Interaction with Hypersonic Crossflow I: Experimental and Numerical Investigations

Applications of transverse jet interaction are widespread in hypersonic aircraft systems. In wind tunnel facilities, it is more convenient to perform jet interaction tests with jet fluids of different compositions and with test models of different sizes from those of the actual aircraft. The paper presents a detailed analysis of the transverse jet interaction with hypersonic crossflow by combining the use of both experimental and numerical tools. A hypervelocity ballistic correlation model HB-1, with a sonic jet nozzle, was designed to provide a simplified flowfield for comparison with the simulations. It is found that the temperature and molecular weight of the injectant gas play an important role in the interactions between the hypersonic flow and transverse jet. The applicability of different scaling parameters for this kind of aerodynamic experiment is discussed at the end of this paper.

Comparison of fluid structure downstream of an evaporative flameholder with subsonic uniform and subsonic-supersonic inflows

Subsonic uniform and supersonic mixing flows are widely present in the combustion chamber of a multi-mode combined scramjet engine, where the significant pressure, velocity, and temperature gradients of the two impose severe limitations on flame propagation. The vortex structures downstream of an evaporative flameholder were experimentally measured under subsonic-supersonic mixing inflow and subsonic uniform inflow. Results indicate that, under subsonic-supersonic mixing inflow conditions, an asymmetric vortex structure in the recirculation zone is formed due to the presence of a subsonic-supersonic shear layer, which exerts compressive or stretching effects on the recirculation zone downstream of the flameholder, in contrast to the symmetric dual-vortex structure observed under subsonic uniform inflow conditions. The asymmetric recirculation zone displays fluid being entrained from one vortex to another, with the degree and direction of entrainment dependent on the supersonic expansion state. Additionally, a simplified flow field structure is proposed for comparing the differences between the subsonic-supersonic mixing inflow and subsonic uniform inflow, along with the introduction of five regions and two stages to describe the flow field structure downstream of the flameholder under subsonic-supersonic mixing inflow. Analysis of the flow characteristics downstream of the flameholder under subsonic-supersonic mixing inflow and subsonic uniform inflow conditions can offer design insights for the combustion scheme of aerospace-integrated propulsion systems.

Benchmarking the Performance of the Actuator-Disk Method for Low-Pressure Axial Flow Fan Simulation

Abstract The actuator-disk method is a cost-effective simulation tool that implicitly represents the rotor of a turbomachine using a blade-element approach combined with two-dimensional (2D) airfoil coefficient input data. Actuator-disk models further rely upon empirical coefficient corrections to modify the 2D input data to better mimic physical blade aerodynamic characteristics. However, the fabrication of high-fidelity, general-purpose corrections remains a formidable challenge, so the limits of actuator-disk model accuracy have never been rigorously tested and remain uncertain. This is especially the case for low-pressure axial flow fan models, given the relative lack of empirical corrections conceived specifically for fan rotor simulation. In this study, benchmark performance limits of the actuator-disk method for axial flow fan analysis are explored. The limits of the modeling approach are interrogated by simulating actuator-disk fan models embedded with accurate physical blade data derived from explicit three-dimensional (3D) fan model computations. It is subsequently shown that even with a near-precise coefficient description, the method remains unreliable. However, using an unconventional actuator-disk model formulation that is based directly on 3D blade force inputs, it is demonstrated that the accuracy of conventional models can be noticeably enhanced if the input coefficient data is artificially manipulated. This nonphysical tuning of the input coefficient data is required to compensate for the simplified flow fields produced by the reduced-order modeling approach. The needed coefficient adjustments, referred to as modeling corrections, are subsequently defined and explained alongside the presentation of new realistic performance targets for future actuator-disk fan model variants.

Numerical simulation of cavitation-vortex interaction mechanism in an advanced rotational hydrodynamic cavitation reactor

Hydrodynamic cavitation (HC), a promising technology for enhancing processes, has shown distinct effectiveness and versatility in various chemical and environmental applications. The recently developed advanced rotational hydrodynamic cavitation reactors (ARHCRs), employing cavitation generation units (CGUs) to induce cavitation, have demonstrated greater suitability for industrial-scale applications than conventional devices. However, the intricate interplay between vortex and cavitation, along with its spatial-temporal evolution in the complex flow field of ARHCRs, remains inadequately elucidated. This study investigated the interaction mechanism between cavitation and vortex in a representative interaction-type ARHCR for the first time using the “simplified flow field strategy” and the Q-criterion. The findings reveal that the flow instability caused by CGUs leads to intricate helical and vortex flows, subsequently giving rise to both sheet and vortex cavitation. Subsequently, utilizing the Q-criterion, the vortex structures are identified to be concentrated inside and at CGU edges with evolution process of mergence and separation. These vortex structures directly influence the shape and dimensions of cavities, establishing a complex interaction with cavitation. Lastly, the vorticity transport equation analysis uncovered that the stretching and dilatation terms dominate the vorticity transport process. Simultaneously, the baroclinic term focuses on the vapor-liquid interface, characterized by significant alterations in density and pressure gradients. These findings contribute to a better comprehension of the cavitation-vortex interaction in ARHCRs.

Effect of the arrangement of cavitation generation unit on the performance of an advanced rotational hydrodynamic cavitation reactor

Hydrodynamic cavitation (HC) is widely considered a promising process intensification technology. The novel advanced rotational hydrodynamic cavitation reactors (ARHCRs), with considerably higher performance compared with traditional devices, have gained increasing attention of academic and industrial communities. The cavitation generation unit (CGU), located on the rotor and/or stator of an ARHCR, is utilized to generate cavitation and consequently, its geometrical structure is vital for the performance. The present work studied, for the first time, the effect of the arrangement of CGU on the performance of a representative ARHCR by employing computational fluid dynamics based on the “simplified flow field” strategy. The effect of CGU arrangement, which was neglected in the past, was evaluated: radial offset distance (c), intersection angle (ω), number of rows (N), circumferential offset angle (γ), and radial spacing (r). The results indicate that the CGU, with an arrangement of a low ω and moderate c, N, γ, and r, performed the highest cavitation efficiency. The corresponding reasons were analyzed by combining the flow field and cavitation pattern. Moreover, the results also exposed a weakness of the “simplified flow field” strategy which may induce the unfavorable “sidewall effect” and cause false high-pressure region. The findings of this work may provide a reference value to the design of ARHCRs.

Study on the Relationship between the Fluidity of Paste in Stope and the Rheological Properties of Paste

Paste should be kept in homogeneous state without segregation or weak segregation during pipeline transportation, which can reduce the occurrence of pipe blockage accidents in engineering application. Only when the paste is homogeneous can the paste have good stope fluidity, and the stope fluidity is also restricted by the geometric size of the stope and other factors. Therefore, it is very important to study the homogeneity of paste and analyze the influence of paste rheological characteristics on stope fluidity. In this paper, it is pointed out that the vertical settling velocity of solid phase materials in paste in the liquid‐phase dense medium is the main factor affecting the homogeneity of paste. Increasing the plastic viscosity and yield stress of the liquid‐phase dense medium is beneficial to increase the drag force of aggregate and then reduce the settling speed of aggregate in the liquid‐phase dense medium. On this basis, the homogeneity of paste can be judged. In the simplified paste flow field, using Cauchy stress equation and flow boundary conditions, the relationship between paste stope flow performance and paste rheological properties was analyzed, and the relationship model between the slope angle of paste flow formation and paste rheological properties was established. Compared with the experimental results, the error of the model results is small, and the slope angle of paste flow can be well predicted.

Optical Diagnostics for Soot Formation Characteristics in Multi-phase Combustion Field Formed in a Simplified Flow Field

Optical Diagnostics for Soot Formation Characteristics in Multi-phase Combustion Field Formed in a Simplified Flow Field

Analysis of turbulent transport characteristic in hydrogen diffusion flames using direct numerical simulation

Non-premixed hydrogen flame is solved to investigate turbulent transport characteristic using direct numerical simulation with768 computing cores. Local and statistical transport behaviors are observed in nonbuoyant and buoyant flames. The absence of buoyancy results in a simplified flow field which can serve as a “cheaner” test to provide an improved understanding of transport process and assess the numerical models. Results show that a wider flame front and a lateral shift of temperature are observed in nonbuoyant flame, where diffusional transport is dominant to mixing process. While, both diffusion and convection are the principal factors in a buoyant flame, which makes flame wrinkle and results in a thinner flame. Besides, statistical information indicates that both gradient transport and counter-gradient transport are observed in hydrogen non-premixed flames, and a transition from gradient transport to counter-gradient transport is captured. A large pressure gradient associated with thermal expansion is responsible for counter-gradient transport in diffusion-controlled flames. However, convective transport is inferred to promote the occurrence of counter-gradient transport in buoyancy-driven flames. It is worth noting that traditional gradient transport model fails to completely describe the intricate scalar transport behaviors in non-premixed hydrogen flames, whether diffusion-driven or buoyancy-driven flames.

Construction modeling and shape prediction of horizontal salt caverns for gas/oil storage in bedded salt

Construction modeling and shape prediction are significant for the parameters optimization and capacity prediction of a horizontal salt cavern gas/oil storage. However, most previous studies concentrate on the middle segment of the cavern, and the insoluble substances are ignored. The actual shapes of the horizontal salt caverns in bedded salt remain unknown. Here a coupled model is proposed for the construction of a horizontal cavern. A composite structural mesh is presented to describe the development of three segments of the cavern. The equations of the salt-brine mass transfer rate and boundary movements are introduced. A simplified flow and concentration field model is presented. The accumulation of the insoluble substances is firstly considered in a horizontal cavern. A VC++ program using the finite volume method is developed to obtain the numerical solution of the model and as a construction design tool for the horizontal salt caverns. The reliability of the model and program has been confirmed by comparisons with laboratory experiments. The shape of a field horizontal cavern HA4-5 in Huai'an, China is successfully simulated. The simulation results coincide well with the field data. It is found that the horizontal cavern is actually U-shaped and there is an undetected open cavern in the middle segment. This cavern is suggested to be used as a non-clogging de-brining pool during gas/oil injection and brine discharge. The brine in the pores of the insoluble sediments will be replaced by gas/oil and the storage capacity will be multiplied. The finding promises a broad prospect for the horizontal salt cavern gas storage in bedded salt.

Numerical simulation of the churning losses in vehicle rear axle hypoid gear

A simplified 3D flow field numerical model of rear axle including passive bevel gear and the differential housing is established, based on the VOF (Volume of Fluid) model and the RNG k-ε turbulence model. With the model, the transient oil flow distribution and the influence law of speed, viccosity and bolt structure on the velocity field and the power losses is obtained. In general, the churning power losses increases with the rotational speed and viscosity while the rotational speed has more impact than viscosity on the power losses. The fluid region and the flow velocity increases with the rotational speed and the bolt structure causes a high speed area with the larger velocity gradient in the backflow surface of the bolts, implying the bolts can generate additional churning torque.

Multiple slit electrochemical micromachining using a single wire and a constant inter-electrode voltage

Fabrication of micro-slit array in difficult to machine materials is a considerable challenge due to its rigorous surface integrity requirements. This paper proposed a novel method for multiple slit electrochemical micromachining using a single wire and a specific apparatus was developed. The wire is spooled through a specific pulley with multiple grooves to generate a multiple-wire face. Thus, the wire tension and the wire travelling direction would be the same in each wire machining branch. A simplified flow field and electric model illustrated that each machining branch has the same electric supply and the same wire travelling status, as well as the same flow field, which is of great benefits for producing arrayed structures with high consistency. Experiments verified that the inter-electrode voltage as well as the slit width decreased with the increase of electrode amount and the maximum electrode feeding rate was limited. To maintain a constant inter-electrode, the output of the pulse generator was adjusted to a suitable value depending upon the wire amount. Compared with a constant output voltage, the largest electrode feeding rate of each single electrode and the total feeding rate are improved. Finally, a task was designed to compare multi-slit ECMM, HS-WEDM, and LS-WEDM in machining efficiency and surface integrity. It indicated multi-slit ECMM is a feasible alternative for machining arrayed structures requiring good surface integrity in high efficiency.

An investigation of seismic anisotropy in the lowermost mantle beneath Iceland

SUMMARYIceland represents one of the most well-known examples of hotspot volcanism, but the details of how surface volcanism connects to geodynamic processes in the deep mantle remain poorly understood. Recent work has identified evidence for an ultra-low velocity zone in the lowermost mantle beneath Iceland and argued for a cylindrically symmetric upwelling at the base of a deep mantle plume. This scenario makes a specific prediction about flow and deformation in the lowermost mantle, which can potentially be tested with observations of seismic anisotropy. Here we present an investigation of seismic anisotropy in the lowermost mantle beneath Iceland, using differential shear wave splitting measurements of S–ScS and SKS–SKKS phases. We apply our techniques to waves propagating at multiple azimuths, with the goal of gaining good geographical and azimuthal coverage of the region. Practical limitations imposed by the suboptimal distribution of global seismicity at the relevant distance ranges resulted in a relatively small data set, particularly for S–ScS. Despite this, however, our measurements of ScS splitting due to lowermost mantle anisotropy clearly show a rotation of the fast splitting direction from nearly horizontal for two sets of paths that sample away from the low velocity region (implying VSH > VSV) to nearly vertical for a set of paths that sample directly beneath Iceland (implying VSV > VSH). We also find evidence for sporadic SKS–SKKS discrepancies beneath our study region; while the geographic distribution of discrepant pairs is scattered, those pairs that sample closest to the base of the Iceland plume tend to be discrepant. Our measurements do not uniquely constrain the pattern of mantle flow. However, we carried out simple ray-theoretical forward modelling for a suite of plausible anisotropy mechanisms, including those based on single-crystal elastic tensors, those obtained via effective medium modelling for partial melt scenarios, and those derived from global or regional models of flow and texture development in the deep mantle. These simplified models do not take into account details such as possible transitions in anisotropy mechanism or deformation regime, and test a simplified flow field (vertical flow beneath the plume and horizontal flow outside it) rather than more detailed flow scenarios. Nevertheless, our modelling results demonstrate that our ScS splitting observations are generally consistent with a flow scenario that invokes nearly vertical flow directly beneath the Iceland hotspot, with horizontal flow just outside this region.

The development and performance of a perforated plate burner to produce vitiated flow with negligible swirl under engine-relevant gas turbine conditions.

The development and performance of a perforated plate burner (PPB) operating using premixed natural gas and air at engine-relevant inlet temperatures and combustor pressures with thermal powers up to 1 MW is discussed. A significant benefit of using burners with simplified flow fields, such as the PPB, for experimental studies in the laboratory is the potential for decoupling the complex fluid dynamics in typical combustors from the chemical kinetics. The primary motivation for developing this burner was to use it as a source of vitiated flow with negligible swirl for reacting jet in vitiated crossflow experiments. The design methodology for the PPB is described, including plate geometry selection and flashback mitigation features. The stable operation of the PPB within a high-pressure test rig was validated: successful ignition, effective use of red-lines for flashback mitigation, and long duration steady-state operation in both piloted and nonpiloted modes were all observed. Exhaust gas emissions measured using a Fourier-transform infrared (FTIR) spectrometer showed very good performance of the PPB in terms of the combustion efficiency (based on measured CO and UHC), and a stability diagram of the PPB was developed as a function of the equivalence ratio and the PPB hole velocity. FTIR measurements also showed very low levels of NOX in nonpiloted operation that were generally within 3 ppm (reported dry and referenced to 15% O2). The capability for steady-state operation, high combustion efficiency, and low levels of NOX makes this PPB an excellent burner candidate for combustion experiments in the laboratory.

Effect of Interdigitated Flow Field Geometry on Redox Flow Cell Performance

A large fraction of the cost of a flow battery installation comes from the high cost of separators and bipolar plates1. Optimizing the use of active area in flow batteries by maximizing power density allows cell stack engineers to reduce the amount of stack materials required. The design of flow fields has a significant impact on cell performance, including peak galvanic power density2,3. In this work, we present an experimental and computational study of interdigitated flow fields with varying dimensions. We demonstrate reduction of resistive and mass transport overpotential losses by altering the interdigitated flow field dimensions to reduce zones of mid-channel stagnant flow within the porous electrode. A two-dimensional model of an interdigitated flow field, coupling fluid dynamics and electrochemistry, is used to guide optimization of the channel, land, and electrode dimensions in the flow field. To study the influence of mass transport on performance, a flow cell is assembled with an unstructured, flow-through porous electrode, with a flat plate replacing the interdigitated flow field. Even with this simplified flow field, discrepancies persist between observed experimental results and predictions from porous electrode theory. We report our progress toward understanding and resolving these discrepancies. 1. V. Viswanathan et al., J. Power Sources, 247, 1040–1051 (2014) 2. M. L. Perry, R. M. Darling, and R. Zaffou, ECS Trans., 53, 7–16 (2013) 3. R. M. Darling and M. L. Perry, J. Electrochem. Soc., 161, A1381–A1387 (2014) Figure 1. Experimental results and computational predictions of overvoltages from a symmetric potassium ferrocyanide/ferricyanide flow cell using one of four different flow fields. The cell consists of one sheet of SGL 39-AA on each side of a Nafion 212 membrane. The active area of each flow field is 5 cm2. Figure 1

Efficient Collision Detection in a Simulated Hydrocyclone

Hydrocyclones enhance oil–water separation efficiency compared to conventional separation methods. An efficient collision detection scheme with Np ln Np dependency on the number of particles is proposed. The scheme is developed to investigate the importance of particle–particle interaction for flow and particle behavior in dilute oil–water separators. However, the proposed scheme can also be used for efficient multigrid soft collision calculation with long-range particle–particle interaction models. The collision detection scheme is first validated using a case of settling spheres. Subsequently, the scheme is applied to a simplified flow field, similar to the flow fields observed in hydrocyclones.

Study on clogging mechanism of fibrous materials in a pump by experimental and computational approaches

Clogging trouble in a sewage pump is a serious problem caused by foreign substances like strings, towels, and cloths. However, mechanism of pump clogging is significantly complicated and it is difficult to clarify factors affecting pump clogging. In this paper, the simplified air duct test and axial fan test are introduced and some parameter studies based on design of experiments (DOE) were conducted. From the DOE results, the sensitivity of each parameter to the clogging was predicted. As the parameters related to the clogging, length and rigidity of the strings, leading shape and lean angle of obstacles, flow velocity in a measuring section were selected. Behavior of string in the pump was also observed in the actual pump apparatus and was compared to that in the air duct. Computational simulation with simplified flow fields and MBD (Multi Body Dynamics) was developed and applied to predict behavior of a string in the pump. These experiments and computation are effective to detect and classify the clogging mechanism. These approaches aided to establish one of the evaluation methods on anti-clogging performance against fibrous materials.

The Simplified Flow Field Analysis Method of Multi-Layers Parallel Plates Perfusion Bioreactor

Multi-layers parallel plates perfusion bioreactor has the potential advantage in cells cultivation of tissue engineering and good scalability for cells cultivation on a large scale. It is necessary to analyze the distribution of flow shear stress (FSS) of bioreactors which has strong influence on the growth of cells. The result of meshing was not satisfactory because of the complexity of multi-layers parallel plates when using computational fluid dynamics (CFD) to analyze the FSS, and the amount of calculation was great and complex especially under the process of influence on FSS caused by analyzing the different structure. The new method of simplified flow field analysis was presented in this paper, which was based on relation between FSS and flow and made the process simpler by analyzing distribution of rate instead of FSS. The simulation result showed that this method can satisfy the requirement of precision and provide reference for the analysis of the flow field which had the established relationship between structural parameters and laminar flow within it.

Characterizing Local O2 Diffusive Losses in GDLs of PEFCs Using Simplified Flow Field Patterns (“2D”,“1D”,“0D”)

By means of advanced diagnostic tools – neutron imaging, pulsed gas method (helox or O2 pulses during air operation) and electrochemical impedance spectroscopy (EIS) – three different flow fields are characterized in a differential (small-size cell operated at high stoichiometry) Polymer Electrolyte Fuel Cell (PEFC) to emulate the local behavior of a full-size cell. The channel-rib structures are designed so as to obtain 2D, 1D and 0D gradients of O2 concentration across the gas diffusion layer (GDL). The important bulk-diffusion losses (evaluated with pulsed helox) of the “2D” cell compared to the “1D” cell indicate the effect of lateral oxygen diffusion in the GDL. Non-bulk diffusion losses (Knudsen and/or thin film diffusion, evaluated with pulsed O2) also play a non-negligible role on the overall diffusive losses. In particular, their increase in dry conditions for the “1D” and the “0D” cells might be a consequence of drying effects in the electrode.EIS spectra can be interpreted consistently with the results of the pulsed gas method: the increase of arc diameter can be correlated with the overall mass transport losses including both bulk and non-bulk diffusion losses.

Numerical Simulation on Internal Flow Field of Rolling Bearing under Oil-Air Lubrication

Heat dissipation and working efficiency of transport air in rolling bearing under oil-air lubrication are closely related to the flowing state of oil-air in bearing chamber. For cylindrical roller bearing NF211, numerical simulation model of oil-air flow field in bearing chamber was established combining with the practical structure features of rolling bearing and ignoring the effect caused by roller rotation. Combining with flow field numerical simulation functions of Fluent software, simulation analysis of the flow state in bearing chamber were carried out. Based on k-ε turbulent model, three-dimensional flow field in the bearing chamber and main feature parameters of inner flow were got analyzed carefully considering the effect of twirling. Comparing with the numerical simulation of simplified flow field, it showed that energy dissipation and axial velocity of the air were influenced by the effect of twirling distinctly. Simulation results were expected to give useful references for the optimization design of the oil-air lubrication system in rolling bearing.

Re‐building brown trout populations in dredged boreal forest streams: in‐stream restoration combined with stocking of young trout

Summary1. Rivers in boreal forested areas were often dredged to facilitate the transport of timber resulting in channels with simplified bed structure and flow fields and reduced habitat suitability for stream organisms, especially lotic fishes. Currently, many streams are being restored to improve their physical habitat, by replacing boulders and gravel and removing constraining embankments. The most compelling justification behind stream restoration of former floatways has been the enhancement of native fish populations, specifically salmonids.2. We examined the success of a stream management programme aimed at re‐building diminished brown trout (Salmo trutta) populations by monitoring densities of young‐of‐year and older trout in 18 managed and three reference streams during 2000–2005. Rehabilitation included in‐stream restoration combined with a 5‐year post‐restoration period of stocking young brown trout. Our space‐for‐time substitution design comprised four pre‐management, four under‐management, 10 post‐management and three reference streams.3. Densities of young‐of‐year brown trout, indicating population establishment, were significantly higher in post‐ compared with pre‐management streams. However, density of young‐of‐year brown trout in post‐management streams was significantly lower compared with near‐pristine reference streams. Furthermore, success of managed brown trout population re‐building varied, indicating stream‐specific responses to management measures. Density of burbot (Lota lota), a native generalist predator, was associated with low recruitment of brown trout.4. Stream‐specific responses imply that rehabilitation of brown trout populations cannot be precisely predicted thereby limiting application. Our findings support the importance of adaptive stream restoration and management, with focus on identifying factor(s) limiting the establishment of target fish populations.