Gap Flow Research Articles

A Three-Dimensional Hydraulic Stack Model for Redox Flow Batteries Considering Porosity Variations in Porous Felt Electrodes and Bypass Flow in Side Gaps

Redox flow batteries provide high flexibility and scalability for large-scale energy storage systems due to their safety, low cost and decoupling of energy and power. While typical flow frame designs usually assume all parts are standard, the industry can suffer from irregularity and manufacturing tolerances of cell components, such as the shape or dimensions of the flow frame and porous electrode. This paper evaluates the impact of side gaps and porosity differences of the graphite felt due to irregularity and manufacturing tolerances on the electrolyte flow in the active cell areas. A three-dimensional hydraulic model with parameterised multi-cell stack geometry has been developed in COMSOL to compare the cell velocity distributions and pressure losses of a vanadium redox flow battery with flow-through electrodes. The results indicate that the side gaps and porosity segments can result in preferential flow within low-resistance areas, leading to significantly lower flow rates for other cell areas compared with standard flow frames. Proposed countermeasures of adjusting channel locations and applying dimples protruding into the cell cavity from the flow frame show good potential to avoid stagnant zones and maintain theoretical flow rates for the active cell areas.

Heat-transfer modeling of the gas gap under a wafer

The wafer temperature is a critical observable in semiconductor manufacturing. One of the various mechanisms determining this temperature is the heat transfer in a gas gap between the wafer and the electrostatic chuck (ESC). Various correlations for this heat transfer are available. However, to calculate more accurately the spatial distribution of this temperature, computing the flow in this gap is necessary. With this motivation in mind, this paper presents a computational fluid dynamics model (CFD) for the flow in the wafer-ESC gap that is designed to be easy to implement in industrial CFD codes. This model is tested in various channel-flow problems and then applied to a generic wafer-ESC configuration. For this configuration, CFD results show that varying the flow rate split between three zones, or the total flow rate, or the rugosity of the wafer affect the heat transfer coefficient and its spatial variation. This is important since controlling this variation would allow to maintain a uniform wafer temperature. The model could be used in more realistic wafer-ESC configurations to consider many other parameter variations, such as the size of injection holes, a radially varying gap distance, or the use of many injection zones. From a broader viewpoint, the model is applicable to vacuum problems other than the wafer-ESC configuration.

Numerical study of two types of rough groove in suppressing the tip clearance cavitation

Tip clearance cavitation (TCC) which is consisted of tip leakage vortex (TLV) cavitation and gap cavitation (occurs inside the gap region) is a common phenomenon in the gap flow filed in axial turbomachinery, and can trigger a series of negative effects. Based on the suppression mechanism of a type of prototypical horizontal casing groove, two types of rough groove structures are proposed in this paper aiming at enhancing the suppression effect on the TCC by utilizing the Coanda effect of the gap leakage jet. The gap flow field is generated via a NACA0009 hydrofoil and a solid wall. The gap size is fixed as 2 mm. The numerical simulation method is adopted. The results show that combining the detached-eddy simulation (DES) approach, SchnerrSauer cavitation model and a type of varRhoTurbVOF method on the OpenFOAM platform, accurate numerical results of cavitating gap flow fields under the small gap conditions can be obtained. The gap flow field analysis reveals that the control ability of the grooves on the leakage flow is enhanced via divergent–convergent flow passage in the roughness groove cases. The leakage rate is reduced while the local turbulence intensity is increased, which weakens the TLV and the tip separation vortex (TSV) dramatically. The rough casing grooves show a superior suppression effect on the TCC. The present work can inspire new casing treatment method for improving the anti-cavitation performance in the axial-flow hydraulic machinery.

Analysis of the Total Leakage Characteristics of Finger Seal Considering Fractal Wear and Fractal Porous Media Seepage Effects

As an advanced flexible dynamic sealing technology, the leakage characteristics of a finger seal (FS) is one of the key research areas in this technology field. Based on the fractal theory, this paper establishes a mathematical model of the FS main leakage rate considering the fractal wear effect by taking into account the influence of the wear height on the basis of the eccentric annular gap flow equation. Based on the Hagen-Poiseuille law and the fractal geometry theory of porous media, a mathematical model of the FS side leakage rate considering the fractal porous media seepage effect is developed. Then, a mathematical model of the FS total leakage rate is established. The results show that the mathematical model of the FS total leakage rate is verified with the test results, the maximum error rate is less than 5%, and the mathematical model of the FS total leakage rate is feasible. With the gradual increase in working conditions and eccentricity, the FS main leakage rate gradually increases. In addition, the effects of the fractal dimension, fractal roughness parameters and porosity after loading on the FS main leakage rate are negligible. As the fractal dimension of tortuosity after loading gradually decreases, the fractal dimension of porosity after loading gradually increases, and the FS side leakage rate gradually increases. As the porosity after loading gradually increases, the FS side leakage rate gradually increases. Under different working conditions, different fractal characteristic parameters and different porosities after loading, the weight of the FS main leakage rate is much greater than that of the FS side leakage rate by more than 95%.

The Dynamic Characteristics of a Piped Capsule Moving in a Straight Pipeline

The hydraulic transportation of piped capsules is a new and energy-saving transportation mode, which is especially suitable for the long-distance and high-stability requirements of material transportation. In this paper, the COMSOL Multiphysics software was used to construct a mathematical model of the dynamic characteristics of a piped capsule moving in a straight pipeline, in which the boundary conditions were redeveloped, the inlet velocity distribution function was defined, and the physical experiment was carried out for verification. The dynamic characteristics were analyzed, and through the calculation of the energy consumption, the optimal piped capsule under the research conditions was obtained. The results show that the simulation results and experimental results for the piped capsule’s average moving velocity, axial velocity, and wall shear stress along the cylinder wall were basically consistent, with a maximum error of 14.22%, 2.62%, and 20.13%, respectively. With a decrease in the diameter-to-length ratio of the piped capsule, the axial velocity of the concentric annular gap flow decreased gradually. The area with a large shear stress was mainly concentrated at the front and rear ends of the cylinder wall, especially the rear area of the support feet of the piped capsule. With the increase in the diameter of the piped capsule, the wall shear stress of the capsule increased. Finally, the superior diameter-to-length ratio for the piped capsule under the research conditions was obtained and shown to be ε = 0.4. The research in this paper will provide a theoretical reference for the structural design and dynamic mechanism analysis of the piped capsule.

Stability characteristics of the spiral Poiseuille flow induced by inner or outer wall rotation

In this paper we reveal, that the stability behavior of the spiral Poiseuille flow in an annular gap with rotating inner or outer cylinder exhibits a striking similarity at low and intermediate rotation rates, while both cases differ significantly at higher rotation rates. Based on the work of Vasanta Ram (2019) we formulate the flow problem by means of the inner swirl parameter (Si), the outer swirl parameter (So) and the curvature parameter (ϵ). Six different values of the curvature parameter {0.005; 0.025; 0.111; 0.25; 0.333; 0.785} and a swirl range of 10−5<Si;So<105 are investigated. We reveal that for the rotating outer cylinder case three characteristic jumps of the critical axial and azimuthal wavenumbers can be identified. Similar features can be identified for the inner rotation case. Based on this, we define three regimes delimited for both the rotating inner and rotating outer cylinder which enables us to perform a systematic comparison. The first regime transition occurs at low values of the swirl parameter (Si; So>10−4) and is accompanied by a rapid decrease of the critical Reynolds number. Our computations reveal that this regime transition does not occur for a small curvature parameter of ϵ= {0.005; 0.025} in the outer rotation case. The second wavenumber jump occurs at higher swirl parameters of around Si>2.5 and So>1.6. While the critical Reynoldsnumber decreases monotonously in the inner cylinder rotating case, we show that in the case of rotating outer cylinder a rapid increase of Rec is observed as So increases further. To gain an insight into the mechanisms involved that induce such wavenumber jumps, a detailed comparative study on the associated Reynolds shear stress distributions is performed. For larger ϵ it is shown that the distributions of the Reynolds shear stresses change significantly over the first jump exhibiting similar features for both flow cases, leading to the conclusion that in both cases the same centrifugal instability is triggered here. Instead, the distributions of the Reynolds shear stresses before and after the second jump are fundamentally different for inner and outer rotation. While the Reynolds shear stresses spread over the whole gap height for the inner rotation case, they are nonzero only close to a small regime near the inner cylinder for the outer rotation case at high rotation rates. Overall, analyzing the shear stresses, we reveal that a similar instability arises in the Spiral Poiseuille flow with inner and outer cylinder rotation at intermediate swirls, but at higher swirl outer rotation seems to give rise to an instability associated with a critical layer that is inherently different to the inner rotation case.

An experimental and numerical study to evaluate performance parameters of GIS circuit breaker during low current interruption

Abstract The basic philosophy of two-stage blast interrupter is combination of self-blast and puffer-interrupter principles. Gas circuit breaker (GCB) with two-stage blast interrupter has two gas chambers. High gas pressure in one chamber generates by using arc energy and in other chamber, by mechanical compression. In such type of breakers, interruption becomes quite difficult at low currents and hence optimization of gas chambers design is essential. In view of above, a numerical program has been developed to calculate SF6 gas pressure-rise in various interrupter volumes and gas flow rates across different passages of interrupter w.r.t. time. To validate model developed in the study, an experimental set-up is established and measured transient gas pressure-rise generated during 420 kV GCB operation. The effect of speed-time characteristics, stroke length and piston diameter on performance parameters of 420 kV GCB model is analyzed. Further, to understand gas flow pattern across insulated nozzle during GCB operation, authors developed a CFX model. By using this numerical model, effect of nozzle profile, gas flow passage areas and profile of gas discharge vents on transient gas pressure and Mach number distribution across inter-electrode gap and gas flow rates through various outlets has been discussed in detail in the paper.

P17: “Vadoscopy” Of Universal Ventricular Assist Device Shut-Off Feature During Pump Operation

Objective: The automatic, universal ventricular assist device has been designed for biventricular support (RVAD or LVAD). Device architecture (UVAD, Fig. 1A, B) enables a life-saving automatic forward flow shut-off function that, in the event of power interruption, helps mitigate patient hazard. Herein, we report the endoscopic visualization (“vadoscopy”) approach used to directly observe the UVAD shut-off function during device activation. Methods: The UVAD implanted in an ex vivo model and connected to a mock circulatory loop was used to simulate pump performance conditions. Device insertions in RVAD and LVAD positions were individually tested. The endoscopic visualization (Karl Storz 5 mm, 30° endoscope; Olympus CH-S190-XZ-E/Q Full HD 3CCD Camera) of the pump rotor occurred during operation and shut-off. The endoscope was placed through left and right atrial appendage and secured with a purse-string, respectively for UVAD used for left (LVAD) and right ventricular (RVAD) support. Results: The circulatory mock loop and ex vivo heart circulation were successfully maintained for both RVAD and LVAD settings. Endoscope was introduced through hermetically sealed insertion point. Direct visualization of the moving UVAD rotor was achieved at various modulated speeds and flows (continuous and pulsatile). The visual confirmation of sub-optimal and full rotor shut-off function (complete closure) was obtained in RVAD and LVAD conditions (Fig. 1C, D, E, F). Conclusion: The UVAD aperture mechanism enabled optimal forward flow shut-off in continuous and pulsatile speed and flow modulation. Direct endoscopic visualization of the UVAD with moving rotor has been feasible. Additional device feature assessment is ongoing.Figure 1. A - UVAD with open aperture; B - UVAD closed aperture; C - Rotor view via inflow; D - Rotor at nominal speed; E - Simulated sub-optimal rotor closure with large gap and regurgitant flow; F - Full rotor shut-off (nearly zero flow) through aperture area.

Enhancing Savonius Vertical Axis Wind Turbine Performance: A Comprehensive Approach with Numerical Analysis and Experimental Investigations

Small-scale vertical-axis wind power generation technologies such as Savonius wind turbines are gaining popularity in suburban and urban settings. Although vertical-axis wind turbines (VAWTs) may not be as efficient as their horizontal-axis counterparts, they often present better opportunities for integration within building structures. The main issue stems from the suboptimal aerodynamic design of Savonius turbine blades, resulting in lower efficiency and power output. To address this, modern turbine designs focus on optimizing various geometric aspects of the turbine to improve aerodynamic performance, efficiency, and overall effectiveness. This study developed a unique optimization method, incorporating a new blade geometry with guide gap flow for Savonius wind turbine blade design. The aerodynamic characteristics of the Savonius wind turbine blade were extensively analyzed using 3D ANSYS CFX software. The optimization process emphasized the power coefficient as the objective function while considering blade profiles, overlap ratio, and blade number as crucial design parameters. This objective was accomplished using the design of experiments (DOE) method with the Minitab statistical software. The research findings revealed that the novel turbine design “OR0.109BS2BN2” outperformed the reference turbine with a 22.8% higher power coefficient. Furthermore, the results indicated a trade-off between the flow (swirling flow) through the gap guide flow and the impact blockage ratio, which resulted from the reduced channel width caused by the extended blade tip length.

Wake Control of Flow Past Twin Cylinders via Small Cylinders

The drag and lift force of a twin-cylinder structure are often greater than those of a single cylinder, causing serious structural safety problems. However, there are few studies on the passive control of twin cylinders. The study aimed to investigate the performance of passive drag reduction measures using small cylinders on twin cylinders at a Reynolds number of 100. The effects of small cylinder height (HD/D = 0~1.0, D is the side length of the twin cylinder) and cross-sectional shape on fluid force and flow structures were studied by direct numerical simulations. The control mechanism was analyzed using high-order dynamic mode decomposition (HODMD). The results showed that significant drag reduction occurred in the co-shedding state, particularly when the gap length of the twin cylinders L/D = 6.0. The small control cylinders with HD = 0.6, by contrast, showed the best performance in reducing the mean drag and fluctuating lift of the twin cylinders. It reduced the mean drag of the upstream cylinder (UC) by 2.58% and the downstream cylinder (DC) by more than 62.97%. The fluctuating lift coefficient for UC (DC) was also decreased by more than 70.41% (59.74%). The flow structures showed that when the flow hit UC under the action of small control cylinders, a virtual missile-like aerodynamic shape was formed at the leading edge of UC. In this way, the gap vortex consisted of two asymmetric steady vortices and the vortex length significantly increased. This was also confirmed by HODMD. The coherence modes in the gap were suppressed and thus the interaction between gap flow and wake flow was mitigated, which resulted in the fluid force reduction.

Balancing Physical Channel Stability and Aquatic Ecological Function through River Restoration

Vortex rock weirs (VRW) are often used in natural channel design applications to maintain channel form and function, provide physical channel stability, and enhance aquatic habitats. A balanced approach is required to address (often) conflicting goals of VRWs, which include providing erosion protection and grade control while facilitating fish passage for target species. This research evaluated a sequence of modified VRWs in a small-scale watercourse in Southern Ontario, Canada. To determine passage suitability for the target fish species, the water level, water temperature, and channel geometries at 10 VRWs and 11 adjacent pools were monitored under different water level conditions. The structural dimensions and velocity at each VRW were compared to the burst swim speed of local small-bodied fish species to determine fish passage suitability and identify the best practices for VRW design and construction. The results concluded that VRWs provided suitable passage for small-bodied fish species through gap and over-weir flow pathways, particularly during low water level conditions. Further, appropriate design considerations based on the VRW gradient, VRW width, keystone size, and pool length contributed to 100% fish ‘passability’ under all water level conditions. The methodology is provided for predicting the velocity and small-bodied fish passage suitability through VRWs, informing the best practices for VRW design and construction while balancing the requirements for channel stability and fish passage, and contributing to fish population management strategies.

Finned Tubular Air Gap Membrane Distillation.

Finned tubular air gap membrane distillation is a new membrane distillation method, and its functional performance, characterization parameters, finned tube structures, and other studies have clear academic and practical application value. Therefore, the tubular air gap membrane distillation experiment modules composed of PTFE membrane and finned tubes were constructed in this work, and three representative air gap structures, including tapered finned tube, flat finned tube, and expanded finned tube, were designed. Membrane distillation experiments were carried out in the form of water cooling and air cooling, and the influences of air gap structures, temperature, concentration, and flow rate on the transmembrane flux were analyzed. The good water-treatment ability of the finned tubular air gap membrane distillation model and the applicability of air cooling for the finned tubular air gap membrane distillation structure were verified. The membrane distillation test results show that with the tapered finned tubular air gap structure, the finned tubular air gap membrane distillation has the best performance. The maximum transmembrane flux of the finned tubular air gap membrane distillation could reach 16.3 kg/m2/h. Strengthening the convection heat transfer between air and fin tube could increase the transmembrane flux and improve the efficiency coefficient. The efficiency coefficient (σ) could reach 0.19 under the condition of air cooling. Compared with the conventional air gap membrane distillation configuration, air cooling configuration for air gap membrane distillation is an effective way to simplify the system design and offers a potential way for the practical applications of membrane distillation on an industrial scale.

Bubble Sliding Characteristics and Dynamics of R134a during Subcooled Boiling Flow in a Narrow Gap

The numerical method was used to study bubble sliding characteristics and dynamics of R134a during subcooled flow boiling in a narrow gap. In the numerical method, the volume of fraction (VOF) model, level set method, Lee phase change model and the SST k − ω turbulent model were adopted for the construction of the subcooled flow boiling model. In order to explore bubble sliding dynamics during subcooled flow boiling, the bubble sliding model was introduced. The bubble velocity, bubble departure diameter, sliding distance and bubble sliding dynamics were investigated at 0.2 to 5 m/s inlet velocities. The simulation results showed that the bubble velocity at the flow direction was the most important contribution to bubble velocity. Additionally, the bubble velocity of 12 bubbles mostly oscillated with time during the sliding process at 0.2 to 0.6 m/s inlet velocities, while the bubble velocity increased during the sliding process due to the bubble having had a certain inertia at 2 to 5 m/s inlet velocities. It was also found that the average bubble velocity in flow direction accounted for about 80% of the mainstream velocities at 0.2 to 5 m/s. In the investigation of bubble sliding distance and departure diameter, it was concluded that the ratio of the maximum sliding distance to the minimum sliding distance was close to two at inlet velocities of 0.3 to 5 m/s. Moreover, with increasing inlet velocity, the average sliding distance increased significantly. The average bubble departure diameter obviously increased from 0.2 to 0.5 m/s inlet velocity and greatly reduced after 0.6 m/s. Finally, the investigations of the bubble sliding dynamics showed that the surface tension dominated the bubble sliding process at 0.2 to 0.6 m/s inlet velocities. However, the drag force dominated the bubble sliding process at 2 to 5 m/s inlet velocities.

An investigation of forces on a representative surface in a pulp flow through rotating and non-rotating grooves

Softwood pulp flow in rotating and non-rotating grooves is numerically simulated in the present study to investigate the fluid flow and the forces acting on a representative surface mounted in the groove. The viscosity of softwood pulp with various consistencies is available from the measurements reported in the literature providing the opportunity to examine the effects of fiber consistency on the velocity and pressure distribution within the groove. The simulations are carried out in OpenFOAM for different values of gap thickness, angular velocity and radial positions from which the pressure coefficient and shear forces values are obtained. It is found that the shear forces within the gap increase linearly with the angular velocity for all fiber consistencies investigated and in both grooves. Also, this behavior can be successfully predicted by modeling the gap flow as a Couette flow in a two-dimensional channel. Meanwhile, a more detailed analysis of the flow kinetic energy close to the stagnation point using Bernoulli’s principle is carried out to provide a better understanding of the pressure coefficient variation with angular velocity in the non-rotating groove. A comparison of pressure coefficients obtained numerically with those calculated by considering the compression effects revealed that the comparison effects are dominating in the pulp flow within the groove.

Vortex shedding analysis of flows past forced-oscillation cylinder with dynamic mode decomposition

Direct numerical simulations are performed for flow past circular cylinders by the lattice Boltzmann method coupled with immersed moving boundary method. By analyzing the flows past a single cylinder at a wide range of cross-flow or in-line oscillation amplitude (0.25≤A/D≤1.5) and frequency (0.5≤fe/f0≤1.5), the results find that the vortex shedding modes inside and outside “lock-in” interval are of significant difference. The vortex shedding mode in the “unlock-in” state is 2S, but C(2S) and P + S shedding modes can be found in the lock-in state. Dynamic mode decomposition is used to analyze characteristic flow features, which shows that mode 1 is the main factor reflecting the flow field structure and mode 2 represents the vortex shedding mode in this work. The vortex shedding modes of flows past a tandem and side-by-side cross-flow double oscillating cylinders are systemically investigated. For tandem double oscillation cylinders, the results of modal decomposition suggest that the shear layer of upstream oscillating cylinder is separated behind the downstream cylinder at a space rate of L/D≤2, but separated behind the upstream cylinder at L/D≥3. Mode 2 at L/D=4 differs from other vortex shedding modes due to the strong inhibition effect by the downstream cylinder on the vortex formation of upstream cylinder. For side-by-side double oscillation cylinders, the wake of two cylinders is a single vortex street at H/D=1, a bistable flow at H/D=2 or 3, a coupled vortex street at H/D=4, and close to a single cylinder at H/D&amp;gt;4. The results of modal decomposition are disordered at H/D=2 due to the interaction between two cylinders and effect of gap flow.

Electromagnetically driven anticyclonic rotation in spherical Couette flow

The electromagnetically driven anticyclonic flow in the wide gap of a rotating concentric spheres system is studied experimentally and numerically in the laminar regime. The working fluid is an electrolyte contained in the spherical gap. The outer sphere rotates at constant angular speed, and the inner sphere is at rest. The electromagnetic stirring is generated due to the interaction of a direct current, which is injected radially through ring-shaped electrodes located at the equatorial zone of each sphere, and a dipolar magnetic field produced by a permanent magnet located inside the inner sphere. Experimental velocity fields in the equatorial plane were obtained with particle image velocimetry. Additionally, full three-dimensional numerical simulations were performed. The high shearing at the equatorial region promotes an instability that can be perceived as a triggering mechanism of four tornado-like vortical structures tilted and entangled in the polar direction. The instability structure rotates in either the cyclonic or the anticyclonic direction, depending on the flow parameters, namely, the angular velocity of the outer sphere and the applied electric current. To the best knowledge of the authors, this is the first time the phenomenon under study is reported with electromagnetic forcing. The numerical results agree quantitatively with the experimental observations.

A Comparison of a Transparent Thermal Insulation System Filled with Refrigerants and a Pig-Fat Based PCM

In this research sustainable refrigerants are tested as filler gases in Transparent Thermal Insulation (TTI) for the first time. These are compared with pig fat, a readily available material with good thermal inertia that is proposed as an organic phase change material (PCM). The aim of this paper is to compare the thermal behaviour of a Hybrid Air Conditioning System (HACS) with TTI filled with R134a, R1233zd and a pig-fat-based PCM. Numerical simulations using the OPAQUE 3 program and two online platforms are used to evaluate the possible application of TTI and PCM as passive systems. Additionally, three TTI models are used to simulate the heat transfer processes of TTI, PCM and R134a. The velocity of the flow in the air gap is also analysed numerically in both laminar and turbulent states. For the assessment, infrared thermographic imagery is used to measure the temperatures in the HACS, giving values of 46.17 °C by day and 38.05 °C at night. The results show that the heat loss and heat gain in the combination TTI filled with refrigerants and pig-fat-based PCM are between 2.22 and 1.51 W/m2. In addition, the HACS was able to keep a small box warm during the night. The flow in the air gap of the HACS can be controlled by installing Ni-Ti wire actuators with a cooling temperature of 23 °C and a heating temperature of 70 °C. The Ni-Ti wire actuators can open and close the dampers at 23 °C and 51 °C, respectively. By installing a 5-watt solar-power fan, the velocity of the flow in the air gap in the HACS can be increased, thus improving the efficiency of the system. In all the experiments, the pig fat proved to be suitable for use in building applications as a non-flammable organic material.

Thermal balance and gap flow of high-pressure, large-scale plunger pairs

Fitting gaps are a critical factor affecting the efficiency of high-pressure, large-scale plunger pumps. In actual work, fitting gaps change under liquid pressure and temperature rise, and the fluid flow in the annular channel changes under the fitting gap, in turn affecting the fitting gap. Therefore, on the basis of the thermo-fluid–structure interaction, a thermodynamic analysis model of plunger friction pairs is established in this work. The model considers the heat production due to plunger pair friction, frictional head loss, local head loss and heat dissipation caused by the fluid and air. Given that heat production and dissipation are greatly affected by annular channel flow, the deformation and leakage equations of the large-scale annular channel are established on the basis of pressure and temperature rise. In consideration of the gap deformation, gap flow, heat production, and heat dissipation of the plunger, a thermal balance equation is established, and the temperature rise of the plunger pair under different initial gaps is analyzed. In addition, a plunger pump experiment system is built to test the temperature rise of the plunger pair. Results show that the model can effectively predict the temperature of the plunger friction pair. The research results can provide a theoretical basis for the design of large-scale, high-pressure gap seals.

Effects of spacing ratio on vortex-induced vibration of twin tandem diamond cylinders in a steady flow

Vortex-induced vibration of twin tandem square cylinders at an inclined angle of 45° to the fluid, i.e., twin diamond cylinders of mass ratio m* = 3, is numerically investigated at Reynolds number Re = 100 and reduced velocity Ur = 3–18. This paper focuses on the effects of cylinders' spacing ratio L* (=L/B, where L is cylinders' center-to-center spacing and B is the characteristic length) ranging from 2 to 6 on the oscillation responses of two-degree-of-freedom cylinders. The results indicate that the wake structure experiences two gap flow patterns, the reattachment and co-shedding regimes, and eight different wake modes. At a small spacing (L* = 2–3), the reattachment regime occurs for the lower or higher Ur with the approximate range of 3 and 16–18. Meanwhile, the reattachment regime mainly occurs for other ranges of Ur at L* = 2–6. The more significant oscillation of each spacing appears in the cross-flow direction, and the maximum cross-flow amplitude of the upstream cylinder is smaller than that of the downstream cylinder. Additionally, although significant cross-flow oscillations occur at small spacings (L* = 2–3) with the Ur ≈ 5–9 and 12–14, the intrinsic mechanisms are entirely different. For the cross-flow oscillation characteristics of larger spacings (L* = 4–6), they are virtually similar.

On the assessment of the carrying capacity of drilling fluids

Methods for evaluating the carrying capacity of drilling fluids for effective hole cleaning are considered. An indicator of the carrying capacity of drilling fluids is proposed, considering the completeness of the flow profile in the annulus. For the rheological models of Newton, Ostwald, Bingham, Herschel – Bulkley and Shulman – Casson, the influence of flow rate and rheological properties on the carrying capacity index in laminar flow in a concentric annular gap is studied. Based on the analysis of field data, the effect of temperature on the carrying capacity of drilling fluids is shown. Keywords: carrying capacity; drilling fluid; laminar flow; rheological model.