Flow Drag Research Articles

Numerical study on drag and heat flux reduction induced by a counterflowing jet for rarefied hypersonic flow over a blunt body

This paper focuses on the drag and heat flux reduction induced by a counterflowing jet located on the leading edge of the blunt body head in rarefied hypersonic flows using the direct simulation Monte Carlo method. Flow structures in the flowfield, such as detached shock wave, Mach disk, contact surface, jet layer, and recompression shock wave, are all weakened gradually with the increase in the freestream altitude, and they eventually disappear at the altitude of 90 km. The increase in the jet pressure provides a great drag reduction by up to 53% when it increases from 800 to 1600 Pa, but the proportion of drag on the blunt body head to the total drag is only affected slightly by the jet pressure. A noteworthy finding is that further increasing jet pressure almost have no effect on heat flux variation when it is larger than 1200 Pa. On the whole, jet temperature has a quite weak influence on both flow structures and drag, while heat flux on the blunt body head is closely related to jet temperature. The results suggest that jet temperature should vary with that of blunt body surface, and moreover, the optimal jet temperature should be moderately lower than the wall surface temperature. In addition, increasing freestream altitude can provide excellent performance of drag reduction, but it causes non-monotonic variation of heat flux. In view of this, it is worth noting that heat flux on the blunt body head actually increases with altitude when the blunt body is in a severely rarefied atmospheric environment, such as the altitude H > 70 km.

Dielectrophoretic cell sorting with high velocity enabled by two-layer sidewall microelectrodes extending along the entire channel

Dielectrophoresis has been widely applied in microfluidic cell separation, and it enables label-free and size-independent cell sorting based on their dielectric fingerprints which is prerequisite for downstream biomedical analysis. The linear flow velocity and cross-sectional area are the two key factors proportional to the flow rate and thus directly related to the sample processing throughput. Aiming at higher flow rate, the state-of-art techniques mostly focus on the cross-sectional area, either by using wide chamber with angled electrode digits or 3D volumetric electrodes with expanded channel depth. On the other hand, the velocity is always carefully limited because cells have very squeezed time to interact with the DEP force that drives them to migrate, and the flow drag induced by high velocity potentially poses adverse effect against the DEP-driven cell migration. In this paper, we present DEP-activated cell sorting at high-velocity being 1–2 orders of magnitude higher than the existing DEP separators, which is enabled by easily-fabricated two-layer 3D sidewall electrodes extending along the entire flow channel. Such electrodes project DEP force fully decoupled from the flow drag, and acting on cells continuously, allowing sufficient time for cell migration. The two-layer design leads to cell migration across a shorter distance between the upper and lower layers, with a unique bridge adaptor evolved at the downstream for the collection of the cells vertically separated. The DEP device achieves separation of live and dead yeast cells at high velocity up to 78.3 mm/s, more than a full order of magnitude higher compared to any reported DEP platforms. This velocity corresponds to a competitive flow rate of 3.1 mL/h given the fairly small cross-sectional area, where the averaged collection rate is over 95 %. For the separation between the live and dead Hela cells, our device enables a high velocity up to 50.5 mm/s with an averaged collection rate of ∼85 %. A high throughput of 1.79 × 1 0^(5) cells/min for the live and dead Hela cell separation has been achieved with 89.7 % collection for live cells and a velocity of 30.3 mm/s.

Modeling water transport in polymer electrolyte membrane electrolyzers using a one-dimensional transport model

In this paper, we present a one-dimensional transport model for water transport across Nafion® membranes in a proton exchange membrane electrolyzer. This physics-based model is based on the three main water transport mechanisms: diffusion, electro-osmotic drag, and pressure-driven flow. A good agreement is obtained between the model predictions using the measured values of the physical parameters as inputs and the previously reported experimental data. Further, with the help of this simple transport model, the numerical analysis is performed to delineate the effect of electrolyzer operating conditions on the net water transport across the membrane, water condensation at the cathode, individual contribution of the transport fluxes, and electrolyzer design. Finally, the model is exercised to simulate the dependence of water transport as a function of membrane thickness. This confirms the validity of the current approach of using thin reinforced membranes by electrolyzer fabricators.

Spatial discretization effects in spanwise forcing for turbulent drag reduction

Wall-based spanwise forcing has been experimentally used with success by Auteri et al. (Phys. Fluids, vol. 22, 2010, 115103) to obtain large reductions of turbulent skin-friction drag and considerable energy savings in a pipe flow. The spatial distribution of the azimuthal wall velocity used in the experiment was not continuous, but piecewise constant. The present study is a numerical replica of the experiment, based on a set of direct numerical simulations (DNS); its goal is the identification of the effects of spatially discrete forcing, as opposed to the idealized sinusoidal forcing considered in the majority of numerical studies. Regardless of the discretization, with DNS the maximum drag reduction is found to be larger: the flow easily reaches complete relaminarization, whereas the experiment was capped at 33 % drag reduction. However, the key result stems from the observation that, for the piecewise-constant forcing, the apparent irregularities of the experimental data appear in the simulation data too. They derive from the rich harmonic content of the discontinuous travelling wave, which alters the drag reduction of the sinusoidal forcing. A detailed understanding of the contribution of each harmonic reveals that, whenever for example technological limitations constrain one to work far from the optimal forcing parameters, a discrete forcing may perform very differently from the corresponding ideal sinusoid, and in principle can outperform it. However, care should be exercised in comparison, as discrete and continuous forcing have different energy requirements.

Numerical investigation of effect of the design parameters of the counter-flow jet for drag reduction in hypersonic low-Reynolds number regime

Due to the correlation of design parameters of the counter-flow jet in addition to the complexity of the flow field, understanding the mechanism of the counter-flow jet for drag reduction and flow control remains challenging. Furthermore, to satisfy the demands of the space transportation system, investigating the counter-flow jet's suitability for a range of flight conditions is critical. To solve these problems, a study was performed by varying the pressure ratio (PR) and exit Mach number of the counter-flow jet at hypersonic low-Reynolds number regime. For numerical simulations, laminar, axisymmetric Navier–Stokes equations were solved by the total variation diminishing scheme with second-order accuracy in space and the explicit strong stability preserving the Runge–Kutta method. With given numerical conditions, the flow field was categorized as the long penetration mode (LPM) based on the penetration length and the fluctuation of the flow field at the high-Reynolds number regime. By reducing the free-stream flow Reynolds number while keeping other parameters unchanged, the flow field transitioned from the LPM to a stable LPM, short penetration mode, or long penetration with periodically oscillation mode. The critical Reynolds number for the transition of the flow field is highly dependent on the exit Mach number of the counter-flow jet and PR. The extended jet layer was the primary reason for the fluctuation of the drag coefficient. Furthermore, with the counter-flow jet at certain flight conditions, drag can be reduced by up to 78% regardless of the stability of the flow field.

Meteoroid rotation and quasi-periodic brightness variation of meteor light curves

Meteor light curves are sometimes known to display flickering: rapid, quasi-periodic variations in brightness. This effect is generally attributed to the rotational modulation of the ablation rate, which is caused by the time-varying cross section area presented by a nonspherical rotating meteoroid to the oncoming airflow. In this work we investigate the effects that the rotation of a meteoroid of given shape (spherical, cubic, or cylindrical) has on the meteor’s light curve, given state-of-the-art experimental laboratory estimates of the drag and lift coefficients of hypersonic flow (Mach number > 5) around various shaped objects. The meteoroid’s shape is important in determining these two forces, due to the different response of the drag and lift coefficients according to the angle of attack. As a case study, the model was applied to a fireball observed on 2018 April 17 by the PRISMA network, a system of all-sky cameras that achieves a systematic monitoring of meteors and fireballs in the skies over the Italian territory. The results show that this methodology is potentially able to yield a powerful diagnostic of the rotation rate of meteoroids prior to their encounter with the atmosphere, while also providing essential information on their pre-fall actual shapes.

An analysis of drag reduction using spanwise forcing on rough walls

Spanwise opposed wall-jet forcing has been shown to reduce the skin-friction drag of wall-bounded turbulent flows by suppressing the near-wall turbulent motion (Yao et al., 2018). In the present work, the response of this drag reduction mechanism to the presence of surface roughness is studied. To this end, direct numerical simulations of flow in smooth and rough plane channels at a matched friction Reynolds number (Reτ=180) are carried out. The roughness is generated by distributing discrete roughness elements at two different values of frontal solidity, namely 0.021 and 0.045. The roughness elements height is k+=18 (＋indicates viscous scaling). By varying the forcing amplitude A+, it is shown that, when other forcing parameters are fixed, maximum drag reduction in a rough channel is achieved at a considerably larger A+ than in a smooth channel. Notably, the strength of the wall-jet at which the maximum drag reduction is achieved, is comparable for the smooth and both rough cases. Additionally, it is observed that the maximum drag reduction values attained in rough channels are smaller compared to those observed in smooth channels, for both lower and higher frontal solidity cases — 2.4% and 2%, respectively. The reduced drag reduction potential originates from two observations; firstly, drag on roughness elements, which is dominated by pressure drag, is not reduced by the method in question. Secondly, the drag on channel floor is reduced to a lesser extent when roughness elements are present. The latter can be attributed to the observation that, while in all cases the forcing suppresses streamwise random turbulent fluctuations and shear stress, this suppression is less pronounced when roughness elements are present.

Free-surface channel flow around a square cylinder

The free-surface channel flow around a square cylinder is analysed, over a wide range of blocking ratios, using three-dimensional simulations. The state of the flow is characterised in terms of the Froude number upstream and downstream of the square cylinder. The simulations confirm the presence of the subcritical and choked states, and provide new insight into the supercritical state and band-gap through an analysis of how the momentum flux varies with Froude number along the channel. The influence of the blocking ratio on the flow state and drag force is analysed and shows the significant rise of drag in the choked regime.

Flow analysis of screw extrusion in three-dimensional concrete printing

Recent advances in three-dimensional concrete printing necessitated the detailed understanding of the operation and performance of screw extruders. This paper shows that the volumetric output rate can be approximated using the rotating barrel and stationary screw assumption (drag flow equation), which is used routinely in polymer melt extrusion calculations. Verification is provided by comparisons to experimental results available in the literature and to computer flow simulations for fluids with yield stress. Significant insight is obtained using fully three-dimensional simulations. This includes particle pathlines, which form “a helix within a helix,” axial pressure profiles, and the effect of yield stress, which is relatively small on the output rate but large on torque and power. The computer simulation also predicts unyielded flow zones in the extruder channel at low screw rotation speeds.

Aero-spike and aero-disk effects of on wave drag reduction of supersonic flow past over blunt body

PurposeThe purpose of this study is to explore the effects of aerospike and hemispherical aerodisks on flow characteristics and drag reduction in supersonic flow over a blunt body. Specifically, the study aims to analyze the impact of varying the length of the cylindrical rod in the aerospike (ranging from 0.5 to 2.0 times the diameter of the blunt body) and the diameter of the hemispherical disk (ranging from 0.25 to 0.75 times the blunt body diameter). CFD simulations were conducted at a supersonic Mach number of 2 and a Reynolds number of 2.79 × 106.Design/methodology/approachICEM CFD and ANSYS CFX solver were used to generate the three-dimensional flow along with its structures. The flow structure and drag coefficient were computed using Reynolds-averaged Navier–Stokes equation model. The drag reduction mechanism was also explained using the idea of dividing streamline and density contour. The performance of the aero spike length and the effect of aero disk size on the drag are investigated.FindingsThe separating shock is located in front of the blunt body, forming an effective conical shape that reduces the pressure drag acting on the blunt body. It was observed that extending the length of the spike beyond a specific critical point did not impact the flow field characteristics and had no further influence on the enhanced performance. The optimal combination of disk and spike length was determined, resulting in a substantial reduction in drag through the introduction of the aerospike and disk.Research limitations/implicationsTo predict the accurate results of drag and to reduce the simulation time, a hexa grid with finer mesh structure was adopted in the simulation.Practical implicationsThe blunt nose structures are primarily employed in the design of rockets, missiles, and re-entry capsules to withstand higher aerodynamic loads and aerodynamic heating.Originality/valueFor the optimized size of the aero spike, aero disk is also optimized to use the benefits of both.

Hitchhiker: A Quadrotor Aggressively Perching on a Moving Inclined Surface Using Compliant Suction Cup Gripper

Perching on the surface of moving objects, like vehicles, could extend the flight time and range of quadrotors. Suction cups are usually adopted for surface attachment due to their durability and large adhesive force. To seal on a surfaces, suction cups must be aligned with the surface and possess proper relative tangential velocity. However, quadrotors’ attitude and relative velocity errors would become significant when the object surface is moving and inclined. To address this problem, we proposed a real-time trajectory planning algorithm. The time-optimal aggressive trajectory is efficiently generated through multimodal search in a dynamic time-domain. The velocity errors relative to the moving surface are alleviated. To further adapt to the residual errors, we design a compliant gripper using self-sealing cups. Multiple cups in different directions are integrated into a wheel-like mechanism to increase the tolerance to attitude errors. The wheel mechanism also eliminates the requirement of matching the attitude and tangential velocity. Extensive tests are conducted to perch on static and moving surfaces at various inclinations. Results demonstrate that our proposed system enables a quadrotor to reliably perch on moving inclined surfaces (up to <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$1.07m/s$</tex-math> </inline-formula> and <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$90^\circ$</tex-math> </inline-formula> ) with a success rate of <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$70\%$</tex-math> </inline-formula> or higher. The efficacy of the trajectory planner is also validated. Our gripper has larger adaptability to attitude errors and tangential velocities than conventional suction cup grippers. The success rate increases by 45% in dynamic perches. <italic xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">Note to Practitioners</i> —This paper was motivated by the problem of perching on moving inclined surfaces using quadrotors. It can be used for perching on the various angled surfaces of an automobile to save energy and enlarge flight distance. It can also be exploited in air-ground cooperative tasks. In recent years, various grippers and trajectory planning methods have been devised to enable quadrotors to perch on static inclined surfaces. These strategies can not be used for perching on moving inclined surfaces. The task poses high requirements for the efficiency of trajectory planning and the grippers’ adaptability to pose errors. This paper proposes a real-time planning method to generate trajectories. The trajectories respect kinematic and motor lift constraints. They allow large attitude maneuvering of quadrotors. A multimodal search in a dynamic time-domain is developed to seek the minimum feasible time for the trajectory planner. Considering large pose errors in dynamic perching, a compliant gripper based on multiple independent suction cups is designed. The wheel and multidirectional layout of cups increase the adaptability to attitude and velocity errors. Results suggest that the proposed system is effective and reliable. In practice, our multiple cups would add weight penalty and increase the drag. The suction cups should be placed close to a quadrotor’s central axis to reduce the drag and disturbance in flow. The proposed system relies on an external motion capture system. In future research, we will focus on onboard sensing and control methods to achieve perching on moving inclined surfaces outdoors.

Evaluation of Various Flow Control Methods in Reducing Drag and Aerodynamic Heating on the Nose of Hypersonic Flying Objects

Effective deduction of air heating load and drag is a critical issue in hypersonic vehicle engineering applications. In this research, seven various geometrical models have been proposed to study and compare the effect of each configuration on the flow field, drag, and aerodynamic heating deduction under the same flow conditions. The presented configurations in this study: (a) blunt-body geometry as a reference of comparison, (b) blunt-body geometry with a spike, (c) blunt-body geometry with an counter flow jet, (d) blunt-body geometry with a spike and counter flow jet, (e) blunt-body geometry with a spike and aerodisk, (f) blunt-body geometry with a spike, aerodisk, and root counter flow jet, (g) blunt-body geometry with a spike, four aerodisks and root counter flow jet. The Reynolds-Averaged equations have been solved using the Finite Volume Method (FVM) along with the shear stress turbulence model (k-ω SST). The flow is assumed compressible, steady-state, and axisymmetric with a free stream Mach number of 6. According to the study of each configuration’s performance related to the parameters of drag, maximum pressure, and maximum heat flux factors on the blunt-body walls, (g) configuration with a drag factor of 0.2699, maximum pressure factor of 209.8, and maximum heat flux factor of 25.1, has the most deduction on the blunt-body walls among the seven configurations. The deduction percentage of drag, maximum pressure, and maximum heat flux factors of (g) configuration to (a) configuration are %72.1, %94.5, and %79.9, respectively, which significantly diminished drag and heat flux. Also, the best configuration scenarios for drag and aerodynamic heating deduction are geometrical models of g, f, d, e, c, b, and a, respectively.

Digital core: study of textural heterogeneities in a rock

Relevance. When real fluid flows through channels of limited dimensions, it experiences a constant loss of mechanical energy (hydraulic resistance) due to the action of viscous friction. Hydraulic drag has two components: drag along the length of the flow and local drag. The drag along the length of the flow arises because the fluid encounters a force per unit area perpendicular to the direction of the flow. Local drag results from the viscous interaction between the fluid and the channel walls. Local hydraulic resistances include sudden and gradual constrictions/expansions (diffusers/confusions) of the fluid channel, angular and choke gaps. The value of the hydraulic resistance coefficient depends on the type of channel geometry, conditions of fluid entry (wettability of the rock texture) and on the flow regime. When fluid flows through channels of different cross-section, as the channel narrows, the flow velocity increases and the pressure, based on Bernoulli's equation, decreases. Objects. Polymictic sandstones of the Tyumen Formation. Methods. The developed algorithms for identifying areas of contraction/expansion (confusers/diffusers) in the rock texture (polymictic sandstone) are based on neural network reconstruction of core data. The study of geometric diversity of geological rock images is based on generalization of real spatial forms in combination with the axiom of hyperplanes. A methodical approach for finding distribution of inertial exciters (confusers/diffusers) affecting the character of multiphase flow front changes in the rock texture is based on the metrics for estimating possible distributions. Results. The paper introduces the initial stages of core digital transformation data. It was determined that the main problem of geological data digital projection on a large scale is the need to take into account the systems of inertial effects, both in the available volume of the rock and in the extrapolated space of it. In order to adequately represent the character of multiphase fluid flow for different scales, the authors have developed the algorithms for segmentation of geological images of fluid space with further identification of distribution laws of inertial exciters (confusers/diffusers) in the rock texture. For training and testing of the developed algorithms we used the computer tomography images of core material and detailed resolution images of rock sections. The source code of neural network algorithms was written in the Python programming language with the additional use of non-commercial libraries. The algorithms were tested on real data, confirming by experimental studies in the laboratory of digital research in oil and gas as part of the technological project "Digital Core" (Industrial University of Tyumen, Tyumen), the laboratory of the V.I. Shpilman Research and Analytical Center for Rational Subsoil Use (Khanty-Mansiysk), Core Research Laboratory (University of Tyumen, Tyumen).

(Digital Presentation) Modeling Water Transport in Proton Exchange Membrane Electrolyzers through First Principles

The efforts to decarbonize transportation and heavy industries, like agricultural chemicals, steel, etc., have increased over the past decade [1-2]. A practical solution to these efforts is using renewable energy (wind, solar, hydro) to produce hydrogen to replace fossil fuels in these energy-intensive applications [1]. The production of hydrogen via water electrolysis has been receiving tremendous attention over the past few years [1-3]. There are mainly three technologies that are employed for water electrolysis – alkaline, polymer electrolyte membrane (PEM), and solid oxide [3]. Intermittent renewable energy provides an advantage to PEM electrolysis due to the robustness of the technology capable of multiple start-stops and variation in applied power [4].PEM electrolyzer technology is improving and becoming market-ready in several countries [3]. The current focus on PEM electrolyzers ranges from improving lifetimes and electrical efficiency, reducing cost by the development of advanced materials/designs, and simplifying the balance of plant/systems designs [1]. The mathematical modeling of PEM electrolyzers is thus an active area of research for substantially reducing development costs and timelines and improving electrolyzer designs [3-4]. However, there are certain phenomena with PEM electrolysis where the current models are semi-empirical and simple first principles-based mechanistic quantifications are lacking.Figure 1 shows a simplified schematic of the physical and chemical phenomena during the operation of a PEM electrolyzer. The anode and cathode are divided by a membrane that is capable of exchanging cations across. The membrane is coated with catalyst layers on both sides to facilitate the reactions. Typical membranes used in electrolyzers belong to the Nafion® family and are perfluorosulfonic acid-based materials [3-7]. Porous transport layers are used to facilitate the interaction between gas and liquid phases in the cell. Water is fed to the anode side of the electrolyzer and is oxidized to produce oxygen gas. Typically, no water is fed to the cathode, since the anode is maintained at a high pressure (~3-70 bar), liquid water condenses and exits the electrolyzer with the hydrogen. The hydrogen gas is fed through a dryer before it is deemed suitable for use. The oxygen gas is also dried and used where needed.In this work, we present a first principles-based water transport model to quantify the movement of water across the electrolyzer cell and predict the quality of the hydrogen gas produced as a function of varying operating conditions and cell designs [4,8,9]. To model the movement of water on the cathode side, three sets of equations are obtained – I. material balances for hydrogen and water in the flow channel (z-direction), II. water movement across the membrane in the x-direction, and III. expressions for variable membrane properties to serve as inputs for I and II [4,5,6,8]. For water movement across the membrane, we consider the contributions from diffusion, electro-osmotic drag, and pressure-driven flow. The condensation of water at the cathode is also modeled to understand the respective transport contributions from the vapor and liquid phases. The coupled equation sets are solved using a Runge-Kutta ODE routine with appropriate boundary conditions. The results of the modeling case studies will be compared with the experimental data available for water transport [9]. References Satyapal, H2@Scale R&D Consortium Kickoff Meeting H2@Scale Overview Chicago, IL – August 1, 2018.Pivovar, 2019 DOE Hydrogen and Fuel Cells Program Review, April 30, 2019.S. Kumar and V. Himabindu, Material Science and Energy Technologies, 2, 442, (2019)Ma et al., International Journal of Hydrogen Energy, 46, 17627, (2021).E. Springer et al., Journal of the Electrochemical Society, 138, 2334, (1991).M. Bernardi and M. W. Verbrugge, AIChE Journal, 37, 1151, (1990).A. Zawodzinski et al., Journal of the Electrochemical. Society, 140, 1981, (1993).Marangio et al. International Journal of Hydrogen Energy, 34, 1143, (2009).Medina and M. Santarelli, International Journal of Hydrogen Energy, 35, 5173, (2010). Figure 1

High-molecular-weight linear polymers improve microvascular perfusion after extracorporeal circulation.

High-molecular-weight linear polymers (HMWLPs) have earned the name "drag-reducing polymers" because of their ability to reduce drag in turbulent flows. Recently, these polymers have become popular in bioengineering applications. This study investigated whether the addition of HMWLP in a venoarterial extracorporeal circulation (ECC) model could improve microvascular perfusion and oxygenation. Golden Syrian hamsters were instrumented with a dorsal skinfold window chamber and subjected to ECC using a circuit comprised of a peristaltic pump and a bubble trap. The circuit was primed with lactated Ringer solution (LR) containing either 5 ppm of polyethylene glycol (PEG) with a low molecular weight of 500 kDa (PEG500k) or 5 ppm of PEG with a high molecular weight of 3,500 kDa (PEG3500k). After 90 min of ECC at 15% of the animal's cardiac output, the results showed that the addition of PEG3500k to LR improved microvascular blood flow in arterioles and venules acutely (2 h after ECC), whereas functional capillary density showed improvement up to 24 h after ECC. Similarly, PEG3500k improved venular hemoglobin O2 saturation on the following day after ECC. The serum and various excised organs all displayed reduced inflammation with the addition of PEG3500k, and several of these organs also had a reduction in markers of damage with the HMWLPs compared to LR alone. These promising results suggest that the addition of small amounts of PEG3500k can help mitigate the loss of microcirculatory function and reduce the inflammatory response from ECC procedures.NEW & NOTEWORTHY High-molecular-weight linear polymers have gained traction in bioengineering applications. The addition of PEG3500k to lactated Ringer solution (LR) improved microvascular blood flow in arterioles and venules acutely after extracorporeal circulation (ECC) in a hamster model and improved functional capillary density up to 24 h after ECC. PEG3500k improved venular hemoglobin O2 saturation and oxygen delivery acutely after ECC and reduced inflammation in various organs compared to LR alone.

Flow over a step cylinder using partially-averaged Navier-Stokes equations with application towards dynamic subsea power cables

Understanding the flow characteristics of buoyancy sections in power cable and umbilical configurations is crucial for analyzing floating offshore wind turbine systems. Buoyancy sections decouple the motions from the floater and the destination point to ensure the integrity of the system. Using accurate drag coefficients in their design is essential, as they significantly impact the overall behavior of the configuration. Therefore, Computational Fluid Dynamic (CFD) analysis is performed in the present study on a single buoyancy module attached to a power cable represented by a step cylinder configuration. The Partially-Averaged Navier-Stokes (PANS) turbulence model is applied. Its accuracy is validated against experimental findings of a wall-mounted cantilever cylinder and an infinite cylinder. The numerical results reveal that the larger diameter cylinder (LDC) section reduces the drag experienced by the smaller diameter cylinder (SDC) section near the junction. The LDC has a sharp, chamfered, and filleted edge replicating the various shapes of buoyancy modules. The edge design of the LDC affects the drag forces and flow patterns. The SDC has an 8% lower drag coefficient in the fillet edge case than the sharp edge case. The drag coefficient is 3.5% lower for the LDC in the filleted edge case than the sharp edge case. The sharp edge causes a significant separation of the fluid upstream of the SDC. However, this separation is notably reduced when the LDC has a chamfered or filleted edge. Detailed drag coefficients for buoyancy section analysis of power cable configurations have been deduced from the presented results.

Direct numerical simulation of compression ramp shock wave/boundary layer interaction controlled by plasma actuator array

Direct numerical simulation of compression ramp shock wave/boundary layer interaction controlled by plasma actuator array is performed to gain a comprehensive understanding of its control outcome and mechanism in this paper. The computation model adopts a 24-deg compression ramp at Mach number Ma∞ = 2.0, whose simulation setup all refers to the experimental reports. First, the plasma actuator array is introduced into the Navier–Stokes (N–S) equation by a heating source model, and the corresponding numerical validation is proved in detail. Then, the flow control effect of plasma actuator array is systematically revealed, including the wave drag reduction and flow separation suppression. The results show that the mean shock wave drag can be decreased by 15% and the characteristic length of separation bubble can be reduced by 28%. At last, a new shock wave/boundary layer interaction control mechanism, called as 3D structure reconstruction, is concluded to guide the compression ramp flow control. Namely, by manipulating the quasi-2D shock wave and separation bubble into a 3D structural morphology in spanwise direction, the overall negative effect induced by shock wave/boundary layer interaction can be mitigated.

Dynamics of single bubble ascension in stagnant liquid: An investigation into multiphase flow effects on hydrodynamic characteristics using computational simulation

Industrial operations often involve multiphase flows, where the motion of bubbles plays a crucial role in determining hydrodynamic properties. In the context of modern practices, advanced computational solvers are increasingly employed to simulate the interactions and trajectories of bubbles within complex flows. This study stands out in its unique examination of the computational software COMSOL's capability to accurately simulate the upward motion of a single bubble within quiescent water, akin to a liquid-like environment. The novelty lies in the comprehensive coverage of various container diameters (ranging from 10 to 80 mm) and heights (from 25 to 300 mm) relative to the air bubble diameter (ranging from 0.5 to 8 mm). Through meticulous comparisons between nine empirical formulas and numerically projected bubble ascent through a water column, a remarkable level of agreement emerges, underscoring the precision and consistency of the simulations. These simulations unveil intriguing findings, shedding light on the intricate interplay of forces governing bubble behavior. Notably, variations in drag forces induce changes in bubble shapes as a function of diameter, while the ascent of bubbles is accompanied by distinctive vortices, resulting in fascinating asymmetry. Furthermore, vorticity concentrates within the bubble, particularly in lighter fluid regions characterized by reduced pressure. The study also unveils how larger aspect ratios minimize flow drag, consequently boosting ascent velocities, and demonstrates the influence of container diameter on the rising velocity. Gravitational forces are found to reduce ascent velocities at greater column heights, while the rate of air bubble rise escalates with its size. This meticulous exploration of bubble dynamics in multiphase flows yields invaluable insights for diverse industrial applications, ultimately enhancing our understanding of this complex phenomenon. The strong alignment observed between empirical formulations and numerical simulations within the COMSOL framework underscores the utility of such computational tools for the study and design of multiphase flows' intricate dynamics.

Improving the rheological characteristics of fly ash water slurry using aqueous extract of Diascorea hispida

ABSTRACT Transportation of high-concentration slurry is a specialized field that requires a deep understanding of the rheological behavior of slurry. Fly ash, a fine, powdery residue of pulverized coal combustion in power plants, is transported in pipelines as a slurry with water as a carrier medium and flow-improver additives. The current research aims to compute the rheological properties of the fly ash-water slurry by adding 0.1 to 1 g/mL of Diascorea hispida extract as a natural drag reducer or flow improver. In addition to the effect of D. hispida, other parameters such as temperature, addition of fly ash, and fly ash-bottom ash mixture on the rheological performance of fly ash-water slurry have been investigated. The economic effect of the developed dispersant, D. hispida, has been examined by applying Darby – Melson combined equation by varying the slurry flow rate from 1 m/s to 3 m/s at a fixed 250 mm pipe diameter. The slurry head loss, solids conveying rate, hydraulic power need, and specific power consumption at different .D hispida concentrations were calculated.

Heat transfer enhancement and temperature uniformity improvement of microchannel heat sinks with twisted blade-like fins

To enhance heat and fluid exchange between main flow and near-wall flow in microchannels, a twisted blade-like fin with an advantage in stimulating both spanwise and normalwise secondary flow is proposed. The cross sections of the fin are low-drag airfoils in different orientations. The flow and heat transfer performances of microchannels with fins are numerically investigated at Re = 50–700. The results show that the twisted blade-like fin improves heat transfer process significantly, especially wall temperature uniformity, and reduces the flow separation region behind the fin, resulting in an obvious small pressure penalty. At small inflow (Re = 50), the best heat transfer and lowest pressure penalty are obtained in the microchannel with a single fin. When Re > 150, better heat transfer is obtained in the microchannel with three fins, and the highest comprehensive thermal performance (TP) reaches 3.58. The twisted direction of fins has a significant effect on heat transfer but less on flow drag. Due to the twisted fins, left and right walls experience an overall decline in temperature, and the temperature uniformity of top and bottom walls is improved. Compared with a smooth microchannel, the average and maximum temperature of the investigated microchannels are reduced by 48.1 K and 49.0 K at most respectively.