scholarly journals Numerical Investigation of High-Reynolds-Number Air-Ventilated Water Flow under Solid Body with Surface Geometry Variations

Fluids ◽  
2021 ◽  
Vol 6 (5) ◽  
pp. 174
Author(s):  
Konstantin I. Matveev ◽  
Jeffrey M. Collins
Keyword(s):  
Stability Boundary ◽  
Practical Interest ◽  
The Body ◽  
Surface Geometry ◽  
Air Cavity ◽  
Operational Conditions ◽  
Marine Vehicles ◽  
Air Supply ◽  
High Reynolds ◽  
Air Cavities

Air-ventilated cavities formed under or around the hulls of marine vehicles can reduce water drag. Hull configurations with partial air ventilation where air cavities reattach to body surfaces are of special practical interest, since the required air supply rates to achieve significant drag reduction can be made rather low. However, formation and stability of such air cavities are sensitive to the hull geometry and operational conditions. In this study, an attempt is made to numerically simulate one setup with a partial air cavity that was previously tested experimentally at high Reynolds numbers, above 50 million. A computational fluid dynamics software Star-CCM+ has been employed for numerical modeling. Stable and unstable states of the air-cavity setup, characterized by long and collapsing air cavities, respectively, were modeled at two air supply rates near the stability boundary. Numerical results were similar to experimental data at the optimal water speed for the tested geometry, when a long air cavity was sustained at a minimal air supply rate. For water speeds that were substantially higher or lower than the optimal case, a stable cavity could not be maintained with small air supply rates for the given hull geometry. Numerical simulations demonstrated how alterations of the body surface could help sustain long air cavities across a broader speed range using air supply rates that were similar to the optimal case. These findings suggest that morphing hull surfaces can potentially be used for control of drag-reducing air cavities and expand the viable operating range for their application to marine vehicles.

Control of Air-Ventilated Cavity Under Ship Hull in Abnormal Waves

2019 ◽  
Author(s):  
Konstantin I. Matveev
Keyword(s):  
High Amplitude ◽  
Maritime Transportation ◽  
Practical Implementation ◽  
Frictional Drag ◽  
Air Cavity ◽  
Fuel Savings ◽  
Ship Hulls ◽  
Reduction Techniques ◽  
Air Supply ◽  
Air Cavities

Abstract Practical implementation of ship drag reduction techniques can lead to substantial fuel savings and lessening environmental impacts of maritime transportation. One of such technologies is based on injecting air underneath ship hulls, which results in the formation of thin air cavities that decrease the wetted hull surface and hence its frictional drag. In realistic sea wave conditions, however, these cavities become unsteady and may easily disintegrate upon interaction with high-amplitude abnormal waves. In this study, the air-cavity dynamics in such situations is simulated with a potential flow model and empirical correlations. A method for controlling the air cavity by varying the air supply rate is numerically investigated. It is shown that degradation of the air-cavity power savings in the event of a rogue wave passing can be partly mitigated by briefly boosting the air supply right after the abnormal wave occurrence. For one considered example, it is found that 20% of power savings is lost in a condition with abnormal waves and constant air supply. In case of temporary augmentation of air injection, the overall decrease of power savings is reduced to 10%.

scholarly journals Simplified model for unsteady air cavities under ship hulls

2019 ◽  
Vol 234 (1) ◽  
pp. 100-107
Author(s):  
Konstantin I Matveev
Keyword(s):  
System Optimization ◽  
Practical Implementation ◽  
Air Cavity ◽  
Ship Hulls ◽  
Air Supply ◽  
Cavity System ◽  
Air Pockets ◽  
Air Cavities ◽  
Solid Walls ◽  
Unsteady Conditions

The drag reduction technique involving air cavities under ship hulls is a promising energy-saving technology. Understanding the air cavity dynamics in unsteady conditions and developing methods for the air cavity system optimization are critically important for practical implementation of this technology. In this study, a potential-flow theory is applied for modeling the air cavities under solid walls in water flow with fluctuating pressure. The present modeling approach incorporates detachment of macroscopic air pockets from the cavity tail. For specific configurations considered in this article, it is found that a change of the rate of air supply into the cavity can partly mitigate degradation of the overall power savings by the air cavity system in unsteady conditions.

Static Control of Drag-Reducing Air Cavities with Variable Cavitator Shape

2019 ◽  
Vol 161 (A1) ◽  
Keyword(s):  
Drag Reduction ◽  
High Performance ◽  
Wedge Angle ◽  
Flow Speed ◽  
Frictional Drag ◽  
Air Cavity ◽  
Operational Conditions ◽  
Ship Hulls ◽  
Air Ventilation ◽  
Air Cavities

Air ventilation of submerged surfaces of ship hulls is a promising technique for drag reduction. To ensure high performance of air cavities in a broad range of operational conditions, the cavity properties can be controlled with help of compact hydrodynamic actuators. In this study, a potential flow theory is applied to model an air cavity formed behind a wedge-shaped cavitator under a horizontal wall imitating a ship bottom. By varying the wedge angle, it is possible to achieve states with maximum drag reduction at given operational conditions. The dependence of the optimal wedge angle on Froude number and hull trim is investigated. The air-cavity ability to reduce frictional drag is found to increase with rising flow speed and bow-down hull trim.

STATIC CONTROL OF DRAG-REDUCING AIR CAVITIES WITH VARIABLE CAVITATOR SHAPE

2021 ◽  
Vol 161 (A1) ◽  
Author(s):  
K I Matveev
Keyword(s):  
Drag Reduction ◽  
High Performance ◽  
Wedge Angle ◽  
Flow Speed ◽  
Frictional Drag ◽  
Air Cavity ◽  
Operational Conditions ◽  
Ship Hulls ◽  
Air Ventilation ◽  
Air Cavities

Air ventilation of submerged surfaces of ship hulls is a promising technique for drag reduction. To ensure high performance of air cavities in a broad range of operational conditions, the cavity properties can be controlled with help of compact hydrodynamic actuators. In this study, a potential flow theory is applied to model an air cavity formed behind a wedge-shaped cavitator under a horizontal wall imitating a ship bottom. By varying the wedge angle, it is possible to achieve states with maximum drag reduction at given operational conditions. The dependence of the optimal wedge angle on Froude number and hull trim is investigated. The air-cavity ability to reduce frictional drag is found to increase with rising flow speed and bow-down hull trim.

scholarly journals Movement of a finite body in channel flow

2016 ◽  
Vol 472 (2191) ◽  
pp. 20160164 ◽  
Author(s):  
Frank T. Smith ◽  
Edward R. Johnson
Keyword(s):  
Reynolds Number ◽  
Channel Flow ◽  
Flow Instability ◽  
Flow Direction ◽  
Finite Size ◽  
The Body ◽  
Thin Body ◽  
Shear Stresses ◽  
Unsteady Interaction ◽  
High Reynolds

A body of finite size is moving freely inside, and interacting with, a channel flow. The description of this unsteady interaction for a comparatively dense thin body moving slowly relative to flow at medium-to-high Reynolds number shows that an inviscid core problem with vorticity determines much, but not all, of the dominant response. It is found that the lift induced on a body of length comparable to the channel width leads to differences in flow direction upstream and downstream on the body scale which are smoothed out axially over a longer viscous length scale; the latter directly affects the change in flow directions. The change is such that in any symmetric incident flow the ratio of slopes is found to be cos ⁡ ( π / 7 ) , i.e. approximately 0.900969, independently of Reynolds number, wall shear stresses and velocity profile. The two axial scales determine the evolution of the body and the flow, always yielding instability. This unusual evolution and linear or nonlinear instability mechanism arise outside the conventional range of flow instability and are influenced substantially by the lateral positioning, length and axial velocity of the body.

Apparent and True Charges in the Partial Discharge Model at Different Defective Domain Sizes

2021 ◽  
Vol 64 (6) ◽  
pp. 94-100
Author(s):  
Nikolay Ignatev ◽  
◽  
Sergey Tetiora ◽  
Dmitry Turkin ◽  
◽  
...  
Keyword(s):  
Equivalent Circuit ◽  
Partial Discharge ◽  
Diagnostic Value ◽  
Partial Discharges ◽  
Active Resistance ◽  
Air Cavity ◽  
Discharge Ignition ◽  
Discharge Model ◽  
Air Cavities ◽  
Solid Insulation

A model of the partial discharges in a sample of solid insulation with air cavity is presented. The material of the insulation is cross-linked polyethylene. The model is based on an active-capacitive equivalent circuit, in which the resistance of the air cavity at the instant of a partial discharge ignition is shunted by the active resistance of the spark. The model takes into consideration the delay of the discharge development. The evaluation of the diagnostic value of the apparent and true charges is carried out. The results of the apparent and true charges relationship with the dimensions and proportions of the air cavity are presented. It is shown that the same magnitude of both true and apparent charges can correspond to the different volumes and shapes of the air cavities in the insulation.

Antecedents of horizontal logistics collaboration in agri-food supply chains

2021 ◽  
Vol ahead-of-print (ahead-of-print) ◽  
Author(s):  
Ismail Badraoui ◽  
Ivo van der Lans ◽  
Youssef Boulaksil ◽  
Jack G.A.J. van der Vorst
Keyword(s):  
Supply Chains ◽  
Food Supply ◽  
Structural Equation ◽  
The Body ◽  
Equation Modeling ◽  
Content Type ◽  
Operational Conditions ◽  
Food Supply Chains ◽  
The Impact ◽  
The Relationship

PurposeThis study investigates the impact of agri-food supply chains (AFSCs) characteristics on the antecedents of horizontal logistics collaboration (HLC). Specifically, the study compares the relationship between collaboration activities and outcomes for companies in and outside AFSCs.Design/methodology/approachFirst, a survey was used to collect data from different industries. Second, confirmatory factor analysis and structural equation modeling were applied to compare the measurement and structural models from different industry categories.FindingsThe results support the premise that collaboration improves trust and commitment in the relationship, which in turn enhance satisfaction. The results also show the existence of a minor influence of AFSCs characteristics on HLC antecedents, in the form of an indirect impact of dedicated investments on commitment.Practical implicationsThe factors having a significant influence on the collaboration outcomes and their respective effects are generally similar across food and nonfood supply chains, providing opportunities for interdisciplinary and collaboration experiences.Originality/valueThis research contributes to the body of knowledge on interfirm collaboration by considering the specificities of HLC. It also highlights the importance of conducting contingency research on collaborative experiences, as firms from different industry contexts operate under distinct operational conditions.

scholarly journals Thermal bridge effect of vertical diagonal tie connectors in precast concrete sandwich panels: an experimental and computational study

2020 ◽  
Vol 172 ◽  
pp. 08001
Author(s):  
Paul Klõšeiko ◽  
Reimo Piir ◽  
Marti Jeltsov ◽  
Targo Kalamees
Keyword(s):  
Computational Study ◽  
Precast Concrete ◽  
Sandwich Panels ◽  
Building Envelope ◽  
Heat Fluxes ◽  
Air Cavity ◽  
Thermal Transmittance ◽  
Thermal Bridge ◽  
Thermal Bridging ◽  
Air Cavities

The purpose of this work was to quantify the thermal bridge effect of vertical diagonal tie connectors in precast concrete sandwich panels (PCSPs). Special interest was in cases where the use of rigid insulation (e.g. PIR) would leave air gaps between insulation boards and diagonal ties, thus intensifying the thermal bridge. A climate chamber experiment using 5 different joint types was performed to gather reference data for CFD model validation. In the experiment, natural convection was observed in joints where no additional insulation was used, i.e. in air cavities. Significantly larger heat fluxes were measured in these cavities compared to insulated joints. The thermal bridging effect was evaluated for a typical PCSP (thermal transmittance without thermal bridges U = 0.11 W/(m²·K)) using CFD software taking into account 3D heat conduction and convection. Simulation results indicate that diagonal ties without adjacent air cavities increased the average thermal transmittance (U-value) of the envelope by 8%, diagonal ties with a 6 mm air cavity – 19...33% and diagonal ties with a 10 mm air cavity – 45...56%. In conclusion, it was found that the joints in insulation caused by diagonal ties affect the overall thermal performance of the building envelope significantly when efforts are not made to fill the air cavities around the connectors.

scholarly journals The temporal evolution of neutral modes in the impulsively started flow through a circular pipe and their connection to the nonlinear stability of Hagen–Poiseuille flow

2002 ◽  
Vol 457 ◽  
pp. 339-376 ◽  
Author(s):  
ANDREW G. WALTON
Keyword(s):  
Poiseuille Flow ◽  
Multiple Scales ◽  
Stability Boundary ◽  
Circular Cross Section ◽  
Flow Characteristics ◽  
Internal Flow ◽  
Bypass Transition ◽  
Flow Through ◽  
High Reynolds ◽  
Wave Limit

The linear stability of the impulsively started flow through a pipe of circular cross-section is studied at high Reynolds number R. A crucial non-dimensional time of O(R7/9) is identified at which the disturbance acquires internal flow characteristics. It is shown that even if the disturbance amplitude at this time is as small as O(R−22/27) the subsequent evolution of the perturbation is nonlinear, although it can still be followed analytically using a multiple-scales approach. The amplitude and wave speed of the nonlinear disturbance are calculated as functions of time and we show that as t → ∞, the disturbance evolves into the long-wave limit of the neutral mode structure found by Smith & Bodonyi in the fully developed Hagen–Poiseuille flow, into which our basic flow ultimately evolves. It is proposed that the critical amplitude found here forms a stability boundary between the decay of linear disturbances and ‘bypass’ transition, in which the fully developed state is never attained.

Experimental Testing and Numerical Modeling of Small-Scale Boat With Drag-Reducing Air-Cavity System

2021 ◽  
Author(s):  
Jeffrey M. Collins ◽  
Phillip R. Whitworth ◽  
Konstantin I. Matveev
Keyword(s):  
Adaptive Mesh Refinement ◽  
Experimental Testing ◽  
Hydrodynamic Performance ◽  
Small Scale ◽  
Video Monitor ◽  
Data Logger ◽  
Time Step ◽  
Experimental Conditions ◽  
Air Cavity ◽  
Air Supply

Abstract Hydrodynamic performance of ships can be greatly improved by the formation of air cavities under ship bottom with the purpose to decrease water friction on the hull surface. The air-cavity ships using this type of drag reduction are usually designed for and typically effective only in a relatively narrow range of speeds and hull attitudes and sufficient rates of air supply to the cavity. To investigate the behavior of a small-scale air-cavity boat operating under both favorable and detrimental loading and speed conditions, a remotely controlled model hull was equipped with a data acquisition system, video camera and onboard sensors to measure air-cavity characteristics, air supply rate and the boat speed, thrust and trim in operations on open-water reservoirs. These measurements were captured by a data logger and also wirelessly transmitted to a ground station and video monitor. The experimental air-cavity boat was tested in a range of speeds corresponding to length Froude numbers between 0.17 and 0.5 under three loading conditions, resulting in near zero trim and significant bow-up and bow-down trim angles at rest. Reduced cavity size and significantly increased drag occurred when operating at higher speeds, especially in the bow-up trim condition. The other objective of this study was to determine whether computational fluid dynamics simulations can adequately capture the recorded behavior of the boat and air cavity. A computational software Star-CCM+ was utilized with the VOF method employed for multi-phase flow, RANS approach for turbulence modeling, and economical mesh settings with refinements in the cavity region and near free surface. Upon conducting the mesh verification study, several experimental conditions were simulated, and approximate agreement with measured test data was found. Adaptive mesh refinement and time step controls were also applied to compare results with those obtained on the user-generated mesh. Adaptive controls improved resolution of complex shedding patterns from the air cavity but had little impact on overall results. The presented here experimental approach and obtained results indicate that both outdoor experimentation and computationally inexpensive modeling can be used in the process of developing air-cavity systems for ship hulls.

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