Fundamentals and Knowledge Relevant to the Drag Reduction Through Air Cavitation of Ship's Hulls

2021 ◽  
Vol 163 (A3) ◽  
Author(s):  
E Amromin
Keyword(s):  
Drag Reduction ◽  
Froude Number ◽  
Model Test ◽  
Ideal Fluid ◽  
Test Results ◽  
Trailing Edge ◽  
Air Supply ◽  
Fluid Theory ◽  
Design Algorithms ◽  
Wave Induced

Numerous experiments with ship drag reduction by air bottom cavitation in diverse countries have exhibited very different achievements. Therefore, a paper clarifying mechanics of this drag reduction and describing the proven design algorithms is appropriate.  Solutions of an ideal fluid problem existing in diverse ranges of Froude number are compared and the solutions suitable for ship drag reduction are considered in more detail. It is emphasized in this paper that a cavity locker at the trailing edge of the bottom niche (recess) assigned for the cavity is necessary to reduce the necessary air supply to the cavity and to mitigate the cavity tail pulsation resulting in a drag penalty. It is also pointed out that the bottom niche depth must allow for cavity withstanding under impact of waves in seaways. Bottom cavitation may even reduce wave-induced loads on the hull. With taking into account the above-mentioned design aspects, the energy spent on the air supply can be minimized. An algorithm of bottom design based on ideal fluid theory is also explained in the paper. Comparisons with several model test results are provided to illustrate the algorithm employment.

Ship Drag Reduction by Air Bottom Ventilated Cavitation in Calm Water and in Waves

2011 ◽  
Vol 55 (03) ◽  
pp. 196-207
Author(s):  
Eduard Amromin ◽  
Gabor Karafiath ◽  
Bryson Metcalf
Keyword(s):  
Drag Reduction ◽  
Air Entrainment ◽  
Fine Tuning ◽  
Wave Impact ◽  
Air Supply ◽  
Bottom Cavity ◽  
Calm Water ◽  
Fluid Theory ◽  
Moderate Range ◽  
Wave Induced

The goal herein is ship drag reduction by air bottom cavitation in the moderate range of Froude number Fr (0.4 < Fr < 0.65) in both calm water and in waves. A ship hull with a bottom niche terminating in a cavity locker/seal (suppressing cavity tail oscillations and reducing the air escape from the cavity) was designed using nonlinear ideal fluid theory. The wave impact on the cavity shape and drag reduction was estimated with a novel analytical approach that takes into account the air compressibility in the cavity and air entrainment by the water. The model drag was measured in the Naval Surface Warfare Center linear tow tank at different drafts in calm water and in waves. The baseline configuration was with the niche closed by a flat cover. The attained total drag reduction at 0.45 < Fr < 0.63 was up to 25%, whereas the air supply power was under 4% of the gain in the required propulsion power. The air cavity was stable in waves (up to sea state 5 for a 90 meter ship) and the effectiveness of drag reduction by cavitation in seaway was greater than in calm water due to smaller wave-induced additional drag of the ship with air bottom cavity. Two identical models were built and tested also as a seatrain. However, the percentage drag reduction due to cavity ventilation in the seatrain configuration was less than for a single hull. The need for fine tuning the air supply distribution between the hulls was found.

Experimental Study on the Influence of Appendages on a Yacht Rolling at Zero Froude Number

2004 ◽  
Vol 41 (04) ◽  
pp. 200-206
Author(s):  
Kim Klaka ◽  
Martin Renilson
Keyword(s):  
Experimental Study ◽  
Froude Number ◽  
Model Test ◽  
Roll Motion ◽  
Forward Speed ◽  
Test Results

Yachts tend to roll uncomfortably while at anchor, causing discomfort to the crew and passengers, generating additional stresses on equipment, and making such operations as embarking and disembarking hazardous activities. Currently, there is a dearth of data regarding roll motions at zero forward speed for hull shapes dominated by large appendages. Hence, an experimental study into the effect of large appendages on roll motion was undertaken. The model test results are presented, showing how changes in appendage geometry alter the roll response.

scholarly journals FAILURE OF SUBMARINE SLOPES UNDER WAVE ACTION

1972 ◽  
Vol 1 (13) ◽  
pp. 80 ◽  
Author(s):  
R.J. Mitchell ◽  
K.K. Tsui ◽  
D.A. Sangrey
Keyword(s):  
Model Test ◽  
Bentonite Clay ◽  
Wave Action ◽  
Test Results ◽  
Infinite Slope ◽  
Glass Wall ◽  
A Value ◽  
The Stability ◽  
Wave Induced ◽  
Theoretical Analyses

The results of model tests, carried out to evaluate the stability of submarine slopes under wave action are presented. A Bentonite clay was sedimented in a glass walled tank 6 feet long by 0.5 feet wide by 2.5 feet deep. The sedimentation and consolidation processes were studied and sediment densities were measured at various depths in the profile. Vane shear strength profiles were also measured afvarious average degrees of consolidation. Plastic markers were placed in the sediment adjacent to a glass wall so that the soil movements under both gravity and wave induced slides could be documented by photography. Dimensional similitude is discussed and the model test data are presented in a dimensionless form. All instabilities were observed to be of the infinite slope type. Analysis of the data shows that wave action is instrumental in initiating downslope mass movements in gently to steeply sloping off-shore sediments. General lack of agreement between the model test results and published theoretical analyses was found but there was close similarity in the depths and form of failure under wave action and under gravity stresses alone. The loss of stability under wave action is analyzed on the concept that failure is gravity controlled and the soil strength is reduced to a value commensurate with gravity sliding by the cyclic shearing stresses imposed by progressive waves. A method of evaluating the stability of prototype slopes using a model test correlation and field vane strength measurements is proposed.

UJI TARIK HIDRODINAMIKMODEL KAPAL BERSAYAP WiSE DENGAN LAMBUNG DASAR BERSTEP

2013 ◽  
Vol 12 (3) ◽  
Author(s):  
Iskendar Iskendar ◽  
Andi Jamaludin ◽  
Paulus Indiyono
Keyword(s):  
Comparative Study ◽  
Model Test ◽  
Hydrodynamic Model ◽  
Surface Effect ◽  
Center Of Gravity ◽  
Test Results ◽  
Flat Bottom ◽  
Towing Tank ◽  
Test Models ◽  
Wetted Surface Area

This paper describes hydrodynamic model tests of Wing in Surface Effect (WiSE) Craft. These craft  was fitted with  stephull  form in different location on longitudinal flat bottom (stepedhull planning craft) to determine the influences of sticking and porpoising motion performances. These motions are usually occured when the craft start to take-off from water surfaces. The test models with scale of 1 : 7 were comprised of 4 (four) stephull models and 1 (one) non-stephull model  as a comparative study. The hydrodynamic  tests were performed with craft speed of 16 – 32 knots (prototype values) in Towing Tank at UPT. Balai Pengkajian dan Penelitian Hidrodinamika (BPPH), BPPT, Surabaya. The resistance (drag) was measured by dynamo meter and the trim of model (draft changing at fore and aft  of model due to model speed) was measured by trim meter. By knowing the value of model trim, the wetted surface area can be determined. Then, the lift forces were calculated based on these measured values. The model test results were presented on tables and curves.  Test results show that models  with step located far away from center of gravity of the WiSE craft tend to porpoising and sticking condition, except if the step location on the below of these center of gravity. While model without step tends to sticking conditions.

scholarly journals Enhanced Drag Reduction Performance of Nanocomposites: The Effects of Surface Modification of Nano Silica

2017 ◽  
Vol 26 (4) ◽  
pp. 096369351702600 ◽  
Author(s):  
Xiaodong Dai ◽  
Guicai Zhang ◽  
Bing Li ◽  
Jijiang Ge ◽  
Xuewu Wang ◽  
...  
Keyword(s):  
Surface Modification ◽  
Drag Reduction ◽  
Rotating Disk ◽  
Shear Resistance ◽  
Test Results ◽  
Nano Silica ◽  
Surface Hydroxyl ◽  
Adsorption Analysis ◽  
Oil Adsorption ◽  
Loop 2

In this paper, nanocomposite was synthesized with nano silica and poly-α-olefin, and the effects of surface modification to the nano silica on its drag reduction performance were investigated. The dosage coupling agent, Y-aminopropyltriethoxysilane, and the modification temperature were studied intensively through surface hydroxyl and oil adsorption analysis. The test results indicated that the hydroxyl number of the silica was decreased by Y-aminopropyltriethoxysilane modification, with improved lipophilicity and oil adsorption. At 50°C, the optimum Y-aminopropyltriethoxysilane dosages were 15% for Nano-Si-10, 5% for Nano-Si-20, and 10% for Degussa-R972. The modification significantly changed the nano silica surface properties and enhanced the interaction with poly-α-olefin. Through drag reduction and shear resistance tests by rotating disk 40 mins degradation and testing loop 2 times shearing, it was shown that the nanocomposite possessed good drag reduction and excellent shear resistance properties.

Inclined Loading Capacity of Embedded Suction Anchors in Clay

2009 ◽  
Author(s):  
Y. S. Kim ◽  
K. O. Kim ◽  
Y. Cho ◽  
S. Bang ◽  
K. D. Jones
Keyword(s):  
Analytical Solution ◽  
Model Test ◽  
Inclination Angle ◽  
Load Application ◽  
Centrifuge Model Test ◽  
Loading Capacity ◽  
Test Results ◽  
Pullout Capacity ◽  
Centrifuge Model ◽  
Inclined Loading

An analytical solution has been developed to estimate the inclined pullout capacity of an embedded suction anchor in clay seafloor. Validation has been made through comparisons with a limited number of centrifuge model test results. Results indicate that the inclined pullout capacity of an embedded suction anchor in clay decreases as the load inclination angle to the horizontal increases. As the point of the load application moves downward, the inclined pullout capacity increases, reaches its peak, and then starts to decrease.

Development and Verification of Modeling Practice for CFD Decay Calculations to Obtain Roll Damping of FPSO

2021 ◽  
Author(s):  
Arjen Koop ◽  
Pierre Crepier ◽  
Sebastien Loubeyre ◽  
Corentin Dobral ◽  
Kai Yu ◽  
...  
Keyword(s):  
Model Test ◽  
Test Results ◽  
Cfd Modelling ◽  
Roll Damping ◽  
Natural Period ◽  
Computational Mesh ◽  
Safety Studies ◽  
Offshore Industry ◽  
Decay Tests ◽  
Set Up

Abstract Estimates for roll damping are important input parameters for simulation studies on vessels operating at sea, e.g. FPSO mooring in waves, wind and current, workability and operability investigations, Dynamic Position studies, ship-to-ship operations and safety studies of vessels. To accurately predict the motions of vessels this quantity should be determined with confidence in the values. Traditionally, model experiments in water basins using so-called decay tests are carried out to determine the roll damping. With recent advancements in CFD modelling, the offshore industry has started using CFD as an alternative tool to compute the roll damping of FPSO’s. In order to help adopt CFD as a widely accepted tool, there is a need to develop confidence in CFD predictions. Therefore, a practical CFD modelling practice is developed within the Reproducible CFD JIP for roll decay CFD simulations. The Modelling Practice describes the geometry modelling, computational mesh, model set-up and post-processing for these type of CFD calculations. This modelling practice is verified and validated by three independent verifiers against available model test data. This paper provides an overview of the developed modelling practice and the calculated CFD results from the verifiers. The CFD modelling practice is benchmarked against available model test results for a tanker-shaped FPSO. By following this modelling practice, the CFD predictions for the equivalent linear damping coefficient and natural period of the roll motions are within 10% for all verifiers and within 10% from the model test results. Therefore, we conclude that when following the developed modelling practice for roll decay simulations, reliable, accurate and reproducible results can be obtained for the roll damping of tanker-shaped FPSOs.

FROUDE’S LAW OF SIMILITUDE – CONTEMPORARY AND FUTURE SEAKEEPING TESTING

2021 ◽  
Vol 153 (A2) ◽  
Author(s):  
R P Dallinga ◽  
R H M Huijsmans
Keyword(s):  
Model Test ◽  
Scale Effects ◽  
Higher Order ◽  
Air Pressure ◽  
Test Results ◽  
Viscous Effects ◽  
Two Phase ◽  
Scale Models ◽  
Non Linear

Historically “scale effects” in the interpretation of tests with scale models in waves using Froude’s Law of Similitude are mostly associated with viscous effects. Nowadays, with a much more complete modelling of reality and a focus on higher order non-linear phenomena, scaling of model test results implies a wider range of assumptions than the validity of Froude’s Law. Our contribution to the conference is a visionary review of contemporary and future problems in the interpretation of these tests. In this context we will discuss the developments in test techniques, including the development of a new Two-Phase Laboratory facilitating seakeeping and sloshing tests at reduced air pressure.

Outlier analysis of sloshing impact loads on liquid ship cargo

2022 ◽  
pp. 147509022110695
Author(s):  
Sang-Yeob Kim ◽  
Yonghwan Kim ◽  
Yang-Jun Ahn
Keyword(s):  
Model Test ◽  
Present Method ◽  
Treatment Method ◽  
Liquefied Natural Gas ◽  
Impact Loads ◽  
Test Results ◽  
Outlier Analysis ◽  
Load Prediction ◽  
Conventional Procedure

This paper introduces an outlier analysis which can improve the convergence of the statistical analysis results of sloshing model test data. The paper classify possible outliers in the sloshing model test into three categories and present a treatment method for each outlier. The developed outlier analysis is adapted to the model test results for the cargo of the liquefied-natural-gas (LNG) carrier in operation. The results of the present new method are compared with those of the conventional procedure, particularly focusing on long-term sloshing prediction. Through this study, the effectiveness of the present method is observed, and it is found that the present method provides is robust and reliable results in the application of experimental data for load prediction.

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