Towing Velocity Research Articles

Investigation on the induced electrical wakes generated by underwater vehicles

The induced electrical wake generated by underwater vehicles in the conductive seawater subject to the geomagnetic field can offer significant information for non-acoustic detection. However, the crucial demand in actual detection requires the sophisticated detection technology to capture the variations in the wake, which has not been experimentally explored. In this paper, an underwater measurement platform, which is capable of accurately capturing the weak electrical signature, is developed to verify the detectability of the induced electric field in the wake. Besides, a towing system is designed and established to generate the wake driving the motion of the conducting seawater in the current experiment. The results indicate that the induced electric field distributions in the wake demonstrate prominent peak characteristics. The detectable intensity of the measured electric signatures in the wake can range from 1 to 3 μV/m, primarily concentrated within the extremely low frequency band below 1 Hz. Additionally, the induced electric field intensity is positively correlated with the towing velocity and demonstrates a similar evolutionary trend. Subsequently, the numerical simulations are performed to not only cross-validate the measurement results but also further demonstrate the detailed distribution of the electrical wake evolution. These findings illustrate the feasibility of detection based on the induced electric field disturbance signal in the wake, which holds potential for non-acoustic detection and tracking of underwater targets.

Vibration behavior prediction of submerged nanobeams with axially traveling supports: numerical, analytical, and machine learning approaches

This work surveyed the vibration behavior and stability analysis of an underwater moving flexible nanosize beam, implementing numerical and analytical methods as well as tree-based machine learning (ML) algorithms. Dynamic modeling is performed based on the nonlocal stress-strain gradient theory (NSSGT), incorporating variable environmental conditions, surface energy, and rotational inertia effects. The natural frequencies and instability threshold are computed numerically. The exact closed-form mathematical expression for the critical towing velocity of the nanosize beam is determined analytically. Also, decision tree regression and least-squares boosting tree (LSBT) regression algorithms are exploited to predict stability boundaries and the fundamental vibration frequency, and their efficiency is surveyed accordingly. Comparative studies and parametric investigations are conducted. The impressions of geometry, added mass coefficient, scale ratio parameter, surface layer characteristics, fluid mass ratio, humidity, temperature rise, and external magnetic field intensity on the nanosystem dynamics are examined and illuminated. It is detected that the results of the analytical method and ML-based approaches are consistent with those reported by the numerical technique and literature. The results asserted that the proposed ML algorithms have acceptable performance and excellent effectiveness in computational time and cost. It is comprehended that the dynamic features of submerged traveling nanobeams drastically depend on the rotational inertia effects and thickness-dependent scale effects. The stability of the immersed movable nanoscale beams is perceived to improve by increasing the scale ratio parameter and the added mass coefficient. From the perspective of the optimal design of engineering instrumentation tools, the outcomes of the current article can be applied as an inclusive benchmark.

Determinants of towing effectiveness in water rescue.

Purpose: Drownings are a societal phenomenon occurring worldwide, hence the importance of rescue skills, including directly towing a victim to a safe place. The purpose of this study was to evaluate the most effective towing techniques based on kinematic parameters, considering different types of drowning cases, for their recommendation for widespread use in water rescue. Methods: The research involved 18 water lifeguards aged 18-25 years. The evaluation included speed tests in towing a mannequin over a distance of 50 m using the Extended Arm Tow (EAT), Double Armpit Tow (DAT), "Sailor" Technique Tow (STT) and with a rescue tube (RT), accompanied by video recording to measure in the designated measurement area the number of cyclic paddling movements by the lower limbs, angles of the body attack, towing velocity, and its decrease during towing. Results: Number of cyclic paddling movements by the lower limbs, towing with a RT was considered the most beneficial, and least beneficial was the DAT. In the DAT, the lifeguard swam with the smallest body angle, in contrast to the STT, where this angle was the largest. The effect of the number of cyclic paddling movements and the body angle by the lifeguard was the velocity, with the highest value recorded in towing using a RT; in other techniques, velocity were similar. Conclusions: Institutions associated with water rescue should recommend towing using a RT for direct rescue actions in the water, as its use shortens the time, while simultaneously increasing safety for both the rescuer and the victim.

Study of Two Ships Approaching Process and Towing Motion under Wave Action

The rescue of ships in distress at sea relies mainly on rescue vessels, which includes the approaching process and the towing motion of the two ships. Ship rescue can be reduced to a two-ship problem, primarily involving the relative motion between ships during rescue. We established a mathematical model of ship motion based on the viscous fluid N-S equation, and the approaching process of two Wigley ships under waves was simulated in a two-dimensional plane. Then, according to three-dimensional potential flow theory, the coupled motion response model of the ships under six degrees of freedom was constructed, and calculation models of wind, waves, currents and other environmental disturbance factors were established to numerically calculate the towing motion. The results show that the upstream vessel has a lower heave motion amplitude and higher roll motion amplitude during the approaching process; A ratio of 1.5 times the width of the ship is the critical area where the motion of the two ships interacts with each other; For the berthing process, the faster the motion of the active vessel is, the lower the motion amplitude will be for both upstream and downstream vessels. When towing under rough sea conditions, changes in wave height and towing velocity have a large influence on the coupling effect of the towing system. When towing, head-on waves and low sea conditions are preferred, and the velocity should be sufficiently high to reduce the influence of cable self-weight on the towed ship. According to the simulations, the recommended velocity range is 3–5 m/s. Finally, a ship model scaling model was developed based on the similar quasi-Froude number of the ship model test, and the simulation results were verified by conducting parallel and towing tests of the model in a basin with a spherical wave-maker device.

Optimal Chassis Suspension Design for Towbarless Towing Vehicle for Aircraft Taxiing

<div class="section abstract"><div class="htmlview paragraph">The towbarless aircraft taxiing system (TLATS) consists of the towbarless towing vehicle (TLTV) and the aircraft, of which the nose wheel is lifted and fixed by the picking up and holding system of the TLTV. The aircraft is towed to the destination in a long distance with a high speed of about 40 km/h only driven by the TLTV, which has the advantages of efficiency and economy by comparing with the traditional towing operation at a low speed of 5 km/h and with a short towing distance of about 50 m. However, the increase of the towing velocity leads to an deteriorate vibration problem in terms of the uncomfortable of the driver and reduction of the structure safe life limit, due to the lack of the chassis suspension for the vibration isolation of the TLTV. Therefore, the air suspension is introduced to the TLTV to alleviate the vibration. The dynamic model of the TLATS with air suspensions is derived. The chassis bounce natural frequency is presented, and the initial stiffness and damping coefficients are determined. The parameters of the air spring, i.e., the initial pressure, volume, effective area under the working condition, are optimally designed. The ride performances of the TLATS with the air suspensions under the random road excitation are compared with those with coil spring suspension and without suspension. The simulation results show that the optimal air suspensions could alleviate the TLATS’ vibration during the high-velocity towing of the aircraft.</div></div>

Analysis of the Deep Sea Mining Pipe Transverse Vibration Characteristics Based on Finite Element Method

This paper analyzes the transverse vibration laws of 5000 m ladder-shaped mining pipe under different towing velocities and accelerations in the ocean, thinking of the pipe as the beam model, discretized based on the FEM. The algorithm is used to solve the problem to obtain the transverse vibration law. The research shows that the mining pipe overall transverse vibration trend decreases first and then increases, the minimum vibration value occurs at 3000 m, and the maximum occurs at the top. Increasing the towing velocity, acceleration, and ore bin weight will increase the transverse vibration value. The vibration intensity produced by the same acceleration in the constant acceleration and deceleration stages is different, and the damping effect after adding the same damping is also different. In the range of 0.01 m/s2–0.1 m/s2, the vibration reduction effect after adding damping in the constant deceleration stage is more significant, and in the range of 0.1 m/s2-0.2 m/s2, the vibration reduction effect after adding damping in the constant acceleration stage is more significant. In the stage of the constant acceleration or deceleration, when adding the same damping, the vibration intensity generated by the large acceleration is still far greater than the vibration intensity generated by the small acceleration, so the mining ship should keep the small acceleration for towing motion.

Comparative Analysis of Longitudinal and Transverse Vibration Characteristics of Ocean Mining Pipe

In this paper, the 5000 m mining pipe is taken as the research object, and the transverse and longitudinal vibration laws of the pipe under different working conditions are analyzed. Based on the finite element method (FEM), the pipe is discretized and calculated by the Wilson-θ Wilson - θ integral method; finally, the corresponding vibration laws of the mining pipe are obtained. The research shows that the mining pipe vibration responses are irregular motion, with the obvious oscillation phenomenon, and the overall vibration trend decreases first and then increases from the top to the bottom; the maximum vibration response occurs at the pipe top. Under the same working conditions, increasing the towing velocity will decrease the overall longitudinal vibration amplitude and increase the overall transverse vibration amplitude. While the ore bin weight will increase the longitudinal vibration amplitude and decrease the transverse vibration amplitude, increasing the mining pipe large diameter stepped section length and damping will decrease the longitudinal and transverse vibration simultaneously. When the towing velocity is between 0–2.8 m/s, the longitudinal vibration intensity is large, which is the main vibration mode. When the towing velocity is 2.8 m/s, the critical point is reached, and the longitudinal and transverse vibrations have the same intensity. When the towing velocity is greater than 2.8 m/s, the transverse vibration intensity is gradually greater than the longitudinal vibration intensity; at this time, the control of the transverse vibration should be appropriately increased.

Snare Performance for Sunken and Submerged Oil Detection and Monitoring

ABSTRACT - 687127 Most oil spill response strategies, tactics, and equipment are designed to address floating oil. Previous research and historic events have shown that spilled oil can suspend (i.e., submerged oil) or sink (i.e., sunken oil) as a function of the oil's density relative to that of the receiving waters. Processes such as wave action or current velocity, sediment entrainment, and oil weathering (e.g., evaporation) may change the buoyancy of floating oils causing them to submerge or sink. Non-floating oil is more difficult and expensive to detect and poses significant challenges for containment and cleanup. Many existing detection techniques for non-floating oils rely on oleophilic sorbents, such as snare, which are weighted depending upon the oil's location in the water column and then towed behind a vessel in designated transects. Currently, there is no quantitative method to relate the amount of oil collected by snare to the amount of oil encountered during towing. In addition, the dynamics and interactions of towed snare and oil remain largely unknown. To address these knowledge gaps, various components of snare performance have been evaluated since 2016 by the Coastal Response Research Center (CRRC) at the University of New Hampshire (UNH). The research has evaluated: (1) the impacts of temperature, salinity, oil type, and tow velocity on adsorption and desorption of oil to snare, (2) snare dynamics and position in the water column as a function of tow velocity, (3) the impacts of material type and potential alternatives to snare (e.g., mosquito and fishing nets, plastic debris) for lesser developed countries (LDCs), and (4) the interaction of snare with sunken and submerged oil. The results determined: (1) adsorption of oil to snare was best for less viscous oils (No. 6 Fuel Oil) and lower water temperatures (5°C) and desorption was greatest at low temperatures (6°C) and low current velocities (< 1 knot), while salinity had no significant effect. (2) Tow depth for snare arrays decreased with increased velocity unless a vane was used. (3) Optimal spacing of snare on a chain is a function of tow and current velocity, and drag forces on the tow chain. (4) Snare alternatives with greatest potential for sunken oil detection in LDCs were nylon mosquito netting and plastic bags. The findings from this research improves understanding of the behavior of snare and how it interacts with sunken and submerged oil and can improve towing techniques used by oil spill responders, leading to more effective detection.

THE COST OF TRANSPORT (COT) OF A HIGH ENERGY EFFICIENCY HYBRID ROBOT

Purpose: The cost of transport is one of the most important values to the efficiency and operation autonomy of a walking robot. This analysis involves factors as the weight, consumption of the actuators, speeds, accelerations, work surfaces, step cycle model or distance travelled, which must be studied in detail to produce stable and energy-efficient locomotion. This paper presents the results obtained for the cost of transport of a hybrid robot with two front legs and two rear wheels, with a total weight of 50 kg in different scenarios. Methodology/approach - The transportation cost of the proposed hybrid robot is obtained by carrying out a detailed analysis of the kinematics, dynamics, stability and energy consumption. Findings - A satisfactory value of efficiency has been obtained, in terms of cost of transport, owing to a gravitationally decoupled design of the legs. The cost of transport of the robot proposed is between 0.11 and 0.24, depending on the work environment in which it operates, that is, walking on a smooth horizontal plane without additional load. Originality/value – This work presents a new design of a gravitationally decoupled robotic leg by means of a new scheme in which the leg is composed of three four-bar mechanisms that can be synthesized independently. These three mechanisms involve frontal and vertical movement within the same plane of movement. One mechanism generates a horizontal path for tow, while another generates a vertical path and a third has the specific mission of making the tow velocity constant when the corresponding motor is operated at a constant velocity. The overall goal of the mechanisms is to improve robot's efficiency. Key Words: Cost of transport, gravitationally uncoupled motion, energy efficiency, experimental validation, hybrid robot.

Hydrodynamic Loads on Net Panels With Different Solidities

Abstract Drag forces on nets represent the largest contribution to hydrodynamic loads on traditional fish farms and will have a large impact on total loads on new designs utilizing netting as containment method. Precise methods for estimation of drag loads are needed. This article gives new knowledge on hydrodynamic forces acting on aquaculture nets. It presents results from towing tests, including updated drag and lift coefficients for Raschel-knitted netting materials used in nets for aquaculture, and quantify wake effect. The results include high solidity nets and high towing velocities. It was found that drag loads were close to proportional with the netting solidity for netting solidities ranging from 0.15 to 0.32. The wake effect is quantified through the average velocity reduction factor, which is given as a linear function of solidity. Much of the previously published data are close to the data found through these tests. However, for high solidity nets, the deviation is significant. Therefore, previously published data and models may overestimate drag loads for high solidity nets.

Fluid-structure interactions (FSI) behaviour of two unequal-diameter flexible cylinders in tandem configuration

Fluid-structure interactions (FSI) of multiple cylindrical structures frequently occur in crossflows. Due to the different wake interference regions of flexible cylinders with different diameters, the interaction between multiple cylinders is complicated and sensitive to the centre-to-centre spacing ratio and diameter ratio. Limited information has been presented on the flow-induced vibration (FIV) of two unequal-diameter flexible cylinders in a tandem arrangement. In this paper, an experimental investigation was carried out in a towing tank to study the FIV of two tandem flexible cylinders with different diameters that can vibrate in both the cross-flow (CF) and in-line (IL) directions. There were two IL spacing ratios (T/d = 12.0 and 6.0, where T is the centre-to-centre distance between two cylinders, and d is the diameter of the small cylinder), and the diameter ratio (d/D, where D is the diameter of the large cylinder) was nearly 0.5. The aspect ratios/mass ratios of the small cylinder and the large cylinder were 350/1.90 and 181/1.47, respectively. The towing velocity varied from 0.05 to 1.00 m/s with an interval of 0.05 m/s. The Reynolds numbers for the small and large cylinder were 800–16000 and 1600–32000, respectively. The CF vibration of the upstream large cylinder with T/d = 6.0 was suppressed at Vrs = 16.28–18.79, whereas the CF vibration of the large cylinder with T/d = 12.0 was not influenced. The downstream cylinder exhibited more complicated behaviour due to the significant wake effect from the upstream cylinder, which is consistent with the two equal-diameter cylinders. The FIV response of the downstream small cylinder was more sensitive to the IL spacing ratio. Because of the wake shielding effect of the large cylinder, the downstream small cylinder vibrated at low IL dominant frequencies, and the IL to CF frequency ratio (fIL/fCF) was approximately 1.0. Moreover,wake-induced flutter was observed on the downstream small cylinder.

Response of Two Unequal-Diameter Flexible Cylinders in A Side-by-Side Arrangement: Characteristics of FIV

Till now, little information is available on the flow-induced vibration (FIV) of multiple flexible cylinders with unequal diameters. Some FIV characteristics of unequal-diameter cylinders can be predicted based on the knowledge of equal-diameter cylinders, while there are still other features remaining unrevealed. In this paper, the FIV characteristics of two flexible cylinders with unequal diameters arranged side-by-side are experimentally investigated. The diameter ratio of the small cylinder (Small Cyl.) to the large cylinder (Large Cyl.) is nearly 0.5. The aspect ratios and mass ratios of the two flexible cylinders are 350/181 and 1.90/1.47, respectively. The centre-to-centre spacing ratio in the cross-flow (CF) direction is kept constant as 6.0 and the two cylinders can oscillate freely in both the CF and in-line (IL) directions. The towing velocity varies from 0.05 m/s to 1.00 m/s. The dominant modes and frequencies, CF and IL displacement amplitudes and response trajectories are discussed. Compared with the case of two identical cylinders in our previous study, the FIV responses demonstrate some similarities and differences. The similarities are as follows. Both cylinders exhibit multi-mode vibration features and they interact with each other. Meanwhile, the IL FIV shows a more complex behaviour than that in the CF direction. The difference is that as the diameter of one cylinder is increased, the effect on the smaller cylinder becomes more significant. For Large Cyl., the FIV response is similar to its isolated counterpart, which indicates that Small Cyl. has a negligible effect on the FIV of the larger one. Whereas Large Cyl. perplexes the FIV of Small Cyl. during the vibration process. The spacing would change when both cylinders are oscillating. Proximity interference between the two cylinders and wake shielding effect of the Large Cyl. may occur. The dominant frequencies of Small Cyl. are reduced and the wake-induced flutter of Small Cyl. is observed from the response trajectories at different measuring points.

Flow-induced vibration of two identical long flexible cylinders in a staggered arrangement

The effects of spatially staggered arrangements on the flow-induced vibration (FIV) characteristics of two identical flexible cylinders were experimentally studied in a towing tank. Flexible cylinders with an aspect ratio of 350 and a mass ratio of 1.90 were free to vibrate in both the cross-flow (CF) direction and the in-line (IL) directions. During the experiments, the towing velocity varied from 0.05 m/s to 1.0 m/s with an interval of 0.05 m/s, corresponding to Reynolds numbers in the range of 800–16,000. A total of twelve different staggered arrangements for two-cylinder models were selected (S/D=2, 3, 4, 6 and T/D=4, 6, 8, where S and T are the center-to-center separation distances between the two cylinders in the CF and IL directions and D is the cylinder diameter). The experimental results show that the multi-mode vibrations of both flexible cylinders are influenced by the complicated flow interference. The Strouhal number St is sensitive to the CF spacing ratio (S/D) for both flexible cylinders. The displacements of the upstream cylinder are larger than those of the isolated cylinder due to proximity interference. The dynamic characteristics of the downstream cylinder are significantly affected by the wake interference. Wake-induced flutter can act on the downstream cylinder, leading to a large IL displacement and an elliptical motion trajectory. Two prominent frequency components are observed in the IL displacement response. The higher frequency component is related to the vortex shedding of the downstream cylinder. The lower frequency component, which approximates to the CF dominant frequency, is typical of wake-induced flutter.

Laboratory investigations on the resonant feature of \u2018dead water\u2019 phenomenon

Interfacial internal wave excitation in the wake of towed ships is studied experimentally in a quasi-two-layer fluid. At a critical ‘resonant’ towing velocity, whose value depends on the structure of the vertical density profile, the amplitude of the internal wave train following the ship reaches a maximum, in unison with the development of a drag force acting on the vessel, known in the maritime literature as ‘dead water’. The amplitudes and wavelengths of the emerging internal waves are evaluated for various ship speeds, ship lengths and stratification profiles. The results are compared to linear two- and three-layer theories of freely propagating waves and lee waves. We find that despite the fact that the observed internal waves can have considerable amplitudes, linear theories can still provide a surprisingly adequate description of subcritical-to-supercritical transition and the associated amplification of internal waves. We argue that the latter can be interpreted as a coalescence of frequencies of two fundamental stable wave motions, namely lee waves and propagating interfacial wave modes.Graphic abstract

Cross-Flow Vortex-Induced Vibration (VIV) Responses and Hydrodynamic Forces of a Long Flexible and Low Mass Ratio Pipe

Laboratory tests were carried out to investigate the cross-flow (CF) dynamic responses and hydrodynamic forces of a flexible pipe that subjected to vortex-induced vibration (VIV). The pipe had a critical mass ratio of 0.54 and an aspect ratio of 181.8. The uniform flow environment was realized by towing the pipe along a towing tank. The towing velocity ranged from 0.1–1.0 m/s with an interval of 0.05 m/s. Two axial pre-tension cases (200 N and 300 N) were enforced. The structural strains were measured at seven positions evenly distributed along the pipe. Then a modal analysis method was applied to reconstruct the displacement responses. It is revealed that the maximum CF displacement amplitude reached up to 2.18 pipe diameter and the strain response exhibited higher harmonic components. The CF dominant frequency gradually rises with the increase of reduced velocity and up to a three-order vibration mode can be observed. In addition, mean drag coefficient, lift force coefficient and added mass coefficient were also calculated to further investigate the fluid force feature of a low mass flexible pipe undergoing VIV.

Dynamic response of three long flexible cylinders subjected to flow-induced vibration (FIV) in an equilateral-triangular configuration

A three-cylinder system with an equilateral-triangular configuration represents one of the most common, basic units of multi-cylinder systems in many engineering fields. In this paper, an experimental investigation of the flow-induced vibration (FIV) of a three-cylinder system in an equilateral-triangular arrangement was conducted in a towing tank to study their response characteristics. Three identical flexible cylinder models with a centre-to-centre distance of 6.0 times the cylinder diameter were towed along the tank to generate a uniform flow. The Reynolds number ranged from approximately 1600 to 16000 depending on the towing velocity, which was in the range of 0.10–1.00 m/s. The time-varying bending strains of the flexible cylinders in the cross-flow (CF) and in-line (IL) directions were measured using strain gauges at different measurement positions. The displacement responses were reconstructed by a modal analysis method based on the acquired strain signals. The experimental results (e.g., displacement amplitudes, response frequencies, dominant modes and motion trajectories of the flexible cylinders) were presented to reveal the FIV response features. Three typical cases of the equilateral-triangular configuration (A, B and C) corresponding to flow incidence angles of 0°, 30°, and 60° were selected and discussed. (The incidence angle is the angle between the flow direction and the line joining the centre of one cylinder to the centre of the equilateral-triangular configuration.) In Cases A and B, the upstream cylinder is insignificantly affected by the downstream cylinders and behaves like an isolated flexible cylinder; meanwhile, the upstream cylinders in Case C vibrate similarly to two side-by-side flexible cylinders with weak mutual interference. Striking departures exist in the IL oscillations of the downstream cylinders due to the notable effects of the upstream cylinders. The phenomenon of mixed modes appears in the IL vibrations of the downstream cylinders. Furthermore, the IL response spectra always contain a robust lower frequency component equal to the dominant frequency in the CF direction. Moreover, eight-, half-moon- and C-shaped vibration figures are comparatively uncommon for the downstream cylinders.

An experimental study on flow-induced vibration of three and four side-by-side long flexible cylinders

Multiple flexible cylinder structures, such as marine risers and cables, are key equipment for the production and transportation of oil and gas in offshore engineering. Till now, little information is available on flow-induced vibration (FIV) of more than two side-by-side flexible cylinders. Though some FIV characteristics of multiple flexible cylinders can be predicted based on the knowledge of two flexible cylinders undergoing vortex shedding, there are still some other characteristics that remain unrevealed, which require further experimental investigations on more than two flexible cylinders. In this paper, the FIV of three- and four-cylinder system in a side-by-side arrangement with a distance of 6.0D between cylinder centerlines was experimentally studied in a towing tank. The flexible cylinder models with mass ratio of 1.90 and aspect ratio of 350 were mounted horizontally and towed along the tank to simulate the uniform flow. The towing velocity ranged from 0.05 to 1.0 m/s and the corresponding Reynolds number varied from 800 to 16000. The experimental results were discussed in detail, including the maximum RMS displacement amplitudes, dominant frequencies, dominant modes, time-varying displacements at the measurement points, displacement response spectra, spanwise evolutions of the RMS displacement and space-time varying displacement contour. It can be found that the multi-mode response characteristics of the three-cylinder system in a side-by-side arrangement are obviously distinct from those of the four-cylinder system due to the effect of complex wake flow behind the cylinders. These findings are very meaningful for the design of a group of flexible cylinders in engineering applications.

Passive VIV Reduction of An Inclined Flexible Cylinder by Means of Helical Strakes with Round-Section

A series of experimental tests of passive VIV suppression of an inclined flexible cylinder with round-sectioned helical strakes were carried out in a towing tank. During the tests, the cylinder models fitted with and without helical strakes were towed along the tank. The towing velocity ranged from 0.05 to 1.0 m/s with an interval of 0.05 m/s. Four different yaw angles (a=0°, 15°, 30° and 45°), defined as the angle between the axis of the cylinder and the plane orthogonal of the oncoming flow, were selected in the experiment. The main purpose of present experimental work is to further investigate the VIV suppression effectiveness of round-sectioned helical strakes on the inclined flexible cylinder. The VIV responses of the smooth cylinder and the cylinder with square-sectioned strakes under the same experimental condition were also presented for comparison. The experimental results indicated that the round-Sectioned strake basically had a similar effect on VIV suppression compared with the square-sectioned one, and both can significantly reduce the VIV of the vertical cylinder which corresponded to the case of a=0°. But with the increase of yaw angle, the VIV suppression effectiveness of both round- and square-section strakes deteriorated dramatically, the staked cylinder even had a much stronger vibration than the smooth one did in the in-line (IL) direction.

Use of Helical Strakes for FIV Suppression of Two Inclined Flexible Cylinders in A Side-by-Side Arrangement

The experimental studies on flow-induced vibrations (FIV) reduction of two side-by-side flexible cylinders inclined at 45° by using the helical strakes were carried out in a towing tank. The main aim of the experiment is to check whether the helical strakes with a pitch of 17.5D and a height of 0.25D, which is considered as the most effective vibration suppression device for the isolated cylinder undergoing vortex-shedding, still perform very well to reduce FIV of two inclined flexible cylinders in a side-by-side arrangement. The vibration of two identical inclined cylinders with a mass ratio of 1.90 and an aspect ratio of 350 was tested in the experiment. The center-to-center distance between the two cylinders was 3.0D. The uniform flow was simulated by towing the cylinder models along the tank. The towing velocity varied from 0.05 to 1.0 m/s with an interval of 0.05 m/s. The maximum Reynolds number can be up to 1.6×104. Three cases were experimentally studied in this paper, including two side-by-side inclined smooth cylinders, only one smooth cylinder fitted with helical strakes in the two side-by-side inclined cylinders system and both two cylinders attached with helical strakes. The variations of displacement amplitude, dominant frequency, FIV suppression efficiency and dominant mode for the two side-by-side inclined cylinders with reduced velocity were shown and discussed.

Internal waves generated by a stratified wake: experiment and theory

This paper presents experimental and theoretical results on the internal waves emitted by a bluff body moving horizontally in a linearly stratified fluid. Three different bluff bodies (a sphere, a spheroid and a cylinder) have been used in order to study the effect of the shape of the bluff body, although most of the results are obtained for the sphere. Two types of internal waves have been observed experimentally: large wavelength lee waves generated by the bluff body itself and small wavelength coherent wake waves generated by the turbulent wake. First, the lee waves are separated from the wake waves by averaging the experimental measurements in the frame moving with the bluff body. The velocity amplitude of the lee waves scales as the inverse of the Froude number $F=2U_{B}/(ND)$ for $F>2$ (where $U_{B}$ is the towing velocity, $D$ the diameter and $N$ the buoyancy frequency). This scaling proves that the internal waves are related to the drag of the bluff body which is due to the separation of the flow behind the bluff body. This separation is usually not taken into account in the classical models which assume that the flow is dipolar. The drag can be modelled as a point force in the Navier–Stokes equations, which gives a correct prediction of the structure and the amplitude of the lee waves. Second, the wake waves have been separated from the lee waves by averaging the velocity fields in the frame moving at the phase velocity of the waves. The phase velocity and the wavelength scale as $F^{-2/3}$ and $F^{1/3}$ respectively which correspond to the velocity and distance between same sign vortices of the von Kármán vortex street. A simplified model is derived for the internal waves emitted by the double row of moving point vortices of the von Kármán street. The amplitude of the wake waves is measured experimentally and seems to depend on the Reynolds number.