Propeller Jet Research Articles

Three-dimension efflux velocity characteristics of marine propeller jets

Determining the efflux velocity in a ship's propeller jet is the key to calculating the velocity at any other location within the diffusing jet. Current semi-empirical equations used to calculate the magnitude of the efflux velocity have been based on studies that employed a limited range of propeller characteristics. This paper reports on the findings of an experimental investigation into the magnitude of the efflux velocities of the jets produced by four different propellers, where the characteristic of the blade geometry has been chosen to extend the range of applicability of the outcomes. Measurements of velocity have been made using a three-dimensional laser Doppler anemometry system, with the test propellers operating at a range of rotational speeds that bound typical operational values. Comparisons are made with current predictive theories and, to aid engineers in the design of marine infrastructure, methods are presented by which the three-dimensional efflux velocity components, as well as the resultant efflux value, can be more accurately determined.

Three-dimension efflux velocity characteristics of marine propeller jets

Three-dimension efflux velocity characteristics of marine propeller jets

Erratum for “Scour Caused by a Propeller Jet” by Jian-Hao Hong, Yee-Meng Chiew, and Nian-Sheng Cheng

Erratum for “Scour Caused by a Propeller Jet” by Jian-Hao Hong, Yee-Meng Chiew, and Nian-Sheng Cheng

Numerical Analysis of Temperature Influence on Evaporation of Liquid Propellant Leaking Jet

In order to analyze the influence of temperature on evaporation of liquid propellant leaking jet, and to find measures to decrease the evaporation quantity, the mathematical model of the jet evaporation was established, and an example calculation was carried out by use of Fluent. The result shows that the evaporation quantity of propellant jet reduces obviously when the environmental temperature drops down, and it can be also reduced by using propellant with high boiling point.

Analytical wake model of tidal current turbine

Prediction of the wake structure is important to understand the lee flow of a tidal current turbine. The proposed analytical wake model consists of several equations derived from the theoretical works of a ship propeller jet. Axial momentum theory was used to predict the minimum velocity at the immediate plane of the lee wake and followed by the proposed recovery equation to determine the minimum velocity at various lateral sections along the rotation axis. Gaussian probability distribution was used to predict the velocity distribution of lateral sections in a wake. Entire wake is able to be illustrated through the calculation of the efflux equation, recovery equation and lateral distribution equations. Authors' previous works proposed a simplified one-dipped velocity profile and this works were being extended to predict the two-dipped velocity profile with the consideration of hub effects. The wake model is validated by using the well-accepted experimental measurements and the goodness-of-fit test. The results demonstrated that the wake model is able to predict the wake profile under various ambient turbulence conditions of TI (turbulence intensity) = 3%, 5%, 8% and 15%.

Methods for predicting seabed scour around marine current turbine

Abstract Marine energy sources are able to make significant contributions to future energy demands. Marine current has huge potential to supply renewable energy as compared to the other energy sources. Marine environment is harsh for the installation and operation of marine current turbine (MCT). Seabed scour around marine current turbine is induced when the flow suppression occurs at the seabed. Seabed scour is widely recognised as a difficult engineering problem which is likely to cause structural instability. The study found that the previous works mainly focus on the bridge piers, wind turbines and ship propeller jets induced scour. Little information to date was found to predict the MCT induced scour. The current paper proposes the potential equations to predict the MCT induced scour. The study also recommends the consideration of the rotor into the existing equations for future research.

The effect of the propeller jet on pile groups

The objective of this study is to determine the effect of jet propeller on the damage of berthing structures combined of armoured slope with pile groups. For this purpose, scour measurements were performed for four types berthing structures, which were armoured slope with tandem arrangements of piles for two and three piles and with side by side arrangements of piles for two and three piles. The effect of gap between piles on damage was investigated. The damage level induced by propeller jet between piles was determined. The gaps were 1, 2, 3, and 4 times the pile diameter. Three different values of R pm (690, 820, and 950) were chosen for the tests. The diameter of circular piles is 40 mm. The slope ratio was 1/3 and the diameter of propeller was 10 cm.

Scour Caused by a Propeller Jet

The scour hole induced by a propeller wash is experimentally investigated with noncohesive sediments. The associated scour profile at the asymptotic condition, which can be expressed by a combination of three polynomials, comprises (1) a small scour hole beneath the propeller, (2) a primary scour hole downstream of the small scour hole, and (3) a deposition mound farther downstream of the primary scour hole. The time-dependent maximum scour depth induced by propeller jets is closely related to the densimetric Froude number, a reference time scale, offset height relative to the propeller diameter, and sediment size. A semiempirical formula, which shows good agreement with the experimental data, is proposed to describe evolution of the maximum scour depth. An equation for the determination of the time to initiate scour is also proposed. Moreover, empirical equations for the maximum asymptotic scour depth induced by a propeller and three-dimensional submerged offset jets agree well with the experimental data. The critical condition for the initiation of scour caused by propeller and offset jets is also presented.

Influence of a boundary on the development of a propeller wash

When vessels operate within harbours or over a density interface in an estuary, the seabed or interface may be close to the tip of the propeller blades. The presence of this boundary will have an effect on the propeller wash and this can affect the erosion of the boundary. The influence of such a boundary on the characteristics of a propeller wash was studied in experiments using a horizontal fixed boundary to confine a propeller jet. Detailed velocity measurements within the jet were obtained using a 3D Particle Image Velocimetry (PIV) system. The bottom stream of a propeller jet was found to expand at a faster rate due to the reduction in pressure beneath the jet caused by the suppression of the replacement fluid. The boundary was found to significantly increase the axial velocities close to it, and reduce the rate of decay of the maximum axial velocity due to the confinement, reducing the height of the jet. Three zones within the propeller wash were identified, the first being before the jet impacted the boundary, the second in which the boundary layer developed at the fixed boundary, followed by a fully developed boundary layer region. Predictive equations to estimate the influence of the boundary have been developed and are presented.

Linear stability analysis of a slightly viscoelastic liquid jet

Gel propellant is one of the non-Newtonian viscoelastic liquids. The study of breakup of a gel propellant jet emanating from an injector has always been a fundamental task for a propulsion system with gel propellants. In this paper, a linear instability analysis method has been used to investigate the breakup of a slightly viscoelastic liquid jet. The disturbance wave growth rate of the axisymmetric mode has been determined by solving the dispersion equation of a slightly viscoelastic liquid jet, which was obtained by combining the linear instability theory and the linear viscoelastic model. Moreover, the maximum growth rate and corresponding dominant wave number have been observed, according to which maximum growth rate curves and dominant wave number curves have been plotted; this theory has been verified in the experiments of other authors. The time constant ratio, zero shear viscosity, liquid jet velocity, surface tension, liquid jet radius, liquid density and the gas density have been tested for their influence on the instability of the viscoelastic liquid jet. The results show that the effect of gas density is the greatest on the breakup of the viscoelastic jet, the jet velocity is affected second and the zero shear viscosity is least affected. However, the effects of other parameters are limited.

OPEN TYPE QUAY STRUCTURES UNDER PROPELLER JETS

In recent years, dramatically increases in ship dimensions and installed engine power, introduction of new type of special purpose ships and use of roll-on/roll-of, ferries, container ships can cause damage which in many cases threatens to undermine berth structures. Vessel jets of these types of ships can change flow area and cause erosion and scour around foundation of berth structures. Due to the damages in berth structures maintenance and repair cost may increase and also cause management losses. For this reason vessel jet induced the flow area around the berth structures during ships berthing and un-berthing operations are extremely important factor for the port structure design. This study is related with investigation of the flow characteristics at the sea bed around the pile, experimentally. Vessel jets were simulated both as circular wall jet and also propeller jet. The objective of this study is to determine the sea bed shear stress and velocity profiles along the jet axis for open type wharf structures (around a cylindrical piles and also on the slopes). Hot film anemometers were used to measure the magnitude of the bed shear stresses. The results from propeller jet experiments explained the erosion over the slopes. Bed shear and velocity profile measurements were carried out on the rigid bed conditions.

An effective method for comparing the turbulence intensity from LDA measurements and CFD predictions within a ship propeller jet

The mean velocity and turbulence intensity are the two main inputs to investigate the ship propeller induced seabed scouring resulting from a vessel is manoeuvring within a port where the underkeel clearances are low. More accurate data including the turbulence intensity is now available by using the laser doppler anemometry (LDA) measurement system and computational fluid dynamics (CFD) approach. Turbulence intensity has a loose definition, which is the velocity fluctuation as the root mean square (RMS) referenced to a mean flow velocity. However, the velocity fluctuation and mean velocity can be the overall value including x, y and z directions or the value of a single component. LDA and CFD results were obtained from two different acquisition systems (Dantec LDA system and Fluent CFD package) and therefore the outputs cannot be compared directly. An effective method is proposed for comparing the turbulence intensity between the experimental measurements and the computational predictions within a ship propeller jet. The flow patterns of turbulence intensity within a ship propeller jet are presented by using the LDA measurements and CFD results from turbulence models of standard k−ε, RNG k−ε, realizable k–ε, standard k–ω, SST k–ω and Reynolds stresses.

Propeller induced scour around the pile type berth structures

Scour caused by propeller jets around harbour structures results in considerable damage. Scour protection for these structures has been of increasing importance in their design. This paper addresses an experimental investigation into the scour on armoured slopes under berth structures. Three different cases were considered for this study, including an armoured layer with an apron, an armoured layer without an apron and an armoured layer with an apron that included a pile on the slope. Propeller jet velocity and scour measurements were conducted in these experiments. Additionally, armoured layer damages on the slope were defined using a damage level parameter. The critical densimetric Froude Number (Frd,cr) was defined for the start of the damage.

The effect of slope angle on propeller jet erosion near quays

The problem of propeller jet-induced erosion has significantly risen during the past three decades due to an increase in the manoeuvrability of ships. Therefore, propeller-induced scours need to be considered in the design of quay structures. In this study, the effect of slope angle on propeller jet erosion was examined. Three different slope angles of m = 2·5, 2·0 and 1·5 were considered. Additionally, three different cases were considered, including an armoured layer with and without a toe apron and pile on an armoured layer with a toe apron. Flow velocity and scour measurements were performed in the experiments. For each slope angle, armour layer damage on the slope was defined by use of a damage level parameter. In addition, critical densimetric Froude numbers (Frd,cr) were defined for the start of damage at different slope angles.

Design Considerations for Pile Groups Supporting Marine Structures with Respect to Scour

Piles supporting marine structures such as jetties, relieving platforms, quay walls and fixed offshore structures are subjected to lateral loads due to berthing and mooring forces, wind, waves, storm surges and current forces. This paper presents some factors that affect the design of pile groups supporting marine structures founded in cohesionless soils. Some main aspects that should be considered in the pile group design are addressed such as pile batter angle, pile group arrangement, pile spacing, pile slenderness ratio and magnitude of lateral static loading. Numerical analyses were conducted to investigate these design aspects with and without impact of scour. Different scour depths were considered to cover the possible root causes of scour around pile groups such as waves, current and ship propeller jets. The study revealed that scour has greater impact on lateral loading of pile groups compared to its impact on single piles. Pile groups with side-by-side arrangement exposed to scour are more critical than single piles and piles groups with tandem arrangement due to the combined effect of scour and pile-soil-pile interaction. It is also concluded that scour protection is not always required. More attention and considerations should be given to scour protection around piles especially if the piles are closely spaced, arranged side-by-side and if slenderness ratio is less than 12.5.

Experimental investigation of the decay from a ship’s propeller

In the present study, an experimental investigation of the decay of the maximum velocity and its turbulent characteristics behind a ship propeller, in “bollard pull” condition (zero speed of advance), is reported. Velocity measurements were performed in laboratory by use of a Laser Doppler Anemometry (LDA) measurement system. Earlier researchers described that the maximum axial velocity is constant at the initial stage of a ship’s propeller jet (Fuehrer and Romisch, 1977; Blaauw and van de Kaa, 1978; Berger et al., 1981; Verhey, 1983) as reported in a pure water jet (Albertson et al., 1950; Lee et al., 2002; Dai, 2005), but a number of researchers disagreed with the constant velocity assumption. The present study found that the maximum axial velocity decays in the zone of flow establishment and the zone of established flow with different rates. The investigation provides an insight into the decays of both the maximum velocity and the maximum turbulent fluctuation in axial, tangential and radial components and the decay of the maximum turbulent kinetic energy. Empirical equations are proposed to allow coastal engineers to estimate the jet characteristics from a ship’s propeller.

Investigation of a ship's propeller jet using momentum decay and energy decay

Previous researchers use the velocity decay as an input to investigate the ship’s propeller jet induced scour. A researcher indicated that most of the equations used to predict the stability of various protection systems are often missing a physical background. The momentum decay and energy decay are currently proposed as an initial input for seabed scouring investigation, which are more sensible in physics. Computational fluid dynamics (CFD) and laser Doppler anemometry (LDA) experiments are used to obtain the velocity data and then transforming into momentum and energy decays. The findings proposed several exponential equations of velocity, momentum and energy decays to estimate the region exposed to the seabed scouring.

Observations of the initial 3D flow from a ship's propeller

The present paper was aimed at presenting the time-averaged velocity and turbulence intensity at the initial plane from a ship's propeller. The flow characteristics of a ship's propeller jet are of particular interest for the researchers investigating the jet induced seabed damage as documented in the previous studies. Laser Doppler Anemometry (LDA) measurements show that the axial component of velocity is the main contributor to the velocity magnitude at the initial plane of a ship's propeller jet. The tangential component contributes to the rotation while the radial component which contributes to the diffusion, are the second and third largest contributors to the velocity magnitude. The maximum tangential and radial velocity components at the initial plane are approximately 82% and 14% of the maximum axial velocity component, respectively. The axial velocity distribution at the initial plane shows two peaked ridges with a low velocity core at the rotation axis. The turbulence intensity distribution shows a three-peaked profile at the initial plane.

3D non-hydrostatic modelling of bottom stability under impact of the turbulent ship propeller jet

New three-dimensional numerical non-hydrostatic model with a free surface that was designed for modelling the bottom and bank stability subjected by ship propeller jets is presented. Unlike all known models, it describes three-dimensional fields of velocities generated by ship propellers, turbulence intensity and length scale in the given domain of arbitrary bottom and coastal topography. Results of simulations are compared with the laboratory experiments.

Jet scour around vertical piles and pile groups

Ship-induced local scours by propeller jets cause considerable damage around harbour structures. This study investigated the local scour around piles and pile groups. In the case of pile clusters, group effects become important, as does the angle of orientation of the pile groups to the prevailing direction of waves and currents. The main factors to consider are flow interference leading to enhanced flow speeds or turbulence at adjacent piles in some cases and sheltering of piles in others. The propeller jet was simplified using a circular water jet on the cohesionless sediment bottom in this study. The study showed that the maximum equilibrium scour depth around a pile was primarily a function of the densimetric Froude number. The scouring around the pile groups was also found to be dependent on densimetric Froude number (Fr d), the distance between the piles to pile diameter defined as clearance ratio ( a/ D) and the relative size of the pile to circular water jet diameter ( D/ d o).