Hydrodynamic Surfaces Research Articles

BEM modelling of a hydrodynamic lifting surface working in ground proximity

Present paper aims to assess the performance and limits of boundary element method for the reliable simulation of ground effect phenomena on hydrodynamic lifting surfaces. Benefitting from the ground effect, especially in extreme ground proximity, suggests a promising concept for lifting bodies. On the other hand, the more pronounced the ground effect, the more complicated flow physics it exhibits, making the problem harder to handle numerically. In this study, a potential-based boundary element method (BEM) is applied to the flow past a 3D rectangular wing within a wide range of ground clearances corresponding to out of ground effect (OGE), in ground effect (GE) and extreme ground effect (EGE) conditions. Both positive and negative angles of attack are tested to examine the lift and downforce producing cases. The potential flow around the airfoil is computed using the mixed constant-strength source and constant-strength dipole based panel method. The results show that BEM successfully predicts the lift force variation for a wing working in ground proximity (GE). However, approaching the bounds of extreme ground effect is seen to introduce a progressive decrease of the fidelity of the method. A similar tendency is observed in the downforce generating conditions, which is also an important scenario for ground vehicle aerodynamics. BEM remarkably overestimates the magnitude of the downforce when the ground clearance reaches up to the EGE conditions, still, it produces quite satisfactory results for the rest of the tested configurations. The pressure distributions are also provided to examine the flow field in the aforementioned cases. Overall, the results lead us to conclude that BEM stands as a robust and reliable prediction tool for ground effect on 3D wings but the extreme ground effect condition represents the limit of its accuracy range.

RANS-based optimization of a T-shaped hydrofoil considering junction design

Hydrodynamic lifting surfaces usually include junctions. High-fidelity simulations are necessary to capture critical physics near these regions, such as separation, junction vortices, and cavitation. We present RANS-based hydrodynamic optimizations of a T-shaped hydrofoil, including changes in the junction geometry. The optimized hydrofoils avoid separation and delay cavitation compared to the baseline. The full optimization design with planform, cross-section, and junction geometry variables yields a total drag reduction of 6.4%. The optimized results show that the relative locations of the maximum foil thickness and the maximum strut thickness significantly impact the junction cavitation. Including the translation between the strut and the foil and more strut geometric variables as design variables will provide further improvement. The comparison between optimized designs demonstrates that optimizing planform and detailed junction geometry provides further improvement in addition to designing the cross-sectional geometry. The hydrostructural analyses show that the optimized T-foils have lower stress at the junction than the baseline because of the resultant junction fairing. However, these hydrodynamic-only optimized T-foils have higher deformation and maximum stress, which could result in accelerated fatigue, highlighting the need for hydrostructural responses in design optimization. The results demonstrate that the developed methodology is useful for designing next-generation complex hydrodynamic lifting surfaces.

Fabrication of Salvinia-inspired surfaces for hydrodynamic drag reduction by capillary-force-induced clustering

For decades, bioinspired functional materials have been attracting the interest of many researchers for their remarkable characteristics. In particular, some plant leaves are well known for their inherent superhydrophobic nature. Salvinia molesta, a free-floating aquatic fern, has egg-beater-shaped hierarchical trichomes on its surface of leaves. Due to the unique structure and complex wettability of the hairs, this plant has the ability to maintain a stable thick air layer upon the structure when it is submerged underwater. Often referred to as the “Salvinia Effect,” this property is expected to be suitable for use in hydrodynamic drag reduction. However, due to the complex shape of the trichome, currently applied fabrication methods are using a three-dimensional printing system, which is not applicable to mass production because of its severely limited productivity. In this work, artificial Salvinia leaf inspired by S. molesta was fabricated using a conventional soft lithography method assisted with capillary-force-induced clustering of micropillar array. The fabrication method suggested in this work proposes a promising strategy for the manufacturing of Salvinia-inspired hydrodynamic drag reduction surfaces.

Tangential developable and hydrodynamic surfaces for early stage of ship shape design

ABSTRACT In this paper, formation of ship hull shapes from tangent developable and hydrodynamic surfaces given by analytical formulas is analysed. Six analytical methods of construction of tangential developable surfaces are presented and every method is illustrated by a specific surface. In this study, thirteen analytical surfaces suitable for the application on early stage of ship shape design are presented. Eight tangential developable and six hydrodynamic surfaces suitable for ship and boat hull forms are presented. The main cross-sections of hydrodynamic surfaces are in three coordinate planes: the design waterline, the midship section, and the maim buttock line, and the shapes of the lines are determined by predetermined conditions. The analytical definition of the surfaces of ship and boat hulls allows for the optimization of ship shapes, the creation of well-streamlined shapes, the construction of ship hull skin developments on the plane, and the simplification of strength problem solutions.

Scaling the dynamic response and stability of composite hydrodynamic lifting surfaces

Imperfectly scaled models are commonplace in aerodynamics and hydrodynamics because few facilities can meet all the flow and structural scaling requirements. The objectives of this work are to (1) derive and numerically validate scaling relations for the steady-state and dynamic hydroelastic response and stability of hydrodynamic composite lifting surfaces and (2) to investigate the effect of imperfect scaling on the steady-state and dynamic response, as well as hydroelastic stability of said surfaces. We derive proper scaling laws and validate them through numerical simulations using unsteady potential flow and composite beam theory. We investigate the prediction of inception of a new, low-frequency mode that only develops at sufficiently high speeds using scaled models. We demonstrate scaling effects on the modal frequencies, damping coefficients, and hydroelastic instability boundaries by comparing cases with fully enforced scaling laws and cases with relaxed similarities. Using the same material for both the prototype and reduced length-scale models results in imperfect scaling; the steady-state and dynamic hydroelastic responses are not scaled, and smaller length-scale models exhibit delayed hydroelastic instabilities. The conclusion is that using appropriate materials for model scale tests to satisfy Cauchy number similarity is necessary for similar steady-state response, but dynamic similarity additionally requires similar solid-to-fluid added mass ratio. Proper scaling allows one to accurately interpret and assess the performance and stability of full-scale flexible composite lifting surfaces such as foils, propulsors, turbines, fins, rudders, energy saving devices, and energy harvesters.

Hydrodynamic surfaces with midsection in the form of Lame curve

General representation of ship geometry is given by the method of slicing the ship hull by three mutually perpendicular planes: vertical symmetry plane which runs along the middle of hull width, horizontal plane which divides the hull into underwater and abovewater parts, and vertical plane perpendicular to the other two which coincides with midsection. By taking the same three predefined sections of the theoretical hull shape, it is possible to obtain three algebraic surfaces of different order, which are called hydrodynamic in this article. By introducing alphabetic parameters to signify orders of ship skeleton main curves and then by giving them various numerical values, it is possible to consider a large number of hull shapes, having only three explicit surface equations. Method of deriving the equations, obtained by other authors, using only three explicit algebraic equations is demonstrated. The proposed technique is illustrated on six new ship hull shapes.

Analytical surfaces for ship hulls

The choice of optimal shape of ship hull surface is one of the main problems of ship architects and designs. A choice of the form is based on empirical formulae or on intuition of designers. In the article a method of determination of explicit algebraic equations of theoretical shape of ship hull with three main cross-sections given in advance and coinciding with the design waterline, the midship section, and with the main buttock line is given. The forms of the lines in the main cross-sections are chosen from conditions taken in advance. These surfaces are called hydrodynamic. A method is illustrated for three threes of main cross-sections of the ship hulls, i.e. nine hydrodynamic surfaces were constructed. All algebraic equations were converted to parametrical form for comfort of computer modelling. With their help, all nine ship surfaces proposed for the introduction were visualized. Having changed constants containing in the surface equations, i.e. correcting the forms of three main geometric parameters of ship hull, one can select the most rational shape of hull surface for the first approach. Further, it is possible to begin planning parallel middle bodies or to combine bow and stern extremities of a ship from different fragments of algebraic surfaces but with the same midship sections. In a paper, only geometrical problems of design of theoretical hull shape are described.

Shape optimization of hydrodynamic textured surfaces for enhancing load-carrying capacity based on level set method

In order to maximize the load-carrying capacity (LCC) of the textured surfaces, the level set method is introduced to conduct the shape optimization of surface textures under the cavitating hydrodynamic lubrication condition. The optimized surface texture shapes are evolved by the Hamilton-Jacobi equation based on the shape derivative obtained by the shape sensitivity analysis. In the optimization procedure, the velocity regularization method and the re-initial equations are applied to make sure a smooth evolution process. The optimization method is validated by two slider bearing optimization problems. Then the optimal surface texture shapes of a textured surface and a parallel thrust bearing are analyzed. The results show that the optimization method is stable and the optimal geometries are the chevron-type shapes.

3-D high-fidelity hydrostructural optimization of cavitation-free composite lifting surfaces

Design and manufacturing technologies have promoted the use of composites in hydrodynamic lifting surfaces. Composites, even with state-of-the-art coatings, are susceptible to cavitation erosion damage. This work aims to design a cavitation-free composite lifting surface to maximize efficiency while ensuring structural integrity. We optimize a canonical hydrofoil using a gradient-based optimization framework that couples a Reynolds-averaged Navier–Stokes solver with a finite-element structural solver. The optimization reduces the weighted drag coefficient by 1.2% over lift coefficients between 0.2 and 0.6, and increases the cavitation inception speed by 82% at the nominal condition compared to the baseline. The optimized design shows higher cavitation inception speeds than an Eppler hydrofoil (E1127, known for cavitation-free operation at moderate lift conditions) with the same baseline planform at CL≥0.2. The improvement from the E1127 hydrofoil was due mostly to delaying tip vortex cavitation induced by 3-D effects. The optimization finds the fiber angle to balance the bend-twist coupling and modifies the directional strength to reduce the susceptibility to excessive deformation and material failure when sweep presents.

Hydrodynamic surfaces

Hydrodynamic surfaces

A comprehensive investigation on micro-structured surfaces for underwater drag reduction

Micro-structured surfaces are desirable in achieving good drag reduction performance for underwater applications. In the study, comprehensive investigation including numerical study, application analysis, precision manufacturing and accurate drag measurement of micro-structured surfaces have been taken for better understanding of drag reduction mechanisms. Five types of micro grooves are firstly proposed and comparisons of respective hydrodynamic performance reveal that the rectangle grooves perform the best, followed by the semicircular ones, the triangle ones, shark skin, and the U-shaped grooves with 5° attack angle has the least effect. Theoretical calculation of optimal groove width has been conducted for application analysis and the optimal groove width decreases dramatically with travel speed, while as increases slightly along with the increasing vehicle length. Considering both drag reduction ability and manufacturing feasibility, the semicircular grooves are emphasized and micro fly milling is adopted for high-precision machining four groups of grooves. Drag reduction tests of these grooves are conducted by a specially designed measuring system. Experimental results show that the smaller lateral spacing of grooves, the better hydrodynamic performance and S4 surfaces exhibits the maximum drag reduction rate with 27.7%. In case fluid velocities in the range of 0.5 m/s and 4.5 m/s, the averaged drag reduction rate is 13.05%.

Nanotechnology to Improve the Performances of Hydrodynamic Surfaces

Nature continues to inspire scientists to adapt solutions in order to satisfy human needs, mainly in the maritime domain with metallic surface corrosion and its mechanical friction. In this research, the source of innovation comes from the lotus leaf and its well-known super-hydrophobicity. In this study, we have investigated the lotus leaf as a model for a super-hydrophobic maritime surface. The hydrothermal technique, which is considered to be a simple, low-cost, and scalable coating method, is applied to create zinc oxide (ZnO) nanorods (NRs), and an evaporation method is used to apply octadecyltrimethoxysilane (ODS). We apply such eco-green coatings onto commercial epoxy paints. Superhydrophobic surfaces (SHS) are obtained on maritime aluminum substrates. The characterization of SHS indicates improved behavior of water droplets on the treated surface: higher water static contact angles (WCA) from 98° to more than 152° and reduced sliding angles (SA) from 46° to 7°. Sliding speeds (SS) have been largely raised from 54 in the epoxy case to 1300 mm·s−1 after treatment. These results clearly demonstrate the real opportunity to apply ZnO-based nanomaterials onto existing commercial maritime coatings.

Additively Manufactured Single-Use Molds and Reusable Patterns for Large Automotive and Hydroelectric Components

Additive manufacturing (AM) has the potential to reduce costs in the casting industry, particularly for large parts (i.e., components with any dimension > 12 in.). Here, pilot-scale production case studies of hydroelectric and automotive parts were used to evaluate two AM-enabled casting methods: the direct printing of sand molds by binder jet and printing of reusable cope and drag patterns for sand casting. An additional benefit was found when additively manufactured patterns were used in combination with heat-treat free aluminum alloys, to produce castings of complex hydrodynamic surfaces. These parts previously required production via subtractive machining due to part distortion induced by the quench step of heat treatments. The machine types used for the three case studies are sand-binder jet, high-resolution polymer fused deposition modeling (FDM), and big area additive manufacturing (BAAM) FDM in combination with CNC machining. Each method demonstrated distinct advantages over traditional casting practices in particular use cases. Single-use molds show great reductions in start-up cost to produce one-off or legacy parts, and additively manufactured impression patterns show promise for innovating the tooling design process for complex geometry and large castings.

Maneuver Simulation and Optimization for AC50 Class

The stability and the dynamic behavior are integral parts of designing hydrofoil supported sailing vessels, such as the America's Cup (AC) 50 class. The foil design and the control systems have an important influence on the performance and stability of the vessel. Both foil and control system design also drive the maneuverability of the vessel and determine maneuvering procedures. The AC50 class requirements lead to complex foil control systems and the maneuvering procedures become sophisticated and multifaceted. Sailing and maintaining AC50 class yachts is a complex, expensive and time-consuming task. A dynamic velocity prediction program (DVPP) for the AC50 is therefore developed to assess the dynamic stability of different foil configurations and to simulate and optimize maneuvers. The goal 143 is to evaluate certain design ideas and maneuvering procedures with this simulator so that sailing time on the water can be saved. The paper describes the principal concepts of developing an AC50 model in the DVPP FSEquilibrium. The force components acting on the yacht are defined based on physical principles, computational fluid dynamics (CFD) simulations and experimental investigations. The control systems for adjusting the aero- and hydrodynamic surfaces are modeled. Controllers are utilized to simulate the human behavior of performing sailing tasks. Maneuvers are then defined as sequences of crew actions and crew behaviors. In the paper examples of utilizing the DVPP in preparation for the 35th America's Cup in Bermuda are described. The DVPP is for example used to investigate the effect of different boat set-ups on stability and handling during maneuvers. With the sailing team, maneuver procedures are developed and tested. Procedures such as dagger board and rudder elevator movement and crew position are investigated and evaluated to minimize the distance lost during tacking and gybing. The DVPP is also employed for trajectory optimization during maneuvers.

Fabrication and characterization of folded foils supporting streamwise traveling waves

A body of work has grown around the use of small amplitude traveling waves on aerodynamic and hydrodynamic surfaces for boundary layer control. In particular, when the traveling wave speed exceeds the free stream velocity, significant drag reductions have been shown in simulation. Building viable prototypes to test these hypotheses, however, has proven challenging. In this paper, we describe a candidate system for constructing structural airfoils and hydrofoils with embedded electromagnetic actuators for driving high velocity traveling waves. Our approach relies on the fabrication of planar substrates which are populated with electromagnetic components and then folded into a prescribed three dimensional structure with actuators embedded. We first specify performance characteristics based on hydrodynamic requirements. We then describe the fabrication of fiber-reinforced polymer composite substrates with prescribed folding patterns to dictate three dimensional shape. We detail the development of a miniaturized single-phase linear motor which is compatible with this approach. Finally, we compare the predicted and measured force produced by these linear motors and plot trajectories for a 200 Hz driving frequency.

Viscous fluid–structure interaction response of composite hydrofoils

Composite materials are increasingly used in hydrodynamic lifting surfaces due to their higher specific strength and favorable fatigue properties. A set of parametric studies are performed to investigate the influence of fiber orientation on the vibration characteristics and load-dependent bend-twist coupled behavior of composite hydrofoils in viscous flow. A 3-D Reynolds-averaged Navier–Stokes (RANS) solver is coupled with a 3-D finite-element method (FEM) to predict the fluid–structure response of cantilevered composite hydrofoils made of unidirectional carbon fiber reinforced polymer (CFRP). Fiber orientation changes the modal characteristics of composite hydrofoils, as well as the hydroelastic response. The bending-up and nose-down material bend-twist coupling leads to lower hydrodynamic load coefficients with increasing flow speed, as well as delayed separation, stall, and static divergence. The opposite trend is observed when the fiber orientation results in a bending-up and nose-up material bend-twist coupling. Material failure index contours show that the fiber orientation affects the location of failure. These parametric studies provide guidance for future design and optimization of composite hydrodynamic lifting surfaces.

THE ANALYSIS OF THE STABILITY OF THE COMMUNICATION SUPPORT STABILIZED BY GABION WALL WITH VARIABLE CONFIGURATION

The subject of the article concerns the stabilization of communication earthworks using the so-called lightweight retaining structures. The stability of the embankment with the embankment built with the gabion wall is analysed, assuming a uniformly distributed load on the crown and a case of unilateral water pressure in the event of a flood. Final results of stability calculations made with the Fellenius and Bishop method in the form of slip surface with the smallest value of the coefficient were presented. Then, assuming the case of damming up of flood water, discretization of the filtration area, velocity distribution and hydrodynamic grating and slip surfaces were developed. The variable parameter is the wall configuration.

Testing and Characterization of a Fixed Wing Cross-Domain Unmanned Vehicle Operating in Aerial and Underwater Environments

This paper presents test results and performance characterization of the first fixed-wing unmanned vehicle capable of full cross-domain operation in both the aerial and underwater environments with repeated transition and low-energy loitering capabilities. This vehicle concept combines the speed and range of an aircraft with the persistence, diving capabilities, and stealth of a submersible. The paper describes the proof-of-concept vehicle including its concept of operations, the approaches employed to achieve the required functions, and the main components and subsystems. Key subsystems include a passively flooding and draining wing, a single motor and propeller combination for propulsion in both domains, and aerodynamic–hydrodynamic control surfaces. Experiments to quantify the vehicle performance, control responses, and energy consumption in underwater, surface, and flight operation are presented and analyzed. Results of several full-cycle tests are presented to characterize and illustrate each stage of operation including surface locomotion, underwater locomotion, water egress, flight, and water ingress. In total, the proof-of-concept vehicle demonstrated 12 full-cycle cross-domain missions including both manually controlled and autonomous operation.

Multiphysics Modeling of Surface Finishing Performance in Pulsed-Waveform Electrochemical Machining

Electrochemical machining (ECM) is a manufacturing technology that allows metal to be precisely removed by electrochemical oxidation and dissolution into an electrolyte solution. ECM is suited for machining parts fabricated from “difficult to cut” materials and/or parts with complicated and intricate geometries. In ECM, the workpiece is the anode and the tool is the cathode in an electrochemical cell; by relative movement of the shaped tool into the workpiece, the mirror image of the tool is “copied” or machined into the workpiece. One notable difficulty with ECM is an inability to predict a priori the tool and process parameters required in order to satisfy the final specifications of the fabricated part [[1]]. Accordingly, there is potential value in development of a physical phenomenon-based design platform to predict optimal ECM tool shape. Such a capability is anticipated to dramatically shorten the process/tooling development cycle. A further goal of ECM is to simultaneously achieve a target surface finish on the machined part. It is thus of interest to develop the capability also to predict the distribution of local surface finish resulting from ECM processing. Modeling of the changes in local surface finish intrinsically operates on a different length scale from that of bulk material removal (μm, versus mm or cm), and thus is most easily treated separately. The physicochemical phenomena involved in the evolution of surface finish during ECM processing are strongly coupled, and include the electric field itself (primary current distribution), surface polarization and electrochemical kinetics (secondary current distribution), and fluid flow and mass transfer (tertiary current distribution). Of particular interest is modeling of pulsed-waveform ECM, for which significant practical advantages have been demonstrated [[2],[3],[4]]. While an extensive body of literature exists analyzing pulsed electrodeposition [[5],[6]], comparatively little work has been published to date on pulsed ECM [[7],[8]]. This talk will discuss recent modeling work seeking to develop a solid foundation for a predictive understanding of the surface finishing aspects of ECM processes. The work described herein encompasses time-dependent modeling of concentration profiles and other relevant physical quantities under the application of pulse(-reverse) current ECM waveforms, starting from simulations assuming a locally flat surface and working toward quantitative descriptions of the transient and steady-periodic behavior on structured substrates. Prior work (see, e.g., Ref. 3) has demonstrated the value in differential pulsed-ECM processing of surfaces with features of size comparable to or larger than the hydrodynamic boundary layer thickness (“macroprofiles”) versus surfaces with features much smaller than the boundary layer thickness (“microprofiles”). Figure 1 (left) plots a schematic of the pulsating concentration profiles at the end of the pulse on-time in forward-only pulsed ECM, assuming 100% current efficiency. Here, the pulse period (ton + toff) and the peak applied current density (jpeak) are assumed constant, with the curves corresponding to different values of ton. The saturation concentration of the dissolved metal is Cs, and the bulk metal concentration Cb equals zero. The quantity τ is the “transition time,” which is the value for ton for which the metal concentration at the surface rises exactly to Cs at the end of the forward pulse. As can be seen, for ton < τ, metal dissolution is not mass transfer limited, but for any ton > τ the rate of dissolution is constrained by the diffusion of the dissolved material, approaching the DC mass transfer rate for sufficiently high ton. Figure 1 (right) illustrates that preliminary simulations performed with COMSOL Multiphysics® exhibit behavior consistent with this qualitative concept.

FAUSST: bridging the gap between steel and fibre reinforced materials

Multi-material design is commonly used within lightweight applications to meet certain design constraints. One common challenge across the different industry fields is the joining of materials of different material classes. Bonding and mechanical joining are generally used to overcome this issue. However, in some application fields, such as in the shipbuilding industry, where the interest of applying fibre-reinforced materials is increasing, these processes are not currently feasible due to regulatory and current technical constraints. One potential solution is FAUSST, a textile based transition joint. FAUSST is a hybrid knitted fabric, which is composed of 100% steel on one side and on the other of 100% glass fibres. The steel side is welded to a flat steel and the transition element is subsequently integrated via lamination processes within a fibre-reinforced component. Afterwards, this component is e.g. joined to a steel structure by welding. Depending on the design of the transition element, loads of up to 120 kN per meter joint can be transferred in the presented design with an overlap length of only 10 mm. This transition element, therefore, may lead to more lightweight designs with smoother surfaces for aesthetical, aerodynamic or hydrodynamic surfaces.