Water Tunnel Research Articles

Approximate uniformity of the size distribution in the crescent-shaped dune swarm: A downsized experimental reinvestigation

Crescent-shaped dunes, or barchans, usually move in swarms in nature, where their sizes become closer to each other while migrating downstream collectively. A major cause of such a phenomenon is the collision among the dunes. Therefore, it is of importance to elucidate the pattern of the local dune interaction process, i.e., the double-dune collision. In this study, based on a downsized water tunnel experiment that adopts two colors of sand samples, the double-dune collision pattern is parameterized, which is investigated as a function of the initial mass ratio of the two dunes. The results depict from a distinctive perspective that the double-dune collision is essential in realizing the uniformity of the size distribution in a dune swarm. The proposed concepts could aid in the development of a comprehensive parameterization for the double-dune collision system.

Dual-coupling drag reduction inspired by tuna skin: Fan-shaped imbricated fish scale composited with flexible coating

The excellent hydrodynamic properties of fishes enable them to respond rapidly to exterior excitation in water. In this study, three types of bionic surfaces were designed and manufactured based on the construction and mechanical properties of the tuna skin. The surface and coating features of these bionic surfaces were analyzed, and their drag reduction performance was investigated in a circulating water tunnel. The results revealed that the drag reduction effect was proportional to the flexible coating thickness, and a maximum drag reduction of 7.22% was achieved for the dual-structure coupling surface with a flexible coating thickness of 140 µm. The simulation results indicated that the “vortex stretching” effect formed near the fish scale wall further decreased the high-velocity regions, leading to low- and high-velocity streaks inside the boundary layer close to the bionic fish scale surface along the streamwise direction. As the flexible coatings could absorb turbulent fluctuations, they formed larger low-pressure areas near the surface, enabling a better drag reduction effect. The drag reduction mechanism for the dual-structure coupling surfaces arose from the combined actions of the flexible coatings and fan-shaped imbricated fish scales. This study may provide an ideal alternative for drag reduction and antifouling properties in underwater vehicle planning.

The mechanism of flip-flops in a AOT lamella: A molecular dynamics study

A long 5000 ns run was conducted on sodium dioctyl sulfosuccinate (AOT)/water lamellar system to understand the unusual behavior. During the simulation run, we track up to 24 AOT molecules translocated from upper leaflet (UL) to lower leaflet (LL) and vice-verse. AOT molecule takes ~2–17 ns in average to move from one leaflet to other. The potential energy barrier of AOT interlayer translocation calculated as sum of Lennard-Jones and electrostatic energies is estimated to be around ~100–120 kJ/mol and the free energy barrier about 90 kJ/mol. A long-scale simulation is accompanied with undulations and large curvation of bilayer. We argue that the transbilayer flip-flop process is a pore-mediated, i.e. the events are accompanied with water pores (defects) formation. On the other side, the size of “water tunnels” is also important, although the successful transbilayer flip-flop events are not depends on the size and lifetime of the water pores.

Experimental investigation of time-invariant eddy viscosity in wave-current interaction

An experimental study of the turbulent boundary layer, where waves propagate with a current, is presented in this paper. A wide range of test conditions have been covered, namely, flows over a rough bed and a smooth bed, combined flows in a large-scale oscillatory water tunnel and combined waves and currents in two flumes with different scales. Particle Image Velocimetry and Laser Doppler Velocimetry were employed to obtain the velocity field. Detailed analysis of eddy viscosity profiles calculated from the experiments leads to the conclusion that previous assumed profiles do not always accurately describe eddy viscosity distributions in a combined wave-current flow. The distributions of eddy viscosity are categorised into two types (two-layer or three-layer), based on the influence of wave motions superimposed. For those cases in the current-dominated regime, eddy viscosity profiles are similar to unidirectional turbulent currents. When combined flows are in the wave-dominated regime, three-layer eddy viscosity distributions are observed. For both types, a linear eddy viscosity profile is found to be present in the bottom 10 per cent of the turbulent boundary layer. Above this, the classic parabolic profile is observed, over the whole turbulent boundary layer for the first type and over 40 per cent of the turbulent boundary layer thickness for the second type. An empirical eddy viscosity distribution in the outer region is proposed for the second type. This newly developed eddy viscosity distribution provides guidance for numerical modellers in the field of wave-current interaction and for coastal engineers wishing to predict sediment transport.

Enhancing Dynamic Response of Cantilevered Rectangular Prism Using a Splitter Plate as a Passive Turbulence Control in Water Tunnel

The effects of splitter plate as a passive turbulence control on the transverse vibration characteristics of a cantilevered rectangular prism with the different finite length (aspect ratio L/H) and depth ratios (side ratio D/H of 0.2 and 0.5) was investigated in a water tunnel. A splitter plate was installed behind the prism with a small gap and sufficient length to prevent vortex shedding intrusion. It enhances the response amplitude ascended linearly with respect to non-dimensional velocity. In the case of the prisms with critical points, interferences of amplitude signal spectra for the period of displacement vanished, yielding a uniform waveform on the response amplitude. Inserting a splitter plate improves the increment rate of the dynamic response of the test models. A brief numerical calculation was performed to find the quantities of flow features around a plain and rectangular prism with a detached splitter plate. The alteration of flow wake characteristics is found by diminishing unsymmetrical vortices shedding and reduce flow entrainment at the trailing edge. The essential findings of the prism with a detached splitter plate are the base pressure improvement, reducing drag force, and vanishing sharp peaks of wake FFT spectra.

Experimental investigation of ventilation bubble dynamics around a vertically moving cylinder under reduced ambient pressure

Ventilation bubble is widely used to reduce friction drag of object travelling in fluid. A large number of experimental studies are performed on the ventilation bubble in water tunnel, while little is known about it around vertically moving cylinder under reduced pressure. We aim to investigate dynamics of the ventilation bubble in vertically moving case. To support the study, a specialized experiment set-up is designed, based on which images of ventilation bubble around vertically moving cylinder under reduced ambient pressure can be captured and experiments are conducted to study the influence of velocities and flow rate on the ventilation bubble dynamics. In detail, we first describe the development of ventilation bubble and details of re-entrant jet. In addition to that, two types of re-entrant jets are observed, and the maximum velocity of re-entrant jet is obtained. Besides, four modes of development of ventilation bubbles’ closure patterns are concluded. Finally, geometric features of ventilation bubble are obtained through image processing which are then described and analyzed in detail. The above results contribute to the research on the control of vertically moving cylinder through ventilation bubble.

A Low Cost Oscillating Membrane for Underwater Applications at Low Reynolds Numbers

Active surface morphing is a nonintrusive flow control technique that can delay separation in laminar and turbulent boundary layers. Most of the experimental studies of such control strategy have been carried out in wind tunnels at low Reynolds numbers with costly actuators. In contrast, the implementation of such a control strategy at low cost for an underwater environment remains vastly unexplored. This paper explores active surface morphing at low cost and at low Reynolds for underwater applications. We do this with a 3D printed foil submerged in a water tunnel. The suction surface of the foil is covered with a magnetoelastic membrane. The membrane is actuated via two electromagnets that are positioned inside of the foil. Three actuation frequencies (slow, intermediate, fast) are tested and the deformation of the membrane is measured with an optosensor. We show that lift increases by 1%, whilst drag decreases by 6% at a Strouhal number of 0.3, i.e., at the fast actuation case. We demonstrate that surface actuation is applicable to the marine environment through an off the shelf approach, and that this method is more economical than existing active surface morphing technologies. Since the actuation mechanism is not energy intensive, it is envisioned that it could be applied to marine energy devices, boat appendages, and autonomous underwater vehicles.

Turbulent flow around circular arcs

The flow around a circular arc is governed by the effect of the sharp leading edge and the arc's curvature. There is a range of incidences where a leading-edge separation bubble (LESB) is formed on the convex side of the arc, and the reattached boundary layer separates further downstream. Akin to foils and cylinders, for increasing values of the Reynolds number, the boundary layer turns from laminar to turbulent resulting in a step change in the forces, here termed force crisis. This phenomenon is characterized experimentally for an arc with a camber-to-chord ratio of 0.22 and for a range of the Reynolds number from 53 530 to 218 000. Forces are measured both in a towing tank and in a water tunnel, and particle image velocimetry is undertaken in the water tunnel. In stark contrast to cylinders, where the force crisis is associated with the laminar-to-turbulent transition of the boundary layer, here, it is found to be associated with the suppressed relaminarization of the boundary layer. In fact, the LESB is always turbulent at the tested conditions, and relaminarization occurs up to a combination of critical angles of attack and critical Reynolds numbers. The critical angle of attack varies linearly with the Reynolds number. These results may contribute to the design of thin cambered wings, sails, and blades at a transitional Reynolds number such as the wings of micro aerial vehicles, swept wings in subsonic flight, turbomachinery blades, and the sails of autonomous sailing vessels.

Topology and cavitation number characteristics of the gaseous jet-induced tail cavity under co-flow

The tail cavity as induced by submerged gaseous jets on the bottom of axisymmetric vehicles is studied in a water tunnel over Froude numbers, which denote co-flow velocities, between 4.8 < Fr < 16.0, and the jet stagnation to ambient pressure ratio in a convergent–divergent nozzle between 1.7 < p0/p∞ < 14.5. The shadowgraph topology of the induced tail cavity falls into three consecutive categories on the regime map: intact cavity, partially broken cavity (PBC), and pulsating foamy cavity. Both the Fr and p0/p∞ are essential drivers for the topology and length of the tail cavity within the current scope. The length of PBC holds a logarithmic relationship to p0/p∞. The fluctuation intensity of the cavitation number is under heavy influence from Fr and p0/p∞ and is regarded as a direct and reliable measurement of jet/closure interaction intensity. Under high Fr, the Rayleigh–Taylor instability is dominant at the cavity surface, which leads to a significant amount of bubble shedding and adds to the total cavity gas leakage. From the perspective of the vehicle, the characteristics of its bottom pressure depend on the cavity regime. The characteristics of jet-induced tail cavity established in the presented study should be helpful for the design and control of submarine vehicles propelled by rockets and supersonic jets.

Experimental study of the dispersion of cough-generated droplets from a person going up- or downstairs.

The dispersion of cough-generated droplets from a person going up- or downstairs was investigated through a laboratory experiment in a water tunnel. This experiment was carried out with a manikin mounted at inclination angles facing the incoming flow to mimic a person going up or down. Detailed velocity measurements and flow visualization were conducted in the water tunnel experiments. To investigate the influence of the initial position on the motion of particles, a virtual particle approach was adopted to simulate the dispersion of particles using the measured velocity field. Particle clustering, which is caused by the unsteadiness of the flow, was observed in both flow visualization and virtual particle simulation. For the case of going upstairs, particles are concentrated below the person’s shoulder and move downward with a short travel distance. For the case of going downstairs, particles dispersing over the person’s head advect over for a long distance. We also found that the motion of the particles is closely related to the initial position. According to the results in this study, suggestions for the prevention of respiratory infectious disease are made.

Hairpin vortex shedding from axisymmetric bodies

Characteristics of hair-pin vortex shedding from axisymmetric bodies, such as a sphere and a circular disk, were investigated by flow visualization. The wakes behind a sphere and a circular disk at Re = 650, 800 were visualized by water tunnel experiment with fluorescence dye. The hair-pin vortex was continuously separated from the axisymmetric bodies. However, the orientation of hair-pin vortex shedding in various mode was depended on the Reynolds number. In this study, the azimuthal angle of the head of hair-pin vortex were observed from downstream direction. In the case of the sphere wake, the azimuthal angle of the hair-pin vortex was almost constant direction of 270 degree at Re = 650. The angle was gradually scattered at Re = 650. For the circular disk, it was found that the shedding angle of the hairpin vortex was more unstable as the Reynolds number increased than the case of sphere.

Experimental and Numerical Research on the Formation Conditions and Entrainment Characteristics of Supercavity by Rear Jet Reflux

Aiming at better controlling the ventilated supercavity flow for drag reduction, the experimental and numerical researches of supercavity by rear gas reflux are proposed in this paper. Several experiments with different test bodies have been carried out to study the formation and collapse conditions of jet-reflux supercavity. An open-circulation water tunnel for ultra-high-speed jet experiment and air jet system is employed to form jet-reflux supercavity around the bodies installed in the forward strut. The experiment results show that the supercavity can be maintained by the reflux of tail jet flow when an initial supercavity covering the jet outlet is formed. However, the supercavity will be destroyed when the jet intensity is further enhanced. Under the same jet coefficient, the scale of jet-reflux supercavity extends as the increase of the body length, while the critical jet coefficient for the collapse of the supercavity decreases as the increase of the body length. The multiphase flow model coupling the VOF model and the level-set method is applied to capture the air-water interface. Then, the flow field characteristics of the jet-reflux supercavity are analyzed and compared with the ventilated supercavity. The streamline inside the cavity presents considerable three-dimensional asymmetry inflating flow characteristics. The variation of the gas reflux coefficient along the axial direction is obtained, which indicates that a handful of reflux gas are required to sustain the head cavity. Therefore, the jet-reflux supercavity can be formed within a certain range of the tail jet intensity. Although jet intensities are not equal to each other, the scale of cavity head is roughly maintained under the same reflux coefficient. When the supercavity gets closed to the nozzle outlet, the maximum scale of cavity is decreased, which leads to a weaker reflux at the outlet. The cavity interface will be impinged by the high-speed gas and mixture liquid, which obviously causes deformation and final collapse. In order to improve the stability of the jet-reflux supercavity, it is necessary to use the gas re-directed structure to control the reflux.

Flowing Water Algorithm: A New Approach for Combinatorial Optimization Problems

Being greatly inspired by the natural flowing regulation of water, we propose a new meta-heuristic algorithm — <i>Flowing Water Algorithm</i> (FWA) for the solution of combinatorial optimization problems (COPs). Since the solution space of COPs is multidimensional, complex and has many local extreme values, according to our proposed method, it appears to be similar to an endless hilly area with mountains, valleys and plateaus. The downward-flowing water in such area finds its way to the lowest point in the hill. Water always flows downward and eventually converges at the lowest place without any outside intervention except for gravity. Such a flowing course can be deemed as a process for the water to seek for the lowest point. The proposed algorithm is derived from such a water flow process. This algorithm combines a local search strategy with a population-based search strategy to improve both local and global search abilities. Four operators, including the local search, water overflow, drilling water tunnel and evaporation-rain are included in FWA, making this algorithm successfully perform tabu search, positive feedback, “survival of the fittest”, and local optimum escape. Two examples of its application in the traveling salesman problem (TSP) show that FWA outperforms the benchmark methods for both solution quality and convergence speed.

Enhanced thermal performance with high-amplitude intermittent impingement cooling

The advances of many future engineering applications rely on effective cooling techniques. Beyond the traditional thermal management solutions, the design potential of unsteady impingement cooling is still under-explored. As a combined experimental and numerical study, this paper reports new findings on high-amplitude intermittent impingement cooling with controlled unsteady patterns. Specifical attention was paid on the intermittent flow close time ratio. The experimental work involved unsteady cooling performance measurement with a small-scale water tunnel system. Unsteady Reynolds Averaged Navier-Stokes Simulation (URANS) was conducted to illustrate the unsteady flow physics, and to evaluate the cooling performance at a wider range of flow conditions (average Reynolds number 2800 < Rem < 10,000, pulsating frequency 0.1 Hz < f < 2 Hz, close time ratio 0.2 < γ < 0.8). Both experimental and numerical data confirm a remarkable improvement of overall cooling efficiency by high-amplitude intermittent impingement flow. Especially around the wall jet region, the enhancement can reach as high as 50%. The generation and interaction of vortex rings break the development of thermal boundary layer, and enhance the generation of near wall turbulence, especially for the wall jet region. Saving in coolant consumption with high-amplitude intermittent impingement cooling technique in practice is also demonstrated. The novel concept presented in this paper can be applied to a wide range of applications including electronic cooling, deicing, gas turbine blade cooling, etc.

Study on the Law of Harmful Gas Release from Limnoperna fortunei (Dunker 1857) during Maintenance Period of Water Tunnel Based on K-Means Outlier Treatment

It is of great significance for air pollution control and personnel safety guarantee to master the release characteristics of harmful gases in the process of Limnoperna fortunei corruption. In view of the lack of research on the environmental pollution caused by the corruption of Limnoperna fortunei, a model experiment was designed to study the three harmful gases of NH3, H2S, and CH4 in the putrid process of Limnoperna fortunei by considering the density of Limnoperna fortunei and the time of leaving water. The results show that: (1) The recognition and processing of outliers based on wavelet decomposition and K-means algorithm can effectively reduce the standard deviation and coefficient of variation of the data set and improve the accuracy of the data set. (2) The variation of NH3 and H2S gas concentrations with the time of water separation satisfies polynomial linear regression (R2 &gt; 99%). (3) At a density of 0.5–7.0 × 104 mussels/m2, the highest concentration of NH3 reached 47.9777–307.9454 mg/m3 with the increase in the density of Limnoperna fortunei and the extension of the time away from water, far exceeding the occupational exposure limit of NH3 of 30 mg/m3, potentially threatening human health and safety. The highest detection value of H2S concentration is 0.1909–5.0946 mg/m3, and the highest detection concentration of CH4 is 0.02%, both of which can be ignored.

Innovative Energy-Saving Propulsion System for Low-Speed Biomimetic Underwater Vehicles

This article covers research on an innovative propulsion system design for a Biomimetic Unmanned Underwater Vehicle (BUUV) operating at low speeds. The experiment was conducted on a laboratory test water tunnel equipped with specialised sensor equipment to assess the Fluid-Structure Interaction (FSI) and energy consumption of two different types of propulsion systems. The experimental data contrast the undulating with the drag-based propulsion system. The additional joint in the drag-based propulsion system is intended to increase thrust and decrease energy input. The tests were conducted at a variety of fins oscillation frequencies and fluid velocities. The experiments demonstrate that, in the region of low-speed forward movement, the efficiency of the propulsion system with the additional joint is greater.

LOW REYNOLDS NUMBER PERFORMANCE OF A MODEL SCALE T-FOIL

Submerged T-foils are an essential forward component of the ride control systems of high speed ferries. A model scale T-Foil for a 2.5m towing tank model of a 112m INCAT Tasmania high-speed wave-piercer catamaran has been tested for both static and dynamic lift performance. The tests were carried out using a closed-circuit water tunnel to investigate the lift and drag characteristics as well as frequency response of the T-Foil. The model T-Foil operates at a Reynolds number of approximately 105, has an aspect ratio of 3.6 and a planform which is strongly tapered from the inboard to outboard end. All of these factors, as well as strut and pivot interference, influence the steady lift curve slope ( of the model T-foil which was found to be 61% of the value for an ideal aerofoil with elliptic loading. The T-foil dynamic performance was limited primarily by the stepper motor drive system and connection linkage. At the frequency of maximum motion of the 2.5 m catamaran model (about 1.5Hz) the model T-foil has approximately 5% reduction of amplitude and 15 degrees of phase shift relative to the low frequency response. Only very small limitations arose due to the unsteady lift as predicted by the analysis of Theodorsen. It was concluded that the model scale T-foil performed adequately for application to simulation of a ride control system at model scale.

Vortex Trapping Cavity on Airfoil: High-Order Penalized Vortex Method Numerical Simulation and Water Tunnel Experimental Investigation

This paper presents a two-dimensional implementation of the high-order penalized vortex in cell method applied to solve the flow past an airfoil with a vortex trapping cavity operating under moderate Reynolds number. The purpose of this article is to investigate the fundamentals of the vortex trapping cavity. The first part of the paper treats with the numerical implementation of the method and high-order schemes incorporated into the algorithm. Poisson, stream-velocity, advection, and diffusion equations were solved. The derivation, finite difference formulation, Lagrangian particle remeshing procedure, and accuracy tests were shown. Flow past complex geometries was possible through the penalization method. A procedure description for preparing geometry data was included. The entire methodology was tested with flow past impulsively started cylinder for three Reynolds numbers: 550, 3000, 9500. Drag coefficient, streamlines, and vorticity contours were checked against results obtained by other authors. Afterwards, simulations and experimental results are presented for a standard airfoil and those equipped with a trapping vortex cavity. Airfoil with an optimized cavity shape was tested under three angles of attack: 3°, 6°, 9°. The Reynolds number is equal to Re = 2 × 104. Apart from performing flow analysis, drag and lift coefficients for different shapes were measured to assess the effect of vortex trapping cavity on aerodynamic performance. Flow patterns were compared against ultraviolet dye visualizations obtained from the water tunnel experiment.

PRESSURE DISTRIBUTION DUE TO STERN TAB DEFLECTION AT MODEL SCALE

Trim tabs form an important part of motion control systems on high-speed watercraft. By altering the pitch angle, significant improvements in propulsion efficiency can be achieved by reducing overall resistance. For a ship in heavy seas, trim tabs can also be used to reduce structural loads by changing the vessel orientation in response to encountered waves. In this study, trials have been conducted in the University of Tasmania hydraulics laboratory using a closed- circuit water tunnel to measure model scale trim tab forces. The model scale system replicates the stern tabs on the full- scale INCAT Tasmania 112 m high-speed wave-piercer catamaran. The model was designed for total lift force measurement and pressure tappings allowed for pressures to be measured at fixed locations on the underside of the hull and tab. This investigation examines the pressures at various flow velocities and tab deflection angles for the case of horizontal vessel trim. A simplified two-dimensional CFD model of the hull and tab has also been analysed using ANSYS CFX software. The results of model tests and CFD indicate that the maximum pressure occurs in the vicinity of the tab hinge and that the pressure distribution is long-tailed in the direction forward of the hinge. This accounts for the location of the resultant lift force, which is found to act forward of the tab hinge.

MEASURED LOADING RESPONSE OF MODEL MOTION CONTROL STERN TABS

&#x0D; &#x0D; &#x0D; Active trim tabs are commonly used as part of the ride control systems of high-speed craft. This paper investigates the lift characteristics of rectangular stern tabs that are commonly fitted to INCAT wave-piercer catamarans. A test apparatus was developed to enable the testing of a model scale trim tab in a circulating water tunnel in the University of Tasmania hydraulics laboratory. The magnitude and location of the lift force produced by the tab were measured over a range of tab angles and flow velocities. From this the lift coefficient of the tab was calculated and the performance of the tab under varying conditions was analysed. The lift force produced by the tab was shown to increase with velocity and tab angle as expected, with the lift coefficient of the tab increasing linearly with tab angle and remaining relatively constant with increases in flow velocity. The magnitude of the measured lift coefficient was lower than had been previously estimated in shallow water tests and the force was found to act forward of the tab hinge, indicating that much of the lift force generated by the tab is due to the increased pressure on the underside of the hull forward of the tab.&#x0D; &#x0D; &#x0D;