Water Tunnel Tests Research Articles

Advanced heave control of a hydrofoil-based autonomous surface vehicle: CFD modeling with integrated variable propeller revolution control

An innovative control strategy employing the Proportional-Integral-Derivative (PID) algorithm is proposed to address the heave stability challenges in the in-house developed hydrofoil-based Autonomous Surface Vehicle (HASV). This method regulates the HASV's heave motion by adjusting the propeller's revolution speed. After confirming that the aerodynamic load on the superstructure contributes less than 2% compared to the hydrodynamic load, a detailed numerical simulation study and recirculating water tunnel test were conducted to validate the reliability of the established Unsteady Reynolds-Averaged Navier-Stokes (URANS) CFD model for the HASV substructure. The integration of the PD controller for propeller revolutions into the CFD free-running model, which incorporates fully nonlinear and viscous effects, was achieved using the ‘Region-Moving’ method for the background domain and the Body Force Method (BFM) for the propeller domain. By analyzing the impact of various PD coefficients on heave control, optimal settings for the HASV were determined. Subsequent studies examined the vehicle's capability in draft correction and draft maintenance under wave conditions. The results of direct CFD-PD simulations demonstrate that the proposed heave control method effectively stabilizes the HASV under complex nonlinear environmental conditions, offering valuable insights for addressing similar heave control challenges in hydrofoil-based vehicles.

Water tunnel testing of downwind yacht sails

Downwind yacht sails, such as spinnakers, are low-aspect-ratio highly cambered wings with a sharp leading edge. They are characterised by substantial three-dimensional flow separation and are thus modelled with difficulty with numerical simulations. Furthermore, accurate full-scale validation data are not available. The first quantitative flow measurements have only recently been achieved in water tunnels. In this study, we aim to provide guidelines on this emerging sail testing methodology. We consider six model-scale rigid models at average-chord-based Reynolds numbers ranging from 5 870 to 61 870. A critical Reynolds number is identified, below which relaminarisation of the reattached boundary layer downstream of the leading edge separation bubble occurs. Both lift and drag increase monotonically at subcritical Reynolds numbers while remaining about constant at transcritical Reynolds numbers. The critical Reynolds number decreases with increasing incidence and is insensitive to the blockage ratio. Spinnakers are normally sailed in front of the mainsail, whose circulation is found to generate an approximately 3∘\\documentclass[12pt]{minimal} \\usepackage{amsmath} \\usepackage{wasysym} \\usepackage{amsfonts} \\usepackage{amssymb} \\usepackage{amsbsy} \\usepackage{mathrsfs} \\usepackage{upgreek} \\setlength{\\oddsidemargin}{-69pt} \\begin{document}$$3^{\\circ }$$\\end{document} upwash on the spinnaker and higher flow velocity on both sides of it. These findings provide guidelines for the experimental testing of spinnaker-like wings in water tunnels and provide new insights into the flow and experimental testing of highly cambered wings with massive flow separation at low Reynolds numbers.

Experimental investigation of flow-induced vibration and flow field characteristics of a flexible triangular cylinder

Flow-induced vibration (FIV) of a flexible cylinder with a triangular cross-section, allowed to oscillate in the cross-flow, inline and torsional direction, is studied experimentally through water tunnel tests. The dynamic response of the cylinder was studied for three different angles of attack ( $0^\circ, 30^\circ, 60^\circ$ ), at reduced velocities of 0.9–16.27, corresponding to Reynolds numbers of 364–3600. At the angle of attack of $0^\circ$ , vortex-induced vibration at low reduced velocity was observed, which transitioned to galloping at higher reduced velocities. At the angles of attack of $30^\circ$ and $60^\circ$ , galloping-type response was observed over the range of the reduced velocities tested. Our results show that the cylinder's torsional oscillation breaks the system's symmetry and affects the structural response at higher reduced velocities regardless of the angle of attack. The FIV response of the flexible triangular cylinder is distinct from that of a rigid flexibly mounted triangular cylinder due to torsional oscillation, spanwise flexibility and the two fixed boundary conditions limiting the amplitude of oscillation. Flow field analysis in the wake of the cylinder was done qualitatively and quantitatively using hydrogen bubble flow visualisation and time-resolved volumetric particle tracking velocimetry techniques, respectively. Our results show the existence of highly three-dimensional vortex structures in the wake of the cylinder. We studied the coupling between the vortex shedding modes in the wake of the cylinder and the structural vibration modes through the spatiotemporal mode analysis using the proper orthogonal decomposition technique to distinguish between different types of the FIV response observed.

Multifunctional Oil‐Entangled Matrix Spray for Maritime Sustainability

AbstractMaritime transport, as the backbone of international trade, contributes to 3% of global greenhouse gas emissions. One major challenge to the sustainability of marine shipping is the fuel consumption associated with flow resistance to ship hulls. While passive approaches like superhydrophobic or lubricant‐infused surfaces hold promise for drag reduction, their scalability, stability, and durability are limited, particularly in oceanic scenarios. Here, an oil‐entangled matrix spray (OEMs) coating is developed to overcome the limitations of conventional surface modification. To create a drag‐reduction surface, liquid lubricant is rationally integrated with a polymer matrix, enabling the firm retention of the lubricant while maintaining high adhesion to various substrates. The resulting slippery interface achieves up to 48% drag reduction compared to an unmodified surface in a water tunnel test. Additionally, the hydrophobic, chemically inert, smooth, and slippery interface of OEMs enables multifunction of the coated substrates, including corrosion resistance to seawater and acid, reduced frosting accretion (by up to 68%), and low shear adhesion to ice (<11 kPa). Furthermore, the OEMs provides a universal platform to integrate customized functions such as photothermal effects, further enhancing interface performances by synergizing solar thermal energy and surface slipperiness. This study presents a promising surface modification strategy for improving the sustainability of marine vehicles where scalable, durable, and multifunctional surfaces are desired to achieve sustainable drag reduction under oceanic cruising and extreme weather conditions.

Numerical simulation study on hydrodynamic time-delay characteristics of supercavitating vehicle

Due to the existence of the time-delay effect of the supercavity, the fluid dynamics of the supercavitating vehicle in the maneuvering will exhibit extremely strong unsteady characteristics, which will bring great challenges to the hydrodynamic acquisition, motion performance analysis and control scheme design and evaluation of the vehicle. Therefore, it is of great significance to study the hydrodynamic characteristics and formation mechanism of the vehicle in supercavitating state. In this paper, a three-dimensional numerical model for continuous motion of the supercavitating vehicle at varying angles of attack is established by using the dynamic mesh technique. The accuracy of the model is verified by using the water tunnel test results. Through simulation, the unsteady cavitation development and hydrodynamic characteristics of the supercavitating vehicle at different speeds, swing frequencies and preset rudder angles are obtained. The results show that the hydrodynamics of the supercavitating vehicle under unsteady motion has time-delay characteristics, and the prediction results of the conventional hydrodynamic calculation method of the vehicle have large errors. The cavitation time-delay effect and the dynamic development of the attached cavitation are the fundamental reasons for the formation of the unsteady hydrodynamic time-delay characteristics of the supercavitating vehicle. The hydrodynamic time-delay characteristics of the supercavitating vehicle is significantly enhanced with the increasing of velocity and weakened with the increasing of swing frequency. The preset rudder angle of the cavitator results in different time-delay characteristics when the vehicle swings at positive and negative angles of attack.

Hub Flow Near-Wake Validation Using CREATETM-AV Helios and UMD Mercury Framework

Experimental near-wake flow field measurements from two Penn State University water tunnel tests of a defeatured helicopter hub are compared with two unsteady computational fluid dynamics (CFD) analyses: CREATETM-AV Helios and the University of Maryland Mercury. Both CFD frameworks employ an unstructured/Cartesian multimesh paradigm and turbulent Spalart–Allmaras detached eddy simulation (SA-DES) modeling. Experimental velocimetry measurements of mean wake velocities, harmonic content, and Reynolds stresses provide valuable validation data for CFD. Overall, the two CFD solvers were in good agreement with each other and qualitatively captured the mean and harmonic content of the wake structures with accuracy. Flow feature dissimilarities between the advancing and retreating sides were differentiated and indicated dominant regions of harmonic flow disturbances biased towards the retreating side, in good agreement with experimental observations. Quantitatively, some variation in velocity deficits and downwash were noted, either in profile character, magnitude, and/or location. Encouragingly, there was little tendency of excessive dissipation in the CFD near wake, and the harmonic content actually tended towards a common overprediction. Reynolds number effects were minimal, and grid density effects were studied but inconclusive. These efforts were part of the Third PSU Rotor Hub Flow Workshop in 2020.

Investigating the Effects of Blockage Ratio on the Performance of a Surface-piercing Propeller in Free Surface Water Tunnel Tests

This paper investigates the effect of the immersion ratio parameter on the hydrodynamic performance of three surface-piercing propellers with diameters of 0.125, 0.132 and 0.140m at different advancing speeds. Experimental tests have been carried out in the free surface water tunnel of the Babol Noshirvani University of Technology. The results showed that the maximum thrust coefficient of three propellers occurs in the velocity range of 3-3.5 m/s. This interval represents the transition area of the three propellers. Also, the effect of the blockage ratio on the hydrodynamic coefficients of three propellers relative to the advance coefficient has been studied. By increasing the immersion depth raises the propeller's wet surface and increases the thrust and torque hydrodynamic coefficients. However, growing the propeller's diameter to 0.140m causes the effect of the blockage ratio parameter by increasing the immersion and the propeller's torque experiences a decreasing trend. Therefore, maximum propeller efficiency value with diameter 0.140m in immersion ratio 0.60 and 0.70, incresing 38% and 44%, respectively; relative to other proepllers. Also, the curve of the efficiency gradient of three propellers in the optimum immersion ratio of 0.40 compared to the advancing coefficient shows that the maximum efficiency gradient occurs in the range of 0.7 to 0.9.

High-blockage corrections for circular arcs at transitional Reynolds numbers

Model-scale testing might suffer from blockage effects due to the finite dimensions of the test section. Measurements must be corrected to predict the forces that would have been measured in unconfined conditions. Blockage corrections are well-established for streamlined and bluff bodies, while more data is needed to develop corrections for bodies that generate both high lift and large wakes. In this work, towing tank and water tunnel tests of two-dimensional circular arcs are employed to develop a correction for a blockage ratio, i.e. the ratio of the frontal area of the geometry to the cross-sectional area of the test section, up to 0.2477. Experiments are conducted at positive incidences between the ideal angle of attack and deep-stall at transitional Reynolds numbers from 53530 to 218000. The results show that a linear blockage correction can be devised for the whole range of tested blockage ratios. Furthermore, the critical angle of attack and Reynolds number at which the force crisis occurs is independent of the blockage ratio. These results may allow extending the range of model sizes that can be tested in water and wind tunnels and may contribute to the accurate accounting of blockage effects at transitional Reynolds number conditions.

Combined Passive/Active Flow Control of Drag and Lift Forces on a Cylinder in Crossflow Using a Synthetic Jet Actuator and Porous Coatings

This paper combines a synthetic jet actuator (SJA) and a leeward porous coating to alter the aerodynamic forces on a cylinder in crossflow at Re = 4.2 × 104. While SJAs and porous coatings are known to be effective flow control methods in isolation, their combined effect has not been studied. A 2D numerical model was created of a cylinder with a SJA at 90° and 100° leeward porous coating. The model was validated using accompanying water tunnel tests. The combined model was tested for dimensionless frequencies 0.15 <f+< 4 and compared to reference models. For f+< 1, using only the SJA increases the cylinder drag coefficient (Cd). Combining a porous coating with the SJA in that regime lowers the Cd values by 15–21%, and causes an overall reduction in Cd compared to the smooth cylinder baseline case. However, using only the porous coating causes a superior 35% reduction in Cd. For f+> 1, the combined SJA and porous coating configuration did not differ from the SJA only configuration, achieving the largest drag reduction of 45% at f+ = 4. The flow control mechanisms of the SJA and porous coating do not combine constructively in this current setup. However, the porous coating is beneficial for f+< 1, causing an overall drag reduction even when the active SJA tends to increase drag.

Blockage effect of a wall on the hydrodynamic characteristics of a supercavitating vehicle's aft body

When the hydrodynamic characteristics of a supercavitating vehicle are tested in a water tunnel, the blockage effect of the wall compresses the diameter of the supercavity, distorting the hydrodynamic force curves. In this paper, computational fluid dynamics (CFD) simulations are used to investigate the hydrodynamic characteristics of a supercavitating vehicle model for various angles of attack, and the simulation results are validated experimentally. When the blockage ratio is less than 0.002295, the blockage effect on the cavity can be neglected. However, for available water tunnels and model sizes, the blockage effect is unavoidable, changing the free and critical angles of attack in the hydrodynamic force curves. The simulation results and conclusions are useful for correcting the results of water tunnel test results influenced by the blockage effect.

Unsteady load mitigation through a passive trailing-edge flap

There are a wide range of applications in which it is desirable to mitigate unsteady load fluctuations while preserving mean loading. This is often achieved with active control systems, but passive systems are sometimes more desirable for enhancing reliability. This is the case, for example, for wind and tidal turbines, where unsteady loading limits the fatigue life of the turbine and results in power peaks at the generator. Here, we consider the unsteady load mitigation that can be achieved through a foil with a trailing-edge flap that is connected to the foil via a torsional spring. We develop a theoretical model and show that the preload can be tuned to preserve the mean foil loading. The spring moment that maximises the unsteady load mitigation is approximately constant, and the load fluctuation reduction is linearly proportional to the ratio of the flap to the full chord of the foil. We verify this relationship through water tunnel tests of a foil with a hinge at 25% of the chord from the trailing edge. As theoretically predicted, we measure unsteady load mitigation of up to 25%, without any variation in the mean load. In highly unsteady flow conditions, when boundary layer separation occurs, the unsteady load reduction decreases. Overall we conclude that passive trailing-edge flaps are effective in alleviating unsteady load fluctuations and their effectiveness depends on their size relative to the foil.

Modeling of the impact of laminar-turbulent transition on cavitation inception

The main incentive to study cavitation inception has been the accompanying jump of flow-induced noise. The water tunnel tests of cavitation inception have been carried out at Reynolds numbers that are substantially smaller than those which occurs to most of the marine applications. As a result, there are significant scale effects. Employment of trips to enhance laminar-turbulent transition in boundary layers of tested models may seem to be useful to reduce these effects, but the experiment of Arakeri and Acosta showed that the trip use can lead to the trend opposite to the trends observed in flows with the natural transition. This paper gives the theoretical explanation of the difference of trends. The explanation is obtained via computations based on a viscous-inviscid interaction procedure with employing a multi-zone flow model. The viscous part of the flow includes attached boundary layers and viscous separation zones. Possibilities of laminar-turbulent transitions and relaminarization are taken into account, as well as capillarity effect on the surfaces of cavities.

The hydroelastic analysis of marine propellers considering the effect of the shaft: Theory and experiment

At present, although there are a large number of studies on the hydroelastic analysis of ship propellers, almost all of them neglect the shaft, thus ignoring the influence of the structural coupling effect between the propeller and the shaft. Therefore, considering this coupling effect, a new hydrodynamic analysis model of propellers is established by using the boundary element method (BEM) coupled finite element method (FEM). And then the model is verified by some experiments on the water tunnel test bench. Finally, based on this model, the influence of the structural coupling effect on propellers' hydroelastic performance is mainly studied. The results show that when the structural coupling effect is ignored, the relative errors of the first-order jellyfish mode of the blade, the dynamic stress at the blade root, and the longitudinal bearing force of the propeller are as high as 22.78%, 28.04%, and 20.37% respectively. Therefore, the structural coupling effect between the propeller and the shaft should be considered when analyzing the propellers’ hydroelastic response. Further, when designing the propeller-shaft system, this coupling effect can also be fully utilized to reasonably match the parameters of the propeller and the shaft to optimize the hydroelastic performance of propellers.

Biomimetic Flow Sensor for Detecting Flow Rate and Direction as an Application for Maneuvering Autonomous Underwater Vehicle

Attaining the information of the hydrodynamic flow rate and direction is essential to the maneuvering of autonomous underwater vehicles (AUVs). This work presents a pillar-based flow sensor that measures hydrodynamic flow rate and direction. We propose a design that mimic the working principle of the neuromast, a ubiquitous organ in fishes that sense the water flow. By utilizing advances in piezo-resistive pressure sensors and 3D printing technology, sensor fabrication becomes fast and cost-effective, owing to reductions in labor and material cost for small batches. Measured results showed that the sensor sensitivity was 9.24 mV/m/s in the single mode and 20.3 mV/m/s in the differential mode. The resolution of the flow sensor was measured to be 4.93 mm/s in the water tunnel testing. The angular resolution of the flow sensor was 2.25°.

Identification of Fixed-Wing Micro Aerial Vehicle Aerodynamic Derivatives from Dynamic Water Tunnel Tests

A micro air vehicle (MAV) is a class of miniature unmanned aerial vehicles that has a size restriction and may be autonomous. Fixed-wing MAVs are very attractive for outdoor surveillance missions since they generally offer better payload and endurance capabilities than rotorcraft or flapping-wing vehicles of equal size. This research paper describes the methodology applying indicial function theory and artificial neural networks for identification of aerodynamic derivatives for fixed-wing MAV. The formulation herein proposed extends well- known aerodynamic theories, which are limited to thin aerofoils in incompressible flow, to strake wing planforms. Using results from dynamic water tunnel tests and indicial functions approach allowed to identify MAV aerodynamic derivatives. The experiments were conducted in a water tunnel in the course of dynamic tests of periodic oscillatory motion. The tests program range was set at high angles of attack and a wide scope of reduced frequencies of angular movements. Due to a built-in propeller, the model’s structure test program was repeated for a turned-on propelled drive system. As a result of these studies, unsteady aerodynamics characteristics and aerodynamic derivatives of the micro-aircraft were identified as functions of state parameters. At the Warsaw University of Technology and the Air Force Institute of Technology, a “Bee” fixed wings micro aerial vehicle with an innovative strake-wing outline and a propeller placed in the wing gap was worked. This article is devoted to the problems of identification of aerodynamic derivatives of this micro-aircraft. The result of this research was the identification of the aerodynamic derivatives of the fixed wing MAV “Bee” as non-linear functions of the angle of attack, and reduced frequency. The identification was carried out using the indicial function approach.

Performance and wake flow characterization of a 1:8.7-scale reference USDOE MHKF1 hydrokinetic turbine to establish a verification and validation test database

Abstract As hydrokinetic turbine technologies continue to advance towards commercialization, public datasets on the performance characteristics for these devices and their flow field effects are invaluable to advance our understanding of these technologies and to validate analytical and numerical models. The Applied Research Laboratory at The Pennsylvania State University (ARL Penn State) collaborated with Sandia National Laboratories and the University of California at Davis to design, fabricate (at a 1:8.7 scale), and experimentally test a novel hydrokinetic turbine rotor design to provide an open platform and dataset for further study and development. The water tunnel test of this three-bladed, horizontal-axis rotor recorded power production, blade loading, the near-wake flow, cavitation effects, and noise generation. These state-of-the-art measurements demonstrate much of the complex physics associated with the flow through an unducted, horizontal-axis turbine, and they elucidate the performance characteristics and flow field effects at an unprecedented fidelity, accuracy and resolution. Measurements of power coefficients (power, torque and thrust) as a function of tip-speed-ratio were performed. The dataset also includes unsteady measurements of driveshaft loading, blade strain, tower pressures, and radiated noise. Detailed flow mapping using laser Doppler velocimetry, and planar and stereo particle image velocimetry includes measurements of mean velocity and Reynolds stresses. Although the wake measurements are limited to less than half a diameter, they reveal the complex flow patterns in the near-wake structure of the rotor. The full database, available at the United States Department of Energy’s marine and hydrokinetic data repository, includes tunnel and model Computer Aided Design geometry files and inflow data sufficient for a “Model-the-Test” computational Verification and Validation study.

REMOVED: Investigating the sound power level of a simplified underwater vehicle induced by flow separation

Abstract Fluid flowing along underwater vehicles often acquire unsteady flow types such as flow separation and vortex shedding. The characteristics of unsteady flows around the underwater vehicle cause high frequency fluctuating pressure and noise on the vehicle rendering the underwater vehicle easily exposed. Studying the characteristics of flow separation on underwater vehicles, establishing accurate prediction methods for flow-induced noise and fast prediction formula based on test and simulation data are important for better understanding of the mechanism of flow separation-induced noise as well as fast prediction and control of underwater noise. In this study, the flow separation characteristics and fluctuating pressure induced by flow separation of underwater vehicles are studied. A simplified model composed of a cylinder, cone and other simple geometry is taken as the research object for water tunnel test and numerical analysis. We also discuss the appropriate numerical simulation method for predicting the flow separation and fluctuating pressure. The vortex distribution characteristics and flow separation mechanisms around the vehicle are analyzed and described based on the numerical method. The fitting function of pressure at different positions is established based on the flow characteristics, the experimental and numerical prediction data. Subsequently, the prediction formula of sound power level is verified. Results show that the flow separation characteristics and the numerical strategy employed in this study efficiently captured the flow filed and the surface fluctuating pressure characteristics of the simplified underwater vehicle. Findings from the numerical results and water tunnel data showed good prediction accuracy. The prediction results reflected the varying relationship of amplitude, frequency and the trend of data distribution.

Performance characteristics of shrouded horizontal axis hydrokinetic turbines in yawed conditions

During operations, river and tidal hydrokinetic turbines encounter changes in flow direction that decrease their performance. Evaluating hydrokinetic turbines in yaw conditions contributes to estimating turbine performance, power stability, and power delivered to the grid. Water tunnel tests using a 19.8 cm diameter horizontal axis model turbine in yaw operation show the performance reduction using three designs: no shroud, a convergent-divergent shroud, and a diffuser shroud. Experimental results obtained at three Reynolds numbers of 1.38 × 105, 1.77 × 105, and 2.17 × 105 show that the output power decreases in off-axis flows for all designs investigated. The reduction is initially negligible up to a 10° yaw angle, but then increases with increasing the yaw angle. The convergent–divergent shroud experiences significantly less performance loss compared to the other two designs. The maximum performance loss of the convergent-divergent shroud design is 6% on average for the Reynolds numbers investigated. Based on the experimental results and in analogy to the linear momentum theory cosine cubed rule, we propose new cosine relations for shrouded turbines in yaw operations. Depending on the shroud's geometry, the maximum power of a shrouded turbine decreases either with the cosine or the cosine squared of the yaw angle.

Bi-Modal Failure Mechanism of Rolling Contact Bearings

The theory of failure of rolling contact bearings is based on fluctuating high level loading and material fatigue. This theory is unimodal, considering only the solid components of the bearing, and ignoring the liquid phase, which is the lubricant. Bearing life is rather dispersed, reaching a ratio of 20 between the extreme values. Since this theory was established, several exceptional phenomena were detected that could not be explained by it, such as: 1) Pitting damage beyond the contact path; 2) Detrimental effect of a minute quantity of water in the lubricant on bearing life. 25 ppm of water in the lubricant brought about shorter bearing life by over than 30%. The bimodal failure theory considers both solid and liquid bearing components. The damaging process of the lubricant evolves from its cavitation. During this process vapor filled cavities are formed in low pressure zones. When these cavities reach high pressure zones they implode exothermally. These implosions cause local high pressure pulses reaching 30,000 at accompanied by a temperature rise of about 2000 degrees K [1]. This paper includes cavitation erosion test results on stainless steel samples by vibratory and water tunnel test rigs. Various methods of lubricant dehydration are presented and evaluated. The main conclusion from this analysis is the use of water-free lubricants, for long life of RC bearings and more uniform service life thereof.

The origin of patch cavitation

The initial form of partial cavitation on axisymmetric bodies in some water tunnel tests is a regular system of patches with the approximately constant azimuthal distance between them. The described numerical simulation shows that the origin of such three-dimensional system is in the relatively small non-axisymmetric regular perturbations of the inflow caused by the shapes of non-circular water tunnel cross sections (these sections were squares or hexagons). Computational prediction of cavitation inception number for patch cavitation is carried out by a viscous-inviscid interaction method that combines three-dimensional analysis of inviscid flows with quasi-two-dimensional analysis of viscous flows. Computational results are compared with the experimental data obtained in 1980s in water tunnels of Applied Physics Laboratory and David Taylor Research Center. The satisfactory agreement of computed and measured data is considered as the proof of the described explanation of the origin of patch cavitation.