Pure Pitch Research Articles

Maneuverability characteristics of a fouling submarine near the seabed

Motion stability of a rough submarine within the vertical plane near the seabed is investigated in this study. The oblique towing tests, pure heave tests and pitch tests are numerically simulated based on a fully appended SUBOFF. Adaptive mesh refinement and overlapping mesh technologies are utilized. The numerical results are validated by comparing the simulated hydrodynamic coefficients with those provided by Defense Advanced Research Projects Agency. Effects of roughness height, roughness distribution, and seabed distance on hydrodynamic responses, hydrodynamic coefficients, and motion stability are analyzed. The findings reveal that both proximity to the seabed and hull roughness undermine the submarine motion stability. Specifically, as the rough submarine approaches the seabed, its motion stability decreases by 85.75%. Besides, the increasing roughness height results in a slower reduction in motion stability. Furthermore, the stern one-third hull fouling in infinite water depth leads to the most significant decrease in motion stability, with a reduction of 57.83% when roughness height is 489 μm. Conversely, the forward one-third and middle one-third hull fouling lead to reductions in motion stability by 17.04% and 10.00%, respectively. Thus, minimizing stern one-third hull fouling is identified as a beneficial measure for enhancing motion stability.

Study on hydrodynamic performance of multi-link cycloidal propeller

Cycloidal propellers are special propellers that can obtain a wide range of thrust in any direction without changing the propeller rotation speed or requiring the assistance of a rudder. They are typically installed on ships with high-mobility requirements. The implementation of pure cycloidal blade pitch control is complex, and there are a few control mechanisms capable of realizing these blade pitch motions. In this paper, a multi-link mechanism is designed to control the movement of the blades, and the relationships between blade pitch angle, linkage lengths, and their adjustment angles are established. The linkage lengths and their adjustment angles are solved using a genetic algorithm. The blade pitch angle controlled by the multi-link mechanism matches well with the pure cycloidal pitch angle after adjusting the linkage lengths and adjustment angles. The layout of linkages for the six-blade cycloidal propeller is designed to ensure that there is no motion interference between the linkages and their supporting shafts during operation. The open water hydrodynamic performance of the cycloidal propeller controlled by the multi-link mechanism and pure cycloidal pitch motion is analyzed using computational fluid dynamics. The simulation results indicate that the cycloidal propeller can achieve similar performance with different control mechanisms.

Efficient biodiesel production by sulfonated carbon catalyst derived from waste glycerine pitch via single-step carbonisation and sulfonation

Glycerine pitch is a highly alkaline residue from the oleochemical industry that contains glycerol and contaminants, such as water, soap, salt and ash. In this study, acidic heterogeneous glycerol-based carbon catalysts were synthesised for biodiesel production via single-step partial carbonisation and sulfonation using pure glycerol and glycerine pitch, producing products labelled as SGC and SGPC, respectively. Carbon materials were obtained by heating glycerol and concentrated sulfuric acid (1:3) at 200℃ for 1 h. The produced SGC and SGPC displayed high densities of sulfonic group (–SO3H), i.e. 1.49 and 1.00 mmol·g−1, respectively, alongside carboxylic (–COOH) and phenolic (–OH) acid. In the catalytic evaluation, excellent oleic acid conversions of 96.0 ± 0.4 % and 92.4 ± 0.5 % were achieved using SGC and SGPC, respectively, under optimised reaction conditions: 1:10 M ratio of oleic acid to methanol, 5 % (w/w) catalyst, 64℃ and 5 h. SGPC was found to be recyclable with 68.5 % conversion after the 6th cycle, which was attributed to the loss of –SO3H and catalyst deactivation by the deposition of oleic acid on its surface. Remarkably, despite the impurities present in the glycerine pitch, the obtained results demonstrated that the reactivity of SGPC is comparable to SGC and superior to that of commercial solid acid catalysts, which demonstrated that the presence of impurities appears to have minimal impact on the production of carbon materials and their properties.

Coexistence of passive vortex-induced vibrations and active pitch oscillation triggered by a square cylinder attached with a deformable splitter plate

To understand passive vortex-induced vibrations (VIV) coexisting with active structure motions, this paper numerically investigates the use of pure pitch oscillation to control a square cylinder mounted with a deformable splitter plate at the Reynolds number of 333. The oscillation is enforced with an amplitude of 3° and different frequencies from 0 to 6 Hz. Direct numerical simulations using a partitioned method with a semi-implicit coupling algorithm are performed. According to the trajectories of the splitter-plate tip displacement with respect to the lift or drag force coefficient, a specific lock-in regime determined by the frequency of the enforced pitch oscillation is identified. Further spectral analyses of the tip displacement and lift force show that the lock-in frequencies are equal to the enforced frequencies. Next to the lock-in regime, semi-lock-in regimes with narrow bandwidths are distinguished, exhibiting both lock-in and non-lock-in features. In the non-lock-in regimes, the frequencies of the most predominant peaks in the spectra are found near the natural frequency of the splitter plate of 3.236 Hz, and the frequencies of the two secondary peaks are distributed along the characteristic lines following the ratios of these frequencies to the enforced frequency, which are ±1. Thus, the interaction is dependent on the combined effects of the passive VIV and the actively enforced pitch oscillations. Moreover, the intersection points of the characteristic lines are located close to the upper and lower frequency limits of the lock-in regime, inferring the conditions for the lock-in onset.

The Effect of Alpha-Lipoic Acid in the Treatment of Chronic Subjective Tinnitus through the Tinnitus Handicap Inventory Scores.

Tinnitus affects millions of adults. Many therapies, including complementary and alternative medicine and tinnitus retraining therapies, have been trialed, but an effective option, particularly for chronic subjective tinnitus (CTS), is still lacking. This study investigated the effects of alpha-lipoic acid (600 mg. per day for two months) on two groups of patients using a questionnaire. One group (A) was affected by tinnitus associated with likely cochlear dysfunction and metabolic syndrome, and the other (B) was composed of subjects with acoustic nerve lesions. All the patients were asked to complete the Italian version of the tinnitus handicap inventory (THI) to determine the overall degree of perceived annoyance at the beginning and end of therapy. Pure tone averages for speech frequencies and for high frequencies were computed, and psychoacoustic pitch and loudness matches were determined for each subject before and after treatment. The pure tone audiometry, pitch, loudness, and THI scores of both groups were reported. In group A, statistically significant differences were observed for the "functional" and "emotional" subscales. The total score of THI and the loudness of tinnitus were also significantly reduced. No statistically significant differences were observed in group B. These findings suggest a possible contribution of the antioxidant effect to the organ of Corti in subjects with metabolic syndrome and CST.

Loads and hull-pressure measurements on a generic submarine in different stages of model build-up

This is an experimental study of loads and surface pressure on a generic BB2 submarine in different stages of model build-up. The build-up is from a bare hull to a fully-appended hull consisting of a casing, 4 aft control surfaces in an X-form configuration, a vertical sail fin and 2 horizontal sailplanes. The test is carried out in the Defence Science and Technology Group low-speed wind tunnel on a 2 m long hull at a freestream velocity of 60 m/s, where the hull-length Reynolds number is 8 million. The load trends are established for incidence angles up to 12∘, where the measured forces are up to 89 N for pure pitch and 163 N for pure yaw. The drag force is up to 14 N. Overall, the load trends are well approximated by high-order polynomials. From constructed pressure-gradient maps of the hull, the effect of model build-up can be observed in the changing pressure footprint and interpreted by using flow topology.

Prediction of the Pitch and Heave Motions in Regular Waves of the DTMB 5415 Ship Using CFD and MMG

According to the requirement of high-performance development of modern ships, it is necessary to quickly and accurately predict the maneuverability of ships under wave conditions. In this paper, based on the CFD (Computational Fluid Dynamics) method in the commercial software STAR-CCM+, the numerical simulation of roll decay motion, pure heel motion, pure heave motion, and pure pitch motion of ship model 5415 is carried out. The relevant hydrodynamic derivatives are obtained, and the results are in good agreement with the experimental values. The equations of motion related to the heave and pitch motions are established according to the MMG (Maneuvering Motion Equation Research Group) model. Then, based on the above dynamic equations, the wave force module is added to successfully simulate and predict the pitch and heave responses of the ship under regular wave conditions, and it is concluded that the simulation model for rapid prediction is also applicable under waves.

Flutter of a finite rectangular Wing’D mill in pitch and heave

Exact solutions of flutter are explored for a mode suitable for an oscillating 2D hydrofoil version of the summarised FlutterWing windpump which radically surpasses the old rotary fanpump. The imaginary part G of the 1D Theodorsen wake lift factor T rises singularly from the steady to allow 1D pure pitch flutter around the leading edge at low frequency. But its logarithmic infinite negative pitch damping by the shed vorticity is capped by the log of the 2D span. The downwash of an oscillating trailing (tip) vortex is reduced by a function of the reduced frequency based on spanwise distance. Its lag damps pitch and further raises the pitch unstable aspect ratio A to above 36. All binary pitch & heave neutral flutter frequency contours in (inertia, imbalance) space still pass through a nexus of the same total pitch inertia and imbalance as the corrected virtual mass concentrated at ¾ chord, a very high total in water. The internal bound windwise vortex from ¼ chord to trailing edge reduces as A−2 the 1D lift slope at all frequencies. It expands the 1D T = ½ double root of high frequency elliptical contours kissing the low frequency limit line at the nexus to make them dip below the line to the right of the nexus as hyperbolae with the nexal ray as left asymptote. Thus low A discovers a new wedge of high frequency binary flutter with real mass moments less than the virtual, as in water.

The estimation of hydrodynamic coefficients of an autonomous underwater vehicle by comparing a dynamic mesh model with a horizontal planar motion mechanism experiment

This study aims to establish a dynamic mesh model based on the horizontal planar motion mechanism (HPMM) test and provide a comprehensive database of hydrodynamic coefficients for the autonomous underwater vehicle (AUV) numerically and experimentally. A series of HPMM tests in National Cheng Kung University's towing tank validated the simulation results. The dynamic mesh model can be performed on oscillation motions of the yaw mode, the pitch mode, and the roll mode by adding a series of comprehensive user-defined functions (UDFs). The test of straight-line motion with constant drift angles, the pure surge test, the pure yaw test, the pure pitch test, the pure yaw test with a constant drift angle, and the pure pitch test with a constant drift angle are all available for the yaw and pitch modes. In addition, the pivot roll test is included in the roll mode. After measuring forces and moments from an individual HPMM test, the hydrodynamic coefficients can be derived by means of Fourier analysis and the least-square method. Since most numerical results are reasonably in good agreement with the experimental data, applying the proposed dynamic mesh model to the simulation of the HPMM tests in the towing tank can be demonstrated.

Nonlinear Stall Flutter Suppression of Wind Turbine Blade Based on LMI Method

Aiming at the failure of stall flutter in aeroelastic system of wind turbine blade, the active control process of stall flutter based on linear matrix inequality (LMI) design is described. The structural model is a typical blade section model based on spring-mass-damper, and the aerodynamic force is the ONERA stall aerodynamic model suitable for pure pitch motion. Based on the state variables, the nonlinear aeroelastic equations are expanded by Taylor series and linearized by low order approximation. The state feedback gain is calculated through LMI, and the time domain response stability analysis and stall flutter suppression method based on linearization are studied. The simulation results show that the maximum amplitude is greatly reduced after flutter control, and the system can be stabilized in a short time.

Flutter Suppression of Wind Turbine Blade based on Adaptive Fuzzy Sliding Mode Control

Aiming at the divergent unstable vibration of wind turbine blades, an adaptive fuzzy sliding mode control algorithm based on sliding mode control is proposed to solve the classic flutter fracture failure problem of blades. The structural model is a typical blade section model based on spring-mass-damper, and the aeroelastic equation is obtained by combining the steady aerodynamic model suitable for pure pitch motion. The adaptive fuzzy sliding mode controller gives the pitch signal, and the second-order pitch actuator performs the pitch action to suppress flutter. The saturation function with variable boundary layer thickness is used instead of symbolic function, and the approach rate parameter is adjusted appropriately by fuzzy rules, so as to realize variable-parameter adaptive fuzzy sliding mode control. Simulation results show that, compared with the traditional sliding mode controller, the proposed control algorithm can not only suppress the classical flutter, but also effectively reduce the amplitude and frequency of chattering, and has good robustness.

RESPONSE OF A HIGH-SPEED WAVE-PIERCING CATAMARAN TO AN ACTIVE RIDE CONTROL SYSTEM

Ride control systems on high-seed vessels are an important design features for improving passenger comfort and reducing motion sickness and dynamic structural loads. To investigate the performance of ride control systems a 2.5m catamaran model based on the 112m INCAT catamaran was tested with an active centre bow mounted T-Foil and two active stern mounted trim tabs. The model was set-up for towing tank tests in calm water to measure the motions response to ride control step inputs. Heave and pitch response were measured when the model was excited by deflections of the T-Foil and the stern tab separately. Appropriate combinations of the control surface deflections were then determined to produce pure heave and pure pitch response. This forms the basis for setting the gains of the ride control system to implement different control algorithms in terms of the heave and pitch motions in encountered waves. A two degree of freedom rigid body analysis was undertaken to theoretically evaluate the experimental results and showed close agreement with the tank test responses. This work gives an insight into the motions control response and forms the basis for future investigations of optimal control algorithms.

Understanding the control characteristics of electric vertical take-off and landing (eVTOL) aircraft for urban air mobility

The aim of the present paper is to present an understanding of the control characteristics for representative eVTOL (Electric Vertical Takeoff and Landing) quadcopters having the same properties of interest as the eVTOL configurations developed presently in Urban Air Mobility. In this context, the paper concentrates on eVTOL quadcopters with pure rpm control, pure propeller pitch control and both rpm and propeller pitch control. Dynamic effects of electric motors can potentially have significant effects on the flight characteristics of these vehicles. The paper will demonstrate for a pitch manoeuvre that while varying purely the rpm of the rotors results in a system that behaves as an acceleration-control system, varying purely the propeller pitch corresponds more to a velocity-control system. The rotor pitch control is more sensitive to the coupling rotor-motor and its transients dynamics. Varying both the rpm and the rotor pitch for controlling the pitch manoeuvre is the best option as this brings the system more towards velocity-control behaviour and dampens the transients due to the motor dynamics.

Experimental study on the maneuvering derivatives of a half-scale SUBOFF model in the vertical plane

The maneuverability of a submersible in the vertical plane is investigated. For this purpose, extensive experiments are performed on a half-scale SUBOFF model in the circulating water channel (CWC) with the planar motion mechanism (PMM). Oblique towing tests containing high attack angles, pure heave tests and pure pitch tests are involved. The validity of the experiments is identified by the accordance of the present hydrodynamic coefficients and the ones of Defense Advanced Research Projects Agency (DARPA) . In order to fit the measured data for cases with high attack angles, this paper develops the expression of dimensionless pitch moment M′ by the piecewise function. Besides, the blockage effects are corrected by Tamura equation. In addition, the uncertainty analysis (UA) is conducted referring to the recommended procedure 7.5-02-06-04 by 27th ITTC (2014b). The results show that, the developed equation turns out to be superior to the standard one. And the total uncertainties of the hydrodynamics are less than 2%, which reveals the accuracy and credibility of the experimental measurement.

Numerical Analysis of the Resistance Force of Floater of Wave Glider and Flapping Motion of Single Wave Glider NACA-0012 Foil

In 2020, the Agency for the Assessment and Application of Technology – Indonesia (BPPT) is designing wave glider systems for Tsunami Early Warning System. The wave glider systems consist of two components, the floater at the water surface and the wave glider submerged 2 meters or more below the water surface. Wave glider contains several foils that can flap up and down due to the heave and pitch motion of the floater so that the system affords the force to move forward. This paper describes the numerical analysis of the resistance of floater and the selection of the pivot position of the foil where it flaps. The speed of the floater is set from 0.3 m/s up to 1 m/s. NACA-0012 is selected as the first choice of the foil. It flaps in pure pitch motion and coupled pitch and heave motion in the different pivot positions. The flapping frequency is set to 0.1 Hz since the peak period of the wave about 10 seconds. The surge forces due to the flapping motion are then analysed to determine the best position of the pivot. The numerical simulation shows that the pivot position at 2.5% - 5% of chord length from the leading edge of NACA-0012 generates the optimum surge force.

Lift Equivalence and Cancellation for Airfoil Surge–Pitch–Plunge Oscillations

A NACA 0018 airfoil in freestream velocity is oscillated in longitudinal, transverse, and angle-of-attack directions with respect to the freestream velocity, known as surge, plunge, and pitch. The lift-based equivalence method introduces phase shifts between these three motions to construct in-phase sinusoidal components for maximum lift, waveform construction. Lift cancellation is also determined with the exact negative pitch and plunge motion amplitudes found from the equivalence method to achieve out-of-phase wave destruction. Lift cancellation occurs when a combination of these motions is sought to obtain a constant lift magnitude throughout the oscillation cycle. To achieve both equivalence and cancellation of lift, a prescribed pure pitch amplitude through the Theodorsen theory equates the corresponding equivalent plunge amplitude and pitch–plunge phase shift. These Theodorsen, linear superposition findings of pitch–plunge are leveraged toward the Greenberg theory to determine a closed-form, surge–pitch–plunge solution through the addition of a surge–plunge phase shift and optimal surge amplitude for lift cancellation. The lift cancellation surge–pitch–plunge amplitudes define the equivalence amplitude investigated here and theoretically limit the experiment to combinations of the first lift harmonic of the Greenberg theory. The analytical results are then compared with experimental lift force measurements and dye visualization. The normalized lift differences due to unsteady wake and boundary-layer behavior are examined to explore the extents of the Greenberg theory for these cases of lift-based equivalence and cancellation.

Effects of a propulsor on the maneuverability of an autonomous underwater vehicle in vertical planar motion mechanism tests

The effects of a propulsor on the hydrodynamic forces acting on an autonomous underwater vehicle (AUV) were investigated by towing tank model tests. A mathematical maneuvering model using a whole vehicle model was established, and captive model tests using vertical planar motion mechanism equipment were performed to obtain maneuvering coefficients. The Reynolds number of the full scale test model in the operation condition was 4.12 × 106, and the revolution rate of the propulsor was fixed at the self-propulsion point. The forces on the body and control fins were measured in static drift, control fin deflection, pure heave and pure pitch tests with and without the propulsor. The propulsor effects on the hydrodynamic force and moment were identified as spatially irregular thrust, flow acceleration around the control fins, and added mass of the propulsor. In the simulation using the component model where the propulsor effect considered as a constant thrust, the maneuverability of the AUV was overestimated compared to that of the whole vehicle model. Thus, the mathematical maneuvering model should consider propulsor-hull interactions.

Characteristics of the mesophase and needle coke derived from the blended coal tar and biomass tar pitch

In this work, coal tar pitch was co-carbonized with biomass tar pitch to manufacture hybrid mesophase pitch as precursor for the preparation of needle coke. The properties of mesophase pitches were characterized using a polarized light optical microscope, apparent viscometer, FT-IR, and 1H NMR. After adding biomass tar pitch into coal tar pitch, the mesophase pitches had more flow domain-like texture due to its lower viscosity and softening points. Besides, the addition of biomass tar pitch altered the chemical structure and functional groups of the resulting mesophase pitch, accompanied by the growth of alkyl and naphthenic compounds. Accordingly, the needle coke prepared from the hybrid mesophase pitches showed more flow domain textures and were better aligned uniaxially than the counterpart derived from pure coal tar pitch. The coefficient of thermal expansion of needle coke was reduced from 0.93 × 10−6 °C-1 to 0.42 × 10−6 °C-1. The low viscosity of the mesophase pitches caused by the addition of biomass tar pitch favored the development of mesophase and highly uniaxial arrangement. The increase in the alkyl and naphthenic group compounds greatly improved the performance of the needle coke.

A numerical study on hydrodynamic maneuvering derivatives for heave-pitch coupling motion of a ray-type underwater glider

We used a numerical method to estimate the hydrodynamic maneuvering derivatives for the heave-pitch coupling motion of an underwater glider. It is very important to assess the hydrodynamic maneuvering characteristics of a specific hull form of an underwater glider in the initial design stages. Although model tests are the best way to obtain the derivatives, numerical methods such as the Reynolds-averaged Navier-Stokes (RANS) method are used to save time and cost. The RANS method is widely used to estimate the maneuvering performance of surface-piercing marine vehicles, such as tankers and container ships. However, it is rarely applied to evaluate the maneuvering performance of underwater vehicles such as gliders. This paper presents numerical studies for typical experiments such as static drift and Planar Motion Mechanism (PMM) to estimate the hydrodynamic maneuvering derivatives for a Ray-type Underwater Glider (RUG). A validation study was first performed on a manta-type Unmanned Undersea Vehicle (UUV), and the Computational Fluid Dynamics (CFD) results were compared with a model test that was conducted at the Circular Water Channel (CWC) in Korea Maritime and Ocean University. Two different RANS solvers were used (Star-CCM+ and OpenFOAM), and the results were compared. The RUG’s derivatives with both static drift and dynamic PMM (pure heave and pure pitch) are presented.

The structural properties of chemically derived graphene nanosheets/mesophase pitch-based composite carbon fibers with high conductivities

A series of mesophase pitch-based composite carbon fibers with various contents of chemically derived graphene nanosheets (CGNSs) were prepared through melt-spinning, stabilization, carbonization, and graphitization. By means of scanning electron microscopy, X-ray diffraction, single-filament testing, and resistivity testing, the effect of CGNSs content on the structures and properties of the composite carbon fibers were investigated. And the thermal conductivity of the composite carbon fibers was predicted by indirect measurement. The results show that the presence of CGNSs can effectively avoid the generation of unwanted radial transverse texture and wedge cleavages of the carbon fibers prepared from pure mesophase pitch. Although the crystalline structure of CGNSs/mesophase pitch-based composite carbon fibers is not apparently improved, appropriate CGNSs content can enhance the tensile strength as well as the conductivity of the composite carbon fibers. When the CGNSs content is 0.1 wt%, the tensile strength, breaking elongation, electrical resistivity and thermal conductivity of the composite carbon fibers can attain 2.12 GPa, 0.35%, 0.95 μΩ m, and 1322 W m−1 K−1, respectively. And the successful preparation of the composite carbon fibers provides a new solution for the cost reduction of mesophase pitch-based carbon fibers with high thermal conductivity.