Heave Pitch Research Articles

Experimental Small-Amplitude Self-Sustained Pitch–Heave Oscillations at Transitional Reynolds Numbers

An experimental investigation of a free-mounted rigid NACA0012 airfoil in pitch–heave is conducted at transitional Reynolds numbers. Previously, small-amplitude self-sustained oscillations in pure pitch were observed for a specific range of Reynolds numbers: . They were attributed to laminar boundary-layer separation, which leads to negative aerodynamic damping at a 0 deg angle of attack. In this phase of the research, the investigation is extended to 2 deg of freedom by enabling the heave motion. Depending on initial conditions, two competing stable limit-cycle oscillations, small and large, are observed. It is determined that the origin of the large-amplitude limit-cycle oscillations is coalescence flutter for which the exponentially growing amplitude is limited by flow separation at large angles of attack. On the other hand, the small-amplitude oscillations are the same laminar separation flutter type as the pitch-only case. For the most part, the heave motion plays no fundamental role in the dynamics, as the amplitude of the pitch oscillations is not drastically affected in comparison with the single-degree-of-freedom pitch-only case nor is the range of Reynolds numbers at which the limit-cycle oscillations are observed. Nonetheless, the heave motion contributes to the net transfer of energy from the flow to the structure, as the mean kinetic energy present in the oscillations is increased significantly compared to the pitch-only case.

Simulations of Pitch–Heave Limit-Cycle Oscillations at a Transitional Reynolds Number

The present study investigates the nonlinear feedback coupling mechanism between unsteady low-Reynolds-number aerodynamics and structural response. Structural-response models addressing one-degree-of-freedom and two-degrees-of-freedom aeroelastic oscillations were coupled with an in-house-developed computational fluid dynamics code to perform large-eddy simulations for flows past a rigid airfoil in free-to-rotate and free-to-rotate-and-heave conditions at a low Reynolds number. As observed in the experiments, the numerical simulations confirmed the presence of self-sustained low-amplitude limit-cycle oscillations of a NACA 0012 airfoil. It was understood that this behavior in the transitional Reynolds-number regime resulted from the unsteadiness of the laminar boundary-layer separation and its delayed recovery when compared to the corresponding static conditions. The feedback coupling mechanism between the laminar-separation-bubble behavior and the structural response caused negative aerodynamic damping; thus, the aerodynamic forces did positive work and fed energy from the flow to the airfoil, sustaining the low-amplitude aeroelastic oscillations. The amplitude in pitch of the limit-cycle oscillations was of the order of 5–7 deg. As the operating angles of attack in unmanned-air-vehicle flights are within close range of the limit-cycle-oscillation occurrence, understanding and controlling the limit-cycle oscillation of the wings could ensure unmanned-air-vehicle stability so that it could acquire high-quality camera images and other data, for instance.

Numerical study on global motion of Truss Spar in frequency and time domains for the Liwan 3-1 area

Using frequency and time domain analysis, the authors analyzed the hydrodynamics and motion behavior of a Truss Spar platform at a water depth of 1500 m in the Liwan 3-1 area of the South China Sea. Firstly, the seakeeping ability is acquired in the frequency domain by calculating the hull’s hydrodynamics and comparing with a semi-submersible platform. The random wave analysis for 100-year, 10-year and 1-year return periods in Liwan 3-1 distinctly shows lower heave but larger surge and pitch responses of the Truss Spar than those of a semi-submersible. Secondly, 3-hour motions of the Truss Spar are predicted and compared in the time domain under 100-year return period conditions in Liwan 3-1 and the Gulf of Mexico. Thirdly, the hull/mooring line coupled and uncoupled models are compared. Finally, the responses of the Truss Spar under 10-year and 1-year return period conditions are assessed. The results reveal that the mooring line damping reflected by the coupled model distinctly decreases the low frequency motion. The maximum heave response for 100-year return period waves is 1.23m and below 0.1m for the case of 1-year return period.

Active Damper Control System Based on LMFC

All electric-damper is now a fashionable method for vehicle balance as the development of the high power linear and motor drive system allow the fast and efficient solution to dampers. The conventional damper is then replaced by active motor drive that provides faster response than the conventional system. In this paper, an LMFC method is proposed to provide a better control method for the four dampers that is located in the vehicle. The 4-coordinate control is difficult as they are closely related to each other. The heave position, pitch and roll angle are the main concerns to keep consistent for the passenger’s comfort and ride performance. To achieve this goal, an effective LMCF is investigated and applied to the active damper system, with its simplicity and reliable for implementation. For the active damper system, the inner loop is with PD control to eliminate the disturbances and nonlinearities, while the outer loop is with linear model following control to track the required dynamic performance. The computed results verify the proposed control methods, and the dynamic response of the active damper systems is studied by optimization

System identification of coupled heave–pitch motion of ships with forward speed in random ocean waves

Ship dynamics in ocean waves involve frequency-dependent added mass and radiation damping which can be estimated either by potential theory-based calculations or by experiments. With uniform forward speed, the added mass and damping matrices become asymmetric. In this paper, we attempt to use a system identification approach, specifically the reverse multiple input–single output (R-MISO) method, for coupled heave–pitch response (two degrees of freedom) of a ship moving with uniform forward speed in random ocean waves. The system mass matrix has both frequency-independent and frequency-dependent components, whereas its damping matrix has only frequency-dependent components. The frequency-dependent components of the mass and damping matrices are asymmetric. The excitation force and moment due to linear monochromatic waves which act on the system are assumed to be known and can be either calculated or obtained from experiments. For numerical illustration, a ship whose hydrodynamic behaviour has been computed by strip theory has been considered. The motion, as well as the loading, is simulated assuming Pierson–Moskowitz spectrum in conjunction with response amplitude operators from the strip theory code, and these results are analysed by the R-MISO method yielding frequency-dependent asymmetric coupled added mass and radiation damping coefficients satisfactorily.

Use of Pitch and Heave Motion Cues in a Pitch Control Task

During pitch rotation of the aircraft, a pilot, seated in front of the aircraft center of gravity, is subjected to rotational pitch and vertical heave motion. The heave motion is a combination of the vertical motion of the aircraft center of gravity and the heave motion as a result of the pitch rotation. In a pitch tracking task, all of these cues could potentially have a positive effect on performance and control behavior, as they are all related to the aircraft pitch attitude. To improve the tuning of flight simulator motion filters, a better understanding of how these motion components are used by the pilot is required. First, the optimal use of the different motion components was evaluated using an optimal control analysis. Next, an aircraft pitch attitude control experiment was performed in the SIMONA Research Simulator, investigating the effects of pitch rotation, pitch heave, and center of gravity heave on pilot control behavior. Pilot performance significantly improved with pitch motion, with an increased crossover frequency for the disturbance open loop. The increase in performance was a result of an increased visual gain and a reduction in visual lead, allowed for by the addition of pitch motion. Pitch heave motion showed similar but smaller effects. The center of gravity heave motion, although taking up most of the simulator motion space, was found to have no significant effects on performance and control behavior.

Evolution of the Bimodal Directional Distribution of Ocean Waves*

Abstract Recent results of numerical wave models have shown that the presence of a bimodal directional spreading is a robust feature at wavenumbers above the spectral peak. This directional bimodality is controlled mainly by directional transfer of energy through nonlinear wave–wave interactions. The bimodal feature has also been observed in the directional spectra derived from the spatial topography of ocean surface waves acquired by stereo-photography, image radars, and an airborne scanning lidar system. In this study, a comprehensive data analysis of the evolution of the wave directional distribution during two active wave growth periods in Lake Michigan is conducted. The wind and wave measurements are acquired by two heave–pitch–roll buoys moored at a nearshore and an offshore station. An empirical method averaging the results of the maximum likelihood method and maximum entropy method is used to estimate the directional distribution from buoy measurements. The study shows that the bimodal distributio...

On the identification of ship coupled heave–pitch motions using neural networks

This paper outlines a procedure for the derivation of the differential equations describing the free response of a heaving and pitching ship from its stationary response to random waves. The coupled heave–pitch motion of a ship in random seas is modelled as a multi-dimensional Markov process. The partial differential equation describing the transition probability density function, known as the Fokker-Planck equation, for this process is derived. The Fokker-Planck equation is used to derive the random decrement equations for the coupled heave–pitch motion. The parameters in these equations are then identified using a neural network approach. The method is validated using numerical simulations and experimental results. The experimental data was obtained using an icebreaker ship model heaving and pitching in random waves. It is shown that the method produces good results when the system is lightly damped. An extension for using this method to identify couple heave–pitch motion in realistic seas is suggested.

Observations of the Directional Spectrum of Fetch-Limited Waves

Abstract Estimates of the wave directional spectrum of fetch-limited sea states are made from measurements made with a heave–pitch–roll buoy at the Maui location off the west coast of New Zealand. The fetch-limited sea states have significant wave heights between 0.5 and 4.5 m and are observed during persistent southeast wind events, which have a well-defined fetch of 200 km. The integrated properties of the estimated angular spreading distributions are in general agreement with those observed in previous studies. However, the angular distributions estimated for the Maui location are bimodal at frequencies greater than the spectral peak frequency. This result for deep water ocean waves is in contrast to the generally accepted unimodal angular distribution for wind seas, but it supports recently reported measurements of the angular distribution of fetch-limited waves in Lake George, Australia. Parametric relationships that describe the characteristics of the bimodal distributions are derived, and the impor...

General Analysis of Directional Ocean Wave Data from Heave Pitch Roll Buoys

General Analysis of Directional Ocean Wave Data from Heave Pitch Roll Buoys

General Analysis of Directional Ocean Wave Data from Heave Pitch Roll Buoys

General Analysis of Directional Ocean Wave Data from Heave Pitch Roll Buoys