Harmonic Motion Research Articles

Mitigation of laminar separation flutter using active oscillation of local surface

This work explores the mitigation effect of laminar separation flutter (LSF) using active oscillation of a local surface at Re = 77 000. Aerodynamic forces and flow field of a National Advisory Committee for Aeronautics (NACA) 0012 airfoil are obtained by numerical simulations using the γ−Reθ transition model. The local oscillation is considered as a harmonic motion of a single mode varying both in temporal and spatial domains. The control effects of oscillation positions, amplitudes, and frequencies on the pitch instability are investigated. The work done by aerodynamic moment per motion cycle when the airfoil undergoes prescribed harmonic pitch motion is defined to evaluate the control performance. The mitigation effect of LSF is demonstrated by a fluid–structure interaction (FSI) method. The results show that the oscillation region should be placed on 0.2c for small oscillation amplitude while the optimal oscillation position is 0.6c for large oscillation amplitude. The flow control mechanism is analyzed in terms of the flow structure. A series of vortices are formed and moved downstream under the effect of active oscillation of the local surface. These vortices suppress the formation of a large-scale laminar separation phenomenon, which improves the pitch stability. The frequency lock-in phenomenon occurs in a certain oscillation frequency range, and it can improve the control performance on the pitch instability. The results of FSI show that the active oscillation of the local surface can completely eliminate the LSF.

An Investigation of Difficult Topics Encountered by Students in Interaction of Matter, Space and Time in Senior Secondary School Physics Curriculum

Despite the description of curriculum on what, why and how well students should learn in a systematic way. Physics curriculum has not been said to have actualize such description when compared with students’ performance in physics. The performance of students in physics has been on the decline over the years. Decline in Physics students’ performance as highlighted by the chief WAEC examiners report should not be a thing of neglect by both teacher and researchers in science education. Thus, this paper investigated difficult topics encountered by students in interaction of matter, space and time in senior secondary school Physics. Two research questions and two null hypotheses guided the study. Descriptive survey research design was employed for the study. 21,974 senior secondary school one to three (SSS1-3) physics students in 292 senior secondary schools in Anambra state made up the population of the study. The sample size of the study comprised of 393 drawn from the population (that is; 290- urban and 103- rural; 238- male and 155- female). The sample size was drawn using Yaro Yamane formular. Multi stage sampling procedure were used to compose the sample size. Researchers developed instrument titled, ‘Interaction of Matter, Space and Time Difficulty Identification Questionnaire (IMSTDIQ)’ was used for data collection. The instrument was subjected to face validity by experts from research measurement and evaluation unit, physics education unit, physics and astronomy unit. Cronbach Alpha formula (α) was used to obtain reliability index for IMSTDIQ at 0.79. The reliability index obtained indicated that the instrument is reliable. Research questions 1 & 2 were answered using mean and standard deviation. Independent sampled t-test was used to test hypotheses 1 & 2. All the hypotheses were tested at 0.05 level of significance. Findings revealed that speed and velocity, rectilinear acceleration, equation of uniformly accelerated motion, and simple harmonic motion are the specific difficult topics encountered in interaction of matter, space and time in senior secondary school physics curriculum based on gender and school location. Keywords: Physics, Interaction of Matter, Space and Time DOI: 10.7176/JEP/13-18-12 Publication date: June 30 th 2022

Studi Eksperimen Bandul dalam Menentukan Percepatan Gravitasi Bumi dan Memahami Konsep Gerak Harmonik Sederhana

This research is a pendulum experimental study in determining the acceleration of Earth's gravity and understanding the concept of simple harmonic motion. This research was conducted at the Physics Laboratory of the University of Muhammadiyah Lamongan. This study aims to examine the benefits of conducting pendulum experiments with the simple pendulum swing method and in determining the acceleration of Earth's gravity, the period of oscillation and concepts in simple harmonic motion with variations in the mass of the pendulum, namely 150 grams, 75 grams and 35 grams. The results of the analysis of this study obtained the acceleration of Earth's gravity 9.79 m/s where the mass and length of the pendulum do not affect the value of gravity. While in simple harmonic motion in the form of pendulum oscillation is influenced by the length of the string. The pendulum oscillation will experience energy dissipation in maintaining its equilibrium position due to the gravity of the earth.

Investigation of the floating IEA wind 15‐MW RWT using vortex methods Part II: Wake impact on downstream turbines under turbulent inflow

AbstractThe manuscript presents a novel numerical investigation on the impact of the wake of a floating IEA wind 15‐MW reference wind turbine (RWT) on downstream machines using a state‐of‐the‐art vortex solver coupled to a multi‐body code. The coupling enables to account for the flexibility of the different turbine components as well as to include the effect of the controller and the dynamics of the floating support structure. First, the turbine is mounted on the WindCrete spar‐buoy platform, and the wake impact on a second turbine positioned at different downstream positions is investigated and compared with the impact of the wake generated by a bottom‐fixed machine. It is found that the faster breakdown of the vortex structures triggered by the motion of the floater in the upstream turbine increases the power production of a downstream machine, as well as its mean thrust level relative to normal operation downstream a bottom‐fixed machine. It is demonstrated that this effect is drastically reduced with the increase of the turbulence intensity (TI). Further, simulations with a prescribed harmonic motion of the upstream turbine in surge and pitch under different turbulence levels are presented. It is found that the motion of the floater has a strong impact in the generated wake and consequently in the operation of downstream machines. In particular, downstream turbines experienced considerably higher blade loading that led to an increase of the aerodynamic power. Finally, aero‐hydro‐servo‐elastic simulations of five turbines in a row have shown that the interaction between multiple floating machines is more dynamic than between bottom‐fixed turbines. This has been mainly observed at high wind speeds, where the pitch and surge motions of the floater in turbines located deep inside the farm can be resonantly excited by the interaction with the wind farm flow. In practice, this means that the power and thrust variations increase with the turbine location depth inside the farm. Overall, the study highlights the importance of an accurate flow and wake modeling for the prediction of turbine‐to‐turbine interaction in a floating context.

Young’s modulus determination for a vibrated metal ruler using Arduino

The article introduces a low-cost Arduino sensor into the Young’s modulus determination laboratory for physics university students. A stainless steel ruler is used as a cantilever beam. Its free end attached a mass is slightly bent and released to make it oscillate as a simple harmonic motion. The Arduino sensor detects the moving mass’s frequency at different loaded forces. This technique shows an acceptable value of Young’s modulus for the used ruler.

An engineering modification to the blade element momentum method for floating wind turbines

The design of next-generation offshore wind rotors for floating applications carries large uncertainties that make it harder to bring costs down. A significant share of this uncertainty is associated with the use of Blade Element Momentum (BEM) methods to run aeroelastic design and loadcase calculations, because the assumptions underlying the momentum theory are violated by the floater motion. This work presents an engineering method for BEM that allows to model generic floater motions and improves the aerodynamic modelling of floating wind turbines. The new model overcomes the challenge of estimating the apparent wind caused by the platform motion in a simple way, which proves to be very effective for rigid aerodynamic simulations. To verify its impact, BEM results with and without the proposed correction are compared to high-fidelity free vortex wake simulations, imposing the same harmonic motion first to the pitch and then to the yaw degree of freedom of the platform. While little differences are found for the yaw motion, the new floating model clearly improves predictions for the platform pitch case in terms of both aerodynamic loads and induction, finding a very good match with the high-fidelity results. An analysis of the verification data provides new valuable insights on the effect of these motions on aerodynamic loads and performance of a floating wind turbine.

Wind tunnel investigation of the wake-flow response for a floating turbine subjected to surge motion

Floating wind turbines undergo large translation motions allowed by the high compliancy of the support structure. Large motions affect the rotor aerodynamic response and wake. This paper examines wake measurements of a wind tunnel experiment with a scale model of the DTU 10MW subjected to imposed harmonic surge motion. The effect of surge motion on the wake is detected by means of spectral analysis of velocity data. It is found that when the rotor undergoes surge motion, periodic forcing is introduced into the fluid, which produces a harmonic variation of the wake speed. The velocity perturbation around the rotor is then convected downstream at about the average axial velocity in the wake. The amplitude of the velocity pulsation at 2.3D is radially-dependent, and is correlated to the amplitude of the variation of the normal-force distribution along the blade.

Pulsation curves strengthen the high gradient magnetic separation process: Experimental study and simulation explanation

The pulsating slurry high gradient magnetic separator (PHGMS) has been widely used for the separation of feebly magnetic materials. As the main competing force, pulsating slurry effectively overcomes matrix blockage and affects the buildup behavior of particles. At present, the used pulse shape in PHGMS constitutes a sinusoidal harmonic vertical motion in which the duration and intensity of the impulsion and suction strokes are similar. However, other pulsation profiles, such as, trapezoidal shape and saw-tooth shape pulses, can also be obtained depending on the stratification mechanism. There is little literature to focus on how pulse shape influences the performance of PHGMS. The change of curve could be a method worth exploring to improve the separation. In this paper, the asymmetry pulsating curves are obtained by switching the velocity of the step motor during one cycle. Then, the single-wire experiments, single-wire 2D numerical simulation and multi-wires experiments are presented to indicate the effect of different pulse shapes. In single-wire experiments, compared with the traditional sinusoidal harmonic curve, the asymmetry pulsating curves have a higher grade, finer particle buildup, but lower accumulation volume. The curves of the rapid downward stroke and slow upward stroke are better than the inverse movement curve in asymmetry pulsation. The results of separating the up and downstream accumulations on the magnetized wire and numerical simulation reveal that these effects can be attributed to the direction of net force and vortex strengthening. The trajectory simulation of multi-component obtains a similar result with single-wire experiments to confirm the superiority of rapid downward stroke and slow upward stroke in selectivity. However, in multi-wires experiments, due to the matrix blockage the curves of rapid upward stroke and slow upward stroke show the greater potential advantages in both grade and weight compared with other curves at condition of high feed velocity.

Development of A Problem-Based Physics E-Module with A Flipped Classroom Approach Using Sigil Software as An Alternative Learning Media During the COVID-19 Pandemic

This study aims to develop a problem-based physics e-module with a flipped-classroom approach using sigil software that is fit for use as an alternative media of learning physics simple harmonic motion material for students of class XI MIPA SMA Muhammadiyah 2 Bandar Lampung during the COVID-19 pandemic. The type of research used is research and development, with an ADDIE development model consisting of analysis, design, development, implementation, and evaluation. The data collection instruments used were interview guide sheets, observation guide sheets, and questionnaire sheets. The research data obtained were then analyzed descriptively qualitatively. Based on the results of the feasibility assessment and data analysis carried out, it was concluded that the problem-based e-module using Sigil software with a flipped-classroom approach was feasible to use

Numerical estimation of longitudinal damping derivatives of a flying wing micro aerial vehicle

Longitudinal damping derivatives, [Formula: see text], of an aerial vehicle is important from an aerodynamic stability point of view. Experimental calculation of longitudinal damping derivatives using wind tunnel is not a cost-effective method; therefore, researchers have developed numerical solutions as an alternative. In this research, the longitudinal damping derivatives of a flying wing micro aerial vehicle (FWMAV) were calculated using numerical simulations by adopting pull-up maneuver and forced harmonic motion in pitch axis. Pull-up maneuver with four steady rotational rates was simulated to obtain pitch rate derivative, C mq. Combined derivative, [Formula: see text], was obtained by simulating forced harmonic motion of FWMAV around a mean angle of attack of 0° with amplitude of oscillation of ± 3° using four reduced frequencies (0.02, 0.03, 0.04, and 0.05). Unstructured surface and volume mesh was used in a spherical domain engulfed inside a large cuboid domain for moving reference frame strategy. Reynolds number taking mean aerodynamic chord as a reference length was 2.33 × 105. Spalart–Allmaras turbulence model was used. Pitch rate derivative, combined derivative, and acceleration derivative were found as − 0.03/rad, − 7.39/rad, and − 7.36/rad, respectively, by the use of a phase method at a reduced frequency of 0.03. During flight dynamic analysis, it was found that [Formula: see text] has a significant contribution on damping in short period mode with no effect on Phugoid mode. The research concluded that for tailless configurations, acceleration derivative [Formula: see text] can exist and can provide necessary damping in the longitudinal flight mode.

Analysis of viscoelastic tapered pylons used in transmission lines due to ground vibrations including soil-structure-interaction effects

This work reports on the influence that mass attachments and soft soil support conditions have on the vibratory motion of flexible pylons with non-uniform cross-section, both for axial and flexural vibrations induced by ground motion. The use of such structures is pervasive, both in civil and in mechanical engineering. In particular, the tapered pylon considered here is used in electrified railways to support transmission lines and possibly other types of apparatus. We first present an analytical solution to the governing equations of motion of an inhomogeneous beam, which entails the recovery of the natural frequencies in the presence of an attached top mass and of a compliant base support represented by soil springs. This is followed by the complete solution for the case of harmonic ground motion. Results are presented in term of transmissibility functions, defined as the ratio of the top pylon displacement amplitude to that of the base motion. The aim here is to identify the frequency range for which the presence of a top mass, which may also act as a passive damper, and of compliant soil support, which may also act as a base isolator, is either beneficial or detrimental in terms of minimizing the kinematic field induced in the pylon by ground vibrations.

Co-deriving the Formulas for Centripetal Acceleration and Mass-Spring Period

There is a close connection between simple harmonic motion and uniform circular motion. This connection is widely taught and included in standard textbooks. Here, we exploit this connection to simultaneously derive two results from introductory mechanics: the period of a mass- spring system and the centripetal acceleration formula.Previously published derivations of the centripetal acceleration formula often rely on calculus, either via explicit computation or arguing geometrically about limits. One approach relies entirely on the kinematics of vectors, without invoking limits. Our approach likewise does not use limits, but is rooted in dynamics.

Exploration of Physics Concepts in Several Game Rides at Surabaya Prestasi Park

The purpose of this research is to find out the concept of physics in the game rides in Surabaya Prestasi Park and provide information through photos taken directly to readers. In this article the experimental method used is direct observation, documentation, and analysis (literature study) as a form of data collection. In this article, four physics concepts are found in the game rides, namely swings, slides, and seesaws in the form of simple harmonic motion concepts, energy, force, and levers. When playing swing there is a concept of physics that occurs in the motion of the swing, namely simple harmonic motion and the energy contained therein. In the slide there are physics concepts namely frictional force, energy concept and inclined plane. On the seesaw there is the concept of a lever. From this article it can be concluded that the rides at Surabaya Prestasi Park have a lot to do with physics concepts, including the concepts of simple harmonic motion, energy, force, and levers through personal documentation with direct observation so that apart from playing, children indirectly have learned the concept of physics so as to increase knowledge and insight

Molecular Dynamics Simulation and Experimental Analysis of the Effect of Ultrasonic Disposal on the Compatibility of NanoAsphalt

Based on LAMMPS molecular dynamics simulation of nano-silica(nano-SiO2) and asphalt molecular motion trajectory in the ultrasonic environment, the nano-SiO2 modified asphalt mixed model was proposed, and then the ultrasonic vibration process was simulated by the periodic displacement method. The solubility parameter and viscosity of the mixed model were simulated and calculated to reveal the compatibility changes of the modified asphalt from a microscopic perspective. Different temperatures and ultrasonic frequencies were achieved by changing the temperature parameter and the period parameter of the simple harmonic motion equation. Besides, to characterize the effect of ultrasonic vibration on the promotion of nano-SiO2-asphalt compatibility, the prepared nano-SiO2 modified asphalt was subjected to viscosity testing through viscosity change. The results show that the simulation could accurately predict the experimental phenomena, and the molecular simulation can be used as an effective method to study the properties of asphalt materials. The compatibility of nano-SiO2 and asphalt is positively correlated with ultrasonic temperature and ultrasonic frequency to some extent. The compatibility effect is best at 130 °C, 40 KHz. When the ultrasound frequency exceeds a certain value, the effect of promoting compatibility is not obvious.

Effect of Liquefaction Induced Lateral Spreading on Seismic Performance of Pile Foundations

Seismically active areas are vulnerable to liquefaction, and the influence of liquefaction on pile foundations is very severe. Study of pile-supported buildings in liquefiable soils requires consideration of soil-pile interaction and evaluation of the interaction resulting from movement of soil surrounding the pile. This paper presents the results of three-dimensional finite difference analyses conducted to understand the effect of liquefiable soils on the seismic performance of piles and pile groups embedded in stratified soil deposits using the numerical tool FLAC3D. A comparative study has been conducted on the performance of pile foundations on level ground and sloping ground. The soil model consists of a non-liquefiable, slightly cemented sand layer at the top and bottom and a liquefiable Nevada sand layer in between. This stratified ground is subjected to 1940 El Centro, 2001 Bhuj (India) earthquake ground motions, and harmonic motion of 0.3g acceleration. Parametric studies have been carried out by changing the ground slope from 0° to 10° to understand the effects of sloping ground on pile group response. The results indicate that the maximum bending moments occur at boundaries between liquefiable and non-liquefiable layers, and that the bending moment increases with an increase in slope angle. The presence of a pile cap prevents horizontal ground displacements at ground level. Further, it is also observed that the displacements of pile groups under sloping ground are in excess of those on level ground due to lateral spreading. Doi: 10.28991/CEJ-SP2021-07-05 Full Text: PDF

Unsteady physics-based reduced order modeling for large-scale compressible aerodynamic applications

A physics-based reduced order model is presented as a viable approach to accurately predict unsteady flowfield solutions at a fraction of the computational time that is required by high fidelity computational fluid dynamics solvers. Such a model could be useful during the preliminary phases of the aircraft design process when an accurate prediction of unsteady loads is crucial to reduce the uncertainty of design choices. This paper extends the formulation of a previously defined technique to unsteady aerodynamic problems involving motion excitation in both subsonic and transonic conditions. In particular, the proposed unsteady-residual-based model defines a reduced solution space by applying proper orthogonal decomposition to a set of snapshots collected from the unsteady high fidelity simulations. Approximate flow solutions for unseen motion signals is then obtained in a time-marching fashion where, at each time step, the prediction coefficients are computed via L2-norm minimization of the unsteady residual vector as provided from the underlying flow solver. As suggested by previous studies, a further reduction of computational complexity is attempted by including a hyperreduction node selection, specifically the missing point estimation routine, in the prediction framework. Different model reduction and hyperreduction levels for the flowfield prediction of harmonic pitching motion of a 2D airfoil and a 3D aircraft in both subsonic and transonic conditions are investigated to determine their influence on the solution accuracy as well as on the required computational cost. Results show that the proposed technique accurately predicts unsteady solutions (maximum prediction error of about 3%) with a computational cost that is around 10% of the time needed to perform the equivalent high fidelity simulations. This performance achievement substantiates the idea that a reduced order model based on unsteady residual minimization is viable replacement for expensive flow solver evaluations in various applications where the computational budget is constrained.

Power-Optimal Control of a Stirling Engine's Frictional Piston Motion.

The power output of Stirling engines can be optimized by several means. In this study, the focus is on potential performance improvements that can be achieved by optimizing the piston motion of an alpha-Stirling engine in the presence of dissipative processes, in particular mechanical friction. We use a low-effort endoreversible Stirling engine model, which allows for the incorporation of finite heat and mass transfer as well as the friction caused by the piston motion. Instead of performing a parameterization of the piston motion and optimizing these parameters, we here use an indirect iterative gradient method that is based on Pontryagin’s maximum principle. For the varying friction coefficient, the optimization results are compared to both, a harmonic piston motion and optimization results found in a previous study, where a parameterized piston motion had been used. Thus we show how much performance can be improved by using the more sophisticated and numerically more expensive iterative gradient method.

Demonstrating Mechanical Beats on Phosphorescent Foil in Real Time

The interference pattern between two harmonic oscillations with slightly different frequencies are called beats. The beats, as a combined motion, have two different periods, one approximately equal to the period of the original oscillations, and another that is significantly longer and is related to the variable amplitude of the motion. The main objective of this presented work is to engage students’ attention and demonstrate in a simple way the concept of mechanical beats as a resultant motion of combined oscillations, but also to experience it through a visual projection. The proposed experimental setup is constructed in a way that satisfies both objectives and it consists of two parts that allow us to demonstrate in real time the combination of two harmonic motions. In the first part, the resultant motion is generated by the oscillations of two parallel rulers equipped with mirrors, and in the second part the visual demonstration of the beat occurs on a phosphorescent rotating foil by using a blue violet laser beam.

Roll–Yaw Lock: Aerodynamic Motion Instabilities of Floating Offshore Wind Turbines

Abstract This article presents an investigation of a newly discovered motion instability phenomenon for floating wind turbines, first observed in numerical aero-hydro-servo-elastic simulations. The source of instability is identified as antisymmetric aerodynamic stiffness coupling terms in roll and yaw caused by the turbine thrust force. Analytical expressions for the roll–yaw and yaw–roll stiffness coupling terms are derived, using the actuator disk analogy of the rotor plane. It is demonstrated that a two degrees-of-freedom linear equation of motion without explicit external forcing qualitatively captures the instability. Based on this model, an analytical stability criteria is derived. An intuitive physical explanation of the phenomena is provided, considering the energy flow of the system. An important finding is that dissipative forces, in the form of aerodynamic and hydrodynamic damping, are needed to fully explain the observed instability. It is demonstrated, counterintuitively, that increased damping in yaw reduces the stability margin. This is explained by the fact that dissipative forces result in a phase difference between the harmonic roll and yaw motion in the coupled roll–yaw modes.

Teaching by active learning: A laboratory experience on fundamentals of vibrations

This article shows the design and construction of a test bench for experiments on mechanical systems vibrations. The experimental setup is finalized to the development of a project work proposed to the students of the course "Mechanical System Vibrations", within the BSc faculty of Mechanical Engineering. The principal motivation that leads to the enrichment of the educational programme of the aforementioned course is the willingness to involve the students in a valuable learning-by-doing activity, aligned to the recent paradigm of active learning. From a didactic perspective, such an approach improves the engagement of the students and promotes the discussion with their peers. In this framework, the curiosity towards the practical activity enhances the acquisition of knowledge. The project regards the theoretical modelling of a 4-storeys building exploiting a lumped-parameters approach, and the deployment of some laboratory sessions. Acceleration data gathered during laboratory activities are employed in analyses in both the time and the frequency domain, exploiting the procedure of “Experimental Modal Analysis”. Conclusively, the project requires the students to design and construct two different types of Tuned Mass Dampers whose effectiveness will be directly tested on the building, subjected to a condition of harmonic motion imposed by an actuated sled. The final deliverable of this practical activity is a report that students should submit by the end of the course.