scholarly journals Hovering in oscillatory flows

2016 ◽  
Vol 804 ◽  
pp. 531-549 ◽  
Author(s):  
Yangyang Huang ◽  
Monika Nitsche ◽  
Eva Kanso
Keyword(s):  
Oscillation Amplitude ◽  
Intrinsic Property ◽  
Vortex Sheet ◽  
Flow Speed ◽  
Rigid Bodies ◽  
Oscillating Flows ◽  
Flow Acceleration ◽  
Physical Insight ◽  
Oscillation Frequencies ◽  
Surrounding Fluid

We investigate the hovering dynamics of rigid bodies with up-down asymmetry placed in oscillating background flows. Recent experiments on inanimate pyramid-shaped objects in oscillating flows with zero mean component demonstrate that the resulting aerodynamic forces are sufficient to keep the object aloft. The mechanisms responsible for this lift production are fundamentally unsteady and depend on the shed vorticity. Here, we consider a model system of a two-dimensional flyer and compute the unsteady, two-way coupling between the flyer and the surrounding fluid in the context of the vortex sheet model. We examine in detail the flow properties (frequency and speed) required for hovering and their dependence on the flyer’s characteristics (mass and geometry). We find that, at low oscillation frequencies, a flyer of a fixed mass and shape requires a constant amount of flow acceleration to hover, irrespective of the frequency and speed of the oscillating flow. Meanwhile, at high oscillation frequencies, the flow speed required to hover is constant. In either case, the aerodynamic requirements to hover (flow acceleration or flow speed) are an intrinsic property of the flyer itself. This physical insight could potentially have significant implications on the design of unmanned air vehicles as well as on understanding active hovering of live organisms that can manipulate their flapping motion to favour a larger oscillation amplitude or frequency.

Harbour oscillations generated by shear flow

1995 ◽  
Vol 282 ◽  
pp. 203-217 ◽  
Author(s):  
A. L. Fabrikant
Keyword(s):  
Growth Rate ◽  
Shear Flow ◽  
Wave Reflection ◽  
Wind Waves ◽  
Vortex Sheet ◽  
Flow Speed ◽  
Natural Mode ◽  
Exciting Forces ◽  
Wavenumber Spectrum ◽  
Harbour Oscillations

A new mechanism that could be responsible for excitation of long-period oscillations in partially enclosed harbours is discussed. This mechanism is based on the interaction between a shear flow and the harbour-basin natural mode and does not suppose any external exciting forces caused by wind waves, tsunami, etc. The growth rate of harbour oscillations is found in terms of a plane-wave reflection coefficient integrated on the wavenumber spectrum of the oscillating outflow field near the harbour entrance. Analytical considerations for simple shear flows (vortex sheet and jet) show that the growth rate changes its sign depending on the ratio of oscillation frequency to flow speed.

scholarly journals Flexible organization of grip force control during movement frequency scaling

2019 ◽  
Vol 122 (6) ◽  
pp. 2304-2315
Author(s):  
Francis M. Grover ◽  
Sarah M. Schwab ◽  
Paula L. Silva ◽  
Tamara Lorenz ◽  
Michael A. Riley
Keyword(s):  
Force Control ◽  
Grip Force ◽  
Sagittal Plane ◽  
Oscillation Amplitude ◽  
Load Force ◽  
Force Load ◽  
Grip Force Control ◽  
Force Coupling ◽  
Tightly Coupled ◽  
Oscillation Frequencies

The grip force applied to maintain grasp of a handheld object has been typically reported as tightly coupled to the load force exerted by the object as it is actively manipulated, occurring proportionally and consistently in phase with changes in load force. However, continuous grip force-load force coupling breaks down when overall load force levels and oscillation amplitudes are lower (Grover F, Lamb M, Bonnette S, Silva PL, Lorenz T, Riley MA. Exp Brain Res 236: 2531–2544, 2018) or more predictable (Grover FM, Nalepka P, Silva PL, Lorenz T, Riley MA. Exp Brain Res 237: 687–703, 2019). Under these circumstances, grip force is instead only intermittently coupled to load force; continuous coupling is prompted only when load force levels or variations become sufficiently high or unpredictable. The current study investigated the nature of the transition between continuous and intermittent modes of grip force control by scaling the load force level and the oscillation amplitude continuously in time by means of scaling the required frequency of movement oscillations. Participants grasped a cylindrical object between the thumb and forefinger and oscillated their arm about the shoulder in the sagittal plane. Oscillation frequencies were paced with a metronome that scaled through an ascending or descending frequency progression. Due to greater accelerations, faster frequencies produced greater overall load force levels and more pronounced load oscillations. We observed smooth but nonlinear transitions between clear regimes of intermittent and continuous grip force-load force coordination, for both scaling directions, indicating that grip force control can flexibly reorganize as parameters affecting grasp (e.g., variations in load force) change over time. NEW & NOTEWORTHY Grip force (GF) is synchronously coupled to changing load forces (LF) during object manipulation when LF levels are high or unpredictable, but only intermittently coupled to LF during less challenging grasp conditions. This study characterized the nature of transitions between synchronous and intermittent GF-LF coupling, revealing a smooth but nonlinear change in intermittent GF modulation in response to continuous scaling of LF amplitude. Intermittent, “drift-and-act” control may provide an alternative framework for understanding GF-LF coupling.

Chip Breaking Parameter Selection for Constant Surface Speed Machining

2013 ◽  
Author(s):  
Luke Berglind ◽  
John Ziegert
Keyword(s):  
Dynamic Capabilities ◽  
Tool Path ◽  
Selection Process ◽  
Oscillation Amplitude ◽  
Parameter Selection ◽  
Work Piece ◽  
Chip Breaking ◽  
Part Surface ◽  
Breaking Parameter ◽  
Oscillation Frequencies

Modulated tool path (MTP) chip breaking is a lathe machining technique which produces chips of a predetermined length to reduce the risk of damage to the tool or the work piece caused by chip entanglement. Individual chips are formed by repeatedly interrupting chip formation through CNC commanded tool oscillations superimposed in the tool feed direction. Previous work has shown that the chip length and part surface finish quality are dependent on the tool oscillation frequency relative to the spindle speed (OPR), and the oscillation amplitude relative to the global feed per revolution (Raf). Apart from chip length and surface quality, the dynamic capabilities of the machine must be considered when selecting MTP parameters, OPR and Raf. The dynamic limitations of the machine will limit the available range of tool oscillation frequencies and amplitudes. In this paper, the factors which affect the MTP parameter selection process are discussed, and a process for selecting these parameters automatically based on multiple constraints and criteria is presented for constant surface speed MTP machining.

Experimental and numerical investigation into the wake of a supercritical airfoil in a compressible unsteady flow

2016 ◽  
Vol 231 (3) ◽  
pp. 419-434 ◽  
Author(s):  
B Beheshti Boroumand ◽  
M Mani ◽  
N Fallahpour
Keyword(s):  
Numerical Study ◽  
Oscillation Amplitude ◽  
Experimental Tests ◽  
Experimental Investigations ◽  
Hot Wire ◽  
Supercritical Airfoil ◽  
Signal Energy ◽  
Test Conditions ◽  
Power Spectral ◽  
Oscillation Frequencies

Experimental investigations were carried out to study the wake characteristics of a pitching supercritical airfoil at Mach number of 0.6. Flow field inside the wake was measured by a hot-wire anemometry at downstream distances from trailing edge of 0.25 and 0.5 times the chord length. All data were taken at mean incidence angles of 0 and 3°. The amplitudes of oscillation were 1 and 3° while the oscillation frequencies were 3 and 6 Hz. Output signals acquired from sensors were analyzed besides the effects of such parameters as frequency and oscillation amplitude. Moreover, a comprehensive numerical study was carried out for the same airfoil under similar experimental test conditions; then, the results of numerical simulations were analyzed and compared with those of experimental tests. Results of the present research could be summarized as: observation of hysteresis and how it is affected by frequency and amplitude variations, observation of increasing turbulence intensity by root mean square investigation and also increasing signal energy by means of power spectral density diagram for those sensors lied inside the wake, and finally, study of correlation between wake’s interior sensors and exterior ones.

Oscillation of a Net Panel With Bending Stiffness

2013 ◽  
Author(s):  
Østen Jensen ◽  
Lars Gansel ◽  
Martin Føre ◽  
Karl-Johan Reite ◽  
Jørgen Haavind Jensen ◽  
...  
Keyword(s):  
Numerical Analysis ◽  
Numerical Model ◽  
Rigid Body ◽  
Polyethylene Terephthalate ◽  
Oscillation Amplitude ◽  
Bending Stiffness ◽  
Oscillatory Motion ◽  
Product Certification ◽  
Net Cages ◽  
Oscillation Frequencies

The behaviour of net panels with bending stiffness is dependent on the stiffness and potentially also the density of the material when exposed to oscillatory motions. This needs to be taken into account when net cages are product certified according to NS9415 (Standard Norge 2009). Experiments using two different net panels with bending stiffness were conducted to investigate the behaviour of nets with bending stiffness in oscillatory motion. For low oscillation frequencies the panels moved in a close to rigid body manner. When the oscillation frequencies where increased, however, there was a distinct difference between the copper and the Polyethylene Terephthalate (PET) nets, with a significant increase in the resulting oscillation amplitude for the copper net and a decrease for the PET net. The proposed numerical model predicts this behaviour well in terms of oscillation amplitudes. It is, however, important to establish good estimates of the net panels bending stiffness in advance, particularly if product certification is the purpose of the numerical analysis.

scholarly journals Ice front and flow speed variations of marine-terminating outlet glaciers along the coast of Prudhoe Land, northwestern Greenland

2018 ◽  
Vol 64 (244) ◽  
pp. 300-310 ◽  
Author(s):  
DAIKI SAKAKIBARA ◽  
SHIN SUGIYAMA
Keyword(s):  
Air Temperature ◽  
Satellite Images ◽  
Flow Speed ◽  
Glacier Retreat ◽  
Velocity Increase ◽  
Flow Acceleration ◽  
Atmospheric Warming ◽  
Year 2000

ABSTRACTSatellite images were analyzed to measure the frontal positions and ice speeds of 19 marine-terminating outlet glaciers along the coast of Prudhoe Land, northwestern Greenland from 1987 to 2014. All the studied glaciers retreated over the study period at a rate of between 12 and 200 m a−1, with a median (mean) retreat rate of 30 (40) m a−1. The glacier retreat began in the year ~2000, which coincided with an increase in summer mean air temperature from 1.4 to 5.5 °C between 1996 and 2000 in this region. Ice speed near the front of the studied glaciers ranged between 20 and 1740 m a−1 in 2014, and many of them accelerated in the early 2000s. In general, the faster retreat was observed at the glaciers that experienced greater acceleration, as represented by Tracy Glacier, which experienced a retreat of 200 m a−1 and a velocity increase of 930 m a−1 during the study period. A possible interpretation of this observation is that flow acceleration induced dynamic thinning near the termini, resulting in enhanced calving and rapid retreat of the studied glaciers. We hypothesize that atmospheric warming conditions in the late 1990s triggered glacier retreat in northwestern Greenland since 2000.

scholarly journals Catalytic Resonance Theory: SuperVolcanoes, Catalytic Molecular Pumps, and Oscillatory Steady State

2019 ◽  
Author(s):  
M. Alexander Ardagh ◽  
Turan Birol ◽  
Qi Zhang ◽  
Omar Abdelrahman ◽  
Paul Dauenhauer
Keyword(s):  
Steady State ◽  
Forced Oscillation ◽  
Oscillation Amplitude ◽  
Catalytic Reactions ◽  
Forced Oscillations ◽  
Heat Of Reaction ◽  
Turnover Frequency ◽  
Resonance Theory ◽  
Intermediate Energies ◽  
Oscillation Frequencies

Catalytic reactions on surfaces with forced oscillations in physical or electronic properties undergo controlled acceleration consistent with the selected parameters of frequency, amplitude, and external stimulus waveform. In this work, the general reaction of reversible A-to-B chemistry is simulated by varying the catalytic (heat of reaction, transition state and intermediate energies) and oscillation parameters (frequency, amplitude, endpoints, and waveform) to evaluate the influence on the overall catalytic turnover frequency and steady state extent of conversion. Variations of catalytic cycle energies are shown to comprise a superVolcano of superimposed individual Balandin-Sabatier volcano plots, with variations in linear scaling relationships leading to unique turnover frequency response to forced oscillation of the catalyst surface. Optimization of catalytic conditions identified a band of forced oscillation frequencies leading to resonance and rate enhancement as high as 10,000x above the static Sabatier maximum. Dynamic catalytic reactions conducted at long times achieved oscillatory steady state differing from equilibrium consistent with the imposed surface oscillation amplitude acting as a ‘catalytic pump’ relative to the Gibbs free energy of reaction.

scholarly journals Catalytic Resonance Theory: SuperVolcanoes, Catalytic Molecular Pumps, and Oscillatory Steady State

2019 ◽  
Author(s):  
M. Alexander Ardagh ◽  
Turan Birol ◽  
Qi Zhang ◽  
Omar Abdelrahman ◽  
Paul Dauenhauer
Keyword(s):  
Steady State ◽  
Forced Oscillation ◽  
Oscillation Amplitude ◽  
Catalytic Reactions ◽  
Forced Oscillations ◽  
Heat Of Reaction ◽  
Turnover Frequency ◽  
Resonance Theory ◽  
Intermediate Energies ◽  
Oscillation Frequencies

Catalytic reactions on surfaces with forced oscillations in physical or electronic properties undergo controlled acceleration consistent with the selected parameters of frequency, amplitude, and external stimulus waveform. In this work, the general reaction of reversible A-to-B chemistry is simulated by varying the catalytic (heat of reaction, transition state and intermediate energies) and oscillation parameters (frequency, amplitude, endpoints, and waveform) to evaluate the influence on the overall catalytic turnover frequency and steady state extent of conversion. Variations of catalytic cycle energies are shown to comprise a superVolcano of superimposed individual Balandin-Sabatier volcano plots, with variations in linear scaling relationships leading to unique turnover frequency response to forced oscillation of the catalyst surface. Optimization of catalytic conditions identified a band of forced oscillation frequencies leading to resonance and rate enhancement as high as 10,000x above the static Sabatier maximum. Dynamic catalytic reactions conducted at long times achieved oscillatory steady state differing from equilibrium consistent with the imposed surface oscillation amplitude acting as a ‘catalytic pump’ relative to the Gibbs free energy of reaction.

Numerical Simulation of Vortex Induced Vibration on an Elastically Mounted Cylinder

2004 ◽  
Author(s):  
Juan B. V. Wanderley ◽  
Carlos A. Levi
Keyword(s):  
Stokes Equations ◽  
Oscillation Amplitude ◽  
High Sensitivity ◽  
Flow Speed ◽  
Navier Stokes ◽  
Experimental Measurements ◽  
Navier Stokes Equations ◽  
Vortex Induced Vibration ◽  
Set Up ◽  
Lock In

The Vortex-induced vibration on a circular cylinder is investigated by the numerical solution of the unsteady Reynolds Average Navier-Stokes equations and results are compared with experimental measurements obtained by different authors. The Beam and Warming implicit factored scheme is used to solve the governing equations and the Baldwin and Lomax model is used to simulate the turbulent flow in the wake of the cylinder. The cylinder is laterally supported by a spring and a damper and is free to oscillate in the transversal direction in an initially uniform flow for the first flow speed investigated. For the subsequent speeds, the final condition obtained for the previous speed is used as initial condition to reproduce the actual experimental set up. In that case, the measurements are done by progressive increments of the flow speed retaining the fluid memory effect. The complexity and high sensitivity of the flow phenomenon at this configuration requires a very accurate and robust numerical model. Most of the known algorithms failed to duplicate the available experimental measurements. The numerical results for the transversal oscillation amplitude are compared to experimental data showing a fairly precise agreement at the difficult to simulate regime of the lock-in phenomenon.

scholarly journals Oceanic mechanical forcing of a marine-terminating Greenland glacier

2012 ◽  
Vol 53 (60) ◽  
pp. 181-192 ◽  
Author(s):  
Jacob I. Walter ◽  
Jason E. Box ◽  
Slawek Tulaczyk ◽  
Emily E. Brodsky ◽  
Ian M. Howat ◽  
...  
Keyword(s):  
Tide Gauge ◽  
Greenland Ice Sheet ◽  
Time Lapse ◽  
Flow Speed ◽  
Ice Flow ◽  
Flow Acceleration ◽  
Order Of Magnitude ◽  
Seismic Records ◽  
Oceanic Forcing ◽  
Continuous Gps

AbstractDynamics of marine-terminating major outlet glaciers are of high interest because of their potential for drawing down large areas of the Greenland ice sheet. We quantify short-term changes in ice flow speed and calving at a major West Greenland glacier and examine their relationship to the presence of the sea-ice melange and tidal stage. A field campaign at the terminus of Store Gletscher (70.40˚N, 50.55˚W) spanning the spring and summer of 2008 included four broadband seismometers, three time-lapse cameras, a tide gauge, an automatic weather station and an on-ice continuous GPS station. Sub-daily fluctuations in speed coincide with two modes of oceanic forcing: (1) the removal of the ice melange from the terminus front and (2) tidal fluctuations contributing to speed increases following ice melange removal. Tidal fluctuations in ice flow speed were observed 16km from the terminus and possibly extend further. Seismic records suggest that periods of intensive calving activity coincide with ice-flow acceleration following breakup of the melange in spring. A synchronous increase in speed at the front and clearing of the melange suggests that the melange directly resists ice flow. We estimate a buttressing stress (~30–60 kPa) due to the presence of the ice melange that is greater than expected from the range of observed tides, though an order of magnitude less than the driving stress.

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