1,966 research outputs found
Coriolis Effect on Dynamic Stall in a Vertical Axis Wind Turbine at Moderate Reynolds Number
The immersed boundary method is used to simulate the flow around a two-dimensional rotating NACA 0018 airfoil at sub-scale Reynolds number in order to investigate the separated flow occurring on a vertical-axis wind turbine. The influence of dynamic stall on the forces is characterized as a function of tip-speed ratio. The influence of the Coriolis effect is also investigated by comparing the rotating airfoil to one undergoing a equivalent planar motion, which is composed of surging and pitching motions that produce an equivalent speed and angle-of-attack variation over the cycle. When the angle of attack of a rotating airfoil starts to decrease in the upwind half cycle, the Coriolis force leads to a wake-capturing phenomenon of a vortex pair at low tip-speed ratio. This effects occurs at a slightly different phase in each cycle and leads to a significant decrease in the average lift during the downstroke phase. Moreover, the wake-capturing is only observed when the combination of surging, pitching, and Coriolis force are present. Finally, an actuator model is placed at an appropriate location on the suction side of the airfoil surface to control the wake-capturing phenomenon. Based on preliminary simulations, a momentum coefficient above 0.02 was able to increase the average lift by more than 70% over the upwind-half cycle
Factorised steady states for multi-species mass transfer models
A general class of mass transport models with Q species of conserved mass is
considered. The models are defined on a lattice with parallel discrete time
update rules. For one-dimensional, totally asymmetric dynamics we derive
necessary and sufficient conditions on the mass transfer dynamics under which
the steady state factorises. We generalise the model to mass transfer on
arbitrary lattices and present sufficient conditions for factorisation. In both
cases, explicit results for random sequential update and continuous time limits
are given.Comment: 11 page
Unsteady low-Reynolds number flow control in different regimes
Peer Reviewedhttp://deepblue.lib.umich.edu/bitstream/2027.42/106476/1/AIAA2013-353.pd
Valence state of Mn in Ca-doped LaMnO3 studied by high-resolution Mn K ß emission spectroscopy
Mn K ß x-ray emission spectra provide a direct method to probe the effective spin state and charge density
on the Mn atom and is used in an experimental study of a class of Mn oxides. Specifically, the Mn K ß line
positions and detailed spectral shapes depend on the oxidation and the spin state of the Mn sites as well as the
degree of d covalency/itinerancy. Theoretical calculations including atomic charge and multiplet effects, as
well as crystal-field splittings and covalency effects, are used as a guide to the experimental results. Direct
comparison of the ionic system MnF2 and the covalent system MnO reveals significant changes due to the
degree of covalency of Mn within atomic-type Mn K ß simulations. Moreover, comparisons of measurement
with calculations support the assumed high spin state of Mn in all of the systems studied. The detailed shape
and energy shift of the spectra for the perovskite compounds, LaMnO3 and CaMnO3, are, respectively, found
to be very similar to the covalent Mn^(3+)-Mn2O3 and Mn^(4+)-MnO2 compounds thereby supporting the identical
Mn-state assignments. Comparison to the theoretical modeling emphasizes the strong covalency in these
materials. Detailed Mn K b x-ray emission results on the La1_xCaxMnO3 system can be well fit by linear
superpositions of the end member spectra, consistent with a mixed-valent character for the intermediate compositions.
However, an arrested Mn-valence response to the doping in the x<0.3 range is found. No evidence
for Mn^2+ is observed at any x values seemingly ruling out proposals regarding Mn^3+ disproportionation
Factorised Steady States in Mass Transport Models on an Arbitrary Graph
We study a general mass transport model on an arbitrary graph consisting of
nodes each carrying a continuous mass. The graph also has a set of directed
links between pairs of nodes through which a stochastic portion of mass, chosen
from a site-dependent distribution, is transported between the nodes at each
time step. The dynamics conserves the total mass and the system eventually
reaches a steady state. This general model includes as special cases various
previously studied models such as the Zero-range process and the Asymmetric
random average process. We derive a general condition on the stochastic mass
transport rules, valid for arbitrary graph and for both parallel and random
sequential dynamics, that is sufficient to guarantee that the steady state is
factorisable. We demonstrate how this condition can be achieved in several
examples. We show that our generalized result contains as a special case the
recent results derived by Greenblatt and Lebowitz for -dimensional
hypercubic lattices with random sequential dynamics.Comment: 17 pages 1 figur
Equilibration and Dynamic Phase Transitions of a Driven Vortex Lattice
We report on the observation of two types of current driven transitions in
metastable vortex lattices. The metastable states, which are missed in usual
slow transport measurements, are detected with a fast transport technique in
the vortex lattice of undoped
2H-NbSe. The transitions are seen by following the evolution of these
states when driven by a current. At low currents we observe an equilibration
transition from a metastable to a stable state, followed by a dynamic
crystallization transition at high currents.Comment: 5 pages, 4 figure
Neurophysiology
Contains research objectives, summary of research and reports on six research objectives.National Institutes of Health (Grant 5 ROl NB-04985-05)National Institutes of Health (Grant NB-07501-02)National Institutes of Health (Grant NB-06251-03)National Institutes of Health (Grant NB-07576-02)U. S. Air Force (Aerospace Medical Division) under Contract AF33(615)-3885Bell Telephone Laboratories IncorporatedNational Institutes of Health (Grant 5 TO1 GM-01555-02
Dynamic separation on a pitching and surging airfoil as a model for flow over vertical axis wind turbine blades
Vertical axis wind turbine (VAWT) blades undergo dynamic separation due to the large angle of attack variation they experience during a turbine rotation. The flow over a single
blade was modeled using a sinusoidally pitching and surging airfoil in a constant free stream flow at a mean chord Reynolds number of 10^5. Two-dimensional, time resolved velocity fields were acquired using particle image velocimetry (PIV). Vorticity contours were used to visualize shear layer and vortex activity. A low order model of dynamic separation was developed using Dynamic Mode Decomposition (DMD). A primary and secondary dynamic separation mode were identified as the critical drivers for the unsteady flow field
Ageing memory and glassiness of a driven vortex system
Many systems in nature, glasses, interfaces and fractures being some
examples, cannot equilibrate with their environment, which gives rise to novel
and surprising behaviour such as memory effects, ageing and nonlinear dynamics.
Unlike their equilibrated counterparts, the dynamics of out-of- equilibrium
systems is generally too complex to be captured by simple macroscopic laws.
Here we investigate a system that straddles the boundary between glass and
crystal: a Bragg glass formed by vortices in a superconductor. We find that the
response to an applied force evolves according to a stretched exponential, with
the exponent reflecting the deviation from equilibrium. After the force is
removed, the system ages with time and its subsequent response time scales
linearly with its age (simple ageing), meaning that older systems are slower
than younger ones. We show that simple ageing can occur naturally in the
presence of sufficient quenched disorder. Moreover, the hierarchical
distribution of timescales, arising when chunks of loose vortices cannot move
before trapped ones become dislodged, leads to a stretched-exponential
response.Comment: 16 pages, 5 figure
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