541 research outputs found
Transition to turbulence in pulsating pipe flow
Fluid flows in nature and applications are frequently subject to periodic
velocity modulations. Surprisingly, even for the generic case of flow through a
straight pipe, there is little consensus regarding the influence of pulsation
on the transition threshold to turbulence: while most studies predict a
monotonically increasing threshold with pulsation frequency (i.e. Womersley
number, ), others observe a decreasing threshold for identical
parameters and only observe an increasing threshold at low . In the
present study we apply recent advances in the understanding of transition in
steady shear flows to pulsating pipe flow. For moderate pulsation amplitudes we
find that the first instability encountered is subcritical (i.e. requiring
finite amplitude disturbances) and gives rise to localized patches of
turbulence ("puffs") analogous to steady pipe flow. By monitoring the impact of
pulsation on the lifetime of turbulence we map the onset of turbulence in
parameter space. Transition in pulsatile flow can be separated into three
regimes. At small Womersley numbers the dynamics are dominated by the decay
turbulence suffers during the slower part of the cycle and hence transition is
delayed significantly. As shown in this regime thresholds closely agree with
estimates based on a quasi steady flow assumption only taking puff decay rates
into account. The transition point predicted in the zero limit equals
to the critical point for steady pipe flow offset by the oscillation Reynolds
number. In the high frequency limit puff lifetimes are identical to those in
steady pipe flow and hence the transition threshold appears to be unaffected by
flow pulsation. In the intermediate frequency regime the transition threshold
sharply drops (with increasing ) from the decay dominated (quasi
steady) threshold to the steady pipe flow level
Tripartite Entanglement and Quantum Correlation
We provide an analytical solution from the correlators of the generalized
-matrix in the 3-qubit pure states. It provides the upper bound to the
maximum violation of Mermin's inequality. For a generic 2-qubit pure state, the
concurrence characterizes the maximum violation of Bell's inequality from the
-matrix. Therefore, people expect that the maximum violation should be
proper to quantify Quantum Entanglement. The -matrix shows the maximum
violation of Bell's operator. For a general 3-qubit state, we have five
invariant entanglement quantities under local unitary transformations. We show
that the five invariant quantities describe the correlation in the generalized
-matrix. The violation of Mermin's operator is not a proper diagnosis by
observing the dependence for entanglement measures. We then classify 3-qubit
quantum states. Each classification quantifies Quantum Entanglement by the
total concurrence. In the end, we relate the experiment correlators to Quantum
Entanglement.Comment: 14 pages, 4 figures, minor changes, reference change
T-linear resistivity in the strange-metal phase of cuprate superconductors due to umklapp scattering from a spin excitation
The strange-metal phase of cuprate superconductors exhibits a linear in
temperature resistivity, however, the origin of this remarkable anomaly is
still not well understood. Here the linear temperature dependence of the
electrical resistivity in the strange-metal phase of cuprate superconductors is
investigated from the underdoped to overdoped regimes. The momentum dependence
of the transport scattering rate arising from the umklapp scattering between
electrons by the exchange of the spin excitation is derived and employed to
calculate the electrical resistivity by making use of the Boltzmann equation.
It is shown that the antinodal umklapp scattering leads to the linear in
temperature resistivity in the low-temperature with the temperature linear
coefficient that decreases with the increase of the doping concentration,
however, the nodal umklapp scattering induces a deviation from the linear in
temperature resistivity in the far lower temperature, and then the quadratic in
temperature resistivity in the far lower temperature is generated by both the
antinodal and nodal umklapp scattering. The theory also shows that the same
spin excitation that acts like a bosonic glue to hold the electron pairs
together also mediates scattering of electrons in the strange-metal phase of
cuprtae superconductors responsible for the linear in temperature resistivity
and the associated electronic structure.Comment: 16 pages, 11 figure
Provably Accelerating Ill-Conditioned Low-rank Estimation via Scaled Gradient Descent, Even with Overparameterization
Many problems encountered in science and engineering can be formulated as
estimating a low-rank object (e.g., matrices and tensors) from incomplete, and
possibly corrupted, linear measurements. Through the lens of matrix and tensor
factorization, one of the most popular approaches is to employ simple iterative
algorithms such as gradient descent (GD) to recover the low-rank factors
directly, which allow for small memory and computation footprints. However, the
convergence rate of GD depends linearly, and sometimes even quadratically, on
the condition number of the low-rank object, and therefore, GD slows down
painstakingly when the problem is ill-conditioned. This chapter introduces a
new algorithmic approach, dubbed scaled gradient descent (ScaledGD), that
provably converges linearly at a constant rate independent of the condition
number of the low-rank object, while maintaining the low per-iteration cost of
gradient descent for a variety of tasks including sensing, robust principal
component analysis and completion. In addition, ScaledGD continues to admit
fast global convergence to the minimax-optimal solution, again almost
independent of the condition number, from a small random initialization when
the rank is over-specified in the presence of Gaussian noise. In total,
ScaledGD highlights the power of appropriate preconditioning in accelerating
nonconvex statistical estimation, where the iteration-varying preconditioners
promote desirable invariance properties of the trajectory with respect to the
symmetry in low-rank factorization without hurting generalization.Comment: Book chapter for "Explorations in the Mathematics of Data Science -
The Inaugural Volume of the Center for Approximation and Mathematical Data
Analytics". arXiv admin note: text overlap with arXiv:2104.1452
Prioritized Planning for Target-Oriented Manipulation via Hierarchical Stacking Relationship Prediction
In scenarios involving the grasping of multiple targets, the learning of
stacking relationships between objects is fundamental for robots to execute
safely and efficiently. However, current methods lack subdivision for the
hierarchy of stacking relationship types. In scenes where objects are mostly
stacked in an orderly manner, they are incapable of performing human-like and
high-efficient grasping decisions. This paper proposes a perception-planning
method to distinguish different stacking types between objects and generate
prioritized manipulation order decisions based on given target designations. We
utilize a Hierarchical Stacking Relationship Network (HSRN) to discriminate the
hierarchy of stacking and generate a refined Stacking Relationship Tree (SRT)
for relationship description. Considering that objects with high stacking
stability can be grasped together if necessary, we introduce an elaborate
decision-making planner based on the Partially Observable Markov Decision
Process (POMDP), which leverages observations and generates the least
grasp-consuming decision chain with robustness and is suitable for
simultaneously specifying multiple targets. To verify our work, we set the
scene to the dining table and augment the REGRAD dataset with a set of common
tableware models for network training. Experiments show that our method
effectively generates grasping decisions that conform to human requirements,
and improves the implementation efficiency compared with existing methods on
the basis of guaranteeing the success rate.Comment: 8 pages, 8 figure
Dynamic modeling and control of a novel XY positioning stage for semiconductor packaging
This paper presents the dynamic modeling and controller design of an XY positioning stage for semiconductor packaging. The XY stage is directly driven by two linear voice coil motors, and motion decoupling between the X and Y axes is realized through a novel flexible decoupling mechanism based on flexure hinges and preloaded spring. Through bond graph method, the dynamic models of X- and Y-axes servomechanisms are established, respectively, and the state space equations are derived. A control methodology is proposed based on force compensations and the performance of the XY stage is investigated by simulations and experimental tests. The results show that the XY stage has good performance. When the reference displacements are defined as 2 mm, the settling time of the X-axis movement is 64 ms, and the overshoot is 0.7%. Y-axis settling time is 62 ms, and the overshoot is 0.8%. X-axis positioning accuracy is 1.85 μm and the repeatability is 0.95 μm. Y-axis positioning accuracy and repeatability are 1.75 μm and 0.9 μm, respectively. In addition, the stage can track linear, circular and complex trajectories very well
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