980 research outputs found
An advection-robust Hybrid High-Order method for the Oseen problem
In this work, we study advection-robust Hybrid High-Order discretizations of
the Oseen equations. For a given integer , the discrete velocity
unknowns are vector-valued polynomials of total degree on mesh elements
and faces, while the pressure unknowns are discontinuous polynomials of total
degree on the mesh. From the discrete unknowns, three relevant
quantities are reconstructed inside each element: a velocity of total degree
, a discrete advective derivative, and a discrete divergence. These
reconstructions are used to formulate the discretizations of the viscous,
advective, and velocity-pressure coupling terms, respectively. Well-posedness
is ensured through appropriate high-order stabilization terms. We prove energy
error estimates that are advection-robust for the velocity, and show that each
mesh element of diameter contributes to the discretization error with
an -term in the diffusion-dominated regime, an
-term in the advection-dominated regime, and
scales with intermediate powers of in between. Numerical results complete
the exposition
A 58-year-old Man with Abdominal Pain; Acute Appendicitis due to an Appendicolith
Case presentation: A 58-year-old man presented to the emergency department with abdominal pain, nausea and loss of appetite for the last 8 hours. He reported diffuse pain that had been localized to the right lower quadrant (RLQ). Physical examination revealed muscular defense and tenderness in the RLQ. Computed tomography (CT) of the abdomen and pelvis confirmed luminal distension with a thickened enhancing wall with an appendicolith.
Learning points: Appendicitis may be developed by an appendicolith, a calcified deposit within the appendix. It may be an incidental finding on an abdominal radiograph, ultrasound (US) examination or CT. It appears as echogenic focus and casts an acoustic shadow on US examination and manifests as a calcified mass on plain radiograph or CT. The incidence of appendicolith is higher among patients with a retrocaecal appendix. In our patient, a clinical diagnosis of acute appendicitis was made and the patient was immediately transferred to the operating room and an appendectomy was performed
A 27-years-old Man with Abdominal Pain; Lead Toxicity
Case presentation: A 27-year-old man came to our emergency department with chief complaints of abdominal pain, nausea and vomiting, colicky pain in all area of abdomen without any radiation and generalized myalgia. In his background, he had no previous medical problem. In his social history he had worked in an automobile battery-reclaiming factory for 5 years. During his physical examination, his appearance was pale with perioral priority, ill and agitated but not toxic with a blood pressure of 127/85 mmHg and a pulse of 80 beats/min, respiratory rate of 14 breaths/min and oral temperature of 37.3 °C, mild generalized abdominal tenderness without rebound. No obvious signs of sensory and motor neuropathy were found. In the head and neck examination, we found lead-lined teeth.
Learning points: The most common cause of chronic metal poisoning is lead. Exposure occurs through inhalation or ingestion. Both inorganic and organic forms of lead that exist naturally produce clinical toxicity. Gastrointestinal manifestations occur more frequently with acute rather than with chronic poisoning, and concurrent hemolysis may cause the colicky abdominal pains. Patients may have complained of a metallic taste and, with long-term exposure, have bluish-gray gingival lead lines. In addition, constitutional symptoms, including arthralgia, generalized weakness, and weight loss raises the possibility of lead toxicity
Hybrid Simulator for Space Docking and Robotic Proximity Operations
In this work, we present a hybrid simulator for space docking and robotic
proximity operations methodology. This methodology also allows for the
emulation of a target robot operating in a complex environment by using an
actual robot. The emulation scheme aims to replicate the dynamic behavior of
the target robot interacting with the environment, without dealing with a
complex calculation of the contact dynamics. This method forms a basis for the
task verification of a flexible space robot. The actual emulating robot is
structurally rigid, while the target robot can represent any class of robots,
e.g., flexible, redundant, or space robots. Although the emulating robot is not
dynamically equivalent to the target robot, the dynamical similarity can be
achieved by using a control law developed herein. The effect of disturbances
and actuator dynamics on the fidelity and the contact stability of the robot
emulation is thoroughly analyzed
Optimal Control of Legged-Robots Subject to Friction Cone Constraints
A hierarchical control architecture is presented for energy-efficient control
of legged robots subject to variety of linear/nonlinear inequality constraints
such as Coulomb friction cones, switching unilateral contacts, actuator
saturation limits, and yet minimizing the power losses in the joint actuators.
The control formulation can incorporate the nonlinear friction cone constraints
into the control without recourse to the common linear approximation of the
constraints or introduction of slack variables. A performance metric is
introduced that allows trading-off the multiple constraints when otherwise
finding an optimal solution is not feasible. Moreover, the projection-based
controller does not require the minimal-order dynamics model and hence allows
switching contacts that is particularly appealing for legged robots. The
fundamental properties of constrained inertia matrix derived are similar to
those of general inertia matrix of the system and subsequently these properties
are greatly exploited for control design purposes. The problem of task space
control with minimum (point-wise) power dissipation subject to all physical
constraints is transcribed into a quadratically constrained quadratic
programming (QCQP) that can be solved by barrier methods
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