Scaling laws of plume-induced granular cratering.

Extraterrestrial landing often requires firing a high-speed plume towards a planetary surface, and the resulting gas-granular interactions pose potential hazards to the lander. To investigate these jet-induced cratering dynamics, an experiment campaign covering a range of gas and granular properties relevant to the lunar and Martian environments was conducted in a large-scale vacuum chamber. Despite the variations in jet Mach number, mass flow rate, and composition of the granular phase investigated in this work, the observed time evolution of crater depth displays a consistent transition from an early-stage linear to a late-stage sublinear growth. To explain these scaling relations, a model that relates the kinetic energy gained by the particles per unit time to the power of the impinging jet is introduced. From this model, erosion rates and the critical depth at which the transition occurs can be extracted, and they are shown to depend on the gas impingement pressure, which was varied by changing ambient pressure, jet Mach number, mass flow rate, and nozzle height above the surface. These results highlight key mechanisms at work in the dynamics of plume-induced cratering and help to develop an understanding of optimal rocket engine firing times for future landings.

Isolated tear of the pectoralis minor tendon in a high school football player.

Multiple pectoralis major tendon tears have been reported in the literature; however, isolated rupture of the pectoralis minor tendon is rare and has been reported 3 times (4 patients).This article describes a case of an isolated pectoralis minor tendon tear in a male high school football player after a traumatic injury. The patient was injured while making a tackle and leading with his arm and chest. He presented with left anterior shoulder and chest wall pain with direct tenderness on palpation over the coracoid. Magnetic resonance imaging of the chest revealed an isolated tear of the pectoralis minor tendon with slight retraction and significant edema in the muscle belly. The patient returned to full activities after conservative management.Although rare, the diagnosis of pectoralis minor tendon rupture should be considered in patients who sustain a contact injury to the shoulder with tenderness on palpation over the coracoid. The mechanism of injury can be related to a direct anterior force to the shoulder, forced external rotation of the arm in slight abduction, or with the arm in extension and shoulder in flexion (eg, blocking in football). The diagnosis can be confirmed with magnetic resonance imaging when edema exists on the medial aspect of the coracoid and extends into the muscle belly. Physical therapy with scapular stabilization exercises and avoidance of abduction and active adduction can be successful in returning these patients to their previous activity levels.