2015年9月12日
托福阅读第一篇
题材划分： 动物类
主要内容：讲关于鸟类动物的两种行为：gliding and soaring。鸟如果遇到两种情况可以不用glide，一种是遇到热气流，一种是，后面几段重点讲热气流是如何形成的，提到了热量不均匀，热空气会上升，又有新的冷空气来填补，然后循环进行。鸟可以利用热气流来维持高度然后碰到另一个再换位置往前走，这样长距离飞行节省体力。飞行员和生物学家看到的热气流是不一样的。
相似TPO练习推荐
TPO28 Buck Rubs and Buck Scrapes
相关背景知识：
Soaring is a flying technique used to gain energy by repeatedly crossing the boundary between air masses of significantly different velocity. Such zones of high wind gradient are generally found close to obstacles and close to the surface, so the technique is mainly of use to birds and operators of radio-controlled gliders, but glider pilots have occasionally been able to soar dynamically in meteorological wind shears at higher altitudes. The highest speeds reported are by radio controlled gliders at 505 mph (813 km/h).
While different flight patterns can be employed in dynamic soaring, the simplest example to explain the energy extraction mechanism is a closed loop across the boundary layer between two air masses in relative movement. The gain in speed can be explained in terms of airspeed or groundspeed:
The glider gains airspeed twice during the loop, when it pierces the boundary layer at an acute angle. Since the 180° turns retain most of the airspeed the glider completes the loop within the initial air mass at a higher airspeed.
The gain in groundspeed occurs when the glider performs a 180° downwind turn within the moving air mass. Since the opposite 180° turn is done within the stationary air mass the groundspeed gain is not reversed.
The energy is extracted by reducing the velocity difference between the two air masses during the 180° turns which accelerate air in opposite directions. The following animation is a simplification of what actually happens. In practice, there is a turbulent mixing layer between the moving and stationary air masses. In addition, drag forces are continually slowing the plane once it has crossed the shear layer, so the airspeed gained by crossing the boundary is not all retained through the 180° turns. Higher speed gives rise to higher drag forces, so there is a maximum speed that can be attained, typically around 10 times the wind speed for efficient glider designs.
......
相关推荐: