Drag Coefficient Prediction Chapter 1 （阻力系数预测第1章）.pdf

1 Drag Coefficient Prediction Chapter 1 The ideal force acting on a surface positioned perpendicular to the airflow is equal to a dynamic pressure, denoted by ‘q ’, times the area of that surface. Dynamic pressure is one-half the square of the flow velocity times the density of the fluid. In equation form this is: 1 2 q r V 2 Where, 2 4 r Air density, lb .-sec / ft V Flow velocity, ft/sec Imagine putting your hand or a flat plate out the window of a car such that the flat surface is positioned normal to the airflow. The total force required to hold it in position when meeting the oncoming airflow will be approximately equal to that defined above. Realistically, the actual force is d ependent on the shape of the object, and the 3-dimensional flow characteristics of the fluid (i.e.; flow relief, turbulent and/or laminar flow, etc.). Often these influences are summed up in a single coefficient known as an aerodynamic coefficient. In our particular case, we are interested in a drag force ‘D’, and likewise a drag coefficient ‘CD ’. Depending on equation formulation and reference area, C can take on different values. For D rockets, C is typically based on the rocket’s maximum cross-section area. Most rockets are circular in D cross -section, therefore its cross-section area is described by the equation for an area of a circle. In general, the equation for drag ‘D ’ is given by: D qC