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Opposition to movement is a multi-fold process The surfaces of encounter between the swimmer and the fluid provide resistance to efficient motion Energy is sucked off as fluids change from laminar to turbulent flow Flow may also become diverted inward creating rotations called vortices that project downstream from the swimmer Drag depends on inertia the tendency to stay put Inertia is proportional to mass Fluid density or mass per unit volume is the important factor for our discussion Drag comes in two important varieties Form drag is determined by shape Friction drag is determined by the nature of the surface Friction is discussed in terms of viscosity or how readily one layer slips past another An important concept in the study of aerodynamics concerns the idea of streamlines A streamline is a path traced out by a massless particle as it moves with the flow It is easiest to visualize a streamline if we move along with the body as opposed to moving with the flow The figure shows the computed streamlines around an airfoil The flow proceeds from left to right Since the streamline is traced out by a moving particle at every point along the path the velocity is tangent to the path Since there is no normal component of the velocity along the path mass cannot cross a streamline The mass contained between any two streamlines remains the same throughout the flowfield We can use Bernoullis equation to relate the pressure and velocity along the streamline Since no mass passes through the surface of the airfoil or cylinder the surface of the object is a streamline The lower the viscosity the thinner the stationary layer The thickness of the stationary layer also varies with speed The faster the movement the thinner the stationary layer Because of this relative thickness of stationary layer how smooth an object must be to reduce friction depends on its speed The faster it
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