Aerodynamics is the study of moving of fluids, mainly air flows, and its possible effects on the motion of surrounding solid objects. It qualifies the appearance of a moving body in the air and its resistance to advancement. It is not a terminology limited to the phenomena occurring in aviation but present everywhere in our daily life. It can be found not only in vehicles (e.g. automobiles, bicycles, railways) and building and construction (e.g. skyscrapers, bridges), but also in propulsion systems (e.g. propellers, rotors, turbines, reactors) and renewable energy production (e.g. wind turbines).
Since the dawn of time, humans have sought to create, invent and improve tools and techniques that are vital for survival. With the advancement of technology, the quest for competitivity and ecology lead the scientists to find the best parameter that can enhance the performance of current apparatus.
The drag is the impedance that prevents humans from reaching optimal performance and thus is often studied to ameliorate its negative effects. Athletes, designers and scientists are all unanimous on devising means to break world records, to be more efficient or even to become more ecological by reducing drag.
In fluid dynamics, drag crisis or Eiffel Paradox of a two- or three-dimensional object is a very counterintuitive phenomenon where the drag coefficient of the body decreases drastically while the velocity of flow increases. This phenomenon has been investigated using the models of simple shapes such as spheres and cylinders.
Unfortunately, sometimes many parameters are given and cannot be changed. this is how the roughness makes its entrance. Scientists have already shown in the past that roughness can be beneficial on bluff body to a given Reynolds number range.
In the current research, a teardrop-shaped foil which is a combination of a cylinder and a symmetry airfoil was used in a range of Reynolds number 3 x 104 < Re < 8 x 104. All the experiments were carried out at the National Cheng Kung University (NCKU, Taiwan) in an open jet tunnel. Initially, a drag measurement experiment identified the Re range of the phenomenon with the angle of attack (AOA) = 0°.
The investigation has been done with three different cases (smooth teardrop and two different sample) to study the effect of textile roughness on the aerodynamic flow around the teardrop. The first one was the investigation of drag-lift and mean pressure over a smooth teardrop. Subsequently, the same experiment was conducted with the teardrop wrapped with two different samples of textile. A laser scanning microscope has been also used to determine the roughness of each textile sample.

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