Reducing the drag coefficient in an automobile improves the performance of the vehicle as it pertains to speed and fuel efficiency. The correct answer is A, “viscosity.” Is a higher drag coefficient better?Īerodynamic drag increases with the square of speed therefore it becomes critically important at higher speeds. What is Reynolds number affected by?Īlthough friction factor and temperature do affect inertial forces and viscosity to some degree, and therefore, Reynolds number, viscosity has the most direct effect upon the value of the Reynolds Number. In aerodynamics, flow separation results in reduced lift and increased pressure drag, caused by the pressure differential between the front and rear surfaces of the object. The fluid exerts a constant pressure on the surface once it has separated instead of a continually increasing pressure if still attached. And since a turbulent boundary layer has more energy to oppose an adverse pressure gradient, engineers often force the boundary layer to turn turbulent over fuselages to reduce overall drag.
#Reynolds number airfoil skin
Pressure drag is more significant than skin friction drag on large bodies – like your fuselage and nacelles. Lift decreases and drag increases when Reynolds number is decreased. (1)Reynolds number plays a very major role in the airfoil aerodynamics for the NACA0012 airfoil without and with Gurney flap. It is taken into account via the sphericity coefficient, which is the ratio of the sphere surface area of the same volume as the body relative to the body’s surface area. Drag coefficient is strongly affected by a body’s shape. Is drag coefficient a function of Reynolds number?ĭrag coefficient for cylinders (1) and spheres (2) as a function of Reynolds number (Re). For a given frontal area and velocity, a streamlined body will always have a lower resistance than a bluff body. For a bluff body, the dominant source of drag is pressure drag. What type of drag force is more dominant for a blunt body eg sphere and a streamlined slender body eg airfoil )? Explain why?įor streamlined bodies, frictional drag is the dominant source of air resistance. That is, the inertial forces are negligible. The drag coefficient decreases at low Reynolds numbers because the flow is in a regime called creeping flow. Why does drag coefficient decrease with Reynolds number? Between Re = 1000 and Re = 200000, the point of separation remains very near the equator so the drag coefficient remains approximately constant at CD = 0.4. Beyond a Reynolds number of 1000, the skin friction is negligible. How is drag coefficient related to Reynolds number?Īs the Reynolds number is increased, the form drag increases relative to the skin fric- tion. Why are cylinders and spheres considered bluff bodies?.
#Reynolds number airfoil how to
How to study the effect of shape on Drag?.Is the drag coefficient of a turbulent flow the same?.How does the Reynolds number affect the drag coefficient?.Does drag increase with Reynolds number?.Is drag coefficient a function of Reynolds number?.What type of drag force is more dominant for a blunt body eg sphere and a streamlined slender body eg airfoil )? Explain why?.Why does drag coefficient decrease with Reynolds number?.How is drag coefficient related to Reynolds number?.The results from PIV and smoke flow visualization showed that the flow stayed fully attached to the airfoil surface from Re as low as 56,000 at an angle of attack of 8° and maintained a fully attached flow up to 14° angle of attack for Re as low as 75,000. The lift coefficient increased from 0.41 to 1.05 at Re = 38,000 in the α range of 0–18°, in which no stalling was documented. At the Reynolds numbers of 75,000, 128,000 and 205,000, maximum lift coefficients of 1.72, 1.81 and 1.86 respectively were obtained at the stall angle of 14°. Particle Image Velocimetry (PIV) together with smoke flow visualization were used to study the flow around the airfoil. A CFD analysis was also performed to get additional information on the flow characteristics. Pressure distributions were obtained over the surface of the airfoil and the lift and drag forces were measured with a dynamometer at different angles of attack, α. Experiments were performed on the improved airfoil (AF300) in an open circuit wind tunnel at Reynolds numbers of 38,000, 75,000, 128,000 and 205,000. A low Reynolds number airfoil was designed for applications in small horizontal axis wind turbines to achieve better startup and low wind speed performances.