Investigation Of Synthetic Jet Flow Control Parameters For The Mitigation Of Laminar Boundary Layer Separation

Investigation of Synthetic Jet Flow Control Parameters for the Mitigation of Laminar Boundary Layer Separation

An experimental study was performed to elucidate the effects of forcing parameters on the mitigation of boundary layer separation on an airfoil at low Reynolds number. Post- stall flow at a Reynolds number of 100,000 and angle-of-attack 12 degrees on a NACA 0025 airfoil served as the baseline for control with a synthetic jet actuator. This baseline flow is characterized by two dominant instabilities: the large scale vortex shedding in the wake of the airfoil, and the roll-up of vortices in the separated shear layer. The forcing parameters that were investigated were the blowing ratio, excitation frequency, and the chordwise forcing location. The results concerning the effects on aerodynamic performance showed that for both drag reduction and lift increase, the benefits of control saturated with increasing blow- ing ratio. Initial improvements to lift and drag were due to the formation of a laminar separation bubble, followed by fully attached flow once a threshold blowing ratio was met. Positioning the slot at the most upstream location resulted in the lowest thresh- old blowing ratio and produced the largest lift-to-drag ratios. A monotonic increase in threshold blowing ratio and decrease in lift-to-drag was observed as the slot location moved downstream. It was also found that while forcing at a frequency corresponding to the wake instability led to maximum lift increase, forcing in the range of the separated shear layer instability led to maximum drag reduction. High-frequency forcing, where the time scales of control are much smaller than those of the flow, was found to be least effective for improving performance. The controlled flow dynamics revealed the presence of large vortices passing over the suction surface and highly unsteady flow when forcing at the wake instability frequency, whereas forcing in the range of the shear layer instability led to the production of a larger number of much smaller vortices. The latter case led to a thinner boundary layer in the time-averaged sense. Extraction of coherent and turbulent velocity fluctuations showed that the controlled flow was steady in time with high-frequency forcing.

Recent Developments in Aerodynamics

Aerodynamics is a branch of fluid mechanics that deals with the motion of air and other gaseous fluids and the forces acting on bodies in motion relative to such fluids. Aerodynamics are classified according to Mach number into incompressible subsonic, compressible subsonic, transonic, supersonic and hypersonic aerodynamics. Aerodynamics can be divided into different forms of internal or external aerodynamics. For example, the study of flow properties inside a gas turbine or rocket engine is an example of internal aerodynamics that is important in accurately estimating thrust force. Examples of external aerodynamics include the flow around the body of airplanes, helicopters, space capsules, cars, rockets and missiles, trains, ships, wind turbines, and even such structures as bridges and tall buildings, which often have to withstand strong winds. Since the first human flights until today, aerodynamics has been growing and developing rapidly. Aerodynamics is one of the applied sciences in engineering that pursues several goals such as estimating forces, moments on objects moving in the air, or estimating heat transfer from these objects. Aerodynamics investigates how gases interact with moving bodies. The main purpose of aerodynamics in most cases is to reduce drag forces and increase lift force, which can lead to less fuel consumption in vehicles and achieving the greatest speed. Of course, in some special cases, such as hypersonic aerodynamics, there is a greater interest in increasing drag to reduce the adverse effects of aerodynamic heating. Another goal of studying aerodynamics is to obtain the details of the flow around flying objects. For example, the correct operation of a control rudder or flap in a supersonic aircraft, or the recognition of the phenomenon of blackout during spacecraft reentry, which occurs under the influence of aerothermodynamic heating and plasma formed around the antenna, is of great importance.

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