Drag Reduction with High-Frequency Repetitive Side-On Laser Pulse Energy Depositions

Abstract

The present study addresses the drag reduction due to the repetitive laser induced energy deposition over a flat-nosed cylinder. Irradiated laser pulses are focused by a convex lens installed in side of the in-draft wind tunnel of Mach 1.94. The maximum frequency and power of the energy deposition is limited up to 50 kHz and 400 W. Time-averaged drag force is measured using a low friction piston which was backed by a load cell in a cavity as a controlled pressure. Stagnation pressure history, which is measured at the nose of the model, is synchronized with corresponding sequential Schlieren images. Amount of drag reduction is linearly increased with input laser power. The power gain only depends upon the pulse energy. A drag reduction about 21% which corresponds to power gain of energy deposition of approximately 10 was obtained. n the past decade, advanced technology using energy deposition, which is produced by laser beam, micro wave or electric spark has been suggested actively to attempt the flow control. The energy deposition technique is applied mainly in fluid engineering filed such as a modification of shock structure, active control of boundary layer, lift enhancement and wave drag reduction. Of these local flow control techniques using energy deposition, this study is contributed to reduce the wave drag of supersonic flight. So far, various approaches to develop drag reduction technologies have been explored since the beginning of high-speed aerodynamics in order to realize economic supersonic flight. Aerodynamic drag force can be classified into friction and pressure drag. Friction drag is determined entirely by state boundary layer, and does not change greatly between subsonic and supersonic flight. However, pressure drag rapidly increases near the transonic flow due to shock wave generated by the aircraft body. Drag force due to shock wave is called wave drag. Shock waves have been a detriment for the development of supersonic aircrafts, which have to overcome high wave drag and surface heating from additional friction. The design for high-speed aircraft tends to choose slender shapes to reduce the drag and cooling requirements. Although this profile is adequate for fighter planes and missiles, it becomes engineering tradeoff between volumetric and fuel consumption efficiencies. In particular, this tradeoff significantly increases at the operating condition of commercial supersonic aircraft, which is preferred to be widebody capable of carrying hundreds of people. Since structure change of flight body for drag reduction reaches the limit, possibility of energy deposition technique was proposed to modify further aerodynamic performance. The intensive plasma generated by laser beam focusing is useful to control the supersonic flow field. When the laser energy is deposited into the oblique shock wave, blast waves and residual hot-spot interacts with bow shock wave in front of supersonic flight. Thereafter, plasma is transmitted to shock wave, and vortex is generated by well known baroclinic effects. This energy deposition technique has received much attention recently, and related investigations have been much conducted. Knight 1 characterizes the energy deposition scheme by using a deposited energy, pulse duration and pulse interval in respective dimensionless forms. If the pulse interval is long enough, flow after a pulse energy deposition is independent from previous pulses. Several numerical studies were conducted to examine the drag reduction due to energy deposition over bluntbody. Riggins et al. 2 investigated the drag reduction with the help of a computational fluid dynamics method using 2-D, axisymmetric Navier-Stokes equations. According to their study, drag force was reduced to value as low as