Yasuto Sunada

Experimental Research on Blunt Trailing-Edge Airfoil Sections at Low Reynolds Numbers

The wind-tunnel test results of five airfoil sections with 28.57% chord thickness at chord Reynolds number of 33,000, 66,000, and 99,000 are presented. Lift and drag forces have been measured and flow visualizations have been made. It is shown by cutting off the trailing edge of the section and by making its trailing edge blunt at low Reynolds numbers that the total drag can be reduced, the maximum lift increased, the linearity of lift curve with incidences improved, and the maximum lift-to-drag ratio increased. The structural merits of blunt trailing-edge sections are also discussed. Nomenclature C = chord length, 84 mm CD = drag coefficient CL = lift coefficient Cvir = chord length of the original section Re = chord Reynolds number, UC/v t = maximum thickness, 24 mm U = wind-tunnel freestream velocity ot = angle of attack, deg

Airfoil Stall Suppression by Use of a Bubble Burst Control Plate

To suppress the stall on an NACA 0012 airfoil, a thin plate (hereafter referred to as a burst control plate) was attached on the airfoil to delay the burst of laminar separation bubbles formed at the stall angle. The burst control plate is used to enhance the vortical structures in the separated shear layer. Wind-tunnel tests were conducted at a chord Reynolds number of 1.3 x 10 5 . Flow visualization tests and surface pressure measurements showed that the burst control plate placed on the airfoil suppresses the bubble burst at a wide range of angle of attack and therefore, both the stall angle and the maximum lift coefficient are increased. The particle image velocimetry measurements indicated that when the angle of attack of the airfoil with the burst control plate is set higher than the stall angle of the original airfoil, a similar flow structure of the original airfoil was observed.

Surface pressure distributions on 4% circular arc airfoil at low Reynolds number

V ERY small aircraft, called micro air vehicles (MAVs), are of high interest because electronic equipment can be miniaturized to allow for the easy manufacture of a vehicle whose entire mass is only a few dozen grams. Small-sized MAVs operate at chord Reynolds numbers below 1:0 10. Because of the influence of viscosity, thin and sharp leading edge airfoils with thickness ratios less than about 5%offer better aerodynamic characteristics than thick and blunt edge airfoils with Reynolds numbers lower than 1:0 10 [1]. Mueller has measured the aerodynamic forces acting on a circular arc airfoil for MAVs; however, compared with thick and blunt edge airfoils, very little literature is available for thin and sharp leading-edge airfoils [1,2], which indicates that more research needs to be done for the low Reynolds number region. Cosyn and Vierendeels [3] numerically studied the low Reynolds number aerodynamics of a flat plate and an S5010 airfoil, which is an airfoil with a 10% thickness ratio. They have also pointed out that low Reynolds number flows exhibit complex flow phenomena, such as laminar separation, which was described by Mueller and DeLurier [4]. For the design and manufacture of MAVs, it is important to know the details of their aerodynamic characteristics, such as surface pressure distribution. However, there is no space inside of the circular arc airfoil for plumbing the pipes from the static pressure port to the pressure transducer. Thus, as far as the authors know, there are no experimental data measuring surface pressure distributions over a circular arc airfoil except for [2], which used luminescent, pressuresensitive paint. In this study, to measure the surface static pressure of the circular arc airfoil, wemade a 4% cambered-airfoil sectionmodel with a 1% thickness ratio by soldering copper pipes. We also conducted surface flow visualizations using the oil flow technique. The present experimental results at a chord Reynolds number of Re 62; 000 are expected to provide useful information for understanding the flowfield of the circular arc airfoil at a low Reynolds number aswell as for confirming the accuracy of numerical estimations concerning a circular arc airfoil.

Three-Dimensional Separated Flow on a Flat Plate with Leading-Edge Serrations

Serrations, which imitate the sawteeth-like structures in the leading edge of owl wing, enable to reduce noises and alter the aerodynamic performance. However, the mechanism is not yet completely understood because the flow fields have three-dimensionality. In this study, in order to investigate three-dimensional mean flow structures on a flat plate with leadingedge serrations, flow fields viewed from the spanwise and vertical directions were measured by two-dimensional time-resolved PIV and stereo PIV at the Reynolds number of 5.0×10. When the angle of attack was 6 degree, the interval of the flow pattern was equal to the width of one sawtooth of the serrations. Whereas the flow over the trough was separated, and the flow over the vertex passed along the surface of the flat plate. This is because pairs of longitudinal vortices induced by the serrations suppress the flow over the vertex on the uppersurface of the flat plate. When the angle of attack was 14 degree, several flows over the vertexes were gathered, and specific flow patterns were formed. The flow over the vertex passed along the surface of the flat plate, so that separation points viewed from the spanwise direction moved to the downstream side. Reverse-flow regions behind the troughs could be divided into two categories, “open” and “closed” ones. Pairs of longitudinal vortices were seen in the closed reverse-flow regions only, and growth of the open reverse-flow regions forces the vortices to move upwards.

Airfoil Stall Suppression by Use of Rectangular Cross Section Burst Control Plates

Based on our previous experimental works conducted on airfoil stall suppression using a burst control plate, wind tunnel experiments have been carried out to deepen the understandings and to improve the performance of this device. Surface pressure measurements, flow visualizations and instantaneous velocity measurements were made for an NACA 0012 airfoil at a chord Reynolds number of 1.3x10 5 . The burst control plate is a device that enhances vortical structures formed inside the laminar separation bubble and suppresses bubble burst. Hence, it can accomplish airfoil stall suppression. Here, rectangular cross section burst control plates with different widths and combinations were tested. The results indicate that the rectangular cross section burst control plate is an effective device in the suppression of airfoil stall. It performs better than the previously tested burst control thin plate as long as the chordwise leading-edge and trailing-edge positions of the plate are carefully selected.

Experimental and Numerical Research on Aerodynamic Characteristics of Rectangular Fin Mounted Vertically over the Wing

The purpose of this paper is to clarify the aerodynamic effect of rectangular fins vertically mounted over the main wing. First, a Computational Fluid Dynamics (CFD) analysis was performed to check the potential aerodynamic improvement of the fins, and then further investigation of the fin characteristics was performed through a wind tunnel testing. In order to test many configurations with the fins, the main wing was divided into blocks. Each block was manufactured using a 3D printer. The design variables for the fin were the spanwise location and the fin angle. The lift, drag forces and the pitching moment were measured varying the angle-of-attack (AoA) of a model aircraft. The interference between the fins and the main wing was investigated. Additionally, the Oswald efficiency factor of the aircraft was obtained in order to evaluate the aerodynamic performance. By selecting the optimum fin angle, the Oswald efficiency factor can be improved significantly.

Airfoil Stall Suppression Using Rectangular Bubble Burst Control Plate and Sensor Actuators

A burst control plate attached to the leading-edge of an airfoil is a device that enhances vortical structures formed inside the laminar separation bubble and can suppress bubble burst. In our previous work, a thin plate type burst control plate was incorporated into a sensor actuator system to work as an autonomous airfoil stall suppression system. In this paper, a rectangular cross section burst control plate, which performs better than the thin plate type, was combined with the modified sensor actuator system to demonstrate that our system works effectively for stall angle delay, lift enhancement and drag reduction. The sensor actuator system uses a pressure transducer and a feedback controller to operate and monitor the actuators in an autonomous mode. Experimental studies of surface pressure and force measurements were conducted for an NACA 0012 airfoil at a chord Reynolds number of 1.3x10 5 . In the first section, the system was operated manually. Results indicated that the present manually operated system is an effective device to suppress airfoil stall. Second, a feedback control method for the system was modified and constructed. Finally, the system was operated in the autonomous mode and checked the benefit of the system. It was demonstrated that the present combination of a sensor actuator system and the rectangular cross section burst control plate is an effective means of bubble burst control and airfoil stall suppression.