Experimental investigation of boundary layer transition via vortex generators

Abstract

The structure and properties of the flow is an extremely important issue for engineers and system designers working on a flow system. Knowing the flow details in the boundary layer of many structures in aviation technology is important for solving many problems in aerodynamics. These problems are skin friction drag of an object, the development of a wing stall, the heat transfer that occurs in high velocity flight, and the performance of a high velocity airplane entry. An object's boundary layer begins as laminar, and when the critical angle of attack is exceeded in aircraft wings, the airflow becomes turbulent and the lift decreases rapidly. Free stream turbulence and pressure gradient are thought to be the most effective parameters in boundary layer transition from laminar state to turbulent state. There are many passive and active control methods to prevent or delay the transition in the boundary layer, one of them is vortex generators. Vortex Generators have a mechanism that prevents or delays boundary layer separations under the effect of reverse pressure gradient by increasing the energy of the boundary layer with the vortices they produce. The vortex generator increases the energy level of the layer by transporting energy from the outer flow to the boundary layer, thereby increasing the performance of many vehicles. Vortex generators are frequently used in the aerodynamic design of many land and aircraft because they can significantly prevent flow separation. Vortex generators are a common flow tool, especially in aviation technology, because they improve flow characteristics and most importantly save energy. In this thesis, the transition from laminar to turbulent flow in the boundary layer by means of vortex generators is experimentally investigated. The purpose of using vortex generators in this study is to explain in detail how they affect the flow in the boundary layer. Within the scope of the thesis, first of all, literature research was carried out about the vortex generators and the transition from laminar to turbulent flow in the boundary layer. In line with the researches, a spherical ball with a diameter of 1 mm was chosen as the vortex generator. The studies were carried out in the Trisonic Research Laboratory of the Faculty of Aeronautics and Astronautics of Istanbul Technical University. Experiments were carried out on a modular air flow bench (AF10) experimental setup using a 27 cm long brass flat plate. AF10 consists of a small scale wind tunnel with adjustable airflow control and an electric fan. A flattened Pitot tube measures the total pressure at various distances from the plate surface by positioning the Pitot tube using a micrometer. The results are obtained by connecting the Pitot tube to the Setra brand model 239 pressure transducer. This transducer response time is lower than 10ms. During data acquisition mean value of 10000 data is used for each measurement point. Additionally, A hot wire anemometer was used for velocity measurements, another experimental setup. Dantec StreamLine system was used for speed measurement. The 55P15 boundary layer type probe was used in the study. This probe is a constant temperature probe designed for use in boundary layers and is used via a probe support. The probe needs to be calibrated prior to experiments. Velocity profiles were measured at various locations on the surface. Experiments with and without a vortex generator were performed and the boundary layer flow under investigation was subjected to a positive pressure gradient and these experiments were compared. Experiments were carried out for 10 different ratios in the range of vortex generator height divided by boundary layer thickness, that is, the ratio h/δ, between 0.4 and 0.8. According to the results of the experiments for flow velocities 8,9,10,12 and 15 m/s without the vortex generator, it was determined that the flow in the boundary layer is laminar since the boundary layer thickness values found overlap with the Blasius solution in many locations. After positioning the vortex generators, it was determined that the flow of VGs kept the flow laminar for 4 or 5 cm and then the flow transitioned. It was determined that the positive pressure gradient effect increased the boundary layer thickness in the experiments. It was also found that this effect makes the flow turbulent faster in experiments with vortex generators.Finally, where the vortex generators will be placed on the plate is a very important issue which determines transition onset and transition length.