CHAPTER 1 INTRODUCTION Wind tunnel is a large tube with air moving inside which is used to copy the actions of an object in flight. In wind tunnel air moves around an object, so the object seems like really flying. Although the form of a wind tunnel can vary, all wind tunnels have a high pressure system, heater, settling chamber and a test section. Air is compressed and stored in the high pressure system. It is released through a pressure regulating valve to create the desired pressure in the settling chamber. In high speed wind tunnels, heater is used to heat the air while passing through the heater bed to avoid liquefaction when it is expanded through the nozzle to get high Mach numbers. Scaled models of aircraft or space craft are placed in the test section for testing as shown in Figure 1.1. Some wind tunnels are big enough to hold full- scale versions of vehicles. Figure 1.1: Wind tunnel with the model of a plane. (Courtesy : Wikipedia) 1
Air is blown or sucked through a duct equipped with a viewing port and instrumentation where models or geometrical shapes are mounted for study. The movement of air through the tunnel is done by using a series of fans, which are usually powered by stationary turbo rather than electric motors. Vertical and horizontal air vanes make the turbulent air flow smooth, before reaching the test section. The cross section of the tunnel is circular to provide a smoother air flow. The smooth inner surface reduces the surface drag. The circular walls of the tunnel are usually embedded with light, which shines through the windows. Pressure taps are included in the model for measuring pressure. Lift, drag, yaw, roll, lateral forces and pitching moments over a range of angle of attack can be measured by a force balance mounted on the model. The movement of air around the model is difficult to observe directly, since air is transparent. Different flow visualization methods have been developed for testing in a wind tunnel. Tufts attached to the model during testing can be used to gauge air flow. Another method is by using evaporating suspensions, where the liquid evaporates leaving behind the clay in the pattern characteristic of the air flow. Applying oil to the surface of the model can show the transition from laminar to turbulent flow as well as flow separation. Smoke or bubbles of liquid can be introduced into the airflow upstream of the test model, so that their path around the model can be photographed. 1.1 TYPES OF WIND TUNNEL Wind tunnels can be classified based on air flow speed in test section and based on shape. Flow speed in wind tunnel is generally referred in terms of Mach number. Mach number is a dimensionless quantity representing the ratio of speed of an object moving through a medium and the local speed of 2
sound. Based on flow speed, wind tunnels are classified as Subsonic, Transonic, Supersonic and Hypersonic. In Subsonic or low speed wind tunnels flow speed in terms of Mach number comes out to be around 0.4. These types of wind tunnels are most cost effective due to the simplicity of the design and low wind speed. Generally low speed wind tunnels are used in schools and universities because of low budget. Maximum velocity in test section of transonic wind tunnels can reach up to speed of sound i.e. 340m/s or Mach number of 1. They are very common in aircraft industry as most aircrafts operate around this speed. Velocity of air in test section of Supersonic wind tunnels wind tunnels can be up to Mach 5. This is accomplished using convergent or divergent nozzles and the power requirements for such wind tunnels are very high. Hypersonic wind tunnels can have wind velocity in test section between Mach 5 and Mach 15. This is also achieved using convergent - divergent nozzles. They are used to test ultra fast air craft and space vehicles. The technological problems in designing and constructing a hyper-velocity wind tunnel are supply of high temperatures and pressures for times long enough to perform a measurement, reproduction of equilibrium conditions, structural damage produced by overheating, fast instrumentation and power requirements to run the tunnel. Based on shape, wind tunnels are classified as Open circuit wind tunnel and Closed circuit wind tunnel. Open circuit wind tunnel is open at both ends. The chances of dirt particles entering with air are more and so more honeycombs (mesh to clean incoming air) are required for cleaning the air. Open type wind tunnels can further be divided into two categories: Suckdown tunnel and Blower tunnel. In Suckdown tunnel, the inlet open to atmosphere and the axial fan or centrifugal blower is connected after the test 3
section. These types of wind tunnels are not preferred because incoming air enters with significant swirl. In blower tunnel a blower is installed at the inlet of wind tunnel which throws the air into wind tunnel. For supersonic and hypersonic type wind tunnel, the desired speed cannot be attained with the help of a blower. Hence a tank with high air pressure is maintained and the flow is obtained by releasing the air from this high pressure system. Outlet of closed circuit wind tunnel is connected to inlet and so the same air circulates in the system in a regulated way. The chances of dirt entering the system are very low. Closed wind tunnels have more uniform flow compared to open type. Closed tunnel is usually a choice for large wind tunnels as these are more costly than open type wind tunnels. 1.2 RELEVANCE OF WIND TUNNEL One of the main aims of using a wind tunnel is to learn more about air planes and how things move through air and thereby improving air transportation. It helps scientists to make aircraft better and safer. New materials or shapes for airplane parts can be tested before flying in a wind tunnel to make it sure it will fly as it should. Wind tunnels are also used to test spacecraft and rockets, which are made to operate in space, where there is no atmosphere. They have to travel through the atmosphere before entering the space. Vehicles that take humans into space also must come back through the atmosphere to Earth. The wind tunnel is a major money saver. It allows us to make a reusable prototype and test it in the tunnel at low cost. Wind tunnels are important in making the new Ares rockets and Orion space shuttle which are vehicles that will take astronauts into space. Ares 4
rockets were planned for using to launch Orion, which is the spacecraft intended for NASA human spaceflight missions. NASA engineers tested ideas for the design of Ares in wind tunnels, to see how well Ares would fly. Engineers tested Orion models to know what would happen to different designs when the spacecraft came back through the atmosphere. Wind tunnels can even help engineers design spacecraft to work on other worlds. Mars has a thin atmosphere and it is important to know what the Martian atmosphere will do to vehicles that are landing there. Parachutes and spacecraft designs are tested in wind tunnels set up to be like the Martian atmosphere. In modern automotive design, the aerodynamic design plays an important role, which determines vehicle s fuel consumption, stability, engine cooling, interior cabin noise and windshield-wiper performance. Wind tunnel simulations allow engineers to study these aerodynamic loads during a newly designed vehicle s development process. Wind tunnels are used in Wind Engineering, to measure the velocity around and pressure or forces upon structures. Specialized atmospheric boundary layer wind tunnels are used to analyze the wind speed and turbulence profile acting on very tall buildings, buildings with complicated or unusual shapes like a hyperbolic or parabolic shape, cable suspension bridges or cable stayed bridges. Aerodynamics are important in almost every sports ie, from ball games like golf, baseball, soccer, football and tennis to athletics, alpine skiing, crosscountry skiing, ski jumping, cycling, motor sport and many others, where the performance results in the optimal motion of the athlete (multi-jointed mechanical system) and/or is equipment (solid system) in the air. Athletes can 5
assess the aerodynamic efficiency of the motor task performed by him with accuracy and in almost real time in a wind tunnel, which help them to focus on specific aspects of his technical behavior to improve his performance. Thus wind tunnels are an excellent example of a technological innovation that supports aircraft and space craft design. They are not simply a tube or box with a fan blowing air but are carefully designed and constructed (and often very large) laboratory instruments. Without constant research in wind tunnel technology, aeronautical research would have ground to a standstill decades ago. 1.3 NEED FOR PROPER CONTROLLER IN A WIND TUNNEL The wind tunnel in this thesis is Hypersonic Blowdown type, which can hit hypersonic speeds of up to 12 Mach numbers. Here there is a high pressure system, from which air is released using a pressure regulating valve, to get the desired flow in the test section. To get the hypersonic speed in the test section, the mass flow rate of the air in the test section has to be maintained constant. Mass flow rate is the mass of a substance which passes through a given surface per unit of time. To attain constant mass flow rate, the pressure in the settling chamber, whose output is given to the test section has to be maintained constant. This can be achieved by controlling the opening of pressure regulating valve. So it is needed to design a controller, which will regulate the operation of the control valve, to get a constant pressure in the settling chamber. The controller has to act within few seconds, since the system exists for very short duration. To obtain accurate, stable air flow and to provide proper operation of the HWT system an efficient controller is required. As the system behavior is changing with time, the controller has to adapt itself to the changes in 6
operating environment. PI or PID controllers (conventional controllers) are suitable for linear systems. For nonlinear systems, we can use conventional controllers only if the operating range is limited. It will give satisfactory response only at the point of linearization. Its performance degrades as the operating point moves away from the point of linearization. Artificial Neural Network based adaptive controllers are not suitable as they take more time to learn the system behavior through weight updation. Scheduled adaptive controllers are not practicable in this case since scheduling the control by forming a lookup table is tedious. As the system behavior is changing from time to time, a perfect lookup table can only be constructed with infinite entries. Self tuning regulators (STR) are based on a parameter estimator and its performance heavily depends on the robustness of the estimation algorithm used. Model reference adaptive controllers can be a reasonable solution in such a situation. But its performance also depends on the reference model used. If we incorporate fast adaptation techniques with conventional controllers it can be a reasonable solution. 1.4 OBJECTIVES The main objectives of this thesis work are To simulate the operation of a Hypersonic Wind Tunnel (HWT) and to study its behavior and non linearity. To design and implement different controllers for the process which satisfies the following requirements. o To bring the pressure in the settling chamber of a HWT to the desired value in less than 10 sec. o To increase the test duration as longer as possible. 7
o To make the overshoot of the system less than 20%. o To minimize the error during test time. o To reduce the effect of external disturbance as low as possible in terms of error and settling time. And finally, to suggest a controller which meets all/ most of the above requirements for the HWT. 1.5 METHODOLOGY Several procedural steps are followed to achieve the objective of this research work. Initially the mathematical model of the HWT is simulated using the simulink tool in MATLab. The model developed is validated and nonlinearity of the system is analyzed. Further, different non linear controllers for the process are designed and simulated, for different set points. Performances of all these controllers were evaluated through the servo and regulator operation. A detailed analysis of the performance parameters were done for all these different controllers and finally, the optimum controller for the process is suggested. Figure1.2. The framework for solving the problem defined above is shown in 8
Figure 1.2 Block diagram of proposed work 1.6 ORGANIZATION OF THE THESIS The rest of the thesis is organized as follows. A comprehensive review of literature on the wind tunnel and various controllers are presented in Chapter 2. The literature survey on historical background of wind tunnel is also included in this chapter. 9
Chapter 3 provides the detailed information on the system model of hypersonic wind tunnel along with the validity of the model and its analysis of nonlinearity. In this chapter the need for controller and its analysis are also discussed. Chapter 4 describes the PI controller design methodology for Hypersonic Wind Tunnel. The performance evaluation of the PI controller is done and is presented along with the simulation results. Chapter 5 discusses the application of Fuzzy technique for assisting the PI controller. Performance analysis and evaluation of the controller is also made here. Chapter 6 covers the Backstepping controller design techniques for regulating pressure in a HWT. The techniques discussed in this chapter are the tracking and stabilizing designs of Backstepping controller, and also the combination of Backstepping with Fuzzy controller. The servo and regulator operation of the controller is simulated and the results are tabulated for performance evaluation. Chapter 7 contains the design and evaluation of the single stage and two stage Cascade controllers. The chapter summarizes the performance of two stage cascade controller. Chapter 8 presents performance analysis of the Model Reference Fuzzy Adaptive PI Controller along with Model Reference Fuzzy Cascade Controller. 10
The performance evaluations of these controllers were done by applying servo and regulator operations for different set points. The simulation results of each controller are analyzed in the respective chapters. The chapter 9 gives the comparative analysis of the controller s performance, summary of work done and the contribution of the work. 11