AERODYNAMICS AND AEROELASTICITY Lecture : Introduction to Selected Research Topics in Mechanical Engineering- I What Is Aerodynamics? Aerodynamics is the study of forces and the resulting motion of objects through the air [1]. Anything that moves through air reacts to aerodynamics. The rules of aerodynamics explain how an airplane is able to fly. A rocket blasting off the launch pad and a kite in the sky react to aerodynamics. Aerodynamics even acts on cars, since air flows around cars. [2] Figures from ref. [3] 2 The generation of force (lift and drag) and moment on airfoils, wings, fuselages, engine nacelles and most importantly whole airplane configurations. Estimation of the wind force on building, ships, and other vehicles. The generation of hydrodynamic forces on surface ships, submarines and torpedoes. Calculation of aerodynamic heating of flight vehicles. Calculation of the flow properties inside rocket and air breathing jet engines and also calculation of the engine thrust. [4] 3 Aerodynamic forces and moments on the body are due to only two basic sources [4] : 1. Pressure distribution over the body 2. Shear stress distribution over the body surface Bodies subjected to fluid flow classified as streamlined and blunt as depending on its shape [3] . Blunt body = a body where most of the drag is pressure drag. Streamlined body= a body where most of the drag is skin friction drag. [4] Figures from ref. [2] 4 What Is Lift? Lift is the push that lets something move up. It is the force that is the opposite of weight. Everything that flies must have lift. For an aircraft to move upward, it must have more lift than weight. A hot air balloon has lift because the hot air inside is lighter than the air around it. Hot air rises and carries the balloon with it. A helicopter's lift comes from the rotor blades. Their motion through the air moves the helicopter upward. Lift for an airplane comes from its wings. [2] What Is Drag? Drag is a force that pulls back on something trying to move. Drag provides resistance, making it hard to move. For example, it is more difficult to walk or run through water than through air. Water causes more drag than air. The shape of an object also affects the amount of drag [2]. Drag is usually an undesirable effect like friction and we always want to minimize it. Because reduction of drag is closely related with reduction of fuel consumption in automobiles and aircraft [3]. 5 To reduce drag: Submarines mimic large fish. Swimmers with long hair cover their head with a tight and smooth cover to reduce head drag. They also wear well-fitting one-piece swimming suits. Horse and bicycle riders lean forward as much as they can to reduce drag. Airplanes, which look somewhat like big birds, retract their wheels after takeoff in order to reduce drag. [3] 6 Figures from ref. [3] One way of changing lift and drag of an airfoil is to change angle of attack. Another one is changing shape by moveable flaps as in done aircrafts. [3] 7 Aeroelasticity is the study of the interaction of inertial, structural and aerodynamic forces on aircraft, buildings, surface vehicles [5]. Figure from ref. [5] 8 Why aeroelasticity is important? Because interaction between aerodynamic, structural and inertial forces can cause several undesirable phenomena: - Divergence (static aeroelasticy) - Flutter (dynamic aeroelasticy) - Limit cycle oscillations (nonlinear aeroelasticity) - Vortex shedding, galloping, buffeting (unsteady aeroelasticity) From Ref. [5] 9 Static aeroelasticity is the study of the deflection of flexible aircraft structures under aerodynamic loads, where the forces and motions are considered to be independent of time. These loads cause the wing to bend and twist, so changing the incidence and consequently the aerodynamic flow, which in turn changes the loads acting on the wing and the deflections, and so on until an equilibrium condition is usually reached. The interaction between the wing structural deflections and the aerodynamic loads determines the wing bending and twist at each flight condition, and must be considered in order to model the static aeroelastic behaviour [6]. The airspeed at which the elastic twist increases rapidly to the point of failure is called the ‘divergence airspeed [7]. Simply stated, divergence occurs when a lifting surface deforms under aerodynamic loads in such a way as to increase the applied load, and the increased load deflects the structure further eventually to the point of failure. Such a failure is not simply the result of a load that is too large for the structure as designed; instead, the aerodynamic forces actually interact with the structure to create a loss of effective stiffness [8]. 10 Flutter is arguably the most important of all the aeroelastic phenomena (Collar, 1978; Garrick and Reid, 1981) and is the most difficult to predict. It is an unstable self-excited vibration in which the structure extracts energy from the air stream and often results in catastrophic structural failure. At some critical speed, known as the flutter speed, the structure sustains oscillations following some initial disturbance. Below this speed the oscillations are damped, whereas above it one of the modes becomes negatively damped and (often violent) unstable oscillations occur [6]. Because of this, structures exposed to aerodynamic forces—including wings and airfoils but also chimneys and bridges must be carefully designed to avoid flutter. In complex systems in which neither the aerodynamics nor the mechanical properties are fully understood, the elimination of flutter can be guaranteed only by through testing. Of the various phenomena that are categorized as aeroelastic flutter, lifting surface flutter is most often encountered and most likely to result in a catastrophic structural failure. As a result, it is required that lifting surfaces of all flight vehicles be analyzed and tested to ensure that this dynamic instability will not occur for any condition within the vehicle’s flight envelope [7]. 11 An example for flutter from wind tunnel experiment [9] 12 The most important phenomena arising from the presence of nonlinearity in aeroelastic systems are Limit Cycle Oscillations (LCO), which are self-induced oscillations at constant or approximately constant amplitude. Limit cycle oscillation (LCO) behavior is characterized by constant amplitude, periodic structural response at frequencies that are those of the aeroelastically-loaded structure. LCO is typically limited to a narrow region in Mach number or angle-of-attack signaling the onset of flow separation. 13 Some examples of these problems: Tacoma Narrows Bridge NASA test using Piper PA-38 From ref. 12 14 The 1940 Tacoma Narrows Bridge, the first Tacoma Narrows Bridge, was a suspension bridge in the U.S. state of Washington that spanned the Tacoma Narrows strait of Puget Sound between Tacoma and the Kitsap Peninsula. It opened to traffic on July 1, 1940, and dramatically collapsed into Puget Sound on November 7 of the same year. At the time of its construction (and its destruction), the bridge was the third-longest suspension bridge in the world in terms of main span length, behind the Golden Gate Bridge and the George Washington Bridge. The bridge's main span finally collapsed under 40-mile-per-hour (64 km/h) wind conditions the morning of November 7, 1940. The bridge's collapse had a lasting effect on science and engineering. In many physics textbooks, the event is presented as an example of elementary forced resonance, with the wind providing an external periodic frequency that matched the bridge's natural structural frequency, though many believe the actual cause of failure was aeroelastic flutter. Its failure also boosted research in the field of bridge aerodynamics- aeroelastics, the study of which has influenced the designs of all the world's great long-span bridges built since 1940 [10]. 15 TACOMA NARROWS BRIDGE [11] Another example [12] 16 The failure of the original Tacoma Narrows Bridge is an example of aeroelastic oscillation. If the wind speed and the mode and frequency of the structurel oscillation are such that energy can be absorbed from the wind by the structure and if the energy absorbed is larger than that dissipated by the structural damping, the amplitude of oscillation will continue to increase and will finally lead to destruction. A lesson learned from the Tacoma Bridge is the recognition of the importance of aeroelastic investigations in structural design. The conventional design procedure focuses mostly on the strength of a structure where as the aeroelastic design focuses on the rigidity, damping characteristics and the aerodynamic shape [13]. 17 Theodore von Kármán (May 11, 1881 – May 6, 1963) was a Hungarian American mathematician, aerospace engineer, and physicist who was active primarily in the fields of aeronautics and astronautics. He is responsible for many key advances in aerodynamics, notably his work on supersonic and hypersonic airflow characterization. He is regarded as the outstanding aerodynamic theoretician of the twentieth century. 18 What can be done to avoid these phenomena: - Aeroelastic design - Wind tunnel testing - Flight flutter testing 19 Wind Tunnel principles The loads exerted by static air on a moving object are equal to those exerted by moving air on a static object, as long as he relative velocities between the air and the body are the same in both cases. For a truly representative wind tunnel experiment , the body must have its true size and the wind must have the speed that the object would have if it was moving. These conditions are not always possible. Several scaling laws can be used. From Ref.[5] 20 From Ref.[5] 21 From Ref.[5] 22 From Ref.[5] 23 24 25 Open type wind tunnels Advantages: -Cheaper to build -Pollutants are purged ( for example flow visualization by smoke) Disadvantages: - The size of the tunnel must be compatible to the size of the room, there must be enough space for air to return) -Noisy -More expensive to run when compared with closed type.