ME 425: Aerodynamics

Transcription

1 ME 45: Aerodynamics Dr. A.B.M. Toufique Hasan Professor Department of Mechanical Engineering Bangladesh University of Engineering & Technology (BUET), Dhaka Lecture-0 Introduction toufiquehasan.buet.ac.bd Aerodynamics Aerodynamics is the study of flow dynamics of air around and within a moving object and its interaction with the atmosphere. This field of engineering deals with the aerodynamic forces namely lift (L) and drag (D) and moments and the heat transfer rates acting on a vehicle in flight. Jumbo jet of st century These mechanical parameters greatly depends on the pattern of flow around the vehicle. And the resultant flow pattern depends on: the geometry of the vehicle (mostly the shape of the airfoil: NACA, NASA SC, RAE, ONERA, BGK, NLR, DLR, OAT, Eppler ) its orientation with respect to undisturbed free-stream (angle of attack-aoa), flight speed (Mach No.) and flight altitude (density) at which the vehicle is moving. st airplane made of composite material (full)

2 Lift and drag Lift Flow Drag Wing y z x Wing consists of series of airfoils placed along the spanwise direction (z-axis) Flow Flow Flow Asymmetric airfoil NACA 00 airoil x (National Advisory Committee for Aeronautics) RAE 8 airoil (Royal Aircraft Establishment, UK) NASA multiement airoil (National Aeronautics and Space Administration) y 3 Airfoil flow dynamics Airfoil AOA Airfoil streamlines Flow separation Chord line AOA Airfoil Flow separation Fig. Flow past an airfoil Fig. Separation bubble from SD7003 airfoil at Re=66000 Journal of Fluid Mechanics 009; pp-9-53 AOA= Angle of Attack 4

3 The complete flight of a vehicle/object is mostly comprises of the following matters: Aerodynamics Propulsion and Power Structural mechanics/dynamics Stability and control Aerodynamics is the fundamental of aerospace engineering (aeronautics). A great scope is available for propulsion technology (Jet propulsion and rocketry) and structural dynamics. 5 Classification of aerodynamics Aerodynamics can be broadly classified to two groups-. External aerodynamics. Internal aerodynamics External aerodynamics. Helicopter. Civil aircraft/passenger aircraft 3. Cargo aircraft 4. Military aircraft (most advanced) 5. Rockets 6. UAV 7. Spaceships 8. Wind turbine 9. Racing car aerodynamics 0. High speed train aerodynamics Internal aerodynamics. Rocket nozzle (C-D nozzle). Supersonic/hypersonic inlet 3. Scramjet engine 4. Compressor/turbine blade 5. Fan blade 6. Butterfly valve 7. Cascade 6 3

4 Military aerodynamics Lockheed Martin F-A Raptor Avro Vulcan Bombar A-0 Thunderbolt II Military aircraft (s) Flying faster than the speed of sound (M>) 7 Propulsion nozzles Space Shuttle Space Shuttle Main Engine (SSME) JAXA spacecraft nozzle 8 4

5 UAV Vehicles with wing spans less than approximately 6 m and masses less than 5 kg are usually considered as UAV (Unmanned air vehicle). Unmanned air vehicle (UAV) UAV are either remotely piloted or autonomous with artificial intelligence. Requirements for a typical low-altitude small UAV include Long flight duration at speeds between 0 and 00 km/h Cruise altitude 3 to 300 m Light weight All-weather capabilities Because of recent availability of very small sensors, video camera, and control hardware, systems as small as 5 cm with mass of 80 g, referred to as micro-air vehicle (MAV), are now possible for limited missions. 9 VL Re Fig. Reynolds number range for flight vehicles* The most important parameter in fluid dynamics/aerodynamics is the Reynolds number, Re which is the ratio of inertia force to viscous force. * 0 5

6 The physics of aerodynamics can be learnt from Theoretical aerodynamics (governing equations, simplification, theoretical modeling etc.) Experimental aerodynamics (wind tunnel, flow visualization-piv, LIF, Schlieren, instrumentation, measurements etc.). ME, BUET has subsonic and supersonic (M~.) wind tunnels. Computational aerodynamics (governing equations, numerical techniques, computational resources, CFD) Since the powerful computers are available now-a-days, aerodynamic problems are conveniently solved using CFD software, such as ANSYS FLUENT (ME, BUET has license) ANSYS CFX FASTRAN STAR CCM+ OpenFoam (free GPL) This course (ME 45) will cover the basics of aerodynamics based on theoretical approach (theoretical aerodynamics). Wind Tunnel Wind Tunnel is a system which produces the desired aerodynamic flow field over an object with controlled conditions. It is used to get reliable testing data to analyze the performance of an aerodynamic object. The basic principle is: The loads exerted by static air on a moving body are equal to those exerted by moving air on a static body, as long as the relative velocities between the air and the body are the same in both cases. Wind tunnel In wind tunnels, the true size of the aerodynamic object is not always possible to set; however, several scaling laws can be used in order to render representative experiments where the size or airspeed have been scaled. Dynamic similarity (same Reynolds number, Re and Mach number, M) must exist between model and prototype. 6

7 Open circuit Wind Tunnel (low speed) screen At the intake, a screen/wire mesh is used to reduce the turbulence in incoming flow. Open-circuit wind tunnel schematic Settling chamber () to test section (): A V V A According to Bernoulli principle: p V p V g g (continuity equation) p V p V ( p p) V V 3 Open circuit Wind Tunnel (low speed) screen Using the continuity relation: V A V V A The test section velocity can be found as: Open-circuit wind tunnel schematic ( p p ) A V A V ( p p) A A The area ratio A /A is a fixed quantity for a wind tunnel of given design. Therefore, the test section velocity is governed by the pressure difference, (p -p ). The fan driving the wind tunnel creates this pressure difference. 4 7

8 Open circuit Wind Tunnel (low speed) screen z y A=yz Test-section Open-circuit wind tunnel schematic Power required at the test section with velocity, V : m V QV 3 AV V AV Fans Test section The losses due to tunnel wall friction, turning of air at intake and the efficiency of the fan result in an actual tunnel power requirement of 60-70% higher than this value. 5 Problem: Consider an airship 5 m in diameter and 60 m in length moves at 5 m/s. What air speed be required in a wind tunnel test of a /8 scale model of the airship? It may be assumed that the standard air temperature and pressure exist for airship operation and model testing. What will be the power requirement of the wind tunnel fan if the model is tested in a test section size of m m? Assume an overall efficiency of the tunnel is 60%. Hints: Dynamic similarity must exist between model and airship. Re Re m airship VL VL m airship V 40 m/s m So, the required test section velocity is 40 m/s. 3 AV 53.6kW Wind tunnel fan power required = 56 kw 6 8