* Your assessment is very important for improving the workof artificial intelligence, which forms the content of this project
Download Vortex Shedding
Survey
Document related concepts
Airy wave theory wikipedia , lookup
Hemodynamics wikipedia , lookup
Derivation of the Navier–Stokes equations wikipedia , lookup
Drag (physics) wikipedia , lookup
Navier–Stokes equations wikipedia , lookup
Boundary layer wikipedia , lookup
Flow measurement wikipedia , lookup
Hydraulic machinery wikipedia , lookup
Wind-turbine aerodynamics wikipedia , lookup
Lift (force) wikipedia , lookup
Coandă effect wikipedia , lookup
Flow conditioning wikipedia , lookup
Compressible flow wikipedia , lookup
Computational fluid dynamics wikipedia , lookup
Reynolds number wikipedia , lookup
Aerodynamics wikipedia , lookup
Transcript
기계공학실험 Ch. 12 Measurements for Flow Passing over a Cylinder Introduction Purpose: Understanding and observation on flows behind a circular cylinder Boundary layer Flow separation Vortex shedding Vortex shedding frequency Experimental setup Pitot‐tube & micro‐manometer (기압계) Hot‐wire probe & hot‐wire anemometer (풍속계) A/D convertor Circular cylinder Wind tunnel References Frank M. White, “Viscous Fluid Flow,” 2nd Ed., McGraw‐Hill, New York, 1991. Robert W. Fox and Alan T. Mcdonald, “Introduction to Fluid Mechanics,” 4th Ed., John Wiley & Sons, 1994. 기계공학실험 Physical Phenomena Boundary layer Flow separation Flows behind a circular cylinder Wake flow Mountain Halla: cylindrical obstacle in flow Jeju island Flow past a cylinder at Re (ρVD/μ) = 2,000 Numerical simulation Alternating low‐pressure vortices on the downstream side of the object The object will tend to move toward the low‐pressure zone. Eventually, if the frequency of vortex shedding matches the resonance frequency of the structure, the structure will begin to resonate Can damage or destroy the chimney 기계공학실험 Flow Separation Bernoulli’s theorem Flows over the surface of a cylinder Increase of flow velocity Decrease of pressure Decrease of flow velocity Increase of pressure 2 2 V1 p V p 1 gh1 2 2 gh2 2 g 2 g Symmetric stream lines Pressure force momentum shear force (zero viscosity) • Since the pressure decreases continuously between points A and B, a fluid element inside the boundary layer experiences a net force in the direction of flow. • In the region between A and B, this net force is sufficient to overcome the resisting shear force (μ(∂V/∂n)), and the motion of the element in the flow direction is maintained. • After point B, since the pressure increases in the direction of flow, the fluid element experiences a net pressure force opposite to its direction of motion. • At some point (point C), the momentum of the fluid in the boundary layer is insufficient to carry the element further into the region of increasing pressure. • The fluid layers adjacent to the solid surface are brought to rest and the flow separates from the surface at the point C. • Boundary layer separation results in the formation of a relatively low‐pressure region behind a body; this region, which is deficient in momentum, is called the wake. 기계공학실험 Pressure and Drag Coefficients Pressure along the cylinder surface Drag vs Re (Velocity) D CD A U 2 2 Low form drag High form drag (inviscid flow) Laminar flow High form drag Early separation Turbulent flowLow form drag Late separation Turbulent flow Small wake area of low pressure Drag decrease Rough surface/Turbulent freestream Small wake area of low pressure Drag decrease 기계공학실험 Influence of the “Dimple” on the Golf Ball Influence on drag Influence on lift Without dimple effect With dimple effect 기계공학실험 Experimental Process Pressure hole #20 Cylinder Pressure holes Pressure hole #1 Flow Flow Pressure tubes Selection box + ‐ + ‐ Micromanometer AD converter / PC Sectional view of the cylinder Pressure distribution Flow separation point Drag coefficient 기계공학실험 Vortex Shedding Vortex shedding Repeating pattern of swirling vortices caused by the unsteady separation of flow of a fluid over bluff bodies The wake consists of pairs of vortices shed alternately from the upper and lower part after separation point (Karman vortex streets). Timeline and streamlines at Re=170 The effect of the Re on the flow past a cylinder Strouhal number St s fs D U f s : shedding frequency U : free stream velocity D : cylinder diameter As Re increases, the wake becomes more complex and turbulent, but the alternating shedding can still be detected at Re=107. 기계공학실험 Vortex Shedding of Chimney Laminar flow: Large area of wake flow High force on chimney Vortex shedding: resonance Danger ↑ Turbulent flow: Small area of wake flow Low force on chimney Vortex shedding: resonance Danger ↓ 기계공학실험 Vortex Shedding Frequency Strouhal number Experiments region St s 0.2 f s 0.2 U D The shedding cycle takes place during the time that the free stream moves approximately five cylinder diameters. Experimental process Hot‐wire U Velocity change Frequency of vortex shedding 기계공학실험 Review Questions 1. 2. 3. 4. 5. 6. 7. The pressure at A is ( ) than at B. The pressure at B is ( ) than at C. The wake has a very ( ) pressure, and consequently ( ). Dimples on golf ball cause a ( ) size of wake, and consequently low ( ). The wake consists of ( ), which shows a phenomenon known as ( ). Vortex shedding frequency can be expressed in the term of the dimensionless ( ) number. What is the title of this music and who is its composer? 기계공학실험