02-post - Lecture 2 Flow Fields Applied Computational Fluid...

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1 Lecture 2 - Flow Fields Applied Computational Fluid Dynamics Instructor: André Bakker © André Bakker (2002-2006) © Fluent Inc. (2002)
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2 Important variables Pressure and fluid velocities are always calculated in conjunction. Pressure can be used to calculate forces on objects, e.g. for the prediction of drag of a car. Fluid velocities can be visualized to show flow structures. From the flow field we can derive other variables such as shear and vorticity. Shear stresses may relate to erosion of solid surfaces. Deformation of fluid elements is important in mixing processes. Vorticity describes the rotation of fluid elements. In turbulent flows, turbulent kinetic energy and dissipation rate are important for such processes as heat transfer and mass transfer in boundary layers. For non-isothermal flows, the temperature field is important. This may govern evaporation, combustion, and other processes. In some processes, radiation is important.
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3 Post-processing Results are usually reviewed in one of two ways. Graphically or alphanumerically. Graphically: Vector plots. Contours. Iso-surfaces. Flowlines. Animation. Alphanumerics: Integral values. Drag, lift, torque calculations. Averages, standard deviations. Minima, maxima. Compare with experimental data.
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4 A flow field example: the football Regulation size american football. Perfect throw. Ball is thrown from right to left. Flow field relative to the ball is from left to right. Shown here are filled contours of velocity magnitude.
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5 Football animation Time dependent calculation. Images were saved at every time step. Transparency for the football surface was set to 50% so that the leather strips are also visible when in the back. This helps showing the motion of the ball.
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6 Football animation Velocity vectors. Watch the flow separation behind the leather strips.
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Vector Plot on Iso-Surface of Constant Grid. Irregular Looking.
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Vector Plot on Bounded Sample Plane. Slower but Regular Looking!
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9 Fluid motion In a fluid flow field, each fluid element undergoes three different effects: 1. Translation. 2. Deformation. 3. Rotation. Translation and deformation
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10 Methods to show translation Translation can be shown by means of: Velocity vectors. Flowlines: Streamlines. Pathlines. Streaklines. Timelines. Oilflow lines.
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11 Flow around a cylinder - grid
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12 Flow around a cylinder – grid zoomed in
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13 Flow around a cylinder – velocity vectors
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14 Flow around a cylinder – velocity magnitude
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15 Flow around a cylinder – pressure field
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16 Pressure Pressure can be used to calculate forces (e.g. drag, lift, or torque) on objects by integrating the pressure over the surface of the object.
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