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Transcript of Wind Engineering - Lecture 1 - Bluff Body Aerodynamics1
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WIND ENGINEERING BLUFF BODY
AERODYNAMICSLecture 1
MEC 4459
Department of Mechanical and Aerospace Engineering
Wind environment and engineering
Mr David Burton
Tel: +61 3 990 55865
mailto:[email protected]:[email protected] -
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The following lectures have been updated
(please download again!);
Lecture 2: Slide 8
Lecture 3: Slide 20-21, 27, 33
Lecture 5: Slide 5
Update
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Bernoullis equation
Bluff bodies
Pressure coefficients Forces and moments
Drag coefficients
2 dimensional objects
Reynolds Number Effects
Lecture Overview
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What is a bluff body? Blunt / bluff geometry
Separated flow
Drag forces dominated by pressure drag
Large wake
Contrast to airfoil /streamlined body
What are the relevant parameters? Body forces (force coefficientspeak and average)
Local pressures (surface pressures, distribution, peak andaverage)
Flow regime (spatial variation velocity, turbulence)
What affects these parameters (non-exhaustive)? Body geometry (including location of measurement point)
Wind direction
Reynolds Number
Jensen Number (boundary layer profile)
Turbulence Intensity (u, v and w)
Turbulence Length Scale (referenced to the body geometry)
Bluff Bodies
Top: http://www.cg.tuwien.ac.at/courses/Visualisierung/2007-2008/Beispiel2/banova_alsallakh/index.htm
Bottom: http://www.dept.aoe.vt.edu/~devenpor/aoe3054/manual/expt1/fig7.jpg
http://www.cg.tuwien.ac.at/courses/Visualisierung/2007-2008/Beispiel2/banova_alsallakh/index.htmhttp://www.cg.tuwien.ac.at/courses/Visualisierung/2007-2008/Beispiel2/banova_alsallakh/index.htmhttp://www.cg.tuwien.ac.at/courses/Visualisierung/2007-2008/Beispiel2/banova_alsallakh/index.htmhttp://www.cg.tuwien.ac.at/courses/Visualisierung/2007-2008/Beispiel2/banova_alsallakh/index.htmhttp://www.cg.tuwien.ac.at/courses/Visualisierung/2007-2008/Beispiel2/banova_alsallakh/index.htmhttp://www.cg.tuwien.ac.at/courses/Visualisierung/2007-2008/Beispiel2/banova_alsallakh/index.htm -
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Bluff Bodies (examples)
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Bluff Bodies (examples)
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Bernoullis Equation (steady, irrotational, inviscid
and incompressible)
Bernoullis equation
Static pressure
Dynamic pressure
Hydrostatic
pressure
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Pressure Coefficient:
, = ,
,
:pressure (at point 1)
,: reference static pressure
,=
: density of fluid (air ~ 1.2kg/m^3 in Melbourne)
Pressure coefficient is a way of expressing the pressure at a point(usually at a surface) independently of the flow properties
Pressure Coefficients
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Ignoring gravity effects then
for irrotationalflow:
+1
2 = +
1
2
, =
,
, = 1
Bernoullis equation
tagnation point
= 0 , = 1
= , = 0
> , < 0
Only holds for
irrotational /inviscidflows
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`Stagnation pressure
FLOW AROUND A THREE-DIMENSIONAL BLUFF BODY S. Krajnovic and L. Davidson, 9TH INTERNATIONAL SYMPOSIUM ON FLOW
VISUALISATION, 2000
Wind Direction
Wind Direction
Pressure Iso-surfaces Surface Pressure Coefficients
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Force Coefficients:
, =
,
In wind engineering drag forces areconventionally resolved indirection parallel (drag force),perpendicular (side and lift force)to the direction of the wind.
However, note it is often moreconvenient to reference the forcecoefficients to the body axis of themodel (or structure)
Force Coefficients
D
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`Force Coefficients
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`Moment Coefficients
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Irrotational flow:
Cylinder Flow
Batchelor, G. K. (1970). An Introduction to Fluid Mechanics, Cambridge University Press
= 1
, = 1 4()
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`Cylinder Flow
Batchelor, G. K. (1970). An Introduction to Fluid Mechanics, Cambridge University PressHeat Transfer by A. Mills
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`Cylinder / Sphere Drag Coefficient
http://www.intechopen.com/source/html/16897/media/image
2.jpeg-W. H. Bell, 1983 turbulence vs drag-some future
consideration, Ocean Engng, 10 1 4763 .
http://www.intechopen.com/source/html/16897/media/image2.jpeghttp://www.intechopen.com/source/html/16897/media/image2.jpeghttp://www.intechopen.com/source/html/16897/media/image2.jpeghttp://www.intechopen.com/books/wind-tunnels-and-experimental-fluid-dynamics-research/the-importance-of-turbulence-reduction-in-assessment-of-wind-tunnel-flow-quality#B6http://www.intechopen.com/books/wind-tunnels-and-experimental-fluid-dynamics-research/the-importance-of-turbulence-reduction-in-assessment-of-wind-tunnel-flow-quality#B6http://www.intechopen.com/books/wind-tunnels-and-experimental-fluid-dynamics-research/the-importance-of-turbulence-reduction-in-assessment-of-wind-tunnel-flow-quality#B6http://www.intechopen.com/books/wind-tunnels-and-experimental-fluid-dynamics-research/the-importance-of-turbulence-reduction-in-assessment-of-wind-tunnel-flow-quality#B6http://www.intechopen.com/books/wind-tunnels-and-experimental-fluid-dynamics-research/the-importance-of-turbulence-reduction-in-assessment-of-wind-tunnel-flow-quality#B6http://www.intechopen.com/books/wind-tunnels-and-experimental-fluid-dynamics-research/the-importance-of-turbulence-reduction-in-assessment-of-wind-tunnel-flow-quality#B6http://www.intechopen.com/books/wind-tunnels-and-experimental-fluid-dynamics-research/the-importance-of-turbulence-reduction-in-assessment-of-wind-tunnel-flow-quality#B6http://www.intechopen.com/books/wind-tunnels-and-experimental-fluid-dynamics-research/the-importance-of-turbulence-reduction-in-assessment-of-wind-tunnel-flow-quality#B6http://www.intechopen.com/books/wind-tunnels-and-experimental-fluid-dynamics-research/the-importance-of-turbulence-reduction-in-assessment-of-wind-tunnel-flow-quality#B6http://www.intechopen.com/books/wind-tunnels-and-experimental-fluid-dynamics-research/the-importance-of-turbulence-reduction-in-assessment-of-wind-tunnel-flow-quality#B6http://www.intechopen.com/books/wind-tunnels-and-experimental-fluid-dynamics-research/the-importance-of-turbulence-reduction-in-assessment-of-wind-tunnel-flow-quality#B6http://www.intechopen.com/books/wind-tunnels-and-experimental-fluid-dynamics-research/the-importance-of-turbulence-reduction-in-assessment-of-wind-tunnel-flow-quality#B6http://www.intechopen.com/books/wind-tunnels-and-experimental-fluid-dynamics-research/the-importance-of-turbulence-reduction-in-assessment-of-wind-tunnel-flow-quality#B6http://www.intechopen.com/source/html/16897/media/image2.jpeghttp://www.intechopen.com/source/html/16897/media/image2.jpeghttp://www.intechopen.com/source/html/16897/media/image2.jpeg -
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Curved surfaces
Coanda effect
Adverse pressuregradient (positive
pressure gradient
in flow direction)
Flow Separation
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Where there is a significant
gradient of velocity this
layer is termed a shear
layer A boundary layer is an
attached shear layer
Can also have a free shear
layer
Shear Layers
Boundary-Layer Theory, Herrmann Schlichting, K. Gersten, Klaus Gersten, p663
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`Sharp edged body
Melbourne, B. Lecture Notes, W Eng
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`Rectangular bodies (2D)
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`Force Coefficients and Reynolds
Number
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`Force Coefficients and Reynolds
Number
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`Force Coefficients and Reynolds
Number
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`Drag Coefficients 2d
Scruton, Introduction to wind effects on structures
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`Drag Coefficients 2d
Scruton, Introduction to wind effects on structures