Wind Engineering - Lecture 1 - Bluff Body Aerodynamics1

8/11/2019 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

[email protected]
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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Number


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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

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Wind Engineering - Lecture 1 - Bluff Body Aerodynamics1