Msc thesis Anagnostopulos Danai Iris

The Aerodynamic Behaviour of
Super Slender Monolithic Towers
Danai Iris Anagnostopulos
S. Cammelli, BMT Fluid MechanicsG. Piccardo, Università di Genova
31/03/2016

Introduction
D. Anagnostopulos The Aerodynamic Behaviour of Super Slender Monolithic Towers
Super slender towers
432 Park Avenue in New York
111 West 57th Street in New York
 Impressively high ratio
between height and
width
 More and more popular
among the residential
real estate market
 Lighter, flexible, less
structural damping:
intrinsic vulnerability to
wind action
Slenderness ratio 15:1
Slenderness ratio 24:1

3
Introduction
A tall building is like a swing
 Resonant condition:
Critical wind speed 𝑣𝑐𝑟 for which 𝑓𝑠 =𝑓𝑏,𝑖
 Low fundamental frequencies 𝑓𝑏,𝑖 thus
require lower critical wind speed for
resonance
 Possible vortex shedding excitation on:
• First order mode of vibration
• Second order mode of vibration
The importance of vortex shedding
,
,
b im
s cr i
f BSt v
f v
B St

  
wt flow

4
Introduction
 Coherent shedding of vortices excites building into motion
 Turbulence is beneficial: contributes to disrupt coherent
shedding of vortices
 Low levels of turbulence near very tall buildings
 The solution: techniques to frustrate coherent shedding
• Variation of cross section with height
• Softening sharp corners
• Create openings in building
• Add spoilers, spiral bands
The effect of turbulence in vortex-induced excitation
Burj Khalifa in Dubai

5
Aim of the work
Ideal structure and its location
Pressure data analysis
The design of aeroelastic and pressure models
Structural response analysis
Setup of aeroelastic tests
Outline of this presentation
Investigate the importance of vortex shedding excitation in supertall slender monolithic
towers at various mean wind velocities associated with different return periods
Wind tunnel tests
• on scaled pressure model: static
• on scaled aeroelastic model: able to reproduce the main dynamic characteristics of
the structure
Preliminary conclusions and prospects of this work

6
Structural Properties
Full Scale Properties
B [m] 20.00
B [m] 20.00
r [m] 2.00
Floor Area [m2] 396.57
Height of building [m] 400.00
Volume [m3] 158626.55
Density [Kg/m3] 250.00
Mass x unit height[Kg/m] 99141.59
Frequency Frequency
1st order mode 2nd order mode
(Hz) (Hz)
0.08 0.24
𝑛2𝑜𝑟𝑑𝑒𝑟
𝑛1𝑜𝑟𝑑𝑒𝑟
≈ 3.0
Dynamic Properties
Slenderness ratio
20:1

7
Aerial view of London
Vb= 20 m/s (zref=10 m, z0ref=0.03 m)
ESDU Item 01008 - logaritmic
model to simulate boundary
layer
 Target mean wind velocity
profiles
 Target turbulence intensity
profiles
Roughness z0=0.7 m

8
Devices used to simulate the boundary
layer in the wind tunnel
Experimental profiles

9
Design of the aeroelastic model
Scaling parameters
Froude Number
Cauchy Number
Reynolds Number
Density parameter
Damping Ratio
Quantity Scaling Factor
Length 𝑓𝐿=𝐿 𝑚/𝐿 𝑝=1:400
Air density 𝑓∆=1:1
Mass 𝑓∆x𝑓𝐿
3
=1:4003
Structural Damping Ratio f=1:1
Fundamental Frequencies 𝑓𝑇= 𝑓𝑚/𝑓𝑝
Wind Velocity 𝑓𝑉= 𝑓𝑇x𝑓𝐿
Time 𝑓𝑡=1/𝑓𝑇
Force 𝑓𝐹= 𝑓𝛥x𝑓𝑉
2
x𝑓𝐿
2
Bending Moment 𝑓 𝑀= 𝑓𝛥x𝑓𝑉
2
x𝑓𝐿
3
Acceleration 𝑓𝐴=𝑓𝐿/𝑓𝑡
2
Full Scale Properties Model Scale Properties
B [m] 20.0 B [m] 0.05
B [m] 20.0 B [m] 0.05
r [m] 2.0 r [m] 0.01
Floor Area [m2] 396.6 Floor Area [m2] 0.002
Height of building [m] 400.0 Height of building [m] 1.00
Volume [m3] 158626.5 Volume [m3] 0.002
Density [Kg/m3] 250.0 Density [Kg/m3] 250.0
Mass x unit height[Kg/m] 99141.6 Mass x unit height[Kg/m] 0.6
Similitudine requirements
 Approach used only for very important
structures
 Most reliable approach to predict
building’s dynamic response to wind
action
 Simulates: mass, stiffness, damping of
actual structure
 Flexes in response to the wind as the
real structure

10
Construction of the
tapered brass spine
 Brass Spine: internal structural part reproduces the stiffness properties and part of the mass
 Outer Shell: reproduces the exterior geometry and part of the mass
Aeroelastic model composed by:

11
TABLE: Modal Periods And Frequencies
OutputCase StepType StepNum Period SAPFrequency
Text Text Unitless Sec Cyc/sec
MODAL Mode 1 0.09 11.09
MODAL Mode 2 0.07 14.88
MODAL Mode 3 0.03 29.98
MODAL Mode 4 0.03 39.66
MODAL Mode 5 0.02 47.60
Design of Brass Spine with the
support of FE program SAP2000
Modal Shapes
Frequencies
First order modes Second order modes

12
Construction
Outer Shell
Complete aeroelastic model
Density SLS thikness Vtot shell Mass shell
[kg/m3] [m] [m3] [kg]
900 0.002 0.0004 0.330
 Derived by selective laser sintering
 Formed by 10 vertical segments
 1mm gap in between the segments
Instrumentation
Accelerometers  2 at the top
 2 at the antinode

13
Design of the static model
2
0,5ρ
refP P
cp
v


Material: Balsa wood Instrumented with
120 pressure taps
Pressure coefficients obtained

14
Calibration
Mode shapes
check
Frequency check
SAP Model WT Model
Mode Direction Frequency [Hz] Frequency [Hz]
1 X-Along 11.1 12.4
1 Y-Cross 14.9 13.8
2 X-Along 30.0 28.7
2 Y-Cross 39.7 44.0
𝑛2𝑜𝑟𝑑𝑒𝑟𝑊𝑇
𝑛1𝑜𝑟𝑑𝑒𝑟𝑊𝑇
= 3.3

15
Damping check
ξ 𝑠 =
𝐿𝑜𝑔𝐷𝑒𝑐
2𝜋
Tecniques to augment damping:
good results with tape
Log Dec % Critical
Mode 1x 0.04 ≈ 0.63
Mode 1y 0.04 ≈ 0.58
Mode 2x 0.03 ≈ 0.45
Mode 2y 0.03 ≈ 0.49
“String burn” technique to determine
the logarithmic decrement in wind-off
conditions

16
Aeroelastic and pressure tests
BMT’s Boundary Layer Wind Tunnel
4.8 m
2.4 m
15 m

17
Test in Turbulent Flow

18
Summarizing
Aeroelastic Test
 Turbulent Flow: Range of wind velocities (model scale) 3-20 m/s with 0.25 m/s step crosswind;
Sample time 1 min for each run
 Smooth Flow: Range of wind velocities (model scale) 3-20 m/s with 0.5 m/s step crosswind; Sample
time 1 min for each run
 Turbulent Flow with adjuncted damping: Range of wind velocities (model scale) 3-20 m/s with 0.5
m/s step crosswind; Sample time 1 min for each run
Approximately 12 hours of acquisition in the wind tunnel
Pressure Test
 Turbulent Flow: Run at a velocy (model scale) 17.5 m/s with angle of attacks 0°, 90°, 180°, 270°.
Approximately 1 hour of acquisition in the wind tunnel
Test in Smooth Flow

19
Pressure
coefficients,
mean values

The crosswind correlation
becomes very weak at a distance
equal to 120 m (i.e. 6 times the
cross-section side), similarly to
classic formulation in the
technical literature (e.g., the
harmonic method in EC1/CNR)
z Correl coeff Correl coeff
[m] cross along
340 1.00 1.00
300 0.64 0.73
220 0.12 0.46
140 0.01 0.33
,
,
( )i j
i j
i j
Cpp

 

,
1
1
( ) ( )( )
n
i j hi hi hj hj
h
Cpp cp mcp cp mcp
n 
  

21
𝑆𝑐𝑖 =
4𝜋𝑚 𝑒,𝑖ξ 𝑖
𝜌𝐵2
m ξ ρ B Sc,i
[Kg/m] [-] [Kg/m3] m [-]
mode 1y 99141 0.0058 1.25 20 14
mode 2y 99141 0.0049 1.25 20 12
Scruton Number
CNR
Low structural frequencies lead to resonance for both first and second order mode of vibration
Critical velocities migrate along the axis of the structure:
 First order mode excited by vortex shedding for low wind speeds, at the top
 Second order mode excited by higher velocities and has to be analysed for more critical
positions with regards to vortex shedding
Low values of Scruton Number show that the structure
is sensible to vibrations induced by vortex shedding

Cross Wind
1st peak:
Resonance
with First
order Mode
2nd peak:
Resonance
with Second
order Mode
Peaks correspond to Vortex Shedding

23
Videos in Turbulent Flow
Left Run at a Velocity (full
scale) of 16 m/s
Right Run at a Velocity
(full scale) of 42 m/s
shows there is vortex
shedding also in the along
wind direction
Data have been filtered to highlight the contribution of different modes
wt flowwt flow
For velocities higher than 30 m/s the contribution of second order mode is dominant

24
Values of peak factor < 3 indicates that
vortex shedding is occurring; here the
response is harmonic (deterministic)
Typical peak factor values for tall
buildings around 3-4

25
Preliminary conclusions and prospects of the work
Utility of the static pressure test
 Qualitative behaviour of the section
 Calculation of correlation coefficient
Utility of dynamic aeroelastic test
 Very interesting experimental results that highlight the importance of the secod order
mode of vibration for the design of the structure have been obtained, in particular for
the turbulent flow
 Some aspects need to be analyzed more in depth, for example the possible coupling
alongwind-crosswind and the total damping (for the moment, has been obtained with the
random decrement tecnique)
Not all the obtained data have been processed and some obtained results need to be
interpreted more in depth
At the present moment there is no specific scientific literature on this subject
Prospects of this work
It seems very interesting to:
 investigate the behaviour at the antinode of the 2nd order mode
 compare experimental results with the theory of slender elements (3D gust effect factor)

Msc thesis Anagnostopulos Danai Iris

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Msc thesis Anagnostopulos Danai Iris