Diapositiva 1

The Aerodynamic Behaviour of Super Slender Monolithic Towers

Danai Iris Anagnostopulos

S. Cammelli, BMT Fluid Mechanics

G. Piccardo, Universit di Genova31/03/2016

IntroductionD. AnagnostopulosThe Aerodynamic Behaviour of Super Slender Monolithic TowersSuper slender towers

432 Park Avenue in New York111 West 57th Street in New YorkImpressively 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 actionSlenderness ratio 15:1Slenderness ratio 24:1

3 IntroductionD. AnagnostopulosThe Aerodynamic Behaviour of Super Slender Monolithic TowersA tall building is like a swing

The importance of vortex shedding

wt flow

4 IntroductionD. AnagnostopulosThe Aerodynamic Behaviour of Super Slender Monolithic TowersCoherent 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 heightSoftening sharp cornersCreate openings in buildingAdd spoilers, spiral bands

The effect of turbulence in vortex-induced excitationBurj Khalifa in Dubai

5D. AnagnostopulosThe Aerodynamic Behaviour of Super Slender Monolithic Towers Aim of the workIdeal structure and its locationPressure data analysisThe design of aeroelastic and pressure models Structural response analysis

Setup of aeroelastic tests

Outline of this presentationInvestigate the importance of vortex shedding excitation in supertall slender monolithic towers at various mean wind velocities associated with different return periodsWind tunnel tests on scaled pressure model: staticon scaled aeroelastic model: able to reproduce the main dynamic characteristics of the structure

Preliminary conclusions and prospects of this work

6D. AnagnostopulosThe Aerodynamic Behaviour of Super Slender Monolithic Towers Ideal structure and its locationStructural Properties

Dynamic PropertiesSlenderness ratio 20:1

7D. AnagnostopulosThe Aerodynamic Behaviour of Super Slender Monolithic Towers Ideal structure and its locationAerial 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

8D. AnagnostopulosThe Aerodynamic Behaviour of Super Slender Monolithic Towers Ideal structure and its locationDevices used to simulate the boundary layer in the wind tunnel

Experimental profiles

9D. AnagnostopulosThe Aerodynamic Behaviour of Super Slender Monolithic Towers Design of the aeroelastic modelScaling parametersFroude Number

Cauchy Number Reynolds Number

Density parameter

Damping Ratio

Similitudine requirementsApproach used only for very important structuresMost reliable approach to predict buildings dynamic response to wind actionSimulates: mass, stiffness, damping of actual structureFlexes in response to the wind as the real structure

10D. AnagnostopulosThe Aerodynamic Behaviour of Super Slender Monolithic Towers Design of the aeroelastic modelConstruction of thetapered brass spineBrass Spine: internal structural part reproduces the stiffness properties and part of the massOuter Shell: reproduces the exterior geometry and part of the massAeroelastic model composed by:

11D. AnagnostopulosThe Aerodynamic Behaviour of Super Slender Monolithic Towers Design of the aeroelastic model

Design of Brass Spine with the support of FE program SAP2000

Modal ShapesFrequenciesFirst order modesSecond order modes

12D. AnagnostopulosThe Aerodynamic Behaviour of Super Slender Monolithic Towers Design of the aeroelastic modelConstructionOuter ShellComplete aeroelastic model

Derived by selective laser sinteringFormed by 10 vertical segments 1mm gap in between the segmentsInstrumentationAccelerometers2 at the top2 at the antinode

13D. AnagnostopulosThe Aerodynamic Behaviour of Super Slender Monolithic Towers Design of the static model

Material: Balsa woodInstrumented with 120 pressure tapsPressure coefficients obtained

14D. AnagnostopulosThe Aerodynamic Behaviour of Super Slender Monolithic Towers Setup of aeroelastic testsCalibrationMode shapes checkFrequency check

15D. AnagnostopulosThe Aerodynamic Behaviour of Super Slender Monolithic Towers Setup of aeroelastic testsDamping checkTecniques to augment damping: good results with tape

String burn technique to determine the logarithmic decrement in wind-off conditions

16D. AnagnostopulosThe Aerodynamic Behaviour of Super Slender Monolithic TowersAeroelastic and pressure testsBMTs Boundary Layer Wind Tunnel

4.8 m2.4 m15 m

17D. AnagnostopulosThe Aerodynamic Behaviour of Super Slender Monolithic TowersAeroelastic and pressure testsTest in Turbulent Flow

18Aeroelastic and pressure testsD. AnagnostopulosThe Aerodynamic Behaviour of Super Slender Monolithic TowersSummarizing Aeroelastic TestTurbulent Flow: Range of wind velocities (model scale) 3-20 m/s with 0.25 m/s step crosswind; Sample time 1 min for each runSmooth 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 runApproximately 12 hours of acquisition in the wind tunnel

Pressure TestTurbulent 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

19D. AnagnostopulosThe Aerodynamic Behaviour of Super Slender Monolithic Towers Pressure data analysis

Pressure coefficients, mean values

D. AnagnostopulosThe Aerodynamic Behaviour of Super Slender Monolithic Towers Pressure data analysisThe 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)

21D. AnagnostopulosThe Aerodynamic Behaviour of Super Slender Monolithic Towers Structural response analysis

Scruton NumberCNRLow structural frequencies lead to resonance for both first and second order mode of vibrationCritical velocities migrate along the axis of the structure:First order mode excited by vortex shedding for low wind speeds, at the topSecond order mode excited by higher velocities and has to be analysed for more critical positions with regards to vortex sheddingLow values of Scruton Number show that the structure is sensible to vibrations induced by vortex shedding

D. AnagnostopulosThe Aerodynamic Behaviour of Super Slender Monolithic Towers Structural response analysisCross Wind1st peak: Resonance with First order Mode2nd peak: Resonance with Second order ModePeaks correspond to Vortex Shedding

23D. AnagnostopulosThe Aerodynamic Behaviour of Super Slender Monolithic Towers Structural response analysisVideos in Turbulent FlowLeft 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 flow

wt flowFor velocities higher than 30 m/s the contribution of second order mode is dominant

24D. AnagnostopulosThe Aerodynamic Behaviour of Super Slender Monolithic Towers Structural response analysis

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

25D. AnagnostopulosThe Aerodynamic Behaviour of Super Slender Monolithic Towers Preliminary conclusions and prospects of the workUtility of the static pressure testQualitative behaviour of the section Calculation of correlation coefficientUtility of dynamic aeroelastic testVery 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 flowSome 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 subjectProspects of this workIt seems very interesting to: investigate the behaviour at the antinode of the 2nd order modecompare experimental results with the theory of slender elements (3D gust effect factor)