Wind tunnel tests of aerodynamic interference between two high-rise buildings

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1 EACWE 5 Florence, Italy 19 th 23 rd July 2009 Wind tunnel tests of aerodynamic interference between two high-rise buildings Flying Sphere image Museo Ideale L. Da Vinci 1 st A.Flaga, 2 nd G.Bosak 1 st Lublin University of Technology and Cracow University of Technology LIWPK@interia.pl Jana Pawla II 37/3a, Cracow, Poland 2 nd Cracow University of Technology g.bosak@windlab.pl Jana Pawla II 37/3a, Cracow, Poland Keywords: aerodynamic interference, high-rise buildings, wind tunnel tests. ABSTRACT The paper presents selected results of experiments on a model of a high-rise building carried out in boundary layer wind tunnel of the Wind Engineering Laboratory at the Cracow University of Technology. The examined building is of 150 m height above the ground and is characterised by various cross-section shape on different height levels. Not far the building, it is planned to be erected another one of a similar height. Influence of aerodynamic interference between the two structures on the response of the examined building has been the main aim of aerodynamic analyses. 1. WIND TUNNEL TESTS The experiments on a model of a high-rise building were carried out in boundary layer wind tunnel of the Wind Engineering Laboratory at the Cracow University of Technology, Poland. The basic dimensions of the wind tunnel working section are: 2.20 m (width),1.40 m (height), 10.00m (length). Formation of the mean wind velocity profile and atmospheric turbulence takes place in the first part of the working section at the length of 6 m by use of respective turbulence generators: barriers, spires Contact person: 2 nd G. Bosak, Cracow University of Technology, Jana Pawla II 37/3a, Cracow, Poland, phone , FAX g.bosak@windlab.where

The axial fan, single stage of efficiency 08-0.9, of the outer diameter 2.72 m and of the velocity of the end of the fan blade of 100 m/s, is located on the suction side of the wind tunnel.

2 and blocks of respective geometry and a mechanically controlled height. In the working section of the tunnel there is a round rotational table of 2 m in diameter which makes possible the change of a wind inflow direction on an examined model. The axial fan, single stage of efficiency , of the outer diameter 2.72 m and of the velocity of the end of the fan blade of 100 m/s, is located on the suction side of the wind tunnel. Maximum mean flow velocity in the working section is equal to 40 m/s. 1.1 Brief description of the building. The examined 45th storey building is of 150 m height above the ground and is characterised of a various cross-section shape on different hieght levels (Fig. 1a). Not far the building, it is planned to be erected another one of a similar height (Fig. 1b.). a) b) Figure 1:Computer visualization: a)case of the one building, b)case of two buildings. The load bearing system of the building consists of reinforced concrete columns, walls and a staircase core. The system of structural elements is irregular. The reinforced concrete plates are fixed to the columns, walls and walls of the staircase core by means of stiff joints forming a space structure, which is able to take all horizontal loads. 1.2 Methodology of wind tests. Experiments have been performed on a model of the building of a scale of 1:300, which has been manufactured of various plastic materials. In the working section of the wind tunnel a nearest neighbourhood of the building has been modelled (Fig. 2). During the measurements, two models of buildings have been used: an active model equipped with pressure sensors on its outer surface and a passive model without any measurement system. Wind pressure distributions and adequate wind pressure coefficients on the outer walls of the active model have been measured in two different situations. In the first situation, only the active model with its neighbourhood has been placed in the wind tunnel working section. In the second one, the passive model has been placed together with the active one. The influence of aerodynamic interference has been evaluated on the basis of comparison of mean wind pressure distributions on outer surface of the active model in the first and the second measurement situations, respectively. Experiments have been performed at the following conditions: 1. Power law exponent of the wind profile α=0.20; 2. Intensity of turbulence I v =20 %; 3. Reference velocity V ref =20 m/s. The measurements have been accomplished using the active model of the building equipped with pressure sensors distributed on various heights above the ground on the outer walls of the model. The examined cross-sections have been placed on m, 67.82m, m and m above

3 the ground corresponding to the real scale. In aech cross-section sixteen pressure sensors were distributed around the perimiter. The sensors have been connected to the pressure scanners making possible simultaneously collection of instantaneous wind pressure time series. The scan rate of each series has been equal to 200 Hz and the collection of data has been performed for 30 s. During the measurements, 64 pressure channel scanner of parallel type has been used. The scanner is built on base of piezoresistive pressure sensors Motorola MPX2010. a) b) 201 o 180 o 135 o C x C y C m O Passive model 90 o 69 o Active model Figure 2: a) Model of the building in the working section of the wind tunnel- case of two high-rise buildings, b) wind attack directions during experiments in the interference situation. The passive model has not any measurement systems. It is used to arrange interference situations. Firstly, the active model, placed on a rotational table in the wind tunnel working section, has been examined for different wind inflow directions. Secondly, the interference configuration of the active and the passive models has been arranged and the measurements have been complited only for wind attack directions presented on Fig. 2. The choice of the wind directions for the interference measurement situations has been connected with a wind rose of strong winds in the building site and the neighbourhood of the second high-rise building. A set of aerodynamic force coefficients: C x - drag coefficient, C y - lift coeficient and C m - aerodynamic moment coefficient have been calculated as a function of an angle of an wind attack on the base of the mean pressure distributions in the various cross-sections of the building. During the calculations of aerodynamic force coefficients the following statements have been received (comp. Fig. 3.): a) dynamic pressure p i (t) is established in measurement point i (pp i ) and the pressure p i (t) is equal in every point along the fragment s i of the perimiter of the cross-section; b) the direction of dynamic pressure p i (t) along the fragment s i of the perimiter of the cross-section is parallel to unite vector n i [n xi, n yi ], which is perpendicular to the perimiter in measurement point i; c) vector r i [r xi, r yi ] determinates the coordinates of the measurements point i; d) the number of the measurements points (pp) which well enough characterized pressure distribution is equal to n. As a result of the statements, the wind force on the fragment s i of the perimiter of the cross-section could be caculated as: F i (t)=p i (t)s i n i (1) The global aerodynamic force F(t) in the cross-section plane of the structure element is obtained by summing up the n forces F i (t) along the perimiter: F(t)[F x (t),f y (t)] where F x n ( t) = Fxi ( t), Fy t) = i= 1 n ( F ( t) (2) i= 1 yi

4 The global aerodynamic moment M z (t) is calculated as: M z n ( t) = M ( t) where M zi (t)= - F xi (t) r yi+ F yi (t) r xi (3) i= 1 zi Y s i F i (t)=p i (t)s i n i n i [n xi, n yi ] pp i p i (t) F(t) [F x (t), F y (t)] D pp n-1 pp n pp 1 M z (t) α(t) G r i [r xi, r yi ] X pp 2 Figure 3: Methodology scheme of calculation of aerodynamic forces F x, F y and aerodynamic moment M z on a base of pressure distribution on outer surface of structure slender element On the base of mean values of the obtained forces, aerodynamic coefficients (C x - aerodynamic drag coefficient; C y - aerodynamic lift coefficient; C m - aerodynamic moment coefficient) have been calculated as functions of wind attack angle according to the formulae: F x( α ) F y( α ) M z( α ) C x( α ) =, C y( α ) =, Cm( α ) = (4) 2 q D q D q D ref V(t) ref where:α- wind attack angle, D- characteristic dimension, q ref - reference pressure. ref 1.3 Exemplary results of measurements. Exemplary functions of aerodynamic force coefficients C x, C y, C m of the building cross-section on the level m above ground (40 th floor) in the two considereted situations (one building and two buildings) are presented in Fig. 4. The angles of wind attack have been changed from 69 o to 201 o with the step of 3 o. The measurements have been completed for 4 cross-sections on different levels above the ground which have corresponded with the particular floors of the building.

5 a) b) one building two buildings c) one building two buildings one building two buildings Figure 4: Functions of aerodynamic coefficients a) drag coefficient C x, b) lift coefficients C y, c) moment coefficient C m for the cross-section of the building on the level m above ground (40 th floor) in the two considered situations (one building and two buildings) obtained from wind tunnel tests. Distributions of external mean pressure coefficients for the cross-section of the building on the level m above ground (40 th floor) in the two considered situations (one building and two buildings) obtained from wind tunnel tests are presented in Fig.5. The wind attack angle is equal to 135 o.

6 a) b) -0.1 C x Neighbour building C y M x O M z M y Wind 135 o C y C x M x O M z M y Wind 135 o Figure 5: Distributions of external mean pressure coefficients for the cross-section of the building on the level m above ground (40 th floor) in the two considered situations: a) one building, b) two buildings, obtained from wind tunnel tests (the wind attack angle is equal to 135 o ). The set of the aerodynamic coefficients functions for various cross-section shapes of the building has made possible the definition of wind action on the structure according to the quasi-steady theory. 2. NUMERICAL CALCULATIONS OF DYNAMIC RESPONSE OF THE BUILDING Analyses of the influence of aerodynamic interference on structural response of the high-rise building, in conditions of strong winds, has been conducted by numerical calculations. The model of wind load has been adopted in agreement with the quasi-steady concept, which takes into cosideration usteady air onflow. The aerodynamic feedbacks have been neglected as well as wind load caused by vortices. 2.1 Description of numerical calculation cases. A model of wind action has been adopted in agreement with the quasi-steady concept. The formulae of wind forces take into consideration aerodynamic turbulence. The formulae presented below describe respectively: along-wind F x (t), across-wind F y (t) and aerodynamic moment M z (t) wind load components for the structure fragment of the length of L. 2 F ( t ) = q( t ) C ( α ( t )) D L, F ( t ) = q( t ) C ( α ( t )) D L, M ( t ) = q( t ) C ( α ( t )) D L (5) x x y y where:α- time variable wind attack angle, D- characteristic dimension, L- length of the fragment of the building considering as a slender structure (the heigth of the storey of the building), q(t)- wind velocity pressure, C x, C y, C m - aerodynamic coefficients obtained from the wind tunnel tests. Time variable wind velocity pressure field q(t) has been adopted by numerical simulation of along-wind u(t) and horizontal across-wind v(t) components of time variable wind velocity V(t). As a result of the simulation the wind velocity pressure has been calculated in the various15 points along the height for the building according to the formula (6): z m

1 2 1 2 2 q( t ) = ρ V( t ) = ρ [u( t ) + v( t ) ] (6) 2 2 Time series of the wind velocity components, u(t) and v(t), obtained by the numerical simulation for two exemplary heigth levels are

7 q( t ) = ρ V( t ) = ρ [u( t ) + v( t ) ] (6) 2 2 Time series of the wind velocity components, u(t) and v(t), obtained by the numerical simulation for two exemplary heigth levels are presented in Fig.6. a) 40 u(t) [s] v(t) [s] b) 40 u(t) [s] v(t) [s] Figure 6: Time series of wind velocity components, u(t) and v(t), obtained by numerical simulation: a) on the heigth level of m above the ground (11th floor), b) on the height level of m above the ground (40th floor). The numerical calculations have been complited in ROBOT v.21.0 MES system by using lienear procedures. The MES model is presented in Fig.7. Figure 7: The MES model of the building

8 2.2 Results of numerical calculation. Responses of the building in condition of aerodynamic interference, as well as, without it, have been determined and analyzed. Below, chosen results of the numerical analyses are presented. In Fig.8 comparison of time series of the component displacement u x, u y of the reperentative point of the top building floor for two considered situations, without and with aerodynamic interference, are presented. Below, in Fig. 9, the coresponding acceleration components a x, a y are given. Planar motion of the representative point of the top building floor calculated without aerodynamic interference (without neighbouring bulding) and with effect of aerodynamic interference (neighbouring bulding is present) are presented in Fig.10. The numerical analyses have allowed to select a group of considerable stressed structure elements. A localization of a few of them is presented in Fig. 11. Simultaneously, a comparison of extreme stresses in the structure elements obtained as a result of numerical dynamic calculations under wind action and extreme stresses received from analyses of combinations of dead and service load (without wind action) has been inserted there. a) b) Figure 8: Comparison of time series of the component displacements u x, u y of the geometrical centre of the 44 building floor: a) calculations without aerodynamic interference, b) calculations with effect of aerodynamic interference.

a) b) Figure 9: Comparison of time series of the component accelerations a x, a y of the geometrical centre of the 44 building floor: a) calculations without aerodynamic interference, b) calculations

9 a) b) Figure 9: Comparison of time series of the component accelerations a x, a y of the geometrical centre of the 44 building floor: a) calculations without aerodynamic interference, b) calculations with effect of aerodynamic interference. a) b) Mean wind direction 135 o Neighbour Figure 10: Planar motion of the representative point of the top building floor: a) calculations without aerodynamic interference (without neighbouring bulding), b) calculations with effect of aerodynamic interference (with neighbouring bulding).

9001 5007 9002 9003 5113 5005 Extreme stresses Combinations of loads Element Wind action (without wind number action) S max [MPa] S min [MPa] S max [Mpa] 5005 1.66-4.64 29.10 5007 4.64-1.30 21.

44 Figure 11: Comparison of extreme stresses in chosen structure elements obtained as a result of numerical dynamic calculations under wind action and extreme stresses received from analyses of

10 Extreme stresses Combinations of loads Element Wind action (without wind number action) S max [MPa] S min [MPa] S max [Mpa] Figure 11: Comparison of extreme stresses in chosen structure elements obtained as a result of numerical dynamic calculations under wind action and extreme stresses received from analyses of combinations of dead and service load (without wind action). CONCLUSIONS On the base of experimental results and theoretical analyses following conclusions could be formulated: 1) A comparison of aerodynamic coefficients of the cross-sections of the building in two experimental situations shows that the aerodynamic interference has not a significant disadvantageous influence. It has not been observed any considerable increase in a level of the wind action on the building. 2) Comparison presented in Fig.10, which presented the planar motion of the representative point on the top level of the building, leads to the conclusion that, the effect of an aerodynamic wake of the neighbour building strongly reveals decrease in the vibration amplitudes. 3) The analyses have confirmed correctness of the design project in an aspect of wind action on the building in relation to ultimate limit states as well to serviceability limit states. REFERENCES Flaga A. (2008). Wind engineering. Fundamentals and Applications. Arkady, Warszawa (in Polish) Simiu E., Scanlan R. (1996). Wind effects on Structures. John Wiley & Sons, New York. Krishnaswamy T.N. Rao G.N., Durvasula S. Reddy K.R. (1975): Model observations of interference effects on oscilatory response of two identical stacks. Procedings of 4 th International Conference on Wind Effects on Buildings and Structures, London, Heaththrow,

J. A. Jurado, S. Hernández, A. Baldomir, F. Nieto, School of Civil Engineering of the University of La Coruña.

EACWE 5 Florence, Italy 19 th 23 rd July 2009 Flying Sphere image Museo Ideale L. Da Vinci Aeroelastic Analysis of Miradoiros Bridge in La Coruña (Spain) J. A. Jurado, S. Hernández, A. Baldomir, F. Nieto,