Investigation in Wind Tunnel Flow Interaction with Models of Two High-Rise Apartment Houses E.A

International Conference on Methods of Aerophysical Research, ICMAR 2008

INVESTIGATION IN WIND TUNNEL FLOW INTERACTION WITH MODELS OF TWO HIGH-RISE APARTMENT HOUSES E.A. Zikov1, A.V. Liabchuk1, A.D. Obukhovsky2, S.D. Salenko2, J.V. Telkova2 1Architect's office “Tectonics” LTD, 660018, Krasnoyarsk, Russia 2 Novosibirsk State Technical University, 630092, Novosibirsk, Russia

During the design of buildings with height more than 75...100 m interaction the buildings with wind flow becomes the important factor. This factor determines safety of the upkeep of buildings and comfort of people situated inside and near the buildings [1, 5]. The purpose of the work is the wind-tunnel investigation of the flow around the two high-rise buildings under interference conditions, the determination of the total and distributed wind load acting on the building’s surface of housing estate, the investigation of influence of the high-rise buildings on the neighbouring buildings. The experiments were carried out in a wind tunnel T-503 at the industrial aerodynamics laboratory of NSTU. The department of aerohydrodynamics has many years experience of studies on the concerned research area [2, 3, 4]. In particular, the modeling technique of surface boundary layer in a wind tunnel was worked out by the staff of the department and damping devices for five bridge spans were developed. The high-rise buildings under consideration (Fig. 1) had a constant width of the facade Bb=32 м along the height and a variable width along the height at the side view. Height of the buildings was 104.7 m (above a ground apparatus floor); width of the maximum section was 22×32 m.

Fig. 1. Overall view of the high-rise buildings models and experimental assembly

Section 1

The models of the high-rise buildings were made from a plastic on 3D-printer according to given CAD-geometry. Accepted scale (1:250) and the application of 3D-printer technology promoted to reconstruct on the dummy a complex structure of the high-rise buildings surface including balconies, recessed balconies, jogs and other elements, important for shaping of flow. Lay of land, buildings and forest-plantations surrounded the high-rise complex were reproduced on the dummy. For simulation of the atmosphere boundary layer a grate with varied array spacing along a height was installed at the nozzle of wind tunnel. During the work a flow around the high-rise buildings was visualized by smoke stream technique, smoking wire method and wool-tuft technique using all-round blowing of the dummy. In this case a form of streamlines, characteristics of separated shear-layer areas and vortex flows were investigated. In ruling southwestern and opposite northeast wind directions one of the buildings is completely located in the wake of another (Fig. 2). In this case between the buildings throughout entire height is formed the stagnation zone with the intensive mixing inside the zone (Fig. 2b, Fig. 3) and the weak exchange with the external flow. This fact must be considered at design regimes of the work of the ventilation and conditioning systems of buildings. In the southeastern (Fig.4) and northwestern wind directions is observed the strong constriction of flow stream between the high-rise buildings, which causes an increase in the rate of flow and dynamic pressure. The growth of the latter can lead to the increased aerodynamic loadings on the attached facade systems of buildings in the sections of the minimum distance between the high-rises. In the wind of north (Fig. 5) and close to it directions high-rise buildings flow around, practically as unified whole, forming after themselves wide wake. In this case lower part of buildings and pedestrian zones fall into the wake, which appears with the flow around located above along the flow of city block construction. This must lead to decrease average wind speed and increase pulsating component in these zones. In the southern and close to it wind directions area relief renders essential influence to the nature of flow. The wind velocity profile above the relatively smooth aqueous surface becomes more filled, then with the flow upward along the slope of the bank of river occurs the compression of stream filaments and, therefore, should be expected increase in the speed in the lower part of the building and in the pedestrian zones. In this case, on the contrary, many buildings of city block fall into the aerodynamic shadow of high-rises. At streamlining of buildings (especially in the wind directions the approximately perpendicular to any wall) the extensive tear-off zone appears above the roofing (Fig. 6), in diagonal wind directions the pair of the vortices of counterrotation is formed above the roofing. These phenomena can present danger to the takeoff and landing maneuvers of helicopters. Flow pattern in the pedestrian zones has very complex nature. It is possible to mark the stagnation zones (Fig. 3), the sections of direct and reverse flows, vortex and screw flows. Into flow in the pedestrian zones has a strong effect the contraction of stream filaments with the air flow between the high-rises (Fig. 4), the horseshoe vortices in the lower part of the buildings, downflows along the windward walls (Fig. 7). The knowledge of flow structure in the environment of the buildings in different wind directions will make it possible to more deliberately solve the problems of the calculation of the local stiffenings of wind loads with the design of attached facade systems, ventilation of buildings, guarantee of safe operation of helicopter areas, comfort in the pedestrian zones.

International Conference on Methods of Aerophysical Research, ICMAR 2008

a) b) Fig. 2. Flow visualization around the high-rise buildings in the southwestern wind direction

a) b) Fig. 3. Flow visualization around the high-rise buildings in the southwestern wind direction

Fig. 4. Flow visualization in Fig. 5. Flow visualization in the southeastern wind direction under northern wind direction

Section 1

a) b) Fig. 6. Flow visualization above the roofing of buildings in horizontal (a) and vertical (b) arrangement of the smoking wire

a) b) Fig. 7. Flow visualization in the lower part of the buildings in northern (a) and northwestern (b) wind direction.

As is known, regular eddy formation from the bluff body surface can lead to appearance of one of the forms of the aeroelastic oscillations of construction - wind resonance. It occurs when the frequency of trailing vortexes it coincides with one of the natural frequencies of the construction. In this case the intensive oscillations of construction across the flow appear. Trailing vortex from the surface of the bluff bodies is characterized by Strouhal number: Sh=fB/V, where f - the frequency of trailing vortexes, V - velocity of incident flow, B - width of the matter across the flow. For determining characteristic Strouhal numbers the measurements of the stream-velocity fluctuations in the environment of buildings with the circular scavengings were made. The pulsations of speed were measured with the aid of the double hot-wire anemometer of the fixed resistance, whose the sensor was established usually on the 0.75 of buildings height. The results of studies showed that independent of wind direction in the environment of the pair of buildings the frequency spectrum of the pulsations was washed away (Fig. 8c), that it does not make it possible to clearly mark one prevailing frequency and number Sh corresponding to it. It is known that after the single cylindrical body, located perpendicularly to the direction of the steady flow, is formed the Karman vortex street, and in the spectrum the clear peak of the frequency of trailing vortexes is observed. However, many factors influence the spectrum of the frequencies of two high-rise buildings being investigated: the exponential low of the distribution of vertical speed, the increased flow turbulence, the underlying surface, surrounding urban building, the variable the buildings section width at the height, and also the interference of two closely spaced buildings. Therefore for the development of the major factors, which lead to “erosion” of frequency spectrum, it was decided to conduct the number of additional studies with the similar prismatic body. Measurements were conducted on the screen in the flow without the grate and after the grate. Thus, the influence all of factors enumerated above consecutively was excluded. Experiments were

International Conference on Methods of Aerophysical Research, ICMAR 2008 conducted at Reynolds numbers Re = 0.5·105 – 1.4·105. It was established that in the flow after the single prism in the uniform flow is observed the clear peak of frequency corresponding to Strouhal numbers Sh = 0..09, when prism was established across the flow and Sh = 0.15, when prism was established along the flow. These results will agree rather well with literature data, given for the prisms of small lengthening and similar cross section. Analogous results are obtained also for the single high-rise building, which has in comparison with the prismatic body the section variable on the height (Fig. 8b). For determining the influence of the distribution of the flow parameters, typical for the atmospheric boundary layer, prismatic body was investigated in the flow above the screen after the irregular grate with Reynolds numbers Re = 0,4·105 – 1,1·105. Results showed that in the frequency spectrum also was present the clearly expressed peak, which corresponds to Strouhal numbers Sh = 0.11, when prism was established across the flow and Sh = 0,13, when prism was established lengthwise. Model of one of the high-rise buildings, which has section variable on the height in comparison with the prismatic body, was investigated at the same conditions. In the flow after the building, just as after the single prism in the uniform flow, was observed sufficiently clear peak (Fig. 8b). Reynolds numbers in this case changed in the range from 0.3·105 до 0..9·105, and Strouhal numbers took values of 0.12 and 0.11 during the longitudinal and transverse arrangement of building respectively. According to the obtained results it is possible to draw the conclusion that in itself the flow turbulization by the irregular grate and the variable width of the buildings section along the height do not have a strong effect on the nature of spectrum. The measurements of pulsations in the flow after the single high-rise building were also carried out on the screen with the relief after the turbulence generating irregular grate. Obtained Strouhal numbers practically coincided with those indicated above. For the pair of high-rise buildings, in spite of strongly “diffuse” nature of frequency spectrum, on the basis carried out experiences it is possible to recommend the average value of Strouhal number of order 0.1.

Fig. 8. Spectrum of velocity pulsations in the environment of the high-rise buildings a) schematize prismatic building in uniform flow, b) single high-rise building, c) two high-rise buildings with the simulation of the surrounding situation.

Thus, should be drawn the conclusion that the major factor, which destroys regular trailing vortex from the surface of high-rise buildings, is their mutual influence, i.e., the aerodynamic interference. The successful architectural solution led to the fact that the vortex structures, formed by each building, during interaction with each other are decomposed. Vortices with the different parameters induce the wide frequency spectrum of pulsations in the flow. It is possible to assume that the duct of air between the high-rise buildings also exerts the damping influence on the eddy formation. The absence of the prevailing frequency practically leads to impossibility of the appearance of wind resonance for under study pair of high-rise buildings. Nevertheless it is necessary to note that

Section 1

this specific case “positive” interference is random success. With the different versions of mutual arrangement for the construction with the close geometric parameters of cross sections ше шы possible the most significant strengthening of vortex excitation, in comparison with the isolated body. For investigating total and distributed average components of aerodynamic loadings on the surface of habitable complex the drained models of high-rise buildings were prepared at scale 1:250 (Fig. 9). The surfaces of models were drained in three horizontal sections throughout the height of building, which correspond to the levels of z=0,5H, z=0,75H and z=0,95H, at the same time in each section the drain holes of at a distance 1,5 m (for the full-scale object) from the angles of buildings were executed. The roofs was also drained on the outline at a distance 1,5 m (for the full- scale object) from the edge of roofs. In each of the indicated sections was executed not less than 20 drain holes with a diameter of 0,8 mm. in addition to this, were executed the additional drain holes in the assumed places of the increased local loads (in the region of the rounding of the edges of roofing, in the edges of buildings adjacent to each other). Altogether on the surface of habitable complex model were executed more than 200 drain holes.

Fig. 9. The general view of the microdistrict model in the wind tunnel with the drained models of high-rise buildings

For measurement of average components of static pressure on the surface of habitable complex drain holes were connected with the multichannel liquid manometer. In the course of experiments the microdistrict model with the drained models of high-rise buildings were blown out by flow around from 32 directions. In this series of experiments the characteristic speed recognize as speed of flow at the height of the roofing of the building above the zero level (under the actual conditions Href = 108 m). An error in the determination of the coefficients of mean pressure was approximately ±0,02 for the windward faces and near ±0,05 for the leeward. Relatively high error is connected with flow separation and, as a result, with the high pulsations of pressure. In certain cases on the roofing was observed the intermittence of flow - flow pattern, when in an unpredictable manner (approximately through 0,5… 2 sec under the conditions for experiment) a change in the flow pattern with the flow around the elements of building with the constant boundary entrance conditions into the working

International Conference on Methods of Aerophysical Research, ICMAR 2008 part of the pipe occurs. In the case of the intermittence of the flow the change of mean pressure coefficients at some points reached ±0.2. An example of the diagram of the distribution of the pressure coefficients on the surface of high-rise buildings in the southeastern wind direction is given in Fig. 10.

Fig. 10. Distribution of the pressure coefficients on the surface of the buildings (red color - positive overpressure, dark-blue - negative overpressure)

Experiments showed that in the most unfavorable case the value of resulting average component of wind load on 14% exceeds that calculated employing the procedure of the existing Construction Norms and Regulations (CNR). But at the same time the bending moment in the base of high-rise building to 3% is less, calculated employing procedure CNR, because of the more favorable load distribution along the vertical coordinate. For investigating pulsating components of aerodynamic loadings on the surface of habitable complex were used the same drained models as during the study of average components of loads (Fig. 9). For measuring the overpressure the pressure sensors, located in immediate proximity from the drainage points, were used. The length of each realization composed 2048 measurements during approximately 5 s, sampling frequency - about 400 Hz, which under the actual conditions at the time scale 1:250 is about 20 minutes and 1,6 Hz, respectively. An error in the determination of pulsating component of the pressure coefficient was approximately ±0,04 for the windward faces and near ±0,06 for the leeward. Relatively high error is connected with flow separation and, as a result, with the high pulsations of pressure.

Section 1

In the process of experiments averaged and pulsating components of wind load, Strouhal number for pulsating components were determined, the action of high-rise buildings on the adjacent buildings is investigated. The values of pulsating components of wind load are obtained and the correlations of pressure pulsations at different points on the surface of buildings are calculated. From the point of view of fastening the elements of enclosure, hook-up elements of facade systems it is necessary to investigate maximum of the local pressure coefficient values. The information about the values of average and peak components of the pressure coefficient for the points, located near the local contraction of the flow between the houses, was analyzed for the development of the possible negative influence of aerodynamic interference between the houses. It would seem, the local contraction of flow can lead to the growth of dynamic pressure and, therefore, to a considerable increase of negative pressure. However, accordingly experimental data, this it does not occur: the coefficient of average pressure component is not less (in the worst case) approximately Ср=-1.1. Experiments showed that the maximum negative pressure is observed on the roofing of high- rise buildings. The value of the coefficient of average pressure component attains the value Ср=- 2,42, that to 20% exceeds the recommended by CNR value Ср= -2. In the rounded off sections of roofing the coefficient of the maximum negative pressure Ср=-1,83, on the walls of building is not more than Ср=-1,83. Thus, interference interaction between the high-rise buildings will not lead to increase in the tearing off efforts in the places of the flow contraction between the buildings to the levels, which exceed recommended by CNR. Experiments showed that this is correct not only for the averages, but also for the peak values of pressure. Sections on the edge of the roofing, where the exceeding in comparison with the recommendations of CNR composes 20%, are most dangerous. The carried out aerodynamic investigations made it possible to precise aerodynamic loadings both on the high-rise buildings as a whole and on the elements of enclosure, hook-up elements of facade systems. These results will make it possible to ensure safety of buildings under the effect of the high winds and comfort of people inside and near the buildings. As a whole the carried out work showed that from the point of view of aerodynamics the accepted architectural-planning scheme should be recognized as successful.

Литература

1. Besprozvannaya I. M., Socolov A.G., Fomin G. M. Wind effect on high air-tight construction. - M.: Stroyizdat, 1976. - 183 p. 2. Salenko S.D. Calculation Technique For Aeroelastic Oscillations Of Multibeam Constructions // J. of Aplied Mech. And Techn. Physics. – 2001. – Vol. 42, № 5. – P. 872–877. 3. Salenko S.D., Kuraev A.A., Akopov V.I. and Kanunnicov A.B. Dampers of aeroelastic vibrations of bridgework cantilevers// Stroitel’stvo i rekonstructsiya zheleznodorozhnykh i avtodorozhnykh mostov. 1996. No. 1. P. 53-58. 4. Salenko S.D., Obukhovsky A.D., Gorban R.A. Investigation of the separation flow in the vicinity of multi–beam prismatic structures // Proc. of Xth Intern. Conf. “Мethods of aerophysical research”. – Novosibirsk, 2000. – Part 2. – P. 153–158. 5. Simiu E., Scanlan R. Wind effects on Structures // An Introduction to Wind Engineering. – N.-Y., 1978.