Theoretical And Experimental Research On Wind Load And Wind Induced Response Of Large Hyperbolic Cooling Towers

Nowadays, with rapid development of power, nuclear, and mining industry, many large hyperbolic cooling-towers which can produce recycled cooling water are widely used. The volume of cooling-towers becomes large and the distribution density becomes dense. Because of the characteristics of tall, large, light weight, compressed frequency of the structure, it is very wind-sensitive, and the wind load becomes the main problem of structural design. With the complexity relationship of large cooling-tower structures and its air flow around, still now, a general theory framework of wind load distribution and wind-induced dynamic responses for a common hyperbolic cooling-tower has not been established. This dissertation tries to study deeply the characteristics of wind-induced surface load distribution, the interference effects and wind-induced dynamic response for the structure, by the away of Computational Fluid Dynamics(CFD) simulation, single tower wind tunnel test, two or three adjacent cooling-towers interference wind tunnel test and wind-induced structural dynamics response analysis in the frequency domain and time domain and other aspects.1. Numerical simulation of air flow around large hyperbolic cooling-tower. Basic theory of CFD is elucidated, some commonly-used simulated methods of air flow around cooling-tower are provided in detail. With help of commercial CFD software, the air flow around and in the cooling-tower is simulated, the wind-induced pressure coefficient distribution on inner and outer surface of the structure is obtained. Based on the characteristics of the flow field around the structure, the nature of fringe effect on top and bottom pressure coefficient is explained, the reason of stability of inner and outer leeward pressure coefficient is discussed.2. Investigation of external roughness effects on the mean wind pressure distribution on hyperbolic cooling-towers. A contraction scale model of large hyperbolic cooling-tower is made, the model with various height and distribution of external roughness is studied in wind tunnel test, different pressure coefficient distributions are obtained. Based on the comparison of different distributions under various external roughness, we find that, the minimum pressure coefficient is sensitive to the external roughness, the value increases when the roughness increases, nevertheless, the external roughness has smaller effects on leeward steady pressure coefficient and position of separation point. By summarizing the wind tunnel data and former researchers' results, an approximate formula is proposed to help the design work, which can estimate the pressure coefficients around the circumference of the structure based on the characteristics of the external roughness.3. Deep study of the wind pressure coefficient around the whole hyperbolic cooling-tower. Based on the sound analysis of the wind tunnel test data, the variation of pressure coefficient along with meridian and circumference is discussed, along the circumference three area is proposed: windward area, separation area and leeward area. Based on the comparison of wind pressure spectra along meridian and circumference, the similarity of the normalized spectra along meridian is found, but the the normalized spectra along circumference is quite different, because the spectra near the separation point are significantly influenced by vortex shedding. By circumferential and meridional pressure correlation study, we point out that, the correlation between windward area and leeward area is very small. Comparison between normalized spectra of drag force and lift force is made, it shows drag force spectra is relatively broad-band, and lift force spectra is relatively narrow-band.4. Study of wind-induced interference effects on two adjacent cooling-towers. The results of experimental test carried out in the boundary layer wind tunnel on a model of two adjacent cooling-towers are presented, it shows that, when two towers are in tandem configuration, there is a critical distance between them, which should be the minimum distance we advise in the design work, when two towers are in side-by-side configuration, the block effect is more remarkable than the "Venturi" effect, when two towers are in staggered configurations, it can be divided into three regions: wake influence region, weak influence region and side influence region. Based on the comparison of drag force and lift force under various configurations, an approximate formula is proposed, which can estimate the total force interference coefficient when the distance between two towers is given. At last, the envelop of geometry coefficient is drawn, and the Fourier formula coefficient is given.5. Study of wind-induced interference effects on three adjacent cooling-towers. Some results of experimental test carried out in the boundary layer wind tunnel on a model of three adjacent cooling-towers are discussed by the study of three parameters, namely, distance between interference towers, distance between measure tower and interference towers, and wind attack angle. Mean and dynamic total wind force interference coefficients are calculated, the results show that, strong effects are restricted to small ranges of the flow angle, for the other wind directions, the surrounding towers provide a shelter effect. The shelter effect not only lead to smaller mean total wind force, but also increases the fluctuating wind force, so the dynamic total wind force would be much higher. Meanwhile, the influenced tower's wind force spectrum is quite different from that of isolated tower.6. Study of the wind-induced dynamic response of large hyperbolic cooling-tower structures in the frequency domain. Some frequency analysis methods of wind-induced dynamic response are provided in detail, we adjust them to hyperbolic cooling-tower thin-walled structure. By parameter(tower total height, shell thickness, meridional curvature) analysis of cooling-tower natural frequency, we find that, the basic natural frequency of cooling tower is sensitive to the top meridional curvature, an approximate formula is proposed based on the parameters, which can estimate the basic natural frequency of cooling tower. With help of wind tunnel test data and CQC method, the dynamic response of cooling-tower in frequency domain is obtained, by comparison of displacement results under various mode level, we suggest that, prior 50 mode is necessary in order to gain a convergence result, higher mode is neglectable. In the end, an approximate function is proposed to simulate the original scattered spectrum of wind pressure based on the wind tunnel test data.7. Study of the wind-induced dynamic response of large hyperbolic cooling-tower structures in the time domain. Some commonly-used simulated methods of wind fluctuation are provided in detail, it shows the AR models with 1 or 2 order are appropriate for simulating the wind pressure series. With the help of POD method, more wind pressure series can be regenerated, by the way of finite element analysis, dynamic displacement and meridional stress response of cooling-tower shell is obtained. The dynamic coefficients with respect to various height and response are calculated, by considering the rationality of all the dynamic coefficients, we suggest that the total dynamic coefficient is about 1.9.