Wind Tunnel Investigations on Models of RC Chimneys and Cooling Towers including Group Effects
Several practical applications call for a better understanding of the flow phenomena around structures to assess wind loads and wind induced behaviour of structures, when they are located in isolation or in a group. These include structures surrounded by other structures, viz. tall chimneys, cooling towers,other surrounding power plant structures, etc. The responses of these structures to wind in their real environments can be considerably different from those measured/evaluated in the isolated condition. Neighbouring structures may either decrease or increase the wind-induced forces on a structure, commonly called as interference effects, and mainly depend on the geometry, spacing, type of arrangement (tandem or staggered), angle of wind incidence, on-coming turbulence due to upstream terrain conditions, etc. Proper assessment of the interference effects is very important for the design of wind sensitive structures. The existing provisions in various codes of practices on wind effects on structures offer limited guidance to the designer, in assessing the effects of interference.
Hence many wind tunnel investigations have been carried out to evaluate the interference effects on industrial tall RC Chimneys, Cooling towers etc., due to similar structures or other surrounding structures. The following are some of the outcomes from these investigations:
- Guidelines to estimate the interference factors between tall RC chimneys arranged in tandem configuration.
- Guidelines to estimate the interference factorsfor tall RC chimneys surrounded by power plant structures.
- Guidelines on aerodynamic coefficients and interference factors for cooling towers
- Studies on effectiveness of strakes on Chimney both in isolated and grouped conditions
- Studies on effectiveness of ribs on cooling towers
Wind Tunnel Investigations on Tall Rectangular Building Models
- Most of the national/international codes on wind loads provide mean drag coefficient values for tall rectangular buildings in the form of uniform pressure/force coefficients.
- The coefficients provided in IS 875 (Part 3) -1987 are mainly based on wind tunnel model studies under uniform smooth flows, whereas many of the current international standards consider the boundary layer flow effects on the local force coefficients for tall buildings.
- Further these codes do not explicitly address the effect of terrain condition and the effect of oblique angles of wind incidence on these coefficients.
- Hence, systematic investigations have been carried out on a set of tall rectangular building models under simulated open and sub-urban terrain conditions in the boundary layer wind tunnel.
Based on these investigations, design guidelines on force coefficients for rectangular buildings have been formulated and presented to BIS codal committee
Full-scale Investigations on Free Standing and Guyed Lattice Towers
Lattice steel towers are used for a number of diverse purposes, namely, telecommunications, radio and television broadcasting, observation, power transmission and lighting supports, etc. The need to design a lattice tower for resonant dynamic response due to wind load arises when the natural vibration frequency (fundamental frequency) of the structure is low enough to be excited by the turbulence in the natural wind.
Full-scale investigation on a 52 m tall free standing lattice tower has been carried out using anemometers, accelerometers and strain gauges to measure wind characteristics and structural dynamic response characteristics.
- The GRF values for measured base bending moment are found compare well with those obtained from the IScode (with and without f) and AS code (with gB and with gu) and fall between these codal values. Hence, it is necessary to consider the `f’ value in the evaluation of GRF values based on the IS code, irrespective of the terrain category.
Guyed masts are used for wireless communication, meteorological measurements, and recently, even for power transmission. The behaviour of the mast is non-linear due to its slenderness and compliant ‘guy-support’ system. The guys also exhibit nonlinear behaviour especially at low values of pretension due to possible multimodal excitations and dynamic response to wind turbulence. Full-scale investigations on a 50 m tall guyed lattice tower has been carried out using anemometers and accelerometers to measure wind characteristics and dynamic response characteristics.
- The r.m.s. values of displacements evaluated for the measured wind speeds using the patch load patterns suggested by Davenport and Spalding, which avoid complex non-linear transient dynamic analysis of guyed masts for evaluation of peak ﬂuctuating responses, were observed to be comparable to the measured values observed during the measurement programme.
Wind Tunnel Investigations on Curved Roof Structure Models
- Systematic investigations using pressure measurements have been carried out on a curved roof structure model with different wall-height to span ratios, accounting for the effect of angles of wind incidence and terrain condition. These aspects are not covered in IS 875 (Part 3) -1987, which provides the pressure coefficients under uniform flow condition.
Based on these investigations, design guidelines on pressure coefficients for curved roof structures have been formulated.
2-D CFD Investigations on Bluff-body and Bridge Deck Sections
A series of 2-D CFD investigations have been carried out on various geometries, viz. square, rectangular, circular, triangular, L-angle, 4 L-angles, H-section, Trapezoid, curved roofs, etc. to evaluate aerodynamic pressure coefficients and drag & lift force coefficients. These evaluated coefficients have been compared with experimental results to validate the performance of different turbulence models using commercially available software (FLUENT 6.3).
Full-scale Investigations on Wind Characteristics during Normal and Extreme Winds
The annual average number of tropical cyclones occurring all over the world is around 92. The Indian ocean is one of the six major tropical cyclone prone regions of the world. Every year, four to ﬁve tropical cyclones occur in the Indian ocean, 80% of which cross the east coast of India. A large number of buildings and structures, including some well-engineered structures, have been reported to be damaged during tropical cyclones.The characteristics of tropical cyclone winds are quite different from those of normal winds.This stresses the need to study the various characteristics of tropical cyclone winds. CSIR-SERC has carried out many full-scale field investigations on wind characteristics during normal and extreme wind conditions using propeller anemometers and sonic anemometers.
- Based on these investigations, the turbulence intensities are observed to be more during cyclone wind conditions than those under normal wind conditions. Hence, the gust velocity factor values are also more during cyclone wind conditions.
- Further, some of the extreme wind conditions indicated non-Gaussian and non-Stationary characteristics also.
- During one of the cyclone wind condition, the turbulence wind spectrum indicated higher energy levels in high frequency region than the conventional turbulence spectrum.
Failure Investigations on Microwave Lattice Towers during Cyclones
Along the coastal regions of India, many microwave lattice towers have collapsed during cyclones with lesser wind speeds than the design basic wind speed. Hence, failure investigations have been carried out to identify the possible failure reasons by considering the cyclone wind characteristics, viz. turbulence intensity and turbulence spectrum.
- The investigations indicated that the Gust Effectiveness Factor values increased due to higher turbulence during the cyclone wind conditions, which lead to the increased dynamic response of the lattice towers and causing their failure.
- Based on this investigation, a new formulation has been proposed to evaluate Gust Effectiveness Factor for lattice towers in cyclone prone regions.
Post-Cyclone Structural Damage Surveys
The coastal belt of peninsular India, especially the east coast, experiences frequent cyclones. Such cyclones coupled with storm surges cause loss of lives and inflict severe damage to a variety of structures, houses, commercial buildings, industrial structures and many life-line installations. CSIR-SERC has been conducting post-disaster damage surveys on buildings and structures ravaged by cyclones from time to time to identify most common types of failures. Detailed surveys are undertaken after the occurrence of every severe cyclone in the peninsular India since 1977. The most common types of failures observed during the recently occurred tropical cyclones, viz. Thane, Phailin, HudHud, are listed below:
- Blowing of Roof sheets and false ceilings of fuel filling stations
- Blowing of roof covering metal sheets and conventional asbestos sheets –Airport building (large roof), steel plant structures and many residential and industrial buildings.
- Broken Glass Cladding as well as metal sheet external cladding of many car show rooms, residential buildings, IT (Information Technology) Buildings.
- Rooftop lattice towers, ground based communication / transmission-line lattice towers.
- Steel trusses with asbestos or G.I. sheet roofing of industrial sheds (either mono or double sloped).
- total / partial collapse of semi / non-engineered structures like compound walls, roofs of dwelling units (kutcha / thatched houses).
Wind Tunnel Investigations on Buildings with Unconventional /Non-Prismatic Shapes
In order to study the differences in aerodynamic characteristics between tall building models with unconventional / non-prismatic plan shape and tall buildings with conventional and prismatic rectangular plan shape, pressure measurement studies have been carried out on various tall building models with tapering, setback, twisted, corner cut, elliptical features. The evaluated aerodynamic coefficients have been compared to identify favourable or adverse effects with reference to buildings with conventional / prismatic shapes.
Wind Tunnel Investigations on Cyclone Shelter Models
CSIR-SERC has carried out pressure measurement studies on scaled models of two cyclone shelters under simulated cyclonic characteristics using an in-house developed auxiliary device. The pressure coefficient distributions on various sections of the cyclone shelter model for 0° angle of wind incidence (in which longer dimension of the cyclone shelter is exposed to wind) are obtained. Based on these investigations, design recommendations have been formulated for the evaluation of wind loads on cyclone shelters.
Formulation of Semi-Empirical Expression for Evaluating Across-wind Response of RC Chimneys
A semi-empirical method was formulated for computation of across-wind response of circular chimneys due to vortex shedding under lock-in region Further, the spectrum of generalized lift force is modified through a non-dimensional parameter ‘fact’ to account for the enhanced effect of lock-in phenomenon, and only by using the structural damping with the principles of conventional structural dynamics, the response is evaluated. A value of 0.45 was suggested for the parameter ‘fact’.
3-D CFD Investigations on Buildings including Group Effects
A series of 3-D CFD investigations have been carried out on various buildings, viz. tall square building, two square buildings in tandem, tall rectangular buildings, Tall twisted rectangular building, Tall elliptical building, Tall rectangular building with set-back, Tall tapered square building, solar panel with reflectors, array of solar panels, etc.to evaluate aerodynamic pressure coefficients and drag & lift force coefficients, and interference factors. These evaluated coefficients and factors have been compared with experimental results to validate the performance of different turbulence models using commercially available software (FLUENT 6.3).