School of Design and Engineering Kanbar College of Design, Engineering & Commerce Philadelphia University
A report on
Aerodynamics of Buildings
Course:
ENGR 311 Fluid Mechanics
Students: Cody G. Borders Curran J. Kneebone
Fall 2013
Abstract: The stresses on buildings from constant wind forces are easily able to fatigue a structure if not constructed properly. The wind forces being observed are considered to act under the principles of fluid dynamics. Fluid dynamics takes into consideration a moving fluid and the forces acting on that fluid. This provides the behavior of the interaction between the air and an object. Using the principles of fluid dynamics, we analyze the aerodynamics and fluid flow around buildings. Our focus is primarily on tall buildings because their construction is highly dependent on the behavior in high winds. We render these tall buildings in the drafting program SolidWorks. SolidWorks is programmed with a flow simulation, which allows us to change multiple factors of the environment and structure of the building and observe the structures’ reaction to these changes. Introduction: Over the past century buildings have risen to new heights, and what used to be considered science fiction in the world of engineering has become a reality. With the undertaking of such bold skyscraper projects, new problems for engineers have risen. One of these main problems is the wind. At such great heights these buildings are subjected to constant extreme loading by the wind. Due to this continued stress many new technologies have risen to help engineers deal with the wind problem and continue to build higher and higher. Some of these technologies include continued development of shear walls, the exterior shape of buildings, and materials used in construction. Engineers still struggle with overcoming wind loading in construction, but with these technological advancements we are able to build these impressive structures yet higher and higher. As buildings have risen higher and higher the development of shear walls has been very important in facilitating this growth. In construction, shear walls are used for resisting the lateral forces applied by the wind (Ali, 2001). Shear walls work by absorbing the incident force of the wind at the floor of each level of the building and transferring this force to the girders and columns in the building, therefore dissipating the load (Ali, 2001). By dissipating this force, the building is less likely to oscillate back and forth, which would result in additional strain and failure of different members of the building. In the past, shear walls were completely solid and typically made of concrete; however, as buildings grew taller it was found that the completely solid structure of shear walls was too heavy and used excessive material. Therefore, shear walls which were only composed of the frame of the shear wall were constructed using steel beams rather than solid concrete walls (Ali, 2001). Today shear walls can be found in different locations of high rise buildings depending on the structural needs of the building. Some are found in the center of the building surrounding the elevator shaft, encompassing the building as an exterior bracing system, or acting as partitions throughout the building (Ali, 2001).
Every object behaves uniquely in fluids. The shape of each object determines how the fluid behaves when in contact with its surface. Buildings may be subject to large amounts of drag force simply due to their shape. We try optimizing the external shape in order to reduce the force applied by wind. This is particularly important in tall buildings where a force applied to the highest point of that building will create a large moment at its base. We are able to analyze the shapes of buildings and assign a drag coefficient which is the relation of force to the frontal area of the shape. Typically, blunt shapes with large frontal areas have a very poor drag coefficient. However, shapes can be streamlined to decrease pressure drag by delaying separation which just reduces the pressure difference between the front and back of the object (Cimbala, 607-643). Fortunately many architects have taken aerodynamics into account when designing larger buildings. Other than the pressure of the building’s upper floors compiling on lower floors, environmental forces, mainly wind, apply the most stress and can cause failure. Several adaptations which are employed in high rises include chamfered corners, fins, setbacks, buttresses, horizontal through building openings, and tapering (Kim, 2002). It has been found that these designs reduce vortex shedding, and ultimately reduce the load the building experiences. In tests it has been found that chamfered corners on a building can reduce wake excited responses in a building by up to 30% (Kim, 2002). Another method employed to reduce wind induced excitations of buildings are active and passive mass dampening systems (Xu, 1996). When wind exerts a force on the building, part of the building, specifically the higher portions of the building, and its occupants will feel an acceleration in the direction the force of the wind is acting. The dampening systems works to counteract that. If there is a force on the left side of the building pushing to the right, a mass dampening system will shift and push to the left in order to cancel out the acceleration due to the wind (Xu, 1996). In all the frequencies and amplitudes of the force created by the mass damping system should be equal to the forces of the wind on the building. Throughout the years, the adaptations of the exterior of buildings, as well as mass dampening systems, have proven effective and allowed us to continue to build higher and higher. The development of materials used in the construction of buildings has also helped engineers continue to build higher. With old construction designs, buildings were too heavy to be built high, and there was too much congested steel work in buildings, however, over the years this has changed. As stated previously, rather than building solid concrete shear walls, steel frames have been used to reduce weight as well as cross sectional area (Nehdi, 2013). Due to advances in the development of concrete, it is still used today in low and high rise structures. Today a common type of concrete used is high elastic modulus concrete, since it offers strength and stiffness, but can also deflect under a load (Nehdi, 2013). Due to this the structure is strong enough to stand, but at the same time can deflect under excessive loading in order to not experience a brittle failure.
Experiment: This experiment was conducted to analyze the aerodynamics of airflow around tall buildings. In the construction of skyscrapers, the exterior shape of the building will decide how much load the building will take from the wind, and in turn how much wind load the building must be able to withstand before failure. Several common adaptations of skyscraper structure to withstand wind include, tapering of the building, chamfered corners, or stepback construction of the building. We render these multiple exterior designs in SolidWorks to test their simulations. Using the flow simulation integrated into the program SolidWorks, we study the airflow at varying speeds and can determine which design is best for dissipating wind loads. This is useful to find which design will use less building material and be more cost effective to build. Data: In order to determine the best building design for dissipating wind loads, several building designs can be compared when exposed to a 30m/s wind. The selected styles examined will be a building with tapered corners of 10%, a building with chamfered corners of 10 degrees, and a building of stepback construction with stepbacks decreasing the top surface area of the building by 1 m2 every 30 m of elevation. By determining the drag coefficient of each style and comparing the calculated values, we can deduce which form of architecture dissipates wind the best and experiences the least drag. To find the drag coefficient, we used the following equation: 𝐹𝑑 𝐶𝑑 = 1 (1) 𝜌𝑣 2 𝐴 2 where 𝐹𝑑 is the resulting drag force, 𝜌 is the density of the fluid which for our purposes is air, 𝑣 is the air velocity since our object is stationary, and A is the frontal area of the object. We are able to measure the drag force in SolidWorks by setting goals which compute the normal force applied to the front and rear surfaces of the building. The velocity and density can simply be programmed into our fluid simulation. To obtain the final solution we solve the simulation for given conditions. The program then computes over one-hundred iterations that provide highly accurate results of the normal forces applied and our resulting drag coefficient.
Figure 1: depicts the velocity trends of air around a building with chamfered corners of 10 degrees.
Figure 2: depicts the velocity trends of air around a building of stepback construction (progression of stepbacks can be found in Table1)
Figure 3: depicts the velocity trends of air around a building of tapered construction (tapering at 10%)
Table 1: displays the data collected through our analysis of the selected building architectures in Solidworks. Through the usage of SolidWorks we were able to determine the drag coefficients (Cd) of each style of building and therefore determine which building shape experiences the least drag forces. Chamfered Corner Building Normal Force (N) Front Back Velocity (m/s) Air Density (kg/m^3) Frontal Area (m^2) Cd Tapered Building Normal Force (N) Front Back Velocity (m/s) Air Density (kg/m^3) Frontal Area (m^2) Cd
54900 220 30.0 1.23 128 0.783
92.7 3.10 30.0 1.23 0.20 0.87
Stepback Building Normal Force (N) Front Back Velocity (m/s) Air Density (kg/m^3) Frontal Area (m^2) Cd
75.0 34.4 30.0 1.23 0.193 1.03
Conclusion: Through our analysis we have determined that the building with the chamfered corners experiences the least drag force. We can deduce this from our data by noting that the chamfered building has the smallest drag coefficient. This comes as a surprise to us since we initially hypothesized that the tapered building would dissipate wind forces more efficiently. Although we can rationalize this because claiming that the chamfers “round” the corners of the building and reduce the pressure difference which is known to lower the drag force. As we consider our possible sources of error, we acknowledge that we only studied a very limited selection of designs. We only analyzed three types of buildings and each of those buildings only had one variation. Such as the degrees of taper and chamfer. To measure more accurately we would need to analyze a vast number of combinations of exterior design.
Works Cited Ali, Mir M. “Evolution of Concrete Skyscrapers: from Ingalls to Jin mao.” Electronics Journal of Structural Engineering 1.1 (2001): Web. Cimbala, John M. "External Flow:Drag and Lift." Fluid Mechanics: Fundamentals and Applications. By Yunus A. Çengel. 3rd ed. New York: McGraw-Hill, 2014. 60743. Print. Kim, Young-Moon. “Dynamic responces of a tapered tall building to wind loads.” Journal of Wind Engineering and Industrial Aerodynamics 90.12-15 (2002): 1771-1782. Web. Nehdi, Moncef L. “Only tall things cast shadows: Opportunities, challenges and research needs of self consolidating concrete in super-tall buildings.” Construction and Building Materials 48 (2013): 80-90. Web. Xu. Y. L. “Parametric study of active mass dampers for wind-excited tall buildings.” Engineering Structures 18.1 (1996): 64-76. Web.
