Computational Fluid Dynamics - Essay Example

COMPUTATIONAL FLUID DYNAMICS FOR EXTERNAL AERODYNAMICS OF BUILDINGS INTRODUCTION In this fast moving world the need to control and predict the movement of fluids is a common problem. Scientists have worked hard on this are an so far the study of this area is called fluid dynamics and the systems that are studied range from weather patterns, through aircraft aerodynamics to the way blood circulates. Since IT has a massive role in the current globalization process engineers have devised systems called as the CFD or Computational fluid dynamics. Computational Fluid Dynamics (CFD) uses current IT to solve these problems using the fundamental of physics. Due to the advancements in almost every field CFD and its application is a rapidly developing discipline due to the continuous development in the capabilities of commercial software and the growth of computer power. The system has been a success and therefore CFD is already widely used in industry and its application is set to spread. CFD technicality We have learned that the physics which governs fluids is relatively simple; the laws of motion and thermodynamics. However, for designing a system of this scope the solutions are very complex and this makes analytical methods largely unusable for industrial applications. An approach to solve such complex dilemmas is to replace the problem with a number of smaller less complex problems. With the advent of computers this approach became practical and in the late 1960s Computational Fluid Dynamics was born. The Logic What does the CFD do To explain the idea behind CFD lets cite an example of an airplane. As the plane moves along the air must move out of its way. This means that the airplane has to make its way shearing through the wind. The way in which the air flows depends on the plane's shape. The flow can be smooth but more likely it will contain vortices, shockwaves and other disturbances. We will have to consider the air as the fluid present. To model the behavior of the fluid the volume is split into many smaller sub-volumes, called a mesh. A mesh can be simply the same sub-volume repeated throughout the space or, more usually, it can be molded around the object that is being modeled and so can be complicated. There are problems associated with the meshing mechanism as it easy to say the word splitting but in reality it is extremely difficult. Therefore the skill is to produce a mesh with exactly the right sub-volumes. If the sub volumes are too big then the solution will have errors; if the volumes are too small then the calculation will take too long to be useful in a design process. The past tells that in 1990 CFD was an activity for owners of CRAY super-computers, but by 2000 the airflow is analyzed by starting from an initial flow, which can be either a guess at the solution or a specific initial condition. This has lead to a change such that using this initial flow the conservation equations are used to predict the flow a short time later. Every time a new prediction is then made from the newly calculated flow. This is a simpler way to solve the evolution of the airflow. In the rapid changing world most applications of CFD involve steady flows because they need less computer control. This explanation of using the conservation equations as a flow solver is a generalization; there are other more complex solvers. However, these methods fabricate similar outputs and, more importantly the draw back is, CFD systems look the same to a user regardless of the solution technique. After having a flow solution, the user is presented with the flow at every point in the mesh. The last phase of the CFD progression is to extract from these data the information that the user actually wants. However the dynamic solutions suffer from one big draw back that is accuracy. For many applications, such as building design, the level of accuracy required is very low and CFD results are more than sufficient. CFD usually gets the qualitative picture correct, which is useful in helping people understand what is happening within a flow. In real meaning, the Computational Fluid Dynamics process requires specialist involvement at every stage. Automation of the method is only possible where almost indistinguishable calculations are being carried out. Software solutions, which reduce the level of knowledge needed to use a CFD system, will prove to be an important aspect of future CFD systems. The basic flow solver technologies have changed little in recent years. The main progress has been to take flow solvers that were formerly only proficient in solving low-speed flows, and extend their range to include high-speed flows, and vice-versa. It seems unlikely that there will be any major advances in this respect. The improvement at the moment is in the enhancement of models that have been used to correspond to the physics associated with different applications. Many new models have appeared in commercial packages, widening the range of problems to which they can be applied, including combustion models, multi-phase flows, particulate modeling and support for non-Newtonian fluids. Results for CFD Results for a well judged CFD is based upon the information cited. Therefore it is very important to think about the information needed and how it will be extorted. For a better picture and a good display of the results of a CFD calculation most systems come with a post-processing and visualization package. CFD is a rapidly developing tool, which when coupled with the right systems has the potential to significantly impact and add value to the processes within organizations dealing with moving fluids. Buildings and CFD Now moving ahead we describe how CFD is used in order to escalate the thermal comfort of a inhabited building. The main problem for using CFD is to examine whether the arrangement of air-conditioning and heating machines could be sufficient to keep the house comfortably cool in the summer and warm in the winter. One basic assumption is that by ensuring that all devices are properly sized and located, it is possible to minimize the energy costs of operating the system, thereby minimizing both the environmental and economic impact of the development. As discussed above the CFD are relatively easier for buildings but validation of air flow prediction in buildings using CFD is inherently difficult. One problem is that studies are usually completed long before the building is constructed, except there is an air flow problem with a completed building. This test is further compounded by the differences between the air flow input techniques or assumptions involved in the computation. In this context CFD is applied to vast range of building types and ventilation strategies throughout the design process. Many would say that CFD is traditionally thought of as a specialist application, its use is becoming more widespread and mainstream throughout the building industry opening up to non-expert and expert users alike. If the building is to be a largely open-plan and has an irregular layout. In this case the heat generation is different and therefore the thermal environment of the resulting large volume occupied zone is difficult to manage. The irregular design of the building dislocates the usual insulating near the roof air layer so that the volume is subject to outsized solar gains through the top during the summer and conversely large heat loses through the roof during the winter. However one important application of CFD in the built environment is the modeling of office spaces. Surely there will be different airflows present and they can thus be classified into two ways. Firstly, usual freshening flows are those generated by air movement through openings in the faade. To simplify it can be said that this air movement can be buoyancy-driven with internal heat gains creating temperature differentials between inside and outside and/or wind-driven by the external pressure field. On the other hand, flows may be driven by perfunctory ventilation systems. The generated air flow has different regions of energy sources and mixing with heated or cooled air interacting with other sources of buoyancy within the space thereby yielding complex flow patterns which would be hard to predict. In general natural ventilation can be combined with mechanical ventilation easily either by design or with user interaction. A typical feature of internal flows is that the driving forces are usually small. By use of software preferably AUTO CAD Representative, low detail, furniture is included in the model in order to determine how much influence it has on the predicted thermal environment. Air conditioning units are simulated using coupled pairs of inlet and outlet boundary conditions, controlled via user coding. The mass flow and the temperature of the air entering the room are calculated from the air-conditioning unit data sheet. The radiators are simulated using baffle cells with an exchange area that is equivalent to those of the real heaters. Appropriate levels of thermal resistance are applied to the windows, walls and roof. Adiabatic boundary conditions are prescribed at all internal walls. The analysis of CFD After the inclusion of the modeling software several calculations are to be performed. The initial simulations will be used to tune the modeling parameters to optimize the convergence behavior of subsequent calculations. In order to accurately model the effects of buoyancy, a transient solution method may be employed, using a pressure correction under-relaxation factor of 0.8, double precision and a small time step. Conclusions The final word is that these examinations vividly show that, even in a simple building geometry, it is possible to predict thermal effects that might not have otherwise been apparent until the building is built. With the help of latest available software such as Cad it is possible to generate a good computational mesh in a small amount of time. Recalculating the simulations with air-conditioning units and radiators in different positions takes just a few hours. Considering the above cases one of the fundamental measures is occupant thermal comfort. Furthermore, one of the typical necessities for thermal console is low air speeds and a fairly narrow band of air temperatures within the occupied zones. The case is that many buildings are adequately designed and constructed without remedy to CFD analysis. It is true that only a little can be achieved by testing the complexity of the system. However, there are cases with an unusual air distribution design, for example, where an evaluation of the likely presentation of the planned design may be obligatory in order to grant assurance in the design. The design development may use CFD with or without other calculation techniques such as active thermal modeling or hand calculations. Computational Fluid Dynamics simulations are emerging as a promising technology for supporting building construction and other industry related assessments, in part due to the advancing power of computational hardware and software. CFD simulations have the prospective to give in more correct solutions than other modeling methodologies because they are a solution of the fundamental physics equations and include the effects of detailed three-dimensional geometry and local environmental conditions. The problem is that the tools are not well validated for ecological modeling and therefore best-practice methodologies have not been established. The CFD designs are based on pictures of the buildings. Photogrammetry derived buildings are based on analyses of a series of photographs. Improved methods for creating working building geometry and mesh are key elements for improvement in order to minimize the overall time required to develop a CFD application in real metropolitan areas. However the procedure is much smoother for well formulated building shapes. Continuous tests are to be conducted to determine what level of building details is necessary. In the rapid developments of computer technology, many simulation tools are available for use, which falls on a very wide spectrum in terms of sophistication and applicability. During the CFD computation process once good building geometry is applicable, then the set up turns to defining mesh refinement and defining the surface boundary conditions, the turbulence model, and the options for selecting code parameters. This is what technology has offered and with the introduction of more power computers the core will become easier for CFD. Bibliography 1. Leatham, M, Stokes, S, Shaw, JA, Cooper, J, Appa, J, Blaylock, TA,( , 2000) 'Automatic mesh generation for rapid response Navier-Stokes calculations', AIAA 2000-2247 2. Hunt, DL, Childs, M, Maina, M, (2001) 'QUACC, a novel method for predicting unsteady flows- including propellers and store release', The Aeronautical Journal. 3. A. Jameson, (2001)"A perspective on computational algorithms for aerodynamic analysis and design", Progress in Aerospace Sciences, vol. 37, pp 197-243. 4. M. Van Dyke, (1982) "An album of fluid motion", Parabolic Press, Read More