Cfd / Computational fluid dynamics: the life of humanity has become much easier thanks to the developing and still developing technology today. This developing technology has provided and continues to help people in communication, easy access to information and many other areas. Well, in what other areas has this developing technology made our work easier?
The answer to this question lies in the design, testing and production stages of the products we see around us. In other words, the blessings of developing technology are widely used in every manufacturing sector. If we give an example right away, we can say welding robots used in manufacturing factories (especially automotive) or gigantic CNC welding machines. The quality of the resulting weld increases with the process performed here.
It also saves a lot of time. In addition to this, since we know the damage of the welding process to human health, we can calculate how great the benefit of this developed welding technology is. Or as another example, technical designs that used to be drawn by hand for hours can now be finalized in a very short time with computer software.
In this context, the benefits of technology to the manufacturing sector can of course be multiplied, but if I need to give an example that will be compatible with the part I will explain in my article, I can start by talking about advanced simulation and analysis programs.
If I give an example of the simulation and analysis programs used thanks to the developed technology, AYE (Fluid Structure Interaction), CFD ( Computational Fluid Dynamics) and SEM (Finite Element Method) programs. So what kind of benefits do these analysis software provide us?
In other words, it allows long and energy-intensive tests to be done easily on the computer. It also eliminates the need for materials required for these tests. It is very important and time-saving actions to identify problems and make rapid design changes in the process of simulating a product that we manufacture. At this point, these simulation programs are quite a savior. If I talk about 3 basic analysis programs that I know, first of all, AYE (Fluid Structure Interaction) is a simulation program that has emerged recently.
CFD (Computational Fluid Dynamics) and SEM (Finite Element Method) programs are also ranked 2nd and 3rd. The AYE program is organized as a mix of CFD and SEM principles. (I will talk about AYE and SEM principles in my later articles). Then, I will touch on the place and use of the CFD system in the aviation industry. Afterwards, I will talk about the CFD analysis and stages that I used in a part of the turbine blade design, which is one of the turbo jet engine components. Hope you like it. Happy reading 🙂
CFD (Computational Fluid Dynamics) Analysis
Physical prototype production and tests are needed for design verification, design improvement and optimization purposes at the design stage in manufacturing. In addition to this prototype production, difficult and expensive tests are also required. Computer-aided simulation programs are quite savior in cases where tests cannot be performed.
CFD comes first among these simulation and analysis programs. This analysis program, also known as CFD (Computational Fluid Dynamics) in the industry, examines the motion and behavior of fluids, as its name suggests. Of course, it is a system that makes this analysis with a numerical approach based on mathematics and physics.
So what is meant by the concept of studying the motion and behavior of fluids? We can make it more understandable as follows. It is a general analysis of momentum, heat transfer, turbulence, matter and phase transfers of the part to be manufactured. In addition, it includes many chemical reaction analysis. For example, the combustion reaction. From here, we can reach the following conclusion about CFD analysis, it makes definitions based on more than one sub-discipline and produces a comprehensive analysis that includes the mathematical details of these processes. This is a pretty big win. Now let’s move on to some technical details.
CFD analysis, like other engineering applications, consists of several stages. If we generalize these stages, we can evaluate them in 3 main frameworks:
- Pre Processing
- Solution (Solution/Run)
- Post Processing
At this stage, the first action we will take before starting the analysis is to identify a healthy problem. Specify exactly what we want from the analysis; anticipate the difficulties that will be encountered throughout the whole process and make the necessary constraints; our evaluation of all physical processes that will take place in the system; We should estimate the effects of processes such as momentum, turbulence, and heat transfer on each other and on the overall performance evaluation, and if there are points that can be neglected, we must clearly state these points.
I will explain in detail in the last part of my article, but if I need to give an example on the subject at this point, we considered one-third of a cylinder as the flow volume in the diameter and length values we determined according to the data obtained in the jet engine turbine blade CFD analysis and placed the blade profile we determined inside this flow volume. We also assumed constant incoming turbulent air flow.
The reason why we assumed the flow to be constant (8.56m/s at a single value) was realized during the initial problem determination phase, when it was determined that the air was turbulent and this could cause problems. In other words, we can see from here that the problem determination, which is included in the initial pre-processes, forms the basis of the decisions we will make in the next steps. For this reason, we can say that it is a very important stage that requires a very long and detailed consideration.
The next step after problem detection is the geometric modeling phase. Using computer aided design programs, 2 or 3 dimensional models are created according to the need. Since CFD analysis can provide both 2D and 3D results, it is always more advantageous to extract 2D analyzes at this stage. We can create the geometry in computer-aided programs and transfer it to the CFD cache with the iges extension, or we can create it in CAD.
After this stage, it comes to determine the domain of the problem that we determined at the beginning. After the domain is determined, a detailed calculation mesh should be created. So what is this stage? We can simply explain it like this: We have to divide the domain of the problem into certain small regular geometric sub-areas or volumes. Then, we have to perform mathematical operations for all the node points of these shapes separately and then evaluate all the results obtained and reach the final result.
Let me give an example so that we can understand the situation simply. The two- and three-dimensional CFD results shown above are from a 200 millimeter wire-guided torpedo. Here, the problem area is expressed with a small regular geometric shape, and the green dots are the nope points. Calculation is made for all these green parts and the general problem mesh is obtained. Limitations, physical processes and omissions are then determined based on the broad results obtained. Thus, the first stage of CFD analysis is completed.
Finally, I would like to mention another important point about this stage. The biggest problem I encountered while performing CFD analysis on the jet engine turbine blade in the project I was working on was to keep the problem area too wide. It is tempting to keep the problem area wide at the beginning because it seems to guarantee the results, but in CFD analysis, the geometry should only belong to the region that was determined at the beginning because modeling outside the regions of interest is a serious waste of time.
Unnecessary parts and gaps should not be included in the analysis. In the geometry simplification phase, we need to pay attention to these parts the most. Because the solution time, the amount of digital network (mesh) and quality vary so much that you may find yourself in a bottomless calculation grid and experience serious time losses.
In fact, the first stage is the hardest and longest stage. All the next steps are easier in CFD analysis. In the solution phase, the general solution of the above mesh problem is taken. In other words, the problem is presented in the form of data analysis in general terms. The point we should not miss at this stage is to follow the results of both physical and numerical values step by step, evaluate and intervene when necessary.
The better we carry out the follow-up and data recording, the higher our chance of responding to the error. At the end of this solution stage of the CFD analysis, the conservation values (momentum, energy, etc.) determined in the pre-processes should be checked, that is, whether they are preserved or not. If the equality is broken, it means that we had trouble in tracking the physical and numerical results.
After the solution phase is over, the part where the calculations are checked and the outputs are taken is the post processing phase. As a principle, with the start of the last operations, it is first questioned how accurate the results obtained are, and whether they are suitable for the conditions and expected situations. The items that we output the most in the last transactions can be listed as follows:
- Temperature values
- speed vectors
- flow lines
- Time dependent animations
About Turbo-Jet Engine Turbine Blade End Analysis (Project Scope)
As a result of CFD analysis, I mentioned that if there is any problem in the process and design of the manufactured product, it is necessary to simulate this problem in detail and correct or minimize it. This program, also known as CFD, which has a higher accuracy than other simulation-analysis programs, is also frequently used in aviation. These simulation and analysis programs correspond to recent technology.
Since the aviation industry is an area that includes the developing current technology, it includes such analysis programs. As part of a project on aviation engines, we performed CFD analysis on the main components such as turbo jet engine turbine blades, compressors, nozzles and fans, as well as intermediate components. Now, in the last part of my article, I will talk about why we needed CFD analysis in the turbine blade part, what are the points considered in CFD analysis, what are the limitations and principles.
We know that using classical fluid mechanics equations, we can obtain realistic values as a result of simple forms such as flat plates or circular pipes. Fluid mechanics, which is also given as a course within the scope of the course content of the department of mechanical engineering, can analyze simple forms in the classical sense, we are quite familiar with this. However, when we tried to use classical fluid mechanics equations in the geometric form of turbine blades in our project, we lost a lot of time and could not reach the solution.
In other words, classical fluid mechanics equations did not solve us in this part. At this point , we needed computational fluid dynamics analysis (CFD). The CFD analysis, which simulates the flow region of the fin and the differential equation of this flow, shows the behavior of the time dependent fluid, pressure, temperature and velocity distributions as a result, has been very useful.
We performed the analysis using the CF-TURBO program, which we encountered in the literature research and which we have just met. In fact, it can also be done using Ansys, but since our project belongs to the aviation engine design group, we used the CF-TURBO program, which is the user interface specifically in this field, and we performed the CFD analysis through this program.
Now, if I need to talk about some of the CFD analysis of the turbine blade of our project for the sake of example:
While performing the analysis, first of all, the control volume of the turbine blade was determined. In order to see the values more accurately, a mesh network (account mesh) was created by enlarging it 2 times. If I were to elaborate, a solution network was created with an average of 40000 solid elements for a blade, and then a solution network consisting of an average of 2 million solid elements was formed in order to fully see the differences in deformation and compression-tensile values.
After the control volume was determined, a dense mesh loop was performed to perform the calculations. (node point)
The internal flow was modeled as compressible because it had to meet the ambient conditions in high altitude subsonic flights.
- The flow analysis here was repeated for different angles of attack.
- Improvements were made according to the outputs obtained as a result of the last processes.
In other words, these simulation-analysis programs, which are a recent technology, play a role as a great savior in engineering applications. In addition to this, it provides problem solving or even eliminating it completely without wasting time. In this article, I have included CFD analysis, also known as CFD, and some stages of this analysis that I used in my engine design project.