CFD and its Use in Hydraulic Machines

What is CFD?

      CFD stands for Computational Fluid Dynamics. It is a branch of CAE which stands for computer-aided engineering. It is based on the Navier-Stokes equations and its basic principles. It gives numerical approximation to the equation that governs fluid motion. It deals with heat transfer problems and fluid flow problems. CFD analysis allows engineers to visualize, test and analyze their product designs for problems related to fluid flow, heat transfer, turbulence, non-Newtonian materials, etc. by simulating exact working conditions. A CFD model gives you a good idea of ​​how the system will act at the design stage. It shows what will work and what won't, and allows you to intervene early before anything has been constructed.

Fig 1. CFD

     CFD technology is an advanced virtual design method, which simulates the influence of the structure on its performance by simulating the flow field inside the mechanical equipment. It can simulate the flow field inside the mechanical equipment, including the speed distribution, pressure distribution, concentration and temperature distribution of the flow of various detailed and intuitive information.

How does CFD work?

Steps:

• Pre-processing: 

     In this starting phase of the process, the problem statement is broken down. The type of fluid flow, whether inviscid or viscous, incompressible or compressible, and unsteady or steady, is mentioned along with the initial boundary conditions. Because CFD uses a finite element technique, the part is discretized into countless smaller elements as part of the process.

Steps involved in pre-processing are:

1. Formulate the Flow Problem.
2. Model the Geometry and Flow Domain.
3. Establish the Boundary and Initial Conditions.
4. Generate the Grid.
5. Establish the Simulation Strategy.
6. Establish the Input Parameters and Files.
7. Perform the Simulation.
8. Monitor the Simulation for Completion.

•   Solving: 

     Using numerical equations, the solver performs the actual calculation to derive the result according to the specified conditions. First, we need to generate the numerical analogy to solve the governing equations of the fluid motion. Discretization is the process which is used to generate the numerical analogy of governing equations. This process uses partial differential equation to describe the flow. In this process every term within this equation is written in such a way that the computer can be programmed to calculate.

There are four different discretization techniques: 

(i)             Finite Difference Method: The FDM discretizes the NSE on a set of points that hold the physical variables of the fluid e.g., pressure and velocity. For the easy implementation, these points are distributed regularly. The Taylor expansion is used to compute the derivatives. 

(ii)           Finite Element Method:  In this method, the considered fluid domain is divided into finite number of sub-domains which are called as elements. For the variation of each variable inside each element, simple function is assumed. The summation of variation of the 12 variables in each element is used to describe the whole flow field.

Fig 2. Elements

(An Introduction to Computational Fluid Dynamics)

(iii)         Finite Volume Method: The basic idea of the FVM is to subdivide the whole domain into small control volumes. These volumes exchange fluxes with their neighbours over the respective connecting face. Regular grids are used for the efficiency of the computation in the finite volume method. Although, these days, irregular grids have become popular for simulating the flow in complex geometry.

(iv)        Spectral Method: Another method of generating a numerical analogy of a differential equation is by using Fourier series or series of Chebyshev polynomials to approximate the unknown functions. This method is called as the Spectral method.

• Post-processing: 

     Finally, the fabricated results are examined by experts. Conclusions are drawn by examining simulated dynamic images, static images and graphs.

Fig 3. Steps involved in CFD

(The development and application of CFD technology in mechanical engineering)

What is the purpose of using CFD?

     In the design process, CFD has a capability to save time. Thus, for data acquisition, CFD is considered as faster and inexpensive as compared to conventional testing. Moreover, in actual testing, numbers of quantities measured at once are finite. On the other hand, in a CFD analysis, all the required quantities can be measured at once, and with a high resolution in time and space.

     As CFD analyses approximate a real physical solution, it cannot completely exclude physical testing. To verify the results of CFD, we should perform a physical testing.

 

How accurate is CFD?

     Accuracy of the CFD solution depends on many factors. The mesh generated on the geometry is key. A CFD simulation is approximation of reality. If the mesh is rough, you will get the less accurate results. If your mesh is fine then you will get more accurate results. Results are not realistic after certain accuracy level.

Fig 4. Commonly used element form in building a mesh  

(An Introduction to Computational Fluid Dynamics)

Where is it used?

     For aerospace industry in 1960s, the first codes of CFD were developed. After that the usage of CFD has increased to all industries which are related to fluid mechanics directly or indirectly. Other industrial users of CFD are:

1. Automotive
2. Power turbomachinery
3. Chemical
4. Power generation
5. Manufacturing
6. Process engineering
7. Product design and Optimization

Use of CFD in Hydraulic Machines:

In fluid system, CFD is used:

1. To simulate actual behaviour
2. To point out the critical areas of concern
3. To find out different alternatives
4. To develop an optimized solution

     Also, CFD is used for predicting performance of hydraulic machinery at different operating conditions. We can simulate turbulent fluid flow, chemical reactions using CFD. Simulation of heat and mass transfer is also possible by using CFD analysis.

Applications of CFD in Hydraulic Machines:

1) Hydraulic Turbines:

     Hydraulic turbines play very important role in the functioning of hydro projects. Therefore the design of these machines should be in such a manner that it can sink with the electric grid demand efficiently and economically. It is possible to see the mechanisms of almost every hydrodynamic phenomenon which occurs at the time of  turbine operation to get more efficient designs by using CFD. Every minute detail of flow features like Fluid Structure Interaction (FSI), PVC can be made available by using CFD. These features are very important while designing highly interactive components such as distributor, runner and draft tube. 

• Francis Turbine:  

     While manufacturing a turbine, it is important to see the results in advance in order to know the feasibility of the turbine. CFD method requires less money and also less time compared to the experimental method which needs model fabrication and test rig set up for getting results. CFD analysis gives pressure and velocity distribution in the form of pressure and velocity profile across whole turbine region while performing the analysis on turbines. While designing a turbine, there is one more important factor which we need to consider is cavitation. Turbine design should not have cavitation effect in it. To check whether the design has cavitation effect or not one should know the distribution of the pressure on both the sides of blades.

 

Fig 5. Complete assembly of a Francis turbine, and Mesh of a Francis turbine runner

(Utility of CFD in the design and performance analysis of hydraulic turbines — A review)

2) Off-highway equipment: 

     Due to the high power density of hydraulic technology, it is the most preferred technology in off-highway equipment. A typical hydraulic machinery contains pumps, cylinders, hoses and motors. System’s efficiency, it’s reliability and power density depend on the design of these parts. Therefore while designing these products, CFD is used to achieve the required outcomes.

Fig 6. CFD model of a complete pump with a rotating propeller (pump housing removed for clarity); colours indicate pressure on the blades and other internal surfaces.

(Engineering & Expertise Hydraulic modelling Computational fluid dynamics)

3) Optimizing pump station design through CFD: 

     Critical factor while designing the pump station is understanding the fluid flow inside hydraulic structures. To design optimized solutions, experience and intuition are not always enough. CFD is an excellent modelling tool to simulate different design alternatives, to find out problems in flow and to develop solutions. CFD modelling can be used for the better understanding of pump station hydraulics. With this understanding, we can check intake conditions in pump stations. Improvement in the design of pump station can also be done by using CFD in less time and cost compared to the physical modelling.
 

Fig 7. Pump Station Design 

(Engineering & Expertise Hydraulic modelling Computational fluid dynamics)

Opportunities: 

     In the future, the development of CFD technology in the field of mechanical engineering should focus on the development of multidisciplinary coupling calculation, general engineering software and the extraction of massive flow field data, so as to improve the calculation precision and efficiency of CFD and the ability to process data. For more realistic simulations sinking with available computational capabilities, the development of high-resolution numerical schemes is one of the most important needs of the hour.

Challenges:

     While designing hydraulic turbines, there are so many approximations involved in boundary conditions and initial guess. Because of that predicting the results of cavitation accurately by using CFD is still a challenge.

Conclusion:

     Computational Fluid Dynamic gives a solution to unavoidable problems faced by engineers and designers which are originated from mankind’s efforts to understand the extensive power of natural elements such as winds, heat, and waves. It plays a key role in the evolution and development of new products and components. The one who has the ability to extract the high efficacy of computational fluid dynamics can save resources such as time, money and manpower.

References

1. An Introduction to computational Fluid Dynamics by Nasser Ashgriz & Javad Mostaghimi Department of Mechanical & Industrial Eng. University of Toronto Toronto, Ontario
2. The development and application of CFD technology in mechanical engineering by Yufeng Wei 2017 IOP Conf. Ser.: Mater. Sci. Eng. 274 012012 
3. Utility of CFD in the design and performance analysis of hydraulic turbines — A review by Gyanendra Tiwari  , Jitendra Kumar  , Vishnu Prasad , Vivek Kumar Patel, Applied Mechanics Department, MNNIT Allahabad, Prayagraj, 211004, India b Department of Civil Engineering, MANIT Bhopal, 462003, India.
4. Engineering & Expertise Hydraulic modelling Computational fluid dynamics 
5. CFD analysis of 3-D flow for francis turbine

 

Authors:

Adwait Gaikwad     

Aryan Khandekar    

Soham Babar          

Sakshi Bade            

Vivek Bagul