BME402 CFD LAB 1 - Lab PDF

Preview

Summary

Lab...

Description

Dept of Mechanical and Biomedical Engineering CFD Lab BME402 Student Name

Class

Student ID Number:

Biomedical Engineering

Assignment Number & Title: Lab1 - Blood Flow and Plaque Submission Date:

17/11/19

Academic Integrity and Plagiarism Plagiarism is the act of copying, including or directly quoting from, the work of another without adequate acknowledgement. All work submitted by students for assessment purposes is accepted on the understanding that it is their own work and written in their own words except where explicitly referenced using the correct format. For example, you must NOT copy information, ideas, portions of text, figures, designs, CAD drawings, computer programs, etc. from anywhere without giving a reference to the source. Sources include the internet, other students’ work, books, journal articles, etc.

You must a ensure that you have read the University Regulations relating to plagiarism, which can be found on the NUIG website: http://www.nuigalway.ie/engineering/plagiarism/ I have read and understood the University Code of Practice on plagiarism and confirm that the content of this document is my own work and has not been plagiarised. Student’s signature (typed if submission on blackboard)

1. Problem Description: The aim of this lab is to analyse the effect of a build-up of plaque on an arterial wall disrupting the blood flow through the vessel as shown in figure.1. Ansys software is used to depict to the effects of the blockage within the system, that is the vessel, and analyse each variable which has been affected by fluid flow. Figure.1 displays the system as a whole, with a vessel length of 25 mm and a diameter of 4 mm. The plaque is shown as the half spherical shape impeding into the walls of the vessel. This plaque build-up clearly causes a large blockage in the vessel.

Fig.1: Schematic Diagram of the blood flow and plaque blockage within the vessel.

2. Analysis method and choice of element: First, the Ansys WorkBench is opened and the geometry tab was added onto the workspace. The fluid flow(fluent) was then selected and dragged onto the WorkBench area as well. The WorkBench was then named as ‘Blood Flow’ and saved. A file containing the CAD drawings of the model, as shown in fig.1, was provided on BlackBoard and opened to display the model. A geometry of the model was created by selecting the body of the model. Named selections were already created and applied to the model. A mesh was then created to find a solution to the problem using finite element methods. In the mesh step, the sizing of each component was created. The inflation of the wall of the vessel was then investigated. For this model, a fine mesh is not required therefore, a medium mesh is used with the default values provided by Ansys. The correct units were applied prior to entering any values. When the correct units were selected, the height of the first layer was set to 0.1mm in the inflation step on the outer wall of the vessel. The mesh was the generated after the ‘generate mesh’ button was selected and the ‘update’ button was selected on the workspace of WorkBench. The set up for the system was created next including the input and output of the system. In the viscous model dialog box, the parameters were set to a viscous and laminar flow. The fluid material selected for this experiment was set to liquid water with a temperature of 25℃ and a pressure of 1atm. Cell zone conditions were also converted from air to liquid water. Boundary conditions were then created at the inlet of the vessel with a velocity of 0.2m/s. A surface monitor was created to measure the output surface and the mass weighted average velocity of the vessel. This is carried out to examine the varying velocity from the inlet to the outlet of the vessel. The x-velocity is set to 0.2m/s which equates to the blood flow along the vessel. The stimulation is then performed using the run calculation tool. The number of iterations is then set to 1000. Iterations can be described as the steps in solving a problem. Ensure

the simulation has run up to convergence. When this has been completed, it was ensured that the average velocity at the outlet reached a stable solution before creating the plots.

3. Results and Figures: Figure.2 below details the magnitude vs the iterations of the model and is a plot of equation residuals along the x, y and z direction. Each line reduces the absolute error in the simulation providing more accurate values for each iteration as the graph develops. The lowest order of magnitude reaches a value of 0.00001 on the orange and green line. The convergence was performed for the velocity variables within the system.

Fig.2: plot of equation residuals.

Figure.3 represents the plot of the area weighted average velocity at the outlet. This graph was created by applying the boundary condition to the outlet surface. A maximum area weighted average of approximately 1.9950m/s is recorded up to an iteration of approximately 420. Initially the area weighted average increases rapidly before staggering slightly until it becomes steady and moved in a straight line.

Fig.3: Plot of average velocity outlet.

Figure.4 represents a contour plot on the pressure of a plane along the centreline of the blood vessel. The maximum pressure recorded is 1618Pa which is recorded near the build-up of plaque at the inlet of the blood vessel. The pressure towards to outlet of the vessel reaches a value of between -1170Pa and -2720Pa at the walls. This indicated the no slip conditions at the wall of the vessel. The negative values are discussed in the discussion and conclusion section.

Fig.4: contour plot of pressure on a plane along the centreline of the blood vessel.

Figure.5 displays a similar image to that shown in figure.4, however, figure.4 details the pressure at the centreline of vessel, whereas figure.5 displays the pressure at the wall of the vessel. Just like the previous figure, the pressure is similar at the plaque build-up with a pressure of 1618Pa and a pressure of -2720Pa towards the outlet of the vessel.

Fig.5: contour plot of wall shear stress on the surface of the blood vessel and the plaque.

Figure.6 is a vector plot of the fluid velocity along the centreline of the vessel. The maximum velocity is recorded at 3.713m/s which occurs just at the edge of the blockage. Similar to the pressure plots, towards the outlet of the vessel, the velocity decreases to between 1.856m/s and 2.785m/s. The velocity at the walls of the vessel beyond the blockage is close to zero. This is due to the low pressure are that becomes trapped in this area.

Fig.6: vector plot of fluid velocity on the plane along the centreline of the blood vessel.

4. Discussion and Conclusion: The purpose of this lab was to analyse the effect a build-up of plaque has on the blood flow in a blood vessel. By defining the geometry and assigning the mesh, a job was simulation was successfully created. The results were then transferred into graphs where they could be visually seen. As expected, there is a higher pressure applied to the area just before the blockage which decreases as the fluid moves past the blockage towards the outlet. The rise in pressure occurred due to the decrease in the vessel diameter at the blockage, similar to the decrease in pressure was due to the increase in the diameter of the vessel. This pressure difference is due to the Venturi Effect. The Venturi Effect states that the velocity of fluid passing through a constricted area will increase and its pressure will decrease. This has been demonstrated in the contour plots above. This effect is partially due to the Bernoulli Equation of energy conservation. In this system, kinetic energy and pressure energy is analysed. Due to the conservation of energy law, the kinetic energy always increases when the pressure is decreased. This means that the pressure decrease is converted to kinetic energy. This is similar to the vector plot of the magnitude of the velocity as shown in figure 6. The velocity is high where the blockage is with a result of 3.713 ms-1. However, juts after the blockage, the velocity is at zero as shown by the blue shading of the plot The values of the graphs in the legend boxes in figure.4 and figure.5, don’t seem real istic due to the minus values that are present in pressure. The analysis was repeated however, the values generated after the re-analysis also did not align with what the values should be. This may due to incorrect selections on the software or incorrect values entered into incorrect sections. In the legend, there is a value of -9845 in between -1170 to -1852. This is clearly incorrect as the values should be decreasing in order, however, this value is out of order and should not be displayed. Incorrect calculations or steps applied to the model is the cause of this unknown error. The distribution of velocity and pressure throughout a blood vessel is a vital aspect to understanding the physiology of how it affects the wall shear stress on the walls of the vessel. When a clot occurs the wall shear stress increases in the blocked vessel however, it decreases and becomes the same as the wall shear stress in a vessel without a blockage. The wall shear stress generally increases at the start of the blockage and decreases afterwards [1] which has been shown in the above figures. Overall, it is clear to see that a blockage in a blood vessel will affect the pressure, velocity and stress of the vessel causing irregularities.

5. References: [1] Bhatia, R., Vashisth, S. and Saini, R. (2016). Wall Shear Stress Analysis in Stenosed Carotid Arteries with Different Shapes of Plaque. International Journal of Computer Applications, 145(4), pp.9-12....