Microfluidics Lab 1

Engineering 1282H

Spring 2015

Team Y1

Mahnoor Naqvi

Spandan Shah

Stefan Heglas, Wednesday 3:00 PM

Date of Submission: 03/09/15

Memorandum

To: Stefan Heglas

From: Team Y1 (Mahnoor Naqvi and Spandan Shah)

Date: 03/07/2015

Re: Computational Fluid Dynamics

I. Introduction

Flow simulations are crucial in understanding the dynamics of fluid flow within various

spaces. They allow for a controlled environment and flexibility in channel shape, structure,

and fluid type. The importance of such simulations becomes crucial in micro and nano

systems where it is difficult to gain an accurate understanding of fluid flow through

physical experimentation due to the size of such systems.

The purpose of this lab was to create a fluid simulation of water within a channel flow in

order to gain an understanding characteristics such as velocity, shear stress and pressure

across a specific flow channel. Modeling the flow enhances understanding of experimental

results by showing where the how the pressure and velocity deviates across the chip

channel.

The SolidWorks simulation uses the same parameters as the Fluid Mechanics program to

determine the flow rate and change in pressure in the channel. While in the program, only

one point in the channel could be analyzed at a time, the SolidWorks simulation could show

results across the entire channel depending on the size of the mesh cells.

II. Results and Description

This analysis was conducted in two parts. The first part looked at the effect of mesh size on

the accuracy of the fluid flow model. The second part looked at modeling flow in the entire

channel of the standard chip utilizing the information gained in the first part of the

analysis.

The parameters and settings for both parts of the analysis are summarized in Table 1 and

Table 2 in the “Figures and Tables” section in the attachments.

III. Discussion

In order to check the validity of the solution in the SolidWorks Flow Simulation several

basic characteristics of fluid flow were compared to that of the simulation to check for a

resemblance. The first check was whether the velocity was greatest at the center of the

channel. This condition was met as evidenced by the 3D velocity contour in Figure 7 in the

Figures and Tables, which shows that the velocity was the greatest at the center of the

channel. The second check was whether the velocity decreased as one went away from the

center to the edges. The 3D velocity contour can be referenced once again the confirm this

check. The third characteristic was that the pressure decreases at the fluid flows across the

channel. Figure 14 in the Figures and Tables, which denotes the “Surface Contours of

Pressure” shows the pressure at the entrance being the highest and as the fluid moves

across the length of the channel, the pressure linearly decreases.

The linear decrease in the pressure can be inferred from the “Surface Contours of Pressure”

figure. There is a consistent decrease from the entrance where the pressure is 102325 Pa to

the exit where the pressure is 101323 Pa. This represents as 1000 Pa pressure difference

across the 25 cm channel.

Another point of validity can be evaluated by looking at Figure 11 in the Figures and Tables,

which is the shear stress plot. The velocity gradient value at the edges of this figure was

relatively low compared to the velocity value at the center of the channel, but it was not

exactly zero. Therefore, it is not entirely consistent with the no-slip condition based on the

parameters of the simulation. The no-slip condition states that the velocity of a fluid at the

edges is the same as the velocity of the edges. In both simulations, the edges were

unmoving. This means that in order for the simulation to be valid, the velocity at the edges

must be zero.

There were slight discrepancies in the SolidWorks simulation when compared to the

expectations of the flow. The primary discrepancy arose from the inconsistency of the

simulation the the no-slip condition. The correct result would have shown a velocity of zero

at the edges of the channel. The reason why the simulation isn’t fully consistent is because

the accuracy of the simulation depends on the mesh size. The smaller the mesh size, the

more accurate the model. However, in order to get an absolutely accurate model, the mesh

size would need to be infinitely small, which is extremely impractical.

Figure 5 in the Figures and Tables proves this by showing the velocity cut plot after the

mesh has been changed to be less coarse. The mesh was changed by increasing the cells in

every direction which caused the simulation to run longer. The best mesh was the one with

the most accurate results and a reasonable run time. Figure 6 also shows the refined mesh

velocity has higher velocity at the center of the plot and lower velocity around the edges is

slower as compared to Figure 2 where the mesh is coarse.

The parabolic 3D plot of the refined velocity contours Figure 7 in the Figures and Tables

shows there is a fully developed flow at 0.010m from the end of the channel. At this point,

the entrance length is at a distance far enough away to allow the water velocity in the

center to reach the fastest velocity and the edge velocities to reach the slowest velocities.

Figure 8 in the Figures and Tables shows the difference of velocities at the three distances

shows the change in velocity of the water as the position in the channel changes. The area

where the water velocity is fastest is shown to increase as the distance from the channel

increases. This shows that as you get farther away from the entrance, the entrance effects

change to fully developed flow effects.

Applications of using the Flow Simulation could be to determine where the the flow is fully

developed in the design channels and whether the test simulations run experimentally

match the ones done on SolidWorks.

IV. Conclusions

The purpose of this lab was to create a SolidWorks simulation mesh that was small enough

to provide an accurate representation of the velocity, shear stress, and pressure in the

channel. This was done testing a coarse mesh, and then adding cells to the mesh so it

became more refined. By creating this model in SolidWorks, we can see where the water

flow should be fastest due to shear stress and pressure. Applying this to the experimental

results gives a better idea of how the flow in the channels work. Furthermore, testing the

results from the simulation can help confirm the results from the experiment. Using the

simulation also analyzes the flow rate and the change in pressure across the channel

whereas the fluid mechanics program can only analyze the parameters at a point on the

channel.

Attachment: Figures and Tables

Figures and Tables

Table 1: Channel Dimensions

Length 25 mm Width 3 mm Height 200 μm

Table 2: Flow Parameters

Pressure Head (ΔP) 1000 Pa Dynamic Viscosity (μ) 0.0010014 Pa-s

Density 998.16 kg/m3

Figure 1: Goals Plot

Figure 2: Pressure Contours

Figure 3: Velocity Contours

Figure 4: Velocity Vectors

Figure 5: Flow Trajectories

Figure 6: Refined Mesh Velocity Contour

Figure 7: Refined Mesh 3D Velocity Contour

Figure 8: Velocity Contours at Channel Entrance

Figure 9: Shear Stress along the Bottom of the Channel

Figure 10: Delta Value of Goals Plot

Figure 11: Lateral Cut Plot

Figure 12: Transverse Cut Plot

Figure 13: Surface Contours of Pressure

Figure 14: Sheer Stress Surface Plot

References

[1] Microfluidics Lab 1 Procedure – Part 1. 2015, March 9. www.carmen.osu.edu [2] Microfluidics Lab 1 Procedure – Part 2. 2015, March 9. www.carmen.osu.edu