Paper ID #28873

Design and Assembly of a Large-Scale Cost-efficient Wind Tunnel ViaComputational Simulations as Capstone Projects

Dr. Xiuhua April Si, California Baptist University

Dr. Xiuhua (April) Si professor and Chair of Aerospace, Industrial, and Mechanical engineering depart-ment at California Baptist University. Her research interests are applications of electromagnetic fields inmaterials, heat transfer, fluid flow, water quality, and drug delivery in the respiratory system. She haspublished more than forty papers in heat transfer, materials science, and simulations in drug delivery andrespiratory disease diagnosis.

Dr. Ziliang Zhou, California Baptist University

Ziliang Zhou is a professor of Mechanical Engineering at California Baptist University

c©American Society for Engineering Education, 2020

Computational Simulations Directed Design and Assembly of a

Large-Scale Cost-efficient Wind Tunnel

Abstract

With the development of computer technology, more and more powerful computer software is

available to run computational simulations for engineering design. Numerical analysis has become

one of the most important steps of an engineering design. It can simulate the real dynamic

situations, to test the feasibility, and to optimize the design. This step bridges the design theory

and the application. It saves time and reduces the cost. It prevents the design flaws through the

simulation and enhance the design quality through the optimization process through simulation. It

has been incorporated by most the major engineering design companies. It becomes one of the

most important skills the current engineers should have. However, it is still not a required course

in most of the undergraduate engineering programs. In order to help our engineering students in

acquiring this important skill, we now required students to use Comsol Multi-Physics, Solidworks,

or Fluent in their senior capstone design and illustrate the implementation through the wind tunnel

project.

The wind tunnel is a vital engineering equipment to conduct fluid mechanics and aerodynamics

experimental tests. Wind tunnels are commercially available but can be expensive for small

engineering programs. Considering its fairly simple structure, it can be an attractive design

project for senior undergraduate engineering students. It provides sufficient technical challenges

and ample enlightening opportunities to allow students to apply their knowledge in fluids,

materials and manufacturing, CAD/CAM, and economics. With this consideration, the college of

engineering decided to design and manufacture a subsonic wind tunnel. It was planned as senior

capstone design project that spanned three consecutive years. The first-year team finished the

design based on the constraints of budget, space, power supply, and specific function needs.

Some parts of the wind tunnel were also manufactured. The second-year team checked the

design of the previous team and made appropriate adjustments/improvements based on their

computational modeling results. Following the approval of the final design, fans, motors, and

other necessary materials were ordered to assemble the tunnel.

The first year team studied and compared the closed and open-circuit wind-tunnels. The closed

design was selected for its compact size and relatively low noise level, as the tunnel will be

installed in a small room. The team made its design based on the size of the test section. No flow

simulation of the closed or open-circuit wind tunnel was conducted. To accelerate their progress,

the motor and fan for the wind tunnel were ordered in the first year. All major flow conduits

were manufactured, except the test section. The second-year team started with a simulation of the

flows within the tunnel using Comsol Multiphysics and Solidworks Fluids to check the design of

the first team. The simulation results showed that the design of the closed-circuit tunnel wouldn’t

function as expected; guiding panes in the four corners of the tunnel were needed to generate

uniform airflows in the test section, as well as to reduce power and noise. However,

manufacturing the guiding panes was beyond the capability of the university machine shop.

Outsource the manufacturing to other companies would drive up the cost and delay the project.

Consequently, the alternative open-loop design was chosen by the team. The test section

dimension was kept the same, and the fan purchased by the first team was also implemented.

Simulations of the fluid flow and the generated noise levels of the open-loop design were

conducted again. The simulation results showed that the flow in the test section was sufficiently

uniform, and the sound power level with the highest flow speed was within the acceptable range.

After the open-loop wind tunnel design was finalized based on the simulation results, all parts

were manufactured and assembled. The smoke tests demonstrated that the flow in the test section

was uniform, and the sound volume with the highest speed was lower than the simulation results.

In conclusion, CFD simulations played a significant role in shaping the wind tunnel design and in

reducing the manufacturing cost as well as timing. The students on the design team gained valuable

experiences in using CFD as an effective tool in design analysis and modification, an important skill

for their future career. This report summarized what was accomplished during the first two years.

Key words: computational simulation, cost-efficient, wind-tunnel, Open-circuit tunnel, closed-

circuit tunnel

Introduction

Computer simulation is a mathematical modelling process performed on computers, to predict the

behavior of and the outcome of a real physical system. Simulation of a system is represented as

the running of the system's model. It can be used to explore and gain new insights into

new technology and to estimate the performance of systems too complex for analytical solutions

[11].

Computer simulation bridges the mathematical theories and the real experimental testing. It saves

design time, reduces design failure and flaws, and optimizes the design. All these process greatly

saves design cost. It has become one of the modern design norms in most of the engineering design

companies. Computational design skills have become one of the required skills modern engineers

should have.

In most undergraduate engineering program curriculums, very few computational analysis and

simulation software have been taught. In most programs, it is not required during undergraduate

studies. It is usually taught in graduate programs. Simulation-based design is still new to most of

the undergraduate engineering students. When being asked to run the simulation software, many

undergraduate students might be intimidated initially because it’s new to them and appears

complicated. They’d rather check directly the handbook or using standard equations for calculating

the design parameters. In our program, we have seen students intentionally avoiding the

computational simulation. They think it is hard and don’t want to waste their time and delay the

manufacturing and assembly process, which is generally considered the fun part of the capstone

process. They want to apply the parameters directly and get the physical design done in a prompt

manner, and hence, missing out the optimal design, or worse yet, ending in a defected design.

In order to help them to gain this important skill, our mechanical engineering students were

exposed to two simulation software in two different required courses. These two software is

Solidworks and Comsol Multiphysics. While taking the courses, the students did not realize the

value of this tool until the capstone project.

In this following described wind-tunnel design project, the students involved in the project

witnessed first-hand the values of computational modeling and simulation in gaining insights into

the wind tunnel aerodynamics, performances comparison of various designs, and the optimization

of the design. Students who were required to use computational simulations in this design not only

practiced their simulation skill, realized the importance of simulation in engineering design, but

also were well prepared for their future engineering design job.

Wind tunnel is a device to generate controlled uniform air streams to study aircraft, automobiles,

wind turbines and other objects in the fluids. It has been widely used in aerospace and automobile

industries and universities for both research and education purposes. As one of the major tools in

studying aerodynamics of aircraft, automobiles, boats, trains, bridges, sports devices, and building

structures, it has become a standard lab equipment for most mechanical/aerospace engineering

programs.

There are two different types of wind tunnels available [9, 10]: Open-circuit and closed-circuit.

Open-circuit tunnel has a simpler structure where air travels slowly through a large-bore section

of the tunnel, is accelerated in the nozzle like test section, and slows again in the large-bore diffuser

section before being released into the atmosphere. There is little control over the pressure,

temperature, and humidity of the air. It also uses more space because of the long structure. By

contrast, airflow in a closed-circuit design is contained in the circular or rectangular tunnel, which

is accelerated by fans, go through the test section, and cycles back to the fans. Air velocity is

controlled by changing the speed of the rotating fans or by adjusting the angle of the fan blades.

Power consumption is also lower since the air circulates inside the tunnel. Depending on the

airflow speed in the test section, multiple types of wind tunnels are classified, such as low-speed,

high-speed, subsonic (80 percent of the speed of sound/0.8 Mach), transonic (1 Mach), supersonic

(1 Mach to 6 Mach), hypersonic (6Mach to 12 Mach), and hypervelocity (> 12 Mach).

Wind tunnels are commercially available. The price ranges from $20K to a few hundred thousand

dollars depending on the size, flow speed, and accessories, such as electronics data acquisition

devices. It can be expensive for some small engineering programs. Considering its fairly simple

structure, it can be an attractive design project for undergraduate engineering students. It provides

sufficient technical challenges and ample opportunities to allow students to apply their knowledge

in fluids, materials, manufacturing, CAD/CAM, and engineering economics. Nearly all

engineering programs require students to undertake a capstone project in their senior year. The

budget for each project ranges from a few hundred dollars to thousands of dollars, depending on

the nature of the projects. If using the design of a wind-tunnel as a capstone project, the cost of the

wages of the design engineer and the utility will be saved, and the budget for the project cost will

be used to invest into a major lab equipment. The only cost is the material costs. It will not only

save money for the college, but also provide a practical design experience for the students involved

in the project. This design experience will enhance their engineering analysis skills for their future

engineering profession.

The Department of Mechanical Engineering decided to let the senior capstone teams to design a

subsonic wind-tunnel with automatic data acquisition systems. It was planned as a multiple year

project involving three capstone teams working consecutively over three years. This report will

describe what was accomplished during the first two years, and particularly, the impact of CFD

analysis in shaping the design changes and improvement. We plan to write another report to

conclude the work after the third year.

The basic design of a generic wind-tunnel has been well established with various handbooks and

references exist as guidance of the design [1,2,3,4,5,6]. Those key parameters from the references

are either theoretical, empirical or concluded from experiments. With the rapid development of the

computational power and engineering software, computational simulation has become a standard

part of the engineering design process. The simulation results can first help to check the feasibility

of various designs and then optimize it. The simulation can greatly reduce the cost of the design

and save time and materials cost by filtering out the unfeasible and defect designs, the final design

will generally meet and optimize the design purposes. It is also the only way to check the design

before the fabrication of the actual device [6].

Through this design practice, the students involved in the project witnessed first-hand the values

of computational modeling and simulation in gaining insights into the wind tunnel aerodynamics,

performances comparison of various designs, and the optimization of the design.

Design without simulation (first year project)

The first-year team aimed to finish the design based on the constraints of budget, space, power

supply, and specific functional needs, manufactured some parts of the wind tunnel as time allowed.

And then the second-year team will check the design and make necessary corrections or

improvement, and finish the manufacturing and assembly.

The first year team studied and compared the closed and the open-circuit, and the blow down wind-

tunnels. The closed design was selected for its compact size, relatively low noise level, and

comparatively lower power supply requirements, as the tunnel will be installed in a small room.

The team made the rest of the design based on the size of the test section (2’ by 1’) requirement,

the limit of the wind speed (70 mph), and the budget. The excel spread sheet downloaded from

NASA [1] was used to calculated all the design features as shown below in Fig. 1.

Fig. 1 calculation of the design parameters with excel spread sheet.

Fig 2. The schematic closed circuit wind-tunnel assembly: #3. Settling chamber with honey

comb, #4: Fan, #5: test chamber

Fig 3. 3D model of the closed-circuit wind-tunnel

The closed circuit wind tunnel designed by the first team would be stretched to 30’ long and 8’

high as shown in Fig 2 and 3. To expedite their progress, the team ordered the motor and fan.

Because of the large size of the sheet metal, they outsourced the manufacturing of the steel tunnel

parts to SMS manufacturing with their own design.

Because of the prolonged ordering process and delayed delivery of parts, the team did not finish

manufacturing the test chamber and supporting structures. The second- team’s duty was to finish

building the test chamber, design and manufacture the support structure, assemble the tunnel, and test

it.

Simulation-based design and optimization

In order to check if the design of the first team is feasible and meet the design specifications, the second

design team was asked by the project advisors to computationally simulate the flow before they start

designing the rest. They were required to study and explore different simulation tools such as

Solidworks Flow simulation, Comsol Multiphysics CFD, and Ansys Fluent, all available on campus.

The first simulation in Fig 6a shows the flow in the testing chamber is not uniform as the design

intention. It varies from bottom to top in the range of 18 to 40 m/s. This velocity variance is caused by

Fig. 4 The fabricated sections of the closed circuit wind tunnel

the sharp ninety degree flow direction change. The distance between the test chamber and the ninety

degree angle not long enough to allow the flow to fully develop. Obviously, 45 degree angled turning

vanes needed to guide the flow gradually to the test chamber. The next three simulations in Fig 6b, c,

and d show how the different number and the geometry of the turning vanes affected the flow in the

testing chamber. It shows that eight turning vanes will be adequate to guide the flow smoothly to the

chamber with uniform flow. The simulation in Fig 6c shows that the tailed guiding vanes does not

significantly improve the flow compared with the flow guided by regular shaped turning vanes from

Fig 6b. It is not necessary to have tailed longer guiding vanes inside the corner. Comparing Fig 6b and

Fig 6d, it is not necessary to have 16 vanes. 8 vanes are adequate to lead to a fully developed uniform

flow in the testing chamber. The 3D flow simulation results shown in Fig 7 and 8 more clearly

demonstrated the necessity of the turning vanes.

From the simulation results in Fig 6, turning vanes are necessary parts in order to generate uniform

flow in the testing chamber. However, the first team did not think them necessary. The 90 degree

corners 7 and 12 in Fig 2 had been manufactured without the vanes. To add the vanes, two brand new

pieces need to be made from scratch. It is technically challenging to custom manufacture eight perfectly

smooth and angled vanes inside the channel as shown in Fig 5. The university machine shop does not

have the machine to manufacture them. It will be much more costly and take a much longer time to

outsource it to other big manufacturing firm.

Fig 5. 3D Solidworks model of the guiding vanes

a. b.

c.

Fig 6. 2D Flow simulations without/with turning vanes in different number and geometry with Comsol

Multiphysics: a: no turning vanes; b: 8 regular turning vanes; c: 8 tailed turning vanes; d: 16 regular turning

vanes.

d.

The simulation results brought the team to rethink the original purpose of a closed circuit wind

tunnel, which is to save space and to control the noise level, in addition to budget and time

constraints. However, in order to adding the turning vanes, the materials and manufacturing cost

will be much higher than planned and it will take much longer time to complete.

The team reviewed the progresses made so far, such as the sections of the wind tunnel completed

already (Sections 1, 3, and 6 are ready, section 5 in Fig. 2 the testing chamber is redesigned and

manufactured due to the unsteady wooden structure in the previous design ). With only the four

sections, and connect section 4 to section 2 then 2 to 6, the team came to the realization of the

possibility of an open circuit design. After careful examination, the team realized that the open

Fig 8. The wind speed profile from simulation in the cross section of the test chamber without/with turning vanes

Fig 7. The 3D model simulation of the flow without/with turning vanes

circuit design will add no more cost and the previous materials and manufactured parts could be

fully utilized, and students wouldn’t need to wait for any other delayed orders. This time, the

students learned a valuable lesson through numerical simulation. Before jumping into action,

they decided to check their ideas, with numerical simulation! In the following days, they

simulated the flow in the test chamber with the open circuit wind-tunnel composed of sections 1,

2, 3, 5, and 6 as shown below in Fig 9.

Fig 9. The 3D solidworks prototype of the open circuit wind tunnel

Fig 10. The 2D simulation of the wind speed in the open circuit wind tunnel

The flow simulation results shown in Fig 10 and 11 demonstrate that the velocity in the test

chamber is uniform and reached to the design potential of 45 m/s (100 mph > 70 mph) in the

middle.

The other concern for the open circuit design is the noise. Normally a lawn mower’s noise level

is 90 dB. Study shows that noise above 85 dB are harmful depending on how long and how often

people are exposed to the noise. When students use this tunnel for labs, it will not run

continuously for more than 15 minutes. As long as the noise level is lower than 90dB, it is

tolerable. In order to make the noise lower, a silencer was added to this tunnel because it is inside

the building next to offices and classrooms. The flow induced noise level and frequency is

related to the size of the tunnel, the wind speed, and the relative scale and the angle of attack of

the testing models [7, 8]. Due to the large diameter of the diffuser, the designed low wind speed,

and no sharp corner which cause flow-structure collision, the noise shouldn’t be higher than the

previous design without the no turning vanes in the closed circuit tunnel. The simulation was

done with Ansys Fluent as shown in Fig 12. The noise without considering the motor and fan is

lower than 50 dB. It was verified by measuring the noise level when the finished tunnel was

Fig 11. The 3D simulation of the wind speed in the open circuit wind tunnel

turned on with a 70 mph wind speed. The noise level at different location shown in Fig 13. The

noise produced by the wind tunnel is lower than 90 dB with the silencer installed. The noise is

further lowered to undetectable level in the neighboring classrooms and offices.

Fig 14. The finished design of the open circuit wind tunnel with a mobile support structure

Fig 12. The simulation results of the

noise level inside the tunnel

generated by the wind

Fig 13. Noise dBs generated by the running

wind tunnel in different location

surrounding the tunnel mobile support

structure

Item Description Quantity Unit price ($) Total cost ($)

1 Fabrication and materials of

the tunnel body

1 8100 8100

2. Fan 1 2107 2107

3 silencer 1 2520 2520

4 Fabrication and materials of

test chamber

1 1581 1581

5 One phase to three phase

VFD

1 300 300

6 Support structure materials

and miscellaneous

1 3500 3500

Total 18108

Total Cost of manufacturing

Fig. 14 is the picture of the completed student designed open circuit wind tunnel. The total cost is

listed in the table above Table 1. The total cost is less than twenty thousand including the

supporting structure. The average budget for a senior design team is about two thousand dollars.

This total cost subtract the average design cost of four thousand dollars resulted in less than

fourteen thousand dollars which is barely enough to get a very small wind tunnel available

commercially. This design greatly saved the cost for the college to get a major engineering

equipment.

Because of the limitation of time, the second team finished assembling the tunnel, but they did not

have enough time to design and install the automatic data acquisition system. Mechanical and

aerospace engineering classes and labs will have to wait for the third senior design team to finish

their data acquisition system design before using for various experimental applications.

Discussion

Simulation before fabrication has become an engineering design norm. The simulation results will

help the designer to screen out the mistakes and infeasible design, and help to optimize the design.

It greatly saves time and resources. The first team did not perform computer simulation before

fabrication and manufacturing. They won’t be able to find the design problems until the test of the

Table 1. Total cost of the wind tunnel

final product. Since they could not find any design issue until the project in the testing phase. It

would add both cost and time and delay the use of the equipment. In fact, it was calculated that the

first year design mistakes costed nearly $5000 extra and delayed one year to the completion of the

entire project.

CFD simulation is now a common practice tool in industry. However, it was not a required skill

for most of the undergraduate engineering programs. Most of the students did not have

opportunities to learn and practice computational simulations in their study. However, it is an

important and required engineering skill in practice. It is desirable for engineering students to

acquire this skill by graduation for their future engineering design jobs.

This wind-tunnel design process demonstrated the importance of computational simulation in

engineering design process. The second year students learned a valuable lesson through their

design activities involving numerical simulation and were able to correct the design mistakes from

the first year team. These group of students were better prepared for their future career.

Conclusion

The outcome of this project is a great demonstration of the importance of computer simulationin

engineering design. Computational simulation directed design is more efficient and reliable.

Engineering programs will gain benefits by adding computational simulation into the

undergraduate curriculum and in their design activities. It would be even more beneficial if it can

become a required engineering skill for all engineering students and in all capstone design projects.

In order to prepare our mechanical engineering students to grasp this skill, we now require our

senior capstone design students to perform computational simulation to test their design if possible.

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