Effect of Water Droplets crossing the Boundary Layer in a
Stagnation Point Configuration
Emilio Borges
NASA Glenn Research Center
Major: Computer Science and Engineering
2016 Summer Session
Date: 11 August 2016
This final report has been reviewed and approved by Mario Vargas to ensure
information is accurate and does not contain sensitive data.
Signature LTI0/11 Aug. 2016
Mentor Name & Org Code/Date
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Effect of Water Droplets crossing the Boundary Layer in a
Stagnation Point Configuration
Emilio Borges1 University of Toledo
Dr. Mario Vargas2 NASA Glenn Research Center
Cleveland, OH 44135
Dr. Andy Broeren 3 NASA Glenn Research Center
Cleveland, OH 44135
Dr. Stephen McClain4 Baylor University
Waco, TX 76798
NASA Glenn Research Center Summer Internship 2016 Funded by the Ohio Space Grant Consortium through the Ohio Aerospace Institute
Aircraft icing is a dangerous phenomenon that is studied in-depth by
NASA’s Icing Branch in order to improve safety for passengers and crew of
aircraft operating in icing conditions. One of NASA’s most widely used tools
is icing prediction codes, such as LEWICE. Within LEWICE, the modeling
of ice roughness and its effects on convective heat transfer can be improved,
furthering LEWICE’s attractiveness to potential customers. Building on
several previous studies, this study examines the effect different sized water
droplets have on airflow within the boundary layer in stagnation region
flows. Using the Vertical Icing Studies Tunnel (VIST) at NASA Glenn
Research Center, a test plate representing the leading 2% chord of a NACA
0012 was subject to various flow conditions. A hot-wire anemometer is used
to measure the VIST’s boundary layer flow characteristics at varying flow
speeds to build a control group of data before proceeding with modifying the
tunnel to install a spray water nozzle. To select the best nozzle for the
experiment, special software was designed to model the flow pattern of the
nozzle within the varying flow conditions of the VIST. Once the nozzle is
installed, another set of hot-wire anemometer data is to be collected to study
the effect the nozzle had on the VIST’s boundary layer. The data gathered
during this study will ultimately be used to improve ice accretion codes, such
as LEWICE, in an effort to improve their ability to match real icing events
and allow more accurate prediction of ice accretions on aircraft surfaces
without the need for full-scale testing.
1 Bachelor’s of Computer Science and Engineering at The University of Toledo, Toledo, OH 43606 2 Aerospace Engineer, Icing Branch, 21000 Brookpark Rd., AIAA Associate Fellow. 3 Aerospace Engineer, Icing Branch, 21000 Brookpark Rd., AIAA Associate Fellow. 4 Associate Professor, Department of Mechanical Engineering, One Bear Place #97356, AIAA Senior Member.
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Nomenclature
𝐴𝑜 = Nozzle orifice area
𝑄 = Volumetric flow rate capacity
𝑉𝑜 = Orifice flow velocity
𝜃𝑠 = Nozzle spray angle
𝜌𝐴 = Air density
𝜌𝑊 = Water density
𝜇 = Dry air dynamic viscosity
𝐴𝑑 = Droplet cross sectional area
𝑉𝑜𝑙𝑑 = Droplet volume
𝑀 = Droplet mass
𝐺 = Gravity
𝑉𝑉𝐼𝑆𝑇 = VIST flow velocity
𝑉𝑠𝑙𝑖𝑝 = Droplet slip velocity
𝑅𝑒 = Reynolds number
𝐶𝑑 = Drag coefficient
I. Introduction
he effects of in-flight ice accretions on aircraft surfaces have been studied by NACA and
NASA since the mid 1940’s [1]. Following the high loss rate of cargo aircraft flying supply
missions over the Himalayas during the Second World War, the United States government began
construction of the Icing Research Tunnel in 1944. Since then, the Icing Branch at NASA Glenn
Research Center has utilized a wide variety of tools to study the effects of icing and the methods
of its formation in order to improve aircraft safety. As demonstrated in Figure 1, as ice forms on
the leading edge of an airfoil, the lift and stall margin decrease while the skin friction drag and
weight increase. As a result of such serious reduction in performance of an aircraft in icing
conditions, the Federal Aviation Administration has published a set of certification requirements
for flying in such icing conditions known as Appendix C. All aircraft must undergo certification
to fly in Appendix C icing conditions. As a result, commercial and military aircraft alike utilize
extensive testing to ensure their aircraft’s ability to operate in such conditions. In addition to pure
research, the Icing Research Tunnel at NASA Glenn Research Center, in-flight testing, and
computational ice prediction codes are all utilized by customers in order to certify their aircraft.
LEWICE, developed by researchers at NASA Glenn, is one of the leading software
applications in industry and provides valuable data to customers without the need for full-scale
testing. LEWICE couples computational fluid dynamics with heat transfer to predict the
formation of ice accretions on airfoils [2]. It is widely used throughout the aeronautics industry
and is currently the best predictor of ice accretions on aircraft surfaces. However, this does not
mean LEWICE is perfect. There are still many areas within the LEWICE source code that need
improvement. In order to validate such improvements, experimental tests need to be conducted
to verify LEWICE’s results match with experimental results. The objective of the author’s ten-
week summer internship at NASA Glenn Research Center is to determine the validity of the
following hypothesis in a stagnation point configuration: water droplets crossing a boundary
layer will create turbulent spots that will accelerate the transition from laminar to turbulent. The
T
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experimental results from this hypothesis will then go on to improve LEWICE’s ice accretion
predictions on aircraft surfaces. However, due to the short duration of the ten-week summer
internship, the author was not able to obtain experimental data for this hypothesis. Instead, all the
necessary requirements to get the experiment set up and ready to gather data were completed.
Figure 1 - Negative effects on aircraft performance due to ice accretion.
II. Experiment
All testing in this study was performed at NASA Glenn Research Center in Building 5,
Room CW-5 utilizing the Vertical Icing Studies Tunnel. The following sections will outline the
testing apparatus, testing equipment used, spray nozzle selection, spray nozzle bread table
experiment, designing the spray nozzle installation, and how data was gathered.
A. Testing Apparatus
The Vertical Icing Studies Tunnel is a closed loop, atmospheric tunnel with a 7.2:1
contraction ratio, a 4 in. wide throat, and a 3 HP DC motor with a max speed of 1750 rpm. The
VIST’s fan enables throat velocities ranging from 2 m/s (6.5 ft/s) to 25 m/s (82 ft/s). The VIST
was designed in 2005 by Dr. Ed White and first presented as a viable test apparatus shortly
thereafter in a publication by White and Oliver [3]. It was originally design to study water
droplet impingement on a 737 midspan at the stagnation region. The VIST’s design to study the
stagnation region of airfoils made it an ideal testing apparatus for this study. Unlike conventional
wind tunnels, the VIST utilizes an instrumented flat plate to model the airfoil desired. In order to
model the desired airfoil, the side walls of the VIST were contoured and instrumented with
pressure taps to ensure the test plate was subjected to the same accelerating flows experienced by
the airfoil in question. The original test plate installed in the VIST was a 30 in. by 60 in. flat,
aluminum plate instrumented with pressure taps along its surface. The stagnation point on the
test plate was directly in the center, with the flow impinging downward on the test plate at the
center and accelerating as the flow continued outward as shown in Figure 2.
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Figure 2 - VIST schematic showing side wall and test plate
position.
Figure 3 - Vertical Icing Studies Tunnel
B. Testing Equipment
To get an accurate measurement of the boundary layer flow, a TSI 1218-10 standard
boundary layer hot-wire anemometer, shown in Figures 4 and 5, was positioned inside the VIST
at varying distances from the stagnation point. A hot-wire anemometer measures air flow
velocity by electrically heating an extremely thin tungsten wire above ambient temperature. As
air flows past the wire, the temperature of the tungsten changes, which changes the resistance of
the wire. This resistance is then measured and compared against a relationship between
resistance and flow velocity [4]. With a sensitive hot-wire anemometer, such as the TSI 1218-10,
it is possible to measure turbulence within the boundary layer flow.
Figure 4 - TSI 1218-10 Standard Boundary Layer Hot-Wire
Anemometer Probe
Figure 5 - TSI 1218-10 probe hundredth of an inch above the VIST test plate
In order to measure minute changes in resistance, the TSI 1218-10 probe is paired up
with the TSI IFA-300 Constant Temperature Anemometer anemometry system. The IFA-300
provides the necessary hardware to configure the probe, collect precise measurements, and
remove any unwanted signal noise. The output from the IFA-300 then feeds into a National
Instruments Data Acquisition system, which allows LabView software to monitor and collect
data from the probe.
Lastly, to control the position of the probe, a Velmex VXM Stepping Motor Controller
was utilized. Through a LabView program, the probe position can be finely controlled to
thousandth of an inch. This level of control was especially helpful when lowering the probe to
hover just above the plate.
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C. Spray Nozzle Selection
Generating water droplets inside the VIST of the right size and flow pattern requires a
specialized water nozzle with a well-understood and predictable operation while in varying wind
velocities. It was known that a flat spray nozzle was needed, but specifically which flat spray
nozzle would work best with the VIST was to be determined. Because the test plate inside the
VIST represents the leading 2% chord of a NACA 0012, the nozzle needed to produce water
droplets between 200 and 5000 micrometers in diameter. The biggest factor, however, when
searching for a nozzle was making sure the inner walls of the VIST would not get wet while
running the experiment. To get a better idea of which nozzles worked best, specialized software
was written to calculate the nozzle’s outer flow trajectory given the nozzle’s flow rate, spray
angle, and orifice diameter parameters, constant VIST wind velocity, and water pressure into the
nozzle.
From the given information, the software then calculates the following:
𝐴𝑜 = 𝜋 (𝑑
2)
2
where d is the orifice diameter
𝑄 = a power curve fit of a linear-to-power transformation of a linear regression on a linear
fit of a power-to-linear transformation of the given nozzle flow rate capacity values
𝑦 = 𝑎𝑥𝑏 → ln(𝑦) = 𝑏 ln(𝑥) + ln(𝑎) → 𝑦 = 𝑒ln(𝑎)𝑥𝑏
where y = nozzle flow rate, x = given water pressure, ln(𝑎) = linear y-intercept, a =
power y-intercept, b = slope
𝑉𝑜 = 𝑄/𝐴𝑜
𝜃𝑠 = logarithmic least squares fitting of the given nozzle spray angle at varying water
pressures [5]
𝑦 = 𝑎 + 𝑏 ln 𝑥
𝑏 =𝑛 ∑ (𝑦𝑖 ln 𝑥𝑖) − ∑ 𝑦𝑖
𝑛𝑖=1 ∑ ln 𝑥𝑖
𝑛𝑖=1
𝑛𝑖=1
𝑛 ∑ (ln 𝑥𝑖) − (∑ ln 𝑥𝑖𝑛𝑖=𝑖 )2𝑛
𝑖=1
𝑎 =∑ 𝑦𝑖
𝑛𝑖=𝑖 − 𝑏 ∑ (ln 𝑥𝑖)
𝑛𝑖=𝑖
𝑛
where y = 𝜃𝑠, x = water pressure
𝜌𝐴 = 1.200227 kg/m3 at 70ºF [6]
𝜌𝑊 = 998.02 kg/m3 at 70ºF [7]
𝜇 = 1.77E-5 at 70ºF [8]
𝐴𝑑 = 𝜋 (𝑑
2)
2
where d is the droplet diameter
𝑉𝑜𝑙𝑑 = 4
3𝜋 (
𝑑
2)
3
where d is the droplet diameter
𝑀 = 𝑉𝑜𝑙𝑑 ∗ 𝜌𝑊
𝐺 = 9.80665 m/s2
𝑉𝑠𝑙𝑖𝑝 = 𝑉𝑜 − 𝑉𝑉𝐼𝑆𝑇
𝑅𝑒 = 𝑉𝑠𝑙𝑖𝑝 𝑑
𝜇/𝜌𝐴 where d is the droplet diameter
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𝐶𝑑 = 24
𝑅𝑒+
2.6(𝑅𝑒
5)
1+(𝑅𝑒
5)
1.52 +0.411(
𝑅𝑒
263,000)
7.94
1+(𝑅𝑒
263,000)
−8 +𝑅𝑒0.8
461,000 [9]
After calculating the above values, the software then uses the Runge-Kutta numerical
method for first order differential equations [10] to solve for the outer droplet’s X and Y
velocities from the following acceleration equations:
𝑥′ = −𝐶 √𝑥2 + 𝑦2 𝑥
𝑦′ = 𝑔 − 𝐶√𝑥2 + 𝑦2 𝑦
where 𝐶 =𝜌𝐶𝑑𝐴𝑑
2𝑀 is the constants value extracted and reworked from the drag force equation
𝐹𝑑 = 𝑀 𝐴𝑐𝑐𝑑 =1
2𝜌𝐴𝑉𝑠𝑙𝑖𝑝
2 𝐶𝑑𝐴𝑑 where 𝐴𝑐𝑐𝑑 is the droplet acceleration. Using the droplet X and
Y velocities over time, the X and Y displacements are calculated and used to plot the graph
shown in Figure 6.
Figure 6 - VIST Nozzle Spray Area Software Interface; Control Panel (top), VIST Water Nozzle Predicted Spray Area (left), Droplet Velocity vs. Time (right)
Once completed, the above software was then ran with various nozzle parameters in
various VIST conditions to see which nozzle worked best for the experiment. In the left graph of
the figure above, the orange vertical lines represent the side walls of the VIST, the blue lines
represent the nozzle spray area, the zero-x axis represents the VIST test plate, and the y axis is
the height of the nozzle from the test plate. The goal of this software was to determine which
nozzles gave us the maximum spray area without getting the side walls wet. With the results
from the software, two nozzles from Spray Systems Co. were selected; an air atomizing 16860-
1/8JJAU-SS spray nozzle [11] and a hydraulic 1/8TT-SS spray nozzle [12] both equipped with a
UniJet 800050 TPU spray tip.
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D. Spray Nozzle Bread Table Experiment
Before installing the spray nozzle into the VIST, a bread table experiment was designed to test and operate the nozzle in order to work out any issues and write the operating procedure. As shown in Figure 7, the nozzle control interface is fed pressurized shop air at 120 psi, which is redirected into an EMERSON Tescom pressure regulator system. The Tescom system is what controls the water pressure feeding into the nozzle through a computer interface (not shown). Once the water tank reaches the desired pressure, the water valve is opened and pressurized water flows through the flow meter and into the nozzle. This experiment allowed for the understanding of operating the water nozzle and paved way for designing how the nozzle should be installed inside the VIST.
Figure 7 - 1/8TT-SS spray nozzle bread table experiment; nozzle control interface (left), nozzle and flow meter (right)
E. Designing Spray Nozzle Installation
With a nozzle selected, the next challenge is designing a frame to hold the nozzle inside
the center of the VIST above the contraction phase. The decision was made to use the acrylic
glass window, shown in Figure 8 near the top, as the location to hold and interface with the
nozzle.
Figure 8 – VIST front; acrylic glass window (top)
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The design for the frame was constrained to minimal machinign and easy assembly. The final design, shown in Figure 9, includes pre-made parts ordered from McMaster-Carr and acrylic sheets machined to shape.
Figure 9 – Nozzle Frame CAD Assembly; (left) nozzle frame without nozzle, (middle) nozzle frame with nozzle, (right) complete nozzle frame assembly with acrylic glass window
F. Gathering Data
Given the short duration of the ten-week internship, the set of boundary layer airflow data
collected is from the unmodified VIST without the nozzle installed. These measurements act as a
control to compare against future measurements when the nozzle is installed. The measurements
were made at varying heights above the VIST test plate near the stagnation point at varying flow
velocities. Six different velocities were measured: 15 ft/s, 25 ft/s, 35 ft/s, 40 ft/s, 45 ft/s, 55 ft/s.
For each velocity, the TSI 1218-10 hot-wire anemometer probe collected 50,000 measurements
per second for 2.5 seconds at eighty-one different heights above the VIST test plate totaling to
10,125,000 measurements per velocity. Before collecting data, the hot-wire anemometer was
carefully lowered to be as close as possible to the test plate. As the test ran, the Velmex system
would raise the probe slightly until eighty-one different heights were measured in the two inch
height boundary the probe was set to measure. Through the IFA-300 and NI DAQ, the
measurements were recorded on a computer in a standard .txt file. The values recorded were
voltages representing the boundary layer flow at different times.
III. Results and Discussion
Following the collection of data, the measurements were run through a MATLAB script,
developed by Dr. Stephen McClain, to interpret the voltage measurements as meaningful data.
For each velocity measured, the script produced two graphs; boundary layer flow velocity vs.
height, as shown in Figure 10, and turbulence intensity vs. height, as shown in Figure 11.
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Figure 10 – Boundary layer flow velocity (m/s) vs. height (in.)
at 15 ft/s
Figure 11 – Boundary layer turbulence intensity (%) vs.
height (in.) at 15 ft/s
Figure 12 – Boundary layer flow velocity (m/s) vs. height (in.)
at 35 ft/s
Figure 13 – Boundary layer turbulence intensity (%) vs.
height (in.) at 35 ft/s
Figure 14 – Boundary layer flow velocity (m/s) vs. height (in.)
at 55 ft/s
Figure 15 – Boundary layer turbulence intensity (%) vs.
height (in.) at 55 ft/s
From the graphs, it is clear the VIST’s boundary layer flow contains too much turbulence
for droplet measurements to be useful. From the turbulence intensity graphs above, the average
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turbulence intensity is around 2%. The acceptable turbulence should be between 0.5% and 1%
intensity.
IV. Conclusion
Due to the high level of turbulence inherent in the VIST, experimental measurements
cannot continue until the turbulence is lowered below 1%. It is thought that the centrifugal fan
used in the VIST is the cause for such high turbulence, but this suspicion has not been verified.
The VIST already includes honeycomb and mesh to improve the flow field, but it is not enough
to lower the turbulence. A study needs to be conducted on what specifically is causing the high
turbulence and develop a plan of action to correct the issue(s).
Acknowledgements
The study outlined in this paper was performed under the NASA Space Grant Internship
funded by the Ohio Space Grant Consortium through the Ohio Aerospace Institute at NASA
Glenn Research Center. The assistance of Mr. Robert Clark, Mr. Kurt Rusmisel, Mrs. Katelyn
McCormick, and Mrs. Marivell Baez at NASA GRC is greatly appreciated. Finally, the
assistance of fellow interns Joaquin Martinez and Aaron Tallman in work on the VIST and
development of spray trajectory software, respectively, is also greatly appreciated. Any opinions
presented in this paper are those of the authors and do not reflect the views of NASA or the
United States government.
References 1Leary, W. M. (2002). We Freeze to Please: A History of NASA's Icing Research Tunnel and the
Quest for Flight Safety (No. NAS 1.21: 4226,). 2Wright, W.B., “User Manual for the NASA Glenn Ice Accretion Code LEWICE,” NASA/CR-
2002-211793, 2002. 3White, E. B. and Oliver, M. J., (2005), “Experiments on Surface Roughness Effects in Ice
Accretion,” Presented at the AIAA 4th Theoretical Fluids Meeting, June 22-25, 2005,
Toronto, ON, AIAA-2005-5190. 4"Hot-wire Anemometer explanation". eFunda. Archived from the original on 10 October 2006.
Retrieved 18 September 2006. 5WolframMathWorld, “Least Squares Fitting--Logarithmic," Retrieved June 27, 2016, from
mathworld.wolfram.com/LeastSquaresFittingLogarithmic.html 6The Engineering ToolBox, “Air Density and Specific Weight,” Retrieved June 28, 2016, from
www.engineeringtoolbox.com/air-density-specific-weight-d_600.html 7The Engineering ToolBox, “Water Density and Specific Weight,” Retrieved June 29, 2016,
from www.engineeringtoolbox.com/water-density-specific-weight-d_595.html 8The Engineering ToolBox, “Dry Air Properties,” Retrieved June 28, 2016, from
www.engineeringtoolbox.com/dry-air-properties-d_973.html 9Faith A. Morrison, “Data Correlation for Drag Coefficient for Sphere,” Department of Chemical
Engineering, Michigan Technological University, Houghton, MI,
www.chem.mtu.edu/~fmorriso/DataCorrelationForSphereDrag2013.pdf 10Bahr. “Solving Ordinal Differential Equation Using Ms. Excel.” Web log post. Bahrfly’s Blog.
N.p., 16 Feb. 2010. Web. 5 July 2016. 11Spray Systems Co. “Automatic & Air Atomizing Spray Nozzles.” Catalog. pp. A4, A6, B16,
C8. http://www.spray.com/cat76/automatic/
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12Spray Systems Co. “Industrial Hydraulic Spray Products.” Catalog. pp. A6, A8, C24-C31.
http://www.spray.com/cat75/hydraulic/ 13White, Edward B. "Design and Construction of an Icing Physics Flow Laboratory." Proposal.
Case Western Reserve University, 2004. Print.