Effect Of Concentrated Roughness On Transition

Effect Of Concentrated Roughness On Transition
Location At Transonic Speed
A. d’Argenio1 and F. D’Errico2,
University of Naples, Federico II, Italy, 80100
A. Marino3, C. Izzo4
CIRA (Italian Aerospace Research Centre), Capua, Italy, 81043
and
C.de Nicola5
University of Naples, Federico II, Italy, 80100
This paper describes the Design of Experiment to study the laminar-turbulence
transition phenomenon induced by trips of different height at different axial
positions on a 10° AEDC cone in Transonic-Supersonic regime. The huge numbers
of factors (as bluntness, distributed roughness, Mach and Reynolds numbers, etc.),
that affect the laminar-turbulent transition, makes this phenomenon very complex
and expensive to study.
The principal objective of this activity is evaluate the correlation between infrared
thermography measurements and high frequency pressure measurements for
transition analysis.
Nomenclature
CIRA
AEDC
TPS
PT1
PSD
γ
σ
λ
Re
Re*
Rext
ReT
Ret
Rek
Rex
1
=
=
=
=
=
=
=
=
=
=
=
=
=
=
=
Italian Aerospace Research Centre
Arnold Engineering Development Centre
Thermal Protection System
Pilot Tunnel 1
Power Spectral Density
Summit Angle
Standard deviation
Precision requirement for the response surface
Reynolds number
Logarithm of Reynolds number
Reynolds number calculated using the transition abscissa as reference length
Reynolds number calculated at the begin of the transition
Reynolds number calculated at the end of the transition
Reynolds number based on roughness height and local flow conditions at top of roughness
Reynolds number based on length of x from leading edge to roughness station and on conditions
Engineer, University of Naples Federico II, [email protected].
Engineer, University of Naples Federico II, [email protected]
3
Researcher/ Test Engineer, Transonic Testing Laboratory, CIRA, [email protected].
4
Transonic Testing Laboratory head, CIRA, [email protected].
5
Prof of Aircraft Aerodynamic, University of Naples Federico II, [email protected]
1
American Institute of Aeronautics and Astronautics
2
Rer
xT
xt
xL
xU
M
K
K*
η
ηk
=
=
=
=
=
=
=
=
=
=
outside boundary layer
Reynolds number calculated using the nose radius as reference length
Transition abscissa
Trip position
Equivalent distance referred to the lower point Reynolds number
Equivalent distance referred to the upper point Reynolds number
Mach number
Roughness Height
Non-dimensional roughness height
Non-dimensional height in boundary layer based on distance above surface
Non-dimensional height in boundary layer based on roughness height
I. Introduction
Transition phenomena are a critical issue which involve several aerothermodynamic fields such as thermal
protection system (TPS) of re-entry vehicles. The spaceships returning on Earth (as Shuttle) are subjected to
extremely high convective heat causing severely increase of wall surface temperature of the vehicle or as
aerodynamic performance of a wing.
For experimental activity in wind tunnels, the determination of the transition position is very important for
numerical-experimental comparison. Generally to avoid any uncertainty about transition position, tip transition
(generally with carborundum grain) are put on the model to force transition at the trip position. In literature, are
present several method to evaluate the “critical height” of the trip transition inducing transition, but results are
quite conservative. Moreover, knowledge of the minimum transition trip height inducing transition on a wind
tunnel model is very important to reduce drag increase caused by friction resistance due to trip transition presence
involving less accuracy in extrapolation from Wind Tunnel to flight.
In general the most important factors which influence transition in transonic regime are Mach number,
Reynolds number, geometry and Roughness. The effect on transition due to roughness is probably the most
difficult to be controlled and to be recognized.
In order to improve the understanding of the transition phenomena, a dedicated experimental activity was
performed in the CIRA PT-1 Transonic tunnel at subsonic/transonic Mach number (0.35≤M≤1.1), on a 10° AEDC
cone model equipped with 10 pressure taps, 12 Kulite high frequency pressure sensor and covered with a matt
black low thermal conductivity coating for infrared thermal recording. Several transition trips of different
roughness size and dimension was applied on the model in order to evaluate the trip effect on transition position..
The huge number of factors (as concentrated roughness size, transition trip position, Mach and Reynolds numbers),
that affect the laminar-turbulent transition, makes this phenomena very complex and expensive to study.
Finally the use of Kulite (high frequency pressure transducers) sensors and thermography technique allowed to
evaluate the correlation between measurements carried out by infrared thermography and Kulite, for transition
analysis.
II. Aim of the Experiments
In next years several experiments will be performed by CIRA on a cone in supersonic/hypersonic regime in
order to improve the understanding of the transition mechanisms in hypersonic flow and, at the same time, to
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improve the effectiveness of the existing transition prediction criteria taking into account the effects of the
presence of distributed roughness and of the bluntness (Ref.[16]).
Within this context, a special test activity has been developed in the CIRA PT-1 Wind Tunnel with two main
purposes: the first was to develop experience, software and techniques for transition detection that will be used
during the future test campaign, the second was to improve the understanding of the transition mechanism in
transonic/supersonic flow considering the effects of concentrated roughness height and of trip position taking into
account Reynolds and Mach number variation.
Finally, the determination of the critical trip height inducing transition in the PT-1 wind tunnel was another
important aim of the activity.
I.
Background
A. Mach Number effect on laminar- turbulent transition position
Mach number is certainly an important
parameter on the transition position. Figure
1 (Ref.[8]) shows the Reynolds number
estimated at the end of transition as a
function of Mach number, on a 10° cone in
the NTF (National Transonic Facility at
NASA
Langley).
In
the
figure
RT
correspond to Rext before defined.
Figure 1 shows that the Rext at which
the transition occurs rises with Mach.
In particular, it is possible to notice an
initial constant trend followed by a rapid
increase of transition Reynolds number. A
critical Mach number (M=0.6) can be
identified,
where
this
trend
change
Figure 1: Estimated end of transition Reynolds number as a
function of the Mach number on a 10°-cone in the NTF [8]
happens.
B. Reynolds number effects
As can be seen in Figure 2 (Ref.[9]) the transitional Reynolds number on a 10° cone geometry decreases with the
increasing of the Reynolds number, according to the physical phenomenon. It is also possible to see that there is no
evidence of the dependence between the two parameters. In fact, despite of the great number of data present in
literature, the relation between this two variable is not clearly defined.
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Figure 3 (Ref.[8]) gives a graphical
representation of the estimated transition
point xT for different Mach numbers, as
function of the unit-Reynolds number.
Moreover, the same figure shows the
equivalent distance xL, referred to the lower
point Reynolds number ReL=ReT-(ReT-Ret),
and the equivalent distance xU referred to
the
upper
point
Reynolds
number
ReU=ReT+(ReT-Ret) where ReT and Ret are
respectively the transition Reynolds number
calculated at the begin and at the end of the
transition.
C. Transition tripping position effect
The effect of the transition trip position
on the AEDC cone, is not so amply
described
in
literature.
However,
as
reference, this dependence can be obtained
Figure 2: Transition Reynolds number as a function of unit
Reynolds number on a 10° cone [9]
by some tests on other geometries. Figure 4
(Ref.[10])
shows
transition
Reynolds
number as a function of transition tripping
position, on a NLF(2)-0415 airfoil. In the
figure, RTR correspond to Rext.
Figure 4 shows that the Rext, at which
the transition occurs, has a complicated
dependence respect to transition position. In
particular as the transition trip position
grows, the Rext first decreases up to a critical
value and after grows in a not well defined
manner.
D. Trip Transition height effect
Trip Transition height is certainly an
Figure 3: Estimated end of transition length as a function of the
unit Reynolds number on a 10° cone in the NTF, assuming
RT=f(M) [8]
important parameter for the transition position.
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Despite it can be expected that the trip
transition size anticipate the transition, this
effect is not so amply described in literature
on a AEDC cone and a well known law
between the roughness height and transition
position is not available.
There
are
very
few
transition
measurement results pertaining to roughness
elements locally distributed on a surface, so
it is very difficult to find a reference to
establish a relation between trip transition
height and transitional Reynolds number.
In Ref.[11] is reported that when the
Reynolds number obtained in function of
Figure 4: Transition Reynolds number and transition tripping
dependence [10]
the roughness height ks exceeds a critical value, the transitional Reynolds number drops greatly.
Ulks
= 120
ν
(1)
Figure 5 (Ref.[11]), shows the ratio of the transitional Reynolds number on a flat plate at zero incidence with
single roughness elements respect to transitional Reynolds number on a smooth plate as function of the trip
transition height (referred to the boundary
layer thickness). In particularly, when the trip
transition height increases, the Reynolds
number ratio decreases. Then the transition
on the plate with roughness elements
anticipate respect to smooth one.
For the current activity, it is also useful to
define the minimum roughness size inducing
transition at the trip position (Ref.[13]), this
value is named “Critical Height”. At the
same manner the critical roughness Reynolds
number Rek,t (Reynolds based on critical
roughness
height) can
particular,
roughness
be
defined.
Reynolds
In
number
smaller than the critical value induce no
disturbance of sufficient magnitude, in
boundary layer, to induce transition at the trip
Figure 5: Ratio of the critical Reynolds number at a flat plate
at zero incidence with single roughness elements to that of the
smooth plate [11]
position but have only the effect to anticipate the transition position. Whereas roughness particles equal the critical
size, the formation of turbulent spots initiate near the roughness that coalesce into a continuously turbulent flow
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somewhat downstream of the roughness. In this
condition, only a small increase in roughness
Reynolds number above the critical value is
required to move the fully developed turbulent
boundary
layer
substantially
just
after
the
roughness particles.
From literature the “Critical Height” inducing
transition for a given Mach number, unit Reynolds
number and roughness location, can be calculated
once that the Reynolds number based on the length
of x from leading edge to roughness station, are
defined. (Figure 6): for a selected value of Rek (in
figure Rek=Rk) and for a given Mach number, unit
Reynolds number, and roughness location, the
value of the nondimensional roughness height ηk,
inducing transition, can be determined once
calculated the value of the Reynolds number ratio
Re k
defined as:
Re x
u ν
Re k
K
= 
Re x  k  0
Re x  x
 U  ν k



(2)
Experimental tests showed that Rek,t values, for
which transition happens just after the roughness
particles, was measured between 250 and 600 at
Mach numbers up to 2 (Ref.[13]). Consequently,
for the investigation in which a fully developed
turbulent
boundary
layer
occurring
at
the
roughness position is desired, values of Rk,t slightly
Figure 6: Critical height inducing for 3D conical bodies at
Mach number from 0 to 5 [Ref.[13]]
larger than 600 should be used to calculate the roughness height inducing transition.
Lets notice that, currently, this method is widely used at PT-1 wind tunnel to choose the roughness height to be
install on models that surely induce transition (just after roughness particles).
II. Test Setup and Test Instrumentations
A. Test facility
PT-1 Transonic Research tunnel is part of a brand new complex which includes ground testing facilities and
dedicated service utilities, making up the Italian
Aerospace Research Centre (CIRA). The introduction of
Transonic Wind Tunnel PT-1 was the aim to support industrial and research programmes with a versatile and high
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flow
quality
aerodynamic
testing
platform, over a wide speed range, for a
variety of small scale test articles. The
pictures in Figure 7 shows a view of the
facility. The PT-1 is a pressurized wind
tunnel, which operates in a closed-circuit.
It has two drive systems: a 145 kW fan,
for continuous subsonic tests, in the low
subsonic
range
(M<0.35),
and
a
compressed air injection system for
intermittent
transonic
and
supersonic
Figure 7: PT-1 Transonic Wind Tunnel [12]
operation, in the high subsonic-transonic
(0.35≤M≤1.1) and supersonic (M=1.4)
ranges, with a maximum total pressure of
1.85 bar and useable test time of 150
seconds. The fan is connected to an
external drive motor providing the motive
power
to
continuous
the
wind
subsonic
tunnel
during
operations.
A
Cooling System (CS) is present to remove
the heat generated by the fan during
continuous operations. Cooling water
Figure 8: PT-1 M-Re operating envelope (Reference length 1m)
[12]
flow is variable, up to 5 lt/s. A heat exchanger dissipates the energy delivered to the stream by the wind tunnel
drive fan and it is designed for low pressure loss, spatially uniform flow and small variations in bulk air
temperature across its face. The facility is equipped with two nozzle blocks: a convergent nozzle for Mach
numbers below 1.1 and a convergent-divergent nozzle to reach M=1.4. The operating envelope of PT-1 is shown in
Figure 8.
The tunnel is equipped with two test sections, one with solid walls for subsonic use and the second, with
perforated walls, for transonic and supersonic tests. In both cases the test section is 0.45m wide, with a height of
0.35m and a useable length of 0.60m. The transonic section porous walls with 60° inclined holes and 6.23% open
area on all four sides. are equipped with appropriate model support systems. For two dimensional wall-to-wall
models, a pair of turntables is used for position control, while for three dimensional models there is an option of a
fixed sting or remotely controllable vertical rotation support.
B. Test Article
The cone used for the experimental activities, has a summit angle γ of 10° (Figure 9). The basic configuration has a
length of 0.22m, an extension system is able to reach the extended configuration up to 0.31m. For this test the
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extended configuration is used. The
nose has a radius less than 0.1mm. The
superficial
roughness
is
less
than
1.6µm.
The model is equipped with 10
pressure
taps
for
steady
pressure
measurements and 13 inserts for high
Figure 9: AEDC Cone
frequency pressure transducer (type
“kulite”). Table 1 reports pressure taps positions and kulites locations.
In order to obtain IR measurements, the model made of RAMAX AISI 420 is covered with a matte black low
thermal conductivity coating (type DEVCON) with maximum thickness of 3mm. The coating extends along 180°
in azimuth direction starting from φ=0°. Near the cone apex the DEVCON thickness decreases according to the
little local diameter.
C. Test Configuration
AEDC
cone
in
extended
configuration is placed at 0° angle of
attack in the 3D transonic test section
with
porous
walls.
The
correct
alignment of the cone model with wind
tunnel flow was performed by means of
an high precision (0.001°) bubble
inclinometer (model Borletti K0-60),
positioning the axis of AEDC cone, in
extended configuration, at 0° angle of
attack with respect to the wind tunnel
flow direction, in the 3D transonic test
section with porous walls (Figure 10).
Figure 10: AEDC Cone installed in the 3D porous test section.
D. Instrumentation And Data Acquisition
1.
Static and Total pressure in PT-1 Wind Tunnel
The data acquisition system was a UNITED SENSOR Pitot tube, type USNH-A-122, installed in the settling
chamber to measure free stream total pressure. Free stream total pressure is acquired via a RUSKA 7222 absolute
transducer with a full scale of 26 PSI and an accuracy of 0.01% FS.
Free stream reference static pressure was measured at the tunnel wall in the test section inlet via 0.6mm diameter
pressure taps distributed on the 4 sides of the test section. Each pressure tap communicates with the pressure
transducer via a 6mm diameter tube. As with the total pressure, the transducer used is a RUSKA 7222 with the same
scale and accuracy as noted above.
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2.
Atmospheric pressure
Atmospheric pressure was measured
using a DRUCK DPI 740 barometer with
accuracy ±15 Pa. Total temperature is
measured using a UNITED SENSOR
USNH-B-106 total temperature probe
installed in the settling chamber.
3.
Static Pressures on AEDC cone
All pressures from flow–field were
acquired using the PSI 8400 system
electronic differential pressure scanners
ID
P1
P2
P3
P4
P5
P6
P7
P8
P9
P10
PRESSURE TAPS
x [mm ] φ [degrees ]
77.50
180°
93.00
180°
124.00
180°
155.00
22 °
186.00
225°
215.00
225°
275.00
225°
127.00
0°
127.00
70°
127.00
90°
KULITE TRASDUCERS
ID
x [mm ] φ [degrees ]
K1
8 .50
135°
K2
94.75
135°
K3
122.00
135°
K4
139.30
135°
K5
156.50
135°
K6
173.80
135°
K7
191.00
135°
K8
208.30
135°
K9
232.50
135°
K10
242.80
135°
Table 1: Pressure Taps Position for Static Pressure Measurements
and Kulite Location for High Frequency Pressure Measurements
(EPS), referenced to wind tunnel total or
static pressure, or to atmospheric pressure,
in accordance with the specific necessity.
A number of pressure scanners with
different full scale values are available,
whose accuracy is 0.05% FS for full
scales greater or equal to 5 psi, and 0.1%
FS for lower full scales.
The data acquisition is performed
using a in–house developed interactive
Figure 11: Pressure coefficient on AEDC cone at zero incidence
[15]
system; it integrates a number of different
sub-systems, allowing their configuration
synchronizing their outputs. Specifically, it
P1
Generatrix
φ
[degrees]
180°
0-± 5
Typology
(Differential/
Absolute)
Differential
manages
calibration,
P2
180°
93.0
0-± 5
Differential
calibration control and data acquisition and
P3
180°
124
0-± 5
Differential
Static
Gall ry
Static Gallery
P4
225°
155
0-± 5
Differential
Static Gallery
in
a
common
environment
configuration,
and
storage of both the pressure and analog
Taps
[-]
Axial
Position
[cm]
77.5
Pressure
Range
[psi]
Referen e
Pressure
Static Gallery
P5
225°
186
0-± 5
Differential
Static Gallery
input modules of the PSI 8400 system,
P6
225°
215
0-± 5
Differential
Static Gallery
data acquisition and storage of the RUSKA
P7
225°
263
0-± 5
Differential
Static Gallery
P8
0°
127
Static Gallery
7222 pressure sensor and the DPI740
0-± 5
Differential
P9
90°
127
0-± 5
Differential
Static Gallery
barometer, and configuration, actuation,
P10
270°
127
0-± 5
Differential
Static Gallery
position acquisition and storage of the
Table 2: Pressure taps set up
turntables control system. In Table 2 is
reported the position and setting are reported for each pressure taps.
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4.
Unsteady Pressure Measurements on the Cone
High frequency pressure is
measured by 9 high frequency
pressure transducers, type Kulite
with a body length of 3.5mm or
9mm and diameter of 1.7mm. In
particular a 15 psi full scale kulites
(XCS-062-15D) have been chosen.
Figure 12: Model during the set-up.
In order to acquire a more accurate
signal the atmospheric pressure has
been chosen as reference.
Channel
Figure 12 shows the model
(Ch)
Low
Sensor
Feed
Gain
[V]
Pass
Filter
[kHz]
during the set-up of Kulite sensors.
Pressure
Tipology
Range
(Differential/
[psi]
Absolute)
Reference
Pressure
k1
10.0
100
100
0 - ± 15
Differential
Atmospheric
k2
10.0
100
100
0 - ± 15
Differential
Atmospheric
kulites with a constant tension of
k3
10.0
100
100
0 - ± 15
Differential
Atmospheric
10VDC
filter
k4
10.0
100
100
0 - ± 15
Differential
Atmospheric
(Ref.[5]). The output signal is
k5
10.0
100
100
0 - ± 15
Differential
Atmospheric
sampled by NI-PXI-1044 system
k6
10.0
100
100
0 - ± 15
Differential
Atmospheric
with
k7
10.0
100
100
0 - ± 15
Differential
Atmospheric
k8
10.0
100
100
0 - ± 15
Differential
Atmospheric
k9
10.0
100
100
0 - ± 15
Differential
Atmospheric
The
cards
of
GLE/SGA-4
system are set up in order to feed
a
with
a
sample
100kHz
frequency
of
300Ks/sec/ch.
The kulites are linked to a
“junction box” made up by 16
interface cards; each of ones can be
Table 3: Set up conditioner and kulites
connected to 4 sensors and to a connector DB25, placed behind the box and named Board#N; N is between 1 and 16
and represent the progressive number of the interface cards in the junction box.
The output signal is amplified with a gain of 100 in order to have an output signal between [-5Volt;+5Volt].
Table 3 resume all the data and the settings for each kulite.
5.
Infrared Thermography System
In order to detect the transition position, an infrared camera system, type ThermoVision A230G (Figure 13), is
used. The advantage of infrared thermography is the ability to identify the heating footprint of complex threedimensional flow phenomena that are extremely difficult to resolve by discrete measurement technique.
The A320G is designed to deliver accurate thermographic imaging and repeatable temperature measurements in
a wide range of applications. Each thermal image is built from over 76,000 pixels that are sampled by the camera’s
on-board electronics and firmware. With a standard Ethernet interface, the camera can be fully configured from a
PC, allowing command, control and collection of full frame data from the A320G in real time.
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The A320G is equipped with an RJ45 Gigabit Ethernet connection that
supplies 16-bit 20x240 images at 60 Hz,
and linear temperature data.
Its straightforward 3-sided mounting
allows quick installation, and it can be
easily
moved
when
application
requirements change; in fact it is very
practical
since
it
is
small
Figure 13: ThermoVision A230G
(170x70x70mm) and light (0.7kg).
The camera field of view is built in 25°x18.8°/0.4m; the detector is made up of a Focal Plane Array (FPA) and
an uncooled microbolometer with pixel resolution of 320x240. The camera is able to work in temperature ranges
of –20°C to +120°C , 0°C to +350°C and optional up to +1200°C, with an accuracy of ±2°C.
The infrared thermocamera is located on an appropriate external support at the top of the test section, sloping
of 35°(Figure 13). in order to see the region of interest through a Germanium window. In order to have a good
infrared image resolution a conventional optic is used, but this implicates the lost of about 1cm of the cone
summit.
III. Analysis Techniques
A. Infrared Analysis Description
The acquisition of infrared
images
was
performed
ThermaCAM
with
Researcher
Professional software, able to
make
studies
on
high/medium/slow speed thermal
events
depending
on
the
hardware
configuration.
Note
that
apparent
the
trend
of
Figure 14: 2D-3D Correlation
temperature obtained on a single midline tracked along cone length, as done by ThermaCAM Researcher
Professional software is not sufficient to detect transition with the required accuracy. For this reason, taking into
account the axial symmetric geometry of the cone, an average of measured apparent temperatures located at the
same x-position was calculated.
Thus a final trend of mean apparent temperatures on model surface was used to evaluate the transition position
(if present) on the cone.
Infrared thermography system acquires 2D thermal images of AEDC cone, so transition can be visually detected and
approximately positioned in the image. Since the model is 3D, a correlation between 2D representation and real cone
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is necessary in order to exactly
locate laminar-turbulent transition
on the test model. In order to find
the
2D-3D
correspondence
a
dedicated program was created,
using MATLAB®. The aim of the
code is to evaluate the trend of
apparent temperature along the
cone model in order to find peaks
of temperature and/or variations in
slope trend.
After 2D-3D correlation and
apparent
temperature
trend
evaluation a useful criterion of
transition detection was recognized
comparing temperature trend of
different runs.
Apparent temperature trend
and relative infrared images of
Figure 15: Laminar trend
(Re=9,598,877 - M=0.380 - Xt/L=0.47 - K=0)
three runs characterized by flux
respectively laminar (Figure 15),
and
with
laminar-turbulent
transition (Figure 16) are showed in
the figures at right side. In each
infrared image it is possible to
notice colour variation representing
flow condition in test section;
laminar to turbulent transition is
showed with dark colours since
flows passes from hot to cold
temperature.
infrared
Finally,
images
in
each
previously
reported, it is possible to notice a
reflection area (coloured in yellow)
and its location at the end of the
cone is indicated by a dramatically
rise
of
apparent
temperature
Figure 16: Transition on the cone model
(Re=17,101,440 - M=0.946 - Xt/L=0.47 - K=0.045mm – XT/L=0.35)
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reported in each correspondent trend.
Analysis performed with ThermaCAM Researcher Professional software, Matlab code and Microsoft Excel
application allowed to find output parameters. In particular, for each runs, transition position and distance between
transition location and transition trip position were detected.
Then non-dimensional transition position was plotted as a function of Mach number, transition trip roughness
height and Reynolds number, at different transition trip position. The aim was to analyse and recognize Mach
number, roughness height and Reynolds number effect on transition position. The most significant plots are shown
as follow.
B. High
Frequency
Analysis
Description
Power
Spectral
Densities
(PSD) were calculated for the
Kulite data to show the changing
frequency
contributions
during
transition. Kulite spectra were
calculated for 3.3 s time samples
using Welch's method (Ref.[5]).
Figure 17 represents the typical
PSD measured in the wind tunnel.
Figure 17: PSD
Two different kind of PSD were
calculated. The first covers all the
frequencies and used a window size
of approximately 3900 points, 50%
overlap with approximately 1024
FFT's were averaged; the second
one considers only the frequencies
up to 30 kHz and used a window
size of 7800 points, 50% overlap
with approximately 1024 FFT's
were averaged. The reason to
determine a PSD only for the
frequencies between 0 and 30kHz
is that the transition phenomena
should be interested frequencies
near 13kHz (Ref.[5]).
Finally a color map which
Figure 18: Color map from PSD measured
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reports the non-dimensional cone abscissa on vertical axis, the frequencies on horizontal axis and the intensity of
the PSD expressed in Db/Hz is
represented by the color-bar. This
was
obtained
interpolating
the
different PSD trends measured by
each Kulite along the cone axis.
Using this map is possible to
observe how the intensity of the
PSD change at the same frequency
moving along the cone axis and for
a specific cone station, how the
intensity of the PSD change with
the frequencies.
Figure 18 represent the color
map
obtained
using
the
PSD
reported in Figure 17.
Figure 19: Color map of a typical laminar flow
IV. Analysis of Results
A. High Frequency Analysis
Different
color
maps
are
reported to explain the correlation
between the quality of the flow and
the Kulite measurements.
Figure 19 represents a typical
color map of a laminar flow: it is
possible to see that there are no
amplifications of the PSD at the
same frequency despite of the
presence of the transition trip that
is indicated in figure with the black
line. It is also possible to see that
for the same position there is a
continuous
reduction
of
the
intensity of the PSD.
Figure 20 is referred to an
Figure 20: Color map of a typical transitional flow
experiment in which transition
occurred, this is demonstrated by the thermographic image. From the same figure is possible to observe an
increasing of the intensity of the PSD yet before the trip position and it is possible to note the amplification of this
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perturbation after the trip position. When transition occurred there is a propagation of the perturbation also at the
low frequencies.
The frequency that interest the propagation of the disturb is around 13 kHz; this result is in accord to literature
(Ref.[5]).
It is also possible to note that where transition occurred there is an increasing of the intensity of the PSD from
low frequencies to frequencies around 13 kHz and after the PSD decrease; in laminar flow the trend of the PSD is
continuously decreasing with the increase of the frequency.
B. MAIN EFFECTS
Analysis performed with thermographic images and Kulite pressure measurements allowed to find output
parameters. In particular, for each runs, were evaluated transition position, transitional Reynolds number and
distance between transition location and transition trip position were detected.
The aim was to analyse and recognize Mach number, roughness height and Reynolds number effect on
transition position. The most significant plots will be shown below.
1.
Mach number effect
Mach
number
effect
on
transition phenomena is prevalent in
comparison with other parameters
influence. Following figures report
plots representing non-dimensional
transition distance towards Mach
number distribution in different
configurations of transition trip
position and roughness height.
Figure 21 and Figure 22 shows
Figure 21: Non-dimensional transition distance towards Mach number
clearly as the transition move
towards the leading edge of the
cone increasing Mach number; it is
also possible to observe as the
transition
anticipate
with
the
increasing of the Reynolds number.
Figure 23 and Figure 24 show
the influence of the trip position on
the transition position. Comparing
figures showed, it also possible to
observe that the distance of the
transition from the tripping position
Figure 22: Non-dimensional transition distance towards Mach number
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decrease with increasing of the
roughness height and the transition
appears at lower Mach number
increasing the roughness height.
Concluding it is possible to
affirm that when roughness height
increase and transition trip moves
backward along the cone model
axis, laminar to turbulent transitions
is induced at lower Mach number.
Furthermore transition position shift
forward along the cone can be
Figure 23: Non-dimensional transition distance towards Mach number
(K=0.067 (F180))
considered as a consequence of
Mach number rise.
2.
Roughness height effect
For the analysis of roughness
height
effect,
transition
versus
non-dimensional
distance
was
plotted
roughness
height
dimensional value. Configurations
with
different
transition
trip
positions and roughness heights, in
the same condition of Mach number
and Reynolds number, are showed
Figure 24: Non-dimensional transition distance towards Mach number
(K=0.129 (F100))
below.
In Figure 25 it is possible to
see a completely absence of
transition at 0.4 Mach number.
At Mach number included in
0.70-0.74 range, it is possible to
observe a parabolic dependence
between
non-dimensional
transition distance and roughness
height as reported in Ref.[11].
Figure 25: Non-dimensional transition distance towards roughness height
Mach[0.38-0.42] - Re [10*10^6 - 15*10^6]
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Increasing
values,
Mach
smaller
number
transition trip
roughness height is necessary to
induce
laminar
to
turbulent
transition.
Finally it is possible to assert
that when Mach number increases,
roughness height necessary to
induce
laminar
to
turbulent
Figure 26: Non-dimensional transition distance towards roughness height
Mach[0.70-0.74] - Re [17*10^6 - 20*10^6]
transition on AEDC cone model
decreases,
and this
effect
is
amplified by transition trip shift
backward.
3.
Reynolds number effect
Reynolds number effect on
laminar to turbulent transition is
less evident than Mach number
and
roughness
eight
one.
Figure 27: Non-dimensional transition distance towards roughness height
Mach[0.94-0.97] - Re [16*10^6 - 18*10^6]
Moreover this effect is not simple
to analyse because it is very
difficult to compare runs fixing
Mach
number
and
varying
Reynolds number.
An evaluation of Reynolds
number effect was performed,
plotting
transition
non-dimensional
distance
versus
Reynolds number, at variable
Mach number; these plots are
reported in the figure below.
Figure
28
and
Errore.
Figure 28: Non-dimensional transition distance towards Reynolds number
(K=0-smooth configuration)
L'origine riferimento non è stata trovata. shows as increasing Reynolds number, transition moves afterward also
for smooth configuration, but only for high Mach number. It is not possible to observe a dependence between
Reynolds number and transition position due to the strict relation between Reynolds and Mach number that not
permit to have many points at the same Mach number with different Reynolds number.
C. Minimum Transition trip Height inducing Transition.
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In wind tunnel investigation
with 3D and 2D models is often
desirable to locate artificially the
position
of
boundary-layer
transition
from
turbulent
flow.
laminar
As
said,
to
a
satisfactory method of inducing
transition is the use of a strip of
distributed
particles
roughness.
In
of
particular,
roughness particles smaller than
the critical size have been found
Figure 29: Non-dimensional transition distance towards Reynolds number
to introduce no disturbance of sufficient magnitude to influence transition. Whereas roughness particles equal or
greater than a critical size give rise to turbulent spots that coalesce into a continuously turbulent flow downstream of
the roughness. Therefore, for wind tunnel measurements, the knowledge of the minimum roughness size able to
induce transition is very important to minimize the drag coefficient increment at transonic/supersonic speed due to
transition trip presence on the model.
Currently, in CIRA PT-1 wind tunnel, roughness height used for transition induction on 3D model is the greatest
one among transition trip heights calculated from literature (Ref.[13]). This, to be sure that the roughness height
ensures laminar to turbulent transition. Of course, this choice causes “small” negative effects as the increasing of the
aerodynamic drag measured in wind tunnel.
Figure 30 shows the
region in which the critical
height of transition trip is
expected
(grey region),
according to literature Test
points relative to transition
trips equally located on
the cone surface, but with
a
different
size
of
roughness, were analyzed
at
increasing
Mach
Figure 30: Dimensional roughness height towards Mach number
number. The output is
reported in Figure 30 too.
The area fill with grey colour represents the region in which critical roughness height (inducing transition) is
expected according to literature (Ref.[14]). In particular round points indicate minimum roughness heights that
could be able to induce transition. Squared points represent, instead, the transition trip heights that surely are able to
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induce transition. Squared points represent today the values of roughness height used in PT-1 wind tunnel during
experiment on 3D models to surely induce transition.
Triangular and asterisk points show the effective measured roughness height values inducing transition
respectively with the trip located at 0.47 and 0.59 non-dimensional abscissa.
These results are very important because it allows to choose smaller values of transition trip height respect to
transition trip height currently used (squared points). Performing data linear interpolation it is possible to notice
that rise in Mach number involves a reduction of roughness height necessary to induce transition for both trip
locations.
Thanks to result plotted in Figure 30, the PT-1 wind tunnel will use smaller transition trip heights which
involve laminar to turbulent transition on 3D model reducing the negative effect of the trip presence :This outcome
implies, of course, a vantage since lower heights of roughness reduce drag coefficient increment due to transition
trip application with respect smooth configuration, involving much accurate Wind Tunnel data.
A final consideration to be reported consists in a trend variation of linear interpolation straight lines between
transition trips located at 0.47 and 0.59 non-dimensional abscissa. This effect means that moving transition trip
towards arrears positions on model surface, grater height values of transition trip located are necessary to induce
transition; on the contrary, moving transition trip forward, smaller roughness height can be used. At Mach number
approximately equal to 0.75 transition trip position seems non to be influential, since roughness height necessary to
induce transition is in any case included in a region delimited by 0.078mm and 0.083mm.
V. Conclusions
The understanding of the mechanisms leading to transition and the development of reliable transition prediction
methods are recognized as critical issues in aerothermodynamics.
An experimental activity was planned in CIRA in order to investigate concentrated roughness effects on a 5-deg
half-angle sphere-cone model, taking into account Reynolds and Mach number and trip position variations. The aim
of the activities is to improve the understanding of the transition mechanisms in transonic flow and, at the same
time, to improve the effectiveness of the existing transition prediction criteria.
In the present work the test setup and the main results have been illustrated.
The ability to detect transition by using high frequency pressure measurements is showed and also a correlation
with thermographic measurements has been presented to demonstrate the precision of the results obtained by
pressure data.
Finally the minimum transition height inducing transition for the PT-1 wind tunnel has been evaluated and the
results has been compared with literature.
Acknowledgments
The authors wish a sincere and grateful acknowledgement to Eng. Antonio Schettino and CLAE Project which
financed this activity.
A special thank goes to CIRA PT-1 Technician, Vincenzo Fiorillo, who participated to the test campaign
execution.
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Finally, the authors wish a sincere and grateful acknowledgment to Eng. Francesco Valenza whose suggestions
were important for the test activities.
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Computer Software
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