laminar-turbulent transition of hypersonic boundary layer...
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Laminar-Turbulent Transition of Hypersonic Boundary Layer Affected by Surface Roughness Xiaolin Zhong, Professor Ph.D. Students: Danny Fong, Clif Mortensen, Yuet Huang, Chris Haley, Mike Miselis Mechanical and Aerospace Engineering Department, UCLA Presentation at AFRL-UCLA Basic Research Collaboration Workshop January 20, 2015 Research Supported by NASA/AFOSR National Center for Hypersonic Laminar-Turbulent Transition Research (2009–Dec. 2013)
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Hypersonic Boundary-Layer Transition
Laminar-turbulent transition of boundary layer is complex and not well understood at hypersonic speeds.
Hypersonic boundary-layer transition significantly increases surface thermal heating and aerodynamic drag.
The ability to predict or delay transition and maintain laminar flow can have a significant pay off in terms of the reduction in aerodynamic heating and drag.
Our research group at UCLA has been doing numerical simulation studies of receptivity and transition mechanisms for hypersonic boundary layers (http://cfdhost.seas.ucla.edu).
In this presentation, I will mainly talk about a new transition suppression/control technique based on new understanding of surface roughness effects on transition.
Transition on a sharp cone at Mach 4.31 (Schneider 2004)
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Surface Roughness-Induced Transition
Surface roughness elements have been used for decades as a tripping device for boundary layer transition by placing them relatively upstream with relatively large heights at hypersonic speeds.
3-D roughness is most effective in tripping.
Meanwhile, a few past experiments reported that roughness can also have the opposite effects by delaying transition under some circumstances. But the mechanisms and the specific conditions under which the delay occurs were unknown. Most of them did not receive much attention in the transition research community.
2D and 3D roughness on a cone
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Holloway and Sterrett (1964): surface roughness can delay transition when its height
is less than the local boundary-layer thickness, under some flow conditions.
Past Experiments of Roughness-Induced Transition Delay
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Fujii (2006): a wavy roughness can suppress the transition of a Mach 7.1 boundary
layer over a sharp cone.
Past Experiment of Roughness-Induced Transition Delay
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A More Recent Simulation & Experiment
Egorov, Novikov and Fedorov (2010): Numerical simulation of Mach 6 flow over a flat
plate showed a wavy wall, or “a shadow grooved plate,” can suppress second mode
instability by creating a short separation region.
Bountin et al. (2013) verified the simulation results by experiment.
Pressure disturbance amplitude
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Research at UCLA on Roughness Effects on Hypersonic Boundary Layer Receptivity and Second Mode Growth
Original development of high order cut-cell DNS method to simulate finite surface roughness
Unexpected results: finite roughness suppresses second mode when roughness is placed after the synchronization point.
Subsequently extensive parametric studies consistently confirm our initial finding
1) Different roughness location and height with disturbance at a fixed frequency.
2) Fixed roughness location with a continuous frequency spectrum of disturbances.
3) Multiple roughness elements on a flat plate. 4) Different roughness width and spacing.
The new finding of the role of synchronization point points to a simple way to design a passive control scheme of hypersonic boundary later transition by roughness.
X(m)
Y(m
)
0.182 0.184 0.186 0.188 0.19
0.002
0.004
0.006
V
22.3573
8.62547
-5.10632
-18.8381
-32.5699
-46.3017
-60.0335
-73.7653
Publications: • Duan, et al., AIAA papers: 2008-3732, 2009-1337, 2010-1450 • Duan: PhD thesis, UCLA, 2009 • Duan, et al.: Journal of Computational Physics, 2010 • Fong, et al.: AIAA Paper 2012-1086, ICCFD7-1502, 2012 • Duan, et al.: AIAA Journal, 2013 • Fong, et al.: AIAA 2013-2985 & 2015-0837. Computers and Fluids, 2014
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Mean Flow Solutions for Mach 5.9 Flow Over a Flat Plate with One or More 2-D Roughness Elements
Pressure contours and streamline patterns around an isolated roughness
element with different roughness heights.
Flow conditions:
5.92M 48.69KT
742.76Pap Pr 0.72
6Re 14.13 10 m 0 / T 1.11wT
Isothermal wall 350KwT
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Wave Mode Mechanisms in Hypersonic Boundary Layer by LST
Evolution of wave velocities of fast (u+c) and
slow (u-c) modes as dimensionless frequency
(and x) increases. The modes synchronize
when they intersect in the figure.
a
0.05 0.1 0.15 0.20.8
0.85
0.9
0.95
1
1.05
1.1
1.15
1 M1
1 M1
x*= 0.189 m
x*= 0.159 m
mode S
mode
F
mode S
mode
FSynchronization point
Fast
mode
Slow
mode
Fast mode (F) and Slow mode (S) are the most important discrete wave modes inside hypersonic boundary layers (Mach number > 4). They are tuned to slow and fast acoustic waves, respectively, near the leading edge.
Mode S and mode F have the same phase velocities and similar eigenfunctions at the synchronization point, even though their growth rates are different.
After synchronization point, mode S belongs to acoustic waves (2nd mode), unique for hypersonic flow. It becomes unstable and could lead to transition.
Location of the synchronization point in the coordinate along flat plate or cone moves upstream as the disturbance frequency increases.
We have done extensive simulation studies on the role of synchronization point to boundary layer receptivity (Annual of Fluid Mechanics 2012) and on second mode suppression by surface roughness.
Flow structure of the flat-plate hypersonic boundary layer disturbed by surface roughness.
Evolution of growth
rates for mode F and
mode S.
Unsteady simulation by imposing periodic mode F or S of a single frequency into the meanflow
Mode S disturbances, periodic in time, at 100 kHz is imposed into the meanflow with one isolated
finite-height roughness element treated by the high order cut cell method.
Weak Mach waves are also generated behind the roughness. They have little effects on boundary-
layer instability because they are outside the boundary layer and decay fast.
X
Y
0.15 0.2 0.25 0.3-0.00
0.01
0.02
0.03
0.04
0.05
0.06
0.07
0.08
0.09
0.10
0.11
0.12
0.13
0.14
X
Y
0.15 0.16 0.17 0.18 0.19 0.2 0.210.00
0.01
0.02
0.03
0.04
0.05
0.06
Roughness
Roughness
Pressure disturbance contours
Zoom in of pressure disturbance contours
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Four cases of different roughness location; one roughness at a time
A sample movie for interaction between periodic mode S and roughness
The sample video above shows the interaction of mode S disturbance,
which is periodic in time, and the roughness.
The strongest disturbance is always in the near wall region.
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X
Y
0.15 0.16 0.17 0.18 0.19 0.2 0.210.00
0.01
0.02
0.03
0.04
0.05
0.06
X(m)
Ma
x|p
'|
0.15 0.2 0.250
1E-06
2E-06
3E-06
4E-06
5E-06
6E-06
7E-06
8E-06
9E-06
1E-05
1.1E-05
1.2E-05
1.3E-05
0.625h
0.5h
0.375h
0.25h
No roughness
Roughness
X(m)
Ma
x|p
'|
0.1 0.150
5E-07
1E-06
1.5E-06
2E-06
2.5E-06
3E-06
3.5E-06 0.625h
0.5h
0.375h
0.25h
No Roughness
Roughness
FFT result – Mode S for case 1 and case 2 (roughness is upstream of the sync pt at 0.331 m for f=100 kHz)
Amplitudes of pressure disturbance evolution on the wall for two cases: (a) Case 1, roughness at 0.1101 m; (b) Case 2, roughness at 0.185 m.
(a) (b)
Case 1 and case 2
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X(m)
Ma
x|p
'|
0.375 0.4 0.425 0.450
5E-05
0.0001
0.00015
0.0002
0.625h
0.5h
0.375h
0.25h
No roughness
Roughness
X(m)
Ma
x|p
'|
0.3 0.325 0.35 0.3750
1E-05
2E-05
3E-05
4E-05
5E-05
0.625h
0.5h
0.375h
0.25h
No roughness
Roughness
FFT result – Mode S for case 3 and case 4 (roughness is at downstream of sync pt at 0.331 m for f=100 kHz)
Pressure disturbance evolution on the wall for two cases: (a) case 3 roughness at 0.331 m; (b) Case 4 roughness at 0.410 m.
(a) (b)
Case 3 and case 4
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phase velocity
a0.05 0.1 0.15 0.2
0.8
0.85
0.9
0.95
1
1.05
1.1
1.15
1 M1
1 M1
x*= 0.189 m
x*= 0.159 m
mode S
mode
F
mode S
mode
F
Interaction of Mode S and Mode F disturbance and roughness - Some Observations
Mode F synchronizes with Mode S and become
the unstable mode after the synchronization point.
Roughness effects on mode F and mode S are the
same:
Cases 1&2: Upstream of synchronization
point, roughness always amplify disturbances in
front of a roughness. The overall amplification
of disturbances increases with roughness height.
Cases 3&4: The trend reverses when
roughness is placed at or downstream of the
synchronization point: roughness damps
disturbance with a strength depending on
roughness height. Taller roughness has a
stronger damping effect.
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Case 1 and case 2
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How about other Frequencies? Interaction of Roughness with a Wave Package of up to 1 MHz
In order to better understand the relation between
disturbance frequency and roughness, we also impose a
pulse perturbation with a Gaussian shape in time, which
consists of a continuous spectrum of frequency, into the
meanflow.
The disturbance is imposed via a hole on the flat plate.
The disturbance is sinusoidal in space which ensures that
no net mass would be introduced into the flow.
FFT analysis shows that the pulse used has an
approximate range of frequency from 0 Hz to 1 MHz.
Time history for disturbance (Gaussian) Snapshot for the disturbance in space FFT result for the Gaussian disturbance
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A sample movie for the interaction of perturbations and roughness
The perturbations are not periodic in time and consist of a single wave
packet of many frequencies traveling downstream.
FFT shows that it has a board frequency spectrum in the range of 0-1
MHz.
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Simultaneous surface pressure perturbations of the wave package at different locations
Instantaneous time history of wall pressure perturbations at various locations, both upstream (left) and downstream (right) of the roughness.
The frequency spectra are obtained by FFT.
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The growth of pressure perturbations at selected frequencies (120KHz, 130KHz, 140KHz)
x (m)
(|d
p|/
|p|)
/(|d
V|/
|U|)
0.15 0.175 0.2 0.225 0.25 0.2750.00
1.00
2.00
3.00
4.00
5.00
6.00
7.00
8.00
9.00 120KHz
120KHz - No roughness
(a)x (m)
(|d
p|/
|p|)
/(|d
V|/
|U|)
0.15 0.175 0.2 0.225 0.25 0.2750.00
1.00
2.00
3.00
4.00
5.00
6.00
7.00
8.00
9.00 130KHz
130KHz - No roughness
(b)
x (m)
(|d
p|/
|p|)
/(|d
V|/
|U|)
0.15 0.175 0.2 0.225 0.25 0.2750.00
1.00
2.00
3.00
4.00
5.00
6.00
7.00
8.00
9.00 140KHz
140KHz - No roughness
(c)
Roughness location is fixed at
0.185 m, corresponding to the
synchronization point for
133.26 KHz.
120 KHz perturbations are
amplified by roughness (The
roughness is before the
synchronization point).
Both 130 KHz and 140 KHz
perturbations are damped by
roughness (The roughness is
after the synchronization
point).
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Frequency (Hz)
(|d
P|/
|P|)
/(|d
V|/
|U|)
0 100000 200000 3000000.00
0.50
1.00
1.50
2.00
2.50
3.00
3.50
4.00
4.50
5.00
5.50
6.00
6.50
7.00
7.50
8.00
8.50 0.19635m
0.21m
0.22185m
0.23685m
0.24885m
0.26385m
133.26KHz
(a)Frequency (Hz)
(|d
P|/
|P|)
/(|d
V|/
|U|)
0 100000 200000 3000000.00
0.50
1.00
1.50
2.00
2.50
3.00
3.50
4.00
4.50
5.00
5.50
6.00
6.50
7.00
7.50
8.00
8.50 0.19635m
0.21m
0.22185m
0.23685m
0.24885m
0.26385m
(b)
Frequency spectra at fixed locations on plate surface downstream of roughness at 0.185 m
Perturbations at lower then sync pt frequency (100KHz~120KHz) is highly amplified downstream
of roughness (The roughness is located before the synchronization point).
Perturbations at higher then sync pt frequency (>133KHz) is suppressed downstream of
roughness (The roughness is located after the synchronization point).
The damped disturbance around the sync pt frequency is still damped downstream of roughness
compared with no roughness case.
With roughness No roughness
21
What about two or more roughness elements?
x(m)
(|d
P|/
|P|)
/(|d
V|/
|U|)
0.15 0.175 0.2 0.225 0.25 0.2750.00
0.50
1.00
1.50
2.00
2.50
3.00
3.50
4.00
4.50
5.00
5.50
6.00
6.50
7.00
7.50
140KHz
Roughness
(c)
x(m)
(|d
P|/
|P|)
/(|d
V|/
|U|)
0.15 0.175 0.2 0.225 0.25 0.2750.00
0.50
1.00
1.50
2.00
2.50
3.00
3.50
4.00
4.50
5.00
5.50
6.00
6.50
7.00
7.50
120KHz
Roughness
(a)
120KHz - no roughness
140KHz - no roughness
x(m)
(|d
P|/
|P|)
/(|d
V|/
|U|)
0.15 0.175 0.2 0.225 0.25 0.2750.00
0.50
1.00
1.50
2.00
2.50
3.00
3.50
4.00
4.50
5.00
5.50
6.00
6.50
7.00
7.50
130KHz
Roughness
(b)
130KHz - no roughness Roughness spacing is about 10
roughness width. First roughness
location: 133.26 KHz. Second
roughness location: 119.26 KHz.
120 KHz perturbation is amplified by
the first roughness since it is locates
upstream of its sync pt. However, it is
damped by the second roughness
since the second roughness is close to
its synchronization point.
Both roughness are located at or
downstream of the sync point of 130
KHz and 140 KHz perturbations.
Therefore, the disturbances of these
two frequencies are damped by both
roughness.
22
Effects of roughness width and spacing
Comparing with roughness height, the effects of
roughness width are relatively small.
AIAA 2015-0837: ideal spacing of about 10 BL
thickness
Frequency (Hz)
(|dP
|/|P
|)
/(|d
V|/
|U|)
0 100000 200000 300000
0.5
1
1.5
2
2.5
0.5 BL wide roughness
1 BL wide roughness
2 BL wide roughness
4 BL wide roughness
133.26 KHz
Frequency (Hz)
(|dP
|/|P
|)
/(|d
V|/
|U|)
100000 200000 3000000.0
0.5
1.0
1.5
2.0
2.5
3.0
0.5 BL wide roughness
1 BL wide roughness
2 BL wide roughness
4 BL wide roughness
133.26 KHz
3.4 mm downstream of roughness 3.4 mm upstream of roughness Frequency spectrum for different roughness width
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Summary of Simulations and Analysis
Evolution of wave velocities of fast (u+c) and
slow (u-c) modes as dimensionless frequency
(and x) increases. The modes synchronize
when they intersect in the figure.
a
0.05 0.1 0.15 0.20.8
0.85
0.9
0.95
1
1.05
1.1
1.15
1 M1
1 M1
x*= 0.189 m
x*= 0.159 m
mode S
mode
F
mode S
mode
F
Synchronization point
Fast
mode
Slow
mode
Mack’s 2nd mode is amplified if roughness is upstream of sync location; damped if downstream.
Multiple roughness elements are more effective in damping perturbations.
Effect of roughness width is insignificant compared with roughness height. Ideal spacing of about 10 thickness.
Synchronization point is frequency dependent: moving upstream at higher frequency.
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Overall Summary
Our research results have shown that placing roughness elements
behind the synchronization location can suppress the 2nd mode
disturbances that cause transition.
All our simulation results (different roughness height, location,
width, spacing, and the pulse disturbance cases) have the same
conclusions.
Based on the new understanding of the flow mechanisms, a new
passive flow control method has been developed to control and
suppress transition on hypersonic vehicle.
The new method has potential significant practical applications in
the development of hypersonic vehicles or rockets (Mach > 4)
because they are passive and robust.