Characterization of a Hypersonic Low-density

Transcription

Characterization of a Hypersonic Low-density Flow for Separated
Flow Investigations
S. O’Byrne1 ,
S. L. Gai1
N. R. Deepak1
Y. Krishna1
J. N. Moss2
1
School of Engineering & IT
University of New South Wales
Australian Defence Force Academy
Canberra, Australia
2
NASA Langley Research Center
Hampton, VA, USA
US Air Force Aerothermodynamics and Turbulence Portfolio Review, July 2013
S. O’ Byrne (University of New South Wales, Australian Defence Force Academy)
1 / 27
Introduction
Motivation
Motivation
High Enthalpy, Low-Density Separated Flows
Better understanding of viscous flow in thermal nonequilibrium — critical to
successful vehicle design
Typical flow separation configurations — compression corners, flat-plates with steps,
blunt bodies
Configurations all have a pre-existing boundary layer at separation — increased
complexity and not accounted for in analytical descriptions
‘Tick’ (or ‘Check’) Geometric Configuration
Capable of producing separation at leading-edge without pre-existing boundary layer
Originally proposed by Chapman et al. (1958) for high Re and low M∞ flows
Considered here for low-density laminar high-enthalpy hypersonic conditions
Approach to the Problem
Comparisons between 3D Navier-Stokes and 2D Direct Simulation Monte Carlo
(DSMC) calculations
Using a suite of flowfield diagnostics to investigate thermal state of flowfield
2 / 27
Introduction
Flow Features — Leading Edge Separation
Leading-Edge Separation
Expansion fan
Flow separation
Characterized by a strong expansion at
the leading edge
Reattachment
Flow separation very close to the leading
edge forming a recirculation region
between A, B and C
Re-compression shock wave
Reattachment on the compression surface
Recirculating region
3 / 27
Introduction
Scope
Goal of Research Project
Improved Understanding of Viscous Aerothermodynamics
A flow configuration without boundary layer development before separation
Testing of Chapman’s isentropic recompression theory to estimate the base pressure
To aid in designing the experiments based on numerical results
Investigate the flowfield using the following techniques
Tuneable diode laser absorption spectroscopy of H2 O
Time-resolved characterization of freestream temperature and H2 O concentration on each
tunnel run.
Resonantly enhanced shearing interferometry (RESI)
Visualizes the time-evolution of the low-density flow
NO PLIF two-line thermometry (rotational and vibrational) and velocimetry
Determines rotational, translational and NO vibrational temperatures, and 2 velocity
component map.
Dual-pump CARS
Determines vibrational temperatures of N2 and O2 in freestream
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Introduction
Scope
Techniques
(a)
RESI (Hruschka et al. Appl. Opt.
2008)
(c)
(b)
NO PLIF Thermometry (Hruschka et al. Exp. Fluids
2011)
NO PLIF Velocimetry (Hruschka et al. Exp.
(d)
Fluids 2011)
CARS (Fraval et al. (Proc. ISSW, 1999)
5 / 27
Computational Approach
Navier-Stokes & Direct Simulation Monte Carlo
Numerical Codes & Models
Navier-Stokes (N-S) Solver — Eilmer-3 (Jacobs and Gollan, 2010)
In-house solver, time-dependent, viscous, chemically reactive
Finite-volume, cell-centred, 3D/axisymmetric discretization
Second order spatial accuracy: modified van Albada limiter and MUSCL
Mass, momentum & energy flux across the cells: AUSMDV algorithm
Time Integration: Explicit time integration
Direct Simulation Monte Carlo (DSMC) — DS2V (Bird, 2006)
Uses probabilistic (Monte Carlo) simulation of physical collisions
Models fluid flow using simulated molecules which each represent a large number of
real molecules
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Geometric Configurations
Leading-Edge Separation
Geometry
(a) Geometry
(b) Model
Configuration Details
Surface
A ←→ B (S-1; expansion)
B ←→ C (S-2; compression)
Length, mm
19.730
44.776
Angle
θ − 1=30.5◦
θ − 2=23.7◦
Horizontal x distance from A ←→ C = 58 mm
Total wetted surface (s) length A 7−→ B 7−→ C = 64.506 mm
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Leading-Edge Flow Conditions
T-ADFA Freestream Conditions
Flow Parameter
Test gas
Re [1/m]
M∞
u∞ [m/s]
T∞ [K]
p∞ [Pa]
ρ∞ [kg/m3 ]
H0 [MJ/kg]
Condition E
Air
1.34 × 106
9.66
2503
165
290
0.006
3.8
Condition A
Air
2.43 × 105
7.25
3730
593
377
0.002
13.3
Two conditions are produced using the same facility and nozzle
Condition E
Low enthalpy — near perfect gas behaviour
Continuum assumptions valid
Condition A
Higher-Knudsen number and significant slip in temperature and velocity at surface
Significant thermal nonequilibrium
8 / 27
Chapman’s Isentropic Re-compression Theory
Chapman et al. (1958) proposed a separated flow model and developed a theory to
estimate the base pressure (Pd )
Experimental evidence at high supersonic Mach numbers suggests that the model
works remarkably well even for pre-existing boundary layer in estimating base
pressure
In hypersonic high temperature flows, the efficacy of Chapman’s isentropic
recompression model is not “rigorously” verified
Here, the same leading edge separation model used by Chapman is considered
9 / 27
Computational Results
Knudsen number
Local Knudsen (Kn) number
(a) DSMC: Cond E
(b) Navier-Stokes: Cond E
(c) DSMC: Cond A
(d) Navier-Stokes: Cond A
 n
λ
Kn = 

∂ρ 2
∂x
+
ρ
o1/2 


∂ρ 2
∂y
local
10 / 27
Results: Condition E
Pressure, Skin Friction & Heat Flux: Condition E Ho =3.8 MJ/kg
Strong expansion at leading edge;
followed by flow separation
Separation: N-S (s/Le = 0.145);
DSMC (s/Le = 0.08)
Reattachment: N-S (s/Le = 2.42);
DSMC (s/Le = 2.46)
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Results: Condition A
Pressure, Skin Friction & Heat Flux: Condition A Ho =13.3 MJ/kg
Between 0.05 ≤ s/Le ≤ 0.25 in N-S,
rate of pressure reduction decreases
with near constant pressure:
Indicative of boundary layer growth
N-S: Separation at s/Le = 0.56;
Reattachment at s/Le = 1.87
DSMC: No indication of
separation/reattachment
12 / 27
Results: Further remarks
Differences between N-S and DSMC
Significant differences between N-S and DSMC for condition A
DSMC predicts almost no separation (except for an infinitesimally small region at the corner)
Flow over most of the expansion surface is in slip flow regime (Moss et al., 2012)
slip velocity = uw (s) = λw
∂u
∂y
slip temperature = Tg − Tw = (∆T )w =
M √
Rarefaction parameter(V ) = √
C
Res
=
w
λw
τw (s)
µw
2γ
(µw cp )−1 λw k
γ+1
ρus
; Res =
µ
dT
dy
w
ρw µw
;C =
ρe µe
Criterion for slip flow (Talbot, 1963) for Condition A
Location
s/Le = 0.25
s/Le = 0.5
s/Le = 1.0
Rarefaction parameter, V
N-S
0.143
0.1009
0.0715
Rarefaction parameter, V
DSMC
0.332
0.223
0.166
13 / 27
Comparison with Champan’s Theory
Comparison with Chapman’s Isentropic Re-compression Theory
From Navier-Stokes Simulations
Average pressure in the recirculation region or dead air region (Pd )
Pressure (P 0 ) and Mach number (M 0 ) downstream of reattachment
Mach number at the edge of the mixing layer (Me)
M 02 = (1 − ud∗2 )Me2
pd
=
p0
Flow
condition
E
A
and
ud∗ = ud /ue
)M 02
1 + ( γ−1
2
γ/(γ−1)
1 + ( γ−1
)M 02 /(1 − ud∗2 )
2
pd /p 0
N-S simulations
0.088
0.08
pd /p 0
DSMC simulations
0.09
-
pd /p 0
Theory
0.330
0.345
0 0
0
ReLe
= u µρ 0Le
32.88 ×103
8.14 ×103
Simple isentropic flow assumption does not appear to hold in hypersonic flow
Streamlines in the shear layer do not recompress isentropically at reattachment, rather
extend over a finite region
Steep isentropic recompression assumption in theory seems unrealistic in low Re flows with
thick shear layers
14 / 27
Streamlines
Streamlines, Separation and Reattachment Angles
(a) Condition E
(b) Condition A
Comparison of measured angle with Oswatitsch
(1957) theory
dτw /ds
v
tanθs = limx ,y →0 u = −3 dpw /ds
s
Angle
Separation
Reattachment
Condition A
Theory
37◦
7.5◦
Condition A
Measured
35◦
10◦
Condition E
Theory
47◦
1.4◦
Condition E
Measured
40◦
4◦
15 / 27
Computational Visualization
Computational Visualization: Resultant Velocity
Condition E and Condition A
(a) Navier-Stokes
(b) Direct simulation Monte Carlo
(c) Navier-Stokes
(d) Direct simulation Monte Carlo
16 / 27
Driver Gas Detection
Driver Gas Contamination Measurements
Characteristics of Contamination
Contamination is strongly dependent on piston
motion tuning and interaction of reflected shock
wave and contact surface between driver and
test gases
Test time is significantly shorter than mass flux
alone would indicate
Test time may fluctuate from one tunnel run to
the next
Driver gas is argon/helium mixture — no
absorption lines
Use absorption in water vapour as an inverse
measurement
Gai (1992)
17 / 27
TDLAS arrangement
Diode Laser Absorption Arrangement
Characteristics
Scan rates over 2 transitions at more than 50 kHz, with a single laser
Can measure average temperature, velocity and H2 O concentration
Even when bottled dry air is used,measurable signal can be obtained
Opposed retroreflector geometry ensures precise velocity measurement
18 / 27
TDLAS arrangement
Diode Laser Absorption Arrangement
1
Nozzle
4
Mirror
2
Pitch fiber
5
Collecting lens
3
Collimating lens
6
Catch fiber
19 / 27
TDLAS Measurements
Diode Laser Absorption Measurements
Assumptions
The water vapour concentration is constant in the test gas
Reduction in water concentration produces an equivalent increase in driver gas
Lines are Doppler broadened
At 280–380 Pa, this is a good assumption
FWHMDoppler = 3.12 pm, Voigt fit only 3 % different
No change in temperature during the test time
Not true, but can be corrected for with simultaneous measured temperature
20 / 27
TDLAS Measurements
Contamination vs time
(a) Condition E
(b) Condition A
Driver gas time
There is a region of constant concentration where the test gas passes through the
nozzle
A gradual decrease followed by an increase in H2 O at later times
The increase occurs due to test gas reflecting from the end of the dump tank
Test time is considerably shorter than the mass flux estimates of 5 ms and 2.5 ms for
conditions E and A respectively
21 / 27
Repeatability
Repeatability
(a) Condition E
(b) Condition A
Repeatability
Condition E: Test gas duration = 1120 ± 60µs — CFD: 700 µs
Condition A: Test gas duration = 700 ± 130µs — CFD: 600 µs
22 / 27
Velocity
Velocity Measurements
(a) Doppler-shifted peaks
(b) Velocity decay
23 / 27
Conclusions
Conclusions
Computations
Lower enthalpy (higher freestream density) condition E : DSMC predicted a 15 %
larger separated region. Pressure, shear stress and heat flux show similar features.
Higher enthalpy (lower freestream density) condition A : N-S results predicted a
clearly separated region whereas the DSMC indicated a much smaller region
DSMC smaller separation for condition A is attributed to the fact that the DSMC
calculations take slip effects into account
Isentropic recompression theory of Chapman may not be adequate in hypersonic
high enthalpy low Reynolds number flows
Experiments
We have developed a method for measuring H2 O concentration and inferring driver
gas contamination using tuneable diode laser absorption spectroscopy. This is the
first of a number of techniques to be brought to bear on this problem.
Provides on-line measurement on each tunnel run, while other experiments are
running
Flow duration at condition E is sufficient, but is borderline at condition A. May need
to reduce forebody size to compensate for this.
24 / 27
Conclusions
Challenges
Challenges
Things to consider. . .
Need more collaboration from Navier-Stokes method experts
Would be interested in co-operating with larger/higher-enthalpy facilities on this
configuration
Need to develop good information transfer between experimental and computational
investigators
Need to know what measured properties are particularly important for testing
computational techniques
Previous experimental data shows early transition to turbulence in free shear layers
Main practical problem is manpower
CARS will take a long time to set up and characterize, and requires development of
nonequilibrium CARS spectral models. It is a low-yield method.
PLIF velocimetry requires lots of tunnel runs
25 / 27
Thanks and Acknowledgments
Thank you
Acknowledgments
US Air Force AOARD (Dr I. Wysong)
Australian Space Research Program
Dr. Peter Jacobs (University of Queensland) — Eilmer
Prof. Graeme Bird — DS2V
UNSW Silver Star Research Grant
For more information
Dr. Sean O’Byrne
[email protected]
Dr. Deepak Ramanath
[email protected]
Mr. Yedhu Krishna
[email protected]
School of Engineering & IT
University of New South Wales
Australian Defence Force Academy
Canberra, Australia
26 / 27
References
References
Bird, G. A. (2006), ‘DS2V: Visual DSMC Program for Two-Dimensional and Axially
Symmetric Flows’.
URL: http://gab.com.au/index.html
Chapman, D. R., Kuehn, D. M. and Larson, H. K. (1958), Investigation of Separated
Flows in Supersonic and Subsonic Streams with Emphasis on the Effect of Transition,
Technical Report 1356, NACA.
Jacobs, P. A. and Gollan, R. J. (2010), The Eilmer3 Code, Technical Report Report
2008/07, Department of Mechanical Engineering, University of Queensland.
Moss, J. N., O’ Byrne., S., Deepak, N. R. and Gai, S. L. (2012), Simulation of
Hypersonic, High-Enthalpy Separated Flow over a ’Tick’ Configuration, 28th
International Symposium on Rarefied Gas Dynamics, Zaragoza, July 9-13th, 2012.
Oswatitsch, K. (1957), Die Ablosungsbedingung von Grenzschichten, in ‘Boundary Layer
Research: International Union of Theoretical and Applied Mechanics Symposium,
Freiburg’, Springer–Verlag, Berlin, pp. 357–367.
Talbot, L. (1963), ‘Criterion for Slip near the Leading Edge of a Flat Plate in Hypersonic
Flow’, AIAA Journal 1(5), 1169–1171.
27 / 27

Characterization of a Hypersonic Low-density

Transcription

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