Afosr cm august 9 10, 2011 rockwell 081611

ON EXPERIMENTS IN FLUID MECHANICS
AND FLOW-STRUCTURE INTERACTION

D. Rockwell

Lehigh University

AFOSR Flow Interactions and Control Program Review

D. Smith

Air Force Office of Scientific Research

August 9-10, 2011

081611 D. Rockwell, AFOSR Review
August 9, 2011

ON EXPERIMENTS IN FLUID MECHANICS
AND FLOW-STRUCTURE INTERACTION

AIMS

• Provide a viewpoint on selected factors that influence experiments
• Raise issues and make preliminary remarks as a basis for discussion
• Suggest approaches for early career researchers
• Focus primarily on case studies and examples outside of, but having
direct analogies with, AFOSR research.

D. Rockwell, AFOSR Review
August 9, 2011

ON EXPERIMENTS IN FLUID MECHANICS AND
FLOW-STRUCTURE INTERACTION

OVERVIEW

• Ideas and objectives
• Theoretical concepts
• Physics- based framework
• Diagnostics
• Instrumentation
• Collaboration
• Future prospects

August 9, 2011

IDEAS AND EXPERIMENTAL OBJECTIVES

Advice communicated from different sources:
• Aim for research that will yield a transformational contribution, rather than an
incremental contribution.
• Avoid the "doing it better" syndrome, i.e., new experiments that improve upon
previous experiments, without new physical insight.

What are the major elements that lead to fundamentally new (highly “creative”)
accomplishments?

August 9, 2011

Dean Keith Simonton
Cambridge University Press
2004

Chance, logic, genius, and Zeitgeist* can be integrated into a single coherent
theory of creativity in science.
But for this integration to succeed, chance must be elevated to the status of
primary cause**.
Chance may be enhanced by:
• Dedication and diligence (e.g., substantial number of publications)
• Exposure to diverse influences (literature, technical sessions, and
colleagues outside the theme of interest)
• Thinking unconventionally (outside the box)
*The spirit of the time; general trend of thought or feeling characteristic of a particular period of time — n the
spirit, attitude, or general outlook of a specific time or period, esp as it is reflected in literature, philosophy, etc
** From cover page of Creativity in Science D. Rockwell, AFOSR Review
August 9, 2011

THEORETICAL CONCEPTS TO GUIDE EXPERIMENTS

CASE STUDIES
Timing of experiments in relation to development of theoretical concepts
• Experiments are closely coordinated with theoretical advances
• Experiments over a long period precede theoretical advances,
which then lead to fundamentally new types of experiments

What can be learned from these scenarios?

August 9, 2011


THEORETICAL ADVANCES CLOSELY COORDINATED WITH EXPERIMENTS

INSTABILITY AND VORTEX FORMATION IN MIXING LAYERS AND JETS

• 1960’s and 70’s Highly collaborative group of
experimental and theoretical colleagues had
formed at Technical University of Berlin
(Wille, Michalke, Berger, Fiedler , Fernholz,
Bechert, Pfizenmaier, Freymuth)

• Experiments closely guided by linear stability
R. Wille and A. Michalke in Van Dyke (1982) theory
Vortex formation arises from
OBSERVATIONS
convectively unstable shear
- Successive empirical/untargeted experiments
layer.
were precluded
- Limitations of theory (due to nonlinear and
Linear region of initial instability
nonparallel effects) defined during experiments,
remains detectable during
providing basis for subsequent investigations
vortex formation

Test: transient disturbance is exponentially
amplified as it is swept downsream.
August 9, 2011

EXPERIMENTAL EFFORTS BEFORE AND AFTER NEW THEORETICAL CONCEPT

VORTEX SHEDDING • Up to 1980’s Extensive experiments on quasi
FROM STATIONARY two-D vortex shedding; selected experiments
CYLINDER yielded important insight. Origin of vortex
formation not addressed.

• Mid to late 80’s Emergence of theoretical
developments related to absolute (global)
instability. (Concept originated in area of
By Taneda in Van Dyke (1982) plasma physics)

Vortex street arises from • 80’s and 90’s Important, new types of
absolutely (globally) unstable experiments account for absolute (global)
wake. instability
Linear state of initial instability
not detectable in fully-evolved • Observations
vortex street - Prolonged periods of experimentation may
occur in absence of a theoretical framework
Test: Impulse (transient) disturbance spreads - Very relevant theoretical developments may
upstream and downstream and contaminates the
entire parallel flow occur in unexpected disciplines (plasma physics)

August 9, 2011

PHYSICS-BASED FRAMEWORK FOR EXPERIMENTS

Early, highly cited experiments in a given line of research typically:
• Emphasized results of qualitative visualization
• Focussed on new physics

STRUCTURE OF MIXING LAYER (1974)
Vortex interactions in a high Reynolds number
mixing layer visualized with Schlieren technique
(Brown and Roshko, 1974) Also: Interactions at low
Reynolds number visualized with dye (Winant and
Browand, 1974)

Patterns of flow transition in Couette flow between
STRUCTURE concentric rotating cylinders visualized with
OF COUETTE aluminum particles (Coles, 1965)
FLOW (1965)

STRUCTURE OF Streaks in a turbulent boundary layer
TURBULENT visualized with hydrogen bubble technique
BOUNDARY (Kline, Reynolds, Schraub and Runstadtler
LAYER (1967) (1967)
August 9, 2011

ESTABLISHMENT OF A PHYSICS-BASED FRAMEWORK FOR EXPERIMENTS

What is the best path to ensure that experiments aim for new physics?
A possible approach:
• Temporarily neglect existence of advanced experimental techniques
• Anticipate what new, unrevealed physics might evolve
• Perform a sort of thought experiment (Gedankenexperiment*) for various
scenarios
• Decide if potential for new physics warrants a detailed experiment

*Gedankenexperiment consists in asking contrafactual questions - “What if . . .?”. It all started
when Galileo wondered “What if air resistance did not exist?” and discovered the law of free fall.
This strategy was later fruitfully extended and popularised in physics, in particular by Einstein.
Statement by Jean-Marc L´evy-Leblond

August 9, 2011

DIAGNOSTIC (PRELIMINARY) EXPERIMENTS AND
ASSESSMENT OF UNDESIRABLE EFFECTS

Those who have experimental experience might suggest the following:
Diagnostic experiments provide the opportunity to
a) Determine if the planned experiment can be sufficiently controlled, free of
undesirable influences
b) Exploit the flexibility of the experimental facility to perform unplanned
experiments that may yield unanticipated observations

From the published literature, we rarely know if the final results evolved from:
(a) alone; (a) influenced by (b); or (b) alone.

But (b) requires an informed perspective:
“In the fields of observation chance favors only the prepared mind”
Louis Pasteur

August 9, 2011

ASSESSMENT OF PRELIMINARY (DIAGNOSTIC) EXPERIMENTS:
POSSIBLE UNDESIRABLE EFFECTS

Observations
• A range of undesirable/unexpected effects may influence the outcome of
an experiment.
• Typically, an investigator has not had experience with all significant
possibilities

Examples
• End effects that may influence the flow across the entire span of the test
section
• Coupling of an unsteady event with an acoustic mode of the test section

August 9, 2011

ASSESSMENT OF DIAGNOSTIC (PRELIMINARY) EXPERIMENTS:
FLOW PAST A CYLINDER:
END EFFECTS

Without end plates (Re = 90), spanwise
structure of vortices has a chevron-like
pattern. Axes of vortices are inclined with
respect to axis of cylinder

Without end plates

With end plates, entire spanwise structure is
altered; vortices are parallel to axis of
cylinder

With end plates
Williamson (1988)
.

August 9, 2011

ASSESSMENT OF DIAGNOSTIC (PRELIMINARY) EXPERIMENTS:

FLOW PAST A PLATE: COUPLING WITH
RESONANT MODE OF TEST SECTION Resonant coupling occurs between: (i)
vortex formation; and (ii) acoustic mode of
plate-test section configuration.

Hourigan and Tan ( 2001) Consequence of coupling is highly ordered
vortex formation from leading- and trailing-
ACOUSTIC MODE
edges of plate (see image at left)
Acoustic
Plate amplitude
Even low level amplitudes resulting from
Test section wall coupling may significantly influence
experimental outcome.
Parker modes due to Parker (1967)

Early Investigation: Batchelor, G. K., and
Townsend, A. A., (1945) Singing Corner Vanes: A
Note on a Peculiar Double Resonant Sustained
Oscillation Occurring in a Wind Tunnel, Council for
Scientific and Industrial Research,Division of
Aerodynamics, Note 62
August 9, 2011

ASSESSMENT OF PRELIMINARY (DIAGNOSTIC) EXPERIMENTS:

UNDESIRABLE EFFECTS IN FLOW SYSTEMS: LACK OF COHERENT
KNOWLEDGE BASE
• Archival publications typically contain no information on what undesirable/
contaminating effects were overcome prior to final experiment, and much
information is lost to the community.
• Presentations at technical meetings typically do not address undesirable/
contaminating effects.
An exception: AIAA 2010 (January, Orlando) Low Re FDTC presentations
on challenges in overcoming undesirable effects during force
measurements were informative.

August 9, 2011

EXECUTION OF EXPERIMENT: ADVANCED INSTRUMENTATION
ULTIMATE IMAGING SYSTEM*
Volume imaging with high spatial and temporal resolution.
HIGH CAPABILITY IMAGING SYSTEMS
Various combinations of: (i) volume vs. planar imaging; (ii) high vs. low temporal
resolution; and (iii) high vs. moderate spatial resolution
OPINIONS
• Continuing development and implementation of ultimate systems is critical;
they will yield important insight
• But high capability systems will continue to lead to major contributions if
right research paths are defined
Example: Quantitative
identification of hairpin
vortices in turbulent
boundary layer

R. Adrian, C.D. Meinhart,
and C.D. Tomkins JFM,
2000, vol.422, pp. 1-54
Instantaneous images with
large time spacing

D. Rockwell, AFOSR Review August 9, 2011

COLLABORATION WITH COMPUTATIONAL GROUPS

Nearly all in our community have had productive collaborations. Would most
agree on the following?
• Verification/validation of computations (and experiments):
- Obviously essential and a major objective.
• ParalleI (interactive) experiments and computations:
- More fruitful outcomes than comparing end results
- Possibility for comparing unanticipated observations

Some examples of collaboration:
• Experiment drives computation
• Experiment in parallel with computation

August 9, 2011

Wave

h COLLABORATION WITH COMPUTATIONAL GROUPS
Laser
COMPUTATIONS DRIVEN BY EXPERIMENTAL IMAGING*
Sheet

DEEP WATER WAVE-STRUCTURE INTERACTION
Orbital Trajectory

Challenge: Incident wave (gust) includes history of all previous cycles of
reciprocating wave motion
Approach: Wave–structure interaction simulated using POD by directly
employing eigenmodes extracted from PIV images.
Queries
• What analogous approaches have been successful within or outside of
our community?
• Will increasingly powerful experimental and computatlonal techniques
enhance the possibilities for highly integrated experimental-computational
efforts, or are parallel endeavors more effective?

*DPIV-driven flow simulation: a new computational paradigm X. Ma, G.E. Karniadakis,
H. Park and M. Gharib Proc.R. Soc. Lond. A 2003 459, 547-565
*Wave-structure interaction: simulation driven by quantitative imaging S. Sirisup, G.E.
Karniadakis, Y. Yang and D. Rockwell Proc.R. Soc. Lond. A 2004 460, 729-755

PARALLEL COMPUTATIONAL-EXPERIMENTAL INVESTIGATION

HIGH FIDELITY COMPUTATIONS AND EXPERIMENTAL IMAGING

THREE- DIMENSIONAL VORTEX FORMATION ON A PLUNGING WING

COMPUTATIONS (AFRL) EXPERIMENTS (LEHIGH
M. Visbal UNIVERSITY)
FDL3DI COMPUTATIONS T. Yilmaz and D. Rockwell
AIAA Paper 2011-219 PIV VOLUME IMAGING

EXPERIMENTAL
COMPUTATIONAL

Nominally two-dimensional
leading-edge vortex rapidly
evolves to a highly three-
dimensional form during
plunging maneuver

Evolution to an arch vortex
first characterized by
Visbal (2011)

August 9, 2011


SEQUENCE OF EXPERIMENTAL AND COMPUTATIONAL EVENTS
1. Joint Experimental-Computational
• Define flow and motion parameters for most efficient use of resources
• Aim for generic, new flow structure
• Stay within the scope of the RTO Program
2. Experimental
• Dye visualization of flow structure to determine new aspects of flow
physics
• Quantitative sectional imaging in crossflow planes.
• Construction of space-time volumes (yzt) of vorticity and crossflow
velocity.
3. Computational
• Compute entire three-D flow patterns
• Compare sectional flow patterns with experimental images.
• Define time evolution of volume (xyz) representations of three-D arch
vortex
August 9, 2011


SEQUENCE OF EXPERIMENTAL AND COMPUTATIONAL EVENTS
4. Experimental
• Construction of phase-referenced volume images guided/verified by
computations
• Assessment of uncertainty of volume construction for defined vorticity
gradients via existing three-D theory (Hill’s spherical vortex)
5. Joint Experimental-Computational
• Discussions of issues resulting from direct comparisons, including
possibilities for further post-processing

August 9, 2011

FUTURE PROSPECTS

What events are likely to enhance experimental advances during the next
decade?
How about:
• Discovery of new flow physics and flow-structure interactions and highly
efficient control of physics
• Understanding and transforming physical concepts from other disciplines.
Example: Nature-inspired flight What are other possibilities?
• Technological advances in computational capabilities and instrumentation
(e.g., illumination and image acquisition).
• Technological progress in other disciplines, which may not be directly
applicable, but can trigger new ideas:
Science (AAAS) 17 December 2010 (p.1612)
Big ideas of the past 10 years and the technologies that
made them possible:
“Many of the decade’s most useful new tools were
……advances in sensing and imaging”

August 9, 2011

Afosr cm august 9 10, 2011 rockwell 081611

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