Study of Turbulent Flows using Vectored
Momentum and Energy Addition
Subrata Roy, University of Florida
Collaborator : Stephen Wilkinson, NASA Langley
(through SAA1-18097)
Applied Physics Research Group, UF, Gainesville
http://aprg.mae.ufl.edu
AFOSR: Drs. R. Ponnappan, D. Smith, J. Schmisseur (UTSI)
DARPA: J. Sponable, D. Barnhart (USC)
AFRL: Drs. M. Visbal, J. Poggie and D. Gaitonde (OSU)
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Contents
 Motivation
 Background
 Preliminary Results
 Proposed work
Applied Physics Research Group, UF, Gainesville
http://aprg.mae.ufl.edu
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Big Picture
 Restricting the turbulent motion at the local
scale can reduce turbulent friction.
 Adhesive microscopic riblets
 DC plasma heating in C-5M shows 0.5-1% L/D
improvement (Aviation Weekly, 2014)
 Basic understanding of near wall forcing and
wall heating in growth and decay of turbulent
flow structures is needed.
 We aim to identify the mechanism through
which vortices can be generated for controlling
receptive modes of a given flow and tune our
actuators to amplify or dampen those modes to
optimally control the flow.
Applied Physics Research Group, UF, Gainesville
http://aprg.mae.ufl.edu
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Objectives
 An improved understanding of mechanisms
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through which a canonical set of vortices interacts
and modifies near-wall and large-scale flow
structures.
A thorough investigation of the effect of vortices
in the vicinity of a surface heat source.
Stimulation (separation control) and annihilation
(drag reduction) of boundary-layer turbulence
using high-fidelity numerical simulations.
Separation of heating and forcing effects of plasma
actuation.
Exploration of receptive location from leading
edge (flat plate), momentum injection with and
without the heating load to find the most
receptive location for transition delay or onset.
Applied Physics Research Group, UF, Gainesville
http://aprg.mae.ufl.edu
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Motivation
 Robust control of drag and heat transfer is possible using transient-growth-based vortex
generation with streamwise independent forcing (Schoppa & Hussain, 2002). A single largescale forcing can stabilize numerous streaks simultaneously. An x-independent forcing of
only 6% of the centerline velocity, produces a significant sustained drag reduction: 20% for
imposed counter-rotating streamwise vortices and 50% for colliding, spanwise wall jets.
 Mechanisms to amplify the growth of sinuous streak waviness (nonlinear Streak Transient Growth )
is needed for flow control authority
 Plasma actuators (namely, serpentine class) exhibit this property of nonlinear growth (Wang
and Roy, 2009; Durscher and Roy, 2012; Jukes and Choi, 2013).
 Plasma actuators generate hairpin and lambda vortices which can penetrate into the
flowfield beyond the buffer layer (Riherd and Roy, 2013a, 2013b, 2014).
 A linear analysis (Chernyshenko 2014) predicts a few percent of drag reduction for a passive
swept wavy wall sized to match the spanwise shear stress in the spatial Stokes layer.
 The resolvent formulation proposed by McKeon et al. (2010, 2013) found that walls that
suppress structures energetic in natural turbulence may cause detrimental effects elsewhere
in spectral space. Specifically, slow-moving spanwise-constant structures are particularly
susceptible to further amplification.
Applied Physics Research Group, UF, Gainesville
http://aprg.mae.ufl.edu
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Motivation
 Lifted streaks – nonlinear growth of instabilities
 Streak lift angles should cross the stability threshold to
obtain sinuous streak instability
 Lifted streaks crossing the threshold might not create
instability if
 Not present outside viscous sublayer
Schoppa and Hussain, 2002
 Not elongated enough in streamwise to permit growth
 Studying streaklines formed using serpentine actuator is
necessary to understand this growth mechanism and
structures responsible.
 Effect of serpentine actuators on drag reduction over a plate
at U = 1.8m/s have been studied and show 22% viscous
drag reduction (Jukes et al, 2006).
 However more study needs to be done at higher velocities
Applied Physics Research Group, UF, Gainesville
http://aprg.mae.ufl.edu
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Serpentine Plasma Actuator
Riherd and Roy, JAP 2013
Applied Physics Research Group, UF, Gainesville
http://aprg.mae.ufl.edu
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Background
 Serpentine actuators – rectangular, circular,
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horseshoe, triangular, comb etc (Roy and
Wang,2009, Riherd and Roy, 2013, Durscher
and Roy,2012)
These actuators allow near wall three
dimensional flow control
Modify the boundary layer thickness by
vectored momentum addition
Plasma modelled using body force
distribution based on first principles
simulation (Singh and Roy, 2008).
Numerical and experimental (stereo PIV)
show the vectored momentum addition and
streamwise vorticity (Riherd and Roy, 2013)
Applied Physics Research Group, UF, Gainesville
http://aprg.mae.ufl.edu
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Background
 The vectored momentum injection occurs due to the
alternate spreading and pinching effects.
 The spreading points accelerate the flow forward like
the standard linear actuators
 Pinching point vectors the flow upwards
 This pinching and spreading
mechanism generates 3D vortices
 Impingement angle – Serpentine
actuators up to 43 while linear gives
only 12 for 14kVpp
 This allows higher momentum
injection into the bulk flow.
Pinch
Spread
Linear
Serpentine
Applied Physics Research Group, UF, Gainesville
http://aprg.mae.ufl.edu
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Background
 Serpentine actuators generate both spanwise
and streamwise vorticity – momentum
injection in x, y and z.
 Generate corkscrew like structures in the
flowfield (Durscher and Roy, JoPD (2012)
 Different serpentine
geometries give similar flow
structures with different
magnitudes and footprint.
Rectangular
Serpentine
Circular
Serpentine
Applied Physics Research Group, UF, Gainesville
http://aprg.mae.ufl.edu
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Background
 Serpentine actuators make TS waves
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more sinuous compared to linear
actuators.
The amplitude study of the serpentine
actuators also gives useful insight
instability growth rate
They introduce spanwise disturbances
at the actuator location which allows
rapid transition.
For a SD7003 airfoil a pulsed (45%)
serpentine actuator shows comparable
PSD of spanwise constant modes (‘a’)
to linear actuator
However the PSD of spanwise varying
modes (‘b’) are drastically altered
right after the actuation
Linear
Serpentine
Riherd and Roy, 2013
Actuator
Location
Applied Physics Research Group, UF, Gainesville
http://aprg.mae.ufl.edu
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Background
Riherd and Roy, 2013
Normalized boundary layer streak profiles based on the standard
deviation of the streamwise velocity across the span of the boundary
layer for g0 = up/u∞ = (a) 1%, (b) 2.5%, (c) 5%, and (d) 10%.
Applied Physics Research Group, UF, Gainesville
http://aprg.mae.ufl.edu
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Preliminary experiment at UF
 Flow over a backward step in a small NASA tunnel
 PIV measurement before the step around the
actuator
 Freestream velocity - 15m/s
 Use of serpentine actuators has thickened the
boundary layer
 The baseline measurement follows close to the
1/7th power law
 The significant change (22%) in slope proves that
skin friction is reduced with the application of
serpentine plasma
Applied Physics Research Group, UF, Gainesville
http://aprg.mae.ufl.edu
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Preliminary experiment at UF
 Pressure data also collected after the step at 16 different locations
 Both pulsed (125Hz AM, 1kHz) and continuous modes are tested
for linear as well as serpentine comb actuator
 Pulsed serpentine actuation performs the best but for continuous
mode serpentine actuator performs better than linear actuator.
Applied Physics Research Group, UF, Gainesville
http://aprg.mae.ufl.edu
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Windtunnel experiment at NASA
 Subsonic tests on NASA LaRC 7  11  40 low speed wind tunnel
 Closed-loop, 40m/s at 2750 rpm
 Boundary layer is tripped using 0.91mm rod
 Drag measurements on 8  30 flat plate at different rpms were performed with linear
and serpentine actuators
 Different frequencies and voltages for actuation were also tested (16, 18 and 20kVpp
with 7, 6 and 5kHz respectively)
 Drag balance –
 Range : 50g
 Output: 2VDC
 Response: 20ms
Blanco, Underwood and Wilkinson (2014) shared with permission
Applied Physics Research Group, UF, Gainesville
http://aprg.mae.ufl.edu
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Windtunnel experiment at NASA
 Preliminary results show up to 29% viscous drag reduction on a flat plate using
serpentine actuators (at 1200 rpm wind tunnel speed) while linear showed 6%
 However due to either actuator degradation with long time use the data was not
repeatable using the same actuator.
 The data showed that if the input
voltage is increased % drag
reduction also increases
 Since the actuators did not cover
the entire span of the plate some
of the drag reduction
contributions might be because of
the actuator end effects.
Blanco, Underwood and Wilkinson (2014) shared with permission
Applied Physics Research Group, UF, Gainesville
http://aprg.mae.ufl.edu
Linear
Serpentine
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Proposed Work
 DNS and LES simulations will be carried out
for laminar and turbulent subsonic flow using
FDL3Di for horseshoe and serpentine
actuation.
 An in-house plasma kinetic code MIG and the
AFRL turbulence code FDL3Di will be loosely
coupled to validate 3-D plasma flow coupling
with experimental data.
 Study the effect of wavelength and amplitude
of horseshoe and serpentine actuators on
turbulent structures and to delay or accelerate
transition
 Find effects of heat addition with and without
plasma actuation on turbulent structures
Applied Physics Research Group, UF, Gainesville
http://aprg.mae.ufl.edu
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Proposed Work (Continued)
 Study the effect of superposition of two or more
actuators on the scaling of vortices
 Explore the concept of (momentum) vectored heat
addition
 Study the effects of pulsed actuation
 Experimental Validation (to be performed by NASA
LaRC collaborator at 7x11 facility )
Applied Physics Research Group, UF, Gainesville
http://aprg.mae.ufl.edu
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Proposed Work (Continued)
 Explore the concept of (momentum) vectored heat addition
Standard actuator
Serpentine
Serpentine
with heating
Serpentine
Applied Physics Research Group, UF, Gainesville
http://aprg.mae.ufl.edu
Proposed Work (Continued)
 Study the effect of superposition of two or more
actuators on the scaling of vortices
 Study the effects of pulse modulated actuation
Applied Physics Research Group, UF, Gainesville
http://aprg.mae.ufl.edu
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NASA Langley Experiments
 Experimental Validation by LaRC collaborator
 Fabrication of proof-of-concept plasma actuator designs
 Testing these actuators under flow conditions at 7”x11” low speed
wind tunnel facility
 Exploration of receptive location from leading edge (flat plate),
momentum injection with and without the heating load to find the
most receptive location for transition delay or onset.
Applied Physics Research Group, UF, Gainesville
http://aprg.mae.ufl.edu
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Expected Outcome
 Improved flow control at a lower cost and weight penalty.
 Improved actuator designs for low speed drag reduction
 Evaluation of validated concepts for high speed viscous drag
reduction studies in NASA’s ARMD Supersonic Project in the 20
Inch Supersonic Wind Tunnel (~ Mach 2).
 Transition developed knowledge to AFRL and to the greater
scientific community through NATO-STO AVT-254.
Applied Physics Research Group, UF, Gainesville
http://aprg.mae.ufl.edu
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