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School of somethingFACULTY OF OTHER
School of Mechanical EngineeringFACULTY OF ENGINEERING
Flow Simulation for Improved Engineering Design
Dr Harvey Thompson
Institute of Engineering Thermofluids, Surfaces & Interfaces (iETSI)
(With particular thanks to: Dr Nik Kapur, Dr Jon Summers, Dr Mark Wilson, Dr Sergii Veremieiev, Dr Yeawchu Lee, Prof. Phil Gaskell)
School of somethingFACULTY OF OTHERSummary
Applications of Computational Fluid Dynamics (CFD) Design and Analysis in Free Surface Film Flows
•2D Simulations: Industrial Coating
•3D Simulations: Flows over real surfaces
Bio-pesticide application methods
Several different precision coating processes
produce a range of important products,
including:
• Polymer films and packaging materials
• Ink-jet printing and imaging media
• Large Area Printed Electronic Devices
• etc...
2D Simulations: Industrial Coating Flows
Slot Coating
Roll Coating
Key Problem for Coating Engineers – avoid formation of ‘streak-lines’ which destroy product quality – caused by eddies in the flow
2D Simulations: Industrial Coating Flows
Coating web direction
Thin ‘streak-line’
destroys product quality
Industrial Coating Flows Difficult to Simulate
(even in 2D!):
• Surface-tension dominated, free surface flows
• Static and dynamic wetting lines where
coating is formed
• Highly curved flow domains
Finite Element (FE) Methods suitable
for such problems
2D Simulations: Industrial Coating Flows
2D Simulations: Industrial Coating Flows
w eb
upper free surface
lower free surface
recircu la tion reg ion
(a) = 25 S
o
(b) = 45 S
o
in te rna lin te rface
Avoidance of ‘Streak-lines’ (1):
Classic Moffatt 1964 JFM paper – strong effect of static contact line on eddy size and strength
Increase static angle to reduce streak-lines
Practical solution: coat cascade surface with PTFE – reduced product wastage by £4M over three years!
2D Simulations: Industrial Coating Flows
Avoidance of ‘Streak-lines (2): Slot exit flow.
Slot exits often used to supply pre-metered coatings such as slide and curtain
2D Simulations: Industrial Coating Flows
Avoidance of ‘Streak-lines’ (2): Slot exit flow.
Back-wetting of uppermost slot may occur during start-up – can lead to defect-causing solids deposits due to degradation in recirculation regions.
Back wetting at the upper slot: (a) experimental, (b) CFD prediction
2D Simulations: Industrial Coating Flows
Avoidance of Streak-lines (2): Slot exit flow
Merging flow out of slot exits – effect of chamfering lower corner
Chamfer can remove
eddies in both liquid
layers simultaneously!
2D Simulations: Industrial Coating Flows
Stirring and Mass Transfer in Continuously Modulated Forward Roll Coating, M.C.T. Wilson et al, JFM, 2006.
Upstream Downstream
Rweb
UwebWeb
Dynam iccontact line 2H 0
Uapp
R app
Substrate
Bath
2D Simulations: Industrial Coating Flows
Navier-Stokes velocity field
Galerkin Finite Element method; spine method
guuuu
ˆ)/(. 2 CaBopt
SrRe
)),,(),,,(( tyxvtyxuu
‘Tracer’ particle trajectories
Runge-Kutta scheme; trigonometric interpolation of nodal
and spinal data allows evaluation of u,v at arbitrary t
),,(1
tyxuSrdt
dx ),,(
1tyxv
Srdt
dy
2D Simulations: Industrial Coating Flows
Stirring and Mass Transfer in Continuously Modulated Forward Roll Coating, M.C.T. Wilson et al, JFM, 2006.
Enables residence times and effects of time-dependent forcing to be analysed, e.g. Roll eccentricity or variable roll speeds.
Experiment CFD
3-D Film and Droplet Flows over Topography
Plant disease control
Several important practical applications: e.g. film flow in the eye,
electronics cooling, heat exchangers,
combustion chambers, etc...
Focus on: precision coating of micro-scale
displays and sensors, Tourovskaia et al,
Nature Protocols, 3, 2006.
Pesticide flow over leaves, Glass et al,
Pest Management Science, 2010.
3D Film Flow over Topography
solid
topographic substrate
spin coatliquid
conformalliquid
coating
cure film
levellingperiod
> 50μm
Stillwagon, Larson and Taylor, J. Electrochem. Soc. 1987
For displays and sensors, coat liquid layers over functional topography – light-emitting species on a screen
Key goal: ensure surfaces are as planar as possible – ensures product quality and functionality – BUT free surface disturbances are persistent!
3D Film Flow over Topography
Key Modelling Challenges:
•3-D surface tension dominated free surface flows are very complex – Commercial CFD codes are limited and Navier-Stokes solvers at early stage of development (see later)
•Surface topography often very small (~100s nm) but influential – need highly resolved grids?
•No universal wetting models exist
•Large computational problems – adaptive multigrid, parallel computing?
•Very little experimental data for realistic 3D flows.
3D Film Flow over Topography
Finite Element methods not as well-established for 3-D free surface flow. Promising alternatives include Level-Set, Volume of Fluid (VoF), Lattice Boltzmann etc… but still issues for 3D surface tension dominated flows – grid resolution etc...
Fortunately thin film lubrication
low assumptions often valid
provided: ε=H0/L0 <<1 and
capillary number Ca<<1
Enables 3D flow to be modelled
by 2D systems of pdes.
x
y
h(x,y)
s(x,y)
gravity
inflow
outflow
L0
H0
3D Film Flow over Topography
Comparison between experimental free surface profiles and those predicted by solution of the full Navier-Stokes and Lubrication equations.
Agreement is very good
Lubrication theory is accurate for thin film flows with small topography and inertia.
Unfortunately not always the case!
Decre & Baret, JFM, 2003: Flow of Water Film over a Trench Topography
3D Film Flow over TopographyThin Film Flows with Significant Inertia
Free surfaces can be strongly influenced by inertia: e.g. free surface instability, droplet coalescence,... standard lubrication theory can be extended to account for significant inertia – Depth Averaged Formulation of Veremieiev et al, Computer & Fluids, 2010.
Film Flows of Arbitrary Thickness over Arbitrary Topography
Need full numerical solutions of 3D Navier-Stokes equations!
Depth-Averaged Formulation for Inertial Film Flows
100 LH 1Ca
1. Reduction of the Navier-Stokes equations by the long-wave approximation:
3. Assumption of Nusselt velocity profile to estimate unknown friction and dispersion terms:
2213 uu 2213 vvh
sz
2. Depth-averaging stage to decrease dimensionality of unknown functions by one:
f
s
udzh
tyxu1
,, f
s
vdzh
tyxv1
,,
,
),(,,,, yxstyxftyxh
Restrictions: 1, yxs
Restrictions: no internal recirculations
23
cot25
6
5Re
22
3
h
ushsh
Caxy
uv
x
uu
t
h
h
u
t
u
2
23 3
cot25
6
5Re
h
vshsh
Cayy
vv
x
vu
t
h
h
v
t
v
0
y
vh
x
uh
t
h
1,0,
3
2,, 0xhvu
0,,,, ,0
pp wylx hvuy
hvux
DAF system of equations:
Boundary conditions:
1.Inflow b.c.
2.Outflow (fully developed flow)
For Re = 0 DAF ≡ LUB
Depth-Averaged Formulation for Inertial Film Flows
Flow over 3D trench: Effect of Inertia
Gravity-driven flow of thin water film: 130µm ≤ H0 ≤ 275µm over trench topography: sides 1.2mm, depth 25µm
bow wavesurge
comet tail
Accuracy of DAF approach
Gravity-driven flow of thin water film: 130µm ≤ H0 ≤ 275µm over 2D step-down topography: sides 1.2mm
Max % Error vs Navier-Stokes (FE)
Error ~1-2% for Re=50 and s0 ≤0.2
s0=step size/H0
Free Surface Planarisation
Noted above: many manufactured products require free surface disturbances to be minimised – planarisation
Very difficult since comet-tail disturbances persist over length scales much larger than the source of disturbances
Possible methods for achieving planarisation include:
• thermal heating of the substrate, Gramlich et al (2002)
• use of electric fields
Electrified Film Flow
Gravity-driven, 3D Electrified film flow over a trench topography
Assumptions:
• Liquid is a perfect conductor
• Air above liquid is a perfect dielectric
Film flow modelled by Depth Averaged Form
Fourier series separable solution of Laplace’s equation
for electric potential above the film is coupled to film flow
by Maxwell free surface stresses.
Electrified Film Flow
Effect of Electric Field Strength on Film Free Surface
No Electric Field With Electric Field
Note: Maxwell stresses can planarise the persistent, comet-tail disturbances very effectively.
Computational Issues
Real and functional surfaces are often extremely complex.
Multiply-connected circuit topography:
Lee, Thompson and Gaskell, International Journal for Numerical Methods in Fluids, 2008
Flow over a maple leaf topographyGlass et al, Pest Management Science, 2010
Need highly resolved grids for 3D flows
Adaptive Multigrid Methods
• Full Approximation Storage (FAS) Multigrid methods very efficient.
• Spatial and temporal adaptivity enables fine grids to be used only where they are needed.
E.g. Film flow over a substrate with isolated square, circular and diamond-shaped topographies
Free Surface Plan View of Adaptive Grid
Parallel Multigrid Methods
Parallel Implementation of Temporally Adaptive Algorithm using:
• Message Passing Interface (MPI)
• Geometric Grid Partitioning
Combination of Multigrid O(N) efficiency and parallel speed up very powerful!
BUT Parallelisation of spatially adaptive algorithm very challenging!
3D FE Navier-Stokes Solutions
Remember:
Lubrication and Depth Averaged Formulations invalid for flow over arbitrary topography and unable to predict recirculating flow regions
As seen earlier important to predict eddies in many applications:
E.g. In industrial coating
3D FE Navier-Stokes Solutions
Mixing phenomena
E.g. Heat transfer enhancement due to thermal mixing, Scholle et al, Int. J. Heat Fluid Flow, 2009.
•Commercial CFD codes still rather limited for these type of problems•Finite Element methods are still the most accurate for surface tension dominated free surface flows – grids based on Arbitrary Lagrangian Eulerian ‘Spine’ methods
Spine Method for 2D Flow Generalisation to 3D flow
3D FE Navier-Stokes Solutions
Gravity-driven flow of a water film over a trench topography: comparison between free surface predictions
3D FE Navier-Stokes vs DAF Solutions
Gravity-driven flow of a water film over a trench topography: particle trajectories in the trench•3D FE solutions can predict how fluid residence times and volumes of
fluid trapped in the trench depend on trench dimensions•Will be extended to analyse ‘thermal mixing’ in 3D flows
3D FE Navier-Stokes Solutions
Droplet Flows: Bio-pesticides
Droplet Flow Modelling and Analysis
Application of Bio-pesticides
Changing EU legislation is limiting use of
chemically active pesticides for pest control in crops.
Bio-pesticides using living organisms (nematodes, bacteria etc...) to kill pests are increasing in popularity but little is known about pesticide motion over leaves
Working with Food & Environment Research Agency in York and Becker Underwood Ltd to understand the dominant flow mechanisms
Nematodes
Nematodes are a popular bio-pesticide control
method - natural organisms present in soil
typically up to 500 microns in length.
• Aggressive organisms that attack the pest by entering body openings
• Release bacteria that stops pest feeding – kills the pest quickly
• Mixed with water and adjuvants and sprayed onto leaves
What do we want to understand?
• Why do adjuvants improve effectiveness – reduced
evaporation rate?
• How do nematodes affect droplet size distribution?
• How can we model flow over leaves?
• How does impact speed, droplet size and orientation affect droplet motion?
Droplet spray evaporation time: effect of adjuvant
Size of droplets
Concentration (%)
Initial mass (mg)
Mass fraction left after 10 min (%)
Evaporation time (min)
large 0 130.3
36.3 26.3
0.01 138.0
36.6 24.0
0.1 161.0
48.7 36.0
small 0 87.3 13.3 16.30.01 92.5 9.7 16.00.1 138.
333.3 25.7
Droplet size distribution for bio-pesticides
Teejet XR110 05 nozzle with 0.8bar
Matabi 12Ltr Elegance18+ knapsack sprayer
Malvern Spraytec Laser Diffraction System
VMD of the bio-pesticide spray depending on the concentration of adjuvant
Substance
Dv50 (μm)
c = 0%
c= 0.01%
c = 0.03%
c = 0.1%
c = 0.3%
water+adjuvant 273.3 275.1 269.4 330.5 352.9
water+carrier material
285.9 276.1 297.3 329.2 360.8
water+commercial product
(biopesticide)271.0 272.8 282.6 307.5 360.6
addition of bio-pesticide does not affect Volume Mean Diameter of the spray
Droplet flow over a leaf: simple theory
2nd Newton’s law in x direction:
Stokes drag:
Terminal velocity:
Velocity:
Contact angle hysteresis:
Relaxation time:
theoretical expressions from Dussan (1985):
Volume of smallest droplet that can move:
Droplet flow over a leaf: simple theory vs. experiments
Podgorski, Flesselles, Limat (2001) experiments:
Dussan (1985) theory:
Le Grand, Daerr & Limat (2005), experiments:
47V10 silicon oil drops flowing over a fluoro-polymer FC725 surface:
Linearity of graph => droplet flow is governed by this law (until pearling!)
Droplet flow over a leaf (θ=60º): effect of inertia
For: V=10mm3, R=1.3mm, terminal velocity=0.22m/s
Lubrication theory Depth averaged formulation
Droplet flow over a leaf (θ=60º): effect of inertia
For: V=20mm3 R=1.7mm terminal velocity=0.45m/s
Lubrication theory Depth averaged formulation
Droplet flow over a leaf (θ=60º): summary of computations
V, mm3
R, mm
BosinθCa a, m/s Ca a, m/s Ca a, m/s
ExperimentComputation
Re=0Computation
Re=10
0.27 0.4 0.06 0 00.000
30.02
0.0001
0.007
10 1.3 0.62 0.003 0.13 0.005 0.21 0.005 0.2220 1.7 0.99 0.006 0.24 0.010 0.42 0.009 0.4030 1.9 1.30 0.008 0.33 0.012 0.54 0.011 0.4840 2.1 1.57 0.011 0.48 0.014 0.62 0.012 0.55
Grid density has a big influence on accuracy of predictions; need at least
512x512 nodes
Koh et al, Eur. Phys. J., 166, 2009: 4millionx4million!
Droplet flow over a leaf: theory shows small effect of initial velocity
Relaxation time:
Initial velocity:
Velocity:
Droplet flow over a leaf: computation of influence of initial condition
V=10mm3 R=1.3mm
a=0.22m/s
Bosinθ=0.61
v0=0.69m/s
Bosinθ init =1.57
V=10mm3 R=1.3mm a=0.22m/s
Bosinθ=0.61
v0=1.04m/s
Bosinθ init =2.49
this is due to the relaxation of the droplet’s shape
Droplet flow over (θ=60º) vs. under (θ=120º) a leaf: computation
V=20mm3 R=1.7mm a=0.45m/s
Bosinθ=0.99
θ=60º
V=20mm3 R=1.7mm a=0.45m/s
Bosinθ=0.99
θ=120º
Bio-pesticides: initial conclusions
Addition of carrier material or commercial product (bio-pesticide) does not affect the Volume Mean Diameter of the spray.
Dynamics of the droplet over a leaf are governed by gravity, Stokes drag and contact angle hysteresis; these are verified by experiments.
Droplet’s shape can be adequately predicted by lubrication theory, while inertia and initial condition have minor effect.
Simulating realistically small bio-pesticide droplets is extremely computationally intensive: efficient parallelisation is needed ( see e.g. Lee et al (2011), Advances in Engineering Software) BUT probably does not add much extra physical understanding!
General Conclusions
Free surface film and droplet flows are everywhere! Lots of important industrial applications.
They are very difficult to model: surface tension-dominated, complex geometry and governing physics. Commercial CFD still quite primitive.
Simplifications such as lubrication theory can be very useful but must be careful about validity. Methods for 3D Navier-Stokes beginning to emerge.
Simulating realistic 3D flows requires highly efficient numerical methods and access to High Performance Computing
Finally: desperate need to more experimental data!