Durrell Rittenberg, Ph.D.

Leveraging STAR-CCM+

for Aircraft Applications

Icing in aerospace

– Common applications

– Impact of icing on Aircraft safety

– Common icing conditions and Mechanism of ice accretion

– Types of ice accretion and impact of aerodynamic performance

Leveraging simulation for ice accretion prediction

– Ice Accretion Simulation Considerations

– Physics

Icing and FAA certification

– Changes to the FAA Icing certification regulations

– Impact to Airplane makers

Overview of Icing with STAR-CCM+

• Ice build-up results in significant changes to the aerodynamics of

the vehicle

• This degrades the performance and controllability of the aircraft

Icing in the aerospace sector

Ground Icing In Flight Icing

• ATR-72: Roselawn, IN; October 1994 –– 68 fatalities, hull loss

– NTSB findings: probable cause of accident was aileron hinge moment reversal due to an ice ridge that formed aft of the protected areas

• EMB-120: Monroe, MI; January 1997 – 29 fatalities, hull loss – NTSB findings: probable cause of accident was loss-of-control due to ice

contaminated wing stall

• EMB-120: West Palm Beach, FL; March 2001 – 0 fatalities, no hull loss, significant damage to wing control surfaces

– NTSB findings: probable cause was loss-of-control due to increased stall speeds while operating in icing conditions (8K feet altitude loss prior to recovery)

• Bombardier DHC-8-400: Clarence Center, NY; February 2009 – 50 fatalities, hull loss

– NTSB findings: probable cause was captain’s inappropriate response to icing condition

• ATR-72 Tyumen Russia February 2012– 12 fatalities, no hull loss

– Crashed after take-off due to icing

Aircraft Icing is a real danger

Prediction of ice build-up for

rime, glaze and mixed icing conditions

Accurately predict the

locations of first ice formation

Accurately predict ice

accretion on critical areas on

vehicle

Determine size and shape of ice

accretion on vehicle

Calculate the pressure loss

due to ice build-up on critical aerodynamic

surfaces

What are the motivators for our icing

capabilities?

• New standards for Supercooled Large

Droplets and High Altitude Ice Crystals

ingestion in engines

• The FAA estimates these requirements will

cost the aerospace industry over $63 million

per year.

• Industry looking for ways to reduce costs

The FAA recently extend the icing certification

requirements [Nov 2014]

Typical design and compliance process

CFD and Icing Simulation

Low cost fast turn around

Pro

ject

Co

st

Icing tunnel testingHigh cost 8 month lead time

Project Timeline

Natural flight testing

Very high cost requires build

Typical design and compliance process

CFD and Icing Simulation

Low cost fast turn around

Pro

ject

Co

st

Icing tunnel testing

High cost 8 month lead time

Project Timeline

Natural flight testing

Reduce cost

Leverage Icing Simulations

Early in the design cycle

The basics of ice accretion and

aircraft icing

Mechanism of icing

Icing Basics• Invisible moisture (cloud & precip) • Temperature range around -20° to +2°C • Cloud contains supercooled liquid

water, ice crystals

Ice Accretion Parameters: • Velocity• Drop Size (MVD)• Liquid Water Content• Temperature • Accretion Time

2000 mm

Freezing Rain Freezing Drizzle Micro-ice crystals

15-50 mm500 mm

Types of ice formation

• Observed below -20° C

• Generally white like snow

• Does not create horns

• Drops freeze on impact

• Smooth surfaces

• Well understood

• Observed near 0° C

• Clear like ice

• Horns appear

• Drops don’t freeze on impact

• Rough surfaces

• Not well understood

• Observed between 0° and -20 ° C

• Clear like ice with white on edges

• Horns may appear

• Complex physics

Reduce maximum Lift

– Increase stall speed

– Stall warn system may not compensate for ice

Increases Drag

– Reduces Climb rate

– Reduces max speed

– May reduce speed to the point of stall.

Increases Weight

– Usually not significant, fuel burn will offset

Thrust

– Increased thrust required, due to drag increase

– GA aircraft are, typically, power limited

Effect of icing on airfoil performance

Data from NASA Glenn Icing Tunnel

Aircraft Icing considerations widespread

Ice Accretion Simulation

ConsiderationsThe Physics of Ice Accretion with formulations

Icing simulation is the joining of multiple

physics regimes

Determine the Flow Physics

Compute water droplet trajectories, giving collection efficiency

Determine thermodynamic

balance at the wall and

compute local ice

accretion rate

CF

DD

MP

/LM

PC

HT

Grow “delta” ice shape

• Single-shot method [Low cost]

• Multi-shot method [Medium cost]

• Fully unsteady flow field [High

cost]

• DMP: Dispersed Multiphase

droplets [Low cost]

• LMP: Lagrangian Multiphase

droplets [High cost]

• Fluid film model

• Evaporation and condensation

• Solidification models

• Mesh morphing

Icing simulation is the joining of multiple

physics regimes

Determine the Flow PhysicsC

FD

• STAR-CCM+®, is a Navier Stokes solver that has been extensively validated for aerodynamic applications. We include a coupled flow solver (implicit and explicit) as well as a Segregated flow solver

• For ice accretion there are three solving modes1. Single shot: Fluid flow is solved once and is assumed to be

unchanged through out the accretion [Low cost, fast, least accurate]2. Multi-shot: Fluid flow is updated periodically as the ice accumulates

[Medium cost, fast, more accurate]3. Fully Transient: Fluid flow is updated after each time step in the

accretion simulation [High cost, most accurate]

Icing simulation is the joining of multiple

physics regimes

Compute water droplet trajectories, giving collection efficiency

Dro

ple

t M

odelin

g

• Lagrangian Multiphase (LMP)– Individually track particles

– Can be run fully coupled with flowfield or with frozen flowfield

– Single particle size or distribution (such as Langmuir-D)

– Injection locations are arbitrary and customizable

• Dispersed Multiphase (DMP)– Lightweight one-way-coupled Eulerian approach

– Freestream is modeled as a multiphase mixture

– Mass, momentum and energy equations solved for each phase

– Better model of the cloud than LMP• Concentration is solved everywhere in the flowfield

• Shadow zones identified

– Can be run fully coupled with flowfield or with frozen flowfield

– Single particle size or distribution (such as Langmuir-D)

– No injection locations: particles exist throughout the freestream flow

Icing simulation is the joining of multiple

physics regimes

Determine thermodynamic

balance at the wall and

compute local ice

accretion rateC

HT

• Thin Film Modeling

– Droplet deposition from DMP / LMP

– Run-back

– Heat transfer

– Freeze / Thaw / Evaporation / Sublimation

– Edge- and wave-based stripping to LMP

• Conjugate Heat Transfer

– Simultaneous, coupled solution for fluid and solid thermal

– A comprehensive set of tools for the modeling of radiative heat transfer from simple surface to surface transfer through to discrete ordinate modeling (DOM) for participating media.

STAR-CCM+ efficiently determines collection efficiency

Collection Efficiency (β): The measure of a configuration's ability to capture incoming water defined as the local mass flux velocity.

𝛽 =𝛼𝜌𝑤𝑢𝑖𝐿𝑊𝐶 𝑈∞

𝐴𝑖|𝐴|

𝛼 = volume fraction of water

𝜌𝑤 = density of water

𝑢𝑖 = velocity of air

LWC = liquid water content

𝑈∞ = speed of the free stream

𝐴𝑖 = local area normal

Determining 3D collection efficiency (β)

External Airflow Super-Cooled Droplets

Collection efficiency computed

on slices around the nacelle

Data collected as a function of

surface distance from the LE

Collection efficiency validation of 747 inlet

Physics of fluid films used in icing

The fluid film accepts mass from impinging Lagrangian or Dispersed Multiphase droplets

Droplets can also be shed into the Lagrangian phases from the film due to

– Wave and Edge Stripping

The momentum of the fluid film is determined by the forces on the film

– Shear

– Gravity

– Surface roughness

Mass Transfer through Evaporation/Boiling, Condensation

Gravity

Eulerian Multi-Component Gas

Condensation

Evaporation

Impingement

Multi-component gas considerationsThe physics of fluid films in STAR-CCM+

Within a timestep, iteratively finds the mass that freezes:

– Computing a relative solid volume fraction

• 0 above 273.15K or 1 below 273.15K

– Updating the thickness of film to be removed in timestep

– At convergence, either

• All liquid film is removed (rime conditions)

• There is a liquid remainder at 273.15K (glaze conditions)

– Morph the solid boundary according to newly formed ice

The physics of melting and solidification for

ice accretion

Animated Detail of Morphed Mesh

Ice accretion validation for Rime ice conditions

2D CT Airfoil – Run 142: 2 Minutes

2D CT Airfoil – Run 112: 6 Minutes

2D CT Airfoil – Run 107: 22.5 Minutes

Commercial Transport Airfoil

₊ Mach = 0.45

₊ Airspeed = 279 kts

₊ Tstatic = -20.2 C

₊ α = 0.0°

₊ LWC = 0.295 g/m3

• Both LMP and DMP methods were

considered and show high

agreement with both LEWICE and

icing tunnel test data (shown here)

• Several 3D airfoil cases were run

and compared to 2D cross-

sectional test data with high

agreement

STAR-CCM+ was able to reproduce basic ice horn shapes

Horn shapes dependent on initial conditions

– Relative humidity

– Surface roughness

– Liquid water content

– Droplet size

– Evaporation Model setting

• Requires multi-component film and gas phases

Current work:

– Determine mesh sensitivities

– Correlate ice shapes to initial conditions and develop ice accretion best practices

– Extend results to 3D

Preliminary ice accretion validation for Glaze

ice conditions

Run 124Chord 0.9 mAirspeed 130 m/sAoA 0.7 degTstatic -9.49 o C0.563 g/m3 LWCAccretion time 294 sMVD 21μm

Run 144Chord 0.9 mAirspeed 130 m/sAoA 0.7 degTstatic -11.6o C0.4 g/m3 LWCAccretion time 180 sMVD 42μm

Reference: Harrold E, Addy, Jr, “Ice Accretions and Icing Effects for Modern Airfoils”, NASA/TP-2000-210031

Ice accretion capabilities in STAR-CCM+

LEVEL 1 - Measuring Collection Efficiency

External

Airflow

Super-Cooled

DropletsExternal

Airflow

Super-Cooled

Droplets

Fluid Film Film Solidification

(morpher)

LEVEL 2 – Predicting the Ice Shape

Collection Efficiency

Ice Shape Prediction

STAR-CCM+ unified process for ice accretion

Flowfield (3D Navier-Stokes)

Dispersed Phase

Fluid Film Freeze/Melt

Update Ice Shape

Mesh Morph / Remesh

Single Shot

Multi-Shot

Fully Transient

Three Dimensional/Implicit Unsteady

Segregated Flow, Segregated Temperature

Ideal Gas

Turbulence SST K-Omega

Dispersed Multiphase

Lagrangian Multiphase

Fluid Film

– Melting/solidification

– Solidified Film Removal, Internal Morphing,

– Solid Density from empirical correlation (Equation 9 from [4])

Multi-Phase Interactions:

1. Dispersed Multiphase-Physics Continuum: (Schiller-Neumann Drag Force, Pressure Gradient Force, Ranz Marshall

Heat Transfer)

2. Dispersed Multiphase – Fluid Film : Impingement

3. Fluid Film – Lagrangian Multiphase: Film Stripping enabled

Optional models

– Fluid Film – Physics Continuum: Evaporation Model (requires multi-component fluid film and gas phases)

Physics of ice accretion in STAR-CCM+

STAR-CCM+ is a complete solution for Aerospace Icing

Applications :

– Simulating the external airflow (CFD)

– Tracking Water Droplets

– Measuring Collection Efficiency

– Predicting Runback, Melting and Evaporation of Water

– Predicting the Ice Shape

– Including Rotating bodies

– Conjugate Heat Transfer

Single Tool, Simple Workflow

Review Summary

Appendix and other

technical details

Mass conservation

d

d𝑡න

𝑉

𝜌𝑓d𝑉 + න

𝐴

𝜌𝑓 𝐯𝑓 − 𝐯𝑔 ∙ d𝐚=න

𝑉

𝑠𝑐ℎ𝑓

d𝑉

Momentum conservation

d

d𝑡න

𝑉

𝜌𝑓𝐯𝑓d𝑉 + න

𝐴

𝜌𝑓𝐯𝑓 𝐯𝑓 − 𝐯𝑔 ∙ d𝐚=න

𝐴

𝐓𝑓 ∙ d𝐚 − න

𝑨

𝑝𝑓 d𝐚+න

𝑉

(𝐟𝑏+𝐬𝑚ℎ𝑓

) d𝑉

Energy conservation

d

d𝑡න

𝑉

𝜌𝑓𝐸𝑓d𝑉 + න

𝐴

𝜌𝑓𝐻𝑓 𝐯𝑓 − 𝐯𝑔 ∙ d𝐚 = න

𝐴

𝐪𝑓 ∙ d𝐚 − න

𝑨

(𝐓𝑓∙ 𝐯) ∙ d𝐚 + න

𝑉

(𝐟𝑏 ∙ 𝐯𝑓 +𝑠𝑒ℎ𝑓

)d𝑉

Basic Mathematical Model of Fluid Film

Enthalpy of liquid-solid film

𝐻𝑓∗ = 𝐻𝑓 + (1 − 𝛼𝑠

∗)𝐻𝑙𝑎𝑡

Relative solid volume fraction

𝛼𝑠∗ =

ℎ𝑠ℎ𝑓

=

1 𝑖𝑓 𝑇∗ < 0𝑓(𝑇∗) 𝑖𝑓 0 < 𝑇∗ < 1

0 𝑖𝑓 1 < 𝑇∗

𝑓 𝑇∗ is fraction solid curve and normalized temperature 𝑇∗ is defined as

𝑇∗ =𝑇 − 𝑇𝑠𝑜𝑙𝑇𝑙𝑖𝑞 − 𝑇𝑠𝑜𝑙

Film Melting and Solidification

Liquid Film

Solid Film

ℎ𝑓

ℎ𝑠

Corrections to the solid film increment

∆ℎ𝑠′ =

ℎ𝑓𝛼𝑠∗ if 𝑇 < 𝑇𝑠𝑜𝑙

0 if 𝑇 > 𝑇𝑠𝑜𝑙

Displacement of a cell face

𝐝𝑓𝑎𝑐𝑒 = 𝐾∆ℎs 𝐧𝑓𝑎𝑐𝑒

K is time factor used to speed up calculations and 𝐧𝑓𝑎𝑐𝑒 is unit vector normal to the face.

Vertex displacement 𝐝𝑣𝑒𝑟𝑡are obtained interpolating face displacements 𝐝𝑓𝑎𝑐𝑒.

Removal of Solidified Mass

Liquid Filmℎ𝑓

∆ℎ𝑠

Solidified

mass to be

removed

𝐝𝑓𝑎𝑐𝑒

Wall boundary

is moved.

Liquid Film

𝐝𝑣𝑒𝑟𝑡 𝐝𝑣𝑒𝑟𝑡

ℎ𝑓