Leveraging STAR-CCM+ for Aircraft Applications. Durrell Rittenberg, Ph.D.

Transcription

1 Leveraging STAR-CCM+ for Aircraft Applications Durrell Rittenberg, Ph.D.

2 Overview of Icing with STAR-CCM+ 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

3 Icing in the aerospace sector Ice build-up results in significant changes to the aerodynamics of the vehicle This degrades the performance and controllability of the aircraft Ground Icing In Flight Icing

4 Aircraft Icing is a real danger ATR-72: Roselawn, IN; October 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 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 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 fatalities, hull loss NTSB findings: probable cause was captain s inappropriate response to icing condition ATR-72 Tyumen Russia February fatalities, no hull loss Crashed after take-off due to icing

5 What are the motivators for our icing capabilities? 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 buildup on critical aerodynamic surfaces

6 The FAA recently extend the icing certification requirements [Nov 2014] 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

7 Project Cost Typical design and compliance process Low cost fast turn around CFD and Icing Simulation Project Timeline High cost 8 month lead time Icing tunnel testing Very high cost requires build Natural flight testing

8 Project Cost Typical design and compliance process Low cost fast turn around CFD and Icing Simulation Project Timeline High cost 8 month lead time Leverage Icing Simulations Early in the design cycle Icing tunnel testing Reduce cost Natural flight testing

9 The basics of ice accretion and aircraft icing

10 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 500 mm mm Freezing Rain Freezing Drizzle Micro-ice crystals

11 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

12 Effect of icing on airfoil performance 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 Data from NASA Glenn Icing Tunnel

13 Aircraft Icing considerations widespread

14 Ice Accretion Simulation Considerations The Physics of Ice Accretion with formulations

15 CHT DMP/LMP CFD Icing simulation is the joining of multiple physics regimes Single-shot method [Low cost] Multi-shot method [Medium cost] Fully unsteady flow field [High cost] Determine the Flow Physics DMP: Dispersed Multiphase droplets [Low cost] LMP: Lagrangian Multiphase droplets [High cost] Compute water droplet trajectories, giving collection efficiency Fluid film model Evaporation and condensation Solidification models Mesh morphing Determine thermodynamic balance at the wall and compute local ice accretion rate Grow delta ice shape

16 CFD Icing simulation is the joining of multiple physics regimes Determine the Flow Physics 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 modes 1. 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]

17 Icing simulation is the joining of multiple physics regimes Droplet Modeling Compute water droplet trajectories, giving collection efficiency 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

18 Icing simulation is the joining of multiple physics regimes CHT Determine thermodynamic balance at the wall and compute local ice accretion rate 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.

19 Determining 3D collection efficiency (β) External Airflow Super-Cooled Droplets Collection Efficiency (β): The measure of a configuration's ability to capture incoming water defined as the local mass flux velocity. β = αρ wu i A i LWC U A STAR-CCM+ efficiently determines collection efficiency α = volume fraction of water ρ w = density of water u i = velocity of air LWC = liquid water content U = speed of the free stream A i = local area normal

20 Collection efficiency validation of 747 inlet Collection efficiency computed on slices around the nacelle Data collected as a function of surface distance from the LE

21 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 Impingement The physics of fluid films in STAR-CCM+ Multi-component gas considerations Condensation Eulerian Multi-Component Gas Evaporation Gravity

22 The physics of melting and solidification for ice accretion Within a timestep, iteratively finds the mass that freezes: Computing a relative solid volume fraction 0 above K or 1 below K 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 K (glaze conditions) Morph the solid boundary according to newly formed ice Animated Detail of Morphed Mesh

23 Ice accretion validation for Rime ice conditions Commercial Transport Airfoil + Mach = Airspeed = 279 kts + T static = C + α = LWC = g/m 3 Both LMP and DMP methods were considered and show high agreement with both LEWICE and icing tunnel test data (shown here) 2D CT Airfoil Run 112: 6 Minutes Several 3D airfoil cases were run and compared to 2D crosssectional test data with high agreement 2D CT Airfoil Run 142: 2 Minutes 2D CT Airfoil Run 107: 22.5 Minutes

24 Preliminary ice accretion validation for Glaze ice conditions 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 Run 144 Chord 0.9 m Airspeed 130 m/s AoA 0.7 deg T static o C 0.4 g/m 3 LWC Accretion time 180 s MVD 42μm Run 124 Chord 0.9 m Airspeed 130 m/s AoA 0.7 deg T static o C g/m 3 LWC Accretion time 294 s MVD 21μm Reference: Harrold E, Addy, Jr, Ice Accretions and Icing Effects for Modern Airfoils, NASA/TP

25 Ice accretion capabilities in STAR-CCM+ LEVEL 1 - Measuring Collection Efficiency LEVEL 2 Predicting the Ice Shape Collection Efficiency External Airflow Super-Cooled Droplets External Airflow Super-Cooled Droplets Ice Shape Prediction Fluid Film Film Solidification (morpher)

26 STAR-CCM+ unified process for ice accretion Flowfield (3D Navier-Stokes) Dispersed Phase Single Shot Multi-Shot Fully Transient Fluid Film Freeze/Melt Update Ice Shape Mesh Morph / Remesh

27 Physics of ice accretion in STAR-CCM+ 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)

28 Review Summary 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

29 Appendix and other technical details

30 Basic Mathematical Model of Fluid Film Mass conservation d dt න V ρ f dv + න A ρ f v f v g da = න V s c h f dv Momentum conservation d dt න V ρ f v f dv + න A ρ f v f v f v g da = න A T f da න A p f da+ න V (f b + s m h f ) dv Energy conservation d dt න V ρ f E f dv + න A ρ f H f v f v g da = න A q f da න(T f v) da + න(f b v f + s e )dv h f A V

31 Film Melting and Solidification Enthalpy of liquid-solid film H f = H f + (1 α s )H lat Relative solid volume fraction α s = h s h f = h f h s 1 if T < 0 f(t ) if 0 < T < 1 0 if 1 < T Liquid Film Solid Film f T is fraction solid curve and normalized temperature T is defined as T = T T sol T liq T sol

32 Removal of Solidified Mass Corrections to the solid film increment h s = h fα s if T < T sol 0 if T > T sol Displacement of a cell face d face = K h s n face K is time factor used to speed up calculations and n face is unit vector normal to the face. Vertex displacement d vert are obtained interpolating face displacements d face. h f h s h f Solidified mass to be removed Wall boundary is moved. d vert Liquid Film d face Liquid Film d vert