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URANS CFD Simulations of Scramjet Flow Path Transients

CCAS Review – May 2015

Logan Riley and Datta V. Gaitonde

AFRL Contacts: Dr. Jeff Donbar and Dr. Mark Hagenmaier

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Bio •  Education

– B.S. Mechanical Engineering from U. Akron [Spring 2012] – Aerospace PhD Program [Fall 2013-Fall 2017 (expected)]

•  DAGSI Fellowship [Fall 2014-Present] – AFRL sponsor: Dr. Jeff Donbar – AFRL intership: Summer 2015

• Research interests: – Hypersonic air-breathing propulsion – Roughness-induced transition

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Background: Mode Transition • Unsteady RANS (URANS) used to explore mode transition in the

HIFiRE-2 scramjet (Yentsch et al. 2013, 2015)

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Image reproduced from Jackson et al., 2011 http://www.nasa.gov/topics/aeronautics/ features/hifire.html

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Background • Unsteady RANS (URANS) used to explore mode transition in the

HIFiRE-2 scramjet (Yentsch et al. 2013, 2015)

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Image reproduced from Jackson et al., 2011 http://www.nasa.gov/topics/aeronautics/ features/hifire.html

Mode-Transition

Wikipedia

Reproduced from Yentsch 2013

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Background • Unsteady RANS (URANS) simulations used to explore mode

transition in the HC-fueled HIFiRE-2 scramjet flowpath (Yentsch et al. 2013, 2015)

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Image reproduced from Jackson et al., 2011 http://www.nasa.gov/topics/aeronautics/ features/hifire.html

HDCR mode-transition characterized by loss of flame-holding in inboard primary-injectors (PI)

Dual-Mode Scramjet-Mode

Reproduced from Yentsch 2013

Flight mode-transition preserves flame-holding in inboard PI because of inlet distortion

Ground Test Flowpath (HDCR), Primary Injectors

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Objectives •  Extend (URANS) capabilities to understand evolution of unstart

events in a hydrocarbon-fueled, dual-mode scramjet • Want to capture large scale transients

–  Identify precursors to unstart – Quantify SBLI sensitivity to variations in fuel input (variable fuel flow rates)

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Can unstart be anticipated by measuring away from the wall?

Can quantities other than pressure be used to predict unstart?

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AFRL Flowpath •  CFD work based on experiments of Donbar et al. 2010 •  Ethylene-fueled combustor •  1kHz wall pressure measurements •  Different fuel-staging conditions

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Reproduced from Donbar et al. 2010

Isolator Pressure Measurements

Experimental Flowpath

Data-rich experiments selected to ground simulations

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AFRL Flowpath •  CFD work based on experiments of Donbar et al. 2010 •  1kHz wall pressure measurements •  Ethylene-fueled combustor •  Different fuel-staging conditions

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Reproduced from Donbar et al. 2010

Experimental Flowpath: Injector Detail

Hybrid mesh necessary to resolve detailed flowpath geometry

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Operating Conditions Constant mass flow rate

• Two operating points – Fixed equivalence ratio – For validation of numerical

approach

Variable mass flow rate

• Ramp from baseline to unstart fueling conditions to study unstart process

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Injector Set Baseline Unstart

B2 (body-side) 0.20 0.40

B6 (body-side) 0.33 0.24

C3 (cowl-side) 0.37 0.26

Fuel-staging selected to highlight primary phenomena within parameter space

Equivalence Ratio

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Metacomp, Inc.’s CFD++ •  Unified grid architecture •  Coupled multi-physics •  Min-mod TVD limiter •  HLLC Riemann Solver

Turbulence Closure •  Cubic k-epsilon

Chemistry •  Finite-rate kinetics •  Taitech-Princeton (TP2)

Ethylene mechanism Mesh Development •  Hybrid structured/unstructured •  Half symmetry •  O(5M) Cells

Models successfully employed in previous mode-transition studies

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Validation • Mean isolator pressure distributions used to validate numerical

approach – Fueled (tare) – Unfueled

• Calibrate modeling parameters – Grid convergence study, O(2-7)M cells – Turbulent Schmidt number calibration (fueled conditions), 𝑆𝑐↓𝑡 ∈0.5−0.9 – Wall heat-transfer:

›  Adiabatic ›  1-D resistance model

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Tare mode operation, cowl-side pressure distribution

Validation: Tare Operation

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Mesh resolution sufficient to resolve shock train

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Fueled operation: Isosurface of M=1 colored by height, with fuel injection streamlines

Validation: Fueled Operation (Baseline) •  Isolator pressure

distribution: – Over-predicted in initial

simulations ›  Sensitive to 𝑆𝑐↓𝑡  ›  Wall treatment

•  Bias of supersonic fluid: – Result of inlet

distortion/side wall flow-separation

–  Similar to work of: ›  Gaitonde, et al. 2003 ›  Yentsch et al. 2013

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H  is  the  height  at  the  start  of  the  isolator

Examining 3-D flowfield helps identify discrepancies between fueled validation simulation and experiment

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H  is  the  height  at  the  start  of  the  isolator

𝐼𝑠𝑜𝑠𝑢𝑟𝑓𝑎𝑐𝑒  𝑜𝑓  𝑀=1   𝑐𝑜𝑛𝑑𝑖𝑡𝑖𝑜𝑛  𝑐𝑜𝑙𝑜𝑟𝑒𝑑  𝑏𝑦  ℎ𝑒𝑖𝑔ℎ𝑡,   𝑤𝑖𝑡ℎ  𝑠𝑢𝑟𝑓𝑎𝑐𝑒  𝑜𝑖𝑙  𝑓𝑙𝑜𝑤𝑠

Preliminary Transient Results

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𝐽↓𝑚 : mass flux 𝐽↓𝑚,𝑚𝑎𝑥 : baseline

Initial results indicate sensitivity of unstart to boundary layer thickness

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Looking Ahead • Explore upstream effect of combustor in dual-mode operation • Connect measured pressure variations to flow structures • Understand influence of inlet distortion on different flow paths (e.g. HIFiRE 2) • Leverage analysis tools (e.g. POD, DMD) to identify fluid structures of interest • Explore application of hybrid LES/RANS methods

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Q&A

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HIFiRE-2 Experiments Ground (HDCR) Configuration Flight Configuration

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Reproduced from Jackson et al., 2008 Reproduced from Gruber et al., 2008

Difference in inlet geometry significantly affects flow field

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HIFiRE-2 Experiments Ground (HDCR) Configuration Flight Configuration

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Reproduced from Jackson et al., 2008 Reproduced from Gruber et al., 2008

Primary (x8)

Secondary (x8)

Outboard

Inboard

Outboard

144 static pressure ports 19 surface thermocouples

4 heat flux gauges

Ground test pressure data used for validation

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Inlet Distortion Effects (HIFiRE 2)

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Low energy region near centerline preserves inboard PI flame-holding in flight

Reproduced from Yentsch et al. 2013

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Inlet Distortion Effects (Double Fin)

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Reproduced from Gaitonde et al., 2003

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Fueled operation: Isosurface of M=1 colored by height, with fuel injection streamlines

Combustion Operation (Baseline) •  Isolator pressure distribution:

–  Sensitive to grid refinement near injectors

–  Sensitive to 𝑆𝑐↓𝑡  •  Bias of supersonic fluid

–  Result of inlet distortion/side wall flow separation

–  Similar to work of: ›  Gaitonde, et al. 2003 ›  Yentsch et al. 2013

–  Influence on fuel penetration

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Combustion Operation (Baseline)

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M>1 Flow suppresses injector penetration