DURGESH CHANDEL

University of Minnesota

August 10th, 2017

Entry Systems and Technology DivisionNASA Ames Research Center

Computations of high enthalpy

shock-waves in EAST using US3D

Motivation

• High-speed entry vehicles experience immense surface-heating and ablation.

• Radiative heating is important, requires knowledge of gas-state and reaction kinetics.

10.16 cm dia. (blue)

Test section at 8 m

76 cm dia. (orange)East operational range:

V = 1.3 - 46.0 km/s

P = 0.002 - 76.87 psia

High-fidelity computations are required to interpret EAST data.

Typical conditions:

M = 23-32

V = 8-12 km/s

P = 3.87×10-3 psia

h0 = 33-60 MJ/kg

Shock-tube experiments inform about,

• Spectroscopic and Kinetics models

• Validation and uncertainty quantification for flight data

all images from nasa.gov

Earth

2

EAST flow

• Current interpretation: Flow over blunt-body

• Previous attempts

• Improved approach in US3D

Fluid Solver results

• Early evolution of shock

• Set-up for Radiation Calculations

Radiation solver results

Summary and Path forward

Contents

EAST: Flow over blunt-body

• Shock-deceleration and varied shock-strength causes strong non-equilibrium.

• Experimental data deviates from numerical predictions at certain conditions.

• Stagnation streamline analogous to shock-tube centerline.

• Shock-speed is matched with an appropriately chosen shock stand-off distance.

4

us d

Flow over cylinder

us =10 km/s

Test section

Shock-tube flow

10.4 km/s10.6 km/s10.7 km/s

~ 1.5m

d

Air-shocks

EAST: Previous Attempts

Contact-front lacks thermal equilibrium

7 days for 2m shock-travel on 2000 CPUs

• Kotov et al.(JCP 2014), 1T model, 2D duct

• Low-dissipation, high order shock-capturing

9.86 km/s Equilibrium

Shock

Front

Contact

Front

• Barnhardt et al., attempted full facility DPLR

simulation, 2009

• Aggressive CFL ramping caused instability

Radiance monotonically decreases behind shock.

Lower CFL cases in DPLR also get unstable.

COOLFluiD simulations are stable but expensive

(Bensassi, K.)

Time-accurate EAST simulations are expensive.

5

Ionization-fraction, near Test-section

tube d

ia.

x= 6.85mx= 6.8m

EAST: Improved Approach in US3D

Goal: 2D axi-symmetric flow simulations w/ real-gas effects.

(time-accurate yet computationally feasible!)

Required modifications:

• Implicit system of equations needs higher accuracy than stagnant shock-problems.

(more ‘kmax’ sub-iterations in computing Flux-Jacobians in each time-step)

• Catalytic recombination BC at wall

• Moving Grid and/or Numerical interpolation introduces errors.

• Moving frame-of reference? Frame-acceleration requires additional modeling.

6

Shock-frame is the best framework for moving-shock problems in US3D.

Moving-frame w/ constant frame-speed:

1. US3D code is heavily tested and verified for stagnant bow-shock problems.

2. Shock-frame calc. found to be more accurate than Lab-frame calc.

(moving-shock in perfect gas at M = 3, 5, 14)

3. Standing shock simulations at M = 20 keep the shock stagnant as expected.

Numerical set-up: Shock-frame calc.

Flux scheme 1st order MSW

Time-integration 1st order implicit FMDP

Frame-of-reference Shock-frame

Grid size Variable grid, 1.8 M

Grid resolution (min.) Δx = 10 µm near shock

Δr = 1 µm near wall

99.9% He + N2 79% N2 + O2

1.10546 kg/m3 3.096d-04 kg/m3

6000 K 300 K

Wave motion in shock tube at time t>0

7

The whole frame is moved with a constant speed close to the shock-speed

Driver gas (x<0) Driven gas (x>0)

r (m

)

x (m)Shock starts at x=0 at time, t = 0

Fluid Solver Results: US3D

Early evolution of shock

9

r (m

)

x (m)

• Shock-deceleration profile consistent w/ EAST.

• Translational and Vibration-electronic modes relax with

time.

r (m

)

x (m)

Shock-velocity

Translational Temperature

Shock-deceleration is stronger in 2nd order flux-scheme

Flow-field at later time

10

Axial-profiles are well-behaved near regions of strong gradients

shock-front

contact-surface

Centerline profiles

Lab-frame

Shocked-gas

~ 5cm wide

r (m

)

Set-up for Radiation Calculations

• Shock reached at ~1.29m

• Shock-front is kept in the

refined region by changing

the frame-speed.

Us = 10.02 km/s

• Thermo-chemical equilibrium is not fully

established.

• Radiation properties would be different

than the measurements at the Test-section.

11

t = 125µs LOS extraction using Bala’s code, Shock-frame

y (

m)

d

shock-front

contact-surface

Te

mp

era

ture

(K

)S

pe

cie

s m

ass-f

rac.

Radiation Solver Results: NEQAIR

VUV Radiation

LOS on Temperature contours, Shock-frame

d

EAST spectrum of interest

VUV = 120-215 nm

UV/Vis = 190-500 nm

Vis/NIR = 480-900 nm

IR = 700-1650 nm

13

Qualitative behavior is similar.

experiment

Us =10.3 km/s

Us =10 km/s

Radiation Spectra

Us = 10.02 km/s

P = 0.2 Torr

14

d = 2.48 cm

experiment

Us = 9.98 km/s

Vis/NIRUs =10.27 km/s

UV/Vis

Us = 9.98 km/s

IR

Summary

Progress in Summer:

• Improved approach in US3D gives stable solution w/ real-gas effects.(shock-front has to be kept in the refined region)

• Flow solution and Radiation profiles are consistent w/ test.

Path Forward:

• Propagate the shock till test-section. (Projected time: 14 days, 480 CPUs; 8 times cheaper than Kotov et. al.)

• Resolve BL features

• Include capability for variable frame-speed in US3D.

• Validate results w/ test-data.

Credits

Mentoring and Support

• Dr. A. Brandis, Dr. B. Cruden, Dr. D. Hash; (NASA Ames)

• Prof. G. V. Candler, Dr. I. Nompelis; (UMN)

Valuable discussions

• Dr. K. Bensassi, Dr. J. Schulz, Dr. R. L. Jaffe; (NASA Ames)

• Ames co-interns: Narendra, Bala, Maitreyee and others.

Questions?