Speaker
Description
Introduction
The study of thermochemical non-equilibrium effects in hypersonic flows is valuable for the development of predictive tools for atmospheric entry vehicles and high-speed propulsion systems. Facilities like the Electric Arc Shock Tube (EAST) at NASA Ames Research Center play a vital role in investigating these phenomena by generating high-enthalpy flows. Recent experiments conducted by Tibère-Inglesse et al. at EAST with an expanding nozzle revealed notable discrepancies between computational fluid dynamics (CFD) predictions and experimental measurements of radiance. Underpredictions were observed in both the ultraviolet (UV) and visible spectra, attributed to inaccuracies in modelling N$_2$$^+$ densities and limitations in the kinetic models used for number density and temperature predictions.
Previous shock tube experiments in air at 10 km/s have demonstrated shock deceleration effects. This raises questions about the accuracy of current simulation methodologies and their ability to capture the complex flow physics present in shock tube experiments.
Recent studies by Collen et al. and Satchell et al. have emphasised the critical role of shock speed attenuation in determining test slug properties. Their research demonstrates that variations in shock speed can cause substantial changes in pressure, temperature, and species densities within the test gas. To address these complexities, the Lagrangian Shock Tube Analysis (LASTA) 2.0 code has been recently developed. This new code incorporates non-equilibrium thermochemistry effects, offering a more comprehensive tool for analysing shock tube flows.
This work aims to address whether shock speed variation significantly effects predictions of expanding flow experiments previously conducted in the EAST facility. The intended outcome of this work is to develop the understanding of high-enthalpy flow physics and improve the accuracy of computational models used in hypersonic research and design, ultimately contributing to the advancement of hypersonic technologies. To investigate shock speed variation effects, the Eilmer CFD code is used as the simulation platform, with additional input conditions derived from the Non-Equilibrium Shock Solver (NESS) and LASTA to account for non-equilibrium effects in the shock tube prior to the expanding nozzle section. Experimental data from EAST Test 63 will be presented for comparison, which used shock velocity targets of 10 km/s and 11 km/s at the nozzle entrance. Shock speeds within the expanding nozzle section will be compared, along with the use of NASA's NEQAIR radiation analysis code to compare simulation results with experimental measurements of radiance and electron number density from EAST.
Current work has been focused on refining the CFD setup with input conditions from NESS to ensure high-quality numerical results. Further simulations with LASTA conditions will commence shortly after, followed by analysis of these datasets with EAST data.
Experimental Overview
Facility Description
The EAST facility has been described by Cruden, with further details relevant for Test 63 from Tibère-Inglesse. Some details are repeated here for completeness.
The facility used a driver section with a trigger wire and pin assembly connected to a 1.3 mF capacitor bank storing 1.2 MJ of energy with a 40 kV electric potential.
An electric arc across the wire and pin initiates current flow as high as 1 MA to the ground electrode, heating the driver gas of choice to ultimately generate a shock wave downstream. The driver assembly was connected to an aluminium tube with a diameter of 101.6 mm. An expanding nozzle with a length of 1.9 m and a half-angle of 10 degrees was connected to the driven tube, with the nozzle entrance located 12.5 m downstream of the primary diaphragm. The nozzle diameter linearly expanded from 101.6 mm at the nozzle entrance to 762 mm at the nozzle exit. Rectangular window ports of length 148.6 mm are situated 292.3 mm downstream of the nozzle entrance, creating a parallel-wall section in the horizontal plane only for the length of the ports. These ports were coupled to four spectrometers, each capturing a different wavelength region, corresponding to Vacuum Ultraviolet ("VUV", 120-200 nm), UV/Visible ("Blue", 200-500 nm), Visible/Near-Infrared ("Red", 500-900 nm), and Mid-Wave-Infrared ("IR", 1600-5500 nm), respectively. Piezoelectric transducers (PCB 132A) were mounted along the length of the shock tube to measure the shock velocity with respect to time, enabling the calculation of the shock velocity at the nozzle entrance. A pitot rake was mounted 517 mm downstream of the nozzle entrance to measure the shock profile. The shock velocity at the window is calculated the shock positions with respect to time measured at 60 mm and 430 mm from the nozzle entrance, using a PCB and/or the pitot rake.
Test Conditions
EAST Test 63 focused on conditions relevant for lunar return missions. Shock velocities of 10 km/s and 11 km/s were targeted, with a free-stream pressure of 0.2 Torr. The test gas used was synthetic air. Test conditions varied from the target conditions due to variability associated with the electric arc system.
Numerical Overview
CFD simulations of the EAST facility with the expanding nozzle are currently in progress. The flow domain considered consists of the expanding nozzle, the test window section, and a section of the shock tube upstream of the expanding nozzle entrance to allow for appropriate flow development. The numerical setup also considers a 2D axisymmetric grid. Two grids will be considered; one with the test window section and one without, reproducing two extremes of the real geometry. Current simulations are being conducted using Eilmer 4 with input conditions from NESS to model a steady shock entering the shock tube with non-equilibrium thermochemistry and shock curvature. Beyond this, input conditions from LASTA will be used to model shock speed variation in the test slug. The completion date is expected to be in August 2024 for the simulations.
Summary
This paper investigates the effects of shock speed variation on expanding flow experiments conducted in NASA's Electric Arc Shock Tube (EAST) facility. Recent experiments with an expanding nozzle in EAST revealed discrepancies between computational fluid dynamics (CFD) predictions and experimental measurements of radiance. To address these discrepancies, this study employs the Eilmer CFD code with input conditions from the Non-Equilibrium Shock Solver (NESS) and Lagrangian Shock Tube Analysis (LASTA) code to account for non-equilibrium effects and shock speed variations. The research focuses on EAST Test 63, which targeted shock velocities of 10 km/s and 11 km/s in synthetic air. Simulations will compare shock speeds within the expanding nozzle section and use NASA's NEQAIR radiation analysis code to compare results with experimental measurements of radiance and electron number density. This work aims to improve understanding of high-enthalpy flow physics and enhance the accuracy of computational models used in hypersonic research and design.
