Influence of thermochemistry on Mach reflection in hypersonic flow

Real gas thermochemistry can significantly impact the aerodynamics of hypersonic systems. For example, shock stand-off distance in front of a blunt body has been shown to depend on the degree of chemical dissociation. High temperature effects can also alter shock-shock interaction phenomena, but the degree of the modification and its consequences can be challenging to predict. Sanderson et al. experimentally investigated oblique shock impingment on a bow shock (Edney type IV configuration) in a flow with significant gas dissociation. Previous studies had suggested significant increase in heat transfer at jet impingement due to real gas effects, however, experiments showed no dependence of peak heat transfer rate on stagnation enthalpy. The influence of nonequilibrium gas chemistry on Mach and regular shock reflection has been investigated in a number of numerical studies. Burtschell et al. numerically investigated a wedge geometry located in a Mach 7 free stream, a setup similar to that used in the present experimental work. Mach stem height and hysteresis behavior was examined. Burtschell et al. found a strong dependence of transition angles, Mach stem height and location on the gas flow model. For a given wedge angle, the inclusion of real gas chemistry led to a significant decrease in Mach stem height. Chemical-vibration coupling, however, slightly increased the height of the Mach stem. Direct Monte-Carlo simulations of a shock reflection with and without real gas effects carried out by Gimelschein et al. also found a substantial effect on Mach stem height and transition angle. However, an experimental study in dissociating nitrogen and carbon dioxide, ionizing argon and frozen argon could detect no effect on the transition condition due to finite relaxation length at the conditions of the experiment. In the present work, we experimentally investigate a Mach reflection generated by two opposing wedges in a Mach 7.1 free stream. The main goal of this work is to determine directly what kinds of real gas effects occur behind a normal shock in a Mach reflection configuration for a previously selected run condition. Experiments are carried out in an expansion tube facility which is capable of simulating high enthalpy hypersonic flight conditions, and a significant degree of vibrational excitation and chemical dissociation are expected behind the normal shock. In high enthalpy gas flows, emission spectroscopy can be used to characterize the test gas composition and thermodynamic state. As impulse facilities, expansion tubes produce a challenging experimental environment for probe measurements with issues such as short test times, high temperatures and velocities, and diaphragm fragmentation. The non-intrusive nature of spectroscopy makes it an attractive technique for determining flow field properties in impulse facilities. Spectrally resolved studies have been previously used as a means towards characterizing high-enthalpy run conditions. Work completed at the X1 and X2 superorbital expansion tube facilities used emission spectroscopy to measure electron number density behind a bow shock and to identify sources of visible radiation. Time-resolved spectral methods were used in the JX1 expansion tube facility to determine the useful test time. Using the CARS technique, temperature profiles were determined for a hypervelocity blunt body flow field using the T3 shock tunnel facility. Using the free piston shock tube/tunnel facility TCM2, laser spectroscopy was used for species identification and shock front temperature profile diagnostics and spontaneous Raman spectroscopy was used to analyze the self-luminosity of nitrogen hypersonic flows for varying enthalpy conditions. In the current experiments, asymmetric wedges are used to generate a Mach stem, with a free shear layer at each triple point. Imaged spectroscopic measurements behind the Mach stem are presented. The spectra confirms flow dissociation and verifies the appropriateness of a run condition which in the future is to be used towards investigating high-temperature effects upon shear layer structure in hypersonic flow

NO and OH Spectroscopic Vibrational Temperature Measurements in a Post-Shock Relaxation Region

In this paper, spatial temperature profiles are examined in the nonequilibrium relaxation region behind a stationary shock wave in a hypervelocity air Mach 7.42 freestream. The normal shock wave is established through a Mach reflection from an opposing wedge arrangement in an expansion tube facility. Schlieren images confirm that the shock configuration is steady and the location is repeatable. Emission spectroscopy is used to identify dissociated species and to make vibrational temperature measurements using both the nitric oxide and the hydroxyl radical A-X band sequences. Temperature measurements are presented at selected locations behind the normal shock. LIFBASE is used as the simulation spectrum software for OH temperature-fitting; however, the need to access higher vibrational and rotational levels for NO leads to the use of an in-house developed algorithm. For NO, results demonstrate the contribution of higher vibrational and rotational levels to the spectra at the conditions of this study. Very good agreement is achieved between the experimentally measured NO vibrational temperatures and calculations performed using an existing state-resolved, three-dimensional forced-harmonic oscillator thermochemical model. The measured NO vibrational temperatures are significantly higher than the OH temperatures

Absorption of Energy at a Metallic Surface due to a Normal Electric Field

The effect of an oscillating electric field normal to a metallic surface may be described by an effective potential. This induced potential is calculated using semiclassical variants of the random phase approximation (RPA). Results are obtained for both ballistic and diffusive electron motion, and for two and three dimensional systems. The potential induced within the surface causes absorption of energy. The results are applied to the absorption of radiation by small metal spheres and discs. They improve upon an earlier treatment which used the Thomas-Fermi approximation for the effective potential.Comment: 19 pages (Plain TeX), 2 figures, 1 table (Postscript

The space shuttle launch vehicle aerodynamic verification challenges

The Space Shuttle aerodynamics and performance communities were challenged to verify the Space Shuttle vehicle (SSV) aerodynamics and system performance by flight measurements. Historically, launch vehicle flight test programs which faced these same challenges were unmanned instrumented flights of simple aerodynamically shaped vehicles. However, the manned SSV flight test program made these challenges more complex because of the unique aerodynamic configuration powered by the first man-rated solid rocket boosters (SRB). The analyses of flight data did not verify the aerodynamics or performance preflight predictions of the first flight of the Space Transportation System (STS-1). However, these analyses have defined the SSV aerodynamics and verified system performance. The aerodynamics community also was challenged to understand the discrepancy between the wind tunnel and flight defined aerodynamics. The preflight analysis challenges, the aerodynamic extraction challenges, and the postflight analyses challenges which led to the SSV system performance verification and which will lead to the verification of the operational ascent aerodynamics data base are presented

Diagnostic Modelling of an Expansion Tube Operating Condition for a Hypersonic Free Shear Layer Experiment

Computational simulations of the AIR-1 test condition in the University of Illinois’ Hypervelocity Expansion Tube were conducted to verify facility operation and to obtain free stream properties that are otherwise difficult to measure. Two types of simulation were undertaken. The first was a one-dimensional simulation of the entire facility and the second was a hybrid simulation, combining a one-dimensional simulation of the shock tube section with a two-dimensional simulation of the acceleration tube. The one-dimensional simulation matched the experimental data well, however the two-dimensional simulation did not initially match the experimental measurements of shock speed and test gas pitot pressure. Further investigation showed the shock speed discrepancy was consistent with air contamination into the acceleration tube and subsequent two-dimensional simulations assuming 10% air contamination showed reasonable agreement with experimental data. Using data taken from the two-dimensional simulation of the expansion tube as a transient inflow condition, modelling was undertaken of a free shear layer experiment being conducted in the facility. Results from equilibrium, finite rate, and perfect gas models were compared. The finite rate simulation provides the best agreement with experimental Schlieren images, with the simulation capturing the major flow structures seen in experiments

"How May I Help You?": Modeling Twitter Customer Service Conversations Using Fine-Grained Dialogue Acts

Given the increasing popularity of customer service dialogue on Twitter, analysis of conversation data is essential to understand trends in customer and agent behavior for the purpose of automating customer service interactions. In this work, we develop a novel taxonomy of fine-grained "dialogue acts" frequently observed in customer service, showcasing acts that are more suited to the domain than the more generic existing taxonomies. Using a sequential SVM-HMM model, we model conversation flow, predicting the dialogue act of a given turn in real-time. We characterize differences between customer and agent behavior in Twitter customer service conversations, and investigate the effect of testing our system on different customer service industries. Finally, we use a data-driven approach to predict important conversation outcomes: customer satisfaction, customer frustration, and overall problem resolution. We show that the type and location of certain dialogue acts in a conversation have a significant effect on the probability of desirable and undesirable outcomes, and present actionable rules based on our findings. The patterns and rules we derive can be used as guidelines for outcome-driven automated customer service platforms.Comment: 13 pages, 6 figures, IUI 201

Do Hadronic Charge Exchange Reactions Measure Electroweak L = 1 Strength?

An eikonal model has been used to assess the relationship between calculated strengths for first forbidden beta decay and calculated cross sections for (p,n) charge exchange reactions. It is found that these are proportional for strong transitions, suggesting that hadronic charge exchange reactions may be useful in determining the spin-dipole matrix elements for astrophysically interesting leptonic transitions.Comment: 14 pages, 5 figures, Submitted to Physical Review

Spectroscopy of 13B via the 13C(t,3He) reaction at 115 AMeV

Gamow-Teller and dipole transitions to final states in 13B were studied via the 13C(t,3He) reaction at Et = 115 AMeV. Besides the strong Gamow-Teller transition to the 13B ground state, a weaker Gamow-Teller transition to a state at 3.6 MeV was found. This state was assigned a spin-parity of 3/2- by comparison with shell-model calculations using the WBP and WBT interactions which were modified to allow for mixing between nhw and (n+2)hw configurations. This assignment agrees with a recent result from a lifetime measurement of excited states in 13B. The shell-model calculations also explained the relatively large spectroscopic strength measured for a low-lying 1/2+ state at 4.83 MeV in 13B. The cross sections for dipole transitions up to Ex(13B)= 20 MeV excited via the 13C(t,3He) reaction were also compared with the shell-model calculations. The theoretical cross sections exceeded the data by a factor of about 1.8, which might indicate that the dipole excitations are "quenched". Uncertainties in the reaction calculations complicate that interpretation.Comment: 11 pages, 6 figure

Expansion Tube Investigation of Shock Stand-Off Distances in High-Enthalpy CO_2 Flow Over Blunt Bodies

The shock standoff distance in front of a blunt body is sensitive to the thermochemical state of the free stream. Recently, experimental and numerical studies have reported significantly different bow shock profiles in high-enthalpy carbon dioxide flows, a discrepancy that may result from non-equilibrium processes during flow acceleration in ground-based facilities. In this work, an expansion tube is used to create a Mach 5.7 carbon dioxide flow, matching the stagnation enthalpy and the velocity of previous studies. Images of shock layers are obtained for spherical geometries and a scaled model of the Mars Science Lander. Different sphere diameters are used in order to access non-equilibrium and equilibrium stagnation line shock profiles predicted by theory. Mars Science Lander profiles at zero angle of attack are in good agreement with available data from the LENS X expansion tunnel facility, confirming results are facility-independent for the same type of flow acceleration, and indicating the flow velocity is a suitable first-order matching parameter for comparative testing. Heat transfer measurements on the Mars Science Lander are also presented for the three different angle of attacks, and the results are consistent with previous studies. Initial results from a proposed organo-metallic based emission spectroscopy technique for bow shock layer interrogation are also presented