Introduction
Thesis outline
Experimental measurements obtained in the two facilities are compared with stagnation streamline theory and simulations considering three reaction rate models. Measurements of the fuze in optically thick conditions, measurements of the expansion tube in optically thin conditions at various model orientations, and direct measurements of the free flow are reported.
Historical Background
- Standoff Distance in High Enthalpy CO 2 Flows
- CO 2 Radiative Heating
All investigated models predicted the flow to be close to vibrational equilibrium in the test section. For the MSL mission, consideration of radiation reduced the error in the forebody heat load by 33%.
Theoretical Background
- Bow Shock Standoff Distance
- Stagnation Streamline Modeling
- Radiative Processes
The freestream conditions are extracted from the nozzle simulation output on the exit face. The experimental measurement and simulation results of the leeward side MSL1 condition for the outer radii of the collection volume are shown in Figure 5.21.
Facilities
Comparison of the Facilities
Having access to both the T5 reflected shock tunnel and the hypervelocity expansion tube (HET) enables a greater range of conditions and services to be achieved. If the flow is optically thin, there is no absorption and the emitted radiation is imaged without interference from the surrounding flow.
T5 Reflected Shock Tunnel
- T5 Freestream Calculations
- T5 Experimental Input Sensitivity Analysis
The primary shock velocity for the reservoir calculation is extracted from the arrival time of the primary shock traveling 2.4 m between stations 3 and 4 (Figure 2.4). The tank pressure is the average of the two pressures located directly upstream of the end wall of the nozzle (Figure 2.5).
Hypervelocity Expansion Tube (HET)
- HET Freestream Calculations
- Vibrational Nonequilibrium in the Expansion Fan
- Sources of Freestream Uncertainty
The free stream conditions for the HET are calculated using the MATLAB Expansion Tube Solver (METS). For the MSL1 to MSL4 conditions, the free stream condition uncertainty due to the accelerator initial pressure is 8.2%.
Experimental Freestream Diagnostics and Measurements
- Fast Response Surface Mounted Thermocouples
- Pitot Measurements
The solution for the rise of the surface temperature with respect to the heat flux is given by the convolution integral. LAURA: Totally catalytic LAURA: Super catalytic. a) Experimental heat flux of Shot 1491 MSL1 compared to simulated heat flux. A comparison of experimental heat flux measurements is made with heat flux obtained from LAURA [1] simulations with a fully catalytic recombination model at Wall and Johnston chemical rates.
Conclusions of the T5 and HET Characterization Efforts
To prevent diffraction of the laser light, a step cut-off Schlieren filter is used, as shown in Figure 3.4 [101]. The integrated emissivity for the shock tube condition with no appreciable absorption in the boundary layer at the model surface, ST0_4, is within 5% of the integrated simulated emissivity. These locations are shown in Figure 6.6 along with the receiver cones of the forward fuselage and wake probes.
Numerical and Experimental Setup
Numerical Setup
- Reacting Flow Simulations: LAURA
- HARA
Johnston's rates are the default CO2 kinetic rates implemented in the LAURA 2016 version used in this work. In the optically thin limit,αλx→ 0, the radiative transport equation simplifies to Iλ,thin(0)=. Using the tangent plate approximation, the radiation transfer equation for an obliquely incident beam can be written as. 3.9) In terms of equations 3.8 and 3.9, the radiation from an obliquely incident ray at the optically thin limit is related to the radiation from the normal ray from.
Experimental Setup
- Facility Data Acquisition System
- Model Geometries
- Schlieren Flow Visualization
- Spectrally Resolved Mid-wave Infrared Radiation
- Spectroscopic Calibration Techniques
The numerical aperture can be related to the acceptance half-angle or critical angle of the fiberθc, given by NA = n sinθc. Care is taken to remove the blackbody source quickly to prevent heating of the fiber. The number of counts detected on the camera Nλ can be related to the total heat flux via the sensitivity of the system Sλ, given by .
Introduction
- Freestream Conditions
- Simulations
- Grid Sensitivity Study
- Standoff Distance Measurements
The final sphere meshes have 64 cells in the plane of symmetry and 256 cells in the wall normal direction. The final sphere-cone meshes have 32 cells in the plane of symmetry and 197 cells in the wall normal direction. The experimental stopping distance is defined as the location of the peak intensity in the images, corresponding to the maximum density gradient measured with the Schlieren technique [ 59 ].
Conical Nozzle Correction
Conical inlet flow divergence reduces the shock distance by a factor approaching unity as L0 approaches infinity. Considering the curvature change, the standoff distance correction factor from conical to parallel flow for sphere cone impacts can be expressed as. The uncertainty in the shock curvature is calculated in the experimental uncertainty of the stand distance using the error propagation formula.
The Sphere
For the low-pressure state of T52889, all three simulations are within the measured uncertainty bounds on the experimental offset distance. The high-pressure T5 experiment with a larger binary scaling parameter of 2.5 showed notable differences in the numerical distance between the different chemical reaction models. The experimental limits of the offset distance are within the numerical offset distance using the Cruden mechanism.
The Spherically-blunted Cone
Hornung et al. have recently derived a unified analytical expression for the shock offset distance as a function of two parameters for flow over a sphere-cone geometry. Sphere-cone experiments and simulations with three different kinetic mechanisms are directly compared with the results summarized in Table 4.6. In experiment T52892, the experimental measurement of the 7" sphere-cone is corrected using a correction factor of 1.282, Section 4.2.
Stagnation Streamline Analysis
- Results of the Stagnation Streamline Analysis
The cumulative invisible contributions are calculated by integrating the density profiles resulting from individual contributions shown in Figure 4.22 from the end of the numerical shock region xs/2 to the edge of the boundary layer. The stagnation streamline analysis is used to show the relative importance of the contributions to the average density for comparison of different chemical reaction models. The combination of the numerical shock region, vibrational non-equilibrium and convection contributions to the total mean density are within ¯ρ/ρ∞= 0.2 for the three different chemistry models.
Conclusions
Pitot pressure traces are shown in Figure 5.4 for the MSL1 condition and in Figure 5.5 for the MSL2 condition. Two replicate measurements of the 0◦AOA wake are obtained for the ExoMars state, as shown in Figure 6.10. Direct free stream measurements resulted in integrated radiation ratios of 2.51 and 3.41 for the MSL1 and MSL2 conditions, respectively.
HET CO 2 Radiation Measurements
Introduction
- Freestream Conditions
- Experimental Setup
- Experimental Timing
- Pitot Measurements
The predicted free stream conditions for the corresponding shock tube and expansion tube conditions are shown in Table 5.2. The pitot probe is located directly below the model or at the center line for the free stream probe. Direct free stream measurements are obtained at two different camera exposure time windows for the MSL1 and MSL2 conditions.
Shock Shape Measurements
The black dotted lines represent the exposure time limits of the freestream data acquisition of 70-150 µs for the MSL1 condition and 50-130 µs for the MSL2 condition. The corresponding measured pitot pressures are 158.4 kPa±5.8 kPa for the MSL1 condition and 87.8 kPa±5.5 kPa for the MSL2 condition. The 16◦ AOA schlieren images are shown for the MSL1 condition, Figure 5.6a, and MSL2 condition, Figure 5.6b.
Shock Layer Simulations
- Accounting for Freestream Radiation
- Shock Tube 16 ◦ AOA Stagnation Point Measurements
- Expansion Tube 16 ◦ AOA Stagnation Point Measurements . 121
These simulations are compared with the experimental measurements and with simulations that do not take into account the expansion fan in the probe's line of sight. For the MSL1 condition, the peak spectral irradiance increases by 2.2% and 5.0% for the fan simulations at the beginning and end of the measurement, respectively, compared to the baseline simulation that did not include the expansion fan. For the MSL2 condition, the peak spectral irradiance increases by 7.8% and 15.5% for the fan simulations at the beginning and end of the measurement, respectively, compared to the baseline simulation that did not include the expansion fan.
Expansion Tube 16 ◦ AOA Lee Side Measurements
- Tube-Wall Boundary Layer in the LOS
This is consistent with the radiation measurement showing poorer agreement for the 0.254 m beam simulation of the lee side (25.1% underprediction) compared to the 0.2m beam simulation of the stagnation point (14.5% underprediction). Predictions in Section 2.3.3 using Mirel's theory calculated a boundary layer size of 7.1 mm and 8.2 mm for the MSL1 and MSL2 conditions, respectively. Using Crocco-Busemann relations, a temperature profile is estimated and a 25 mm laminar boundary layer is projected onto the 0.095 m beam for the MSL1 mode radiation measurement.
Expansion Tube 0 ◦ AOA Measurements
This simulation increases the peak spectral radiance by 7.6% compared to the simulation without a wall boundary in the line of sight. The larger difference for the 0◦ AOA stagnation measurements compared to the 16◦ AOA stagnation point measurements (7.1% and 15.2%) can be attributed to less shock layer absorption for the 0◦ AOA stagnation point measurements, Section 5.9. The peak emissivity calculated assuming the Fridman mechanism is less than the Johnston calculations by 10.9% and 21.9% for the MSL1 and MSL2 conditions, respectively.
Expansion Tube Freestream Probe Measurements
- Delayed Freestream Measurements
- Freestream Radiation Sensitivities to Pressure and Temper-
The later exposure time window captures a portion of the less stretched gas observed from pitot tracks starting 188 µs and 160 µs after arrival at the contact surface for the MSL1 and MSL2 conditions, respectively. With temperature held constant at the nominal free stream temperature, the sensitivity to free stream pressure is shown in Figures 5.32 and 5.33 for the MSL1 and MSL2 conditions, respectively. The free-stream radiation sensitivity to temperature is shown in Figures 5.32 and 5.33 for the MSL1 and MSL2 conditions, respectively.
Simulations Assuming a Reflected Shock Processed Free Stream
In the optically thin limit, the pressure is linearly proportional to the number density of CO2, and doubling the pressure doubles the spectral radiance. With the pressure held constant at the nominal free stream pressure, performing free stream simulations with increasing temperatures shows that there is a much greater dependence of the spectral radiance on temperature than on pressure. An upper limit on the free-stream temperature to match the experimentally measured maximum radiation is shown to be 1700 K for the MSL1 condition and 1600 K for the MSL2 condition, higher than the predicted perfect gas temperature of 1221 K and 1042 K.
Integrated Measurements
The largest discrepancy occurs with the freestream probe measurements and the discrepancy is larger for the MSL2 condition than for the MSL1 condition. The larger differences observed in the expansion tube conditions are partly due to free-flow characteristics, such as the expansion fan and the wall boundary layer in the line of sight, which have not been accounted for in the simulations. In the expansion tube, Fridman chemistry simulations result in a better match in the distance to the shock, compared to Johnston chemistry simulations, discussed in Chapter 4.
Conclusions
The soot is softened by an extensive cleaning process of the shock tube as detailed in the thesis of Jewell [52]. The repeat experiments show good agreement, with the shape being consistent for the majority of the wavelengths providing confidence in the shot-to-shot repeatability of T5 radiation measurements in the forebody. There was no evidence that vibration freezing in the T5 nozzle significantly affected the shock standoff distance in any of the test conditions.
T5 CO 2 Radiation Measurements
Introduction
Attached to the end of the stinger is a flexible PVC tube secured with hose clamps to ensure that the fiber is protected during the test. To ensure that the fiber probe is looking at the back of the blackbody source, the model is mounted on the table and aligned with the blackbody source using a laser level. Bench top calibration of the 16◦AOA stagnation point resulted in Sλ,cal= 756 counts/(W/cm2-sr-µm), within ±72 counts/(W/cm2-sr-µm) of the measurement distribution in - here.
Shock Shape Measurements
Shock Layer Radiation
The integrated radiation intensity of the MSL state at the wall is 33.4% below the optically thin limit, and a decrease in radiation intensity near the wall indicates that boundary layer absorption is present in the experiments. The coordinate system is shown in figure 3.3, where x is the distance in the body and y is the distance in the radial direction. Repetitive experiments show excellent agreement with shape matching and peak irradiance varying by 2.0% providing confidence in the shot-to-shot repeatability of subsequent in-body T5 irradiance measurements.
Conclusions
These simulations account for the interaction of the bow shock with the nozzle shear layer, which is in the field of view of the probe. To resolve these discrepancies, the bow shock offset distance was studied by theory, simulations and experiments in the T5 Reflected Shock Tunnel and compared to the Hypervelocity Expansion Tube experiment to assess the object independence of the results. In the HYPULSE expansion tube plant, Bakos and Morgan [4] used pressure transducers placed 76 mm below the secondary diaphragm to observe the reflected surge.
Conclusions and Future Work
Concluding Remarks
Experiments covered the binary scaling and enthalpy of the LENS I Run 8 condition [65] using different conical nozzle throat sizes, but the large offset distance anomaly observed in the LENS I device was not observed. The sensitivity of the spacer distance to the chemical contribution was examined by comparing three kinetic models. To account for free-stream radiation in the ray-tracing simulations, the free-stream plate was extended to lengths that represented the cutoff lengths of the rays within the fiber probe's receiving cone reaching the object boundary.
Future Work
For each shot, the recoil of the nozzle and the initial distance of the model from the nozzle should be recorded. Performance data of the new free-piston shock tunnel at GAL-CIT.28th Joint Propulsion Conference and Exhibit, AIAA Paper. An estimate of the chemical kinetics behind normal shock waves in mixtures of carbon dioxide and nitrogen for conditions typical of Martian intrusion.
All HET Shots
HET Radiation Experiment Shots
- T5 Radiation Campaign Summary of Conditions
- Freestream conditions for the sphere-cone tests analyzed
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HET shot list part 1
HET shot list part 2
HET shot list part 3
HET radiation shot list
T5 900:1 Conical Nozzle Shots
T5 100:1 Conical Nozzle Shots
T5 Contour Nozzle Shots
