Chapter VI: T5 CO 2 Radiation Measurements
6.3 Shock Layer Radiation
system [1]. This approach does not properly capture wall boundary layer absorption but is useful for evaluating the appropriate optical thickness regime.
Figure 6.4: T5 ExoMars and MSL condition 0◦ AOA stagnation point LOS shock layer simulations compared to simulations in the optically thin limit.
The ExoMars condition integrated radiative intensity at the wall is 4.0% below the optically thin limit and can be considered optically thin. The MSL condition integrated radiative intensity at the wall is 33.4% below the optically thin limit and a decrease in radiative intensity next to the wall indicates boundary layer absorption is present in the experiments. Figure 6.5 shows the freestream density vs. enthalpy for the two HET conditions, the ExoMars and MSL T5 conditions and flight trajectories, and contour nozzle condition. For all of the conditions considered in the radiation work, the freestream density is an order of magnitude greater than flight trajectories.
Background Radiation
A background radiation signal can be observed in all of the T5 measurements that is not present in any of the HET expansion tube and shock tube experiments on the spectra between 3850 nm to 4150 nm. In the schlieren images, particles can sometimes be observed as in Figure 6.3a. These particles are a possible source of the background radiation. The particles are believed to be due to soot formed in the CO2reservoir at high pressures greater than 300 atm. The soot is mitigated by an extensive cleaning process of the shock tube outlined in the thesis of Jewell [52]. For
Figure 6.5: Freestream density vs. total enthalpy comparison of the T5 Exomars and MSL conditions to flight trajectories of two recent missions. The HET MSL1 and MSL2 conditions and a contour nozzle condition are also plotted.
future investigations into the source of this experimental artifact, the background signal is fit to be a black body described by Planck’s law (Equation 1.37) and the effective temperature ranges from 590 to 1530 K for selected tests.
6.4 0◦AOA Radiation Measurements
The coordinates (x/R, y/R) of the two probe locations used when the model is mounted at a 0◦AOA are at the nose on the forebody (0,0) and close to the shoulder in the afterbody (0.557, 0.861). The coordinate system is shown in Figure 3.3, where x is the distance into the body and y is the distance in the radial direction. These locations are shown in Figure 6.6 together with the acceptance cones of the forebody and wake probes. The solid lines indicate the outside rays of the acceptance cone and the dashed lines indicate the normal ray.
Forebody Measurements
Two repeat 0◦ AOA stagnation point measurements are obtained at the ExoMars condition as shown in Figure 6.7. The repeat experiments show good agreement, with the shape matching for the majority of the wavelengths providing confidence in the shot-to-shot repeatability of T5 radiation measurements in the forebody. The largest difference occurs at the peak where there is a 4% difference in radiance.
Figure 6.6: 0◦AOA T5 MSL model schematic with rays emanating from the fiber probe locations. The forebody and afterbody probes have acceptance cone half angles of 17.5◦and 11.5◦, respectively.
The 0◦ AOA stagnation point measurements obtained at the MSL and contour nozzle condition are shown in Figures 6.8 and 6.9, respectively. As predicted in the shock layer simulations, absorption features due to the boundary layer on the model surface appears for these two conditions. The ExoMars, MSL, and contour condition measurements with freestream temperatures of 855 K, 1651 K, and 1793 K, Table 6.2, are fit to blackbody spectra to obtain blackbody curve temperatures at 760 K, 1020 K, and 1530 K.
Wake Measurements
Two repeat 0◦AOA wake measurements are obtained for the ExoMars condition as shown in Figure 6.10. The repeat experiments show excellent agreement with the shape matching and the peak radiance differing by 2.0% providing confidence in the shot-to-shot repeatability of T5 radiation measurements in the afterbody. The offset blackbody temperature in the wake is 590 K which is less than 760 K extracted from
the forebody measurement.
Figure 6.7: T52891 and T52892 ExoMars condition 0◦AOA stagnation point mea- surements. The blackbody curve that best fits the background radiation is indicated by the black-dashed line.
Figure 6.8: T52901 MSL condition 0◦ AOA stagnation point measurement. The blackbody curve that best fits the background radiation is indicated by the black- dashed line.
Figure 6.9: T52902 contour nozzle condition 0◦AOA stagnation point. The black- body curve that best fits the background radiation is indicated by the black-dashed line.
Figure 6.10: T52895 and T52896 ExoMars condition 0◦AOA wake measurements.
The blackbody curve that best fits the background radiation is indicated by the black-dashed line.
6.5 16◦AOA Radiation Measurements
The coordinates of the two 16◦ probe locations (x/R, y/R) are at the 16◦ AOA stagnation point (0.125,0.433) and in the wake (0.557, 0.861). The coordinate system is shown in Figure 3.3, where x is the distance into the body and y is the distance in the radial direction. The two fiber probe locations are used to obtain radiation measurements at three orientations with the model at a 16◦AOA as shown in Figure 6.11. The acceptance cones of the forebody and wake probes are also shown with the solid lines indicating the outside rays of the acceptance cone and the dashed lines indicate the normal ray. The red and black lines are on the wind side of the model and the green lines are on the lee side. The two wake orientations are accessible by turning the sting 180◦in the mount.
Figure 6.11: 16◦ AOA T5 MSL model schematic with rays emanating from the fiber probe locations. The forebody and afterbody probes have acceptance cone half angles of 17.5◦and 11.5◦, respectively.
The three experiments are conducted at the MSL condition at the stagnation point, lee side, and wind side locations. These measurements are shown in Figures 6.12, 6.13, and 6.14, respectively. A boundary layer absorption feature that dips below the estimated blackbody background can be observed for the stagnation point mea- surement. The background blackbody temperature is 960 K, similar to the value observed for the 0◦AOA measurement, 1020 K.
For the wake measurements, the spectra are similar for the wind and lee side with the peak radiance between the wake and lee side measurements only differing by 2.5%. However, the background blackbody temperature is larger on the wind side (1530 K) than on the lee side (720 K).
Figure 6.12: T52900 16◦AOA MSL condition stagnation point measurement. The blackbody curve that best fits the background radiation is indicated by the black- dashed line.