Introduction
Overview of Previous Work
For absorption calculations, the ground state (or absorbing state) population must be considered (v00,J00). The γ(0,1) vibrational band can again be used to study the number density profile of NO (A) that forms downstream of the shock.
Contributions of Present Work and Outline
Experimental Facility and Diagnostics
The Hypervelocity Expansion Tube Facility
The arrival of helium after the shock provides a uniform Pitot pressure of 3.8±0.4 kPa in M7-H8-He. Underprediction of the Pitot pressure of the propellant gas was also observed in the work of McGilvray et al.
Model Geometries
Schlieren Imaging
Since the discharge time is long, the lamp must be triggered 200 µs before the arrival of the test gas. A delay of 1,305 ms is set using a Berkeley Nucleonics Model 577 pulse generator, with an additional delay of 200 µs set between the camera and the lamp.
Emission Spectroscopy of Hypervelocity Flow
Therefore, it is useful to consider the span length of the test article when designing the z-type configuration. Since the spectrometer slits are oriented vertically, the horizontal image must be reshaped to focus along the length of the slit.
Calibration of Spectroscopic Measurements
Along the surface of the cylinder model, image distortion due to the boundary layer is visible in the images.
Radiative Processes of Nitric Oxide Electronic Transitions
Computing the NO A-X Spectrum
For the ground state, the energy is just the sum of the rotational and vibrational energies. This is due to the fact that the peak population in the Boltzmann distribution has been pushed towards larger rotating quantum numbers.
Radiative Transfer
If the ground number density and the first excited state are known, Boltzmann. The integral of the line profile is defined to be equal to 1 for the entire frequency range. However, the ratio of the speed to the speed of light is insignificant in most cases.
By introducing the relationship between intensities, the relationship between the optically thin intensity and the full radiative transport intensity is simplified. The interpretation of Rii is more straightforward as it is a measurement of the deviation from the optically thin case.
Kinetics of Electronically Excited NO
The methodology used in cases M7-H8-A can be easily applied to the first 1 mm downstream of the stroke. From the high-speed schlieren videos, the instability in the bow shock location is present for most frames. This is likely the result of feedback between the separation shock and the angle of the rejoining shock.
UV Radiation of Hypervelocity Flow Over Circular Cylinders
Single Shot Schlieren
The shock image contains a larger gradient in pixel intensities for M5-H6-A, indicating higher post-shock densities. Because the shock distance measurement is made relative to the mounting surface far downstream, it makes a non-negligible contribution to the error. This is expected, as the standoff distance scales as the ratio of the free-flow density to the post-shock density.
To obtain an estimate of the average density along the shock layer, the two-temperature reactive Landau-Teller model is used to calculate the density profile along the stagnation streamline. The non-dimensional standoff distance is observed to vary linearly as a function of density ratio, which is consistent with the linear dependence of the standoff scale.
Radiation Along Stagnation Streamline for an Air Test Gas
The starting point of the position axis is set on the surface of the cylinder and is marked with a white dashed line. No radiation is observed before the shock, as the temperature of the free stream is low relative to the post-shock gas. The radiation ends at the location of the surface of the cylinder, which is set as the origin.
After the NOγ radiation peak, the profile of all vibrational bands decreases linearly to the surface of the cylinder. The origin of the position axis is placed on the surface of the cylinder, indicated by a dashed white line.
Spectrum Fitting For NO A-X Transitions
The summation is performed over the pixel range of the detector relevant to the wavelengths used in the fit. The error associated with the normalized intensity values can be deduced to be of the form For each of the two regions, a scaling factor is applied to the synthetic spectrum which reduces goodness of fit.
There is a greater uncertainty in the choice of fitTv due to the shape of the contour plot. Error bars of two replicate experiments represent fit uncertainty due to experimental noise using the Tibère-Inglesse et al.
Radiation Measurements of Oxygen/Nitrogen Mixtures
Like the radiation from the iron hollow cathode lamp, atomic radiation is formed as discrete lines in the spectrum. A likely source of the silicon contamination is the vacuum grease used to prepare the O-rings for a vacuum seal. The change in detector intensity is due to the reduced quenching of the excited states by other species such as O2.
Normalization is performed using the peak value of the γ(0,1) vibrational band for air in both plots. The integrated emission of other vibrational bands can be plotted for the lowest oxygen mole fraction to study variations of the population of electronically excited NO.
Spectrum Fitting For A-X, B-X, and C-X Systems
To accurately match experiment, the addition of the electronic excitation temperature, Tex, is necessary to describe the distribution of the upper electronic states of NO. Specair version 3.0 is used to calculate the convoluted NO radiation γ, β and δ of the NO molecule, with Tex used to describe the Boltzmann distribution of excited states. In an arcjet study by Hyun, Park and Chang, the contributions of the upper electronic states of NO were investigated [40].
It was found that the total intensity of the NO band was weaker than that of the NO δ and γ bands. The procedure is applied to the first extracted airframe spectrum of the M7-H8-A state.
Two-Temperature Calculation of Integrated Intensity for Mixtures
Number density of NO(A) is therefore an indication of the amount of radiation expected in an optically thin gas. When the oxygen mole fraction is further reduced, the calculation predicts a drop in NO(A) number density for the case of XO2 = 0.063. It is possible to directly compare the measured signal from the experiment with the predicted number density of NO(A).
Downstream of the peak, the experimental profiles of XO2 = 0.273 and XO2 = 0.210 are bounded by the optically thin profile and the Iν profile. The delay in NO(A) is most notable in the calculation of XO2 =0.063, where the number density and Iν remain below 20% of the peak value at a streamwise location of x = 1 mm.
Concluding Remarks
Due to the larger wave angle, the perfect gas pressure is 89.1 kPa downstream of the reaffirmation shock. Along this line, the emission of the reconfirmation shock and part of the post-oblique/separation shock is recorded. The slow movement of the reconfirmation shock along the slits can be easily observed in Fig.
Unlike the reattachment shock region, signal intensities downstream of the plumb shock are similar in the two time intervals. Another set of emission measurements was performed in the after-lead oblique shock and reattachment shock region of the double wedge flow.
Optical Measurements in a Shock/Boundary-Layer Interaction
High Speed Schlieren Imaging
However, footage of the accelerator gas was captured and used to indicate the start of the test time in the videos. For all double wedge experiments, the onset of the test time (t = 0 µs) is determined by the arrival of the contact surface in the pitot track. Upon arrival of the contact patch (t = 0 µs), the separation region extends nearly 10 mm upstream from the hinge location along the first wedge.
The initial separation results in the development of an accelerating gas after the impact 100 µs before the arrival of the contact surface. The final separation length of the accelerator gas is transferred to the test gas and is shown in Fig.
Tracking of Major Features in Schlieren Videos
However, the positioning of the separation shock and the triple point occur at different points in the test time than in the case of the M7-H8-He. The location of the shock is plotted outside the theoretical test time, with the bow shock moving upstream for the second time. The other defining feature of the double-wedge current is boundary layer separation.
The separation of the boundary layer is indicated by an area of low pixel intensity in the images. The upstream location of the separation region is measured by setting a threshold in pixel intensity from the background value.
Emission Measurements in the Post-Bow Shock and Shear Layer
Dashed lines represent regions used in binning spectra downstream of the measured shear layer location. Due to the existence of the second wedge, streamlines are forced to rotate upwards towards the bow shock. The appearance of the shear layer and reflected waves also add complexity to the measurement.
Between the shear layer and the location of the reflected wave, the temperature decreases dramatically. Nevertheless, the measured temperatures are observed to decrease downstream of the shear layer similar to the interval of 35-45 µs.
Two-Temperature Calculations of Oblique Shock System
As expected, little dissociation occurs downstream of the lead oblique shock due to low post-shock temperatures. After several compressions, the streamlines eventually merge and are closely spaced downstream of the reassembly shock. The most notable differences between the two-shock and three-shock cases are the translational-rotational temperature and pressure immediately downstream of the reassembly shock.
As a result, both the frozen-translational-rotational temperature and the pressure downstream of the reassembly shock decrease later in the test time. A perfect gas calculation of the reassembly shock results in a shock angle of βR = 48.0◦ , significantly larger than the measured one.
Emission Measurements in Post-Lead Oblique Shock and Reattach-
This is due to the low number density of ground state NO molecules present downstream of the reattachment shock. Therefore, longer exposure times can be used without worrying about the shock of reattachment fouling the detector. A gain of 80 and an exposure time of 25 µs are selected for reattachment shock radiation measurements.
However, the weak signal of the post-oblique shock region can be correlated to yield a noisy spectrum. With the addition of the separation shock, the number density of NO (A) is predicted to decrease downstream of the reconnection shock.
Concluding Remarks
Freestream unsteadiness, three-dimensionality of the bow shock, and increased relaxation along the stream are considered likely causes of the drop in postshock temperatures. High-resolution schlieren images of the cylinder flow were acquired for accurate measurements of the offset distance. However, we observe a slow relaxation of the gas after the shock compared to the steady-state translational-rotational temperature calculation.
The radiation is measured downstream of the shear layer and the location of the reflected shock, where the temperature drop compared to the bow shock gas is measured. Increased relaxation along the streams due to the downward motion of the triple point and instabilities in the free stream have been discussed as possible causes.
Conclusions
Future Work
If a light source is too dim to use with the experimental camera settings, a second approach is to determine the reciprocity factor of the amplified camera. In the present work, exposure times of the deuterium lamp are on the order of 100 ms, several orders of magnitude greater than in the experiment. The coupling of the radiative transport to the fluid mechanics is then necessary to accurately model the spatial and temporal dependence of the emission.
The capabilities of the emission spectroscopy diagnostic can be further extended by investigating other wavelength ranges and measuring additional species. By changing the magnification of the collection optics, a larger portion of the flow can be imaged.
