Chapter V: Optical Measurements in a Shock/Boundary-Layer Interaction
5.2 Tracking of Major Features in Schlieren Videos
Figure 5.4 contains frames of a high speed schlieren of M7-H8-A flow over a double wedge geometry. Out of all past and present experiments of the double wedge in the expansion tube, M7-H8-A is the only condition where the test gas arrives with minimal initial separation. In Fig. 5.4, the test gas begins as a two-shock system, where separation starts at the hinge. As time progresses, the separation shock becomes visible and the separation length grows, moving upstream along the first wedge. Development of other flow features is qualitatively similar to what was described by Swantek [90]. However, the positioning of the separation shock and the triple point occur at different points in test time than in the M7-H8-He case.
Effect of Facility Startup on Double Wedge Flow
Figure 5.5 compares two frames from the M7-H8-He and the M7-H8-A condition at t =45 µs after the arrival of the contact surface. Background images are subtracted from the raw images to highlight the important features of the shock structure.
Although the perfect gas freestream conditions are similar between the cases, the location of flow features have shifted. Define Lsep as the length of the separation region and x1as the distance of the separation front from the wedge tip. The non- dimensional length of the separation region is Lxsep
1 = 5.2×10−2±0.6×10−2 for the M7-H8-A case compared to Lxsep
1 = 9.3×10−2±0.7×10−2for the M7-H8-He condition at the same period in test time. This is likely due to the initial separation present in the M7-H8-He case. In addition, the series of waves behind the bow shock is further upstream in the helium case when compared to the air acceleration case.
These changes in the evolution of the flow field are critical when comparing time resolved heat flux data to simulations, since flow separation and shock impingement heavily determine where peak heat flux occurs on the test article. Careful attention must be placed in correctly modeling the startup processes of any high enthalpy facility, as this will modify the initial conditions of such a time sensitive flow.
(d) 58 µs (e) 68 µs (f) 78 µs
(g) 88 µs (h) 98 µs (i) 108 µs
(j) 118 µs (k) 128 µs (l) 138 µs
(m) 148 µs (n) 158 µs
Figure 5.4: High speed schlieren images of M7-H8-A, with air used as an accelera- tion gas. Time interval between frames is 10 µs.
(a) M7-H8-He (b) M7-H8-A
Figure 5.5: Background subtracted frames of two freestream conditions taken 45 µs after the contact surface.
the knowledge gained from radiation measurements of cylinder flow, it is expected that the M7-H8-A freestream will provide the strongest ultraviolet signal in the post-bow shock region. Spectroscopic measurements will only be performed with this high enthalpy condition, providing the best case scenario for short exposure measurements. As a result, the focus of discussion will be on the analysis of M7-H8-A schlieren images.
Before every experiment, a background video is recorded at the same camera settings as the test with no flow present. The light source of the flash lamp diminishes in intensity over time and will result in global changes in pixel intensities. The image processing software ImageJ is used to perform mathematical operations on the series of images. The raw schlieren videos are subtracted with background videos using the subroutine "difference" in the image calculator of ImageJ. This allows for imperfections from optics to be removed and to minimize the intensity variations of the light source during test time. Figure 5.6 is a background subtracted frame with all flow features labeled. The origin of the coordinate system is selected to be at the location of the tip of the first wedge.
Bow shock standoff distance is measured using the same method outlined for ana- lyzing cylinder schlieren images. The procedure is applied to a series of background subtracted images along a horizontal line yt = 42.62 mm above the leading wedge tip. Later on, this will be one of two locations used for emission measurements.
Figure 5.7 is the xt bow shock location along yt = 42.62 mm. Vertical lines rep- resent the establishment time and the end of theoretical test time. As observed in Fig. 5.4, the bow shock travels upstream early in the flow development process. At
Figure 5.6: Labeled diagram of major features in a shock/boundary-layer interaction.
A M7-H8-A freestream is shown with the frame takent = 100 µs after the contact surface.
t =70 µs, the bow shock arrives at a mean location with oscillations present. After approximately t = 137 µs, the bow shock reverses direction and partially moves downstream until the theoretical test time ends. The location of the shock is plotted beyond the theoretical test time, where the bow shock moves upstream a second time.
The temporal evolution of the bow shock location demonstrates the potential for smearing on the detector throughout test time. This is of great concern early in test time, where the bow shock travels with a mean horizontal velocity of 83 ± 6 m/s. In addition, oscillations of the shock can generate peak-to-peak movement of approximately 1.5 mm, a displacement large enough for observable changes on a detector. It is the movement of the bow shock that ultimately governs the selected exposure time for emission measurements.
The other defining feature of the double wedge flow is the separation of the boundary layer. Swantek [90] and Knisely [48] have performed measurements of the separation length for double wedge flows in both the HET and the Caltech T5 reflected shock tunnel. However, the location of separation was only measured using still schlieren
Figure 5.7: Measured bow shock location along a horizontal line located yt = 42.62 mm as a function of test time. Background subtracted images are used for measurements.
images. With background subtracted images such as Fig. 5.6, the separation zone can be easily identified in the movies and tracked using similar methods applied to bow shocks. Through ImageJ, the background subtracted images are rotated 30 degrees using a bilinear interpolation to orient the leading wedge horizontally. A region 0.79 mm thick (equivalent to 4 pixels) is binned along the wedge surface to obtain average pixel intensity versus the streamwise coordinate st. The separation of the boundary layer is indicated by a region 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. The threshold is set to 80% of the mean pixel intensity where no flow is present in the images. The location where binned pixel intensity drops below the threshold is selected as the upstream location of the separation zone.
Figure 5.8 plots the streamwise location of the start of the separation region as a function of time. As previously identified in the schlieren videos, the separation of the boundary layer starts approximately 1.9 mm from the hinge location after the flow has established and propagates upstream throughout test time. Outside test time, the boundary layer separation continues to grow until reaching a mean location 260 µs after the arrival of the contact surface. An important outcome from this measurement is the deceleration of the separation length motion over time.
Because a shock is formed at the separation front, the movement of the separation shock also decays with time. The mechanism driving the decay of the separation
Figure 5.8: Front of separation region measured along leading wedge surface.
Vertical lines mark the startup process and end of test time. Horizontal line marks the hinge location from the tip along the wedge face. Background subtracted images are used for measurements.
growth is likely to originate downstream. The most plausible flow feature to cause this deceleration is the reattachment shock behind the separation bubble. Based on the wake model of Roshko, the post-reattachment state of the gas determines the pressure downstream of the separation bubble, ultimately governing the growth of the region [82].
Figure 5.9: Measured lead oblique shock and reattachment shock locations along a horizontal line at yt =33.91 mm as a function of test time. Background subtracted images are used for measurements.
The last remaining features that can be readily tracked are the lead oblique shock and the reattachment shock. A horizontal line of pixels located yt = 33.91 mm are binned, intersecting the oblique shock features in the background subtracted images. This is chosen as the second location used for emission measurements. The procedure for locating shocks is applied to the line. Figure 5.9 plots thextlocations of the lead oblique shock and the reattachment shock at a fixed yt = 33.91 mm line. During startup, the lead oblique shock experiences extreme oscillations. Once the flow has established, the oblique shock exhibits movement throughout test time.
This is confirmed in the series of images in Fig. 5.4, where a frame corresponding to t = 58 µs captures unsteadiness of the downstream portion of the lead oblique shock. This movement is likely due to freestream disturbances. Movement of the reattachment shock is measured, where the shock location propagates downstream with time relative to the horizontal line. This is due to the combination of wave angle change and the streamwise moment of the shock itself.
5.3 Emission Measurements in the Post-Bow Shock and Shear Layer Region