Defense Industry

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Defense industry typically involves applications having targets that are extremely fast and small which require very accurate and fast monitoring methods. We have experience in imaging of various defense applications such as different kinds of ballistic objects, collisions, shockwaves and explosion studies. CAVILUX laser illumination enables extremely fast, even ultra-high-speed lighting which is essential for acquiring high-quality images of these phenomena.

Ballistics

Explosions

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Measurements of hypersonic boundary-layer instabilities using a pulsed-laser schlieren technique

Author: Stuart Laurence, Department of Aerospace Engineering, University of Maryland, College Park

1 Introduction

When a hypersonic vehicle travels through the atmosphere, a boundary layer develops in the air close to the vehicle surface. Initially (close to the nose of the vehicle) this boundary layer is laminar, but typically will transition to turbulence at some point downstream. A turbulent boundary layer produces significantly larger heat flux and frictional drag at the vehicle surface than a laminar one, so to be able to accurately predict vehicle performance, knowledge of the laminar-to-turbulent transition process is important. There are a variety of boundary-layer instabilities whose growth and breakdown can lead to transition; for slender planar or axisymmetric bodies and small incidences, a key instability mechanism is the second or Mack mode, which can be thought of physically as acoustic waves that become trapped within the boundary layer. Second-mode waves typically exhibit very high frequencies – around 100 kHz or even higher – which makes their measurement very difficult with conventional techniques. Here I describe measurements of the second-mode instability using a schlieren system incorporating a CAVILUX pulsed-diode laser.

2 Experimental configuration

Experiments were performed at two hypersonic wind tunnels: the High Enthalpy Shock Tunnel Goettingen (HEG) of the German Aerospace Center (DLR), and Hypervelocity Tunnel 9 of the Arnold Engineering Development Center (AEDC) at White Oak, Maryland. HEG is capable of reproducing the extremely high flow velocities typical of atmospheric reentry (up to 7 km/s), though for very short test periods (~1 ms). In the present experiments, the flow velocity and density were 4.4 km/s and 0.0175 kg/m3. Tunnel 9, on the other hand, can produce high Mach numbers with longer test periods (around 1 s), but with lower flow velocities. In the present Tunnel 9 experiments, a variety of flow conditions were used, all having a Mach number of approximately 14 and a flow speed of 2 km/s.

In both cases the test article was a slender, 7° half-angle cone. The flow within the boundary layer over the cone was visualized using a conventional Z-fold schlieren arrangement, as shown in figure 1.

Figure 1: Z-fold schlieren visualization set-up used in the experiments described here.

Schlieren is a technique used to visualize flow features in compressible flows: a density gradient at some location in the imaging plane within the test section (in a direction normal to the knife edge placed in front of the camera) will result in a change in intensity at the corresponding location on the image taken by the imaging device (in this case a high-speed camera). In the HEG experiments the light source was a CAVILUX Smart pulsed-diode laser and the camera was a Vision Research Phantom v1210, recording at 200 kHz. The laser was run in ultra-high-speed mode, with a repeated 4-pulse pattern as shown in figure 2.

Figure 2: Laser pulse pattern used in the HEG experiments: Δt12 is 2 μs and Δt34 is 3 μs.

This pattern was necessary because the characteristic frequency of the second-mode disturbance in this case (~600 kHz) was significantly higher than the recording frequency, so closely spaced pulse pairs were used to unambiguously resolve the wave motion (further details to be provided shortly). In the Tunnel 9 experiments, a CAVILUX HF laser providing, uniformly spaced pulses at approximately 70 kHz, was used together with a Phantom v2512 camera. The laser pulse width in the two experiments varied between 20 ns and 50 ns; such short pulse widths were necessary to freeze the high-speed flow structures in images.

3 Results

The HEG experiments were particularly challenging because of the high flow velocity (meaning high second-mode frequencies) and low density (meaning weak intensity variations in the schlieren images). An example of a visualized second-mode wave packet, visible from its oblique “rope-like” structures close to the surface, is shown as it propagates within a sequence of schlieren images in figure 3. The propagation speed of the wave packet is constant – the apparently uneven motion is a result of the laser pulse pattern. By performing two-dimensional image correlations, it is possible to recover the propagation speed – in this case it is 3.8 km/s. The unequal spacing between the two pulse pairs in figure 2 avoids problems with aliasing in these correlations. By then taking the Fourier transform of rows of pixels parallel to the cone surface, wavenumber spectra can be constructed; these can subsequently be converted into frequency spectra using the propagation speed calculated earlier.

Figure 3: Sequence of reference-subtracted schlieren images showing the propagation of a second-mode wave packet (flow is left to right)

Plots of the averaged power spectral density (PSD) at three locations downstream are shown in the left plot of figure 4. Here we see a strong peak at approximately 600 kHz – this corresponds to the second-mode frequency at these conditions. The peak grows rapidly as we move downstream, showing strong amplification of the second mode. A more detailed picture of this growth is shown in the right plot of figure 4, which is a contour plot of the PSD versus distance downstream. Further details of these measurements can be found in Laurence et al. (2016).

Figure 4: (Left) Plots of the schlieren power spectral density (PSD) near the surface at three locations downstream (s is the distance along the cone from the nose); (right) contour plot of the PSD versus distance downstream.

An example of a propagating second-mode wave packet in one of the Tunnel 9 experiments is shown in figure 5. Again we see the characteristic “rope-like” structures, though now the disturbance energy appears to be less concentrated towards the cone surface than it was in the HEG experiments.

Figure 5: Propagation of a second-mode wave packet in a Tunnel 9 experiment

In the Tunnel 9 experiments, the schlieren system was calibrated by placing a long-focal-length lens in the imaging plane and recording images of it. This enabled a calibration curve relating image intensity to the density gradient to be established. From this calibration curve, one can then quantify the growth rate of the second-mode instability. A contour plot of the integrated growth rate, or N-factor, versus distance downstream and frequency is shown in figure 6.

Figure 6: Contour plot of N-factor versus distance downstream and frequency in Tunnel 9 experiment.

Again we see a strong second-mode contribution, but now at a much lower frequency of approximately 100 kHz. The decrease in this frequency moving downstream is associated with the thickening of the mean boundary layer. Such quantitative measurements are very important as they provide data against which numerical simulations and stability analysis computations can be compared. Further details of the Tunnel 9 experiments can be found in Kennedy et al. (2017).

4 Conclusions

The experiments described here demonstrate that it is possible to use high-speed schlieren techniques to perform quantitative measurements of extremely high-frequency instability waves in hypersonic boundary layers. The capabilities of CAVILUX pulsed-diode laser light sources proved instrumental in enabling these measurements.

5 References

Kennedy, R., Laurence, S., Smith, M., and Marineau, E. (2017), “Hypersonic Boundary-Layer Transition Features from High-Speed Schlieren Images”, 55th AIAA Aerospace Sciences Meeting, AIAA SciTech Forum, AIAA Paper 2017-1683

Laurence S., Wagner, A., and Hannemann, K. (2016), “Experimental study of second-mode instability growth and breakdown in a hypersonic boundary layer using high-speed schlieren visualization”, Journal of Fluid Mechanics, vol. 797, pp. 471-503

About the author

Stuart Laurence (Ph.D) completed his graduate studies at the Graduate Aeronautical Laboratories, California Institute of Technology, in the area of hypersonic flows. He currently is Assistant Professor at the Department of Aerospace Engineering, University of Maryland, College Park

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Shotgun bullets

Shotgun bullets

Visualization of shotgun bullets shot at a protective armor – front illumination with Cavitar’s CAVILUX illumination laser. Video taken at 40.000 frames per second.

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Visualization of bullet impact using CAVILUX HF laser illumination

Bullet impact

Author: Hannu Eloranta, Pixact Ltd

1 Introduction

The imaging of high-speed objects, such as bullets, constitutes several technical challenges for the imaging equipment. Besides accurate timing of the imaging sequence, either a very short exposure time or pulsed illumination is required to freeze the motion of objects. The combination of high velocity and high magnification favors pulsed light sources since typically exposure times less than 1µs are required to avoid motion blur. Pulsed light sources also typically provide higher light output than cw light sources. Since the energy output is concentrated on short pulses, also the heating effect of the illumination is minimized.

CAVILUX diode lasers offer several attractive features for high-speed imaging. They can be programmed rather freely to deliver pulses or bursts with desired frequency and pulse energy. The laser head is coupled with fiber optics, which allows easy alignment of the illumination. Spot illumination is easily optimized by adjusting the lens system attached to the fiber light guide. In contrast to other laser light sources, the light provided by CAVILUX diode lasers is essentially incoherent. Thus, any special optical elements are not needed for speckle-free illumination. In some cases also the monochromatic nature of the laser illumination is important, because optical bandpass filters can be used to block radiation generated by explosions and hot objects.

This paper presents the visualization of bullets impacting with metal, plastic and wood plate structures. Image sequences are recorded with a high-speed camera and CAVILUX HF diode laser.

2 The imaging setup

Bullets are shot from a distance of 5 m to a test piece with a .375 Magnum handgun. The bullet speed varies between 220 and 300m/s. The impact of the bullet with the test piece is visualized from various directions. The camera and the illumination are located on single stand at a distance of 3 m from the test piece. A schematic drawing of the setup is presented in Fig. 1.

The camera used to record the high-speed sequences of the impacts is Photron FASTCAM SA1.1 equipped with Nikon 300mm f/#4 lens. The lens aperture is adjusted to f/#8. The imaging frequency of 50 kHz is chosen. At this high framing rate the camera resolution is limited to 512 x 208 pix2. The original bit depth of the images is 12bit, but the images are stored in 8bit mode with 2 bit shift lower. CAVILUX HF is driven in the slave mode triggered by the camera exposure start signal. Standard CAVILUX lens optics is used to produce a spot illumination matching with the camera field of view. The lens optics is located on top of the camera. Due to the laser duty cycle limit of 2%, the pulse duration at 50 kHz frequency is limited to 0.4µs, which yields 0.2mJ energy/pulse. In this example only one laser head is used. To further optimize the illumination, to minimize shadows etc., several laser heads can be combined. However, one laser head is enough to create high-quality images of the bullet even though the lens aperture is set to f/#8 increasing the depth of focus.

ballistics_measurement_setup

Figure 1. Imaging setup.

3 Results

In Fig. 2 examples of frames captured after the impact are shown. In Figs 2a and 2b the bullet deformation and the type of splinters after impact can be seen. In both images the illumination is from the same direction, but in the upper image the plate is painted black in order to obtain better contrast between the bullet and the background. In this way small splinters are easier to detect. In Fig. 2c a three image sequence from the back side of the wood test piece is shown. In this sequence a pressure wave moving away from the point of impact in the wood plate can be seen. Fig. 3 presents image-by-image sequences of selected impacts.

Bullet impact

Bullet impact

Bullet impact
Figure 2. Examples of images acquired after the impact.

Bullet impact
Bullet impact
Bullet impact

Figure 3. High-speed image sequences of impacts.

A jitter-free timing of the light pulses enables the analysis of bullet speed. Image analysis is used to determine the bullet location in each frame. After scaling the camera to world coordinates, the displacement of the bullet between two frames can be used to evaluate the bullet speed before and after the impact. Vertical lines in Fig. 4 represent the location of the bullet tip in the image series. The impact with the plate clearly decelerates the bullet.

figure_4
Figure 4. Examples of images acquired after the impact.

4 Conclusion

In high-speed imaging the design and properties of illumination is the key to achieve high quality image data. The role of illumination is pronounced for small objects requiring high magnification. Exposure times below 1µs are typically required in order to freeze the motion of bullets and other high-velocity targets. Even if there are cameras with sub-µs shutter speeds, a more convenient solution is to utilize pulsed light sources since high power illumination is needed anyway. Especially lasers can provide a very high light output over a short period of time. The pulse duration is typically adjusted in the ns range. However, a common problem related with most lasers is the coherence of light, which results in nonuniform beam profile with hot spots and other defects (speckle). In this respect CAVILUX diode lasers offer a highly attractive alternative. They provide a high-quality beam which is easily converted to spot with extremely uniform intensity profile. The high-quality beam combined with very flexible programming of the laser pulse frequency and pulse duration makes these diode lasers the obvious choice for most challenging high-speed imaging applications.

About the author

Hannu Eloranta (Dr.Tech.) has been working as a researcher in the Group of Experimental Fluid Dynamics at Tampere University of Technology. His research topics include several aspects of flow dynamics, such as turbulence, fluid-structure interactions and flow control. In addition he has been developing optical measurement techniques and analysis methods for fluid mechanics. Currently he is working in Pixact Ltd – a company specialized in developing optical measurement techniques.

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Explosion of banger

Explosion of banger

Visualization with Cavitar’s CAVILUX illumination laser of the explosion of a banger – front illumination. Video taken at 40.000 frames per second.

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Ignition of Rocket

Combustion of rocket

Visualization of the launch of a combustion rocket with Cavitar’s CAVILUX illumination laser. The video was taken at 1.000 fps.

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