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Focusing-schlieren visualization in a dual-mode scramjet

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Abstract

Schlieren imaging is particularly suited to measuring density gradients in compressible flowfields and can be used to capture shock waves and expansion fans, as well as the turbulent structures of mixing and wake flows. Conventional schlieren imaging, however, has difficulty clearly capturing such structures in long-duration supersonic combustion test facilities. This is because the severe flow temperatures locally change the refractive index of the window glass that is being used to provide optical access. On the other hand, focusing-schlieren imaging presents the potential of reduced sensitivity to thermal distortion of the windows and to clearly capture the flow structures even during a combustion test. This reduced sensitivity is due the technique’s ability to achieve a narrow depth of focus. As part of this study, a focusing-schlieren system was developed with a depth of focus near ±5 mm and was applied to a direct-connect, continuous-flow type, supersonic combustion test facility with a stagnation temperature near 1200 K. The present system was used to successfully visualize the flowfield inside a dual-mode scramjet. The imaging system captured combustion-induced volumetric expansion of the fuel jet and an anchored bifurcated shock wave at the trailing edge of the ramp fuel injector. This is the first time successful focusing-schlieren measurements have been reported for a dual-mode scramjet.

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Abbreviations

a :

Unobstructed source image height

C :

Contrast of image

f # :

f-Number of lens

F :

Focal length of the schlieren lens

H :

Ramp injector-surface-normal height

I ij :

Local image intensity

\(\hat{I}\) :

Spatial-average intensity in an interrogation area of a schlieren image

l :

Distance from schlieren lens to object

l′:

Distance from schlieren lens to imaging plane

L :

Distance from schlieren lens to source grid

L′:

Distance from schlieren lens to cutoff grid

pk:

Kernel size for Gaussian filter

P 0 :

Stagnation pressure

P W :

Static wall pressure

T 0 :

Stagnation temperature

x, y, z :

Streamwise, heightwise, and spanwise directions

α :

Filter control parameter

ε :

Refraction angle due to density gradient

Φ :

Fuel equivalence ratio

σ g :

Standard deviation for Gaussian filter

\(\hat{\sigma }\) :

Standard deviation of image intensity in an interrogation area of a schlieren image

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Acknowledgments

The authors appreciate the assistant of Dr. Roland Krauss and Mr. Roger Reynolds in operating the supersonic combustion facility. This work was supported by Young Researcher Oversea Study Program of the Japan Society for the Promotion of Science. This research was also supported by the National Center for Hypersonic Combined Cycle Propulsion Grant FA 9550-09-1-0611. The technical monitors on the Grant were Chiping Li (AFOSR), and Aaron Auslender and Rick Gaffney (NASA).

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Authors and Affiliations

  1. Department of Mechanical and System Engineering, Okayama University, Tsushimanaka 3-1-1, Kita, Okayama, Okayama, 700-8530, Japan

    Toshinori Kouchi

  2. Department of Mechanical and Aerospace Engineering, University of Virginia, 570 Edgemont Rd., Charlottesville, VA, 22904-4248, USA

    Christopher P. Goyne, Robert D. Rockwell & James C. McDaniel

Authors
  1. Toshinori Kouchi
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  2. Christopher P. Goyne
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  3. Robert D. Rockwell
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  4. James C. McDaniel
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Correspondence to Toshinori Kouchi.

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Kouchi, T., Goyne, C.P., Rockwell, R.D. et al. Focusing-schlieren visualization in a dual-mode scramjet. Exp Fluids 56, 211 (2015). https://doi.org/10.1007/s00348-015-2081-9

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