SPIE Proceedings [SPIE 18th Intl Congress on High Speed Photography and Photonics - Xian, China (Sunday 28 August 1988)] 18th Intl Congress on High Speed Photography and Photonics

Visualization of shock waves in layered media

H. Reichenbach

Fraunhofer -Institut für Kurzzeitdynamik, - Ernst -Mach -Institut -4 Eckerstraße, 7800 Freiburg, FRG

ABSTRACT

The airblast precursor is a shock refraction effect formed by the interaction of ashock wave with a high- sound -speed layer. Helium was injected through a porous ceramicplate to create such a layer near the wall of a small shock tube. The flow was madevisible by shadow, schlieren and interferometer photography. Incident shock Mach numbersof M = 1.7 were used. A prominent feature of the flow is an unstable supersonic wall jetthat develops large -scale, turbulent structures.

1. INTRODUCTION

Shock refraction phenomena occur if shock waves interact with layered gaseous media ofdifferent sound velocities. If a plane shock wave propagates along a layer of a gas withhigher sound speed than of the ambient gas, the wave refracts ahead into this layer,forming a precursor shock (see Figure 1). The layer gas is deflected upwards by the obliqueprecursor, which forces the formation of a supersonic wall jet. A turning shock deflectsthe flow towards the ground surface. Incident and precursor shock form an angle. Thiscauses a wave pattern very similar to a Mach reflection.

One of the first attempts to simulate this refraction -phenomenon in a shock tube wasreported by White (1952). The shock refraction patterns formed by the interaction of ashock wave with a discontinuous change in sound speed were investigated by Abd -el- Fattahet al. (1976). Much of our understanding of the precursor phenomenon comes from recenttwo -dimensional nonsteady gasdynamic calculations (e.g., Glowacki et al. 1986) and alsofrom experiments, for example, of our institute.

The topic of this paper is the description of the experimental apparatus and especiallyof the visualization techniques that we used to investigate the complicated flow fieldbehind the precursor shock.

2. EXPERIMENTAL APPARATUS

2.1. Shock tube system

The experiments were performed in a shock tube shown schematically in Figure 2. Itconsists of a 1.8 m -long driver section and a 8.8 m -long driven section, both with acircular cross section of 20 cm inner diameter. The driver can be pressurized up to 16bars with various gases. Ambient pressure was used in the driven section. Sheets ofcellulose -acetat served as diaphragm. They were punctured pneumatically by a needle toinitiate the experiment. In tests reported here incident shocks with a shock Mach numberof Ms = 1.7 were used.

The test section is 4 cm wide by 11 cm high by 20 cm long. It is fitted with optical -quality windows.

2.2. Techniques for creating high- sound -speed layers

The precursor shock structure is fundamentally a gasdynamic reaction to a high- sound-speed layer along the surface. Three successful methods (see Figure 3) were developed tocreate a layer of high sound speed: (1) helium layers contained by thin membranes, (2)

helium injected through a ceramic porous plate, (3) hot air layer, formed by a heated foiltechnique.

Of the experiments described in this paper only the helium injection method wasapplied. In this case, a diffusion zone is formed between air and helium. Some precautionmust be applied to produce relatively thin layers of high helium concentration.

2.3. Optical system

For most of the optical investigations a Cranz -Schardin camera was used (see Figure 4).It consists of 24 point- source sparks, each of which is focused on one of the 24 objectivelenses of the camera by the same concave mirror. In the sketch only 3 sparks and lenses

SPIE Vol. 1032 High Speed Photography and Photonics (1988) / 837

Visualization of shock waves in layered media

H. Reichenbach

Fraunhofer-Institut fur Kurzzeitdynamik, - Ernst-Mach-Institut - 4 EckerstraBe, 7800 Freiburg, FRG

ABSTRACT

The airblast precursor is a shock refraction effect formed by the interaction of a shock wave with a high-sound-speed layer. Helium was injected through a porous ceramic plate to create such a layer near the wall of a small shock tube. The flow was made visible by shadow, schlieren and interferometer photography. Incident shock Mach numbers of M = 1.7 were used. A prominent feature of the flow is an unstable supersonic wall jet that develops large-scale, turbulent structures.

1. INTRODUCTION

Shock refraction phenomena occur if shock waves interact with layered gaseous media of different sound velocities. If a plane shock wave propagates along a layer of a gas with higher sound speed than of the ambient gas, the wave refracts ahead into this layer, forming a precursor shock (see Figure 1). The layer gas is deflected upwards by the oblique precursor, which forces the formation of a supersonic wall jet. A turning shock deflects the flow towards the ground surface. Incident and precursor shock form an angle. This causes a wave pattern very similar to a Mach reflection.

One of the first attempts to simulate this refraction -phenomenon in a shock tube was reported by White (1952). The shock refraction patterns formed by the interaction of a shock wave with a discontinuous change in sound speed were investigated by Abd-el-Fattah et al. (1976). Much of our understanding of the precursor phenomenon comes from recent two-dimensional nonsteady gasdynamic calculations (e.g., Glowacki et al. 1986) and also from experiments, for example, of our institute.

The topic of this paper is the description of the experimental apparatus and especially of the visualization techniques that we used to investigate the complicated flow field behind the precursor shock.

2. EXPERIMENTAL APPARATUS

2.1. Shock tube system

The experiments were performed in a shock tube shown schematically in Figure 2. It consists of a 1.8 m-long driver section and a 8.8 m-long driven section, both with a circular cross section of 20 cm inner diameter. The driver can be pressurized up to 16 bars with various gases. Ambient pressure was used in the driven section. Sheets of cellulose-acetat served as diaphragm. They were punctured pneumatically by a needle to initiate the experiment. In tests reported here incident shocks with a shock Mach number of MS = 1.7 were used.

The test section is 4 cm wide by 11 cm high by 20 cm long. It is fitted with optical- quality windows.

2.2. Techniques for creating high-sound-speed layers

The precursor shock structure is fundamentally a gasdynamic reaction to a high-sound- speed layer along the surface. Three successful methods (see Figure 3) were developed to create a layer of high sound speed: (1) helium layers contained by thin membranes, (2) helium injected through a ceramic porous plate, (3) hot air layer, formed by a heated foil technique.

Of the experiments described in this paper only the helium injection method was applied. In this case, a diffusion zone is formed between air and helium. Some precaution must be applied to produce relatively thin layers of high helium concentration.

2.3. Optical system

For most of the optical investigations a Cranz-Schardin camera was used (see Figure 4). It consists of 24 point-source sparks, each of which is focused on one of the 24 objective lenses of the camera by the same concave mirror. In the sketch only 3 sparks and lenses


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are shown. Since there are no moving parts in this system, the optical resolving power islimited primarily by the aperture of the objective lenses. Parallax is minimized by thelarge focal length mirror (f = 350 cm) and by the narrow test sections (4 cm).

The precursor wave system was made visible by the shadow method (see Figure 4) whichrequires a point light source (less than 1 mm diameter here) and focusing on a referenceplane. Mostly, we focused the objective lenses on a plane of r = 40 cm from the center ofthe test section. The schlieren method was used to make density gradients visible. In thiscase, schlieren knives (razor blades proved excellent) were placed at the image plane ofthe light source near the lenses.

Frequently, 12 light paths of the Cranz -Schardin camera were used to produce schlierenpictures and the others to produce alternatively shadow pictures of the same test. Thissimple trick helped significantly to analyze the flow field phenomena.

In addition, single -frame Mach -Zehnder interferograms were made. Usually, a mono-chromatic light source is used. In this case, an interferogram consists of black and mono-chromatic colored fringes. The order of the fringes cannot be determined because they arenondiscernible. In contrast, at a white light source (light of all wavelengths) only onesharp fringe exists with a few colored satellite fringes. The zero -order fringe - wherethe optical path length in both interferometer light beams is the same - is easilydetectable. But the number of the fringes is too small to analyze a flow field.

A good solution used for a long time at EMI is filtered light with a transmittance oftwo different wave length bands. In this case, colored interferograms develop with enoughfringes, where the order of each of them can be determined. The fringes differ from eachother in the color borders.

Interferograms proved a good tool to measure the sound speed distribution in thehelium layer as well as the density distribution in the flow field.

3. PRECURSOR RESULTS

3.1. Sound velocity profiles

To activate a homogenous helium layer, the gas flow through a porous ceramic plate wasfound to be best suited. But to control the sound speed distribution which depends on thehelium concentration, quantitative measurements were made interferometrically. Figure 5shows an interferogram. The sound velocity profile can be measured from the fringe shift.Helium concentrations of more than 90 % were found. This equals a sound speed ratio heliumto air of about c1 /co - 2.3.

3.2. Precursor evolution

The startup of the precursor is shown in the schlieren and shadow pictures of Figure 6.To visualize the basic effects, a relatively thick helium layer was chosen. The lightregion near the wall contains essentially pure helium and the dark band above it is amolecular diffusion layer. The shock front velocity in the helium layer is higher than inair. Therefore, a gas flow with a component in upward direction develops and forms theprecursor. By interaction of the incident shock wave with the leading edge of the heliumlayer a reflected wave (A) (acoustic wave) is created. A triple point (TP') and a slipline(SL') are formed together with a further shock wave which we call Mach wave. The precursor(P) and the Mach wave (M) form an angle. By gasdynamic requirements, a second triple point(TP) is created. This triple point is the origin of another slipline SL and a shock wave T(turning shock). Between the two sliplines, gas is transported to the bottom and turnedand pushed into a forward -orientated direction. So .a jet is formed. Note the laminarcounter -clockwise rollup of helium layer gas above the wall jet gas.

The startup phase of the precursor proceeds to the transition phase if the acousticwave A and the slipline (SL') are reflected at the upper wall of the test chamber, seeFigure 7. If the velocity difference at a slipline is high enough, the laminar sliplinechanges to a turbulent one. The jet causes small shock waves (shocklets) to develop. TheMach lines produced by the rough surface of the ceramic plate indicate a supersonic flowin the jet. An interferogram of the transition phase is shown in Figure 8. The flow behindthe precursor wave is laminar but at later times it starts to get turbulent (see Figure 9).Coherent structures are developing.

3.3. Vorticity sources and turbulence mechanism

Based on the experiments the flow field of a helium layer generated precursor is shownschematically in Figure 10. There are many vorticity sources, some of them are identified.A first group is related to the density layer gradient whereas a second group is caused by

838 / SP /E Vol 1032 High Speed Photography and Photonics (1988)

are shown. Since there are no moving parts in this system, the optical resolving power is limited primarily by the aperture of the objective lenses. Parallax is minimized by the large focal length mirror (f = 350 cm) and by the narrow test sections (4 cm) .

The precursor wave system was made visible by the shadow method (see Figure 4) which requires a point light source (less than 1 mm diameter here) and focusing on a reference plane. Mostly, we focused the objective lenses on a plane of r = 40 cm from the center of the test section. The schlieren method was used to make density gradients visible. In this case, schlieren knives (razor blades proved excellent) were placed at the image plane of the light source near the lenses.

Frequently, 12 light paths of the Cranz-Schardin camera were used to produce schlieren pictures and the others to produce alternatively shadow pictures of the same test. This simple trick helped significantly to analyze the flow field phenomena.

In addition, single-frame Mach-Zehnder interferograms were made. Usually, a mono chromatic light source is used. In this case, an interferogram consists of black and mono chromatic colored fringes. The order of the fringes cannot be determined because they are nondiscernible. In contrast, at a white light source (light of all wavelengths) only one sharp fringe exists with a few colored satellite fringes. The zero-order fringe - where the optical path length in both interferometer light beams is the same - is easily detectable. But the number of the fringes is too small to analyze a flow field.

A good solution used for a long time at EMI is filtered light with a transmittance of two different wave length bands. In this case, colored interferograms develop with enough fringes, where the order of each of them can be determined. The fringes differ from each other in the color borders.

Interferograms proved a good tool to measure the sound speed distribution in the helium layer as well as the density distribution in the flow field.

3. PRECURSOR RESULTS

3.1. Sound velocity profiles

To activate a homogenous helium layer, the gas flow through a porous ceramic plate was found to be best suited. But to control the sound speed distribution which depends on the helium concentration, quantitative measurements were made interferometrically. Figure 5 shows an interferogram. The sound velocity profile can be measured from the fringe shift. Helium concentrations of more than 90 % were found. This equals a sound speed ratio helium to air of about c-/c ~ 2.3.

3.2. Precursor evolution

The startup of the precursor is shown in the schlieren and shadow pictures of Figure 6. To visualize the basic effects, a relatively thick helium layer was chosen. The light region near the wall contains essentially pure helium and the dark band above it is a molecular diffusion layer. The shock front velocity in the helium layer is higher than in air. Therefore, a gas flow with a component in upward direction develops and forms the precursor. By interaction of the incident shock wave with the leading edge of the helium layer a reflected wave (A) (acoustic wave) is created. A triple point (TP 1 ) and a slipline (SL 1 ) are formed together with a further shock wave which we call Mach wave. The precursor (P) and the Mach wave (M) form an angle. By gasdynamic requirements, a second triple point (TP) is created. This triple point is the origin of another slipline SL and a shock wave T (turning shock). Between the two sliplines, gas is transported to the bottom and turned and pushed into a forward-orientated direction. So a jet is formed. Note the laminar counter-clockwise rollup of helium layer gas above the wall jet gas.

The startup phase of the precursor proceeds to the transition phase if the acoustic wave A and the slipline (SL 1 ) are reflected at the upper wall of the test chamber, see Figure 7. If the velocity difference at a slipline is high enough, the laminar slipline changes to a turbulent one. The jet causes small shock waves (shocklets) to develop. The Mach lines produced by the rough surface of the ceramic plate indicate a supersonic flow in the jet. An interferogram of the transition phase is shown in Figure 8. The flow behind the precursor wave is laminar but at later times it starts to get turbulent (see Figure 9). Coherent structures are developing.

3.3. Vorticity sources and turbulence mechanism

Based on the experiments the flow field of a helium layer generated precursor is shown schematically in Figure 10. There are many vorticity sources, some of them are identified. A first group is related to the density layer gradient whereas a second group is caused by

838 / SPIE Vol. 1032 High Speed Photography and Photonics (1988)


the wall jet interactions.

The velocity of the wall jet is larger than the precursor velocity, hence the tip ofthe jet is forced to fold back on itself. This flow contains vortex streets (e.g., SL, SL',WBL), and the turning of this flow by the pressure gradient RW1 causes these layers to gounstable and become turbulent.

4. CONCLUSIONS

The interaction of a shock wave with a gas layer of higher sound speed than of theambient air is cause of a very complicated flow field. The understanding of the differentphenomena was considerably improved by applying classical optical methods such as shadow,schlieren and interference optical techniques. The experiments with helium layers werevery successful, demonstrating and visualizing the following.

- Interaction of the leading edge- Startup phase- Evolution of the precursor- Transition phase- Transition to turbulence at the slipline- Transition to turbulence in the jet flow- Vorticities sources- Turbulence mechanism

We are about to perform additional experiments to quantify the complete evolution of theprecursor also in the three -dimensional case.

5. ACKNOWLEDGEMENTS

I would like to thank Dr. Allen Kuhl, RDA, for his advice, his encouraging discussionsand his continual interest. Dr. George Ullrich, DNA, is thanked for his encouragement andsupport. Without the careful conducting of the experiments by Dipl. -Ing. (FH) W. Schätzleand Mr. W. Gehri, the results could not have been achieved.

This work was partially supported by the US -DNA under contract no. DNA -001 -86 -C -0075.

6. REFERENCES

1. A.M. Abd -el- Fattah, L.F. Henderson and A. Lozzi, "Precursor Shock Waves at a Slow -Fact Interface ", J. Fluid Mech. 76, 157 - 176 (1976).

2. W. J. Glowacki, A.L. Kuhl, H.M. Glaz and R.E. Ferguson, "Shock Wave Interaction withHigh- Sound -Speed Layers ", Shock Tubes and Waves, 187 - 194 (1985).

3. H. Reichenbach and A.L. Kuhl, "Techniques for Creating Precursors in Shock Tubes ",Shock Tubes and Waves, 847 - 853 (1987).

4. D.R. White, "An Experimental Survey of the Mach Reflection of Shock Waves ", Proc.2nd Mid -West Conf. on Fluid Dynamics (1952).

J

Air

Co

HSL CI> Co

J

R

Figure 1. Shock wave propagation into a layered gaseous medium.

HSL


the wall jet interactions.

The velocity of the wall jet is larger than the precursor velocity, hence the tip of the jet is forced to fold back on itself. This flow contains vortex streets (e.g., SL, SL 1 , WBL) , and the turning of this flow by the pressure gradient RV\L causes these layers to go unstable and become turbulent.

4. CONCLUSIONS

The interaction of a shock wave with a gas layer of higher sound speed than of the ambient air is cause of a very complicated flow field. The understanding of the different phenomena was considerably improved by applying classical optical methods such as shadow, schlieren and interference optical techniques. The experiments with helium layers were very successful, demonstrating and visualizing the following.

- Interaction of the leading edge- Startup phase- Evolution of the precursor- Transition phase- Transition to turbulence at the slipline- Transition to turbulence in the jet flow- Vorticities sources- Turbulence mechanism

We are about to perform additional experiments to quantify the complete evolution of the precursor also in the three-dimensional case.

5. ACKNOWLEDGEMENTS

I would like to thank Dr. Alien Kuhl, RDA, for his advice, his encouraging discussions and his continual interest. Dr. George Ullrich, DNA, is thanked for his encouragement and support. Without the careful conducting of the experiments by Dipl.-Ing. (FH) W. Schatzle and Mr. W. Gehri, the results could not have been achieved.

This work was partially supported by the US-DNA under contract no. DNA-001-86-C-0075.

6. REFERENCES

1. A.M. Abd-el-Fattah, L.F. Henderson and A. Lozzi, "Precursor Shock Waves at a Slow- Fact Interface", J. Fluid Mech. 76, 157 - 176 (1976).

2. W. J. Glowacki, A.L. Kuhl, H.M. Glaz and R.E. Ferguson, "Shock Wave Interaction with High-Sound-Speed Layers", Shock Tubes and Waves, 187 - 194 (1985).

3. H. Reichenbach and A.L. Kuhl, "Techniques for Creating Precursors in Shock Tubes", Shock Tubes and Waves, 847 - 853 (1987).

4. D.R. White, "An Experimental Survey of the Mach Reflection of Shock Waves", Proc. 2nd Mid-West Conf. on Fluid Dynamics (1952).

Air

Co

HSL Ci>•C 0

3ET

Figure 1. Shock wave propagation into a layered gaseous medium.



1800

driver section

6980

driven section

test chamber200 110

Figure 2. Shock tube.

a. Membrane Technique

measures in mm

b. Porous Plate TechniquePressure 9uuye Suc tical

c. Heated Foil Technique240

/onflwi 3512+0

Electrode

O 2

±1 Electrode

insulate,-

Figure 3. Test pictures for high -speed layers.

840 / SPIE Vol 1032 High Speed Photography and Photonics (1988)

SPriny

1800

driver section

6980

diaphragm driven section

test chamber / 200 - 110

10680

741 1670

Figure 2. Shock tube.

measures in mm

a. Membrane Technique X/jFrame

/̂~---^^\.

AJW _ He Sucti'oZ

=/J?

^-_ /b. Porous Plate Technique

Pressu Sue tic

c. Heated Foil Technique

Figure 3. Test pictures for high-speed layers.



shadow principle

point source test screenspecimen

---- point sparks

r --

film

camera

lenses f = 55cm

reference plane

Figure 4. Cranz -Schardin camera (principle).

test chamber of theshock tube

Figure 5. Interferogram of a helium layer.

spherical mirrorf = 350cm


shadow principle

point sparks

point source test screen specimen

reference plane

film lenses f = 55cm test chamber of the shock tube

spherical mirror f = 350cm

Figure 4. Cranz-Schardin camera (principle).

Figure 5. Interferogram of a helium layer.



Diffusion La er

Figure 6. Startup period of the precursor.

Figure 7. Transition phase.


Figure 6. Startup period of the precursor.

Figure 7. Transition phase.



Figure 8. Interferogram of the transition phase.

Figure 9. Interferogram of an extended helium layer. Turbulent jet flow.

SPIE Vol 1032 High Speed Photography and Photonics (1988) / 843

Figure 8. Interferogram of the transition phase.

Figure 9. Interferogram of an extended helium layer. Turbulent jet flow.



SP W8 RWy

Figure 10. Interaction of a plain

I = Incident shock waveTP,TP' = Triple pointP = PrecursorTL = Layer of high sound speedSL,SL' = Slipline

RW B

shock wave with a layered gaseous medium.

844 / SPIE Vo% 1032 High Speed Photography and Photonics (1988)

T=RW =B =W. =S =BL =

Turning shockRarefaction waveBow shockVorticity sourceMoving stagnation pointBoundary layer

Figure 10. Interaction of a plain shock wave with a layered gaseous medium.

I = Incident shock waveTP,TP' = Triple pointP = PrecursorTL = Layer of high sound speedSL,SL' = Slipline

T = Turning shockRW = Rarefaction waveB = Bow shockW. = Vorticity sources£ = Moving stagnation pointBL = Boundary layer



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SPIE Proceedings [SPIE 18th Intl Congress on High Speed Photography and Photonics - Xian, China (Sunday 28 August 1988)] 18th Intl Congress on High Speed Photography and Photonics