Installation of a System for Laser-Generated Perturbations in...

14th Int Symp on Applications of Laser Techniques to Fluid Mechanics Lisbon, Portugal, 07-10 July, 2008

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Installation of a System for Laser-Generated Perturbations in Hypersonic Flow

Dirk Heitmann1, Christian J. Kähler

2, Rolf Radespiel

3

1: Institute of Fluid Mechanics, Technische Universität Braunschweig, Germany, [email protected]

2: Institute of Fluid Mechanics, Technische Universität Braunschweig, Germany, c.kaehler@ tu-braunschweig.de 3: Institute of Fluid Mechanics, Technische Universität Braunschweig, Germany, r.radespiel@ tu-braunschweig.de Abstract An optical method for generating localized, controlled perturbations in hypersonic flows has been arranged to analyze the transition mechanism at Mach 6. The setup of the optical perturber consists of two frequency-doubled Nd:YAG double-pulse lasers. By combining these four pulses a wave packet can be created. The perturbations can be inserted at various positions on the model surface or in the free stream. This technique offers the possibility to generate disturbances with adjustable shape, frequency and amplitude. The disturbance can be inserted at practically any position on the model surface or in the free stream. By means of this perturbation method the receptivity of the boundary layer and the spatial evolution of the disturbance can be examined. The installation and setup of this optical perturber is described in the present paper. Preliminary measurements in air at rest are presented. The ambient pressure is taken into account corresponding to the conditions in the hypersonic facility. Two examples with artificial disturbances in the flow on a flat plate model are presented and the possibility to excite selective frequencies is shown. These frequencies are either damped out or amplified depending on shape, frequency and amplitude of the disturbance.

1. Introduction The stability and transition process in super- and hypersonic boundary layers has been studied during the last decades. The transition scenario consists of four elements, corresponding to the most important aspects of the transition problem:

(i.) receptivity, (ii.) linear instability, (iii.) nonlinear instability and (iv.) final laminar-flow breakdown.

A large variety of transition prediction models have been developed, but a physics-based prediction model, which describes all four elements, is not available yet. Experiments concerning the receptivity and transition process in ground-based test facilities are very difficult. E.g. the transition Reynolds number on cone models in different wind tunnels show large scatterings (Schneider, 2004) - mainly due to acoustic noise radiated from turbulent nozzle wall boundary layers. An approach for studying these processes, in spite of this problem, is to generate artificial, controlled disturbances. The transition in hypersonic flows is dominated by the second mode instability waves at frequencies in the range of 100-250 kHz (Mack, 1984), which are not possible to generate mechanically. Only a concept with a high voltage electric discharge is reported in the literature (Maslov et al., 1990). Another approach – the use of an optical perturber –should be capable of generating such high frequency disturbances as well. In Kähler & Dreyer, 2004 and Kähler, 2005 such a perturber was described and successfully tested in subsonic flows. In the latter reference, only single pulses were used. To obtain the ability to adjust the frequency a combination of several lasers is necessary to generate a small wave packet. Such a disturbance method offers the possibility to adjust all relevant parameters, like amplitude, frequency and shape. Furthermore, this method has the advantage of being non-intrusive. No roughness element is created and the position of the


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disturbance can be changed easily – even to the free stream. The action of laser radiation generates a plasma formation if sufficient power densities (q>108W/cm²) are existing. Such a plasma absorbs the laser radiation almost completely. Due to the intensified absorption, a plasma is ignited followed by a shock wave which changes into an acoustic wave after a few microseconds. In the beginning, the plasma front stays ahead of the shock wave and the plasma acquires a shape, extended toward laser radiation. The shape of the plasma depends mainly on the focal length of the employed lens combination. Later, the shock wave separates from the plasma. Due to the finite size of the laser spot on the surface, the shape of the shock wave can be different from a hemisphere. However, when the shock wave distance is an order of magnitude larger than the plasma dimension, the shock wave can be assumed hemispherical with a high degree of accuracy. Depending on the intensity of the laser pulse, the shock wave, the acoustic disturbance or the heated air can be used for the perturbation (see e.g. Breitling et al., 1999; Kähler, 2005).

2. Facility and Model The Hypersonic Ludwig tube of the University of Brunswick has been described previously (Estorf et al., 2004; Wolf et al., 2005). It is a wind tunnel which runs for about 80 ms in a Mach number range between Ma∞ = 5.8-5.95 depending on the axial position in the test section and on the unit Reynoldsnumber. The latter can be varied between [3-20] ×106 1/m. The driver tube pressure p0 is measured prior to each run with an accuracy of ±1%. The ratio of initial pressure to total pressure after the expansion p0/p1t is known from previous measurements. The total temperature is measured during each run by two fast thermocouples, one of them on the upper side of the storage tube and the other on the lower side. Temperature stratification with a difference of at most 30 K has been measured. The uncertainties in Mach number, pressure and temperature result in an uncertainty of the Reynoldsnumber of not more than 3%. The noise level of the facility, defined as normalized Pitot pressures PitPit p/'p , is between 1.3 and 1.5% (Heitmann et al., 2008).

A flat plate with sharp leading edge, provided by the Institute of Aerodynamics and Gasdynamics (IAG), is used for the experiments. Round inserts can be placed at three positions. Each of them offers different positions to mount the sensors. The model was already used for measurements of second mode instability waves at natural transition (see Rödiger et al., 2008).

3. Measurement Equipment, Data Acquisition and Data Processing The flat plate was instrumented with four high-frequency pressure sensors of type PCB M131A32. The sensors were flush mounted in the surface at different positions. The diameter of the active area is 3.18 mm. Power was supplied by a PCB 482A22 instrument which, at the same time, performed signal conditioning. The sensors have sensitivities between 140.5 and 169 mV/psi. For further details the reader is referred to the manufacturer PCB Piezotronics, 2007. The data of the pressure sensors was sampled with a Spectrum M2i.4652 transient recorder. The sampling rate was set to 3 MHz and the data was stored in 16 bit format. The schlieren pictures were made with a CMOS camera (LaVision FM3S). The laser beam profiles were examined with a C-Cam BCi5-1300 camera and MrBeam software from Laser-Laboratorium Göttingen e.V. (LLG).


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Fig. 2: Beam profile of laser No. 2 (2D-FFT filtered to reduce interference pattern)

4. Perturbation System

4.1 Setup

Two Quantel Brilliant Nd:YAG double pulse lasers are used to generate the disturbances. The employed beam combination is shown in Fig. 1. The infrared beams are combined using a dielectric polarizer at the Brewster angle (4), which reflects the p-polarized beam while transmitting the s-polarized beam. A retardation plate (5) transforms both linearly polarized beams into circularly polarized light before it enters the frequency doubler (6). Here, the second harmonic (532 nm) is generated from the fundamental wavelength (1064 nm). The dichroic mirror (7) separates the two wavelengths by reflecting the second harmonic and transmitting the 1064 nm-wavelength. The beams of lasers 3 and 4 are reflected outside the laser housings, with two mirrors in such that the beams have an offset of only a few millimeters to the beams of lasers 1 and 2. After an optical path length of a few meters this offset is negligible. The pulse length is approximately 4.2 ns with a maximum energy of 150 mJ/pulse at a wavelength of 532 nm. Basis for the following experiments is the introduction of an exactly defined and extremely reproducible artificial disturbance. Therefore, the laser properties have to be measured, because these properties depend strongly on the state of the laser (age of the flashlamps and the resonator, impurity and microscopic damages on optical components) and deviate from the manufacturer's data, because these are measured only with maximum energy and not with the pulse power interesting here. Due to the low power the thermal equilibrium might deviate significantly from the equilibrium at maximum energy, which could cause a decrease of the beam quality. To check for such a decrease beam profiles were measured for different energies per pulse. No significant change in the intensity distribution was found for different energies per pulse. Fig. 2 shows exemplarily the beam profile of laser 2. Several intensity peaks are visible, displaying the multi-modes of the laser. A spectral filter was applied on the picture to reduce the interference pattern. The energy and pointing stability of the four lasers was examined using a C-Cam BCiI5 camera. The energy stability was found to be between 2.7 and 7.3%. The pointing stability was measured to be between 35-135 µm, measured at a distance of about 1m to the laser exit. In the experiments, one of the first pulses was used and it appeared reasonable to check for an influence of the thermal equilibrium of the laser on the beam parameters. Thus, the first 250 pulses of laser No. 3 at medium power corresponding to the experiments were recorded. No differences between the first and later pulses could be noticed.

Fig. 1: Four-pulse laser system: 1, λ/2-retardation-plate; 2, Rmax mirror at 1064 nm; 3, Beam dump; 4, Dielectric polarizer; 5, λ/4-retardation-plate; 6, Crystal doubler housing; 7, Dichroic mirror; 8, Mirror


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4.2 Perturbation in resting air

When the surface is hit by the laser pulse, the material is locally heated, melted and vaporized. Due to the intensified absorption a plasma is ignited followed by a shock wave which changes into an acoustic wave after a few microseconds. Either the shock wave, the acoustic disturbance or the heated air can be used for the perturbation (see e.g. Breitling et al., 1999; Kähler, 2005). In the latter reference the expansion of the shock wave was examined by means of schlieren pictures, similar to the shadow pictures shown in Fig. 4. In Kähler & Scholz, 2003 the perturbation method was examined with BOS pictures. The velocity of the shock wave directly after separation from the plasma (∆t = 3 µs) was measured to be about 700 m/s. After 15 µs the velocity has decreased to the speed of sound. At this time the shock wave has covered a distance of approximately 9 mm. In Fig. 3 the time shapes of the pressure pulses of an optical breakdown are shown. The shock wave reaches the first sensor after 30 µs. The propagation of the shock wave in Fig. 3 corresponds well with the propagation measured from schlieren pictures in Kähler, 2005. The propagation speed could be measured to be 348.0 m/s, which matches the velocity calculated from schlieren pictures at those distances and at an ambient pressure of 1 atm.

Only the propagation of the shock wave could be resolved with the schlieren pictures. The pressure sensors give additional insight into the flow phenomena inside the shock wave. After the shock wave has passed, the pressure is, for a short time, lower than before the disturbance, followed by a damped oscillation. Chumakov et al., 2003 observed a similar behavior in their experiments and gave the following explanation. Due to an acoustic dispersion of the shock wave, acoustic waves are radiated towards the center. The acoustic dispersion becomes apparent when examining the length of the pressure peak of the shock wave. The pressure peak of the shock wave in Fig. 3 broadens with increasing running length from about 7 µs (∆x = 16 mm) to 14 µs (∆x = 232 mm). The broadening of the shock wave peak corresponds to the emission of acoustic waves toward the center. After a very short time the plasma becomes a volume of high temperature and low density gas. These acoustic waves are transformed into a shock wave when they hit each other in the volume of hot and low density gas in the center, which is reflected from the center and forms the oscillations shown in the pressure traces in Fig. 3. The frequency of the damped oscillation was

Fig. 4: Shadow pictures of perturbation method in air at rest

Fig. 3: Pressure traces of an optical air breakdown

∆∆∆∆t = 35 µµµµs

∆∆∆∆t = 75 µµµµs

∆∆∆∆t = 35 µµµµs


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Fig.6: Time difference between laser pulse and pressure peak as function of counter pressure

measured to about 18 kHz and seems to depend mainly on the energy of the laser pulse. In Fig. 3 the propagation was shown for an ambient pressure of 1 bar. In the wind tunnel there are static pressures in the range of 10 mbar. Therefore, Fig. 5 shows the shock wave –in resting air as well – for different pressures ranging from 800 to 11 mbar. The figure shows the pressure signal at a distance of 96 mm from the focussed beam. The plasma was ignited at 0 µs. At lower counter pressures the amplitude is smaller by a factor of ten. Furthermore, the shock wave reaches the sensor after a smaller ∆t. This is predicted from different semi-analytic formulas. The Sedov-Taylor scaling is given as example below (see e.g. Jeong et al., 1999). This scaling predicts a power law dependence.

( )5.0

05.2SS

ρ

Eαrrt∆

−

∞

= , with

This behaviour is apparent in the measurement shown in Fig. 6, as well. However, the measurement shows better agreement to 12.0pt∆ ∝ .

So far only single laser pulses have been shown. In Fig. 7 a wave packet test, consisting of four pulses, is shown. The power of the lasers was adapted using different Q-Switch settings. The delay between the single shots was set to 14.29 µs, which represents a frequency of 70 kHz. This test was performed at an ambient pressure of 1 bar. All three sensors could detect the four pulses. The measured pressure corresponds again well with the distance to the plasma. However, looking at the sensor closest to the plasma, it is apparent that every pulse causes a bigger pressure rise than the subsequent pulses. The sensors with 76 and 92 mm distances show higher amplitude in the third pulse than in the second pulse. Fig. 8, which shows shadow pictures of double pulses, explains this behaviour. The first plasma ignites at the model surface. The plasmas of the subsequent pulses ignite when the laser rays are in the region of the shock wave of the previous pulse. The amplitude

Fig. 5: Pressure signal of a shock wave by a single focused laser pulse

E0 energy at point-explosion α constant (1.175 for spherical propagation) ρ∞ density of ambient medium rS radius of shock wave ∆t time difference until shock wave reaches sensor


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of the first pulse is twice as high as that of the subsequent ones because it propagates only as a half sphere. Additionally, the distance from the sensor to the shock wave origin changes. The problem of the plasma igniting above the model results from the lens combination with a large effective focal length. This focal length is necessary to position the lenses outside of the test section. Another parameter for the position of the plasma ignition is the ambient pressure. In the tests in hypersonic flow with static pressures of about 10 mbar, which are shown later, all plasmas ignited at the surface as planned.

5. Application of Perturbation System to hypersonic flow In this section two cases with introduction of disturbances into the flow are shown. The flow conditions for both cases were the same (Ma 6, p0 = 5bar, T0 = 500 K, Re/l = 4.3 × 106 1/m). The disturbance position was 100 mm from the leading edge. In previous experiments without disturbances the amplitude growth could only be detected at the rear positions of the model for this Reynoldsnumber. This low Reynoldsnumber was chosen to clearly separate the effects of natural transition from the effects of the perturbation. In the first test a single point-shaped pulse with high energy (E ≈ 120 mJ) was inserted on the surface. Fig. 9 shows the pressure signals of three sensors at different distances to the location where the laser hit the surface. The laser pulse is fired at t = 0. The sensors show two pressure peaks. The first corresponds to the propagation of the spherical wave in flow direction (U∞ + c; U∞: free stream velocity; c: propagation speed of perturbation) and the second one to the propagation against the flow direction (U∞ - c). These two peaks are shown in Fig. 9 for the sensor at 16 mm distance. Immediately after the plasma ignites the velocity is much higher than the free stream speed in the facility. I.e. the shock wave propagates partially against the flow direction, before it slows down to the speed of sound. Therefore this part of the shock wave interacts with the heated model, where the laser hit the surface. This interaction deforms the shock wave and decreases its amplitude and velocity. For this reason it is not possible to determine the free stream and

Fig. 8: Shadow pictures of a double pulse ∆t = 14.29 µs between the pulses

Fig. 7: Pressure traces of a wave packet of four pulses

∆∆∆∆t = 20 µµµµs

∆∆∆∆t = 20 µµµµs

∆∆∆∆t = 60 µµµµs


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perturbation velocity from the time difference between the two pressure peaks of a single sensor. It is more reasonable to take only the first pressure peak (U∞ + c) of the different sensors into account. The shock wave reaches the first sensor after 7 µs, the second after 19 µs and the third after 287 µs. This results in the following velocities U∞ + c:

U∞ + c [m/s] Calculated from

2079 ∆t (Laser – sensor 1) 1486 ∆t (sensor 1 – sensor 2) 1119 ∆t (sensor 2 – sensor 3)

When the disturbance reaches the sensors 2 and 3 (distance to disturbance 32 mm and 332 mm) the propagation velocity of the wave should have decreased to the speed of sound. The speed of sound is approximately 155 m/s (T∞ ≈ 60 K). So the convection velocity of the wave is about 964 m/s. This is slightly higher than the free stream velocity in the facility. The high velocity is believed to result from a small angle of attack of the model. The pressure trace immediately behind the laser pulse shows the pressure peak clearly. The signal after the peak looks different from the signal in air at rest. The signal does not oscillate around zero. Instead, it remains at a higher level for about 300 ms. The damped oscillation, at about 18 kHz for air at rest, is no longer visible. An oscillation of about 25 kHz can allusively be seen, but it is not clear if it results from the laser pulse or from the flow. Fig. 11 shows a time resolved Fourier spectrum of the signal from the sensor with 16 mm distance. It was computed using a windowed Fourier transformation. Therefore the signal was divided into windows of 512 data points (170 µs). These windows were multiplied with a normalized Blackman window and Fourier transformed. A spectrum of a signal prior to the tunnel run was subtracted, to reduce the noise of the measurement chain. The windows overlapped with 256 data points. The figure shows that the disturbance of the flow is confined to the very short time period, when the shock wave flows over the sensor. Practically all frequencies are excited. The large amplitude at 300 kHz is sensor specific and appears in all signals. Frequencies in the range of 0-10 kHz show a large amplitude for about 300 µs. The schlieren pictures in Fig. 10 show the existence of a Mach line starting at the point of disturbance. This line persists in all schlieren pictures (pictures were made up to ∆t = 100 µs). After the laser pulse the model is locally heated. This results in an increased pressure which causes the Mach line. The pressure increase of the sensor with ∆x = 16 mm is likely caused by this Mach line. In Fig. 12, an extract from the time-frequency plane is taken for the sensors at distance of 16 and 32 mm. There are some especially excited frequencies (35 kHz and the harmonics at 75, 110, 150, … kHz). The sensor at 32 mm measured an amplified frequency at 35 kHz. The other frequencies are damped out. The harmonics of the 35 kHz peak are a little bit more slowly damped. This first test showed the possibility to introduce a disturbance into the flow. A single point-shaped pulse with large amplitude results in an excitation of all frequencies. There appear some dominant frequencies; however the frequencies are damped out with increasing running length.


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To apply the method of artificial disturbances on measurements of stability theory, the disturbances should obviously be as small as possible instead of being as large as in the example shown above. Furthermore, the disturbance should resemble the instability waves of natural transition. Second mode waves, according to theory, have no slip angle and the amplified frequencies were measured in previous tests to be in a range between 70 and 130 kHz (Roediger et al., 2008). The second example therefore covers a test with a four pulse wave packet (f = 70 kHz) of line-shaped disturbances. The energy per pulse was set to 49 mJ/pulse. Due to the increased focused area, this energy appeared to be sufficiently small for stability tests. In Fig. 13 time-frequency planes for the

Fig. 12: Spectra of pressure fluctuations for two sensors with different distance to a point-shaped disturbance

Fig. 11: Time-frequency plane of a perturbation with a single point-shaped laser pulse (measured at 16 mm distance)

Fig. 10: Schlieren pictures of a single point-shaped laser pulse in hypersonic flow (a, flat plate; b, shock wave; c, Mach line emanating from disturbance location; d, Mach line from leading edge)

Fig. 9: Pressure trace of a single point-shaped laser pulse in hypersonic flow

∆∆∆∆t = 50 µµµµs

∆∆∆∆t = 20 µµµµs

a

b

a

b

c

d

U∞


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sensors at distances of 16 and 32 mm from the disturbance are shown. They were obtained in the same way as Fig. 11. The start of the wave packet is again at t = 0 µs. In the first picture a small disturbance can be seen at 70 kHz. The second sensor shows a slightly larger amplitude, showing the amplification of the disturbance.

6. Conclusion An experimental setup to study hypersonic boundary layer stability was presented. The disturbances were generated optically, with the advantages of being non-intrusive and allowing all relevant parameters to be changed easily, such as frequency, amplitude, shape of the disturbance and perturbation position. The perturbation method was investigated concerning the pointing and energy stability of the different lasers. First results in air at rest showed the possibility to generate wave packets with adjustable frequency and amplitude. The formation of a shock wave additionally results in the excitation of another frequency (18 kHz), which is probably caused by acoustical dispersion of the shock wave. However, in stability experiments with low amplitude of the wave packet, the introduced oscillation is very small. One example with high energy per pulse was given so that the peak of the shock wave could be detected by all sensors. This example allowed calculating the velocity of the disturbance in the flow. Additionally the frequency spectra during the disturbance exhibited the existence of special excited frequencies in the boundary layer. The low-frequency oscillation could only be detected, i.e. it has no disturbing influence. A second example was given with the aim to generate an amplified disturbance. The frequency of mutually amplified disturbances was then introduced into the flow with a wave packet consisting of four pulses with low amplitude. It was shown that an excitation of selective frequencies is possible with this method. This disturbance method seems promising for further stability experiments. In future experiments it is planned to change additional parameters, such as position on the model or Reynolds number, to obtain a larger portion of the stability diagram. Beyond this, it seems reasonable to generate

Fig. 13: Time-frequency planes of a perturbation with a wave packet of four line-shaped pulses (E = 49 mJ/pulse, f = 70 kHz)


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disturbances outside the boundary layer. By this, the thermal disturbance does not contribute to the perturbation, and the receptivity of the boundary layer to pressure disturbances can be investigated.

Acknowledgements The authors would like to thank H. Knauss, T. Rödiger and E. Krämer, who provided the model and parts of the measuring equipment. This research was supported by the German Research Foundation (DFG) within the project KA 1808/2-1.

References BREITLING, D.; SCHNITTENHELM, H.; BERGER, P.; DAUSINGER, F., HÜGEL, H.,

Shadowgraphic and interfermometric investigations on Nd:YAG laser- induced vapour /

plasma plumes for different processing wavelength, Applied Physics A, pp. 505-508, 1999. CHUMAKOV, A. N.; PETRENKO, A. M.; BOSAK, N. A., Features of the time shape of the

pressure pulses of optical air breakdown, J. of Engineering Physics and Thermophysics, Vol. 76, No. 4, 2003.

ESTORF, M; WOLF, T; RADESPIEL, R, Experimental and numerical investigations on the

operation of the Hypersonic Ludwieg Tube Braunschweig, 5th European Symposium on Aerothermodynamics for Space Vehicles, 2004.

HEITMANN, D.; KÄHLER, C.J.; RADESPIEL, R.; RÖDIGER, T.; KNAUSS, H.; KRÄMER, E., Disturbance-Level and Roughness-Induced Transition Measurements in a Conical Boundary

Layer at Mach 6, Paper AIAA-2008-3951, 26th AIAA Aerodynamic Measurement Technology and Ground Testing Conference, Seattle, USA, 2008.

JEONG, S.H.; GREIF, R.; RUSSO, R.E., Shock wave and material vapour plume propagation during excimer laser ablation of amuninium samples, J. Phys, D: Appl. Phys. 32, pp. 2578-2585, 1999.

KÄHLER, C.J., Perspective of Non-Intrusive Flow Control with Lasers, CEAS/KATnet Conference on Key Aerodynamic Technologies, Bremen, 2005.

KÄHLER, C.J.; DREYER, M., Dynamic 3D stereoscopic PIV and schlieren investigation of

turbulent flow structures generated by laser induced plasma, 12th International Symposium on Applications of Laser Techniques to Fluid Mechanics, Lisbon, Portugal, July 12–15 2004.

KÄHLER, C.J.; SCHOLZ, U., Investigation of laser-induced flow structures with time-resolved

PIV, BOS and IR technology, 5th International Symposium on Particle Image Velocimety, Busan, Korea, Sept. 22–24 2003.

MACK, L. M., Boundary Layer Linear Stability Theory, Special Course on Stability and Transition

of Laminar Flow, AGARD Rept. 709, pp. 379-409, March 1984. MASLOV, A.A.; KOSINOV, A.D.; SHEVELKOV, S.G., Experiments on the stability of

supersonic laminar boundary layers, J. Fluid Mech., 219, pp. 621-633, 1990. PCB Piezotronics, “Pressure and Force Sensors Division, Pressure Catalog,”

http://www.pcb.com/Linked Documents/Pressure/PFScat.pdf, 2007. RÖDIGER, T.; HEITMANN, D.; KNAUSS, H.; SMORODSKY, B.V.; BOUNTIN, D.A.;

MASLOV, A.A.; RADESPIEL, R.; KRÄMER, E., Hypersonic instability waves measured on a

flat plate at Mach 6, 14th International Conference on Methods of Aerophysical Research (ICMAR 2008), Novosibirsk, Russia, 2008.

SCHNEIDER, S., Hypersonic Laminar-Turbulent Transition on Circular Cones and Scramjet

Forebodies, Prog. Aerosp. Sci. Vol.40, pp1-50, 2004. WOLF, T.; ESTORF, M.; RADESPIEL, R., Simulation of the Time-Dependent Flow Field in the

Hypersonic Ludwieg Tube Braunschweig, 4th Atmospheric Reentry Vehicles & Systems, 2005.

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