# 6-Ultrasonically Absorptive material

**OCTRA as Ultrasonically Absorptive Thermal Protection Material for Hypersonic Transition Suppression analyzed on a 7° Cone**

*Viola WARTEMANN ^{1}*, Alexander WAGNER^{2}, Divek SURUJHLAL^{3}, *

*Giannino PONCHIO CAMILLO*

^{4},*Christian DITTERT*

^{5}, Carolin RAUH^{6}** Corresponding author*

^{1* }*German Aerospace Center (DLR), Spacecraft Department, Institute of Aerodynamics and Flow Technology, **Lilienthalplatz 7, 38108 **Braunschweig, Germany, Viola.Wartemann@dlr.de*

^{2 }*German Aerospace Center (DLR), Spacecraft Department, Institute of Aerodynamics and Flow Technology, Bunsenstraße 10, 37073 Göttingen**, Alexander.Wagner@dlr.de*

^{3 }*German Aerospace Center (DLR), Spacecraft Department, Institute of Aerodynamics and Flow Technology, Bunsenstraße 10, 37073 Göttingen**, **Divek.Surujhlal@dlr.de*

^{4 }*German Aerospace Center (DLR), Spacecraft Department, Institute of Aerodynamics and Flow Technology, Bunsenstraße 10, 37073 Göttingen**, **Giannino.PonchioCamillo@dlr.de*

^{5,6 }*German Aerospace Center (DLR), Department of Space System Integration, Institute of Structures and Design, **Pfaffenwaldring 38-40, 70569 Stuttgart **Carolin.Rauh@dlr.de*

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**Keywords: **Ultrasonically Absorptive Thermal Protection Material, OCTRA, Hypersonic Transition Suppression, Stability Analyses, Second Mode Instability, High Enthalpy Shock Tunnel Göttingen (HEG)

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**Abstract**

The increase of the laminar portion of a boundary layer is of critical importance to the design and optimization of future hypersonic transport vehicles. This motivates the development of concepts to control hypersonic boundary layer transition. In the present paper an ultrasonically absorptive porous coating with random microstructure is used to passively control boundary layer transition.

The second mode instability, commonly referred to as Mack mode [1], is the dominant mode for essentially 2D boundary layers at high local Mach number (M_{e} > 4) and/or cold walls. A strong stabilization effect of the second mode instability above porous surface models with regular, cylindrical pores was shown theoretically and experimentally by Fedorov et al. [2] and Rasheed et al. [3]. Analogous results were presented by e.g. Lukashevich et al. [4], who investigated randomly structured felt metal.

First studies with randomly structured carbon-carbon ceramic (C/C) were conducted by Wagner et al. [5] in the HEG and were compared with numerical linear stability theory, short LST, predictions by Wartemann et al. [6]. In all cases a stabilization effect on the second mode instability was observed, resulting in a significant delay of transition onset.

The starting material C/C (Wagner et al. [5]) has two main disadvantages: its limited oxidation resistance and the low mechanical strength, which could be critical for real hypersonic flight applications.

Carbon fiber reinforced silicon carbide (C/C-SiC) is highly suitable as thermal protection material (TPS) and was successfully tested as TPS during multiple flight test programs (see e.g. Weihs et al. [7]). This dense C/C-SiC TPS material was not developed for the application as an acoustic absorber.

To close the gap between the porous C/C and the dense C/C-SiC, a new material based on C/C-SiC was developed [8]. Contrary to the dense C/C-SiC materials, the new C/C-SiC material, also known as OCTRA (Optimized Ceramic for Hypersonic Applications), has a porosity and permeability like C/C. The present paper addresses the numerical rebuilding of the OCTRA absorber behavior using an impedance boundary conditions together with linear stability analysis.

The numerical results are compared with wind tunnel tests, which were performed in the HEG at Mach 7.5 and different unit Reynolds numbers. A 7° half-angle cone model with a nose radius of 2.5 mm and a total length of about 1.1 m was used. The measurements are compared with the numerical calculations of the original C/C material and the improved OCTRA material. The influence on the second modes and the transition itself are investigated.

Figure 2 provides exemplary for the abstract the second modes at the selected PCB sensor positions at a free stream unit Reynolds number of Re_{m} =1.4 × 10^{6}/m.:

- a) x = 0.650 m
- b) x = 0.785 m

On the right ordinate figure 2 shows the measured amplitude spectral density (ASD). It was derived by conducting a discrete Fourier transformation of the measured pressure fluctuations recorded in the test time, as a function of the measured frequency marked as symbols.

The measurements on the smooth surface are marked with black color, the measurements on the porous C/C surface with red and on the porous OCTRA surface with blue. The same colors are used for the LST calculations (e^{N} = f(f)). As expected, the calculated/measured second mode is amplified in streamwise direction.

Due to the increase of the boundary layer thickness the frequencies of the second modes are shifted to lower values, which can be explained with the following relation between the boundary layer thickness δ and the wavelength λ [1]: λ ≈ 2 δ.

The measured ASD data cannot be compared directly with the numerically calculated e^{N}-values. However, it is possible to compare the damping of the second mode and its frequency range.

The comparison of the calculated/measured frequency range, using the maximum of the functions, shows a difference of less 5% for the smooth wall (comparing the maximum of the black lines with the black symbols) as well as for the C/C material (comparing the maximum of the red dashed lines with the red symbols), which is in good agreement.

Analyzing the damping of the second modes due to the C/C material, was already analyzed in [6]. These data are added here for completeness and to demonstrate the improved behavior of the new OCTRA material: The measured/calculated damping of second mode is higher on the OCTRA surface as on the C/C surface.

In the experiments at the selected sensor positions the second modes are completely damped or in the range of the background noise level. The LST delivers the same trend: the damping of the second mode from the numerically-rebuilt OCTRA material is higher than for the C/C material. Nevertheless, applying OCTRA the measured damping is higher than the predicted. These differences and the corresponding transition shift will be discussed in detail in this paper.

**References **

[1] Mack, L.M.: Boundary layer linear stability theory, AGARD – Special Course on Stability and Transition of Laminar Flow (1984)

[2] Fedorov, A.V., Malmuth, N.D., Rasheed, A., Hornung, H.G.: Stabilization of hypersonic boundary layers by porous coatings, AIAA 39(4), 605–610 (2001)

[3] Rasheed, A., Hornung, H.G., Fedorov, A.V., Malmuth, N.D.: Experiments on passive hypervelocity boundary-layer control using an ultrasonically absorptive surface, AIAA 40(3), 481–489 (2002)

[4] Lukashevich, S.V., Morozov, S.O., Shiplyuk, A.N.: Experimental study of the effect of a passive porous coating on disturbances in a hypersonic boundary layer, Journal of Applied Mechanics and Technical Physics 54(4), 572–577 (2013). doi:10.1134/S002189441304007X

[5] Wagner, A., Hannemann, K., Wartemann, V., Giese, T.: Hypersonic Boundary-layer Stabilization by Means of Ultrasonically Absorptive Carbon-carbon Material, 51st AIAA Aerospace Sciences Meeting, Texas (2013). doi:10.2514/6. 2013-270

[6] Wartemann, V., Wagner, A., Kuhn, M., Eggers, T., Hannemann, K.: Passive hypersonic boundary layer transition control using an ultrasonically absorptive coating with random microstructure, Procedia IUTAM 14 (2015). doi:10.1016/j.piutam.2015.03.068

[7] Weihs, H.: Sounding Rockets for Entry Research: SHEFEX Flight Test Program, 21st ESA Symposium on European Rocket and Balloon Programmes and Related Research, Vol. SP-721 (2013)

[8] Dittert, C., Kütemeyer, M.: Octra – Optimized Ceramic for Hypersonic Application with Transpiration Cooling, Advances in High Temperature Ceramic Matrix Composites and Materials for Sustainable Development, Vol. 263, 2017. doi:10.1002/9781119407270.ch37