Hypersonic turbulent quantities in support of Morkovin’s hypothesis
Nature CommunicationsTL;DR
Imagine a super-fast airplane flying, five or six times the speed of sound. The air flowing over its skin is incredibly chaotic and turbulent, like a raging river. Back in the 1960s, a scientist named Morkovin proposed a clever idea: if you just account for how the air gets squeezed and stretched (its density changes), this super-fast, chaotic air actually behaves a lot like the slow-moving, well-understood flow of water in a pipe. This makes it much easier to predict things like friction and heat. The problem was, nobody could properly measure one of the key 'up-and-down' wobbles in this chaotic flow to prove it. This study used a special laser technique with krypton gas to finally measure that wobble. They found it matched Morkovin's old idea perfectly, confirming a foundational principle of high-speed flight.
This paper presents boundary-layer profiles of streamwise mean and streamwise/wall-normal fluctuation data ( \(\overline{u},{u}_{\,{{\rm{RMS}}}}^{{\prime} },{v}_{{{\rm{RMS}}}\,}^{{\prime} }\) ) recorded with Krypton Tagging Velocimetry (KTV) at 100 kHz in a hypersonic, turbulent, zero-pressure-gradient boundary layer. The edge Mach number, wall-to-recovery temperature ratio, and friction Reynolds number are (\(M∞ = 6.4, Tw/Tr = 0.54, Reτ = 450\), and (\(M∞ = 6.0, Tw/Tr = 0.17, Reτ = 780\)), for the ‘cold-flow’ and ‘enthalpy-matched’ conditions, respectively. The KTV data agrees with direct numerical simulation (DNS) within the error bounds of the experiment down to as low as 10% of the boundary-layer thickness (\(y/δ ≈ 0.1\)). The KTV and DNS data agree with incompressible laser-doppler anemometry (LDA) data after applying the Morkovin scaling, which accounts for mean density differences across the boundary layer. Therefore, the experimental data presented are supportive of Morkovin’s hypothesis, which is fundamental to our understanding of supersonic and hypersonic compressible turbulence. These are the first such wall-normal fluctuation measurements to support the hypothesis first proposed in 1962. Morkovin’s hypothesis establishes a comparison between incompressible and compressible flows and is essential for understanding supersonic and hypersonic turbulence. In this work, the authors present the measurements of wall-normal fluctuations that support the hypothesis proposed in 1962.
- 1First wall-normal fluctuation measurements supporting Morkovin’s hypothesis.
- 2Krypton Tagging Velocimetry used to measure hypersonic turbulent boundary layers.
- 3Agreement with DNS and LDA data underlines the validity of Morkovin scaling.
- 4Findings are essential for understanding compressible turbulence in supersonic flows.
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