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According to Peter Kampf's answer here, soft skin can damp a (vibrational?) mode that cause laminar flow to become turbulent. Thus, it could help delay boundary layer transition. My question is, for a modern large transonic jet (737+), how much may this reduce drag?

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This has mostly been examined for aquatic flow, e.g. with the soft skin of dolphins. Studies were done with test setups because measuring the boundary layer around live dolphins is exceedingly hard.

One study compared a rigid cylinder with a soft one of the same dimensions, using a polyurethane coating. Below is a diagram from that study with 3 denoting the rigid and 6 the soft cylinder. As you can see, the difference is small but consistent over the range of Reynolds numbers tested. Picture source here.

Friction drag coeffcients measured in a water tank

For dolphins, active regulation of muscle tension might be responsible for a resonant vibration of the skin, but much of the literature is still rather speculative. From the study:

Based on experimental research on live dolphins, a hypothesis has been formulated (see Chapter 1); this is that by the regulation of the skin muscle tension dolphins promote resonant interaction with a flow. This leads to interaction of fluctuations of the boundary layer and natural frequency of fluctuation of the dolphin skin. As a result, the skin coverings vibrate with resonant frequency practically without energy consumption. Fluctuations of this coating occur at a frequency that controls the frequency of bursting from the viscous sublayer. This is also the reason for the abnormally low resistance, which has been discovered in the authors’ modeling experiments.

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  • $\begingroup$ This is also the reason for the abnormally low resistance, which has been discovered in the authors’ modeling experiments. What the heck, so dolphins massage the flow around them to reduce drag? $\endgroup$ May 29, 2020 at 12:39
  • $\begingroup$ @Abdullah: Sounds like it. But resonance means that almost no work is done and I think this finding is still rather speculative. $\endgroup$ May 29, 2020 at 12:48
  • $\begingroup$ This may be where Reynolds number can get tricky. Take a look at the face of someone with exposed skin in a rocket sled. Flutter? Anything soft has the potential to deform, store energy, recoil, etc. But birds may have some success with shaping their body form in the airstream with feathers. But at higher "velocity dominated Reynolds numbers" (just made that up) soft and pliable may not be the way to go. $\endgroup$ May 29, 2020 at 16:11

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