Abstract The influence of localized nitrogen transpiration on second mode instabilities in a hypersonic boundary layer is experimentally investigated. The study is conducted using a $$7^\circ$$ 7 ∘ half-angle cone with a length of 1100 mm and small nose bluntness at $$0^\circ$$ 0 ∘ angle-of-attack. Transpiration is realized through a porous Carbon/Carbon patch of $$44 \times 82$$ 44 × 82 mm located near the expected boundary layer transition onset location. Transpiration mass flow rates in the range of 0.05–1% of the equivalent boundary layer edge mass flow rate were used. Experiments were conducted in the High Enthalpy Shock Tunnel Göttingen (HEG) at total enthalpies around 3 MJ/kg and unit Reynolds numbers in the range of $$1.4 \cdot 10^6 \,$$ 1.4 · 10 6 to $$6.4 \cdot 10^6 \, {\text {m}}^{-1}$$ 6.4 · 10 6 m - 1 . Measurements were conducted by means of coaxial thermocouples, Atomic Layer Thermopiles (ALTP), pressure transducers and high-speed schlieren. The present study shows that the most amplified second mode frequencies were shifted to lower values as nitrogen is transpired into the boundary layer. In some cases the instability amplitudes were found to be significantly reduced. The observed frequency reduction was verified to correlate with the change of the relative sonic line height in the boundary layer. The amplitude damping was observed to occur only until the most amplified frequencies were reduced to around 50% of their undisturbed values. When transpiration within this limit was performed shortly upstream of the natural boundary layer transition onset, a transition delay of approximately 17% could be observed. Graphic abstract

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