Investigations of internal and external flow at high speeds have paramount importance in arriving at a sustainable propulsion and effective aerodynamics for supersonic/hypersonic transit. Mixing gaseous fuel and air mixture at a higher velocity is difficult to predict and model, owing to the complicated non-linear process involved in scalar mixing. Similarly, fluctuations propagating upstream due to combustor back pressure variations in a hypersonic inlet could be violent and render the shock systems to remain in a self-sustained oscillating state under certain operating conditions. In the present talk, representative cases of internal and external flows at tri-sonic speeds from my research activities will be summarized. Firstly, findings of mixing length in a supersonic confined jet will be examined through different experiments. The tonal noise suppression using rear face modifications in the transonic cavity flow will be further discussed through both experiments and computations. Later, shock-related unsteadiness arising from the leading-edge separation in external flows at both supersonic and hypersonic Mach numbers will be elaborated through experimental and computational findings.