Development in device, structure, design, and space component mechanisms is essential for performance and cost. Direct human interaction and monitoring are complicated in the aeronautical and space industry for morphing the structure-like winglet movement according to environmental change, which currently takes place by electromechanical sensing and response. This kind of system demands timely maintenance and control. Alteration of this bulkiness is provided by a viable technology called 4D printing, which incorporates the interdisciplinary study of three-dimensional printing, intelligent materials, stimulus, and design. In contrast to static-natured additive manufactured or 3D printed structures, four-dimensional (4D) printing enables transformation in objects under application of the correct environmental command like temperature, ohmic energy, water, light, magnetic energy, and much more. The insight of 4D printing is due to focus-researched intelligent materials such as shape memory polymers (SMP), shape memory alloys (SMA), and their composites. Shape memory polymer composites (SMPC) are an emerging class of insolent structural substances that can be inelastically abnormal and regain its standard shape by an external environmental change which serves in demanding applications like medical, origami, sensors, and robotics, and in aerospace structures such as solar arrays, deployable panels, cells, booms, self-deployable structures, reflector antennas, and more. Herein, this review amalgamates progress of four-dimensional printing with a combination of 3D printing, shape memory polymers, additive effect on its structural as well as shape-memorizing qualities, SMPC, composite fabrication methods, actuation medium, mechanism, and focus on SMPC in future applications with natural stimuli like solar energy, light, vibration, water, and more in self-actuation robots, wearable electronics, satellite hinges, in whole, aero-mechanical sectors, soft robotics industries, innovative electronics areas, defense sectors, and so on.

In earlier work, we have explored the relevance of hydrodynamic stability theory to fully developed turbulent wall flows. Using an extended OrrSommerfeld Equation, based on an anisotropic eddy-viscosity model, it was shown that there exists a wide range of unstable wave numbers (wall modes), which mimic some of the key features of turbulent wall flows. Here we present experimental confirmation for the same. There is good qualitative and quantitative agreement between theory and experiment. Once the dominant coherent structure is obtained from stability theory, control of turbulence would be the next logical step. As shown, the use of a compliant wall shows considerable promise.We also present some theoretical work for bypass transition (Klebanoff/Kmodes), wherein the receptivity of a laminar boundary layer to a vortex sheet in the freestream has been studied. Further, it is shown that triadic interaction between K-modes, 2D TS waves and 3D TS waves can lead to rapid algebraic growth. A similar mechanism seems to carry over to inner wall structures in wall turbulence and perhaps this is the "root cause" for sustenance of turbulence.

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