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Decoding the complexity of turbulence at its origin

Final Report Summary - TURBOFLOW (Decoding the complexity of turbulence at its origin)

The project “TURBOFLOW” had the aim to elucidate the origins of turbulence in simple shear flows such as pipes and channels. From a practical point of view in these fluid flows turbulence is responsible for the major part of friction losses and hence pumping costs. In this study we could eventually explain the nature of the transition, a problem that had been studied since the late 19th century. As demonstrated for a flow between two moving walls, the transition to turbulence corresponds to a spreading process where turbulent clusters compete with the laminar background flow. More precisely it can be characterized as a phase transition that falls into the so called directed percolation universality class and it shares universal features with seemingly unrelated problems such as models of the spreading of viruses in populations or forest fires. While this phase transition explains how patches of turbulence become sustained it does not encompass the transition to fully turbulent flows. As shown in a collaborative study fully turbulent flows arise at yet another transition encountered at slightly larger flow speeds. At this transition the refractory nature that inhibits the growth of turbulent clusters at lower speeds ceases to be relevant and turbulence can now expand continuously. Eventually we could make use of these insights to design control methods for turbulent flows at even larger flow speeds: By altering the mean velocity profile we could for the first time demonstrate that fully turbulent flow can be reverted into laminar flow. In the process we could reduce the friction losses in pipe flow by more than 90%.
We also exploited the insights obtained into the transition mechanism to shed light on related problems, like the transition to turbulence in pulsating flows (i.e. related to blood flows in large blood vessels) and transition in dilute polymer solutions.