Two kinds of nonlinearities coexist in viscoelastic fluid flows, i.e., inertia and elasticity, which can engender different types of chaotic states including inertial turbulence (IT), drag-reducing turbulence (DRT), elastic turbulence, and elasto-inertial turbulence (EIT). The state of maximum drag reduction (MDR), the ultimate state of DRT of viscoelastic fluids, is recently regarded as EIT. This Letter quantitatively demonstrates the role of IT and EIT in drag-reducing turbulent flows passing through the parallel plane channels via the contributions of Reynolds shear stress and the nonlinear part of elastic shear stress to flow drag. The nature of DRT is reexamined under a wide range of flow conditions covering a series of flow regimes from the onset of DR to MDR with the Oldroyd-B model. We argue that EIT-related dynamics appears in DRT long before settling to MDR state and competitively coexists with IT in both spatial and temporal domains at moderate and high Reynolds number (Re). More specifically, under a low DR condition, EIT first emerges close to the channel walls. With the increase in elasticity, low-drag EIT gradually replaces a high-drag IT from channel walls to center, resulting in a drastic decrease in flow drag comparing with IT. When EIT dynamics dominates the whole channel, MDR phenomenon occurs. Our findings provide evidence that DRT phenomenon is the result of IT and EIT interaction.

Recently, the nature of viscoelastic drag-reducing turbulence (DRT), especially the maximum drag reduction (MDR) state, has become a focus of controversy. It has long been regarded as polymer-modulated inertial turbulence (IT), but is challenged by the newly proposed concept of elasto-inertial turbulence (EIT). This study is to repicture DRT in parallel plane channels by introducing dynamics of EIT through statistical, structural and budget analysis for a series of flow regimes from the onset of drag reduction to EIT. Some underlying mechanistic links between DRT and EIT are revealed. Energy conversion between velocity fluctuations and polymers as well as pressure redistribution effects are of particular concern, based on which a new energy self-sustaining process (SSP) of DRT is repictured. The numerical results indicate that at low Reynolds number ( $Re$ ), weak IT flow is replaced by a laminar regime before the barrier of EIT dynamics is established with the increase of elasticity, whereas, at moderate $Re$ , EIT-related SSP can get involved and survive from being relaminarized. This further explains the reason why relaminarization phenomenon is observed for low $Re$ while the flow directly enters MDR and EIT at moderate $Re$ . Moreover, with the proposed energy picture, the newly discovered phenomenon that streamwise velocity fluctuations lag behind those in the wall-normal direction can be well explained. The repictured SSP certainly justifies the conjecture that IT nature is gradually replaced by that of EIT in DRT with the increase of elasticity.

Commercial tritium resources available are too scarce to fully supply the future fusion reactors after International Thermonuclear Experimental Reactor (ITER). Tritium self‐sufficiency, ITER fails to fully validate, was regarded as one of the most important issues needed to be solved in the pathway of achieving fusion energy. After ITER, several concepts of fusion engineering test reactors and fusion demonstration reactors have been proposed worldwide, for example, Chinese Fusion Engineering Test Reactor (CFETR), Fusion Nuclear Science Facility (FNSF), DEMOnstration fusion reactor (DEMO) in European Union and Korea. CFETR is in the engineering design phase and would be hopefully completed around 2020. Tritium resources for the reactor start‐up and tritium self‐sufficiency are two primary issues besides the steady‐state operation for CFETR. The objectives of this work are as follows: (a) to introduce the preliminary fuel cycle concept and available tritium resources for CFETR, (b) to evaluate and discuss the tritium demand for CFETR start‐up (phase I: 200 MW) and the feasibility of DD start‐up, (c) to identify the possible pathways to tritium self‐sufficiency through sensitivity analysis based on the design baseline of CFETR, (d) to evaluate the consequences in case of failing tritium self‐sufficiency, and (e) to identify future R&D needed for tritium self‐sufficiency. It is expected to give insights into the question on how to start the reactor in a more economical way, into the feasibility of tritium self‐sufficiency, and into the question on what will happen in case of failing tritium self‐sufficiency.