Large eddy simulations of a turbulent mixing layer periodically excited with fundamental and third harmonic frequency

A better understanding of the mixing behavior of excited turbulent mixing layers is critical to a number of aerospace applications.Previous studies of excited turbulent mixing layers focused on single frequency excitation or the excitation with fundamental and its second harmonic frequency.There is a lack of detailed studies on applying low and higher frequency excitation.In this study,we have performed large-eddy simulations of periodically excited turbulent mixing layers.The excitation consists of a fundamental frequency and its third harmonic.We have used phase-averaging to identify the vortex structure and strength in the mixing layer,and we have studied the vortex dynamics.Two different vortex paring mechanisms are observed depending on the phase shift between the two excitation frequencies.The influence of these two mechanisms on the mixing of a passive scalar is also studied.It is found that exciting the mixing layer with these low and high frequencies has initially an adverse influence on the mixing process;however,it improves the mixing further downstream of the splitter plate with the excitation using a phase shift ofΔφ=πshowing the best mixing performance.The present works shed lights on the fundamental vortex dynamics,and has great potential for aeronautical,automotive and combustion engineering applications.

P2-type layered high-entropy oxides as sodium-ion cathode materials

P2-type layered oxides with the general Na-deficient composition Na_(x)TMO_(2)(x<1,TM:transition metal)are a promising class of cathode materials for sodium-ion batteries.The open Na+transport pathways present in the structure lead to low diffusion barriers and enable high charge/discharge rates.However,a phase transition from P2 to O2 structure occurring above 4.2 V and metal dissolution at low potentials upon discharge results in rapid capacity degradation.In this work,we demonstrate the positive effect of configurational entropy on the stability of the crystal structure during battery operation.Three different compositions of layered P2-type oxides were synthesized by solid-state chemistry,Na_(0.67)(Mn_(0.55)Ni_(0.21)Co_(0.24))O_(2),Na_(0.67)(Mn_(0.45)Ni_(0.18)Co_(0.24)Ti_(0.1)Mg_(0.03))O_(2) and Na_(0.67)(Mn_(0.45)Ni_(0.18)Co_(0.18)Ti_(0.1)Mg_(0.03)Al_(0.04)Fe_(0.02))O_(2) with low,medium and high configurational entropy,respectively.The high-entropy cathode material shows lower structural transformation and Mn dissolution upon cycling in a wide voltage range from 1.5 to 4.6 V.Advanced operando techniques and post-mortem analysis were used to probe the underlying reaction mechanism thoroughly.Overall,the high-entropy strategy is a promising route for improving the electrochemical performance of P2 layered oxide cathodes for advanced sodium-ion battery applications.