Integrated optimization analyses of aerodynamic/stealth characteristics of helicopter rotor based on surrogate model

Abstract Based on computational fluid dynamics （CFD） method, electromagnetic high-frequency method and surrogate model optimization techniques, an integration design method about aerody- namic/stealth has been established for helicopter rotor. The developed integration design method is composed of three modules： integrated grids generation （the moving-embedded grids for CFD sol- ver and the blade grids for radar cross section （RCS） solver are generated by solving Poisson equa- tions and folding approach）, aerodynamic/stealth solver （the aerodynamic characteristics are simulated by CFD method based upon NavieStokes equations and Spalart-Allmaras （S-A） tur- bulence model）, and the stealth characteristics are calculated by using a panel edge method combining the method of physical optics （PO）, equivalent currents （MEC） and quasi-stationary （MQS）, and integrated optimization analysis （based upon the surrogate model optimization technique with full factorial design （FFD） and radial basis function （RBF）, an integrated optimization analyses on aerodynamic/stealth characteristics of rotor are conducted. Firstly, the scattering characteristics of the rotor with different blade-tip swept and twist angles have been carried out, then timfrequency domain grayscale with strong scattering regions of rotor have been given. Meanwhile, the effects of swept-tip and twist angles on the aerodynamic characteristic of rotor have been performed. Furthermore, by choosing suitable object function and constraint condition, the compromised design about swept and twist combinations of rotor with high aerodynamic performances and low scattering characteristics has been given at last.

Recent Rapid Decline of the Arctic Winter Sea Ice in the Barents–Kara Seas Owing to Combined Effects of the Ural Blocking and SST

This study investigates why the Arctic winter sea ice loss over the Barents–Kara Seas（BKS） is accelerated in the recent decade. We first divide 1979–2013 into two time periods： 1979–2000（P1） and 2001–13（P2）, with a focus on P2 and the difference between P1 and P2. The results show that during P2, the rapid decline of the sea ice over the BKS is related not only to the high sea surface temperature（SST） over the BKS, but also to the increased frequency,duration, and quasi-stationarity of the Ural blocking（UB） events. Observational analysis reveals that during P2, the UB tends to become quasi stationary and its frequency tends to increase due to the weakening（strengthening） of zonal winds over the Eurasia（North Atlantic） when the surface air temperature（SAT） anomaly over the BKS is positive probably because of the high SST. Strong downward infrared（IR） radiation is seen to occur together with the quasi-stationary and persistent UB because of the accumulation of more water vapor over the BKS. Such downward IR favors the sea ice decline over the BKS, although the high SST over the BKS plays a major role. But for P1, the UB becomes westward traveling due to the opposite distribution of zonal winds relative to P2, resulting in weak downward IR over the BKS. This may lead to a weak decline of the sea ice over the BKS. Thus, it is likely that the rapid decline of the sea ice over the BKS during P2 is attributed to the joint effects of the high SST over the BKS and the quasi-stationary and long-lived UB events.

Some limit theorems of killed Brownian motion

In this paper, we prove some limit theorems for killed Brownian motion during its life time. The emphases are on quasi-stationarity and quasi-ergodicity and related problems. On one hand, using an eigenfunction expansion for the transition density, we prove the existence and uniqueness of both quasi-stationary distribution (qsd) and mean ratio quasi-stationary distribution (mrqsd). The later is shown to be closely related to laws of large numbers (LLN) and to quasi-ergodicity. We further show that the mrqsd is the unique stationary distribution of a certain limiting ergodic diffusion process of the BM conditioned on not having been killed. We also show that a phase transition occurs from mrqsd to qsd. On the other hand, we study the large deviation behavior related to the above problems. A key observation is that the mrqsd is the unique minimum of certain large deviation rate function. We further prove that the limiting diffusion process also satisfies a large deviation principle with the rate function attaining its unique minimum at the mrqsd. These give interpretations of the mrqsd from different points of view, and establish some intrinsic connections among the above topics. Some general results concerning Yaglom limit, moment convergence and LLN are also obtained.