No abstract

A numerical study was made of flow behind a circular cylinder in a uniform flow, where the cylinder was rotationally oscillated in time. The temporal behavior of vortex formation was scrutinized over broad ranges of the two externally specified parameters, i.e., the dimensionless rotary oscillating frequency (0.110рS f р0.220) and the maximum angular amplitude of rotation ͑ max ϭ15°, 30°, and 60°͒. The Reynolds number (ReϭU ϱ D/) was fixed at Reϭ110. A fractional-step method was utilized to solve the Navier-Stokes equations with a generalized coordinate system. The main emphasis was placed on the initial vortex formations by varying S f and max. Instantaneous streamlines and pressure distributions were displayed to show the vortex formation patterns. The oscillatory forcing was in the vicinity of the lock-on range, which can be applied to flow feedback control afterwards. The vortex formation modes and relevant phase changes were characterized by measuring the lift coefficient (C L) and the time of negative maximum C L (t ϪC L max) with variable forcing conditions.

Accurate and precise ionization energies of methylamines (CH 3 NH 2 and CH 3 ND 2 ) are determined to be 9.0422Ϯ0.0012 and 9.0532Ϯ0.0012 eV, respectively, by ͑1ϩ1Ј͒ two-photon mass-analyzed threshold ionization ͑MATI͒ spectroscopy. From selective ionizations from specified intermediate quantum states, fundamental frequencies of amino-wagging and CH 3 -rocking modes of CH 3 NH 2 ϩ (CH 3 ND 2 ϩ ) in D 0 states are determined to be 738 ͑573͒ and 1013 ͑1024͒ cm Ϫ1 , respectively. The frequency of the amino wag is largely blueshifted from that of the neutral S 1 state, while the CH 3 -rocking frequency is little shifted from that of S 1 . Internal rotational constants associated with the nearly free internal rotation of the top ͑amino group͒ with respect to the frame ͑methyl group͒ about the C-N axis are accurately determined, from which the geometries of methylamine ions are revealed. Barrier heights for torsional motion in CH 3 ND 2 ϩ are determined to be 25Ϯ5 and 34Ϯ5 cm Ϫ1 at the origin and first ND 2 -wagging bands, respectively. Mode-resolved spectroscopy using the MATI signal for resolving overlapped spectral features in the intermediate state is employed for clarifying the vibrational assignment of the intermediate state. An ab initio calculation at the QCISD level is carried out, giving good agreement with the experiment.

Resonance enhanced two photon ionization study in a pulsed molecular beam identified spin-forbidden transitions of the 2 3Πu(1u)–X 1Σg+(0g+) and 2 3Πu(0u+)–X 1Σg+(0g+) of Rb2. Λ-doubling and interference-induced rotational branch intensity anomalies are observed for 2 3Πu(1u)–X 1Σg+(0g+) transitions. The rotational constant for the 2 3Πu state was determined to be Be=0.018 932 5±0.000 007 4 cm−1 for Rb285, giving Re=4.5796±0.0009 Å. Other molecular parameters for Rb285 are determined as Te=19 784.254±0.009 cm−1, ωe=42.200±0.006 cm−1, ωexe=0.171±0.001 cm−1, and ωeye=−0.001 06±0.000 06 cm−1 for the 2 3Πu(0u+) state and Te=19 785.74±0.01 cm−1, ωe=42.297±0.005 cm−1, ωexe=0.1725±0.0009 cm−1, and ωeye=−0.000 78±0.000 04 cm−1 for 2 3Πu(1u) state. These Re, ωe, and Te values agree well with ab initio calculated values. The spin–orbit coupling constant A and Λ-doubling parameters p and q are determined by simultaneous fitting of the rotational structures of both 2 3Πu(1u)−X 1Σg+ and 2 3Πu(0u+)−X 1Σg+ transitions.

A systematic numerical analysis is performed for the quasi-periodicity in the wake where a circular cylinder is rotationally oscillated in time. The main emphasis is placed on the identification of frequency selection subjected to the controlled perturbations in the vicinity of lock-on. The frequency responses are scrutinized by measuring the lift coefficient (CL). A direct numerical simulation is made to portray the unsteady dynamics of wake flows at Re = 110. It is found that, after the shedding frequency is bifurcated at the boundary of lock-on, one frequency follows the forcing frequency and the other gradually converges to the natural shedding frequency. The asymptotic convergence phenomena are observed by solving the Van der Pol equation and the circle map. A new frequency selection formula is proposed. The quasi-periodic states are interpreted in terms of the forcing frequency, shedding frequency and modulated frequencies by employing the torus concept and the CL(t) diagram. In the quasi-periodic state, the variation of magnitudes and relevant phase changes of CL with forcing phase are examined.