Enabling Micro Synthetic Jet Actuators in Boundary Layer Separation Control Using Flow Instability

Research on synthetic jet actuators (SJAs) has shown great potential of using SJAs in control of boundary layer flow separation to reduce the drag and increase the efficiency of aerodynamic devices. The challenge lies in developing an actuator not only small, light, robust and economic, but also capable of reaching the control objectives. This paper presents an idea of using the flow instability to enhance the actuation of a SJA. In the case of controlling laminar separation, the SJA is used to trigger frictional Tollmien-Schlichting (T-S) instability. At a forcing frequency strategically determined, the triggered T-S instability which is originally weak can be enhanced by the frictionless Kelvin- Helmholtz (K-H) instability of the baseline flow, until the T-S instability becomes substantially strong to resist the separation. The effective actuation of a SJA in resisting laminar separation caused by adverse pressure gradient in a boundary layer is demonstrated by experimental results of profiles of mean and fluctuating velocities. The orifice diameter, which is the characteristic dimension, of the SJA is 500 μm. The forcing voltage is only ±7.5V., and the forcing frequency is 100 Hz. The Reynolds number is in a range of 1.78x105~2.24x105. Boundary layer properties are used to understand the associated physics, and disturbance intensity is first time used to evaluate the effectiveness of the SJA. Analysis of the experimental results led to the conclusion that flow instability plays a critical role in enabling a micro SJA and also in making the control effectiveness less dependent or independent of the detailed structure and size of the actuator

Semileptonic decays of BcB_c meson to S-wave charmonium states in the perturbative QCD approach

Inspired by the recent measurement of the ratio of $B_c$ branching fractions to $J/\psi \pi^+$ and $J/\psi \mu^+\nu_{\mu}$ final states at the LHCb detector, we study the semileptonic decays of $B_c$ meson to the S-wave ground and radially excited 2S and 3S charmonium states with the perturbative QCD approach. After evaluating the form factors for the transitions $B_c\rightarrow P,V$, where $P$ and $V$ denote pseudoscalar and vector S-wave charmonia, respectively, we calculate the branching ratios for all these semileptonic decays. The theoretical uncertainty of hadronic input parameters are reduced by utilizing the light-cone wave function for $B_c$ meson. It is found that the predicted branching ratios range from $10^{-6}$ up to $10^{-2}$ and could be measured by the future LHCb experiment. Our prediction for the ratio of branching fractions $\frac{\mathcal {BR}(B_c^+\rightarrow J/\Psi \pi^+)}{\mathcal {BR}(B_c^+\rightarrow J/\Psi \mu^+\nu_{\mu})}$ is in good agreement with the data. For $B_c\rightarrow V l \nu_l$ decays, the relative contributions of the longitudinal and transverse polarization are discussed in different momentum transfer squared regions. These predictions will be tested on the ongoing and forthcoming experiments.Comment: 12 pages, 3 figures, 5 table