A Karman-Vortex Generator for passive separation control in a conical diffuser

Flow separation in a conical diffuser with large divergence angle （29.14°） and large area ratio （3.533） is eliminated by a novel passive flow control device called Karman-Vortex Generator （KVG）. The effect of the KVG is verified and investigated by the URANS, DES and DDES methods based on the SST model. CFD results show that the performance coefficient of the diffuser can be doubled by the KVG, and the total pressure recovery coefficient can be improved by about 1.2%. DES and DDES re- suits show that the KVG can introduce a Karman-vortex street frequency in the diffuser. This frequency decays rapidly, and could not be detected in the ending plane of the expansion section, Different KVG configurations with different locations and dimensions are numerically simulated and compared. Some suggestions are provided.

Simulation and Instability Investigation of the Flow around a Cylinder between Two Parallel Walls

The two-dimensional flows around a cylinder between two parallel walls at Re=40 and Re=100 are simulated with computational fluid dynamics(CFD). The governing equations are Navier-Stokes equations. They are discretized with finite volume method(FVM) and the solution is iterated with PISO Algorithm. Then, the calculating results are compared with the numerical results in literature, and good agreements are obtained. After that, the mechanism of the formation of Karman vortex street is investigated and the instability of the entire flow field is analyzed with the energy gradient theory. It is found that the two eddies attached at the rear of the cylinder have no effect on the flow instability for steady flow, i.e., they don't contribute to the formation of Karman vortex street. The formation of Karman vortex street originates from the combinations of the interaction of two shear layers at two lateral sides of the cylinder and the absolute instability in the cylinder wake. For the flow with Karman vortex street, the initial instability occurs at the region in a vortex downstream of the wake and the center of a vortex firstly loses its stability in a vortex. For pressure driven flow, it is confirmed that the inflection point on the time-averaged velocity profile leads to the instability. It is concluded that the energy gradient theory is potentially applicable to study the flow stability and to reveal the mechanism of turbulent transition.

Mechanism of Tonal Noise Generation from Circular Cylinder with Spiral Fin

The pitch of the spiral finned tube influences seriously to the acoustic resonance in the heat exchanger.In this research,the flow characteristics in relating to the aeolian tone from the finned cylinder are studied by the numerical simulation.It is observed that the tonal noise generated from the finned tube at two pitch spaces.The ratio of the fin pitch to the cylinder diameter is changed at 0.11 and 0.27.The tone level increases and the frequency decreases with the pitch shorter.The separation flow from the cylinder generates the span-wise vortices,Karman vortices,and the separation flow from the fin generates the stream-wise vortices.When the fin pitch ratio is small,the stream-wise vortices line up to span-wise and become weak rapidly.Only the Karman vortices are remained and integrate in span.So the Karman vortex became large.This causes the low frequency and the large aeolian tone.

Sub-ensemble study of pressure gradient effects on the variation of Karman constant

Using data from direct numerical simulation （DNS） of incompressible and compressible channel flow, we develop a method of sub-ensemble decomposition to investigate the pressure gradient effect on the Karman constant and the additive constant B characterizing the mean velocity profile （MVP）. The sub-ensemble decomposition is defined according to the magnitude of vertical fluctuation velocity, which mimics coherent motions like ejection and sweep. DNS data analysis shows that each sub-ensemble displays a distinct Karman constant, with a variation which mimics effects of pressure gradient. The latter is demonstrated by a relation between sub-ensembles＇ km and Bm similar to empirical data under various pressure gradients. A set of global parameters, k0-pg=0.39 ＆ B0-pg=5.5, are then derived for interpreting two constants observed by Nagib et al.

Pharmacological modulation of brain Nav1.2 and cardiac Nav1.5 subtypes by the local anesthetic ropivacaine

Objective The present study was aimed to investigate the pharmacological modulatory effects of ropivacaine,an amide-type local anesthetic,on rat Nav1.2（rNav1.2）and rNav1.5,the two Na＋channel isoforms heterologously expressed in Xenopus oocytes and in HEK293t cell line,respectively.Methods Two-electrode voltage-clamp（TEVC）and whole-cell patchclamp recordings were employed to record the whole-cell currents.Results Ropivacaine induced tonic inhibition of peak Na＋ currents of both subtypes in a dose-and frequency-dependent manner.rNav1.5 appeared to be more sensitive to ropivacaine.In addition,for both Na＋channel subtypes,the steady-state inactivation curves,but not the activation curves,were significantly shifted to the hyperpolarizing direction by ropivacaine.Use-dependent blockade of both rNav1.2 and rNav1.5 channels was induced by ropivacaine through a high frequency of depolarization,suggesting that ropivacaine could preferentially bind to the 2 inactivated Na＋channel isoforms.Conclusion The results will be helpful in understanding the pharmacological modulation by ropivacaine on Nav1.2 subtype in the central nervous system,and on Nav1.5 subtype abundantly expressed in the heart.