Strength Of Shear Layer Research Articles

Effect of superhydrophobicity on the wake of a pitching foil across various Strouhal numbers

Particle image velocimetry-based measurements have been performed to study the flow over a pitching superhydrophobic NACA—National Advisory Committee for Aeronautics—0012 foil (SphNACA0012 foil) and to compare the same with NACA0012 foil. This is perhaps the first experimental study on a pitching SphNACA0012 foil. The objective is to understand the effects of superhydrophobicity on NACA0012 foil pitching about its leading edge at various Strouhal numbers, St = 0.08–0.65, with a constant pitching amplitude, θ0 = 4°, and Reynolds number, Re = 2400. Superhydrophobicity is found to significantly affect three different flow regimes: 2S von-Ka´rma´n wake, symmetric wake, and asymmetric wake regimes at smaller, intermediate, and larger values of St. In the 2S von-Ka´rma´n wake regime, although the wake structures of both the foils are quite comparable, a reduction in the shear layer strength and early rolling-up of vortices are observed for SphNACA0012 foil due to a partial slip effect. With a further increase in St, a transition to the symmetric wake regime is observed for both the foils and another transition that leads to a symmetry breaking of the vortex street is found to be earlier for SphNACA0012 foil, which results in the onset of lift generation at lower St than that of NACA0012 foil. The shear layer strength is weaker for the SphNACA0012 foil except at St = 0.24, which is a critical St for the jet deflection point and results in a 13% larger thrust coefficient for SphNACA0012 foils as compared to NACA0012 foils. These results on the larger lift as well as thrust coefficient at low St suggest improved maneuverability with high propulsive efficiency when superhydrophobicity is employed on the propulsor of underwater vehicles.

Shear Layer Dynamics in a Reacting Jet in Crossflow

This study explores the effect of heat release on the growth of the shear layer vortical structures in a reacting jet in crossflow. Jets composed of mixtures of hydrogen, helium and nitrogen were used to independently vary the momentum flux ratio (J), jet to crossflow density ratio (S) and heat release. Velocity fields were obtained from 10 kHz high-speed stereoscopic particle image velocimetry (SPIV) and regions of elevated temperature/combustion products from simultaneous OH planar laser induced fluorescence (OH-PLIF). The shear layer vortices (SLV) originating from instabilities in the windward and leeward shear layers were identified using vortex identification indicator functions in order to track their spatial location and strength. The results show that the asymmetries in shear layer strength between the windward and leeward shear layers are dependent primarily on J, for both reacting and non-reacting flow-fields. The SLV growth rate dependencies on J and S is found to match trends noted by previous studies for non-reacting jets, where SLV growth rates increase with degree of global instability of the JICF. Heat release is also shown to suppress the SLV growth rates relative to non-reacting cases with the same jet parameters. Related to this point, the degree of lifting of the flame also has a significant impact on SLV growth. As flame lifting is directly related to autoignition times, this point shows strong coupling between kinetic rates and jet hydrodynamic stability.

Estimation of the plasma blob occurrence rate

As part of the blob duty cycle, the blob detection rate, also called the blob occurrence rate or simply blob frequency, is of critical importance to predict the impact of plasma blobs on plasma wall interaction and modification of the scrape-off layer such as the scrape-off layer width. This rate is estimated based on extrapolation from measurements on present devices. A recent physics-based prediction of the blob detection rate [G. Fuchert et al., Plasma Phys. Controlled Fusion 58, 054006 (2016)] has been compared by means of gyrofluid simulations with varying collisionality and shear layer strength. The prediction shows a robust agreement within a factor of two despite varying generation positions and driving instabilities.

The mechanism investigation of two-degree-of-freedom vortex-induced vibration with electro-magnetic forces

The electro-magnetic forces generated by electromagnetic field take control of the flow in the electrolyte solution. In this paper, the mechanism of two-degree-of-freedom vortex-induced vibration controlled by electro-magnetic forces is investigated numerically. With the coordinate at the moving cylinder, the stream function-vorticity equations, the initial and boundary conditions and distribution of hydrodynamic force are deduced in the exponential-polar coordinate. The equation of vorticity transport is solved by the alternative-direction implicit algorithm. The equation of stream function is integrated by means of a fast Fourier transform algorithm. The cylinder motion is calculated by the Runge-Kutta method. The flow field, pressure, lift/drag and cylinder displacement are interacted along the transverse and streamwise direction, where the instantaneous variations are discussed. The derivation shows that the vibration displacement in one direction, whose effects on the flow field influence the vortex-induced forces in both directions, affects the inertial force only in the corresponding direction and is independent of that in the other direction. The numerical calculations show that the vortex-induced vibration is affected by two factors, i.e., the vortex shedding and the cylinder shift. Both of the two factors have influences on the shear layers in the transverse direction and the secondary vortex in the streamwise direction, which further leads to the variations of lift/drag and the cylinder motion. Along the transverse direction, the strength of shear layer on one side is increased by the vortex shedding while the strength of shear layer on the other side is increased by the cylinder shift. Along the streamwise direction, the pressure of cylinder tail is varied with the effect of shedding vortex on the secondary vortex while the effect of cylinder shift on the secondary vortex is also opposite to that of shedding vortex. Notably, the effect of cylinder shift prevails over the effect of shedding vortex so that the former is dominated in the total effects. The flow separation and vortex shedding are suppressed as the fluid of boundary layer is accelerated under the action of electro-magnetic forces. Meanwhile, the vibration displacements decrease gradually along both the transverse and streamwise directions, which also suppresses the effects of pressure/suction sides. Therefore, the vibration is suppressed and the cylinder turns steady rapidly. In addition, the thrust generated by the wall electro-magnetic force counteracts the drag generated by the fluid electro-magnetic force, which means that the final position is at the upstream of the initial position. The experimental results show that the vortexes on the cylinder are suppressed fully and the flow field is steady under the action of electro-magnetic force, which agrees well with the numerical results.

Suppression of vortex-induced vibration of a circular cylinder by Lorentz force

The electro-magnetic control of vortex-induced vibration (VIV) of a circular cylinder is investigated numerically in the exponential-polar coordinates attached on the moving cylinder for Re=150 in the paper. Compared with the fixed cylinder, the vibration of cylinder leads to the shift of stagnation point, the shear layer strength and the inertial force, which affects the hydrodynamic forces on the cylinder. The effects of the instantaneous wake geometries and the corresponding cylinder motion on the hydrodynamic forces for one entire period of vortex shed are discussed in the drag-lift phase diagram. The Lorentz force for controlling the vibration cylinder is classified into the field Lorentz force and the wall Lorentz force. The field Lorentz force decreases the lift oscillation, and in turn, suppresses the VIV, whereas the wall Lorentz force has no effect on the lift.

Rotordynamic Forces Due to Turbine Tip Leakage: Part I—Blade Scale Effects

An experimental and theoretical investigation has been conducted on rotordynamic forces due to nonaxisymmetric turbine tip leakage effects. This paper presents an actuator disk model that describes the flow response to a finite clearance at the rotor tip. The model simplifies the flow field by assuming that the radially uniform flow splits into two streams as it goes through the rotor. The stream associated with the tip clearance, or the underturned stream, induces radially uniform unloading of the rest of the flow, called the bladed stream. Thus, a shear layer forms between the two streams. The fraction of each stream and the strength of shear layer between the two are found as functions of the turbine loading and flow parameters without resorting to empirical correlations. The results show that this model’s efficiency predictions compare favorably with the experimental data and predictions from various correlations. A companion paper builds on this analysis to yield a model of the three-dimensional disturbances around an offset turbine and to predict the subsequent cross forces.