Optimization of a columnar vortex generator installed over an aircraft carrier ski-jump ramp

Aircraft performances over aircraft carriers are essential in modern navies. Take-off operation is critical due to the short runway available. The ski-jump ramp is a useful system that allows to operate under safe conditions. However, the sharp edge at the end of the runway provokes a region with recirculation bubble and low velocity producing strong flow disturbances. Hence, the aircraft performances are affected and the pilot’s workload is augmented. Previous researches showed that columnar vortex generator reduces the recirculation bubble generated over the end of flight deck. This article presents an in-depth experimental study performed by wind tunnel testing in order to determine the relation between the columnar vortex generator size and the recirculation bubble reduction. Particle image velocimetry is used to investigate the flow field velocity and flow structure around the ski-jump ramp as a non-intrusive experimental technique. Encouraging results were found for the biggest columnar vortex generator studied.

Columnar vortex generator for flow control over a ski-jump ramp

Purpose Aircraft carriers are essential for modern naval operations. Takeoff maneuver is critical because of the short runway distance. The ski-jump ramp is a system which increases the angle of attack of the aircraft, so an extra lift is obtained. Regarding the flow configuration over the ski-jump ramp at ahead wind conditions, the complex aerodynamic environment generated by the ramp configuration influences aircraft operations. This flow field is mainly characterized by a low velocity recirculation bubble that reduces aircraft performances. The purpose of this paper is to find a solution to reduce these adverse effects, by means of flow control devices, which opens a wide field of research. Design/methodology/approach This paper presents wind tunnel tests performed to study the flow configuration in the vicinity of the ski-jump ramp and the flow control devices effects. A 1:100 scaled ship model was built to develop experimental tests by using flow control devices fabricated by means of additive manufacturing. Particle image velocimetry technique was used to measure the velocity flow field and the turbulence intensity maps. Findings Interesting results were obtained when the angle between the intersection of the ski-jump ramp and the columnar vortex generator (CVG) is modified. The results showed a high reduction of the recirculation bubble generated over the flight deck. Originality/value CVG has presented encouraging results as a passive flow control device. A study of the variation of CVG geometrical parameters has been developed.

Buoyancy-induced, columnar vortices

A buoyancy-induced, columnar vortex is deliberately triggered in the unstably stratified air layer over a heated ground plane and is anchored within, and scales with, an azimuthal array of vertical, stator-like planar flow vanes that form an open-top enclosure and impart tangential momentum to the radially entrained air flow. The columnar vortex comprises three coupled primary flow domains: a spiraling surface momentum boundary layer of ground-heated air, an inner thermally driven vertical vortex core and an outer annular flow that is bounded by a helical shear layer and the vanes along its inner and outer edges, respectively, and by the spiraling boundary layer from below. In common with free buoyant columnar (dust devil) vortices that occur spontaneously over solar-heated terrain in the natural environment, the stationary anchored vortex is self-sustained by the conversion of the potential energy of the entrained surface-heated air layer to the kinetic energy of the induced vortical flow that persists as long as the thermal stratification is maintained. This conversion occurs as radial vorticity produced within the surface boundary layer is tilted vertically near the vortex centreline by the buoyant air to form the core of the columnar vortex. The structure and dynamics of the buoyant vortex are investigated using high-resolution stereo particle image velocimetry with specific emphasis on the evolution of the vorticity distributions and their effects on the characteristic scales of the ensuing vortex and on the kinetic energy of the induced flow.

Analogies and differences between the stability of an isolated pancake vortex and a columnar vortex in stratified fluid

In order to understand the dynamics of pancake shaped vortices in stably stratified fluids, we perform a linear stability analysis of an axisymmetric vortex with Gaussian angular velocity in both the radial and axial directions with an aspect ratio of ${\it\alpha}$. The results are compared to those for a columnar vortex (${\it\alpha}=\infty$) in order to identify the instabilities. Centrifugal instability occurs when $\mathscr{R}>c(m)$ where $\mathscr{R}=ReF_{h}^{2}$ is the buoyancy Reynolds number, $F_{h}$ the Froude number, $Re$ the Reynolds number and $c(m)$ a constant which differs for the three unstable azimuthal wavenumbers $m=0,1,2$. The maximum growth rate depends mostly on $\mathscr{R}$ and is almost independent of the aspect ratio ${\it\alpha}$. For sufficiently large buoyancy Reynolds number, the axisymmetric mode is the most unstable centrifugal mode whereas for moderate $\mathscr{R}$, the mode $m=1$ is the most unstable. Shear instability for $m=2$ develops only when $F_{h}\leqslant 0.5{\it\alpha}$. By considering the characteristics of shear instability for a columnar vortex with the same parameters, this condition is shown to be such that the vortex is taller than the minimum wavelength of shear instability in the columnar case. For larger Froude number $F_{h}\geqslant 1.5{\it\alpha}$, the isopycnals overturn and gravitational instability can operate. Just below this threshold, the azimuthal wavenumbers $m=1,2,3$ are unstable to baroclinic instability. A simple model shows that baroclinic instability develops only above a critical vertical Froude number $F_{h}/{\it\alpha}$ because of confinement effects.

The numerical simulation of draft tube cavitation in Francis turbine at off-design conditions

Purpose – The vortex ropes in draft tube of Francis turbine always cause fluctuation and vibration, which consequently threaten the safety and stability of hydro turbines. The purpose of this paper is to use a cavitation flow computational method to simulate spiral vortex ropes under part load conditions and columnar vortex ropes under high-load conditions in draft tube. The unsteady cavitating flow characteristics in draft tube and its interaction with runner cavitation were analyzed. Design/methodology/approach – The calculation method was verified by cavitation simulation around a 3D hydrofoil. The results show that the Large Eddy Simulation (LES) turbulence model with the Zwart-Gerber-Blemari cavitation model have comparative advantage in cavitation simulations whether from capture of cavity shape or prediction of pressure changes. So it was chosen to simulate the two-phase cavitation flow in Francis turbine. The boundary conditions for inlet and outlet were set to inlet total pressure and outlet static pressure. The finite volume method with the central difference was adopted to discretize the equations. Findings – The calculated Thoma number agreed well with the experimental data. The vortex rope diameter and length increased with the cavitation development for both of the two types of vortex ropes conditions. The maximum peak-to-peak values of pressure pulsations located in the draft tube elbow part under all of the Thoma numbers conditions. Under spiral vortex rope conditions, the pressure pulsation in the same section of draft tube cone show obviously phase shift. The vortex rope affects the development of runner cavitation, which induces the symmetric and axisymmetric cavitation region in the suction side of blades for spiral and columnar vortex rope condition, respectively. Research limitations/implications – The mesh independence had been checked only in non-cavitation flow; in addition, the mesh density did not well satisfy the requirements of LES due to the limitations of computing power. The higher mesh density on a simplified model with one blade flow path and the entire draft tube may be helpful for obtaining more precise results. Originality/value – The spiral and columnar vortex ropes in a Francis turbine were compared and analyzed. The annular hydraulic jump appeared in the columnar vortex rope conditions has little effects on the pressure pulsations. The uneven flow field caused by spiral vortex led to the asymmetric cavitation development.

Vortex Characteristics of Heliostats’ Surface Wind Pressure under Resting Condition

Flat rectangular heliostats’ surface wind pressure distribution under resting condition was analyzed via wind tunnel experiment results. The flow field characteristics of the mirror in parallel with the ground were combined to reveal the distribution cause of fluctuating pressure. Phenomena of columnar vortex and conical vortex formed from mirror’s flow field under wind angles 0° and 45° were aimed to explain their internal structure using theory of point vortex model. With power spectrum of peculiar points’ fluctuating wind pressure analyzed, the surface vortex characteristics were further elaborated. The above work revealed heliostats’ surface wind pressure characteristics under resting conditions, and established the theoretical basis of its design study.