Henda Djeridi

Investigation of thickness effects on 2D NACA symmetric foils

A numerical and experimental study on the hydrodynamic behaviour of 2D NACA (15, 25, 35%) symmetric hydrofoils at high chord (/spl ap/10/sup 6/) Reynolds numbers and high angles of attack is performed. The focus is devoted to the comparison between results particularly for stall conditions corresponding to unsteady separated flow. The effect of thickness on the hydrodynamic coefficients and the topology of the flow are studied and the capacity of commercial code to predict the hydrofoil stall by Reynolds-Averaged Navier Stokes equations is analysed. The hydrodynamic coefficients are measured using a force balance and the flow patterns are investigated using LDV techniques. The cavitating incipient flow has been investigated to determine the minimum pressure coefficient and correlations are proposed to join these results to the thickness effect. All the calculations (steady and unsteady) have been performed using FLUENT/spl reg/ code. The emphasis is on turbulence modelling, grid size and boundary layer conditions on the foil. The computation is validated with very detailed experimental data. A hysteretic behaviour of force coefficient is highlighted and the development of leading edge vortices before and after stall occurrence is characterised.

Effect of longitudinal vortex on boundary layer state and separation on Naca symmetric foil

Vortex generators have been widely used in aerodynamics to control the separation of boundary layers. In such application (Angele and Muhammad, 2005) vortex generators are embedded in the boundary layer and the vortex height, with regards to the wall, is of the boundary layer thickness. The objective of this configuration is obviously far from being the effects of a single longitudinal vortex (generated upstream by an elliptical plan form profile) on the turbulent boundary layer shape over a Naca0015 symmetric foil at different incidences at high Reynolds number 5 105 . The vortex is situated outside the boundary layer (ten times the BL thickness over the wall) taking into account the small value of the thickness in our hydrodynamic application. Obviously, this situation is optimum as the vortex delays separation and increases the maximum lift but introduces drag penalty at small incidence. This is nevertheless frequently encountered in hydrodynamic applications (hub vortex upstream of a rudder) and of interest. To point out the mechanism of the boundary layer manipulation, both global efforts using gauge balance and velocity measurements using LDV and PIV have been performed and compared with and without vortex. The base flow is an APG boundary layer characterized by a predominant wake area. Effect of the vortex is analyzed via the shape factor both in inflow and outflow regions. The longitudinal vortex suppress the hysteretic loop classically described in this Reynolds number range (Djeridi et al., 2009) but an increase of the drag is observed in the range of incidence just before stall. Velocity measurements indicated that, for incidences near the stall appearance, the shape factor is decreased both in the inflow and in the outflow regions. Even for large incidences, in the inflow region the value of the shape factor is equivalent to the one found in the turbulent BL over a flat plate. In this region the vortex modifies the equilibrium state of the BL as attested by the Clauser parameter. Even for large distances between the vortex and the wall, the ability of the vortex to suppress the detachment of the BL is observed on the evolution of the backflow coefficient. This effect is greater pronounced in inflow area near the trailing edge region where the flow is locally reattached due to the high momentum fluid displacement.Copyright

Bubble Effect on the Structures of Weakly Turbulent Couette Taylor Flow

In industrial applications, rotating flows have been recognized to enhance mixing and transfer properties. Moreover, bubbly flows are also used to improve transfers. Therefore, it is interesting to study the effects of the dispersed phase on the structure of a Couette Taylor flow. Experiments are conducted for the quasi-periodic (Ta = 780) and the weakly turbulent (Ta = 1000) flow regimes. Bubbles (0.035 times as small as the gap) are generated by agitation of the upper free surface (ventilated flow). Larger bubbles (0.15 times as small as the gap) are generated by injection at the bottom of the apparatus and by applying a pressure drop (cavitating flow). Void fraction, bubble size and velocity, as well as axial and azimuthal velocity components of the liquid are investigated. For α>0.1%, there is evidence of bubble induced modifications of axial wavelength and liquid velocity gradients, depending on the bubble location in the gap and their size. For particular conditions of the flow, a premature transition to weakly turbulent flow is observed.© 2005 ASME