In this paper, the performance of sub boundary layer vortex generators and conventional vortex generators in controlling the Separation bubble has been compared and the resultant highly three-dimensional flow has been studied. Two pairs of vortex generators mounted symmetrically along the spanwise direction are placed upstream of Separation point to produce counter-rotating vortices. Effect of these three-dimensional vortex generators on the Separation bubble and the flow downstream has been examined. The simulations show that the length of the Separation bubble is reduced by sixty two per cent due to the deployment of vortex generators of height 0. 33 δ while the original Separation bubble is completely eliminated by the vortex generators of height 0. 66 δ . However the presence of larger height vortex generators by itself causes a small mean Separation bubble downstream. The flow downstream of vortex generators is highly three-dimensional and zones of recirculation can be observed between regions of attached flow. Presence of adverse pressure gradient results in greater interaction between counter-rotating vortices, leading to their early breakup and higher vortex decay rate compared to the zero pressure gradient case. Further, it is seen from the simulations that the counter-rotating array of vortices does not move away from the wall even far downstream.

In the present paper, an experimental and numerical investigation of fluid flow and heat transfer in the case of wall injection besides main flow through a circular sudden enlargement are studied. The injected flow is achieved through an annular slot placed around the inner side wall of the step. The static pressure variation along the sudden enlargement length is measured and calculated at different values of injection ratio (Q) and injection flow angles. The average heat transfer with Reynolds number (ReJ) of injected flow at different values of the inlet flow angle is obtained. The velocity, turbulent kinetic energy and temperature contours are presented in this study. Reynolds number of injected flow is varied between 320 and 840, Reynolds number of main flow is between 5895 and 8450 and the injection flow angles are 0o, 15o, 30o, 45o and 60o. In the injection case, the results indicate that, the pressure recovery coefficient increases by decreasing the injection ratio and increasing the flow angle. The average heat transfer coefficient increases as both injection Reynolds number and the injection flow angle increase. The numerical results showed that two recirculation zones generate behind the step between the injection flow and the main flow. The size of these recirculation zones decreases by increasing the injection flow rate. The turbulent kinetic energy increases within region between the recirculation zones and main zone also, it decays by injecting flow in the recirculation zone. The length for higher value of flow temperature decreases by injecting flow in the recirculation zone, and that length increases as the injection flow rate increases. The comparison between the experimental results and the numerical results gives good agreement using the k-e model with Leschziner and Rodi correction.

The effects of a Trapped Vortex Cavity (TVC) on the aerodynamic performance of a NACA 0024 airfoil at a constant angle of attack (AoA) of 14◦ were investigated in this study. It was observed that mass suction (MFR) was required to stabilise the vortex within the cavity segment. Lift to drag ratio (L/D) and MFR were chosen as performance objectives, along with a fully attached flow constraint (flow Separation at X/c ≥ 95% ). Parametric analysis was carried on the baseline airfoil with and without suction and compared to the airfoil with TVC with and without suction. It was observed that L/D increases as MFR increases for a baseline airfoil, and flow Separation is delayed at high suction values (MFR = 0. 2 kg/s). The TVC modifies the pressure distribution on the baseline airfoil when MFR is applied to the cavity section and there is a significant increase in lift, thus, L/D increases and flow Separation is delayed. A lower value of MFR = 0. 08 kg/s is sufficient to stabilise the vortex and improve the efficiency of the TVC airfoil. The findings of these parametric studies were used to do a multi-objective optimisation using a genetic algorithm to attain the desired cavity shape while achieving the largest L/D and the lowest MFR (that is proportional to the power required for control) with a fully attached flow constraint. It was found that mass suction and cavity shape both had an equal influence on flow control. The Pareto optimal front yielded a series of optimum designs. One of them was subjected to an off-design analysis in order to validate its performance at other incidences. It was observed that it performs better than the baseline airfoil, with an improved L/D and an increase in stall angle from 10◦ to 14◦.

In this study, a novel fluidic jet actuator is designed to control flow Separation on a NACA0015 airfoil at various angles of attack. The U-shaped jet actuator has two rectangular slots implemented near the leading edge of the airfoil. It is driven by a piston mechanism and operates at three excitation frequencies. Depending on the motion of the mechanism, a synchronized jet flow is generated by blowing and suction at the dual exits of the actuator slots. The experimental studies are carried out in a subsonic wind tunnel. The unsteady 2D Computational Fluid Dynamics simulations are performed by Detached Eddy Simulation with the SST k-ω turbulence model where measured jet velocities at the exits of the actuator slots are imposed as boundary conditions to mimic motion of the piston. The results at the on-mode and off-mode of the actuator are evaluated in terms of surface pressure coefficient distributions on the airfoil and averaged aerodynamic force coefficients. At low angles of attack, there is an adequate match between numerical and experimental results for the base flow without any control. At higher angles of attack, flow Separation becomes considerably dominant and stall prevention by active flow control is detected especially at high excitation frequencies‎.

The passive control of flow-Separation at averaged Reynolds Number (Re=3.42×105) using self-adapting flexible-flaps in the upper side of the wing, is presented. The two-way Fluid-Structure Interaction (FSI) in an elastic-layer up on the airfoil (NACA 0012) is investigated numerically by Coupling between the Transient Structural and Fluid Flow (Fluent) in ANSYS-Workbench14.0. During the fluid-structure interaction, the transient deformation of the elastic-layer provokes the modification of the flow topology at large-scale. There are reductions of the size and intensity of the vortex-shedding and an increase in the Strouhal number. This explains the increase of the lift-to-drag ratio. The study of the flap flexibility shows that the deformation of the elastic-layer and the variation of aeronautical efforts are inversely proportional to the Young Modulus.