A direct numerical simulation investigation of the synthetic jet frequency effects on separation control of low-Re flow past an airfoil

Abstract

CitationA direct numerical simulation investigation of the synthetic jet frequency effects on separation control of low-Re flow past an airfoil 2015, 27 (5) We present results of direct numerical simulations of a synthetic jet (SJ) based separation control of flow past a NACA-0018 (National Advisory Committee for Aeronautics) airfoil, at 10• angle of attack and Reynolds number 10 4 based on the airfoil chord length C and uniform inflow velocity U 0 . The actuator of the SJ is modeled as a spanwise slot on the airfoil leeward surface and is placed just upstream of the leading edge separation position of the uncontrolled flow. The momentum coefficient of the SJ is chosen at a small value 2.13 × 10 −4 normalized by that of the inflow. Three forcing frequencies are chosen for the present investigation: the low frequency (LF) F + = f e C/U 0 = 0.5, the medium frequency (MF) F + = 1.0, and the high frequency (HF) F + = 4.0. We quantify the effects of forcing frequency for each case on the separation control and related vortex dynamics patterns. The simulations are performed using an energy conservative fourth-order parallel code. Numerical results reveal that the geometric variation introduced by the actuator has negligible effects on the mean flow field and the leading edge separation pattern; thus, the separation control effects are attributed to the SJ. The aerodynamic performances of the airfoil, characterized by lift and lift-to-drag ratio, are improved for all controlled cases, with the F + = 1.0 case being the optimal one. The flow in the shear layer close to the actuator is locked to the jet, while in the wake this lock-in is maintained for the MF case but suppressed by the increasing turbulent fluctuations in the LF and HF cases. The vortex evolution downstream of the actuator presents two modes depending on the frequency: the vortex fragmentation and merging mode in the LF case where the vortex formed due to the SJ breaks up into several vortices and the latter merge as convecting downstream; the discrete vortices mode in the HF case where discrete vortices form and convect downstream without any fragmentation and merging. In the MF case, the vortex dynamics is at a transition state between the two modes. The low frequency actuation has the highest momentum rate during the blowing phase and substantially affects the flow upstream of the actuator and triggers early transition to turbulence. In the LF case, the transverse velocity has a 1%U 0 pulsation at the position 18%C upstream of the actuator. C 2015 AIP Publishing LLC.

BiGlobal linear stability analysis on low-Re flow past an airfoil at high angle of attack

“…The simulations are performed using an in-house code that has been validated in our previous works. [16][17][18] The unsteady flow pattern is presented by the instantaneous vorticity field in Figure 2. The periodic shedding of the flow is clearly observed downstream of the airfoil.…”

“…However, the generalization is accompanied by the high computational cost which restricts this method in relatively simple geometries (e.g., the work of Tezuka and Suzuki 13 and Bagheri et al 14 ). For most numerical simulations of flow past an isolated airfoil or other configurations using either the high-fidelity DNS or the large-eddy simulation (LES) approach, the computational domain is normally assumed straight in the spanwise direction and periodic boundary condition is prescribed for all primitive variables (e.g., the work of Jones et al, 7 Kitsios et al, 15 Zhang et al, 16 and Zhang and Samtaney 17,18 ). The variation of the flow along the homogeneous spanwise (z-) direction is significantly weaker than the other two inhomogeneous directions, i.e., ∂φ/∂z ≪ ∂φ/∂ x and ∂φ/∂z ≪ ∂φ/∂ y in which φ is any flow variable.…”

Assessment of spanwise domain size effect on the transitional flow past an airfoil

“…The coefficient is normally computed by averaging over combinations of proven that fluctuations in the spanwise direction become uncorrelated when the aspect ratio is sufficiently large [3,23,28,30,51,52].…”

“…airfoil is rescaled to unit chord length C and extended to include a sharp trailing edge and 6C, which are confirmed to be sufficiently large for such simulations [43,51,52].…”

Effects of Boundary Layer Control Method on Hydrodynamic Characteristics and Tip Vortex Creation of a Hydrofoil

“…Also, Gardner et al [24] experimentally measured the variation of stall angle caused by high-pressure blowing. Moreover, Zhang and Samtaney [25] employed Direct Numerical Simulation (DNS) to investigate the effects of frequency on the boundary layer separation control. Furthermore, Meng et al [26] studied the plasma-laminar separation bubble control over aerofoils.…”