C. W. Wong

Control of Separated Flow on a NACA 0015 Airfoil using Three-Dimensional Plasma Actuator

Experimental study has been conducted to determine the effect of a three-dimensional plasma actuator, whose exposed and encapsulated electrodes geometry are designed with serrated configuration, on a NACA 0015 airfoil for flow separation control. The electrodes of the serrated configuration were arranged in a manner that the sawtooth are opposite to each other and horizontally separated by a dielectric material, hence the electrode gap is modified along the spanwise direction. In order to understand the characteristics of the threedimensional flow originated from the plasma actuator, measurements including the vorticity, longitudinal velocity and induced flow angle generated by the serrated configuration, were conducted in the absence of free air-stream using PIV technique. The mechanisms of vorticity generation for the plasma actuator are discussed. Lift and drag measurements on the airfoil were conducted at low Reynolds number (77 × 10 3 ). The plasma actuator showed effective control authority by generating both the streamwise flow and the spanwise-periodic vorticity, and it was found to lead to stall delay by 5° and increase in lift coefficient by about 6%.

Influence of Axial-Flow Turbulence Intensity on Fluid-Structure Interaction for a Flexible Cylinder

This work aims to investigate experimentally the fluid-structure interaction FSI for a flexible cylinder subjected to axial turbulent flows. Two configurations are considered; a solitary flexible cylinder with simple support ends and is free to vibrate in the transverse direction, another configuration is a non-flexible cylinder which installed parallel and adjacent to the flexible cylinder in various cylinder centre-to-centre pitch P * (=P/D, where D is the diameter of the flexible cylinder). Investigation on the effect of axial-flow turbulence intensity T u at 0.7 and 2.9% on the FSI for the two configurations over a range of freestream velocity U∞ (0.19–2.14 m/s) is performed by means of simultaneous measurement of flexible cylinder vibration and velocity field adjacent to the cylinder. We found that T u has a strong effect on the flexible cylinder vibration in both configurations. At high ∞ and T u , we observed substantial interaction of flow structures between cylinders which provokes the buckling of the flexible cylinder at a lower critical velocity compared with its counterpart of a solitary cylinder.

Turbulence Intensity Effect on Axial-Flow-Induced Cylinder Vibration

A numerical study is conducted on the effect of inlet turbulent intensity on the axial-flow-induced vibration of an elastic cylinder subjected to axial tubular flow. The cylinder with fix-supported ends is free to vibrate in the lateral direction. While a large eddy simulation is used to calculate the turbulent flow field, the Ansys mechanical + Fluent two-way coupling has been deployed to capture the fluid-structure interaction. The calculation agrees qualitatively with experimental data. Various inlet turbulence intensities, T u , i.e., 0, 0.3, 5.0 and 10.0%, are examined at two non-dimensional flow velocities, \( \overline{U} \), i.e., 3.30 and 7.62. The results show that T u has a significant effect on the cylinder vibration. At \( \overline{U} \) = 3.30, the maximum displacement grows with T u and the vibration is classified as the subcritical vibration; the instability of cylinder is not induced with increasing T u . At \( \overline{U} \) = 7.62, the buckling occurs at T u = 0%, while the flutter takes place at T u = 0.3%; both are associated with an asymmetric pressure distribution around the cylinder.

Slope-Seeking Control for Stall Mitigation of a NACA0015 Airfoil

This paper aims to investigate the response of the slope seeking with extended Kalman filter (EKF) deployed in a closed-loop system for airfoil aerodynamics control. A novel dielectric barrier discharge (DBD) sawtooth plasma actuator is used to manipulate the flow around the NACA 0015 airfoil. The sawtooth plasma actuator leads to a delay in the stall angle by 5° and an increase in the maximum lift coefficient by about 9 %, appreciably better than that with the “traditional” DBD plasma actuator of the same input power. For the closed-loop control, the convergence time t c of the lift force F L is investigated for two control algorithms from Re = 4.4 × 104 to 7.7 × 104. The t c is about 70 % less under the slope seeking with EKF than that under the slope seeking with high-pass (HP) and low-pass (LP) filters at Re = 7.7 × 104. The slope seeking with EKF shows excellent robustness over a moderate Re range; that is, the voltage amplitude determined by the control algorithm promptly responds to a change in Re, much faster than that of the conventional slope seeking with HP and LP fliters.

Effect of Nose Shape on Separation Bubble and Surface Pressure on a Cylinder in Axial Flow

The flow characteristics is experimentally studied over the leading edge of a circular cylinder with blunt, conical, and hemispherical nose shapes, for a range of Reynolds number Re D = 2.5 × 103− 4.2 × 104. While mean and fluctuating pressure (C p , C p ′) around the leading edge are measured with a pressure transducer, PIV and flow-visualization experiments are performed to assimilate how an increase in Re D influences the shear layer reattachment length x R , shear layer transition length x Tr , and bubble width W. The results reveal that x R , x Tr , and W all shrinking significantly with Re D up to Re D = 104. Their shrinkage is however inconsequential for Re D > 104. At a given Re D , when the nose changes as blunt, conical and hemispherical, x R and W shorten, but x Tr enlarges. Following the separation bubble size, C p and C p ′ in the bubble are highly sensitive to Re D for Re D 104.

Reynolds Number Effect on Flow Classification Behind Two Staggered Cylinders

This work aims to investigate based on the Strouhal number St and the flow structure, the dependence of flow classification on the Reynolds number Re in the wake of two staggered cylinders, with Re varying from 1.5 × 103 to 2.0 × 104. The cylinder center-to-center pitch, P * = P/d examined is 1.2–6.0 (d is the cylinder diameter), and the angle (α) between the incident flow and the line through the cylinder centers is 0°–90°. Two single hotwires were used to measure simultaneously the St behind each of the two cylinders at streamwise positions from x * = x/d = 2.5–15. While the present data reconfirms the flow structure modes previously reported, the dependence of the flow modes on P * and α exhibits an appreciable dependence on Re. The observation is connected to the Re effect on the generic features of a two-cylinder wake such as flow separation, boundary layer thickness, gap flow deflection, and vortex formation length.