• Journal of Mechanical Science and Technology
• /
• v.16 no.12
• /
• pp.1561-1575
• /
• 2002

A reduced-order linear feedback controller, which is used to control the linear disturbance in two-dimensional plane Poiseuille flow, is applied to a boundary layer flow for stability control. Using model reduction and linear-quadratic-Gaussian/loop-transfer-recovery control synthesis, a distributed controller is designed from the linearized two-dimensional Navier-Stokes equations. This reduced-order controller, requiring only the wall-shear information, is shown to effectively suppress the linear disturbance in boundary layer flow under the uncertainty of Reynolds number. The controller also suppresses the nonlinear disturbance in the boundary layer flow, which would lead to unstable flow regime without control. The flow is relaminarized in the long run. Other effects of the controller on the flow are also discussed.

• Proceedings of the KSME Conference
• /
• 2002.08a
• /
• pp.775-778
• /
• 2002

In order to analyse the mechanism of a flow tone around a cavity, the correlations between the flow in the cavity and the boundary layer flow in front of the cavity are studied experimentally in this paper. The instability In the boundary layer forms the vortex at the front edge of the cavity and the flow tone is occurred by the vortex breakdown at the rear edge of the cavity Therefore, the boundary layer measurement is important in the cavity flow control. We measure the velocity of the boundary layer at the entrance of the cavity using hot-wire anemometry and the flow tone around the cavity by microphone. The boundary layer characteristic is changed by the various angle of the flap on the front edge of the cavity, while it is less influenced by the ratio of length and depth of the cavity.

• Journal of Mechanical Science and Technology
• /
• v.20 no.11
• /
• pp.1993-2001
• /
• 2006

The piezoelectric bimorph film, which, as an actuator, can generate more effective displacement than the usual PVDF film, is used to control the turbulent boundary-layer flow. The change of wall pressures inside the turbulent boundary layer is observed by using the multi-channel microphone array flush-mounted on the surface when actuation at the non-dimensional frequency $f_b^+$:=0.008 and 0.028 is applied to the turbulent boundary layer. The wall pressure characteristics by the actuation to produce local displacement are more dominantly influenced by the size of the actuator module than the actuation frequency. The movement of large-scale turbulent structures to the upper layer is found to be the main mechanism of the reduction in the wall- pressure energy spectrum when the 700$700{\nu}/u_{\tau}$-long bimorph film is periodically actuated at the non- dimensional frequency $f_b^+$:=0.008 and 0.028. The biomorph actuator is triggered with the time delay for the active forcing at a single frequency when a 1/8' pressure-type, pin-holed microphone sensor detects the large-amplitude pressure event by the turbulent spot. The wall-pressure energy in the late-transitional boundary layer is partially reduced near the convection wavenumber by the open-loop control based on the large amplitude event.

• 한국가시화정보학회:학술대회논문집
• /
• 2002.11a
• /
• pp.117-120
• /
• 2002

An experimental research has been carried out for flow control of the shock wave/turbulent boundary-layer interaction utilizing aeroelastic mesoflaps. Various shapes and thicknesses of the mesoflap are tested to achieve different deflections of the flap, and ail the results are compared to the solid-wall reference case without flow-control mechanism. Quantitative variation of skin friction has been measured downstream of the interactions using the laser interferometer skin friction meter, and qualitative skin friction distribution has been obtained by observing the interference fringe pattern on the oil-film surface. A strong spanwise variation in the fringe patterns with a narrow region of separation near the centerline is noticed to form behind the shock structure, which phenomenon is presumed partially related to three-dimensional flow structures associated with both the sidewalls and the bottom test surface. The effect of the shape of the cavity is also observed and it is noticed that the shape of the cavity is not negligible.

• Journal of computational fluids engineering
• /
• v.10 no.3 s.30
• /
• pp.19-26
• /
• 2005

The purpose of this paper is the study on the characteristics of an inlet system with shock/boundary layer interactions by using various types of bumps which are substituted for the conventional bleeding system in supersonic inlet. in this study a comprehensive numerical analysis has been performed to understand the three-dimensional flow field including shock/boundary layer interaction and growth of turbulent boundary layer that might occur around a three-dimensional bump in a supersonic inlet. The characteristics of boundary layer seen in the current numerical simulations indicate the potential capability of a three-dimensional bump to control shock/boundary layer interaction in supersonic inlets.

• Wind and Structures
• /
• v.35 no.1
• /
• pp.1-20
• /
• 2022

This research work presents an experimental study's outcomes to reveal the impact of an O-ring on the flow control over a sphere placed in a turbulent boundary layer. The investigation is performed quantitatively and qualitatively using particle image velocimetry (PIV) and dye visualization. The sphere model having a diamater of 42.5 mm is located in a turbulent boundary layer flow over a smooth plate for gap ratios of 0≤G/D≤1.5 at Reynolds number of 5 × 10³. Flow characteristics, including patterns of instantaneous vorticity, streaklines, time-averaged streamlines, velocity vectors, velocity fluctuations, Reynolds stress correlations, and turbulence kinetic energy (), are compared and discussed for a naked sphere and spheres having O-rings. The boundary layer velocity gradient and proximity of the sphere to the flat plate profoundly influence the flow dynamics. At proximity ratios of G/D=0.1 and 0.25, a wall jet is formed between lower side of the sphere and flat plate, and velocity fluctuations increase in regions close to the wall. At G/D=0.25, the jet flow also induces local flow separations on the flat plate. At higher proximity ratios, the velocity gradient of the boundary layer causes asymmetries in the mean flow characteristics and turbulence values in the wake region. It is observed that the O-ring with various placement angles (𝜃) on the sphere has a considerable alteration in the flow structure and turbulence statistics on the wake. At lower placement angles, where the O-ring is closer to the forward stagnation point of the sphere, the flow control performance of the O-ring is limited; however, its impact on the flow separation becomes pronounced as it is moved away from the forward stagnation point. At G/D=1.50 for O-ring diameters of 4.7 (2 mm) and 7 (3 mm) percent of the sphere diameter, the -ring exhibits remarkable flow control at 𝜃=50° and 𝜃=55° before laminar flow separation occurrence on the sphere surface, respectively. This conclusion is yielded from narrowed wakes and reductions in turbulence statistics compared to the naked sphere model. The O-ring with a diameter of 3 mm and placement angle of 50° exhibits the most effective flow control. It decreases, in sequence, streamwise velocity fluctuations and length of wake recovery region by 45% and 40%, respectively, which can be evaluated as source of decrement in drag force.

• International Journal of Fluid Machinery and Systems
• /
• v.1 no.1
• /
• pp.24-32
• /
• 2008

Bleeding away a part of the boundary layer next to the wall is an effective method for controlling boundary-layer distortions from incident shock waves or curvature in geometry. When the boundary-layer flow is supersonic, the physics of bleeding with and without an incident shock wave is more complicated than just the removal of lower momentum fluid next to the wall. This paper reviews CFD studies of shock-wave/boundary-layer interactions on a flat plate with bleed into a plenum through a single hole, three holes in tandem, and four rows of staggered holes in which the simulation resolves not just the flow above the plate, but also the flow through each bleed hole and the plenum. The focus is on understanding the nature of the bleed process.

• International Journal of Aerospace System Engineering
• /
• v.2 no.1
• /
• pp.6-11
• /
• 2015

An active flow control technique using blowing and distributed suction on low Reynolds airfoil is investigated. Simultaneous blowing and distributed suction can recirculate the jet flow mass, and reduce the penalty to propulsion system due to avoiding dumping the jet mass flow. Energy is injected into main flow by blowing on the suction surface, and the low energy boundary flow mass is removed by distributed suction, thus the flow separation can be successfully suppressed. Aerodynamic lift to drag ratio is improved significantly using the flow control technique, and the energy consumption is quite low.

• Journal of the Korean Society of Visualization
• /
• v.9 no.2
• /
• pp.61-65
• /
• 2011

Several previous works on rocket nozzle flows have revealed the existence of the transition from FSS to RSS and the occurrence of asymmetric flow associated with the boundary layer separation, which can cause excessive side-loads of the propulsion system. Thus, it is of practical importance to investigate the asymmetric flow behaviors of the propulsion nozzle and to develop its control method. In the present study, the asymmetric flow control method using a cavity system was applied to supersonic nozzle flow. Time-dependent asymmetric flow was experimentally investigated with the rate of change of the nozzle pressure ratio. The results obtained showed that the cavity system installed on nozzle wall would be helpful in fixing the unsteady motions of the boundary layer separation, consequently reducing the possibility of the occurrence of the asymmetric flow.

• Proceedings of the KSME Conference
• /
• 2002.08a
• /
• pp.161-164
• /
• 2002

In the present study, a passive control method, using the porous wall and cavity system, is applied to the shock wave/boundary layer interactions in transonic moist air flow. The two-dimensional, unsteady, compressible Navier-Stokes equations, which are fully coupled with a droplet growth equation, are solved by the third-order MUSCL type TVD finite difference scheme. Baldwind-Lomax turbulence model is employed to close the governing equations. In order to investigate the effectiveness of the present control method, the total pressure losses of the flow and the time-dependent behaviour of shock motions are analyzed in detail. The computed results show that the present passive control method considerably reduces the total pressure losses due to the shock/boundary layer interaction in transonic moist air flow and suppresses the unsteady shock wave motions over the airfoil, as well. It is also found that the location of the porous ventilation significantly influences the control effectiveness.