Moving Surface Boundary-layer Control Research Articles

Moving Surface Boundary-Layer Control on the Wake of Flow around a Square Cylinder

In this paper, the entire process of the flow around a fixed square cylinder and the moving surface boundary-layer control (MSBC) at a low Reynolds number was numerically simulated. Two small rotating circular cylinders were located in each of the two rear corners of the square cylinder, respectively, to transfer momentum into the near wake behind the square cylinder. The rotations of the two circular cylinders were realized via dynamic mesh technology, when the two-dimensional incompressible Navier–Stokes equations for the flow around the square cylinder were solved. We analyzed the effects of different rotation directions, wind angles θ, and velocity ratios k (the ratio of the tangential velocity of the rotating cylinder to the incoming flow velocity) on the wake of flow around a square cylinder to evaluate the control effectiveness of the MSBC method. In the present work, the aerodynamic forces, the pressure distributions, and the wake patterns of the square cylinder are discussed in detail. The results show that the high suction areas near the surfaces of the rotating cylinders can delay or prevent the separation of the shear layer, reduce the wake width, achieve drag reduction, and eliminate the alternating vortex shedding. For a wind angle of 0°, the inward rotation of the small circular cylinders is the optimal arrangement to manipulate the wake vortex street behind the square cylinder, and k=2 is the optimal velocity ratio between the control effectiveness and external energy consumption.

Moving Surface Boundary Layer Technique For NACA 0012 Airfoil At Ultra-Low Reynolds Number

Application of moving surface boundary layer control technique has been confined to relatively high Reynolds numbers. The present paper reports a numerical study of application of the above flow technique in the ultra-low Reynolds number range. A two dimensional incompressible unstructured grid based Navier Stokes solver has been used for conducting the numerical studies. Moving surface has been applied at three different portions on the airfoil surface, firstly, in the form of a rotating leading edge portion of the airfoil, secondly, a continuous moving surface from leading edge of airfoil to 57% of the chord along the leeward surface of the airfoil and thirdly a continuous moving surface from leading edge to 97% of the chord along the leeward surface of the airfoil. All the moving surface configurations show improvement of aerodynamic performance of the airfoil through enhancement of lift and decrement of drag as compared to a fixed surface one.

Flow control over a circular cylinder using virtual moving surface boundary layer control

Flow control study of a circular cylinder is carried out using symmetric dielectric barrier discharge (DBD) plasma actuators at the Reynolds number of 10,000. Here, two symmetric DBD plasma actuators are located at the top and bottom of the circular cylinder, respectively, each of which induces pairs of counter-rotating starting vortices on both sides of exposed electrodes. The downstream starting vortices soon take the form of a wall jet along the freestream direction. On the other hand, the upstream starting vortices interact with the incoming flow remain for some time, bringing in high momentum from the freestream to near-wall region, enabling the boundary layer to withstand the adverse pressure gradient and suppressing the separation around the circular cylinder. The rotating vortical structures around the circular cylinder created by the plasma actuators lead to a reduction in the drag coefficient of up to 25%, providing a similar effect to moving surface boundary layer control (MSBLC). This configuration of symmetric DBD plasma actuator, which is studied for the first time in this investigation, is, therefore, called virtual MSBLC. Our results also indicated that the control effect of virtual MSBLC can be enhanced with an increase in the momentum coefficient of plasma jet. Unlike traditional MSBLC devices, the virtual MSBLC based on symmetric DBD plasma actuators do not have profile drag and are without complicated mechanical systems.Graphical abstract

FLOW CONTROL USING MOVING SURFACE AT THE LEADING EDGE OF AEROFOIL

Flow control is a significant topic of research in the field of aviation. Researchers in this field are continuously trying their best to find various flow control strategies in order to extract aerodynamic benefits by applying them. Applying moving surface at the leading edge of aerofoil is a type of strategy among the various types of active flow control strategies. In the present research work a rotating cylinder is added on the leading edge of the aerofoil as a moving surface in order to control the flow over its surface. The moving surface boundary layer control is applied to NACA 0018 airfoil for investigating its aerodynamic benefits and effectiveness. The moving surface is created by rotating a smooth cylinder at the leading edge of the aerofoil. The peripheral velocity of the cylinder injects momentum to the upper surface boundary layer of the aerofoil and thus delays its separation. This results in the gain in both the maximum lift coefficient and the stall angle. The work has been done at four different Reynolds Number i.e., at Re = 1.4 X 10^5, 1.85 X 10^5, 2.3 X 10^5, 2.8 X 10^5 at different angles of attack.

Large Eddy Simulation of Flow Control Around a Cube Subjected to Momentum Injection

The concept of Momentum Injection (MI) through Moving Surface Boundary layer Control (MSBC) applied to a cubic structure is numerically studied using Large Eddy Simulation at a Reynolds number of 6.7×104. Two small rotating cylinders are used to add the momentum at the front vertical edges of the cube. Two configurations are studied with the yaw angle of 0° and 30°, respectively, with ratio of the rotation velocity of cylinders and the freestream velocity of 2. The results suggest that MI delays the boundary layer separation and reattachment, and thus reduces the drag. A drag reduction of about 6.2 % is observed in the 0° yaw angle case and about 44.1 % reduction in the 30° yaw angle case. In the case of 0° yaw angle, the main change of the flow field is the disappearance of the separation regions near the rotating cylinders and the wake region is slightly changed due to MI. In the 30° yaw angle case, the flow field is changed a lot. Large flow separations near one rotating cylinder and in the wake is significantly reduced, which results in the large drag reduction. Meanwhile, the yaw moment is increased about 50.5 %.

Suppression of vortex-induced vibration using moving surface boundary-layer control

Experimental results of flow around a circular cylinder with moving surface boundary-layer control (MSBC) are presented. Two small rotating cylinders strategically located inject momentum in the boundary layer of the cylinder, which delays the separation of the boundary layer. As a consequence, the wake becomes narrower and the fluctuating transverse velocity is reduced, resulting in a recirculation free region that prevents the vortex formation. The control parameter is the ratio between the tangential velocity of the moving surface and the flow velocity (Uc/U). The main advantage of the MSBC is the possibility of combining the suppression of vortex-induced vibration (VIV) and drag reduction. The experimental tests are preformed at a circulating water channel facility and the circular cylinders are mounted on a low-damping air bearing base with one degree-of-freedom in the transverse direction of the channel flow. The mass ratio is 1.8. The Reynolds number ranges from 1600 to 7500, the reduced velocity varies up to 17, and the control parameter interval is Uc/U=5–10. A significant decreasing in the maximum amplitude of oscillation for the cylinder with MSBC is observed. Drag measurements are obtained for statically mounted cylinders with and without MSBC. The use of the flow control results in a mean drag reduction at Uc/U=5 of almost 60% compared to the plain cylinder. PIV velocity fields of the wake of static cylinders are measured at Re=3000. The results show that the wake is highly organized and narrower compared to the one observed in cylinders without control. The calculation of the total variance of the fluctuating transverse velocity in the wake region allows the introduction of an active closed-loop control. The experimental results are in good agreement with the numerical simulation studies conducted by other researchers for cylinders with MSBC.

Exploration in optimal design of an airfoil with a leading edge rotating cylinder

Based on the theory of moving surface boundary layer control (MSBC), a concept of an airfoil having a rotating cylinder at the leading edge has been developed and experimentally proven to have good aerodynamic performance even at large angles of attack. Thus, this research aims to give guidance on optimizing the design of this kind of airfoil with high lift coefficients. Using computational fluid dynamics (CFD) technique, the CFD simulation results have been compared with the experimental results available in the literature, and then the SST two-equation model is selected as the appropriate turbulence model. At a given cylinder surface velocity ratio, the cylinder diameter d, the drop height of trailing edge δ and the curvatures of the pressure and suction surfaces of the airfoil are regarded as the optimal design parameters and the airfoil lift coefficient is considered as the optimization objective function. Therefore, using orthogonal optimization method, we herein develop a new design of airfoil favorable for having a rotating leading edge. It has been numerically proven that the resulting airfoil has good capability of achieving a substantially superior performance when compared to the airfoils of the prior art.

An assessment of turbulence models for the prediction of flow past a circular cylinder with momentum injection

Reynolds averaged Navier–Stokes equations (RANS) are solved to simulate the flow past a circular cylinder. Momentum injection through Moving Surface Boundary-layer Control (MSBC) with zero net mass injection is implemented to achieve wake control. Popular eddy viscosity based closure models are assessed for their predictive capability of turbulent wake characteristics. For the present simulation, sub-critical Reynolds number ( Re) of 3900 is chosen, where extensive validations are available. A stabilization approach is proposed to model, predict and control vortex shedding behind a circular cylinder. Along with the mass, momentum conservation, turbulent kinetic energy (TKE) and its rate of dissipation equations are solved with the objective of achieving the annihilation of wake structures. To enable momentum injection, two simple rotary type control cylinders are located at 120°, behind the main cylinder. The ratio of the main cylinder, control cylinder and gap between them are fixed at D: 0.1D: 0.01D, respectively. These control cylinders, which are like externally controllable actuators, are assessed for their ability to influence momentum injection and hence wake patterns. The popular finite volume based Semi Implicit Pressure Linked Equations (SIMPLE) scheme is employed for the numerical calculations. Detailed assessment of different eddy viscosity based turbulence models viz., standard k – ε , Renormalization Group (RNG) k – ε , realizable k – ε and k – ε version of Kato–Launder (KL) is carried out. As a precursor, validation of turbulence statistics such as, mean streamwise velocity along the wake centerline, mean pressure coefficient on the cylinder surface and time averaged Reynolds stresses etc. is carried out against known experimental and numerical computations. The role of externally controllable actuators on the fluid flow patterns past a circular configuration is assessed with the help of streaklines, streamlines, vorticity, Reynolds stress contours etc. Complete suppression of vortex shedding is observed for an injection parameter (defined as the ratio of control cylinder velocity ( U c ) to that of the freestream ( U ∞ )) ξ = 6.0 . The results clearly demonstrate the effectiveness of a rather simple momentum injection strategy in suppressing turbulent vortex structures.

Fluid dynamics of a cubic structure as affected by momentum injection and height

The concept of moving surface boundary-layer control (MSBC) as applied to a cubic structure is investigated experimentally at a subcritical Reynolds number of 6.7×10 4. Two high-speed rotating cylinders replaced adjacent vertical edges (W) of the cube's nominal upstream face and controlled the momentum injection parameter U c/ U, where U c is the cylinder surface velocity and U is the free-stream velocity. Three different configurations of the cube are considered: (i) the basic cube resting on a horizontal surface ( H= W; H, vertical distance from the ground to the top face of the cube); (ii) H=2 W; and (iii) H=3 W. The focus is on the momentum injection and cube height on the fluid dynamical parameters represented by the surface pressure, drag and side force. The results suggest significant effect of the MSBC and the height on the boundary-layer separation and reattachment, thus affecting the fluid dynamical parameters. For the basis cube (Case (i)), the maximum drag reduction was observed to be around 67%, while the peak increase in the side force was as high as 900%. For H=2 W (Case (ii)), the effect of U c/ U was to reduce the drag at virtually all angles of attack, however, the decrease was substantially less compared to that observed for the basic cube (Case (i)). This is primarily due to the lateral flow created on the side faces of the cube because of the gap formed by the proximity of the ground. However at the higher height ( H=3 W, Case (iii)), the trend reverses. Now the reduction in drag is significantly higher, even compared to that for the basic cube case. The fundamental information should prove useful to industrial aerodynamicists and practicing engineers.

Pressure distribution on a roof in presence of the Moving Surface Boundary-layer Control

The paper studies effect of momentum injection, accomplished through circular cylindrical rotating elements, on the pressure distribution at the roof of a model house. Extensive wind tunnel tests, complemented by flow visualization, suggest that such Moving Surface Boundary-layer Control (MSBC) effectively delays separation and significantly increases the pressure, both on leeward and windward surfaces. This would assist in protection of the roof against wind storms and snow accumulation.

Fluid dynamics of flat plates and rectangular prisms in the presence of moving surface boundary-layer control

Abstract Fluid dynamics of a family of two-dimensional flat plates and rectangular prisms (aspect ratio of 0.27, 0.5 and 1.0) with moving surface boundary-layer control (MSBC) was carried out employing three complementary approaches: (i) extensive wind tunnel test program; (ii) finite element-based numerical integration of the complete Navier–Stokes equations; and (iii) flow visualization. A comparison between the drag of rectangular prisms of varying aspect ratios with sharp and rounded corners reveal substantial drag reduction, even in absence of the MSBC. It suggests fundamentally different fluid mechanical processes involved. The surface pressure distribution, resultant forces, streamline patterns and wake characteristics suggest significant increase in the wake pressure, with associated decrease in the drag, in presence of the MSBC. Results show increase in the Strouhal number indicating effective decrease in the bluffness. Both the numerical and flow visualization results show narrowing of the wake with increasing momentum injection and eventual suppression of the vortex shedding. In case of the flat plate, classical quasi-steady analysis suggests significant improvement against galloping type of instability. Implications of the present study to alleviate flow-induced instabilities such as vortex resonance and galloping are apparent.

High-Performance Airfoil with Moving Surface Boundary-Layer Control

All airfoils are known to stall at high angles of attack as a result of flow separation, resulting in a sudden loss of the lift force. To avoid flow separation, it is necessary to introduce some form of boundary-layer control. The present study focuses on the performance of an airfoil with moving surface boundary-layer control (MSBC). Effects of the angles of attack, rate of momentum injection, as well as rotating cylinder surface condition on the surface pressure distribution and aerodynamic coefficients are assessed. A comprehensive study involving wind-tunnel investigation, numerical simulation, and flow visualization clearly demonstrates that the momentum injection through MSBC results in a significant delay in the stall angle (from 10 to 50 deg) and an increase in the lift coefficient by more than 200% at high angles of attack (α 30 deg). A multielement panel method, modeling the flow separation using free vortex lines, predicts the overall aerodynamics of an airfoil with the MSBC quite accurately. The airfoil performance can be improved further by judicious selection of the rotating cylinder surface condition

AERODYNAMICS AND DYNAMICS OF RECTANGULAR PRISMS WITH MOMENTUM INJECTION

A structural member with rectangular cross-section is frequently encountered in civil, mechanical and ocean engineering applications. Structural loading in terms of steady and unsteady pressure fields, susceptibility to flow-induced instabilities, etc., for tall buildings, bridges, offshore platforms, marine risers and a wide variety of other configurations have been of interest to engineers. Recent progress in engineering materials and computer-aided design has led to structures with reduced stiffness, making them prone to wind, earthquake, as well as ocean-wave and current-excited oscillations. The present research is aimed at understanding these issues at a fundamental level through a comprehensive study of moving-surface boundary-layer control (MSBC) as applied to two-dimensional rectangular prisms with reference to drag reduction and suppression of flow-induced vibrations. Extensive wind-tunnel tests, complemented by quasisteady analysis and flow visualization study, suggest that the concept of MSBC represents a versatile tool for drag reduction and suppression of both vortex resonance and galloping type of instabilities.

MOVING SURFACE BOUNDARY-LAYER CONTROL: A REVIEW

The paper briefly reviews developments in the exciting field of the moving surface boundary-layer control (MSBC). To begin with, application of the concept to a family of two-dimensional airfoils, investigated experimentally, is briefly summarized. The moving surface was provided by rotating cylinders located at the leading edge and/or trailing edge as well as the top surface of the airfoil. Results suggest that the concept is quite promising, leading to a substantial increase in lift and a delay in stall. Depending on the performance desired, appropriate combinations of cylinder location and speed can be selected to obtain favourable results over a wide range of the angle of attack. Next, the effectiveness of the concept in reducing drag of bluff bodies such as a two-dimensional flat plate at large angles of attack, rectangular prisms, and three-dimensional models of trucks is assessed. Results show that injection of momentum through moving surfaces, achieved here by introduction of bearing-mounted, motor-driven, hollow cylinders, can significantly delay separation of the boundary layer and reduce the pressure drag. The momentum injection procedure also proves effective in arresting wind-induced vortex resonance and galloping type of instabilities, suggesting possible application in the next generation of civil engineering structures. Now the attention is directed towards the role of computational fluid mechanics to this class of problems. The system performance, as predicted by results obtained using two distinctly different numerical procedures, shows good correlation with the wind tunnel data. Finally, results of a flow visualization study, conducted in a closed-circuit water tunnel using slit lighting and polyvinyl chloride tracer particles, are touched upon. They show, rather dramatically, the effectiveness of the MSBC.

Suppression of wind-induced vibrations of tall structures through moving surface boundary-layer control

Abstract The suppression of aerodynamic response of square-section tall structures, like pylons of long-span bridges and very high buildings, is studied. The method used is the moving surface boundary-layer control achieved through rotating cylinders at the leading edge of the structure. Effectiveness of the approach in suppressing the aerodynamic instabilities of a square prism was confirmed in earlier studies by the authors. The method also proves successful in suppressing torsional vibrations. Effective and economical arrangements of rotors are investigated with reference to prototype tall structures.

On the boundary-layer control through momentum injection: Studies with applications

The concept of moving surface boundary-layer control, as applied to a Joukowsky airfoil, is investigated through a planned experimental programme complemented by numerical studies. The moving surface was provided by rotating cylinders located at the leading edge and/or trailing edge as well as top surface of the airfoil. Results suggest that the concept is quite promising, leading to a substantial increase in lift and a delay in stall. Depending on the performance desired, appropriate combinations of cylinder geometry, location and speed can be selected to obtain favourable results over a wide range of angle of attack. Next, effectiveness of the concept in reducing drag of bluff bodies such as a two-dimensional flat plate at large angles of attack, rectangular prisms and three-dimensional models of trucks is assessed through an extensive wind tunnel test-programme. Results show that injection of momentum through moving surfaces, achieved here by introduction of bearing-mounted, motordriven, hollow cylinders, can significantly delay separation of the boundary-layer and reduce the pressure drag. The momentum injection procedure also proved effective in arresting wind-induced vortex resonance and galloping type of instabilities. A flow visualization study, conducted in a closed-circuit water tunnel using slit lighting and polyvinyl choride tracer particles, adds to the wind-tunnel and numerical investigations. It shows, rather dramatically, the effectiveness of the moving surface boundary-layer control (MSBC).

On the suppression of aerodynamic instabilities through the moving surface boundary-layer control

Several methods have been reported for suppression of the aerodynamic vibrations of structures under wind action. Most of them are passive in character as against the proposed active approach. Results show that galloping and vortex-excited vibrations of a square prism can be suppressed by the injection of momentum into the boundary-layer.

Drag reduction of bluff bodies through momentum injection

Effectiveness of the moving surface boundary-layer control in increasing lift and/or reducing drag at a subcritical Reynolds number is studied, using a two-dimensional wedge-shaped airfoil and a flat plate at high angles of attack, through an extensive wind-tunnel test program. Results suggest that injection of momentum, achieved here by introduction of bearing mounted, motor driven, hollow cylinders, can significantly delay separation of the boundary layer, resulting in a narrow wake and the associated reduction in the pressure drag. Results show that both the cylinder surface velocity as well as the surface roughness have significant effect on the boundary-layer control. The wedge airfoil showed an increase in CLICD from 2 to 80, whereas the flat plate at 90 deg showed a reduction in drag coefficient by around 75%. A flow visualization study, conducted in a closed-circuit water tunnel using slit lighting and polyvinyl chloride tracer particles, complements the windtunnel tests. It shows, rather dramatically, the effectiveness of the moving surface boundary-layer control.

Moving surface boundary-layer control - Studies with bluff bodies and application

The concept of moving surface boundary-layer control has proved quite successful in increasing lift and delaying stall of slender bodies like airfoil sections. We assess effectiveness of the concept in reducing drag of bluff bodies, such as a two-dimensional flat plate at large angles of attack, rectangular prisms, and three-dimensional models of trucks, through an extensive wind-tunnel test program.

Moving surface boundary-layer control as applied to two-dimensional and three-dimensional bluff bodies

The concept of moving surface boundary-layer control has proved quite successful in increasing lift and delaying stall of slender bodies such as airfoil sections. This paper assesses the effectiveness of the concept in reducing drag of bluff bodies such as a two-dimensional flat plate at large angles of attack, rectangular prisms and three-dimensional models of trucks through an extensive wind tunnel test program. Results suggest that injection of momentum through moving surfaces, achieved here by the introduction of bearing-mounted, motor-driven, hollow cylinders, can significantly delay separation of the boundary layer and reduce the pressure drag. A flow visualization study, conducted in a closed-circuit water tunnel using slit lighting and polyvinylchloride tracer particles, complements the wind tunnel tests. It shows, rather dramatically, the effectiveness of the moving surface boundary-layer control.