Effect of passive control devices on the three-dimensional vortex structure of a rectangular jet with an aspect ratio of 2

Computations were performed to investigate the utility of three passive devices for controlling obliqueshock-wave/boundary-layer interactions on a flat plate. All three devices involve a cavity with two slots one upstream of the incident shock where blowing takes place and another downstream of it where bleeding takes place. The blowing and bleeding rates were determined passively by the pressure difference across the shock. Results are presented which show the nature of the flow field induced by passive blowing and bleeding. This study is based on the ensemble-averaged conservation equations of mass, momentum (compressible Navier-Stokes), and energy closed by a low Reynolds number k-oo model of turbulence. Solutions were obtained by using a cell-centered finite-volume method based on flux-difference splitting of Roe with slope limiter of Chakravarthy and Osher and a diagonalized alternating-direction implicit scheme with multigrid on a patched/ overlapped grid system. * Graduate student. ** Graduate student. Now, research staff at Sterling Software/NASA Ames Research Center. Professor. Senior Member AIAA. Research Engineer. Member AIAA. Copyright © 1997 by the American Institute of Aeronautics and Astronautics, Inc. No copyright is asserted in the United States under Title 17, U.S. Code. The U.S. Government has a royaltyfree license to exercise all rights under the copyright claimed herein for government purposes. All other rights are reserved by the copyright owner. INTRODUCTION Effective control of shock-wave/boundary-layer interactions to prevent flow separation and minimize flow distortions is important in a number of applications. These include airframes of supersonic aircraft, external and mixed-compression inlets, and wind tunnels. For some of these applications, shock-wave induced boundary-layer separations are not just annoyances because of reductions in efficiency but rather a very serious operational hazard. As an example, shock-wave induced separations in mixed-compression inlets can lead to the unstart condition requiring the entire propulsion system to undergo a restart sequence during flight. A number of techniques have been developed to control the detrimental effects of incident shock waves on boundary layers. Of these, bleeding and blowing have been found to be highly effective in eliminating both flow separation and improving boundary-layer profile. In terms of implementation, blowing and bleeding have been operated mostly in an active mode. By active, it is meant that a source of high or low pressure must be maintained in order to ensure blowing or bleeding. Since maintaining such a source requires additional energy which also reduces efficiency, a passive mode of operation is desirable. In 1979, Bushnell and Whitcomb (see Ref. 11) proposed a passive control device made up of a porous surface over a cavity or plenum. It was suggested that placing this device over a shock-wave/boundary-layer interaction region will produce a flow through the cavity from downstream to upstream of the shock because of the static pressure rise across the shock. Since that Copyright© 1997, American Institute of Aeronautics and Astronautics, Inc. time, a number of investigators have studied passive devices for controlling shock-wave/boundary-layer interactions. They found success in reducing shock drag over transonic airfoils (see review in Ref. 11). Recently, passive control was also found to be successful in controlling normal shock/boundary-layer interactions about an external compression inlet. So far, passive blowing and bleeding have been applied only to flows with low supersonic speeds (Mach numbers less than 1.4) so that the resulting shocks have been relatively weak. Also, no studies computational or experimental have been reported that show the nature of the multidimensional flow field about passive control devices. The objective of this study is to investigate computationally the utility of three passive devices for controlling oblique-shock-wave / boundary-layer interactions on a flat plate in which the freestream Mach number is much higher than 1.4 and the incident shock wave can induce boundary-layer separation in the absence of control. The focus is on understanding the nature of the flow and the usefulness of passive bleed in controlling strong shocks. In the next section, the problem studied along with the three passive control devices investigated are described. Afterwards, the formulation, numerical method of solution, and results obtained are presented. DESCRIPTION OF PROBLEM Figure 1 shows a schematic diagram of the passively controlled oblique-shock-wave/boundary-layer interaction problem being studied. It involves a supersonic turbulent boundary layer flowing past a flat plate with an incident oblique shock wave and blowing/bleeding from a passive device. Figures 2 to 4 show details of the three passive devices analyzed. The reasons for selecting these three configurations are given in the results section. The physical dimensions in Figs. 1 to 4 are as follows: LI = 57.34 cm, H = 25.4 cm, L = 7.65 cm, al = 1.14 cm, a2 = 0.76cm, a3 =0.38 cm, bl = 1.62 cm, b2 = 1.08 cm, b3 = 0.54 cm, cl = 8.6 cm, c2 = 7.65 cm, cl = 6.06 cm, c2' = 5.11 cm, h = 0.0381 cm, and s = 0.635 cm. Othei dimensions are given later in this section. Fig. 1. Schematic of problem studied. Fig. 2. Passive device 1: flap. Fig. 3. Passive device 2: big ellipse. ff{\_ JL-' — i l IBlcalin Fig. 4. Passive device 3: small ellipse. For the problem shown in Fig. 1, the computational domain is the region bounded by the dashed lines. The fluid that enters the domain above the flat plate is air Copyright© 1997, American Institute of Aeronautics and Astronautics, Inc. with a specific-heats ratio (y) of 1.4. The freestream Mach number (Moo), static pressure (Poo), and static temperature (T») are 2.46, 10.736 KPa, and 131.93 K, respectively. This supersonic flow has a turbulent, boundary layer next to the flat plate. At the inflow boundary, the thickness of that boundary layer (5) is 3.244 cm. The displacement thickness (8*) is 0.196 cm. A shock-wave generator characterized by L2 and a causes an oblique shock wave to strike the turbulent, boundary layer on the flat plate, a was set at 8° which produced a shock wave that was strong enough to induce flow separation on the flat plate in the absence of bleed or blowing. L2 was chosen so that the shock would incident on the flat plate at a distance of 34.4 cm measured from the inflow boundary under inviscid flow conditions (i.e., zero boundary-layer thickness). Aside from studying the three passive control devices shown in Figs. 2 to 4, the following were investigated: two locations of the incident shock wave relative to the passive control device (one just upstream of the bleed slot as shown in Figs. 2 to 4 and another just downstream of the blowing slot as shown in Figs. 5 and 6) and two boundary-layer heights at the inflow boundary (one finite as shown in Figs. 2 to 5 and the other zero as shown in Fig. 6). Note that when the shock impingement location change, the passive control device moved not the flat plate (i.e., L2 remains constant). Also, note when the boundary-layer height is zero at the inflow, the wall was kept inviscid until reaching the passive device. All cases studied are summarized in Table 1. Table 1 Summary of Cases Studied*

Probabilistic assessment of steel buildings installed with passive control devices under multi-hazard scenario of earthquake and wind

Numerical simulation of different passive control devices in hybrid configurations have been done to control dynamic stall. Previous studies have been carried out by researchers on different passive devices in isolation(vortex generators, gurney flap, droop leading edge etc) and by extracting the positive effects of these passive devices, researchers further focused on introducing hybrid configurations (mostly droop leading edge and gurney flap) on the airfoil and optimized their physical parameters to achieve the best flow control. In the present research, parametric study was carried out and then a combination of these three passive control devices have been used with an intent to extract the positive effects of each of these for better flow control. 2D computational analysis on an oscillating NACA 0012 airfoil was carried under deep dynamic stall conditions. Computational analysis was carried out on different passive devices; vortex generators (VGs), Fixed Droop Leading Edge (FDLE) and gurney flap (GF), in Single Passive Device (SPD) configuration. Results indicated that the height and location of the vortex generator had a profound effect on dynamic stall phenomenon while the fixed droop showed excellent dynamic stall control. The gurney flap contributed in increasing the lift but at the expense of increased drag and moment coefficient. In the Triple Passive Device (TPD) configuration case VG (H=7.6mm at 10%c), which was somewhat successful in Single Passive Device (SPD) configuration, with FDLE of 20 O and GF 1%c failed to contribute towards successful flow control, however the results improved when the same VG was moved to 15%c and 40%c. Although VG (H=2.5mm at 0.5%c and 10%c) failed to alter the flow in Single Passive Device (SPD) configuration, it proved successful when used in hybrid combination with FDLE 20 O and GF 1%c.

We study the control effect for a 20-story benchmark building and apply passive or semi-active control devices to the building. First, the viscous damping wall is selected as a passive control device which consists of two outer plates and one inner plate, facing each other with a small gap filled with viscous fluid. The damping force depends on the interstory velocity, temperature and the shearing area. Next, the variable oil damper is selected as a semi-active control device which can produce the control forces by little electrical power. We propose a damper model in which the damping coefficient changes according to both the response of the damper and control forces based on an LQG feedback and feedforward control theory. It is demonstrated from the results of a series of simulations that the both passive device and semi-active device can effectively reduce the response of the structure in various earthquake motions.

In this paper, we study the control effect for a 20-story benchmark building and apply passive and semiactive control devices to the building. First, we take viscous damping walls as a passive control device which consists of two outer plates and one inner plate, facing each other with a small gap filled with viscous fluid. The damping force is related to the interstory velocity, temperature, and the shearing area. Next, we take a variable oil damper as a semiactive control device which can produce the control forces by little electrical power. We propose a damper model in which the damping coefficient changes according to the response of the damper and control forces calculated by the controller based on a linear quadratic Gaussian control theory. It is demonstrated from the results of some simulations that both passive device and semiactive device can effectively reduce the response of the structure in various earthquake motions.

The three-dimensional vortical structures in a rectangular jet were determined by measuring the phase-average fluctuating static pressure, and the characteristics of the structure were discussed. The experiments were carried out for an air jet issuing from a sharp-edged rectangular orifice. The jet was excited in the interaction mode, in which the stable interaction of vortices occurred. The phase-average pressure was measured over the flow field with a static pressure probe. The three dimensional contours of phase-average pressure provided useful information on complicated vortical structures. The structures were characterized by stretching, splitting and cut-and-connect of vortices. The results suggest that the direct measurements of fluctuating static pressure are very useful for detecting three-dimensional complicated vortical structures.

Direct numerical simulation of coherent structures in the three-dimensional transitional jet with a moderate Reynolds number of 5000 was conducted. The finite volume method was used to discretize the governing equations in space and the low-storage, three-order Runge-Kutta scheme was used for time integration. The comparisons between the statistical results of the flow field and the related experimental data were performed to validate the reliability of the present numerical schemes. The emphasis was placed on the study of the spatial evolution of the three-dimensional coherent vortex structures as well as their interactions. It is found that the evolution of the spanwise vortex structures in three-dimensional space is similar to that in two-dimensional jet. The spanwise vortex structures are subject to three-dimensional instability and induce the formation of the streamwise and lateral vortex structures. Going with the breakup and mixing of the spanwise vortex structures, the streamwise and transverse vortex tubes also fall to pieces and the mixing arranged small-scale structures are formed in the flow field. Finally, the arrangement relationship among the spanwise, the streamwise and the lateral vortex structures was analyzed and their interactions were also discussed.

A synthetic jet is a promising technique to promote the mixing and to control the flow separation because of the periodic flow disturbance without a net mass injection. The synthetic jet has three types of vortex structures in quiescent fluid. An experimental investigation on the three-dimensional flow structure of the synthetic jet in quiescent fluid has been conducted in order to classify the regime of the vortex structure. Jet orifice diameter is 1.0 mm, Stokes number is 6.0a 12.7, and dimensionless stroke length is 0.25a 10. The phase-averaged three-dimensional flow structures of the synthetic jet were measured by scanning stereoscopic PIV. The vortex structure of the synthetic jet is changed according to Stokes number and stroke length. The parameter map for classification of vortex regime was obtained.

We investigate experimentally the effect of aspect ratio (AR) on the unsteady, threedimensional vortex structure of low-AR, flat-plate wings rotating from rest with a 45 angle of attack. This configuration is a simplified model of a flapping-wing hovering half-stroke. The objectives are to quantitatively characterize the evolution of the detailed, threedimensional vortex structure and its variation with AR. The experiments are conducted in a glass tank facility containing a mixture of glycerin and water. Plates of AR = 2 and 4 are tested, using a trapezoidal velocity program with a tip Reynolds number of 5,000 for each and a total rotation of 120°. The unsteady, three-dimensional, volumetric velocity data are reconstructed from phase-locked and phase-averaged stereoscopic digital particle image velocimetry measurements in multiple, closely-spaced chordwise planes. The threedimensional vortex formation is characterized using the Q-criterion and the helicity density. For each AR we find that the overall vortex structure is a loop consisting of a connection among the leading-edge, tip, and trailing-edge vortices. For both AR’s the leading-edge vortex (LEV) is larger with increasing span, i.e. conical, which is more pronounced for AR = 4. The LEV for each AR is attached over the inboard portion of the plate up to about 50% span throughout the motion. However, after approximately 30° of rotation it detaches from the plate in the outboard region near the tip, forming an arch-like structure. The arch is anchored at the tip due to the influence of the tip vortex (TV). A second LEV then forms in front of the arch, close to the leading edge. For AR = 2 the overall LEV continues to move with the plate and does not exhibit shedding into the wake. In contrast, for AR = 4 the flow structure in the tip region breaks down significantly and the flow appears to be fullyseparated for the remainder of the run. The emergence of discrete vortices is observed in the separated shear layers at the tip and trailing edges for both AR’s. The smaller vortices of the instability wrap around the primary trailing-edge vortex (TEV) and TV, forming a somewhat helical structure. For AR = 2 the helicity density is significant throughout the vortex loop, indicating a highly three-dimensional structure with flow velocity along the vortex. The AR = 4 case has substantially less helicity. The spanwise (root-to-tip) velocity is higher for AR = 2, in part due to the higher spanwise velocity gradient. The spanwise flow distribution within and near the LEV is complex, exhibiting both positive (outboard) and negative velocity. Significant positive spanwise velocity is distributed over portion of the plate aft of the LEV and within the TEV flow. Overall the AR = 2 and 4 flows are more similar with angular position than chord lengths traveled at the tip.

Laminar-to-transitional evolution of three-dimensional vortical structures in a low-aspect-ratio rectangular synthetic jet

Floating offshore wind turbines (FOWT) are subjected to strong loads, mainly due to wind and waves. These disturbances cause undesirable vibrations that affect the structure of these devices, increasing the fatigue and reducing its energy efficiency. Among others, a possible way to enhance the performance of these wind energy devices installed in deep waters is to combine them with other marine energy systems, which may, in addition, improve its stability. The purpose of this work is to analyze the effects that installing some devices on the platform of a barge-type wind turbine have on the vibrations of the structure. To do so, two passive control devices, TMD (Tuned Mass Damper), have been installed on the platform of the floating device, with different positions and orientations. TMDs are usually installed in the nacelle or in the tower, which imposes space, weight, and size hard constraints. An analysis has been carried out, using the FAST software model of the NREL-5MW FOWT. The results of the suppression rate of the tower top displacement and the platform pitch have been obtained for different locations of the structural control devices. They have been compared with the system without TMD. As a conclusion, it is possible to say that these passive devices can improve the stability of the FOWT and reduce the vibrations of the marine turbine. However, it is indispensable to carry out a previous analysis to find the optimal orientation and position of the TMDs on the platform.

Frequency band-wise passive control of linear time invariant structural systems with H∞ optimization

Direct numerical simulation of a particle-laden low Reynolds number turbulent round jet

Summary Structures today may be equipped with passive structural control devices to achieve some performance criteria. The optimal design of these passive control devices, whether via a formal optimization algorithm or a response surface parameter study, requires multiple solutions of the dynamic response of that structure, incurring a significant computational cost for complex structures. These passive control elements are typically point-located, introducing a local change (possibly nonlinear, possibly uncertain) that affects the global behavior of the rest of the structure. When the structure, other than these localized devices, is linear and deterministic, conventional solvers (e.g., Runge–Kutta, MATLAB's ode45, etc.) ignore the localized nature of the passive control elements. The methodology applied in this paper exploits the locality of the uncertain and/or nonlinear passive control element(s) by exactly converting the form of the dynamics of the high-order structural model to a low-dimensional Volterra integral equation. Design optimization for parameters and placement of linear and nonlinear passive dampers, tuned mass dampers, and their combination, as well as their design-under-uncertainty for a benchmark cable-stayed bridge, is performed using this approach. For the examples considered herein, the proposed method achieves a two-orders-of-magnitude gain in computational efficiency compared with a conventional method of comparable accuracy. Copyright © 2016 John Wiley & Sons, Ltd.

The purpose of these investigations is to study the possibilities to improve performances of a typical vertical axis wind turbines (VAWT), of Darrieus‐type, using passive devices (mounted “on” or “inside” the blades of VAWT) to control dynamic flow separation (dynamic stall phenomenon). The passive devices considered for investigations are the Gurney flap, the slot or thin channel, and the turbulence promoters. Studies are performed numerically using computational fluid dynamics and where it is possible the results are compared with existent experimental or numerical data. (© 2014 Wiley‐VCH Verlag GmbH & Co. KGaA, Weinheim)