Role of Klebanoff modes in active flow control of separation: direct numerical simulations

2018 ◽  
Vol 850 ◽  
pp. 954-983 ◽  
Author(s):  
Shirzad Hosseinverdi ◽  
Hermann F. Fasel
Keyword(s):  
Flow Control ◽  
Numerical Simulations ◽  
Active Flow Control ◽  
Low Frequency ◽  
Secondary Instability ◽  
Direct Numerical Simulations ◽  
Striking Difference ◽  
Computational Domain ◽  
Low Levels ◽  
Active Flow

Our previous research has shown that an active flow control strategy using two-dimensional (2-D) harmonic blowing and suction with properly chosen frequency and amplitude can significantly reduce the separation region, delay transition to turbulence and can even relaminarize the flow. How such effective flow control for transition delay and relaminarization is affected by free-stream turbulence (FST) remains an open question. In order to answer this question, highly resolved direct numerical simulations (DNS) are carried out where very low-amplitude isotropic FST fluctuations are introduced at the inflow boundary of the computational domain. With FST the effectiveness of the flow control is not diminished, and the extent of the separated flow region is reduced by the same amount as for the zero FST case. However, a striking difference observed in the DNS is the fact that in the presence of even very low levels of FST, the flow transitions shortly downstream of the reattachment location of the bubble, contrary to the case without FST. It appears that this different behaviour for even very small levels of FST is caused by an interaction between the high-amplitude 2-D disturbances introduced by the flow control forcing and 3-D Klebanoff modes (K-modes) that are generated by the FST. The streamwise elongated streaks due to the K-modes cause a spanwise-periodic modulation of the basic flow and subsequently of the primary 2-D wave. The disturbances associated with this modulation exhibit strong growth and initiate the breakdown process to turbulence. Linear secondary instability investigations with respect to low-frequency 3-D disturbances are carried out based on the linearized Navier–Stokes equations. The response of the forced flow to the low-frequency 3-D disturbances reveals that the time-periodic base flow is unstable with respect to a wide range of 3-D modes. In particular, the wavelength associated with the spanwise spacing of the K-mode falls into the range of, and is in fact very close to, the most unstable 3-D disturbances. Results from the secondary instability analysis and the comparison with DNS results, support the conjecture that the forcing amplitude has a major impact on the onset and amplification rate of the K-modes: lowering the forcing amplitude postpones the onset of the growth of the K-modes and reduces the growth rate of the K-modes for a given FST intensity. The net effect of these two events is a delay of the transition onset. Nevertheless, the instability mechanism that governs the transition process is the same as previously identified, i.e. interaction of the K-mode and 2-D primary wave. Furthermore, for low levels of FST, the amplitude of the low-frequency K-modes scales linearly with the FST intensity in the approach boundary layer up to the secondary instability regime.

Active Flow Control With an Oscillating Backward Facing Step in a Shallow Open Channel

2012 ◽  
Author(s):  
Sertac Cadirci ◽  
Hasan Gunes
Keyword(s):  
Boundary Layer ◽  
Flow Control ◽  
Flow Separation ◽  
Open Channel ◽  
Recirculation Zone ◽  
Active Flow Control ◽  
Computational Domain ◽  
Backward Facing Step ◽  
Induced Flow ◽  
Active Flow

An oscillatory, zero-net-mass flux actuator system, Jet and Vortex Actuator (JaVA), is implemented on the step wall of a backward facing step. JaVA can energize the boundary layer by creating jets or vortices thus it may delay flow separation when used properly. The main part of JaVA is a rectangular cavity with a moving actuator plate. The actuator plate is mounted asymmetrically inside the cavity of the JaVA box, such that there are one narrow and one wide gap between the plate and the box. The main governing parameters are the actuator plate’s width (b), the amplitude (a) and the operating frequency (f). The main target of the control with active jets on the step wall is to influence directly the main recirculation zone, thus as the actuator plate or the step’s vertical wall moves periodically in horizontal direction, a jet emerges into the recirculation zone. Non-dimensional numbers such as the scaled amplitude (Sa = 2πa/b) and the jet Reynolds number (ReJ = 4abf/ν) as well as the cross flow parameter characterize the JaVA-induced flow types and the effects on the recirculation zone. One period consists of one blowing and one suction phase into the recirculation zone. Boundary layer profiles extracted from time-averaged flow fields of the not actuated (f = 0) and actuated cases at various operating frequencies indicate the effect of active flow control. The interaction between JaVA-induced flow regimes and the boundary layer is investigated numerically in an open channel with a BFS. The computational domain consists of a moving zone along the channel and the motion of the actuator plate is generated by a moving grid imposing appropriate boundary conditions with User-Defined-Functions and the calculations are carried out by a commercial finite-volume-based unsteady, laminar, incompressible Navier-Stokes solver. Numerical simulations and comparisons reveal the JaVA-boundary layer interaction for various governing parameters. Reynolds numbers based on the step height for the shallow open channel flow are Reh = 225 and 450. The proposed control method based on suction and blowing with an oscillating vertical step seems to be effective in shortening the recirculation zone length and delaying the flow separation downstream of the backward facing step.

Reducing Energy Consumption of Ground Vehicles by Active Flow Control

2010 ◽  
Author(s):  
Miles Bellman ◽  
Ramesh Agarwal ◽  
Jonathan Naber ◽  
Lee Chusak
Keyword(s):  
Experimental Data ◽  
Flow Control ◽  
Numerical Simulations ◽  
Fuel Consumption ◽  
Aerodynamic Drag ◽  
Active Flow Control ◽  
Ground Vehicles ◽  
Ground Vehicle ◽  
Jet Actuators ◽  
Active Flow

In U.S, the ground vehicles consume about 77% of all (domestic and imported) petroleum; 34% is consumed by automobiles, 25% by light trucks and 18% by large heavy duty trucks and trailers. It has been estimated that 1% increase in fuel economy can save 245 million gallons of fuel/year. Additionally, the fuel consumption by ground vehicles accounts for over 30% of CO2 and other greenhouse gas (GHG) emissions. Moreover, most of the usable energy from the engine goes into overcoming the aerodynamic drag (53%) and rolling resistance (32%); only 9% is required for auxiliary equipment and 6% is used by the drive-train. 15% reduction in aerodynamic drag at highway speed of 55mph can result in about 5–7% in fuel saving. The goal of this paper is to demonstrate by numerical simulations that the active flow control (AFC) technology can be easily deployed /retrofitted to reduce the aerodynamic drag of ground vehicles by 15–20% at highway speed. For AFC, we employ a few oscillatory jet actuators (also known as synthetic jet actuators) at the rear face of the ground vehicle. These devices are easy to incorporate into the existing vehicles with very modest cost. The cost may come down significantly for a large volume — in hundreds of millions, especially for ground vehicles. Numerical simulations are performed using the Unsteady Reynolds-Averaged Navier-Stokes (URANS) equations on solution adaptive structured grids in conjunction with a two-equation realizable k-ε turbulence model. The commercially available grid generator “GAMBIT” and the CFD solver “FLUENT” are employed in the simulations. Three generic ground vehicle configurations are considered in the simulations; the experimental data has been available for these configurations without and with AFC. The numerical simulations are in good agreement with the experimental data. These studies clearly demonstrate that the AFC techniques can be effectively employed to achieve significant reduction (10–15%) in aerodynamic drag of ground vehicles thereby reducing the fuel consumption by 5–7%.

Active Flow Control for Reduction of Fluctuating Aerodynamic Forces of a Blunt Trailing Edge Airfoil

2009 ◽  
Author(s):  
Arash Naghib Lahouti ◽  
Horia Hangan
Keyword(s):  
Flow Control ◽  
Numerical Simulations ◽  
Control Mechanism ◽  
Active Flow Control ◽  
Three Dimensional ◽  
Leading Edge ◽  
Aerodynamic Forces ◽  
Trailing Edge ◽  
Blunt Trailing Edge ◽  
Active Flow

Vortex shedding from the base of two dimensional bluff bodies is accompanied by three dimensional wake instabilities. These instabilities manifest as streamwise and vertical vorticity components which occur at a certain spanwise wavelength. The spanwise wavelength of the instabilities (λz) depends on several parameters, including profile geometry and Reynolds number. The present study aims to determine λz for a blunt trailing edge airfoil, which is comprised of an elliptical leading edge, followed by a rectangular section. Results of numerical simulations of flow around the airfoil at Re(d) = 500, 800, 1200, and 17,000, and flow visualization at Re(d) = 2200 indicate that λz has an average value of 2.2d. An active flow control mechanism based on the three dimensional wake instabilities is proposed, to attenuate the fluctuating aerodynamic forces of the airfoil. The mechanism is comprised of trailing edge injection ports distributed across the span, with a spacing equal to λz. Injection of a secondary flow leads to excitation of the three dimensional instabilities and disorganization of the von Ka´rma´n vortex street. Numerical simulations at Re(d) = 500 and 17,000 indicate that the flow control mechanism can attenuate the fluctuating aerodynamic forces significantly, and reduce mean drag using a relatively small injection mass flow rate.

Direct Numerical Simulations of a Laminar Separation Bubbles on a Curved Plate: Part 2 — Flow Control Using Pulsed Vortex Generator Jets

2013 ◽  
Author(s):  
Wolfgang Balzer ◽  
Hermann F. Fasel
Keyword(s):  
Flow Control ◽  
Numerical Simulations ◽  
Large Scale ◽  
Separation Bubble ◽  
Hydrodynamic Instability ◽  
Active Flow Control ◽  
Vortex Generator ◽  
Direct Numerical Simulations ◽  
Laminar Separation ◽  
Vortex Generator Jets

Highly-accurate direct numerical simulations (DNS) are employed to investigate active flow control of laminar boundary layer separation by means of pulsed vortex generator jets (VGJs), i.e. by injecting fluid into the flow through a spanwise array of small holes. The uncontrolled flow configuration is represented by a laminar separation bubble developing on a curved-plate geometry modeling the convex suction-side curvature of the Pratt&Whitney “PackB” research blade. The simulation setup and uncontrolled flow results were presented in part I of the present paper. In this second part, particular focus is directed towards identifying the relevant physical mechanisms associated with VGJ control of low Reynolds number separation, as for example encountered in low-pressure turbine applications. The numerical results confirm findings of earlier flat-plate simulations, which showed that the control effectiveness of pulsed VGJs can be explained by the fact that linear hydrodynamic instability mechanisms are exploited. When pulsing with frequencies to which the (uncontrolled) separated shear layer is naturally unstable, instability modes are shown to develop into large-scale, spanwise-coherent structures. These structures provide the necessary entrainment of high-momentum fluid causing a much sooner reattachment of the separated flow compared to the uncontrolled flow and consequently leading to a significant reduction in performance losses.

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