Session P23: Flow Control V

Vortex Dynamics of Pitched Orifice Synthetic Jets in Quiescent Fluid

The influence that the orifice pitch angle and downstream lip radius have on the vortex dynamics of an aspect ratio, AR = 20, rectangular synthetic jet actuator issuing into quiescent fluid are investigated experimentally using particle image velocimetry measurements. Comparisons are made for an orifice pitch angle of α = 45° and dimensionless downstream lip radii of R* = 0, 5.1, and 7.8 at conditions of Re = 910 with St = 0.097. The time- and phase-averaged velocity fields were recorded on the centerline of the orifice major axis in addition to two off-centerline spanwise positions. Analysis of the phase-averaged vorticity field showed that the vortex formation is strongly influenced by the lip radius, which noticeably alters the flow field development. Specifically, increasing the downstream lip radius increases the asymmetry in the strength of the elongated vortex that is formed. This in turn increases the momentum near the wall aligned in the direction tangential to the wall. Additionally, all geometries exhibited a highly three-dimensional jet development which is the result of vortex dynamics associated with: 1) spatial variability in self-induction, 2) circulation asymmetry, and 3) possibly interactions with the wall image vortex system.   

Effects of Exit Nozzle Geometric Parameters on Sweeping Jet Actuator Performance

A sweeping jet actuator (SJA) emits a self-induced and self-sustained continuous but spatially oscillating jet at the outlet when pressurized with a fluid.  The SJA increases the local flow momentum without moving parts and is a promising way to suppress aerodynamic flow separation. However, in practical applications, the integration of SJA with curved aerodynamic surfaces results in different exit nozzle geometries from an isolated SJA. This talk will discuss the effects of exit nozzle geometry on flow physics in a quiet environment. The geometric parameters that we considered are nozzle angle, length, asymmetry, and curvature. A set of time-dependent flow fields are obtained using unsteady Reynolds Averaged Navier-Stokes simulations. Time history of velocity and pressure is recorded inside the SJA feedback channels and downstream of the SJA outlet. The jet oscillation frequency is obtained by employing fast Fourier transform (FFT) for all data sets and compared against the baseline numerical and experimental results. We show that the geometric variations of nozzle exit have a negligible impact on the oscillation frequency. However, there are notable effects on the oscillation amplitude and flow direction, indicating sensitivity to the variation of the exit nozzle. We will provide a brief overview of current modeling efforts and the preliminary results of LES simulation.    

Flow Physics and Scaling for Discrete Jet Forcing on a Wall-Mounted Hump

An experimental study is conducted to explore flow physics and scaling parameters (e.g., aspect ratio, exit area, spacing) for various types of fluidic oscillators in support of the development of active flow control technology. Various actuation modules are tested on the NASA hump geometry. Experiments are carried out at a chord-based Reynolds numbers of 1.0 x 10⁶ (Ma = 0.09). Time-averaged pressure is measured along both the chord and span of the model. Stereoscopic PIV is performed downstream of the actuation location to investigate the underlying control mechanisms in detail. Separation control using various spatially distributed fluidic oscillators is tested with spacings of Δz/c = 2.27%, 4.55%, 6.82%, & 9.09%. Performance curves are generated based on a novel method to experimentally obtain the momentum coefficient. These curves reveal regions of different efficiency and can be associated with boundary layer and circulation control depending on their slope. Stereoscopic PIV data is used to connect the mean and unsteady flow structure to separation control efficacy using various data analysis techniques.   

Reattachment of a Separation Cell using a Discrete Jet Array

The flow topology and turbulent structure of a separation cell formed in the adverse pressure gradient over a nominally 2-D surface, similar to forward stall cells over airfoils, are investigated experimentally in the absence and presence of fluidic control.  Actuation is effected by a spanwise array of surface-tangential, fluidically oscillating jets located upstream of the cell formation.  The effects of the actuation on the streamwise evolution of the cell structure are assessed using stereo particle image velocimetry in several streamwise-normal planes.  The evolution of concentrations of streamwise vorticity is assessed with specific emphasis on its turbulent organization within the cell’s global topology by the presence of the jet actuation.  It is shown that flow reattachment between the cell’s bounding outboard edge vortices is facilitated by the formation of a spanwise array of segmented reattachment cells that scale with the actuation jets and are each bounded by pairs of vorticity concentrations with predominantly opposite sense.

Supported by VLRCOE at Georgia Tech.   

Not Participating

Traditional circulation control (CC) for lift enhancement utilizes a nominally '2-D' control jet / sheet issued tangentially over the curved trailing edge of an airfoil.  It utilizes the Coanda effect to entrain and turn the outer flow and alter the stagnation point and stagnation streamline, thereby augmenting the airfoil's circulation and lift.  The present flow control approach introduces the segmented '3-D' array of flow control jets for the first time in place of the '2-D' control with the main objective of reducing the flow control input requirement for a given increment in lift.  A scaling relative to the active flow control area between the 2-D and 3-D flow control approaches indicates the 3-D flow control higher effectiveness in the lift increment relative to the active flow control area.  It is the tradeoff between the increased 3-D jets effectiveness and reduced total active area compared to the 2-D jet that determines which approach effects the higher total lift increments.  Depending on this balance, it is shown that the 3-D flow control may require tens of precents less flow control input than its 2-D counterpart.  Lastly, the proposed scaling for the 3-D flow control effectiveness indicates the guiding parameters for the flow control elements sizing and distribution.   

Control of Aerodynamics Instabilities of a Slender Axisymmetric Body at High Incidence

The aerodynamic loading and stability of a wire-mounted slender cylinder (L/D = 9) with an ogive forebody are investigated in wind tunnel experiments at high angles of incidence (45° < α < 60°).  Within this incidence range, the counter rotating vortex pair that normally forms over the forebody evolves asymmetrically in a manner that depends on its azimuthal orientation such that the interaction between the vortex pair and the cylinder's near wake vortices induces a nominally stable side force whose sense depends on the forebody's orientation.  However, within a critical range of incidence angles and forebody orientations the interaction between the forebody and wake vortices undergoes unstable mode switching that is accompanied by time-periodic side forces which couple to strong unstable, predominantly lateral oscillations of the cylinder within its body-yaw plane.  The present investigations have demonstrated the effectiveness of fluidic actuation using a synthetic jet placed at the juncture of the forebody and the cylinder for decoupling between the interactions of the forebody unstable vortex pair and the wake and thereby suppressing the side force oscillations and the lateral yaw instability.  It is shown that the actuation is effective both at fixed incidence and during pitch up/down maneuvers of the cylinder.   

Flow Control on a Swept Back Cylinder of Finite Aspect Ratio

Force and moment measurements on a 60º swept back cylinder of finite aspect ratio indicated that large flow asymmetry is created by local injection of momentum in a direction that is perpendicular to the cylinder’s axis and inclined at 20º to the surface. The cylinder contains two actuators that divided the span into three segments of similar length and whose azimuthal location could be changed at will. The experiments were performed at Reynolds number of 3 x 10⁵ and roughness dots were distributed throughout the surface to ensure that the flow was fully turbulent. The interaction was investigated using surface pressure measurements, stereoscopic PIV and oil flow visualization. Since the jet was injected from a small nozzle representing a point source, its azimuthal location on the cylinder’s surface was very significant. When the actuator was azimuthally close to the natural separation line, the emitted jet was redirected by the strong spanwise flow on the cylinder. The jet-crossflow interaction seems to have reattached one vortex (in the lee of the cylinder) to the cylinder surface, generating a sufficiently large side force capable of yawing the cylinder to a new equilibrium location.   

Not Participating

The aerodynamic loads on a representative 3-D wing model with a trailing edge flap are controlled in wind tunnel experiments using distributed air bleed that is driven through arrays of ports in the airfoil surface by pressure differences between the pressure and suction surfaces and is regulated by low-power, surface-integrated louvers for improved temporal response compared to conventional control. Interaction between the bleed and the local cross flow over the surface induces large-scale changes in the global flow field which lead to modification of the aerodynamic loads without varying the angle of attack.The aerodynamic load variation occurs on the convective timescale of the flow, which is significantly faster than the movement of conventional aerodynamic control surfaces. The present research program focuses on the elucidation of the receptivity of the flow over a 3-D wing model to manipulation by the bleed flow and the flow mechanisms that link these interactions to controlled variations in the global aerodynamic loads. High-resolution stereo particle image velocimetry measurements are used to characterize the variation of the distribution of streamwise vorticity in the wake, and the corresponding variation of the spanwise loading of the wing.   

Aerodynamic Active Flow Control using Hybrid, Momentum-based Actuation

The aerodynamic characteristics of a Clark-Y airfoil with a rounded trailing edge are modified

using active flow control (AFC) to enable aerodynamic control in the absence of moving control

surfaces. Using AFC, small-scale local perturbations are engendered near the airfoil surface which

lead to larger-scale changes in the flow around the airfoil. Distributed aerodynamic bleed, a form

of AFC in which a flow is driven through the airfoil interior by pressure differences between the

pressure and suction surfaces and regulated by integrated lateral louvers, is located near mid-

chord and used to reduce lift and increase drag. Blowing from fluidic jets is used at the leading

edge of the suction surface to mitigate separation at high angle of attack, leading to increased lift,

and on both surfaces near the trailing edge to form Coanda jets for varying lift bi-directionally. By

using hybrid actuation consisting of multiple forms of AFC, the aerodynamic effects can be

combined. For instance, lift can be varied bi-directionally with changes in pitching moment using

trailing edge blowing from opposite surfaces, or without altering pitching moment using

aerodynamic bleed with leading edge blowing.   

Experimental and Numerical Investigations of Flow Entrainment and Aerodynamic Performance for Small Aspect Ratio Wings with Wingtip Jets

Wind tunnel and CFD experiments were conducted using a NACA 0012 wing model to investigate effects of wingtip jets on flow and aerodynamic loads. Tests were conducted at 5, 10, and 15 m/s at 7.5° angle of attack. Average aerodynamic forces and moments were obtained using a six-component external balance. CFD simulations were performed using the RANS solver to investigate conditions similar to the physical experiment. This was done to better understand the effects on the flow field and wing performance.

Changes in spanwise velocity show critical differences between the jet on and off cases and provide insight into the difference in lift. The jet causes a negligible change to the spanwise velocity below the airfoil, and significantly reduces and even reverses the spanwise velocity above the airfoil. This reversal of the spanwise flow is due to the air entrainment caused by the jet. Under the steady blowing, the wingtip vortex is displaced upward and outward from the wingtip.

Results show that the blowing jet from the wingtip reduces the pressure on the top of the wing whereas the effect on the pressure on the bottom of the wing is minimal. Observed changes in pressure distribution explain forces and moments changes, specifically the total lift and drag increase.   

On the Control of a Complex Multi-Stream Supersonic Nozzle

The need to increase aircraft performance in terms of efficiency and maneuverability necessitates the design of increasingly complex nozzle configurations often yielding unwanted phenomena that clash with the benefits of the design. This study focuses on a multi aperture rectangular SERN , represented experimentally as a core (M = 1.6) and bypass (M = 1.0) flow that coalesce behind a splitter plate and exit onto an aft deck. Preliminary simulation and experiment have indicated that the instability associated with the splitter plate leads to unwanted tones in the flow field. To mitigate this, a spanwise wavenumber has been introduced to the splitter plate trailing edge as a form of passive control. This study extends this effort by laying the framework for active control of the flow via micro jets embedded within the splitter plate. Flow through the jet actuators is to be adjusted using the duty cycle of valves within the pressurized lines. Both open and closed loop techniques are proposed utilizing pressure sensors within the aft deck plate as well as acoustic microphones in the far field. PIV measurements quantify the effect of the control techniques and POD analysis provides insight into the optimal placement of sensors to feed back the most pertinent information of the flow.   

Separation delay and drag reduction through contoured transverse grooves in a boat-tailed bluff body with vortex shedding

The reduction of the aerodynamic drag of blunt-based bluff bodies with a wake characterized by alternate shedding of vortical structures is interesting for several applications. One well-known method to reduce the drag of this type of body is a geometrical modification called boat-tailing, consisting in a gradual reduction of the body cross-section before a sharp-edged base. We combine boat-tailing with properly contoured transverse grooves to further delay boundary-layer separation and reduce drag.

The considered geometry cross-section has an elliptical forebody and a rectangular main body followed by a circular-arc boat tail. The effectiveness of the contoured grooves is herein assessed through experiments in addition to simulations. The introduction of a single transverse groove leads to a significant increase of the pressure on the body base and, consequently, to a reduction of drag compared with the boat-tailed body without the groove. This is due to a delay of flow separation over the boat tail. A steady local recirculation is present inside the groove and downstream its reattachment the boundary layer is thinner and has higher momentum than in the case with no groove, allowing separation to be delayed.