Adam Edstrand

Topology of a Trailing Vortex Flow Field with Steady Circulation Control Blowing

Trailing vortices adversely affect many applications and are robust to control efforts. In this study, a 1 mm slot placed along the suction-side of the airfoil tip spans 70% of the chord that blows pressurized air in the spanwise direction in an attempt to oppose the vortex motion via the Coanda effect. First, this experimental study uses stereo particle image velocimetry to observe the effects of different control levels along surface of the wing. These results show that the control jet appears to behave more akin to a jet in crossflow, bending backwards and rolling up with the vortex rather than oppose the vortex motion. The vortex separates from the surface further upstream (relative to the baseline case) with increased turbulent kinetic energy within the core. Investigation into the intermediate field shows a larger, more diffuse vortex relative to the baseline case. This diffuse vortex reduces the wake-hazard by as much as 40% in the first 5 chords downstream, potentially improving aircraft safety.

An Experimental Investigation of Unsteady and Steady Circulation Control for an Elliptical Airfoil

c K = . The experimental results demonstrate that unsteady circulation control with phase shifts of 45° and 90° at an appropriate frequency (102.4 Hz ) near the baseline vortex shedding frequency of the uncontrolled model and duty cycle ( 50% ) can improve lift generation in comparison with steady blowing and with reduced mass flow rates. However, the noise generated by the present actuators is significant, and unsteady circulation control produce considerably more noise than its steady counterpart for similar lift. Finally, substantial noise reduction is demonstrated via appropriate phase shift operation of the valves. Nomenclature

Investigation of the Instabilities of Supersonic Impinging Jets Using Unsteady Pressure Sensitive Paint

At given nozzle to plate spacings, the flow field of high speed impinging jets is known to be characterized by a resonance phenomenon. Large coherent structures that convect downstream and impinge on the surface create strong acoustic waves that interact with the inherently unstable shear layer at the nozzle exit. This feedback mechanism, driven by the coherent structures in the jet shear layer, can either be axisymmetric or helical in nature. Fast response pressure sensitive paint is applied to the impingement surface to map the unsteady pressure distribution associated with these resonant modes. Phase-conditioned results at several kHz are obtained using a flush mounted unsteady pressure transducer on the impingement plate as a reference signal. Tests are conducted at nozzle to plate spacings of x/Dj = 4 and 4.5. The resulting phase-conditioned pressure distribution reveals dramatically di↵erent flow fields at the corresponding impingement heights. The existence of a purely axisymmetric mode is identified at x/Dj = 4.5 characterized by concentric rings of higher/lower pressure that propagate radially with increasing phase. Two simultaneous modes are observed at x/Dj = 4. One being a dominant symmetric mode and the second a sub-dominant helical mode exhibiting a unique ‘yin-yang’ pressure distribution. Phaseconditioned Schlieren images are also given to visualize the flow structures associated with each mode. Results at other impingement heights not shown here are also discussed in connection with the existence of axisymmetric and helical modes.

Active attenuation of a trailing vortex inspired by a parabolized stability analysis

Designing effective control for complex three-dimensional flow fields proves to be non-trivial. Often, intuitive control strategies lead to suboptimal control. To navigate the control space, we use a linear parabolized stability analysis to guide the design of a control scheme for a trailing vortex flow field aft of a NACA0012 half-wing at an angle of attack

On the mechanism of trailing vortex wandering

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Investigation of impinging jet resonant modes using unsteady pressure‑sensitive paint measurements

and a chord-based Reynolds number

Characterization of an Aeroacoustic Wind Tunnel Facility

Re=1000

A parallel stability analysis of a trailing vortex wake

. The stability results show that the unstable mode with the smallest growth rate (fifth wake mode) provides a pathway to excite a vortex instability, whereas the principal unstable mode does not. Inspired by this finding, we perform direct numerical simulations that excite each mode with body forces matching the shape function from the stability analysis. Relative to the uncontrolled case, the controlled flows show increased attenuation of circulation and peak streamwise vorticity, with the fifth-mode-based control set-up outperforming the principal-mode-based set-up. From these results, we conclude that a rudimentary linear stability analysis can provide key insights into the underlying physics and help engineers design effective physics-based flow control strategies.