Session G08: Flow Control: Passive (5:00pm - 5:45pm CST)

Geometric Modifications to Complex Supersonic Nozzle Configurations Guided by Machine Learning

This study focuses on a Multi Aperture Rectangular Single Expansion Ramp Nozzle. A MARS configuration consists of a core and bypass flow separated by a splitter plate that exits onto an aft deck on one side, mimicking a nozzle integrated into an air frame. In an effort to implement passive control to the system via geometric modifications, two highly receptive regions have been identified by way of simulation and experiment: the splitter plate trailing edge and the aft deck. Modifying the aft deck is the primary area of focus in this study. Changes are made to the aft deck trailing edge (ADTE), deck length, and width. An Artificial Neural Network is used to predict sound pressure levels for decks with varying lengths over a range of operating conditions to determine a minimum noise case, as well as create an acoustic model for a variable deck length. Particle Image Velocimetry (PIV) measurements at the deck trailing edge guide the implementation of various modifications to the ADTE. All aft deck changes are tested experimentally for two splitter plates, a nominal case and one in which a wave number has been introduced to the trailing edge. Near and far field pressure probes, as well as PIV act as the measurements used to identify differences among the configurations tested.   

Experimental Evaluation of Splitter Plate Trailing Edge Modifications for Passive Control in a Supersonic Multi-Stream Nozzle

Particle Image Velocimetry (PIV) and pressure measurements are performed for two configurations of a Multi-Aperture Rectangular Single Expansion Ramp Nozzle (MARS) to explore geometric modifications as a form of passive control. The nozzle consists of a supersonic core stream and a sonic bypass stream separated by a splitter plate. Previous studies of the MARS have shown the effectiveness of the bypass stream as a thermal and acoustic barrier can be impaired by a vortex shedding instability at the splitter plate trailing edge (SPTE). Simulation efforts indicate that an introduction of a spanwise wavenumber to the SPTE induces streamwise vorticity which allows for the break up of the shedding structures. This study seeks to validate the findings from Large Eddy Simulations (LES) by executing experiments for a wavenumber of β = 0.8. A stereoscopic PIV campaign is used to compare the flow structures of the nominal and wave SPTEs along with simultaneous pressure measurements in the near and far-field.   

Control of Supersonic Flow Over an Open Cavity with a Leading-Edge Spanwise Tab Array

Flow over an open cavity presents a dynamically rich flow dominated by broadband and tonal resonant effects. In the current study, passive control via a spanwise array of tabs along the cavity leading edge is performed on an open cavity of L/D $=$ 6 at Mach 1.4. The spanwise wavelength of the fences is one cavity depth while the height was determined to match the penetration extent of spanwise-arranged leading edge slots of the same wavelength utilized in [1] in the same flow facility. Significant fluctuating surface pressure reductions of up to 45 {\%} of $P_{rms} $are observed along the cavity floor and rear wall. Particle Image Velocimetry (PIV) is conducted to compare the velocity fields of the baseline flow with the controlled case. Two-component PIV measurements in streamwise aligned (x-y) planes revealed that fences produce lifting of the shear layer along with modifications of the recirculation characteristics. Stereoscopic PIV at cross-stream aligned (y-z) planes reveal the presence of counter-rotating vortices causing increased mixing of the flow. These observations are consistent with findings using leading edge blowing control in [1]. [1] Lusk et al., Exp Fluids (2012) 53:187-199   

Roughness-like modulation of canonical wall-bounded turbulence using effective boundary conditions.

Small-scale surface inhomogeneities result in a displacement of the turbulent and mean quantities of wall-bounded flows such that they perceive smooth walls at different virtual origins of ${l_U}^+$ and ${l_T}^+$ respectively. Recent direct numerical simulations of geometry-resolved flows over drag-reducing surfaces have shown that the turbulence perceives an origin at a shallower depth than the mean flow and that after setting the origin at ${l_T}^+$ the shift of the mean velocity profile becomes $\Delta U^+ = {l_U}^+-{l_T}^+$. Such a displacement of smooth-wall-like turbulence also extends to drag increasing surfaces such as roughness where the resulting increase in drag is due to the turbulence origin being deeper than the mean flow. We use a set of boundary conditions designated the transpiration-resistance model (TRM) to reproduce this behavior. The TRM accounts for both tangential slip and wall-normal transpiration, the latter of which is essential for emulating turbulent flow over structured surfaces, through the constitutive parameters ${l_x}^+, {l_z}^+$ and $m^+$. We also demonstrate that streamwise slip has a negligible effect on the turbulence displacement while transpiration is strictly coupled to the spanwise crossflow induced at the boundary.   

Tesla's fluidic diode exploits early turbulence and pulsatile flows

Nikola Tesla is known for his ingenious inventions for controlling and transforming electrical currents, but he also dabbled in fluidics or flow control. His design for a no-moving-parts valve or diode is an asymmetric conduit that allows flow to pass more easily in one direction than in reverse. Through extensive experimental tests of this device, we find that its diodic function turns on abruptly by triggering turbulence at unusually low Reynolds numbers. Further, we observe significant boosts in performance for oscillatory or pulsatile flows, which are transformed into one-way flows by a network of diodes. This fluidic analog of AC-to-DC conversion or rectification is a realization of Tesla's vision for the device.   

Drag reduction of a circular cylinder using a slot and axially arranged holes

In this study, a slot and axially arranged holes (AAH) with the same width or diameter are used to reduce the drag on a circular cylinder at a subcritical Reynolds number of Re $=$ 32,000. We measure the drag force and velocity through wind tunnel experiments with varying $\alpha $, where $\alpha $ is the angle between the slot or AAH and the freestream flow direction. The cylinder with the slot and AAH show similar variations in the drag coefficient with respect to $\alpha $: At low $\alpha $, both the slot and AAH generate self-issuing jet, which effectively push the near wake to far downstream, resulting in the drag reduction compared to a base cylinder. On the other hand, at high $\alpha $, the asymmetric flow separation is caused by blowing-and-suction through the slot and AAH. Thus, the alternate vortex shedding occurs in the very near wake, leading to the drag increase. As compared to the slot, AAH show less drag reduction but can reduce the drag on the cylinder in a wider range of $\alpha $. Some more details will be discussed in the presentation. Supported by NRF (NRF-2019R1F1A1064066) and Civil-Military Technology Cooperation Program (18-CM-AS-22).   

Sizing passive biomimetic surfaces within a turbulent boundary layer for separation control

The detriments of flow separation have led researchers to explore methods of separation control with inspiration from nature. Water tunnel studies have shown that shark skin samples can passively reduce flow separation. Microscopic denticles are able to passively actuate to heights of 1-5{\%} of the boundary layer. Recent attempts have been made to reproduce this mechanism in air by covering a NACA 0012 airfoil with an array of additively manufactured passive microflaps which have geometry similar to shortfin mako shark denticles. The microflaps actuated to over eight percent of the boundary layer and were found to be ineffective for separation control at this scale. In the current study, a microflap array is placed on the suction side of a trailing edge flap behind a variable length flat plate to increase the boundary layer height over the microflap array. Boundary layer thicknesses range from approximately 2 cm to 4.5 cm at the onset of the trailing edge flap. In these conditions, microflaps can actuate within the bottom five percent of the boundary layer height. Surface flow visualization is observed and force data is acquired for baseline and biomimetic flaps at multiple angles of flap deformation. Results will be presented as to the effectiveness of separation control over the flap with respect to microflap and boundary layer sizing.   

Bioinspired Passive Flaps for Separation Control: Shark Scales v. Bird Feather.

Water tunnel PIV studies have shown that the passively actuatable denticles of shortfin mako sharks are able to control flow separation. In a similar way, covert feathers on birds are able to passively actuate to delay flow separation. These two biological solutions to flow separation operate at different scales and in different fluids, and the relationship between them is not fully understood. Several iterations of a mechanical surface mimicking the geometry and kinematics of shortfin mako shark skin have been developed. In a low-speed wind tunnel, the performance of a NACA-0012 airfoil covered with over 6000 individually hinged microflaps has been characterized at a Reynolds number of 160000. At these conditions, the microflap array experiences similar dynamic pressures and Reynolds numbers to natural mako skin, but occupies an intermediate size between denticles and covert-feather flaps. Lift and drag measurements show that flaps of this size are ineffective for separation control, but the microflaps do exhibit passive actuation in response to flow separation. This provides insights into the relationship between denticles and covert feathers, suggesting that intermediately sized flaps may not be able to take advantage of either passive flow control mechanism.