Session J22: Flow Control: Passive - Bluff Bodies and Lifting Surfaces

Dynamics and Wakes of Kirigami Sheets in Flow

We present the behavior and wake patterns of various kirigami patterned flexible sheets placed perpendicular to the flow, such that the fluid is forced through the pattern. These sheets can display static and dynamic flow-induced instabilities that can result in buckling or limit cycle oscillations, or undergo significant elongation while remaining stable, depending on their pattern and the flow velocity. These sheets exhibit a variety of wake patterns as a result of the varied structural responses, including vortex shedding at multiple scales and the formation of jets that redirect the incoming flow. The mean drag force triggers the three-dimensional shape of the kirigami pattern without any need for other external actuations. While similar kirigami patterns display similar structural behavior and create similar wakes, the specifics of the response can be manipulated by adjusting the geometric parameters within each category of patterns, and therefore the wake produced can be tuned as desired. This creates a potential for novel applications in flow control.

 

Acknowledgement: This work is partially supported by the National Science Foundation, grant number CBET 2320300.   

Vortex Shedding Frequency Over Seal Whisker-Inspired Geometries of Varying Wavelength

The introduction of surface undulations to smooth bluff body geometry has important effects on flow. In particular, undulated cylinder geometry inspired by seal whiskers has been shown to alter shedding frequency and reduce fluid forces significantly compared to a smooth cylinder. In this work a rigid computational whisker model is used to systematically investigate the effects of varied undulation wavelength on unsteady lift force and shedding frequency. Prior research has parametrized the whisker-inspired geometry and demonstrated the relevance of geometric features on force reduction properties, but an analysis of the effects of wavelength variation alone has yet to be performed. A set of five whisker-inspired models at varying wavelengths are simulated at Reynolds number 250 and compared to a hydrodynamically equivalent elliptical cylinder. It is shown that nondimensional wavelength values of 3 and above result in reduced frequency and amplitude of vortex shedding and a reduction of oscillating lift force. Examining the flow physics as a function of span, the overall shedding frequency results from specific vortex structures that produce distinct local shedding modes with a dependence on undulation wavelength. The interaction of these modes produces a sophisticated lift frequency spectrum at wavelength values corresponding with maximum reduction in forces.   

Mean Kinetic Energy in Arrays of Undulated Cylinders

Extensive studies of fluid flow over basic cylinders have limited the complex three-dimensional undulated topographies of seal whiskers (vibrissae) to biological studies and single cylinder experiments or simulations. The present study experimentally investigates the momentum deficit, and its interactions, between nine undulated elliptic cylinders with various streamwise spacings and spanwise staggering. Mean kinetic energy budgets are calculated for various configurations and axial locations. The undulated specimens were 3D printed using high-resolution fused deposition modelling (FDM) and were smoothed and painted afterwards. The cylinders were scaled to a mean chord length (C) of 3.36 cm, a mean thickness of 1.75 cm, and a total length (L) of 60 cm. The array was mounted onto a grid with 1C spacing in the Portland State University wind tunnel measuring 5 m long, 1.2 m wide, and 0.8 m high. Flow visualization behind the array was performed using 2D-3C Stereo Particle Image Velocimetry (SPIV). Measurements were taken at increments of five streamwise chord lengths (C) with a Reynolds number ranging between 13,000 to 23,000. To study downstream development, ensemble averages were generated using 2,000 images per each of two imaging planes. The bio-inspired engineering model has broad applications in column structure designs that aim to minimize momentum deficits and diminish vibrational response, chemical mixing, biosensors, and behavioural biological research.   

Resilient and Scalable Bio-Inspired Metamaterial for Passive Noise Control

Aerodynamic noise poses a significant challenge in urban environments, impacting the quality of life for residents. Interaction between an object's surface and air causes an adverse pressure gradient, resulting in flow separation and vortices around aircraft, contributing to aerodynamic noise. Experimental investigations were conducted using turbulent jet flow on cylindrical bodies, coated with periodically arranged, mushroom shaped micro-structures. The jet was tested at three different Mach numbers, ranging from the subsonic to transonic regions. The micro-structure coatings have different spacing-to-height ratios of 0.7, 1.0, and 1.8, along with an uncoated cylinder. The findings indicate a substantial reduction in overall noise, ranging from 2 to 6 dB (30-75%) when using the three coatings in comparison to the base cylinder. This reduction is especially noticeable at lower frequencies within the human hearing range of 20 Hz to 20,000 Hz. The surface modification with shark skin denticles facilitated the reduction in pressure fluctuations leading to remarkable attenuation of sound transmission. The 0.7 and 1.0 coatings show a larger reduction in pressure fluctuations compared to the 1.8 ratio. A suppression of vortex shedding intensity is believed to the cause of reduction, but there is no delay in flow separation. The study's results can significantly contribute to quieter and more efficient systems in various industries, offering noise reduction strategies based on biomimetic principles.   

Adaptive-passive control of flow-induced vibrations of a sphere

While active methods have proven effective for controlling flow-induced vibrations (FIV) of a sphere, they require power input, and often present major implementation challenges. In contrast, passive flow control methods are easily implemented, do not require power input, and are more suitable for practical applications. However, despite their immense potential, the utilization of passive methods for controlling the FIV of spheres has remained largely unexplored. In this study, a series of systematic experiments were performed to study the effect of a simple passive device of a trip wire on the FIV of a sphere for a wide range of Reynolds numbers of ~ 5000-30000. It was found that a strategically placed trip wire can effectively control (suppress or enhance) the sphere vibrations. However, the optimal size of the trip wire varies with the reduced velocity. To achieve optimal control over a wide range of reduced velocities (Mode I to Mode III of sphere vibrations), we propose an adaptive-passive mechanism involving a trip wire attached to a spring that adjusts its size based on flow velocity without using power.  This approach could potentially be applied to optimize energy output in renewable energy harvesting systems based on FIV or reduce maintenance costs for offshore structures.   

Separation control on airfoil and wing with finite boundary layer trips

The influence of boundary layer trips of various aspect ratios on aerodynamic performance and flow development over a NACA 0018 airfoil and wing is investigated experimentally. Direct force and particle image velocimetry measurements are performed at a chord Reynolds number of 60000 and an angle of attack of 5 degree. The effect of trip length relative to the model span is considered over a range of trip aspect ratios from 0.08 to 1 relative to the model span, with all trips centred at the midspan plane of the models. The baseline stalled flow development is shown to be augmented significantly for all the cases examined, producing substantial changes to aerodynamic performance. The results demonstrate that trip segments affect the flow over a significantly wider spanwise regions compared to the size of the trip. While the trip spanwise extent is shown to have a prominent influence on the overall flow development, comparable gains in aerodynamic performance can be achieved over a relatively wide range of trip aspect ratios.   

Investigation of Riblets on the Aerodynamic Performance of Wind Turbine Blades

Effective control of turbulent flow over airfoil surface is important for air vehicles to increase flight efficiency and improve the performance of wind turbines. During the recent decades, extensive studies have demonstrated that engineered surface, such as riblets, can effectively modify the structure of near wall turbulence and reduce the skin-friction over a flat surface. This study seeks to quantify the effects of riblets on improving the aerodynamic performance of a DU-91-W2-250 airfoil with a 0.4 m chord length and 0.8 m span. With the riblet design optimized based on the tested Reynolds number and flow statistics over the suction side of the airfoil, the results highlight that the riblet surface treatment can effectively reduce drag across a wide range of angle of attack, as well as enhancing the lift coefficient under high angle of attacks. Additional measurements for the flow statistics over the riblet surface using micro particle image velocimetry revealed that these microstructures can modify the near-wall turbulent statistics and mitigate the shear stress.   

The effect of porous surfaces on wings in unsteady flows

The porous structure holds great potential for modifying the performance of various aerodynamic applications as a passive flow control method. The focus of this study was to investigate the effect of porous surfaces on wings in unsteady flows caused by wing motion, active flow control actuation, and their combination, utilizing both experimental methods and numerical simulations. Wind tunnel experiments of a nominally 2-D wing undergoing unsteady motion with plasma actuators were conducted, and a 2-D Direct Numerical Simulation (DNS) on the same setup was carried out. Our analysis will compare the aerodynamic loads, such as lift, drag, and pitching moment, between wings with and without porous surfaces. Additionally, flow field analysis will be conducted to explore the associated flow characteristics and provide insights for the future design of wings incorporating porous surfaces.   

Passive flow control of phase change material using baffle for convection enhancement

The aim of this research is to enhance convective heat transfer within phase change materials (PCM) using baffles on a vertical wall. PCMs have major drawbacks such as low thermal conductivity, high viscosity, and limited natural convection. To address these issues, we investigated a passive approach to dissipate the heat with baffles. The visualization experiments including shadowgraphy and particle image velocimetry (PIV) were carried out to compare between the case with baffle (WB) and the case without baffle (NB). The baffles are horizontal plates that divide the vertical wall into zones and are designed to have a gap near the heat source to facilitate filling of the PCM and absorb volume changes. The average temperature difference in the horizontal direction was lowered by 39.68 % due to the presence of the baffles. And, the temperature difference in the vertical direction was reduced by 50 % in the WB instance against the NB case. Furthermore, when comparing WB with NB, the convection coefficient increased by up to 28 %, 34 % and 27 % at the heat power levels of 4, 6, and 8 W, respectively. As a result, the baffles on the vertical wall improved convective heat transfer performance by changing the internal flow pattern.