Session Z17: Flow Control: Passive

Resilient and Scalable Quasicrystal Coatings for Quiet and Efficient Urban Air Mobility

With the rapid increase in planning and designing of Urban Air Mobility (UAM), one of the major concerns is noise pollution. Aviation noise from UAMs result in different broadbands of noise that can be very loud to the surrounding community. This is a challenge to introduce and maintain a network of urban air transportation at lower altitudes as a part of daily life. Phononic crystal is a functional acoustic material that has excellent potential in the practical engineering of noise reduction. Optimizing the microscale texturing in selected locations of a UAM vehicle is challenging due to the different physics involved in the noise absorption and decomposition within the micro patterns. We performed exploratory experiments using incompressible and transonic turbulent jet flow on cylindrical bodies coated with periodically arranged micro-structures, classified as low and high strength fibers. Preliminary results show a 2-4 dB of overall noise reduction in the case of high strength fibers compared to the base cylinder for a low-pass filter. The surface modification enabled a band gap that generated substantial attenuation of sound transmission. Various canonical and geometry-specific flow diagnostics measurements are underway to inform an optimized process.   

Flow Structure Dynamics in Arrays of Undulated Cylinders

Fluid flow over simple cylinders has been well characterized through experimentation and simulation. However, complex seal-whisker inspired undulated-cylinder topographies have been limited to biological studies, single cylinder simulations, or single cylinder visualization experiments. The present study experimentally investigates the wake, and wake-interactions, between one to nine undulated elliptic cylinders with varying spacial orientations, including axial shifting to study the effects of the geometry along the length of the whiskers. Further, the drag and lift components of each cylinder were measured at each spatial orientation using load-cell sensors. The undulated cylinders were 3D printed using a high-resolution fused deposition modelling (FDM) printer, surface texture was smoothed and coated, and scaled up to a mean chord length (C) of 3.36 cm, a mean thickness of 1.75 cm, and a length (L) of 60 cm. The specimens were mounted onto a modular grid with 1C incremental spacing and placed into a 5 m long wind tunnel with 1.2 m width and 0.8 m height. Stereo particle image velocimitry (SPIV) was used to visualize the spanwise plane directly behind the cylinder arrays at increments of five chord lengths, C, with a corresponding Reynolds number ranging between 13,000-23,000. Ensemble averages were generated using 2,000 images per each of the three downstream planes to visualize wake behaviour far down stream. The results of the analysis may be applied to a variety of column structure designs requiring reduced wake growth, reduced vibrations, as well as behavioural biological research.

    

Swept Back Angle Induced Spanwise Force Variation on a Seal Whisker Inspired Geometry

Seals possess undulated whiskers, distinct from the smooth whiskers of other mammals, that aid them in navigation and prey tracking. Results show that the opposing dual spanwise undulations on the whisker surface are responsible for a 10.4% reduction in drag and 91.2% reduction in oscillating lift forces resulting from the modified vortex shedding as compared to a streamlined ellipse of equivalent dimensions. Although most prior research has investigated flow perpendicular to the whisker span, seal whiskers experience a wide range of orientations with respect to incoming flow as the animals navigate their environment. The focus of this work is the exploration of whisker swept flow angles resulting from both the deliberate relaxation and protraction of whiskers and by material bending in response to drag forces. The complex surface undulations of the whisker create three-dimensional flow behaviors and modification of forces that vary significantly with sweep angle. This variation has implications in seals’ sensing behavior as well as the potential applications of whisker geometry for drag and lift force reduction. To understand the effects of this orientation change, direct numerical simulations of flow over model whisker surfaces at swept back angles are performed. These simulations are used to extract force and shedding frequency data across spanwise segments to understand the mechanisms responsible for the significant reduction in forces and modification to shedding behavior uniquely attributed to this geometry at sweep angles.

    

Elucidating the mechanism of surface perforation on the delay of boundary layer separation for flow over a cylinder

Surface perforation is an effective method of passive flow control. It has been demonstrated that perforated surface on a bluff body such as a cylinder with perforation holes connected to a common internal hollow space significantly reduces the unsteady aerodynamic loading known to cause drag and lift fluctuations, structural vibrations and acoustic noise (Sudalaimuthu and Liu, 2019). It is hypothesized that perforation coupled with the internal connection of the upstream and downstream flow fields helps reduce the magnitude of the adverse pressure gradient, thus effectively delaying and suppressing flow separation so as to alleviate the effect of the unsteady loading. This paper aims to elucidate this pressure gradient mechanism on boundary layer separation delay through Particle Image Velocimetry measurements of the velocity field around a perforated cylinder. The mean pressure gradient field is evaluated through the balance of the Reynolds-Averaged Navier-Stokes equation with each velocity-related term resolved with converged measurement data. The pressure field is further reconstructed with the Rotating Parallel Ray Omni-Directional method. The experiment is performed at a Reynolds number of 1.0 x 10⁵ based on a tunnel speed of 10 m/s and a cylinder diameter 0.152 meters.

    

Reduced-complexity models for turbulent flows over anisotropic porous materials

Structured porous materials have the potential to reduce skin friction, delay separation, and regulate heat transfer in turbulent flows. The design of porous materials for these flow control applications often relies on the use of models grounded in the Darcy-Brinkman-Forchheimer (DBF) framework, whereby the effect of the porous materials is introduced via bulk properties such as permeability. These bulk representations provide incomplete characterizations of the flow in regions with strong spatial inhomogeneity, such as the interface between a porous substrate and an unobstructed flow. This talk considers the use of the DBF framework for resolvent analysis of turbulent flows over anisotropic porous substrates. Using bulk representations, resolvent analysis yields reasonable predictions for the emergence of spanwise-coherent rollers resembling Kelvin-Helmholtz vortices. However, reproducing the effect of the porous substrate on the energetic near-wall cycle requires consideration of the interfacial geometry and its impact on the mean velocity profile. Simple phenomenological models that can capture the effect of interfacial geometry for mean profile predictions and subsequent analyses are developed and tested.   

Supersonic Pre-Transitional Disturbances in Boundary Layers on Porous Surfaces

The effect of wall permeability on the response of pre-transitional supersonic boundary layers subject to low-amplitude, free-stream vortical disturbances is investigated via asymptotic methods and numerically. A porous wall with regularly-spaced cylindrical pores couples the pressure and wall-normal velocity fluctuations at the wall when the spanwise diffusion is negligible, thereby reducing the growth of low-frequency laminar streaks. Highly-oblique Tollmien-Schlichting waves that develop further downstream are instead enhanced by the porous set. This finding is confirmed by a triple-deck analysis, which shows that the onset of Tollmien-Schlichting waves shifts upstream over a permeable wall. For boundary layers on concave permeable walls, the initial amplitude of the Görtler vortices is attenuated, while their growth rate is initially enhanced and then attenuated further downstream.   

Aerodynamic performance analysis of the wavy wing with varying spanwise waviness characteristics

Present research work aim to analyse the impact of varying spanwise waviness parameters (i.e wavelength and amplitude) on the aerodynamic performance of the rectangular aircraft wing. Performance of wavy wing model were compared with smooth (i.e baseline) wing model. For that purpose two wavy wing models were generated one with increasing waviness wavelength and amplitude from root to tip λ3050h510 and second one with decreasing waviness parameters from root to tip λ5030h105. Wing models were developed by using NACA0021 airfoil profile and simulation were performed at Reynolds number Re=1.2x10^5. Numerical simulations were performed at Angle of Attack (AoA) from 0 to 20^o with the interval of 4^o. Simulation results were validated against published experimental work. Computational Fluid Dynamics (CFD) simulation results reveal that, wavy wing lift performance degrade in pre-stall regime, whereas drag performance improved significantly. Additionally, lift and drag performance of wavy wing have shown substantial improvement in post stall regime. Results revealed that wavy wing model λ3050h510 performed better as compared wing model λ5030h105. It is estimated that, wavy wing model experience maximum decrement of 17.9% in lift at 4^o AoA, whereas 9.2% decrease in drag coefficient at 12^o AoA in comparison of baseline wing. Moreover, maximum increment of 16.3% in lift coefficient and 13.6% decrement in drag coefficient were noticed at 20^o AoA. Flow behavior analysis reveal that reduction in leading edge suction area results decrease in wavy wing lift performance, whereas restriction to the flow separation at waviness peak is the major reason behind increment in wavy wing performance in post stall regime. Moreover, it is hypothesis that use of wavy trailing edge is responsible for decrease in drag coefficient in pre-stall region.   

Control of wall-mounted bluff body wake by a porous substrate

Unsteady separation and vortex shedding in the wake of bluff bodies result in many adverse effects such as drag increase, vibration, and noise. Flow separation along the wall in the wake of wall-mounted bluff bodies are common in engineering devices and aero/hydro vehicles. Porous substrates have been explored as means of passive control in wall bounded flows since the wall permeability enhances momentum exchange and affects the flow structure over the porous wall, and that potentially affect the formation of a separation bubble as well. A previous experimental study has shown that the recirculation bubble behind a 3D obstacle in the channel vanishes when placed over a porous substrate with high permeability. In the present study, the control of the wall-mounted bluff body wake by using a porous substrate is investigated via flow simulations. The flow over and through the porous medium is simulated by solving the volume averaged Navier-Stokes equations employing the sharp-interface immersed boundary method and the Darcy-Forchheimer model. The effects of the substrate porosity and its size and depth on the strength and stability of the separation are investigated for the flow over a rib in the channel as well as the flow over a wall-mounted dome.   

ASYMMETRIC DISTURBANCE EFFECT ON BLUFF BODY FLOW USING LARGE EDDY SIMULATION

The present study performed the large eddy simulation for the flow around the asymmetric wavy (ASW) cylinders with different wavelengths at the Reynolds number of 3000. The ASW disturbance is introduced to achieve the drag reduction and the lift fluctuation as the passive control flow. Moreover, the flows around the smooth cylinder and the symmetric wavy (SW) cylinder are also considered for the purpose of the comparison. The ASW cylinders revealed about the same variations of the drag and the lift fluctuation according to the wavelength with the SW cylinder, regardless of the asymmetric degree. In the long wavelength regime, the drag and the lift fluctuation become small with increasing the asymmetric degree. Therefore, the asymmetric disturbance provided the addition reduction of the drag and the suppression of the lift fluctuation in comparison with the SW cylinder with the optimal wavelength in the long wavelength range. When the asymmetric degree increases, the flow near the wake in the short wavelength part is more inclined to the long wavelength part, which contributes the vortex dislocation. However, this effect gives different roles to the drag and lift fluctuation.