Session A35: General Fluid Dynamics: Flow Over Obstacles

Mean wake transitions of wall mounted long prisms at low Reynolds numbers

Transitions of wake structures behind small aspect-ratio (height-to-width) wall-mounted prisms is a multivariate function. It depends on flow parameters, such as Reynolds number, boundary layer thickness, and prism geometry, e.g.- Aspect-ratio and depth-ratio. Transitions in the wake of prisms with changing depth-ratios (length-to-width) remains unexplored in literature. In this study, we numerically investigate the transitions in wake of wall-mounted finite prisms with an aspect-ratio of 1 and varying depth-ratios between 0.016-4 at Reynolds number between 50-500. Preliminary results indicate the suppression of unsteady mechanism with increasing depth-ratio, which restores flow symmetry in a steady wake. The mean wake topology transitions between quadrupole, dipole, and no-pole type wakes. The threshold depth-ratio for such a transition increases with increasing Reynolds number. We aim to expand this study by classifying and characterizing the wake based on observed distinct topologies, which is then used to establish a mechanism of transition in the wake due to changing depth-ratio.   

Wind Flow Around the Toumba Building at Lefkandi: Experimental and Numerical Analysis

This paper presents a part of an interdisciplinary study of an ancient Greek archaeological monument, the 10th-century BC Toumba Building at Lefkandi. The Toumba Building is one of the earliest-known monumental structures built in Greece following the end of the Bronze Age. Only the base of its walls is preserved, and scholars have proposed different reconstructions for the missing parts. The study presented herein is aimed at informing a structural analysis that will test a new breakthrough reconstruction hypothesis proposed by the team. In particular, the roof pitch angle remains controversial and a wind analysis was therefore critical to assessing the validity of such reconstruction. This research studies the turbulent flow generated around a scaled-physical-building model through wind tunnel experiments, using 2-dimensional planar PIV. The flow data was utilized to validate a series of CFD simulations aimed at reconstructing the pressure distribution around the building and thus estimate the wind load. Three different roof-pitch-angles and three different angles of attack were considered to identify the worse-case scenario. The CFD model was successfully validated and the critical wind load was estimated. Preliminary structural analysis suggests that the building was likely able to withstand a much stronger load that previously proposed. Further analysis is ongoing.   

Experimental Investigation of Flow Characteristics around a Novel Structure for Passive Flow Control

A novel structure built of two side walls and a top wall is analyzed to understand its flow control capability. The structure is designed to create control inputs of flow acceleration, deceleration, and vortex generation by manipulating the angle of attack of the top and side walls. Arrays of the structure on a small or micro-scale are expected to decorate a surface to control the flow over it. The current study is performed with a large-scale mock-up of the structure to understand the detailed flow characteristics around it. Twenty variants are 3D printed with different cross-section geometries. The angle of attack of the top and side walls varies between zero, five, and ten degrees both in positive (nozzle-type) and negative directions (diffuser-type). The experiment is conducted in a wind tunnel using 2D Particle Image Velocimetry. Instantaneous and time-averaged velocity, vorticity, and turbulent kinetic energy are investigated. Results show that a flat plate structure perturbs the flow the least. Vortex generation is confirmed from all the variants tested. Additionally, the nozzle-type structures are shown to decelerate airflow through them, while the diffuser-type samples accelerate airflow. Recommendations on an idealized sample geometry for smaller-scale testing are made.   

Super-resolution PIV study of the flow behind square-shaped and triangular-shaped bluff bodies.

The paper presents the results and analyses of two cases of the flow behind the square-shaped and triangular-shaped bluff bodies under a moderate Reynolds number (Re=5000). Experimental values have been obtained with Particle Image Velocimetry (PIV) methods in an open wind tunnel. Two different PIV methods were used: traditional PIV based on cross-correlation (CC) algorithms and Artificial Intelligence PIV (AI PIV) based on Deep Learning and Convolutional Neural Networks, which offers super-resolution velocity fields with down to 1 vector per pixel. In addition, HWA measurements were conducted at a few points for verification and validation of the results.

These two selected flow cases differ in the nature of the downstream wake. Triangle bluff body wake is characterized by a higher vorticity and Strouhal Number (St) compared to that of a square body. This made it possible to assess the impact of the advanced PIV data post-processing methodology. It should be noted that AI PIV gives a more precise and a higher spatial resolution of the flow field so that we can see more details from the flow maps. Both applied methods demonstrated a predominance of Karman vortex shedding; however, with AI PIV, many small-scale vortices were also identified. Comparing successive snapshots made it possible to see, on the one hand, the process of formation and, on the other, the merging of smaller vortices, especially those at a further distance behind the object. The obtained results showed a more precise velocity field with a higher spatial resolution using AI PIV. The full paper will provide not only qualitative but also quantitative comparisons between the two methods.

    

Drag Reduction by Air Layer on Superhydrophobic Surface in a Taylor-Couette Flow

When a superhydrophobic surface (SHPS) is immersed to an aqueous solution, an air layer is often formed depending on the surface characteristics. A slip condition formed at the interface of the air and the solution is known to reduce skin friction effectively. However, many previous studies on the drag reduction have been conducted at a low Reynolds number (Re) or in a narrow range of Re. We developed a Taylor-Couette flow device for precise and detailed measurements of torque and velocity vectors for Re ranging from 1,000 through 20,000 using a single working fluid. We also developed a SHPS with a porous structure that can retain an air layer against a high shear. Detailed results of torque measurements and preliminary velocity field data obtained by particle image velocimeter will be presented. In addition, we will discuss the drag reduction capabilities of the SHPS with increasing Re and compare mean and turbulent properties of the T-C flows with and without the SHPS.   

Longevity of plastron layer in underwater vapor stable superhydrophobic surfaces under different atmospheric condition and drag measurements

The use of superhydrophobic surfaces for marine and submarine vehicles aids in drag reduction. The drag reduction is achieved due to the formation of a thin layer of air called a "plastron" inside the textured surface between the solid and liquid phases. But the plastron layer is unstable due to the effects of hydrostatic pressure, diffusion of air in water and instability due to the shear nature of the flow over the surface. It was attempted to make vapor stable surfaces by varying the surface morphology of micro, nano, hierarchical and re-entrant geometries, to increase the longevity of the plastron layer and reduce drag under flow conditions. A low-cost scalable fabrication method was utilized to make superhydrophobic surfaces with highly re-entrant geometry and hierarchical surface morphology. Fabricated surfaces showed increased longevity of the plastron layer at different pressures and temperature due to the increased stability of air or vapor in the textures. Initial results indicate significant drag reduction on these surfaces under laminar flow conditions at atmospheric pressure.   

Drag reduction in high Weissemberg number turbulent wall-bounded flows of realistic polymer solutions

Tom’s effect, namely the drag-reducing capability of polymers in turbulence, has been debated since its discovery in the 40s. Viscoelastic models achieve drag reduction in numerical simulations of prototypal flows. These models require unrealistically large concentrations to obtain drag reduction, well beyond the model limit of validity. Furthermore, many laboratory-scale experiments are performed at Weissenberg numbers unreachable by the viscoelastic models. In this context, Direct Numerical Simulations of DNA macromolecules, modeled as FENE dumbbells, fully coupled to the Newtonian fluid are discussed for a turbulent flow in a circular pipe using realistic parameters for the solution. A hybrid Eulerian-Lagrangian approach is used to evolve every dumbbell and the Exact Regularized Point Particle method (Gualtieri et al. 2015) is used to precisely account for the momentum coupling between the two phases. The oral presentation will provide methodological details and physical insight into the drag reduction mechanism of these high Weissemberg number flows. 

References

F. Serafini, F. Battista, P. Gualtieri, and C.M. Casciola. Drag reduction in turbulent wall-bounded flows of realistic polymer solutions. Submitted 2022.

P.S. Virk. Drag reduction fundamentals. AIChE Journal, 1975.    

An experimental study of sustainable drag reduction in Taylor-Couette flow

Researchers have shown that superhydrophobic (SH) surfaces can decrease frictional drag force. In our study, patterned SH surfaces were used in laminar and turbulent flow of a bespoke Taylor-Couette apparatus to comprehend their drag reduction mechanism. Our Taylor-Couette cell was mounted on a rheometer device that could measure the torque on the inner rotor as a function of rotational speed. We applied SH coatings on an inner cylinder to evaluate the drag reduction performance of various coatings in different flow regimes and calculate the effective slip length. We also studied drag reduction in patterned structures on the cylinder, and here it was revealed that triangular-shaped riblets achieved effective surface drag reduction. Furthermore, we examined combined surface modifications, SH coating and V-grooves in the Taylor-Couette flow. The performance of these surfaces was compared in terms of different dimensionless parameters; their surface roughness, the riblets sharpness and the Reynolds number. Our experimental results showed a reduction in measured torque of up to 45%. Also, durability tests indicated that our designed surfaces could maintain their hydrophobicity over repeated tests.   

Body interaction effects on mean drag forces for an infinite array of cylindrical cruciform shapes with normal inflow

The move of aquaculture farms to open ocean areas poses great challenges to aquaculture farming technologies and operations due to the harsh offshore wave and current conditions. Understanding the drag and lift responses of netted aquaculture structures is key for accurately predicting nonlinear sea loads and seakeeping responses. Our particular interest is understanding the drag force on a single net element idealized as a cylindrical cruciform, with and without a canonical knot, to better inform turbulent viscous drag forces on the full netted aquaculture cages. We use high-resolution 3D direct numerical simulation of a cylindrical cruciform shape with doubly periodic conditions and constant normal inflow to study the body interaction effects on the mean drag forces on the net element. The numerical method uses Boundary Data Immersion Method (BDIM) to solve the incompressible Navier-Stokes equations. We analyze the mean drag forces of the individual net element for a large range of cylinder diameter-to-length (d/l) ratios (or the net solidity) for cruciforms with and without spherical knots at the center at two different Reynolds numbers. For small d/l, we show that the presence of the second cylinder in a cruciform arrangement produces a drag coefficient less than that of a single cylinder in infinite domain. We identify the d/l above which where body interaction effects become important to the mean drag force. By analyzing the pressure coefficient on the net element at different locations, we establish that the body interaction effect, similar to solid blockage effect in tunnel experiments, is the relevant influence on the drag coefficient as opposed to wake blockage. Finally, we show that the overall presence of the knot, below a certain knot diameter, reduces the overall drag on the net element and that the above results are independent of a cylinder-diameter-based Reynolds number.