Nature-inspired solutions to bluff body aerodynamic problems: A review

This review investigates the nature-inspired techniques for the optimization of the aerodynamic forces on bluff bodies. To provide a rich understanding of these nature-inspired phenomena, three distinct zones of the species fishes (nektons), birds (avians) and the fast running land animals are considered. This allows contextualizing different capabilities of the species in different environmental necessities. The review follows a trend in which drag reduction capabilities of individual parts of these species, including body shape & size, tails, fins, surface structure, wings, and wingtips, have been explored in detail. By focusing on specific parts, the review examined the methods and physics involved, which provides space to narrate the development of ideas and our current understanding of the nature-inspired drag reduction and their application to bluff body aerodynamics. Consequently, nature-inspired promising areas for future endeavor related to the bluff body has been discussed in detail. It was found that, though, aerospace field has found several bird inspired application but the bluff body flow modification have only few. Similar is the case with fishes and land animals which have not been explored yet for aerodynamic use on the bluff bodies. The crucial importance of passive devices are also highlighted along with the review of their application on the bluff bodies inspired by nature. Furthermore, several of nature-inspired techniques are proposed and compared to facilitate the research in this direction. It provides a fundamental method to develop nature-inspired flow control devices for the bluff bodies.

Proximity Effects on Characteristics of Flow around Three Inline Square Cylinders

This work presents the numerical investigations performed to study the proximity effects on fluid flow characteristics around three inline square cylinders using the lattice Boltzmann method. For this purpose the gap spacing (g) is systematically varied in the range 0.5 to 16 diameters of cylinder by keeping Reynolds number fixed at 200. Five different flow patterns are observed at different values of spacing: bluff body flow, gap trapped flow, irregular flow, alternate shedding, and modulated shedding. These patterns have a significant effect on flow induced forces and vortex shedding frequency. The spacing value g = 2 is found to be critical due to sudden changes in fluid flow characteristics. The flow parameters of first cylinder are found to be closer to single cylinder values but for middle and third cylinder the differences confirm the wake interference effect even at large values of spacing.

Flow Structures and Vortex Dynamics in the Wake of Three Tandem Prisms

A study of the flow around three tandem square prisms may provide us a better understanding of complicated flow physics related to multiple closely spaced structures. In this paper, a numerical investigation on the flow around three tandem prisms at Reynolds number Re = 150 is conducted for L/W = 1.2 ∼ 10.0, where L is the prism center-to-center spacing and W is the prism width. Four distinct flow regimes and their ranges are identified, viz., single bluff-body flow (L/W < 3.0), alternating reattachment flow (3.0 < L/W < 4.3), synchronized coshedding flow (4.3 < L/W < 7.3) and desynchronized coshedding flow (7.3 < L/W ≤ 10.0). The synchronized coshedding flow can be further subdivided into two regimes: single St flow (4.3 < L/W < 5.1) and dual St flow (5.1 < L/W < 7.3). A secondary vortex street following the primary vortex street is observed for the dual St flow and the desynchronized coshedding flow. The detailed physics of the evolution of the primary vortex street to the secondary is imparted. The inherent frequency associated with the secondary vortex street is smaller than that with the primary. The evolution process of the primary vortex street to the secondary leads to a tertiary frequency. The DMD (dynamic mode decomposition) analysis for the first time is proposed as a useful and quantitative tool to identify the secondary vortex street and its onset position.

Instantaneous fluid force acting on a circular cylinder control by a small rod

Two unique bluff body flow control methods using a small rod have been proposed in previous studies. The first is the forced reattachment method, which is a type of separated shear layer control. This method reduces drag and generates a lift. The second is the front rod method, which involves the placement of a small rod upstream of the bluff body to control the incoming flow and reduce drag. This paper describes the features of the instantaneous fluid force achieved using these flow control methods. These methods were experimentally applied to a cylinder, and the instantaneous pressure field and the flow visualizations of these methods are presented. When the forced reattachment method was applied, a lift force was generated, and the vibration was less than that in the case of the front rod method. When the front rod method was applied, the drag force was reduced by over 50% in comparison with those in the uncontrolled case and the case with the forced reattachment method.

TEMPORO-SPECTRAL COHERENT STRUCTURE OF TURBULENCE AND PRESSURE USING FOURIER AND WAVELET TRANSFORMS

Studying the spatial distribution in coherent fields such as turbulent and turbulent-induced force ones is important to model and evaluate turbulent-induced forces and response of structures on the turbulent flows. Turbulent field-based coherent function is commonly used for the spatial distribution characteristic of induced forces in the frequency domain. This paper will focus to study spectral coherent structure of turbulence and forces in not only the frequency domain using conventional Fourier transform-based coherence, but also temporo-spectral coherent one in the time-frequency plane thanks to wavelet transform-based coherence for more understanding of the turbulence and force coherences and their spatial distributions. Effects of spanwise separations, bluff body flow and flow conditions on coherent structures of turbulence and induced pressure, comparison between turbulence and pressure coherences as well as intermittency of coherent structure in the time-frequency plane will be investigated here.