Bluff Body Research Articles

Frequency-domain quadrupole correction for the permeable-surface Ffowcs Williams and Hawkings integration

The permeable-surface Ffowcs Williams and Hawkings (FW–H) integration for computing the far-field sound has the advantage of encapsulating the sources and nonlinear propagation inside the integral surface. However, it suffers from spurious sound when the volume integral for quadrupole term outside the permeable surface is conventionally ignored. The spurious sound is often suppressed by using two distinct approaches, which modifies the FW–H integration and acoustic variables/sources, respectively. This work clarifies the connection between the two approaches by analyzing the integral of the quadrupole sources. We show that the modification of the acoustic sources can be reformulated as a modification of the FW–H integration, which means that the two distinct approaches are interconvertible. A new quadrupole correction model for the FW–H integration is proposed by delicately modifying the acoustic sources. The modified acoustic sources consist of the filtered Lighthill stress tensor, where a convection operator is used to filter out the acoustically inefficient components. The proposed quadrupole correction model is consistent with the previous work on the modification of the FW–H integration under special conditions with the uniform convection velocity. The proposed model is validated by computing the sound pressure generated by laminar and turbulent flows over bluff bodies. It is found that the sensitivity of the acoustic pressure to the FW–H surface's position is suppressed and the accuracy of the predicted sound is improved. The results suggest that the modification of acoustic variables/sources can be a powerful method to construct new quadrupole correction models for the permeable FW–H integration.

The effect of a textured surface in the form of triangular prisms on the occurrence and development of cavitation behind the cylinder in microchannels

Cavitation flow in a microchannel behind a cylinder with a smooth and textured surface is investigated using mathematical modeling methods. The textured cylinder has 72 triangular prisms on its surface. The height of the prism, normal to the surface of the bluff body, was 100 nm. Profiles of the flow velocity and volume fraction of vapor are constructed. The monitoring of the velocity and pressure at five points behind both the smooth and the textured cylinder was carried out, while the inlet pressure changed up to 30 bars. It is shown that there is no restructuring of the vortex street to a symmetrical form at quite high-pressure values at the inlet to the channel for a rough cylinder. Namely, roughness prevents the flow stabilization. A pressure jump in the microchannel is observed when a cavity appears with a uniform increase in the flow rate. Two pulsation frequencies are determined for each of the flow modes. The first pulsation frequency ranges from 480 to 2200 Hz and is associated with the formation of cavitation. The second pulsation frequency is associated with the hydrodynamic flow around the cylinder and its values range from 26 to 95 kHz. An increase in surface roughness leads to a growth of the cavitation pulsation frequency and intensifies cavitation. At that, the velocity pulsations in the flow before the onset of cavitation increase, and the frequency of hydrodynamic pulsations after its onset decrease. The paper provides an analysis of the drag coefficient of a hydraulic section with rough and smooth cylindrical bluff bodies. The effect of roughness on the change in the hydrodynamic characteristics of the flow is described.

A novel method of reconstructing planar flow field from given distribution of single velocity component: comparison to conventional divergence-free approach

Measurements of a single flow velocity component are still prevalent due to their reasonable costs and some difficulties in multiple-component measurements. If the transverse component can be obtained additionally by a numerical technique, qualitative features of the flow will be understood more effectively. In this context, methods based on the 2-dimensonal divergence-free assumption have been widely used for problems in which a single velocity component is measured over a planar domain. In this study, the authors proposed a method of approximating the second planar velocity component by minimising an objective function expressed with divergence and vorticity so that the mass transport in the out-of-plane direction could be taken into consideration. The present method was tested with numerically produced 3-dimensional flows in a hexahedral chamber and a flow around a bluff body measured by particle image velocimetry. There was a tendency that the present method calculated the second velocity component with smaller errors than existing divergence-free approaches. It was also shown that the present method had a high capability to locate strong suction and generation caused by the mass transport in the out-of-plane direction. The present method is deemed promising for many one-component flow measurements in engineering and medicine.

Deep reinforcement learning-based active flow control of an elliptical cylinder: Transitioning from an elliptical cylinder to a circular cylinder and a flat plate

We study the adaptability of deep reinforcement learning (DRL)-based active flow control (AFC) technology for bluff body flows with complex geometries. It is extended from a cylinder with an aspect ratio Ar = 1 to a flat elliptical cylinder with Ar = 2, slender elliptical cylinders with Ar less than 1, and a flat plate with Ar = 0. We utilize the Proximal Policy Optimization (PPO) algorithm to precisely control the mass flow rates of synthetic jets located on the upper and lower surfaces of a cylinder to achieve reduction in drag, minimization of lift, and suppression of vortex shedding. Our research findings indicate that, for elliptical cylinders with Ar between 1.75 and 0.75, the reduction in drag coefficient ranges from 0.9% to 15.7%, and the reduction in lift coefficient ranges from 95.2% to 99.7%. The DRL-based control strategy not only significantly reduces lift and drag, but also completely suppresses vortex shedding while using less than 1% of external excitation energy, demonstrating its efficiency and energy-saving capabilities. Additionally, for Ar from 0.5 to 0, the reduction in drag coefficient ranges from 26.9% to 43.6%, and the reduction in lift coefficient from 50.2% to 68.0%. This reflects the control strategy's significant reduction in both drag and lift coefficients, while also alleviating vortex shedding. The interaction and nonlinear development of vortices in the wake of elliptical cylinders lead to complex flow instability, and DRL-based AFC technology shows adaptability and potential in addressing flow control problems for this type of bluff body flow.

Koopman mode analysis on discovering distributed energy transfer of post-transient flutter of a bluff body

The flutter response of a rectangular cylinder model with an aspect ratio of 5 is investigated by direct numerical simulation for various Reynolds numbers (Re) and flow angles of attack (α). The results show that the lower α and Re, the larger the torsional amplitude at the critical reduced flow velocity (Ur) corresponding to the occurrence of flutter. High-order Koopman mode analysis method (HOKM) was developed to synchronously capture the inherent flow and structural modes. The element-mode-based energy transfer between cylinders and flow modes was used to reveal the underlying mechanism. HOKM analysis shows that only odd-order modes contribute to the occurrence of flutter, with an augmentation in Ur enhancing this phenomenon. By analyzing the work done in odd-order modes, it is found that the primary mode always occupies the main contribution to the work done. The leading-edge endpoint portion is always where the maximum work is done. An escalation in Re and α notably influences both the spatially relevant part (WΦ) and the temporally relevant part (Wc), while an increase in Ur predominantly impacts WΦ. This study offers universal guidance on the safety and protection of ocean engineering structures and flow control strategies.

Numerical Study of Vortex Effect on Splitter Plate under Turbulence Flow over Circular Cylinder

Vortex shedding is commonly observed in both natural and artificial situations. Two key variables, the blockage ratio and the Reynolds number, determine the flow characteristics around a confined bluff body. The goal of the current study was to apply numerical simulation to examine the development of vortices in a continuous turbulence flow at the distanced position of a single splitter plate. As a starting point, the flow around a fixed circular cylinder and plate is examined at various spaced position ratios, where *L ratios are defined as the distance between the cylinder's centre and the plate's front edge. In the wake of the cylinder, when *L is raised, positive and negative vortices are generated. The pressure distribution following the vortex sheddingcore strike is felt at the plate face tip (starting edge). Compared to the other two positions, the CL created by L2 is larger and receives better cortex hits.

Leidenfrost spheres, projectiles, and model boats: Assessing the drag reduction by superhydrophobic surfaces

Superhydrophobic surfaces are expected to reduce drag on bluff bodies moving in water due to the introduction of a thin air layer around the solid and the resulting partial slip boundary condition. The use of Leidenfrost vapor layers, sustained on the surface of a heated metal body is a reliable method to estimate the maximum drag reduction possible due to such air layers. In the past such an approach was used to estimate the drag reduction on a free-falling heated sphere, in which case the form drag is the lead component of the drag force. Here, we extend this approach to evaluate the effect of the thin gas layers on the hydrodynamic drag of free-falling streamlined projectiles and towed model boats, where the form drag is minimal, and the skin friction drag is the lead component of the drag force. By comparing the drag for streamlined bodies with and without sustained air-layers, we see only incremental drag reductions, for the sub-critical Reynolds number tested herein. The same is true for towed model boats. These results hold both for superhydrophobic surface treatments and Leidenfrost vapor layers. Thus, we concluded that for the investigated range of sub-critical Reynolds numbers, the skin friction drag is less sensitive to the effect of the thin gas layers compared to the form drag. These novel findings have important implications for the practical potential of energy savings using gas layers sustained on superhydrophobic surfaces.

Effect of aspect ratio on flow-induced vibration of oblate spheroids and implications for energy generation

This study experimentally investigates the influence of aspect ratio on cross-flow flow-induced vibration (FIV) of elastically mounted oblate spheroids. The aspect ratio (ϵ=b/a) of an oblate spheroid, defined as the ratio of the major diameter (b) in the cross-flow direction to the minor diameter (a) in the streamwise direction, was varied between 1.00 and 3.20. The FIV response was characterized over a range of reduced velocity, 3.0⩽U∗=U/(fnwb)⩽12.0, where U is the free-stream velocity and fnw is the natural frequency of the system in quiescent water. The corresponding Reynolds number varied over the range 4730⩽Re⩽20120. It was found that in addition to the vortex-induced vibration (VIV) Mode I and Mode II responses observed for a sphere, on increasing the aspect ratio to ϵ=1.53 and 2.0, a galloping-dominated response, denoted by G-I, was encountered at high reduced velocities. With a further increase in aspect ratio to ϵ=2.50, the body vibration exhibited an additional VIV-like response (V-I) following the sequential appearance of Mode I, Mode II and G-I, with smooth transitions between these modes. In the case of the largest aspect ratio considered in the present study, ϵ=3.20, the spheroid intriguingly exhibited only a pure VIV Mode I before transitioning to a VIV-dominated mode, namely V-II. The largest vibration amplitude observed was 2.17b, occurring at the highest tested reduced velocity of U∗=12.0 for ϵ=2.5. Furthermore, the maximum time-averaged power coefficient was observed to be 0.165 for the thinnest oblate spheroid tested, ϵ=3.20, approximately 660% higher than that observed for VIV of a sphere. This shows the relevance of geometry for FIV energy harvesting from oblate spheroids. The findings highlight the distinctive nature of FIV responses of 3D oblate spheroids compared to 2D bluff bodies such as elliptical, D-section, and square cylinders.

Bladeless Wind Turbine Triboelectric Nanogenerator for Effectively Harvesting Random Gust Energy

AbstractIn the environment, there is an abundance of gust energy which is challenging to harvest with conventional rotating wind turbines, such as the gusts generated by passing vehicles along roadsides. Addressing the irregular and low‐frequency characteristics of gusts, a bladeless wind turbine triboelectric nanogenerator (BWT‐TENG) with enhanced aerodynamic performance is proposed, enabling effective harvesting of random gust energy. First, a bladeless wind turbine with a cylindrical bluff body shape is designed, and its aerodynamic principles under gust‐driven conditions are elucidated through the computational fluid dynamics method. Subsequently, parameter optimization is conducted for the multilayered TENG. Systematic experiments demonstrated that the BWT‐TENG achieved a peak power density of 4.08 W m−3 driven by a gust of 10 m s−1, and can even operate at frequencies as low as 0.1 Hz. Finally, experiments showcased the BWT‐TENG powering a warning light in a simulated rainfall environment and harvesting gust energy from vehicles passing by real roadside to power wireless gyroscopic sensors, thereby achieving self‐powered structural health monitoring of roads or bridges. This work provides a novel strategy for utilizing TENGs in the harvest of environmental gust energy and demonstrates the vast potential of TENGs in the field of self‐powered structural health monitoring.

Connections between propulsive efficiency and wake structure via modal decomposition

We present experiments on oscillating hydrofoils undergoing combined heaving and pitching motions, paying particular attention to connections between propulsive efficiency and coherent wake features extracted using modal analysis. Time-averaged forces and particle image velocimetry measurements of the flow field downstream of the foil are presented for a Reynolds number of $Re=11\times 10^3$ and Strouhal numbers in the range $St=0.16\unicode{x2013}0.35$ . These conditions produce 2S and 2P wake patterns, as well as a near-momentumless wake structure. A triple decomposition using the optimized dynamic mode decomposition method is employed to identify dominant modal components (or coherent structures) in the wake. These structures can be connected to wake instabilities predicted using spatial stability analyses. Examining the modal components of the wake provides insightful explanations into the transition from drag to thrust production, and conditions that lead to peak propulsive efficiency. In particular, we find modes that correspond to the primary vortex development in the wakes. Other modal components capture elements of bluff body shedding at Strouhal numbers below the optimum for peak propulsive efficiency and characteristics of separation for Strouhal numbers higher than the optimum.

Virtual physical framework reveals vortex-induced vibration “Soar” and “Death” for a bluff body at high Reynolds numbers

Vortex-induced vibration (VIV) of a bluff body is well known as a canonical phenomenon in complex fluid–structure interactions (FSI). However, our understanding and physical theories of this phenomenon are still restricted to a lower Reynolds number (Re) range owing to the limitations of experimental fluid mechanics. Herein, we describe a virtual physical framework (VPF) capable of editing and controlling the physical parameters of an actual physical system with high fidelity by combining numerical disciplinary and mechanical actuators. We demonstrated the effectiveness of applying a VPF to address the experimental fluid mechanical challenges of vortex-induced vibration (VIV) for a bluff body at a high Re by virtualizing a large, flexibly mounted rigid cylindrical system. We observed the VIV evolution of the bluff body at different Re values. Notably, the VIV appears in its “soar” stages with unexpected amplitudes of 2.4 D, nearly 200% higher than the traditionally acknowledged value at approximately Re = 2.0E5, while suddenly entering into its “death” stages with no vibrations over the critical Re region. The dissynchrony changes in the energy dissipation and input with Re caused by drag and excitation forces in the fluid system were further found to result in these VIV phenomena. This discovery upsets previous perceptions of VIVs and raises profound questions regarding related engineering projects. The VPF also opens a promising paradigm in experimental research for accelerating scientific discovery and investigating unaddressed classical physical phenomena.

Biomimetic swallowtail V-shaped attachments for enhanced low-speed wind energy harvesting by a galloping piezoelectric energy harvester

Efficient low wind speed energy harvesting is pivotal in expanding the reach of sustainable wind power to regions with limited wind resources. This paper introduces an innovative approach to enhance wind energy harvesting efficiency in low wind speed environments through the development of a biomimetically inspired design featuring V-shaped attachments on a square bluff body. Drawing inspiration from the swallowtail, these attachments are engineered to manipulate airflow, creating an asymmetrical pressure field that significantly improves the vibration amplitude of the harvesting device. The proposed design is tested in wind tunnel experiments, demonstrating a 32 % reduction in cut-in wind speed and a significant increase in power output at the lowest tested wind speed of 1.7 m/s, compared to traditional designs. Computational fluid dynamics simulations further elucidate the mechanism behind the enhanced energy harvesting, highlighting the role of V-shaped attachments in generating stronger and more stable vortex shedding, crucial for efficient energy harvesting. The proposed attachments not only substantially reduce the cut-in wind speed of galloping but also significantly increase the device's power output, demonstrating the potential of this design for expanding the viability of wind energy in regions with limited wind resources.

The Effect of Adding a Square Disturbance on the Flow Characteristics across Circular Cylinders Arranged in Tandem

Circular cylinder is one form of bluff body that is often used in engineering and industrial applications. The addition of a square placed in front of the circular cylinder aims to accelerate the flow from laminar to turbulent so that flow separation can be delayed more slowly so that the resulting pressure drag is smaller. The research method used was experimental using two circular cylinders arranged in tandem with a diameter (D) of 25 mm and variations in the distance between cylinders (L/D) 2;2,5;3;3,5;4 and two square-shaped square bodies (SDB) in front of each cylinder with dimensions of 4 mm. Position of SDB angle is 30o and the gap distance is 0.4 mm. The Reynolds number used is 2.3x104. This study aims to obtain the distribution of pressure coefficient (CP), drag pressure coefficient (CDP) and velocity profile behind the test specimen. The results showed that the addition of two square-shaped square bodies on each cylinder can reduce the drag force on the cylinder which causes a difference in values, where the largest pressure coefficient value occurs at a distance of L/D 4 with a value of -1.073 and the lowest value of drag pressure coefficient (CDP) at a distance of L/D 2 on the upstream cylinder with a value of 0.0786. While the downstream cylinder is located at a distance of L/D 3 with a value of -0.079 and the lowest speed value is located in the variation of L/D 4 with a value of 9.52 m/s.

Passive Control of Vortices in the Wake of a Bluff Body

Vortices belong to the most important phenomena in fluid dynamics and play an essential role in many engineering applications. They can act detrimentally by harnessing the flow energy and reducing the efficiency of an aerodynamic device, whereas in other cases, their presence can be exploited to achieve targeted flow conditions. The control of the vortex parameters is desirable in both cases. In this paper, we introduce an optimization strategy for the control of vortices in the wake of a bluff body. Flow modelling is based on RANS and DES computations, validated by experimental data. The algorithm for vortex identification and characterization is based on the triple decomposition of motion. It produces a quantitative measure of vortex strength which is used to define the objective function in the optimization procedure. It is shown how the shape of an aerodynamic device can be altered to achieve the desired characteristics of vortices in its wake. The studied case is closely related to flame holders for combustion applications, but the conceptual approach has a general applicability to vortex control.

Vortex shedding from bluff bodies: a conformal mapping approach

A model to calculate flow around bluff bodies of various geometries is presented. A conformal map between the plane of the bluff body and the plane of a unit circular cylinder is established by using a combination of Karman–Trefftz transformations and Fornberg’s method. Flow in the circle plane is calculated using the authors’ Hybrid Potential Flow (HPF) model and mapped back to the shape plane. By joining this calculated near-body flow with von Karman’s model for a vortex wake, forces due to vortex shedding and shedding frequencies are calculated. In this manner, a complete solution for the flow around bluff bodies of various geometries is established. Results for two shapes are presented, along with recommendations for further work.

Boundary feedback control of Navier–Stokes equations around arbitrary bluff body with prescribed drag and lift coefficients

This paper presents a boundary feedback control method for the two- and three-dimensional Navier–Stokes equations around an arbitrary bluff body, with prescribed drag and lift coefficients. In order to determine the feedback control law, we consider an extended system coupling the equations governing the Navier–Stokes problem with an equation satisfied by the control on the bluff body, which is a part of the domain boundary. We utilize the artificial compressibility method, which approximates the Navier–Stokes equations in the limit, along with a priori estimation techniques, to establish a boundary control. Such a control law ensures that the prescriptions of the drag and lift coefficients are attained. Subsequently, a compactness result allows to let the compressibility parameter tend to zero. The application of the Faedo-Galerkin method is essential in reaching this result.

Corner Vortex Structures Within Recirculation Zones of Confined Bluff-Body Flows

The vortical structures of recirculation zones in turbulent nonreacting and premixed reacting flows around confined equilateral-triangle bluff bodies are investigated using large-eddy simulations. A three-dimensional flow structure is observed within the recirculation zones of reacting and nonreacting cases; the structure includes both the “spanwise” vortices that extend the length of the recirculation zone and “corner vortex structures” that are situated adjacent to the bluff-body trailing edge and spanwise walls. The corner vortex structures enhance the mixing and residence times of fluid inside the recirculation zone. Fluid is circulated between the spanwise vortices and corner vortex structures. Corner vortex structures always appear downstream (relative to the local flow) of the boundary-layer separation location on the spanwise walls within the recirculation zone, and they are absent when boundary-layer formation on the spanwise walls is prevented. Laminar boundary-layer theory predicts the boundary-layer separation locations within ∼12% of the calculated values from the large-eddy simulations in all but one case. These results suggest that 1) boundary-layer separation on the spanwise walls is a necessary condition for the formation of corner vortex structures, and 2) boundary-layer separation on the spanwise walls is caused by an adverse pressure gradient in the recirculation zone.

Experimental investigation of rear flexible flaps interacting with the wake dynamics behind a squareback Ahmed body

We have conducted an experimental study on the use of rear flexible vertical flaps as adaptive solutions to reduce the drag of a squareback Ahmed body, and on the fluid–structure interaction mechanisms at the turbulent wake. To that aim, wind tunnel experiments were conducted to compare the performance of various configurations including the baseline body, the body with rigid flaps and with flexible flaps. These configurations were tested under different aligned and cross-flow conditions. The results reveal that the flexible adaptive devices effectively reduce the drag within for low values of the dimensionless stiffness quantified through the Cauchy number, Ca. Thus, the two-dimensional deformation of the flexible flaps, which undergo progressive inwards reconfiguration (with an averaged tip deflection angle of Θ≃4°), reduces the bluffness of the flow separation at the body base, thus shrinking the recirculation region. This reconfiguration leads to increased base pressure, resulting into a 8.3% decrease in the global drag, CD, under aligned conditions. Similar drag reductions are observed under yawed conditions.Two regimes are identified in terms of the coupled fluid–structure dynamics. For low Ca, the passive reconfiguration of the flaps include small amplitude, periodic oscillations corresponding to the first free deformation mode of a cantilevered beam. Alongside these weak oscillations, the flaps are deformed guided by the changes in the value of the horizontal base pressure gradient, depicting bi-stable behavior which is caused by the synchronization between the Reflectional Symmetry Breaking (RSB) mode, typically present in the wake of three-dimensional bluff bodies, and the flaps deformation. For higher values of Ca, the flexible flaps deflect inwardly by about Θ≃20° on average, but exhibit vigorous oscillations combining the first and second free deformation modes of a cantilevered beam. These large amplitude oscillations excite the flow separation at the model’s trailing edges, leading to significant fluctuations in the separated shear layers and a consequent 31% increase in the global drag. Under yawed conditions, the flaps responses for large values of Ca are different due to the asymmetry of the corresponding recirculation region.

Adaptive detached eddy simulation of turbulent combustion with the subgrid dissipation concept

Detached eddy simulation has become a widely used method in eddy simulations due to its balance between cost and accuracy. The recently developed subgrid dissipation concept (SDC) combustion model [Liu et al., “On the subgrid dissipation concept for large eddy simulation of turbulent combustion,” Combust. Flame 258, 113099 (2023)] is found to be more reasonable and accurate than the conventional eddy dissipation concept model in large eddy simulation (LES). In this paper, the SDC model is adapted to the ℓ2-ω adaptive detached eddy simulation framework, named DES-SDC. The required key quantities, including the fine structure mass fraction and dissipation rate, are appropriately blended across Reynolds-averaged Navier–Stokes and LES regions. The DES-SDC approach is validated using premixed bluff body stabilized flame, non-premixed swirl flame, and premixed swirl flame with complex geometry. It is much more tolerant to coarse mesh resolution than pure LES, yet it preserves the capability of resolving the key unsteady feature critical for the combustion process, as it is designed to be. The DES-SDC approach is relatively insensitive to the grid resolution. The present research provides a promising approach for accurately simulating practical unsteady turbulent combustion problems at an affordable computational cost.

Spectral proper orthogonal decomposition of harmonically forced turbulent flows

Many turbulent flows exhibit time-periodic statistics. These include turbomachinery flows, flows with external harmonic forcing and the wakes of bluff bodies. Many existing techniques for identifying turbulent coherent structures, however, assume the statistics are statistically stationary. In this paper, we leverage cyclostationary analysis, an extension of the statistically stationary framework to processes with periodically varying statistics, to generalize the spectral proper orthogonal decomposition (SPOD) to the cyclostationary case. The resulting properties of the cyclostationary SPOD (CS-SPOD for short) are explored, a theoretical connection between CS-SPOD and the harmonic resolvent analysis is provided, simplifications for the low and high forcing frequency limits are discussed, and an efficient algorithm to compute CS-SPOD with SPOD-like cost is presented. We illustrate the utility of CS-SPOD using two example problems: a modified complex linearized Ginzburg–Landau model and a high-Reynolds-number turbulent jet.