Wake Of Bluff Body Research Articles

Monitoring the Wake of Low Reynolds Number Airfoils for Their Aerodynamic Loads Assessment

Experimental investigations are carried out to explore the aerodynamic performance and vortex shedding characteristics of S5010 and E214 airfoil-based wings to provide guidance for the design of MAVs and other low-speed vehicles. Force and wake shedding frequency measurements are carried out in a subsonic wind tunnel in the Reynolds number (Re) range of 4 × 104 - 1 × 105. The measurements with increasing Re show that the slope of the lift curve in the linear region increases by 14% for S5010, while this increment is 11% for E214. The peak lift coefficient of both airfoils reduces with reducing Re. For lower pitch angles, the influence of Re on drag coefficients is less significant, but at higher angles, the drag increases as the Re drops. Unlike pre-stall mountings, the pitch-down propensity of the airfoil enhances in the post-stall region for high Re flows. Moreover, the frequency of shed vortices reduces with rising angle of attack at a given Re. In contrast, the Strouhal number almost remains constant with varying Re at a fixed angle of attack. For S5010 and E214 airfoils, the Strouhal number is noticed to vary between 0.68 - 0.36 and 0.58 - 0.36, respectively, for pitch angle variation of 12°- 28°. The airfoils show a higher Strouhal number than the bluff body wakes, but this difference decreases for high angles of attack mountings. This finding reveals that the wake structure of the airfoil at a high post-stall angle behaves as bluff body wakes.

Equilibrium of fluid fluxes in the wake of a three-dimensional flat-back bluff body

Equilibrium of fluid fluxes in the wake of a three-dimensional flat-back bluff body

Genetically-Based Active Flow Control And PIV Measurements Of A Circular Cylinder Wake

In the field of flow manipulation, considerable attention is devoted to controlling the wake of bluff bodies, which are characterized by the phenomenon of vortex shedding. Specifically, the control of the wake behind a single circular cylinder has garnered substantial research interest due to its engineering applications, such as reducing aerodynamic drag in the automotive and aerospace industries. Therefore, in the current work, through the application of the planar PIV technique, the fluid dynamic behavior of a circular cylinder wake controlled via synthetic jets (SJ) and Machine Learning (ML) methods is analyzed and discussed. ML algorithms are used to determine the optimal voltage signal for the loudspeaker, which effectively minimizes the aerodynamic drag of the cylinder. This approach overcomes the limitations associated with the a priori assumption of a parametric waveshape. In particular, the gradient-enriched machine learning control (gMLC) algorithm is chosen as optimization tool to search the best open-loop control policy for aerodynamic drag reduction. The cost function for this problem has been defined considering both the drag reduction contribution and the electrical power spent for the actuation. Initial optimization tests are performed by minimizing a cost function considering only the reduction in aerodynamic drag. Subsequently, a term related to the electrical power consumption for actuation is incorporated into the cost function. Once obtained the optimal control policies for these cases (hereinafter renamed CC1 and CC2, respectively), the physical mechanism responsible for reducing the aerodynamic drag is investigated more closely by analyzing the effect of the control laws on the turbulent flow statistics. Figure 1 reports a comparative assessment of the CC1 and CC2 with the baseline configuration and the configuration controlled by a sinusoidal law (CCS). The baseline configuration is characterized by an extended region of localized turbulent fluctuations in the far wake, where the time-averaged signature of von Kármán vortices is found. In contrast, all the controlled configurations show a global reduction in wake turbulence, with a peak of turbulent energy at the rear stagnation point due to the momentum injection from the SJ actuator. Notably, the CCS and CC2 configurations share similar flow field morphologies, while the CC1 configuration shows a greater reduction in turbulent fluctuations compared to the other controlled cases. This control law also achieves the largest drag reduction among all configurations studied reporting a 9.7% of drag reduction, compared to CCS and CC2, which exhibit similar reductions of 8.4% and 8.5%, respectively. Therefore, the optimization procedure has proven the capability of finding control policies able to outperform the traditional sinusoidal control widely studied in literature.

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.

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.

Numerical analysis of an oscillating square and circular bluff body submerged in the wake of an identical static bluff body: A GPU Accelerated Immersed Boundary-Lattice Boltzmann Approach

Hydrodynamic forces acting on marine structures, tubes, and objects can trigger disruptive vibrations. This study is an effort to encapsulate the results of wake induced vibration among tandem bodies. Specifically, impact of a static upstream body on a downstream oscillating body at Reynolds number 100 and constant frequency ratio Fr=1.0. The effects of streamwise displacement are investigated in a tandem arrangement of 1.05≤Lx/D≤12.0. In this study an in-house developed code employed that combines an immersed boundary lattice Boltzmann method with a structural equation and is accelerated using a graphical processing unit. When comparing scenario 1 (both bodies oscillate) to scenario 2 (only the downstream body oscillates), the study finds that the upstream square body motion has little effect on the downstream square body oscillation. However, the upstream circular body motion significantly impacts the downstream body oscillation. In addition, the study observes that the maximum amplitude occurs after reaching the critical spacing in scenario 2, whereas it occurs at and after the critical spacing in scenario 1. The critical spacing ratio is similar in both scenarios but varies for square and circular bodies. Lastly, the study examines the flow physics and hydrodynamic force coefficients of the downstream oscillating body.

Numerical evaluation of the bubble dynamic influence on the characteristics of multiscale cavitating flow in the bluff body wake

Unsteady cavitating flow around the bluff body widely existed in practical engineering fields. This paper aims to investigate the physics involved in the multi-scale cavitating flow in the wake of a wedge-shaped bluff body, with special emphasis on the bubble effects. Numerical simulation is conducted using the traditional Eulerian-Eulerian (E-E) and newly developed Eulerian-Lagrangian (E-L) methods in OpenFOAM. It is found that, compared with the experimental results, the E-L method can predict more accurate cavitation characteristics than the E-E method due to the contribution of discrete bubbles. Bubbles can significantly influence the behaviors of the multi-scale cavitating flow. Specially, analysis of pressure fluctuations indicates that the bubbles induce a stronger power spectral density in the middle- and high-frequency regions. With the bubble effects considered, the vortex structure is stretched and more extensive in the near wake regions. The strength of time-averaged vorticity distribution decreases in the far wake region. Further analysis of the vortex transport equation shows that bubble dynamics significantly increase the intensity of the stretching term by enhancing the spanwise velocity fluctuations. The intensity of the baroclinic torque term predicted by the E-L method is higher than that by the E-E method near the center line due to the influence of bubbles. On the other hand, the analysis of turbulent kinetic energy distribution indicates that bubbles can induce turbulence by increasing cross-stream velocity fluctuations. The increment of the shear Reynold stress, u′xu′y‾, suggests that the bubbles intensify the coupling between the streamwise and cross-stream velocity fluctuations.

Coherent pressure and acceleration estimation from triply decomposed turbulent bluff-body wakes

Coherent pressure and acceleration estimation from triply decomposed turbulent bluff-body wakes

Automatic alignment of underwater snake robots operating in wakes of bluff bodies

This paper presents the development of controllers to automatically align an underwater snake robot (USR) with a wake that forms behind a bluff body, while swimming at a desired distance to the object. The low-level controllers stabilize the joint motion of the USR to a swimming gait while achieving a desired orientation and tangential velocity. The high-level controllers are designed to select references for the orientation and tangential velocity to achieve a desired placement and alignment. The control system is analyzed and proven uniformly practically asymptotically stable (UPAS). The proposed control method is validated through high-fidelity simulations that capture the intricate interaction between the USR and the surrounding fluid, and is seen to perform well in these simulations.

Freestream and shear layer effects in bluff-body-stabilized turbulent premixed flames

The effects of low and high approach flow turbulence levels on a large-scale, confined, premixed bluff-body-stabilized flame are experimentally characterized. The flame and flow are imaged using simultaneous 10-kHz-rate formaldehyde (CH2O) planar laser-induced fluorescence (PLIF), hydroxyl (OH) PLIF, particle image velocimetry (PIV), and OH* chemiluminescence. Elevated approach flow turbulence intensities of 16 % as compared to a lower value of 3 % affects the global and local flame and flow. The elevated approach flow turbulence results in a 22 % reduction in the length of the recirculation zone, despite the mean flame and mean shear layer locations being similar for both approach flow turbulence levels. Increasing the approach flow turbulence intensity does not lead to the mean flame position moving further into the reactant freestream. Both approach flow turbulence levels result in a similar growth of the flame brush thickness moving downstream until approximately 1.5 bluff-body widths, after which the flame brush for the low and high approach flow turbulence case stops growing and continues to linearly increase, respectively. This study describes the mechanisms by which high approach flow turbulence affects the global and local flame and emphasizes that the turbulent flame characteristics are influenced not only by the freestream approach flow turbulence but also by the bluff-body shear layers, recirculation zone, and wake.

Comparative analysis of flag based energy harvester undergoing extraneous induced excitation

This study presents a detailed comparative analysis of the piezoelectric flexible plate under the influence of oscillating wakes of different bluff bodies. The piezoelectric plate is tested to determine the optimal configuration for energy harvesting from the wake flow of bluff bodies with different cross-sections in a low-speed closed-circuit wind tunnel. The Reynolds number is varied from 10000 to 75000 and the distance between the bluff body and plate is changed from 1D to 7D (where D is the diameter of the bluff body). The flutter of the plate without a bluff body is considered as a benchmark case and then subsequently the bluff bodies are introduced to see energy generation behavior. The results show that the bluff body caused a significant reduction of 50% in the flow speed as compared to the flutter case along with an improvement of 847% in energy harvesting output. Furthermore, selecting an appropriate bluff body ensures higher peak-to-peak flapping amplitude, which in turn produces a higher amount of electrical energy due to higher bending curvature and resultant strain produced in the piezoelectric plate. An increase of 125% in flapping amplitude is obtained using a bluff body compared to the flutter case. Furthermore, comparison within the various bluff bodies shows that change in cross-section has an enormous impact on output, and an increase of 515% in output power is obtained compared to the typical circular bluff body, which is more often quoted in the literature. Additionally, bending modes are identified and the variation in output power is linked with the flapping behavior of the piezoelectric plate, which is helpful in understanding the energy generation behavior of the piezo plate.

Reconstructing Three-Dimensional Bluff Body Wake from Sectional Flow Fields with Convolutional Neural Networks

Reconstructing Three-Dimensional Bluff Body Wake from Sectional Flow Fields with Convolutional Neural Networks

Active flow control for bluff body drag reduction using reinforcement learning with partial measurements

Active flow control for drag reduction with reinforcement learning (RL) is performed in the wake of a two-dimensional square bluff body at laminar regimes with vortex shedding. Controllers parametrised by neural networks are trained to drive two blowing and suction jets that manipulate the unsteady flow. The RL with full observability (sensors in the wake) discovers successfully a control policy that reduces the drag by suppressing the vortex shedding in the wake. However, a non-negligible performance degradation ( $\sim$ 50 % less drag reduction) is observed when the controller is trained with partial measurements (sensors on the body). To mitigate this effect, we propose an energy-efficient, dynamic, maximum entropy RL control scheme. First, an energy-efficiency-based reward function is proposed to optimise the energy consumption of the controller while maximising drag reduction. Second, the controller is trained with an augmented state consisting of both current and past measurements and actions, which can be formulated as a nonlinear autoregressive exogenous model, to alleviate the partial observability problem. Third, maximum entropy RL algorithms (soft actor critic and truncated quantile critics) that promote exploration and exploitation in a sample-efficient way are used, and discover near-optimal policies in the challenging case of partial measurements. Stabilisation of the vortex shedding is achieved in the near wake using only surface pressure measurements on the rear of the body, resulting in drag reduction similar to that in the case with wake sensors. The proposed approach opens new avenues for dynamic flow control using partial measurements for realistic configurations.

Unsteady Numerical Simulation of Two-Dimensional Airflow over a Square Cross-Section at High Reynolds Numbers as a Reduced Model of Wind Actions on Buildings

Airflow over a square cross-section at high Reynolds numbers and different angles of incidence is investigated with the aim of providing deeper insight into wind actions on elongated structures and, in particular, tall buildings. The flow around bluff bodies is characterized by separation at sharp corners, as well as possible flow reattachment at side surfaces. The alternate shedding of vortices is also generated in the wake of bluff bodies due to the unsteady nature of flow separation. Two-dimensional (2D) URANS numerical simulations were conducted in order to model transient flow and examine wind actions on a square used as a model of a typical cross-section of a tall building far from its roof and the ground. For validation purposes, the study’s numerical results on drag and lift coefficients, Strouhal numbers, as well as pressure coefficient distribution were found to be in good agreement with available experimental and numerical results in the literature for relatively low Reynolds numbers. The numerical study was then extended to higher Reynolds numbers, approaching values that are pertinent for wind flow around buildings, thus addressing the lack of such results in the literature. On the basis of these results, the impact of Reynolds numbers and angles of incidence on drag and lift coefficients, as well as the pressure coefficient distribution along the walls of the cross-section, is highlighted.

Numerical analyses of the flow past a short rotating cylinder

This work studies the three-dimensional flow dynamics around a rotating circular cylinder of finite length, whose axis is positioned perpendicular to the streamwise direction. Direct numerical simulations and global stability analyses are performed within a parameter range of Reynolds number $Re=DU_\infty /\nu <500$ (based on cylinder diameter $D$ , uniform incoming flow velocity $U_\infty$ ), length-to-diameter ratio ${\small \text{AR}}=L/D\leq 2$ and dimensionless rotation rate $\alpha =D\varOmega /2U_\infty \leq 2$ (where $\varOmega$ is rotation rate). By solving Navier–Stokes equations, we investigated the wake patterns and explored the phase diagrams of the lift and drag coefficients. For a cylinder with ${\small \text{AR}}=1$ , we found that when the rotation effect is weak ( $0\leq \alpha \lesssim 0.3$ ), the wake pattern is similar to the unsteady wake past the non-rotating finite-length cylinder, but with a new linear unstable mode competing to dominate the saturation state of the wake. The flow becomes stable for $0.3\lesssim \alpha \lesssim 0.9$ when $Re<360$ . When the rotation effect is strong ( $\alpha \gtrsim 0.9$ ), new low-frequency wake patterns with stronger oscillations emerge. Generally, the rotation effect first slightly decreases and then sharply increases the $Re$ threshold of the flow instability when $\alpha$ is relatively small, but significantly decreases the threshold at high $\alpha$ ( $0.9<\alpha \leq 2$ ). Furthermore, the stability analyses based on the time-averaged flows and on the steady solutions demonstrate the existence of multiple unstable modes undergoing Hopf bifurcation, greatly influenced by the rotation effect. The shapes of these global eigenmodes are presented and compared, as well as their structural sensitivity, visualising the flow region important for the disturbance development with rotation. This research contributes to our understanding of the complex bluff-body wake dynamics past this critical configuration.

A phase-resolved force analysis of bubble trapping behind cylinders in liquid-gas flows

A liquid-gas flow across a bluff body can result in non-uniform distribution of void fraction due to the interaction of the bubbles with the bluff-body wake. This experimental investigation was conducted to characterize a bubble-trapping region and the clustering dynamics of two distinct bubble sizes in an upward water channel with a rectangular cross-section. Bubble trapping in the near wake of a cylinder is shown as a function of bubble size, liquid flow rate, and gas flow rate through patterns of local void fraction. Particle tracking velocimetry and particle image velocimetry were used to calculate time-averaged trajectories of 3 mm diameter and 0.5 mm diameter bubbles as they flowed around a cylinder. The liquid Reynolds number (Re), based on the cylinder diameter of 9.5 mm, was varied from Re = 100 to Re = 3,000. In addition, the injection of liquid flow tracers and application of particle shadow image velocimetry allowed measurements of the time-averaged liquid velocities in the wake. The phase-resolved tracking results were evaluated to determine the effects of the added mass, pressure gradient, lift, drag, and buoyancy forces acting on the air bubbles. Trapping of both bubble sizes was observed across a range of operating conditions; however, this behavior was not explained solely by the Reynolds number of the flow. The force balance analysis revealed that inertial forces and lift forces acting on the bubbles both contributed to the clustering, which occurred when either the inertial forces or the lift forces acting on the bubble were sufficiently high compared to the bubble drag forces. Both the inertia- and lift-to-drag ratios were necessary to predict the bubble clustering dynamics of the distinct bubble sizes investigated.

Control of a circular cylinder flow using attached solid/perforated splitter plates at deflection angles

Control of a circular cylinder flow by rear-attached solid/perforated splitters has been experimentally investigated using Particle Image Velocimetry (PIV) and far-field microphones, respectively. The Reynolds number is Re = 2.7 × 104–6.8 × 104 based on the cylinder diameter D, and the effects of splitter length L (L/D = 0–4.0), porosity σ (0%–22%), and deflection angle α (0°–30°) on noise and flow characteristics are reported. The literature seriously lacks the combination effects of these parameters on bluff-body wakes, especially the flow mechanism on noise variations. Acoustic results show that for a solid splitter, the optimal noise reduction of 18 dB is achieved when L/D = 1.0–1.5, whereas when 2.0 ≤ L/D ≤ 4.0, the noise increases rather than decreases, consistent with the literature. At L/D = 1.0, deflecting and/or perforating the splitter would not further reduce the noise and, on the contrary, weaken the noise control efficiency. However, at L/D = 2.0, a rear-half perforation with σ ≥ 18.2% or a deflection angle of α ≥ 30° individually reduces noise by more than 10 dB compared to the bare cylinder. Combining both methods at L/D = 2.0 eliminates vortex-shedding noise when σ ≥ 11.6% and α ≥ 20°. PIV measurements reveal that noise reduction at L/D = 1.0 results from reduced vortex strength in the flow, while noise increase at L/D = 2.0 is attributed to vortex-splitter trailing edge interaction, which is mitigated by the perforation and/or the deflection of the splitter. The study suggests that, in general, appropriately perforating and deflecting longer splitter plates (L/D ≥ 2.0) effectively reduce bluff-body noise, while solid, shorter splitter plates (0.5 ≤ L/D ≤ 1.5) positioned streamwise achieve significant noise reduction. These insights offer valuable noise control strategies for various engineering applications.

Conditional space-time POD extensions for stability and prediction analysis

The correlation and extraction of coherent structures from a turbulent flow is a principal objective of data-driven modal decomposition techniques. The Conditional space-time Proper Orthogonal Decomposition (CST-POD) offers insight into transient dynamics, revealing the causes of specific flow phenomena, or events, in a customizable manner. This work targets the temporal evolution of CST-POD modes in a reduced subspace, resulting in new extensions and adaptations that are distinct from capabilities of other decomposition methods. First, the properties of CST-POD and its relation to the ensemble average are instantiated with a simple Lorenz system. Other CST-POD properties are examined with applications to problems displaying intermittent properties that test data-driven methods, including transitional supersonic boundary layers and schlieren video processing of unstarted inlet buzz. These examples also explore the pragmatic concerns of oversampling and offer theoretical connections to the sub-sampling of a larger Hankel space through time-delay embedding. Through these insights, it is demonstrated that the subsequent application of dynamic mode decomposition (DMD) to CST-POD modes, provides a flexible tool to investigate targeted flow instabilities, both absolute (tonal) and convective in nature. It is shown, using a resonating jet as an example, that Spectral POD modes associated with tones can be recovered if the time-horizon of CST-POD is suitably extended. Regarding convective instabilities, a multi-resolution framework (CST-mrDMD) yields a refined “cause and effect” stability analysis, capable of diagnosing the natural forcing mechanisms within the jet, and the resulting unstable shear-layer mode. In the final interpretation, the potential for real-time flow prediction of extreme events is derived from an active sensor correlated to a CST-POD mode using the example of bluff-body wake structures impinging on a channel wall.

Oblique and parallel modes of the bistable bluff body wake

Oblique and parallel modes of the bistable bluff body wake

Computational analysis of inverted flag-based energy harvester in the wake of cylindrical bluff body

In the current study, the penalty immersed boundary method (PIBM) is used to numerically analyze the flapping motion of an inverted flag, placed behind a bluff body using two-dimensional viscous flow. Direct numerical simulations (DNS) are carried out by changing the Reynolds number, bending stiffness, and streamwise gap (which represents the distance from the bluff body to the fixed end of the flag) behind the inverted D-shape cylinder to find their impact on the peak-to-peak amplitude, flapping frequency of the flag. The optimal values of the stated parameters are determined and explained in how the change in geometric and flow parameters affects the flapping behavior which has an ultimate impact on energy output. The addition of a bluff body caused the high strain due to higher bending modes and curvature in the flag. It is also shown that the alternating vortical flow structures have a strong influence on the dynamical behavior of the inverted flag placed inside the wake region of an inverted D-shape cylinder. Relationships between flow velocity, streamwise gap, and the flag's bending rigidity highlight consistent trends for the enhancement of energy production. Additionally, the lateral position of an inverted flag and its effect on vorticity and energy harvesting is discussed in detail with their power spectra to find the maximum amplitude and corresponding drag force. This research would help in concluding the optimal parameters of the inverted flag for the highest energy generation behind the bluff body.