Vortex Shedding Process Research Articles

Numerical Investigation of Flow Control past a Slotted Cylinder for Lower Critical Transitional regime

The phenomenon of fluid flow past a bluff body can be ubiquitously perceived both in nature as well as a multitude of engineering applications. The pragmatic and real-world significance of this problem combined with its simple yet intricate physics has turned the heads of quite a few researchers in the past century. The present study deals with the numerical investigation of flow characteristics around a modified circular cylinder with a slit. The numerical simulations were performed with a finite volume-based solver at a Reynolds number (Re) = 2500 based on the cylinder diameter (D) and the centerline velocity at the inlet, which can be considered as the lower critical value for the dynamic shift from laminar to turbulent flow regime. The upstream and downstream stagnation points on the cylinder surface were connected by a slit. The slits implemented were parallel, converging, and diverging with respect to the direction of flow and amalgamated with one cylinder having no slit, a total of four cases were solved. The windward width of parallel and converging slits was 0.1D and the diverging slit being a mirror of the converging slit. The end-width ratio (Φ) of the converging and diverging slits was 1.5. The results indicated that incorporation of slits in the modified bluff body contributes to drag reduction by the creation of self-issuing jets which affect the wake vortex shedding process downstream of the cylinder. The parallel slit being the most efficacious one leads to the highest reduction of drag due to the formation of smaller sized vortices as compared to convergent or diverging slits in the wake region of the cylinder.

Large amplitude flapping of an inverted elastic foil in uniform flow with spanwise periodicity

An elastic foil interacting with a uniform flow with its trailing edge clamped, also known as the inverted foil, exhibits a wide range of complex self-induced flapping regimes such as large amplitude flapping (LAF), deformed and flipped flapping. In particular, the LAF is of interest for its applications in the development of energy harvesting devices. Here, we perform three-dimensional numerical experiments assuming spanwise periodicity on the LAF response of an inverted foil at Reynolds number Re=30,000 for a relatively low mass-ratio m∗=1.0 using a variational fluid–structure formulation and large-eddy simulation (LES). We examine the role of the vortex structures particularly the counter-rotating periodic vortices generated from the leading and trailing edges of the inverted foil, and the interaction between them on the LAF. For that purpose, we investigate the dynamics of the inverted foil for a novel configuration wherein we introduce a fixed splitter plate at the trailing edge to suppress the vortex shedding from the trailing edge and thereby inhibit the interaction between the counter-rotating vortices. Unlike the vortex-induced vibration of an elastically-mounted circular cylinder, we find that the inhibition of the interaction has an insignificant effect on the transverse flapping amplitudes, due to a relatively weaker coupling between the counter-rotating vortices emanating from the leading edge and trailing edge. The inhibition of the trailing edge vortex generally reduces the streamwise flapping amplitude, the flapping frequency and the net strain energy of foil. To further generalize our understanding of the LAF, we next perform low-Reynolds number (Re∈[0.1,50]) simulations for the identical foil properties to realize the impact of vortex shedding on the large amplitude flapping. Due to the absence of vortex shedding process in the low-Re regime, the inverted foil no longer exhibits the periodic flapping. Nevertheless, the flexible foil still loses its stability through divergence instability to undergo a large static deformation. This study has implications on the development of novel control mechanisms for energy harvesting and propulsive devices.

Vortex-induced vibrations of dual-step cylinders with different diameter ratios in laminar flows

Vortex-induced vibrations of dual-step cylinders in laminar flows at the Reynolds number of 200 based on a larger diameter are investigated through three-dimensional direct numerical simulations. Four larger-to-smaller diameter ratios (D/d) of 4.0, 2.0, 1.43, and 1.19 are considered for the cylinder with a low mass ratio of 2. Numerical results reveal that D/d significantly influences vibration responses and the vortex shedding process. The case with D/d = 1.43 can effectively reduce vibration amplitudes. The wake vortices are shed in cellular patterns along the cylinder span, with a distinct frequency for each cell. A direct and “X-shaped” connection of wake vortex-shedding modes occurs between different vortex cells, and a loop connection appears between the same cell vortices. A new wake pattern entitled the “out-of-phase vortex shedding” is found downstream of the smaller cylinder at D/d = 2.0. This is manifested by the wake vortices not being shed at a fixed frequency and intensity. The essential reason for this phenomenon lies in a non-lock-in condition between the structural vibration and vortex shedding frequencies of the smaller cylinder. As D/d decreases, a coherence between vortex cells is enhanced, leading to a disappearance of the “out-of-phase vortex shedding.”

Lock-in phenomenon of vortex shedding in oscillatory flows: an analytical investigation pertaining to combustors

An analytical investigation is performed to understand the lock-in phenomenon, observed in vortex shedding combustors. Several aeroengine afterburners and ramjets use a bluff body to stabilize the flame. The bluff body sheds vortices. During the occurrence of high-amplitude combustion instability, the frequency of vortex shedding locks in to the frequency of the chamber acoustic field. This phenomenon is termed vortex-acoustic lock-in. In general, there is a two-way coupling between the vortex shedding process and the acoustic field, making analytical investigation difficult. Since the frequency of the latter remains largely unaltered, performing an investigation to study the response of vortex shedding to external excitation not only allows one to gain insights, but also make the problem analytically tractable. We begin with a lower-order model available in the literature to describe the vortex shedding process in non-reacting flows, arising from sharp corners in the presence of upstream velocity excitation. The continuous time domain model is transformed to a discrete map, which connects the time instances of two successive vortex shedding events. The frequency and amplitude of excitation are varied to study the instantaneous vortex shedding time period, as the response of the system. In the absence of forcing, the iterates of the map form a period-1 solution with the frequency equalling the natural vortex shedding frequency. On increasing the amplitude of excitation, quasi-periodic behaviour of the iterates is observed, followed by a period-1 lock-in solution, where vortex shedding occurs at the excitation frequency. On further increasing the amplitude, de-lock-in occurs. From the map, an analytical solution is extracted, which represents the lock-in state. The condition and thereby the region in the frequency–amplitude parameter space where a general$p:1$lock-in occurs is then identified. Several important analytical expressions, such as for (1) critical threshold frequency above which lock-in occurs, (2) boundary of lock-in region in the parameter space, that are of direct importance to the design of quieter combustors are obtained. The study also identifies the transition of higher-order$p:1$to$1:1$lock-in state, through a series of lock-in and de-lock-in steps, whose occurrence could be verified from future experiments.

Effect of Inclination on Vortex Shedding Frequency Behind a Bent Cylinder: An Experimental Study

This paper presents experimental results on the vortex shedding frequency measured behind a bent cylinder. Experiments were conducted in a wind tunnel covering Reynolds numbers between 50 and 500, a range of interest for flow sensing, flow control, and energy harvesting applications. The bent cylinder comprised a vertical leg always oriented at normal incidence with respect to the free-stream flow, and an inclined leg whose inclination was varied during the tests between 90° and 15°. The bent cylinder was oriented in the wind tunnel with the vertical leg upstream and the inclined leg downstream, and the vortex shedding frequency was measured with hot-wire anemometry at several locations behind the inclined leg. The present bent cylinder design improves upon those previously considered by providing a finer control on the upstream boundary condition acting upon the inclined leg, which in the present design is not affected by the yaw angle of the inclined leg. With the exception of free-end effects, only noticeable for certain inclinations and Reynolds number values, inclination effects were surprisingly not observed, and the frequency of vortex shedding measured behind the inclined leg of the bent cylinder was consistent (within a few percent) with the cross-flow vortex shedding frequency at the same flow velocity. The present results corroborate and significantly extend the limited observations on bent cylinders available in the literature, further highlighting the importance of the upstream boundary condition on the vortex shedding process with inclined cylinders.

Passive suction jet control of flow regime around a rectangular column with a low side ratio

A passive jet method was experimentally investigated to manipulate the unsteady vortex shedding around a rectangular column. This method employed a circuit channel connecting the windward and leeward sides of the column to implement the windward passive suction and leeward passive blowing synchronously to modify the flow regime around the column. A bare column model with a side ratio of SR = 1.3 was utilized as a baseline case and the same column models covered with passive suction/jet rings served as controlled cases. Based on different angles of attack (AoAs) of the column models, the Reynolds number varied from 29,300 to 48,200 at an oncoming wind velocity of 10 m/s. At different intervals of suction/jet rings, the surface pressure distributions, aerodynamic forces, and flow structures of the column models were measured to evaluate the control effectiveness of the passive jet method. The results indicated that a notable reduction in the fluctuations of both the surface pressures and aerodynamic forces was achieved. Meanwhile, the mean values of drag forces were also evidently decreased. The flow regime was modified in the vicinity of passive jet holes. The main separation vortices were stretched by the small-scale reverse vortices induced by the jet flow. In addition, the TKE level in the wake flow was considerably decreased in the controlled cases. By extracting eigenvalues and mode coefficients, the POD analysis revealed that the descent of the low-order mode energy corresponded to the suppression of the vortex shedding. The series of phase reconstruction and POD-filtered flow fields demonstrated that the passive jet flow could interact with the shedding vortices and weaken the strength of the vortex shedding process.

Self-Suction-and-Jet Control in Flow Regime and Unsteady Force for a Single Box Girder

Self-suction-and-jet flow control is a passive flow control method involving self-organized suction at the leading edge and a wake jet at the trailing edge of a structure. In this study, it was used in a 1:125 physical model of the Great Belt East Bridge to decrease dynamic wind loads and modify the flow structure around the main girder. Experiments were performed in a wind tunnel at a wind speed of 12 m/s, yielding a Reynolds number of 2.8 × 104 for the height of the main girder model. The surface pressure (and therefore, the aerodynamic force) fluctuated less in the controlled models than in the uncontrolled model. The velocimetry data indicated that self-suction-and-jet flow control effectively changed the vortex shedding process in the wake of the model. The interactions between the self-organized jet flows and shear flows caused the trailing edge vortex shedding to transform from asymmetric to symmetric, eliminating the vortex shedding completely. Linear stability analysis of the mean wake flow structures indicated that self-suction-and-jet flow control pushed the unstable flow region farther downstream.

Proper Orthogonal Decomposition Analysis of Flow Characteristics of an Airfoil with Leading Edge Protuberances

A numerical investigation of the aerodynamic performances and flow mechanisms of an airfoil with leading-edge protuberances is presented within stall region at a Reynolds number of . In detail, a large-eddy simulation has been conducted and then validated through quantitative comparisons with previous experimental and numerical investigations. Based on the numerical results, a proper orthogonal decomposition (POD) analysis has been carried out. The flow control mechanisms within stall regime and physical backgrounds of corresponding POD modes have been uncovered from the view of flow energy. It has been found that the improved aerodynamic characteristics of modified airfoil might be attributed to different flow mechanisms at peaks and troughs, including spanwise momentum transfer process and flow transition within laminar separation bubble. In addition, the fluctuating characteristics of airfoil flow field have also been investigated. Consequently, it has been found that the Karman vortices dominate the rear wake of the smooth airfoil and the first four POD modes are closely related with the shedding process. In contrast, the corresponding vortex shedding process is suppressed by the modified airfoil according to the results of POD analysis.

Control of circular cylinder flow via bilateral splitter plates

We carry out wind tunnel investigations to study the flow of a circular cylinder modified with two rigid splitter plates hinged along its stagnation points. The equal-sized and symmetrically placed splitter plates are both parallel to the incoming airflow, and their single-sided length in the streamwise direction varies from 0 to 2.0D (where D is the cylinder diameter). The wind tunnel experiments are conducted at the Re of 3.33 × 104. In addition to bilaterally arranged plates, two other configurations of splitter plates, i.e., front-plate-only and rear-plate-only, are also investigated. By employing the sectional measurement of surface pressure in the midspan slice, we evaluate typical aerodynamic parameters, including pressure distribution, instantaneous drag and lift forces, frequency spectra of the unsteady lift forces, mean drag, and root-mean-square lift coefficients acting on the cylindrical test models. A particle image velocimetry (PIV) system is used to visualize and quantify the vortex shedding process and the dynamic interactions of the natural and modified cylinders. Experimental results of the surface pressure measurement and PIV measurement results are then combined to reveal the effects of rigid plates with different configurations (bilateral, front-only, and rear-only) on the circular cylinder flow.

Experimental investigation on the noise reduction method of helical cables for a circular cylinder and tandem cylinders

Cylinder noise can be remarkably reduced when the surface is helically wrapped with cables along the cylinder span. In this paper, aeroacoustic and aerodynamic experiments are conducted to reveal the noise reduction mechanism of the helical-cable technique. With the application of helical cables on the single circular cylinder, three-dimensional flow is promoted into the cylinder near wake, leading to the suppression of two-dimensional vortex shedding process and flow fluctuations. Besides, the three-dimensional wake also destroys the in-phase relationship of flow fluctuations along the cylinder span, leading to the reduction of spanwise correlation length and noise radiation efficiency. These two effects together contribute to the cylinder noise reduction. Furthermore, this helical-cable technique is extended to tandem cylinders and the results show that it can also reduce the tandem cylinder noise effectively. Especially for the moderate and large spacing tandem cylinder configurations, the key of the noise reduction is the control on the upstream cylinder.

Wake structures and acoustic resonance excitation of a single finned cylinder in cross-flow

This paper presents an experimental and numerical investigation to clarify the effect of circular fins on the wake structures, the dynamic loading, as well as the flow–sound interaction mechanism downstream of a single finned cylinder in cross-flow. It is revealed that adding circular fins to the cylinder enhances the flow coherence along the cylinder’s span, strengthens the intensity of the vortex shedding process downstream of the cylinder, reduces the vortex formation length, and increases the dynamic lift force acting on the cylinder. Thus, adding circular fins to the cylinder makes the flow more susceptible to acoustic resonance excitation, compared to an equivalent bare cylinder. Three-dimensional numerical simulations of the flow field around the cylinder show that the flow is entrained between the fins which results in the generation of an accelerated jet-like flow in the developing shear layer region. The jet-like flow emanating from the fins leads to a closer and stronger shear layer roll-up, and hence a shorter vortex formation region. This, in turn, increases the fluctuating lift force on the cylinder, as the strong oscillating shear layers occur closer to base of the cylinder. Additionally, with the reduction of the vortex formation length, the base pressure coefficient decreases and therefore the drag force on the cylinder increases. These intrinsic flow features explain the global effect of circular fins on the vortex shedding process and there by on the excitation mechanism of acoustic resonance.

Relationship between bubble characteristics and hydrodynamic parameters for single bubbles in presence of surface active agents

Rising single air bubbles were investigated in aqueous solutions of hexadecylamine (HDA) and methyl isobutyl carbinol (MIBC) as surface active agents at varying concentrations at a constant gas flow rate. Shadowgraphy was applied to determine main bubble characteristics, such as equivalent diameter, morphology, and rising velocity. From these characteristics, critical parameters like the concentration at the minimum bubble velocity were derived. Simultaneous application of Particle Image Velocimetry (PIV) provided information about hydrodynamic parameters, e.g. the induced liquid velocities and vortex shedding.From surface tension measurements, the concentration of adsorbed species on the interface and packing densities of HDA and MIBC on the bubble surface could be calculated. HDA exhibited a better adsorption and a higher packing density on the bubble surface compared to MIBC due to the ionic character and the straight hydrocarbon chain. The bubble characteristics were therefore more strongly affected by HDA than by MIBC.Combining the Shadowgraphy and PIV results it was found that the mean liquid velocity as well as the amount of induced turbulent kinetic energy increased with increasing concentration of surfactants in the solutions, while the investigated bubble characteristics such as equivalent diameter and rising velocity decreased. The increase in mean liquid velocity and induced turbulent kinetic energy could be correlated with the oscillating frequency of the bubble trajectory, which also increased with increasing surfactant concentration. The vortex shedding process could be visualised using Proper Orthogonal Decomposition (POD) revealing the micro-process of energy cascading.

Experimental and numerical investigation of cavitating vortical patterns around a Tulin hydrofoil

The objective of this paper is to investigate the cavitating flow patterns around a specific hydrofoil (Tulin hydrofoil) via combined experimental and numerical studies, and to better understand the vortex-cavitation interactions in transient cavitating flows. The experimental studies were carried out in the closed-loop cavitation tunnel, and a high-speed digital camera is applied to document the cavitation patterns. The numerical investigations are performed using a large-eddy simulation method and the Zwart cavitation model. The results showed that the predicted cavity formation and evolution agree well with the experimental observation. With decreasing of the cavitation number, different cavitating flow patterns occur around the hydrofoil, namely inception cavitation (σ = 1.57), vortex cavitation (σ = 1.27), cavitating vortex street (σ = 1.07), cloud cavitation (σ = 0.87), mixture supercavitation (σ = 0.67), and developed supercavitation (σ = 0.54). Especially, the flow structures of vortex cavitation, cavitating vortex street and cloud cavitation were further investigated with the Euler and Lagrangian method. For vortex cavitation, the cavitating vortex forms and sheds at the leading and trailing edge of the foil alternatively, with a pair of asymmetric vortex structures observed. The trailing edge cavitating vortex has a regular vortex shape and a clear boundary between the vortex structures. For cavitating vortex street, vortex shedding process is not independent, instead, the vortex braid between the trailing edge cavitating vortexes and the so called “cat's eye” vortex row can be obviously observed. The center of circulation region is corresponding to low finite-time Lyapunov exponent (FTLE) value, and the bridges of the FTLE is concentrated in the boundary of the vortex structure and curling in the dynamic surrounding of the circulation region. The trajectory of the points well presents the dynamic flow pattern. For cloud cavitation pattern, the cavity develops to cover the entire suction surface, and the cloud cavity sheds downstream periodically.

Active control of circular cylinder flow with windward suction and leeward blowing

This study is an experimental investigation of the effectiveness and mechanism of a concept for bluff-body control characterized by combined windward suction and leeward blowing (WSLB). Wind tunnel tests are performed at a subcritical Reynolds number (Re) of 3.33 × 104. Open-loop WSLB is used to control the flow around a cylindrical test model. Distributed suction and blowing nozzles are symmetrically arranged at the front and rear stagnation points. Steady suction and blowing are implemented simultaneously on both surfaces of the cylinder. WSLB control is characterized by the dimensionless momentum of the suction/blowing relative to the incoming airflow. Instantaneous pressure distributions on the midspan of the cylinder surface for the baseline and controlled cases are measured to quantify the modifications of WSLB control to the aerodynamic coefficients of the cylinder. In addition to surface pressure measurements, a particle image velocimetry (PIV) system is used to describe the wake flow structures of the baseline and controlled cylinders. Experimental results demonstrate that WSLB control decreases sectional drag at the midspan and reduces the fluctuating amplitudes of dynamic wind loads acting on the cylinder. The Strouhal number to characterize the vortex shedding frequencies of the controlled cylinders is also found to deviate from that of the natural cylinder. PIV measurement results show that the active blowing positioned at the leeward stagnation point forms a pair of vortices into the cylinder wake that modify the original vortex shedding process. As the blowing vortices convect into the wake, they stretch the unsteady shear flows from the upper and lower sides of the cylinder, increase the vortex-formation length and push the alternative vortex shedding further downstream. It is also shown that a steady and symmetric perturbation imposed on the periodic cylinder flow can reduce the von Karman vortices and modify the mode of wake vortex shedding significantly. With active control of windward suction and leeward blowing (WSLB), blowing positioned at the leeward stagnation point forms a pair of vortices in the cylindrical wake that modifies the typical vortex shedding process. As the blowing vortices convect downstream, they contribute to detaching the unsteady shear layers from the cylinder. It is shown that a steady and symmetric perturbation imposed on the periodic cylinder flow can lead to a symmetric mode of wake vortex shedding.

Numerical Investigation of the Effects of Nonsinusoidal Motion Trajectory on the Propulsion Mechanisms of a Flapping Airfoil

The effect of nonsinusoidal trajectory on the propulsive performances and the vortex shedding process behind a flapping airfoil is investigated in this study. A movement of a rigid NACA0012 airfoil undergoing a combined heaving and pitching motions at low Reynolds number (Re = 11,000) is considered. An elliptic function with an adjustable parameter S (flattening parameter) is used to realize various nonsinusoidal trajectories of both motions. The two-dimensional (2D) unsteady and incompressible Navier–Stokes equation governing the flow over the flapping airfoil are resolved using the commercial software starccm+. It is shown that the nonsinusoidal flapping motion has a major effect on the propulsive performances of the flapping airfoil. Although the maximum propulsive efficiency is always achievable with sinusoidal trajectories, nonsinusoidal trajectories are found to considerably improve performance: a 110% increase of the thrust force was obtained in the best studied case. This improvement is mainly related to the modification of the heaving motion, more specifically the increase of the heaving speed at maximum pitching angle of the foil. The analysis of the flow vorticity and wake structure also enables to explain the drop of the propulsive efficiency for nonsinusoidal trajectories.

Two-dimensionalization of a three-dimensional bluff body wake

The three-dimensional flow characteristics of a circular cylinder with synthetic jet control are numerically studied using large eddy simulation. The Reynolds number based on the diameter of the cylinder is Re = 500. The control effects and underlying mechanism are revealed to show how the synthetic jet changes the three-dimensional wake pattern. Analysis of the dynamic control process indicates that the blowing stroke helps the shear layer to assemble vorticity, and then, the suction stroke accelerates the detachment of the concentrated vorticity. The vortex shedding process will be gradually dominated by symmetric actuation of the synthetic jets. Thus, the asymmetric vortex shedding mode could be changed into a symmetric mode several periods after actuation at certain excitation frequencies, leading to significant suppression of lift fluctuations. A periodic pressure variation at the leeward surface of the circular cylinder caused by the changes of the separation point for the flow over a circular cylinder and recirculation region results in a large drag fluctuation. The excitation phase influences only the control process, but not the final state, while the excitation frequency plays an important role in the formation of different wake patterns. It is also found that the synthetic jet can completely suppress the formation of streamwise vortices due to the three-dimensional instability suppression and reduce the deformation of spanwise vortices, resulting in a conversion of the original three-dimensional flow into a two-dimensional one. Such two-dimensionalization can be achieved for both asymmetric and symmetric wake patterns, indicating that it is not influenced by the excitation phase and frequency as long as the actuation is two-dimensional.

Wake control using spanwise-varying vortex generators on bridge decks: A computational study

A passive control device called spanwise-varying vortex generator (VG) is proposed to mitigate vortex shedding of a bridge model, since the 3D spanwise-varying control method shows high efficiency in suppressing vortex shedding behind bluff bodies. Pairs of VGs were arranged on the lower surface of the bridge in the spanwise direction to generate streamwise vortices and perturb two-dimensional spanwise vortex shedding. Delayed detached-eddy simulation and dynamic mode decomposition were carried out to examine their effects on wake control. The results demonstrate that with VG control, the regular spanwise vortices are highly distorted and are accompanied by a spanwise mismatch in the vortex shedding process. Consequently, the regular spanwise vortex shedding is substantially suppressed, resulting in a considerable reduction in vortical strength in the wake and significant increases in the vortex formation length. In addition, the dynamic mode decomposition reveals that VGs can redistribute the energy spectrum and transport the energy from the mode associated with the regular vortex shedding to other low-frequency modes. Therefore, forces induced by vortex shedding are considerably reduced.

Passive noise control technique for suppressing acoustic resonance excitation of spirally finned cylinders in cross-flow

This paper presents an innovative passive noise control technique for suppressing flow-excited acoustic resonance of spirally finned cylinders in cross-flow. This was achieved by maintaining the number of fins, but varying the fin density along the finned cylinder’s span to create a non-uniform finned cylinder. Two non-uniform finned cylinders were investigated with pitch ratios of p(mid)/p(side) = 0.5 and 1.7. To better understand the influence of the non-uniform finned cylinders on the wake structure, bare cylinders with the same effective diameter were investigated. In the non-uniform finned cylinder case, due to the variation in the fin density, the effective diameter along its span changes. Thus, the effective diameter of a non-uniform finned cylinder happens to be a dual diameter cylinder. Three dual diameter cylinders were investigated with diameter ratios ranging between 0.5 ≤ D(mid)/D(side) ≤ 2. These diameter ratios closely correspond to the pitch ratio (p(mid)/p(side)) and effective diameter ratio (Dc(mid)/Dc(side)) of the non-uniform finned cylinders investigated. The results show that the non-uniform finned cylinderscause spatial variations in the vortex shedding process due to dissimilarities in the flow behavior at different fin density regions. Temporal variations were also revealed due to changes in the vortex shedding frequency at different spanwise regions. This resulted in a significant decrease in the spanwise flow coherence and vortex shedding strength. The spatio-temporal variations in the shed vortices at different spanwise regions observed by the non-uniform finned cylinders were found to be analogous to dual diameter cylinders with diameter ratios equivalent to the pitch ratios of the non-uniform finned cylinder. As a result, substantial attenuation in the acoustic pressure during flow-excited acoustic resonance was achieved by the non-uniform finned cylinders. The results presented in this paper show that non-uniform finned cylinders are a viable noise control technique for suppressing flow-excited acoustic resonance in tube bundles of heat exchangers.

Asymmetric characteristics of the shock bifurcation in the reflected shock/boundary layer interaction

PurposeWhen a reflected shock interacts with the boundary layer in a shock tube, the shock bifurcation occurs near the walls. Although the study of the shock bifurcation has been carried out by many researchers for several decades, little attention has been devoted to investigate the instability pattern of the bifurcation. This research work aims to successfully capture the asymmetry of the whole flow field, and attempt to achieve the instability mechanism of the shock bifurcation by a direct numerical simulation of the reflected shock wave/boundary layer interaction at Ma = 1.9. In addition, the reason for the formation of the bifurcated structure is also explored.Design/methodology/approachThe spatial and temporal evolution of the shock bifurcation is obtained by solving the two-dimensional compressible Navier–Stokes equations using a seventh-order accurate weighted essentially non-oscillatory (WENO) scheme and a three-step Runge–Kutta time advancing approach.FindingsThe results show that the formation of shock bifurcation is mainly because of the shock/gradient field interaction, and the height of the bifurcated foot increases with the growth of the shock intensity and the gradient field. The unsteady asymmetry of the upper and bottom shock bifurcated structures is because of the vortex shedding with high frequency in the rear recirculation zone, which leads to the fluctuation of the recirculation area. The vortex shedding process behind the bifurcated structure closely resembles the Karman vortex street formed by the flow around the cylinder. The dimensionless vortex shedding frequency varies between 0.01 and 0.02. In comparison to the scenario at Ma = 1.9, the occurring time of instability is delayed and the upper and bottom bifurcated feet intersect in a relatively short time at Ma = 3.5. The region behind the bifurcated shock is a transitional flow field containing obvious cell structures and “isolated islands.”Originality/valueThis paper discovers an unsteady flow pattern of the shock bifurcation, and the mechanism of this instability in the reflected shock/boundary layer interaction is revealed in detail.

Proper orthogonal decomposition analysis of boundary layer oscillating aspiration controlling separation flows in two-dimensional compressor cascades

Vortex structures of the separation flow fields in two-dimensional compressor cascades controlled by the boundary layer oscillating suction are numerically investigated. The proper orthogonal decomposition method is adopted to present the variation of characteristics owned by large-scale vortices. It is found that introducing unsteady excitations with proper frequencies into the steady aspiration results in a more effective control effect and the optimal oscillation frequency should be in line with the characteristic frequencies of the steady aspirated flow field. The separation flows can be decomposed in to several basic structures with the dominant one being in the manner of Karman vortex street regardless of the control method adopted. The boundary layer oscillating suction not only alleviates the separation by removing low-energy fluid, but also intensifies the harmonic flow element represented by proper orthogonal decomposition modes with others suppressed. The well-organized vortex shedding process could contribute to the loss reduction to some extent.