Shear Layer Reattachment Research Articles

Influences of blockage ratio and Reynolds number on the spatiotemporal dynamics around a rectangular prism

Particle image velocimetry is used to experimentally investigate the influence of blockage ratio (BR) and Reynolds number (Re) on the turbulent flow around a rectangular prism with depth-to thickness ratio of 3. The prism was selected because it falls within the intermediate regime where the turbulent dynamics is sensitive to the incoming boundary condition. The tested blockage ratios were 2.5%, 5%, and 10% at Reynold numbers of 3000 and 7500. The results are analyzed in terms of the mean flow, turbulent kinetic energy (TKE), frequency spectra, reverse flow area, as well as spectral proper orthogonal decomposition (SPOD). The results indicate that as blockage ratio and/or Reynolds number increase, the tendency of reattachment of the separated shear layer onto the surface of the prism increases while the location of maximum TKE over the prism shifts toward the leading edge, indicating earlier transition of the separated shear layer from laminar to turbulence. For the cases without mean reattachment over the side faces of the prisms, the separated bubble over and downstream of the prism exhibits strong tendency of synchronization in terms of the instantaneous areas of the flow reversal, suggesting a global instability mechanism encompassing the entire prism. In cases with mean flow reattachment, conversely, the low-frequency flapping motion manifests over the prism. SPOD analysis further shows that the relevant shedding dynamics are captured in the first mode and the von Kármán shedding structures have the highest energy.

Contribution of Unsteadiness to Solid Fuel Burning Characteristics in a Scramjet Combustor

The contributions of unsteadiness to solid fuel combustion are investigated numerically and validated against new experiments in a deep cavity scramjet. Large-eddy simulations discretized with discontinuous Galerkin (DG) elements are solved with a flamelet manifold approach in three dimensions. A multiphase model that incorporated thermal decomposition inside the foam layer is coupled with stagnation flow flames to determine the combustion manifold and regression rate. The approach accurately models small-scale experiments of convective burning over solid fuel. The inclusion of the manifold into the DG code features two innovations: a polynomial fitting of the pressure to reduce the interpolation dimensions and the coefficient of determination to include non-monotonic manifolds in DG schemes. Model validation was performed using pressure and averaged regression rate data, both of which showed strong agreement with experiments. Three-dimensional pressure modes in the cavity support a substantial increase in regression rates and a broadening of the peak due to oscillations of the impingement point. The majority of fuel is pyrolyzed at the shear-layer reattachment point in a stagnation flow boundary layer. The fuel in the cavity is pyrolyzed by conductive heat transfer from the main shear layer. Poor combustion is observed in the expanding fuel section.

Enhancing hydrodynamic forces through miniaturized control of square cylinders using the lattice Boltzmann method

This study investigates the influence of small control cylinders on the fluid dynamics around a square cylinder using the Lattice Boltzmann Method (LBM). Varying the gaps (L) between the main and control cylinders from 0 to 6, four distinct flow regimes are identified: the solo body regime (SBR), shear layer reattachment (SLR), suppressed fully developed flow (SFDF), and intermittent shedding (IS). The presence of control cylinders results in significant reductions in flow-induced forces, with drag coefficient (CD) and root mean square values of drag and lift coefficients (CDrms and CLrms) decreasing by approximately 31%, 90%, and 81%, respectively. The SFDF flow regime exhibits the lowest fluid forces compared to other regimes. The effects of tiny control cylinders on the fluid flow characteristics of a square cylinder are examined using the Lattice Boltzmann Method (LBM) in this research work. The gaps (L) between the main and control cylinders are varied in the range from 0 to 6. The size of each control cylinder is equal to one-fifth of the primary cylinder. According to the findings, there are four distinct flow regimes as the gap spacing varies: solo body regime (SBR), shear layer reattachment (SLR), suppressed fully developed flow (SFDF), and intermittent shedding (IS) for gap spacing ranges 0 ≤ L ≤ 0.2, 0.3 ≤ L ≤ 0.9, 1 ≤ L ≤ 3, and 3.2 ≤ L ≤ 6, respectively. Additionally, it has been noted that the amplitude of variable lift force is reduced when the gap separation between the main and control cylinders is increased. When compared to solo cylinder values, it is found that the presence of small control cylinders in the flow field results in a considerable reduction of flow-induced forces. The SFDF flow regime was determined to have the lowest fluid forces compared to the other flow regimes studied. Our findings highlight the efficacy of small control cylinders in mitigating flow-induced forces and controlling flow characteristics. The LBM proves to be a valuable computational technique for such fluid flow problems.

Fluid structure interaction problem for flow past three unequal sized square cylinders at different Reynolds numbers

The flow past three square cylinders of unequal size placed in an inline arrangement is studied using the lattice Boltzmann method at different Reynolds numbers [Re = (u∞ d)/ν] within the range of Re = 120, 150, 160, 175, and 200 for various gap spacings (g = s/d), ranging from 1 to 6. This study focused on the symmetric examination of flow behavior for various gap spacing within the three unequal-sized square cylinders. The main objective of this study was to investigate the effects of Reynolds numbers and gap spacing for flow structure mechanism and vortex shedding suppression in between the gap and down-stream position of all three cylinders. Results are obtained in terms of vorticity contours visualization, drag and lift coefficients, Strouhal number, and physical parameters. In vorticity contour visualization, different flow behaviors are observed, known as flow regimes, and are named according to their characteristics, and they are (i) steady flow regime, (ii) shear layer reattachment flow regime (SLR), (iii) fully developed vortex shedding flow regime, (iv) two-row fully developed vortex shedding flow regime, and (v) fully developed irregular vortex shedding flow regime. The present study also includes a discussion on aerodynamic forces, namely the mean drag coefficient (Cdmean), root mean square of the lift coefficient (Clrms), and Strouhal numbers (St) for three cylinders with sizes d = 20, d1 = 15, and d2 = 10, respectively. The maximum value of Cdmean for the first cylinder (C1) is obtained at (Re, g) = (200, 3) that is, 1.5156, where the existing flow regime is the SLR flow regime, while for C2 and C3, the maximum Cdmean values are examined at critical flow behaviors, where the existing flow regime is a fully developed irregular vortex shedding flow regime. Negative values of Cdmean are also examined for cylinders C2 and C3 at some combinations of (Re, g), attributed to the effect of thrust. Furthermore, it is noticed that the values of Strouhal number are increased with an increment in values of gap spacing. The highest value of the Strouhal number for all three cylinders is observed for C1 at (Re, g) = (120, 5), reaching 0.1556 along with a two-row fully developed flow regime. Furthermore, it is investigated from the present problem that the position of unequal sized square cylinders strongly influenced the flow structure mechanism. The information found and discussed in this study could be effective for structure designing arrangement in the case of three square cylinders of unequal size placed in a horizontal arrangement.

Numerical investigation on the effect of bionic fish swimming on the vortex-induced vibration of a tandemly arranged circular cylinder

The effect of bionic fish swimming on the vortex-induced vibration (VIV) of a circular cylinder arranged in tandem at a low Reynolds number of 150 is numerically investigated in this work. The bionic fish placed upstream of the cylinder with gap ratios of 1, 3, and 5 and that located downstream of the cylinder with gap ratios of 3 and 5 are examined in the simulations that were carried out in the reduced velocity range of Ur = 2–15. It is found that both the gap ratio and the reduced velocity have a significant influence on the VIV response and wake flow structure. When the bionic fish is placed upstream, the maximum response amplitude of the downstream cylinder is much greater than that of an isolated one. Two flow regimes are identified in terms of the shear layer reattachment, i.e., the continuous reattachment and the alternate reattachment. Comparing the vortex shedding frequencies of the cylinder and the swimming fish, it is found that the frequency of the cylinder is always locked in the fish swimming frequency, and multiple frequencies occur at Ur = 5. When the bionic fish is arranged downstream, four flow regimes are observed, including the extended-body, continuous reattachment, alternate attachment, and co-shedding regimes. Furthermore, the time-mean energy transfer coefficient of the cylinder is considerably higher at Ur = 5 than that when the fish is placed upstream of the cylinder.

Analysis of the wake mechanism in external flow around tandem bluff bodies with different aspect ratios

The interaction mechanism of external flow with two inline rectangular cylinders having different aspect ratios under the impact of gap spacing (G) is the subject of this research. The gap spacing between the cylinders was varied from 0.25 to 20 times their size. Both cylinders were vertically mounted, with the first having a higher aspect ratio than the second. The results revealed five distinct flow patterns under the influence of G: single slender body, shear layer reattachment, intermittent shedding, binary vortex street, and single-row vortex street. The mean pressure on both cylinders was found to vary due to changes in flow patterns. Both cylinders bore the same shedding frequency but had different pressure variations. The second cylinder placed in the wake of first experienced negative average drag force for some spacing values, while the first cylinder had positive average drag values for all chosen G. Due to the change in flow pattern from shear layer reattachment to intermittent shedding flow, the negative drag force on the second cylinder jumped to a positive drag. It was also observed that the rms values of drag and lift force coefficients, as well as their amplitudes for the second cylinder, were mostly higher than corresponding values for the first cylinder at all selected G. This study revealed that G = 4 and 8 are the critical gap spacing values due to sudden changes in fluid force parameters.

Two-degree-of-freedom flow-induced vibrations of a D-section prism

This paper presents a comprehensive study of flow-induced vibrations of a D-section prism with various angles of attack $\alpha$ ( $= 0^{\circ }\unicode{x2013}180^{\circ }$ ) and reduced velocity $U^*$ (= 2–20) via direct numerical simulations at a Reynolds number ${Re} = 100$ . The prism is allowed to vibrate in both streamwise and transverse directions. Based on the characteristics of vibration amplitudes and frequencies, the responses are classified into nine different regimes: typical VIV regime ( $\alpha = 0^{\circ }\unicode{x2013}30^{\circ }$ ), hysteretic VIV regime ( $\alpha = 35^{\circ }\unicode{x2013}45^{\circ }$ ), extended VIV regime ( $\alpha = 50^{\circ }\unicode{x2013}55^{\circ }$ ), first transition response regime ( $\alpha = 60^{\circ }\unicode{x2013}65^{\circ }$ ), dual galloping regime ( $\alpha = 70^{\circ }$ ), combined VIV and galloping regime ( $\alpha = 75^{\circ }\unicode{x2013}80^{\circ }$ ), narrowed VIV regime ( $\alpha = 85^{\circ }\unicode{x2013}145^{\circ }$ ), second transition response regime ( $\alpha = 150^{\circ }\unicode{x2013}160^{\circ }$ ) and transverse-only galloping regime ( ${\alpha = 165^{\circ }\unicode{x2013}180^{\circ }}$ ). In the typical and narrowed VIV regimes, the vibration frequencies linearly increase with increasing $U^*$ . In the hysteretic and extended VIV regimes, the vibration amplitudes are large in a wider range of $U^*$ as a result of the closeness of the vortex shedding frequency to the natural frequency of the prism because of the shear layer reattachment and separation point movement. In the two galloping regimes, the transverse amplitude keeps increasing with $U^*$ while the streamwise amplitude stays small or monotonically increases with increasing $U^*$ . In the combined VIV and galloping regime, the vibration amplitude is relatively small in the VIV region while drastically increasing with increasing $U^*$ in the galloping region. In the transition response regimes, the vibration frequencies are galloping-like but the divergent amplitude cannot persist at high $U^*$ . Furthermore, a wake mode map in the examined parametric space is offered. Particular attention is paid to physical mechanisms for hysteresis, dual galloping and flow intermittency. Finally, we probe the dependence of the responses on Reynolds numbers, mass ratios and degrees of freedom, and analyse the roles of the shear layer reattachment and separation point movement in the appearance of multiple responses.

Flow and heat transfer of supercritical hydrogen in a regenerative cooling channel with the arc ribs of a rocket engine

This paper reports on the heat transfer characteristics and flow performance of supercritical hydrogen in a regenerative cooling channel with arc-shaped ribs. A 3-D numerical model of hydrogen flowing inside the cooling channel has been established and the data are compared with experimental results. The effect of rib parameters such as the number of ribs n, rib angle θ, rib size (hr × e), and rib orientation (convex or concave) on the cooling channel performance has been explored. The results indicate that arc ribs effectively mitigate the heat transfer deterioration and thermal stratification phenomena by reducing the wall temperature; nevertheless, the pressure loss due to friction would considerably increase. In studied cases, the wall temperature significantly decreases when the rib angle θ increases from 0° to 90°. The results have also shown that increasing the size of square arc ribs while maintaining the same rib spacing p/hr and length of the ribbed section Lr is beneficial for the overall heat transfer performance. Compared to smooth channel, a channel with large 90° arc ribs oriented either convex or concave to the flow direction enhanced the thermohydraulic performance η by up to 45%. This enhancement in performance of channel due to arc ribs is 1.95 and 1.384 times higher than the previously known designs; V-ribs (23%) and cylindrical ribs (32.5%), respectively. The flow field has shown that the evolution and morphology of rib-induced streamwise vortex considerably improve the wall temperature reduction by intense fluid mixing. In addition, it has been shown that the direct impact of the coolant stream on the ribs, the reattachment of the separated shear layer, and the intermittent growth of the boundary layer helps to enhance heat transfer.

Numerical and Experimental Study on Flow Field around Slab-Type High-Rise Residential Buildings

High-rise residential buildings often adopt rectangular cross-sections with large depth-to-width ratios. Moreover, the cross-sections have many grooves and chamfers for better ventilation and lighting. However, related research is lacking. This study performed wind tunnel tests and large eddy simulations (LES) on two typical buildings to analyze the surface wind pressures and flow fields around the buildings. The base moment spectra, along with the wind pressure coefficients, demonstrate that numerical simulation is capable of accurately representing the magnitudes and variations in wind loads along the height of the building. Furthermore, numerical simulation effectively captures the dominant energy distribution characteristics of fluctuating wind loads in the frequency domain. The shear layer separations, vortex shedding and reattachment phenomenon were observed. It was found that in the middle and lower parts of the buildings, the shear layer separation changed dramatically. Buildings with depth-to-width ratios close to 2 are minimally affected by changes in wind direction. However, for buildings with larger depth-to-width ratios, especially when the short side faces the wind, the reattachment of the shear layer and the shedding of wake vortices become crucial factors in generating fluctuating cross-wind loads. This emphasizes the significant impact of wind direction and plan dimensions on flow characteristics and aerodynamic behavior. When the building contained corners and grooves, the low-wind-speed area induced by the shear layer separation shrank and the reattachment point shifted closer to the windward facade.

Numerical study of aerodynamic characteristics and particle motion in square cylinder vortex flow with different void ratios

In actual production practice, blunt block structures usually exist in multiple forms simultaneously, such as buildings, bridges, and offshore platforms. Flow around multi-cylinder structures is often accompanied by complex phenomena, including separation and reattachment of shear layers, vortex-structure interactions, and the evolution of wake vortex streets. These phenomena can worsen fatigue damage or local accumulation of harmful substances on bluff block structures. The main purpose of this study is to analyze the complex flow phenomena of square cylinders with different void ratios and the distribution of particulate matter in the flow. The results show that the drag forces applied to the square cylinder under different void conditions will be very different, the flow will also vary greatly, and the location of particle deposition will also change significantly with the change of void ratio conditions.

Effect of Protrusions on the Surface Flow and Pressure Fluctuations in the Inter-stage Region of A Launch Vehicle Model

Experiments were carried out to assess the effects of geometrically scaled and viscous scaled protrusions on the surface flow and unsteady pressures in the inter-stage region of a typical launch vehicle model in the Mach number range 0.8 to 1.6. Formation of bow-waves due to the protrusions, and three-dimensional boundary layer separations and reattachments of the shear layer were seen from flow visualizations. Measurements of unsteady pressures show that protrusions cause substantial increases in fluctuation levels on the core vehicle. The highest levels on fluctuations were found to be at a location behind the viscous scaled protrusion, close to the shear layer reattachment. The spectra of pressure signals suggest that the reattachment may be periodic in the range 0.90 £ M £ 1.60; the associated frequency was found to vary in the range 1 kHz to 3.2 kHz (corresponding to Strouhal number in the range 0.034 to 0.051) depending on M. Streamlining the viscous-scaled protrusion was found to replace bow-waves with multiple oblique shocks, resulting in reduced levels of pressure fluctuations, comparable to the basic levels.

Hypersonic Fluid–Thermal–Structural Interaction of Cone–Slice–Ramp: Computations with Experimental Validation

The paper presents an investigation of fluid–thermal–structural interaction (FTSI) on a complex geometry in hypersonic flows. The objective is to assess high-fidelity numerical tools for FTSI by validating two numerical frameworks against wind-tunnel data. We choose a cone–slice–ramp geometry with a wedge angle of 30 deg subjected to a laminar inflow at Mach 8. First, the time-averaged and the time-resolved laminar flow solutions are validated and highlight the mean flow features and their unsteady characteristics, including low-frequency bubble breathing and shear-layer flapping at a higher frequency. The numerical analysis reveals the presence of upstream and downstream legs of streamwise vortices. Next, FTSI analysis is performed at a tractable computational cost by decomposing the numerical frameworks into a quasi-steady FTSI (QS-FTSI) analysis and a transient fluid–structure interaction (FSI) analysis based on the disparity of the characteristic timescales. The QS-FTSI results highlight the shear-layer reattachment as the dominant source heating the structure, with nonuniform spanwise variations due to the streamwise vortices. The temperature increase drives the decrease in the structural modal frequencies. The FSI analysis was performed for an unheated, undeformed structure and a nominal heated and deformed structure. The average error in the predicted FSI frequency of the unheated panel is 2.65% in comparison to the experimental data; however, this value increases to 7.42% for the heated panel. The frequency peaks are associated with the structural mode shapes via dynamic mode decomposition analysis and show a close resemblance to natural modes, indicating weak FSI coupling. The FTSI study concludes that for a cone–slice–ramp configuration under the present inflow conditions, the interaction mostly occurs in a one-way coupled manner, where the flowfield drives the thermal and structural response of the panel.

Aeroacoustics of turbulent flow over a forward–backward facing step

This paper presents a combined experimental and numerical study on the aeroacoustics of low Mach number turbulent flow over a forward–backward facing step (FBS). The height of the step is fixed at 50% of the incoming boundary layer thickness. Multiple lengths of the step are considered with aspect ratios (step length to height) equal to 1,2,2.4,4, and 8. The Reynolds number based on the step height ranges from 1.2×104 to 2.8×104 in both experiments and simulations. Wall pressure measurement results show that the largest wall pressure fluctuations occur slightly downstream of the step leading edge, and the downstream flow field is influenced by the leading edge flow disturbance. Large-Eddy Simulation (LES) is performed to visualize the instantaneous and time-averaged flow field. Large-scale vortices formed at the leading edge are found to shed downstream directly when the step length is shorter than the shear layer reattachment length, which leads to lower level wall pressure fluctuations on the top surface. Spectral proper orthogonal decomposition (SPOD) is applied to identify the dynamic behaviours of coherent structures downstream of the step leading edge. Using simulation data, the fluctuating wall pressure spectra are also calculated via Poisson’s equation. The results indicate that both turbulence-mean-shear interaction and turbulence–turbulence interaction contribute to the wall pressure fluctuations in the on-step reattachment region. Aeroacoustic beamforming results show that the dominant noise source is located at the step leading edge, where the highest level of fluctuating wall pressure is recorded. Meanwhile, the integrated far-field acoustic spectra show that the levels of FBS noise are lower when there is no flow reattachment occurring on the top surface. A new scaling law for FBS sound spectra is proposed to address the influence of the step aspect ratio on noise production.

The effect of spacing on the flow-induced vibration of one-fixed-one-free tandem cylinders with a rigid splitter plate

The fluid-structure interaction mechanism of an assembly system, including a fixed upstream cylinder and a transverse oscillating downstream cylinder with a rigid splitter plate, is numerically studied at a low Reynolds number Re = 150 and mass ratio m* = 10.0. The effect of different spacing ratios (G/D = 1.5, 3.0, 5.0, and 8.0) and reduced velocities (Ur = 1.0–40.0) on the dynamic responses, hydrodynamic characteristics, and wake patterns of the downstream cylinder-plate (DCP) are analyzed in detail. The results show that, depending on the spacing ratios and the reduced velocities, different response regimes can be identified: (a) For G/D = 1.5 and 3.0, the response of the DCP presents a steady flow at low Ur where the oscillation is completely suppressed. As Ur increases, the response regime shifts to vortex-induced vibration (VIV), which is characterized by a wider lock-in bandwidth and a larger peak vibration amplitude than those of the single cylinder-plate body. (b) For G/D = 5.0 and 8.0, the response regimes consist of VIV and galloping. For the former, the upstream vortex shedding leads to a significant increase in the lift force on the DCP, and its frequency is highly synchronized with the oscillation. By contrast, multiple harmonic components of the lift force are observed in the galloping-dominated Ur range. Two prominent peaks can be identified in the power spectra. The lower peak, produced by the reattachment of the separated shear layers, is the main contributor to the oscillation. The higher peak originating from the upstream vortex shedding frequency is out of phase with the oscillation, and its contribution to the oscillation is very small.

Effect of splitter plate length on FIV of circular cylinder

The influence of the plate length on the flow-induced vibration (FIV) of a circular cylinder with a rigid splitter plate is numerically studied at a low Reynolds number of 100. The mass and damping ratios of the system are respectively m∗=10 and ζ=0. The reduced velocity (Vr) is varied from 2 to 26 and seven different nondimensional splitter plate length (L∗) values in the range of 0–2 are considered. Three different response patterns are identified in the present research, namely vortex-induced vibration (VIV), combined VIV-galloping and weak VIV-galloping. When the system is undergoing VIV, the frequency synchronisation is delayed and the lock-in range is enlarged with increasing L∗. The onset of galloping is postponed with the kink alleviated and the galloping frequency lowering. Abrupt drops in the vortex and total phases are associated with the initiation of galloping. In the galloping branch, both vortex and total forces remain in phase with the displacement. For the added mass and excitation coefficients (Cay and Cey) of the cylinder-plate assembly which have rarely been reported in the literature, Cay decreases as Vr is increased in the VIV range. Nevertheless, it leaps at the onset of galloping and the galloping response is accompanied by positive Cay values. Cey is negative in most cases and its trough appears at beginning of the VIV lock-in range or around the kink in the galloping branch. Due to the large-amplitude and low-frequency nature of the galloping oscillation, three new multi-vortex wake patterns (4P, 5P and 6P) are found to take place. Overall, for a longer splitter plate in the present study, the shear layer reattachment is observed at lower Vr and the galloping oscillation is associated with more vortex pairs.

Dynamic response of a circular cylinder in the presence of a detached splitter plate: On the gap distance sensitivity

Flow-induced vibration of a circular cylinder fitted with a detached rear splitter plate in a laminar flow with Re = 120 is studied numerically. Effects of the gap distance (G/D= 0–2) and reduced velocity (Ur= 2–18) on the vibration response, flow pattern, and hydrodynamic coefficient are investigated in detail. As the gap distance increases, three vibration responses are identified: IB-resonance-galloping at G/D=0, VIV with a wide lock-in region but low frequencies at G/D=0.5 and 1, and typical VIV at G/D=1.5 and 2. The associated phase difference angles and the added mass coefficients for the VIV responses at G/D= 0.5–2 show a jump from 0° to 180° and a transition from positive to negative, respectively, while they always keep being zero and positive for the IB-resonance-galloping response at G/D=0. At G/D= 0–1, the shear layers from the cylinder surface cannot roll up to form complete vortices in the cylinder wake but reattach to the splitter plate’s surface or rear tip, leading to the overshoot (OS) and shear-layer reattachment (SR) flow pattern, and meanwhile the elongated vortex formation length and delayed vortex shedding. As a consequence, the optimal drag- and lift-reductions are achieved at G/D=1. In contrast, complete vortices can be found both behind the cylinder and plate for the cases of G/D= 1.5–2, resulting in the co-shedding (CS) flow pattern, including CS-I and CS-II pattern based on whether the vortices from the cylinder and plate are merged or not. The 2S vortex shedding modes are generally observed at present Reynolds number, while the 2P modes are identified in the galloping and lock-in region. The splitter plate positively contributes to the total lift forces when the cylinder-plate body gallops, while it presents a negative effect by counteracting the forces on the plate itself or on the circular cylinder in the VIV region.

Research on flow around two tandem rectangular cylinders

Uniform flow around two identical rectangular cylinders arranged in tandem has been investigated via the lattice Boltzmann method. With a fixed Reynolds number of 100, the effects of side ratio (γ = 1/2, 1/4, 1/8 and 1/16) and spacing ratio (S* = 0.25–12) on the fluid dynamics are investigated. Results indicate that both side ratio and spacing ratio have significant influences on the flow structure, vortex shedding frequency and hydrodynamic force. Several distinct vortex wake modes including the single bluff-body (SBB), shear-layer reattachment (SLR) and secondary vortex formation (SVF) modes are observed. The variations of Strouhal number, drag and lift coefficients versus spacing ratio are similar between all the current side ratios. In the branches of small and large spacing ratios, the fluid dynamics are obviously different and there is a critical spacing ratio, beyond which the relevant mechanical parameters jump suddenly as a result of the flow mode transition. A phenomenon of two-stage drag frequency separation between two cylinders occurs in the region of low side ratio and large spacing ratio, and its formation mechanism has also been analysed on the basis of flow characteristics and frequency spectra of force coefficients.

Cross flow over two heated cylinders in tandem arrangements at subcritical Reynolds number using large eddy simulations

This study analyses the heat transfer and flow characteristics of cross-flow over two heated infinite cylinders in a tandem (in-line) configuration. Non-isothermal Large Eddy Simulations (LES) using the dynamic Smagorinsky model were conducted at a fixed Reynolds number of 3,000 (based on the free stream velocity and the cylinder diameter). A range of cylinder gap ratios (1.0≤L/D≤5.0) was investigated (in increments of 0.25) with two different Prandtl numbers Pr=0.1 and 1.0. Results show that the flow structures vary according to the order of the patterns: (i) Extended body regime: without attachment for low L/D (1.0-1.25) where cylinders behave as a single bluff body with top–bottom vortex shedding, (ii) Shear layer reattachment regime: with reattachment for moderate L/D (1.5-3.75) where the detached shear layer from the upstream cylinder reattaches to the downstream cylinder, and (iii) Co-shedding regime: for high gap ratios (3.75≤L/D≤5.0) a phenomenon called “jumping”, where the two cylinders behave as isolated bluff bodies. Furthermore, it was observed that the average Nusselt number of both cylinders experience a drastic variation at a critical spacing ratio (between 3.75≤L/D≤4.0). For L/D≤3.0, the average Nusselt number of the upstream cylinder was found to be higher than that of the downstream one. However, for spacing ratios L/D>3.0, the average Nusselt number was similar for both cylinders. For the downstream cylinder, the maximum Nusselt number was located at the separation angle and was found to be independent of the spacing ratio.

Effects of flexible films on the aerodynamics of a 5:1 rectangular cylinder

The present study demonstrates the aerodynamics of a 5:1 rectangular cylinder with flexible films perpendicularly mounted on its upper surface at both the upstream and downstream ends in wind angle of attack (AOA) up to 20°. Flow and pressure fields around the cylinder are examined by smoke-wire visualizations and pressure measurements in a wind tunnel, respectively. Experimental results show that periodic flapping-induced vortices (FIVs) chiefly caused by the leading-edge flapping film play important roles in the cylinder flow and pressure field properties. The FIVs have a positive effect on the reattachment of the cylinder leading-edge shear layer. For the leading-edge film, the tip of the flapping film would touch down on the cylinder upper surface due to the large amplitude flapping of the film at fD/U∞ ≈ 0.811 (f is the film flapping frequency, D is the model characterized length, and U∞ is the free oncoming flow velocity). Meanwhile, the periodic FIVs dominate the cylinder flow and aerodynamic properties in the range of AoA = [0°, 8°), thus defined as the wall-flapped mode. With an increase in AoA = [8°, 20°], the tip of the flapping leading-edge film would not touch down owing to the small amplitude flapping of the film. Simultaneously, the periodic FIVs have limited effects on the cylinder flow and aerodynamic properties, thus named as the free-flapping mode. Compared to the without flexible films case, the scale of the FIVs is smaller than that of the impinging leading-edge vortices but in a higher frequency, resulting in a quick dissipation of the cylinder impinging shear layer instability, rapid recovery of the pressure on the cylinder upper surface, and reductions in the cylinder drag coefficient, lift coefficient, moment coefficient, and spanwise correlation. For the trailing-edge film, it flaps at fD/U∞ ≈ 0.650 and has a limited effect on the cylinder flow and aerodynamic properties. Finally, the flexible films can significantly mitigate the static stall of the rectangular cylinder, which may stabilize the elongated bluff bodies.

Flow and Heat Transfer Characteristics of Staggered Cylinders

Abstract Determining how the flow structure affects the heat transfer of a staggered cylindrical array was one of the objectives of this numerical study. The major flow patterns and the corresponding characteristics of heat transfer were investigated, including Shear Layer Reattachment (SLR), Induced Separation (IS), Vortex Pairing and Enveloping (VPE), Vortex Impingement (VI), and Synchronized Vortex Shedding (SVS). As the stagger angle is larger than 35°, the mean Nusselt numbers of array systems are larger than that of a single cylinder. The magnitude of Nu/Cd is suggested as a parameter to measure the efficiency of the heat transfer for an array system.