Showing papers on "Flow separation published in 2019"

Numerical investigation of laminar–turbulent transition in laminar separation bubbles: the effect of free-stream turbulence

Abstract: The role of free-stream turbulence (FST) in the hydrodynamic instability mechanisms and transition to turbulence in laminar separation bubbles (LSBs) was investigated using direct numerical simulations (DNS). Towards this end, a set of highly resolved DNS have been carried out, where isotropic FST fluctuations with intensities from 0.1 % to 3 % are introduced to investigate the relevant physical mechanisms governing the interaction of separation and transition in LSBs. For disturbance-free simulations, i.e. without FST, laminar–turbulent transition involves a Kelvin–Helmholtz (KH) instability of the separated shear layer. For LSBs subjected to FST, vortical FST fluctuations penetrate the approaching attached laminar boundary layer upstream of the separation location and induce slowly growing low-frequency disturbances, so-called Klebanoff (K) modes, which cause a spanwise modulation with a distinct spanwise wavelength. Simultaneously, the FST enhances the initial levels of instability waves with frequencies in the frequency range of the KH instability, but at much smaller amplitude levels compared to the K-modes. Results from the calculations based on the linearized Navier–Stokes equations and comparison with DNS results reveal that the K-mode exhibits exponential growth in the separated shear layer until it reaches a peak amplitude. At the same time, two-dimensional (2D) disturbance waves are also exponentially amplified, in fact at larger growth rate compared to the K-mode, due to the primary (convective) shear-layer instability mechanism until they saturate downstream of the peak amplitude associated with the K-mode. Therefore, based on detailed spectral analysis and modal decompositions for the separation bubbles investigated, the transition process is the result of two different mechanisms: (i) strong amplification of high-frequency (order of the shedding frequency), essentially 2D or weakly oblique fluctuating disturbances and (ii) low-frequency, three-dimensional K-modes caused by FST. Depending on the intensity of the FST, one of these mechanisms would dominate the transition process, or both mechanisms act together and contribute simultaneously. The net effect of these two events is an acceleration of transition for an increased level of FST intensity, which in turn leads to a reduction of the extent of the separation bubble in streamwise and wall-normal directions. The ‘roll-up’ into spanwise large-scale vortical structures resulting from the shear-layer instability, and the eventual breakdown of these structures, strongly contribute to the reattachment process. The spanwise coherence of these ‘rollers’ deteriorates due to the presence of large-amplitude K-modes, thus effectively weakening their strength for high levels of FST intensities ( ).

A viscous damping model for piston mode resonance

Abstract: A viscous damping model is proposed based on a simplified equation of fluid motion in a moonpool or the narrow gap formed by two fixed boxes. The model takes into account the damping induced by both flow separation and wall friction through two damping coefficients, namely, the local and friction loss coefficients. The local loss coefficient is determined through specifically designed physical model tests in this work, and the friction loss coefficient is estimated through an empirical formula found in the literature. The viscous damping model is implemented in the dynamic free-surface boundary condition in the gap of a modified potential flow model. The modified potential flow model is then applied to simulate the wave-induced fluid responses in a narrow gap formed by two fixed boxes and in a moonpool for which experimental data are available. The modified potential flow model with the proposed viscous damping model works well in capturing both the resonant amplitude and frequency under a wide range of damping conditions.

Selective upstream influence on the unsteadiness of a separated turbulent compression ramp flow

Abstract: A study of the shock-wave/boundary-layer interaction induced by a compression ramp was carried out using high-fidelity simulations. The objective was to investigate the influence of upstream disturbances on low-frequency separation unsteadiness. Two computations were performed for a 24° compression ramp at Mach 2.25, one highly resolved case and one reduced-resolution case. The reduced-resolution case was run for an extended duration to capture many cycles of low-frequency unsteadiness. Basic flow characteristics, including statistics on the boundary layer, wall pressure, and skin friction, were computed. Frequency spectra were calculated to confirm the presence of low-frequency unsteadiness. The influence of upstream disturbances on large-scale separation unsteadiness was investigated using correlations, filtering, and conditional averaging based on the position of the primary separation shock. Low-frequency unsteadiness was found to be related to structures near the wall (y/δ < 0.5) with a time scale greater than 20δ/U∞, whereas higher frequency separation motion could be attributed to turbulent boundary layer structures with a time scale on the order of δ/U∞. The finding that the separation region responds selectively to certain large-scale, near-wall perturbations in the incoming flow supports a model of separation unsteadiness in which external forcing by certain components of boundary layer turbulence drives a weakly damped global mode of the separation bubble. This contrasts with suggestions that have been made in the literature that the separation region oscillates on its own, as in an amplified global mode.

Quantification of added-mass effects using particle image velocimetry data for a translating and rotating flat plate

Abstract: Added mass characterises the additional force required to accelerate a body when immersed in an ideal fluid. It originates from an asymmetric change to the surrounding pressure field so the fluid velocity satisfies the no-through-flow condition. This is intrinsically linked with the production of boundary vorticity. A body in potential flow may be represented by an inviscid vortex sheet and added-mass forces determined using impulse methods. However, most fluids are not inviscid. It has been theorised that viscosity causes the ‘added-mass vorticity’ to form in an intensely concentrated boundary layer region, equivalent to the inviscid distribution. Experimentally this is difficult to confirm due to limited measurement resolution and the presence of additional boundary layer vorticity, some the result of induced velocities from free vorticity in the flow field. The aim of this paper is to propose a methodology to isolate the added-mass vorticity experimentally with particle image velocimetry, and confirm that it agrees with potential flow theory even in separated flows. Experiments on a flat-plate wing undergoing linear and angular acceleration show close agreement between the theoretical and measured added-mass vorticity distributions. This is demonstrated to be independent of changes to flow topology due to flow separation. Flow field impulse and net force are also consistent with theory. This paper provides missing experimental evidence coupling added mass and the production of boundary layer vorticity, as well as confirmation that inviscid unsteady flow theory describes the added-mass effect correctly even in well-developed viscous flows.

Study on flow separation and transition of the airfoil in low Reynolds number

Abstract: As typical flow characteristics in a low Reynolds number, laminar separation bubbles (LSBs) and transition to turbulence over airfoils have been extensively studied in recent years. In order to analyze their flow mechanism, numerical investigation using the finite volume method to solve the Reynolds averaged Navier-Stokes equations with a transition Shear Stress Transport (SST) four-equation transition model is performed in this work, combined with the experimental study facilitated by the oil film interferometry technique. Specifically, the transition SST four-equation transition model is solved to simulate the separation location and LSB structure at low Reynolds numbers on a Wortmann FX63-137 airfoil. Good agreement is obtained between the numerical simulation and experimental measurements regarding the separation, transition and reattachment location, aerodynamic coefficients, and overall flow structures. At higher Reynolds numbers of 200 000 and 300 000, similar bubble structures on the airfoil surface are observed, and the location of the bubble moves toward the leading edge of the airfoil by increasing the angle of attack. However, in Reynolds numbers ranging from 300 000 to 500 000, significant changes of the laminar flow separation structures emerge. The flow structure changes from the classical laminar separation bubble to the nonclassical separation flow structure that is composed of a major vortex 1(V1) and a minor vortex 2(V2). Due to the small distance between V1 and V2, it is difficult to distinguish the delicate structure of the two separation bubbles from the classical laminar separation bubble by the experimental method.

Role of magnetic field on forced convection of nanofluid in a branching channel

Abstract: Purpose Numerical study of nanofluid forced convection within a branching channel was performed under the influence of a uniform magnetic field. The purpose of this study is to enhance the heat transfer performance of the separated flow at the branching channel with the use of magnetic field and nanofluid. The use of magnetic field and enhancement in both the thermal conductivity and electrical conductivity with the inclusion of the nanoparticles provides favorable thermophysical properties of the nanofluid when it used as a heat transfer fluid in a branching channel. The results of this study may be used to control the thermal performance in a branching channel and further optimization studies in the presence of magnetic field. Design/methodology/approach Galerkin weighted residual finite element method was used for the simulations. The numerical simulation results are performed by changing the inclination angle of the lower branching channel (between 0° and 90°), thermophysical properties of the fluid via inclusion of nanoparticles (between 0 and 0.04), Reynolds number (between 100 and 400) and magnetic field strength (Hartmann number changes between 0 and 15). Findings It was observed that the recirculation zones and reattachment length of the upper and lower branching channels are affected by the variation of those parameters. Reattachment lengths increase with the augmentation of the Reynolds number and deterioration of the Hartmann number. Average Nusselt number becomes higher for higher values of Hartmann number and solid particle volume fraction. Inclusion of the nanoparticle to the base fluid is very effective for the configuration with higher values of Hartmann number. An optimum value of the inclination angle of the lower branching channel is observed, beyond which heat transfer rate is significantly reduced due to the establishment of a large vortex in the upper branching channel and restriction of the fluid motion. Originality/value In this study, forced convection of nanofluid flow in a branching channel under the effect of magnetic field was numerically studied. Magnetic field effects with nanoparticle inclusion to the base fluid on the convective heat transfer was analyzed for various inclination angles of the lower branching channel. Flow separation at the junction of the channels and thus convective heat transfer rate are influenced by the variation of these parameters. There are many studies related to application of the magnetic field with nanofluids, and a few of them are related to configurations with separated flows. To the best of the authors’ knowledge, there exist no studies for the application of nanofluids and magnetic field for the convective heat transfer in a branching channel. This topic is of importance as there are many engineering applications of the branching channels.

Numerical study of wake and aerodynamic forces on two tandem circular cylinders at Re = 103

Abstract: Three-dimensional large eddy simulations were carried out to investigate the flow around two tandem circular cylinders at a subcritical Reynolds number of Re = 103. The cylinder center-to-center spacing ratio L/D is varied from 1.25 to 6, where D is the cylinder diameter. In order to enhance the understanding of flow physics around two circular cylinders, particular attention is devoted to fluctuating forces, shear-layer reattachment, flow separation, wake recirculation, Strouhal number (St), and phase lag (ϕ) between the fluctuating lift of the two cylinders. The flow structure around the cylinders is highly sensitive to L/D. A change in L/D thus leads to overshoot flow (L/D ≤ 1.25), reattachment flow (1.5 ≤ L/D ≤ 3.5), and coshedding flow (L/D ≥ 4). The boundaries are characterized by drastic changes in the flow structure and a discontinuous drop/rise in St and forces. The St drops at the boundary between overshoot and reattachment flow regimes and jumps at the boundary between reattachment and coshedding flow regimes, while fluctuating forces and ϕ both jump at both boundaries. The flow separation on the downstream cylinder is much delayed (122°–128°) in the reattachment flow regime compared to that on the single cylinder (95°) or upstream cylinder (92°–95.5°). The fluctuating pressure on the entire surface of either cylinder is low for the overshoot flow because the two cylinders are enclosed by the upstream-cylinder-generated shear layers having the longest wake recirculation. The ϕ is almost zero in the overshoot flow. With increasing L/D, ϕ linearly increases in the reattachment and coshedding regimes with different gradients, larger in the latter regime than in the former, by nearly twice.

On the unsteady characteristics of turbulent separations over a forward-backward-facing step

Abstract: Turbulent separation bubbles over and behind a two-dimensional forward–backward-facing step submerged in a deep turbulent boundary layer are investigated using a time-resolved particle image velocimetry. The Reynolds number based on the step height and free-stream velocity is 12 300, and the ratio of the streamwise length to the height of the step is 2.36. The upstream turbulent boundary layer thickness is 4.8 times the step height to ensure a strong interaction of the upstream turbulence structures with the separated shear layers over and behind the step. The velocity measurements were performed in streamwise–vertical planes at the channel mid-span and streamwise–spanwise planes at various vertical distances from the wall. The unsteady characteristics of the separation bubbles and their associated turbulence structures are studied using a variety of techniques including linear stochastic estimation, proper orthogonal decomposition and variable-interval time averaging. The results indicate that the low-frequency flapping motion of the separation bubble over the step is induced by the oncoming large-scale alternating low- and high-velocity streaky structures. Dual separation bubbles appear periodically over the step at a higher frequency than the flapping motion, and are attributed to the inherent instability in the rear part of the mean separation bubble. The separation bubble behind the step exhibits a flapping motion at the same frequency as the separation bubble over the step, but with a distinct phase delay. At instances when an enlarged separation bubble is formed in front of the step, a pair of vertical counter-rotating vortices is formed in the immediate vicinity of the leading edge.

Numerical investigation of the effect of airfoil thickness on onset of dynamic stall

Abstract: Effect of airfoil thickness on onset of dynamic stall is investigated using large eddy simulations at chord-based Reynolds number of 200 000. Four symmetric NACA airfoils of thickness-to-chord ratios of 9 %, 12 %, 15 % and 18 % are studied. The three-dimensional Navier–Stokes solver, FDL3DI is used with a sixth-order compact finite difference scheme for spatial discretization, second-order implicit time integration and discriminating filters to remove unresolved wavenumbers. A constant-rate pitch-up manoeuver is studied with the pitching axis located at the airfoil quarter chord. Simulations are performed in two steps. In the first step, the airfoil is kept static at a prescribed angle of attack (). In the second step, a ramp function is used to smoothly increase the pitch rate from zero to the selected value and then the pitch rate is held constant until the angle of attack goes past the lift-stall point. The solver is verified against experiments for flow over a static NACA 0012 airfoil. Static simulation results of all airfoil geometries are also compared against XFOIL predictions with a generally favourable agreement. FDL3DI predicts two-stage transition for thin airfoils (9 % and 12 %), which is not observed in the XFOIL results. The dynamic simulations show that the onset of dynamic stall is marked by the bursting of the laminar separation bubble (LSB) in all the cases. However, for the thickest airfoil tested, the reverse flow region spreads over most of the airfoil and reaches the LSB location immediately before the LSB bursts and dynamic stall begins, suggesting that the stall could be triggered by the separated turbulent boundary layer. The results suggest that the boundary between different classifications of dynamic stall, particularly leading edge stall versus trailing edge stall, is blurred. The dynamic-stall onset mechanism changes gradually from one to the other with a gradual change in some parameters, in this case, airfoil thickness.

Flow structure around and heat transfer from cylinders modified from square to circular

Abstract: This work aims at numerically investigating the influence of corner modification on the flow structure around and heat transfer from a square cylinder at a Reynolds number Re = 150 based on the cylinder width d and freestream velocity. The sharp corners of the square cylinder are rounded with r/d = 0 (square), 0.125, 0.25, 0.375, and 0.5 (circular), where r is the radius of the corner. The rounded corners have a profound effect on the flow structure from the perspective of flow separation, vortex strength, separation bubble, and wake bubble each playing a role in heat transfer from different surfaces of the cylinder. The boundary layer having a higher friction coefficient on the front and side surfaces leads to a higher local heat transfer. A shorter wake bubble renders a higher heat transfer from the rear surface. The increase in r/d from 0 to 0.5 leads to a 33% enhancement in the heat transfer from the cylinder. The enhancement largely results from a shrink in the wake bubble and an increase in vortex strength. The minimum time-mean drag and fluctuating forces are achieved at r/d = 0.25 and 0.125, respectively. The effect of r/d in various Reynolds averaged quantities is discussed.

Uncertainty Estimation Module for Turbulence Model Predictions in SU2

Abstract: With the advent of improved computational resources, aerospace design has shifted from a testing-based process to a simulation-driven procedure, wherein uncertainties in design and operating condit...

Experimental Studies on the Effect of Leading-Edge Tubercles on Laminar Separation Bubble

Abstract: The effect of wavy leading ledge, in the manner seen in humpback whales, known as tubercles, to the leading edge of a NACA 4415 airfoil is investigated experimentally at low Reynolds number of 120,000. Comparisons are carried out against a baseline NACA-4415 airfoil is addressed through surface flow visualization, surface pressure and particle image velocimetry (2D-PIV) measurements. Experiments were carried out at angles of attack of 6° and 18° corresponding to pre-stall and post-stall conditions of the baseline airfoil with free stream velocity of 7.5 m/s. The oil-flow visualization studies reveal interesting complex flow features, repeating behind every tubercle at both angles of attack. At the lower angle of attack the extent of the laminar separation bubble, which is a dominant feature on the baseline airfoil, is significantly altered by the presence of the tubercles, which result in the formation of smaller separation bubbles spread along the span instead of one single bubble. The 2-D PIV and oil flow results angle of attack of 18° show that tubercles are very much effective beyond the stall conditions of the base line airfoil. They maintain attached flow till 50% of the cord at least in regions behind the tubercles, instead of complete separation at the leading edge as noticed for the baseline case. The size of recirculating zone downstream of the airfoil post-stall is also significantly reduced by the tubercles. All these factors contribute to the increased performance of the tubercles compared to the baseline NACA 4415 airfoil. However, post-stall the use of tubercles seems to impose a highly varying separation point and subsequently the associated wake width on the airfoil, the effects of which are yet to be explored in detail.

Effects of nozzle-exit boundary-layer profile on the initial shear-layer instability, flow field and noise of subsonic jets

Abstract: The influence of the nozzle-exit boundary-layer profile on high-subsonic jets is investigated by performing compressible large-eddy simulations (LES) for three isothermal jets at a Mach number of 0.9 and a diameter-based Reynolds number of 5 × 10 4 , and by conducting linear stability analyses from the mean-flow fields. At the exit section of a pipe nozzle, the jets exhibit boundary layers of momentum thickness of approximately 2.8 % of the nozzle radius and a peak value of turbulence intensity of 6 %. The boundary-layer shape factors, however, vary and are equal to 2.29, 1.96 and 1.71. The LES flow and sound fields differ significantly between the first jet with a laminar mean exit velocity profile and the two others with transitional profiles. They are close to each other in these two cases, suggesting that similar results would also be obtained for a jet with a turbulent profile. For the two jets with non-laminar profiles, the instability waves in the near-nozzle region emerge at higher frequencies, the mixing layers spread more slowly and contain weaker low-frequency velocity fluctuations and the noise levels in the acoustic field are lower by 2-3 dB compared to the laminar case. These trends can be explained by the linear stability analyses. For the laminar boundary-layer profile, the initial shear-layer instability waves are most strongly amplified at a momentum-thickness-based Strouhal number St θ = 0.018, which is very similar to the value obtained downstream in the mixing-layer velocity profiles. For the transitional profiles, on the contrary, they predominantly grow at higher Strouhal numbers, around St θ = 0.026 and 0.032, respectively. As a consequence, the instability waves rapidly vanish during the boundary-layer/shear-layer transition in the latter cases, but continue to grow over a large distance from the nozzle in the former case, leading to persistent large-scale coherent structures in the mixing layers for the jet with a laminar exit velocity profile.

Passive separation control of a NACA0012 airfoil via a flexible flap

Abstract: The incorporation of nature-inspired techniques to control or reduce boundary layer separation, to bring about performance enhancements on air/water vehicles, has been an active research area for many years. In this paper, a baseline NACA0012 airfoil is modified using a short flap on its upper surface at a Reynolds number of Re = 1000. The impact of the flap configuration—described by length, attachment position, deployment angle, and material properties, on the aerodynamic performance of the airfoil—quantified by mean and fluctuating forces, is investigated, and the flow field is analyzed. Inspired by the observation of pop-up feathers on a bird’s wing, the flap is first set to be rigid for a range of location, size, and inclination angles. After the optimal location of a rigid flap has been established, the flap is then allowed to be flexible, its motion is coupled to the encircling flow field, and it is tested for a range of mass ratios and bending stiffness values. The fluid motion is obtained by solving the lattice Boltzmann equation, while the dynamics of the flexible flap are calculated using the finite element method and the coupling between the flow and flap handled by the immersed boundary method. For the flexible flap, two flapping patterns are observed and the mechanism of separation control via rigid/flexible flap is explained. Compared to the flapless NACA0012 airfoil case, in the case with a flap of optimal configuration, the mean lift coefficient is improved by 13.51%, the mean drag coefficient is decreased by 3.67%, the mean lift-drag ratio is improved by 17.84%, the maximum lift fluctuation is decreased by 40.90%, and the maximum drag fluctuation is decreased by 56.90%.

Wall-modelled large-eddy simulation of turbulent flow past airfoils

Abstract: We present large-eddy simulation (LES) of flow past different airfoils with , near the turbulent separation, the skin-friction lines show small-scale reversal flows that are similar to those observed in DNS of the flat plate turbulent separation. A notable feature of turbulent separation in flow past an airfoil is the appearance of turbulence structures and small-scale reversal flows in the spanwise direction due to the vortex shedding behaviour.

Control effect of micro vortex generators on leading edge of attached cavitation

Abstract: In this study, we investigate the effects of micro vortex generators (VGs) installed close to the leading edge of a quasi-two-dimensional NACA0015 hydrofoil under cavitating and non-cavitating conditions. Our aim is to improve physical insight into interaction mechanisms of the boundary layer with the formation and stability of partial cavities. Under non-cavitating conditions, the proposed micro VGs effectively suppress laminar separation. However, under cavitating conditions, even very small micro VGs within the boundary layer promote the formation of counter-rotating cavitating vortices. In comparison with the smooth hydrofoil surface (without micro VGs), the cavitation onset is shifted toward the leading edge. Additionally, classical “fingering structures” and Tollmien–Schlichting waves are no longer present. Since the onset of the cavity does no longer appear at (or close to) the laminar separation line, a novel onset mechanism is observed experimentally. The mechanism consists of stable vortex cavitation, followed by vortex break-down into bubbly structures that are finally accumulated into an attached cavity region. By reduction in the height of the micro VGs, a delayed vortex break-down is found, leading to an increase in the length of the cavitating vortex pattern. This allows for enhanced control on the cavity dynamics, especially with respect to the penetration depth of the re-entrant jet. As a result of our investigation, we conclude that well suited micro VGs show a high potential to manipulate and control cavity dynamics.