Boundary Layer Reattachment Research Articles

Effect of gap width on flow structure and reattaching characteristic of two tandem 5:1 rectangular cylinders: Experimental and numerical studies

The flow structure and reattaching characteristic of two tandem rectangular cylinders with aspect ratio being 5:1 have been investigated through wind tunnel experiments and three-dimensional large eddy simulation (LES) methods. The gap width G between the two cylinders varies from 2 times of D to 20 times of D, where D represents the depth of the two cylinders. The surface pressure distribution and aerodynamic forces of each cylinder are obtained via wind tunnel experiments. Two distinct flow patterns are identified with the increasing G through three-dimensional LES methods, and the aerodynamic results are presented in good agreement with the experiments as well. The experimental and numerical results indicate that the flow structure is highly sensitive to the variation in G, leading to alterations in the aerodynamics and vortex-shedding characteristic of two cylinders. Furthermore, the simulation results also capture the shift in the reattaching points as G increases. Additionally, following the simulation findings, a proposed criterion based on the wind tunnel experimental data is presented for predicting the boundary layer reattachment points on two tandem 5:1 rectangular cylinders.

Experimental investigation of non-synchronous vibration and its mechanism analysis by using a cantilever beam-like compressor cascade

With the increasement of unsteady load and the extensive utilization of light-weight material, compressor blade is encountering increasingly serious flow induced vibration problems. Among which, non-synchronous vibration (NSV) has received special attention due to its complex generation mechanism and significant impact on high cycle fatigue. To better understand the physical mechanism causing NSV in compressor, this study designed a cantilever beam-like compressor cascade and made a detailed investigation about its flow separation induced vibration by means of experimental and numerical methods. The experiments were conducted at 9 engine representative conditions and the data of both on-blade pressure and structural response were measured simultaneously. Results show that, the effect of Ma number on the dynamic response will increase with the increasement of incidence angle and there is a strong nonlinearity. The effect of incidence angle on the dynamic response shows obvious bifurcation phenomenon. The results of large eddy simulation (LES) further reveal that the obvious characteristics of the dynamic response are caused by the laminar separation bubble (LSB) of suction surface, and the periodic separation and reattachment of boundary layer provides initial excitation sources for NSV. The unsteady excitation acts on blades in the form of energy and then forces the blades to vibrate with low order modes. The vibration amplitudes are closely related to the separation points and increase as the separation points move forward.

Bi-global stability of supersonic backward-facing step flow

Supersonic backward-facing step (BFS) flow is numerically studied using direct numerical simulation (DNS) and global stability analysis (GSA) with a free stream Mach number of 2.16 and a Reynolds number of 7.938 × 105 based on the flat-plate length L and free stream conditions. Two-dimensional BFS flow becomes unstable to three-dimensional perturbations as the step height h exceeds a certain value, while no two-dimensionally unstable mode is found. Global instability occurs with the fragmentation of the primary separation vortex downstream of the step. Two stationary modes and one oscillatory unstable mode are obtained at a supercritical ratio of L/h = 32.14, among which the two stationary modes originate from the coalescence of a pair of conjugate modes. The most unstable mode manifests itself as streamwise streaks in the reattached boundary layer, which is similar to that in shock-induced separated flow, although the flow separation mechanisms are different. Without introducing any external disturbances, the DNS captures the preferred perturbations and produces a growth rate in agreement with the GSA prediction in the linear growth stage. In the quasi-steady stage, the secondary separation vortex breaks up into several small bubbles, and the number of streamwise streaks is doubled. A low-frequency unsteadiness that may be associated with the oscillatory mode is also present.

Water tunnel testing of downwind yacht sails

Downwind yacht sails, such as spinnakers, are low-aspect-ratio highly cambered wings with a sharp leading edge. They are characterised by substantial three-dimensional flow separation and are thus modelled with difficulty with numerical simulations. Furthermore, accurate full-scale validation data are not available. The first quantitative flow measurements have only recently been achieved in water tunnels. In this study, we aim to provide guidelines on this emerging sail testing methodology. We consider six model-scale rigid models at average-chord-based Reynolds numbers ranging from 5 870 to 61 870. A critical Reynolds number is identified, below which relaminarisation of the reattached boundary layer downstream of the leading edge separation bubble occurs. Both lift and drag increase monotonically at subcritical Reynolds numbers while remaining about constant at transcritical Reynolds numbers. The critical Reynolds number decreases with increasing incidence and is insensitive to the blockage ratio. Spinnakers are normally sailed in front of the mainsail, whose circulation is found to generate an approximately 3∘\\documentclass[12pt]{minimal} \\usepackage{amsmath} \\usepackage{wasysym} \\usepackage{amsfonts} \\usepackage{amssymb} \\usepackage{amsbsy} \\usepackage{mathrsfs} \\usepackage{upgreek} \\setlength{\\oddsidemargin}{-69pt} \\begin{document}$$3^{\\circ }$$\\end{document} upwash on the spinnaker and higher flow velocity on both sides of it. These findings provide guidelines for the experimental testing of spinnaker-like wings in water tunnels and provide new insights into the flow and experimental testing of highly cambered wings with massive flow separation at low Reynolds numbers.

Study of the influence of turbulent flows on the aerodynamic performance of fixed-wing aircraft type UAV MALE

A study of the air behavior in the near-wake region of the NACA0012 airfoil infinite wing model at low Reynolds numbers was conducted. The study consisted of comparing the results obtained from experiments with those from numerical simulations. The numerical results of the velocity profiles in the near-wake region showed good agreement with those provided by the PIV system for most of the simulations and those in the literature. In addition, boundary layer separation and reattachment forming a laminar separation bubble along the airfoil was captured using simulations in Ansys Fluent and compared with a literature database to better confirm the simulation results. The UAVs have a complex cross-section shape of the fuselage. The second part of this paper explores the aerodynamic consequences of its shape, at the operational flight ceiling, in a comparative approach for three different fuselage designs.

The Effects of Mainstream Reynolds Number and Blowing Ratio on Film Cooling of Gas Turbine Vanes

Film cooling performance was evaluated numerically for three mainstream Reynolds numbers and four blowing ratios (BR). A computational model based on finite volume discretization was used to solve an incompressible and transient flow over a NACA 4412 cascade vane. Several passive scalars were included in the model to evaluate the condition of adiabatic temperature and constant temperature for the surface vane. For the adiabatic temperature condition, the film effectiveness mainly depends on the jet trajectory and recirculation zones. For the constant temperature condition, the net heat flux reduction (NHFR) varies according to the boundary layer separation and reattachment. Consequently, misleading conclusions could be drawn if only one of the two approaches is adopted. For instance, the mainstream Reynolds number Re∞ = 3615 reached a maximum average effectiveness lower than 0.3 with an average NHFR of 0.25. However, for Re∞ = 10,845 the maximum average effectiveness was close to 0.45, but with negative average NHFR values. This finding demonstrates the need to explore new indicators like jet trajectory, convective coefficient and skin friction coefficient, as presented in this paper.

Numerical optimisation of the active lift turbines using OpenFoam's overset method

This work presents a preliminary study of the numerical optimization of "active lift turbine" using computational fluid dynamics (CFD). This active lift turbine is designed to improve efficiency of vertical axis energy recovery turbines (wind and tidal turbines). It uses a crank rod system to convert normal forces into momentum in order to improve the yield.
 After a validation work of CFD model in two dimensions, the numerical optimization of the tidal turbine system is carried out. In fact, due to the specific characteristics of the active lift turbine, the blades do not follow a circular trajectory (Fig. \ref{fig:combined}). The OpenFoam overset module is therefore chosen because it allows to control the motion of the blades easily \cite{Chalmers}. The Overset method allows a good performance evaluation and limits the risk of numerical divergence that could be caused by mesh deformation methods; Moreover it limits the computation time compared to remeshing methods. However, it is more expensive than a more traditional Arbitrary Mesh Interface method \cite{Openfoam}, which is not suitable for the active lift turbine simulations.
 The impact of several parameters on the performances is studied. The simulations are performed for a range of operating conditions including different inflow velocities, angles of attack and active flow turbine settings (like the amplitude of the radius variation, see Fig \ref{fig:combined}). The results are evaluated in terms of the turbine power output and efficiency.
 The results show the importance of numerical simulation for the development of new types of energy recovery systems. It allows a better understanding of the interactions between the turbine and the fluid, and of the operation of such systems.
 Further studies are required for the active lift turbine: conventional attempts at optimisation are not perfectly suited to this system. Improving the lift/drag ratio makes less sense when the normal forces should also be taken into account to improves the performances. Furthermore, other aspects of operation should be analysed: variation of the turbulence rate, implementation of boundary layer re-attachment systems...

Instability and transition onset downstream of a laminar separation bubble at Mach 6

Instability measurements of an axisymmetric, laminar separation bubble were made over a sharp cone-cylinder-flare with a$12^{\circ }$flare angle under hypersonic quiet flow. Two distinct instabilities were identified: Mack's second mode (which peaked between 190 and 290 kHz) and the shear-layer instability in the same frequency band as Mack's first mode (observed between 50 and 150 kHz). Both instabilities were measured with surface pressure sensors and were captured with high-speed schlieren. Linear stability analysis results agreed well with these measured instabilities in terms of both peak frequencies and amplification rates. Lower-frequency fluctuations were also noted in the schlieren data. Bicoherence analysis revealed nonlinear phase-locking between the shear-layer and second-mode instabilities. For the first time in axisymmetric, low-disturbance flow, naturally generated intermittent turbulent spots were observed in the reattached boundary layer. These spots appeared to evolve from shear-layer-instability wave packets convecting downstream. This work presents novel experimental evidence of the hypersonic shear-layer instability contributing directly to transition onset for an axisymmetric model.

Vortex-induced vibration and hydrodynamic characteristics of a round-ended cylinder

A round-ended section possesses the advantages of both circular and square sections, i.e. a large span and smooth transition at both ends. Therefore, the two-degree-of-freedom VIV response of a round-ended cylinder is numerically investigated in this paper in the reduced velocity (Ur) range of 2–16, comparing with the response of a circular cylinder of the same mass-damping ratio. The numerical results indicate that the development and separation of boundary layers determine the appearance of the minimum pressure coefficient, the vortex shedding mode and the characteristic sizes of wake structure. The reattachment and second separation of boundary layers are observed in the round-ended cylinder when Ur ≥ 6, contributing to a much downstream final separation point as compared to the circular cylinder. Both the hydrodynamic forces and the VIV response are closely associated with the vortex shedding mode, which is further illustrated by the variations of the added mass coefficient, the phase difference and the transferred energy coefficient. The turning points of the curves of hydrodynamic coefficients and response amplitudes correspond to the transition of vortex shedding mode as well as the switching of VIV branch. Compared to the circular cylinder, the oscillation of the round-ended cylinder presents an elongated lower branch and much smaller vibration amplitudes.

Mapping of the flow structure and hydrodynamic properties of a round-ended cylinder

This paper reports the numerical results of flow past a round-ended cylinder with various incidence angles in a low Reynolds number range of Re = 60–160. Mapping of the flow structure and hydrodynamic properties is examined in the incidence angle range of α = 0°–90° with increment of 15°. Three wake patterns are identified, including the steady and symmetric wake without vortex shedding (Pattern I), the Karman vortex street (Pattern II), and the Karman vortex street with the occurrence of subordinate vortex (Pattern III). The reattachment of boundary layers results in the occurrence of a subordinate vortex and hence the non-single-frequency fluctuation of hydrodynamic coefficients (CL and CD, which are lift and drag coefficients, respectively). Elliptical and figure-eight CL–CD curves are observed, depending on the frequency ratios of the two coefficients and their weights. Non-zero time-averaged CL occurs when 0° < α < 90°, due to the asymmetric boundary layer separation. The backward migration of boundary layer separation point contributes to the reduction of frictional drag and the vortex formation length. The shortening of the vortex formation length results in the enhanced fluctuations of CL and CD.

On the effects of surface waviness upon catalytic steam reforming of methane in micro-structured reactors - a computational study

The effects of wavy channels upon steam methane reforming in catalytic micro-structured reactors, using Nickle as the catalyst, are investigated numerically. Laminar, three-dimensional models of reacting flows, with heterogeneous chemistry, are developed and applied to micro-structured reactors with different wave patterns on their internal walls. It is observed that introduction of surface waves on the walls of reactor, can significantly alter production and consumption of H2 and CH4. This is shown to be due to large fluctuations in convective mass and heat transfer, induced by the surface waviness. In particular, it is shown that separation and reattachment of concentration and thermal boundary layers and the subsequent modifications in Sherwood and Nusselt number can significantly affect the catalytic processes over the walls of reactor. A major local intensification of catalytic processes is observed where Sherwood and Nusselt number are maximised. Similarly, the catalytic process becomes highly inefficient in places with minimal Sherwood (and Nusselt) number. The analyses further reveal that a strategic discrete coating of wavy walls with the catalyst can substantially improve the performance of microstructure. For example, by coating only 25% of the surface area of wavy walls, CH4 conversion rate and selectivity per coated surface area increase by 459% and 308% compared to those of an equivalent fully coated straight channel. This implies the possibility of developing highly efficient and cost-effective catalytic micro-reactors for production of hydrogen from methane.

Flexible skin for measurement of boundary layer state and flight attitude identification on UAV

The wall shear stress and pressure are important for analyzing boundary layer flow and evaluating the aerodynamic performance of the aircraft. In this study, a flexible skin consisting of dual layer hot-film sensors and pressure belts was developed to measure the distribution of wall shear stress and pressure on an unmanned aerial vehicle during increase of angle of attack (AOA), tail spin, and decreases of AOA. The sensitivity of the dual layer hot-film sensor is improved by about 150% due to heat conduction reduced. The relative error of pressure belt is less than 1% at 10 °C–65 °C. The boundary layer separation and reattachment time, separation AOA and time-shift of the flow field changes in flight conditions are determined. The separated AOA of the left and right wing boundary layers are 26.41° and 17.58° respectively. There is a delay about 4 s between the separation of the boundary layer and the entry of the tail spin, which can provide early warning to prevent abnormal flight conditions such as post stall and tail spin.

Experimental analysis of the impact of an acoustic excitation on a separated boundary layer behaviour

The paper examines the effect of acoustic excitation at broadband frequency range (100 - 650 Hz) on separated boundary layer developing on a flat plate subjected to an adverse pressure gradient. The experiment was conducted with a low inlet turbulence intensity level (Tu < 1%) in order to provide a cleaner environment that magnifies the effects of the excitation frequency. It was shown that acoustic excitation at sound pressure levels: SPL=125 and 135 dB, can lead to a more rapid increase in flow instability followed by an earlier laminar-turbulent (l-t) transition. The boundary layer reattachment point was shifted upstream and the size of the separation bubble was reduced in the flow with the acoustic excitation active.

Numerical simulation of dynamic stall flow control using a multi-dielectric barrier discharge plasma actuation strategy

To alleviate the deterioration in wind turbine performance caused by dynamic stall, the flow control of a pitching NACA0012 airfoil is investigated through numerical simulation of an alternating current dielectric barrier discharge (AC-DBD) plasma actuator at a Reynolds number Re = 135 000. To avoid the harmonic oscillations of aerodynamic force caused by unsteady DBD actuation, this work focuses on improving the control potential for steady actuation. The control mechanisms of actuators at various positions are investigated using five groups of actuators mounted at 0%, 3%, 10%, 45%, and 80% chord lengths c above the upper surface of an airfoil. The actuator at 80%c performs more efficiently in terms of lift enhancement in the initial upstroke and the final downstroke. The actuator at 0%c suppresses the growth of the leading-edge vortex and maintains the suction of the dynamic stall vortex (DSV). After the shedding of the DSV, it suppresses the secondary separation to delay the onset of dynamic stall. At the flow reattachment stage, the actuators at 3%c and 10%c accelerate the boundary layer reattachment by momentum injection. From these results, a multi-DBD control strategy is proposed. The scheme selects the optimal actuator in operation at a certain stage of dynamic stall and takes advantage of actuators at different positions to enhance the average and maximum aerodynamic force, delay the onset of dynamic stall, accelerate flow reattachment, and avoid excessive energy consumption.

Unsteady Aerodynamics Around a Pitching Airfoil with Shock and Shock-Induced Boundary-Layer Separation

This study numerically investigated the characteristics of unsteady aerodynamic forces around a pitching airfoil in the transonic flow regime. The unsteady Reynolds-averaged compressible Navier–Stokes (URANS) equations were solved using a Spalart–Allmaras (SA) turbulence model. The pitching NACA64A010 airfoil at mean angles of attack of 0, 2, 4, and 6 deg with the amplitude of 1 deg and reduced frequency of 0.2 was parametrically simulated. The study found that the trend of the unsteady aerodynamic characteristics around the pitching airfoil, particularly that of the unsteady lift force, significantly changes with the mean-angle-of-attack conditions. The shock-induced boundary-layer separation and reattachment during the pitching oscillation have a crucial role in determining the trend of the unsteady aerodynamic forces, such as the phase-delayed or phase-advanced features for the variation in the angle of attack. The boundary-layer separation during the oscillation, observed at a high mean angle of attack of 6 deg, produces the phase-advanced feature of the lift force. Because the phase-advanced feature is associated with a positive damping effect, the result indicates that the boundary-layer separation may serve as a stabilizing effect on the pitching airfoil motion. The boundary-layer reattachment from separation during the airfoil oscillation, observed at an intermediate mean angle of attack of 4 deg, generates an unusual shock wave movement, resulting in a unique lift loop for the variation in the angle of attack. The study also demonstrated a good prediction accuracy of the URANS with the SA model for the unsteady aerodynamics around the pitching airfoil with the shock and shock-induced boundary-layer separation.

Development of Explainable Data-Driven Turbulence Models with Application to Liquid Fuel Nuclear Reactors

Liquid fuel nuclear reactors offer innovative possibilities in terms of nuclear reactor designs and passive safety systems. Molten Salts Reactors (MSRs) with a fast spectrum are a particular type of these reactors using liquid fuel. MSFRs often involve large open cavities in their core in which the liquid fuel circulates at a high speed to transport the heat generated by the nuclear reactions into the heat exchangers. This high-speed flow yields a turbulent field with large Reynolds numbers in the reactor core. Since the nuclear power, the neutron precursor’s transport and the thermal exchanges are strongly coupled in the MSFR’s core cavity, having accurate turbulent models for the liquid fuel flow is necessary to avoid introducing significant errors in the numerical simulations of these reactors. Nonetheless, high-accuracy simulations of the turbulent flow field in the reactor cavity of these reactors are usually prohibitively expensive in terms of computational resources, especially when performing multiphysics numerical calculations. Therefore, in this work, we propose a novel method using a modified genetic algorithm to optimize the calculation of the Reynolds Shear Stress Tensor (RST) used for turbulence modeling. The proposed optimization methodology is particularly suitable for advanced liquid fuel reactors such as the MSFRs since it allows the development of high-accuracy but still low-computational-cost turbulence models for the liquid fuel. We demonstrate the applicability of this approach by developing high accuracy Reynolds-Averaged Navier–Stokes (RANS) models (averaged flow error less than 5%) for a low and a large aspect ratio in a Backward-Facing Step (BFS) section particularly challenging for RANS models. The newly developed turbulence models better capture the flow field after the boundary layer tipping, over the extent of the recirculation bubble, and near the boundary layer reattachment region in both BFS configurations. The main reason for these improvements is that the developed models better capture the flow field turbulent anisotropy in the bulk region of the BFS. Then, we illustrate the interest in using this turbulence modeling approach for the case of an MSFR by quantifying the impact of the turbulence modeling on the reactor key parameters.

High streamwise airfoil oscillations at constant low and high incidence angles

Ratios of streamwise airfoil oscillations to the freestream velocity above 30% have not been well investigated in the literature for a reduced frequency range relevant to unsteady applications. A known departure from the experimental correlation to analytical theory for lower magnitudes of this ratio, known as surge amplitude, motivates a parameter study for constant freestream, at constant low- and high-incidence angles, to understand the circulatory lift dependence on angle of attack, Reynolds number, surge amplitude, and reduced frequency in comparison with theory and higher-order computations. To better understand the increased deviation between theory and experiment with increasing velocity fluctuation, a detailed study of surge amplitude of 0.5 is investigated. The experiment for comparison was a free-surface water tunnel with a NACA (National Advisory Committee for Aeronautics) 0018 airfoil oscillated in the streamwise direction. Force measurements, normalized by instantaneous dynamic pressure, reveal that unsteady lift is dependent on Reynolds number and reduced frequency in both attached and fully separated conditions. In separated conditions, mean and fluctuating lift show a dependency on reduced frequency for larger velocity fluctuations than a relative surge amplitude of 10%. Two-dimensional computations were found to agree well with experimental data for Reynolds number 75 k, low incidence cases, and for high incidence with reduced frequencies less than 0.15, where a fully separated upper surface boundary layer condition occurred. Agreement between computations and experiments was not favorable for reduced frequencies above 0.15 for high incidence cases, where partial upper surface boundary layer reattachment is predicted.

Hypersonic shock wave and turbulent boundary layer interaction in a sharp cone/flare model

A direct numerical simulation of hypersonic Shock wave and Turbulent Boundary Layer Interaction(STBLI) at Mach 6.0 on a sharp 7° half-angle circular cone/flare configuration at zero angle of attack is performed. The flare angle is 34° and the momentum thickness Reynolds number based on the incoming turbulent boundary layer on the sharp circular cone is Reθ = 2506. It is found that the mean flow is separated and the separation bubble occurring near the corner exhibits unsteadiness. The Reynolds analogy factor changes dramatically across the interaction, and varies between 1.06 and 1.27 in the downstream region, while the QP85 scaling factor has a nearly constant value of 0.5 across the interaction. The evolution of the reattached boundary layer is characterized in terms of the mean profiles, the Reynolds stress components, the anisotropy tensor and the turbulence kinetic energy. It is argued that the recovery is incomplete and the near-wall asymptotic behavior does not occur for the hypersonic interaction. In addition, mean skin friction decomposition in an axisymmetric turbulent boundary layer is carried out for the first time. Downstream of the interaction, the contributions of transverse curvature and body divergence are negligible, whereas the positive contribution associated with the turbulence kinetic energy production and the negative spatial-growth contribution are dominant. Based on scale decomposition, the positive contribution is further divided into terms with different spanwise length scales. The negative contribution is analyzed by comparing the convective term, the streamwise-heterogeneity term and the pressure gradient term.

Influence of uncertain inflow conditions on a subsonic compressor cascade based on wind tunnel experiment

The intended inflow condition of compressors is subject to the uncertain inflow Mach number ( Ma1) and incidence angle ( i). To quantify the effects of uncertain inflow conditions, real-time measuring experiments of Ma1 and i are performed under various operating conditions to extract the statistical characteristics of the uncertainties. Statistical analysis shows that Ma1 and i follow Gaussian distribution. 0.005 and 0.125 are chosen as the standard deviation of Ma1 and i, respectively. Based on these data, an in-house code of non-intrusive polynomial chaos and Sobol’ indices are employed to quantify the variability in the aerodynamic performance of a compressor cascade caused by uncertain Ma1 and i. The results show that the inflow uncertainties narrow the steady working range of the compressor cascade and lead to fluctuation in the aerodynamic performance. The fluctuation amplitude of the total pressure loss coefficient increases about 15.8 times and tenfold while the static pressure ratio increases just double and 1.3 times under various Ma1 and i conditions, respectively. An analysis of flow field under various operating conditions shows that the uncertainties change the peak isentropic Mach number on the suction surface alter the location and size of the boundary layer separation and reattachment, and shift the interference between the wake and suction surface separation vortex. The sensitivity analysis reveals that as Ma1 increases from 0.5 to 0.6, the dominant parameter affecting the uncertainty of the flow field changes from i to Ma1. When Ma1 increases to 0.75, the coupling effect of Ma1 and i dominates the flow field.

Review-Heat Transfer Inside Cavity Flows Trends

The complex cavity flow represented by the feedback mechanism and self-sustained oscillations of cavity shear layer are widely investigated in various publications either through analysing the aero-acoustic phenomenon occurring and its control or through examining the heat transfer mechanism in such a flow. Boundary layer separation, turbulence, unsteadiness, recirculation and reattachment complicate the flow phenomena at the cavity and may lead to substantial effects on heat transfer. Besides, in order to enhance the heat transfer in some applications, cavity flow was introduced, and thus the importance of understanding the cavity flow structure and its relation with heat transfer has become vital, where such a phenomenon occurs in many engineering applications. This paper presents a literature review on the studies that focus on examining heat transfer in cavity flow, by evaluating the effect of cavity geometry and inlet flow field on heat transfer at different cavity regions.