Location Of Transition Research Articles

Effect of angle of attack on vortex dynamics in laminar separation bubbles

The dynamics of coherent structures that develop within laminar separation bubbles over a NACA 0018 airfoil at Rec = 100 000 and Angles of Attack (AOA) of 0°, 5°, 8°, and 10° are investigated experimentally using a high-speed flow visualization technique and vortex tracking via embedded microphones. The results identify vortex shedding within the separation bubble for all the cases investigated. The vortices form upstream of mean transition location and break down in the vicinity of mean reattachment, with irregular vortex merging events detected in the aft portion of the separation bubble. As the mean size of the separation bubble decreases with increasing angle of attack, vortex shedding characteristics also change appreciably, with shedding frequency increasing and characteristic streamwise wavelength decreasing. However, vortex roll up and salient aspects of vortex development take place within approximately the same regions downstream of separation location in terms of percentage of the bubble length. For all the cases examined, significant cycle-to-cycle variability is observed in salient vortex characteristics, being particularly pronounced in the aft portion of the bubble where vortex merging occurs and significant spanwise deformations of vortex filaments are expected. Merging of the shear layer vortices is shown to occur irregularly between the mean transition and reattachment location, with merging events separated by a relatively wide range of shedding cycles. As the angle of attack increases from AOA = 5° to 10°, the fraction of primary vortices involved in merging increases up to about 25%. However, enhancement in vortex merging is also noted at lower AOA where shedding occurs close to the trailing edge, which is speculated to be linked to upstream scattering of periodic pressure fluctuations induced at the trailing edge.

Detection of Lambda- and Omega-vortices with the temperature-sensitive paint method in the late stage of controlled laminar–turbulent transition

An experiment investigating the laminar–turbulent transition of a Blasius boundary-layer-like flow was set up in the laminar water channel at the Institute of Aerodynamics and Gas Dynamics, University of Stuttgart. The late stage of controlled transition with K-type breakdown was investigated with the temperature-sensitive paint (TSP) method on the flat-plate surface. Additional velocity measurements in the boundary layer were performed with the hot-film anemometry for better interpretation of the TSP results. The test conditions enable the TSP method to resolve the complete transition process temporally and spatially. Therefore, it was possible to detect the coherent structures occurring in the late stage of laminar–turbulent transition from the visualizations on the flat-plate surface: namely, $$\varLambda$$ - and $$\varOmega$$ -vortices. The transition location is derived from the TSP visualizations with a gradient-based method and with the Turbulence Energy Recognition Algorithm (TERA) from the velocity measurements. The derived average transition location shows good agreement between the two techniques, but the TSP method detected a later beginning and earlier end of transition.

Improved performance of ballasted tracks under impact loading by recycled rubber mats

Ballasted tracks at transition locations such as approaches to bridges and road crossings experience increasing degradation and deformation due to dynamic and high impact forces, a key factor that decreases the stability and longevity of railroads. One solution to minimise ballast degradation at the transition zones is using rubber energy absorbing drainage sheets (READS) manufactured from recycled tyres. When placed beneath the ballast layer, READS distributes the load over wider area and attenuate of the load over a longer duration thus decreasing maximum stress, apart from reducing the energy transferred to the ballast and other substructure components. Subsequently, the track substructure experiences less plastic deformation and degradation. These mats also provide an environmentally friendly and cost-effective alternative. In this study, a series of large-scale drop hammer impact tests was carried out to investigate how effectively the READS could attenuate impact loads and help mitigate ballast deformation and degradation. Soft and stiff subgrade were used to investigate the load-deformation response of ballast (with and without READS), subjected to impact loads from a hammer dropped from various heights (hd = 100–250 mm). Laboratory test results show that the inclusion of READS helps to reduce the dynamic impact load transferred to the ballast layer resulting in significantly less permanent deformation and degradation of ballast, apart from significant attenuation of load magnitude and vibration to the underlying subgrade layers.

A Reliable Experimental Methodology for the Study of Wind-Turbine Rotor Blade Aerodynamics

The aerodynamic performance of airfoils and blades designed for modern wind-turbine rotors, which have diameters of the order of hundred meters, must be examined at chord Reynolds numbers matching those of practical applications. In general, such high Reynolds numbers cannot be achieved in conventional wind tunnels. Moreover, knowledge on the boundary-layer transition location is essential to evaluate airfoil and blade performance at these flow conditions. This work presents an experimental methodology that can be applied at flow conditions reproducing those of real wind-turbine rotor blades and simultaneously provides aerodynamic coefficients and transition locations. The experimental methodology consists of: the Temperature-Sensitive Paint (TSP) technique for global, non-intrusive and reliable transition detection; conventional pressure measurements for the determination of the aerodynamic coefficients; and the High Pressure Wind Tunnel Göttingen (DNW-HDG) to run the experiments at Reynolds numbers matching those of real applications. The obtained results can be used to verify airfoil and blade performance and to validate numerical predictions. In the present work, the experimental methodology was applied to systematically investigate the aerodynamic performance of an airfoil designed for the mid-span sections of modern wind-turbine rotor blades. The examined chord Reynolds numbers were as high as 12 million and the angle-of-attack ranged from -14° to +20°. The presented methodology was here demonstrated to be mature for productive testing.

Recent progresses on hypersonic boundary-layer transition

Boundary-layer transition (BLT) is one of the key problems in developing hypersonic aircrafts. Major aerospace countries all over the world have been heavily supporting relevant researches, and the project funding keeps increasing in recent years. In this paper, wind tunnel experiments, numerical simulations and theoretical analysis are combined to study some hot issues in this area, which include effects of unit Reynolds number, angle of attack and nose bluntness, nonlinear interactions during breakdown, secondary instability of crossflow vortices, etc. Some of our views on understanding hypersonic BLT are summarized. The wind tunnel tests are conducted in the Φ1 m hypersonic wind tunnel of China Aerodynamics Research and Development Center, and the relation between transition location and different unit Reynolds numbers, angles of attack and nose bluntness is obtained. It is well known that hypersonic BLT undergoes an important phenomenon called “nose-bluntness inversal”, but a worldwide accepted reasonable explanation for this phenomenon is still missing. To investigate the “nose-bluntness reversal”, we suggest to consider the influence of front shock waves because free-stream disturbances are changed by shock waves. Meanwhile, we should also consider the following two aspects: blunt-nose receptivity and entropy-layer effect. Some premiliary but encouraging results are presented on these two aspects via direct numerical simulations (DNS) and stablity analysis. During the late stage of transition, a “quiet zone” may appear when the second mode waves reach large amplitude, and this is followed by low-frequency modes. DNS and the dynamic mode decomposition are applied to study this problem. The results indicate that the relation between the second modes and the low-frequency modes is possibly mode transformation. In the end of this paper, a newly discovered secondary unstable mode of saturated crossflow vortices is introduced. This mode has the same level of growth rate as Z mode, but is related to the wall-normal shear stress, thus it is named Y mode.

Laminar-turbulent transition of supersonic boundary layer in the presence of external disturbances

A method for estimating the location of the laminar-turbulent transition is described, which is based on the theory of linear stability under the condition of the acoustic field presence. In the stable region near the stability loss location, the amplitude of the velocity disturbances is determined, which are excited by an external sound wave. By virtue of the proximity of sound wave parameters and proper oscillations, the obtained value of the amplitude is accepted to be equal to the amplitude of the growing wave of the boundary layer. At some downstream point, the Reynolds stresses corresponding to this amplitude become equal to several percents of laminar stresses of the undisturbed boundary layer. Based on experimental data it is accepted in the work that at the transition onset, the Reynolds stresses amount to 12 % of laminar stresses.

Symmetry-based description of laminar-turbulent transition

We report a simple but accurate description of flat plate transitional flow based on a structural ensemble dynamics (SED) theory of wall turbulence. It is postulated and verified by simulation data that a multi-layer stress length (SL) function l12 presents a two-state structure in the streamwise x direction, characterizing the laminar-turbulent transition. The resulting algebraic model (called SED-SL) predicts correctly for the first time the entire streamwise profiles of the friction coefficient, Cf(x), of all seven sets of experimental (of e.g. T3 series) and simulations data of flat plate transitional flows, with varying incoming turbulence intensities Tu (or scales), superior to other closure models, with a two-state correlation for transition location Rex* = 3.3 × 106(1 + (Tu/0.65)4)−1.5/4. The extension to accurate description of transitional flows of engineering interests is discussed.

A transition prediction method for flow over airfoils based on high-order dynamic mode decomposition

This article presents a novel approach for predicting transition locations over airfoils, which are used to activate turbulence model in a Reynolds-averaged Navier-Stokes flow solver. This approach combines Dynamic Mode Decomposition (DMD) with eN criterion. The core idea is to use a spatial DMD analysis to extract the modes of unstable perturbations from a steady flowfield and substitute the local Linear Stability Theory (LST) analysis to quantify the spatial growth of Tollmien–Schlichting (TS) waves. Transition is assumed to take place at the stream-wise location where the most amplified mode’s N-factor reaches a prescribed threshold and a turbulence model is activated thereafter. To improve robustness, the high-order version of DMD technique (known as HODMD) is employed. A theoretical derivation is conducted to interpret how a spatial high-order DMD analysis can extract the growth rate of the unsteady perturbations. The new method is validated by transition predictions of flows over a low-speed Natural-Laminar-Flow (NLF) airfoil NLF0416 at various angles of attack and a transonic NLF airfoil NPU-LSC-72613. The transition locations predicted by our HODMD/eN method agree well with experimental data and compare favorably to those obtained by some existing methods (LST/eN or γ-Reθt‾). It is shown that the proposed method is able to predict transition locations for flows over different types of airfoils and offers the potential for application to 3D wings as well as more complex configurations.

Optimization of total resolved shear stress in AlN single crystals homoepitaxially grown by physical vapor transport method

We investigate the distribution and evolution of the total resolved shear stress (TRSS) in AlN single crystals homoepitaxially grown by the physical vapor transport method based on an in-house three-dimensional thermal-elastic stress code. The thermal stresses of three primary slip systems in 2H-AlN crystals are modeled. The simulation results show that the magnitude of the TRSS increases tremendously after the crystal reaches its critical thickness δc at 6 mm under our specific hotzone and growth conditions. The location of the maximum TRSS transitions from the bottom surface of the crystal to the peripheral region beneath the seed crystal during the growing process. The influences of the power ratio between two resistant heaters on the TRSS and the growth rate are also investigated. The results suggest that a high growth rate could be maintained until the crystal thickness reaches δc and that the temperature gradient inside the growth chamber should be decreased during the later stages by adjusting the power ratio to reduce the overall TRSS and defect propagation. The TRSS could be reduced by 93.31% by rapidly decreasing the temperature differences in AlN crystals immediately after the growth process.

Experimental and numerical study of wing boundary layer behavior in propeller flowfield

In this research, the effects of propeller slipstream on wing boundary layer and transition front were studied through wind tunnel tests and numerical analysis. In this respect, the flow around a NACA 6-series airfoil section was simulated in a computational fluid dynamics solver without and with propeller flowfield. The numerical study, presented in this paper, was concerned with the effect of propeller slipstream on both wing aerodynamics and boundary layer treatment. For experimental tests, oil flow visualization technique was used to determine laminar to turbulent transition location, laminar separation bubble with turbulent reattachment and turbulent separation line over the wing surfaces. Existence of propeller slipstream changed pressure and skin friction distribution over the wing surfaces, in both chordwise and spanwise directions and it hence affected on the wing loading distribution. Also upstream propeller affected on boundary layer characteristics including laminar/turbulent transition onset and separation over the wing. The results showed that the transition location moved toward the leading edge, and the separation bubble was washed out due to propeller slipstream.

Analytical prediction of the hydraulic jump detachment length in front of mounted obstacles in supercritical open-channel flows

Detached hydraulic jumps are major features of supercritical open-channel flows interacting with emerging obstacles. Such a flow pattern exhibits strong similarities with shock waves detached in front of bluff bodies in supersonic aerodynamic flows. This paper aims at evaluating the capacities of an analytical model, adapted from supersonic aerodynamics, to predict the hydraulic jump detachment length. The analytical predictions are compared to the measured hydraulic jumps from two experiments: (i) a uniform supercritical open-channel flow that skirts a mounted and emerging obstacle (with a horseshoe vortex) and (ii) a mounted and emerging obstacle moving at constant velocity in water at rest (without a horseshoe vortex). Moreover, numerical calculations of supercritical flow skirting emerging obstacles are undertaken, with a free-slip condition at the bed to remove the horseshoe vortex, while keeping the detached hydraulic jump. The comparison of the detachment lengths of these experimental, analytical, and computed hydraulic jumps reveals that two types of detachment lengths can be defined. The detachment length visible on experiments corresponds to the toe of the hydraulic jump, while the detachment length predicted by the analytical model rather corresponds to the location of flow regime transition from the supercritical to subcritical regime. The present work thus validates the analytical model for predicting the location of flow regime transition (for configurations without a horseshoe vortex) but not for predicting the toe of the hydraulic jump. We finally confirm the strong connections between two distinct phenomena: a hydraulic jump in water flow and a shock wave in gas flow.

Uncertainty-based design optimization of NLF airfoil for high altitude long endurance unmanned air vehicles

PurposeThe high probability of the occurrence of separation bubbles or shocks and early transition to turbulence on surfaces of airfoil makes it very difficult to design high-lift and high-speed Natural-Laminar-Flow (NLF) airfoil for high-altitude long-endurance unmanned air vehicles. To resolve this issue, a framework of uncertainty-based design optimization (UBDO) is developed based on an adjusted polynomial chaos expansion (PCE) method.Design/methodology/approachThe γ ̄Re-θt transition model combined with the shear stress transport k-ω turbulence model is used to predict the laminar-turbulent transition. The particle swarm optimization algorithm and PCE are integrated to search for the optimal NLF airfoil. Using proposed UBDO framework, the aforementioned problem has been regularized to achieve the optimal airfoil with a tradeoff of aerodynamic performances under fully turbulent and free transition conditions. The tradeoff is to make sure its good performance when early transition to turbulence on surfaces of NLF airfoil happens.FindingsThe results indicate that UBDO of NLF airfoil considering Mach number and lift coefficient uncertainty under free transition condition shows a significant deterioration when complicated flight conditions lead to early transition to turbulence. Meanwhile, UBDO of NLF airfoil with a tradeoff of performances under both fully turbulent and free transition conditions holds robust and reliable aerodynamic performance under complicated flight conditions.Originality/valueIn this work, the authors build an effective uncertainty-based design framework based on an adjusted PCE method and apply the framework to design two high-performance NLF airfoils. One of the two NLF airfoils considers Mach number and lift coefficient uncertainty under free transition condition, and the other considers uncertainties both under fully turbulent and free transition conditions. The results show that robust design of NLF airfoil should simultaneously consider Mach number, lift coefficient (angle of attack) and transition location uncertainty.

Research of transition criterion for semi-empirical prediction method at specified transonic regime

A transonic wind tunnel transition test is implemented on a fuselage-wing configuration with the sweep angle 35∘ for commercial aircraft. The wide range of angle of attack from −3.69∘ to 3.07∘ assures that with increasing angle of attack, the laminar to turbulent transition dominant factor varies from cross-flow (CF) vortices to Tollmien-Schlichting (TS) waves. With linear stability theory, the limiting N-factors are calibrated based on the pressure distribution by experiment or the Reynolds Averaged Navier-Stokes (RANS) solver using the fixed experimental transition location. The pressure distribution of the RANS solver agrees well with the experiment in general except some small discrepancies, which causes deviation by 0.6 for the limiting TS N-factor. The RANS solver and the stability analysis provide the limiting N-factors of 7.0 and 8.7 for CF-vortices and TS-waves at the two sides, respectively. In the between, the TS value decays with the CF value due to their interaction. Thus, the transition criterion for limiting N-factors is established for the laminar prediction tool of eN method at similar transonic wind tunnel. With the transition criterion, the transition location difference for 95% cases between the simulation and the experimental data is lower than 5% chord. The good match illustrates that the transition tool is accurate and robust for engineering applications, and also verifies the reasonability of the limiting N-factors. Therefore, the transition criteria at similar transonic conditions and well-performed eN transition tool can be applied for the future laminar wing design.

The opportunities and tensions of historic preservation and transit oriented development (TOD)

The article examines two research questions concerning historic preservation and TOD. First, what are the opportunities for and challenges to developing TODs in historic areas. Transit locations are often located in older historic areas and a restored or adaptively reused historic building can be an asset to a TOD because of the amenity it affords and its preservation of the linkage to the history of a place. There are other opportunities, such as the TOD capitalizing on an historic area's attractive and unique architectural style. At the same time, there may be tensions because as a TOD aims to maximize density near transit, pressure may mount to demolish lower density historic buildings.Second, what tools have been useful to preservationists, planners, developers, and communities for preserving historic buildings in TOD locations? Case studies of success and failure to combine historic preservation and TOD in six communities nationally in the United States showed that while the synthesis can be challenging it is feasible and results in distinctive and attractive developments. A key strategy included combining the federal historic tax credit and other subsidies. We observed other useful approaches, such as engaging stakeholders and place-making via local art and other means. Varying local conditions, however, argue against one set approach.

Classification of Transitions in Upconversion Luminescence of Lanthanides by Two-Dimensional Correlation Analysis.

Upconversion luminescence bands from Yb3+/Er3+ codoped into a matrix such as NaGdF4 can show a very complex structure on account of multiple intra-f shell transitions occurring in the presence of random crystal fields. We demonstrate that two-dimensional correlation analysis, applied to such time-integrated luminescence spectra measured as a function of excitation power, allows us to gain substantial information about the states involved in transitions, without any additional theoretical input. The detailed correlation analysis allows us not only to identify the location of various transitions but further to club them into groups on the basis of their quantum mechanical origin, and finally subclassify the transitions with each group depending on whether they have a common initial or final state.

Spin Hall effect of Airy beam in inhomogeneous medium

The spin Hall effect of Airy beam in the quadratic-index inhomogeneous medium is investigated based on the matrix optics method. It is known that Airy beam possesses an asymmetrical field distribution and some intriguing propagation properties, such as it is a non-diffracting, self-healing, and self-accelerating beam. We find that the spin Hall effect of an Airy beam in the quadratic-index inhomogeneous medium displays some distinctive polarization-dependent phenomena which are completely different from the traditional spin Hall effect of light. First, there is a polarization-dependent rotation of Airy beam induced by the spin–orbit coupling; second, there is a polarization-dependent phase front deformation of Airy beam induced by the spin–orbit coupling, besides the polarization-dependent shift of the entire phase front of the beam; third, transverse deflection of the center of gravity of the beam in the phase transition locations is determined not just by the polarization of the beam, but also by the position. These results mean that the symmetry of the field distribution of the beam also plays an important role in the spin Hall effect of light.

Phase relations in the system Fe–Ni–Si to 200 GPa and 3900 K and implications for Earth's core

Phase relations in Fe–5 wt%Ni–4 wt%Si alloy was examined in an internally resistive heated diamond anvil cell under high pressure (P) and temperature (T) conditions to about 200 GPa and 3900 K by in-situ synchrotron X-ray diffraction. The hexagonal close-packed (hcp) structure was observed to the highest P–T condition, supporting the idea that the stable iron alloy structure in Earth's inner core is hcp. The P–T locations of the phase transition between the face-centred cubic (fcc) and hcp structures were also constrained to 106 GPa. The transition occurs at 15 GPa and 1000 K similar to for pure Fe. The Clausius–Clapeyron slope is however, 0.0480 GPa/K which is larger than reported slopes for Fe (0.0394 GPa/K), Fe–9.7 wt%Ni (0.0426 GPa/K), and Fe–4 wt%Si (0.0394 GPa/K), stabilising the fcc structure towards high pressure. Thus the simultaneous addition of Ni and Si to Fe increases the dP/dT slope of the fcc–hcp transition. This is associated with a small volume change upon transition in Fe–Ni–Si. The triple point, where the fcc, hcp, and liquid phases coexist in Fe–5 wt%Ni–4 wt%Si is placed at 145 GPa and 3750 K. The resulting melting temperature of the hcp phase at the inner core-outer core boundary lies at 550 K lower than in pure Fe.

Shock Control of a Low-Sweep Transonic Laminar Flow Wing

This paper presents a combined experimental and computational study of a low-sweep transonic natural laminar flow (NLF) wing with shock-control bumps (SCBs). A transonic NLF wing with a relatively low sweep angle of 20 deg was chosen for this study. To avoid the complexity of the flow introduced by perforated/slotted walls commonly used for transonic wind-tunnel tests for reducing the wall interference, both experimental tests and computational simulations were conducted with solid wind-tunnel wall conditions. This allows for like-to-like validation of the computational simulation. Optimization of the shock-control bumps was first conducted to design the wind-tunnel test model with bumps. Two critical parameters of the three-dimensional SCBs for shock control (i.e., bump crest position and bump height) were optimized in terms of total drag reduction at the given design point in the wind tunnel. We show that the strong shock wave on the low-sweep NLF wing can be effective controlled by well-designed SCBs deployed along the wing span. The optimized SCBs result in 18.5% pressure drag reduction with 5% viscous drag penalty, and the SCBs also bring some benefits at off-design conditions. The wind-tunnel tests include pressure measurement, particle image velocimetry, and temperature-sensitive paint to provide detailed insight into the shock-control flowfield and to validate the computational simulations. Comparisons include surface pressure profile, velocity distribution, and transition location.

Use of the Complex Zeros of the Partition Function to Investigate the Critical Behavior of the Generalized Interacting Self-Avoiding Trail Model.

The complex zeros of the canonical (fixed walk-length) partition function are calculated for both the self-avoiding trails model and the vertex-interacting self-avoiding walk model, both in bulk and in the presence of an attractive surface. The finite-size behavior of the zeros is used to estimate the location of phase transitions: the collapse transition in the bulk and the adsorption transition in the presence of a surface. The bulk and surface cross-over exponents, and , are estimated from the scaling behavior of the leading partition function zeros.

Sound radiation by supersonic unstable modes in hypersonic blunt cone boundary layers. I. Linear stability theory

There has been renewed interest in supersonic modes in hypersonic boundary layers, which have been previously thought to be insignificant due to their smaller amplitudes than Mack’s traditional second mode. Supersonic modes are associated with an unstable second mode synchronizing with the slow acoustic spectrum, causing sound to radiate outwards from the boundary layer. Because supersonic modes have not been observed experimentally, the majority of previous investigations either relied on Linear Stability Theory (LST) to study supersonic modes on a flat plate or observed them in the context of other research objectives. This two-part study uses a combined LST and Direct Numerical Simulation (DNS) approach to investigate the mechanism of supersonic modes in Mach 5 flow over a blunt cold-wall cone with thermochemical nonequilibrium effects. Paper I focuses on LST with new shock boundary conditions, whereas Paper II [C.P. Knisely and X. Zhong, “Sound radiation by supersonic unstable modes in hypersonic blunt cone boundary layers. II. Direct numerical simulation,” Phys. Fluids 31, 024104 (2019)] focuses on DNS with the overall goal of investigating the impact of supersonic modes on transition. LST results indicate that supersonic modes exist in the flow with wall-to-free-stream temperature ratio Tw/T∞ = 0.2 and create an abnormal growth pattern. However, supersonic modes were not shown to exist using LST in the case with Tw/T∞ = 0.667. Subsequent DNS analysis in Paper II shows supersonic modes in the Tw/T∞ = 0.667 case, although they are significantly weaker than the second mode and are unlikely to lead to transition. Understanding the mechanism of supersonic modes can yield more accurate transition location predictions leading to improved estimates for drag and heat transfer to the vehicle.