Turbulent Boundary Layer Transition Research Articles

Simulation of the flow past a circular cylinder from sub-critical to super-critical Reynolds numbers using an intermittency-based hybrid model

In this work, we investigate the prediction of the flow around a circular cylinder from sub-critical to super-critical Reynolds numbers using a hybrid approach which combines a dynamic variational multiscale (DVMS) large-eddy simulation (LES) model and a transitional RANS model. In the proposed hybrid approach, the variational multiscale model, aiming to limit the effects of the subgrid-scale (SGS) model to the smallest resolved scales, is combined with the dynamic procedure which provides a tuning of the SGS dissipation in space and time. For representing laminar to turbulent boundary layer transition, the equations of the RANS part of the hybrid approach are completed with a transition model based on an intermittency transport equation. Results are compared to those of other numerical simulations in the literature and with experimental data. They highlight the overall good prediction capabilities of the proposed hybrid strategy for the simulation of such massively separated flows, even with the use of coarse meshes. In particular, the intermittency-based hybrid model was found to be able to predict the drag crisis of a circular cylinder, as well as the sharp increase in vortex shedding frequency, unlike the equivalent hybrid approach when no laminar-turbulent transition model is introduced.

On the laminar–turbulent transition mechanism on megawatt wind turbine blades operating in atmospheric flow

Abstract. Among a few field experiments on wind turbines for analyzing laminar–turbulent boundary layer transition, the results obtained from the DAN-AERO and aerodynamic glove projects provide significant findings. The effect of inflow turbulence on boundary layer transition and the possible transition mechanisms on wind turbine blades are discussed and compared to CFD (computational fluid dynamics) simulations of increasing fidelity (Reynolds-averaged Navier–Stokes, RANS; unsteady Reynolds-averaged Navier–Stokes, URANS; and large-eddy simulations, LESs). From the experiments, it is found that the transition scenario changes even over a single revolution with bypass transition taking place under the influence of enhanced upstream turbulence, for example, such as that from wakes, while natural transition is observed in other instances under relatively low inflow turbulence conditions. This change from bypass to natural transition takes place at azimuthal angles directly outside the influence of the wake indicating a quick boundary layer recovery. The importance of a suitable choice of the amplification factor to be used within the eN method of transition detection is evident from both the RANS and URANS simulations. The URANS simulations which simultaneously check for natural and bypass transition match very well with the experiment. The LES predictions with anisotropic inflow turbulence show the shear-sheltering effect and a good agreement between the power spectral density plots from the experiment and simulation is found in case of bypass transition. A condition to easily distinguish the region of transition to turbulence based on the Reynolds shear stress is also observed. Overall, useful insights into the flow phenomena are obtained and a remarkably consistent set of conclusions can be drawn.

Delay of laminar–turbulent transition by counter-rotating cylindrical roughness elements in a laminar flat plate boundary layer

Delaying laminar–turbulent transition in boundary layers is of great interest since the skin-friction coefficient can be reduced by up to one order of magnitude. In this experimental research, it is shown that counter-rotating cylindrical roughness elements are able to delay transition under realistic flow conditions. Evidence is given by the intermittency, evaluated from hot-film measurements in a laminar water channel. An increase in rotation speed results in a delay of transition of up to {6.5}{%} in the center of the plate. This trend can be explained by the streaks amplified by the rotating cylinders, resulting in a damping of the fluctuation amplitude in the boundary layer. The advantage of this method is that the transition delay can be actively controlled with conventional cylindrical roughness elements.Graphical abstract

Unsteady interaction of crossflow instability with a forward-facing step

Experiments have been conducted on a swept wing model in a low-turbulence wind tunnel at chord Reynolds number of $2.17 \times 10^{6}$ to investigate the unsteady interaction of a forward-facing step (FFS) with incoming stationary crossflow (CF) vortices. The impact of varying the FFS height on the development and growth of primary and secondary CF disturbances and the ensuing laminar–turbulent transition is quantified through detailed hot-wire anemometry and infrared thermography measurements. The presence of the FFS results in either a critical (i.e. moderate transition advancement) or a supercritical behaviour (i.e. transition advancing abruptly to the FFS location). The arrival of the forced stationary CF vortices at the step is accompanied by their amplification. Unsteady analysis for the critical cases indicates temporal velocity fluctuations following closely the development of the baseline configuration (i.e. agreeing with the development of secondary instabilities). Consequently, laminar breakdown originates from the outer side of the upwelling region of the CF vortices. In contrast, for the supercritical FFS, the laminar breakdown unexpectedly originates from the inner side of the upwelling region. Evidence points to an unsteady mechanism possibly supported by locally enhanced spanwise-modulated shears and the recirculation region downstream of the FFS edge. This mechanism appears to govern the abrupt tripping of the flow in supercritical step cases. The findings in this work provide insight into the unsteady FFS–CF vortex interaction, which is pivotal to understanding the influence of an FFS on the laminar–turbulent boundary-layer transition in swept aerodynamic surfaces.

Modeling of Transonic Transitional Three-Dimensional Flows for Aerodynamic Applications

The paper describes a technology designed for computing three-dimensional transonic laminar–turbulent flows at various aerodynamic configurations with the use of the general-purpose computational fluid dynamics code ANSYS Fluent and an integrated special module of computing the laminar–turbulent transition position created on the basis of the autonomous software package LOTRAN 3, developed previously by the authors. Within the framework of this technology, computations with a prolate spheroid and engine nacelle are performed for different Mach numbers ( Reynolds numbers ), and angles of attack ( to ). New results are obtained on the position of the laminar–turbulent transition in boundary layers in transonic flow regimes, and the problem of the dominating transition mechanism is considered. The results obtained in the present study are demonstrated to be in good agreement with experimental data on the position of the laminar–turbulent transition available in the literature.

Effect of micro rib on aerothermal dynamic in channel flow

In the present development of gas turbine, there is a main contradiction between the limited amount of cold air, the pressure head in the gas supply system and the large flow loss caused by the cooling unit inside the turbine blade. It leads to demands for the cooling techniques to provide efficient heat transfer with lower pressure loss. Surface micro turbulator is considered as a potential cooling technique. In this study, numerical simulation and experimental research were undertaken to evaluate the turbulent flow heat transfer and pressure loss in channel with a typical 2-D micro rib on one wall with different rib heights. Investigated rib heights range from 0.0013~0.053 of the channel hydraulic diameter ratio(h/D). The Reynolds number is kept at 20,000 to control the inlet boundary layer thickness, which makes the height of all cases located between the viscous layer and the log-law region in the boundary layer with a dimensionless height h+=2~80. Reynolds-Averaged Navier–Stokes(RANS) and Large Eddy Simulation(LES) results were validated by experiment. The results show that the pressure loss and heat transfer show different trends with the h+, which is mainly due to the enhancement of near-wall turbulence caused by the small rib height and the form drag caused by the large rib height. Furthermore, the analysis of near-wall turbulence characteristics given by LES indicates the enhancement of the normal turbulence fluctuation by the small rib within the turbulent boundary layer transition zone dominates the heat transfer enhancement downstream of the rib. the heat transfer is enhanced and the pressure loss is small when the rib height changes in the boundary layer, Which leads to a better overall thermal performance(OTP). The conclusion affirms that micro rib is a potential cooling technique to attain high heat transfer performance with low pressure loss. which is of great significance to enhance the overall efficiency of gas turbine.

A metamorphosis of three-dimensional wave structure in transitional and turbulent boundary layers

Abstract

Statistical Description of Intermittency in the Transition Region at the Low Degree of Flow Turbulence

The statistical method for describing the intermittency coefficient in the region of laminar–turbulent boundary layer transition at the low level of external perturbations is proposed. The method is based on the assumption that turbulence develops when the velocity deviates locally in space and time from the mean by a threshold value. The analytical formulas are obtained for the frequency of occurrence of the coherent structures (turbulent spots and wedges) and for the intermittency coefficient or the mean fraction of the surface area covered by them in the case of two basic scenarios of transition, namely, the enhancement of steady crossflow instability vortices on the swept wing and the Tollmien–Schlichting waves.

On the thresholds of vortex visualisation methods

Since the discovery of vortex structures in turbulence, research in the area of vortex visualisation methods has been of interest. The prevalent Galilean invariant visualisation methods, like Q, Lambda2, and Omega methods, are based on the local pointwise analysis of invariants of the velocity gradient tensor. When plotting the vortex iso-surfaces, different methods will introduce different thresholds for visualisation purpose. Varying the thresholds can greatly change the appearance of the results and the interpretation can be significantly different for the same flow phenomenon. Using our direct numerical simulation data on late stage laminar turbulent boundary layer transition, we will investigate the similarities and differences among several prevailing vortex visualisation methods caused by the selection of thresholds. The sensitivity of thresholds are further explored analytically in a few flow examples. In addition, we propose an appropriate usage of the threshold which will result in remarkably similar looking vortex structures.

Post-test analysis of the LAPCAT-II subscale scramjet

A subscale flight experiment configuration propelled by a Mach 8 supersonic combustion ramjet (scramjet) was designed within the framework of the European Commission co-funded Long Term Advanced Propulsion Concepts and Technologies II project. The focus of this design exercise was to verify by ground testing the ability of the proposed scramjet engine to produce adequate thrust for hypersonic level flight. Experiments performed in DLR’s High Enthalpy Shock Tunnel Göttingen confirmed precedent CFD predictions of the total thrust and demonstrated the operability of the vehicle. Yet, significant discrepancies between the CFD analyses, which were performed to design the vehicle, and subsequent detailed measurements of the pressure distribution in the combustor were observed and could not be resolved so far. This paper focuses on a further analysis of these residual discrepancies. It was found that the CFD predictions of the combustor pressure distribution are sensitive to the configuration of the intake boundary layer. Particularly, different assumptions for the location of the laminar to turbulent boundary layer transition strongly influence cross-flow structures which develop on the intake and which are able to trigger different combustion modes in the combustor. While the effect on total vehicle performance remains limited, a significant impact on the structure and magnitude of the surface pressure distribution was observed. i.e., the large combustor peak pressures, which occur in the experiment, can be explained by the occurrence of a strong shock train in the vicinity of the combustor wall.

Unit Reynolds number, Mach number and pressure gradient effects on laminar\u2013turbulent transition in two-dimensional boundary layers

The influence of unit Reynolds number (Re_1=17.5times 10^{6}–80times 10^{6},text {m}^{-1}), Mach number (M= 0.35–0.77) and incompressible shape factor (H_{12} = 2.50–2.66) on laminar–turbulent boundary layer transition was systematically investigated in the Cryogenic Ludwieg-Tube Göttingen (DNW-KRG). For this investigation the existing two-dimensional wind tunnel model, PaLASTra, which offers a quasi-uniform streamwise pressure gradient, was modified to reduce the size of the flow separation region at its trailing edge. The streamwise temperature distribution and the location of laminar–turbulent transition were measured by means of temperature-sensitive paint (TSP) with a higher accuracy than attained in earlier measurements. It was found that for the modified PaLASTra model the transition Reynolds number (Re_{text {tr}}) exhibits a linear dependence on the pressure gradient, characterized by H_{12}. Due to this linear relation it was possible to quantify the so-called ‘unit Reynolds number effect’, which is an increase of Re_{text {tr}} with Re_1. By a systematic variation of M, Re_1 and H_{12} in combination with a spectral analysis of freestream disturbances, a stabilizing effect of compressibility on boundary layer transition, as predicted by linear stability theory, was detected (‘Mach number effect’). Furthermore, two expressions were derived which can be used to calculate the transition Reynolds number as a function of the amplitude of total pressure fluctuations, Re_1 and H_{12}. To determine critical N-factors, the measured transition locations were correlated with amplification rates, calculated by incompressible and compressible linear stability theory. By taking into account the spectral level of total pressure fluctuations at the frequency of the most amplified Tollmien–Schlichting wave at transition location, the scatter in the determined critical N-factors was reduced. Furthermore, the receptivity coefficients dependence on incidence angle of acoustic waves was used to correct the determined critical N-factors. Thereby, a found dependency of the determined critical N-factors on H_{12} decreased, leading to an average critical N-factor of about 9.5 with a standard deviation of sigma approx 0.8.

On computing the location of laminar–turbulent transition in compressible boundary layers

Abstract Evolution equations of small disturbances aimed to compute the location of a laminar–turbulent transition in boundary layers by the e N -method taking into account the compressibility and heat transfer are described and justified for aerodynamic applications. The results of computation of N-factors and transition locations with the use of locally-parallel flow assumption for a flat plate exposed to supersonic flow at zero angle of attack are given. The results agree well with known numerical and experimental data.

An algebraic model for bypass transition in turbomachinery boundary layer flows

A simple algebraic model is proposed for laminar to turbulent transition in boundary layers subjected to elevated free-stream turbulence. The model is combined with the newest version of the k-ω RANS turbulence model by Wilcox. The transition model takes into account, in an approximate way, two effects: filtering of high-frequency free-stream disturbances by shear and breakdown of near-wall disturbances into fine-scale turbulence. The model only uses local variables.The model has been tuned for the flat plate T3C cases of ERCOFTAC, relevant for bypass transition and tested for flow through cascades of N3-60 (Re=6·105) steam turbine stator vanes, V103 (Re=1.385·105) compressor blades and T106A (Re=1.6·105) gas turbine rotor blades. The transition model produces good results for bypass transition in attached boundary layer state and in separated boundary layer state for flows of elevated free-stream turbulence, both for 2D steady RANS and 3D time-accurate RANS simulations. Good results are also obtained for transition in separated laminar boundary layers at low free-stream turbulence by 3D time-accurate RANS simulations, thanks to resolution of the boundary layer instability and the beginning of the breakdown of the generated spanwise vortices.

Feather roughness reduces flow separation during low Reynolds number glides of swifts.

Swifts are aerodynamically sophisticated birds with a small arm and large hand wing that provides them with exquisite control over their glide performance. However, their hand wings have a seemingly unsophisticated surface roughness that is poised to disturb flow. This roughness of about 2% chord length is formed by the valleys and ridges of overlapping primary feathers with thick protruding rachides, which make the wing stiffer. An earlier flow study of laminar-turbulent boundary layer transition over prepared swift wings suggested that swifts can attain laminar flow at a low angle of attack. In contrast, aerodynamic design theory suggests that airfoils must be extremely smooth to attain such laminar flow. In hummingbirds, which have similarly rough wings, flow measurements on a 3D printed model suggest that the flow separates at the leading edge and becomes turbulent well above the rachis bumps in a detached shear layer. The aerodynamic function of wing roughness in small birds is, therefore, not fully understood. Here, we performed particle image velocimetry and force measurements to compare smooth versus rough 3D-printed models of the swift hand wing. The high-resolution boundary layer measurements show that the flow over rough wings is indeed laminar at a low angle of attack and a low Reynolds number, but becomes turbulent at higher values. In contrast, the boundary layer over the smooth wing forms open laminar separation bubbles that extend beyond the trailing edge. The boundary layer dynamics of the smooth surface varies non-linearly as a function of angle of attack and Reynolds number, whereas the rough surface boasts more consistent turbulent boundary layer dynamics. Comparison of the corresponding drag values, lift values and glide ratios suggests, however, that glide performance is equivalent. The increased structural performance, boundary layer robustness and equivalent aerodynamic performance of rough wings might have provided small (proto) birds with an evolutionary window to high glide performance.

Adaptive and Model-Based Control Theory Applied to Convectively Unstable Flows

Research on active control for the delay of laminar–turbulent transition in boundary layers has made a significant progress in the last two decades, but the employed strategies have been many and dispersed. Using one framework, we review model-based techniques, such as linear-quadratic regulators, and model-free adaptive methods, such as least-mean square filters. The former are supported by an elegant and powerful theoretical basis, whereas the latter may provide a more practical approach in the presence of complex disturbance environments that are difficult to model. We compare the methods with a particular focus on efficiency, practicability and robustness to uncertainties. Each step is exemplified on the one-dimensional linearized Kuramoto–Sivashinsky equation, which shows many similarities with the initial linear stages of the transition process of the flow over a flat plate. Also, the source code for the examples is provided.

Experiments on passive hypersonic boundary layer control using ultrasonically absorptive carbon–carbon material with random microstructure

For the first time, the influence of ultrasonically absorptive carbon–carbon material on hypersonic laminar to turbulent boundary layer transition was investigated experimentally. A 7° half-angle blunted cone with a nose radius of 2.5 mm and a total length of 1,077 mm was tested at zero angle of attack in the High Enthalpy Shock Tunnel Gottingen of the German Aerospace Center (DLR) at Mach 7.5. One-third of the metallic model surface in circumferential direction was replaced by DLR in-house manufactured ultrasonically absorptive carbon–carbon material with random microstructure for passive transition control. The remaining model surface consisted of polished steel and served as reference surface. The model was equipped with coaxial thermocouples to determine the transition location by means of surface heat flux distribution. Flush-mounted piezoelectric fast-response pressure transducers were used to measure the pressure fluctuations in the boundary layer associated with second-mode instabilities. The free-stream unit Reynolds number was varied over a range of Rem = 1.5 × 106 m−1 to Rem = 6.4 × 106 m−1 at a stagnation enthalpy of h0 ≈ 3.2 MJ/kg and a wall temperature ratio of Tw/T0 ≈ 0.1. The present study revealed a clear damping of the second-mode instabilities and a delay of boundary layer transition along the ultrasonically absorptive carbon–carbon insert.

Application of the Turbulent Potential Model to Heat Transfer Predictions on a Turbine Guide Vane

The accurate numerical simulation of the flow through turbomachinery depends on the reliable prediction of laminar to turbulent boundary layer transition phenomena. The aim of this paper is to study the ability of the turbulent potential model to predict those nonequilibrium turbulent flows for several test cases. Within this model turbulent quantities are described by the turbulent scalar and turbulent vector potentials of the turbulent body force—the divergence of the Reynolds stress tensor. For model validation first flat plate test cases with different inlet turbulence intensities, zero pressure gradient, and nonuniform pressure gradient distributions along the plate were calculated and compared by means of skin friction values measured in the experiments. Finally the model was validated by heat transfer measurement data obtained from a highly loaded transonic turbine guide vane cascade for different operating conditions.

Nonlinear effects in transonic flutter with emphasis on manifestations of limit cycle oscillations

This paper presents flutter and forced oscillation experiments in a transonic wind tunnel. For an aeroelastic supercritical 2-D airfoil configuration we studied typical transonic phenomena in as pure a form as possible. Various manifestations of small-amplitude limit cycle oscillations were observed for different flow conditions as well as coexisting limit cycles. We demonstrated how very small control forces were sufficient to excite or suppress flutter oscillations. Limit cycle oscillations occurred under free and forced turbulent boundary layer transition in a perforated wall test-section. Flutter calculations based on experimental aerodynamic forces yield stability limits which show good agreement with directly measured experimental flutter values. The results indicate that flow separation at the trailing edge, and the interactions between the shock and the marginal region of separated flow beneath it, may be responsible for limiting the amplitude of the observed limit cycle oscillations.

Multiple instabilities of three-dimensional boundary layers along a concave wall

The initial stage of laminar turbulent transition in boundary layers on a swept wing is governed by two types of instability, namely, the Tollmien-Schlichting instability based on the effect of viscosity and the cross-flow instability associated with the twisted velocity distribution of three-dimensional boundary layers. If the flow is along a locally concave surface, there is another possibility, that of Taylor–Görtler instability induced by centrifugal force. The strength of these instabilities mainly depends upon local Reynolds number, magnitude and direction of the cross-flow, and local curvature of the wall. Thus a model system of linear equations is proposed to describe the initial development of disturbances through the three instabilities, and its eigensolutions are used to evaluate stability limits of the Falkner Skan-Cooke flow to those disturbances. The three types of disturbances can be distinguished from each other by definite differences in the wavenumber and frequency regions. Three subregions in the parameter plane of cross-flow magnitude and wall curvature are determined in correspondence to the three instabilities, each of which gives the lowest value of critical Reynolds numbers in its own Subregion.

Free-Stream Turbulence and Pressure Gradient Effects on Heat Transfer and Boundary Layer Development on Highly Cooled Surfaces

Heat transfer and boundary layer measurements were derived from flows over a cooled flat plate with various free-stream turbulence intensities (Tu = 1.6–11 percent), favorable pressure gradients (k = νe/ue2•due/dx = 0÷6•10−6) and cooling intensities (Tw/Te = 1.0–0.53). Special interest is directed towards the effects of the dominant parameters, including the influence on laminar to turbulent boundary layer transition. It is shown, that free-stream turbulence and pressure gradients are of primary importance. The increase of heat transfer due to wall cooling can be explained primarily by property variations as transition, and the influence of free-stream parameters are not affected.