High Free-stream Turbulence Levels Research Articles

Heat Transfer Characteristics of the Multicavity Tips at Different Turbulence Levels

Abstract A comprehensive understanding of the tip heat transfer characteristics is important to the blade tip design. In this work, the heat transfer coefficient (h) data of several multicavity tips was measured using a steady-state method with an infrared camera. The multicavity tips with different cavity numbers were developed by placing ribs into the traditional squealer tip. All the measurements were conducted in a low-speed linear cascade. Moreover, to gain insight into the heat transfer characteristics of the multicavity tips, computational simulations verified by measurements were implemented to acquire the detailed flow structure of these tips. Measurement results show that the h distributions of the multicavity tips are changed by the ribs. Due to the reattachment of the leakage flow, additional high-h regions are observed downstream of the rib. And the h values in the suction-side rim corner increase, especially at near the trailing-edge. A low-h region is found in the corner upstream of the rib. Furthermore, adopting a high freestream turbulence level would enhance the heat transfer. The augment of h values on the tip front portion is more evident than that on the midto-rear part. The validation in a turbine stage indicates that the present data obtained in the stationary condition still could reveal the heat transfer characteristics of these multicavity tips in a rotating condition.

Effects of Upstream Wakes on the Boundary Layer Over a Low-Pressure Turbine Blade

Abstract In the present work, the evolution of the boundary layer over a low-pressure turbine blade is studied using direct numerical simulations, with the aim of investigating the unsteady flow field induced by the rotor-stator interaction. The freestream flow is characterized by the high level of freestream turbulence and periodically impinging wakes. As in the experiments, the wakes are shed by moving bars modeling the rotor blades and placed upstream of the turbine blades. To include the presence of the wake without employing an ad-hoc model, we simulate both the moving bars and the stationary blades in their respective frames of reference and the coupling of the two domains is done through appropriate boundary conditions. The presence of the wake mainly affects the development of the boundary layer on the suction side of the blade. In particular, the flow separation in the rear part of the blade is suppressed. Moreover, the presence of the wake introduces alternating regions in the streamwise direction of high- and low-velocity fluctuations inside the boundary layer. These fluctuations are responsible for significant variations of the shear stress. The analysis of the velocity fields allows the characterization of the streaky structures forced in the boundary layer by turbulence carried by upstream wakes. The breakdown events are observed once positive streamwise velocity fluctuations reach the end of the blade. Both the fluctuations induced by the migration of the wake in the blade passage and the presence of the streaks contribute to high values of the disturbance velocity inside the boundary layer with respect to a steady inflow case. The amplification of the boundary layer disturbances associated with different spanwise wavenumbers has been computed. It was found that the migration of the wake in the blade passage stands for the most part of the perturbations with zero spanwise wavenumber. The non-zero wavenumbers are found to be amplified in the rear part of the blade at the boundary between the low- and high-speed regions associated with the wakes.

Effects of Upstream Step Geometries on Endwall Film Cooling and Phantom Cooling Performances of a Transonic Turbine Vane

Abstract In gas turbine engines, the endwall misalignment (step geometries with various step heights) commonly exists between the combustor exit and the first nozzle guide vane endwall, due to assembly error, metal corrosion, and thermal expansion. The step geometries affect the secondary flow field near the endwall and coolant flow behavior, thereby leading to a significant variation in endwall film cooling, vane surface phantom cooling, and vane passage aerodynamic performances. This paper presents a detailed experimental and numerical study on endwall film cooling and vane pressure side surface phantom cooling at simulated gas turbine operating conditions (high inlet freestream turbulence level of 16%, exit Mach number of 0.85, and exit Reynolds number of 1.7 × 106). The experimental measurements were conducted at Virginia Tech’s transonic blowdown wind tunnel for two configurations: baseline cascade (HR = 0) and forward-facing step geometry (HR = −0.1). The numerical predictions were performed by solving the steady-state Reynolds-averaged Navier Stokes (RANS) with realizable k–ɛ turbulence model. Based on a double coolant temperature model, a novel numerical method for the predictions of adiabatic wall film cooling effectiveness was proposed. The qualitative and quantitative comparisons between the numerical prediction results and experimental data are provided in this paper. The results indicate that this method can reliably predict endwall film cooling distributions and vane pressure side surface phantom cooling distributions, and significantly reduce (more than 50%) numerical prediction errors. The effects of upstream step geometries were numerically studied at the design flow conditions (BR = 2.5, DR = 1.2), by quantizing the endwall film cooling effectiveness, endwall net heat flux reduction (NHFR), vane pressure side surface phantom cooling effectiveness, and total pressure loss coefficients (TPLC) for various upstream step heights: three forward-facing step heights (HR = −0.16, −0.1, and −0.06), a baseline cascade (HR = 0), and four backward-facing step heights (HR = 0.06, 0.1, 0.136, and 0.2). Results show the upstream forward-facing step geometry is beneficial for enhancing endwall film cooling (maximum enhancement level of 15%), endwall net heat flux reduction (maximum enhancement level of 10%), and vane pressure side surface phantom cooling (maximum enhancement level of 66.7%). Nevertheless, the upstream backward-facing step geometry is pernicious to the endwall film cooling, endwall heat flux reduction, and phantom cooling, and these pernicious effects have sharply deteriorated when the upstream step height is larger than a critical value (HR > 0.136). To be specific, the film cooling effectiveness (from 30% to 80%), net heat flux reduction (up to 78%), and phantom cooling effectiveness (completely disappeared) significantly decrease by introducing an overlarge backward-facing step height HR = 0.2. The upstream step geometries lead to deteriorated aerodynamic performance, though the reduction level may be slight (0.36% at HR = 0.136). These suggest that the overlarge backward-facing misalignment will increase the risk of thermal failure and must be avoided during the component assembly and the operational processes of a gas turbine.

Turbine Vane Endwall Film Cooling and Pressure Side Phantom Cooling Performances With Upstream Coolant Flow at Various Injection Angles

Abstract In this paper, the endwall film cooling and vane pressure side surface phantom cooling performances of a first nozzle guide vane (NGV) with endwall contouring, similar to an industry gas turbine, were experimentally and numerically evaluated at the simulated realistic gas turbine operating conditions (high inlet freestream turbulence level of 16%, exit Mach number of 0.85, and exit Reynolds number of 1.7 × 106). A novel numerical method for the predictions of adiabatic wall film cooling effectiveness was proposed based on a double coolant temperature model. The credibility and accuracy of this numerical method were validated by comparing the predicted results with experimental data. Results indicate that the present numerical method can accurately predict endwall film cooling performance and vane surface phantom cooling performance for both the ideal low density ratio (DR = 1.2) and the typical high density ratio (DR = 2.0) conditions. The endwall film cooling effectiveness, vane surface phantom cooling effectiveness, and secondary flow field were compared and analyzed for the axisymmetric convergent contoured endwall at three coolant injection angles (small injection angle of θ = 40 deg, design injection angle of θ = 50 deg, large injection angle of θ = 60 deg), two density ratios (low density ratio of DR = 1.2 and typical high density ratio of DR = 2.0), and the design blowing ratio (M = 2.5), based on the commercial CFD solver ansys fluent. An analysis method of the coolant momentum flux φ (decomposed into the axial component φx and vertical component φz) was proposed to describe and explain the migration and mixing mechanisms of coolant flow. Results indicate that the proposed analysis method of the coolant momentum flux φ can accurately and reliably describe and explain the coolant flow physics, including the coolant lift-off, the interaction between the coolant and the mainstream, and the coolant migration in the vane passage. The increased coolant injection angles result in a deterioration of the endwall film cooling performance and vane pressure surface side phantom cooling performance. Nevertheless, the sensitivity of endwall film cooling effectiveness and phantom cooling effectiveness to coolant injection angles is variable, and is significantly affected by density ratio. This suggests that the coupling effects of the coolant injection angles and density ratio should be taken into account for the prediction of endwall film cooling and phantom cooling performances. It is very necessary for the optimized design of the coolant injection angles and the predictions of film cooling performance at the realistic density ratio.

Effects of freestream turbulence on the wake growth rate of a model wind turbine and a porous disc

This study presents the results of an experimental investigation that focuses on quantifying the differences between the spreading rates of a model wind turbine wake and a porous disc wake at different freestream turbulence intensity levels. Two-dimensional two-component particle image velocimetry (2D2C PIV) measurements are performed within the wakes of a model wind turbine and a porous disc (up to 7D downstream) of the same diameter and a matching thrust coefficient. The wind turbine is operated at a Tip Speed Ratio (TSR) of 2 in order to have matching thrust coefficient conditions for a consistent wake comparison. The results show that the mean wake flow field (both near and far wake) is significantly different for the wind turbine compared to the porous disc even if they are operating at similar, high or low, freestream turbulence levels. The wake of the wind turbine recovers much faster than that of a porous disc with a matching thrust coefficient especially in the far wake region at both low and high freestream turbulence levels. On the other hand, the data shows that the far wake of the turbine operating at low freestream turbulence is very similar to that of the disc operating at high freestream turbulence. This suggests caution and stresses the importance in choosing the freestream turbulence intensity level when using porous discs to represent wind turbines in wind tunnel studies.

The Effects of Axisymmetric Convergent Contouring and Blowing Ratio on Endwall Film Cooling and Vane Pressure Side Surface Phantom Cooling Performance

Abstract To reduce the intensity of endwall secondary flow, the axisymmetric convergent contoured endwall is commonly designed in the first nozzle guide vane (NGV) passage of the real gas turbine engines. This endwall contouring can obviously alter flow field near endwall and affect the coolant flow through the upstream double-row discrete film holes. This leads to a significant influence on the endwall film cooling performance, vane surface phantom cooling, and vane passage aerodynamic performance. In this paper, a detailed numerical investigation on the endwall film cooling and vane pressure side surface phantom cooling was performed, at the simulated realistic gas turbine operating conditions (high inlet freestream turbulence level of 16%, exit Mach number of 0.85, and exit Reynolds number of 1.7 × 106). Based on a double coolant temperature model, a novel numerical method for the predictions of adiabatic wall film cooling effectiveness was proposed. This numerical method was validated by comparing the predicted results with experimental data of endwall Nusselt number, endwall film cooling effectiveness, and flow visualization near endwall. The results indicate that the present numerical method can accurately predict endwall thermal load distributions, endwall film cooling distributions, and vane surface phantom cooling distributions. The endwall heat transfer coefficient, endwall film cooling effectiveness, phantom cooling effectiveness of the vane pressure side surface, and total pressure loss coefficients (TPLC) were predicted and compared for two endwall contouring shapes (flat endwall and axisymmetric convergent contoured endwall) at three different blowing ratios (low blowing ratio of BR = 1.0, design blowing ratio of BR = 2.5, and high blowing ratio of BR = 3.5) with a constant density ratio of DR = 1.2, based on the present novel numerical method. Results show that the axisymmetric convergent endwall contouring leads to a slight enhancement (maximum enhancement level less than 20%) of endwall heat transfer in the entire vane passage (0 < x < 0.65Cx). The axisymmetric convergent endwall contouring has a significantly desired effect on endwall film cooling performance (maximum enhancement level of 67%), phantom cooling performance of the vane pressure side surface (maximum increase level approximately 100%), and aerodynamic loss (maximum reduction level of 1.45%) for all blowing ratio cases, but the benefit enhancement level is obviously affected by the blowing ratio values. This suggests that the optimum of endwall contouring shapes is an effective technical way to improve endwall film cooling performance and decrease the depletion of coolant; the coupled effects of the appropriate axisymmetric convergent endwall contouring and the optimum blowing ratio should be considered in the design process of advance endwall cooling schemes.

Extension of an algebraic intermittency model for better prediction of transition in separated layers under strong free-stream turbulence

A constitutive law describing the Reynolds stresses in boundary layers undergoing laminar-to-turbulent transition, constructed in previous work by elastic-net regression on an experimental data base, is used to improve an algebraic intermittency model for cases with transition in a separated layer influenced by a high level of free-stream turbulence. The intermittency model is combined with a k-ω turbulence model and the basic version, developed in previous work, functions well for bypass transition in attached boundary layers and for transition in separated boundary layers under a low free-stream turbulence level. The basic model version is extended by an additional production term in the transport equation for turbulent kinetic energy. A sensor detects the front part of a separated layer and activates the production term. The term expresses the effect of Klebanoff streaks generated upstream of separation on the Kelvin-Helmholtz instability rolls in the separated part of the layer. The Klebanoff streaks cause faster breakdown by the combined effects of a large adverse pressure gradient and an elevated free-stream turbulence level. The extended model does not alter the results of the basic model version for bypass transition in an attached boundary layer and for transition in a separated boundary layer under a low free-stream turbulence level. The extended model significantly improves the predictions of the previous model version for transition in a separated boundary layer under a high free-stream turbulence level.

An Extended Version of an Algebraic Intermittency Model for Prediction of Separation-Induced Transition at Elevated Free-Stream Turbulence Level

An algebraic intermittency model for boundary layer flow transition from laminar to turbulent state, is extended using an experimental data base on boundary layer flows with various transition types and results by large eddy simulation of transition in a separated boundary layer. The originating algebraic transition model functions well for bypass transition in an attached boundary layer under a moderately high or elevated free-stream turbulence level, and for transition by Kelvin–Helmholtz instability in a separated boundary layer under a low free-stream turbulence level. It also functions well for transition in a separated layer, caused by a very strong adverse pressure gradient under a moderately high or elevated free-stream turbulence level. It is not accurate for transition in a separated layer under a moderately strong adverse pressure gradient, in the presence of a moderately high or elevated free-stream turbulence level. The extension repairs this deficiency. Therefore, a sensor function for detection of the front part of a separated boundary layer activates two terms that express the effect of free-stream turbulence on the breakdown of a separated layer, without changing the functioning of the model in other flow regions. The sensor and the breakdown terms use only local variables.

Bypass transition in boundary layers subject to strong pressure gradient and curvature effects

This paper aims at characterizing the bypass transition in boundary layers subject to strong pressure gradient and curvature effects. A series of highly resolved large-eddy simulations of a high-pressure turbine vane are performed, and the primary focus is on the effects of free-stream turbulence (FST) states on transition mechanisms. The turbulent fluctuations that have convected from the inlet first interact with the blunt blade leading edge, forming vortical structures wrapping around the blade. For cases with relatively low-level FST, streamwise streaks are observed in the suction-side boundary layer, and the instabilities of the streaks cause the breakdown to turbulence. Moreover, the varicose mode of streak instability is predominant in the adverse pressure gradient region, while the sinuous mode is more common in the (weak) favourable pressure gradient region. On the other hand, for cases with higher levels of FST, the leading-edge structures are more irregularly distributed and no obvious streak instability is observed. Accordingly, the transition onset occurs much earlier, through the breakdown caused by interactions between vortical structures. Comparing between different cases, it is the competing effect between the FST intensity and the stabilizing pressure gradient that decides the path to transition and also the transition onset, whereas the integral length scale of FST affects the scales of the streamwise streaks in the boundary layer. Furthermore, while the streaks in the low-level FST cases are mainly induced by leading-edge vortical structures, the corresponding fluctuations show a stage of algebraic growth despite the weak favourable pressure gradient and curvature.

A realizable data-driven approach to delay bypass transition with control theory

The current work presents a realizable method to control streaky disturbances in boundary layer flows and delay transition to turbulence via active flow control. Numerical simulations of the nonlinear transitional regime in a Blasius boundary layer are performed where streaks are excited in the boundary layer via a high level of free-stream turbulence (FST). The occurring disturbances are measured via localized wall sensors and damped via near-wall actuators resembling plasma actuators. The time-varying amplitude of the signal of each actuator is computed by processing signals from the sensors. This processing is the result of two control laws: the Linear Quadratic Gaussian regulator (LQG) and the Inverse Feed-Forward Control technique (IFFC). The use of the first, LQG, requires a state-space representation of the system dynamics, so the flow is described via an operator that captures only the most relevant information of the dynamics and results in a reduced order model (ROM). The ROM is computed via the eigensystem realization algorithm, based on the impulse responses of the real system. Collecting such impulse responses may be unfeasible when considering FST because of the high dimensionality of the input forcing needed for a precise description of such a phenomenon. Here, a new method to identify the relevant system dynamics and generate the needed impulse responses is proposed, in a data-driven approach that would be realizable in experiments. Finally, the effectiveness of the technique in delaying bypass transition is shown. The work (i) presents a systematic way to deal with high dimensional disturbances to build ROMs for a control technique, and (ii) shows that even when considering constraints such as the type and size of actuators and sensors, it is possible to achieve at least as large delay of bypass transition as that obtained in more idealized cases found in literature.

Parallel Large-Eddy Simulation of Subsonic and Transonic Flows with Transition in Compressor Cascade

Laminar–turbulent transition plays a critical role in the aerodynamic performance of compressors and is sensitive to flow conditions and external disturbances. In the present work, a parallel large-eddy-simulation solver is developed and applied to investigate the effects of free-stream turbulence on the mode and location of transition on the blade suction surface of a compressor cascade under subsonic and transonic conditions. Both the parallel accuracy and efficiency of the solver are demonstrated. The transition location is observed to shift upstream as the free-stream turbulence level increases at both the subsonic and transonic conditions. Moreover, increasing the free-stream turbulence level from 0 to 4% causes the transition mode to change from Kelvin–Helmholtz instability-induced transition to bypass transition in the case of subsonic flow, whereas transition occurs in a Kelvin–Helmholtz instability-induced mode even at a high free-stream turbulence level of up to 7% in the case of transonic flow. The work demonstrates the ability of the parallel large-eddy-simulation method to capture the transitional-flow structures in compressor cascades and its potential application to the analysis and design of advanced compressor.

Effects of Upstream Step Geometry on Axisymmetric Converging Vane Endwall Secondary Flow and Heat Transfer at Transonic Conditions

This paper presents a detailed experimental and numerical study on the effects of upstream step geometry on the endwall secondary flow and heat transfer in a transonic linear turbine vane passage with axisymmetric converging endwalls. The upstream step geometry represents the misalignment between the combustor exit and the nozzle guide vane endwall. The experimental measurements were performed in a blowdown wind tunnel with an exit Mach number of 0.85 and an exit Re of 1.5×106. A high freestream turbulence level of 16% was set at the inlet, which represents the typical turbulence conditions in a gas turbine engine. Two upstream step geometries were tested for the same vane profile: a baseline configuration with a gap located 0.88Cx (43.8 mm) upstream of the vane leading edge (upstream step height = 0 mm) and a misaligned configuration with a backward-facing step located just before the gap at 0.88Cx (43.8 mm) upstream of the vane leading edge (step height = 4.45% span). The endwall temperature history was measured using transient infrared thermography, from which the endwall thermal load distribution, namely, Nusselt number, was derived. This paper also presents a comparison with computational fluid dynamics (CFD) predictions performed by solving the steady-state Reynolds-averaged Navier–Stokes with Reynolds stress model using the commercial CFD solver ansysfluent v.15. The CFD simulations were conducted at a range of different upstream step geometries: three forward-facing (upstream step geometries with step heights from −5.25% to 0% span), and five backward-facing, upstream step geometries (step heights from 0% to 6.56% span). These CFD results were used to highlight the link between heat transfer patterns and the secondary flow structures and explain the effects of upstream step geometry. Experimental and numerical results indicate that the backward-facing upstream step geometry will significantly enlarge the high thermal load region and result in an obvious increase (up to 140%) in the heat transfer coefficient (HTC) level, especially for arched regions around the vane leading edge. However, the forward-facing upstream geometry will modestly shrink the high thermal load region and reduce the HTC (by ∼10% to 40% decrease), especially for the suction side regions near the vane leading edge. The aerodynamic loss appears to have a slight increase (0.3–1.3%) because of the forward-facing upstream step geometry but is slightly reduced (by 0.1–0.3%) by the presence of the backward upstream step geometry.

An algebraic intermittency model for bypass, separation-induced and wake-induced transition

An algebraic model using the intermittency concept is proposed for laminar to turbulent transition in attached and separated boundary layers. The model modifies the production terms of the k–ω RANS turbulence model by Wilcox. For attached flows, the model describes bypass transition by taking into account two main effects, which are damping of high-frequency disturbances by a laminar shear layer and breakdown of a near-wall laminar layer perturbed by streaks. For separated flow, the model describes breakdown of a laminar free shear layer. The proposed model is a modified and extended version of an earlier model by the authors for bypass transition only (Kubacki and Dick, Int. J. Heat and Fluid Flow, 58, 68–83, 2016).The primary tuning of the model has been done with flat plate T3C flows of ERCOFTAC with steady approaching flow, relevant for bypass transition and separation-induced transition. Secondary tuning has been done with two cascades with steady approaching flow at high turbulence level, one with N3-60 (Re= 6×105) steam turbine stator vanes and one with V103 (Re= 1.385×105) compressor blades. Model validation has been done for steady approaching flows of the same cascades, but including a case with low level of free-stream turbulence, and for a cascade with T106A (Re= 1.6×105) gas turbine rotor blades for high and low levels of free-stream turbulence. Further validation has been done for flow perturbed by travelling wakes through a cascade of N3-60 vanes at high and low levels of background turbulence level. The transition model produces good results for all cases, both for bypass transition in attached boundary layer state and for transition in separated boundary layer state.

Influence of inflow turbulence intensity on the performance of bare and diffuser-augmented micro wind turbine model

The performance of a wind turbine is directly affected by the site wind condition. In urban-built locality, the wind is typified by fluctuating velocity and direction, and high turbulence intensity (TI). This paper investigates the impact of turbulence intensity on micro wind turbine efficiency in converting the wind energy to power. The performance of bare micro wind turbine (MWT) and diffuser-augmented micro wind turbine (DAMWT) models subject to different level of turbulence intensities is reported. Turbulence intensities ranging from ≈2% to 29% were generated by means of turbulence grids. The turbine performance is assessed in terms of the relationship between the coefficient of performance, CP and tip speed ratio, λ. Computational fluid dynamics (CFD) simulations and wind tunnel tests show that shrouding the turbine with diffuser increases the peak CP by approximately two times. Beyond a certain tip speed ratio, the performance of both MWT and DAMWT is shown to decrease with turbulence intensity, however the Cp of the DAMWT is still greater than bare MWT wind indicating the diffuser augmentation is still achievable even at high level of freestream turbulence.

Effect of free-stream turbulence on boundary layer transition.

This paper is concerned with the transition to turbulence in flat plate boundary layers due to moderately high levels of free-stream turbulence. The turbulence is assumed to be generated by an (idealized) grid and matched asymptotic expansions are used to analyse the resulting flow over a finite thickness flat plate located in the downstream region. The characteristic Reynolds number Rλ based on the mesh size λ and free-stream velocity is assumed to be large, and the turbulence intensity ε is assumed to be small. The asymptotic flow structure is discussed for the generic case where the turbulence Reynolds number εRλ and the plate thickness and are held fixed (at O(1) and O(λ), respectively) in the limit as [Formula: see text] and ε→0. But various limiting cases are considered in order to explain the relevant transition mechanisms. It is argued that there are two types of streak-like structures that can play a role in the transition process: (i) those that appear in the downstream region and are generated by streamwise vorticity in upstream flow and (ii) those that are concentrated near the leading edge and are generated by plate normal vorticity in upstream flow. The former are relatively unaffected by leading edge geometry and are usually referred to as Klebanoff modes while the latter are strongly affected by leading edge geometry and are more streamwise vortex-like in appearance.

Large-eddy simulation of transitional flows using a co-located grid

A large-eddy simulation (LES) of a transitional separated flow over a plate with a semi-circular leading at low (<0.2%) and high (5.6%) free-stream turbulence (FST) has been performed, using a co-located grid with the Rhie–Chow pressure smoothing. A numerical trip is used to produce a high FST level and a dynamic subgrid-scale model is also employed in the current study. The entire transition process leading to breakdown to turbulence has been shown clearly by the flow visualisations using instantaneous spanwise vorticities, and the differences between the low- and high-FST cases are clearly visible. Coherent structures are also visualised using isosurfaces of the Q-criterion, and for the high-FST case, the spanwise-oriented quasi-two-dimensional rolls, which are clearly present in the low-FST case, are not visible anymore. Detailed quantitative comparisons between the present LES results and experimental data and the previous LES results at low FST using a staggered grid have been done and a good agreement has been obtained, indicating that the current LES using a co-located grid with pressure smoothing can also predict transitional flows accurately.

Experimental study of localized boundary layer disturbances at high free stream turbulence level (review)

Experimental study of localized boundary layer disturbances at high free stream turbulence level (review)

Influence of Large-Scale Freestream Turbulence on the Performance of a Thin Airfoil

Micro air vehicles are typically small in size and are remotely controlled or autonomous aircraft that fly slowly (50; 000 < Reynolds number < 200; 000) and very close to the Earth's surface. Due to the combined influences of low Reynolds number effects and the high levels of freestream turbulence, present within the atmospheric boundary layer, micro air vehicles are very difficult to control/fly outdoors.

An experimental study of the forerunners of boundary-layer localized disturbances at an enhanced turbulence level

Turbulence production processes in boundary layer at a high level of free-stream turbulence have been studied. The tests were carried out in the MT-324 subsonic wind tunnel of ITAM, SB RAS, on models of straight and 45° swept wings at Reynolds numbers Rec1 = 97000 and Rec2 = 137000, and also at low (Tu = 0.18 % U ∞) and high (Tu = 0.79 and 2.31 % U ∞) levels of free-stream turbulence. The longitudinal localized disturbances developing in the boundary layer under the action of free-stream turbulence were artificially modeled using local air suction through a slot on the model surface. Wave packets, or forerunners, produced in the boundary layer, in the region preceding the abrupt local change of flow velocity near the localized-disturbance fronts, were examined. The high level of free-stream turbulence was found to accelerate the downstream evolution of the wave packets and their transformation into turbulent spots.

Separated flow structures around a cylindrical obstacle in a narrow channel

A detailed experimental study is performed on the separated flow structures around a low aspect-ratio circular cylinder (pin-fin) in a practical configuration of liquid cooling channel. Distinctive features of the present arrangement are the confinement of the cylinder at both ends, water flow at low Reynolds numbers ( Re = 800, 1800, 2800), very high core flow turbulence and undeveloped boundary layers at the position of the obstacle. The horseshoe vortex system at the junctions between the cylinder and the confining walls and the near wake region behind the obstacle are deeply investigated by means of Particle Image Velocimetry (PIV). Upstream of the cylinder, the horseshoe vortex system turns out to be perturbed by vorticity bursts from the incoming boundary layers, leading to aperiodical vortex oscillations at Re = 800 or to break-away and secondary vorticity eruptions at the higher Reynolds numbers. The flow structures in the near wake show a complex three-dimensional behaviour associated with a peculiar mechanism of spanwise mass transport. High levels of free-stream turbulence trigger an early instabilization of the shear layers and strong Bloor–Gerrard vortices are observed even at Re = 800. Coalescence of these vortices and intense spanwise flow inhibit the alternate primary vortex shedding for time periods whose length and frequency increase as the Reynolds number is reduced. The inhibition of alternate vortex shedding for long time periods is finally related to the very large wake characteristic lengths and to the low velocity fluctuations observed especially at the lowest Reynolds number.