Large eddy simulations in low-pressure turbines: Effect of wakes at elevated free-stream turbulence

The transition of a separated shear layer over a flat plate, in the presence of periodic wakes and elevated free-stream turbulence (FST), is numerically investigated using Large Eddy Simulation (LES). The upper wall of the test section is inviscid and specifically contoured to impose a streamwise pressure distribution over the flat plate to simulate the suction surface of a low-pressure turbine (LPT) blade. Two different distributions representative of a ‘high-lift’ and an ‘ultra high-lift’ turbine blade are examined. Results obtained from the current LES compare favourably with the extensive experimental data previously obtained for these configurations. The LES results are then used to further investigate the flow physics involved in the transition process.In line with experimental experience, the benefit of wakes and FST obtained by suppressing the separation bubble, is more pronounced in ‘ultra high-lift’ design when compared to the ‘high-lift’ design. Stronger ‘Klebanoff streaks’ are formed in the presence of wakes when compared to the streaks due to FST alone. These streaks promoted much early transition. The weak Klebanoff streaks due to FST continued to trigger transition in between the wake passing cycles.The experimental inference regarding the origin of Klebanoff streaks at the leading edge has been confirmed by the current simulations. While the wake convects at local free-stream velocity, its impression in the boundary layer in the form of streaks convects much slowly. The ‘part-span’ Kelvin–Helmholtz structures, which were observed in the experiments when the wake passes over the separation bubble, are also captured. The non-phase averaged space-time plots manifest that reattachment is a localized process across the span unlike the impression of global reattachment portrayed by phase averaging.     

Coherent structures in the boundary layer of a flat thick plate

We use POD and EPOD (extended POD) analysis to extract the main features of the flow over a thick flat plate simulated with an LES. Our goal is to better understand the coupling between the velocity field and the surface pressure field. We find that POD modes based on the full velocity and energy fields contain both flapping and shedding frequencies. Pressure modes are found to be uniform in the spanwise direction and the most intense variations take place at the mean reattachment point. Velocity modes educed from the pressure modes with EPOD are seen to correspond to eddies shed by the recirculation bubble.     

The dynamics of coherent structures in a flat-plate boundary layer

From the data of a direct numerical simulation the three-dimensional coherent structures of an incompressible, spatially evolving flat-plate boundary layer have been calculated using the POD method. By Galerkin projection of the Navier-Stokes equations onto the corresponding system of eigenfunctions then a low-dimensional model of the flow in the form of a system of ODE's has been derived. In a region of the boundary layer just beyond the spike stages of transition this system displays deterministic chaos that has been quantified by determining its Lyapunov exponents.     

Effect of turbulence on propagation of a coherent beam in the boundary layer and mixing layer

A semi-empirical model is developed to study distortions of the phase function of a coherent beam, which are induced by turbulent fluctuations of flow parameters. Large eddy simulations of the boundary-layer and mixing-layer flows and also of related aero-optical effects are performed. Results of numerical calculations are compared with results of physical experiments and with data obtained by solving the Reynolds-averaged Navier—Stokes equations.     

Experimental study on the effect of pulsating inflow to an enclosure for improved mixing

Optimal control of inlet jet flows is of broad interest for enhanced mixing in ventilated rooms. The general approach in mechanical ventilation is forced convection by means of a constant flow rate supply. However, this type of ventilation may cause several problems such as draught and appearance of stagnation zones, which reduces the ventilation efficiency. A potential way to improve the ventilation quality is to apply a pulsating inflow, which has been hypothesised to reduce the stagnation zones due to enhanced mixing. The present study aims at testing this hypothesis, experimentally, in a small-scale two-dimensional water model using Particle Image Velocimetry with an in-house vortex detection program. We are able to show that for an increase in pulsation frequency or alternatively in the flow rate the stagnation zones are reduced in size and the distribution of vortices becomes more homogeneous over the considered domain. The number of vortices (all scales) increases by a factor of four and the swirl-strength by about 50% simply by turning on the inflow pulsation. Furthermore, the vortices are well balanced in terms of their rotational direction, which is validated by the symmetric Probability Density Functions of vortex circulation (Γ) around Γ = 0. There are two dominating vortex length scales in the flow, namely 0.6 and 0.8 inlet diameters and the spectrum of vortex diameters become broader by turning on the inflow pulsation. We conclude that the positive effect for enhanced mixing by increasing the flow rate can equally be accomplished by applying a pulsating inflow.     

Dynamical characteristics of coherent structure in the outer region of turbulence boundary layer

In the present paper the coherent structures in the outer region of turbulent boundary layer were investigated experimentally and analytically. From the observation of the flow field over smooth wall, rough wall and sand wave wall, it was found that the direct effect of wall on the flow structure can reachy ⁺¹≈100, and both lateral and vertical vortices exist in the outer region, but the coherent structures in the outer region are mainly the formation, development and decay of the large-scale lateral vortices. By experimental and dynamical analysis, some influence factors and their relations associated with the dynamical process of lateral vortices were deduced. The project supported by the National Natural Science Foundation of China     

Visualization of flow structures in a turbulent boundary layer using a new technique

In this paper, an experimental investigation on the flow structures in a turbulent boundary layer employing a special laser light sheet-Hydrogen bubble flow visualization technique is described. It is observed that the high/low speed streaks are directly related to the hairpin or horseshoe-like vortices. This observation can give a better understanding of the physical mechanism in the turbulent boundary layer. Fluid Mechanic Institute, BUAA     

A mechanism for excitation of coherent structures in wall region of a turbulent boundary layer

Introduction Themechanismforthegenerationofcoherentstructuresinthewallregionofaturbulent boundarylayerhasalwaysbeeninconcernandinvestigated.AccordingtoTsujimotoand Miyake[1],thecharacteristicsofturbulenceinthewallregionweremainlydeterminedbythe generationandevolutionofcoherentstructures,notbythesmall_scaleturbulence.However, excitationsfromregionofy >60werefoundtobenecessary,otherwisethewallregionwould degeneratetolaminarflow.Therefore,theinvestigationofthemechanismthathowcoherent structuresi…     

Some new results of the observation on the coherent structures of turbulent boundary layer in flow with adverse pressure gradient

By a suitable manipulation of hydrogen bubble generation, some new results were obtained: (1) The long-streaks are generated along the interfaces between low and high-speed streaks. The long-streaks are generally stretching and are moving faster than its neighboring high-speed streaks. The hydrogen bubbles in long-streaks have longer life. (2) The stream-wise vortices are also generated along the interfaces. The project is supported by the National Natural Science Foundation of China.     

Two-dimensional integral equation for a thin wind turbine blade rotating in the round tunnel

The paper presents a mathematical treatment of the aerodynamic problem about a thin wind turbine blade rotating in the round tunnel. The radius of the blade is almost the radius of the tunnel. This permits formulation of the boundary condition on the tip vortex line to the simple slip condition over the surface of the tunnel. By applying a standard technique from potential theory, the problem is reduced to a two-dimensional integral equation whose kernel is connected with a special Green's function satisfying the homogeneous Neumann boundary condition on the tunnel surface. This Green's function is constructed in an explicit analytical form.     

Numerical modeling of coherent structures attendant on impurity propagation in the atmospheric boundary layer over a forest canopy

Three-dimensional large eddy simulation is used to solve the problem for a homogeneous forest canopy. The development of the Kelvin-Helmholtz instability above the canopy leads to the formation of coherent structures in the atmosphere flow, which are reproduced in the calculations. The statistical characteristics of the flow obtained from the numerical modeling are compared with experimental data. The passive admixture transfer from the canopy to the clean atmosphere is studied for two cases, namely, for constant and variable coupled concentration of the impurity in the canopy.     

Instability waves in the wall region of a turbulent boundary layer on a flat plate

Coherent structures and the bursting phenomena in the wall region of a turbulent boundary layer play a very important role in determining the characteristics of the boundary layer. Yet the nature and the origin of the coherent structures are unclear until now. In this paper, nonlinear stability calculations for the wall region of a turbulent boundary layer have been made. It was found that there do exist instability waves which may be responsible for the coherent structures. The project is supported by the National Natural Science Foundation of China.     

Low- and high-speed structures in the outer region of an adverse-pressure-gradient turbulent boundary layer

Large- and very large-scale structures in the form of elongated regions of low and high streamwise momentum have been studied in the outer region of a turbulent boundary layer subjected to a strong adverse pressure gradient. Large sets of streamwise–spanwise instantaneous velocity fields are acquired by particle image velocimetry at three wall-normal positions (0.2δ, 0.5δ, 0.8δ) at three different streamwise locations and at 0.1δ at the last streamwise location which allows us to study the wall-normal and streamwise variations of the structures. Subsequently, a pattern-recognition method and a classification scheme are employed in order to detect, classify and characterize the structures in an efficient and rigorous manner. Like in the case of zero-pressure-gradient turbulent boundary layers, long meandering streaky regions of low and high momentum are observed in the outer region of the present flow but they appear less frequently; especially in the lower part (at 0.1δ and 0.2δ) of the large-velocity-defect zone, i.e. near detachment. The dimensions of these large structures scale on boundary-layer thickness (δ) and are generally comparable to those previously reported for such structures in the overlap region of zero-pressure-gradient turbulent boundary layers. Interestingly, the adverse pressure gradient does not significantly affect the dimensions and arrangement of the large-scale structures in the upper part (at 0.5δ and 0.8δ) a segment of the outer region where the scaled Reynolds stresses also remain fairly self-similar.     

Magnetic effect on the formation of longitudinal vortices in a rotating laminar boundary layer

This paper presents a numerical study of magnetic effect on the formation of longitudinal vortices in a rotating laminar boundary layer. The criterion for the position marking the onset of longitudinal vortices is defined in this paper. The onset position characterized by the rotational Goertler number G_δ,rot, depends on the local rotation number, Reynolds number, the magnetic field parameter, the Prandtl number and the wave number. The results show that positive rotation destabilizes the flow. The flow is found to become more unstable to the vortex mode of instability as the value of magnetic field parameter M increases. The numerical data shows good agreement with the experimental results. Copyright © 2005 John Wiley & Sons, Ltd.     

A DNS study on the effects of convex streamwise curvature on coherent structures in a temporally-developing turbulent boundary layer with supercritical water

Direct numerical simulation (DNS) results are used to establish the effect of convex streamwise curvature on the development of turbulent boundary layers, and the effect of such curvature on the forced-convection heat transfer variations observed at certain supercritical thermodynamic states. The results illustrate the stabilizing effects of this flow geometry through modification of the structure and distribution of hairpin-like vortical flow structures in the boundary layer. Furthermore, enhancement of convective heat transfer realized at a particular heat flux-to-mass flux ratio with the working fluid at a supercritical state is observed to be reduced by the stabilizing effect of convex surface curvature.     

A theoretical model of the coherent structure in the wall region of a turbulent boundary layer

In this paper,the formation of the coherent structures in the wall region of aturbulent boundary layer was studied,using the nonlinear theory of the hydrodynamicstability.The spanwise and streamwise wavelengths of the most amplified unstablewave obtained by this study were found in good agreement with the experiments,whichmakes the distinct feature of this study in the present paper,as the basis of thestability analysis,a more rational velocity profile has been used,which is different fromthat of the turbulent mean flow.And also,the new nonlinear theory was used.Theresult is useful in understanding of the quasi-periodicity of the coherent structure in theturbulent boundary layer.     

An experimental study of the large scale coherent structures in a forced free shear layer

The dynamic characteristics of the large scale coherent structures in a forced free shear layer are experimentally studied by means of flow visualization. The quantitative measurements are acquired by the use of a LDV. It is shown that the development of the coherent structures can be greatly influenced by upstream artificial perturbations and as a result the mixing in the layer can be controlled. Like vortex merging, vortex splitting is also a common evolution pattern in the development of the coherent structures.     

Modelling of fluidelastic instability in a square inline tube array including the boundary layer effect

Flow-induced vibration (FIV) is a design concern in many engineering applications such as tube bundles in heat exchangers. When FIV materializes, it often results in fatigue and/or fretting wear of the tubes, leading to their failure. Three cross-flow excitation mechanisms are responsible for such failures: random turbulence excitation, Strouhal periodicity, and fluidelastic instability. Of these three mechanisms, fluidelastic instability has the greatest potential for destruction. Because of this, a large amount of research has been conducted to understand and predict this mechanism. This paper presents a time domain model to predict the fluidelastic instability forces in a tube array. The proposed model accounts for temporal variations in the flow separation. The unsteady boundary layer is solved numerically and coupled with the structure model and the far field flow model. It is found that including the boundary layer effect results in a lower stability threshold. This is primarily due to a larger fluidelastic force effect on the tube. The increase in the fluidelastic effect is attributed to the phase difference between the boundary layer separation point motion and the tube motion. It is also observed that a non-linear limit cycle is predicted by the proposed model.