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.     

Distributed Roughness Effects on Transitional and Turbulent Boundary Layers

A numerical investigation is carried out to study the transition of a subsonic boundary layer on a flat plate with roughness elements distributed over the entire surface. Post-transition, the effect of surface roughness on a spatially developing turbulent boundary layer (TBL) is explored. In the transitional regime, the onset of flow transition predicted by the current simulations is in agreement with the experimentally based correlations proposed in the literature. Transition mechanisms are shown to change significantly with the increasing roughness height. Roughness elements that are inside the boundary layer create an elevated shear layer and alternating high and low speed streaks near the wall. Secondary sinuous instabilities on the streaks destabilize the shear layer promoting transition to turbulence. For the roughness topology considered, it is observed that the instability wavelengths are governed by the streamwise and spanwise spacing between the roughness elements. In contrast, the roughness elements that are higher than the boundary layer create turbulent wakes in their lee. The scale of instability is much shorter and transition occurs due to the shedding from the obstacles. Post-transition, in the spatially developing TBL, the velocity defect profiles for both the smooth and rough walls collapsed when non dimensionalized in the outer units. However, when compared to the smooth wall, deviation in the Reynolds stresses are observable in the outer layer; the deviation being higher for the larger roughness elements.     

DNS of a Laminar Separation Bubble in the Presence of Oscillating External Flow

A three-dimensional Direct Numerical Simulation (DNS) of a laminar separation bubble in the presence of oscillating flow is performed. The oscillating flow induces a streamwise pressure gradient varying in time. The special shape of the upper boundary of the computational domain, together with the oscillating pressure gradient causes the boundary layer flow to alternately separate and re-attach. When the inflow decelerates, the shear layer starts to separate and rolls up. Simultaneously the flow becomes 3D. After a transient period, the phase-averaged reverse flow inside the separation bubble reaches speeds ranging from 20 up to 150% of the free-stream velocity. During these phases, the flow is absolutely unstable and self-sustained turbulence can exist. When the inflow starts to accelerate, a spanwise roll of turbulent flow is shed from the shear layer. Shortly after this, the remainder of the separation bubble moves downstream and rejoins with the shed turbulent roll. During the flow-acceleration phase, a patch of laminar boundary layer flow is obtained. Along the flat plate, a series of turbulent patches of flow travelling downstream, separated by laminar flow can be observed, reminiscent of boundary layer flow in a turbine cascade with periodically appearing free-stream disturbances.     

A vorticity stretching diagnostic for turbulent and transitional flows

Vorticity stretching in wall-bounded turbulent and transitional flows has been investigated by means of a new diagnostic measure, denoted by Γ, designed to pick up regions with large amounts of vorticity stretching. It is based on the maximum vorticity stretching component in every spatial point, thus yielding a three-dimensional scalar field. The measure was applied in four different flows with increasing complexity: (a) the near-wall cycle in an asymptotic suction boundary layer (ASBL), (b) K-type transition in a plane channel flow, (c) fully turbulent channel flow at Re _τ = 180 and (d) a complex turbulent three-dimensional separated flow. Instantaneous data show that the coherent structures associated with intense vorticity stretching in all four cases have the shape of flat ‘pancake’ structures in the vicinity of high-speed streaks, here denoted ‘h-type’ events. The other event found is of ‘l-type’, present on top of an unstable low-speed streak. These events (l-type) are further thought to be associated with the exponential growth of streamwise vorticity in the turbulent near-wall cycle. It was found that the largest occurrence of vorticity stretching in the fully turbulent wall-bounded flows is present at a wall-normal distance of y ⁺?=?6.5, i.e. in the transition between the viscous sublayer and buffer layer. The associated structures have a streamwise length of ~200–300 wall units. In K-type transition, the Γ-measure accurately locates the regions of interest, in particular the formation of high-speed streaks near the wall (h-type) and the appearance of the hairpin vortex (l-type). In the turbulent separated flow, the structures containing large amounts of vorticity stretching increase in size and magnitude in the shear layer upstream of the separation bubble but vanish in the backflow region itself. Overall, the measure proved to be useful in showing growing instabilities before they develop into structures, highlighting the mechanisms creating high shear region on a wall and showing turbulence creation associated with instantaneous separations.     

Experiments on localized disturbances in a flat plate boundary layer. Part 2. Interaction between localized disturbances and TS-waves

Previous studies on boundary layer transition at moderate levels of free stream turbulence (FST) have shown that the transition process can be promoted by the introduction of Tollmien-Schlichting (TS) waves. In the present work the interaction between localized boundary layer disturbances and controlled TS-waves is studied experimentally. The localized disturbances are generated either from a controlled free stream perturbation, or by means of suction or injection through a slot in the flat plate surface. Both methods result in boundary layer disturbances dominated by elongated streamwise streaks of high and low velocity in the streamwise component. A strong interaction is observed preferably for high frequency TS-waves, which are damped when generated separately, and the interaction starts as a local amplification of a wide band of low-frequency oblique waves. The later stages of the transition process can be identified as a non-linear interaction between the oblique structures, leading to regeneration of new and stronger streamwise streaks.     

The effects of wall roughness on the separated flow over a smoothly contoured ramp

Experimental measurements address the effects on a turbulent boundary layer of wall roughness on a flat plate and a ramp that produces a separation bubble over the ramp trailing edge. A fully rough flow condition is achieved on the upstream flat plate. The main effect of the wall roughness on the outer layer turbulence on a flat plate is to change the friction velocity. The separation region is substantially larger for the rough-wall case. The rough-wall boundary layer turbulence is less sensitive to the onset of an adverse pressure gradient over the ramp, producing substantially smaller Reynolds stress peaks in upstream flat-plate, wall-unit coordinates.     

Vortex dynamics mechanisms of separated boundary layer in a highly loaded low pressure turbine cascade

This paper investigates the vortex dynamics in the suction-side boundary layer on an aero-engine low pressure turbine blade at two different Reynolds numbers at which short and long laminar separation bubbles occur. Different vortical patterns are observed and investigated through large eddy simulation (LES). The results show that at the higher Reynolds number, streamwise streaks exist upstream of separation line. These streaks initiate spanwise undulation in the form of vortex tubes, which roll-up and shed from the shear layer due to the Kelvin–Helmholtz instability. The vortex tubes alternately pair together and eventually distort and break down to small-scale turbulence structures near the mean reattachment location and convect into a fully turbulent boundary layer. At the lower Reynolds number, streamwise streaks are strong and the separated flow is unable to reattach to the blade surface immediately after transition to turbulence. Therefore, bursting of short bubbles into long bubbles can occur, and vortex tubes have larger diameters and cover a part of the blade span. In this case vortex pairing does not occur and vortex shedding process is promoted mainly by flapping phenomenon. Moreover, the results of dynamic mode decomposition (DMD) analysis show a breathing motion as a source of unsteadiness in the separation location, which is accompanied by the flapping phenomenon.     

Loss Production Mechanisms in a Laminar Separation Bubble

The present paper reports the results of a detailed experimental study, carried out by means of a two-component Laser Doppler Velocimeter, aimed at investigating the loss generation mechanisms induced by laminar separation bubble and transition process. Measurements have been performed along a flat plate installed within a double contoured test section, designed to produce an adverse pressure gradient typical of Ultra-High-Lift turbine blade profiles, which induces the formation of a laminar separation bubble. Results were detailed enough to allow calculating laminar and turbulent deformation works in the separated flow region. Normal and shear contributions of both viscous and turbulent deformation works have been analyzed and employed to explain the generation of total pressure losses in the separated flow region, where the generation and amplification of Kelvin–Helmholtz instability induces the separated shear layer roll-up, thus the bubble reattachment. Results obtained for different Reynolds number conditions have been employed for the formulation of a loss scaling procedure involving the separated shear layer thickness, which is directly correlated to the dynamics of Kelvin–Helmholtz roll-up vortices.     

DNS of heat transfer in transitional, accelerated boundary layer flow over a flat plate affected by free-stream fluctuations

Direct numerical simulations (DNS) of flow over and heat transfer from a flat plate affected by free-stream fluctuations were performed. A contoured upper wall was employed to generate a favourable streamwise pressure gradient along a large portion of the flat plate. The free-stream fluctuations originated from a separate LES of isotropic turbulence in a box. In the laminar portions of the accelerating boundary layer flow the formation of streaks was observed to induce an increase in heat transfer by the exchange of hot fluid near the surface of the plate and cold fluid from the free-stream. In the regions where the streamwise pressure gradient was only mildly favourable, intermittent turbulent spots were detected which relaminarised downstream as the streamwise pressure gradient became stronger. The relaminarisation of the turbulent spots was reflected by a slight decrease in the friction coefficient, which converged to its laminar value in the region where the streamwise pressure gradient was strongest.     

The effect of streamwise vortices on the turbulence structure of a separating boundary layer

A high Reynolds number flat plate turbulent boundary layer is investigated in a wind-tunnel experiment. The flow is subjected to an adverse pressure gradient which is strong enough to generate a weak separation bubble. This experimental study attempts to shed some new light on separation control by means of streamwise vortices with emphasize on the change in the boundary layer turbulence structure. In the present case, counter-rotating and initially non-equidistant streamwise vortices become and remain equidistant and confined within the boundary layer, contradictory to the prediction by inviscid theory. The viscous diffusion cause the vortices to grow, the swirling velocity component to decrease and the boundary layer to develop towards a two-dimensional state. At the position of the eliminated separation bubble the following changes in the turbulence structure were observed. The anisotropy state in the near-wall region is unchanged, which indicates that it is determined by the presence of the wall rather than the large scale vortices. However, the turbulence in the outer part of the boundary layer becomes overall more isotropic due to an increased wall-normal mixing and a significantly decreased production of streamwise fluctuations. The turbulent kinetic energy is decreased as a consequence of the latter. Despite the complete change in mean flow, the spatial turbulence structure and the anisotropy state, the process of transfer of turbulent kinetic energy to the spanwise fluctuating component seems to be unchanged. Local regions of anisotropy are strongly connected to maxima in the turbulent production. For example, at spanwise positions in between those of symmetry, the spanwise gradient of the streamwise velocity cause significant production of turbulent fluctuations. Transport of turbulence in the spanwise direction occurs in the same direction as the rotation of the vortices.     

Effects of free-stream turbulence on a transitional separated–reattached flow over a flat plate with a sharp leading edge

The transitional separated–reattached flow on a flat plate with a blunt leading edge under 2% free-stream turbulence (FST) is numerically simulated using the Large-eddy simulation (LES) approach. The Reynolds number based on the free-stream velocity and the plate thickness is 6500. A dynamic subgrid-scale model is employed and the LES results compare well with the available experimental data.It is well known that FST enhances shear-layer entrainment rates, reduces the mean reattachment distance, and causes early transition to turbulence leading to an early breakdown of the separated boundary layer. Many experimental studies have shown that different vortex shedding frequencies exist, specially the so called low-frequency flapping when there is a separation bubble but its mechanism is still not completely understood. The previous study by us without free-stream turbulence (NFST) did not show the existence of such a low-frequency flapping of the shear layer and it is not clear what the effects of FST will have on these shedding modes. Detailed analysis of the LES data has been presented in the present paper and the low-frequency flapping has not been detected in the current study.     

Spectral structure and linear mechanisms in a rapidly distorted boundary layer

The aim of the present work is to investigate the spectral structure of a rapidly distorted boundary layer that develops on a flat plate in presence of a localised patch of roughness or/and grid-generated freestream turbulence. We observe that, at a certain distance downstream of the roughness patch the boundary layer exhibits a bimodal shape in the energy spectrum of the streamwise velocity fluctuations, similar to that found in a fully-turbulent boundary layer at relatively high Reynolds numbers. The physical mechanism that gives rise to the low-wavenumber peak in the spectrum, which represents long streamwise motions or “superstructures”, is identified to be the interaction of the broadband disturbances with the region of high shear near the wall in the boundary layer. We next show that the flat-plate boundary layer combined with surface roughness and grid turbulence can serve as building-block elements towards synthesising the wall-normal structure of a canonical turbulent boundary layer, in the context of large-scale streamwise motions. The rapidly distorted (or “synthetic”) boundary layer presents a simpler environment in which the coherent motions can evolve and therefore can enable a better characterisation of these motions. To further illustrate the utility of the present approach we compare results from our measurements with the predictions of the Rapid Distortion Theory (RDT). We show that the streamwise turbulence energy in the near-wall region of the rapidly distorted boundary layer grows linearly with time consistent with the RDT results on the effect of pure shear on an initially isotropic turbulence. Moreover close to the edge of the boundary layer the large-scale fluctuations experience an enhancement in the streamwise turbulence energy in accordance with the linear blocking model in the RDT framework. The present work thus highlights the importance of linear processes in wall turbulence and can help us identify aspects of it to which the linear theories can be meaningfully applied.     

On the robustness of separation control by streamwise vortices

The robustness of vane-type vortex generators (VGs) for separation flow control was studied in a separating turbulent boundary layer on a flat plate. VG arrays of different sizes and streamwise positions were positioned upstream of the separation bubble and their effect on the flow field was studied with the help of particle image velocimetry (PIV). The extent of the separated region was varied by changing the pressure gradient. Three different separation bubbles were produced and their extent was approximately doubled for each increase in pressure gradient. It was found that the sensitivity of the control effect to changes in the size of the separation bubble is small within the applied range of pressure gradients. Furthermore, the importance of the relative position of the VGs with respect to the separated region is small.     

Application of a Synthetic Jet to Control Boundary Layer Separation under Ultra-High-Lift Turbine Pressure Distribution

The transition and separation processes of the boundary layer developing on a flat plate under a prescribed adverse pressure gradient typical of Ultra-High-Lift low-pressure turbine profiles have been investigated, with and without the application of a synthetic jet (zero net mass flow rate jet). A mechanical piston has been adopted to produce an intermittent flow with zero net mass flow rate. The capability of the device to suppress or reduce the large laminar separation bubble occurring under steady inflow condition at low Reynolds numbers has been experimentally investigated by means of hot-wire measurements. Wall static pressure measurements complement the hot-wire time-resolved velocity results. The paper reports the investigations performed for both steady and controlled conditions. The active device is able to control the laminar separation bubble induced at low Reynolds number conditions by the strong adverse pressure gradient. An overall view of the time-dependent evolution of the controlled boundary layer is provided by the phase-locked ensemble averaging technique, triggered at the synthetic jet frequency. The separated flow transition process, which is detected for the uncontrolled condition, is modified by the synthetic jet in different ways during the blowing and suction phases. Overall, the phase-locked velocity distributions show a reduced separated flow region for the whole jet cycle as compared to the uncontrolled condition. The phase-locked distributions of the random unsteadiness allow the identification of vortical structures growing along the shear layer mainly during the blowing phase.     

Separation control by tangential blowing inside the bubble

 Experiments have been carried out investigating the effectiveness of steady tangential blowing (inside the separation bubble) to control an axisymmetric separated flow at low speeds. Turbulent boundary separation was induced on a contoured afterbody and the separated shear layer reattached on a narrow cylindrical sting. Measurements made consisted of model surface pressures, mean velocity, turbulent shear stress and kinetic energy profiles using a 2-component LDV system. The results explicitly demonstrate that blowing downstream of the separation location, but within the bubble, can be an effective means of separation control, considering both wall and wake flow reversals. Received: 16 October 1998/Accepted: 27 September 1999     

Experimental Study on the Behaviour of Local Laminar Separation Bubble at a Rectangular Wing Section

This paper identifies laminar separation bubbles at the root or span-wise midsection of a rectangular wing using direct surface pressure measurements in the wind tunnel and analyses their behavior at different Reynolds numbers and angles of attack. The separation, transition, and reattachment locations are determined as functions of the angles of attack and the Reynolds number. The transition structure and turbulence characteristics in the separated shear layer are studied using laser Doppler velocimetry. Surface pressure data and simultaneously acquired velocity signals are correlated to show the pattern of growing disturbances in the shear layer. Surface oil flow visualizations clarified the wingtip and separation bubble’s interactions near the leading edge of the wing at the higher angles of attack. Turbulence statistics are also calculated from the streamwise velocity distributions, and an apparent deviation is observed for the skewness and flatness values from the normal distributions in the near-wall region. The separation bubble effect on aerodynamic coefficients of a 3D rectangular wing root section is studied and reported.

     

Transitionally rough zero pressure gradient turbulent boundary layers

Near-wall measurements are performed to study the effects of surface roughness and viscous shear stresses on the transitionally rough regime (5 < k ⁺ < 70) of a zero pressure gradient turbulent boundary layer. The x-dependence is known from the eleven consecutive measurements in the streamwise direction, which allows for the computation of the streamwise gradients in the boundary layer equations. Thus, the skin friction is computed from the integrated boundary layer equation with errors of 3 and 5% for smooth and rough, respectively. It is found that roughness destroys the viscous layer near the wall, thus, reducing the contribution of the viscous stress in the wall region. As a result, the contribution in the wall shear stress due to form drag increases, while the viscous stress decreases. This yields Reynolds number invariance in the skin friction as k ⁺ increases into the fully rough regime. Furthermore, the roughness at the wall reduces the high peak of the streamwise component of the Reynolds stress in the near-wall region. However, for the Reynolds wall-normal and shear stress components, its contribution is not significantly altered for sand grain roughness.     

An Experimental Investigation of the Separated-Flow Transition Under High-Lift Turbine Blade Pressure Gradients

Laminar boundary layer separation, shear layer transition and reattachment have been experimentally investigated on a flat plate installed within a double contoured test section designed to produce an adverse pressure gradient typical of Ultra-High-Lift turbine profiles. Measurements have been performed for the Reynolds number range 70,000 < Re < 200,000, typical of real engine operation. Profile aerodynamic loadings as well as boundary layer velocity profiles have been measured to survey the separation and transition processes. Particle Image Velocimetry measurements allowed the visualization of vortical structures induced by the shear layer instability. Spectral analysis of hot-wire velocity data has been adopted to identify the characteristic frequencies of the phenomena. Distinct energy peaks, associated with the Kelvin–Helmholtz waves generated in the shear layer over the separation bubble, appear in the spectra. In particular the evolution along the shear layer of the energy contents at the characteristic frequencies of the phenomenon has been analyzed. Two frequency ranges have been identified in which the instability waves are amplified within the shear layer over the stagnation area. The inviscid Kelvin–Helmholtz instability is the main mechanism that drives transition, but it starts to be relevant only after that lower frequency oscillations are amplified and reach the saturation.     

The near-wall layer beneath a moderately converging three-dimensional turbulent separated and reattaching flow

Pulsed-wire mean velocity and surface shear stress measurements have been made in a three-dimensional separation bubble in which there is a mild lateral convergence, bounded by side regions of spanwise invariance. Even though the convergence is mild the bubble parameters change considerably with lateral position. Velocity measurements near the surface were made with a special through-wall pulsed-wire probe. The cross-flow layer is substantially thicker than the reverse-flow layer even in the invariant region. Cross-flow and reverse-flow velocity profiles are each remarkably close in shape, though probably not exactly self similar. Surface shear stresses in the cross- and reverse-flow directions conform to local scalings and Reynolds-number dependences based on thickness and `external' velocity. These scalings also apply (quantitatively) downstream of and, it appears, through attachment. The surface shear stress in the cross-flow direction is higher than the streamwise stress, consistent with a distinctly fuller mean velocity profile. There is a striking comparability with three-dimensional boundary layers once the flow directions are transposed, the cross flow taking the part of the primary flow, and the reverse flow the secondary flow.     

FLOW CHARACTERISTICS AROUND AN INCLINED ELLIPTIC CYLINDER IN A TURBULENT BOUNDARY LAYER

The flow characteristics around an inclined elliptic cylinder located near a flat plate were investigated experimentally. The axis ratio of the elliptic cylinder was AR=2. The pressure distributions along the surface of the cylinder and the flat plate were measured by varying the angle of attack of the elliptic cylinder. The velocity profiles behind the cylinder were measured using hot-wire anemometry. When the angle of attack varies, the peak pressure location on the windward cylinder surface moves towards the rear edge of the cylinder, while that on the leeward surface moves towards the front edge of the cylinder. The vortex-shedding frequency also gradually decreases, defining a critical angle of attack for each gap ratio. The location of the minimum pressure on the flat plate surface moves downstream for positive angles of attack, while it moves upstream for negative angles of attack. Negative angles of attack cause a greater disturbance in the boundary layer near the wall compared to positive angles of attack. This shows that the separated wall shear layer from the boundary layer and the lower shear layer of the cylinder wake are strongly merged compared to other cases.