Temporal Structure of the Boundary Layer in Low Reynolds Number, Low Pressure Turbine Profiles

2008 ◽  
Author(s):  
Benigno J. Lazaro ◽  
Ezequiel Gonzalez ◽  
Raul Vazquez
Keyword(s):  
Boundary Layer ◽  
Reynolds Number ◽  
Shear Layer ◽  
Temporal Structure ◽  
Side Wall ◽  
Suction Side ◽  
External Flow ◽  
Viscous Layer ◽  
Perturbation Field ◽  
Laminar Shear

Viscous effects in the suction side of low pressure turbines account for about 1/2 of the total turbine losses. Modern design practices simultaneously include decreasing the profiles’ Reynolds number and increasing their aerodynamic load, thereby compromising the suction side boundary layer flow. The objective of the present investigation is to experimentally elucidate the spatial and temporal structure of the suction side boundary layer in the low Reynolds number regime. Under steady state approaching flow conditions, the boundary layer undergoes laminar separation shortly after the external flow velocity peak is reached. The separation produces a low kinetic energy, uniform pressure fluid region. Between this and the external flow, a laminar shear layer develops. The laminar shear layer undergoes a sudden, non linear instability process when its distance to the suction side wall is compatible with the shear layer most unstable scale. This instability promotes the formation of large scale vortices that reattach the flow to the suction side wall. When the approaching flow includes moving wakes that simulate the previous turbine stage, the above description is modified. The laminar shear layer undergoes a global instability triggered by the passing wakes’ perturbation field. Later on the viscous layer reattaches as the wakes’ perturbation field accelerates the fluid. After each wake passage there is a transient, relaxation period characterized by the growth of the low energy, recirculating fluid region and the simultaneous lift of the shear layer away from the suction side wall. The measurements conducted allow the identification of flow regions, as well as temporal and spatial scales that account for the downstream evolution of the viscous layer integral parameters.

The Effect of Pulsed Blowing on the Boundary Layer of a Highly Loaded Low Pressure Turbine Blade

2013 ◽  
Author(s):  
Marion Mack ◽  
Roland Brachmanski ◽  
Reinhard Niehuis
Keyword(s):  
Boundary Layer ◽  
Reynolds Number ◽  
Mach Number ◽  
Suction Side ◽  
Reynolds Numbers ◽  
Low Pressure ◽  
Trailing Edge ◽  
Low Pressure Turbine ◽  
Wide Range ◽  
Highly Loaded

The performance of the low pressure turbine (LPT) can vary appreciably, because this component operates under a wide range of Reynolds numbers. At higher Reynolds numbers, mid and aft loaded profiles have the advantage that transition of suction side boundary layer happens further downstream than at front loaded profiles, resulting in lower profile loss. At lower Reynolds numbers, aft loading of the blade can mean that if a suction side separation exists, it may remain open up to the trailing edge. This is especially the case when blade lift is increased via increased pitch to chord ratio. There is a trend in research towards exploring the effect of coupling boundary layer control with highly loaded turbine blades, in order to maximize performance over the full relevant Reynolds number range. In an earlier work, pulsed blowing with fluidic oscillators was shown to be effective in reducing the extent of the separated flow region and to significantly decrease the profile losses caused by separation over a wide range of Reynolds numbers. These experiments were carried out in the High-Speed Cascade Wind Tunnel of the German Federal Armed Forces University Munich, Germany, which allows to capture the effects of pulsed blowing at engine relevant conditions. The assumed control mechanism was the triggering of boundary layer transition by excitation of the Tollmien-Schlichting waves. The current work aims to gain further insight into the effects of pulsed blowing. It investigates the effect of a highly efficient configuration of pulsed blowing at a frequency of 9.5 kHz on the boundary layer at a Reynolds number of 70000 and exit Mach number of 0.6. The boundary layer profiles were measured at five positions between peak Mach number and the trailing edge with hot wire anemometry and pneumatic probes. Experiments were conducted with and without actuation under steady as well as periodically unsteady inflow conditions. The results show the development of the boundary layer and its interaction with incoming wakes. It is shown that pulsed blowing accelerates transition over the separation bubble and drastically reduces the boundary layer thickness.

Shear Layer Development, Separation, and Stability Over a Low-Reynolds Number Airfoil

2018 ◽  
Vol 140 (7) ◽  
Author(s):  
Paul Ziadé ◽  
Mark A. Feero ◽  
Philippe Lavoie ◽  
Pierre E. Sullivan
Keyword(s):  
Boundary Layer ◽  
Reynolds Number ◽  
Shear Layer ◽  
Separation Bubble ◽  
Low Reynolds Number ◽  
Base Flow ◽  
Mean Velocity ◽  
Laminar Separation ◽  
Low Reynolds ◽  
Layer Development

The shear layer development for a NACA 0025 airfoil at a low Reynolds number was investigated experimentally and numerically using large eddy simulation (LES). Two angles of attack (AOAs) were considered: 5 deg and 12 deg. Experiments and numerics confirm that two flow regimes are present. The first regime, present for an angle-of-attack of 5 deg, exhibits boundary layer reattachment with formation of a laminar separation bubble. The second regime consists of boundary layer separation without reattachment. Linear stability analysis (LSA) of mean velocity profiles is shown to provide adequate agreement between measured and computed growth rates. The stability equations exhibit significant sensitivity to variations in the base flow. This highlights that caution must be applied when experimental or computational uncertainties are present, particularly when performing comparisons. LSA suggests that the first regime is characterized by high frequency instabilities with low spatial growth, whereas the second regime experiences low frequency instabilities with more rapid growth. Spectral analysis confirms the dominance of a central frequency in the laminar separation region of the shear layer, and the importance of nonlinear interactions with harmonics in the transition process.

Decreasing the Side Wall Contamination in Wind Tunnels

1983 ◽  
Vol 105 (4) ◽  
pp. 435-438 ◽  
Author(s):  
T. Motohashi ◽  
R. F. Blackwelder
Keyword(s):  
Boundary Layer ◽  
Reynolds Number ◽  
Boundary Layers ◽  
Laminar Boundary Layer ◽  
Three Dimensional ◽  
Side Wall ◽  
Wind Tunnels ◽  
Turbulent Fluid ◽  
Side Walls ◽  
Suction Slot

To study boundary layers in the transitional Reynolds number regime, the useful spanwise and streamwise extent of wind tunnels is often limited by turbulent fluid emanating from the side walls. Some or all of the turbulent fluid can be removed by sucking fluid out at the corners, as suggested by Amini [1]. It is shown that by optimizing the suction slot width, the side wall contamination can be dramatically decreased without a concomitant three-dimensional distortion of the laminar boundary layer.

Numerical investigation of the role of free-stream turbulence in boundary-layer separation

2016 ◽  
Vol 801 ◽  
pp. 289-321 ◽  
Author(s):  
Wolfgang Balzer ◽  
H. F. Fasel
Keyword(s):  
Boundary Layer ◽  
Reynolds Number ◽  
Shear Layer ◽  
Free Stream ◽  
Hydrodynamic Instability ◽  
Low Reynolds Number ◽  
Transition To Turbulence ◽  
Laminar Separation ◽  
Free Stream Turbulence ◽  
Low Reynolds

The aerodynamic performance of lifting surfaces operating at low Reynolds number conditions is impaired by laminar separation. In most cases, transition to turbulence occurs in the separated shear layer as a result of a series of strong hydrodynamic instability mechanisms. Although the understanding of these mechanisms has been significantly advanced over the past decades, key questions remain unanswered about the influence of external factors such as free-stream turbulence (FST) and others on transition and separation. The present study is driven by the need for more accurate predictions of separation and transition phenomena in ‘real world’ applications, where elevated levels of FST can play a significant role (e.g. turbomachinery). Numerical investigations have become an integral part in the effort to enhance our understanding of the intricate interactions between separation and transition. Due to the development of advanced numerical methods and the increase in the performance of supercomputers with parallel architecture, it has become feasible for low Reynolds number application ($O(10^{5})$) to carry out direct numerical simulations (DNS) such that all relevant spatial and temporal scales are resolved without the use of turbulence modelling. Because the employed high-order accurate DNS are characterized by very low levels of background noise, they lend themselves to transition research where the amplification of small disturbances, sometimes even growing from numerical round-off, can be examined in great detail. When comparing results from DNS and experiment, however, it is beneficial, if not necessary, to increase the background disturbance levels in the DNS to levels that are typical for the experiment. For the current work, a numerical model that emulates a realistic free-stream turbulent environment was adapted and implemented into an existing Navier–Stokes code based on a vorticity–velocity formulation. The role FST plays in the transition process was then investigated for a laminar separation bubble forming on a flat plate. FST was shown to cause the formation of the well-known Klebanoff mode that is represented by streamwise-elongated streaks inside the boundary layer. Increasing the FST levels led to accelerated transition, a reduction in bubble size and better agreement with the experiments. Moreover, the stage of linear disturbance growth due to the inviscid shear-layer instability was found to not be ‘bypassed’.

Passive Manipulation of Separation-Bubble Transition Using Surface Modifications

2009 ◽  
Vol 131 (2) ◽  
Author(s):  
Brian R. McAuliffe ◽  
Metin I. Yaras
Keyword(s):  
Boundary Layer ◽  
Reynolds Number ◽  
Shear Layer ◽  
Separation Bubble ◽  
Surface Modifications ◽  
Small Scale ◽  
Two Dimensional ◽  
Vortex Pairing ◽  
Separated Shear Layer ◽  
Scale Turbulence

Through experiments using two-dimensional particle-image velocimetry (PIV), this paper examines the nature of transition in a separation bubble and manipulations of the resultant breakdown to turbulence through passive means of control. An airfoil was used that provides minimal variation in the separation location over a wide operating range, with various two-dimensional modifications made to the surface for the purpose of manipulating the transition process. The study was conducted under low-freestream-turbulence conditions over a flow Reynolds number range of 28,000–101,000 based on airfoil chord. The spatial nature of the measurements has allowed identification of the dominant flow structures associated with transition in the separated shear layer and the manipulations introduced by the surface modifications. The Kelvin–Helmholtz (K-H) instability is identified as the dominant transition mechanism in the separated shear layer, leading to the roll-up of spanwise vorticity and subsequent breakdown into small-scale turbulence. Similarities with planar free-shear layers are noted, including the frequency of maximum amplification rate for the K-H instability and the vortex-pairing phenomenon initiated by a subharmonic instability. In some cases, secondary pairing events are observed and result in a laminar intervortex region consisting of freestream fluid entrained toward the surface due to the strong circulation of the large-scale vortices. Results of the surface-modification study show that different physical mechanisms can be manipulated to affect the separation, transition, and reattachment processes over the airfoil. These manipulations are also shown to affect the boundary-layer losses observed downstream of reattachment, with all surface-indentation configurations providing decreased losses at the three lowest Reynolds numbers and three of the five configurations providing decreased losses at the highest Reynolds number. The primary mechanisms that provide these manipulations include: suppression of the vortex-pairing phenomenon, which reduces both the shear-layer thickness and the levels of small-scale turbulence; the promotion of smaller-scale turbulence, resulting from the disturbances generated upstream of separation, which provides quicker transition and shorter separation bubbles; the elimination of the separation bubble with transition occurring in an attached boundary layer; and physical disturbance, downstream of separation, of the growing instability waves to manipulate the vortical structures and cause quicker reattachment.

Coupled Blowing and Suction for Flow Separation Control

2019 ◽  
Author(s):  
M. Tadjfar ◽  
D. J. Kamari
Keyword(s):  
Boundary Layer ◽  
Reynolds Number ◽  
Flow Field ◽  
Turbulence Model ◽  
Flow Separation ◽  
Active Flow Control ◽  
Suction Side ◽  
Separation Control ◽  
Flow Separation Control ◽  
Active Flow

Abstract The effects of applying a coupled unsteady blowing and suction combination over SD7003 airfoil at Reynolds number of 60,000 at an angle of attack of 13°, where a large separation on the suction side of the airfoil existed, was considered to investigate active flow control (AFC) mechanism. URANS equations were employed to solve the flow field and k–ω SST was used as the turbulence model. The unsteady blowing and suction were implemented at an angle to the surface crossing the boundary layer (CBL). The influence of location and frequency of the blowing/suction jets were examined.

Excitation of Shear Layer Due to Surface Roughness Near the Leading Edge: An Experiment

2021 ◽  
Vol 143 (5) ◽  
Author(s):  
Pradeep Singh ◽  
S. Sarkar
Keyword(s):  
Boundary Layer ◽  
Surface Roughness ◽  
Shear Layer ◽  
Leading Edge ◽  
Wall Roughness ◽  
Wall Unit ◽  
Flow Features ◽  
Bubble Length ◽  
Laminar Shear ◽  
Comprehensive Study

Abstract In this paper, a comprehensive study has been performed to address the excitation of a separated boundary layer near the leading edge due to surface roughness. Experiments are performed on a model airfoil with the semicircular leading edge at a Reynolds number (Rec) of 1.6×105, where the freestream turbulence (fst) is 1.2%. The flow features are investigated over the three rough surfaces with the roughness characteristic in the wall unit of 17, 10.5, and 8.4, which are estimated from the velocity profile at a location far downstream of reattachment. The wall roughness results in an early transition and reattachment, leading to a reduction of the laminar shear layer length apart from the bubble length. It is worthwhile to note that although the large-amplitude pretransitional perturbations are apparent from the beginning for the rough surface, the shear layer reflects the amplification of selected frequencies, where the fundamental frequency when normalized is almost the same as that of the smooth wall. The universal intermittency curve can be used to describe the transition of the shear layer, which exhibits some resemblance to the excitation of the boundary layer under fst, signifying the viscous effect.

Unsteady Loss Production Mechanisms in Low Reynolds Number, High Lift, Low Pressure Turbine Profiles

2007 ◽  
Author(s):  
Benigno J. Lazaro ◽  
Ezequiel Gonzalez ◽  
Raul Vazquez
Keyword(s):  
Boundary Layer ◽  
Reynolds Number ◽  
Suction Side ◽  
Low Pressure ◽  
Momentum Thickness ◽  
High Lift ◽  
Recirculation Bubble ◽  
Low Pressure Turbine ◽  
Low Reynolds ◽  
Production Mechanisms

The loss production mechanisms that occur in modern high lift, low pressure turbine profiles operating at low Reynolds numbers and subjected to periodic incoming wakes generated by an upstream located, moving bars mechanism, have been experimentally investigated. In particular, laser-Doppler and hot-wire anemometry have been used to obtain spatially and temporally resolved characterizations of the suction side boundary layer structure at the profile trailing edge. Phase measurements locked to the motion of the upstream moving bars have been used to analyze the effect of the incoming wakes on the suction side boundary layer response, which accounts for most of the profile loss generation. It is observed that the incoming wakes produce a temporal modulation of the boundary layer momentum thickness. This modulation appears to be connected to shedding of rotational flow from the recirculation bubble that develops in the suction side of high lift, low pressure turbine profiles. Furthermore, the momentum thickness reduction and subsequent increase that occurs after the wake passage appears to be related to the unsteady process leading to the recovery of the suction side recirculation bubble. The effect of the wake passage frequency and back surface adverse pressure gradient on the above described mechanisms is also investigated. Conclusions obtained can help understanding the unsteady response of modern low pressure turbine profiles operating in the low Reynolds number regime.

Local Flow and Heat Transfer Behavior in Convex-Louver Fin Arrays

1999 ◽  
Vol 121 (1) ◽  
pp. 136-141 ◽  
Author(s):  
N. C. DeJong ◽  
A. M. Jacobi
Keyword(s):  
Heat Transfer ◽  
Boundary Layer ◽  
Reynolds Number ◽  
Pressure Drop ◽  
Shear Layer ◽  
Local Flow ◽  
Number Range ◽  
Flow And Heat Transfer ◽  
Transfer Behavior ◽  
Strip Geometry

Local and surface-averaged measurements of convection coefficients and core pressure-drop data are provided for an array of convex-louver fins. For a Reynolds number range from 200 to 5400, these data are complemented with a flow visualization study and contrasted with new measurements from a similar offset-strip geometry. The results clarify the effects of boundary layer restarting, shear-layer unsteadiness, spanwise vortices, and separation, reattachment, and recirculation on heat transfer in the convex-louver geometry.

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