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.

Effects of Free-Stream Turbulence on Transition of a Separated Boundary Layer Over the Leading-Edge of a Constant Thickness Airfoil

2015 ◽  
Vol 138 (2) ◽  
Author(s):  
A. Samson ◽  
S. Sarkar
Keyword(s):  
Boundary Layer ◽  
Shear Layer ◽  
Free Stream ◽  
Separation Bubble ◽  
Leading Edge ◽  
Reynolds Numbers ◽  
Constant Thickness ◽  
Free Stream Turbulence ◽  
Transition Mechanism ◽  
Bubble Length

This paper describes the change in the transition mechanism of a separated boundary layer formed from the semicircular leading-edge of a constant thickness airfoil as the free-stream turbulence (fst) increases. Experiments are carried out in a low-speed wind tunnel for three levels of fst (Tu = 0.65%, 4.6%, and 7.7%) at two Reynolds numbers (Re) 25,000 and 55,000 (based on the leading-edge diameter). Measurements of velocity and surface pressure along with flow field visualizations are carried out using a planar particle image velocimetry (PIV). The flow undergoes separation in the vicinity of leading-edge and reattaches in the downstream forming a separation bubble. The shear layer is laminar up to 20% of separation length, and then, the perturbations are amplified in the second-half attributing to breakdown and reattachment. The bubble length is highly susceptible to change in Tu. At low fst, the primary mode of instability of the shear layer is Kelvin–Helmholtz (K-H), although the local viscous effect may not be neglected. At high fst, the mechanism of shear layer rollup is bypassed with transient growth of perturbations along with evidence of spot formation. The predominant shedding frequency when normalized with respect to the momentum thickness at separation is almost constant and shows a good agreement with the previous studies. After reattachment, the flow takes longer length to approach a canonical boundary layer.

Experimental Study on the Effect of Freestream Turbulence on the Development of an Inflectional Boundary Layer From the Semi-Circular Leading Edge of a Flat Plate

2013 ◽  
Author(s):  
A. Samson ◽  
S. Sarkar
Keyword(s):  
Boundary Layer ◽  
Shear Layer ◽  
Flat Plate ◽  
Leading Edge ◽  
Separated Shear Layer ◽  
Bubble Length ◽  
Shedding Frequency ◽  
Velocity Spectra ◽  
Low Speed Wind Tunnel ◽  
Good Agreement

The characteristics of a boundary layer from the semi-circular leading edge of a flat plate has been investigated for two levels of stream turbulence (Tu = 0.5% and 7.7%) in a low-speed wind tunnel. Measurements of velocity and surface pressure were made along with a planar PIV to visualize flow structures for varying turbulence levels at a Reynolds number of 25000 (based on the leading edge diameter). At low stream turbulence the measurements reveal flow undergoes separation in the vicinity of leading-edge with reattachment in the downstream. Velocity spectra illustrates that the separated shear layer is laminar up to 20% of separation length and then the perturbations are amplified in the second half attributing to breakdown and reattachment. It is also evident that the shear layer is inviscidly unstable and the predominant shedding frequency when normalised with respect to the momentum thickness at separation shows a good agreement with previous studies. The bubble length is highly susceptible to change in Tu depicting an attached layer which grows into a fully turbulent profile at high Tu. Here, the spectra for an attached layer depicts a turbulent-like flow with band of frequencies from the beginning.

Swept wing boundary-layer receptivity to localized surface roughness

2012 ◽  
Vol 711 ◽  
pp. 516-544 ◽  
Author(s):  
David Tempelmann ◽  
Lars-Uve Schrader ◽  
Ardeshir Hanifi ◽  
Luca Brandt ◽  
Dan S. Henningson
Keyword(s):  
Boundary Layer ◽  
Surface Roughness ◽  
Practical Interest ◽  
Leading Edge ◽  
Constant Factor ◽  
Computationally Efficient ◽  
Swept Wing ◽  
Predictive Tool ◽  
Free Stream Turbulence ◽  
Streamwise Location

AbstractThe receptivity to localized surface roughness of a swept-wing boundary layer is studied by direct numerical simulation (DNS) and computations using the parabolized stability equations (PSEs). The DNS is laid out to reproduce wind tunnel experiments performed by Saric and coworkers, where micron-sized cylinders were used to trigger steady crossflow modes. The amplitudes of the roughness-induced fundamental crossflow wave and its superharmonics obtained from nonlinear PSE solutions agree excellently with the DNS results. A receptivity model using the direct and adjoint PSEs is shown to provide reliable predictions of the receptivity to roughness cylinders of different heights and chordwise locations. Being robust and computationally efficient, the model is well suited as a predictive tool of receptivity in flows of practical interest. The crossflow mode amplitudes obtained based on both DNS and PSE methods are 40 % of those measured in the experiments. Additional comparisons between experimental and PSE data for various disturbance wavelengths reveal that the measured disturbance amplitudes are consistently larger than those predicted by the PSE-based receptivity model by a nearly constant factor. Supplementary DNS and PSE computations suggest that possible natural leading-edge roughness and free-stream turbulence in the experiments are unlikely to account for this discrepancy. It is more likely that experimental uncertainties in the streamwise location of the roughness array and cylinder height are responsible for the additional receptivity observed in the experiments.

Effect of Elevated Free-Stream Turbulence on Transitional Heat Transfer Over Dual-Scaled Rough Surfaces

2003 ◽  
Author(s):  
Ting Wang ◽  
Matthew C. Rice
Keyword(s):  
Heat Transfer ◽  
Boundary Layer ◽  
Surface Roughness ◽  
Vortex Shedding ◽  
Free Stream ◽  
Leading Edge ◽  
Test Surface ◽  
Dominant Effect ◽  
Free Stream Turbulence ◽  
Heat Transfer Phenomenon

The surface roughness over a serviced turbine airfoil is usually multi-scaled with varying features that are difficult to be universally characterized. However, it was previously discovered in low freestream turbulence conditions that the height of larger roughness produces separation and vortex shedding, which trigger early transition and exert a dominant effect on flow pattern and heat transfer. The geometry of the roughness and smaller roughness scales played secondary roles. This paper extends the previous study to elevated turbulence conditions with free-stream turbulence intensity ranging from 0.2–6.0 percent. A simplified test condition on a flat plate is conducted with two discrete regions having different surface roughness. The leading edge roughness is comprised of a sandpaper strip or a single cylinder. The downstream surface is either smooth or covered with sandpaper of grit sizes ranging from 100 ∼ 40 (Ra = 37 ∼ 119 μm). Hot wire measurements are conducted in the boundary layer to study the flow structure. The results of this study verify that the height of the largest-scale roughness triggers an earlier transition even under elevated turbulence conditions and exerts a more dominant effect on flow and heat transfer than does the geometry of the roughness. Heat transfer enhancements of about 30 ∼ 40 percent over the entire test surface are observed. The vortical motion, generated by the backward facing step at the joint of two roughness regions, is believed to significantly increase momentum transport across the boundary layer and bring the elevated turbulence from the freestream towards the wall. No such long-lasting heat transfer phenomenon is observed in low FSTI cases even though vortex shedding also exists in the low turbulence cases. The heat transfer enhancement decreases, instead of increases, as the downstream roughness height increases.

Interactions of Separation Bubble With Oncoming Wakes by LES

2011 ◽  
Author(s):  
S. Sarkar ◽  
Jasim Sadique
Keyword(s):  
Heat Transfer ◽  
Boundary Layer ◽  
Shear Layer ◽  
Separation Bubble ◽  
Leading Edge ◽  
Flat Plates ◽  
Flow Past A Cylinder ◽  
Periodic Behaviour ◽  
Rotor Stator Interaction ◽  
Multiple Bubbles

The unsteady flow physics and heat transfer characteristics due to interactions of periodic passing wakes with a separated boundary layer are studied with the help of Large-eddy simulations (LES). A flat plate with a semicircular leading edge is employed to obtain the separated boundary layer. Wake data extracted from precursor LES of flow past a cylinder are used to replicate a moving bar that generates wakes in front of a cascade (in this case an infinite row of flat plates). This setup is a simplified representation of the rotor-stator interaction in turbomachinery. With a uniform inlet, the laminar boundary layer separates near the leading edge, undergoes transition due to amplification of the disturbances, becomes turbulent and finally reattaches forming a bubble. In the presence of oncoming wakes, the characteristics of the separated layer have changed and the impinging wakes are found to be the mechanism affecting the reattachment. Phase averaged results illustrate the periodic behaviour of both flow and heat transfer. Large undulations in the phase-averaged skin friction and Nusselt number distributions can be attributed to the excitation of separated shear layer by convective wakes forming coherent vortices, which are being shed and convect downstream. This interaction also breaks the bubble into multiple bubbles. Further, the transition of the shear layer during the wake-induced path is governed by a mechanism that involves the convection of these vortices followed by increased fluctuations.

Experimental Analysis and Prediction of Wake-Induced Transition in Turbomachinery

2004 ◽  
Author(s):  
Witold Elsner ◽  
Stephane Vilmin ◽  
Stanislaw Drobniak ◽  
Wladyslaw Piotrowski
Keyword(s):  
Boundary Layer ◽  
Boundary Layers ◽  
Leading Edge ◽  
Transition Process ◽  
Full Time ◽  
Blade Profile ◽  
Velocity Fluctuations ◽  
Cambridge University ◽  
Flow Features ◽  
Good Agreement

The paper presents an experimental and numerical analysis of the interaction between wakes and boundary layers on aerodynamic blade profiles. The experiment revealed that incoming wakes interact with boundary layers and cause the significant increase of velocity fluctuations in the boundary layer and in consequence shift the transition zone towards the leading edge. The full time evolution of periodic wake induced transition was reproduced from measurements. The numerical simulation of the flow around the blade profile has been performed with the use of the adaptive grid viscous flow unNEWT PUIM solver with a prescribed unsteady intermittency method (PUIM) developed at Cambridge University, UK. The results obtained give evidence that the turbulence transported within the wake is mainly responsible for the transition process. The applied CFD solver was able to reproduce some essential flow features related to the bypass and wake-induced transitions and the simulations reveal good agreement with the experimental results in terms of localisation and extent of wake-induced transition.

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.

Experimental Investigation of a Turbulent Flow in the Vicinity of an Appendage Mounted on a Flat Plate

1991 ◽  
Vol 113 (4) ◽  
pp. 635-642 ◽  
Author(s):  
P. Merati ◽  
H. M. McMahon ◽  
K. M. Yoo
Keyword(s):  
Boundary Layer ◽  
Shear Layer ◽  
Flat Plate ◽  
Secondary Flow ◽  
Three Dimensional ◽  
Leading Edge ◽  
Significant Degree ◽  
Major Effect ◽  
Turbulent Shear ◽  
Mean Flow

Experimental measurements were carried out in an incompressible three-dimensional turbulent shear layer in the vicinity of an appendage mounted perpendicular to a flat plate. The thickness of the turbulent boundary layer as it approached the appendage leading edge was 76 mm or 1.07 times the maximum thickness of the appendage. As the oncoming boundary layer passed around the appendage, a strong secondary flow was formed which was dominated by a horseshoe root vortex. This secondary flow had a major effect in redistributing both the mean flow and turbulence quantities throughout the shear layer, and this effect persisted to a significant degree up to at least three chord lengths downstream of the appendage leading edge.

Transition in a Separation Bubble

1996 ◽  
Vol 118 (4) ◽  
pp. 752-759 ◽  
Author(s):  
E. Malkiel ◽  
R. E. Mayle
Keyword(s):  
Boundary Layer ◽  
Shear Layer ◽  
Separation Bubble ◽  
Boundary Layer Transition ◽  
Turbulent Spot ◽  
Layer Transition ◽  
Lateral Direction ◽  
Intensity Measurements ◽  
Separated Shear Layer ◽  
Laminar Shear

In the interest of being able to predict separating–reattaching flows, it is necessary to have an accurate model of transition in separation bubbles. An experimental investigation of the process of turbulence development in a separation bubble shows that transition occurs within the separated shear layer. A comparison of simultaneous velocity traces from comparison of simultaneous velocity traces from probes separated in the lateral direction suggests that Kelvin–Helmholtz waves, which originate in the laminar shear layer, do not break down to turbulence simultaneously across their span when they proceed to agglomerate. The streamwise development of intermittency in this region can be characterized by turbulent spot theory with a high dimensionless spot production rate. Moreover, the progression of intermittency along the centerline of the shear layer is similar to that in attached boundary layer transition. The transverse development of intermittency is also remarkably similar to that in attached boundary layers. The parameters obtained from these measurements agree with correlations previously deduced from turbulence intensity measurements.

Surface Roughness Effects on Turbulent Boundary Layer Structures

2001 ◽  
Vol 124 (1) ◽  
pp. 127-135 ◽  
Author(s):  
L. Keirsbulck ◽  
L. Labraga ◽  
A. Mazouz ◽  
C. Tournier
Keyword(s):  
Boundary Layer ◽  
Surface Roughness ◽  
Shear Stress ◽  
Turbulent Boundary Layer ◽  
Turbulence Model ◽  
Layer Structure ◽  
Wall Region ◽  
Wall Roughness ◽  
Smooth Wall ◽  
Budget Analysis

A turbulent boundary layer structure which develop over a k-type rough wall displays several differences with those found on a smooth surface. The magnitude of the wake strength depends on the wall roughness. In the near-wall region, the contribution to the Reynolds shear stress fraction, corresponding to each event, strongly depends on the wall roughness. In the wall region, the diffusion factors are influenced by the wall roughness where the sweep events largely dominate the ejection events. This trend is reversed for the smooth-wall. Particle Image Velocimetry technique (PIV) is used to obtain the fluctuating flow field in the turbulent boundary layer in order to confirm this behavior. The energy budget analysis shows that the main difference between rough- and smooth-walls appears near the wall where the transport terms are larger for smooth-wall. Vertical and longitudinal turbulent flux of the shear stress on both smooth and rough surfaces is compared to those predicted by a turbulence model. The present results confirm that any turbulence model must take into account the effects of the surface roughness.

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