Effects of Surface-Roughness Geometry on Separation-Bubble Transition

2005 ◽  
Vol 128 (2) ◽  
pp. 349-356 ◽  
Author(s):  
Stephen K. Roberts ◽  
Metin I. Yaras
Keyword(s):  
Surface Roughness ◽  
Separation Bubble ◽  
Turbine Blades ◽  
Dominant Mode ◽  
Roughness Height ◽  
Height Variation ◽  
Separated Shear Layer ◽  
Roughness Elements ◽  
Negative Skewness ◽  
Typical Range

This paper presents measurements of separation-bubble transition over a range of surfaces with randomly distributed roughness elements. The tested roughness patterns represent the typical range of roughness conditions encountered on in-service turbine blades. Through these measurements, the effects of size and spacing of the roughness elements, and the tendency of the roughness pattern toward protrusions or depressions (skewness), on the inception location and rate of transition are evaluated. Increased roughness height, increased spacing of the roughness elements, and a tendency of the roughness pattern toward depressions (negative skewness) are observed to promote earlier transition inception. The observed effects of roughness spacing and skewness are found to be small in comparison to that of the roughness height. Variation in the dominant mode of instability in the separated shear layer is achieved through adjustment of the streamwise pressure distribution. The results provide examples for the extent of interaction between viscous and inviscid stability mechanisms.

Effects of Surface-Roughness Geometry on Separation-Bubble Transition

2005 ◽  
Author(s):  
Stephen K. Roberts ◽  
Metin I. Yaras
Keyword(s):  
Surface Roughness ◽  
Separation Bubble ◽  
Turbine Blades ◽  
Dominant Mode ◽  
Roughness Height ◽  
Height Variation ◽  
Separated Shear Layer ◽  
Roughness Elements ◽  
Negative Skewness ◽  
Typical Range

This paper presents measurements of separation-bubble transition over a range of surfaces with randomly distributed roughness elements. The tested roughness patterns represent the typical range of roughness conditions encountered on in-service turbine blades. Through these measurements, the effects of size and spacing of the roughness elements, and the tendency of the roughness pattern toward protrusions or depressions (skewness), on the inception location and rate of transition are evaluated. Increased roughness height, increased spacing of the roughness elements, and a tendency of the roughness pattern toward depressions (negative skewness) are observed to promote earlier transition inception. The observed effects of roughness spacing and skewness are found to be small in comparison to that of the roughness height. Variation in the dominant mode of instability in the separated shear layer is achieved through adjustment of the streamwise pressure distribution. The results provide examples for the extent of interaction between viscous and inviscid stability mechanisms.

Numerical Study of Turbulent Spot Development in a Separated Shear Layer

2007 ◽  
Author(s):  
B. R. McAuliffe ◽  
M. I. Yaras
Keyword(s):  
Shear Layer ◽  
Internal Structure ◽  
Numerical Study ◽  
Separation Bubble ◽  
Turbine Blades ◽  
Freestream Turbulence ◽  
Helmholtz Instability ◽  
Turbulent Spots ◽  
Separated Shear Layer ◽  
Vortex Loops

The development of turbulent spots in a separation bubble under elevated freestream turbulence levels is examined through direct numerical simulation. The flow Reynolds number, freestream turbulence level, and streamwise pressure distribution are typical of the conditions encountered on the suction side of low-pressure turbine blades of gas-turbine engines. Based on the simulation results, the spreading and propagation rates of the turbulent spots and their internal structure are documented, and comparisons are made to empirical correlations that are used for predicting the transverse growth and streamwise propagation characteristics of turbulent spots. The internal structure of the spots is identified as a series of vortex loops that develop as a result of low-velocity streaks generated in the shear layer. A frequency that is approximately 50% higher than that of the Kelvin-Helmholtz instability is identified in the separated shear layer, which is shown to be associated with convection of these vortex loops through the separated shear layer. While freestream turbulence is noted to promote breakdown of the laminar separated shear layer into turbulence through the generation of turbulent spots, evidence is found to suggest co-existence of the Kelvin-Helmholtz instability, including the possibility of breakdown to turbulence through this mechanism.

Surface Roughness Impact on Boundary Layer Transition and Loss Mechanisms over a Flat-Plate under a Low-Pressure Turbine Pressure Gradient

2021 ◽  
pp. 1-40
Author(s):  
Heechan Jeong ◽  
Seung Jin Song
Keyword(s):  
Surface Roughness ◽  
Pressure Gradient ◽  
Shear Layer ◽  
Flat Plate ◽  
Turbulence Intensity ◽  
Turbulent Mixing ◽  
Separation Bubble ◽  
Low Pressure Turbine ◽  
Separated Shear Layer ◽  
Profile Loss

Abstract An experimental study has been conducted to investigate the effects of surface roughness on the profile loss of a flat-plate with a contoured wall. All of the measurements have been conducted for the suction side pressure gradient of a high-lift low pressure turbine airfoil at the fixed Reynolds number (Rec) and freestream turbulence intensity (Tu) of 1.2 · 105 and 3.2%, respectively, representing a cruise condition. The time-resolved streamwise and wall-normal velocity fields for three different surface roughness values of Ra/C · 105 = 0.065, 4.417 and 7.428 have been measured with a 2D hot-wire probe. For the smooth surface, a laminar separation bubble forms from about 60% of the chord; and laminar-to-turbulent transition occurs during reattachment. Since the portion of turbulent flow over the flat-plate is relatively small, the overall profile loss is mainly determined by the momentum deficit generated during transition. Increased roughness decreases the maximum height and length of the separation bubble but does not affect the separation bubble onset location. The beneficial effects of increased surface roughness on the profile loss appear in the separated shear layer and reattachment. Increased surface roughness increases turbulent mixing in the separated shear layer. Thus, the shear layer thickness and momentum deficit are reduced. In addition, increased surface roughness reduces the length scale and turbulence intensity of the shed vortices. Consequently, turbulent mixing and momentum deficit during reattachment of boundary layers are decreased, resulting in a lower profile loss.

scholarly journals Weather response to a large wind turbine array

2010 ◽  
Vol 10 (2) ◽  
pp. 769-775 ◽  
Author(s):  
D. B. Barrie ◽  
D. B. Kirk-Davidoff
Keyword(s):  
Surface Roughness ◽  
Wind Turbines ◽  
General Circulation ◽  
Wind Farm ◽  
Turbine Blades ◽  
Circulation Model ◽  
Wind Farms ◽  
The North ◽  
Roughness Elements ◽  
Electrical Generation

Abstract. Electrical generation by wind turbines is increasing rapidly, and has been projected to satisfy 15% of world electric demand by 2030. The extensive installation of wind farms would alter surface roughness and significantly impact the atmospheric circulation due to the additional surface roughness forcing. This forcing could be changed deliberately by adjusting the attitude of the turbine blades with respect to the wind, which would enable the "management" of a large array of wind turbines. Using a General Circulation Model (GCM), we represent a continent-scale wind farm as a distributed array of surface roughness elements. Here we show that initial disturbances caused by a step change in roughness grow within four and a half days such that the flow is altered at synoptic scales. The growth rate of the induced perturbations is largest in regions of high atmospheric instability. For a roughness change imposed over North America, the induced perturbations involve substantial changes in the track and development of cyclones over the North Atlantic, and the magnitude of the perturbations rises above the level of forecast uncertainty.

Boundary-Layer Transition Over Rough Surfaces With Elevated Free-Stream Turbulence

2004 ◽  
Author(s):  
S. K. Roberts ◽  
M. I. Yaras
Keyword(s):  
Boundary Layer ◽  
Surface Roughness ◽  
Free Stream ◽  
Separation Bubble ◽  
Turbine Blades ◽  
Boundary Layer Transition ◽  
Experimental Results ◽  
Layer Transition ◽  
Free Stream Turbulence ◽  
Attached Flow

This paper presents experimental results documenting the combined effects of surface roughness and free-stream turbulence level on boundary-layer transition. The experiments were conducted on a flat surface, upon which a pressure distribution similar to those prevailing on the suction side of turbine blades was imposed. The test matrix consists of four variations in the roughness conditions, at each of three free-stream turbulence levels and two flow Reynolds numbers. The ranges of these parameters considered in the study, which are typical of low-pressure turbines, resulted in both attached-flow and separation-bubble transition. The experimental results show that the transition inception location remains sensitive to surface roughness with increasing free-stream turbulence. Through spectral analysis of the velocity signals, this is shown to be due to earlier appearance and larger amplitude of Tollmien-Schlichting instability waves in both attached-flow and separation-bubble transition. In the test cases in which a separation-bubble is present, the rate of transition is seen to be insensitive to surface roughness, and only mildly sensitive to free-stream turbulence.

scholarly journals Experimental and CFD Analysis of Impact of Surface Roughness on Hydrodynamic Performance of a Darrieus Hydro (DH) Turbine

Energies ◽  
2020 ◽  
Vol 13 (4) ◽  
pp. 928 ◽  
Author(s):  
Mohammad Hassan Khanjanpour ◽  
Akbar A. Javadi
Keyword(s):  
Surface Roughness ◽  
Drag Coefficient ◽  
Output Power ◽  
Negative Impact ◽  
Pressure Coefficient ◽  
Turbine Blades ◽  
Roughness Height ◽  
Hydrodynamic Performance ◽  
Cfd Modeling ◽  
Wide Range

Although improving the hydrodynamic performance is a key objective in the design of ocean-powered devices, there are some factors that affect the efficiency of the device during its operation. In this study, the impacts of a wide range of surface roughness as a tribological parameter on stream flow around a hydro turbine and its power loss are studied. A comprehensive program of 3D Computational Fluid Dynamics (CFD) modeling, as well as an expansive range of experiments were carried out on a Darrieus Hydro (DH) turbine in order to measure reduction in hydrodynamic performance due to surface roughness. The results show that surface roughness of turbine blades plays an important role in the hydrodynamics of the flow around the turbine. The surface roughness increases turbulence and decreases the active fluid energy that is required for rotating the turbine, thereby reducing the performance of the turbine. The extent of the negative impact of surface roughness on the drag coefficient, pressure coefficient, torque, and output power is evaluated. It is shown that the drag coefficient of a turbine with roughness height of 1000 μm is about 20% higher than a smooth blade (zero roughness height) and the maximum percentage of reduction of output power could be up to 27% (numerically) and 22% (experimentally).

Numerical Study of Turbulent-Spot Development in a Separated Shear Layer

2008 ◽  
Vol 130 (4) ◽  
Author(s):  
Brian R. McAuliffe ◽  
Metin I. Yaras
Keyword(s):  
Shear Layer ◽  
Internal Structure ◽  
Numerical Study ◽  
Separation Bubble ◽  
Turbine Blades ◽  
Freestream Turbulence ◽  
Helmholtz Instability ◽  
Turbulent Spots ◽  
Separated Shear Layer ◽  
Vortex Loops

The development of turbulent spots in a separation bubble under elevated freestream turbulence levels is examined through direct numerical simulation. The flow Reynolds number, freestream turbulence level, and streamwise pressure distribution are typical of the conditions encountered on the suction side of low-pressure turbine blades of gas-turbine engines. Based on the simulation results, the spreading and propagation rates of the turbulent spots and their internal structure are documented, and comparisons are made to empirical correlations that are used for predicting the transverse growth and streamwise propagation characteristics of turbulent spots. The internal structure of the spots is identified as a series of vortex loops that develop as a result of low-velocity streaks generated in the shear layer. A frequency that is approximately 50% higher than that of the Kelvin–Helmholtz instability is identified in the separated shear layer, which is shown to be associated with the convection of these vortex loops through the separated shear layer. While freestream turbulence is noted to promote breakdown of the laminar separated shear layer into turbulence through the generation of turbulent spots, evidence is found to suggest coexistence of the Kelvin–Helmholtz instability, including the possibility of breakdown to turbulence through this mechanism.

The Transition Mechanism of Highly-Loaded LP Turbine Blades

2003 ◽  
Author(s):  
R. D. Stieger ◽  
H. P. Hodson
Keyword(s):  
Boundary Layer ◽  
Shear Layer ◽  
Separation Bubble ◽  
Turbine Blades ◽  
Suction Surface ◽  
Transition Mechanism ◽  
Separated Shear Layer ◽  
Lp Turbine ◽  
Laminar Shear ◽  
Highly Loaded

A detailed experimental investigation was conducted into the interaction of a convected wake and a separation bubble on the rear suction surface of a highly loaded low-pressure (LP) turbine blade. Boundary layer measurements, made with 2D LDA, revealed a new transition mechanism resulting from this interaction. Prior to the arrival of the wake, the boundary layer profiles in the separation region are inflexional. The perturbation of the separated shear layer caused by the convecting wake causes an inviscid Kelvin-Helmholtz rollup of the shear layer. This results in the breakdown of the laminar shear layer and a rapid wake-induced transition in the separated shear layer.

The Transition Mechanism of Highly Loaded Low-Pressure Turbine Blades

2004 ◽  
Vol 126 (4) ◽  
pp. 536-543 ◽  
Author(s):  
R. D. Stieger ◽  
H. P. Hodson
Keyword(s):  
Boundary Layer ◽  
Shear Layer ◽  
Separation Bubble ◽  
Turbine Blades ◽  
Low Pressure ◽  
Suction Surface ◽  
Transition Mechanism ◽  
Separated Shear Layer ◽  
Lp Turbine ◽  
Highly Loaded

A detailed experimental investigation was conducted into the interaction of a convected wake and a separation bubble on the rear suction surface of a highly loaded low-pressure (LP) turbine blade. Boundary layer measurements, made with 2D LDA, revealed a new transition mechanism resulting from this interaction. Prior to the arrival of the wake, the boundary layer profiles in the separation region are inflexional. The perturbation of the separated shear layer caused by the convecting wake causes an inviscid Kelvin-Helmholtz rollup of the shear layer. This results in the breakdown of the laminar shear layer and a rapid wake-induced transition in the separated shear layer.

Boundary-Layer Transition in Separation Bubbles Over Rough Surfaces

2004 ◽  
Author(s):  
S. K. Roberts ◽  
M. I. Yaras
Keyword(s):  
Boundary Layer ◽  
Surface Roughness ◽  
Separation Bubble ◽  
Turbine Blades ◽  
Suction Side ◽  
Boundary Layer Transition ◽  
Test Surface ◽  
Layer Transition ◽  
Free Stream Turbulence ◽  
Separation Bubbles

This paper documents the effects of surface roughness on boundary layer transition in separation-bubbles under low free-stream turbulence conditions (<1%). The experiments were performed on a flat surface, upon which a pressure distribution similar to those prevailing on the suction side of low-pressure turbine blades was imposed. The test matrix consists of four variations in the roughness conditions, including a reference test case with a smooth surface. The remaining roughness levels are typical of in-service turbine blades in gas turbine engines. The measurements were performed at flow Reynolds numbers of 350,000 and 470,000, based on the length of the test surface. The separation, transition inception, transition completion, and re-attachment locations, and the streamwise intermittency distributions in the transition region are documented for each of the test cases. Increasing surface roughness is shown to result in earlier transition inception, and consequently, a reduced size of the separation-bubble. However, the presence of surface roughness does not appear to have a significant effect on the rate of transition within the separation-bubble.

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