Measurements and Prediction of Free-Stream Turbulence and Pressure-Gradient Effects on Attached-Flow Boundary-Layer Transition

2003 ◽  
Author(s):  
S. K. Roberts ◽  
M. I. Yaras
Keyword(s):  
Reynolds Number ◽  
Free Stream ◽  
Turbine Blades ◽  
Boundary Layer Transition ◽  
Pressure Gradients ◽  
Layer Transition ◽  
Free Stream Turbulence ◽  
Flow Reynolds Number ◽  
Attached Flow ◽  
Transition Length

This paper presents measurements of free-stream turbulence, streamwise pressure gradients and flow Reynolds number effects on attached-flow transition. The measurements were performed on a flat plate, at free-stream turbulence intensities ranging from 0.5% to 9.0%, four Reynolds numbers, and several streamwise pressure distributions, including ones that are typical of the suction side pressures of axial turbine blades. Based on the results, the extent of upstream movement of transition location with free-stream turbulence, the changes in transition length with variations in the streamwise pressure gradients, and the sensitivity of these trends to flow Reynolds number are quantified. Interpretation of the measurements is based primarily on streamwise and cross-stream intermittency distributions extracted from the velocity traces of hot-wire traverses. The measured transition inception locations and transition lengths are used to evaluate mathematical models available in the published literature. A modification is proposed to a transition length model to improve the prediction of the streamwise intermittency distribution.

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.

Free-Stream Turbulence and Pressure Gradient Effects on Heat Transfer and Boundary Layer Development on Highly Cooled Surfaces

1985 ◽  
Vol 107 (1) ◽  
pp. 54-59 ◽  
Author(s):  
K. Rued ◽  
S. Wittig
Keyword(s):  
Heat Transfer ◽  
Boundary Layer ◽  
Free Stream ◽  
Boundary Layer Transition ◽  
Pressure Gradients ◽  
Layer Transition ◽  
Free Stream Turbulence ◽  
Boundary Layer Development ◽  
Gradient Effects ◽  
Layer Development

Heat transfer and boundary layer measurements were derived from flows over a cooled flat plate with various free-stream turbulence intensities (Tu = 1.6–11 percent), favorable pressure gradients (k = νe/ue2•due/dx = 0÷6•10−6) and cooling intensities (Tw/Te = 1.0–0.53). Special interest is directed towards the effects of the dominant parameters, including the influence on laminar to turbulent boundary layer transition. It is shown, that free-stream turbulence and pressure gradients are of primary importance. The increase of heat transfer due to wall cooling can be explained primarily by property variations as transition, and the influence of free-stream parameters are not affected.

Evaluation of Turbulent Spot Production Rate in Boundary Layers Under Variable Pressure Gradients for Gas Turbine Applications

2019 ◽  
Author(s):  
M. Dellacasagrande ◽  
D. Lengani ◽  
D. Simoni ◽  
M. Ubaldi ◽  
P. Zunino
Keyword(s):  
Reynolds Number ◽  
Production Rate ◽  
Turbulence Intensity ◽  
Free Stream ◽  
Separated Flow ◽  
Pressure Gradients ◽  
Turbulent Spot ◽  
Free Stream Turbulence ◽  
Flow Parameters ◽  
Transition Length

Abstract The paper presents an experimental data base on transitional boundary layers developing on a flat plate installed within a variable area opening endwall channel. Measurements have been carried out by means of time-resolved PIV. The overall test matrix spans 3 Reynolds numbers, 4 free-stream turbulence intensity levels and 4 different flow adverse pressure gradients. For each condition, 16000 instantaneous flow fields have been acquired in order to obtain high statistical accuracy. The flow parameters have been varied in order to provide a gradual shift of the mode of transition from a bypass process occurring with mild adverse pressure gradients at high free-stream turbulence, to separated flow transition, occurring with low Reynolds number, low free-stream turbulence intensity and elevated adverse pressure gradient. In order to quantify the influence of the flow parameter variation on the boundary layer transition process, the transition onset and end positions, and the turbulent spot production rate have been evaluated with a wavelet based intermittency detection technique. This post-processing technique is in fact able to identify the vortical structures developing within the boundary layer, the intermittency function is then automatically evaluated for each tested condition counting the number of such structures and defining the cumulative probability function. The by-pass transition mode has the longest transition length that decreases with increasing the Reynolds number. The transition length of the separated flow case is smaller than the by-pass one, and the variation of the flow parameters has a similar impact. Similarly, the dimensionless turbulent spot production rate reduces when the Reynolds number is increasing. The variation of the inlet turbulence intensity has a small influence on this parameter except for the condition at the highest turbulence intensity, that always shows the lowest turbulent spot production rate because a by-pass type transition occurs. This large amount of data has been used to develop new correlations used to predict the spot production rate and the transition length in attached and separated flows.

Boundary Layer Transition at High Levels of Free Stream Turbulence

1998 ◽  
Author(s):  
Masaharu Matsubara ◽  
P. Henrik Alfredsson ◽  
K. Johan A. Westin
Keyword(s):  
Boundary Layer ◽  
Boundary Layers ◽  
Free Stream ◽  
Turbine Blades ◽  
Boundary Layer Transition ◽  
Transition To Turbulence ◽  
Flow Visualisation ◽  
Mean Flow ◽  
Layer Transition ◽  
Free Stream Turbulence

Transition to turbulence in laminar boundary layers subjected to high levels of free stream turbulence (FST) can still not be reliably predicted, despite its technical importance, e.g. in the case of boundary layers developing on gas turbine blades. In a series of experiments in the MTL-wind tunnel at KTH the influence of grid-generated FST on boundary layer transition has been studied, with FST-levels up to 6%. It was shown from both flow visualisation and hot-wire measurements that the boundary layer develops unsteady streaky structures with high and low streamwise velocity. This leads to large amplitude low frequency fluctuations inside the boundary layer although the mean flow is still close to the laminar profile. Breakdown to turbulence occurs through an instability of the streaks which leads to the formation of turbulent spots. Accurate physical modelling of these processes seems to be needed in order to obtain a reliable prediction method.

Simulating Boundary Layer Transition With Low-Reynolds-Number k–ε Turbulence Models: Part 1—An Evaluation of Prediction Characteristics

1991 ◽  
Vol 113 (1) ◽  
pp. 10-17 ◽  
Author(s):  
R. C. Schmidt ◽  
S. V. Patankar
Keyword(s):  
Boundary Layer ◽  
Reynolds Number ◽  
Free Stream ◽  
Low Reynolds Number ◽  
Turbulence Models ◽  
Boundary Layer Transition ◽  
External Boundary ◽  
Layer Transition ◽  
Free Stream Turbulence ◽  
Low Reynolds

An analysis and evaluation of the capability of k–ε low-Reynolds-number turbulence models to predict transition in external boundary-layer flows subject to free-stream turbulence is presented. The similarities between the near-wall cross-stream regions in a fully turbulent boundary layer and the progressive stages through which developing boundary layers pass in the streamwise direction are used to describe the mechanisms by which the models simulate the transition process. Two representative models (Jones and Launder, 1972; Lam and Bremhorst, 1981) are employed in a series of computational tests designed to answer some specific practical questions about the ability of these models to yield accurate, reliable answers over a range of free-stream turbulence conditions.

scholarly journals Boundary Layer Transition of Strongly Concave Surfaces

1989 ◽  
Author(s):  
Stephen Riley ◽  
Mark W. Johnson ◽  
John C. Gibbings
Keyword(s):  
Boundary Layer ◽  
Free Stream ◽  
Boundary Layer Transition ◽  
Radius Of Curvature ◽  
Pressure Gradients ◽  
Flat Plates ◽  
Layer Transition ◽  
Free Stream Turbulence ◽  
Surface Data ◽  
Correlation Techniques

Boundary layer transition has been studied on two blades of constant 0.5 and 1 metre radius of curvature with free stream turbulence levels of 0.7%, 2.6% and 7.2%. Zero pressure gradients were used throughout. Strong Gortler vortices developed in the boundary layer which led to growth rates of up to ten times the flat plate rate. The boundary layer profile was also highly distorted by the vortices. Transition correlation techniques for flat plates proved totally inadequate for the concave surface data, but a method of obtaining correlations for these surfaces was suggested by considering the inner critical region of the boundary layer alone.

Effects of Periodic Unsteadiness, Free-Stream Turbulence, and Flow Reynolds Number on Separation-Bubble Transition

2003 ◽  
Author(s):  
S. K. Roberts ◽  
M. I. Yaras
Keyword(s):  
Reynolds Number ◽  
Free Stream ◽  
Separation Bubble ◽  
Turbine Blades ◽  
Separated Flow ◽  
Suction Side ◽  
Streamwise Velocity ◽  
Flow Transition ◽  
Free Stream Turbulence ◽  
Flow Reynolds Number

This paper presents measurements of the combined effects of free-stream turbulence and periodic streamwise velocity variations on separation-bubble transition. The measurements were performed on a flat plate at two values of flow Reynolds number, with a streamwise pressure distribution similar to those encountered on the suction side of axial turbine blades. The experiment was designed to facilitate independent control of turbulence and periodic velocity fluctuations in the free-stream. The free-stream turbulence intensity was varied from 0.4% to 4.5%, and the periodic unsteadiness corresponded to Strouhal numbers of 0.0, 2.4 and 4.0. Based on the results, the relative importance of free-stream turbulence and periodic unsteadiness on the streamwise locations of separation, transition and reattachment points are quantified. Existing mathematical models for predicting separated-flow transition and reattachment are then evaluated in this context.

Effects of Adverse Pressure Gradients on the Nature and Length of Boundary Layer Transition

1990 ◽  
Vol 112 (2) ◽  
pp. 196-205 ◽  
Author(s):  
G. J. Walker ◽  
J. P. Gostelow
Keyword(s):  
Pressure Gradient ◽  
Adverse Pressure Gradient ◽  
Basic Mechanism ◽  
Boundary Layer Transition ◽  
Pressure Gradients ◽  
Layer Transition ◽  
Instability Waves ◽  
Free Stream Turbulence ◽  
Transition Length ◽  
Tollmien Schlichting

Existing transition models are surveyed and deficiencies in previous predictions, which seriously overestimate transition length under an adverse pressure gradient, are discussed. A new model for transition in an adverse pressure gradient situation is proposed and experimental results are provided that confirm its validity. A correlation for transition length is advanced that incorporates both Reynolds number and pressure gradient effects. Under low free-stream turbulence conditions the basic mechanism of transition is laminar instability. There are, however, physical differences between zero and adverse pressure gradients. In the former case, transition occurs randomly, due to the breakdown of laminar instability waves in sets. For an adverse pressure gradient, the Tollmien–Schlichting waves appear more regularly with a well-defined spectral peak. As the adverse pressure gradient is increased from zero to the separation value the flow evolves continuously from random to periodic behavior and the dimensionless transition length progressively decreases.

Bypass Transition Modelling: A New Method Which Accounts for Free-Stream Turbulence Intensity and Length Scale

2000 ◽  
Author(s):  
Paul E. Roach ◽  
David H. Brierley
Keyword(s):  
Boundary Layer ◽  
Turbulence Intensity ◽  
Free Stream ◽  
Leading Edge ◽  
Boundary Layer Transition ◽  
Test Surface ◽  
Length Scale ◽  
Pressure Gradients ◽  
Layer Transition ◽  
Free Stream Turbulence

The publication of the present authors’ boundary layer transition data in 1992 (now widely known as the ERCOFTAC test case T3) has led to a spate of new experimental and modelling efforts aimed at improving our understanding of this problem. This paper describes a new method of determining boundary layer transition with zero mean pressure gradient. The approach examines the development of a laminar boundary layer to the start of transition, accounting for the influences of free-stream turbulence and test surface geometry. It is presented as a “proof of concept”, requiring a significant amount of work before it can be considered as a practically applicable model for transition prediction. The method is based upon one first put forward by G.I. Taylor in the 1930’s, and accounts for the action of local, instantaneous pressure gradients on the developing laminar boundary layer. These pressure gradients are related to the intensity and length scale of turbulence in the free-stream using Taylor’s simple isotropic model. The findings demonstrate the need to account for the separate influences of free-stream turbulence intensity and length scale when considering the transition process. Although the length scale has less of an effect than the intensity, its influence is, nevertheless, significant and must not be overlooked. This fact goes a long way towards explaining the large scatter to be found in simple correlations which involve only the turbulence intensity. Intriguingly, it is demonstrated that it is the free-stream turbulence at the leading edge of the test surface which is important, not that found locally outside the boundary layer. The additional influence of leading edge geometry is also shown to play a major role in fixing the point at which transition begins. It is suggested that the leading edge geometry will distort the incident turbulent eddies, modifying the effective “free-stream” turbulence properties. Consequently, it is shown that the scale of the eddies relative to the leading edge thickness is a further important parameter, and helps bring together a large number of test cases.

Boundary-Layer Transition Affected by Surface Roughness and Free-Stream Turbulence

2005 ◽  
Vol 127 (3) ◽  
pp. 449-457 ◽  
Author(s):  
S. K. Roberts ◽  
M. I. Yaras
Keyword(s):  
Boundary Layer ◽  
Surface Roughness ◽  
Free Stream ◽  
Separation Bubble ◽  
Boundary Layer Transition ◽  
Test Cases ◽  
Layer Transition ◽  
Test Matrix ◽  
Free Stream Turbulence ◽  
Attached Flow

This paper presents experimental results documenting the effects of surface roughness and free-stream turbulence 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 low-pressure turbine blades was imposed. The test matrix consists of five variations in the roughness conditions, at each of three free-stream turbulence intensities (approximately 0.5%, 2.5%, and 4.5%), and two flow Reynolds numbers of 350,000 and 470,000. 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 focus of the paper is on separation-bubble transition, but the few attached-flow test cases that occurred under high roughness and free-stream turbulence conditions are also presented for completeness of the test matrix. Based on the experimental results, the effects of surface roughness on the location of transition onset and the rate of transition are quantified, and the sensitivity of these effects to free-stream turbulence is established. The Tollmien–Schlichting instability mechanism is shown to be responsible for transition in each of the test cases presented. The root-mean-square height of the surface roughness elements, their planform size and spacing, and the skewness (bias towards depression or protrusion roughness) of the roughness distribution are shown to be relevant to quantifying the effects of roughness on the transition process.

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