Influence of Freestream Turbulence Intensity on Bypass Transition Parameters in a Boundary Layer

2017 ◽  
Vol 139 (5) ◽  
Author(s):  
Joanna Grzelak ◽  
Zygmunt Wierciński
Keyword(s):  
Turbulence Intensity ◽  
Leading Edge ◽  
Mesh Size ◽  
Bypass Transition ◽  
One Dimensional ◽  
Subsonic Wind Tunnel ◽  
Turbulent Transition ◽  
Turbulence Degeneration ◽  
Grid Mesh ◽  
Transverse Variation

An experimental investigation was carried out to study the turbulent flow behind passive grids in a subsonic wind tunnel. The enhanced level of turbulence was generated by five wicker metal grids with square meshes and different parameters (diameter of the grid rod d = 0.3 to 3 mm and the grid mesh size M = 1 to 30 mm). The velocity of the flow was measured by means of a one-dimensional hot-wire probe. For this purpose, skewness, kurtosis, and transverse variation of the velocity fluctuations were determined, obtaining knowledge of the degree of turbulence isotropy and homogeneity in the flow behind grids of variable geometry, for different incoming velocities U = 4, 6, 10, 15, 20 m/s. Approximately, the isotropic and homogeneous turbulence was obtained for x/M > 30. Next, several correlations for turbulence degeneration law were tested. Finally, as the main goal of the study, impact of turbulence intensity on bypass laminar–turbulent transition parameters (transition inception, shape parameter, and the length of the transition region) on a flat plate was investigated. Parameter ITum was created as an integral taken from the leading edge of the plate to the transition inception, divided by the distance from the leading edge to the transition inception, expressing in this way the averaged value of turbulence intensity.

Evolution of the Laminar Boundary Layer Over a Flat Plate Under a Free Stream Turbulence Intensity of 1.3%

2008 ◽  
Author(s):  
E. J. Walsh ◽  
F. Brighenti ◽  
D. M. McEligot
Keyword(s):  
Boundary Layer ◽  
Flat Plate ◽  
Laminar Boundary Layer ◽  
Turbulence Intensity ◽  
Free Stream ◽  
Early Stage ◽  
Turbine Blades ◽  
Leading Edge ◽  
Bypass Transition ◽  
Free Stream Turbulence

The evolution of the laminar boundary layer over a flat plate under a free stream turbulence intensity of 1.3% is analysed. The effect of free stream turbulence on the onset of transition is one of the important sources leading to bypass transition. Such disturbances are of great interest in engineering for the prediction of transition on turbine blades. The study concentrates on the early part of the boundary layer, starting from the leading edge, and is characterised by the presence of streamwise elongated regions of high and low streamwise velocity. It is demonstrated that the so called “Klebanoff modes” are not entirely representative of the flow structures, due to the time-averaged representations used in most studies. For the conditions of this investigation it is found that the urms and the peak disturbances remain constant in the early stages of the transition development. This region, in which the streaks strength is constant, is problematic for many theories as it is not known where on a surface to initiate a growth theory calculation, and hence the prediction of transition onset is difficult. The observation that a constant urms region exists within the boundary layer under these conditions may be the source of great difficulty in predicting transition onset under turbulence levels around 1%. This region suggests that the streaks are either continuously generated and damped, or do not grow during the early stage of transition, and highlights the importance of continuous influence of the free stream turbulence along the boundary layer edge. This work concludes that the first is more likely, and furthermore the measurements are shown to agree with recent direct numerical simulations.

scholarly journals Experimental Investigation of Transition to Turbulence as Affected by Passing Wakes

2001 ◽  
Author(s):  
Richard W. Kaszeta ◽  
Terrence W. Simon ◽  
David E. Ashpis
Keyword(s):  
Reynolds Number ◽  
Turbulence Intensity ◽  
Free Stream ◽  
Turbine Engines ◽  
Bypass Transition ◽  
Pressure Gradients ◽  
Suction Surface ◽  
Laminar To Turbulent Transition ◽  
Turbulent Transition ◽  
Near Wall

This paper presents experimental results from a study of the effects of periodically passing wakes upon laminar-to-turbulent transition and separation in a low-pressure turbine passage. The test section geometry is designed to simulate unsteady wakes in turbine engines for studying their effects on boundary layers and separated flow regions over the suction surface by using a single suction surface and a single pressure surface to simulate a single turbine blade passage. Single-wire, thermal anemometry techniques are used to measure time-resolved and phase-averaged, wall-normal profiles of velocity, turbulence intensity and intermittency at multiple streamwise locations over the turbine airfoil suction surface. These data are compared to steady-state wake-free data collected in the same geometry to identify the effects of wakes upon laminar-to-turbulent transition. Results are presented for flows with a Reynolds number based on suction surface length and stage exit velocity of 50,000 and an approach flow turbulence intensity of 2.5%. While both existing design and experimental data are primarily concerned with higher Reynolds number flows (Re > 100,000), recent advances in gas turbine engines, and the accompanying increase in laminar and transitional flow effects, have made low-Re research increasingly important. From the presented data, the effects of passing wakes on transition and separation in the boundary layer, due to both increased turbulence levels and varying streamwise pressure gradients are presented. The results show how the wakes affect transition. The wakes affect the flow by virtue of their difference in turbulence levels and scales from those of the free-stream and by virtue of their ensemble-averaged velocity deficits, relative to the free-stream velocity, and the concomitant changes in angle of attack and temporal pressure gradients. The relationships between the velocity oscillations in the freestream and the unsteady velocity profile shapes in the near-wall flow are described. In this discussion is support for the theory that bypass transition is a response of the near-wall viscous layer to pressure fluctuations imposed upon it from the free-stream flow. Recent transition models are based on that premise. The data also show a significant lag between when the wake is present over the surface and when transition begins.

INFLUENCE OF THE PLATE LEADING-EDGE SHAPE AND THICKNESS ON THE BOUNDARY LAYER LAMINAR-TURBULENT TRANSITION IN HYPERSONIC FLOW

2019 ◽  
Vol 50 (5) ◽  
pp. 461-481
Author(s):  
Sergei Vasilyevich Aleksandrov ◽  
Evgeniya Andreevna Aleksandrova ◽  
Volf Ya. Borovoy ◽  
Andrey Vyacheslavovich Gubernatenko ◽  
Vladimir Evguenyevich Mosharov ◽  
...  
Keyword(s):  
Boundary Layer ◽  
Hypersonic Flow ◽  
Leading Edge ◽  
Turbulent Transition ◽  
Laminar Turbulent Transition

One-Dimensional Contact Pressure Distribution of Radial Tires in Motion

1995 ◽  
Vol 23 (2) ◽  
pp. 116-135 ◽  
Author(s):  
H. Shiobara ◽  
T. Akasaka ◽  
S. Kagami ◽  
S. Tsutsumi
Keyword(s):  
Pressure Distribution ◽  
Contact Pressure ◽  
Experimental Studies ◽  
Rolling Resistance ◽  
Leading Edge ◽  
Forward Direction ◽  
Contact Pressure Distribution ◽  
Support Ring ◽  
One Dimensional ◽  
Lowered Pressure

Abstract The contact pressure distribution and the rolling resistance of a running radial tire under load are fundamental properties of the tire construction, important to the steering performance of automobiles, as is well known. Many theoretical and experimental studies have been previously published on these tire properties. However, the relationships between tire performances in service and tire structural properties have not been clarified sufficiently due to analytical and experimental difficulties. In this paper, establishing a spring support ring model made of a composite belt ring and a Voigt type viscoelastic spring system of the sidewall and the tread rubber, we analyze the one-dimensional contact pressure distribution of a running tire at speeds of up to 60 km/h. The predicted distribution of the contact pressure under appropriate values of damping coefficients of rubber is shown to be in good agreement with experimental results. It is confirmed by this study that increasing velocity causes the pressure to rise at the leading edge of the contact patch, accompanied by the lowered pressure at the trailing edge, and further a slight movement of the contact area in the forward direction.

Numerical Investigation on Leading Edge of Film Cooling Blade with Different Turbulence and Blowing Ratios

2014 ◽  
Vol 1070-1072 ◽  
pp. 1731-1734
Author(s):  
Shao Hua Li ◽  
Ge Wu ◽  
Ling Zhang
Keyword(s):  
Temperature Field ◽  
Numerical Investigation ◽  
Turbulence Intensity ◽  
Film Cooling ◽  
Leading Edge ◽  
Cooling Efficiency ◽  
Simulated Temperature

In order to investigate the influence of cooling efficiency of leading edge of film cooling blade with different turbulence intensity and blowing ratios,which use method of N-S equation,various blowing ratios of 1.0、1.5 and 2.0,various turbulence intensity of 5%、12%、20% and 30%,it simulated temperature field in leading edge of film cooling blade.The results show: cooling efficiency decreased when blowing ratios is increased.When turbulence intensity is 5%、12% and 20%,it obtains maximum cooling efficiency blowing ratios of 1.0.When turbulence intensity is 30%,it obtains maximum cooling efficiency blowing ratios of 1.5. In blowing ratios of 1.0,cooling efficiency decreased when turbulence increased.But in blowing ratios of 1.5 and 2.0,cooling efficiency increased when turbulence increased.

scholarly journals Grid Dependency Study for the NASA Rotor 37 Compressor Blade

1997 ◽  
Author(s):  
Andrea Arnone ◽  
Ennio Carnevale ◽  
Michele Marconcini
Keyword(s):  
Total Pressure ◽  
Turbulence Modeling ◽  
Leading Edge ◽  
Substantial Improvement ◽  
Compressor Blade ◽  
Computational Domain ◽  
Temperature Ratio ◽  
Turbulent Transition ◽  
Total Temperature ◽  
The Impact

The NASA Rotor 37 has been computed by several authors in the last few years with relative success. The aim of this work is to present a systematic grid dependency study in order to quantify the amount of uncertainty that comes from the grid density. The computational domain is divided onto several regions (i.e. leading edge, trailing edge, shear layer …) and for each of them, the impact of the grid density is investigated. By means of this analysis, substantial improvement has been obtained in the prediction of efficiency and exit angle. On the contrary, the improvement achieved in total pressure and total temperature ratio is less remarkable. It is believed that only after a systematic grid dependency study can the contribution of turbulence modeling, laminar-turbulent transition, and boundary conditions be analyzed with success.

Stabilisation of subcritical bypass transition in the leading-edge boundary layer by suction

2014 ◽  
Vol 15 (11) ◽  
pp. 795-805 ◽  
Author(s):  
Michael O. John ◽  
Dominik Obrist ◽  
Leonhard Kleiser
Keyword(s):  
Boundary Layer ◽  
Leading Edge ◽  
Bypass Transition

scholarly journals Effect of Discrete Surface Disturbances on Vane External Heat Transfer

1997 ◽  
Author(s):  
R. S. Bunker
Keyword(s):  
Heat Transfer ◽  
Turbulence Intensity ◽  
Pressure Ratio ◽  
Leading Edge ◽  
Suction Side ◽  
External Heat ◽  
Freestream Turbulence ◽  
External Heat Transfer ◽  
Enhancement Factors ◽  
Surface Disturbances

A transonic linear vane cascade has been utilized to assess the effects of localized surface disturbances on airfoil external heat transfer coefficient distributions, such as those which may be created by the spallation of thermal barrier coatings. The cascade operates at an overall pressure ratio of 1.86, with an inlet total pressure of about 5 atm. Cascade Reynolds numbers based on axial chord length and exit velocity range from 2.2 to 4.8 · 106. Surface disturbances are modeled with the use of narrow trip strips glued onto the surface at selected locations, such that sharp forward facing steps are presented to the boundary layer. Surface locations investigated include the near leading edge region on either side of the stagnation point, the midchord region of the pressure side, and the high curvature region of the suction side. Heat transfer enhancement factors are obtained for disturbances with engine representative height-to-momentum thickness ratios, as a function of Reynolds number. Enhancement factors are compared for both smooth and rough airfoil surfaces with added disturbances, as well as low and high freestream turbulence intensity. Results show that leading edge heat transfer is dominated by freestream turbulence intensity effects, such that enhancements of nearly 50% at low turbulence levels are reduced to about 10% at elevated turbulence levels. Both pressure and suction side enhancement factors are dominated by surface roughness caused effects, with large enhancements for smooth surfaces being drastically reduced for roughened surfaces.

Tilt and Interfacial Fluid Pressure Measurements of a Disk Sliding on a Polymeric Pad

2005 ◽  
Vol 127 (1) ◽  
pp. 198-205 ◽  
Author(s):  
Sum Huan Ng ◽  
Len Borucki ◽  
C. Fred Higgs ◽  
Inho Yoon ◽  
Andre´s Osorno ◽  
...  
Keyword(s):  
Negative Pressure ◽  
Reynolds Equation ◽  
Fluid Pressure ◽  
Leading Edge ◽  
Dominant Factor ◽  
Pressure Measurements ◽  
Capacitive Sensing ◽  
One Dimensional ◽  
Pressure Mapping ◽  
Pressure Region

Previous experimental work has shown that negative fluid pressure does develop at the disk/pad interface during chemical mechanical polishing. However, these studies dealt with one-dimensional measurement and modeling. To better understand the problem, two-dimensional pressure mapping is carried out. In addition, the orientation of the disk is measured with a capacitive sensing technique. Results reveal a large negative pressure region at the disk/pad interface that is skewed toward the leading edge of the disk. The disk is also found to be leaning down toward the leading edge and toward the center of the pad. A mixed-lubrication model based on the Reynolds equation and taking into account the disk orientation angles has been developed. Modeling and experimental results show similar trends, indicating the tilting of the disk as a dominant factor in causing the negative pressure phenomenon.

scholarly journals Particle image velocimetry measurements of induced separation at the leading edge of a plate

2016 ◽  
Vol 804 ◽  
pp. 278-297 ◽  
Author(s):  
J. P. J. Stevenson ◽  
K. P. Nolan ◽  
E. J. Walsh
Keyword(s):  
Particle Image Velocimetry ◽  
Shear Layer ◽  
Particle Image ◽  
Reverse Flow ◽  
Leading Edge ◽  
Quadrant Analysis ◽  
Bypass Transition ◽  
Reynolds Stresses ◽  
Free Stream Turbulence ◽  
Image Velocimetry

The free shear layer that separates from the leading edge of a round-nosed plate has been studied under conditions of low (background) and elevated (grid-generated) free stream turbulence (FST) using high-fidelity particle image velocimetry. Transition occurs after separation in each case, followed by reattachment to the flat surface of the plate downstream. A bubble of reverse flow is thereby formed. First, we find that, under elevated (7 %) FST, the time-mean bubble is almost threefold shorter due to an accelerated transition of the shear layer. Quadrant analysis of the Reynolds stresses reveals the presence of slender, highly coherent fluctuations amid the laminar part of the shear layer that are reminiscent of the boundary-layer streaks seen in bypass transition. Instability and the roll-up of vortices then follow near the crest of the shear layer. These vortices are also present under low FST and in both cases are found to make significant contributions to the production of Reynolds stress over the rear of the bubble. But their role in reattachment, whilst important, is not yet fully clear. Instantaneous flow fields from the low-FST case reveal that the bubble of reverse flow often breaks up into two or more parts, thereby complicating the overall reattachment process. We therefore suggest that the downstream end of the ‘separation isoline’ (the locus of zero absolute streamwise velocity that extends unbroken from the leading edge) be used to define the instantaneous reattachment point. A histogram of this point is found to be bimodal: the upstream peak coincides with the location of roll-up, whereas the downstream mode may suggest a ‘flapping’ motion.

Sign in / Sign up

Export Citation Format

Share Document