Effect of boundary layer destabilization by a water jet on thermal and structural behavior of turbulent spot footprints

Abstract High Reynolds flow over a nozzle guide-vane with elevated inflow turbulence was simulated using wall-resolved large eddy simulation (LES). The simulations were undertaken at an exit Reynolds number of 0.5 × 106 and inflow turbulence levels of 0.7% and 7.9% and for uniform heat-flux boundary conditions corresponding to the measurements of Varty and Ames (2016, “Experimental Heat Transfer Distributions Over an Aft Loaded Vane With a Large Leading Edge at Very High Turbulence Levels,” ASME Paper No. IMECE2016-67029). The predicted heat transfer distribution over the vane is in excellent agreement with measurements. At higher freestream turbulence, the simulations accurately capture the laminar heat transfer augmentation on the pressure surface and the transition to turbulence on the suction surface. The bypass transition on the suction surface is preceded by boundary layer streaks formed under the external forcing of freestream disturbances which breakdown to turbulence through inner-mode secondary instabilities. Underneath the locally formed turbulent spot, heat transfer coefficient spikes and generally follows the same pattern as the turbulent spot. The details of the flow and temperature fields on the suction side are characterized, and first- and second-order statistics are documented. The turbulent Prandtl number in the boundary layer is generally in the range of 0.7–1, but decays rapidly near the wall.

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Studies on Bypass Transition of a Boundary Layer Subjected to Localized Periodic External Disturbances

Volume 4: Turbo Expo 2004 ◽

10.1115/gt2004-53305 ◽

2004 ◽

Cited By ~ 1

Author(s):

K. Funazaki ◽

Y. Wakita ◽

T. Otsuki

Keyword(s):

Boundary Layer ◽

Flat Plate ◽

External Disturbance ◽

Transition Process ◽

Bypass Transition ◽

Hot Wire ◽

Turbulent Spot ◽

External Disturbances ◽

Small Sphere ◽

Probe Sensor

This study aims at clarification of wake-induced bypass transition process of a boundary layer on a flat plate with no pressure gradient. Special attention is paid to inception as well as growth of a turbulent spot created by the incoming wake as an external disturbance. To meet this goal a unique wake generator is invented to create an isolated turbulent spot. A multi-probe sensor with seven single-hot-wire probes is used to measure wake-affected boundary layer. The wake generator consists of a disk, pillars and a very thin wire with a small sphere on it. The sphere on the wire generates periodic wakes behind it when it passes across the main flow in front of the test flat plate. These sphere wakes impinge the flat plate in a spatially and timewisely localized manner so that the wakes periodically leave narrow affected zones inside the boundary layer. The observations confirm that an isolated turbulence spot emerges from each of those wake-affected zones. It is also found that the turbulent spot observed in this study bears a close resemblance to the conventional turbulent spot that takes a shape of arrowhead pointing downstream.

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Turbulent spot flow topology and mechanisms for surface heat transfer

Journal of Fluid Mechanics ◽

10.1017/s0022112008002838 ◽

2008 ◽

Vol 612 ◽

pp. 81-105 ◽

Cited By ~ 8

Author(s):

D. R. SABATINO ◽

C. R. SMITH

Keyword(s):

Heat Transfer ◽

Boundary Layer ◽

Turbulent Boundary Layer ◽

Surface Heat ◽

Heated Plate ◽

Surface Shear Stress ◽

Turbulent Spot ◽

Flow Topology ◽

Turbulent Spots ◽

Surface Heat Transfer

The properties of artificially initiated turbulent spots over a heated plate were investigated in a water channel. The instantaneous velocity field and surface Stanton number were simultaneously established using a technique that combines particle image velocimetry and thermochromic liquid crystal thermography. Several characteristics of a spot are found to be similar to those of a turbulent boundary layer. The spacing of the surface heat transfer streak patterns within the middle or ‘body’ of a turbulent spot are comparable to the low-speed streak spacing within a turbulent boundary layer. Additionally, the surface shear stress in the same region of a spot is also found to be comparable to a turbulent boundary layer. However, despite these similarities, the heat transfer within the spot body is found to be markedly less than the heat transfer for a turbulent boundary layer. In fact, the highest surface heat transfer occurs at the trailing or calmed region of a turbulent spot, regardless of maturity. Using a modified set of similarity coordinates, instantaneous two-dimensional streamlines suggest that turbulent spots entrain and subsequently recirculate warm surface fluid, thereby reducing the effective heat transfer within the majority of the spot. It is proposed that energetic vortices next to the wall, near the trailing edge of the spot body, are able to generate the highest surface heat transfer because they have the nearest access to cooler free-stream fluid.

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A ‘turbulent spot’ in an axisymmetric free shear layer. Part 1

Journal of Fluid Mechanics ◽

10.1017/s0022112080000043 ◽

1980 ◽

Vol 98 (1) ◽

pp. 65-95 ◽

Cited By ~ 16

Author(s):

M. Sokolov ◽

A. K. M. F. Hussain ◽

S. J. Kleis ◽

Z. D. Husain

Keyword(s):

Boundary Layer ◽

High Speed ◽

Large Scale ◽

Mixing Layer ◽

Streamwise Velocity ◽

Reynolds Stresses ◽

Free Shear Layer ◽

Turbulent Spot ◽

Air Jet ◽

Phase Average

A three-dimensional ‘turbulent spot’ has been induced in the axisymmetric free mixing layer of a 12.7 cm diameter air jet by a spark generated at the nozzle boundary layer upstream of the exit. The spot coherent-structure signature, buried in the large-amplitude random fluctuating signal, has been educed at three downstream stations within the apparent self-preserving region of the mixing layer (i.e. x/D = 1.5, 3.0 and 4.5) at the jet exit speed of 20 ms−1. The eduction has been performed through digital phase averaging of the spot signature from 200 realizations. In order to reduce the effect of the turbulence-induced jitter on the phase average, individual filtered signal arrays were optimally time-aligned through an iterative process of cross-correlation of each realization with the ensemble average. Further signal enhancement was achieved through rejection of realizations requiring excessive time shifts for alignment. The number of iterations required and the fraction of realizations rejected progressively increase with the downstream distance and the radial position.The mixing-layer spot is a large-scale elongated structure spanning the entire width of the layer but does not appear to exhibit a self-similar shape. The dynamics of the mixing-layer spot and its eduction are more complicated than those of the boundary-layer spot. The spot initially moves downstream essentially at a uniform speed across the mixing layer, but further downstream it accelerates on the high-speed side and decelerates on the low-speed side. This paper discusses the data acquisition and processing techniques and the results based on the streamwise velocity signals. Phase average distributions of vorticity, pseudo-streamlines, coherent and background Reynolds stresses and further dynamics of the spot are presented in part 2 (Hussain, Kleis & Sokolov 1980).

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Studies of Wake-Induced Bypass Transition of a Flat-Plate Boundary Layer: Comparisons Between Two Transition Modes Induced by Small Sphere Wake and Thin Wire Wake

Volume 3: Turbo Expo 2005, Parts A and B ◽

10.1115/gt2005-68355 ◽

2005 ◽

Author(s):

Ken-ichi Funazaki ◽

Takahiro Otsuki

Keyword(s):

Boundary Layer ◽

Flat Plate ◽

Plate Boundary ◽

Transition Process ◽

Bypass Transition ◽

Velocity Data ◽

Turbulent Spot ◽

Transition Modes ◽

The Wire ◽

Flat Plate Boundary Layer

This study aims at deepening the understanding of wake-induced bypass transition process of a flat-plate boundary layer using two types of wake generating objects, which are small spheres and thin wires. Main focus is on emergence of isolated turbulent spots from the influence of the wake passage over the boundary layer. Precursors of the wake-induced turbulent spot, which have not been observed in an explicit manner in any other previous studies, are also of concern in this study. It is expected that wakes from the wires are so weak that an isolated turbulent spot may be induced by the wire wake, while the position of the spot emergence varies randomly along the wire. A multi-channel sensor with 7 hot-wire probes acquires the velocity data of the flow over the flat plate subjected to the wake passage. These velocity data reveal the spot shape and spot generation rate. In addition, the existence of Klebanoff mode in this wake-affected boundary layer is examined.

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Investigation of the Calmed Region Behind a Turbulent Spot

Volume 1: Turbomachinery ◽

10.1115/96-gt-489 ◽

1996 ◽

Cited By ~ 4

Author(s):

J. P. Gostelow ◽

G. J. Walker ◽

W. J. Solomon ◽

G. Hong ◽

N. Melwani

Keyword(s):

Boundary Layer ◽

Pressure Gradient ◽

Axial Flow ◽

Adverse Pressure Gradient ◽

Turbulent Spot ◽

Boundary Layer Flows ◽

Turbulent Spots ◽

Turbulent Layer ◽

Controlled Diffusion ◽

Flow Compressor

Measurements are presented of the calmed region behind triggered wave packets and turbulent spots under a controlled diffusion adverse pressure gradient in a wind tunnel. Similar measurements are also presented from the stator blades of an axial flow compressor, where turbulent spots are induced by the passing of rotor wakes. The purpose is to gain an appreciation of turbulent spot behavior under a strong adverse pressure gradient as a foundation for the more accurate modeling of spots and their environment in predictions of transitional boundary layer flows. Under an adverse pressure gradient the calmed region behind the spot is extensive; its interaction with the surrounding turbulent layer is complex and is dependent on whether the surrounding natural boundary layer is laminar or turbulent. Some insights are gleaned concerning the behavior of the calmed region which will subsequently be used in attempts to model the calmed region. Although these fundamental investigations of the calmed region have been extensive much remains to be understood.

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