scholarly journals Experiments on transient growth of turbulent spots

2017 ◽  
Vol 829 ◽  
Author(s):  
L. Klotz ◽  
J. E. Wesfreid
Keyword(s):  
Experimental Investigation ◽  
Reynolds Number ◽  
Theoretical Analysis ◽  
Poiseuille Flow ◽  
High Amplitude ◽  
Transient Growth ◽  
External Forcing ◽  
Turbulent Spots ◽  
Maximal Gain ◽  
Theoretical Results

We present detailed experiments on transient growth of turbulent spots induced by external forcing in plane Couette–Poiseuille flow, which are studied in the framework of linear transient growth. The experimental investigation is supplemented with full theoretical analysis. We compare quantitatively the experimental and theoretical results, including maximal gain and the time at which it occurs. We also present the limits of validity for the application of the linear theory at high amplitude perturbation and Reynolds number, showing experiments with self-sustained states.

An experimental investigation of turbulent spots and breakdown to turbulence

1960 ◽  
Vol 9 (2) ◽  
pp. 235-246 ◽  
Author(s):  
J. W. Elder
Keyword(s):  
Boundary Layer ◽  
Experimental Investigation ◽  
Reynolds Number ◽  
Hydrodynamic Stability ◽  
Free Stream ◽  
Stream Velocity ◽  
Turbulent Spot ◽  
Turbulent Spots ◽  
The Impact ◽  
Critical Intensity

The theory of hydrodynamic stability and the impact on it of recent work with turbulent spots is discussed. Emmons's (1951) assumptions about the growth and interaction of turbulent spots are found experimentally to be substantially correct. In particular it is shown that the region of turbulent flow on a flat plate is simply the sum of the areas that would be obtained if all spots grew independently.An investigation of the conditions required for breakdown to turbulence near a wall, that is, to initiate a turbulent spot, suggests that regardless of how disturbances are generated in a laminar boundary layer and independent of both the Reynolds number and the spatial extent of the disturbances, breakdown to turbulence occurs by the initiation of a turbulent spot at all points at which the velocity fluctuation exceeds a critical intensity. Over most of the layer this intensity is about 0·2 times the free-stream velocity. The Reynolds number is important merely in respect of the growth of disturbances prior to breakdown.

Compressed Thin-Walled Cold-Formed Steel Members with Closed Cross-Sections

2014 ◽  
Vol 969 ◽  
pp. 93-96 ◽  
Author(s):  
Mohamad Al Ali
Keyword(s):  
Experimental Investigation ◽  
Theoretical Analysis ◽  
Cross Sections ◽  
Theoretical Research ◽  
Loading Process ◽  
Steel Members ◽  
Fundamental Information ◽  
Cold Formed Steel ◽  
Thin Walled ◽  
Theoretical Results

The paper presents fundamental information about experimental-theoretical research oriented to determinate the resistance of thin-walled compressed steel members. The investigated members had closed cross-sections made from homogeneous materials. The theoretical analysis in this research is oriented to determinate the resistance of mentioned members according to relevant standards, while the experimental investigation is to verify the theoretical results and to investigate the behavior of mentioned members during the loading process.

Jets induced by oscillatory motion

1972 ◽  
Vol 53 (2) ◽  
pp. 287-303 ◽  
Author(s):  
B. J. Davidson ◽  
N. Riley
Keyword(s):  
Experimental Investigation ◽  
Reynolds Number ◽  
Oscillatory Motion ◽  
Periodic Oscillations ◽  
Steady Streaming ◽  
Theoretical Results

The flow induced by a cylinder performing transverse periodic oscillations in a fluid which is otherwise at rest is investigated. The motion of the cylinder is not necessarily harmonic. Attention is focused upon the induced steady streaming. In particular, the jet-like flow which arises when the streaming Reynolds number is large is studied. The theoretical results obtained are applied to a class of elliptic cylinders, and a simple experimental investigation is carried out which substantiates the main theoretical conclusions.

scholarly journals Optimal Transient Growth in an Incompressible Flow past a Backward-Slanted Step

Fluids ◽  
2019 ◽  
Vol 4 (1) ◽  
pp. 33 ◽  
Author(s):  
Marco Martins Afonso ◽  
Philippe Meliga ◽  
Eric Serre
Keyword(s):  
Optimal Control ◽  
Incompressible Flow ◽  
Variational Formulation ◽  
Aerodynamic Drag ◽  
Turbulent Wake ◽  
Transient Growth ◽  
External Forcing ◽  
Adjoint Methods ◽  
Recirculation Bubble ◽  
Maximal Gain

With the aim of providing a first step in the quest for a reduction of the aerodynamic drag on the rear-end of a car, we study the phenomena of separation and reattachment of an incompressible flow by focusing on a specific aerodynamic geometry, namely a backward-slanted step at 25 ∘ of inclination. The ensuing recirculation bubble provides the basis for an analytical and numerical investigation of streamwise-streak generation, lift-up effect, and turbulent-wake and Kelvin–Helmholtz instabilities. A linear stability analysis is performed, and an optimal control problem with a steady volumic forcing is tackled by means of a variational formulation, adjoint methods, penalization schemes, and an orthogonalization algorithm. Dealing with the transient growth of spanwise-periodic perturbations, and inspired by the need of physically-realizable disturbances, we finally provide a procedure attaining a kinetic-energy maximal gain on the order of 10 6 , with respect to the power introduced by the external forcing.

Experimental Investigation on Blunt-Edged UTM Delta Wing VFE-2 Configurations at Low Reynolds Number

2018 ◽  
Vol 12 (3) ◽  
pp. 255
Author(s):  
Muhammad Zal Aminullah Daman Huri ◽  
Shabudin Bin Mat ◽  
Mazuriah Said ◽  
Shuhaimi Mansor ◽  
Md. Nizam Dahalan ◽  
...  
Keyword(s):  
Experimental Investigation ◽  
Reynolds Number ◽  
Delta Wing ◽  
Low Reynolds Number ◽  
Low Reynolds

scholarly journals Optimising the AR Engraved Structure on Light-Guide Facets for a Wide Range of Wavelengths

Optics ◽  
2020 ◽  
Vol 2 (1) ◽  
pp. 25-42
Author(s):  
Ioseph Gurwich ◽  
Yakov Greenberg ◽  
Kobi Harush ◽  
Yarden Tzabari
Keyword(s):  
Theoretical Analysis ◽  
Light Guide ◽  
Spectral Band ◽  
Angular Range ◽  
The Past ◽  
Input And Output ◽  
Wide Range ◽  
The Given ◽  
Theoretical Results ◽  
Shape And Size

The present study is aimed at designing anti-reflective (AR) engraving on the input–output surfaces of a rectangular light-guide. We estimate AR efficiency, by the transmittance level in the angular range, determined by the light-guide. Using nano-engraving, we achieve a uniform high transmission over a wide range of wavelengths. In the past, we used smoothed conical pins or indentations on the faces of light-guide crystal as the engraved structure. Here, we widen the class of pins under consideration, following the physical model developed in the previous paper. We analyze the smoothed pyramidal pins with different base shapes. The possible effect of randomization of the pins parameters is also examined. The results obtained demonstrate optimized engraved structure with parameters depending on the required spectral range and facet format. The predicted level of transmittance is close to 99%, and its flatness (estimated by the standard deviation) in the required wavelengths range is 0.2%. The theoretical analysis and numerical calculations indicate that the obtained results demonstrate the best transmission (reflection) we can expect for a facet with the given shape and size for the required spectral band. The approach is equally useful for any other form and of the facet. We also discuss a simple way of comparing experimental and theoretical results for a light-guide with the designed input and output features. In this study, as well as in our previous work, we restrict ourselves to rectangular facets. We also consider the limitations on maximal transmission produced by the size and shape of the light-guide facets. The theoretical analysis is performed for an infinite structure and serves as an upper bound on the transmittance for smaller-size apertures.

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