Near Field Development of Planar Wakes Under the Effect of Asymmetric Initial Conditions

2015 ◽  
Vol 137 (9) ◽  
Author(s):  
Ladan Momayez ◽  
Marouen Dghim ◽  
Mohsen Ferchichi ◽  
Sylvain Graveline
Keyword(s):  
Flat Plate ◽  
Initial Conditions ◽  
Near Field ◽  
Leading Edge ◽  
Reynolds Numbers ◽  
Near Wake ◽  
Field Development ◽  
Large Structures ◽  
Hotwire Anemometry ◽  
Upstream Flow

This paper reports an experimental investigation on the response of a planar wake past a flat plate to various upstream flow conditions. A tripping wire was placed on the upper side of the flat plate downstream the leading edge which resulted in asymmetric boundary layers. The near wake asymmetry was compared to their symmetrical counterpart at two different Reynolds numbers. The near wake dynamics were investigated using hotwire anemometry and flow visualizations. Self-similarity of the asymmetrical wakes was established. Asymmetry seemed to have the largest effect on the convection velocity of the large structures in the asymmetric laminar-turbulent wake.

On the Response of Planar Wakes to Asymmetric Initial Conditions

2014 ◽  
Author(s):  
Ladan Momayez ◽  
Marouen Dghim ◽  
Mohsen Ferchichi ◽  
Sylvain Graveline
Keyword(s):  
Boundary Layer ◽  
Turbulent Boundary Layer ◽  
Flat Plate ◽  
Laminar Boundary Layer ◽  
Initial Conditions ◽  
Leading Edge ◽  
Turbulent Wake ◽  
Lower Side ◽  
Near Wake ◽  
Laminar Wake

This work reports an experimental investigation on the response of a planar wake generated by a profiled flat plate to various upstream flow conditions. A tripping wire was placed on the upper side of the flat plate just downstream of the leading edge of the plate that resulted in asymmetric separating shear layers at the trailing edge. The near wake asymmetry is compared to the symmetrical case at two different Reynolds numbers. Two asymmetric initial conditions resulted, namely, laminar boundary layer on the lower side and a turbulent boundary layer on the upper side, and a turbulent boundary layer on the lower side and tripped turbulent boundary layer on the upper surface. The near wake dynamics were investigated under the effects of the degree of asymmetry using hot-wire anemometry and flow visualizations. The measurements showed when one of the two boundary layers was tripped, the wake shifted towards the tripped side and wake spreading was found to be larger than in the case of the symmetrical wake with the effect being more pronounced in the asymmetric laminar wake. Self-similarity of the asymmetrical wakes was established by properly selecting appropriate similarity variables however, the similarity of the wake was less evident in the tripped laminar boundary layer case. Convection velocity, Uc, of the Von Karman large eddies, estimated using processed flow visualization images seemed to increase with increased Reynolds number and with increased upstream momentum thickness. In the symmetric laminar wake, Uc/U∞ increases from 0.2 and reached an asymptotic value of about 0.85 further downstream. In the presence of perturbation, Uc/U∞ attained a constant value of about 0.83 further downstream compared to the symmetric case. For the turbulent wake, however, asymmetry of the turbulence levels was found to increase the convection speed compared to both the laminar wake and the symmetric turbulent wake reaching a constant value nearly at the same downstream position for both the symmetric and asymmetric turbulent wake.

scholarly journals Flow past a square-section cylinder with a wavy stagnation face

2001 ◽  
Vol 426 ◽  
pp. 263-295 ◽  
Author(s):  
RUPAD M. DAREKAR ◽  
SPENCER J. SHERWIN
Keyword(s):  
Reynolds Number ◽  
Cross Flow ◽  
Leading Edge ◽  
Reynolds Numbers ◽  
Physical Structure ◽  
Near Wake ◽  
Hairpin Vortices ◽  
Independent State ◽  
Flow Past ◽  
Edge Surface

Numerical investigations have been performed for the flow past square-section cylinders with a spanwise geometric deformation leading to a stagnation face with a sinusoidal waviness. The computations were performed using a spectral/hp element solver over a range of Reynolds numbers from 10 to 150.Starting from fully developed shedding past a straight cylinder at a Reynolds number of 100, a sufficiently high waviness is impulsively introduced resulting in the stabilization of the near wake to a time-independent state. It is shown that the spanwise waviness sets up a cross-flow within the growing boundary layer on the leading-edge surface thereby generating streamwise and vertical components of vorticity. These additional components of vorticity appear in regions close to the inflection points of the wavy stagnation face where the spanwise vorticity is weakened. This redistribution of vorticity leads to the breakdown of the unsteady and staggered Kármán vortex wake into a steady and symmetric near-wake structure. The steady nature of the near wake is associated with a reduction in total drag of about 16% at a Reynolds number of 100 compared with the straight, non-wavy cylinder.Further increases in the amplitude of the waviness lead to the emergence of hairpin vortices from the near-wake region. This wake topology has similarities to the wake of a sphere at low Reynolds numbers. The physical structure of the wake due to the variation of the amplitude of the waviness is identified with five distinct regimes. Furthermore, the introduction of a waviness at a wavelength close to the mode A wavelength and the primary wavelength of the straight square-section cylinder leads to the suppression of the Kármán street at a minimal waviness amplitude.

Some Experiments on the Reattachment of a Laminar Boundary Layer Separating From a Rearward Facing Step on a Flat Plate Aerofoil

1960 ◽  
Vol 64 (599) ◽  
pp. 668-672 ◽  
Author(s):  
T. W. F. Moore
Keyword(s):  
Boundary Layer ◽  
Liquid Film ◽  
Flat Plate ◽  
Laminar Boundary Layer ◽  
Reverse Flow ◽  
Leading Edge ◽  
Reynolds Numbers ◽  
Pressure Measurements ◽  
Flow Vortex

Summary:The results of experiments on the reattachment of a laminar boundary layer, separating from a rearward facing step in a flat plate aerofoil, are correlated with the properties of the short leading edge bubble which forms on thin aerofoils near the stall.The experiments, comprising pressure measurements, Pitot explorations, liquid film and smoke studies, indicate that for all Reynolds numbers above the value given by the Owen-KIanfer criterion the reattachment is turbulent behind a stationary air reverse flow vortex bubble. It is also found that the reattachment is laminar for Reynolds numbers below the critical, which further supports Crabtree's interpretation of the Owen-KIanfer criterion in terms of the condition for the growth of turbulent bursts.

Effect of initial conditions on the near-field development of a round jet

2004 ◽  
Vol 37 (1) ◽  
pp. 56-64 ◽  
Author(s):  
P. Burattini ◽  
R. A. Antonia ◽  
S. Rajagopalan ◽  
M. Stephens
Keyword(s):  
Initial Conditions ◽  
Near Field ◽  
Round Jet ◽  
Field Development

scholarly journals Effect of free-stream turbulence on boundary layer transition

2014 ◽  
Vol 372 (2020) ◽  
pp. 20130354 ◽  
Author(s):  
M. E. Goldstein
Keyword(s):  
Reynolds Number ◽  
Flat Plate ◽  
Free Stream ◽  
Leading Edge ◽  
Boundary Layer Transition ◽  
Transition To Turbulence ◽  
Free Stream Turbulence ◽  
Edge Geometry ◽  
Downstream Region ◽  
Upstream Flow

This paper is concerned with the transition to turbulence in flat plate boundary layers due to moderately high levels of free-stream turbulence. The turbulence is assumed to be generated by an (idealized) grid and matched asymptotic expansions are used to analyse the resulting flow over a finite thickness flat plate located in the downstream region. The characteristic Reynolds number R λ based on the mesh size λ and free-stream velocity is assumed to be large, and the turbulence intensity ε is assumed to be small. The asymptotic flow structure is discussed for the generic case where the turbulence Reynolds number εR λ and the plate thickness and are held fixed (at O (1) and O (λ), respectively) in the limit as and ε →0. But various limiting cases are considered in order to explain the relevant transition mechanisms. It is argued that there are two types of streak-like structures that can play a role in the transition process: (i) those that appear in the downstream region and are generated by streamwise vorticity in upstream flow and (ii) those that are concentrated near the leading edge and are generated by plate normal vorticity in upstream flow. The former are relatively unaffected by leading edge geometry and are usually referred to as Klebanoff modes while the latter are strongly affected by leading edge geometry and are more streamwise vortex-like in appearance.

Locomotion of a passively flapping flat plate

2010 ◽  
Vol 659 ◽  
pp. 43-68 ◽  
Author(s):  
JIE ZHANG ◽  
NAN-SHENG LIU ◽  
XI-YUN LU
Keyword(s):  
Flat Plate ◽  
Vortical Structure ◽  
Leading Edge ◽  
Periodic Flow ◽  
Flexible Plate ◽  
Forward Movement ◽  
Propulsion Systems ◽  
Near Wake ◽  
Boltzmann Method ◽  
Flow States

Locomotion of a passively flapping flat plate has been studied numerically by means of a multiblock lattice Boltzmann method. A flexible plate is modelled by a rigid plate with a torsion spring acting about the pivot at the leading edge of the plate. A dynamic model of this kind is called a lumped-torsional-flexibility model. When the leading edge is forced to heave sinusoidally, the plate pitches passively and propels itself in the horizontal direction as a result of the fluid–plate interaction. We have investigated various aspects of the mechanics behind the behaviour of the flapping plate, including the periodic- and non-periodic-flow states, the spontaneous motion of the plate, vortical structure and how they compare to similar propulsion systems in animals. In the periodic-flow regime, two dynamical responses of the passively pitching plate (forward and backward movements) are observed. Which movement will occur depends only on the frequency ratio F of the natural frequency of the system and the heaving frequency associated with the lumped torsional flexibility. It is found that the plate will select the forward movement when F > 1 and the backward movement when F ≤ 1. In the forward-movement regime, analysis of the dynamical behaviours and propulsive properties of the passively pitching plate indicates that the torsional flexibility can remarkably improve the propulsive performance. In addition, four kinds of vortex structures in the near wake are identified, which mainly depend on the forward speed of the plate. Finally the forward movement is compared to the flapping-based locomotion of swimming and flying animals. The results obtained in this study are consistent with the observations and measurements of swimming and flying animals; thus, they may provide physical insights into understanding of the propulsive mechanisms of the flapping wings and fins of animals.

Optimal wavepackets in streamwise corner flow

2015 ◽  
Vol 766 ◽  
pp. 405-435 ◽  
Author(s):  
Oliver T. Schmidt ◽  
Seyed M. Hosseini ◽  
Ulrich Rist ◽  
Ardeshir Hanifi ◽  
Dan S. Henningson
Keyword(s):  
Initial Conditions ◽  
Global Analysis ◽  
Experimental Studies ◽  
Measurement Data ◽  
Leading Edge ◽  
Reynolds Numbers ◽  
Base Flow ◽  
Flow Modification ◽  
Self Similar Solution ◽  
Streamwise Pressure Gradient

AbstractThe global non-modal stability of the flow in a right-angled streamwise corner is investigated. Spatially confined linear optimal initial conditions and responses are obtained by use of direct-adjoint looping. Two base states are considered, the classical self-similar solution for a zero streamwise pressure gradient, and a modified solution that mimics leading-edge effects commonly observed in experimental studies. The latter solution is obtained in a reverse engineering fashion from published measurement data. Prior to the global analysis, a classical local linear stability and sensitivity analysis of both base states is conducted. It is found that the base-flow modification drastically reduces the critical Reynolds number through an inviscid mechanism, the so-called corner mode. A survey of the geometry of the two base states confirms that the modification greatly aggravates the inflectional nature of the flow. Global optimals are calculated for subcritical and supercritical Reynolds numbers, and for two finite optimization times. The optimal initial conditions are found to be self-confined in the spanwise directions, and symmetric with respect to the corner bisector. They evolve into streaks or streamwise modulated wavepackets, depending on the base state. Substantial transient growth caused by the Orr mechanism and the lift-up effect is observed.

Measurements of velocity distributions in the laminar wake of a flat plate

1978 ◽  
Vol 84 (4) ◽  
pp. 705-715 ◽  
Author(s):  
Michio Nishioka ◽  
Tosio Miyagi
Keyword(s):  
Flat Plate ◽  
Outer Part ◽  
Reynolds Numbers ◽  
Near Wake ◽  
Trailing Edge ◽  
Velocity Overshoot ◽  
Theoretical Predictions ◽  
Laminar Wake ◽  
Far Wake ◽  
Good Agreement

The two-dimensional wake of a thin flat plate parallel to the stream was maintained laminar and steady at Reynolds numbers up to 3000 in a low-turbulence wind tunnel. Velocity distributions in the wake were measured in detail for Reynolds numbers from 20 to 3000. One of the interesting results is the appearance of a velocity overshoot, namely that the velocity in the outer part of the shear layer exceeds that of the uniform flow in the vicinity of the trailing edge. Comparisons between the experimental results and Goldstein's theoretical predictions show good agreement in the far wake irrespective of the Reynolds number, but not in the near wake even at higher Reynolds numbers, in particular immediately behind the trailing edge.

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