Secondary instabilities in the wake of an elongated two-dimensional body with a blunt trailing edge

2018 ◽  
Vol 846 ◽  
pp. 578-604 ◽  
Author(s):  
B. Gibeau ◽  
C. R. Koch ◽  
S. Ghaemi
Keyword(s):  
Boundary Layer ◽  
High Speed ◽  
Energy Content ◽  
Streamwise Vortex ◽  
Secondary Instability ◽  
Two Dimensional ◽  
Cylinder Wake ◽  
Trailing Edge ◽  
Piv Measurements ◽  
Blunt Trailing Edge

The secondary instability in the wake of a two-dimensional blunt body with a chord to thickness ratio of 46.5 was experimentally investigated for Reynolds numbers of 3500, 5200 and 7000 based on the blunt trailing edge height $h$. Planar, stereoscopic and high-speed particle image velocimetry (PIV) measurements were performed to characterise the wake and upstream boundary layer. The same mode B secondary instability that is found in the cylinder wake was found to be present in the wake of the elongated body studied here. The most probable wavelength of the secondary instability, defined as the spanwise distance between adjacent streamwise vortex pairs in the wake, was found to range from $0.7h$ to $0.8h$ by applying a spatial autocorrelation to the spanwise–wall-normal instantaneous fields of the $Q$-criterion. The temporal evolution of the secondary wake vortices was investigated using time-resolved stereoscopic PIV measurements and it was shown that the vortices maintain both their directions of rotation and spanwise positions during the primary vortex shedding cycles. In agreement with previous literature, the secondary instability did not greatly change as the upstream boundary layer transitioned from laminar to turbulent. Moreover, any upstream boundary layer structures were found to rapidly evolve into wake structures just past the blunt trailing edge. The wavelength of the secondary instability was shown to match the spanwise distance between adjacent low-speed zones of streamwise velocity in the wake. These undulating velocity patterns proved to be a viable method for determining the secondary instability wavelength; however, this type of analysis is highly sensitive to the energy content used for data reconstruction when proper orthogonal decomposition is applied beforehand.

The Effects of a Tripped Turbulent Boundary Layer on Vortex Shedding from a Blunt Trailing Edge Hydrofoil

2012 ◽  
Vol 134 (5) ◽  
Author(s):  
Philippe Ausoni ◽  
Amirreza Zobeiri ◽  
François Avellan ◽  
Mohamed Farhat
Keyword(s):  
Boundary Layer ◽  
Turbulent Boundary Layer ◽  
Vortex Shedding ◽  
High Speed ◽  
Boundary Layer Transition ◽  
Trailing Edge ◽  
Layer Transition ◽  
Blunt Trailing Edge ◽  
Turbulent Transition ◽  
Reference Length

Experiments on vortex shedding from a blunt trailing edge symmetric hydrofoil operating at zero angle of attack in a uniform high speed flow, Reh=16.1·103-96.6·103, where the reference length h is the trailing edge thickness, are reported. The effects of a tripped turbulent boundary layer on the wake characteristics are analyzed and compared with the condition of a natural turbulent transition. The foil surface is hydraulically smooth and a fully effective boundary layer tripping at the leading edge is achieved with the help of a distributed roughness. The vortex shedding process is found to be strongly influenced by the boundary layer development: the tripped turbulent transition promotes the re-establishment of organized vortex shedding. In the context of the tripped transition and in comparison with the natural one, significant increases in the vortex span-wise organization, the vortex-induced hydrofoil vibration, the wake velocity fluctuations, and the vortex strength are revealed. Although the vortex shedding frequency is decreased, a modified Strouhal number based on the wake width at the end of the vortex formation region is constant and evidences the similarity of the wakes in terms of spatial distribution for the two considered boundary layer transition processes.

Coherent Structures in Turbulent Boundary Layer Flows Over a Shallow Cavity

2017 ◽  
Author(s):  
Khaled J. Hammad
Keyword(s):  
Boundary Layer ◽  
Turbulent Boundary Layer ◽  
Coherent Structures ◽  
Energy Content ◽  
Reynolds Numbers ◽  
Turbulent Energy ◽  
Trailing Edge ◽  
Cavity Depth ◽  
Piv Measurements ◽  
Shallow Cavity

Proper Orthogonal Decomposition (POD) analysis of high-resolution Particle Image Velocimetry (PIV) measurements is used to identify the most energetic coherent structures of the turbulent flow field resulting from the interaction of a turbulent boundary layer with a shallow cavity. The PIV measurements capture the flow structure and turbulence, upstream, over, and downstream of a shallow open cavity. Two sets of PIV measurements corresponding to a turbulent incoming boundary layer and a cavity length-to-depth ratio of four are used. The cavity depth based Reynolds numbers are 21,000 and 42,000. The first six POD modes are found to contain a substantial percentage of the overall turbulent energy, approximately 45.2% and 45.7% for Reynolds numbers of 21,000 and 42,000, respectively. The overall spatial and modal energy content distribution are almost identical for both Reynolds numbers. The spatial flow characteristics of POD modes 1–6 reveal the existence of vortical structures, developing in the mixing region, that grow in size as they approach the trailing edge of the cavity. POD mode number one, containing approximately 20% of the overall turbulent energy, represents non-impinging vortices interaction with the cavity trailing edge, while POD modes 2–6 capture various stages of the impinging vortices type of interaction.

Two-Dimensional Experimental Investigation on the Effects of a Fowler Flap Gap and Overlap Size on the Wake Flow Field

2016 ◽  
Vol 139 (2) ◽  
Author(s):  
David Demel ◽  
Mohsen Ferchichi ◽  
William D. E. Allan ◽  
Marouen Dghim
Keyword(s):  
Experimental Investigation ◽  
Flow Field ◽  
Angle Of Attack ◽  
Wake Flow ◽  
Two Dimensional ◽  
Edge Region ◽  
Suction Surface ◽  
Trailing Edge ◽  
Piv Measurements ◽  
Image Velocimetry

This work details an experimental investigation on the effects of the variation of flap gap and overlap sizes on the flow field in the wake of a wing-section equipped with a trailing edge Fowler flap. The airfoil was based on the NACA 0014-1.10 40/1.051 profile, and the flap was deployed with 40 deg deflection angle. Two-dimensional (2D) particle image velocimetry (PIV) measurements of the flow field in the vicinity of the main wing trailing edge and the flap region were performed for the optimal flap gap and overlap, as well as for flap gap and overlap increases of 2% and 4% chord beyond optimal, at angles of attack of 0 deg, 10 deg, and 12 deg. For all the configurations investigated, the flow over the flap was found to be fully stalled. At zero angle of attack, increasing the flap gap size was found to have minor effects on the flow field but increased flap overlap resulted in misalignment between the main wing boundary layer (BL) flow and the slot flow that forced the flow in the trailing edge region of the main wing to separate. When the angle of attack was increased to near stall conditions (at angle of attack of 12 deg), increasing the flap gap was found to energize and improve the flow in the trailing edge region of the main wing, whereas increased flap overlap further promoted flow separation on the main wing suction surface possibly steering the wing into stall.

Secondary instability and subcritical transition of the leading-edge boundary layer

2016 ◽  
Vol 792 ◽  
pp. 682-711 ◽  
Author(s):  
Michael O. John ◽  
Dominik Obrist ◽  
Leonhard Kleiser
Keyword(s):  
Boundary Layer ◽  
Reynolds Number ◽  
High Speed ◽  
Three Dimensional ◽  
Leading Edge ◽  
Reynolds Numbers ◽  
Secondary Instability ◽  
Bypass Transition ◽  
Wall Suction ◽  
Transition Mechanism

The leading-edge boundary layer (LEBL) in the front part of swept airplane wings is prone to three-dimensional subcritical instability, which may lead to bypass transition. The resulting increase of airplane drag and fuel consumption implies a negative environmental impact. In the present paper, we present a temporal biglobal secondary stability analysis (SSA) and direct numerical simulations (DNS) of this flow to investigate a subcritical transition mechanism. The LEBL is modelled by the swept Hiemenz boundary layer (SHBL), with and without wall suction. We introduce a pair of steady, counter-rotating, streamwise vortices next to the attachment line as a generic primary disturbance. This generates a high-speed streak, which evolves slowly in the streamwise direction. The SSA predicts that this flow is unstable to secondary, time-dependent perturbations. We report the upper branch of the secondary neutral curve and describe numerous eigenmodes located inside the shear layers surrounding the primary high-speed streak and the vortices. We find secondary flow instability at Reynolds numbers as low as$Re\approx 175$, i.e. far below the linear critical Reynolds number$Re_{crit}\approx 583$of the SHBL. This secondary modal instability is confirmed by our three-dimensional DNS. Furthermore, these simulations show that the modes may grow until nonlinear processes lead to breakdown to turbulent flow for Reynolds numbers above$Re_{tr}\approx 250$. The three-dimensional mode shapes, growth rates, and the frequency dependence of the secondary eigenmodes found by SSA and the DNS results are in close agreement with each other. The transition Reynolds number$Re_{tr}\approx 250$at zero suction and its increase with wall suction closely coincide with experimental and numerical results from the literature. We conclude that the secondary instability and the transition scenario presented in this paper may serve as a possible explanation for the well-known subcritical transition observed in the leading-edge boundary layer.

Trailing Edge Noise From Blunt and Sharp Edge Geometries

2008 ◽  
Author(s):  
Daniel W. Shannon ◽  
Scott C. Morris ◽  
William K. Blake
Keyword(s):  
Boundary Layer ◽  
Turbulent Boundary Layer ◽  
Boundary Layer Separation ◽  
Broadband Noise ◽  
Trailing Edge ◽  
Trailing Edge Noise ◽  
Edge Geometry ◽  
Blunt Trailing Edge ◽  
Blunt Edge ◽  
Radiated Sound

The objective of this study was to experimentally investigate the broadband trailing edge noise generated by a sharp trailing edge geometry and an asymmetric blunt edge. The flow field in the vicinity of the sharp trailing edge was found to be equivalent to that of a flat plate turbulent boundary layer. The interaction of the two boundary layers with the edge was responsible for broadband noise generation. The blunt trailing edge geometry exhibited additional complexity, with turbulent boundary layer separation and sound generated by vortex shedding. The measurement program included hot-wire anemometry, unsteady surface pressure, and radiated sound utilizing two microphone arrays. The boundary layer parameters and wall pressure spectra were used to compute the radiated sound from existing scattering theory. These calculations agreed very well with the array data, with differences typically within 2dB over the frequency range considered valid for the theory.

Measurement on the Structure of the Reynolds Stress for the Turbulent Boundary Layer Flow Over a Two-Dimensional Escarpment With Mild Upwind Slope

2011 ◽  
Author(s):  
Bao-Shi Shiau ◽  
Ben-Jue Tsai
Keyword(s):  
Boundary Layer ◽  
Turbulent Boundary Layer ◽  
Reynolds Stress ◽  
High Speed ◽  
Slope Angle ◽  
Turbulent Energy ◽  
Quadrant Analysis ◽  
Two Dimensional ◽  
Spectrum Distribution ◽  
Layer Flow

Experimental measurement study on the structure of the Reynolds stress and turbulence spectrum for wind flows over a two-dimensional escarpment with mild upwind slope (slope angle θ = 15°) were performed in the wind tunnel. The Quadrant analysis was applied to analyze the experimental data and yield the structure of the Reynolds stress. In according to the quadrant analysis, the Reynolds stress is composed of four events of the stress components, i.e. outward interaction, ejection (low-speed fluid upward), inward interaction, and sweep (high-speed fluid downward). Measured results show that: (1) Measurements of the structure of the Reynolds stress reveal that both the sweep and ejection events are the major contributors to the Reynolds stress for flow around the two dimensional escarpment with mild upwind slope. (2) The contributions to the Reynolds stress made by ejection events and sweep events are almost the same at heights Z/Zref greater than 0.2 for different downstream distances along the mild slope of escarpment. Here Zref is the turbulent boundary layer thickness. When flow reached the top of the slope of escarpment, stress fractions of ejection event and sweep event, S2 and S4 increased significantly. (3) The he turbulent energy spectrum distribution was not found very dominant spectrum peak as winds flow over the mild upwind slope and top surface of escarpment.

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