scholarly journals Investigation of the Development of Localized Disturbances in the Boundary Layer of a Flat Plate under Conditions of Moderate Degree of the Incoming Flow Turbulence behind the Surface Step

2021 ◽  
Vol 16 (1) ◽  
pp. 65-80
Author(s):  
Ivan A. Sadovsky ◽  
Mikhail M Katasonov ◽  
Alexander M. Pavlenko
Keyword(s):  
Boundary Layer ◽  
Flat Plate ◽  
Wave Packets ◽  
Low Frequency ◽  
Separated Flow ◽  
Moderate Degree ◽  
Incoming Flow ◽  
Free Stream Turbulence ◽  
Surface Section ◽  
Flow Turbulence

In a wind tunnel on a flat plate in a separated flow behind a rectangular step, the emergence and development of localized disturbances generated by low-frequency impulse deviations of the local surface section under conditions of low and moderate degrees of the incoming flow turbulence is studied. The results were obtained by hot-wire anemometry at low subsonic flow velocity. It was found that impulse deviations of the wall generate disturbances, which are socalled Streaky structures and wave packets of oscillations. The separation of the laminar boundary layer accelerates the growth of wave packets with subsequent turbulization of the near-wall flow. The specific features of the behavior of localized disturbances under conditions of moderate degree of free-stream turbulence are revealed.

scholarly journals Generation of disturbances in the boundary layer of a flat plate by periodic low-frequency motion of the surface section

2019 ◽  
Vol 234 (1) ◽  
pp. 109-114
Author(s):  
Mikhail M Katasonov ◽  
Victor V Kozlov ◽  
Alexandr M Pavlenko
Keyword(s):  
Boundary Layer ◽  
Flat Plate ◽  
Wave Packets ◽  
Three Dimensional ◽  
Plate Boundary ◽  
Low Frequency ◽  
Localized Structures ◽  
Surface Section ◽  
Periodic Impulse ◽  
Low Frequency Motion

The development of localized disturbances, generated by periodic impulse lifting of three-dimensional surface in the flat plate boundary layer at Reδ1 > 400 is experimentally investigated. It is shown that a large amplitude surface lifting leads simultaneously to the formation of two types of perturbations in the boundary layer: longitudinal localized structures and two wave packets. Spatial development of oscillations at the central frequency of the wave packets is consistent with the linear theory of hydrodynamic stability.

Receptivity to free-stream vorticity of flow past a flat plate with elliptic leading edge

2010 ◽  
Vol 653 ◽  
pp. 245-271 ◽  
Author(s):  
L.-U. SCHRADER ◽  
L. BRANDT ◽  
C. MAVRIPLIS ◽  
D. S. HENNINGSON
Keyword(s):  
Boundary Layer ◽  
Flat Plate ◽  
Free Stream ◽  
Three Dimensional ◽  
Low Frequency ◽  
Leading Edge ◽  
Sound Waves ◽  
Streamwise Vorticity ◽  
Low Frequencies ◽  
Free Stream Turbulence

Receptivity of the two-dimensional boundary layer on a flat plate with elliptic leading edge is studied by numerical simulation. Vortical perturbations in the oncoming free stream are considered, impinging on two leading edges with different aspect ratio to identify the effect of bluntness. The relevance of the three vorticity components of natural free-stream turbulence is illuminated by considering axial, vertical and spanwise vorticity separately at different angular frequencies. The boundary layer is most receptive to zero-frequency axial vorticity, triggering a streaky pattern of alternating positive and negative streamwise disturbance velocity. This is in line with earlier numerical studies on non-modal growth of elongated structures in the Blasius boundary layer. We find that the effect of leading-edge bluntness is insignificant for axial free-stream vortices alone. On the other hand, vertical free-stream vorticity is also able to excite non-modal instability in particular at zero and low frequencies. This mechanism relies on the generation of streamwise vorticity through stretching and tilting of the vertical vortex columns at the leading edge and is significantly stronger when the leading edge is blunt. It can thus be concluded that the non-modal boundary-layer response to a free-stream turbulence field with three-dimensional vorticity is enhanced in the presence of a blunt leading edge. At high frequencies of the disturbances the boundary layer becomes receptive to spanwise free-stream vorticity, triggering Tollmien–Schlichting (T-S) modes and receptivity increases with leading-edge bluntness. The receptivity coefficients to free-stream vortices are found to be about 15% of those to sound waves reported in the literature. For the boundary layers and free-stream perturbations considered, the amplitude of the T-S waves remains small compared with the low-frequency streak amplitudes.

scholarly journals Experimental study of perturbations modeled by a membrane in 2D and 3D boundary layers

2021 ◽  
Vol 2119 (1) ◽  
pp. 012001
Author(s):  
I A Sadovskii ◽  
M M Katasonov
Keyword(s):  
Boundary Layer ◽  
Wave Packets ◽  
Experimental Studies ◽  
Moderate Degree ◽  
Circular Membrane ◽  
Linear Stage ◽  
Impulsive Motion ◽  
Longitudinal Structure ◽  
Flow Turbulence ◽  
2D And 3D

Abstract The work is devoted to experimental studies of the dynamics of the development of perturbations introduced by a membrane under various conditions. The studies were carried out under conditions of a low and moderate degree of free-flow turbulence. It is shown that the impulsive motion of the membrane generates a localized longitudinal structure in the boundary layer, as well as wave packets at its fronts. A circular membrane generates wave packets consisting of forward and oblique waves, while a rectangular membrane generates predominantly forward waves. A moderate degree of turbulence inhibits the development of wave packets at the linear stage and intensifies at the nonlinear stage. The separation of the boundary layer stimulates an increase in the amplitude of the wave packets.

Morphing Continuum Theory: Incorporating the Physics of Microstructures to Capture the Transition to Turbulence Within a Boundary Layer

2016 ◽  
Vol 139 (1) ◽  
Author(s):  
Louis B. Wonnell ◽  
James Chen
Keyword(s):  
Boundary Layer ◽  
Flat Plate ◽  
Ad Hoc ◽  
Constitutive Relations ◽  
Turbulence Models ◽  
Continuum Theory ◽  
Transition To Turbulence ◽  
Flow Turbulence ◽  
Good Agreement ◽  
Inherent Advantage

A boundary layer with Re = 106 is simulated numerically on a flat plate using morphing continuum theory. This theory introduces new terms related to microproperties of the fluid. These terms are added to a finite-volume fluid solver with appropriate boundary conditions. The success of capturing the initial disturbances leading to turbulence is shown to be a byproduct of the physical and mathematical rigor underlying the balance laws and constitutive relations introduced by morphing continuum theory (MCT). Dimensionless equations are introduced to produce the parameters driving the formation of disturbances leading to turbulence. Numerical results for the flat plate are compared with the experimental results determined by the European Research Community on Flow, Turbulence, and Combustion (ERCOFTAC) database. Experimental data show good agreement inside the boundary layer and in the bulk flow. Success in predicting conditions necessary for turbulent and transitional (T2) flows without ad hoc closure models demonstrates the theory's inherent advantage over traditional turbulence models.

scholarly journals Low-frequency unsteadiness in the wake of a normal flat plate

1998 ◽  
Vol 370 ◽  
pp. 101-147 ◽  
Author(s):  
F. M. NAJJAR ◽  
S. BALACHANDAR
Keyword(s):  
Shear Layer ◽  
Flat Plate ◽  
Free Stream ◽  
Low Frequency ◽  
Separated Flow ◽  
Phase Mismatch ◽  
Streamwise Vortices ◽  
Long Time ◽  
Drag And Lift ◽  
High Degree

The separated flow past a zero-thickness flat plate held normal to a free stream at Re=250 has been investigated through numerical experiments. The long-time signatures of the drag and lift coefficients clearly capture a low-frequency unsteadiness with a period of approximately 10 times the primary shedding period. The amplitude and frequency of drag and lift variations during the shedding process are strongly modulated by the low frequency. A physical interpretation of the low-frequency behaviour is that the flow gradually varies between two different regimes: a regime H of high mean drag and a regime L of low mean drag. It is observed that in regime H the shear layer rolls up closer to the plate to form coherent spanwise vortices, while in regime L the shear layer extends farther downstream and the rolled-up Kármán vortices are less coherent. In the high-drag regime three-dimensionality is characterized by coherent Kármán vortices and reasonably well-organized streamwise vortices connecting the Kármán vortices. With a non-dimensional spanwise wavelength of about 1.2, the three-dimensionality in this regime is reminiscent of mode-B three-dimensionality. It is observed that the high degree of spanwise coherence that exists in regime H breaks down in regime L. Based on detailed numerical flow visualization we conjecture that the formation of streamwise and spanwise vortices is not in perfect synchronization and that the low-frequency unsteadiness is the result of this imbalance (or phase mismatch).

Boundary layer development after a separated region

1998 ◽  
Vol 374 ◽  
pp. 91-116 ◽  
Author(s):  
IAN P. CASTRO ◽  
ELEANORA EPIK
Keyword(s):  
Boundary Layer ◽  
Boundary Layers ◽  
Free Stream ◽  
Turbulence Models ◽  
Separated Flow ◽  
Leading Edge ◽  
Turbulence Structure ◽  
Outer Flow ◽  
Free Stream Turbulence ◽  
Layer Region

Measurements obtained in boundary layers developing downstream of the highly turbulent, separated flow generated at the leading edge of a blunt flat plate are presented. Two cases are considered: first, when there is only very low (wind tunnel) turbulence present in the free-stream flow and, second, when roughly isotropic, homogeneous turbulence is introduced. With conditions adjusted to ensure that the separated region was of the same length in both cases, the flow around reattachment was significantly different and subsequent differences in the development rate of the two boundary layers are identified. The paper complements, but is much more extensive than, the earlier presentation of some of the basic data (Castro & Epik 1996), confirming not only that the development process is very slow, but also that it is non-monotonic. Turbulence stress levels fall below those typical of zero-pressure-gradient boundary layers and, in many ways, the boundary layer has features similar to those found in standard boundary layers perturbed by free-stream turbulence. It is argued that, at least as far as the turbulence structure is concerned, the inner layer region develops no more quickly than does the outer flow and it is the latter which essentially determines the overall rate of development of the whole flow. Some numerical computations are used to assess the extent to which current turbulence models are adequate for such flows.

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