scholarly journals Secondary Instabilities in Incompressible Axisymmetric Boundary Layers: Effect of Transverse Curvature

2012 ◽  
Vol 134 (2) ◽  
Author(s):  
N. Vinod ◽  
Rama Govindarajan
Keyword(s):  
Boundary Layer ◽  
Laminar Flow ◽  
Axial Flow ◽  
Base Flow ◽  
Transition To Turbulence ◽  
Flow Past A Cylinder ◽  
Incompressible Boundary ◽  
Layer Flow ◽  
Secondary Instabilities ◽  
Long Cylinder

The secondary instability of the incompressible boundary layer in the axial flow past a cylinder is studied. The laminar flow is shown to be always stable at high transverse curvatures to secondary disturbances. Because the primary mode is stable as well, (Tutty et al., 2002, “Boundary Layer Flow on a Long Thin Cylinder,”. Phys. Fluids, 14(2), pp. 628–637), this implies that the boundary layer on a thin long cylinder may undergo transition to turbulence by means very different from that on a flat plate. The azimuthal wavenumber of the least stable secondary modes (m±) are related to that of the primary (n) by m+ = 2n and m− = −n. The base flow is shown to be inviscidly stable at any curvature.

EXPERIMENTAL INVESTIGATION OF FLOW RESPONSE TO STATIONARY DISTURBANCE IN ROTATING-DISK FLOW

2019 ◽  
Vol XVI (2) ◽  
pp. 13-22
Author(s):  
Muhammad Ehtisham Siddiqui
Keyword(s):  
Boundary Layer ◽  
Boundary Layer Flow ◽  
Three Dimensional ◽  
Rotating Disk ◽  
Careful Analysis ◽  
Laboratory Frame ◽  
Transition To Turbulence ◽  
Turbulent Regime ◽  
Mean Velocity ◽  
Layer Flow

Three-dimensional boundary-layer flow is well known for its abrupt and sharp transition from laminar to turbulent regime. The presented study is a first attempt to achieve the target of delaying the natural transition to turbulence. The behaviour of two different shaped and sized stationary disturbances (in the laboratory frame) on the rotating-disk boundary layer flow is investigated. These disturbances are placed at dimensionless radial location (Rf = 340) which lies within the convectively unstable zone over a rotating-disk. Mean velocity profiles were measured using constant-temperature hot-wire anemometry. By careful analysis of experimental data, the instability of these disturbance wakes and its estimated orientation within the boundary-layer were investigated.

Keel Design for Low Viscous Drag

1987 ◽  
Author(s):  
Clifford J. Obara ◽  
C. P. van Dam
Keyword(s):  
Boundary Layer ◽  
Laminar Flow ◽  
Laminar Boundary Layer ◽  
Leading Edge ◽  
Boundary Layer Transition ◽  
Viscous Drag ◽  
Hydrodynamic Characteristics ◽  
Sweep Angle ◽  
Layer Transition ◽  
Layer Flow

In this paper, foil and planform parameters which govern the level of viscous drag produced by the keel of a sailing yacht are discussed. It is shown that the application of laminar boundary-Layer flow offers great potential for increased boat speed resulting from the reduction in viscous drag. Three foil shapes have been designed and it is shown that their hydro­dynamic characteristics are very much dependent on location and mode of boundary-Layer transition. The planform parameter which strongly affects the capabilities of the keel to achieve laminar flow is lea ding-edge sweep angle. The two significant phenomena related to keel sweep angle which can cause premature transition of the laminar boundary layer are crossflow instability and turbulent contamination of the leading-edge attachment line. These flow phenomena and methods to control them are discussed in detail. The remaining factors that affect the maintainability of laminar flow include surface roughness, surface waviness, and freestream turbulence. Recommended limits for these factors are given to insure achievability of laminar flow on the keel. In addition, the application of a simple trailing-edge flap to improve the hydrodynamic characteristics of a foil at moderate-to-high leeway angles is studied.

Study of the Effect of Tangential Point Blowing on the Incompressible Boundary Layer Flow Around a Circular Cylinder

2014 ◽  
Author(s):  
S. K. Ghosh ◽  
S. K. Mukherjea ◽  
B. N. Datta
Keyword(s):  
Boundary Layer ◽  
Circular Cylinder ◽  
Strouhal Number ◽  
Lift Coefficient ◽  
Separation Point ◽  
Pressure Drag ◽  
Two Dimensional Flow ◽  
Incompressible Boundary ◽  
Layer Flow ◽  
Oscillation Frequencies

In the present work, two dimensional flow simulations have been performed to study the effect of tangential blowing on the drag force for the case of flow around a circular cylinder only at Reynolds number (Re) 200. Blowing ports have been placed symmetrically with respect to the horizontal flow axis. The effect of blowing on the boundary layer has been studied with respect to the intensity of blowing as well as port position of blowing. For a particular intensity of blowing, oscillations of the front stagnation point and the separation point was studied. It was found that oscillation frequencies were identical with that of lift coefficient. It was also observed that with the increase of the intensity of the blowing the position of the separation point shifts downstream (separation delay)thus the wake becomes narrower resulting the decrease of drag. Strouhal number was found to increase with the intensity of blowing. It was observed that the Strouhal number as well as the position of the separation point is influenced by the position of the blowing port. Although, for a significant range of the blowing port positions the Strouhal number is observed to be constant. Drag was found to be influenced by the position of the blowing ports. Pressure drag was found to be more significant. Thus total drag is influenced primarily by the pressure drag. It was observed that in the range of 40° to about 110° for the position of the blowing port drag decreased. But beyond 110° it increased again.

scholarly journals Reattachment streaks in hypersonic compression ramp flow: an input–output analysis

2019 ◽  
Vol 880 ◽  
pp. 113-135 ◽  
Author(s):  
Anubhav Dwivedi ◽  
G. S. Sidharth ◽  
Joseph W. Nichols ◽  
Graham V. Candler ◽  
Mihailo R. Jovanović
Keyword(s):  
Boundary Layer ◽  
Streamwise Velocity ◽  
Base Flow ◽  
Transition To Turbulence ◽  
Streamwise Vorticity ◽  
Input Output ◽  
Output Analysis ◽  
Input Output Analysis ◽  
Boundary Layer Interaction ◽  
Compression Ramp

We employ global input–output analysis to quantify amplification of exogenous disturbances in compressible boundary layer flows. Using the spatial structure of the dominant response to time-periodic inputs, we explain the origin of steady reattachment streaks in a hypersonic flow over a compression ramp. Our analysis of the laminar shock–boundary layer interaction reveals that the streaks arise from a preferential amplification of upstream counter-rotating vortical perturbations with a specific spanwise wavelength. These streaks are associated with heat-flux striations at the wall near flow reattachment and they can trigger transition to turbulence. The streak wavelength predicted by our analysis compares favourably with observations from two different hypersonic compression ramp experiments. Furthermore, our analysis of inviscid transport equations demonstrates that base-flow deceleration contributes to the amplification of streamwise velocity and that the baroclinic effects are responsible for the production of streamwise vorticity. Finally, the appearance of the temperature streaks near reattachment is triggered by the growth of streamwise velocity and streamwise vorticity perturbations as well as by the amplification of upstream temperature perturbations by the reattachment shock.

Mechanisms and passive control of crossflow-vortex-induced transition in a three-dimensional boundary layer

2002 ◽  
Vol 456 ◽  
pp. 49-84 ◽  
Author(s):  
PETER WASSERMANN ◽  
MARKUS KLOKER
Keyword(s):  
Boundary Layer ◽  
Passive Control ◽  
Three Dimensional ◽  
Plate Boundary ◽  
Secondary Instability ◽  
Base Flow ◽  
Transition Control ◽  
Dimensional Boundary ◽  
Layer Flow ◽  
Underlying Mechanisms

Crossflow-vortex-induced laminar breakdown in a three-dimensional flat-plate boundary-layer flow is investigated in detail by means of spatial direct numerical simulations. The base flow is generic for an infinite swept wing, with decreasing favourable chordwise pressure gradient. First, the downstream growth and nonlinear saturation states initiated by a crossflow-vortex-mode packet as well as by single crossflow-vortex modes with various spanwise wavenumbers are simulated. Second, the secondary instability of the flow induced by the saturated crossflow vortices is scrutinized, clearly indicating the convective nature of the secondary instability and strengthening knowledge of the conditions for its onset. Emphasis is on the effect of crossflow-vortex-mode packets and of the spanwise vortex spacing on the secondary stability properties of the saturation states. Saturated uniform crossflow vortices initiated by single crossflow-vortex modes turn out to be less unstable than vortices initiated by a packet of vortex modes, and closely spaced saturated vortices are even stable. Third, we investigate the transition control strategy of upstream flow deformation by appropriate steady nonlinear vortex modes as applied in wind tunnel experiments at the Arizona State University. A significant transition delay is shown in the base flow considered here, and the underlying mechanisms are specified.

scholarly journals Entrainment of short-wavelength free-stream vortical disturbances in compressible and incompressible boundary layers

2016 ◽  
Vol 797 ◽  
pp. 683-728 ◽  
Author(s):  
Xuesong Wu ◽  
Ming Dong
Keyword(s):  
Boundary Layer ◽  
Boundary Layers ◽  
Free Stream ◽  
Base Flow ◽  
Bypass Transition ◽  
Leading Order ◽  
Compressible Boundary Layers ◽  
Characteristic Wavelength ◽  
Incompressible Boundary ◽  
Edge Layer

The fundamental difference between continuous modes of the Orr–Sommerfeld/Squire equations and the entrainment of free-stream vortical disturbances (FSVD) into the boundary layer has been investigated in a recent paper (Dong & Wu, J. Fluid Mech., vol. 732, 2013, pp. 616–659). It was shown there that the non-parallel-flow effect plays a leading-order role in the entrainment, and neglecting it at the outset, as is done in the continuous-mode formulation, leads to non-physical features of ‘Fourier entanglement’ and abnormal anisotropy. The analysis, which was for incompressible boundary layers and for FSVD with a characteristic wavelength of the order of the local boundary-layer thickness, is extended in this paper to compressible boundary layers and FSVD with even shorter wavelengths, which are comparable with the width of the so-called edge layer. Non-parallelism remains a leading-order effect in the present scaling, which turns out to be more general in that the equations and solutions in the previous paper are recovered in the appropriate limit. Appropriate asymptotic solutions in the main and edge layers are obtained to characterize the entrainment. It is found that when the Prandtl number $\mathit{Pr}<1$, free-stream vortical disturbances of relatively low frequency generate very strong temperature fluctuations within the edge layer, leading to formation of thermal streaks. A composite solution, uniformly valid across the entire boundary layer, is constructed, and it can be used in receptivity studies and as inlet conditions for direct numerical simulations of bypass transition. For compressible boundary layers, continuous spectra of the disturbance equations linearized about a parallel base flow exhibit entanglement between vortical and entropy modes, namely, a vortical mode necessarily induces an entropy disturbance in the free stream and vice versa, and this amounts to a further non-physical behaviour. High Reynolds number asymptotic analysis yields the relations between the amplitudes of entangled modes.

scholarly journals Optimal Perturbations in Boundary-Layer Flows Over Rough Surfaces

2013 ◽  
Vol 135 (12) ◽  
Author(s):  
S. Cherubini ◽  
M. D. de Tullio ◽  
P. De Palma ◽  
G. Pascazio
Keyword(s):  
Boundary Layer ◽  
Three Dimensional ◽  
Base Flow ◽  
Immersed Boundary ◽  
Boundary Layer Flows ◽  
Instability Theory ◽  
Roughness Elements ◽  
Tollmien Schlichting ◽  
Layer Flow ◽  
Lagrangian Optimization

This work provides a three-dimensional energy optimization analysis, looking for perturbations inducing the largest energy growth at a finite time in a boundary-layer flow in the presence of roughness elements. The immersed boundary technique has been coupled with a Lagrangian optimization in a three-dimensional framework. Four roughness elements with different heights have been studied, inducing amplification mechanisms that bypass the asymptotical growth of Tollmien–Schlichting waves. The results show that even very small roughness elements, inducing only a weak deformation of the base flow, can strongly localize the optimal disturbance. Moreover, the highest value of the energy gain is obtained for a varicose perturbation. This result demonstrates the relevance of varicose instabilities for such a flow and shows a different behavior with respect to the secondary instability theory of boundary layer streaks.

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