scholarly journals The Influence of Slopes of Isolated Three-Dimensional Valleys on Near-Surface Turbulence

2021 ◽  
Author(s):  
Sylvio Freitas ◽  
Frank Harms ◽  
Bernd Leitl
Keyword(s):  
Boundary Layer ◽  
Longitudinal Velocity ◽  
Three Dimensional ◽  
Boundary Layer Flows ◽  
Homogeneous Surface ◽  
Near Surface ◽  
Surface Turbulence ◽  
Stable Atmospheric Boundary Layer ◽  
Recirculation Zones ◽  
Layer Flow

AbstractMotivated by a limited understanding of how valleys affect near-surface turbulence, characterizations of neutrally stable atmospheric-boundary-layer flows over isolated valleys are presented. In particular, the influence of the slopes of the three-dimensional ridges that form the idealized valleys are investigated. Flows over three distinct symmetric valley geometries were modelled in a large boundary-layer wind tunnel. For each valley geometry, the high-resolution measurements from the crests of each of the ridges and the midpoint between them are compared with an undisturbed moderately rough classed boundary-layer flow over flat terrain with homogeneous surface roughness. Flow separation originates above the crests of the first ridges of all geometries and generates recirculation zones. These are characterized by slope-dependent increases in three-dimensional near-surface turbulence when compared with the attached flows further upstream. The recirculation zones longitudinally extend to roughly half the valley width. Above the crests of the second ridges, the longitudinal velocity component decreases and turbulence intensity increases when compared with the flows above the crests of the first ridges. Results also exhibit significant increases of turbulence above the inner-valley regions of all geometries.

scholarly journals The Effects of the Width of an Isolated Valley on Near-Surface Turbulence

Atmosphere ◽  
2021 ◽  
Vol 12 (10) ◽  
pp. 1330
Author(s):  
Sylvio Freitas ◽  
Frank Harms ◽  
Bernd Leitl
Keyword(s):  
Boundary Layer ◽  
High Resolution ◽  
Laser Doppler Velocimetry ◽  
Three Dimensional ◽  
Doppler Velocimetry ◽  
Constant Depth ◽  
Near Surface ◽  
Surface Flows ◽  
Surface Turbulence ◽  
Recirculation Zones

With the aim of ascertaining the effects of the widths (A) of valleys on near-surface turbulence, flows over an isolated symmetric three-dimensional valley of constant depth (H) and slopes are characterized in a large-boundary-layer wind tunnel. Starting at A = 4H, valley widths were systematically varied to A = 12H with constant increments of 2H. High-resolution laser-Doppler velocimetry measurements were made at several equivalent locations above each of the resulting valley geometries and compared with data from undisturbed flows over flat terrain. Flow separation caused by the first ridges generated inner-valley recirculation bubbles with lengths dependent on the valley widths. Secondary recirculation zones were also observed downstream from the crests of the second ridges. Results show that the width modifications exert the strongest effects on turbulence within the valleys and the vicinities of the second ridges. Above these locations, maximal magnitudes of turbulence are generally found for the larger width geometries. Furthermore, lateral turbulence overpowers the longitudinal counterparts nearest to the surface, with maximal gains occurring for the smaller widths. Our data indicate that valley widths are impactful on near-surface flows and should be considered together with other more established geometric parameters of influence.

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.

scholarly journals Optimal Perturbations in Boundary Layer Flows Over Rough Surfaces

2012 ◽  
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. Amplification mechanisms are described which by-pass the asymptotical growth of Tollmien–Schlichting waves. The immersed boundary technique has been coupled with a Lagrangian optimization in a three-dimensional framework. Two types of roughness elements have been studied, characterized by a different height. 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, pointing out the importance of varicose instabilities for such a flow and a different behavior with respect to the secondary instability theory of boundary layer streaks.

Receptivity mechanisms in three-dimensional boundary-layer flows

2009 ◽  
Vol 618 ◽  
pp. 209-241 ◽  
Author(s):  
LARS-UVE SCHRADER ◽  
LUCA BRANDT ◽  
DAN S. HENNINGSON
Keyword(s):  
Boundary Layer ◽  
Free Stream ◽  
Three Dimensional ◽  
Leading Edge ◽  
Nonlinear Process ◽  
Transition To Turbulence ◽  
Boundary Layer Flows ◽  
Free Stream Turbulence ◽  
Dimensional Boundary ◽  
Layer Flow

Receptivity in three-dimensional boundary-layer flow to localized surface roughness and free-stream vorticity is studied. A boundary layer of Falkner–Skan–Cooke type with favourable pressure gradient is considered to model the flow slightly downstream of a swept-wing leading edge. In this region, stationary and travelling crossflow instability dominates over other instability types. Three scenarios are investigated: the presence of low-amplitude chordwise localized, spanwise periodic roughness elements on the plate, the impingement of a weak vortical free-stream mode on the boundary layer and the combination of both disturbance sources. Three receptivity mechanisms are identified: steady receptivity to roughness, unsteady receptivity to free-stream vorticity and unsteady receptivity to vortical modes scattered at the roughness. Both roughness and vortical modes provide efficient direct receptivity mechanisms for stationary and travelling crossflow instabilities. We find that stationary crossflow modes dominate for free-stream turbulence below a level of about 0.5%, whereas higher turbulence levels will promote the unsteady receptivity mechanism. Under the assumption of small amplitudes of the roughness and the free-stream disturbance, the unsteady receptivity process due to scattering of free-stream vorticity at the roughness has been found to give small initial disturbance amplitudes in comparison to the direct mechanism for free-stream modes. However, in many environments free-stream vorticity and roughness may excite interacting unstable stationary and travelling crossflow waves. This nonlinear process may rapidly lead to large disturbance amplitudes and promote transition to turbulence.

scholarly journals Cancellation of Tollmien–Schlichting waves with surface heating

2021 ◽  
Vol 128 (1) ◽  
Author(s):  
Georgia S. Brennan ◽  
Jitesh S. B. Gajjar ◽  
Richard E. Hewitt
Keyword(s):  
Boundary Layer ◽  
Asymptotic Methods ◽  
Three Dimensional ◽  
Exact Formula ◽  
Surface Heating ◽  
Two Dimensional ◽  
Boundary Layer Flows ◽  
Dimensional Boundary ◽  
Tollmien Schlichting ◽  
Layer Flow

AbstractTwo-dimensional boundary layer flows in quiet disturbance environments are known to become unstable to Tollmien–Schlichting waves. The experimental work of Liepmann et al. (J Fluid Mech 118:187–200, 1982), Liepmann and Nosenchuck (J Fluid Mech 118:201–204, 1982) showed how it is possible to control and reduce unstable Tollmien–Schlichting wave amplitudes using unsteady surface heating. We consider the problem of an oncoming planar compressible subsonic boundary layer flow with a three-dimensional vibrator mounted on a flat plate, and with surface heating present. It is shown using asymptotic methods based on triple-deck theory that it is possible to choose an unsteady surface heating distribution to cancel out the response due to the vibrator. An approximation based on the exact formula is used successfully in numerical computations to confirm the findings. The results presented here are a generalisation of the analogous results for the two-dimensional problem in Brennan et al. (J Fluid Mech 909:A16-1, 2020).

scholarly journals How smooth is a dolphin? The ridged skin of odontocetes

2019 ◽  
Vol 15 (7) ◽  
pp. 20190103 ◽  
Author(s):  
Dylan K. Wainwright ◽  
Frank E. Fish ◽  
Sam Ingersoll ◽  
Terrie M. Williams ◽  
Judy St Leger ◽  
...  
Keyword(s):  
Boundary Layer ◽  
Drag Reduction ◽  
Three Dimensional ◽  
Morphological Data ◽  
Boundary Layer Flows ◽  
Surface Geometry ◽  
Average Roughness ◽  
Fluid Resistance ◽  
Layer Flow

Dolphin skin has long been an inspiration for research on drag reduction mechanisms due to the presence of skin ridges that could reduce fluid resistance. We gathered in vivo three-dimensional surface data on the skin from five species of odontocetes to quantitatively examine skin texture, including the presence and size of ridges. We used these data to calculate k + values, which relate surface geometry to changes in boundary layer flow. Our results showed that while ridge size differs among species, odontocete skin was surprisingly smooth compared to the skin of other swimmers (average roughness = 5.3 µm). In addition, the presence of ridges was variable among individuals of the same species. We predict that odontocete skin ridges do not alter boundary layer flows at cruising swimming speeds. By combining k + values and morphological data, our work provides evidence that skin ridges are unlikely to be an adaptation for drag reduction and that odontocete skin is exceptionally smooth compared to other pelagic swimmers.

Three-dimensional laminar compressible boundary layers with large injection

1975 ◽  
Vol 71 (4) ◽  
pp. 711-727 ◽  
Author(s):  
C. S. Vimala ◽  
G. Nath
Keyword(s):  
Boundary Layer ◽  
Swirling Flow ◽  
Three Dimensional ◽  
Large Mass ◽  
Variable Cross Section ◽  
Variable Cross ◽  
Compressible Boundary Layer ◽  
Boundary Layer Flows ◽  
Compressible Boundary Layers ◽  
Layer Flow

The effect of large mass injection on the following three-dimensional laminar compressible boundary-layer flows is investigated by employing the method of matched asymptotic expansions: (i) swirling flow in a laminar compressible boundary layer over an axisymmetric surface with variable cross-section and (ii) laminar compressible boundary-layer flow over a yawed infinite wing in a hypersonic flow. The resulting equations are solved numerically by combining the finite-difference technique with quasi-linearization. An increase in the swirl parameter, the yaw angle or the wall temperature is found to be capable of bringing the viscous layer nearer the surface and reducing the effects of massive blowing.

The Design and Validation of a Thermal Boundary Layer Wall Plate

2019 ◽  
Vol 141 (12) ◽  
Author(s):  
Drummond Biles ◽  
Alireza Ebadi ◽  
Michael P. Allard ◽  
Christopher M. White
Keyword(s):  
Boundary Layer ◽  
Wall Temperature ◽  
Thermal Transport ◽  
Boundary Layer Flow ◽  
Thermal Boundary Layer ◽  
Convective Heat Transfer Coefficient ◽  
Three Dimensional ◽  
Boundary Layer Flows ◽  
Layer Flow ◽  
Nonequilibrium Boundary Layer

A feedback controlled thermal wall plate designed to investigate thermal boundary layer flows is described and validated. The unique capabilities of the design are the ability to modify the thermal boundary conditions in a variety of ways or to hold the wall-temperature fixed even when the flow above the wall is unsteady and strongly three-dimensional. These capabilities allow for the generation and study of thermal transport in nonequilibrium boundary layer flows driven by different perturbations and of varying complexity. The thermal wall plate and the experimental facility in which the thermal wall plate is installed are first described. The wall-plate is then validated in a zero-pressure-gradient (ZPG) boundary layer flow for conditions of a uniform wall temperature and a temperature step. It is then shown that the wall temperature can be held constant even when a hemisphere body is placed on the wall that produces large localized variations in the convective heat transfer coefficient. Last, since the thermal wall plate is intended to support the study of thermal transport in a variety of nonequilibrium boundary layer flow, several possible experimental configurations are presented and described.

Three-dimensional turbulent boundary layers in external flows: a report on Euromech 60

1975 ◽  
Vol 71 (4) ◽  
pp. 815-826 ◽  
Author(s):  
T. K. Fanneløp ◽  
P. Å. Krogstad
Keyword(s):  
Boundary Layer ◽  
Boundary Layers ◽  
Three Dimensional ◽  
Turbulent Boundary Layers ◽  
Boundary Layer Flows ◽  
Special Report ◽  
Flow Problems ◽  
The Subject ◽  
External Flows ◽  
Layer Flow

Euromech 60 is the third in a series of European Mechanics Colloquia dealing primarily with three-dimensional turbulent boundary layers. The Colloquium was held in Trondheim at the Technical University (NTH) from 14–16 April 1975 with forty-two participants from ten different countries, and was organized by L. N. Persen and T. K. Fanneløp. A total of 23 papers were presented, dealing with both experiments on and predictions of turbulent boundary-layer flows and related topics. Those concerned with the development of prediction methods were challenged by a set of boundary-layer flow problems defined well in advance in order to be calculated prior to the Colloquium. The results show close agreement for some calculated variables and surprisingly large discrepancies in others. A brief account of this exercise is included and it will also be the subject of a special report.

scholarly journals Similarity Requirements for Mixed Convective Boundary Layer Flow over Vertical Curvilinear Porous Surfaces with Heat Generation/Absorption

2020 ◽  
Vol 2020 ◽  
pp. 1-9
Author(s):  
Kh. Abdul Maleque
Keyword(s):  
Boundary Layer ◽  
Differential Equations ◽  
Three Dimensional ◽  
Heat Absorption ◽  
Boundary Layer Flows ◽  
Convective Boundary ◽  
Layer Flow ◽  
Surface Similarity ◽  
Lamé Coefficients ◽  
Flow Study

Similarity requirements for three dimensional combined forced and free convective laminar boundary layer flows over the porous inclined vertical curvilinear surfaces with buoyancy effects and heat absorption/generation effects are investigated theoretically. The potential flow in the mainstream and Gabriel lame coefficients outside of the boundary layer are the function of ξ,η. Hence, the external velocity components (Ue, Ve) and Gabriel lame coefficients h1,h2,h3 are independent of ζ. Here, h3ξ,η=1 has been set such that ζ represents actual distance measured normal to the surface. Similarity requirements for an incompressible fluid are sought on the basis of detailed analyses in order to reduce the governing partial differential equations into a set of ordinary differential equations. Finally, different possible cases are exhibited in a tabular form with the inclusion of ΔT variations for onward flow study that are helpful to the future researchers for the flow over the orthogonal curvilinear surfaces.

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