On the generation of spatially growing waves in a boundary layer

1965 ◽  
Vol 22 (3) ◽  
pp. 433-441 ◽  
Author(s):  
M. Gaster
Keyword(s):  
Boundary Layer ◽  
Boundary Layers ◽  
Laminar Boundary Layer ◽  
Wave Number ◽  
Wave System ◽  
Velocity Fluctuations ◽  
Forced Wave ◽  
General Terms ◽  
Complex Wave ◽  
Complex Wave Number

The solution is obtained in general terms for the velocity fluctuations generated in a laminar boundary layer by an oscillating disturbance on the boundry wall. The form of excitation is chosen to represent a vibrating ribbon of the type used by Schubauer to force disturbance in boundary layers. The forced wave system generated by the ribbon is shown to be a spatially growing one, which is described far downstream by an eigenmode of the system which has a complex wave-number.

Flow resistivity, characteristic impedance and complex wave number assessment of coconut fiber with thin thickness samples.

2017 ◽  
Author(s):  
Key Fonseca de Lima ◽  
Nilson Barbieri ◽  
Fernando Jun Hattori Terashima ◽  
Vinicius Antonio Grossl ◽  
Nelson Legat Filho
Keyword(s):  
Characteristic Impedance ◽  
Wave Number ◽  
Coconut Fiber ◽  
Complex Wave ◽  
Flow Resistivity ◽  
Complex Wave Number

Decreasing the Side Wall Contamination in Wind Tunnels

1983 ◽  
Vol 105 (4) ◽  
pp. 435-438 ◽  
Author(s):  
T. Motohashi ◽  
R. F. Blackwelder
Keyword(s):  
Boundary Layer ◽  
Reynolds Number ◽  
Boundary Layers ◽  
Laminar Boundary Layer ◽  
Three Dimensional ◽  
Side Wall ◽  
Wind Tunnels ◽  
Turbulent Fluid ◽  
Side Walls ◽  
Suction Slot

To study boundary layers in the transitional Reynolds number regime, the useful spanwise and streamwise extent of wind tunnels is often limited by turbulent fluid emanating from the side walls. Some or all of the turbulent fluid can be removed by sucking fluid out at the corners, as suggested by Amini [1]. It is shown that by optimizing the suction slot width, the side wall contamination can be dramatically decreased without a concomitant three-dimensional distortion of the laminar boundary layer.

Experimental Analysis and Prediction of Wake-Induced Transition in Turbomachinery

2004 ◽  
Author(s):  
Witold Elsner ◽  
Stephane Vilmin ◽  
Stanislaw Drobniak ◽  
Wladyslaw Piotrowski
Keyword(s):  
Boundary Layer ◽  
Boundary Layers ◽  
Leading Edge ◽  
Transition Process ◽  
Full Time ◽  
Blade Profile ◽  
Velocity Fluctuations ◽  
Cambridge University ◽  
Flow Features ◽  
Good Agreement

The paper presents an experimental and numerical analysis of the interaction between wakes and boundary layers on aerodynamic blade profiles. The experiment revealed that incoming wakes interact with boundary layers and cause the significant increase of velocity fluctuations in the boundary layer and in consequence shift the transition zone towards the leading edge. The full time evolution of periodic wake induced transition was reproduced from measurements. The numerical simulation of the flow around the blade profile has been performed with the use of the adaptive grid viscous flow unNEWT PUIM solver with a prescribed unsteady intermittency method (PUIM) developed at Cambridge University, UK. The results obtained give evidence that the turbulence transported within the wake is mainly responsible for the transition process. The applied CFD solver was able to reproduce some essential flow features related to the bypass and wake-induced transitions and the simulations reveal good agreement with the experimental results in terms of localisation and extent of wake-induced transition.

Theoretical Analysis of Oscillating and Streaming Fields Between Two Parallel Beams and Numerical Investigation of the Cooling Efficiency

2003 ◽  
Author(s):  
Qun Wan ◽  
A. V. Kuznetsov
Keyword(s):  
Boundary Layer ◽  
Nusselt Number ◽  
Boundary Layers ◽  
Standing Wave ◽  
Critical Value ◽  
Core Region ◽  
Wave Number ◽  
Wave Form ◽  
Channel Height ◽  
The Core

The main purpose of this paper is to investigate the oscillating and streaming flow fields and the heat transfer efficiency across a channel between two long parallel beams, one of which is stationary and the other oscillating in standing wave form. The oscillating amplitude is assumed much smaller than the channel height. When the Reynolds number, which is defined by the oscillating frequency and the standing wave number, is much greater than unity, boundary layer structures are found near both beams, which are separated by a core region in the center of the channel. The oscillating fields within the core region and both boundary layers are obtained analytically. Based on the oscillating fields, the streaming fields within both boundary layers are also analytically obtained. Further investigation of boundary layer streaming fields shows that the streaming velocities approach constant values at the edges of the boundary layers and provide slip velocities for the streaming field in the core region. The core region streaming velocity field is numerically obtained by solving the mass and momentum conservation equations in their stream function–vorticity form. The temperature field is also computed for two cases: both beams are kept at constant but different temperatures (case A) or the oscillating beam is kept at a constant temperature and the stationary beam is prescribed a constant heat flux (case B). Cases of different channel heights are computed and a critical height is found. When the channel height is smaller than the critical value, for each half standing wavelength distance along the beams, two symmetric eddies are observed, which occupy the whole channel. In this case, the Nusselt number increases with the increase of the channel height. After the critical value, two layers of asymmetric eddies are observed near the oscillating beam and the Nusselt number decreases and approaches unity with the increase of the gap size. The abrupt change of the streaming field and the Nusselt number as the channel height goes through its critical value may be due to the bifurcation caused by instability of the vortex structure in the fluid layer.

Prediction of the Becalmed Region for LP Turbine Profile Design

1997 ◽  
Author(s):  
Volker Schulte ◽  
Howard P. Hodson
Keyword(s):  
Boundary Layer ◽  
Boundary Layers ◽  
Turbulent Flows ◽  
Laminar Boundary Layer ◽  
Turbine Blades ◽  
Boundary Layer Separation ◽  
Suction Surface ◽  
High Lift ◽  
Lp Turbine ◽  
Profile Loss

Recent attention has focused on the so called ‘becalmed region’ that is observed inside the boundary layers of turbomachinery blading and is associated with the process of wake-induced transition. Significant reductions of profile loss have been shown for high lift LP turbine blades at low Reynolds-numbers due the effects of the becalmed region on the diffusing flow at the rear of the suction surface. In this paper the nature and the significance of the becalmed region are examined using experimental observations and computational studies. It is shown that the becalmed region may be modelled using the unsteady laminar boundary layer equations. Therefore, it is predictable independently of the transition or turbulence models employed. The effect of the becalmed region on the transition process is modelled using a spot-based intermittency transition model. An unsteady differential boundary layer code was used to numerically simulate a deterministic experiment involving an isolated turbulent spot. The predictability of the becalmed region means that the rate of entropy production can be calculated in that region. It is found to be of the order of that in a laminar boundary layer. It is for this reason and because the becalmed region may be encroached upon by pursuing turbulent flows that for attached boundary layers, wake-induced transition cannot significantly reduce the profile loss. However, the becalmed region is less prone to separation than a conventional laminar boundary layer. Therefore, the becalmed region may be exploited in order to prevent boundary layer separation and the increase in loss that this entails. It is shown that it should now be possible to design efficient high lift LP turbine blades.

scholarly journals A Bypass Transition Model for Boundary Layers

1993 ◽  
Author(s):  
Mark W. Johnson
Keyword(s):  
Boundary Layer ◽  
Boundary Layers ◽  
Free Stream ◽  
Transition Model ◽  
Wall Region ◽  
Bypass Transition ◽  
Pressure Gradients ◽  
Turbulence Level ◽  
Velocity Fluctuations ◽  
Laminar Boundary Layers

Experimental data for laminar boundary layers developing below a turbulent free stream shows that the fluctuation velocities within the boundary layer increase in amplitude until some critical level is reached which initiates transition. In the near wall region, a simple model, containing a single empirical parameter which depends only on the turbulence level and length scale, is derived to predict the development of the velocity fluctuations in laminar boundary layers with favourable, zero or adverse pressure gradients. A simple bypass transition model which considers the streamline distortion in the near wall region brought about by the velocity fluctuations suggests that transition will commence when the local turbulence level reaches approximately 23%. This value is consistent with experimental findings. This critical local turbulence level is used to derive a bypass transition prediction formula which compares reasonably with start of transition experimental data for a range of pressure gradients (λθ = −0.01 to 0.01) and turbulence levels (Tu = 0.2% to 5%). Further improvement to the model is proposed through prediction of the boundary layer distortion, which occurs due to Reynolds stresses generated within the boundary layer at high free stream turbulence levels and also through inclusion of the effect of turbulent length scale as well as turbulence level.

Use of the Cionco model to obtain further information on the nature of leaf boundary layers

1968 ◽  
Vol 46 (2) ◽  
pp. 177-178 ◽  
Author(s):  
L. A. Hunt
Keyword(s):  
Boundary Layer ◽  
Boundary Layers ◽  
Laminar Boundary Layer ◽  
Air Flow ◽  
Diffusion Resistance ◽  
Corn Crop

The Cionco model for air-flow within a foliage canopy was used to estimate the diffusion resistance of the boundary layer around the uppermost leaves in a corn crop. The values were compared with others derived from a formula which represents transfer through a laminar boundary layer. These comparisons suggested that the boundary layer was either turbulent, or in a state of transition from laminar to turbulent.

Reimagining full wave rf quasilinear theory in a tokamak

2021 ◽  
Vol 87 (2) ◽  
Author(s):  
Peter J. Catto ◽  
Elizabeth A. Tolman
Keyword(s):  
Boundary Layer ◽  
Boundary Layers ◽  
Collision Frequency ◽  
Velocity Space ◽  
Wave Number ◽  
Temporal Behaviour ◽  
Full Wave ◽  
Effective Collision Frequency ◽  
Space Boundary ◽  
Effective Collision

The velocity dependent resonant interaction of particles with applied radiofrequency (rf) waves during heating and current drive in the presence of pitch angle scattering collisions gives rise to narrow collisional velocity space boundary layers that dramatically enhance the role of collisions as recently shown by Catto (J. Plasma Phys., vol. 86, 2020, 815860302). The behaviour is a generalization of the narrow collisional boundary layer that forms during Landau damping as found by Johnston (Phys. Fluids, vol. 14, 1971, pp. 2719–2726) and Auerbach (Phys. Fluids, vol. 20, 1977, pp. 1836–1844). For a wave of parallel wave number ${k_{||}}$ interacting with weakly collisional plasma species of collision frequency $\nu$ and thermal speed ${v_{\textrm{th}}}$ , the effective collision frequency becomes of order $\nu {({k_{||}}{v_{th}}/\nu )^{2/3}} \gg \nu $ . The narrow boundary layers that arise because of the diffusive nature of the collisions allow a physically meaningful wave–particle interaction time to be defined that is the inverse of this effective collision frequency. The collisionality implied by the narrow boundary layer results in changes in the standard quasilinear treatment of applied rf fields in tokamaks while remaining consistent with causality. These changes occur because successive poloidal interactions with the rf are correlated in tokamak geometry and because the resonant velocity space dependent interactions are controlled by the spatial and temporal behaviour of the perturbed full wave fields rather than just the spatially local Landau and Doppler shifted cyclotron wave–particle resonance condition associated with unperturbed motion of the particles. The correlation of successive poloidal circuits of the tokamak leads to the appearance in the quasilinear operator of transit averaged resonance conditions localized in velocity space boundary layers that maintain negative definite entropy production.

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