Experimental Investigation of Boundary Layer Relaminarization in Accelerated Flow

2018 ◽  
Vol 140 (8) ◽  
Author(s):  
Pascal Bader ◽  
Manuel Pschernig ◽  
Wolfgang Sanz ◽  
Jakob Woisetschläger ◽  
Franz Heitmeir ◽  
...  
Keyword(s):  
Boundary Layer ◽  
Turbulent Flow ◽  
Skin Friction ◽  
Laser Doppler Anemometry ◽  
Plate Boundary ◽  
Mean Velocity ◽  
Local Skin ◽  
Velocity Fluctuations ◽  
Mean Values ◽  
The Mean

Flow in turbomachines is generally highly turbulent. Nonetheless, boundary layers may exhibit laminar-to-turbulent transition, and relaminarization of the turbulent flow may also occur. The state of flow of the boundary layer is important since it influences transport phenomena like skin friction and heat transfer. In this paper, relaminarization in accelerated flat-plate boundary-layer flows is experimentally investigated, measuring flow velocities with laser Doppler anemometry (LDA). Besides the mean values, statistical properties of the velocity fluctuations are discussed in order to understand the processes in relaminarization. It is shown that strong acceleration leads to a suppression of turbulence production. The velocity fluctuations in the accelerated boundary layer flow “freeze,” while the mean velocity increases, thus reducing the turbulence intensity. This leads to a laminar-like velocity profile close to the wall, resulting in a decrease of the local skin friction coefficient. Downstream from the section with enforced relaminarization, a rapid retransition to turbulent flow is observed. The findings of this work also describe the mechanism of retransition.

Flow Characteristics of Transitional Boundary Layers on an Airfoil in Wakes

2000 ◽  
Vol 122 (3) ◽  
pp. 522-532 ◽  
Author(s):  
H. Lee ◽  
S.-H. Kang
Keyword(s):  
Boundary Layer ◽  
Reynolds Number ◽  
Skin Friction ◽  
Velocity Fluctuation ◽  
Plate Boundary ◽  
Flow Characteristics ◽  
Turbulence Models ◽  
Transitional Region ◽  
Mean Velocity ◽  
Measured Velocity

Transition characteristics of a boundary layer on a NACA0012 airfoil are investigated by measuring unsteady velocity using hot wire anemometry. The airfoil is installed in the incoming wake generated by an airfoil aligned in tandem with zero angle of attack. Reynolds number based on the airfoil chord varies from 2.0×105 to 6.0×105; distance between two airfoils varies from 0.25 to 1.0 of the chord length. To measure skin friction coefficient identifying the transition onset and completion, an extended wall law is devised to accommodate transitional flows with pressure gradient and nonuniform inflows. Variations of the skin friction are quite similar to that of the flat plate boundary layer in the uniform turbulent inflow of high intensity. Measured velocity profiles are coincident with families generated by the modified wall law in the range up to y+=40. Turbulence intensity of the incoming wake shifts the onset location of transition upstream. The transitional region becomes longer as the airfoils approach one another and the Reynolds number increases. The mean velocity profile gradually varies from a laminar to logarithmic one during the transition. The maximum values of rms velocity fluctuations are located near y+=15-20. A strong positive skewness of velocity fluctuation is observed at the onset of transition and the overall rms level of velocity fluctuation reaches 3.0–3.5 in wall units. The database obtained will be useful in developing and evaluating turbulence models and computational schemes for transitional boundary layer. [S0098-2202(00)01603-5]

Investigation of Boundary Layer Flows Over a Flat Plate With Different-Size Transverse Square Grooves at s/w = 10

2007 ◽  
Author(s):  
Redha Wahidi ◽  
Walid Chakroun ◽  
Sami Al-Fahad
Keyword(s):  
Boundary Layer ◽  
Flat Plate ◽  
Skin Friction ◽  
Turbulence Intensity ◽  
Viscous Sublayer ◽  
Velocity Profiles ◽  
Boundary Layer Flows ◽  
Mean Velocity ◽  
Uncertainty Range ◽  
The Mean

Turbulent boundary layer flows over a flat plate with multiple transverse square grooves spaced 10 element widths apart were investigated. Mean velocity profiles, turbulence intensity profiles, and the distributions of the skin-friction coefficients (Cf) and the integral parameters are presented for two grooved walls. The two transverse square groove sizes investigated are 5mm and 2.5mm. Laser-Doppler Anemometer (LDA) was used for the mean velocity and turbulence intensity measurements. The skin-friction coefficient was determined from the gradient of the mean velocity profiles in the viscous sublayer. Distribution of Cf in the first grooved-wall case (5mm) shows that Cf overshoots downstream of the groove and then oscillates within the uncertainty range and never shows the expected undershoot in Cf. The same overshoot is seen in the second grooved-wall case (2.5mm), however, Cf continues to oscillate above the uncertainty range and never returns to the smooth-wall value. The mean velocity profiles clearly represent the behavior of Cf where a downward shift is seen in the Cf overshoot region and no upward shift is seen in these profiles. The results show that the smaller grooves exhibit larger effects on Cf, however, the boundary layer responses to these effects in a slower rate than to those of the larger grooves.

Influence of large-scale motions on the frictional drag in a turbulent boundary layer

2017 ◽  
Vol 829 ◽  
pp. 751-779 ◽  
Author(s):  
Jinyul Hwang ◽  
Hyung Jin Sung
Keyword(s):  
Boundary Layer ◽  
Skin Friction ◽  
Large Scale ◽  
Wall Region ◽  
Local Skin ◽  
Velocity Fluctuations ◽  
Vortical Structures ◽  
Change Of Scale ◽  
Local Skin Friction ◽  
Near Wall

Direct numerical simulation data of a turbulent boundary layer ($Re_{\unicode[STIX]{x1D70F}}=1000$) were used to investigate the large-scale influences on the vortical structures that contribute to the local skin friction. The amplitudes of the streamwise and wall-normal swirling strengths ($\unicode[STIX]{x1D706}_{x}$and$\unicode[STIX]{x1D706}_{y}$) were conditionally sampled by measuring the large-scale streamwise velocity fluctuations ($u_{l}$). In the near-wall region, the amplitudes of$\unicode[STIX]{x1D706}_{x}$and$\unicode[STIX]{x1D706}_{y}$decreased under negative$u_{l}$rather than under positive$u_{l}$. This behaviour arose from the spanwise motions within the footprints of the large-scale low-speed ($u_{l}<0$) and high-speed structures ($u_{l}>0$). The intense spanwise motions under the footprint of positive$u_{l}$noticeably strengthened the small-scale spanwise velocity fluctuations ($w_{s}$) below the centre of the near-wall vortical structures as compared to$w_{s}$within the footprint of negative$u_{l}$. The streamwise and wall-normal components were attenuated or amplified around the modulated vortical motions, which in turn led to the dependence of the swirling strength on the$u_{l}$event. We quantified the contribution of the modulated vortical motions$\langle -w\unicode[STIX]{x1D714}_{y}\rangle$, which were related to a change-of-scale effect due to the vortex-stretching force, to the local skin friction. In the near-wall region, intense values of$\langle -w\unicode[STIX]{x1D714}_{y}\rangle$were observed for positive$u_{l}$. By contrast, these values were low for negative$u_{l}$, in connection with the amplification of$w_{s}$and$\unicode[STIX]{x1D706}_{y}$by the strong spanwise motions of the positive$u_{l}$. The resultant skin friction induced by the amplified vortical motions within$u_{l}^{+}>2$was responsible for 15 % of the total skin friction generated by the change-of-scale effect. Finally, we applied this analysis to a drag-reduced flow and found that the amplified vortical motions within the footprint of positive$u_{l}$were markedly diminished, which ultimately contributed to the total drag reduction.

Experimental Investigation of Turbulent Boundary Layers Under Favorable Pressure Gradient

2010 ◽  
Author(s):  
Pranav Joshi ◽  
Joseph Katz
Keyword(s):  
Boundary Layer ◽  
Friction Coefficient ◽  
Pressure Gradient ◽  
Skin Friction ◽  
Velocity Profiles ◽  
Skin Friction Coefficient ◽  
Mean Velocity ◽  
Freestream Velocity ◽  
Favorable Pressure Gradient ◽  
The Mean

The goal of this research is to study the effect of favorable pressure gradient (FPG) on the near wall structures of a turbulent boundary layer on a smooth wall. 2D-PIV measurements have been performed in a sink flow, initially at a coarse resolution, to characterize the development of the mean flow and (under resolved) Reynolds stresses. Lack of self-similarity of mean velocity profiles shows that the boundary layer does not attain the sink flow equilibrium. In the initial phase of acceleration, the acceleration parameter, K = v/U2dU/dx, increases from zero to 0.575×10−6, skin friction coefficient decreases and mean velocity profiles show a log region, but lack universality. Further downstream, K remains constant, skin friction coefficient increases and the mean velocity profiles show a second log region away from the wall. In the initial part of the FPG region, all the Reynolds stress components decrease over the entire boundary layer. In the latter phase, they continue to decrease in the middle of the boundary layer, and increase significantly close to the wall (below y∼0.15δ), where they collapse when normalized with the local freestream velocity. Turbulence production and wallnormal transport, scaled with outer units, show self-similar profiles close to the wall in the constant K region. Spanwise-streamwise plane data shows evidence of low speed streaks in the log layer, with widths scaling with the boundary layer thickness.

The Present State of the Turbulence Problem

1933 ◽  
Vol 1 (1) ◽  
pp. 19-28
Author(s):  
Walter Tollmien
Keyword(s):  
Turbulent Flow ◽  
Flat Plate ◽  
Skin Friction ◽  
Turbulence Theory ◽  
Shearing Stress ◽  
Mean Velocity ◽  
Fully Developed Turbulence ◽  
Developed Turbulence ◽  
The Mean ◽  
Real Goal

Abstract In this survey the author first describes certain types of turbulent flow, following which he deals successively with the production of turbulent motion; the instability of the laminar motion; fully developed turbulence; momentum interchange and mixing lengths; and relations between the shearing stress at the wall and the mean velocity distributions. Finally he takes up the calculation of skin friction for simple cases of fully developed turbulence, especially for that of the flat plate. Although the methods outlined have often led to practically useful results, it is the author’s belief that they should be considered only as advances toward the real goal of the turbulence theory. The derivation of turbulence phenomena from the hydrodynamical equations will, in his opinion, be possible only by the application of statistical methods.

On the Tendency to Self-Preservation in Axisymmetric Ducted Jets

1964 ◽  
Vol 86 (4) ◽  
pp. 765-771 ◽  
Author(s):  
R. Curtet ◽  
F. P. Ricou
Keyword(s):  
Boundary Layer ◽  
Equations Of Motion ◽  
Form Factors ◽  
Velocity Profiles ◽  
Stream Velocity ◽  
Mean Velocity ◽  
Velocity Fluctuations ◽  
Air Jet ◽  
The Mean ◽  
Secondary Stream

If it is assumed that the mean-velocity profiles of a ducted jet are similar in form sufficiently for downstream of the orifice it is possible, as shown in earlier papers [1, 2, 3], to integrate the equations of motion using the boundary-layer approximation and assuming a constant-energy secondary stream. It is necessary to know when and how this limiting profile is reached, and whether a similar tendency to self-preservation of the components of the velocity fluctuations is observed before the jet reaches the duct-wall boundary layer. Measurements have been made in an axisymmetric ducted air jet of the mean and fluctuating velocities, jet width, secondary-stream velocity, ductwall static pressure, and the boundary layer thickness. Results are compared with values predicted by the approximate jet theory. The authors define form factors calculated from measured profiles of mean velocities, of radial and longitudinal components of the velocity fluctuations, and of the shear stress. The variation of these form factors indicates a definite tendency to similarity for the mean velocity profiles; however, departures from similarity persist for the velocity fluctuations to the limit of measurements, about three duct diameters (40 nozzle diameters).

Near-Wall Measurements in Turbulent Boundary Layers Using Laser Doppler Anemometry

2002 ◽  
Author(s):  
T. Gunnar Johansson ◽  
Luciano Castillo
Keyword(s):  
Boundary Layer ◽  
Shear Stress ◽  
Wall Shear Stress ◽  
Pressure Gradient ◽  
Laser Doppler Anemometry ◽  
Reynolds Numbers ◽  
Mean Velocity ◽  
The Mean ◽  
Wall Shear ◽  
Near Wall

Near wall measurements have been performed in a zero pressure gradient turbulent boundary layer at low to moderate local Reynolds numbers using Laser-Doppler Anemometry in order to investigate how accurately the wall shear stress can be determined. Also, scaling problems are particularly difficult at low Reynolds numbers since they involve simultaneous influences of both inner and outer scales and this is most clearly observed in the near-wall region. In order to fully describe the zero pressure gradient turbulent boundary layer at low to moderate local Reynolds numbers it is necessary to accurately measure a number of quantities. These include the mean velocity and Reynolds stresses, and their spatial derivatives all the way down to the wall (y+∼1). Integral parameters that need to be measured are the wall shear stress and boundary layer thickness, particularly the momentum thickness. Problems with the measurement of field properties get worse close to a wall, and they get worse for increasing local Reynolds number. Three different approaches to measure the wall shear stress were examined. It was found that small measurement errors in the mean velocity close to the wall significantly reduced the accuracy in determining the wall shear stress by measuring the velocity gradient at the wall. The constant stress layer was found to be affected by the advection terms. However, it was found that taking the small pressure gradient into account and improving on the spatial resolution in the outer part of the boundary layer made the momentum integral method reliable.

Turbulent flow over large-amplitude wavy surfaces

1984 ◽  
Vol 140 ◽  
pp. 27-44 ◽  
Author(s):  
Jeffrey Buckles ◽  
Thomas J. Hanratty ◽  
Ronald J. Adrian
Keyword(s):  
Boundary Layer ◽  
Turbulent Flow ◽  
Velocity Fields ◽  
Laser Doppler Velocimeter ◽  
Free Shear Layer ◽  
Wave Surface ◽  
Velocity Measurements ◽  
Mean Velocity ◽  
Wavy Surfaces ◽  
The Mean

The laser-Doppler velocimeter is used to measure the mean and the fluctuating velocity for turbulent flow over a solid sinusoidal wave surface having a wavelength λ of 50.8 mm and a wave amplitude of 5.08 mm. For this flow, a large separated region exists, extending from x/λ = 0.14 to 0.69. From the mean velocity measurements, the time-averaged streamlines and therefore the extent of the separated region are calculated. Three flow elements are identified: the separated region, an attached boundary layer, and a free shear layer formed by the detachment of the boundary layer from the wave surface. The characteristics of these flow elements are discussed in terms of the properties of the mean and fluctuating velocity fields.

LDA-Measurements of Transitional Flows Induced by a Square Rib

2001 ◽  
Vol 124 (1) ◽  
pp. 108-117 ◽  
Author(s):  
S. Becker ◽  
C. M. Stoots ◽  
K. G. Condie ◽  
F. Durst ◽  
D. M. McEligot
Keyword(s):  
Boundary Layer ◽  
Turbulent Boundary Layer ◽  
Boundary Layers ◽  
Laser Doppler Anemometry ◽  
Plate Boundary ◽  
Index Of Refraction ◽  
Wall Region ◽  
Reynolds Stresses ◽  
Mean Velocity ◽  
Turbulent Statistics

New fundamental measurements are presented for the transition process in flat plate boundary layers downstream of two-dimensional square ribs. By use of laser Doppler anemometry (LDA) and a large Matched-Index-of-Refraction (MIR) flow system, data for wall-normal fluctuations and Reynolds stresses were obtained in the near wall region to y+<0.1 in addition to the usual mean streamwise velocity component and its fluctuation. By varying velocity and rib height, the experiment investigated the following range of conditions: k+=5.5 to 21, 0.3<k/δ1<1,180<Rek<740,6×104<Rex,k<1.5×105,ReΘ660,−125<x−xk/k<580. Consequently, results covered boundary layers which retained their laminar characteristics through those where a turbulent boundary layer was established shortly after reattachment beyond the forcing rib. For “large” elements, evolution of turbulent statistics of the viscous layer for a turbulent boundary layer y+<∼30 was rapid even in flows where the mean velocity profile still showed laminar behavior.

scholarly journals Turbulent flow in curved channels

2021 ◽  
Vol 931 ◽  
Author(s):  
Geert Brethouwer
Keyword(s):  
Turbulent Flow ◽  
Outer Region ◽  
Curvature Radius ◽  
Normal Velocity ◽  
Mean Velocity ◽  
Velocity Fluctuations ◽  
Turbulence Structures ◽  
Curvature Effects ◽  
Fully Developed Turbulent Flow ◽  
The Mean

Fully developed turbulent flow in channels with mild to strong longitudinal curvature is studied by direct numerical simulations. The Reynolds based on the bulk mean velocity and channel half-width $\delta$ is fixed at $20\,000$ , resulting in a friction Reynolds number of approximately 1000. Four cases are considered with curvature varying from $\gamma = 2\delta /r_c = 0.033$ to 0.333, where $r_c$ is the curvature radius at the channel centre. Substantial differences between the mean wall shear stress on the convex and concave walls are already observed for $\gamma = 0.033$ . A log-law region is absent and a region with nearly constant mean angular momentum develops in the channel centre for strong curvatures. Spanwise and wall-normal velocity fluctuations are strongly amplified by curvature in the outer region of the concave channel side. Only near the walls, where curvature effects are relatively weak, do the mean velocity and velocity fluctuation profiles approximately collapse when scaled by wall units based on the local friction velocity. Budgets of the streamwise and wall-normal Reynolds-stress equations are presented and turbulence structures are investigated through visualizations and spectra. In the case with strongest curvature, the flow relaminarizes locally near the convex wall. On the concave channel side, large elongated streamwise vortices reminiscent of Taylor–Görtler vortices develop for all curvatures considered. The maximum in the premultiplied two-dimensional wall-normal energy spectrum and co-spectrum shifts towards larger scales with increasing curvature. The large scales substantially contribute to the wall-normal velocity fluctuations and momentum transport on the concave channel side.

Sign in / Sign up

Export Citation Format

Share Document