Fluid Dynamic Characteristics of the Flow Over an Array of Large Roughness Elements

1991 ◽  
Vol 113 (4) ◽  
pp. 367-373 ◽  
Author(s):  
S. V. Garimella ◽  
P. A. Eibeck
Keyword(s):  
Reynolds Number ◽  
Laser Doppler Velocimetry ◽  
Fluid Dynamic ◽  
Water Channel ◽  
Leading Edge ◽  
Channel Height ◽  
Doppler Velocimetry ◽  
Reattachment Length ◽  
Roughness Elements ◽  
Large Roughness

Flow visualization and measurements of velocity and turbulence intensity using laser Doppler velocimetry are used to investigate separation and reattachment processes in the flow over an array of protruding elements mounted on the bottom wall of a rectangular water channel. The concept of an array shear layer is introduced to demarcate the region of influence over which the resistance of the array retards the flow. Flow separation at the leading edge of the elements is documented. The confined or interacting nature of the flow in the cavities between elements is established as a function of element spacing. The reattachment length downstream of the element varies from 4 to 1.5 element heights, decreasing both with an increase in Reynolds number and a decrease in channel height.

Symmetric and Asymmetric Turbulent Flows in a Rectangular Duct With a Pair of Ribs

1988 ◽  
Vol 110 (4) ◽  
pp. 373-379 ◽  
Author(s):  
T.-M. Liou ◽  
C.-F. Kao
Keyword(s):  
Boundary Layer ◽  
Reynolds Number ◽  
Turbulent Flows ◽  
Laser Doppler Velocimetry ◽  
Boundary Layer Thickness ◽  
Critical Reynolds Number ◽  
Doppler Velocimetry ◽  
Rectangular Duct ◽  
Reattachment Length ◽  
Mean Velocity

Laser-Doppler velocimetry (LDV) measurements are presented of mean velocity and turbulence intensity for turbulent flows past a pair of ribs in a rectangular duct of aspect ratio 2. The Reynolds number based on the duct hydraulic diameter was varied in the range of 2.0 × 103 to 7.6 × 104. The experiments cover ribs with rib height to duct height ratios from 0.13 to 0.33 and with rib width to height ratios from 1 to 10. The critical rib height above which and the critical Reynolds number below which the flow patterns become asymmetric were determined from the results. In addition, the effects of the rib width and boundary layer thickness on the formation and the size of the separation bubbles on the top surface of the ribs as well as on the reattachment length behind the ribs were documented. Furthermore, the degree of turbulence enhancement was compared between the asymmetric and the symmetric flows.

Acoustic and phase portrait analysis of leading-edge roughness element on laminar separation bubbles at low Reynolds number flow

2021 ◽  
pp. 095441002110443
Author(s):  
Hossein Jabbari ◽  
Esmaeili Ali ◽  
Mohammad Hasan Djavareshkian
Keyword(s):  
Reynolds Number ◽  
Phase Portrait ◽  
Separation Bubble ◽  
Roughness Element ◽  
Low Reynolds Number ◽  
Leading Edge ◽  
Suction Side ◽  
Laminar Separation ◽  
Roughness Elements ◽  
Edge Roughness

Since laminar separation bubbles are neutrally shaped on the suction side of full-span wings in low Reynolds number flows, a roughness element can be used to improve the performance of micro aerial vehicles. The purpose of this article was to investigate the leading-edge roughness element’s effect and its location on upstream of the laminar separation bubble from phase portrait point of view. Therefore, passive control might have an acoustic side effect, especially when the bubble might burst and increase noise. Consequently, the effect of the leading-edge roughness element features on the bubble’s behavior is considered on the acoustic pressure field and the vortices behind the NASA-LS0417 cross-section. The consequences express that the distribution of roughness in the appropriate dimensions and location could contribute to increasing the performance of the airfoil and the interaction of vortices produced by roughness elements with shear layers on the suction side has increased the sound frequency in the relevant sound pressure level (SPL). The results have demonstrated that vortex shedding frequency was increased in the presence of roughness compared to the smooth airfoil. Also, more complexity of the phase portrait circuits was found, retrieved from velocity gradient limitation. Likewise, the highest SPL is related to the state where the separation bubble phenomenon is on the surface versus placing roughness elements on the leading edge leads to a negative amount of SPL.

Critical Reynolds Number in Constant-Acceleration Pipe Flow From an Initial Steady Laminar State

2010 ◽  
Vol 132 (9) ◽  
Author(s):  
Charles W. Knisely ◽  
Kazuyoshi Nishihara ◽  
Manabu Iguchi
Keyword(s):  
Reynolds Number ◽  
Flow Visualization ◽  
Pipe Flow ◽  
Laser Doppler Velocimetry ◽  
Critical Reynolds Number ◽  
Transition To Turbulence ◽  
Doppler Velocimetry ◽  
Constant Acceleration ◽  
Laminar State ◽  
Steady Laminar

The transition to turbulence in a constant-acceleration pipe flow from an initial laminar state was investigated in a custom-made apparatus permitting visual access to the water flow in the pipe. The apparatus allowed both laser Doppler velocimetry measurements and flow visualization using a tracer. The experiment was carried out by accelerating the flow from a steady laminar state to a steady turbulent state. The relation between the critical Reynolds number for transition to turbulence and the acceleration was found to be similar to that in a constant-acceleration pipe flow started from rest. In addition, with increased acceleration, the turbulent transition was found to be delayed to higher Reynolds numbers using flow visualization with simultaneous laser Doppler velocimetry measurements.

scholarly journals Experimental investigations on the sharp leading-edge separation over a flat plate at zero incidence using particle image velocimetry

2020 ◽  
Vol 61 (9) ◽  
Author(s):  
K. Fujiwara ◽  
R. Sriram ◽  
K. Kontis
Keyword(s):  
Reynolds Number ◽  
Large Scale ◽  
Particle Image ◽  
Leading Edge ◽  
Reynolds Numbers ◽  
Reattachment Length ◽  
Large Scale Structures ◽  
Image Velocimetry ◽  
Scale Structures ◽  
Sharp Leading Edge

Abstract Leading-edge separated flow field over a sharp flat plate is experimentally investigated in Reynolds numbers ranging from 6.2 × 103 to 4.1 × 104, using particle image velocimetry (PIV) and its statistics. It was observed that the average reattachment length is nearly independent of Reynolds number and the small secondary bubble observed near the leading edge was found to shrink with increasing Reynolds number. The wall-normal profiles of the statistical values of kinematic quantities such as the velocity components and their fluctuations scaled well with average reattachment length lR and freestream velocity U∞. Their magnitudes compare well with previous investigations even though the current triangular shaped sharp leading edge is different from previous flat-faced or semi-circular ones. The shear layer was observed to exhibit 2 different linear growth rates over 2 distinct regions. Instantaneous PIV realizations demonstrate unsteady nature of the separation bubble, whose origins in the upstream portion of the bubble are analysed. Bimodal nature of the probability density function (PDF) of fluctuating streamwise velocity at around x/lR = 0.08–0.15 indicates successive generation and passage of vortices in the region, which subsequently interact and evolve into multiscale turbulent field exhibiting nearly Gaussian PDF. Shedding of vortices with wide range of scales are apparent in most of the instantaneous realizations. Proper Orthogonal Decomposition (POD) of the velocity fluctuation magnitude field revealed that the flow structures of the dominant modes and their relative energies are independent of Reynolds number. In each of the dominant modes (first 3 modes), the length scales corresponding to the large scale structures and their spacing are the same for all Reynolds numbers, suggesting that their Strouhal number (observed to be ~ 0.09–0.2 at Reynolds number of 6.2 × 103) of unsteadiness should also be independent of Reynolds number. A single large structure- comparable in size to lR—was apparent well before reattachment in a few instantaneous realizations, as compared to multiple small-scale structures visible in most realizations; at Reynolds number of 6.2 × 103, realizations with such large-scale structures occurred approximately after every 20–30 realizations, corresponding to non-dimensional frequency of 0.4–0.6, which is identified to be the “regular shedding”. It was possible to reconstruct the large-scale structure during the instances from just the first 3 POD modes, indicating that the Strouhal number of regular shedding too is independent of Reynolds number. Graphic abstract

scholarly journals Computational Fluid Dynamic Analysis of Vortex Generators on MAV.

2022 ◽  
pp. 58-73
Author(s):  
Vadla Raghavender ◽  
Priyanka Vatte ◽  
V Varun ◽  
M. Pala. Prasad Reddy
Keyword(s):  
Reynolds Number ◽  
Fluid Dynamic ◽  
Low Reynolds Number ◽  
Leading Edge ◽  
Vortex Generators ◽  
Computational Fluid Dynamic Analysis ◽  
Aerial Vehicle ◽  
Low Reynolds ◽  
Low Reynolds Number Airfoil ◽  
And Control

Micro Vortex generators are very small components deployed on the wings to control airflow over the upper surface of the wing to affect the boundary layer over it. These are employed onto a Micro aerial vehicle (MAV) of fixed wing type with an S5010 which is a low Reynolds number airfoil. This airfoil provides good aerodynamic results as compared to many low Reynolds number airfoils. Micro vortex generators are used to enhance the performance through controlling airflow at different speeds and angle of attack. The comparison of a half part of the MAV wing which is designed in CATIA, with and without the vortex generators on its leading edge at 10% of its chord length is done to show how the vortex generators improve the performance and control authority at different speeds and angle of attacks. These are shown with the velocity and pressure distribution around the wing by considering laminar flow in the simulation.

scholarly journals Subcritical versus supercritical transition to turbulence in curved pipes

2015 ◽  
Vol 770 ◽  
Author(s):  
J. Kühnen ◽  
P. Braunshier ◽  
M. Schwegel ◽  
H. C. Kuhlmann ◽  
B. Hof
Keyword(s):  
Reynolds Number ◽  
Hopf Bifurcation ◽  
Laser Doppler Velocimetry ◽  
Basic Flow ◽  
Transition To Turbulence ◽  
Doppler Velocimetry ◽  
Minimum Threshold ◽  
Curved Pipes ◽  
Friction Factors ◽  
Helical Pipe

Transition to turbulence in straight pipes occurs in spite of the linear stability of the laminar Hagen–Poiseuille flow if both the amplitude of flow perturbations and the Reynolds number $\mathit{Re}$ exceed a minimum threshold (subcritical transition). As the pipe curvature increases, centrifugal effects become important, modifying the basic flow as well as the most unstable linear modes. If the curvature (tube-to-coiling diameter $d/D$) is sufficiently large, a Hopf bifurcation (supercritical instability) is encountered before turbulence can be excited (subcritical instability). We trace the instability thresholds in the $\mathit{Re}-d/D$ parameter space in the range $0.01\leqslant d/D\leqslant 0.1$ by means of laser-Doppler velocimetry and determine the point where the subcritical and supercritical instabilities meet. Two different experimental set-ups are used: a closed system where the pipe forms an axisymmetric torus and an open system employing a helical pipe. Implications for the measurement of friction factors in curved pipes are discussed.

Experiments on critical Reynolds number and global instability in roughness-induced laminar–turbulent transition

2018 ◽  
Vol 844 ◽  
pp. 878-904 ◽  
Author(s):  
Dominik K. Puckert ◽  
Ulrich Rist
Keyword(s):  
Reynolds Number ◽  
Critical Reynolds Number ◽  
Water Channel ◽  
Plate Boundary ◽  
Linear Stability Theory ◽  
Global Instability ◽  
Turbulent Transition ◽  
Roughness Elements ◽  
Hot Film ◽  
Laminar Turbulent Transition

The effects of isolated, cylindrical roughness elements on laminar–turbulent transition in a flat-plate boundary layer are investigated in a laminar water channel. Our experiments aim at providing a comparison to global linear stability theory (LST) by means of hot-film anemometry and particle image velocimetry. Although the critical Reynolds number from theory does not match the transition Reynolds number observed in experiments, there are distinct experimental observations indicating a changeover from purely convective to absolute/global instability very close to the critical Reynolds number predicted by theory. Forcing with a vibrating wire reveals the evolution of the system dynamics from an amplifier to a wavemaker when the critical Reynolds number is exceeded. The mode symmetry is varicose for thick roughness elements and a changeover from varicose to sinuous modes is observed at the critical Reynolds number for thin roughness elements. Therefore, most predictions by global LST can be confirmed, but additional observations in the physical flow demonstrate that not all features can be captured adequately by global LST.

Flow Structures of a Radial-Inlet Swirler

2000 ◽  
Author(s):  
Zhanhua Ma ◽  
Dexin Wang ◽  
San-Mou Jeng ◽  
Michael A. Benjamin
Keyword(s):  
Reynolds Number ◽  
Flow Field ◽  
Laser Doppler Velocimetry ◽  
Strong Influence ◽  
Flow Fields ◽  
Flow Structures ◽  
Doppler Velocimetry ◽  
Swirl Chamber ◽  
Radial Profiles ◽  
Downstream Flow

The upstream and downstream flows of a radial inlet swirler were investigated by using a 2-D Laser Doppler Velocimetry (LDV) probe. The radial profiles of axial and tangential velocities at various axial locations were obtained. The effects of Reynolds Number, swirler exit configuration and combustor outlet condition were examined. The results reveal that the combustor outlet condition has strong influence on the downstream flow field of the swirler. The effect of Reynolds Number depends on the swirler exit configurations. The influence of the convergent configuration from swirl chamber to orifice on the flow fields is very small whereas the influence of the divergence at the swirler exit is significant. The upstream and downstream flows of the radial inlet swirler are of sub-critical nature, thus the swirler exit configuration and combustor outlet condition significantly affect the flow fields.

Onset of Transition in the Flow Over a Three-Dimensional Array of Rectangular Obstacles

1992 ◽  
Vol 114 (2) ◽  
pp. 251-255 ◽  
Author(s):  
S. V. Garimella ◽  
P. A. Eibeck
Keyword(s):  
Heat Transfer ◽  
Reynolds Number ◽  
Water Channel ◽  
Three Dimensional ◽  
Reynolds Numbers ◽  
Channel Height ◽  
Circuit Boards ◽  
Three Dimensional Flow ◽  
Transfer Data ◽  
Array Geometry

Onset of transition is investigated in the flow over an array of protruding elements mounted on the bottom wall of a rectangular water channel simulating flow passages between adjacent circuit boards in computers. The element dimensions are held constant while the channel height and the element spacing are varied. Flow visualization and turbulence measurements are used to determine transition Reynolds numbers, which compare well with previous results obtained from heat transfer data. The complicated, three-dimensional flow field causes transition to be a function not only of flow rate and array geometry but also of location in the array. Transition occurs in the fully developed region of the array at a channel height-based Reynolds number of 700 for a channel height of 1.2 element heights, increasing to 1900 for a channel height of 3.6 element heights. However, when Reynolds number is defined based on element height, transition occurs at the same Reynolds number of 550 for all channel heights. Increasing the stream wise spacing between elements causes transition to occur at lower Reynolds numbers.

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