scholarly journals Effects of frontal and plan solidities on aerodynamic parameters and the roughness sublayer in turbulent boundary layers

2015 ◽  
Vol 782 ◽  
pp. 541-566 ◽  
Author(s):  
M. Placidi ◽  
B. Ganapathisubramani
Keyword(s):  
Boundary Layer ◽  
Energy Content ◽  
Roughness Height ◽  
Rough Wall ◽  
Roughness Sublayer ◽  
Image Velocimetry ◽  
Floating Element ◽  
Aerodynamic Parameters ◽  
Pod Analysis ◽  
Proper Orthogonal

Experiments were conducted in the fully rough regime on surfaces with large relative roughness height ($h/{\it\delta}\approx 0.1$, where $h$ is the roughness height and ${\it\delta}$ is the boundary layer thickness). The surfaces were generated by distributed LEGO® bricks of uniform height, arranged in different configurations. Measurements were made with both floating-element drag balance and high-resolution particle image velocimetry on six configurations with different frontal solidities, ${\it\lambda}_{F}$, at fixed plan solidity, ${\it\lambda}_{P}$, and vice versa, for a total of twelve rough-wall cases. The results indicated that the drag reaches a peak value ${\it\lambda}_{F}\approx 0.21$ for a constant ${\it\lambda}_{P}=0.27$, while it monotonically decreases for increasing values of ${\it\lambda}_{P}$ for a fixed ${\it\lambda}_{F}=0.15$. This is in contrast to previous studies in the literature based on cube roughness which show a peak in drag for both ${\it\lambda}_{F}$ and ${\it\lambda}_{P}$ variations. The influence of surface morphology on the depth of the roughness sublayer (RSL) was also investigated. Its depth was found to be inversely proportional to the roughness length, $y_{0}$. A decrease in $y_{0}$ was usually accompanied by a thickening of the RSL and vice versa. Proper orthogonal decomposition (POD) analysis was also employed. The shapes of the most energetic modes calculated using the data across the entire boundary layer were found to be self-similar across the twelve rough-wall cases. However, when the analysis was restricted to the roughness sublayer, differences that depended on the wall morphology were apparent. Moreover, the energy content of the POD modes within the RSL suggested that the effect of increased frontal solidity was to redistribute the energy towards the larger scales (i.e. a larger portion of the energy was within the first few modes), while the opposite was found for variation of plan solidity.

Scale-Dependent Energy Fluxes in a Rough-Wall Turbulent Channel Flow

2010 ◽  
Author(s):  
Jiarong Hong ◽  
Joseph Katz ◽  
Michael P. Schultz
Keyword(s):  
Boundary Layer ◽  
Outer Layer ◽  
Large Scale ◽  
Turbulent Channel Flow ◽  
Roughness Height ◽  
Rough Wall ◽  
Energy Fluxes ◽  
Image Velocimetry ◽  
Non Local ◽  
Scale Turbulence

In this paper, we investigate the characteristics of energy fluxes within rough-wall turbulent boundary layer at different scales in the context of large eddy simulations (LES) utilizing high resolution Particle Image Velocimetry (PIV) data obtained in an optically index-matched facility. The rough surface consists of closely-packed pyramidal elements, and the measurement region includes the entire roughness sublayer and lower portion of outer-layer with a vector resolution of ∼9 wall units and 14% of roughness height. Our recent study has demonstrated that the entire boundary layer is flooded with an excessive number of roughness-scale eddies that are generated near the wall and advected away from it by large scale coherent structures present in the outer layer. Following this observation, the original velocity field is spatially filtered using 2D top-hat filter of length scale Δ = 1k, 3k, 6k (k is roughness height) that represent roughness, intermediate and large scale motions, respectively. In these ranges, the subgrid scale (SGS) energy fluxes show substantial increase with scale and with decreasing distance from the wall. The latter trend persists even when fluxes are scaled with the local TKE production rate. When the fluxes are decomposed to local and non-local contributions, they show a scale-dependent near-wall increase of energy transfer which bypasses the typical cascading process. This non-local flux is even larger than the local one near the wall for the [k, 3k] range. These trends are attributed to interactions of large scale turbulence with the wall roughness and the abundant roughness scale eddies near the wall. The paper also examines the behavior of Smagorinsky model coefficients, and show scale-dependence when trends at filter scales of 1k and 3k are compared to those at 6k. Both dissipation based and dynamic model coefficients show very little variation with height as long as the filter scale is in the 1–3k range, but increase with elevation for Δ = 6k.

A simple model for low-frequency unsteadiness in shock-induced separation

2009 ◽  
Vol 629 ◽  
pp. 87-108 ◽  
Author(s):  
S. PIPONNIAU ◽  
J. P. DUSSAUGE ◽  
J. F. DEBIÈVE ◽  
P. DUPONT
Keyword(s):  
Boundary Layer ◽  
Shock Wave ◽  
Particle Image ◽  
Mixing Layer ◽  
Low Frequency ◽  
Oblique Shock Wave ◽  
Separation Shock ◽  
Image Velocimetry ◽  
Boundary Layer Interaction ◽  
Aerodynamic Parameters

A model to explain the low-frequency unsteadiness found in shock-induced separation is proposed for cases in which the flow is reattaching downstream. It is based on the properties of fluid entrainment in the mixing layer generated downstream of the separation shock whose low-frequency motions are related to successive contractions and dilatations of the separated bubble. The main aerodynamic parameters on which the process depends are presented. This model is consistent with experimental observations obtained by particle image velocimetry (PIV) in a Mach 2.3 oblique shock wave/turbulent boundary layer interaction, as well as with several different configurations reported in the literature for Mach numbers ranging from 0 to 5.

The rough-wall turbulent boundary layer from the hydraulically smooth to the fully rough regime

2007 ◽  
Vol 580 ◽  
pp. 381-405 ◽  
Author(s):  
M. P. SCHULTZ ◽  
K. A. FLACK
Keyword(s):  
Boundary Layer ◽  
Layer Thickness ◽  
Outer Layer ◽  
Boundary Layer Thickness ◽  
Strong Support ◽  
Roughness Height ◽  
Rough Wall ◽  
Root Mean Square Roughness ◽  
Mean Velocity ◽  
Number Range

Turbulence measurements for rough-wall boundary layers are presented and compared to those for a smooth wall. The rough-wall experiments were made on a three-dimensional rough surface geometrically similar to the honed pipe roughness used by Shockling, Allen & Smits (J. Fluid Mech. vol. 564, 2006, p. 267). The present work covers a wide Reynolds-number range (Reθ = 2180–27 100), spanning the hydraulically smooth to the fully rough flow regimes for a single surface, while maintaining a roughness height that is a small fraction of the boundary-layer thickness. In this investigation, the root-mean-square roughness height was at least three orders of magnitude smaller than the boundary-layer thickness, and the Kármán number (δ+), typifying the ratio of the largest to the smallest turbulent scales in the flow, was as high as 10100. The mean velocity profiles for the rough and smooth walls show remarkable similarity in the outer layer using velocity-defect scaling. The Reynolds stresses and higher-order turbulence statistics also show excellent agreement in the outer layer. The results lend strong support to the concept of outer layer similarity for rough walls in which there is a large separation between the roughness length scale and the largest turbulence scales in the flow.

POD Analysis of the Wake Development of a Pivoted Circular Cylinder Undergoing Vortex Induced Vibrations

2017 ◽  
Author(s):  
E. Marble ◽  
C. Morton ◽  
S. Yarusevych
Keyword(s):  
Circular Cylinder ◽  
Degrees Of Freedom ◽  
Structural Response ◽  
Cylinder Wake ◽  
Time Resolved ◽  
Vortex Induced Vibrations ◽  
Image Velocimetry ◽  
Pod Analysis ◽  
Component Particle ◽  
Proper Orthogonal

Vortex Induced Vibrations (VIV) of a pivoted circular cylinder with two degrees of freedom are investigated experimentally, focusing on quantifying the wake topology. Experiments are performed in a water tunnel for a pivoted cylinder with a fixed mass ratio of 10.8, moment of inertia ratio of 87.0–109.5, and a diameter-based Reynolds number of 3100. The reduced velocity was varied from 4.42 to 9.05 by varying the natural frequency of the structure. Velocity measurements were performed via time-resolved, two-component Particle Image Velocimetry (PIV), synchronized with cylinder displacement measurements. Time and phase-averaging are employed to analyze the wake development and relate it to the structural response. Proper Orthogonal Decomposition (POD) is utilized to gain insight into the development of coherent structures in the cylinder wake. The observed shedding patterns agree well with the Morse & Williamson [1] shedding map except for the cases at the boundary between 2P and non-synchronized shedding. The results show that the cylinder follows an elliptical trajectory with equal frequency of oscillation in streamwise and transverse directions. For the 2P regime, the tilt and direction of trajectory affect the formation and development of vortices in the wake. This results in a distinct asymmetry about the wake centerline in time-averaged statistics.

POD Investigation of Near-Wall Turbulent Structures

2002 ◽  
Author(s):  
Gaetano Maria Di Cicca ◽  
Angelo Iollo ◽  
Pier Giorgio Spazzini ◽  
Gaetano Iuso ◽  
Michele Onorato
Keyword(s):  
Experimental Data ◽  
Boundary Layer ◽  
Particle Image ◽  
Digital Particle Image Velocimetry ◽  
Orthogonal Decomposition ◽  
Wall Region ◽  
Turbulent Structures ◽  
Image Velocimetry ◽  
Proper Orthogonal ◽  
Near Wall

Experimental data of a turbulent boundary layer developing over a flat plate, obtained by Digital Particle Image Velocimetry (DPIV) technique, are analyzed making use of proper orthogonal decomposition (POD). Different POD definitions have been used in order to check their ability in educing the various structures dominating the near wall region. Results show a specific sensitivity depending on the POD definition adopted.

scholarly journals PIV Visualization of Flow Around Airfoil Controlled by Synthetic Jet Actuators at Shear-Layer and Wake Instability Frequencies

2021 ◽  
Author(s):  
Eric Yang ◽  
Pierre E. Sullivan
Keyword(s):  
Boundary Layer ◽  
Shear Layer ◽  
Large Scale ◽  
Turbulent Wake ◽  
Synthetic Jet ◽  
Shear Layer Instability ◽  
Image Velocimetry ◽  
Wake Instability ◽  
Proper Orthogonal ◽  
Instability Frequency

Abstract The response of a separated boundary layer to synthetic jet flow control at the global wake instability (F+ ≈ 𝒪(1)) and the shear-layer instability (F+ ≈ 𝒪(10)) measured by particle image velocimetry are presented. The visualization shows that in each of the control cases, coherent vorticity develops and breaks down into a turbulent wake. When the jets are actuated by burst-modulation at the wake instability frequency, they induce regular formation and detachment of large-scale vorticity to form a wide turbulent wake. Excitation at the shear-layer instability frequency, on the other hand, produces a train of alternating velocity fluctuations in the boundary layer which dissipate to a narrower wake. Proper orthogonal decomposition of the velocity fields show that the physical extent of the jet-induced coherent structures is decreased with increasing addition of momentum for both excitation frequencies.

scholarly journals Dynamics of Large Scale Turbulence in Finite-sized Wind Farm Canopy using Proper Orthogonal Decomposition and a Novel Fourier-POD Framework

2020 ◽  
Author(s):  
Tanmoy Chatterjee ◽  
Yulia T. Peet
Keyword(s):  
Boundary Layer ◽  
Power Generation ◽  
Atmospheric Boundary Layer ◽  
Large Scale ◽  
Wind Farm ◽  
Scaling Laws ◽  
Wind Farms ◽  
Orthogonal Decomposition ◽  
Pod Analysis ◽  
Proper Orthogonal

Large scale coherent structures in atmospheric boundary layer (ABL) are known to contribute to the power generation in wind farms. In the current paper, we perform a detailed analysis of the large scale structures in a finite sized wind turbine canopy using modal analysis from three dimensional proper orthogonal decomposition (POD). While POD analysis sheds light on the large scale coherent modes and scaling laws of the eigenspectra, we also observe a slow convergence of the spectral trends with the available number of snapshots. Since the finite sized array is periodic in the spanwise direction, we propose to adapt a novel approach of performing POD analysis of the spanwise/lateral Fourier transformed velocity snapshots instead of the snapshots themselves. This methodology not only helps in decoupling the length scales in the spanwise and the streamwise direction when studying the energetic coherent modes, but also provides a detailed guidance towards understanding the convergence of the eigenspectra. In particular, the Fourier-POD eigenspectra helps us illustrate if the dominant scaling laws observed in 3D POD are actually contributed by the laterally wider or thinner structures and provide more detailed insight on the structures themselves. We use the database from our previous large eddy simulation (LES) studies on finite-sized wind farms which uses wall-modeled LES for modeling the Atmospheric boundary layer laws, and actuator lines for the turbine blades. Understanding the behaviour of such structures would not only help better assess reduced order models (ROM) for forecasting the flow and power generation but would also play a vital role in improving the decision making abilities in wind farm optimization algorithms in future. Additionally, this study also provides guidance for better understanding the POD analysis in the turbulence and wind farm community.

Three Dimensional Volumetric Velocity Measurements in the Inner Part of a Turbulent Boundary Layer Over a Rough Wall Using Digital HPIV

2010 ◽  
Author(s):  
Siddharth Talapatra ◽  
Joseph Katz
Keyword(s):  
Boundary Layer ◽  
Turbulent Boundary Layer ◽  
Three Dimensional ◽  
Sample Volume ◽  
Rough Wall ◽  
Roughness Sublayer ◽  
Uniform Grid ◽  
Roughness Elements ◽  
Particle Pairs ◽  
Particle Images

Microscopic digital Holographic PIV is used to measure the 3D velocity distributions in the roughness sublayer of a turbulent boundary layer over a rough wall. The sample volume extends from the surface, including the space between the tightly packed, 0.45 mm high, pyramidal roughness elements, up to about 5 roughness heights away from the wall. To facilitate observations though a rough surface, experiments are performed in a facility containing fluid that has the same optical refractive index as the acrylic rough walls. Magnified in line holograms are recorded on a 4864×3248 pixel camera at a resolution of 0.67μm/pixel. The flow field is seeded with 2μm silver coated glass particles, which are injected upstream of the same volume. A multiple-step particle tracking procedure is used for matching the particle pairs. In recently obtained data, we have typically matched ∼5000 particle images per hologram pair. The resulting unstructured 3D vectors are projected onto a uniform grid with spacing of 60 μm in all three directions in a 3.2×1.8×1.8 mm sample volume. The paper provides sample data showing that the flow in the roughness sublayer is dominated by slightly inclined, quasi-streamwise vortices whose coherence is particularly evident close to the top of the roughness elements.

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