scholarly journals Dynamic roughness perturbation of a turbulent boundary layer

2011 ◽  
Vol 688 ◽  
pp. 258-296 ◽  
Author(s):  
I. Jacobi ◽  
B. J. McKeon
Keyword(s):  
Boundary Layer ◽  
Turbulent Boundary Layer ◽  
Flow Field ◽  
Mode Shapes ◽  
Small Scale ◽  
Normal Velocity ◽  
Velocity Fluctuations ◽  
Eigenmode Analysis ◽  
Dynamic Perturbation ◽  
Non Equilibrium

AbstractThe zero-pressure-gradient turbulent boundary layer over a flat plate was perturbed by a temporally oscillating, spatial impulse of roughness, and the downstream response of the flow field was interrogated by hot-wire anemometry and particle-image velocimetry. The key features common to impulsively perturbed boundary layers, as identified in Jacobi & McKeon (J. Fluid Mech., 2011), were investigated, and the unique contributions of the dynamic perturbation were isolated by contrast with an appropriately matched static impulse of roughness. In addition, the dynamic perturbation was decomposed into separable large-scale and small-scale structural effects, which in turn were associated with the organized wave and roughness impulse aspects of the perturbation. A phase-locked velocity decomposition of the entire downstream flow field revealed strongly coherent modes of fluctuating velocity, with distinct mode shapes for the streamwise and wall-normal velocity components. Following the analysis of McKeon & Sharma (J. Fluid Mech., vol. 658, 2010, pp. 336–382), the roughness perturbation was treated as a forcing of the Navier–Stokes equation and a linearized analysis employing a modified Orr–Sommerfeld operator was performed. The experimentally ascertained wavespeed of the input disturbance was used to solve for the most amplified singular mode of the Orr–Sommerfeld resolvent. These calculated modes were then compared with the streamwise and wall-normal velocity fluctuations. The discrepancies between the calculated Orr–Sommerfeld resolvent modes and those experimentally observed by phase-locked averaging of the velocity field were postulated to result from the violation of the parallel flow assumption of Orr–Sommerfeld analysis, as well as certain non-equilibrium effects of the roughness. Additionally, some difficulties previously observed using a quasi-laminar eigenmode analysis were also observed under the resolvent approach; however, the resolvent analysis was shown to provide reasonably accurate predictions of velocity fluctuations for the forced Orr–Sommerfeld problem over a portion of the boundary layer, with potential applications to designing efficient flow control strategies. The combined experimental and analytical effort provides a new opportunity to examine the non-equilibrium and forcing effects in a dynamically perturbed flow.

scholarly journals Local transport of passive scalar released from a point source in a turbulent boundary layer

2018 ◽  
Vol 846 ◽  
pp. 292-317 ◽  
Author(s):  
K. M. Talluru ◽  
J. Philip ◽  
K. A. Chauhan
Keyword(s):  
Boundary Layer ◽  
Turbulent Boundary Layer ◽  
Large Scale ◽  
Reynolds Shear Stress ◽  
Phase Relationship ◽  
Passive Scalar ◽  
Streamwise Velocity ◽  
Concentration Fluctuations ◽  
Normal Velocity ◽  
Velocity Fluctuations

Simultaneous measurements of streamwise velocity ($\tilde{U}$) and concentration ($\tilde{C}$) for a horizontal plume released at eight different vertical locations within a turbulent boundary layer are discussed in this paper. These are supplemented by limited simultaneous three-component velocity and concentration measurements. Results of the integral time scale ($\unicode[STIX]{x1D70F}_{c}$) of concentration fluctuations across the width of the plume are presented here for the first time. It is found that$\unicode[STIX]{x1D70F}_{c}$has two distinct peaks: one closer to the plume centreline and the other at a vertical distance of plume half-width above the centreline. The time-averaged streamwise concentration flux is found to be positive and negative, respectively, below and above the plume centreline. This behaviour is a resultant of wall-normal velocity fluctuations ($w$) and Reynolds shear stress ($\overline{uw}$). Confirmation of these observations is found in the results of joint probability density functions of$u$(streamwise velocity fluctuations) and$\tilde{C}$as well as that of$w$and$\tilde{C}$. Results of cross-correlation coefficient show that high- and low-momentum regions have a distinctive role in the transport of passive scalar. Above the plume centreline, low-speed structures have a lead over the meandering plume, while high-momentum regions are seen to lag behind the plume below its centreline. Further examination of the phase relationship between time-varying$u$and$c$(concentration fluctuations) via cross-spectrum analysis is consistent with this observation. Based on these observations, a phenomenological model is presented for the relative arrangement of a passive scalar plume with respect to large-scale velocity structures in the flow.

Statistical structure of momentum sources and sinks in the outer region of a turbulent boundary layer

2008 ◽  
Vol 606 ◽  
pp. 225-237 ◽  
Author(s):  
B. GANAPATHISUBRAMANI
Keyword(s):  
Boundary Layer ◽  
Turbulent Boundary Layer ◽  
Outer Region ◽  
Particle Image Velocimetry Data ◽  
Relative Strength ◽  
Spanwise Direction ◽  
Normal Velocity ◽  
Velocity Fluctuations ◽  
Sources And Sinks ◽  
Normal Distance

The spatial structure of momentum sources and sinks (T > 0 and T < 0; where T is the streamwise component of the Lamb vector) is examined in a turbulent boundary layer by using dual-plane particle image velocimetry data obtained in streamwise–spanwise planes at two wall-normal locations (x2/δ = 0.1 and 0.5, where x2 is the wall-normal location and δ is the boundary layer thickness). Two-point correlations of T indicate that the size of source motions remains relatively constant while the size of sink motions increases with increasing wall-normal distance. The relative strength of sink motions also increases away from the wall. The velocity field in the vicinity of source/sink motions was explored by computing cross-correlations of T with the velocity components. Source-like motions are correlated with elongated low-momentum zones that possess regions of upwash embedded within them and appear to be the strongest in areas where these low-momentum zones meander in the spanwise direction. Momentum sinks appear to be located within low-speed regions that are within larger high-momentum zones. The velocity fluctuations undergo rapid transitions between quadrants in the vicinity of sinks (i.e. both streamwise and wall-normal velocity fluctuations change sign). The length scales, over which the fluctuations change sign, are much larger at x2/δ = 0.5 than at x2/δ = 0.1.

scholarly journals Wall-Normal Velocity Correlations in a Zero Pressure Gradient Turbulent Boundary Layer

2020 ◽  
Vol 142 (8) ◽  
Author(s):  
Sylvain Morilhat ◽  
François Chedevergne ◽  
Francis Micheli ◽  
Frank Simon
Keyword(s):  
Boundary Layer ◽  
Turbulent Boundary Layer ◽  
Pressure Gradient ◽  
Wall Pressure ◽  
Convection Velocity ◽  
Turbulent Shear ◽  
Normal Velocity ◽  
Mean Velocity ◽  
Velocity Fluctuations ◽  
Taylor’S Hypothesis

Abstract An experimental campaign dedicated to the characterization of the wall-normal velocity correlations in a zero pressure gradient turbulent boundary layer was performed. A double set of laser Doppler velocimetry (LDV) benches were used to access two-point two-time correlations of the wall-normal velocity. The measurements analysis confirms several important hypotheses classically made to model wall pressure spectra from the velocity correlations. In particular, the ratio of the wall-normal Reynolds stress to the turbulent shear stress is confirmed to exhibit a large plateau in the logarithmic region. In addition, Taylor's hypothesis of frozen turbulence is well recovered for the wall-normal velocity fluctuations. The convection velocity for the wall-normal velocity fluctuations is also shown to evolve across the boundary layer, according to the mean velocity profile. Furthermore, the decorrelation time scale of velocity correlations appears to be increasing throughout the boundary layer thickness in accordance with the increase of the convection velocity. The results obtained with this original campaign will help improving models for wall pressure spectra, especially those based on the resolution of the Poisson equation for the pressure for which the wall pressure correlations are related to the wall-normal velocity correlations.

Survey on the Indirect Methods of Potential Flow Used for the Optimization of Airships Profile

2014 ◽  
Vol 554 ◽  
pp. 717-723
Author(s):  
Reza Abbasabadi Hassanzadeh ◽  
Shahab Shariatmadari ◽  
Ali Chegeni ◽  
Seyed Alireza Ghazanfari ◽  
Mahdi Nakisa
Keyword(s):  
Boundary Layer ◽  
Turbulent Boundary Layer ◽  
Flow Field ◽  
Drag Coefficient ◽  
Body Surface ◽  
Potential Flow ◽  
The Body ◽  
Indirect Methods ◽  
Singularity Method ◽  
Singularity Distribution

The present study aims to investigate the optimized profile of the body through minimizing the Drag coefficient in certain Reynolds regime. For this purpose, effective aerodynamic computations are required to find the Drag coefficient. Then, the computations should be coupled thorough an optimization process to obtain the optimized profile. The aerodynamic computations include calculating the surrounding potential flow field of an object, calculating the laminar and turbulent boundary layer close to the object, and calculating the Drag coefficient of the object’s body surface. To optimize the profile, indirect methods are used to calculate the potential flow since the object profile is initially amorphous. In addition to the indirect methods, the present study has also used axial singularity method which is more precise and efficient compared to other methods. In this method, the body profile is not optimized directly. Instead, a sink-and-source singularity distribution is used on the axis to model the body profile and calculate the relevant viscose flow field.

Measurements in a Turbulent Boundary Layer Over a d-Type Surface Roughness

1975 ◽  
Vol 42 (3) ◽  
pp. 591-597 ◽  
Author(s):  
D. H. Wood ◽  
R. A. Antonia
Keyword(s):  
Boundary Layer ◽  
Surface Roughness ◽  
Turbulent Boundary Layer ◽  
Turbulent Energy ◽  
Shear Stresses ◽  
Normal Velocity ◽  
Mean Velocity ◽  
Outer Flow ◽  
Intensity Measurements ◽  
Normal And Shear Stresses

Mean velocity and turbulence intensity measurements have been made in a fully developed turbulent boundary layer over a d-type surface roughness. This roughness is characterised by regular two-dimensional elements of square cross section placed one element width apart, with the cavity flow between elements being essentially isolated from the outer flow. The measurements show that this boundary layer closely satisfies the requirement of exact self-preservation. Distribution across the layer of Reynolds normal and shear stresses are closely similar to those found over a smooth surface except for the region immediately above the grooves. This similarity extends to distributions of third and fourth-order moments of longitudinal and normal velocity fluctuations and also to the distribution of turbulent energy dissipation. The present results are compared with those obtained for a k-type or sand grained roughness.

scholarly journals Sonic Eddy Model of the Turbulent Boundary Layer

Fluids ◽  
2022 ◽  
Vol 7 (1) ◽  
pp. 37
Author(s):  
Paul Dintilhac ◽  
Robert Breidenthal
Keyword(s):  
Boundary Layer ◽  
Mach Number ◽  
Turbulent Boundary Layer ◽  
Skin Friction ◽  
Entrainment Rate ◽  
Square Root ◽  
Reynolds Stresses ◽  
Free Shear Layer ◽  
Velocity Fluctuations ◽  
Acoustic Signaling

The effects of Mach number on the skin friction and velocity fluctuations of the turbulent boundary layer are considered through a sonic eddy model. Originally proposed for free shear flows, the model assumes that the eddies responsible for momentum transfer have a rotation Mach number of unity, with the entrainment rate limited by acoustic signaling. Under this assumption, the model predicts that the skin friction coefficient should go as the inverse Mach number in a regime where the Mach number is larger than unity but smaller than the square root of the Reynolds number. The velocity fluctuations normalized by the friction velocity should be the inverse square root of the Mach number in the same regime. Turbulent transport is controlled by acoustic signaling. The density field adjusts itself such that the Reynolds stresses correspond to the momentum transport. In contrast, the conventional van Driest–Morkovin view is that the Mach number effects are due to density variations directly. A new experiment or simulation is proposed to test this model using different gases in an incompressible boundary layer, following the example of Brown and Roshko in the free shear layer.

Resonant flow in small cavities submerged in a boundary layer

1988 ◽  
Vol 420 (1859) ◽  
pp. 219-245 ◽  
Keyword(s):  
Boundary Layer ◽  
Turbulent Boundary Layer ◽  
Large Scale ◽  
Resonant Cavity ◽  
Cavity Flow ◽  
Flow Speed ◽  
Small Scale ◽  
S Waves ◽  
Grooved Surface ◽  
Tollmien Schlichting

The viscosity-dominated unsteady flow in a row of small transverse square cavities lying submerged in a turbulent boundary layer is first considered. Experiments performed primarily with one size of cavities show that the cavity flow can be excited by freestream disturbances in a narrow frequency band that is independent of the flow speed. The turbulent boundary layer in which the cavities are submerged remains transparent to the disturbances. The cavity flow resonates when the depths of the cavity and the Stokes layer are nearly the same, that is when 2π fk 2 / v = 1, where f is the frequency of the resonant cavity flow, k is the cavity height and v is the kinematic viscosity of the fluid. An associated laminar boundary-layer excitation experiment shows that the instability process over the grooved surface also involves the amplification of Tollmien–Schlichting (T–S) waves in much the same manner as in a smooth-wall Blasius profile but the grooves enhance receptivity. A theory is given proposing that the resonant groove flow in the low Reynolds number turbulent boundary layer is driven by highly amplified matched T–S waves. The possible relevance of the observed coupling between the large-scale freestream disturbances and the small-scale cavity flows to the turbulence production mechanism in a smooth flat-plate turbulent boundary layer is also discussed.

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