Surface pressure fluctuations on steps immersed in turbulent boundary layers

2012 ◽  
Vol 712 ◽  
pp. 471-504 ◽  
Author(s):  
Minsuk Ji ◽  
Meng Wang
Keyword(s):  
Boundary Layer ◽  
Shear Layer ◽  
Frequency Spectrum ◽  
Surface Pressure ◽  
Pressure Fluctuations ◽  
Step Height ◽  
Reattachment Length ◽  
Velocity Fluctuations ◽  
Dominant Source ◽  
Separated Shear Layer

AbstractSurface pressure fluctuations induced by turbulent boundary-layer flow at ${\mathit{Re}}_{\theta } = 4755$ over small backward- and forward-facing steps are studied with large-eddy simulation. Four step heights that are 53, 13, 3.3 and 0.83 % of the boundary-layer thickness are considered to investigate the effects of step height on surface pressure characteristics and pressure-source mechanisms. The extent to which turbulent velocity fluctuations in the boundary layer and the separated shear layer contribute to the surface pressure fluctuations is examined with scaling of various pressure statistics and two-point correlations. For larger steps, vortical structures develop in the shear layer and the associated intense velocity fluctuations are the dominant source. Downstream of slightly less than one reattachment length from the step, the root-mean-square pressure is found to scale with the local maximum cross-stream Reynolds normal stress ${ \overline{{v}^{\ensuremath{\prime} \hspace{0.167em} 2} } }_{\mathit{max}} $. The pressure frequency spectrum at the maximum ${p}_{\mathit{rms}} $ location consists of an energy-containing range that scales with the mean reattachment length ${x}_{r} $ and a higher frequency range that rolls off with a slope close to $\ensuremath{-} 7/ 3$. As the step height decreases, the boundary-layer turbulent fluctuations become the dominant source, the ${ \overline{{v}^{\ensuremath{\prime} \hspace{0.167em} 2} } }_{\mathit{max}} $ scaling of ${p}_{\mathit{rms}} $ is no longer valid and the roll-off slope of the frequency spectrum becomes steeper. The downstream recovery of a step-perturbed boundary layer towards an equilibrium boundary layer is investigated from the point of view of surface pressure fluctuations. For steps with a strong separated shear layer, pressure fluctuations are found to decay rapidly for up to three reattachment lengths downstream of the step, within which approximately 60 % of the peak ${p}_{\mathit{rms}} $ is dissipated. Farther downstream, recovery is much slower. The pressure-recovery distances estimated for the largest backward and forward steps are 175 and 295 step heights, respectively.

Mass transfer between a plane surface and an impinging turbulent jet: the influence of surface-pressure fluctuations

1982 ◽  
Vol 119 ◽  
pp. 91-105 ◽  
Author(s):  
K. Kataoka ◽  
Y. Kamiyama ◽  
S. Hashimoto ◽  
T. Komai
Keyword(s):  
Boundary Layer ◽  
Mass Transfer ◽  
Flat Plate ◽  
Stagnation Point ◽  
Surface Pressure ◽  
Velocity Gradient ◽  
Large Scale ◽  
Pressure Fluctuations ◽  
Wall Region ◽  
Local Measurement

Local measurement of the mass-transfer rate and velocity gradient when an axisymmetric jet impinges on a flat plate was carried out using an electrochemical technique. Local measurement of the surface pressure on the flat plate was carried out separately using piezoelectric pressure transducers. The stagnation-point mass-transfer coefficient reaches a maximum when the flat plate is placed at 6 nozzle diameters from a convergent nozzle. It has been confirmed that the mass transfer to the flat plate for a high Schmidt number is greatly enhanced owing to the velocity-gradient disturbances in the wall region of the boundary layer, while the momentum transfer is insensitive to such disturbances. The relative intensity of the velocity-gradient fluctuations on the wall has an extremely large value at and near to the stagnation point, and decreases downstream, approaching a large constant value.These velocity-gradient disturbances are not due to the usual interaction of Reynolds stress with the shear stress of the mean flow, but are due to the interaction with the surface-pressure fluctuations converted from the velocity fluctuations of the oncoming jet.The three co-ordinate dimensions of large-scale eddies are calculated from the auto- and spatial correlations of the surface-pressure fluctuations. It is considered that such large-scale eddies play an important role in the production of a velocity-gradient disturbance in the wall region of the boundary layer from the velocity turbulence of the approaching jet.

scholarly journals Pressure fluctuations induced by a hypersonic turbulent boundary layer

2016 ◽  
Vol 804 ◽  
pp. 578-607 ◽  
Author(s):  
Lian Duan ◽  
Meelan M. Choudhari ◽  
Chao Zhang
Keyword(s):  
Boundary Layer ◽  
Turbulent Boundary Layer ◽  
Surface Pressure ◽  
Free Stream ◽  
Spatial Scales ◽  
Pressure Fluctuations ◽  
Propagation Speed ◽  
Dominant Frequency ◽  
Mean Velocity ◽  
Taylor’S Hypothesis

Direct numerical simulations (DNS) are used to examine the pressure fluctuations generated by a spatially developed Mach 5.86 turbulent boundary layer. The unsteady pressure field is analysed at multiple wall-normal locations, including those at the wall, within the boundary layer (including inner layer, the log layer, and the outer layer), and in the free stream. The statistical and structural variations of pressure fluctuations as a function of wall-normal distance are highlighted. Computational predictions for mean-velocity profiles and surface pressure spectrum are in good agreement with experimental measurements, providing a first ever comparison of this type at hypersonic Mach numbers. The simulation shows that the dominant frequency of boundary-layer-induced pressure fluctuations shifts to lower frequencies as the location of interest moves away from the wall. The pressure wave propagates with a speed nearly equal to the local mean velocity within the boundary layer (except in the immediate vicinity of the wall) while the propagation speed deviates from Taylor’s hypothesis in the free stream. Compared with the surface pressure fluctuations, which are primarily vortical, the acoustic pressure fluctuations in the free stream exhibit a significantly lower dominant frequency, a greater spatial extent, and a smaller bulk propagation speed. The free-stream pressure structures are found to have similar Lagrangian time and spatial scales as the acoustic sources near the wall. As the Mach number increases, the free-stream acoustic fluctuations exhibit increased radiation intensity, enhanced energy content at high frequencies, shallower orientation of wave fronts with respect to the flow direction, and larger propagation velocity.

Passive Manipulation of Separation-Bubble Transition Using Surface Modifications

2009 ◽  
Vol 131 (2) ◽  
Author(s):  
Brian R. McAuliffe ◽  
Metin I. Yaras
Keyword(s):  
Boundary Layer ◽  
Reynolds Number ◽  
Shear Layer ◽  
Separation Bubble ◽  
Surface Modifications ◽  
Small Scale ◽  
Two Dimensional ◽  
Vortex Pairing ◽  
Separated Shear Layer ◽  
Scale Turbulence

Through experiments using two-dimensional particle-image velocimetry (PIV), this paper examines the nature of transition in a separation bubble and manipulations of the resultant breakdown to turbulence through passive means of control. An airfoil was used that provides minimal variation in the separation location over a wide operating range, with various two-dimensional modifications made to the surface for the purpose of manipulating the transition process. The study was conducted under low-freestream-turbulence conditions over a flow Reynolds number range of 28,000–101,000 based on airfoil chord. The spatial nature of the measurements has allowed identification of the dominant flow structures associated with transition in the separated shear layer and the manipulations introduced by the surface modifications. The Kelvin–Helmholtz (K-H) instability is identified as the dominant transition mechanism in the separated shear layer, leading to the roll-up of spanwise vorticity and subsequent breakdown into small-scale turbulence. Similarities with planar free-shear layers are noted, including the frequency of maximum amplification rate for the K-H instability and the vortex-pairing phenomenon initiated by a subharmonic instability. In some cases, secondary pairing events are observed and result in a laminar intervortex region consisting of freestream fluid entrained toward the surface due to the strong circulation of the large-scale vortices. Results of the surface-modification study show that different physical mechanisms can be manipulated to affect the separation, transition, and reattachment processes over the airfoil. These manipulations are also shown to affect the boundary-layer losses observed downstream of reattachment, with all surface-indentation configurations providing decreased losses at the three lowest Reynolds numbers and three of the five configurations providing decreased losses at the highest Reynolds number. The primary mechanisms that provide these manipulations include: suppression of the vortex-pairing phenomenon, which reduces both the shear-layer thickness and the levels of small-scale turbulence; the promotion of smaller-scale turbulence, resulting from the disturbances generated upstream of separation, which provides quicker transition and shorter separation bubbles; the elimination of the separation bubble with transition occurring in an attached boundary layer; and physical disturbance, downstream of separation, of the growing instability waves to manipulate the vortical structures and cause quicker reattachment.

Study on Flow Fields of Boundary-Layer Separation and Hydraulic Jump during Rundown Motion of Shoaling Solitary Wave

2015 ◽  
Vol 09 (05) ◽  
pp. 1540002 ◽  
Author(s):  
Chang Lin ◽  
Ming-Jer Kao ◽  
Guang-Wei Tzeng ◽  
Wei-Ying Wong ◽  
James Yang ◽  
...  
Keyword(s):  
Boundary Layer ◽  
Free Surface ◽  
Solitary Wave ◽  
Shear Layer ◽  
Flow Separation ◽  
Hydraulic Jump ◽  
High Speed ◽  
Flow Fields ◽  
Main Stream ◽  
Separated Shear Layer

The characteristics of flow fields for a complete evolution of the non-breaking solitary wave, having a wave-height to water-depth ratio of 0.363 and propagating over a 1:5 sloping bottom, are investigated experimentally. This study mainly focuses on the occurrences of both flow separation on the boundary layer under an adverse pressure gradient and subsequent hydraulic jump with the abrupt rising of free surface during rundown motion of the shoaling wave, together with emphasis on the evolution of vortex structures underlying the separated shear layer and hydraulic jump. A flow visualization technique with particle trajectory method and a high-speed particle image velocimetry (HSPIV) system with a high-speed digital camera were used. Based on the instantaneous flow images visualized and/or the ensemble-averaged velocity fields measured, the following interesting features, which are unknown up-to-date, are presented and discussed in this study: (1) Flow bifurcation occurring on both offshore and onshore sides of the explicit demarcation curve and the stagnation point during runup motion; (2) The dependence of the diffuser-like flow field, being changed from the supercritical flow in the shallower region to the subcritical flow in the deeper counterpart, on the Froude number during the early and middle stages of rundown motion; (3) The positions and times for the occurrences of the incipient flow separation and the sudden rising of free surface of the hydraulic jump; (4) The associated movement and evolution of vortex structures under the separated shear layer, the hydraulic jump and/or the high-speed external main stream of the retreated flow; and (5) The entrainment of air bubbles from the free surface into the external main stream of the retreated flow.

Experimental Study on the Receptivity of Shear Layer Separating at a Rear-Edge of Boundary-Layer Plate

2007 ◽  
Author(s):  
Masahito Asai ◽  
Takeshi Imai
Keyword(s):  
Boundary Layer ◽  
Shear Layer ◽  
Shear Instability ◽  
Instability Mode ◽  
Free Shear Layer ◽  
Frequency Range ◽  
Acoustic Disturbances ◽  
Rear Edge ◽  
Separated Shear Layer ◽  
Vorticity Fluctuation

Receptivity of the free shear layer developing from a 90-degrees rear-edge of boundary-layer plate to acoustic disturbances is examined experimentally to clarify the dependency of the receptivity coefficient on the rear-edge curvature. The results show that for finite rear-edge curvatures, the receptivity coefficient decreases with increasing the disturbance frequency while it is almost independent of the frequency for the sharp rear-edge over the frequency range examined. The decrease in the receptivity coefficient for the rounded rear-edge is attributed to the fact that the sound-induced Stokes layer which is the vorticity fluctuation developing into the free-shear instability mode is shed into the off-centerline of the separated shear layer.

Direct numerical simulation of a separated turbulent boundary layer

1998 ◽  
Vol 374 ◽  
pp. 379-405 ◽  
Author(s):  
Y. NA ◽  
P. MOIN
Keyword(s):  
Numerical Simulation ◽  
Boundary Layer ◽  
Shear Stress ◽  
Turbulent Boundary Layer ◽  
Direct Numerical Simulation ◽  
Shear Layer ◽  
Stokes Equations ◽  
Pressure Fluctuations ◽  
Shear Stresses ◽  
Computational Domain

A separated turbulent boundary layer over a flat plate was investigated by direct numerical simulation of the incompressible Navier–Stokes equations. A suction-blowing velocity distribution was prescribed along the upper boundary of the computational domain to create an adverse-to-favourable pressure gradient that produces a closed separation bubble. The Reynolds number based on inlet free-stream velocity and momentum thickness is 300. Neither instantaneous detachment nor reattachment points are fixed in space but fluctuate significantly. The mean detachment and reattachment locations determined by three different definitions, i.e. (i) location of 50% forward flow fraction, (ii) mean dividing streamline (ψ=0), (iii) location of zero wall-shear stress (τw=0), are in good agreement. Instantaneous vorticity contours show that the turbulent structures emanating upstream of separation move upwards into the shear layer in the detachment region and then turn around the bubble. The locations of the maximum turbulence intensities as well as Reynolds shear stress occur in the middle of the shear layer. In the detached flow region, Reynolds shear stresses and their gradients are large away from the wall and thus the largest pressure fluctuations are in the middle of the shear layer. Iso-surfaces of negative pressure fluctuations which correspond to the core region of the vortices show that large-scale structures grow in the shear layer and agglomerate. They then impinge on the wall and subsequently convect downstream. The characteristic Strouhal number St=fδ*in/U0 associated with this motion ranges from 0.0025 to 0.01. The kinetic energy budget in the detachment region is very similar to that of a plane mixing layer.

Wavenumber-Frequency Spectrum Analysis of Pressure Fields Around an Automobile

2019 ◽  
Author(s):  
Xifeng Wang ◽  
Kenta Mizushiri ◽  
Hiroshi Yokoyama ◽  
Akiyoshi Iida
Keyword(s):  
Boundary Layer ◽  
Flow Field ◽  
Frequency Spectrum ◽  
Spectrum Analysis ◽  
Reverse Flow ◽  
Pressure Fluctuations ◽  
Flow Region ◽  
Vehicle Body ◽  
Frequency Spectrum Analysis ◽  
Pressure Fields

Abstract In order to evaluate the interior noise caused by the flow around automobiles, it is necessary to clarify the nature of the pressure fluctuations on the surface of vehicle body. The pressure fluctuations around the vehicle which are caused by the fluid motion can be solved by unsteady-compressible Navier-Stokes equation. However, the differences between the scales and intensity of the pressure fluctuations related to the hydrodynamic pressure fluctuation (HPF) of the flow field and the aerodynamic sound (acoustic pressure fluctuation APF) are quite large, these phenomena can be considered separately as two different phenomena. This assumption can help us to understand the contributions of these two components of pressure fluctuations to the structural vibration and interior sound of automobiles. Since both the HPF and the APF are pressure fluctuations, they cannot be separated only by measuring with a single pressure sensor. In this study, we divided these pressure fluctuations by using wavenumber-frequency spectrum analysis. Wind tunnel experiment showed that the HPF and the APF have different wavenumber fields in the wake of a rear-view mirror, and the intensity and wavenumber of the HPF are larger than that of the APF. Flow field was also investigated by using the incompressible flow simulation. As a result of wavenumber-frequency spectrum analysis based on the pressure fields around the vehicle body, the HPF and the APF have different wavenumbers in the case of a boundary layer flow field with no separation such as boundary layer on the vehicle roof. On the other hand, very small wavenumber components of the HPF were observed in the recirculation flow around the rear-view mirror downstream, despite incompressible simulation was done. This is probably due to the flow fields excite the vehicle body in the direction close to the vertical with respect to the vehicle body surface (side shield) in the separated flow region, and the wavenumber vector project on the shield surface apparently become smaller. The wavenumber vector becomes short but the frequency is constant, which leads the speed of pressure propagation apparently increases. In the reverse flow region, even if the uniform flow velocity is smaller than the speed of sound, the HPF may still contribute to vibration and sound generation. At the same time, since the flow velocity is actually slowed in the reverse flow region, large wavenumber components were also observed. Therefore, the wavenumber spectrum was observed in a wide range of the wavelength region. In conclusion, by investigating the wavenumber frequency spectrum, it is possible to estimate the flow field contributing to the interior noise of automobiles.

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