Multiple-arrayed pressure measurement for investigation of the unsteady flow structure of a reattaching shear layer

2002 ◽  
Vol 463 ◽  
pp. 377-402 ◽  
Author(s):  
INWON LEE ◽  
HYUNG JIN SUNG
Keyword(s):  
Flow Field ◽  
Large Scale ◽  
Vortical Structure ◽  
Single Point ◽  
Pressure Fluctuations ◽  
Vortical Flow ◽  
Recirculation Region ◽  
Stream Velocity ◽  
Vortical Structures ◽  
Wall Pressure Fluctuations

Spatio-temporal characteristics of wall pressure fluctuations in separated and reattaching flows over a backward-facing step were investigated through an extensive pressure-velocity joint measurement with an array of microphones. The experiment was performed in a wind tunnel with a Reynolds number of 33 000 based on the step height and the free-stream velocity. Synchronized wavelet maps showed the evolutionary behaviour of pressure fluctuations and gave further insight into the modulated nature of large-scale vortical structures. To see the relationship between the flow field and the relevant spatial mode of the pressure field, a new kind of wavenumber filtering, termed ‘spatial box filtering’ (SBF), was introduced and examined. The vortical flow field was reconstructed using every single-point velocity measurement by means of the conditional average based on the SBF second mode of pressure fluctuations. The flow field showed a well-organized spanwise vortical structure convected with a speed of 0.6U0 and a characteristic ‘sawtooth’ pattern of the unsteady trace of reattachment length. In addition to the coherent vortical structures, the periodic enlargement/shrinkage process of the recirculation region owing to apping motion was analysed. The recirculation region was found to undergo an enlargement/shrinkage cycle in accordance with the lowpass-filtered component of pressure fluctuations. In addition, such modulatory behaviour of the vortical structure as the global oscillation phase was discussed in connection with the conditionally averaged flow field.

An experimental investigation of turbulent flow over a two-dimensional obstacle on a flat plate

2021 ◽  
Vol 263 (2) ◽  
pp. 4459-4470
Author(s):  
Shivam Sundeep ◽  
Xin Zhang ◽  
Siyang Zhong ◽  
Huanxian Bu
Keyword(s):  
Shear Layer ◽  
Large Scale ◽  
Pressure Fluctuations ◽  
Wall Pressure ◽  
Far Field ◽  
Two Dimensional ◽  
Vortical Structures ◽  
Wall Pressure Fluctuations ◽  
Field Noise ◽  
Velocity Spectra

Aeroacoustic and aerodynamic characteristics of the turbulent boundary layer encountering a large obstacle are experimentally investigated in this paper. Two-dimensional obstacles with a square and a semi-circular cross-section mounted on a flat plate are studied in wind tunnel tests, with particular interests in the shear layer characteristics, wall pressure fluctuations, and far-field noise induced by the obstacles. Synchronized measurements of the far-field noise and the wall pressure fluctuations were conducted using microphone arrays in the far-field and flush-mounted in the plate, respectively. Additionally, the streamwise and wall-normal velocity fluctuations behind the obstacle were measured using the X-wire probe. The measured velocity profiles, spectra, and wall pressure spectra are compared, showing that the rectangular obstacle has a significant impact on both the turbulent flow and far-field noise. The large-scale vortical structures shed from the obstacles can be identified in the wall pressure spectra, the streamwise velocity spectra, and the wall pressure coherence analysis. Within the shear layer, the pairing of vortices occurs and the frequency of the broadband peak in the velocity spectra decreases as the shear layer grows downstream. Further eddy convective velocities of large-scale vortical structures inside the shear layer were analyzed based on the wall pressure fluctuations.

scholarly journals The role of large-scale vortical structures in transient convective heat transfer augmentation

2013 ◽  
Vol 718 ◽  
pp. 89-115 ◽  
Author(s):  
David O. Hubble ◽  
Pavlos P. Vlachos ◽  
Tom E. Diller
Keyword(s):  
Heat Transfer ◽  
Flow Field ◽  
Convective Heat Transfer ◽  
Convective Heat ◽  
Large Scale ◽  
Vortical Structure ◽  
Time Resolved ◽  
Vortical Structures ◽  
Heat Transfer Augmentation

AbstractThe physical mechanism by which large-scale vortical structures augment convective heat transfer is a fundamental problem of turbulent flows. To investigate this phenomenon, two separate experiments were performed using simultaneous heat transfer and flow field measurements to study the vortex–wall interaction. Individual vortices were identified and studied both as part of a turbulent stagnation flow and as isolated vortex rings impacting on a surface. By examining the temporal evolution of both the flow field and the resulting heat transfer, it was observed that the surface thermal transport was governed by the transient interaction of the vortical structure with the wall. The magnitude of the heat transfer augmentation was dependent on the instantaneous strength, size and position of the vortex relative to the boundary layer. Based on these observations, an analytical model was developed from first principles that predicts the time-resolved surface convection using the transient properties of the vortical structure during its interaction with the wall. The analytical model was then applied, first to the simplified vortex ring model and then to the more complex stagnation region experiments. In both cases, the model was able to accurately predict the time-resolved convection resulting from the vortex interactions with the wall. These results reveal the central role of large-scale turbulent structures in the augmentation of thermal transport and establish a simple model for quantitative predictions of transient heat transfer.

On the Simulation and Spectral Analysis of Unsteady Turbulence and Transition Effects in a Multistage Low Pressure Turbine

2019 ◽  
Vol 141 (5) ◽  
Author(s):  
Georg Geiser ◽  
Jens Wellner ◽  
Edmund Kügeler ◽  
Anton Weber ◽  
Anselm Moors
Keyword(s):  
Frequency Domain ◽  
Large Scale ◽  
Separation Bubble ◽  
Pressure Fluctuations ◽  
Low Pressure ◽  
Transition Flow ◽  
Low Pressure Turbine ◽  
Wall Pressure Fluctuations ◽  
Frequency Domain Methods ◽  
Transition Effects

A nonlinear full-wheel time-domain simulation of a two-stage low pressure turbine is presented, analyzed, and compared with the available experimental data. Recent improvements to the computational fluid dynamics (CFD) solver TRACE that lead to significantly reduced wall-clock times for such large scale simulations are described in brief. Since the configuration is characterized by significant unsteady turbulence and transition effects, it is well suited for the validation and benchmarking of frequency-domain methods. Transition, flow separation and wall pressure fluctuations on the stator blades of the second stage are analyzed in detail. A strong azimuthal π-periodicity is observed, manifesting in a significantly varying stability of the midspan trailing edge flow with a quasi-steady closed separation bubble on certain blades and highly dynamic partially open separation bubbles with recurring transition and turbulent reattachment on other blades. The energy spectrum of fluctuating wall quantities in that regime shows a high bandwidth and considerable disharmonic content, which is challenging for frequency-domain-based simulation methods.

Experimental and numerical investigation of the transient characteristics and volute casing wall pressure fluctuations of a single-blade pump

2018 ◽  
Vol 233 (2) ◽  
pp. 280-291 ◽  
Author(s):  
Steffen Melzer ◽  
Tim Müller ◽  
Stephan Schepeler ◽  
Tobias Kalkkuhl ◽  
Romuald Skoda
Keyword(s):  
Flow Field ◽  
Boundary Conditions ◽  
Numerical Simulations ◽  
Pressure Fluctuations ◽  
Wall Pressure ◽  
Pressure Amplitude ◽  
Centrifugal Pumps ◽  
Transient Pressure ◽  
Time Step ◽  
Wall Pressure Fluctuations

In contrast to conventional multiblade centrifugal pumps, single-blade pumps are characterized by a significant fluctuation of head and highly transient and circumferentially nonuniform flow field even in the best-efficiency point. For a contribution to a better understanding of the flow field and an improvement of numerical methods, a combined experimental and numerical study is performed with special emphasis on the analysis of the transient pressure field. In an open test rig, piezoresistive pressure sensors are utilized for the measurement of transient in- and outflow conditions and the volute casing wall pressure fluctuations. The quality of the numerical simulations is ensured by a careful adoption of the real geometry details in the simulation model, a grid study and a time step study. While the power curve is well reproduced by the numerical simulations, the time-averaged head is systematically overpredicted, probably due to underestimation of losses. Transient pressure boundary conditions for the numerical simulation show a better prediction of the measured pressure amplitude than constant boundary conditions, whereas the time-averaged head prediction is not improved. For a more accurate prediction of the transient flow field and the time-averaged characteristics, the utilization of scale-resolving turbulence models is assumed to be indispensable.

Reynolds-number dependence of wall-pressure fluctuations in a pressure-induced turbulent separation bubble

2017 ◽  
Vol 833 ◽  
pp. 563-598 ◽  
Author(s):  
Hiroyuki Abe
Keyword(s):  
Reynolds Number ◽  
Pressure Gradient ◽  
Large Scale ◽  
Separation Bubble ◽  
Pressure Fluctuations ◽  
Local Maximum ◽  
Wall Pressure ◽  
Mean Flow ◽  
Frequency Spectra ◽  
Wall Pressure Fluctuations

Direct numerical simulations are used to examine the behaviour of wall-pressure fluctuations $p_{w}$ in a flat-plate turbulent boundary layer with large adverse and favourable pressure gradients, involving separation and reattachment. The Reynolds number $Re_{\unicode[STIX]{x1D703}}$ based on momentum thickness is equal to 300, 600 and 900. Particular attention is given to effects of Reynolds number on root-mean-square (r.m.s.) values, frequency/power spectra and instantaneous fields. The possible scaling laws are also examined as compared with the existing direct numerical simulation and experimental data. The r.m.s. value of $p_{w}$ normalized by the local maximum Reynolds shear stress $-\unicode[STIX]{x1D70C}\overline{uv}_{max}$ (Simpson et al. J. Fluid Mech. vol. 177, 1987, pp. 167–186; Na & Moin J. Fluid Mech. vol. 377, 1998b, pp. 347–373) leads to near plateau (i.e. $p_{w\,rms}/-\unicode[STIX]{x1D70C}\overline{uv}_{max}=2.5\sim 3$) in the adverse pressure gradient and separated regions in which the frequency spectra exhibit good collapse at low frequencies. The magnitude of $p_{w\,rms}/-\unicode[STIX]{x1D70C}\overline{uv}_{max}$ is however reduced down to 1.8 near reattachment where good collapse is also obtained with normalization by the local maximum wall-normal Reynolds stress $\unicode[STIX]{x1D70C}\overline{vv}_{max}$. Near reattachment, $p_{w\,rms}/-\unicode[STIX]{x1D70C}\overline{vv}_{max}=1.2$ is attained unambiguously independently of the Reynolds number and pressure gradient. The present magnitude (1.2) is smaller than (1.35) obtained for step-induced separation by Ji & Wang (J. Fluid Mech. vol. 712, 2012, pp. 471–504). The reason for this difference is intrinsically associated with convective nature of a pressure-induced separation bubble near reattachment where the magnitude of $p_{w\,rms}$ depends essentially on the favourable pressure gradient. The resulting mean flow acceleration leads to delay of the r.m.s. peak after reattachment. Attention is also given to structures of $p_{w}$. It is shown that large-scale spanwise rollers of low pressure fluctuations are formed above the bubble, whilst changing to large-scale streamwise elongated structures after reattachment. These large-scale structures become more prominent with increasing $Re_{\unicode[STIX]{x1D703}}$ and affect $p_{w}$ significantly.

scholarly journals Noise Characteristics Analysis of the Horizontal Axis Hydrokinetic Turbine Designed for Unmanned Underwater Mooring Platforms

2019 ◽  
Vol 7 (12) ◽  
pp. 465 ◽  
Author(s):  
Zhigao Dang ◽  
Zhaoyong Mao ◽  
Baowei Song ◽  
Wenlong Tian
Keyword(s):  
Flow Field ◽  
Large Scale ◽  
Pressure Fluctuations ◽  
Noise Pollution ◽  
Horizontal Axis ◽  
Acoustic Analogy ◽  
Noise Emission ◽  
Suction Surface ◽  
Noise Characteristics ◽  
Hydrokinetic Turbine

Operating horizontal axis hydrokinetic turbine (HAHT) generates noise affecting the ocean environment adversely. Therefore, it is essential to determine the noise characteristics of such types of HAHT, as large-scale turbine sets would release more noise pollution to the ocean. Like other rotating machinery, the hydrodynamic noise generated by the rotating turbine has been known to be the most important noise source. In the present work, the transient turbulent flow field of the HAHT is obtained by incompressible large eddy simulation, thereafter, the Ffowcs Williams and Hawkings acoustic analogy formulation is carried out to predict the noise generated from the pressure fluctuations of the blade surface. The coefficient of power is compared with the experimental results, with a good agreement being achieved. It is seen from the pressure contours that the 80% span of the blade has the most severe pressure fluctuations, which concentrate on the region of leading the edge of the airfoil and the suction surface of the airfoil. Then, the noise characteristics around a single turbine are systematically studied, in accordance with the results of the flow field. The noise characteristics around the whole turbine are also investigated to determine the directionality of the noise emission of HAHT.

Experimental and Computational Investigation of a Simplified Geometry of an Engine/Pylon/Wing Installation at High-Lift Flight Conditions

2013 ◽  
Author(s):  
Matthieu Lucas ◽  
Yannick Bury ◽  
Cyril Bonnaud ◽  
Laurent Joly
Keyword(s):  
Boundary Layer ◽  
Flow Field ◽  
Angle Of Attack ◽  
Leading Edge ◽  
Longitudinal Vortex ◽  
Vortical Flow ◽  
Stream Velocity ◽  
Piv Measurements ◽  
Comprehensive Knowledge ◽  
Oil Flow

This paper focuses on the numerical and experimental characterization of the vortex structures that develop along a simplified geometry of a wing equipped with pylon-mounted engine at low speed/high angle of attack flight conditions. In these conditions, the presence of the engine installation under the wing induces a complex and unsteady vortical flow field at the nacelle/pylon/wing junctions which interacts with the upper wing boundary layer and leads to a drop of aircraft performances. In order to gain insight into the physics driving this interaction, it is proposed to isolate its fundamental mechanisms by simplifying the problem. The parameters of interest that led to the simplification of the model are first described. As a first step into a more comprehensive knowledge of this complex physics, this study is initially conducted at a Reynolds number of 200000, based on the chord wing and on the free stream velocity. Two configurations of angle of attack and sideslip angles (α = 8°/β = 0° and α = 8°/β = 30°) have been investigated. This work relies on unsteady RANS computations, oil flow visualizations and 3C-PIV measurements. The vortex dynamics thus produced is described in terms of vortex core position, intensity, size and turbulent intensity thanks to a vortex tracking post-processing algorithm. In addition, the analysis of the velocity flow field obtained from the PIV measurements will highlight the influence of the longitudinal vortex issued from the pylon/wing junction on the separation process of the boundary layer near the upper wing leading-edge.

Numerical investigation of the flow dynamics past a three-element aerofoil

2013 ◽  
Vol 732 ◽  
pp. 401-444 ◽  
Author(s):  
Sébastien Deck ◽  
Romain Laraufie
Keyword(s):  
Numerical Investigation ◽  
Large Scale ◽  
Free Stream ◽  
Pressure Fluctuations ◽  
Phase Relationship ◽  
Feedback Mechanism ◽  
Flow Dynamics ◽  
Free Stream Velocity ◽  
Stream Velocity ◽  
Detached Eddy Simulation

AbstractA numerical investigation of the flow dynamics around a two-dimensional high-lift configuration was carried out by means of a zonal detached eddy simulation (ZDES) technique for flow conditions corresponding to aircraft approach. Both slat and flap regions have been scrutinized and compared with experimental data available in the literature. It is shown that slat and flap coves behave like shallow cavities. The distance between the upstream cusp and the downstream edge is the relevant length scale for each cove taken separately. Consistently with previous findings, this study indicates that the maximum of the broadband spectrum of slat (respectively flap) pressure fluctuations occurs for Strouhal numbers $0. 5\leq \mathit{St}\leq 4$ when based on slat chord (respectively on flap chord) and free-stream velocity. It is shown that mode $(n)$ of the slat cove and mode $(n+ 1)$ of the flap cove are very close making a coherent phase relationship possible. A large-scale coupled self-sustained oscillations mechanism between slat and flap cavities, evidenced by spectral analysis, occurs at a Strouhal number $\mathit{St}= 3{\unicode{x2013}} 6$ based on the main wing chord and free-stream velocity. This yields to an acoustic feedback mechanism characterized by a normalized frequency depending on the free stream Mach number like $\mathit{St}= (1- { M}_{0}^{2} )/ 2{M}_{0} $. The present result appears to line up with the findings by Hein et al. (J. Fluid Mech., vol. 582, 2007, pp. 179–202) who showed that two types of resonance could exist: surface waves ones, scaling with the total aerofoil length and longitudinal cavity-type resonances, scaling with the slat cove length.

Large scales in the developing mixing layer

1976 ◽  
Vol 76 (1) ◽  
pp. 127-144 ◽  
Author(s):  
F. K. Browand ◽  
P. D. Weidman
Keyword(s):  
Reynolds Number ◽  
Large Scale ◽  
Vortical Structure ◽  
Mixing Layer ◽  
Two Dimensional ◽  
Conditional Sampling ◽  
Pairing Interaction ◽  
Vortical Structures ◽  
Developing Flow ◽  
Moderate Reynolds Number

A new experimental technique is described for the study of the interactions between the large-scale vortical features in the two-dimensional mixing layer. Detector probes above and below the mixing layer are used to monitor the large-scale structure. Conditional sampling is performed in a moderate Reynolds number developing flow, by using phase and amplitude information from these detector probes. It is shown that significant Reynolds-stress production is associated with the pairing interaction in which two vortical structures combine to form a single, larger vortical structure.

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