Excitation Models for Buffeting of Cylinder Bundles in Parallel Flow

2006 ◽  
Author(s):  
Radu Pomiˆrleanu
Keyword(s):  
Swirling Flow ◽  
Spectral Characteristics ◽  
Pressure Fluctuations ◽  
Low Frequency ◽  
Parallel Flow ◽  
Rod Bundles ◽  
Wall Pressure Fluctuations ◽  
Flow Geometry ◽  
Model Predictions ◽  
Selection Of

In this paper, the effect of several models representing the state of the art in wall-pressure fluctuations statistics were applied to the geometry of square rod bundles with P/D = 1.33, and used together with the random vibration theory to yield the spectral characteristics of rod vibration. Comparison of model predictions with experimental data representing rod acceleration allowed the selection of Efimtsov model as better suited for the rod bundles flow geometry and low frequency range. Further assimilating the mixing-vanes induced swirling flow as a disturbance to the parallel flow, two corrections were proposed based on comparison of measured vibration characteristics of vaned and vaneless bundles.

An Experimental Study of the Flow and Wall Pressure Field Around a Wing-Body Junction

1986 ◽  
Vol 108 (3) ◽  
pp. 308-314 ◽  
Author(s):  
M. A. Z. Hasan ◽  
M. J. Casarella ◽  
E. P. Rood
Keyword(s):  
Pressure Gradient ◽  
Pressure Field ◽  
Pressure Fluctuations ◽  
Three Dimensional ◽  
Low Frequency ◽  
Adverse Pressure Gradient ◽  
Horseshoe Vortex ◽  
Wall Pressure ◽  
Frequency Content ◽  
Wall Pressure Fluctuations

The flow and wall-pressure field around a wing-body junction has been experimentally investigated in a quiet, low-turbulence wind tunnel. Measurements were made along the centerline in front of the wing and along several spanwise locations. The flow field data indicated that the strong adverse pressure gradient on the upstream centerline causes three-dimensional flow separation at approximately one wing thickness upstream and this induced the formation of the horseshoe root vortex which wrapped around the wing and became deeply embedded within the boundary layer. The wall-pressure fluctuations were measured for their spectral content and the data indicate that the effect of the adverse pressure gradient is to increase the low-frequency content of the wall pressure and to decrease the high-frequency content. The wall pressure data in the separated region, which is dominated by the horseshoe vortex, shows a significant increase in the low-frequency content and this characteristic feature prevails around the corner of the wing. The outer edge of the horseshoe vortex is clearly identified by the locus of maximum values of RMS wall pressure.

THE PHYSICAL ORIGIN OF SEVERE LOW-FREQUENCY PRESSURE FLUCTUATIONS IN GIANT FRANCIS TURBINES

2005 ◽  
Vol 19 (28n29) ◽  
pp. 1527-1530 ◽  
Author(s):  
R.-K. ZHANG ◽  
Q.-D. CAI ◽  
J.-Z. WU ◽  
Y.-L. WU ◽  
S.-H. LIU ◽  
...  
Keyword(s):  
Swirling Flow ◽  
Pressure Fluctuations ◽  
Low Frequency ◽  
Francis Turbine ◽  
Navier Stokes ◽  
Physical Origin ◽  
Navier Stokes Equation ◽  
Power Stations ◽  
Hydraulic Turbines ◽  
Hydrodynamic Stability Theory

The physical origin of severe low-frequency pressure fluctuation frequently observed in Francis hydraulic turbines under off-design conditions, which greatly damages the structural stability of turbines and even power stations, is analyzed based on the hydrodynamic stability theory and our Reynolds-averaged Navier-Stokes equation simulation (RANS) of the flow in the entire passage of a Francis turbine. We find that spontaneous unsteady vortex ropes, which induce severe pressure fluctuations, are formed due to the absolute instability of the swirling flow at the conical inlet of the turbine's draft tube.

Low-frequency unsteadiness of swept shock-wave/turbulent-boundary-layer interaction

2018 ◽  
Vol 856 ◽  
pp. 797-821 ◽  
Author(s):  
Xin Huang ◽  
David Estruch-Samper
Keyword(s):  
Boundary Layer ◽  
Turbulent Boundary Layer ◽  
High Speed ◽  
Reverse Flow ◽  
Pressure Fluctuations ◽  
Low Frequency ◽  
Boundary Layer Separation ◽  
Frequency Instability ◽  
Pulsation Frequency ◽  
Wall Pressure Fluctuations

High-speed turbulent boundary-layer separation can lead to severe wall-pressure fluctuations, often extending over a swept shock region. Having noted the shear layer’s influence within axisymmetric step flows, tests go on to experimentally assess the unsteadiness of a canonical swept separation, caused by a slanted $90^{\circ }$-step discontinuity (with varying azimuthal height) over an axisymmetric turbulent boundary layer. Results document an increase in shock pulsation frequency along the swept separation region ($\unicode[STIX]{x1D6EC}\leqslant 30^{\circ }$ sweep angles) – whereby the recirculation enables downstream feedback via the reverse flow – as the local streamwise separation length is reduced. A link between the spanwise variation in the separation shock’s low-frequency instability and the downstream mass ejection rate, as large shear-layer eddies leave the bubble, is sustained. The local entrainment-recharge dynamics of swept separation are thereby duly evaluated.

Flow-Feedback Method for Mitigating the Vortex Rope in Decelerated Swirling Flows

2013 ◽  
Vol 135 (6) ◽  
Author(s):  
Constantin Tănasă ◽  
Romeo Susan-Resiga ◽  
Sebastian Muntean ◽  
Alin Ilie Bosioc
Keyword(s):  
Swirling Flow ◽  
Flow Instability ◽  
Pressure Fluctuations ◽  
Water Injection ◽  
Operating Regime ◽  
Experimental Investigations ◽  
Vortex Rope ◽  
Practical Applications ◽  
Wall Pressure Fluctuations ◽  
Hydraulic Turbines

When reaction hydraulic turbines are operated far from the design operating regime, particularly at partial discharge, swirling flow instability is developed downstream of the runner, in the discharge cone, with a precessing helical vortex and its associated severe pressure fluctuations. Bosioc et al. (2012, “Unsteady Pressure Analysis of a Swirling Flow With Vortex Rope and Axial Water Injection in a Discharge Cone,” ASME J. Fluids Eng., 134(8), p. 081104) showed that this instability can be successfully mitigated by injecting a water jet along the axis. However, the jet discharge is too large to be supplied with high pressure water bypassing the runner, since this discharge is associated with the volumetric loss. In the present paper we demonstrate that the control jet injected at the inlet of the conical diffuser can actually be supplied with water collected from the discharge cone outlet, thus introducing a new concept of flow feedback. In this case, the jet is driven by the pressure difference between the cone wall, where the feedback spiral case is located, and the pressure at the jet nozzle outlet. In order to reach the required threshold value of the jet discharge, we also introduce ejector pumps to partially compensate for the hydraulic losses in the return pipes. Extensive experimental investigations show that the wall pressure fluctuations are successfully mitigated when the jet reaches 12% of the main flow discharge for a typical part load turbine operating regime. About 10% of the jet discharge is supplied by the plain flow feedback, and only 2% boost is insured by the ejector pumps. As a result, this new approach paves the way towards practical applications in real hydraulic turbines.

Unsteadiness in a large turbulent separation bubble

2016 ◽  
Vol 799 ◽  
pp. 383-412 ◽  
Author(s):  
Abdelouahab Mohammed-Taifour ◽  
Julien Weiss
Keyword(s):  
Shear Layer ◽  
Separation Bubble ◽  
Pressure Fluctuations ◽  
Pressure Sensors ◽  
Low Frequency ◽  
Wall Pressure ◽  
Medium Frequency ◽  
Wall Pressure Fluctuations ◽  
Turbulent Separation ◽  
The Mean

The unsteady behaviour of a massively separated, pressure-induced turbulent separation bubble (TSB) is investigated experimentally using high-speed particle image velocimetry (PIV) and piezo-resistive pressure sensors. The TSB is generated on a flat test surface by a combination of adverse and favourable pressure gradients. The Reynolds number based on the momentum thickness of the incoming boundary layer is 5000 and the free stream velocity is$25~\text{m}~\text{s}^{-1}$. The proper orthogonal decomposition (POD) is used to separate the different unsteady modes in the flow. The first POD mode contains approximately 30 % of the total kinetic energy and is shown to describe a low-frequency contraction and expansion, called ‘breathing’, of the TSB. This breathing is responsible for a variation in TSB size of approximately 90 % of its average length. It also generates low-frequency wall-pressure fluctuations that are mainly felt upstream of the mean detachment and downstream of the mean reattachment. A medium-frequency unsteadiness, which is linked to the convection of large-scale vortices in the shear layer bounding the recirculation zone and their shedding downstream of the TSB, is also observed. When scaled with the vorticity thickness of the shear layer and the convection velocity of the structures, this medium frequency is very close to the characteristic frequency of vortices convected in turbulent mixing layers. The streamwise position of maximum vertical turbulence intensity generated by the convected structures is located downstream of the mean reattachment line and corresponds to the position of maximum wall-pressure fluctuations.

The wall pressure signature of transonic shock/boundary layer interaction

2011 ◽  
Vol 671 ◽  
pp. 288-312 ◽  
Author(s):  
MATTEO BERNARDINI ◽  
SERGIO PIROZZOLI ◽  
FRANCESCO GRASSO
Keyword(s):  
Boundary Layer ◽  
Dynamic Pressure ◽  
Pressure Fluctuations ◽  
Low Frequency ◽  
Space Time ◽  
Wall Pressure ◽  
Spanwise Direction ◽  
Frequency Spectra ◽  
Wall Pressure Fluctuations ◽  
Time Correlations

The structure of wall pressure fluctuations beneath a turbulent boundary layer interacting with a normal shock wave at Mach number M∞ = 1.3 is studied exploiting a direct numerical simulation database. Upstream of the interaction, in the zero-pressure-gradient region, pressure statistics compare well with canonical low-speed boundary layers in terms of fluctuation intensities, space–time correlations, convection velocities and frequency spectra. Across the interaction zone, the root-mean-square wall pressure fluctuations attain very large values (in excess of 162 dB), with a maximum increase of about 7 dB from the upstream level. The two-point wall pressure correlations become more elongated in the spanwise direction, indicating an increase of the pressure-integral length scales, and the convection velocities (determined from space–time correlations) are reduced. The interaction qualitatively modifies the shape of the frequency spectra, causing enhancement of the low-frequency Fourier modes and inhibition of the higher ones. In the recovery region past the interaction, the pressure spectra collapse very accurately when scaled with either the free-stream dynamic pressure or the maximum Reynolds shear stress, and exhibit distinct power-law regions with exponent −7/3 at intermediate frequencies and −5 at high frequencies. An analysis of the pressure sources in the Lighthill's equation for the instantaneous pressure has been performed to understand their contributions to the wall pressure signature. Upstream of the interaction the sources are mainly located in the proximity of the wall, whereas past the shock, important contributions to low-frequency pressure fluctuations are associated with long-lived eddies developing far from the wall.

Effects of Surface Irregularity on Turbulent Boundary Layer Wall Pressure Fluctuations

1984 ◽  
Vol 106 (3) ◽  
pp. 343-350 ◽  
Author(s):  
T. M. Farabee ◽  
M. J. Casarella
Keyword(s):  
Spectral Density ◽  
Flat Plate ◽  
Pressure Field ◽  
Pressure Fluctuations ◽  
Low Frequency ◽  
Velocity Profiles ◽  
Wall Pressure ◽  
Surface Irregularity ◽  
Wall Pressure Fluctuations ◽  
Frequency Components

Measurements were made of the mean velocity profiles and wall pressure field upstream and downstream of the flow over both a backward-facing and forward-facing step. For each configuration the velocity profiles show that the effects of the separation-reattachment process persist more than 24 step heights downstream of the step. Extremely high values of the RMS wall pressure are measured near reattachment. These values are 5 and 10 times larger than on a smooth flat plate for the backward-facing step and the forward-facing step, respectively. The spectral density of the wall pressure fluctuations in the recirculation region is dominated by low frequency components. Downstream of reattachment there is a reduction in the low frequency content of the wall pressures and an increase in the high frequency components. At the farthest measured position downstream, the spectral density is still higher than that found on a smooth flat plate. These results show that the complex turbulent flow generated by a surface irregularity can significantly increase the localized wall pressure field and these increases persist far downstream of the irregularity. Consequently, a surface irregularity can be a major source of turbulence-induced vibrations and flow noise, as well as a cause of the inception of cavitation in marine applications.

Measurements of Fluctuating Wall Pressure for Separated/Reattached Boundary Layer Flows

1986 ◽  
Vol 108 (3) ◽  
pp. 301-307 ◽  
Author(s):  
T. M. Farabee ◽  
M. J. Casarella
Keyword(s):  
Boundary Layer ◽  
Pressure Field ◽  
Pressure Fluctuations ◽  
Plate Boundary ◽  
Low Frequency ◽  
Wall Pressure ◽  
Boundary Layer Flows ◽  
Backward Facing Step ◽  
Wall Pressure Fluctuations ◽  
Layer Flow

Measurements were made of the wall pressure field beneath separated/reattached boundary layer flows. These flows consisted of two types; flow over a forward-facing step and flow over a backward-facing step. Wall pressure fluctuations from an equilibrium flat plate boundary layer flow were also measured and used as a baseline for comparative purposes. Values of the RMS fluctuating pressure as well as the frequency spectral density, phase velocity, and coherence of the surface pressure field were measured at various locations upstream and downstream of the steps. The experimental results show that the separation-reattachment process produces large-amplitude, low-frequency pressure fluctuations. The measured spectral statistics of the wall pressure fluctuations are consistent with the view that at reattachment there exists a region of coherent highly energized velocity fluctuations located near the wall which, as it convects downstream, decays and diffuses away from the wall. This energized region remains identifiable in the wall pressure statistics as far as 72 step heights downstream of the backward-facing step.

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