Shear-Layer Detection in Poststenotic Flow by Spectrum Analysis of Doppler Signals

1988 ◽  
Vol 110 (4) ◽  
pp. 320-325 ◽  
Author(s):  
T. Tamura ◽  
A. Fronek
Keyword(s):  
Shear Layer ◽  
Spectrum Analysis ◽  
Doppler Shift ◽  
Continuous Wave ◽  
Reverse Flow ◽  
Separated Flow ◽  
Sample Volume ◽  
Doppler Spectrum ◽  
Doppler Signals ◽  
Shift Frequency

Spectrum analysis of the Doppler signals was performed 0.5 tube diameters downstream from an axisymmetric constriction with an area reduction of 80 percent in steady flow at a jet Reynolds number of 2840. Both pulsed and continuous wave (CW) Doppler spectra showed significant reverse flow components in the separated flow. The pulsed Doppler spectra exhibited sudden changes when the sample volume crossed the shear layer between the center jet and the separated flow. A power spectrum equation was theoretically derived from continuity of flow to define the Doppler shift frequency for the shear layer velocity. The CW Doppler spectrum showed a minimum spectrum density at a frequency which equalled the shear layer Doppler shift frequency derived from the equation. The pulsed spectra exhibited the sudden changes at the same frequency as well.

Haemodynamic changes in human kidney allografts following administration of nifedipine: assessment with doppler spectrum analysis

1992 ◽  
Vol 5 ◽  
pp. S17-S20 ◽  
Author(s):  
J. W. S. Merkus ◽  
L. B. Hilbrands ◽  
A. J. Hoitsma ◽  
W. N. J. C. van Asten ◽  
R. A. P. Koene ◽  
...  
Keyword(s):  
Spectrum Analysis ◽  
Human Kidney ◽  
Doppler Spectrum ◽  
Haemodynamic Changes

On the Use of an Inflatable Rubber Lip to Improve the Reverse Thrust Flow Field in a Variable Pitch Fan

2021 ◽  
Author(s):  
David John Rajendran ◽  
Vassilios Pachidis
Keyword(s):  
Flow Field ◽  
Reverse Flow ◽  
Separated Flow ◽  
Geometric Feature ◽  
Engine Operation ◽  
Design Modification ◽  
Variable Pitch ◽  
Aircraft Landing ◽  
Turn Radius ◽  
Reverse Thrust

Abstract The installed Variable Pitch Fan (VPF) reverse thrust flow field is obtained from the flow solution of an integrated airframe-engine-VPF research model for the complete reverser engagement regime during the aircraft landing run. The reverse thrust flow field indicates that the reverse flow out of the nacelle inlet is washed downstream by the freestream. Consequently, reverse flow enters the engine through the bypass nozzle from a 180° turn of the washed-down stream. This results in a region of separated flow at the nozzle lip that acts as a blockage to the reverse flow entry into the engine. To mitigate the blockage issue, a smooth guidance of the reverse flow into the engine can be achieved by using an inflatable rubber lip that would define a bell-mouth like geometric feature with a round radius at the nacelle exit. In nominal engine operation, the rubber lip would be stowed flush within the contours of the nacelle surface. The design space of the rubber lip is studied by considering different rounding radii and locations of the turn radius with respect to the nacelle trailing edge. It is observed that a rounding radius of 0.1x nacelle length is sufficient to reduce the blockage and increase the ingested reverse flow by 47% to 18% in the 140 to 40 knots landing speed range. The inflatable rubber lip represents a design modification that can improve VPF reverse thrust operation, in cases where an augmentation of reverse thrust capability is desired

A novel approach to estimating the Doppler shift frequency from quadrature mixer output

2017 ◽  
Vol 88 (7) ◽  
pp. 073503 ◽  
Author(s):  
M. Y. Wang ◽  
A. D. Liu ◽  
C. Zhou ◽  
J. Q. Hu ◽  
H. Li ◽  
...  
Keyword(s):  
Doppler Shift ◽  
Novel Approach ◽  
Shift Frequency

scholarly journals Scaling of separated shear layers: an investigation of mass entrainment

2017 ◽  
Vol 826 ◽  
pp. 851-887 ◽  
Author(s):  
Francesco Stella ◽  
Nicolas Mazellier ◽  
Azeddine Kourta
Keyword(s):  
Shear Layer ◽  
Large Scale ◽  
Separated Flow ◽  
Layer Growth ◽  
Upper And Lower Bounds ◽  
Stream Velocity ◽  
Physical Parameters ◽  
Separated Shear Layer ◽  
Mass Entrainment ◽  
Recirculation Length

We report an experimental investigation of the separating/reattaching flow over a descending ramp with a $25^{\circ }$ expansion angle. Emphasis is given to mass entrainment through the boundaries of the separated shear layer emanating from the upper edge of the ramp. For this purpose, the turbulent/non-turbulent interface and the separation line inferred from image-based analysis are used respectively to mark the upper and lower bounds of the separated shear layer. The main objective of this study is to identify the physical parameters that scale the development of the separated shear layer, by giving a specific emphasis to the investigation of mass entrainment. Our results emphasise the multiscale nature of mass entrainment through the separated shear layer. The recirculation length $L_{R}$, step height $h$ and free-stream velocity $U_{\infty }$ are the dominant scales that organise the separated flow (and related large-scale quantities as pressure distribution or shear layer growth rate) and set mean mass fluxes. However, local viscous mechanisms seem to be responsible for most of local mass entrainment. Furthermore, it is shown that large-scale mass entrainment is driven by incoming boundary layer properties, since $L_{R}$ scales with $Re_{\unicode[STIX]{x1D703}}$, and in particular by its turbulent state. Surprisingly, the relationships evidenced in this study suggest that these dependencies are established over a large distance upstream of separation and that they might also extend to small scales, at which viscous entrainment is dominant. If confirmed by additional studies, our findings would open new perspectives for designing effective separation control systems.

scholarly journals On the structure of the self-sustaining cycle in separating and reattaching flows

2018 ◽  
Vol 857 ◽  
pp. 907-936 ◽  
Author(s):  
A. Cimarelli ◽  
A. Leonforte ◽  
D. Angeli
Keyword(s):  
Shear Layer ◽  
Rectangular Plate ◽  
Large Scale ◽  
Reverse Flow ◽  
Leading Edge ◽  
Streamwise Velocity ◽  
The Self ◽  
Small Scale ◽  
Spectral Evolution ◽  
Mean Velocity

The separating and reattaching flows and the wake of a finite rectangular plate are studied by means of direct numerical simulation data. The large amount of information provided by the numerical approach is exploited here to address the multi-scale features of the flow and to assess the self-sustaining mechanisms that form the basis of the main unsteadinesses of the flows. We first analyse the statistically dominant flow structures by means of three-dimensional spatial correlation functions. The developed flow is found to be statistically dominated by quasi-streamwise vortices and streamwise velocity streaks as a result of flow motions induced by hairpin-like structures. On the other hand, the reverse flow within the separated region is found to be characterized by spanwise vortices. We then study the spectral properties of the flow. Given the strongly inhomogeneous nature of the flow, the spectral analysis has been conducted along two selected streamtraces of the mean velocity field. This approach allows us to study the spectral evolution of the flow along its paths. Two well-separated characteristic scales are identified in the near-wall reverse flow and in the leading-edge shear layer. The first is recognized to represent trains of small-scale structures triggering the leading-edge shear layer, whereas the second is found to be related to a very large-scale phenomenon that embraces the entire flow field. A picture of the self-sustaining mechanisms of the flow is then derived. It is shown that very-large-scale fluctuations of the pressure field alternate between promoting and suppressing the reverse flow within the separation region. Driven by these large-scale dynamics, packages of small-scale motions trigger the leading-edge shear layers, which in turn created them, alternating in the top and bottom sides of the rectangular plate with a relatively long period of inversion, thus closing the self-sustaining cycle.

scholarly journals Low-frequency unsteadiness in the wake of a normal flat plate

1998 ◽  
Vol 370 ◽  
pp. 101-147 ◽  
Author(s):  
F. M. NAJJAR ◽  
S. BALACHANDAR
Keyword(s):  
Shear Layer ◽  
Flat Plate ◽  
Free Stream ◽  
Low Frequency ◽  
Separated Flow ◽  
Phase Mismatch ◽  
Streamwise Vortices ◽  
Long Time ◽  
Drag And Lift ◽  
High Degree

The separated flow past a zero-thickness flat plate held normal to a free stream at Re=250 has been investigated through numerical experiments. The long-time signatures of the drag and lift coefficients clearly capture a low-frequency unsteadiness with a period of approximately 10 times the primary shedding period. The amplitude and frequency of drag and lift variations during the shedding process are strongly modulated by the low frequency. A physical interpretation of the low-frequency behaviour is that the flow gradually varies between two different regimes: a regime H of high mean drag and a regime L of low mean drag. It is observed that in regime H the shear layer rolls up closer to the plate to form coherent spanwise vortices, while in regime L the shear layer extends farther downstream and the rolled-up Kármán vortices are less coherent. In the high-drag regime three-dimensionality is characterized by coherent Kármán vortices and reasonably well-organized streamwise vortices connecting the Kármán vortices. With a non-dimensional spanwise wavelength of about 1.2, the three-dimensionality in this regime is reminiscent of mode-B three-dimensionality. It is observed that the high degree of spanwise coherence that exists in regime H breaks down in regime L. Based on detailed numerical flow visualization we conjecture that the formation of streamwise and spanwise vortices is not in perfect synchronization and that the low-frequency unsteadiness is the result of this imbalance (or phase mismatch).

scholarly journals Particle image velocimetry measurements of induced separation at the leading edge of a plate

2016 ◽  
Vol 804 ◽  
pp. 278-297 ◽  
Author(s):  
J. P. J. Stevenson ◽  
K. P. Nolan ◽  
E. J. Walsh
Keyword(s):  
Particle Image Velocimetry ◽  
Shear Layer ◽  
Particle Image ◽  
Reverse Flow ◽  
Leading Edge ◽  
Quadrant Analysis ◽  
Bypass Transition ◽  
Reynolds Stresses ◽  
Free Stream Turbulence ◽  
Image Velocimetry

The free shear layer that separates from the leading edge of a round-nosed plate has been studied under conditions of low (background) and elevated (grid-generated) free stream turbulence (FST) using high-fidelity particle image velocimetry. Transition occurs after separation in each case, followed by reattachment to the flat surface of the plate downstream. A bubble of reverse flow is thereby formed. First, we find that, under elevated (7 %) FST, the time-mean bubble is almost threefold shorter due to an accelerated transition of the shear layer. Quadrant analysis of the Reynolds stresses reveals the presence of slender, highly coherent fluctuations amid the laminar part of the shear layer that are reminiscent of the boundary-layer streaks seen in bypass transition. Instability and the roll-up of vortices then follow near the crest of the shear layer. These vortices are also present under low FST and in both cases are found to make significant contributions to the production of Reynolds stress over the rear of the bubble. But their role in reattachment, whilst important, is not yet fully clear. Instantaneous flow fields from the low-FST case reveal that the bubble of reverse flow often breaks up into two or more parts, thereby complicating the overall reattachment process. We therefore suggest that the downstream end of the ‘separation isoline’ (the locus of zero absolute streamwise velocity that extends unbroken from the leading edge) be used to define the instantaneous reattachment point. A histogram of this point is found to be bimodal: the upstream peak coincides with the location of roll-up, whereas the downstream mode may suggest a ‘flapping’ motion.

Doppler spectrum analysis

2012 ◽  
Vol 94-B (3) ◽  
pp. 344-347 ◽  
Author(s):  
T. Wu ◽  
P-Y. Chen ◽  
C-H. Chen ◽  
C-L. Wang
Keyword(s):  
Spectrum Analysis ◽  
Doppler Spectrum
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