scholarly journals Vortical structure in the wake of a transverse jet

1994 ◽  
Vol 279 ◽  
pp. 1-47 ◽  
Author(s):  
T. F. Fric ◽  
A. Roshko
Keyword(s):  
Boundary Layer ◽  
Velocity Ratio ◽  
Adverse Pressure Gradient ◽  
Reynolds Numbers ◽  
Structural Features ◽  
Bluff Bodies ◽  
Wake Vortices ◽  
Transverse Jet ◽  
Wall Boundary Layer ◽  
Crossflow Velocity

Structural features resulting from the interaction of a turbulent jet issuing transversely into a uniform stream are described with the help of flow visualization and hot-wire anemometry. Jet-to-crossflow velocity ratios from 2 to 10 were investigated at crossflow Reynolds numbers from 3800 to 11400. In particular, the origin and formation of the vortices in the wake are described and shown to be fundamentally different from the well-known phenomenon of vortex shedding from solid bluff bodies. The flow around a transverse jet does not separate from the jet and does not shed vorticity into the wake. Instead, the wake vortices have their origins in the laminar boundary layer of the wall from which the jet issues. It is argued that the closed flow around the jet imposes an adverse pressure gradient on the wall, on the downstream lateral sides of the jet, provoking 'separation events’ in the wall boundary layer on each side. These result in eruptions of boundary-layer fluid and formation of wake vortices that are convected downstream. The measured wake Strouhal frequencies, which depend on the jet-crossflow velocity ratio, match the measured frequencies of the separation events. The wake structure is most orderly and the corresponding wake Strouhal number (0.13) is most sharply defined for velocity ratios near the value 4. Measured wake profiles show deficits of both momentum and total pressure.

Wake Vortices in Jets in Cross-Flow

2011 ◽  
Author(s):  
Ivana Milanovic ◽  
Khairul B. M. Q. Zaman ◽  
Tim Bencic
Keyword(s):  
Boundary Layer ◽  
Velocity Ratio ◽  
Cross Flow ◽  
Boundary Layer Separation ◽  
Tunnel Wall ◽  
Wake Vortices ◽  
Wall Boundary Layer ◽  
The Cross ◽  
Excitation Techniques ◽  
And Behavior

The objective of the current experimental study is to investigate unsteady wake vortices of jets in cross-flow in order to (1) explore the effect of various excitation techniques, their parametric dependence, and impact on the flow field, and (2) provide detailed flow visualizations for a range of velocity ratios. The jet passed through a nozzle and entered the cross-flow with a turbulent boundary layer. While mechanical perturbation did not result in any significant periodic organization of the wake vortices, the database obtained for the unperturbed flow provided an insight into their possible origin and behavior. The key finding was the shedding of wake vortices from the ‘swell’ at the lee side of the jet. The ‘swell’ delineated a region of the flow at the intersection of the jet efflux boundary layer and the cross-flow boundary layer. Separation events released upright vortices and these peeled strands of vorticity convected, twisted and stretched for a few diameters along the wall while still being attached to the bottom of the ‘swell.’ They extended from the tunnel wall and penetrated the jet core where they appeared to burst. In no case were the wake vortices seen to originate either from the wall boundary layer or the jet shear layer at downstream locations. Increasing the jet-to-cross-flow velocity ratio influenced the slope of the wake vortices relative to the jet. The upper part of the vortical strand for VR < 4 was almost perpendicular to the cross-flow. Higher velocity ratios featured a tilt of the vortex structure toward the jet. This angle was particularly pronounced for VR = 8 and 10, and could be observed at all downstream distances. The average convection velocity of the wake vortices for the given cross-flow and all velocity ratios was estimated to be about 0.8 Ucf.

Flow Field Measurement in a Crossflowing Elevated Jet

2006 ◽  
Vol 129 (5) ◽  
pp. 551-562 ◽  
Author(s):  
Nejla Mahjoub Said ◽  
Sabra Habli ◽  
Hatem Mhiri ◽  
Hervé Bournot ◽  
Georges Le Palec
Keyword(s):  
Near Field ◽  
Symmetry Plane ◽  
Reynolds Numbers ◽  
Structural Features ◽  
Bluff Bodies ◽  
Measured Velocity ◽  
Topological Features ◽  
Quantitative Characteristics ◽  
Crossflow Velocity ◽  
Flow Field Measurement

Structural features resulting from the interaction of a turbulent round jet issuing transversely into a uniform stream are described with the help of flow visualization and the PIV technique. The jet exits from a rigidly mounted pipe projecting at a distance from the floor of a tunnel. The aim of the present work is to investigate the flow structure in the near-field jet-pipe exit. Jet-to-crossflow velocity ratios from 0.375 to 3 were revealed at Reynolds numbers from 1660 to 6330. Flows in the vertical symmetry plane and horizontal plane across the jet-wake, jet-exit, and pipe-wake regions are investigated. The measured velocity fields present quantitative characteristics of the streamlines, vortices, and topological features of the flow structures. In particular, the origin and formation of the vortices in the wake are described and shown to be fundamentally different from the well-known phenomenon of vortex shedding from solid bluff bodies.

scholarly journals Experiments on a jet in a crossflow in the low-velocity-ratio regime

2019 ◽  
Vol 863 ◽  
pp. 386-406 ◽  
Author(s):  
L. Klotz ◽  
K. Gumowski ◽  
J. E. Wesfreid
Keyword(s):  
Spatial Dependence ◽  
Velocity Ratio ◽  
Reynolds Numbers ◽  
Bluff Bodies ◽  
Threshold Values ◽  
Low Velocity ◽  
Landau Model ◽  
Global Behaviour ◽  
Crossflow Velocity ◽  
Ratio Range

The hairpin instability of a jet in a crossflow (JICF) for a low jet-to-crossflow velocity ratio is investigated experimentally for a velocity ratio range of $R\in (0.14,0.75)$ and crossflow Reynolds numbers $Re_{D}\in (260,640)$. From spectral analysis we characterize the Strouhal number and amplitude of the hairpin instability as a function of $R$ and $Re_{D}$. We demonstrate that the dynamics of the hairpins is well described by the Landau model, and, hence, that the instability occurs through Hopf bifurcation, similarly to other hydrodynamical oscillators such as wake behind different bluff bodies. Using the Landau model, we determine the precise threshold values of hairpin shedding. We also study the spatial dependence of this hydrodynamical instability, which shows a global behaviour.

An approximate theory for the streaming motion past axisymmetric bodies at Reynolds numbers of from 1 to approximately 100

1977 ◽  
Vol 82 (3) ◽  
pp. 583-604 ◽  
Author(s):  
Michael S. Kolansky ◽  
Sheldon Weinbaum ◽  
Robert Pfeffer
Keyword(s):  
Boundary Layer ◽  
Reynolds Number ◽  
Pressure Gradient ◽  
Reynolds Numbers ◽  
Bluff Bodies ◽  
Approximate Theory ◽  
Viscous Term ◽  
Number Range ◽  
Curvature Effects ◽  
Flow Past

In Weinbaum et al. (1976) a simple new pressure hypothesis is derived which enables one to take account of the displacement interaction, the geometrical change in streamline radius of curvature and centrifugal effects in the thick viscous layers surrounding two-dimensional bluff bodies in the intermediate Reynolds number range O(1) < Re < O(102) using conventional Prandtl boundary-layer equations. The new pressure hypothesis states that the streamwise pressure gradient as a function of distance from the forward stagnation point on the displacement body is equal to the wall pressure gradient as a function of distance along the original body. This hypothesis is shown to be equivalent to stretching the streamwise body co-ordinate in conventional first-order boundary-layer theory. The present investigation shows that the same pressure hypothesis applies for the intermediate Reynolds number flow past axisymmetric bluff bodies except that the viscous term in the conventional axisymmetric boundary-layer equation must also be modified for transverse curvature effects O(δ) in the divergence of the stress tensor. The approximate solutions presented for the location of separation and the detailed surface pressure and vorticity distribution for the flow past spheres, spheroids and paraboloids of revolution at various Reynolds numbers in the range O(1) < Re < O(102) are in good agreement with available numerical Navier–Stokes solutions.

Transverse-jet shear-layer instabilities. Part 2. Linear analysis for large jet-to-crossflow velocity ratio

2008 ◽  
Vol 602 ◽  
pp. 383-401 ◽  
Author(s):  
LEONARDO S. DE B. ALVES ◽  
ROBERT E. KELLY ◽  
ANN R. KARAGOZIAN
Keyword(s):  
Shear Layer ◽  
Near Field ◽  
Velocity Ratio ◽  
Base Flow ◽  
Experimental Results ◽  
Dimensional Parameter ◽  
Potential Core ◽  
Transverse Jet ◽  
Spatial Growth ◽  
Crossflow Velocity

The dominant non-dimensional parameter for isodensity transverse jet flow is the mean jet-to-crossflow velocity ratio,R. In Part 1 (Megerianet al.,J. Fluid Mech., vol. 593, 2007, p. 93), experimental results are presented for the behaviour of transverse-jet near-field shear-layer instabilities for velocity ratios in the range 1 <R≤ 10. A local linear stability analysis is presented in this paper for the subrangeR>4, using two different base flows for the transverse jet. The first analysis assumes the flow field to be described by a modified version of the potential flow solution of Coelho & Hunt (J. Fluid Mech., vol. 200, 1989, p. 95), in which the jet is enclosed by a vortex sheet. The second analysis assumes a continuous velocity model based on the same inviscid base flow; this analysis is valid for the larger values of Strouhal number expected to be typical of the most unstable disturbances, and allows prediction of a maximum spatial growth rate for the disturbances. In both approaches, results are obtained by expanding in inverse powers ofRso that the free-jet results are obtained asR→∞. The results from both approaches agree in the moderately low-frequency regime. Maximum spatial growth rates and associated Strouhal numbers extracted from the second approach both increase with decreasing velocity ratioR, in agreement with the experimental results from Part 1 in the range 4<R≤10. The nominally axisymmetric mode is found to be the most unstable mode in the transverse-jet shear-layer near-field region, upstream of the end of the potential core. The overall agreement of theoretical and experimental results suggests that convective instability occurs in the transverse-jet shear layer for jet-to-crossflow velocity ratios above 4, and that the instability is strengthened asRis decreased.

Turbulence in a separated boundary layer

1991 ◽  
Vol 226 ◽  
pp. 91-123 ◽  
Author(s):  
M. Dianat ◽  
Ian P. Castro
Keyword(s):  
Boundary Layer ◽  
Pressure Gradient ◽  
Low Frequency ◽  
Turbulence Models ◽  
Adverse Pressure Gradient ◽  
Spanwise Direction ◽  
Wall Region ◽  
Thin Wall ◽  
Wall Boundary Layer ◽  
Stress Ratios

This paper presents and discusses the results of an extensive experimental investigation of a flat-plate turbulent boundary subjected to an adverse pressure gradient sufficiently strong to lead to the formation of a large separated region. The pressure gradient was produced by applying strong suction through a porous cylinder fitted with a rear flap and mounted above the boundary layer and with its axis in the spanwise direction. Attention is concentrated on the structure of the turbulent flow within the separated region and it is shown that many features are similar to those that occur in separated regions produced in a very dissimilar manner. These include the fact that structure parameters, like Reynolds stress ratios, respond markedly to the re-entrainment of turbulent fluid transported upstream from the reattachment region, the absence of any logarithmic region in the thin wall boundary layer beneath the recirculation zone and the lack of any effective viscous scaling in this wall region, and the presence of a significant low-frequency motion having timescales much longer than those of the large-eddy structures around reattachment.Similarities with boundary layers separating under the action of much weaker pressure gradients are also found, despite the fact that the nature of the flow around separation is quite different. These similarities and also some noticeable differences are discussed in the paper, which concludes with some inferences concerning the application of turbulence models to separated flows.

Transition in Pressure-Surface Boundary Layers

1987 ◽  
Vol 109 (2) ◽  
pp. 296-302 ◽  
Author(s):  
R. I. Crane ◽  
G. Leoutsakos ◽  
J. Sabzvari
Keyword(s):  
Boundary Layer ◽  
Water Channel ◽  
Outer Wall ◽  
Reynolds Numbers ◽  
Surface Boundary ◽  
Mean Velocity ◽  
Pressure Surface ◽  
Wall Boundary Layer ◽  
Transition Criteria ◽  
Dimensional Boundary

Laminar-to-turbulent transition in the presence of Go¨rtler vortices has been investigated experimentally, in the outer wall boundary layer of a curved water channel. Ratios of boundary layer thickness at the start of curvature to wall radius were around 0.05 and core flow turbulence intensities were between 1 and 3 percent. Measurements of intermittency factor were made by hot film probe and of mean and rms velocity by laser anemometer. At Reynolds numbers low enough to allow considerable nonlinear vortex amplification in the laminar region, transition was found to begin sooner and progress faster at a vortex upwash position than at a spanwise-adjacent downwash position. Measured Go¨rtler numbers at transition onset bore little relationship to those often used as transition criteria in two-dimensional boundary layer prediction codes. Little spanwise variation in intermittency occurred at higher Reynolds numbers, where mean velocity profiles at upwash were much less inflected. Toward the end of curvature, favorable pressure gradients estimated to exceed the Launder relaminarization value corresponded with cases of incomplete transition.

Experimental and Numerical Studies of Cold Inflow at the Exit of Buoyant Channel Flows

1987 ◽  
Vol 109 (2) ◽  
pp. 392-399 ◽  
Author(s):  
Vijay Modi ◽  
K. E. Torrance
Keyword(s):  
Boundary Layer ◽  
Numerical Study ◽  
Reynolds Numbers ◽  
Critical Value ◽  
Ambient Fluid ◽  
Buoyant Jet ◽  
Wall Boundary Layer ◽  
Numerical Studies ◽  
Turbulent Flow Regime ◽  
Neutrally Buoyant

Experimental and numerical studies of the separation of a smooth attached buoyant flow from the inner wall of a duct, as the duct discharges into a quiescent environment, are reported. The associated penetration of neutrally buoyant ambient fluid into the duct is called cold inflow. The experimental study was carried out for air flows over ranges of Reynolds and Froude numbers, based on duct radius, of Re = 2400 to 3300 and Fr = 0.68 to 2.69. The experiments provide information on the onset and extent of cold inflow in a turbulent flow regime. Spatial profiles of fluctuating temperature reveal a wedge-shaped cold inflow region at the wall near the exit when Fr is decreased below a critical value. The numerical study examines the influence of Re and Fr on the structure of the cold inflow phenomenon at moderate Reynolds numbers (Re = 200 to 500 and Fr = 1 to 5). Steady-state, two-dimensional, laminar flow solutions reveal a region of downward-flowing cold air near the wall of the duct which leads to premature separation of the wall boundary layer. The separated boundary layer merges into the buoyant jet above the duct exit.

Flow Separation Control in an Annular to Conical Diffuser Using Two-Dimensional and Three-Dimensional Wall Steps

2013 ◽  
Vol 135 (4) ◽  
Author(s):  
Kin Pong Lo ◽  
Christopher J. Elkins ◽  
John K. Eaton
Keyword(s):  
Boundary Layer ◽  
Pressure Gradient ◽  
Flow Control ◽  
Separation Bubble ◽  
Adverse Pressure Gradient ◽  
Boundary Layer Separation ◽  
Core Flow ◽  
Layer Separation ◽  
Wall Boundary Layer ◽  
Wall Boundary

Conical diffusers are often installed downstream of a turbomachine with a central hub. Previous studies showed that nonstreamlined hubs had extended separated wakes that reduced the adverse pressure gradient in the diffuser. Active flow control techniques can rapidly close the central separation bubble, but this restores the adverse pressure gradient, which can cause the outer wall boundary layer to separate. The present study focuses on the use of a step-wall diffuser to stabilize the wall boundary layer separation in the presence of core flow control. Three-component mean velocity data for a set of conical diffusers were acquired using magnetic resonance velocimetry. The results showed the step-wall diffuser stabilized the wall boundary layer separation by fixing its location. An axisymmetric step separation bubble was formed. A step with a periodically varying height reduced the reattachment length of the step separation and allowed the diffuser to be shortened. The step-wall diffuser was found to be robust in a range of core flow velocity profiles. The minimum distance between the core flow control mechanism and the step-wall diffuser as well as the minimum length of the step were determined.

scholarly journals Multi-resolution, time-resolved PIV measurements of a Decelerating Turbulent Boundary Layer near Separation

2021 ◽  
Vol 1 (1) ◽  
Author(s):  
Christian E Willert ◽  
Matteo Novara ◽  
Daniel Schanz ◽  
Reinhard Geisler ◽  
Michael Schroll ◽  
...  
Keyword(s):  
Boundary Layer ◽  
Turbulent Boundary Layer ◽  
Velocity Profile ◽  
Pressure Gradient ◽  
Adverse Pressure Gradient ◽  
Reynolds Numbers ◽  
Time Resolved ◽  
Piv Measurements ◽  
Peak Energy ◽  
Resolution Imaging

We report on measurements of the time-evolving velocity profile of a turbulent boundary layer subjected to a strong adverse pressure gradient (APG) at Reynolds numbers up to Reθ ≈ 55 000 with an upstream friction Reynolds number exceeding Reτ ≈ 10 000. Near the point of flow separation high-resolution imaging at high camera frame rates captured the time evolving velocity profile using the so-called “profile-PIV” technique in a nested imaging configuration of two cameras operating at different image magnifications. One camera used an image magnification better than unity to resolve the viscous scales directly at the wall while the remainder of the roughly 200 mm thick boundary layer is simultaneous captured by the second camera. In the APG the variance of the stream-wise velocity exhibits no “inner peak” commonly found in turbulent boundary layers without pressure gradient influence. Spectral analysis further shows that the peak energy within the boundary layer shifts away from the wall toward lower frequencies. The overlap between the simultaneously imaged areas allows to assess and, to first order, correct for the effect of spatial smoothing on statistical quantities, spectra and related quantities. A multi-frame cross-correlation algorithm was used to process the extensive data base. In addition, a newly developed 2D-2C “Shake-The-Box” algorithm (STB) provided highly resolved particle tracking data beyond the reach of conventional PIV processing.

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