The flow structure of a puff

2008 ◽  
Vol 367 (1888) ◽  
pp. 489-507 ◽  
Author(s):  
Casimir W.H van Doorne ◽  
Jerry Westerweel
Keyword(s):  
Three Dimensional ◽  
Circular Cross Section ◽  
Travelling Wave ◽  
Stereoscopic Particle Image Velocimetry ◽  
Transition To Turbulence ◽  
Streamwise Vortices ◽  
Mean Flow ◽  
Hairpin Vortices ◽  
Time Resolved ◽  
Low Reynolds

From time-resolved stereoscopic particle image velocimetry measurements over the entire circular cross section of a pipe, a first-of-its-kind quasi-instantaneous three-dimensional velocity field of a turbulent puff at a low Reynolds number is reconstructed. At the trailing edge of the puff, where the laminar flow undergoes transition to turbulence, pairs of counterrotating streamwise vortices are observed that form the legs of large hairpin vortices. At the upstream end of the puff, a quasi-periodic regeneration of streamwise vortices takes place. Initially, the vortex structure resembles a travelling wave solution, but as the vortices propagate into the turbulent region of the puff, they continue to develop into strong hairpin vortices. These hairpin vortices extract so much energy from the mean flow that they cannot be sustained. This structure provides a possible explanation for the intermittent character of the puffs in pipe flow at low Reynolds numbers.

Mechanisms for generating coherent packets of hairpin vortices in channel flow

1999 ◽  
Vol 387 ◽  
pp. 353-396 ◽  
Author(s):  
J. ZHOU ◽  
R. J. ADRIAN ◽  
S. BALACHANDAR ◽  
T. M. KENDALL
Keyword(s):  
Channel Flow ◽  
Three Dimensional ◽  
Wall Layer ◽  
Critical Threshold ◽  
Streamwise Vortices ◽  
Mean Flow ◽  
Hairpin Vortices ◽  
Threshold Strength ◽  
Upstream Process ◽  
The Mean

The evolution of a single hairpin vortex-like structure in the mean turbulent field of a low-Reynolds-number channel flow is studied by direct numerical simulation. The structure of the initial three-dimensional vortex is extracted from the two-point spatial correlation of the velocity field by linear stochastic estimation given a second-quadrant ejection event vector. Initial vortices having vorticity that is weak relative to the mean vorticity evolve gradually into omega-shaped vortices that persist for long times and decay slowly. As reported in Zhou, Adrian & Balachandar (1996), initial vortices that exceed a threshold strength relative to the mean flow generate new hairpin vortices upstream of the primary vortex. The detailed mechanisms for this upstream process are determined, and they are generally similar to the mechanisms proposed by Smith et al. (1991), with some notable differences in the details. It has also been found that new hairpins generate downstream of the primary hairpin, thereby forming, together with the upstream hairpins, a coherent packet of hairpins that propagate coherently. This is consistent with the experimental observations of Meinhart & Adrian (1995). The possibility of autogeneration above a critical threshold implies that hairpin vortices in fully turbulent fields may occur singly, but they more often occur in packets. The hairpins also generate quasi-streamwise vortices to the side of the primary hairpin legs. This mechanism bears many similarities to the mechanisms found by Brooke & Hanratty (1993) and Bernard, Thomas & Handler (1993). It provides a means by which new quasi-streamwise vortices, and, subsequently, new hairpin vortices can populate the near-wall layer.

Flow dynamics and enhanced mixing in a converging–diverging channel

2016 ◽  
Vol 807 ◽  
pp. 167-204 ◽  
Author(s):  
S. W. Gepner ◽  
J. M. Floryan
Keyword(s):  
Three Dimensional ◽  
Travelling Wave ◽  
Flow Dynamics ◽  
Streamwise Vortices ◽  
Dimensional Model ◽  
Three Dimensional Model ◽  
Wave Instability ◽  
Unsteady Separation ◽  
Flow Stabilization ◽  
Diverging Channel

An analysis of flows in converging–diverging channels has been carried out with the primary goal of identifying geometries which result in increased mixing. The model geometry consists of a channel whose walls are fitted with spanwise grooves of moderate amplitudes (up to 10 % of the mean channel opening) and of sinusoidal shape. The groove systems on each wall are shifted by half of a wavelength with respect to each other, resulting in the formation of a converging–diverging conduit. The analysis is carried out up to Reynolds numbers resulting in the formation of secondary states. The first part of the analysis is based on a two-dimensional model and demonstrates that increasing the corrugation wavelength results in the appearance of an unsteady separation whose onset correlates with the onset of the travelling wave instability. The second part of the analysis is based on a three-dimensional model and demonstrates that the flow dynamics is dominated by the centrifugal instability over a large range of geometric parameters, resulting in the formation of streamwise vortices. It is shown that the onset of the vortices may lead to the elimination of the unsteady separation. The critical Reynolds number for the vortex onset initially decreases as the corrugation amplitude increases but an excessive increase leads to the stream lift up, reduction of the centrifugal forces and flow stabilization. The flow dynamics under such conditions is again dominated by the travelling wave instability. Conditions leading to the formation of streamwise vortices without interference from the travelling wave instability have been identified. The structure and the mixing properties of the saturated states are discussed.

A numerical study of three-dimensional vortex ring instabilities: viscous corrections and early nonlinear stage

1994 ◽  
Vol 279 ◽  
pp. 351-375 ◽  
Author(s):  
Karim Shariff ◽  
Roberto Verzicco ◽  
Paolo Orlandi
Keyword(s):  
Growth Rates ◽  
Numerical Study ◽  
Initial Perturbation ◽  
Three Dimensional ◽  
Instability Wave ◽  
Mean Flow ◽  
Hairpin Vortices ◽  
Single Wave ◽  
Second Mode ◽  
Radial Structure

Finite-difference calculations with random and single-mode perturbations are used to study the three-dimensional instability of vortex rings. The basis of current understanding of the subject consists of a heuristic inviscid model (Widnall, Bliss & Tsai 1974) and a rigorous theory which predicts growth rates for thin-core uniform vorticity rings (Widnall & Tsai 1977). At sufficiently high Reynolds numbers the results correspond qualitatively to those predicted by the heuristic model: multiple bands of wavenumbers are amplified, each band having a distinct radial structure. However, a viscous correction factor to the peak inviscid growth rate is found. It is well described by the first term, 1 – α1(β)/Res, for a large range of Res. Here Res is the Reynolds number defined by Saffman (1978), which involves the curvature-induced strain rate. It is found to be the appropriate choice since then α1(β) varies weakly with core thickness β. The three most nonlinearly amplified modes are a mean azimuthal velocity in the form of opposing streams, an n = 1 mode (n is the azimuthal wavenumber) which arises from the interaction of two second-mode bending waves and the harmonic of the primary second mode. When a single wave is excited, higher harmonics begin to grow successively later with nonlinear growth rates proportional to n. The modified mean flow has a doubly peaked azimuthal vorticity. Since the curvature-induced strain is not exactly stagnation-point flow there is a preference for elongation towards the rear of the ring: the outer structure of the instability wave forms a long wake consisting of n hairpin vortices whose waviness is phase shifted π/n relative to the waviness in the core. Whereas the most amplified linear mode has three radial layers of structure, higher radial modes having more layers of radial structure (hairpins piled upon hairpins) are excited when the initial perturbation is large, reminiscent of visualization experiments on the formation of a turbulent ring at the generator.

Direct numerical simulations of roughness-induced transition in supersonic boundary layers

2012 ◽  
Vol 693 ◽  
pp. 28-56 ◽  
Author(s):  
Suman Muppidi ◽  
Krishnan Mahesh
Keyword(s):  
Numerical Simulations ◽  
Shear Layer ◽  
Boundary Layers ◽  
Plate Boundary ◽  
Direct Numerical Simulations ◽  
Transition To Turbulence ◽  
Streamwise Vortices ◽  
Mean Flow ◽  
The Mean ◽  
Near Wall

AbstractDirect numerical simulations are used to study the laminar to turbulent transition of a Mach 2.9 supersonic flat plate boundary layer flow due to distributed surface roughness. Roughness causes the near-wall fluid to slow down and generates a strong shear layer over the roughness elements. Examination of the mean wall pressure indicates that the roughness surface exerts an upward impulse on the fluid, generating counter-rotating pairs of streamwise vortices underneath the shear layer. These vortices transport near-wall low-momentum fluid away from the wall. Along the roughness region, the vortices grow stronger, longer and closer to each other, and result in periodic shedding. The vortices rise towards the shear layer as they advect downstream, and the resulting interaction causes the shear layer to break up, followed quickly by a transition to turbulence. The mean flow in the turbulent region shows a good agreement with available data for fully developed turbulent boundary layers. Simulations under varying conditions show that, where the shear is not as strong and the streamwise vortices are not as coherent, the flow remains laminar.

Experimental observation of hairpin auto-generation events in a turbulent boundary layer

2016 ◽  
Vol 795 ◽  
pp. 611-633 ◽  
Author(s):  
Y. Jodai ◽  
G. E. Elsinga
Keyword(s):  
Boundary Layer ◽  
Turbulent Boundary Layer ◽  
Shear Layer ◽  
Transition Process ◽  
Wall Region ◽  
Generation Model ◽  
Streamwise Vortices ◽  
Hairpin Vortices ◽  
Time Resolved ◽  
Near Wall

Time-resolved tomographic particle image velocimetry experiments show that new hairpin vortices are generated within a fully developed and unperturbed turbulent boundary layer. The measurements are taken at a Reynolds number based on the momentum thickness of 2038, and cover the near-wall region below $y^{+}=140$, where $y^{+}$ is the wall-normal distance in wall units. Instantaneous visualizations of the flow reveal near-wall low-speed streaks with associated quasi-streamwise vortices, retrograde inverted arch vortices, hairpin vortices and hairpin packets. The hairpin heads are observed as close to the wall as $y^{+}=30$. Examples of hairpin packet evolution reveal the development of new hairpin vortices, which are created upstream and close to the wall in a manner consistent with the auto-generation model (Zhou et al., J. Fluid Mech., vol. 387, 1999, pp. 353–396). The development of the new hairpin appears to be initiated by an approaching sweep event, which perturbs the shear layer associated with the initial packet. The shear layer rolls up, thereby forming the new hairpin head. The head subsequently connects to existing streamwise vortices and develops into a hairpin. The time scale associated with the hairpin auto-generation is 20–30 wall units of time. This demonstrates that hairpins can be created over short distances within a developed turbulent boundary layer, implying that they are not simply remnants of the laminar-to-turbulent transition process far upstream.

Quantitative measurements of three-dim ensional structures in the wake of a circular cylinder

1994 ◽  
Vol 270 ◽  
pp. 277-296 ◽  
Author(s):  
Hussein Mansy ◽  
Pan-Mei Yang ◽  
David R. Williams
Keyword(s):  
Circular Cylinder ◽  
Secondary Flow ◽  
Three Dimensional ◽  
Streamwise Vortices ◽  
Mean Flow ◽  
Near Wake ◽  
Three Dimensional Flow ◽  
Primary Flow ◽  
Flow Component ◽  
Dimensional Flow

The fine scale three-dimensional structures usually associated with streamwise vortices in the near wake of a circular cylinder have been studied at Reynolds numbers ranging from 170 to 2200. Spatially continuous velocity measurements along lines parallel to the cylinder axis were obtained with a scanning laser anemometer. To detect the streamwise vortices in the amplitude modulated velocity field, it was necessary to develop a spatial decomposition technique to split the total flow into a primary flow component and a secondary flow component. The primary flow is comprised of the mean flow and Strouhal vortices, while the secondary flow is the result of the three-dimensional streamwise vortices that are the essence of transition to turbulence. The three-dimensional flow amplitude increases in the primary vortex formation region, then saturates shortly after the maximum amplitude in the primary flow is reached. In the near-wake region the wavelength decreases approximately like Re−0.5, but increases with downstream distance. A discontinuous increase in wavelength occurs below Re = 300 suggesting a fundamental change in the character of the three-dimensional flow. At downstream distances (x/D = 10-20), the spanwise wavelength decreases from 1.42D to 1.03D as the Reynolds number increases from 300 to 1200.

scholarly journals Properties of a sweeping jet emitted from a fluidic oscillator

2018 ◽  
Vol 857 ◽  
pp. 216-238 ◽  
Author(s):  
Florian Ostermann ◽  
Rene Woszidlo ◽  
C. Navid Nayeri ◽  
C. Oliver Paschereit
Keyword(s):  
Flow Field ◽  
Three Dimensional ◽  
Stereoscopic Particle Image Velocimetry ◽  
Total Force ◽  
Entrainment Rate ◽  
Time Resolved ◽  
Fluidic Oscillator ◽  
Spatial Oscillation ◽  
The Individual ◽  
Visualization Techniques

This experimental study investigates the flow field and properties of a sweeping jet emitted from a fluidic oscillator into a quiescent environment. The aspect ratio of the outlet throat is 1. Stereoscopic particle image velocimetry is employed to measure the velocity field plane-by-plane. Simultaneously acquired pressure measurements provide a reference for phase correlating the individual planes yielding three-dimensional, time-resolved velocity information. Lagrangian and Eulerian visualization techniques illustrate the phase-averaged flow field. Circular head vortices, similar to the starting vortex of a steady jet, are formed repetitively when the jet is at its maximum deflection. The quantitative jet properties are determined from instantaneous velocity data using a cylindrical coordinate system that takes into account the changing deflection angle of the jet. The jet properties vary throughout the oscillation cycle. The maximum jet velocity decays much faster than that of a comparable steady jet indicating a higher momentum transfer to the environment. The entrainment rate of the spatially oscillating jet is larger than for a steady jet by a factor of 4. Most of the mass flow is entrained from the direction normal to the oscillation plane, which is accompanied by a significant increase in jet depth compared to a steady jet. The high entrainment rate results from the enlarged contact area between jet and ambient fluid due to the spatial oscillation. The jet’s total force exceeds that of an idealized steady jet by up to 30 %. The results are independent of the investigated oscillation frequencies in the range from 5 to 20 Hz.

scholarly journals Large-eddy simulation of three-dimensional dunes in a steady, unidirectional flow. Part 1. Turbulence statistics

2013 ◽  
Vol 721 ◽  
pp. 454-483 ◽  
Author(s):  
Mohammad Omidyeganeh ◽  
Ugo Piomelli
Keyword(s):  
Secondary Flow ◽  
Three Dimensional ◽  
Wall Stress ◽  
Two Dimensional ◽  
Streamwise Vortices ◽  
Mean Flow ◽  
Form Drag ◽  
Separated Shear Layer ◽  
Large Eddy ◽  
The Mean

AbstractWe performed large-eddy simulations of flow over a series of three-dimensional dunes at laboratory scale (Reynolds number based on the average channel depth and streamwise velocity was 18 900) using the Lagrangian dynamic eddy-viscosity subgrid-scale model. The bedform three-dimensionality was imposed by shifting a standard two-dimensional dune shape in the streamwise direction according to a sine wave. The statistics of the flow are discussed in 10 cases with in-phase and staggered crestlines, different deformation amplitudes and wavelengths. The results are validated qualitatively against experiments. The three-dimensional separation of flow at the crestline alters the distribution of wall pressure, which in turn may cause secondary flow across the stream, which directs low-momentum fluid, near the bed, toward the lobe (the most downstream point on the crestline) and high-momentum fluid, near the top surface, toward the saddle (the most upstream point on the crestline). The mean flow is characterized by a pair of counter-rotating streamwise vortices, with core radius of the order of the flow depth. However, for wavelengths smaller than the flow depth, the secondary flow exists only near the bed and the mean flow away from the bed resembles the two-dimensional case. Staggering the crestlines alters the secondary motion; the fastest flow occurs between the lobe and the saddle planes, and two pairs of streamwise vortices appear (a strong one, centred about the lobe, and a weaker one, coming from the previous dune, centred around the saddle). The distribution of the wall stress and the focal points of separation and attachment on the bed are discussed. The sensitivity of the average reattachment length, depends on the induced secondary flow, the streamwise and spanwise components of the channel resistance (the skin friction and the form drag), and the contribution of the form drag to the total resistance are also studied. Three-dimensionality of the bed increases the drag in the channel; the form drag contributes more than in the two-dimensional case to the resistance, except for the staggered-crest case. Turbulent-kinetic energy is increased in the separated shear layer by the introduction of three-dimensionality, but its value normalized by the plane-averaged wall stress is lower than in the corresponding two-dimensional dunes. The upward flow on the stoss side and higher deceleration of flow on the lee side over the lobe plane lift and broaden the separated shear layer, respectively, affecting the turbulent kinetic energy.

Onset of three-dimensionality, equilibria, and early transition in flow over a backward-facing step

1991 ◽  
Vol 231 ◽  
pp. 501-528 ◽  
Author(s):  
Lambros Kaiktsis ◽  
George Em Karniadakis ◽  
Steven A. Orszag
Keyword(s):  
Numerical Study ◽  
Three Dimensional ◽  
Reynolds Numbers ◽  
Shear Layers ◽  
Secondary Instability ◽  
Transition To Turbulence ◽  
Single Frequency ◽  
Mean Flow ◽  
Backward Facing Step ◽  
Numerical Predictions

A numerical study of three-dimensional equilibria and transition to turbulence in flow over a backward-facing step is performed using direct numerical solution of the incompressible Navier-Stokes equations. The numerical method is a high-order-accurate mixed spectral/spectral-element method with efficient viscous outflow boundary conditions. The appearance of three-dimensionality in nominally two-dimensional geometries is investigated at representative Reynolds numbers ranging from the onset of three-dimensional bifurcation to later transitional stages. Strongly three-dimensional regions are identified through standard correlation coefficients and new three-dimensionality indices, as well as through instantaneous and time-average streamline patterns and vorticity contours. Our results indicate that onset of three-dimensionality occurs at the boundaries between the primary and secondary recirculating zones with the main channel flow, the latter being the most stable flow component. There is. therefore, strong secondary instability in the shear layers, mainly due to the one emanating from the step corner.The flow further downstream is excited through the action of the upstream shear layers acquiring a wavy form closely resembling Tollmien–Schlichting waves both spatially and temporally with a characteristic frequency f1; upstream, at the shear layer another incommensurate frequency, f2, is present. The two-frequency flow locks-in to a single frequency if external excitations are imposed at the inflow at a frequency close to f1 or f2; the smaller amplitude excitations, however, may cause a strong quasi-periodic response. Such excitations may significantly increase or decrease (by more than 20%) the length of the primary separation zone XR at lock-in or quasi-periodic states. The equilibrium states resulting from the secondary instability at supercritical Reynolds numbers produce a flow modulated in the spanwise direction, with corresponding variations in the reattachment location XR. While three-dimensionality explains partially the discrepancy between numerical predictions and experimental results on XR at higher Reynolds number Re, the main source of discrepancy is attributed to the inflow conditions, and in particular to external disturbances superimposed on the mean flow, the latter being the main reason also for the somewhat earlier transition found in laboratory experiments.

Effect of Blade Profile Contouring on Endwall Flow Structure in a High-Lift Low-Pressure Turbine Cascade

2016 ◽  
Vol 139 (2) ◽  
Author(s):  
Keith Sangston ◽  
Jesse Little ◽  
M. Eric Lyall ◽  
Rolf Sondergaard
Keyword(s):  
Total Pressure ◽  
Three Dimensional ◽  
Low Pressure ◽  
Stereoscopic Particle Image Velocimetry ◽  
Loss Reduction ◽  
Mean Flow ◽  
High Lift ◽  
Computational Tools ◽  
Design Modification ◽  
Low Pressure Turbine

Previous work has shown that low-stagger contouring near the endwall of a nominally high-lift and high-stagger angle front-loaded low-pressure turbine (LPT) airfoil is successful in reducing endwall loss by limiting the development and migration of low momentum fluid associated with secondary flow structures. The design modification that leads to loss reduction in that study was determined from an intuitive approach based on the premise that reducing flow separation near the endwall will lead to reduced loss production. Those authors also relied heavily upon Reynolds-averaged Navier–Stokes (RANS) based computational tools. Due to uncertainties inherent in computational fluid dynamics (CFD) predictions, there is little confidence that the authors actually achieved true minimum loss. Despite recent advances in computing capability, turbulence modeling remains a shortcoming of modern design tools. As a contribution to overcoming this problem, this paper offers a three-dimensional (3D) view of the developing mean flow, total pressure, and turbulence fields that gave rise to the loss reduction of the airfoil mentioned above. Experiments are conducted in a linear cascade with aspect ratio of 3.5 and Re = 100,000. The results are derived from stereoscopic particle image velocimetry (PIV) and total pressure measurements inside the passage. Overall, the loss reduction correlates strongly with reduced turbulence production. The aim of this paper is to provide readers with a realistic view of mean flow and turbulence development that include all the components of the Reynolds stress tensor to assess, at least qualitatively, the validity of high fidelity computational tools used to calculate turbine flows.

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