Interaction of an unstable planar jet with an oscillating leading edge (original) (raw)
Related papers
Unstable jet–edge interaction. Part 1. Instantaneous pressure fields at a single frequency
Journal of Fluid Mechanics, 1986
Despite its central importance, the pressure field at a leading edge has remained uncharacterized for the classical jet-edge interaction at a single predominant frequency. This investigation shows that the force, due to the integrated instantaneous pressure field on the edge, is located a distance downstream of the tip of the edge as much as one-quarter of a wavelength ( A ) of the incident instability; this distance also corresponds to about one-quarter of the geometric length (L) between the nozzle and tip of the edge. Consequently, the traditional assumption that the phase-locking criterion for self-sustained oscillations can be expressed as a ratio of CIA is inappropriate for low-speed jet flows, which have been of primary interest over the past two decades.
Effects of controlled vortex generation and interactions in transverse jets
Physical Review Fluids
This experimental study examined the effects of controlled vortex generation and interactions created by axisymmetric excitation of a transverse jet, with a focus on the structural and mixing characteristics of the flow. The excitation consisted of a double-pulse forcing waveform applied to the jet, where two distinct temporal square-wave pulses were prescribed during a single forcing period. The two distinct pulses produced vortex rings of different strength and celerity, the strategic selection of which promoted vortex ring interactions or collisions in the near field to varying degrees. Jet flow conditions corresponding to a transitionally convectively and absolutely unstable upstream shear layer (USL) in the absence of forcing, at a jet-to-cross-flow momentum flux ratio of J = 10, and to an absolutely unstable USL at J = 7, were explored for a jet Reynolds number of 1800. Acetone planar laser-induced fluorescence imaging was utilized to quantify the influence of different prescribed temporal waveforms. All forcing conditions enhanced the spread, penetration, and molecular mixing of the jet as compared to the unforced jet, though to differing degrees. Interestingly, when the jet was convectively unstable, forcing which promoted vortex collisions provided the greatest enhancement in molecular mixing, whereas the absolutely unstable jet produced the greatest enhancement in mixing when the vortex rings did not interact, with important implications for optimized jet control.
Effect of an external excitation on the flow structure in a circular impinging jet
Physics of Fluids, 2005
The effect of an external excitation on circular impinging jet flow is studied experimentally. Velocity fields in moderate air flows ͑Re= 10 000͒ are investigated by hot-wire anemometry. The basic flow is excited by a small sinusoidal modulation of the nozzle exit velocity. The phase-averaging technique is used to study the behavior of vortex structures in the jet, specifically rolling-up, pairing, and interaction with the wall. The jet is found to be sensitive to excitation in the frequency range characterized by a Strouhal number, St e = f e D / U, from 0.3 to 3. Different flow regimes are identified in the excited impinging jet: a periodic flow regime with the same frequency as the excitation, a regime with a frequency corresponding to a subharmonic of the excitation frequency, a regime alternating between these two frequencies, and two border regimes with more complicated behavior. The low-frequency excitation leads to the formation of vortices, which are larger than those occurring in an unexcited jet. Consequently, the near-wall velocity fluctuations are enhanced and the unsteady flow separation induced by vortex impingement is more pronounced. By contrast, excitation at higher frequencies, characterized by value 0.017 of a Strouhal number based on the shear layer thickness ,S t = f e / U, leads to the roll-up of small vortices and hence to the attenuation of the near-wall velocity fluctuations. In this case, the flow separation is suppressed because the small vortices are unable to induce it. Finally, the vortex merging process is sensitive to the double-frequency excitation with subharmonic components. For example, a combined excitation at St e = 1.60 and 0.53 leads to the merging of three vortices.
Synchronization and dynamics of the axisymmetrically excited jet in crossflow
Physical Review Fluids
This experimental study explored the effects of various axisymmetric temporal excitation waveforms on gaseous transverse jet synchronization and dynamical characteristics, vorticity evolution, and molecular mixing. The study focused on the naturally absolutely unstable transverse jet, with a jet-to-crossflow momentum flux ratio J = 6, employing simultaneous acetone planar laser-induced fluorescence imaging and stereo particle image velocimetry, enabling new perspectives on the nature of jet control and characteristic signatures associated with optimization. Jet excitation with sinusoidal, square-wave, and double-pulse forcing waveforms was examined, the latter two created with differing numbers of underlying Fourier timescales, and the number of Fourier modes was shown to affect the degree of lock-in of the jet's upstream shear layer to the applied excitation, consistent with an analogous Van der Pol oscillator model. Proper orthogonal decomposition (POD) analysis of the jet's evolving velocity field enabled comparisons of the most energetic mode structures for different excitation conditions and, more significantly, comparisons of various phase portraits associated with three-dimensional plots of the three most energetic POD mode coefficients. Phase portraits revealed significantly different topologies for different excitation conditions, ranging from fairly simple periodic (circular) shapes for sinusoidal excitation to more complex single and multiple loop trajectories for double-pulse forcing with varying degrees of temporal spacing between slow and fast pulses. Subsequent quantification of molecular mixing via acetone PLIF showed that the simpler phase portrait topologies tended to have improved molecular mixing as compared with more complex shapes, but all cases with excitation improved mixing over the unforced transverse jet.
Unstable jet-edge interaction. Part 2. Multiple-frequency pressure fields
Journal of Fluid Mechanics, 1986
In Part 1 of this investigation we addressed the instantaneous pressure field at a leading-edge due to single frequency jet-edge interactions ; here we consider edge pressure fields and associated vortex interaction patterns at the edge having a number of (at least six) well-defined spectral components. Each of the spectral components is present along the entire extent of the approach shear layer upstream of the edge; the relative amplitudes of these components are preserved in the conversion process from the free (approach) shear layer to the surface pressure field, the key link being complex, but ordered patterns of vortex interaction at the edge. Moreover, the predominant spectral components can be reasoned on the basis of these visualized vortex interactions by considering the vortex array in the incident shear layer.
Journal of Fluid Mechanics, 1986
The fundamental physics of unstable-shear-layer/leading-edge interactions is investigated experimentally in a recirculating water-channel system with a planar nozzle insert at Re = 600. Primary and secondary vortex formation and interaction in the leading-edge region are studied by means of LDA and flow visualization and related to phase and amplitude variations of pressure measurements obtained with high-sensitivity transducers mounted on and near the leading edge. Results for shear layers with a single frequency and for layers with at least six discrete components are presented in extensive graphs, diagrams, and visualization sketches and characterized in detail; and their implications for leading-edge noise radiation and the formation of turbulent boundary layers are considered.
Hydrodynamics During the Transient Evolution of Open Jet Flows from/to Wall Attached Jets
Flow, Turbulence and Combustion, 2016
Swirl stabilized flows are the most widely deployed technology used to stabilize gas turbine combustion systems. However, there are some coherent structures that appear in these flows close to the nozzle whose occurrence and stability are still poorly understood during transition. The external recirculation zone and the Precessing Vortex Core to/from the Coanda effect are some of them. Thus, in this paper the transition of an Open Jet Flow-Medium Swirl flow pattern to/from a Coanda jet flow is studied using various geometries at a fixed Swirl number. Phase Locked Stereo Particle Image Velocimetry and High Speed Photography experiments were conducted to determine fundamental characteristics of the phenomenon. It was observed that the coherent structures in the field experience a complete annihilation during transition, with no dependency between the structures formed in each of the flow states. Moreover, transition occurs at a particular normalized step size whilst some acoustic shifts in the frequencies of the system were noticed, a phenomenon related to the strength of the vortical structures and vortices convection. It is concluded that a transient, precessing, Coanda Vortex Breakdown is formed, changing flow dynamics. The structure progresses to a less coherent Trapped Vortex between the two states. During the phenomenon there are different interactions between structures such as the Central Recirculation Zone, the High Momentum Flow Region and the Precessing Vortex Core that were also documented.
Vortex pairing in a circular jet under controlled excitation. Part 1. General jet response
Journal of Fluid Mechanics, 1980
Hot-wire and flow-visualization studies have been carried out in three air jets subjected to pure-tone acoustic excitation, and the instability, vortex roll-up and transition as well as jet response to the controlled excitation have been investigated. The centreline fluctuation intensity can be enhanced by inducing stable vortex pairing to a level much higher than even that a t the 'preferred mode', but can also be suppressed below the unexcited level under certain conditions of excitation. The conditions most favourable to vortex pairing were determined as a function of the excitation Strouhal number, the Reynolds number (Re,), and the initial shear-layer state, i.e. laminar or turbulent. It is shown that the rolled-up vortex rings undergo pairing under two distinct conditions of excitation: 'the shear layer mode' when the Strouhal number based on the initial shear-layer momentum thickness (Xt,) is about 0.012, and 'the jet column mode' when the Strouhal number based on the jet diameter (Xt,) is about 0.85. The former involves pairing of the near-exit thin vortex rings when the initial boundary layer is laminar, irrespective of the value of St,. The latter involves pairing of the thick vortex rings a t x / D r 1.75, irrespective of St, or whether the initial boundary layer is laminar or turbulent. For laminar exit boundary layer, pairing is found to be stable, i.e., occurring regularly in space and time, for R e , < 5 x lo4, but becomes intermittent with increasing Re, or fluctuation intensity in the initial boundary layer.
Vortical structures in a laminar V-notched indeterminate-origin jet
Physics of Fluids, 2005
A flow visualization investigation using dye-injection and laser-induced fluorescence techniques has been carried out to understand the vortex dynamics resulting from a Vnotched indeterminate-origin jet with two peaks and two troughs. The laminar jet was studied under forcing and non-forcing conditions to investigate the resultant dynamics of coherent large-and small-scale flow structures. Present experimental observations indicated that the effects of the nozzle peaks and troughs differ from those reported previously. Instead of the peaks producing streamwise vortex-pairs which spread outwards into the ambient fluid and the troughs generating similar vortex-pairs but entrain ambient fluid into the jet flows as indicated by earlier studies, the present experimental observations showed that both peaks and troughs produce outward-spreading streamwise vortex-pairs. Laser cross-sections further showed that the subsequent formation of azimuthal ring-vortices causes these streamwise vortex-pairs to be entrained. This entrainment causes the streamwise vortex-pairs to "rollup" together with the ring vortices, leading to intense flow interactions between them.
The role of the intense vorticity structures in the turbulent structure of the jet edge
Advances in Turbulence XII - Proceedings of the 12th EUROMECH European Turbulence Conference, 2009
The characteristics of the intense vorticity structures (IVSs) near the turbulent/nonturbulent (T/NT) interface separating the turbulent and the irrotational flow regions are analysed using a direct numerical simulation (DNS) of a turbulent plane jet. The T/NT interface is defined by the radius of the large vorticity structures (LVSs) bordering the jet edge, while the IVSs arise only at a depth of about 5η from the T/NT interface, where η is the Kolmogorov micro-scale. Deep inside the jet shear layer the characteristics of the IVSs are similar to the IVSs found in many other flows: the mean radius, tangential velocity and circulation Reynolds number are R/η ≈ 4.6, u 0 /u ≈ 0.8, and Re Γ /Re 1/2 λ ≈ 28, where u 0 , and Re λ are the root mean square of the velocity fluctuations and the Reynolds number based on the Taylor micro-scale, respectively. Moreover, as in forced isotropic turbulence the IVSs inside the jet are well described by the Burgers vortex model, where the vortex core radius is stable due to a balance between the competing effects of axial vorticity production and viscous diffusion. Statistics conditioned on the distance from the T/NT interface are used to analyse the effect of the T/NT interface on the geometry and dynamics of the IVSs and show that the mean radius R, tangential velocity u 0 and circulation Γ of the IVSs increase as the T/NT interface is approached, while the vorticity norm |ω| stays approximately constant. Specifically R, u 0 and Γ exhibit maxima at a distance of roughly one Taylor micro-scale from the T/NT interface, before decreasing as the T/NT is approached. Analysis of the dynamics of the IVS shows that this is caused by a sharp decrease in the axial stretching rate acting on the axis of the IVSs near the jet edge. Unlike the IVSs deep inside the shear layer, there is a small predominance of vortex diffusion over stretching for the IVSs near the T/NT interface implying that the core of these structures is not stable i.e. it will tend to grow in time. Nevertheless the Burgers vortex model can still be considered to be a good representation for the IVSs near the jet edge, although it is not as accurate as for the IVSs deep inside the jet shear layer, since the observed magnitude of this imbalance is relatively small.