Debopam Das

On the Evolution of Counter Rotating Vortex Ring Formed Ahead of a Compressible Vortex Ring

Fig. 1 shows the formation and evolution of counter rotating vortex ring ahead of primary vortex ring generated at the open end of a shock tube, for shock Mach number (M) 1.7. Driver and driven section length of the shock tube are 165mm and 1200mm. The details of the experiments and visualization method are given in Ref. 1. Earlier visualization studies [2-4] using shadowgraph and schlieren do not show the clear structure of the counter rotating vortex ring and its complete evolution due to integral nature of the techniques. The strong embedded shock (Fig. 1e) present at this M is normal along the axis of the vortex ring and oblique near the vortex core [5]. Thus, the flow decelerates more along the axis compared to the outer portion, which causes formation of a strong shear layer that rolls up into a counter-rotating vortex ring. The counter rotating vortex ring rolls over the periphery of the primary ring and moves in the upstream direction (w.r.t to primary ring) due to its self induced velocity and interaction with primary ring. Finally it ejected into the trailing jet where it interacts with the shear layers of the trailing jet, loses its identity and become turbulent structure due to its opposite circulation of shear layer vortices.

A study of the counter rotating vortex rings interacting with the primary vortex ring in shock tube generated flows

The formation and evolution of counter rotating vortex rings (CRVRs) appearing in shock tube-generated flows at high shock Mach numbers (M) have been studied numerically by solving ax symmetric Navier–Stokes equations and compared with experiments. The AUSM + scheme is used for convective terms, and for time stepping a four-stage Runge–Kutta scheme is used. High-speed smoke flow visualizations and optical shadowgraph techniques are employed for verifying the numerical results. It is observed that the strong shear layer formed near the Mach disc in the axial region of the vortex ring plays a dominant role in CRVR formation. A series of CRVRs is formed for longer driver section and higher M as the shear layer persists for longer duration. The interaction of these CRVRs with the primary vortex and trailing jet vortices is studied for (i) different pressure-pulse durations at the open end keeping the amplitude constant, and (ii) varying pulse amplitude when the duration is fixed. Results are also presented comparing a high-amplitude case against a lower-amplitude one with a longer pulse duration. The maximum vorticity inside the first CRVR is found to be higher than the primary vortex ring during its formation.

Suppression of purely elastic instabilities in the torsional flow of viscoelastic fluid past a soft solid

Experiments are performed to explore the role of a soft, deformable solid layer on the purely elastic instability in the torsional flow of polymer solutions between two circular discs. The gel layer is placed on the stationary bottom plate of a rheometer, and the polymer solution is placed between the gel and the rotating top disc. The observed variation of viscosity with shear rate (or shear stress) is correlated with the presence or absence of purely elastic instability in the viscometric flow. Earlier work has shown that with increase in shear rate, the torsional flow of a polymer solution between rigid discs undergoes transition from the simple viscometric flow state to elastic turbulence via a sequence of instability modes. We combine rheological observations and flow visualization to show that the deformable solid has a profound effect on the stability of the torsional flow. In marked contrast to flow between rigid plates (where the fluid shows apparent shear-thickening at the onset of instability),...

On Evolution and Acoustic Characteristics of a Compressible Vortex Ring

Sound generated during formation of a compressible vortex ring at the open end of a shock tube and during its propagation is studied experimentally for shock Mach numbers of 1.28 to 1.61. The occurrence of different events such as primary ring formation, growth of the primary ring, secondary and tertiary vortices formation, and pinching-off are identified as predominant noise producing events. Wavelet analysis of the measured microphone signal identifies the time of occurrence of the above events which is verified using flow visualization pictures. The distributions of acoustic fluctuations are measured at a fixed distance from the exit center of the tube at different angular locations in horizontal diametrical plane and the directivity of the amplitude of sound pressures associated with the evolution of vortex ring is found. It is observed that the vortex rings evolution sound is dominant between 25° to 50° angle from the axis of the shock tube. Experiments with short driver section produce rings with small trailing jet. Sound generated during the initial formation of these rings, after the diffraction of the incident shock at the open end of the shock tube, is dominant than the vortex rings evolution sound. In shock tubes with larger driver section, the primary vortex ring is followed by a relatively longer trailing jet. In these cases, sound generated during formation of subsequent shear-layer vortices and their interaction with the trailing jet is also significant.

Unsteady Wake Characteristics of a Flapping Wing through 3D TR-PIV

AbstractThe unsteady wake characteristics of a butterfly-shaped wing undergoing flapping and feathering motions are studied. A 4-bar quick return mechanism is used to generate the flapping motion. To achieve considerable feathering motion, the leading edge of the wing is fixed to the flapping arm of the model, while the trailing edge of the wing is kept free to bend and deform out of weight and aerodynamic loading. The flapping frequency and wing size considered for this study cover rotational Reynolds numbers of 6,040–10,080 for a zero advance ratio. The three-dimensional (3D) (stereoscopic) time-resolved (TR) velocity field is measured to understand the dynamics of the lift and thrust generation. The conservation of momentum principle has been used to estimate the thrust force from the particle image velocimetry (PIV) results. The mean velocity profiles in the wake show the frequency dependence ejection of the trailing edge vortex, which is believed to be effectively used for thrust production and maneu...

Characteristics of Noise Produced during Impingement of a Compressible Vortex Ring on a Wall

Noise produced during normal impingement of a compressible vortex ring on a flat surface is studied in the shock-Mach number (M) range of 1.31 to 1.55. The compressible vortex ring is generated at the open end of a short driver section shock tube. The far-field noise is decomposed into three major components; (i) sound field due to formation and evolution of the vortex ring, (ii) reflected shock and vortex ring interaction noise and (iii) noise due to impingement of the ring on the wall. The impingement noise consists of fluctuating pressure due to deformation and stretching of the vortex ring, formation and growth of a secondary wall vortex ring, lifting-off of the primary-secondary vortex ring pair. All these events are identified using discrete wavelet transform (DWT) of the sound pressure signal and verified with the laser sheet based flow visualizations. Acoustics fluctuations measured at different angular location shows that the noise due to wall-vortex ring interaction is dominant at 10° ≤ θ ≤ 40°.

On the Wall Interaction of Compressible Vortex Rings and Associated Noise

Sound generated during the interaction of a compressible vortex ring with wall has been studied experimentally. Compressible vortex rings are generated from the open end of a short driver section shock tube for shock Mach numbers 1.31 and 1.55. The intensity of sound produced during the wall interaction of vortex ring is measured and compared with the sound generated from other phenomena such as the formation and evolution of vortex ring and shock-vortex interaction. Noise associated with the distinct phenomena during wall interaction such as vortex stretching, secondary-wall-vortex formation and its interaction with primary ring is identified using Discrete Wavelet Transform of measured acoustic pressure signal. The related flow field is verified with high speed flow visualization pictures. It has been observed that the noise generated during the interaction of vortex with wall is more than the noises generated during the evolution of vortex ring and shock-vortex interaction for higher shock Mach numbers. As the shock Mach number increases wall interaction noise becomes dominant and the acoustic pressure signatures become very sharp like shock signatures.

Experimental Investigation of the Acoustic Characteristics of Shock-Vortex Ring Interaction Process

The acoustic waves generated during the interaction of shock wave with a compressible vortex ring are studied experimentally. All experiments are performed in a short driver section shock tube with helium as a driver section gas for shock Mach numbers 1.30, and 1.55. The effect of different shock Mach numbers and reflected shock strength on the generation of acoustics waves is studied for different ring velocities. The far field noise generated during the interaction of the shock wave with the vortex ring is identified in addition to the sound generated during the evolution of vortex rings using Discrete Wavelet Transform. It has been found that the intensity of acoustic disturbance generated during the interaction of vortex ring with shock is dominant as compared with the sound generated during other key processes such as formation, jet interaction and pinching off noise. The intensity of acoustic fluctuation by shock vortex interaction is dominant in the direction of shock wave which also increases with Mach numbers.

Numerical visualization of shock tube-generated vortex---wall interaction using a fifth-order upwind scheme

Compressible Navier–Stokes equations are solved using a fifth-order upwind scheme in the AUSM+ framework to visualize a compressible vortex ring generated from a shock tube. The ring impinges on a wall kept near the open end of the tube. The vortex ring has an embedded shock, counter rotating vortex rings ahead of it and a number of small-scale shear layer vortices trailing behind. When this complex configuration impinges on a wall, wall vorticity is lifted and begins to interact with the complex system of vortices. The paper focusses on the features of the resulting flow field by visualizing them on increasingly finer grids. It is shown that though the different grids capture a fairly matching description of the initial turbulent vortex system that propagates towards the wall, small differences existing between them magnify with time. During vortex–wall interaction, some key experimentally observed features are identified on all the grids, but the details of the vortical structure look significantly different on different grids.Graphical Abstract

Characteristics of Embedded-Shock-Free Compressible Vortex Rings: A Detailed Study Using PIV

The present study focus on evolution of compressible vortex ring generated at the open end of a shock tube through accurate measurement of velocity field using Particle Image Velocimetry (PIV). To investigate the unsteady characteristics of embedded shock-free, low Mach number vortex rings, two cases (shock Mach numbers, M=1.27 and M=1.37) are considered for PIV measurements. Time-dependent variations of circulation, core and ring diameters, and ring velocity are calculated from the measured velocity field. Pinching-off process is investigated in detail for both cases. Formation time and the time of complete detachment of the vortex ring from the trailing jet are identified from the velocity and vorticity field. The ring formation is complete at about t * (=tU b /D)=1.75 and 1.65 for M=1.27 and 1.37, respectively, where t is time, U b is fluid velocity behind the shock at exit, and D is tube diameter. Complete detachment of the vortex ring from the trailing jet is observed at t ∗ = 2 and 2.9 for M=1.27 and 1.37, respectively.