An official website of the United States government
Experiments were conducted in a large-scale Ward-type tornado simulator to study tornado-like vortices. Both flow velocities and the pressures at the surface beneath the vortices were measured. An interpretation of these measurements enabled an assessment of the mean flow field as well as the mean and fluctuating characteristics of the surface pressure deficit, which is a manifestation of the flow fluctuation aloft. An emphasis was placed on the effect of the aspect ratio of the tornado simulator on the characteristics of the simulated flow and the corresponding surface pressure deficit, especially the evolution of these characteristics due to the transition of the flow from a single-celled vortex to a two-celled vortex with increasing swirl ratio.
more »
« less
This content will become publicly available on May 1, 2026
Influence of swirl ratio on the structure and dynamics of tornado-like vortices
We investigated the flow dynamics of tornado-like vortices, examining the influence of swirl ratio, S, defined as the ratio of tangential to radial momentum at the vortex base, on their structural characteristics. Using a combination of particle image velocimetry (PIV) in a custom-built simulator and large-eddy simulations (LES), we analyzed vortex flows at swirl ratios of S=4.66, 1.25, and 0.33. The results demonstrate that vortex flow characteristics strongly depend on S, with improved agreement between experimental and numerical data when employing flow-based swirl ratio definitions. Vortex wandering was quantified in experiments, and corrections were applied to refine tangential and radial velocity profiles. At S=0.33 and 1.25 in experiments and S=1.25 and 4.66 in simulations, the vortex transitioned from a single-celled to a double-celled structure, with further evolution into multi-celled vortices at the highest swirl ratio, substantially modifying circulation patterns. Proper orthogonal decomposition (POD) characterized the coherent structures governing vortex dynamics and their dependence on swirl ratio, revealing distinct physical features associated with each vortex regime.
more »
« less
- Award ID(s):
- 2207115
- PAR ID:
- 10610999
- Publisher / Repository:
- AIP Publishing
- Date Published:
- Journal Name:
- Physics of Fluids
- Volume:
- 37
- Issue:
- 5
- ISSN:
- 1070-6631
- Format(s):
- Medium: X
- Sponsoring Org:
- National Science Foundation
More Like this
-
-
ABSTRACT We present the first observations of chromospheric swirls using the Hydrogen-alpha Rapid Dynamics camera and Rapid Oscillations in the Solar Atmosphere imaging instruments at the Dunn Solar Telescope. These vortices contribute to heating and dynamics across the solar atmosphere. We analyse the morphology and evolution of 34 swirls and their cospatial bright points (BPs) from the photosphere to the mid-chromosphere. To examine swirl–BP interactions and temporal behaviour, we use image segmentation, Fourier and spectral analysis, and local correlation tracking. The observed swirls have an average lifetime of 7.9 $$\pm$$ 5 min and diameter of 3.6 $$\pm$$ 1 Mm, with a positive correlation indicating smaller swirls tend to be short-lived. 76 per cent are associated with a compact BP appearing 12 s to 9 min after swirl formation. Swirl motion is also closely linked to their BP(s) global motions. The swirls exhibit a mean angular speed of 0.04 rad s$$^{-1}$$, radial speed of 17.7 km s$$^{-1}$$, and period of 180 s. We observe the formation of a spiral-shaped swirl driven by a BP interacting with a large photospheric vortex. The BP is dragged towards the vortex centre, after which the swirl forms. The BP undergoes changes in orientation and elongation that mirror the swirl’s chromospheric development. A time lag of $-42.5$ s between the sudden change in the BP’s orientation and the peak of the swirl’s intensity variation suggests torsional Alfvén waves may contribute to swirl evolution. Our results support a magnetic origin for swirls rooted in motions of photospheric BPs.more » « less
-
We study the emergence of precessing vortex core (PVC) oscillations in a swirling jet experiment. We vary the swirl intensity while keeping the net mass flow rate fixed using a radial-entry swirler with movable blades upstream of the jet exit. The swirl intensity is quantified in terms of a swirl number $$S$$ . Time-resolved velocity measurements in a radial–axial plane anchored at the jet exit for various $$S$$ values are obtained using stereoscopic particle image velocimetry. Spectral proper orthogonal decomposition and spatial cross-spectral analysis reveal the simultaneous emergence of a bubble-type vortex breakdown and a strong helical limit-cycle oscillation in the flow for $$S>S_{c}$$ where $$S_{c}=0.61$$ . The oscillation frequency, $$f_{PVC}$$ , and the square of the flow oscillation amplitudes vary linearly with $$S-S_{c}$$ . A solution for the coherent unsteady field accurate up to $$O(\unicode[STIX]{x1D716}^{3})$$ ( $$\unicode[STIX]{x1D716}\sim O((S-S_{c})^{1/2})$$ ) is determined from the nonlinear Navier–Stokes equations, using the method of multiple scales. We show that onset of bubble type vortex breakdown at $$S_{c}$$ , results in a marginally stable, helical linear global hydrodynamic mode. This results in the stable limit-cycle precession of the breakdown bubble. The variation of $$f_{LC}$$ with $$S-S_{c}$$ is determined from the Stuart–Landau equation associated with the PVC. Reasonable agreement with the corresponding experimental result is observed, despite the highly turbulent nature of the flow in the present experiment. Further, amplitude saturation results from the time-averaged distortion imposed on the flow by the PVC, suggesting that linear stability analysis may predict PVC characteristics for $$S>S_{c}$$ .more » « less
-
Effects of nozzle inlet size and curvature on the flow development in a bidirectional vortex chamberA finite-volume solver is used to describe the cyclonic motion in a cylindrical vortex chamber comprising eight tangential injectors and a variable nozzle size. The simulations are performed under steady, incompressible, and inviscid flow conditions with air as the working fluid. First, we apply a fine tetrahedral mesh to minimize cell skewness, particularly near injectors. Second, this mesh is converted into a polyhedral grid to improve convergence characteristics and precision. After achieving convergence, the velocity components are evaluated and compared to existing analytical solutions. We find that well-resolved numerical simulations can accurately predict the expected forced vortex behavior in the core region as well as the free vortex tail in the outer region. We also confirm that the swirl velocity remains axially invariant irrespective of the outlet radius. Similarly, we are able to ascertain that the axial and radial velocities embody the bidirectional nature of the motion. As for the computed pressure distribution, it is found to agree quite well with both theoretical formulations and experimental measurements of cyclone separators. Then using a parametric trade study, the effect of nozzle variations on the internal flow character, mantle structure, and recirculation zones is systematically investigated. Apart from the exit diameter, we find that the nozzle length and inlet curvature can substantially affect the internal flow development including the formation of backflow regions, recirculation zones, and mantle excursions. Finally, an empirical relation is constructed for the nozzle radius of curvature and shown to effectively suppress the emergence of recirculation and backflow regions.more » « less
-
Precessing vortex cores (PVC), arising from a global instability in swirling flows, can dramatically alter the dynamics of swirl-stabilized flames. Previous study of these instabilities has identified their frequencies and potential for interaction with the shear layer instabilities also present in swirling flows. In this work, we investigate the dynamics of precessing vortex cores at a range of swirl numbers and the impact that turbulence, which tends to increase with swirl number due to the increase in mean shear, has on the dynamics of this instability. This is particularly interesting as stability predictions have previously incorporated turbulence effects using an eddy viscosity model, which only captures the impact of turbulence on the base flow, not on the instantaneous dynamics of the PVC itself. Time-resolved experimental measurements of the three-component velocity field at ten swirl numbers show that at lower swirl numbers, the PVC is affected by turbulence through the presence of vortex jitter. With increasing swirl number, the PVC jitter decreases as the PVC strength increases. There is a critical swirl number below which jitter of the PVC vortex monotonically increases with increasing swirl number, and beyond which the jitter decreases, indicating that the strength of the PVC dominates over turbulent fluctuations at higher swirl numbers, despite the fact that the turbulence intensities continue to rise with increasing swirl number. Further, we use a nonlinear van der Pol oscillator model to explain the competition between the random turbulent fluctuations and coherent oscillations of the PVC. The results of this work indicate that while both the strength of the PVC and magnitude of turbulence intensity increase with increasing swirl number, there are defined regimes where each of them hold a stronger influence on the large-scale, coherent dynamics of the flow field.more » « less
