An exact irrotational solution for a hemispherically bounded cyclonic flowfield

Williams

2021

This work describes an inviscid solution for a cyclonic flowfield evolving in a hemispherical chamber configuration. In this context, a vigorously swirling motion is triggered by a gaseous stream that is introduced tangentially to the inner circumference of the chamber's equatorial plane. The resulting updraft spirals around while sweeping the chamber wall, reverses direction while approaching the headend, and then tunnels itself out through the inner core portion of the chamber. Our analysis proceeds from the Bragg–Hawthorne formulation, which proves effective in the treatment of steady, incompressible, and axisymmetric motions. Then using appropriate boundary conditions, we are able to obtain a closed-form expression for the Stokes streamfunction in both spherical and cylindrical coordinates. Other properties of interest are subsequently deduced, and these include the principal velocities and pressure distributions, vorticity, swirl intensity, helicity density, crossflow velocity, and the location of the axial mantle; the latter separates the outer annular updraft from the inner, centralized downdraft. Due in large part to the overarching spherical curvature, the cyclonic motion also exhibits a polar mantle across which the flow becomes entirely radial inward. Along this lower and shorter spherical interface, the polar velocity vanishes while switching angular direction. Interestingly, both polar and axial mantles coincide in the exit plane where the ideal outlet size, prescribed by the mantle position, is found to be approximately 70.7% of the chamber radius. We thus recover the same mantle fraction of the cyclonic flow analog in a right-cylindrical chamber where an essentially complex-lamellar motion is established.

A quasi complex-lamellar solution for a hemispherically bounded cyclonic flowfield

Williams

2021

Exact Beltramian solutions for hemispherically bounded cyclonic flowfields

Williams

2021

This study focuses on the development of two analytical models that describe the wall-bounded cyclonic flowfield in a hemispherical domain. The closed-form solutions that we pursue are motivated by the need to characterize the swirling bidirectional motion engendered in an upper stage thrust chamber, namely, the VR35K-A VORTEX® engine, conceived and developed by Sierra Nevada Corporation. Our analysis proceeds from the Bragg–Hawthorne formulation, which is quite effective in the treatment of steady, inviscid, and axisymmetric flows. In this work, we show that two rotational solutions may be derived for particular specifications of the stagnation head and tangential angular momentum expressions that appear in the Bragg–Hawthorne equation. Then, with the parental streamfunctions in hand, other properties of interest are deduced and these include the main velocity and pressure variations, vorticities, crossflow velocities, extensional and shearing strain rates, virtual energy dissipation rates, and both axial and polar mantle distributions; the latter consist of pairs of rotating, non-translating interfacial layers, separating the so-called inner and outer bidirectional and bipolar regions, respectively. More specifically, two Beltramian solutions are identified with mantles that appear at 50% and 61.06% of the chamber radius, respectively. By matching the outlet radius of the chamber to the mantle location in the equatorial plane, the outflow is permitted to exit the chamber seamlessly. In both models, the axial and radial velocities vary linearly with the injection speed and a characteristic inflow parameter consisting of a geometric ratio of the inlet area and the chamber radius squared.

Effects of nozzle inlet size and curvature on the flow development in a bidirectional vortex chamber

Sharma

2021

A 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.