Mean Swirl Research Articles

Nonlinear interaction between a precessing vortex core and acoustic oscillations in a turbulent swirling flame

The interaction of a helical mode with acoustic oscillations is studied experimentally in a turbulent swirl-stabilized premixed flame. In addition to a precessing vortex core (PVC), the helical mode features perturbations in the outer shear layer of the burner flow. Measurements of the acoustic pressure, unsteady velocity field and flame emission are made in different regimes including self-sustained combustion oscillations and stable regimes with and without acoustic forcing. The acoustic oscillation and the helical mode create a pronounced rotating heat release rate perturbation at a frequency corresponding to the difference of the frequencies of the two individual mechanisms. Measurements over a wide range of operating conditions for different flow rates and equivalence ratios show that while the helical mode is always present, with a constant Strouhal number, self-excited thermoacoustic oscillations exist only in a narrow region. The interaction can be observed also in cases of thermoacoustically stable conditions when external acoustic modulation is applied to the system. The evolution of the helical mode with the forcing amplitude is examined. High-speed imaging from the downstream side of the combustor demonstrates that the heat release rate perturbation associated with the nonlinear interaction of the helical mode and the acoustic oscillations produces a ”yin and yang” -type pattern rotating with the interaction frequency in the direction of the mean swirl. At unstable conditions, the oscillation amplitude associated with the interaction is found to be significantly stronger in the heat release rate than in the velocity signal, indicating that the nonlinear interaction primarily occurs in the flame response and not in the aerodynamic field. The latter is, however, generally possible as is demonstrated under non-reacting conditions with acoustic forcing. Based on a second-order analysis of the G-equation, it is shown that the nonlinear flame dynamics necessarily generate the observed interaction component if the flame is simultaneously perturbed by a helical mode and acoustic oscillations.

Flow field development in a direct injection diesel engine with different manifolds

This paper reports a study on the effect of helical, spiral, and helical-spiral combination manifold configuration on air motion and turbulence inside the cylinder of a Direct Injection (DI) diesel engine motored at 3000 rpm. Three-dimensional model of the manifolds and the cylinder is created and meshed using the pre-processor GAMBIT. The flow characteristics of these engine manifolds are examined under transient conditions using Computational Fluid Dynamics (CFD) code STAR-CD. The predicted CFD results of mean swirl velocity of the engine at different locations inside the combustion chamber at the end of compression stroke are compared with experimental results available in the literature. We also compared the volumetric efficiency of the modeled helical manifold. The results obtained showed reasonably good agreement with the measured data given in the literature. Further, this paper discusses the predicted flow structure, swirl velocity and variation of turbulent energy inside the cylinder with different manifold. Comparisons of volumetric efficiency with different manifold configuration at 3000 rpm speed are also presented. The turbulence is modeled using RNG k-e model. It is observed that helical-spiral manifold gives the maximum swirl ratio inside the cylinder than helical manifold. But volumetric efficiency observed is less for helical-spiral manifold engine. Swirl inside the engine is important for diesel engine. Hence, for better performance a helical-spiral inlet manifold configuration is recommended. Keywords: Flow structure, spiral manifold, helical manifold, helical-spiral combined manifold, turbulence, swirl, CFD, Diesel engine

Investigation of the turbulent swirl flows in a conical diffuser

Results of the theoretical and experimental investigations of the turbulent mean swirl flows characteristics change along straight conical diffuser of incompressible fluid (air) are presented in this paper. The main swirl flow characteristics review is given. In addition: the specific swirl flow energy, the energy loss, the mean circulation, the swirl flow parameter, the ratio between the swirl and axial flow loss coefficients change along the diffuser are presented. Among other values: the Boussinesq number, outlet Coriolis coefficient and swirl flow loss coefficient dependences on inlet swirl flow parameter are also given. The swirl flow specific energy and outlet Coriolis coefficient calculation procedure are presented in this paper, as well as experimental test bed and measuring procedures. The swirl flow fields were induced by the axial fan impeller. Various swirl parameters were achieved by the impeller openings and rotational speeds.

Noise radiation from a ducted rotor in a swirling-translating flow

This paper investigates the noise radiation produced by a rotor inside a duct, which is convected by a swirling-translating mean flow. The study is based on an extension of Gennaretti's and Morino's boundary element method to the frequency domain for scattering problems in conjunction with a spinning rotor source model in the presence of a swirl flow. The proposed formulation is validated against exact solutions and is further used to investigate the effects of the translating flow Mach number and swirling flow angular velocity on noise radiation to the far field. The scattered sound is highly affected by the convecting mean flow. The modal content of the scattered field increases when increasing the translating flow Mach number, while a swirling flow leads to a reduction of the mode propagation, if co-rotating with respect to the azimuthal order of the spinning source, or an increase of the modal content, if counter-rotating with respect to the source. In general, the mean translating flow moves the main lobes of the directivity patterns downstream, while in some cases the mean swirling flow neglects this effect and the downstream lobe is completely shifted.

Alignment of Hurricane-like Vortices on f and β Planes

Abstract A nonlinear, two-layer, vortex-tracking semispectral model (i.e., Fourier transformed in azimuth only) is used to study the evolution of dry, but otherwise hurricane-like, initially tilted vortices in quiescent surroundings on f and β planes. The tilt projects onto vorticity asymmetries that are dynamically vortex Rossby waves. Since the swirling wind in the principal mean vortex used here decays exponentially outside the eyewall, it has an initial potential vorticity (PV) minimum. The resulting reversal of PV gradient meets the necessary condition for inflectional (i.e., barotropic or baroclinic) instability. Thus, the vortex may be inflectionally stable or unstable. On an f plane, the tilt precesses relatively slowly because the critical radius, where the phase speeds of the waves match the mean swirling flow, is far from the center. An alternative Gaussian-like PV monopole that has a monotonic outward decrease of PV is stable to inflectional instability. It has a smaller critical radius and rapid tilt precession. Generally, vortices with fast tilt precession are more stable, as are stronger vortices in higher latitudes. On a β plane, the interaction between the symmetric vortex and the planetary PV gradient induces β gyres that push the vortex poleward and westward. The interaction between the β gyres and the planetary PV gradient may either create a PV minimum or intensify a minimum inherited from the initial condition. Thus, the nonlinear β effect reduces the ability of the vortex to recover from initial tilt, relative to the same vortex on an f plane. This result contrasts with previous studies of barotropic vortices on f planes, where the linear and nonlinear solutions were nearly identical.

Development of a new swirl-measurement method for an engine cylinder head by automating the swirl-measuring process

Traditional swirl-measurement equipment that adopts a paddle wheel is manually operated to measure the swirl intensity generated from a helical port in an engine cylinder head. The conventional equipment was modified to operate automatically using a step motor to adjust the valve lift and an additional step motor to control the surge tank pressure. The automatic swirl-measuring equipment was operated in either steady or quasi-steady flow conditions. The surge tank pressure was controlled by automatically opening or closing a bypass valve using the step motor when the swirl flow was measured in a steady flow whereas, when the swirl flow was in a quasi-steady flow, the surge tank pressure varied naturally adapting to the valve lift in the conditions of closing the bypass valve. Several sensor signals, such as the paddle speed, differential pressures at the flow nozzle and surge tank, and intake air temperature imperative to the swirl ratio calculation, were measured and recorded automatically using a personal computer and data acquisition systems. The value of the mean swirl ratio from the cylinder head port showed nearly identical values of 2.0 in both the steady and the quasi-steady flow conditions. Compared with the conventional swirl system, the time required for 1 cycle of the swirl ratio measurement for a port in the cylinder head was drastically reduced by adopting the automatic swirl-measuring system. Moreover, in the automatic swirl-measuring system, the measuring time in the quasi-steady flow condition was less than that in the steady condition, because the target value of the surge tank pressure need not be controlled.

Dynamic behavior of a freely-propagating turbulent premixed flame under global stretch rate oscillations

Dynamic features of a freely propagating turbulent premixed flame under global stretch rate oscillations were investigated by utilizing a jet-type low-swirl burner equipped with a high-speed valve on the swirl jet line. The bulk flow velocity, equivalence ratio and the nominal mean swirl number were 5 m/s, 0.80 and 1.23, respectively. Seven velocity forcing amplitudes, from 0.09 to 0.55, were examined with a single forcing frequency of 50 Hz. Three kinds of optical measurements, OH-PLIF, OH ∗ chemiluminescence and PIV, were conducted. All the data were measured or post-processed in a phase-locked manner to obtain phase-resolved information. The global transverse stretch rate showed in-phase oscillations centering around 60 (1/s). The oscillation amplitude of the stretch rate grew with the increment of the forcing amplitude. The turbulent flame structure in the core flow region varied largely in axial direction in response to the flowfield oscillations. The flame brush thickness and the flame surface area oscillated with a phase shift to the stretch rate oscillations. These two properties showed a maximum and minimum values in the increasing and decreasing stretch periods, respectively, for all the forcing amplitudes. Despite large variations in flame brush thickness at different phase angles, the normalized profiles collapse onto a consistent curve. This suggests that the self-similarity sustains in this dynamic flame. The global OH ∗ fluctuation response (i.e. response of global heat-release rate fluctuation) showed a linear dependency to the forcing velocity oscillation amplitudes. The flame surface area fluctuation response showed a linear tendency as well with a slope similar to that of the global OH ∗ fluctuation. This indicated that the flame surface area variations play a critical role in the global flame response.

Vortices, Complex Flows and Inertial Particles

The properties of vortical structures at high Reynolds number in uniform flows and near rigid boundaries are reviewed. New properties are derived by analysing the dynamics of the main flow features and the related integral constraints, including the relations between mean swirl and bulk speed, the relative level of internal fluctuations to bulk properties, and connections between the steadiness and topology of the structures. A crucial property that determines energy dissipation and the transport of inertial particles (with finite fall speed) is the variation across the structure of the ratio of the mean strain rate (Σ) to the mean vorticity (Ω). It is shown how, once such particles are entrained into the vortical regions of a coherent structure, they can be transported over significant distances even as the vortices grow and their internal structure is distorted by internal turbulence, swirling motions and the presence of rigid boundaries. However if the vortex is strongly distorted by a straining motion so that Σ is greater than Ω, the entrained particles are ejected quite rapidly. These mechanisms are consistent with previous studies of entrained and sedimenting particles in disperse two phase flows over flat surfaces, and over bluff obstacles and dunes. They are also tested in more detail here through laboratory observations and measurements of 50–200-μm particles entrained into circular and non-circular vortices moving first into still air and then onto rigid surfaces placed parallel and perpendicular to the direction of motion of the vortices.

The use of micro-orifice nozzles and swirl in a small HSDI engine operating at a late split-injection LTC regime

This paper describes the investigation of low-temperature combustion (LTC) in a small single-cylinder research high-speed direct injection (HSDI) diesel engine. This engine is representative of passenger car turbocharged diesel engines equipped with a particulate trap and an oxidation catalyst. A medium-load operation mode has been evaluated where the diesel fuel is injected in two injection events close to top dead centre, and high levels of exhaust gas recirculation are used (near stoichiometric air-fuel ratios). The combustion is characterized by extremely low emission of nitrogen oxides (of the order of 10–15 parts per million) and low combustion noise. Three injector nozzles have been tested with 6, 12, and 18 orifices. The orifice diameters are in the range 70–120 μM and have been chosen to obtain injector nozzles with the same hydraulic flow. All three nozzles have been tested at a condition without swirl and at a mean swirl number of 1.7. The effects of the nozzle orifice number and swirl have been investigated by means of analysis of fuel spray, combustion, and emissions. It was found that when the number of orifices is increased, the rate of heat release during the first half of combustion is enhanced. However, during the second part of the combustion, interactions between sprays worsen the air-fuel mixing process and reduce the rate of heat release. Swirl was found to amplify the effects of spray interactions. While normally swirl speeds up the combustion in the second half of the combustion, the opposite happens when spray-spray interaction occurs.

Rotor-Stator Interaction Noise in Swirling Flow: Stator Sweep and Lean Effects

An asymptotic technique in the limit of large blade and vane number is employed to study the interaction in mean swirling flow of an unsteady rotor wake with a stator row downstream. The effect of various stator characteristics, including stator sweep, lean and number of vanes, on the tone noise radiated upstream is investigated. The important effects of mean swirl are included in both the rotor wake evolution and the construction of the upstream-radiated sound field. Specifically, we construct the first term in the series expansion of the upstream unsteady pressure in inverse powers of the blade count. We show that sweep and lean in suitable combination (positive sweep and positive lean) can lead to this first term becoming zero, so that the upstream radiation is then approximated by the next (smaller) term in the series, and is therefore necessarily reduced. A simple criterion, based on the form of the incident wake at the stator leading edge, which leads to this first term becoming zero, is identified. This provides a very efficient method for assessing the impact of sweep and lean over a wide range of operating conditions. The analytical nature of the model allows the physical mechanism behind the effectiveness of sweep and lean to be identified.

Transient growth and rotor–stator interaction noise in mean swirling duct flow

The phenomenon of spatial transient growth in swirling duct flow with application to aeroengine duct acoustics is investigated. In a typical aeroengine a region of swirling flow exists between the rotor and the downstream stator. The possibility for optimal transient growth of disturbance energy, and of a norm related to the acoustic power radiated back upstream from the stator is shown. Significant transient growth of both energy and noise norms can be observed over length scales relevant to aeroengine size, and over a wide range of parameter values such as the degree of mean swirl, azimuthal wavenumber and frequency. The optimal disturbances are found to have significant three dimensionality. The growth in energy results from increases in the azimuthal and radial components of the disturbance velocity field as it propagates downstream, whilst increases in the azimuthal velocity bring about non-modal growth of the noise measure.

Effect of mean entropy on unsteady disturbance propagation in a slowly varying duct with mean swirling flow

The propagation of acoustic disturbances in a slowly-varying duct with mean swirling flow and non-uniform mean entropy is considered. This is representative of the region of swirling flow downstream of the rotor in aeroengine applications where variations in mean entropy can be significant. The duct is assumed to vary slowly in axial cross section and a consistent multiple-scales solution is derived. The variation in axial wavenumber and amplitude of the duct modes is determined as part of the solution. Comparisons are made between the cases of isentropic and non-isentropic flow. The cut-on/cut-off characteristics of a given mode are altered by any form of non-uniform mean entropy, with modes pushed closer to cut-off resulting in larger variations in amplitude. The greatest departure from the baseline isentropic result occurs for a positive entropy gradient. Increasing the swirl velocity at the duct inlet leads to increasing levels of mean entropy.

Acoustic scattering in a duct with mean swirling flow

In this paper we consider the diffraction of waves by a sharp edge in three-dimensional flow with non-zero mean vorticity. This is an extension of the famous Sommerfeld problem of the diffraction of waves by a sharp edge in quiescent conditions. The precise problem concerns an infinitely long annular circular cylinder, which contains a concentric semi-infinite circular cylinder which acts as a splitter. The mean flow has both axial and swirl components, and cases in which the splitter is arranged with either a leading edge or a trailing edge relative to the axial flow are considered. This is a model of a number of practical situations in the aeroengine context. We treat both sonic and nearly-convected incident disturbances, and two regimes are considered; one in which the azimuthal order, $m$ , of the incident waves is $O(1)$ , and a second in which $m\,{\gg}\,1$ . A solution for $m\,{=}\,O(1)$ in the case of rigid-body swirl is found using the Wiener–Hopf technique, and special care is needed to handle the infinite accumulation of scattered nearly-convected modes which results from the presence of the mean vorticity. Simplification in the limit $m\,{\gg}\,1$ then allows us to consider more general swirl distributions. A number of effects arise due to the presence of mean vorticity. This includes the generation of sound at a trailing edge due to the scattering of a nearly-convected disturbance, which is to be contrasted with the way in which a convected gust passes a trailing edge silently in uniform mean flow.

Upstream-radiated rotor–stator interaction noise in mean swirling flow

A major component of the noise in modern aeroengines is rotor–stator interaction noise generated when the wake from the rotating fan impinges on a stator row downstream. An analytically based model for the prediction of upstream-radiated rotor–stator interaction noise is described, and includes the important effect of mean swirling flow on both the rotor wake evolution and the acoustic response. The analytic nature of the model allows for the inclusion of all wake harmonics and enables the response at all blade passing frequencies to be determined.An asymptotic analysis based on large rotor blade number is used to model the evolution of the rotor wake downstream in a cylindrical duct carrying mean swirling flow. The equations governing the axial evolution of the wake simplify to three coupled first-order differential equations in the interior, while close to the duct walls, a boundary-layer correction is required in order to satisfy the impermeability conditions at the boundaries. At the stator location, the wake is used as input into a local linear cascade model at each radius. The interaction of each wake harmonic gives rise to acoustic waves of multiple azimuthal order which contribute to the pressure field radiated back upstream. This enables the total acoustic response to be determined in terms of cylindrical duct modes in mean swirling flow.The effect of stator blade geometry (thickness, camber, angle of attack) and rotor–stator separation on the total upstream-radiated noise is determined. Blade geometry is shown to have a significant effect on the noise generated, and increasing the rotor–stator gap can lead to large reductions in noise levels. Asymptotic treatment of the acoustic field, based on large azimuthal order, is also considered and used to identify the dominant contributions to the total pressure field resulting from the rotor–stator interaction. The ray structure of the acoustic modes in swirl is shown to be very different in some cases from that in uniform flow.

The effect of swirl on combustion and exhaust emissions in heavy-duty diesel engines

The effect of in-cylinder air swirl on diesel combustion and exhaust emissions has been studied in a heavy-duty diesel engine. A 1.8 L single-cylinder diesel engine fitted with a device to modify the swirl rate was used. This device allowed variation of the mean swirl ratio, a parameter related to the ratio of the angular velocity of the air within the combustion chamber and crankshaft angular rotation, from 0 to 5.33. Although no single optimum swirl level has been found, the results presented here give an insight into swirl effects on diesel engine combustion and exhaust emissions over a wide range of engine operating conditions.

Turbulent, swirling flow over an axisymmetric, constant radius surface

LDV measurements are presented of isothermal flow in a full-scale cold-model industrial burner. The burner’s quarl is similar to the exit shape of a horned instrument: a curved, diverging surface over which air flows. Measurements were obtained in a turbulent swirling flow in the near flow field both with and without the quarl in place, all at a Reynolds number of about 71,000 and mean swirl number of 0.89. Radial profiles of the axial, radial, and tangential components of mean and fluctuation velocities and cross-correlations are obtained at several axial locations. Structural parameters are considered. The data suggest that when the quarl is in place the flow consist of inner and outer regions. They are characterised by a constant negative axial velocity in the central core and a wall jet that develops along the wall.

Analysis of wake vortex decay mechanisms in the atmosphere

Results of high-resolution numerical simulations of aircraft wake vortex evolution and decay in different regimes and atmospheric conditions are presented. The different cases comprise (i) the near field interaction of a trailing vortex with an exhaust jet, (ii) the evolution of single vortices and counter-rotating vortex pairs in homogeneous isotropic turbulence, as well as (iii) the decay of wake vortices in a turbulent stably stratified atmosphere, and (iv) in a weakly turbulent sheared environment. The different cases are used to analyse common aspects of vortex dynamics and decay mechanisms. In all scenarios the formation of coherent secondary vorticity structures that enclose the primary vortices is observed. These secondary vorticity structures deform and weaken the primary vortices and in some cases lead to rapid vortex decay. It is shown that the mean swirling flow effectively rearranges and intensifies any secondary vorticity by tilting and stretching. The secondary vorticity may either originate from the turbine jet, ambient turbulence or may be produced baroclinically. Based on the observed phenomena, eleven postulates are established that pinpoint fundamental aspects of the observed decay mechanisms.

Interchangeability of vortex-breakdown types

In order to investigate the connection between the bubble and the spiral form of vortex breakdown, experiments were conducted: an external disturbance in the form of an azimuthally spinning waveform was imposed in a pipe. The azimuthal wave number was varied by adjusting the phase difference among four oscillating pistons mounted circumferentially on the pipe. By imposing a disturbance of zero azimuthal wave number, a spiral was transformed into a bubble, and this occurred only for selective piston frequencies; the vortex breakdown which altered from the spiral to the bubble moved upstream, where it remained as a bubble as long as the external disturbance remained. Once the disturbance was removed, the bubble returned to a spiral. By imposing a disturbance of azimuthal wave number +1 (the first circumferential mode rotating in the same direction as the mean swirl), a bubble was transformed into a spiral for selective piston frequencies, and the spiral moved downstream. These preferred frequencies were found to be the same as the unexcited frequencies observed in the spiral in its natural state. As long as the external disturbance was imposed, the breakdown altered from the bubble to the spiral remained as a spiral; once the disturbance was removed, the spiral reverted to a bubble. By imposing a disturbance with azimuthal wave number –1 (the first circumferential mode rotating in the opposite direction to the mean swirl), no change was detected in either a bubble or a spiral. By imposing a disturbance with azimuthal wave number 2 (the second circumferential mode), for selective piston frequencies a bubble was transformed into what appears to be the so-called two-tailed type. Thus, it appears that hydrodynamic instability plays a role in interchanging vortex breakdown types, and a comparison with available stability theories is discussed.

CFD simulation of precession in sudden pipe expansion flows with low inlet swirl

In this paper, three-dimensional, time-dependent calculations are carried out using the finite volume CFD code CFX4 and the VLES approach with standard k– ε model to simulate the turbulent swirl flow in an axisymmetric sudden expansion with an expansion ratio of 5.0 for a Reynolds number of 10 5. This flow is unstable over the entire swirl number range considered between 0 and 0.48, and a large-scale coherent structure is found to precess about the centerline. Compared with the unswirled case, inclusion of a slight inlet swirl (swirl number below 0.23) can reduce the precession speed, cause the precession to be against the mean swirl and suppress the flapping motion. Several modes of precession are predicted as the swirl intensity increases, in which the precession, as well as the spiral structure, reverses direction. Accompanying the transition between different modes, abrupt changes in precession frequency are also experienced. Grid sensitivity and comparison with smaller expansion ratio data are also discussed.

Propagation of unsteady disturbances in a slowly varying duct with mean swirling flow

The propagation of unsteady disturbances in a slowly varying cylindrical duct carrying mean swirling flow is described. A consistent multiple-scales solution for the mean flow and disturbance is derived, and the effect of finite-impedance boundaries on the propagation of disturbances in mean swirling flow is also addressed.Two degrees of mean swirl are considered: first the case when the swirl velocity is of the same order as the axial velocity, which is applicable to turbomachinery flow behind a rotor stage; secondly a small swirl approximation, where the swirl velocity is of the same order as the axial slope of the duct walls, which is relevant to the flow downstream of the stator in a turbofan engine duct.The presence of mean vorticity couples the acoustic and vorticity equations and the associated eigenvalue problem is not self-adjoint as it is for irrotational mean flow. In order to obtain a secularity condition, which determines the amplitude variation along the duct, an adjoint solution for the coupled system of equations is derived. The solution breaks down at a turning point where a mode changes from cut on to cut off. Analysis in this region shows that the amplitude here is governed by a form of Airy's equation, and that the effect of swirl is to introduce a small shift in the location of the turning point. The reflection coefficient at this corrected turning point is shown to be exp (iπ/2).The evolution of axial wavenumbers and cross-sectionally averaged amplitudes along the duct are calculated and comparisons made between the cases of zero mean swirl, small mean swirl and O(1) mean swirl. In a hard-walled duct it is found that small mean swirl only affects the phase of the amplitude, but O(1) mean swirl produces a much larger amplitude variation along the duct compared with a non-swirling mean flow. In a duct with finite-impedance walls, mean swirl has a large damping effect when the modes are co-rotating with the swirl. If the modes are counter-rotating then an upstream-propagating mode can be amplified compared to the no-swirl case, but a downstream-propagating mode remains more damped.