Numerical predictions of the flow characteristics of subsonic jet emanating from corrugated lobed nozzle

Purpose The purpose of this study is to investigate the aerodynamic characteristics of subsonic jet emanating from corrugated lobed nozzle. Design/methodology/approach Numerical simulations of subsonic turbulent jets from corrugated lobed nozzles using shear stress transport k-ω turbulence model have been carried out. The analysis was carried out by varying parameters such as lobe length, lobe penetration and lobe count at a Mach number of 0.75. The numerical predictions of axial and radial variation of the mean axial velocity, u′u′ ¯ and v′v′ ¯ have been compared with experimental results of conventional round and chevron nozzles reported in the literature. Findings The centreline velocity at the exit of the corrugated lobed nozzle was found to be lower than the velocity at the outer edges of the nozzle. The predicted potential core length is lesser than the experimental results of the conventional round nozzle and hence the decay in centreline velocity is faster. The centreline velocity increases with the increase in lobe length and becomes more uniform at the exit. The potential core length increases with the increase in lobe count and decreases with the increase in lobe penetration. The turbulent kinetic energy region is narrower with early appearance of a stronger peak for higher lobe penetration. The centreline velocity degrades much faster in the corrugated nozzle than the chevron nozzle and the peak value of Reynolds stress appears in the vicinity of the nozzle exit. Practical implications The corrugated lobed nozzles are used for enhancing mixing without the thrust penalty inducing better acoustic benefits. Originality/value The prominent features of the corrugated lobed nozzle were obtained from the extensive study of variation of flow characteristics for different lobe parameters after making comparison with round and chevron nozzle, which paved the way to the utilization of these nozzles for various applications.

Bubble and conical forms of vortex breakdown in swirling jets

Experimental investigations of laminar swirling jets had revealed a new form of vortex breakdown, named conical vortex breakdown, in addition to the commonly observed bubble form. The present study explores these breakdown states that develop for the Maxworthy profile (a model of swirling jets) at inflow, from streamwise-invariant initial conditions, with direct numerical simulations. For a constant Reynolds number based on jet radius and a centreline velocity of 200, various flow states were observed as the inflow profile’s swirl parameter $S$ (scaled centreline radial derivative of azimuthal velocity) was varied up to 2. At low swirl ($S=1$) a helical mode of azimuthal wavenumber $m=-2$ (co-winding, counter-rotating mode) was observed. A ‘swelling’ appeared at $S=1.38$, and a steady bubble breakdown at $S=1.4$. On further increase to $S=1.5$, a helical, self-excited global mode ($m=+1$, counter-winding and co-rotating) was observed, originating in the bubble’s wake but with little effect on the bubble itself – a bubble vortex breakdown with a spiral tail. Local and global stability analyses revealed this to arise from a linear instability mechanism, distinct from that for the spiral breakdown which has been studied using Grabowski profile (a model of wing-tip vortices). At still higher swirl ($S=1.55$), a pulsating type of bubble breakdown occurred, followed by conical breakdown at 1.6. The latter consists of a large toroidal vortex confined by a radially expanding conical sheet, and a weaker vortex core downstream. For the highest swirls, the sheet was no longer conical, but curved away from the axis as a wide-open breakdown. The applicability of two classical inviscid theories for vortex breakdown – transition to a conjugate state, and the dominance of negative azimuthal vorticity – was assessed for the conical form. As required by the former, the flow transitioned from a supercritical to subcritical state in the vicinity of the stagnation point. The deviations from the predictions of the latter model were considerable.

Similarity of wake meandering for different wind turbine designs for different scales

The wake meandering characteristics of four different wind turbine designs with diameters ranging from a few centimetres (wind tunnel scale) to a hundred metres (utility scale) are investigated using large-eddy simulation with the turbine blades and nacelle parametrised using a new actuator surface model. Different velocity fields and meandering behaviours are observed at near-wake locations. At far-wake locations, on the other hand, the mean velocity deficit profiles begin to collapse when scaled by the centreline velocity deficit based on the incoming wind speed at turbine hub height, suggesting far-wake similarity across scales. The turbine-added turbulence kinetic energy profiles are shown to also nearly collapse with each other in the far wake when normalised using a velocity scale defined by the thrust on the turbine rotor. Moreover, we show that at far-wake locations, the simulated flow fields for all four turbine designs exhibit similar wake meandering characteristics in terms of (1) a Strouhal number independent of rotor designs of different sizes and (2) the distributions of wake meandering wavelengths and amplitudes when normalised by the rotor diameter and a length scale defined by the turbine thrust respectively.

Turbulent jets with off-source heating

Motivated by anomalous entrainment behaviour in cumulus clouds, Bhat et al. (Exp. Fluids, vol. 7, 1989, pp. 99–102) pioneered a laboratory experiment to study turbulent jets subjected to a volumetric heating away from the momentum source. The study concluded that the use of a constant entrainment coefficient was insufficient for the flow, and that the results did not confirm the analysis of Hunt (Recent Research Advances in the Fluid Mechanics of Turbulent Jets and Plumes, 1994, pp. 309–334, Kluwer Academic), which suggested that an increase in relative turbulent transport of streamwise momentum could lead to a decrease in entrainment. The present paper re-evaluates theoretical aspects of both studies, and includes a decomposition of the factors contributing to entrainment. The reworked analysis is then used to examine three-dimensional numerical simulations of turbulent jets with off-source heating. The data are consistent with previous work, but give deeper insight not easily obtainable through experiment. Specifically, direct measurement of flux integrals shows that previous inference from experimental measurements of centreline velocity and profile widths under the assumption of self-similarity can lead to underestimation of the mass flux by over 50 % in some cases. Radial profiles of temperature, radial velocity and turbulent correlations show significant departures from self-similarity. The flux measurements show that there is actually an increase in the entrainment coefficient with heating, and that it is locally enhanced by positive forcing and decreased by an increase in turbulent transport of streamwise momentum, thereby confirming the essence of the original proposal of Hunt.

The quiescent core of turbulent channel flow

The identification of uniform momentum zones in wall-turbulence, introduced by Adrian, Meinhart & Tomkins (J. Fluid Mech., vol. 422, 2000, pp. 1–54) has been applied to turbulent channel flow, revealing a large ‘core’ region having high and uniform velocity magnitude. Examination of the core reveals that it is a region of relatively weak turbulence levels. For channel flow in the range $\def \xmlpi #1{}\def \mathsfbi #1{\boldsymbol {\mathsf {#1}}}\let \le =\leqslant \let \leq =\leqslant \let \ge =\geqslant \let \geq =\geqslant \def \Pr {\mathit {Pr}}\def \Fr {\mathit {Fr}}\def \Rey {\mathit {Re}}Re_{\tau } = 1000\text {--}4000$, it was found that the ‘core’ is identifiable by regions bounded by the continuous isocontour lines of the streamwise velocity at $0.95U_{CL}$ (95 % of the centreline velocity). A detailed investigation into the properties of the core has revealed it has a large-scale oscillation which is predominantly anti-symmetric with respect to the channel centreline as it moves through the channel, and there is a distinct jump in turbulence statistics as the core boundary is crossed. It is concluded that the edge of the core demarcates a shear layer of relatively intense vorticity such that the interior of the core contains weakly varying, very low-level turbulence (relative to the flow closer to the wall). Although channel flows are generally referred to as ‘fully turbulent’, these findings suggest there exists a relatively large and ‘quiescent’ core region with a boundary qualitatively similar to the turbulent/non-turbulent interface of boundary layers, jets and wakes.

Spatial simulation of channel flow instability and control

A direct numerical simulation is performed on the full time-dependent three-dimensional Navier–Stokes equations in a spatially developing plane-channel flow at a Reynolds number of 10 000. Two-dimensional eigenfunctions based on the solution of the Orr–Sommerfeld equation are introduced at the inflow with random noise added to simulate a vibrating ribbon transition experiment. The flow is allowed to choose a natural path to secondary instability, either K-type (after Klebanoff) or H-type (after Herbert), depending on the amplitude of the two-dimensional disturbance. For low-amplitude two-dimensional disturbances (1 % of the centreline velocity), H-type modes are found to dominate, while a doubling of the amplitude (2 % of the centreline velocity) produces a mixed H-type/K-type disturbance field with explosive growth of the secondary modes. In addition, the use of a suction/blowing slot that is phase lagged with respect to a fixed wall pressure signal is demonstrated to significantly reduce the energy in the primary mode owing to the destruction of phase between the streamwise and wall-normal velocity components. The use of forward finite-time Lyapunov exponents to generate Lagrangian coherent structures as a means of flow visualization is also presented, showing qualitative agreement with previous experimental visualizations, and represents a viable means of identifying characteristic vortical flow structures.

Experimental Investigation of the Flow Front Behind a Liquid-Air Interface for Capillary Flow

An experimental system for understanding the flow field near the meniscus during the capillarity or under capillary action is developed. Capillary flow is one of the mechanisms for driving fluid in a microfluidic device. The literature highlights that a significant amount of work has been done on the theoretical understanding of the capillary transport in rectangular microchannels. However, these models for capillary flow neglect the flow behavior at the liquid-air interface, which may have a significant influence in terms of the velocity field and the transience of the penetration depth in the micro-capillary. The objective of the present study is to understand the flow development during the advancement of the meniscus. The aim is to elucidate the dynamics of the three phase contact line and other micro-scale effects during the capillarity. A μ-PIV technique has been used to study the flow development near the meniscus and the results are further refined using a hybrid μ-PIV/PTV technique. Effects of surface tension in the fully developed flow regime during the advancement of meniscus are studied in detail. Variations in the centreline velocity of the progression of the meniscus and temporal variations in the development of flow are identified as possible areas for departure from theory.