Dimensionless Wavelength Research Articles

Analytical approximations of laminar flow states above a curved wall heated from below

An analytical framework is developed for estimating wall-bounded plane laminar flow states above curved walls subject to external shear and heating. The framework is applicable to arbitrary lower-wall geometries with arbitrary temperature prescriptions. The study relies on perturbation expansions of flow states with respect to the inverse of the dimensionless channel wavelength. Exact closed-form solutions are obtained for systems of perturbation equations up to the third order, which have the freedom to allow for any flow constraint. As an application of the analytical framework, we examine flow states above two specific instances of wall geometries with long channel wavelengths. The approximated flow states shed insights into morphologies of laminar convection states at low shear rates (i.e. low Reynolds numbers).

Numerical investigation of thermal and entropy generation characteristics in a sinusoidal corrugated channel under the influence of sinusoidal wall temperature

The corrugated channels play a vital role in many engineering applications, such as electronic cooling and heat exchanger devices; thus, forced convection heat transfer and entropy generation within the corrugated channels are important subjects to investigate. This study numerically investigated the laminar forced convection heat transfer and entropy generation of water flow within a sinusoidal corrugated channel. The channel walls were subjected to sinusoidal wall temperature heating while varying the wave parameters. The finite element method had been employed to solve the governing equation, and the simulation results were conducted for Reynolds number 5 ≤ Re ≤ 500, dimensionless wall temperature wavelength 0.25 ≤ λth ≤ 2.25, and Prandtl number Pr = 6.93. The main findings were depicted in terms of Nusselt number, skin friction coefficient, heat transfer effectiveness, total entropy generation, and Bejan number contours, as well as the results being compared with the results of uniform heating. In addition, the results demonstrated that sinusoidal heating significantly improved heat transfer compared to uniform heating for λth < 1.75 and all ranges of Reynolds number, while a critical Reynolds number was found, where the uniform heating showed a higher heat rate compared to non-uniform heating at λth ≥ 1.75 . Besides that, it is found that the sinusoidal wall temperature has lower entropy generation than uniform heating, except with λth = 0.25. This investigation recommends that sinusoidal wall temperature heating is more effective in heat transfer than uniform heating by 7.5, 2.78, 1.9, 1.52, and 1.34 factors at λth = 0.25, 0.75, 1.25, 1.75 and 2.25, respectively.

Electromagnetohydrodynamic flow through a periodically grooved channel

This paper harnesses the spectral approach to study electromagnetohydrodynamics (EMHD) flow through an undulating 2D microchannel for a Newtonian fluid. The present study aims to investigate EMHD flow through a periodically patterned channel for large values of corrugation amplitude (surface roughness) relative to the channel height, Hartmann number and a wide range of wavelengths. A mathematical model is developed for the hydraulic permeability and the velocity field using the Fourier approach. By imposing a small pattern amplitude constraint, asymptotic analysis was employed to investigate the model presented in the literature. In the present study, hydraulic permeability is shown to strongly depend on the Hartmann number, pattern amplitude and wavelength. The spectral method predicts a monotonic decrease in hydraulic permeability irrespective of pattern amplitude at a small dimensionless wavelength (λ = 1) and Hartmann number (Ha < 1). At large wavelengths (λ > 3), the hydraulic permeability demonstrates an augmentation corresponding to the pattern amplitude. At intermediate wavelength ( 2.5<λ<3 ), the permeability of the channel firstly decreases and then increases with pattern amplitude. For large Hartmann numbers (Ha≫1) , the prediction from the spectral model exhibits a similar trend as a function of wavelength. Across a range of wavelengths, the spectral model captured the permeability minima for various large Hartmann number values. The spectral model is significantly faster than the finite-element-based numerical simulation, with computational efficiency ranging from 150 to 250 times higher. The existence of a limited pattern amplitude and Hartmann number for small-large wavelength patterns, at which the permeability of the channel is minimized, is one example of a prediction from the spectral model that is beyond the resolution of currently accessible asymptotic theories.

Developing characteristics of subsonic-supersonic shear layers induced by different lobed mixers in the RBCC

The rapid mixing of air and rocket gas is pivotal for enhancing combustion performance in the ejector mode of rocket-based combined-cycle engines. This study experimentally investigated developing characteristics of subsonic-supersonic shear layers induced by sinusoidal- and rectangular-lobed mixers using a subsonic-supersonic shear layer experimental system and an ice-cluster-based planar laser scattering technique. Both sinusoidal- and rectangular-lobed mixers evidently bolstered mixing through the promotion of transverse flow and vortex interactions. Under equivalent conditions, the sinusoidal-lobed mixer offered superior mixing effects. The mixing enhancement of a sinusoidal-lobed mixer exhibited a positive correlation with the wavelength, with excessively small wavelengths potentially impeding mixing. Based on the dimensionless wavelength ratio, a criterion was identified for evaluating the mixing enhancement of the sinusoidal-lobed mixer: a positive mixing enhancement occurred when the dimensionless wavelength ratio was at least 1. Notably, in this study, the shear layer thickness growth rate increased by 12 % when the dimensionless wavelength ratio was 1.905. These findings can offer valuable insights for improving combustion performance via enhanced mixing.

Experimental and numerical investigation of a density-driven instability during a horizontal miscible displacement

We performed an experimental and numerical investigation of a convective buoyancy-driven instability that arises during the injection of a denser miscible fluid into a less dense one in a rectilinear geometry. We visualized the instability using a shadowgraph technique, and we obtained quantitative information using micro-Particle Image Velocimetry. Numerical simulations provided further insights into the three-dimensional (3D) velocity field. We have shown that the instability only occurs above a certain Péclet number, Pe, depending on the Rayleigh, Ra, and Schmidt, Sc, numbers. We suggest scalings of the critical time, TC, and dimensionless wavelength, λ/h, of the instability, both of which increase with increasing Pe and Ra. Finally, we investigated the interactions of the instability vortices with each other and the geometry boundaries.

Global and local analyses of the Görtler instability in hypersonic flow

The stationary Görtler instability in hypersonic flow over a concave wall is systematically investigated across a range of geometric and flow parameters using resolvent analysis, which seeks for the forcing and response pair that maximizes the energy amplification. The optimal forcing takes the form of streamwise vortices, while the optimal response is streamwise streaks. The growth of the optimal disturbance is contributed by both the lift-up and centrifugal mechanisms. The latter becomes dominant as the boundary layer develops, and its growth rate agrees well with that predicted by local stability analysis. In terms of changes in geometric parameters, an increase in curvature destabilizes the Görtler instability, as expected, while the effect of the angle subtended by the concave wall (the turning angle) is shown to be negligible. With respect to changes in flow parameters, the Görtler instability is stabilized at low Reynolds numbers, destabilized under the cold-wall effect, and insensitive to the change in Mach number. The most amplified spanwise wavelength scales with the boundary-layer thickness, which remains mostly unchanged when the freestream Mach number is varied from 3 to 10. A new dimensionless wavelength parameter is proposed to predict the wavelength of the most dangerous Görtler vortices in the compressible flow regime. The resolvent analysis results are confirmed by a three-dimensional numerical simulation, where the hypersonic flow is perturbed by a spatial white noise.

Hydrodynamic performance of slender swimmer: effect of travelling wavelength

The impact of Strouhal number St (= 0.1–1.0), Reynolds number Re (= 50–2000) and dimensionless wavelength λ (= 0.5–2.0) on the hydrodynamic performance of a travelling wavy foil of a constant length is extensively investigated. The relationship of time-mean thrust with St, Re and λ is presented, suggesting that the propulsive force increases with increasing St, Re and λ. As such, the drag–thrust boundary advances as these parameters increase. A shorter λ makes the thrust steadier while a longer λ enhances the maximum instantaneous thrust. The latter is beneficial for prey to escape from a predator. The fluid added mass caused by the foil oscillation increases with St and λ but declines with Re (&lt;500). Seven types of wake structures produced by the foil are identified, discussed and connected to thrust generation, showing how St, Re and λ affect the fluid dynamics, wake transition, vortex strength, wake jet, velocity, added mass, added damping, power input, efficiency and pressure profiles. The outcome of this work renders a physical basis for understanding the swimming of aquatic animals.

Fragmentation and atomization characteristics of near-orifice liquid jet under transverse gas film

In order to understand the fragmentation and atomization characteristics of the liquid jet in transverse gas film, a pintle injection element using air and water as simulants is designed. The two-phase flow large eddy simulation and backlight imaging are used to study the liquid-jet breakup process and spray-field dynamic characteristics in the nearorifice area of pinte injection element under the atmospheric environment. The primary fragmentation process of the liquid jet dominated by surface wave is obtained by large eddy simulation, which reveals the establishment process of the spray field in near-orifice area of the gas-liquid pintle injector. After the subsonic airflow leaves the slit, it expands and accelerates into supersonic state. Then the deceleration and pressurization phenomenon occurs once the supersonic airflow passes through the detached bow shock upstream of the liquid jet. The liquid jet bends downstream due to the difference in pressure between upstream and downstream, and the Rayleigh-Taylor (R-T) unstable surface wave appears on the jet windward surface. As the surface wave develops, the penetration of the wave trough by airflow causes the continuous liquid jet to fragment. Proper orthogonal decomposition (POD) method can effectively reconstruct spray snapshot. The POD mode shows that the low-frequency spray oscillation in near-orifice area is caused by the overall expansion/contraction process of the spray field, while the high-frequency one is due to the “impact wave” movement of the liquid block or liquid mist group on the windward side. The latter is produced by the R-T unstable surface wave before the jet breakup, and can be categorized as traveling wave structure. The dimensionless traveling wave wavelength has a power-law relationship with Weber number.

Natural convection in phase change material: Experimental study

Natural convection in a melting material (hexadecane) is studied experimentally using Magnetic Resonance Imaging. This imaging technique allows to discriminate solid and liquid phases and to measure velocity vectors in the volume sample, even through/in opaque phases. Within our experimental conditions, the convection occurs above Ra≈1430. The thermo-convective flow affects the melting interface leading to higher liquid height in uprising flow and lower liquid height in downward flow regions. Our experiments show that (i) the transient evolution of the mean liquid height h¯ is enhanced with convection since h¯∝t0.8 (h¯∝t12−3β with β=1/4), while h¯∝t0.5 in the conductive regime similarly to the Stefan’s problem; (ii) the steady averaged liquid height h¯ increases 4 times larger in the convective regime than in the conductive regime. In our experiments the range of A, ratio between the solid and liquid heights, is such as A>1.5. Within this range of values we obtain convective pattern under the form of hexagons / polygons. As the melting boundary grows, we observe a decrease in the polygon number. The polygonal pattern is characterized by a constant dimensionless wavelength, i.e. λ/h¯≈2, and thus a constant wavenumber around 3.1 similarly to the primary bifurcation in the classical Rayleigh–Bénard Convection (RBC). Heat transfer is evaluated via the Nusselt number Nu. Close to the onset of convection and above, our results highlight a scaling law Nu∝Raβ with β=1/4 similarly to the classical RBC. The kinetic energy Ec is also evaluated via velocity measurements leading to Ec/E0∝Nu4 and therefore to Re∝Ra2β∝Ra1/2. Within the frame of our experiments, all scaling laws are converging to β=1/4. They reflect clearly an increase in the convective intensity from both thermal and dynamical points of view.

Similarity and Froude Number Similitude in Kinematic and Hydrodynamic Features of Solitary Waves over Horizontal Bed

This study presents, experimentally, similarity and Froude number similitude (FNS) in the dimensionless features of two solitary waves propagating over a horizontal bed, using two wave gauges and a high-speed particle image velocimetry (HSPIV). The two waves have distinct wave heights H0 (2.9 and 5.8 cm) and still water depths h0 (8.0 and 16.0 cm) but identical H0/h0 (0.363). Together with the geometric features of free surface elevation and wavelength, the kinematic characteristics of horizontal and vertical velocities, as well as wave celerity, are elucidated. Illustration of the hydrodynamic features of local and convective accelerations are also made in this study. Both similarity and FNS hold true for the dimensionless free surface elevation (FSE), wavelength and celerity, horizontal and vertical velocities, and local and convective accelerations in the horizontal and vertical directions. The similarities and FNSs indicate that gravity dominates and governs the wave kinematics and hydrodynamics.

Own torsional vibrations of a cylindrical shell in an elastic medium

The article covers that we consider the own axisymmetric torsional vibrations of an infinitely long cylindrical shell in an elastic inertial medium. The dependences of the first shell oscillation frequency in an elastic medium on the dimensionless wavelength are obtained.

Optimization of wavy cylinder for aerodynamic drag and aeroacoustic sound reduction using computational fluid dynamics analysis

Motivated by the requirement to lessen the aerodynamic drag and aeroacoustic sound of the bluff bodies, the present paper is devoted to a numerical analysis of the aerodynamics and aeroacoustics related with the flow past wavy circular cylinders. Based on the efficient flow control method, as has been presented by preceding researchers, the existing work embarks upon an investigation on the wavy cylinder at a various wavelength and amplitude conditions. Computations are performed for a circular cylinder of the length-to-diameter ratio ( L/ D) of 25 at a Reynolds number ( Re) of 97300 using large eddy simulation and Ffowcs Williams- Hawking’s acoustic analogy. Firstly, the cylinder without waviness is subjected to a uniform incoming flow is considered for validation against measurements. Secondly, various collection of wave shape parameters, specifically dimensionless wavelength λ/ D (=1 to 2.5), and wave amplitude a/ D (=0.05 to 0.2) have been taken into consideration. It is disclosed that the proper selection of shape parameters could significantly reduce the drag and sound emission levels, compared to the normal cylinder. Finally, a multi-objective particle swarm optimization was performed using the radial basis neural network to simultaneously reduce the aerodynamic drag and sound emission, with λ/ D and a/ D as design variables. We recognized a critical λ/ D and a/ D for the wavy circular cylinder at the considered subcritical Re.

Numerical study of melting and solidification in a wavy double-pipe latent heat thermal energy storage system

The objective of this paper is to develop the influences of channel waviness on the performance of a latent heat storage system during phase change mechanism. The heat exchanger is a vertically oriented double pipe where the heat transfers to/from the PCM in the annulus by moving the water in the inner tube. Various wavelengths, as well as wave amplitudes, are examined at various fluid Re and water temperatures (Tin) to find the effects of channel waviness on different aspects of melting/solidification time, pressure drop, pumping power and exchanged heat rate. Increasing Re, Tin and amplitude of wavy wall improves the system performance during melting and solidification mechanisms. Besides, it is found that there is an optimum dimensionless wavelength of 0.2 for achieving the minimum melting and solidification times as a result of maximum heat exchanged between the water and PCM. Furthermore, the waviness has an almost negligible effect on the pumping power which is reduced for the dimensionless wavelengths higher than 2.0. In the best scenario, the required time to melt and solidify the PCM reduces by almost 28.6% and 57.63%, respectively, using wavy channels compared with the smooth wall case.

Numerical study of thermo-hydraulic characteristics for forced convective flow through wavy channel at different Prandtl numbers

The present work numerically investigates the thermo-fluidic and entropy generation characteristics for laminar forced convective flow through wavy channel at different Prandtl number (Pr). Results are presented for the following range of parameters: Reynolds number $$5 \le \text{Re} \le 200$$ , Prandtl number $$0.72 \le \Pr \le 100$$ , dimensionless amplitude $$0.3 \le \alpha \le 0.7$$ and dimensionless wavelength $$0.5 \le \lambda \le 1.5$$ . It is observed that with increase in Pr, the thickness of the thermal boundary layer at trough region decreases slowly for smaller Re, whereas at higher Re, the rate of decrement is higher. The average Nusselt number increases with Pr for all amplitude, wavelength and Reynolds number. The relative heat transfer enhancement compared to equivalent plane channel is presented in terms of enhancement ratio (ER), and it shows a non-monotonic variation of ER with Pr at lower Re and a monotonic one at higher Re. The combined alteration of rate of heat transfer and pressure drop as compared to plane channel is enumerated by performance factor (PF), and the variation of PF with Pr shows non-monotonic behaviour at lower Re and monotonic one at higher Re. The variation of PF shows non-monotonic variation with Re for higher Pr and for smaller wavelength, whereas it monotonically decreases for all Pr at higher wavelength. Thermal entropy generation contribution is higher over the viscous one for all the cases considered. The local thermal entropy generation distribution varies with Re, Pr and geometrical configuration of the channel. For smaller amplitude (α = 0.3), the total entropy generation is minimum in the considered range of Re and Pr.

Natural convection heat transfer and entropy generation from a heated cylinder of different geometry in an enclosure with non-uniform temperature distribution on the walls

A numerical investigation has been performed to visualize the natural convective heat transfer and nature of entropy generation from a heated cylinder of two separate geometries (circular and square) situated within a square enclosure subjected to non-uniform temperature distributions on the left vertical and bottom walls. The flow inside the enclosure is steady, incompressible and laminar and the working fluid is Newtonian with constant Prandtl number (Pr = 0.71). The results are discussed in terms of the distribution of streamlines and isotherms, surface-averaged Nusselt number and entropy generation, for a different combination of Rayleigh number (103 to 106) and dimensionless wavelength of sinusoidal temperature distribution ( $$0.1 \le \lambda \le 0.7$$ ). It reveals that sinusoidal temperature with higher wavelength enhances the heat transfer. Moreover, the highest value of Nusselt number is obtained in case of enclosure embraces the circular cylinder. Further, the thermal entropy generation is observed to be minimum for sinusoidal temperature distribution with a wavelength of 0.7 irrespective of Rayleigh number. In addition to this, the geometrical configuration of the cylinder has a negligible effect on the variation of fluid friction and thermal irreversibility at higher Rayleigh number. Finally, the circular shape of the embedded cylinder is found to be optimal since it results in maximum heat transfer with least entropy generation.

Low-frequency pseudo-Rayleigh and pseudo-Scholte waves at an interface of liquid/soft porous sediment with underlying hard porous sediment substrate

Summary The paper focuses on the propagation of low-frequency pseudo-Rayleigh and pseudo-Scholte waves at the liquid/soft porous sediment interface with an underlying hard porous sediment half-space. The overlying liquid is assumed to be ideal compressible medium and the porous sediments are modelled by Biot theory. Based on the boundary conditions, the closed-form dispersion equations of far-field interface waves are deduced using 2-D Helmholtz decomposition theorem and Fourier transform. The velocity and attenuation of pseudo-Rayleigh and pseudo-Scholte waves are determined by Newton iteration in a reasonable rooting interval. The analytical expressions of the displacement field and liquid pressure distribution caused by interface waves are also derived. Then, the dispersion equations for four degenerate systems are derived as special cases by assuming the thickness of the liquid layer or the sandwiched porous soft sediment layer to be zero or infinite. Lastly, numerical examples are used to verify the degeneracy of the system and to analyse the propagation characteristics of pseudo-Rayleigh and pseudo-Scholte waves. They show the dependences of the velocity and displacement field on dimensionless modulus and dimensionless wavelength. When the dimensionless wavelength is small or very large, the phase velocity and displacement field calculated by the present system is the same as the special cases, thus proving the validating of the new system.

Suppression of Cavity Flow Oscillations via Three-Dimensional Steady Blowing

This paper presents an investigation of the flow physics and control of cavity flow oscillations using a three-dimensional (3-D) array of steady jets located near the cavity leading edge. The array injects 3-D, steady flow normal to the freestream to suppress the fluctuating surface pressure in a rectangular cavity with a length-to-depth ratio of 6 for Mach 0.3 to 0.7. Sixteen configurations are assessed for their suppression as a function of the aggregate momentum coefficient , spatial duty cycle , and dimensionless wavelength . Significant reductions of fluctuating surface pressure are observed. Schlieren and particle image velocimetry are performed to investigate the baseline and controlled flows. Companion large-eddy simulations with spanwise periodic boundary conditions at Mach 0.6 generally agree with the experiments, despite a significant difference in the Reynolds numbers. The simulations show that control reduces the pressure fluctuations inside the entire cavity, albeit at a higher level of . Spanwise wavelike structures produced by the control input distort the shear layer, inhibit the growth of large-scale vortical structures, and reduce the strength and length scale of turbulent fluctuations near the impingement region. A slot-configuration design guide is provided, which compares favorably with the limited available data in the literature.

Labyrinthine and secondary wave instabilities of a miscible magnetic fluid drop in a Hele-Shaw cell

A comprehensive experimental study is presented to analyse the instabilities of a magnetic fluid drop surrounded by miscible fluid confined in a Hele-Shaw cell. The experimental conditions include different magnetic fields (by varying the maximum pre-set magnetic field strengths,$H$, and sweep rates,$SR=\text{d}H_{t}/\text{d}t$, where$H_{t}$is the instant magnetic field strength), gap spans,$h$, and magnetic fluid samples, and are further coupled into a modified Péclect number$Pe^{\prime }$to evaluate the instabilities. Two distinct instabilities are induced by the external magnetic fields with different sweep rates: (i) a labyrinthine fingering instability, where small fingerings emerge around the initial circular interface in the early period, and (ii) secondary waves in the later period. Based on 81 sets of experimental conditions, the initial growth rate of the interfacial length,$\unicode[STIX]{x1D6FC}$, of the magnetic drop is found to increase linearly with$Pe^{\prime }$, indicating that$\unicode[STIX]{x1D6FC}$is proportional to the square root of the$SR$and$h^{3/2}$at the onset of the labyrinthine instability. In addition, secondary waves, which are characterised by the dimensionless wavelength$\unicode[STIX]{x1D6EC}=\unicode[STIX]{x1D706}/h$, can only be triggered when the three-dimensional magnetic microconvection is strong enough to make$Pe^{\prime }$exceed a critical value, i.e.$Pe^{\prime }&gt;19\,000$, where$\unicode[STIX]{x1D706}$is the wavelength of the secondary wave. In this flow regime of high$Pe^{\prime }$, the length scale of the secondary wave instability is found to be$\unicode[STIX]{x1D6EC}=7\pm 1$, corresponding to the Stokes regime; meanwhile, in the flow regime of low$Pe^{\prime }$, the flow corresponds to the Hele-Shaw regime introduced by Fernandezet al.(J. Fluid Mech., vol. 451, 2002, pp. 239–260).

Effects of Reynolds number on energy extraction performance of a two dimensional undulatory flexible body

In this paper, a NACA0012 airfoil is used as a two-dimensional simplified model of a fish body undertaking undulating motion. Using numerical simulation and dynamic mesh method combined with the spring-based smoothing and the local remeshing, the effects of Reynolds number on energy extraction efficiency of an undulating flexible body are studied at different wave velocities and wavelengths. Based on unsteady incompressible Navier-Stokes (N-S) Equations, the flow field around an undulating two-dimensional fish-like body was simulated with the aid of CFD software Fluent 15.0. The present numerical results indicate that:1.Given the maximum amplitude, the tendency of the variation energy extraction efficiency with Reynolds number at different dimensionless wave velocity and wavelength are generally similar: as the Reynolds number increases, the energy extraction efficiency increases at first and then decreases. Relatively high efficiency always occurs at moderate Reynolds numbers;2.For each Reynolds number examined, when the wavelength is constant, the energy extraction efficiency first increases and then decreases with increasing dimensionless wave velocity. There is an optimum wave velocity at which the energy extraction efficiency achieve their maximum values;3.For a given Reynolds number, there is a threshold value of dimensionless wave velocity, at which the energy extraction efficiency becomes zero and this is a turning point from energy extraction to energy consumption.

Inertio-elastic instability in Taylor-Couette flow of a model wormlike micellar system

In this work, we use flow visualization and rheometry techniques to study the dynamics and evolution of secondary flows in a model wormlike micellar solution sheared between concentric cylinders, i.e., in a Taylor-Couette cell. The wormlike micellar solution studied in this work contains cetyltrimethylammonium bromide (CTAB) and sodium salicylate (NaSal). This system can be shear banding and highly elastic, nonshear banding and moderately elastic, or nearly Newtonian as the temperature is varied over a narrow range. The effect of elasticity on transitions and instabilities is probed by changing the temperature over a wide range of elasticity (El ≪ 1, El ≈ 1, and El ≫ 1). Elasticity is defined as the ratio of the Weissenberg number to the Reynolds number. For shear banding wormlike micelle solutions where El ≫ 1, a primary transition from the base Couette flow to stationary vortices that are evenly spaced in the axial direction of the shear cell and are characterized by an asymptotic wave-length is observed. The dimensionless wave-length at the onset of this shear banding transition for CTAB/NaSal system turns out to be much larger than those reported for other shear banding wormlike micelle systems. For the same fluid at a temperature where it shear-thins but does not display shear banding, El ≈ 1, and for slow ramp speeds, the primary transition is to distinct structures that are not stationary but rather travel in the axial direction. At low elasticity (El ≪ 1), where the fluid behaves as a nearly Newtonian fluid, several transitions from purely azimuthal Couette flow to modified Taylor vortex flows and finally chaotic regimes are documented. The behavior in the shear-banding and nonshear-banding regimes are discussed and compared with results in related systems. The possibility of hysteresis in the flow transitions as well as the effects of co-rotation and counter-rotation of the cylinders on transitions and instabilities are also examined for a wide range of elasticity.