Laminar Crossflow Research Articles

Experimental Study of the Heat Transfer Enhancement from a Circular Cylinder in Laminar Pulsating Cross-flows

Experiments were designed to investigate the heat transfer enhancement from a single heated circular cylinder in laminar pulsating cross-flows. Several parameters were explored, including Strouhal number between 0.18 and 2.80, Reynolds number between 205 and 822, and pressure amplitude prms between 40 Pa and 276 Pa. It was observed that the heat transfer enhancement factor increased up to 2.10, and an empirical correlation between the convection heat transfer coefficient and pulsating flows was developed. Results found that the heat transfer enhancement factor decreases with Strouhal number. Increasing Reynolds number was found to have a negative impact on the heat transfer enhancement factor when the pulsating frequency is relatively low, but with a higher pulsating frequency, an increasing Reynolds number increases the heat transfer enhancement factor. Larger pressure amplitude was found to continuously produce larger heat transfer enhancement factor when both Strouhal number and Reynolds number are kept constant.

Response and wake patterns of two side-by-side elastically supported circular cylinders in uniform laminar cross-flow

Vortex-induced vibrations (VIV) of two side-by-side elastically supported circular cylinders in a uniform flow with the Reynolds number of 100 are numerically investigated by using the immersed boundary method. The cylinders are constrained to oscillate in the cross-flow direction with a center-to-center spacing ratio T/D ranging from 2 to 5. The structural damping is set to zero to enable large vibration amplitudes in the range of reduced velocity Ur=3−10. It is found that the proximity of the cylinders does not have a significant impact to the lock-in region and cylinder responses, except at a small spacing ratio of T/D=2. The critical spacing ratio is determined as T/D=4 and beyond that the interaction between the cylinders is negligible. The following six near-wake patterns are observed; the irregular pattern, in-phase flip-flopping pattern, out-of-phase flip-flopping pattern, in-phase-synchronized pattern, antiphase-synchronized pattern and the biased antiphase-synchronized pattern. These patterns are plotted in a plane of Ur and T/D, together with approximate borderlines to distinguish one region from the others. The time histories, spectral features and wavelet transform contours of drag and lift forces are presented to elucidate the mechanisms of the in-phase and out-of-phase flip-flopping phenomena. It is established that the in-phase flip-flopping stems from the long-short near-wake pattern and its low-frequency flip-over, whereas the out-of-phase pattern originates from the large vortex shedding from the fictitious bluff-body with an augmented characteristic length.

Physical investigation of square cylinder array dynamical response under single-phase cross-flow

Fluid structure interaction and flow-induced vibration in square cylinder arrangement under single-phase incompressible laminar cross flow are investigated in the present paper. Dynamic instability governed by damping generation is studied without any consideration about mixing with turbulence effects. Conservative and non-conservative effects are pointed out and dynamical stability limit sensitivity to physical parameters is analyzed. Finally the influence of key physical parameters on fluid solid dynamics interaction is quantified.

Application of the meshless finite volume particle method to flow-induced motion of a rigid body

We present a new approach to numerical modelling of incompressible flow of fluid about an elastically mounted rigid structure with large body motions. The solution is based on the Finite Volume Particle Method (FVPM), a meshless generalisation of the mesh-based finite volume method. The finite volume particles are allowed to overlap, without explicit connectivity, and can therefore move arbitrarily to follow the motion of a wall. Here, FVPM is employed with a pressure projection method for fully incompressible flow coupled with motion of a rigid body. The developed extension is validated for Vortex-Induced Vibration (VIV) of a circular cylinder in laminar crossflow. To minimise computational effort, non-uniform particle size and arbitrary Lagrangian–Eulerian particle motion schemes are employed, with radial basis functions used to define the particle motion near the cylinder. Close agreement is demonstrated between the FVPM results and a reference numerical solution. Results confirm the feasibility of FVPM as a new approach to the modelling of flow with strongly coupled rigid-body dynamics.

Numerical Study of Fluid Flow and Heat Transfer Enhancement of Nanofluids over Tube Bank

In the present work, laminar cross flow forced convective heat transfer of nanofluid over tube banks with various geometry under constant wall temperature condition is investigated numerically. We used nanofluid instead of pure fluid ,as external cross flow, because of its potential to increase heat transfer of system. The effect of the nanofluid on the compact heat exchanger performance was studied and compared to that of a conventional fluid.The two-dimensional steady state Navier-Stokes equations and the energy equation governing laminar incompressible flow are solved using a Finite volume method for the case of flow across an in-line bundle of tube banks as commercial compact heat exchanger. The nanofluid used was alumina-water 4% and the performance was compared with water. In this paper, the effect of parameters such as various tube shapes ( flat, circle, elliptic), and heat transfer comparison between nanofluid and pure fluid is studied. Temperature profile, heat transfer coefficient and pressure profile were obtained from the simulations and the performance was discussed in terms of heat transfer rate and performance index. Results indicated enhanced performance in the use of a nanofluid, and slight penalty in pressure drop. The increase in Reynolds number caused an increase in the heat transfer rate and a decrease in the overall bulk temperature of the cold fluid. The results show that, for a given heat duty, a mas flow rate required of the nanofluid is lower than that of water causing lower pressure drop. Consequently, smaller equipment and less pumping power are required.

Active flow control applications with a jet and vortex actuator in a laminar cross flow

In this study the effect of an oscillatory, zero-net-mass flux device called a Jet and Vortex Actuator (JaVA) on the laminar boundary layer is investigated. The JaVA can be utilized to energize the boundary layer over a flat plate by creating jets and vortices thus it can delay or prevent boundary layer separation if it is used properly.It appears to produce qualitatively different flow regimes depending on its actuation parameters, e.g. frequency and amplitude or subtle changes in geometry such as the position of the actuator plate with respect to the JaVA-cavity. This latter effect was only discovered recently. Since little is known about the underlying fluid dynamics and because of a complete lack of unsteady data, a device is designed and built for experiments in water. Unsteady flow fields have been recorded for visualization and furthermore quantitative evaluation by means of Particle Image Velocimetry (PIV) has been carried out in addition to numerical simulations. Results show that the JaVA-induced vortices ejected into the flat plate boundary layer significantly enhance the velocity profiles and its characteristics such as the displacement thickness and the momentum thickness if the plate is oscillating at high frequencies as it is flush-mounted or inside the cavity. But if the plate is extracted out of the cavity then there is no improvement in the flow fields hence separation can be delayed or prevented for long downstream distances only if the actuation parameters and plate positions are selected properly.

Three-dimensional numerical study of jet-in-crossflow characteristics at low Reynolds number

A numerical study of a square jet in a cross flow is carried out at a Reynolds number of 100. The flow field and heat transfer characteristic downstream of the jet have been explored by solving three-dimensional unsteady Navier–Stokes equations and energy equation using higher order spatial and temporal discretization. The projection of vortical structure on a plane is seen to give the component of vortex normal to the plane. Four combinations of velocity profile namely (1) uniform crossflow and uniform jet, (2) laminar boundary layer crossflow and uniform jet, (3) uniform crossflow and parabolic jet profile, and (4) laminar boundary layer crossflow and parabolic jet are compared at same phase to see their effect on the flow field and heat transfer characteristic. All the four cases are seen to exhibit unsteadiness but the jet with parabolic profile is seen to show stronger unsteadiness. The instantaneous vortical structures of all the cases at the same phase show that the structures are more complex for the jet with parabolic velocity profile. The temperature field is seen to be correlated with the vortical structures. Comparison of the time averaged flow field reveals that the jet penetration is the highest for the jet having parabolic profile and boundary layer crossflow. The adiabatic effectiveness is observed to be more for the jet with uniform velocity profile and uniform crossflow and was least for the jet with parabolic velocity profile and boundary layer crossflow.

Modeling of flux enhancement in presence of concentration polarization by pressure pulsation during laminar cross flow ultrafiltration

A theoretical study for the flux enhancement by pulsation of transmembrane pressure is presented for osmotic pressure controlled ultrafiltration under laminar flow regime. The transient velocity profile is solved analytically using Green`s function method. Time dependent convective diffusive equation is solved to quantify the membrane surface concentration and the permeate flux, numerically. The effects of the amplitude and frequency of pulsation on flux, surface concentration and observed retention are studied.

Large-Eddy Simulation of Jet Mixing in Supersonic Crossflows

Large-eddy simulation of an underexpanded sonic jet injection into supersonic crossflows is performed to obtain insights into key physics of the jet mixing. A high-order compact differencing scheme with a recently developed localized artificial diffusivity scheme for discontinuity-capturing is used. Progressive mesh refinement study is conducted to quantify the broadband range of scales of turbulence that are resolved in the simulations. The simulations aim to reproduce the flow conditions reported in the experiments of Santiago and Dutton [Santiago, J. G., and Dutton, J. C., Velocity Measurements of a Jet Injected into a Supersonic Crossflow, Journal of Propulsion and Power, Vol.132,1997, pp. 264―273] and elucidate the physics of the jet mixing. A detailed comparison with these data is shown. Statistics obtained by the large-eddy simulation with turbulent crossflow show good agreement with the experiment, and a series of mesh refinement studies shows reasonable grid convergence in the predicted mean and turbulent flow quantities. The present large-eddy simulation reproduces the large-scale dynamics of the flow and jet fluid entrainment into the boundary-layer separation regions upstream and downstream of the jet injection reported in previous experiments, but the richness of data provided by the large-eddy simulation allows a much deeper exploration of the flow physics. Key physics of the jet mixing in supersonic crossflows are highlighted by exploring the underlying unsteady phenomena. The effect of the approaching turbulent boundary layer on the jet mixing is investigated by comparing the results of jet injection into supersonic crossflows with turbulent and laminar crossflows.

The interaction between small clusters of cohesive particles and laminar flow: Coupled DEM/CFD approach

A computational method for the simulation of systems where close coupling exists between a flowing fluid and mobile or stationary cohesive particles is presented in this work. The method is based on the solution of particle motion dynamics by the Discrete Element Method (DEM) coupled with the flow field calculation by a Computational Fluid Dynamics (CFD) approach. The method is demonstrated on two examples inspired by the problem of sand production in oil exploration, namely the stability of a random packed layer of cohesive particles under gravity and under laminar cross-flow. Regime maps suitable for the estimation of limiting conditions for the onset of particle erosion have been generated computationally.

Large eddy simulation of a swirling transverse jet into a crossflow with investigation of scalar transport

The flow field of a turbulent jet emerging from a straight round pipe into a laminar crossflow is investigated by means of large eddy simulations. The concentration of a passive scalar, introduced with the jet, is calculated in order to quantify the mixing of the jet and the crossflow. In the jet, swirl is introduced by means of body forces and a range of jet swirl numbers from S=0 up to S=0.6 is studied. The impact of the jet swirl on the flow field, on the coherent structures, and on the mixing efficiency is investigated and quantified by means of various analyses. It is found that for all swirl numbers larger than zero a clear asymmetry appears in all quantities studied. Additional to the two hanging vortices at both sides of the jet a third vortex is introduced by the swirling pipe flow which interacts with the former. This feature is described in detail as it is not mentioned in the literature. For the strongest swirl investigated a recirculation zone near the jet exit is observed. Despite the asymmetry and even with a recirculation zone at the outlet, the counter-rotating vortex pair still exists in all cases in the downstream flow, where it entrains a large amount of crossflow fluid into the jet. The near field, however, is altered by the jet swirl in several respects. The jet more and more approaches the bottom wall with increasing swirl. As a result, the entrainment is gradually attenuated due to the larger blocking of the secondary flow by the wall. Increased swirl increases both the turbulent kinetic energy in the pipe and the vorticity of the average flow field near the jet exit, and thus stimulates the mixing in these regions. However, this stimulating effect is overwhelmed by the closer position of the jet trajectory to the wall of the channel with increasing swirl, which in turn reduces entrainment of fresh crossflow fluid into the jet. As a final result of these two competing effects, the overall mixing efficiency of a jet into a crossflow is merely unchanged with the addition of swirl. Various mixing indices, both spatial and temporal, are used for this analysis. Their respective advantages and disadvantages are discussed and detailed illustrations provide a sound understanding of their behavior.

Modelling, parameterisation and simulation-based optimisation of a microflow-control actuator

A synthetic jet is considered to control microflows, where the Knudsen number is between 0.001 and 0.1. The flow is modelled with the compressible, 2D Navier–Stokes equations. The wall boundary conditions are modified for the slip velocity and the temperature jump encountered for this Knudsen number range. The membrane motion is modelled as a moving boundary. After a validation using experimental results available only for a macroflow over a hump, the present study focuses on developing a design optimisation methodology for micro-synthetic jets in micro-scale, laminar crossflow. First, single-variable optimisations are performed. As compared to the baseline case, the optimisations yield 2, 15, 15 and 200% increase in actuation efficiency for the cases varying the orifice width, the orifice height, the cavity height and the frequency, respectively. Then, multi-variable shape optimisation is performed. Compared to the baseline case, the optimisation using shape parameters results in a 7-fold increase in the actuation efficiency, while the optimisation with Bezier polynomials results in more than a 10-fold increase.

Numerical simulation of laminar flow of oil and heat transfer near a circular cylinder with arched guide plates

We have investigated the intensification of the heat transfer from a cylinder in an oil medium due to the setting of arched guide plates on the basis of the numerical solution of the Navier-Stokes and energy equations with the help of multiblock computer-aided technologies realized in the VP2/3 package. Calculations have been made for the laminar nonstationary cross flow around a heated cylinder in a viscous medium with a strong temperature dependence of its thermophysical characteristics. Comparison has been made between the numerical forecasts of the drag, local heat transfer, thermohydraulic efficiency, and total Nusselt number for a cylinder with plates and for a single cylinder.

Direct numerical simulation of passive scalar transport in transverse jets

Direct numerical simulation is used to study passive scalar transport and mixing in a round turbulent jet, in a laminar crossflow. The ratio of the jet velocity to that of the crossflow is 5.7, the Schmidt number of the scalar is 1.49, and the jet-exit Reynolds number is 5000. The scalar field is used to compute entrainment of the crossflow fluid by the jet. It is shown that the bulk of this entrainment occurs on the downstream side of the jet. Also, the transverse jet entrains more fluid than a regular jet even when the jet has not yet bent into the crossflow. The transverse jet's enhanced entrainment is explained in terms of the pressure field around the jet. The acceleration imposed by the crossflow deforms the jet cross-section on the downstream side, which sets up a pressure gradient that drives downstream crossflow fluid toward the jet. The simulation results are used to comment on the applicability of the gradient–diffusion hypothesis to compute passive scalar mixing in this flow field. Computed values of the eddy diffusivity show significant scatter, and a pronounced anisotropy. The near field also exhibits counter gradient diffusion.

Application of the PIV technique to measurements around and inside a forming drop in a liquid–liquid system

A particle image velocimetry (PIV) method has been developed to measure the velocity field inside and around a forming drop with a final diameter of 1 mm. The system, including a microscope, was used to image silicon oil drops forming in a continuous phase of water and glycerol. Fluorescent particles with a diameter of 1 μm were used as seeding particles. The oil was forced through a 200 μm diameter glass capillary into a laminar cross-flow in a rectangular channel. The velocity field was computed with a double-frame cross-correlation function down to a spatial resolution of 21 × 21 μm. The method can be used to calculate the shear stress induced at the interface by the cross-flow of the continuous phase and the main forces involved in the drop formation process.

Direct numerical simulation of round turbulent jets in crossflow

Direct numerical simulation is used to study a round turbulent jet in a laminar crossflow. The ratio of bulk jet velocity to free-stream crossflow velocity is 5.7 and the Reynolds number based on the bulk jet velocity and the jet exit diameter is 5000. The mean velocity and turbulent intensities from the simulations are compared to data from the experiments by Su & Mungal (2004) and good agreement is observed. Additional quantities, not available from experiments, are presented. Turbulent kinetic energy budgets are computed for this flow. Examination of the budgets shows that the near field is far from a state of turbulent equilibrium – especially along the jet edges. Also – in the near field – peak kinetic energy production is observed close to the leading edge, while peak dissipation is observed toward the trailing edge of the jet. The results are used to comment upon the difficulty involved in predicting this flow using RANS computations. There exist regions in this flow where the pressure transport term, neglected by some models and poorly modelled by others, is significant. And past the jet exit, the flow is not close to established canonical flows on which most models appear to be based.

Heat transfer from a rotating disk in a parallel air crossflow

The heat transfer from a rotating disk in an air stream parallel to the plane of rotation is of importance in the assessment of disk brake performance. Numerically determined heat transfer coefficients and correlations are accordingly presented for a large range of rotational and crossflow velocities. These were obtained by means of large-eddy-simulations (LES). The extreme conditions of a stationary disk in an air crossflow and a rotating disk in still air are also considered. It is found that a critical ratio between the rotational and the crossflow Reynolds numbers exists with respect to rotational heat transfer augmentation. Only above this critical value, rotational heat transfer augmentation sets on in case of laminar crossflow Reynolds numbers. This phenomenon is directly linked to a flow instability that leads to a periodic vortex generation, and which can be described by the classical Landau model. For higher angular velocities, the wake becomes fully turbulent, and the transition is very rapid.

Concentration field and turbulent fluxes during the mixing of two buoyant plumes

In this work an experimental study of mixing of two identical plumes, carried out in a turbulent neutral boundary layer generated in a wind tunnel, is presented. Measurements have been performed with fast flame ionisation detectors (FFIDs) and a two-component Laser-Doppler Anemometer system. Results allow the study of both the average and the fluctuating concentration field, including the turbulent vertical and longitudinal mass fluxes, in single plumes and during the interaction of two identical plumes. This information gives insight into the details of the mixing phase of the two plumes. Results of trajectories and additional rise (due to plume interactions) have been compared with previous measurements carried out in laminar cross-flows, showing similar behaviour. Concentration distributions in plume cross-sections in turbulent cross-flows differ from those measured in laminar cross-flows. Average vertical and longitudinal velocity measurements into the plume core show the strength of the shielding effect of the upwind plume and some details of interaction between the counter-rotating vortex pairs (CVPs). For large values of the alignment angle φ, between the line joining the stacks and the cross-flow, an average negative vertical velocity is measured in the middle of the plume even if its centre of mass is rising. This downward velocity is induced by the slow interaction of the CVPs and generates a vertical stretching of the plume and negative rise enhancement. Vertical turbulent fluxes change sign on the plume centreline and are of opposite sign with respect to the longitudinal turbulent fluxes. Results indicate a good linearity between vertical turbulent fluxes and concentration gradients, with different proportionality for the top and bottom parts of the plume (especially in the near field) indicating that dispersion could be described by a gradient-transfer model.

The influence of temperature gradient on the Strouhal–Reynolds number relationship for water and air

This paper focuses on the wake flow behind a heated circular cylinder in the laminar vortex shedding regime. The phenomenon of vortex shedding from a bluff body is an interesting scientific and engineering problem. Acquisition of reliable experimental data is considered an indispensable step toward a deeper physical understanding of the topic. An experimental study of the wake flow behind a heated cylinder in the forced convection regime is performed using water as the working fluid. Firstly, qualitative visualization experiments were performed and the parallel vortex shedding mode was adjusted. Next, hot-wire anemometry was used for St–Re data acquisition. Data analysis confirmed the so-called thermal effect in water: cylinder heating increases the vortex shedding frequency and destabilizes the wake flow. The effective temperature concept was used and the St–Re data were successfully transformed to the St–Re eff curve. Furthermore, a comparison with air as the working fluid was discussed (cylinder heating decreases the vortex shedding frequency in air, thus stabilizing the wake flow). The formula to determine the effective temperature in water was experimentally derived from the present data, while the data and formula for air is already known. The relationship between the Strouhal number and the effective Reynolds number for water and air is represented by the same, universal formula: St = 0.2660 - 1.0160 Re eff - 0.5 , where Re eff is calculated at the effective temperature. Finally, the measurement results were compared to the thermodynamic St–Re equation derived by Maršík et al. [F. Maršík, Z. Trávníček, R.H. Yen., A.-B. Wang, Fluid dynamics concept for the critical Reynolds number of a heated/cooled cylinder in laminar crossflow, in preparation]. A satisfactory agreement between the derived equation and experimental data for both fluids (water and air) was achieved.

Analysis of Laminar Forced Convection of Air Crossflow in In-Line Tube Banks with NonSquare Arrangements

ABSTRACT Numerical analysis is made of forced-convection heat transfer in laminar two-dimensional steady crossflow in banks of plain tubes in square and nonsquare in-line arrangements. A finite-volume method with a nonorthogonal, boundary-fitted grid and co-located variable storage is used to solve the Navier–Stokes equations and energy conservation equation for a tube bundle with five longitudinal rows, including inlet and outlet sections. Local and overall heat transfer and fluid flow results are presented at combinations of transverse and longitudinal pitch-to-diameter ratios of 1.25, 1.5, and 2.0 at Reynolds numbers of 100 and 300 for a Prandtl number of 0.71. A comparison of the present study results with well-established experiments and empirical correlations showed good overall agreement. New equations are proposed for a correction factor for the effects of nonsquare arrangements on average friction factor.