Crossflow Fluid Research Articles

Experimental study on the vortex formation and entrainment characteristics for a round transverse jet in shallow water

The vortex formation and entrainment characteristics for a round transverse jet in shallow water were experimentally investigated by means of a combination of LIF flow visualization and PIV measurement. A scarf vortex wrapped around the main body of the jet is formed in the near-wall region due to the interaction between the resulting wall jet and sufficiently shallow crossflow, with some more or less unsteady flow properties and with spreading ranges as functions of both the velocity ratio and the water depth within the near field. The entrainment of the ambient crossflow fluid into the jet main body is closely associated with the time-evolving features of the shear layer between the jet and surrounding fluid as well as the induced vortical structures near the wall. In the case of slight impingement upon the wall, the interaction between the jet shear layer and the weak, unstable scarf vortex gives rise to an appreciable local entrainment enhancement, confined in the near-wall region in the vicinity of the stagnation point. While in the case of intense impingement upon the wall, the well-organized and stable scarf vortex gives rise to a greatly enhanced entrainment and a greatly increased lateral spreading rate nearly throughout the overall near field as compared to the conventional wall jet. In addition, the entrainment of the ambient crossflow fluid by the scarf vortex in this case occurs largely on the surface of the unique spiral roller structure by itself due to the presence of smaller and unorganized eddies, and accordingly the scarf vortex is likely to keep its spiral roller structure steadily to a relatively great downstream distance within the near field.

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.

Dynamics and mixing of vortex rings in crossflow

Direct numerical simulation is used to study the effect of crossflow on the dynamics, entrainment and mixing characteristics of vortex rings issuing from a circular nozzle. Three distinct regimes exist, depending on the velocity ratio (ratio of the average nozzle exit velocity to free-stream crossflow velocity) and stroke ratio (ratio of stroke length to nozzle exit diameter). Coherent vortex rings are not obtained at velocity ratios below approximately 2. At these low velocity ratios, the vorticity in the crossflow boundary layer inhibits roll-up of the nozzle boundary layer at the leading edge. As a result, a hairpin vortex forms instead of a vortex ring. For large stroke ratios and velocity ratio below 2, a series of hairpin vortices is shed downstream. The shedding is quite periodic for very low Reynolds numbers. For velocity ratios above 2, two regimes are obtained depending upon the stroke ratio. Lower stroke ratios yield a coherent asymmetric vortex ring, while higher stroke ratios yield an asymmetric vortex ring accompanied by a trailing column of vorticity. These two regimes are separated by a transition stroke ratio whose value decreases with decreasing velocity ratio. For very high values of the velocity ratio, the transition stroke ratio approaches the ‘formation number’. In the absence of trailing vorticity, the vortex ring tilts towards the upstream direction, while the presence of a trailing column causes it to tilt downstream. This behaviour is explained. In the absence of crossflow, the trailing column is not very effective at entrainment, and is best avoided for optimal mixing and entrainment. However, in the presence of crossflow, the trailing column is found to contribute significantly to the overall mixing and entrainment. The trailing column interacts with the crossflow to generate a region of high pressure downstream of the nozzle that drives crossflow fluid towards the vortex ring. There is an optimal length of the trailing column for maximum downstream entrainment. A classification map which categorizes the different regimes is developed.

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.

Large-eddy simulation of film cooling flows at density gradients

The present paper investigates the impact of the velocity and density ratio on the turbulent mixing process in gas turbine blade film cooling. A cooling fluid is injected through an inclined pipe at α = 30° into a turbulent boundary layer at a freestream Reynolds number of Re ∞ = 400,000. This jet-in-a-crossflow (JICF) problem is investigated using large-eddy simulations (LES). The governing equations comprise the Navier–Stokes equations plus additional transport equations for several species to simulate a non-reacting gas mixture. That is, gases of different density are effused into an air crossflow at a constant temperature. An efficient large-eddy simulation method for low subsonic flows based on an implicit dual time-stepping scheme combined with low Mach number preconditioning is applied. The comparison of the numerical findings with experimental velocity data from two-component particle-image velocimetry (PIV) measurements shows an excellent agreement. The results evidence the dynamics of the flow field in the vicinity of the jet hole, i.e., the recirculation region and the inclination of the shear layers, to be mainly determined by the velocity ratio. However, evaluating the cooling efficiency downstream of the jet hole the mass flux ratio proves to be the dominant similarity parameter, i.e., the density ratio of the jet and crossflow fluid has to be considered.

Large-eddy simulation of three-dimensional vortical structures for an impinging transverse jet in the near region

The three-dimensional vortical structures for an impinging transverse jet in the near region were numerically investigated by means of large-Eddy Simulation (LES). The LES results reproduced the skewed jet shear layer vortices close to the jet nozzle and the scarf vortex in the near-wall zone, in good agreement with the experimental observations. Different vortical modes in the skewed jet shear layer close to the jet nozzle were identified depending upon the velocity ratio between jet and crossflow, namely changing from an approximately axisymmetric mode to a helical one with the velocity ratios varying from 20 to 8. Moreover, the scarf vortex wrapped around the impinging jet in the near-wall zone showed distinct asymmetry with regard to its bilateral spiral legs within the near region. And the entrainment of the ambient crossflow fluids by the scarf vortex in the near-wall zone was appreciably influenced by its asymmetry and in a large part occurred on the surface of the spiral roller structures in the course of spreading downstream.

Optimal Design of Tube Banks in Crossflow Using Entropy Generation Minimization Method

An entropy generation minimization, EGM, method is applied as a unique measure to study the thermodynamic losses caused by heat transfer and pressure drop for a fluid in crossflow with tube banks. The use of EGM allows the combined eect of heat transfer and pressure drop to be assessed through the simultaneous interaction with the tube bank. A general dimensionless expression for the entropy generation rate is obtained by considering a control volume around a tube bank and applying the conservation equations for mass and energy with the entropy balance. Analytical/empirical correlations for heat transfer coecients and friction factors are used, where the characteristic length is used as the diameter of the tubes and reference velocity used in Reynolds number and pressure drop is based on the minimum free area available for the fluid flow. Both in-line and staggered arrangements are studied and their relative performance is compared for the same thermal and hydraulic conditions. A parametric study is also performed to show the eects of dierent design variables on the overall performance of tube banks. It is shown that all relevant design parameters for tube banks, including geometric parameters and flow conditions can be simultaneously optimized.

Second law analysis for optimal thermal design of radial fin geometry by convection

This paper presents a second law analysis for the optimal geometry of fin array by forced convection. The analytical analysis involves the achievement of a balance between the entropy generation due to heat transfer and entropy generation due to fluid friction. In the design of a thermal system, it is important to minimize thermal irreversibilities because the optimal geometry will be found when the entropy generation rate is minimized. In this paper, the entropy generation rate is discussed and optimum thickness for fin array is determined on the basis of entropy generation minimization subjected to the global constraint. In addition, the influence of cost parameters on the optimum thickness of fin array is also considered and presented in graphical form. It has been found that the increase in cross flow fluid velocity will enhance the heat transfer rate that will reduce the heat transfer irreversibility.

Effects of jet velocity profiles on a round jet in cross-flow

This paper reports the results of an experimental investigation on the effects of jet velocity profiles on the flow field of a round jet in cross-flow (JICF) using laser-induced fluorescence and digital particle-image velocimetry techniques (DPIV). Tophat and parabolic jets were considered, with the momentum ratios (MRs) ranging from 2.3 to 5.8. Results show that the thicker shear layer associated with a parabolic JICF is able to delay the formation of leading-edge and lee-side vortices when compared to the tophat JICF at the corresponding MR. As a result, there is an increase in jet penetration and a reduction in the near-field entrainment of cross-flow fluid by a parabolic JICF. Also, the less coherent nature of the leading-edge and lee-side vortices in a parabolic JICF is more likely to break up sporadically into smaller-scaled vortices. In addition, DPIV results show that a parabolic JICF exhibits not only a faster velocity recovery of cross-flow fluid at the jet lee-side than the corresponding tophat JICF, it also consistently registers a higher magnitude of the peak average vorticity than the tophat JICF for all MR considered. Despite these differences, the time-averaged flow topology for both cases share many salient features.

Near-field pairing of leading-edge vortices in elliptic jets in cross flow

MCCDPIV measurements of the in-plane velocity components and out-of-plane vorticity for two elliptic jets of aspect ratio of 2 and 3 exhausting normally into a cross flow with velocity ratios ranging from 3 to 5 have been carried out. The purpose is to obtain quantitative data on the vortex pairing process which occurs along the leading edge of the cross flow/jet shear layer interface. The results show that for elliptic jets in cross flow with VR⩾3, which have their major axis aligned in the direction of the cross flow, there is an increase in the local entrainment of the cross flow fluid during the vortex pairing.

Near wake of a circular cylinder submitted to blowing – I: Boundary layers evolution

The dynamical and thermal behavior of the flow around a circular cylinder submitted to blowing is experimentally investigated. The blowing is applied through the whole surface of a porous cylinder submitted to a heated turbulent main fluid in cross-flow. Velocity, pressure and temperature profiles are determined for different injection rates and main flow temperatures for Reynolds numbers ranging from 3900 to 14 000. The velocity and temperature profiles are similarly modified with the blowing, while the injection thickens the boundary layers and modifies their shape. These evolutions lower the transfer coefficients, both dynamical (shear stress) and thermal (heat transfer), and are dependent on the location around the cylinder. The blowing impact on the wall temperature is also quantified, exibiting an excellent effectiveness of the cooling with blowing.

Structural features and surface heat transfer associated with a row of short-hole jets in crossflow

Abstract Surface streak experiments coupled with flow visualization and numerical simulations were used to investigate short length-to-diameter injection holes for geometries of interest to the gas turbine industry. The flow field results were also related to measured surface heat transfer coefficients in the downstream region of the jets. Geometric surface topology maps inferred from “oil and lampblack” surface streak experiments give insights into the flow field, as well as boundary layer disruption, and ultimately the convective heat transfer coefficient enhancement in the wake region of the jets. A secondary counter-rotating vortex pair (CRVP) with the opposite sense of rotation as the well-documented main CRVP is present in some configurations and is found to noticeably enhance the attachment event in the wake region and correspondingly enhance the convective heat transfer coefficient. The separation and attachment regions corresponding to the separation and attachment of boundary layer fluid brought about by jet–crossflow interaction was found to strongly affect the heat transfer on the surface downstream of the jets. The relative strength of the attachment events, and therefore the effect on h , is dependent on the trajectory of the CRVP, presence or absence of the secondary CRVP, and the attachment of crossflow fluid that has accelerated around the issuing jet. In addition to providing information about the crossflow boundary layer disruption, the surface topological results relate the signature of the surface shear stress to the coherent flow structures in the flow field being investigated.

Étude expérimentale d'une interaction de jets obliques avec un écoulement transversal compressible. II. Effets du taux d'injection sur les transferts thermiques en surface

Experimental study of inclined jets cross flow interaction in compressible regime. II. Effect of injection rate on surface heat transfer. An experimental study of a row of jets entering in a transonic mainstream ( M=0.72) is carried out in order to investigate the influence of the injection rate, defined as the ratio of the mass fluxes per unit surface of the jet to the cross flow fluids, on the interaction of the two flows and on the wall heat transfer. The velocity field is derived from total pressure and temperature measurements. Infrared thermography technique is used to identify the wall surface heat transfer. Experiments are carried out for three values of the injection rate R=0.24, 0.37 and 0.50. The experiments indicate the existence of an optimum film effectiveness for a value of R close to 0.37. A higher injection rate leads to a higher penetration of the jet in the cross flow with an associated reduction of the film cooling effect due to the injection of cross flow fluid under the jets. A lower injection rate induces a higher mixing of the jet fluid with the incoming boundary layer flow which also reduces the film efficiency.

Second-law analysis on a flat plate-fin array under crossflow

Second-law analysis on a flat plate-fin array under crossflow was conducted, from which the entropy generation rate was evaluated. Increase in the crossflow fluid velocity would enhance the heat transfer rate (or equivalently, reduce the heat transfer irreversibility). Nevertheless, owing to the simultaneous increase in drag force exerting on the fin bodies, the hydrodynamic irreversibility increases as well. An optimal Reynolds number thereby exists over wide operating conditions. Optimal design/operational conditions were searched for on the basis of entropy generation minimization.

Penetration and Mixing of Fully Modulated Turbulent Jets in Crossflow

Fully-modulated, incompressible, turbulent transverse jets were studied experimentally over a range of pulsing frequencies, duty-cycles, and at two jet-to-crossflow velocity ratios. The jet flow was completely modulated by operating a solenoid valve resulting in the shut off of jet supply during a portion of the cycle. The planar laserinduced fluorescence technique was used to determine the penetration, dilution, and structural features of the pulsed jets. The molecular mixing rate was quantified through a chemical reaction between the jet and crossflow fluids. Short injection times resulted in creation of vortex ring structures whereas long injection times produced axially elongated turbulent puffs, similar to a segment of the steady jet. The latter case resulted in only modest enhancement of the jet penetration depth and dilution. Pulsed jets dominated by vortex ring had penetration depths significantly greater than a steady jet with the same velocity ratio. Penetration of up to about 5 times the steady jet value at 50 jet diameters downstream of the jet exit was observed with 200 ms pulses. Duty-cycle had a significant effect on the performance of pulsed jets with short injection times. Increasing the duty-cycle for a fixed injection time diminished the jet penetration. The dilution and mixing rates of pulsed jets with short injection time were also increased over the steady jet. The greatest reduction in the mixing rate was approximately 50% for well-separated pulses with short injection times.

Large-eddy simulations of a round jet in crossflow

This paper reports on a series of large-eddy simulations of a round jet issuing normally into a crossflow. Simulations were performed at two jet-to-crossflow velocity ratios, 2.0 and 3.3, and two Reynolds numbers, 1050 and 2100, based on crossflow velocity and jet diameter. Mean and turbulent statistics computed from the simulations match experimental measurements reasonably well. Large-scale coherent structures observed in experimental flow visualizations are reproduced by the simulations, and the mechanisms by which these structures form are described. The effects of coherent structures upon the evolution of mean velocities, resolved Reynolds stresses, and turbulent kinetic energy along the centreplane are discussed. In this paper, the ubiquitous far-field counter-rotating vortex pair is shown to originate from a pair of quasi-steady ‘hanging’ vortices. These vortices form in the skewed mixing layer that develops between jet and crossflow fluid on the lateral edges of the jet. Axial flow through the hanging vortex transports vortical fluid from the near-wall boundary layer of the incoming pipe flow to the back side of the jet. There, the hanging vortex encounters an adverse pressure gradient and breaks down. As this breakdown occurs, the vortex diameter expands dramatically, and a weak counter-rotating vortex pair is formed that is aligned with the jet trajectory.

Instantaneous Discharge of Buoyant Fluid in Cross-Flow

Artificial instantaneous discharge of buoyant fluid in ambient waters is usually carried out at the time of maximum cross-flow so that the resulting dilution is optimal. The determination of the characteristics of this problem is important for environmental impact study but has seldom been studied. To investigate the transport, mixing, and dilution of an isolated cloud of buoyant fluid in cross-flow ambient, a three-dimensional buoyancy extended k-e model is employed. The computed results show that two flow regimes exist. First is the initial dumping regime in which the effect of crosscurrent and the associated turbulence on the longitudinal width and the average dilution is insignificant as compared to the effect of buoyancy on those quantities. The flow and mixing characteristics in this regime are close to those in the case of stagnant ambient. Second is the ultimate regime in which the effect of buoyancy on the longitudinal width and the average dilution is small as compared to the effect of cross-flo...

Trajectory and entrainment of a round jet in crossflow

This paper examines the trajectory and entrainment characteristics of a round jet in crossflow. A series of large eddy simulations was performed at Reynolds numbers of 1050 and 2100 and at jet to crossflow velocity ratios of 2.0 and 3.3. Trajectories, which are defined based on the mean streamlines on the centerplane, all collapse to a single curve far from the jet exit, and this curve can be represented with a power law fit. Within this power law region, entrainment of crossflow fluid is shown to be the primary mechanism by which the jet trajectory is determined. Upstream of the power law region, near the jet exit, jet trajectory varies from changes in pressure drag and from differences in the turbulence intensities in the incoming pipe flow.

Manipulation of a jet in a cross flow

Abstract The effect of forcing on the mixing and velocity field of a jet in a cross flow is investigated for a jet-to-cross flow velocity ratio of R=1.0 using reactive-Mie-scattering flow-visualization and two-color Particle Image Velocimetry (PIV) techniques. Reactive-Mie-scattering images are used to assess the effect of forcing on molecular mixing, whereas double-exposed two-color PIV images are employed to obtain instantaneous velocity and corresponding vorticity distributions. The jet emanates from a square conduit and is manipulated by four piezoelectric actuators – one mounted along each side of the jet conduit near the jet-exit plane. Time-and phase-averaged distributions of velocity and rms velocity fluctuations in the x–y plane z=0 are also obtained. A novel approach is employed for processing jet and cross-flow fluids based on the size of the seeding particle. The present results show that when the jet is unforced, the “wake” of the jet contains a domain in which the magnitude of the velocity is very low compared to that of the cross-flow velocity, suggesting that the fluid within this domain is almost stagnant. When the jet is excited, the low-velocity domain in its wake is substantially reduced through vigorous mixing. Manipulation of the upstream and downstream segments of the jet shear layer also leads to an increase in the jet penetration into the cross flow (up to 30% as compared to the unforced case) and substantial mixing enhancement. Unlike in streamwise forcing where the jet fluid remains mainly in the central portion of the spanwise surface, spanwise forcing induces strong spanwise jet undulations.

Demineralization of Protein Hydrolysates from Enzymatic Hydrolysis of Leather Shavings Using Membrane Diafiltration

ABSTRACT Pressure-driven membrane separations operated in the diafiltration mode appear promising as an efficient demineralization method of waste protein hydrolysates from leather production. The conclusion drawn from our experimental results is that nanofiltration membranes should be used rather then ultrafiltration ones as they show complete passage of the salts with high retention of the protein hydrolysates. In addition to the classical membrane separation operating parameters, which are crossflow fluid velocity, transmembrane pressure difference, and temperature,the required residual salt concentration as well as acceptable protein losses affect the effectiveness of the process.