Three-dimensional Vortical Structures Research Articles

NUMERICAL STUDY OF HEAT TRANSFER IN TWO-ROW HEAT EXCHANGERS HAVING EXTENDED FIN SURFACES

This paper reports on a three-dimensional study of air through two-row cylinder tubes. The analysis is intended to present a comparison of numerical and experimental data to validate the laminar flow postulation. The current study explores the influence of four perforated fin surfaces on the pressure drop and heat transfer rate. To gain further insight into the three-dimensional vortical flow structure, we conduct a topological study of the velocity field. Examination of the surface flow topology and the flow patterns at cross-flow planes sheds some light on the complex interaction of the cylinder tube with the mainstream flow. This study clearly reveals a saddle point in front of the first row of cylinder tubes. Also clearly revealed by the computed solutions is a flow reversal found in the wake of the tube. The character of the critical-point-induced flow is also addressed. This study shows that the addition of perforated fins is not without deficiency. There is, in fact, a trade-off between the benefit of having an improved heat transfer and the penalty of having an increased pressure drop.

Velocity Perturbations Induced by the Longitudinal Vortices in a Cylinder Wake

This paper presents data showing the three-dimensional vortical structures in the near wake region of circular cylinders. The in-plane velocity field was measured using a digital Particle Image Velocimetry (PIV) technique. The vortical structures are found to include inclined counter-rotating longitudinal vortices in the braids joining consecutive Ka´rma´n vortices. A simple vortex-pair model is proposed to estimate velocity perturbation induced by the longitudinal vortices in the near wake region. The perturbation resulting from the longitudinal vortices is shown to induce spanwise velocity modulation and a velocity spike of a nominally two-dimensional vortex street.

Three-Dimensional Vortical Structures in a Rectangular Jet.

The three-dimensional vortical structures in a rectangular jet were determined by measuring the phase-average fluctuating static pressure, and the characteristics of the structure were discussed. The experiments were carried out for an air jet issuing from a sharp-edged rectangular orifice. The jet was excited in the interaction mode, in which the stable interaction of vortices occurred. The phase-average pressure was measured over the flow field with a static pressure probe. The three dimensional contours of phase-average pressure provided useful information on complicated vortical structures. The structures were characterized by stretching, splitting and cut-and-connect of vortices. The results suggest that the direct measurements of fluctuating static pressure are very useful for detecting three-dimensional complicated vortical structures.

Digital-Particle-Image-Velocimetry (DPIV) in a scanning light-sheet: 3D starting flow around a short cylinder

Scanning-Particle-Image-Velocimetry Technique (SPIV), introduced by Brucker (1992) and Brucker and Althaus (1992), offers the quantitative investigation of three-dimensional vortical structures in unsteady flows. On principle, this technique combines classical Particle-Image-Velocimetry (PIV) with volume scanning using a scanning light-sheet. In our previous studies, single scans obtained from photographic frame series were evaluated to show the instantaneous vortical structure of the respective flow phenomena. Here, continuous video recordings are processed to capture also the temporal information for the study of the set-up of 3D effects in the cylinder wake. The flow is continuously sampled in depth by the scanning light-sheet and in each of the parallel planes frame-to-frame cross-correlation of the video images (DPIV) is applied to obtain the 2D velocity field. Because the scanning frequency and repetition rate is high in comparison with the characteristic time-scale of the flow, the evaluation provides a complete time-record of the 3D flow during the starting process. With use of the continuity concept as described by Robinson and Rockwell (1993), we obtained in addition the out-of-plane component of the velocity in spanwise direction. This in view, the described technique enabled the reconstruction of the three-dimensional time-dependent velocity and vorticity field. The visualization of the dynamical behaviour of these quantities as, e.g. by video, gave a good impression of the spanwise flow showing the “tornado-like” suction effect of the starting vortices.

A Numerical Study of Three Dimensional Vortex Behavior of a Side Ported Rotary Engine.

This numerical study focuses on the three-dimensional vortical structure of a flow field inside a side-ported rotary engine. Since both two-dimensional numerical simulation and experimental visualization methods may not be applicable to side-ported rotary engine where the flow field inside is essentially three-dimensional, the authors have developed a three-dimensional computer simulation code based on the compressible Navier-Stokes equations. A Chakraverthy third-order TVD scheme devised in the code successfully calculates a flow field in a light load region at 3000 rpm, since the macroscopic stroke characteristics show appropriate behaviors. The velocity vector fields alone do not seem to provide fundamental images of the flow fields, thus characteristic vortex tubes are sought. Although it is not easy to extract vortex tubes closely surrounded by the moving boundary walls, they are capable of providing useful information which otherwise may not be given. The shapes and their change in vortex tubes, in fact, refine perspectives on the flow fields, and hence the effects of the intake flow and the rotor motion on the flow structure are well understood.

Numerical Simulation on 2D/3D Reacting Free Shear Layer.

The evolution of two- and three-dimensional vortical structures in an initially nonpremixed temporally developing reacting mixing layer is simulated by solving the time-dependent, compressible Navier-Stokes equations. The result of the numerical simulations shows that, as the evolution of hydrodynamic instability is suppressed, and the vorticity and the amount of chemical product are reduced by the baroclinic effect and volumetric expansion due to reaction heat release. In the case of temperature dependent diffusivities, the vorticity is reduced as well, in comparison with the temperature-independent case. Structures of the cores, ribs and legs are observed in the three dimensional mixing layer. Similarities between the distributions of spanwise vorticity and product mass fraction, and between those of streamwise vorticity and reaction rate, can be identified when the pairing process is saturated which may imply that the streamwise vortical dynamics plays an essential role in the progress of the chemical reaction.

Three-Dimensional Vortical Structures in the Wake behind a Hemisphere on Open Channel Bed

Instantaneous three-dimensional velocity fields in the wake of a hemisphere located on an open channel bed are obtained by using flow visualization in multi-sections and Mass-Consistent model. The instantaneous pictures of the coherent structure are taken at short time interval and their behaviors are investigated at high Reynolds number. The secondary hairpin vortex has been generated on the upstream side of the large hairpin vortex and coalesced into the large hairpin. The streamwise distances between hairpin vortices are irregular. The three-dimensional distributions of the instantaneous Reynolds stress and pressure are also shown.

Base drag reduction by control of the three-dimensional unsteady vortical structures

The present paper deals with the wake of a 2D body equipped with a drag reduction device. The device is a 3D trailing edge consisting of alternate segments of blunt base and spanwise cavity. The aerodynamic mechanisms acting on the near wake are studied in a water tunnel from schlieren observations by thermally marking large scale structures. The results show that the efficiency of the device is directly related to the presence of longitudinal vortices. An optimization of the shapes in subsonic compressible flow had led to a decrease of more than 40% of the total drag of the profile.

The coherent structure in turbulent mixing layers (structure in the core and rib regions)

To make clear the coherent structure in turbulent mixing layers, a large eddy simulation was performed on a temporally growing turbulent mixing layer with a hyperbolic tangent velocity profile. From this study, the development of a two-dimensional large vortical structure (core) and a three-dimensional streamwise vortical structure (rib) were observed. Also, it was made clear that rib structure was connected in the core region. Furthermore, the high Reynolds number flow case was calculated, and it was made clear that although the two-dimensional vortical structure was deformed and tearing was observed, the spanwise vortical structure and the rib structure could be observed in the high Reynolds number flow.

Three-dimensional vortical structure in lid-driven square cavity flows.

A three-dimensional vortical structure of a lid-driven square cavity flow was studied by flow-visualization techniques. Several sectional patterns of the flow visualized by a thin laser sheet showed that the structure was characterized by cells periodically aligned in the spanwise direction of the cavity. The cell consisted of both a pair of two vortex rings whose sense of rotation was opposite each other and a pair of two U-shape secondary vortices at corners. The vortex rings were a part of a primary vortex, and similar to the Taylor vortices between two coaxially rotating cylinders. The cells fluctuated in the spanwise direction with a long period of time 56H/U0, U0 being the velocity of the lid. A time-mean interval of the cells was found as a function of Reynolds number which was defined in terms of velocity U0 and the height H of the cavity in a range from 500 to 3000. The time-mean interval was independent of the aspect ratio A of the cavity, if A was greater than or equal to 3.

Three-dimensional vortical structures of transition in plane channel flow

Three-dimensional visualization of flow field structures in transitional plane channel flow obtained from a numerical simulation is presented at two Reynolds numbers. At the one-spike stage, independent of Reynolds number, it is seen that the flow is characterized by a multilayer vortex system. In the upper layer, total vorticity vector plots indicate a high-shear layer dominated by spanwise vorticity, whereas the middle layer (corresponding approximately to the critical layer) forms a vortex loop (λ vortex) consisting of strong streamwise and spanwise vorticity components. At the three-spike stage prior to breakdown, the vortical structure becomes concave in the flow direction. This process is accompanied by intense vortex liftup activity near the wall, which seems to originate at the legs of the vortex loop trailing the high-shear layer. Finally, at the five-spike stage, it is suggested that high-vorticity regions (vortical structures) develop into horseshoe eddies in planes inclined to the main flow direction.

Flow characteristics of orifice and tapered jets

Two types of small-aspect-ratio slot jets were investigated. An orifice slot jet with a 3:1 aspect ratio was shown to have features similar to an elliptic jet with a well-designed contraction. A larger spreading rate and enhanced turbulent mixing, relative to a circular jet, can be obtained using this orifice nozzle, which is advantageous for many practical applications. By adding a conical contraction to the slot jet (tapered slot jet) the characteristics were totally altered. The axes switching, typical to an elliptic jet, did not occur and the spreading rate at the major axis plane was larger than at the minor axis plane. The turbulence amplification in the initial core region was higher than both in the circular and elliptic jets. The transition from the conical contraction of the nozzle to the slot shape of the outlet section produced three-dimensional vortical structures. These structures interacted with the asymmetric vortices shed from this noncircular configuration to enhance the small scale turbulence in the core region as well as in the jet circumference. The large spreading rate of the two configurations of slot jets is beneficial to increase the bulk mixing with the surrounding fluid through entrainment. The augmented turbulence level of the tapered slot jet is important for reacting flows where small scale mixing is required.

Energy separation in a vortex street

When a bluff body is placed in a crossflow, the total temperature in its wake can become substantially less than the incoming one, as manifested by the fact that the recovery factorRon its rearmost surface takes negative values at high subsonic flow: this is the phenomenon referred to here as the Eckert-Weise effect. Although a vortex street has been a suspected cause, the issue of whether this is so, and what the mechanism is, has remained unsettled. In this experimental and theoretical investigation, we first examine the cause of the Eckert-Weise effect by enhancing the vortex shedding through acoustic synchronization: resonance between the vortex shedding and transversely standing acoustic waves in a wind tunnel. At the lowest synchronization, where a ringing sound emanates from the wind tunnel,Rat the rearmost section of the cylinder is found to become negative even at a Mach number of 0.2; the base pressure (Cpb) takes dips correspondingly, indicative of the intensification of the vortex street. At this lowest acoustic resonance, the decrease ofRandCpb, uniform along the span, agrees with the expectation based on the spanwise uniformity of the lowest standing wave. At the next acoustic resonance where the standing wave now varies along the span, the corresponding dips inRandCpb, non-uniform along the span, reveals an interesting ‘strip-theory’-like behaviour of the vortex intensities in the vortex street. These results correlating the change inRwithCpbconfirm that the Eckert-Weise effect is indeed caused by the vortex shedding, the mechanism of which is examined theoretically in the latter half of the paper.A simple theoretical argument, bolstered by a full numerical simulation, shows that the time-varying static pressure field due to the vortex movement separates the instantaneous total temperature into hot and cold spots located around vortices; once time-averaged, however, the total temperature distribution conceals the presence of hot spots and takes the guise of a colder wake, the Eckert-Weise effect. Therefore the correct explanation of the Eckert-Weise effect, a time-averaged phenomenon, emerges only out of, and only as a superposition of, instantaneous total temperature separation around vortices. Such a separation is not confined to the outside of vortex cores; every vortex in its entirety becomes thermally separated. Nor is it limited to the far downstream equilibrium configuration of the Kármán vortex street but applies to the important near-wake vortices, and to any three-dimensional vortical structure as well. For low subsonic flows in particular, this dynamical explanation also leads to a similar separation of total pressure; these features may thus be potentially exploited as a general marker to identify and quantify vortices.

Flow structure and mass transfer for a wavy channel in transitional flow regime.

The relationship between flow structure and mass transfer in a wavy channel was investigated in the range from laminar to turbulent flow. Laminar flow has a steady two-dimensional structure, but turbulent flow has an unsteady three-dimensional vortical structure. In particular, the flow field in a large recirculation vortex within the furrow of a wavy wall shows an intermittent reversed flow and a nonuniformity of flow in the spanwise direction for turbulent flow. The flow intermittency is closely related to the mass transfer, and a remarkable increment of mass transfer rate is induced near the flow reattachment point in the large recirculation vortex, which suggests a renewal of the concentration boundary layer because of the flow intermittency.