Flow Structure In Cavity Research Articles

Review-Heat Transfer Inside Cavity Flows Trends

The complex cavity flow represented by the feedback mechanism and self-sustained oscillations of cavity shear layer are widely investigated in various publications either through analysing the aero-acoustic phenomenon occurring and its control or through examining the heat transfer mechanism in such a flow. Boundary layer separation, turbulence, unsteadiness, recirculation and reattachment complicate the flow phenomena at the cavity and may lead to substantial effects on heat transfer. Besides, in order to enhance the heat transfer in some applications, cavity flow was introduced, and thus the importance of understanding the cavity flow structure and its relation with heat transfer has become vital, where such a phenomenon occurs in many engineering applications. This paper presents a literature review on the studies that focus on examining heat transfer in cavity flow, by evaluating the effect of cavity geometry and inlet flow field on heat transfer at different cavity regions.

Controlling nucleation during unseeded THM growth of CdZnTe crystal

There are always undesired grains formed at the beginning of unseeded THM growth which deteriorate the crystal quality greatly. In this paper, several ingots with the diameter of 27 mm were grown by unseeded THM to investigate the nucleation process. The growth temperature in the experiments was 750 °C and the growth rate was 5 mm/day. The influence of crucible bottom geometry and temperature gradient on nucleation are studied. Numerical simulations of thermal and concentration fields were also performed for the THM process to study the growth process. The simulation results show that square cavity flow's structure and parabolic wall temperature profile are the main reason for secondary nucleation in the radial direction. Dummy crystal with cold finger was introduced in order to stabilize the thermal field and modify the lateral temperature gradient. Subsequent experimental results indicate that a large longitudinal temperature gradient with small lateral temperature gradient is more favor to suppress secondary nucleation and maintain stable growth.

Effects of location and angle of primary injection on the cavity flow structure of a trapped vortex combustor model

In these years, many works have been done on the combustion performance and flow structure of a trapped vortex combustor (TVC). However, to the best of our knowledge, little works have been done so far to unravel the effect of combination of various factors on cavity flow structure. In this study, a TVC has been studied to understand the effects of the angle and location of primary injection on the cavity flow structure with both experimental and numerical methods. The results indicate that the computational results match well with the experimental results. Two different flow structures have been found out with different combinations of θ and H, which are termed as “broken” and “closed”. Besides, when the distance between the primary injection and the bottom wall of the cavity (H) increases and the inlet angle of the primary injection (θ) is kept constant, the flow structure will be “broken”. If θ increases at this moment, the flow structure changes from “broken” to “closed”. The results are helpful for providing a good deal of insight into the flow structure and flow field organization of TVC.

Characterization of turbulent natural and mixed convection in confined enclosures equipped with a heat source

In this numerical study two configurations are considered, the first configuration in natural convection corresponds to a closed cavity equipped with a heating bar and the second mixed configuration corresponds to the same cavity but ventilated. The flow is turbulent (Gr H = 1,2.10 8 ), a choice of model is very important. The turbulence model chosen for natural convection is the low-Reynolds k-e model. A comparison of the turbulence models led us to choose the RNG k-e model for mixed convection study, because it is the suitable model for flows in ventilated cavities as well as flows with recirculation. Ventilation effect on natural flow has been studied by analyzing flow dynamic and thermal structure. Nusselt average number on each bar face is found to be improved by jet injection into the ventilated cavity, from about 50% to 60%. This comparison reveals the different velocities influence of the incoming air jet on the confined cavity flow structure, or this jet succeeds in breaking the single-cell flow of natural convection case into a multicellular flow for the other case of mixed convection at high velocities above the heating bar, of which it is the main purpose of this study.

Characterization of turbulent natural and mixed convection in confined enclosures equipped with a heat source

In this numerical study two configurations are considered, the first configuration in natural convection corresponds to a closed cavity equipped with a heating bar and the second mixed configuration corresponds to the same cavity but ventilated. The flow is turbulent (Gr H = 1,2.10 8 ), a choice of model is very important. The turbulence model chosen for natural convection is the low-Reynolds k-e model. A comparison of the turbulence models led us to choose the RNG k-e model for mixed convection study, because it is the suitable model for flows in ventilated cavities as well as flows with recirculation. Ventilation effect on natural flow has been studied by analyzing flow dynamic and thermal structure. Nusselt average number on each bar face is found to be improved by jet injection into the ventilated cavity, from about 50% to 60%. This comparison reveals the different velocities influence of the incoming air jet on the confined cavity flow structure, or this jet succeeds in breaking the single-cell flow of natural convection case into a multicellular flow for the other case of mixed convection at high velocities above the heating bar, of which it is the main purpose of this study.

Investigation of fluid flow structure in open type cavity under stationary and pulsatile flow conditions

A study of fluid flow in structured channels plays an important role in solving most fluid dynamics problems. The knowledge of the flow structure in separate cavities can be used to intensify mixing in microreactors and microelectrochemical systems (MEMS) as well as to increase heat transfer in heat sinks. Besides, knowledge about fluid flow in micro cavities could be useful to mitigate the symptoms of aneurysms. In this study, a 2D micro-particle image velocimetry (µPIV) system was used to investigate water flow in an open-type microcavity located in a straight, square cross-section (0.5 × 0.5 mm2) microchannel. Two identical micro cavities of the same cross-section as the main channel and the depth of 1 mm are oppositely located in the middle part of the channel. Because of symmetry, the flow structure of the primary vortex was investigated in only one cavity. Measurements were carried out in different planes over the entire height of the cavity at Reynolds number in the range from 100 to 3000. The flow in the channel was stationary and pulsating. Results indicate that flow in the cavity is three-dimensional. This is confirmed by velocity profile shape changes in its different transversal planes. In addition, it is determined that flow structure in the cavity depends on flow regime and flow pulsation characteristics. Velocity distribution shows that in the transverse direction there is the minimum velocity zone corresponding to the centre of the primary vortex generated by the channel flow, also the same zone is observed along the cavity in its axial plane. At low Re, such velocity distribution occurs in pulsating flow earlier than in the case of stationary flow. With increasing Re, velocity distribution in pulsating flow flattens out also more. It means that disturbed flow in the channel accelerates the formation of a stable flow structure in the cavity.

Effect of structure parameters of the flow guide vane on cold flow characteristics in trapped vortex combustor

The cold flow characteristics are investigated to show the effect of the structural parameters of the flow guide vane on the trapped vortex combustor (TVC). The results show that the structural parameters have significant effects on the TVC. As a/H increases, the total pressure loss, the wall shear stress at the bottom of the cavity and the turbulent intensity in the main combustion zone increase. b/B does not have a significant effect on the cavity flow structure and the total pressure loss, and the wall shear stress at the bottom of the cavity increases as increases. There is no significant increase of the turbulent intensity with the increase of b/B. The increase of c/L has little effect on the total pressure loss, and it is not conducive to a stable combustion. As c/L increases, the wall shear stress at the bottom of the cavity decreases. When a/H = 0.4, b/B = 0.4, c/L = 0.1, a desirable dual-vortex structure is formed with an acceptable pressure loss to achieve a stable combustion. Moreover, to ascertain that the flame is stable for different values of Vma with the optimal structural parameters, the effect of Vma on the flow field is discussed. Results suggest that the dual-vortex structure has no relationship with the increase of Vma. Furthermore, an unsteady simulation is conducted to show the generation and the development of the dual-vortex.

Convection-Enhanced Transport into Open Cavities : Effect of Cavity Aspect Ratio.

Recirculating fluid regions occur in the human body both naturally and pathologically. Diffusion is commonly considered the predominant mechanism for mass transport into a recirculating flow region. While this may be true for steady flows, one must also consider the possibility of convective fluid exchange when the outer (free stream) flow is transient. In the case of an open cavity, convective exchange occurs via the formation of lobes at the downstream attachment point of the separating streamline. Previous studies revealed the effect of forcing amplitude and frequency on material transport rates into a square cavity (Horner in J Fluid Mech 452:199-229, 2002). This paper summarizes the effect of cavity aspect ratio on exchange rates. The transport process is characterized using both computational fluid dynamics modeling and dye-advection experiments. Lagrangian analysis of the computed flow field reveals the existence of turnstile lobe transport for this class of flows. Experiments show that material exchange rates do not vary linearly as a function of the cavity aspect ratio (A = W/H). Rather, optima are predicted for A ≈ 2 and A ≈ 2.73, with a minimum occurring at A ≈ 2.5. The minimum occurs at the point where the cavity flow structure bifurcates from a single recirculating flow cell into two corner eddies. These results have significant implications for mass transport environments where the geometry of the flow domain evolves with time, such as coronary stents and growing aneurysms. Indeed, device designers may be able to take advantage of the turnstile-lobe transport mechanism to tailor deposition rates near newly implanted medical devices.

諫早湾口北部周辺の流動特性について

Ten years or more have passed since large-scale poor cultured laver harvests which arose in the head of Ariake Bay in 2000 winter.However, except for cultured laver, fish catches including a bivalve has decreased and does not show even a sign of recovery. When the drainage gate is opened over a long period of time, it is thought indispensable towards the Ariake Bay reproduction to investigate the change and influence of the aquatic environment of Isahaya Bay and Ariake Bay before and after opening the drainage gate.In this study, the field observation on flow regime at the northern mouth of Isahaya Bay was carried out under the stratified and mixing condition twice in 2010. As a result, structure of cavity flow appeared at the mouth of Isahaya Bay in all condition of both flood and ebb tide. Now, various flow regime has produced on various conditions at the northern mouth of Isahaya Bay, if potential flow will be recovered by opening the gate, it is expected flow regime changes a lot.

Jet-cavity interaction: effect of the cavity depth

A two-dimensional turbulent wall jet over a shallow cavity is studied numerically. The present paper investigates the wall jet outer layer effect on a shallow cavity flow structure. This layer is an important turbulence source which is added to that of the inner shear layer. It was found that the reattachment process seems to be accelerated in the turbulent wall jet incoming flow compared to that of the cavity in duct flow. The present study shows that the flow structure is very sensitive to the cavity depth to nozzle height ratio; the increasing of this ratio induces a reattachment length decrease.

Numerical simulation of cavity flow structure in an axisymmetric trapped vortex combustor

Reacting and non-reacting flow structure in a three dimensional (3D) trapped vortex combustor (TVC) cavity was investigated numerically by using shear stress transport (SST) k–ω model and eddy dissipation combustion model. In order to validate the present numerical model, the predicted results were compared with the experimental results available in literature for the same conditions. The pressure drop for various cavity aspect ratios, and the flow field predicted by the numerical model matched reasonably well with the experimental data. Non-reacting flow studies revealed that the secondary air jet momentum does not have significant effect on cavity flow structure, however, fuel–air mixing was observed to be improved for the higher momentum flux ratio (MFR) case. In contrast, under reacting flow conditions, the cavity flow structure was highly sensitive to secondary air jet momentum. For higher MFR cases, the cavity flow was characterized by multiple vortices, which were caused due to the volume expansion associated with combustion. Hence, it could be concluded that, under reacting flow conditions, cavity flow structure is highly dependent on the momentum flux ratio between the cavity and the mainstream flow. Besides this, the influence of turbulence associated with flow accompanied with the generation of baroclinic torque might also be a cause for the vortex breakdown, which has to be investigated extensively.

Influence of upstream flow characteristics on the reattachment phenomenon in shallow cavities

The influence of the upstream flow characteristics on the behavior of the flow over a shallow cavity and on the reattachment phenomenon is examined in this paper. Accordingly, a comparison of the cavity?s flow structure is performed for two different upstream flows: the wall jet flow and the boundary layer flow. The wall jet possesses a particular structure with two regions: an inner layer analogous to that of a boundary layer and an outer layer similar to that of a free jet; this layer is an additional source of turbulence production in addition to that of the inner shear layer. The present study interested to the effect of this external layer on the shallow cavity?s flow. The numerical approach is based on the low Reynolds stress-omega turbulence model. Fluent 6.3 and the pre-processor Gambit 2.3 are used for the computation. The numerical results indicate that the flow structure is very sensitive to the upstream flow?s characteristics. Indeed, for the same Reynolds number and the same boundary layer thickness at the cavity leading edge, the cavity flow structure in a wall jet upstream flow case differs considerably from that of a boundary layer upstream flow. The most important finding is the earlier reattachment process in the wall jet inflow case, where an important reduction of the reattachment length is observed compared to that of a cavity under a boundary layer flow.

Influence of Stator-Rotor Interaction on the Aerothermal Performance of Recess Blade Tips

This paper investigates the influence of stator-rotor interaction on the stage performance of three blade tip geometries. A reference flat tip is used to assess two different recess blade geometries. The study is made in the context of the realistic turbine stage configuration provided by the ETHZ 1.5-stage LISA turbine research facility. This numerical investigation describes the details of unsteady recess cavity flow structure and confirms the beneficial effects of the improved recess geometry over the flat tip and the nominal recess design both in terms of stage efficiency and tip heat load. The tip flow field obtained from the improved recess design combines the advantages of a nominal recess design (aerodynamic sealing) and the flat tip configuration. The turbine stage capacity is almost unchanged between the flat tip and the improved recess tip cases, which simplifies the design procedure when using the improved recess design. The overall heat load in the improved recess case is reduced by 26% compared with the flat tip and by 14% compared with the nominal recess. A key finding of this study is the difference in effects of the upstream stator wake on the recess cavity flow. Where cavity flow in the nominal design is only moderately influenced, the improved recess cavity flow shows enhanced flow unsteadiness. The tip Nusselt number from a purely steady-state prediction in the nominal recess case is nearly identical to the time-average prediction. The improved design shows a 6% difference between steady-state and time-average tip Nusselt number. This is due to the strong influence of the wake passing on the recess cavity flow. In fact, the wake enhances a small flow difference at the leading edge of the recess cavity between the nominal and improved recess cavities, which results in a completely different flow field further downstream in the recess cavity.

Simulation of laminar flow in a three‐dimensional lid‐driven cavity by lattice Boltzmann method

PurposeThe purpose of this paper is to apply lattice Boltzmann equation method (LBM) with multiple relaxation time (MRT) model, to investigate lid‐driven flow in a three‐dimensional (3D), rectangular cavity, and compare the results with flow in an equivalent two‐dimensional (2D) cavity.Design/methodology/approachThe second‐order MRT model is implemented in a 3D LBM code. The flow structure in cavities of different aspect ratios (0.25‐4) and Reynolds numbers (0.01‐1000) is investigated. The LBM simulation results are compared with those from numerical solution of Navier‐Stokes (NS) equations and with available experimental data.FindingsThe 3D simulations demonstrate that 2D models may predict the flow structure reasonably well at low Reynolds numbers, but significant differences with experimental data appear at high Reynolds numbers. Such discrepancy between 2D and 3D results are attributed to the effect of boundary layers near the side‐walls in transverse direction (in 3D), due to which the vorticity in the core‐region is weakened in general. Secondly, owing to the vortex stretching effect present in 3D flow, the vorticity in the transverse plane intensifies whereas that in the lateral plane decays, with increase in Reynolds number. However, on the symmetry‐plane, the flow structure variation with respect to cavity aspect ratio is found to be qualitatively consistent with results of 2D simulations. Secondary flow vortices whose axis is in the direction of the lid‐motion are observed; these are weak at low Reynolds numbers, but become quite strong at high Reynolds numbers.Originality/valueThe findings will be useful in the study of variety of enclosed fluid flows.

On the boundary-layer structure of cavity flow in a porous medium driven by differential heating

This paper describes the boundary-layer structure of flow through a porous medium in a two-dimensional rectangular cavity driven by differential heating of the upper surface. The lower surface and sidewalls of the cavity are thermally insulated. In the limit of large Darcy–Rayleigh number, the solution involves a horizontal boundary layer near the upper surface where the main thermal gradients occur. For a monotonic temperature distribution at the upper surface, these drive fluid to the colder end of the cavity where it descends within a narrow vertical boundary layer before returning to the horizontal layer. The horizontal and vertical layers form an interactive system which is solved by a combination of asymptotic analysis and numerical computation. A complete solution is obtained for the case of a quadratic temperature distribution at the upper surface. The solution of the interactive boundary-layer system determines the almost constant temperature in the core region below the horizontal and vertical layers, which contains relatively weak variations in both the thermal and velocity fields.

B205 高速ビデオPIVによる軸対称キャビティー内流れの時系列計測

The cavity flow is not only a fundamental problem in fluid dynamics, but also an important engineering problem. One of the actual problems of the cavity flow is pressure disturbance or acoustic noise generated in a duct with a cavity. The fundamental mechanism of pressure disturbance or noise generation in the cavity flow has been investigated. However, the structure of cavity flow depends on the shape of it and the flow condition in the duct. The flow in an axially symmetric cavity is not the same with the flow in a rectangular cavity. In this report, the flow in an axially symmetric cavity is investigated experimentally by using high-speed-video PIV. The results obtained by time series measurements of the flow show that the unsteadiness of the cavity flow depends on the cavity ratio and the circumferential structure of the flow must be considered.