Recirculation Flow Structure Research Articles

A two-cell vortex model

A two-cell vortex solution to the Navier–Stokes equations is derived in this article. The theoretical analysis provides expressions for the velocity components, vorticity, and pressure distributions of the two-cell vortex flow. A key feature of this new modeling approach is the presence of a “vortex eye” region, which is absent in classical single-cell vortex models. The derived tangential velocity profile is kept general, allowing for the recovery of the single-cell profile as a special case. The model parameters are constrained by the boundary conditions inherent to two-cell vortices. The model explicitly demonstrates that the two-cell vortex flow exhibits two regions of recirculating flow structures in the meridional plane. The analysis shows that near the vortex core, fluid descends from above and diverges outward. This downward flow meets the incoming flow from the vortex's outer region at a distance known as the “eye-wall” radius. The merging of these two streams results in an upward deflection, generating the two-cell fluid motion. Unlike in single-cell vortices, where vorticity reaches its maximum at the vortex center, the present results indicate that in two-cell vortices, the vorticity peaks at the eye-wall radius.

Turbulent Coherent Flow Structures to Predict the Behavior of Particles With Low to Intermediate Stokes Number Between Submerged Obstacles in Streams

AbstractUnderwater obstacles are identified as local hotspots of various particulate matters in streams. As the transport of particles is dependent on surrounding flow, we expect that flow structures created in the vicinity of obstacles provide a favorable condition for high particle concentration. We quantify the particle behaviors at small to intermediate range of Stokes number as they move past two obstacles forming a gap of varying length, based on three indices: the ratio of particles entering the gap, the time span between entry and exit of particles, and the integral time scale of flow. We performed laboratory experiments in a recirculating racetrack flume using quantitative imaging, particle image velocimetry to obtain flow velocity fields, and particle tracking velocimetry to track full trajectory of particles. As the gap length increases, the interaction between inner and outer flows of the gap increases, which is followed by an increase of the ratio of particles entering the gap and a decrease of the time span of particles and the integral time scale. Noticeably, indices converged when the gap length reached a critical value and recirculating flow structures were fully developed in between the gap, which indicates that the transport of particles with small to intermediate Stokes number (St = 0.1–0.5) is related to the large‐scale flow motions. Understanding these patterns allow us to select specific zones to sample and monitor both organic (seeds, eggs, and larvae of endemic aquatic species), and inorganic (sediment and microplastics) particles for better management of aquatic environments.

Influence of key geometrical features on the non-reacting flow of a Lean Direct Injection (LDI) combustor through Large-Eddy Simulation and a Design of Experiments

Lean Direct Injection (LDI) emerged as an interesting concept to limit NOx emissions in aero engines at the cost of operating close to the flame lean blow-off limit. In this technology, fuel is injected into a swirled airstream that generates recirculating flow structures that stabilize the flame. It is then of paramount importance at the design stage to understand the effect of various features on these structures. The present investigation makes use of Eulerian-Lagrangian Large-Eddy Simulations (LES) previously validated against existing experimental data for a reference condition to study the liquid non-reacting flow inside the CORIA Spray LDI burner with the help of Adaptive Mesh Refinement (AMR). A Design of Experiments (DoE) is proposed to analyze the significance of several geometrical features on the flow field, namely the combustor width, the air swirler vane angle, the number of swirler vanes and the axial location of the fuel injector tip. The study covers the qualitative appearance of the flow and the quantitative characterization of the spray dispersion and fuel-air mixing process. In this way, the chosen response variables include the size of the relevant coherent flow structures (Central Toroidal Recirculation Zone induced by the Vortex Breakdown Bubble, Corner Recirculation Zone and Swirled Jet) and their associated velocities, spray features (global drop sizes and spray penetration), pressure drop across the swirler and induced swirl number. Besides, the Precessing Vortex Core (PVC) relevance and frequency content is studied through Proper Orthogonal Decomposition (POD). Results from the statistical analysis show that the number of swirler vanes and their angle are the geometrical parameters that most importantly influence the flow features: stronger recirculation zones leading to an improved atomization and mixing have been found both when decreasing the number of swirler blades and increasing the blade angle. However, both solutions also increase the pressure losses across the swirler. As far as the spectral analysis is concerned, the number of swirler vanes is the most influencing factor on both the frequency and intensity of the PVC modes, being crucial for the possible activation and the energetic content of a double-helix PVC mode.

Experimental study on two-dimensional cross-flow impeller with a minimal splitter plate instead of casings

In the present study, the authors investigate the simplest model of cross-flow impellers, in order to provide the fundamental flow features of cross-flow impellers. The model is a two-dimensional cross-flow impeller with a very-high aspect ratio, and rotates in stationary fluid. More strictly, we occasionally place a splitter plate with minimal influence instead of casings for more precise observations and measurements. Instantaneous measurements of flow-velocity vectors on the mid-span plane are conducted using a CCD camera and a double-pulse YAG laser with a particle-image-velocimetry technique, in addition to conventional flow visualisations and hot-wire-velocimetry measurements. As a result, the authors show the revolutional motion of such an eccentric vortex as a large-scale re-circulating flow structure inside the cross-flow impeller, and reveal the detail of flow features at Reynolds number Re = 300-2500. Furthermore, the authors show the relations between Re and out-flow rate from the impeller.

An experimental and numerical investigation of forced convection in high porosity aluminum foams subjected to jet array impingement in channel-flow

This paper reports an experimental and numerical investigation on flow and thermal transport in high porosity aluminum foams (ε~0.94–0.96) placed in a channel of square cross-section and subjected to jet array (5 × 5) impingement. The plate issuing the jets was flushed with the metal foam face. Experiments were conducted for three different values of jet-to-jet spacing (x/dj=y/dj) of 2, 3 and 5, and three foam samples with pore-densities 10, 20, and 40 PPI (pores per inch). Values of steady-state heat transfer rate and pressure drop were calculated for Reynolds number (based on channel hydraulic diameter and channel inlet velocity) ranging from 2500 to 13,500. The main findings are as follows: first, for all the test runs, the impingement configurations had higher heat transfer rate, h, compared to that for the baseline configuration of metal foams in a channel flow. Second, for all values of the jet-to-jet spacing, an increase in pore-density was accompanied by an increase in heat transfer. The enhancement (h/h0) varied between 26 and 48, with the highest gain observed for the widest jet (x/d=2) and the highest pore density (40 ppi) configuration, where h0 is the heat transfer coefficient for developed turbulent flow in a circular duct. The numerical computations reveal interesting recirculating flow structures immediately downstream of the jet exits and the extent of flow penetrating in the foam. Collectively, these flow patterns play a dominant role in determining the volume of metal foam volume participating in the thermal transport and the net convective heat transfer rate.

Spatial variation of flow characteristics in a subarctic meandering river in ice‐covered and open‐channel conditions: A 2D hydrodynamic modelling approach

AbstractTo be able to understand year‐round river channel evolution both at present and in the future, the spatial variation of the flow characteristics and their sediment transport capabilities under ice cover need to be detected. As the measurements done through cross‐sectional drill holes cover only a small portion of the river channel area, the numerical simulations give insight into the wider spatial horizontal variation of the flow characteristics. Therefore, we simulate the ice‐covered flow with a hydrodynamic two‐dimensional (2D) model in a meandering subarctic river (Pulmanki River, Finland) in mid‐winter conditions and compare them to the pre‐winter open‐channel low flow situation. Based on the simulations, which are calibrated with reference measurements, we aim to detect (1) how ice‐covered mid‐winter flow characteristics vary spatially and (2) the erosion and sedimentation potential of the ice‐covered flow compared to open‐channel conditions.The 2D hydrodynamic model replicated the observed flow characteristics in both open‐channel and ice‐covered conditions. During both seasons, the greatest erosional forces locate in the shallow sections. The narrow, freely flowing channel area found in mid‐winter cause the main differences in the spatial flow variation between seasons. Despite the causes of the horizontal recirculating flow structures being similar in both seasons, the structures formed in different locations depended on whether the river was open or ice covered. The critical thresholds for particle entrainment are exceeded more often in open‐channel conditions than during ice‐covered flow. The results indicate spatially extensive sediment transport in open‐channel conditions, but that the spatial variability and differences in depositional and erosional locations increase in ice‐covered conditions. Asymmetrical bends and straight reaches erode throughout the year, whereas symmetrical, smaller bends mainly erode in open‐channel conditions and are prone to deposition in winter. The long ice‐covered season can greatly affect the annual morphology of the submerged channel. © 2019 John Wiley & Sons, Ltd.

Effect of offset-jets arrangement on leading edge hot-air heating effectiveness of engine inlet guide strut

An experimental investigation is conducted to study the effect of offset-jets arrangement on the conjugated heat transfer performance for a specifically wedge-shaped concave surface subjected to external cold flow and internal hot jet impingement. The external flow Reynolds number (Rec) defined on the base width of wedge-shaped concave surface is fixed as 54,000. The jet Reynolds number (Rej) is varied in the range of 7900–31,700. Four non-dimensional jet-hole offset distances (L/d) ranging from 0 to 2.5 and four non-dimensional jet-to-leading edge distances (H/d) ranging from 6 to 15 are considered respectively. In order to illustrate the effect of offset-jets arrangement on the flow and heat transfer features inside the wedge-shaped concave cavity, numerical simulations are also performed. The results show that the offset-jets arrangement generally has a significant impact on improving the overall heating effectiveness in the vicinity of concave wall leading edge due to the reduction of normal jet impinging distance and the formation of recirculation flow structure inside the concave cavity. In current study, L/d=1.5 is confirmed to be a superior offset-jets arrangement for achieving the highest specified area-averaged heating effectiveness. It is also noted that the effect of offset-jets arrangement on the leading edge hot-air heating effectiveness is tightly dependent on the normal jet-to-leading edge distance. Under H/d=15, the offset-jets distance has a weak effect of on the internal hot-jet impingement heat transfer. In this situation, the offset-jets arrangement has nearly no effect on improving the hot-jet heating effectiveness.

Flow measurements in the exhaust system of a motorized engine

Stereo Particle Image Velocimetry (SPIV) is used to measure the highly turbulent flow in the upstream region of the catalytic converter for a four stroke IC engine. These experiments are conducted for a motorized engine to investigate the feasibility of using SPIV for exhaust flow measurements in a firing IC engine in our future research. The results obtained here can also be used for validation of the CFD models. The measured flow is highly three dimensional, non-uniform with large magnitudes of turbulence indicating recirculating flow structures. These structures show signatures of jet flows coming out of the exhaust valves at all exhaust crank angles. The triangular shape and location for the end face of each exhaust runner pipe, the length and geometry of the runners also affect the flow mixing process upstream of the catalytic converter contributing to the complexity of this flow. Although majority of the exhaust flow passes through the catalytic converter, some will recirculate due to impingement of the exhaust jets on the surface of the catalyst.

Active Flow Control of a Low Reynolds Number S809 Wind Turbine Blade Model under Dynamic Pitching Maneuvers

A low Reynolds number wind turbine blade model based on the S809 airfoil was tested in a subsonic wind tunnel to study the structural vibration of the blade under dynamic pitching maneuvers. Piezoelectric-based synthetic jet actuators were embedded inside the blade and activated with a synthetic jet momentum coefficient, Cμ of 2.30 × 10-3. Structural vibration was quantified for a range of unsteady angles undergoing “pitch up and down” and “sinusoidal pitch” maneuvers at a Reynolds number of 5.28 × 104. The blade tip deflection amplitude and frequency were acquired utilizing a pair of strain gauges mounted at the root of the model. Using active flow control vibration reduction was more effective during the pitch up portion of the blade motion cycle compared to the pitch down portion. This effect is due to dynamic stall, where a leading edge vortex is shed during the pitch up motion and contributes to higher lift compared to static angles of attack and lower lift when the blade is pitched down. Dynamic stall was measured with phase-locked stereoscopic particle image velocimetry (SPIV), where global mean flow measurements reveal a shift in location and reduction in the size of a recirculating flow structure near the suction surface of the blade during the pitch up motion compared to the pitch down.

환기부족 구획화재에서 수직 개구부의 형상 및 위치가 화재특성에 미치는 영향

To investigate numerically the effects of geometry and location of vertical opening on the thermal and chemical fire characteristics in full-scale under-ventilated compartment fires, the ventilation factor () to estimate a theoretical maximum inflow of ambient air and the mass loss rate in a heptane pool fire were fixed for all cases. It was shown that variations in door geometry affected significantly the change in thermal and chemical characteristics inside the compartment. Variations in window location resulted in the complex change in additional fire characteristics including the fire duration time and recirculating flow structure. These results were analyzed in details by the multi-dimensional flow and fire characteristics including the vent flow and fuel/air mixing phenomena.

3D Numerical Combustion Simulation of Designed Low Heating Value Fuel Combustor for Further Detail Modulation

This study applied three-dimension numerical combustion simulation method to design the specifications of a low heating value (LHV) fuel combustor. According to the discussion on design parameter system, quantity of main jets is the governing factor for flame stabilization of the primary combustor zone. Combustor outlet temperature factor improves with increase of load, and can be reduced to 0.33 under full load. However, load increase may slightly change recirculation flow structure and weaken flame stabilization. The findings showed that the combustion strength of near-field area greatly increased, and the recirculation flow structure center moved to the liner well when combustor used propane as fuel. This also caused deterioration of temperature factor.

Design and simulation validation of LHV fuel combustor

The purpose of this study is to design a LHV (Low Heating Value) fuel combustor and apply it in a 10KW gas turbine engine for power generation. The source of LHV fuel is produced from gasifi cation in oxygen-defi cient combustion of waste plastic; its heating value is about 6.7 MJ/m3. Composition, adiabatic fl ame temperature, flammable range, burning rate of this fuel are analyzed in this paper. In combustor design, turbine air jet flow initiated recirculation flow structure is adopted to strengthen mixing and flame holding eff ect. The fuel is supplied through axial vortex to enhance local mixing of fuel and air. The current design is evaluatedaccording to previous experience and one-dimensional flow-fi eld characteristic. The detailed flow structure was obtained from numeric simulation analysis of 3D isothermal flow-fi eld. The preliminary conclusion is that better air / fuel mixing effect can be achieved by adopting configuration of co-swirlers recirculation flow.

On the Inconsistencies Related to Prediction of Flow into an Enclosing Hood Obstructed by a Worker

The recirculating flow structures formed in the wake of a worker standing in front of an enclosing fume hood were numerically investigated. Two- and three-dimensional, unsteady, laminar/turbulent computations were performed for a Reynolds number (Re) range of 1.0 × 103 − 1.0 × 105. The standard k − ϵ, Renormalization group (RNG) k − ϵ, and Shear Stress Transport (SST) k − ω models were used in Unsteady Reynolds Averaged Navier-Stokes (URANS) computations, and the results were compared with each other and also with the previous predictions reported in the literature. Numerical issues regarding the grid convergence and the inadequacies of turbulence models that may come into play at low Reynolds numbers were addressed. On the whole, SST k − ω model was found to be promising for qualitatively accurate prediction of both steady and unsteady recirculatory flow patterns in the wake of the worker. On the other hand, the standard and RNG k − ϵ models failed in prediction of anticipated unsteadiness at low Reynolds numbers. In a more realistic three-dimensional simulation with SST k − ω model, the anticipated unsteady and recirculating flow field in the wake of the worker was captured. Present results seem to qualitatively agree with the deductions made from experimental analyses in the literature while conflicting with some aspects of the previously reported numerical results. The apparent inconsistencies observed between the current results and those published in the literature were elucidated.

Internal recirculation flow structure in vertical upflow gas-solids suspensions part I. A core-annulus model

Internal recirculation flow structure in vertical upflow gas-solids suspensions part I. A core-annulus model

MODELING OF JET-GENERATED RECIRCULATING FLOWS IN A MINIATURE COMBUSTOR

The complicated flow fields in miniature annular combustors are investigated numerically. The flow parameters studied include the jet tube length and momentum ratio of the primary jet. From the numerical results it is found that four different recirculations, namely, the primary, second, third, and dilution recirculations, might be generated in the combustor owing to the interaction between the mainstream and the lateral jets. In all cases studied, the primary recirculation can be observed, although its strength, size, and location are changed with the flow parameters. The jet tube length produces a significant effect on the recirculation flow structure in the primary zone. The increase of the momentum ratio of the primary jet results in the suppression and enhancement of the primary and second recirculations, respectively.

Internal recirculation flow structure in vertical upflow gas-solid suspensions Part II. Flow structure predictions

A predictive two-region model previously developed by D.-R. Bai, J.-X. Zhu, Y. Jin and Z.-Q. Yu, Powder Technol., 85 (1995) 171, is used to illustrate the characteristics of the core-annulus internal recirculation flow structure in circulating fluidized beds. The variations of the flow parameters with operating conditions are examined using the simulation results. While most predictions agree well with the available experimental data, other simulation results are yet to be confirmed when more relevant experimental results become available.

Internal recirculation flow structure in vertical upflow gas-solids suspensions Part I. A core-annulus model

A comprehensive, predictive two-region model has been developed to describe the core-annular internal recirculation flow structure in vertical gas-solids upward flowing suspensions. This new model is more flexible by allowing axial variations of the core radius and the gas and particle velocities in both the core and the annular regions. The validity of the model has been confirmed by good agreement between the predicted values and the various experimental measurements reported in the literature, including the radius of core region, voidage in the core region, solids recirculation ratio and deposition coefficient.

Measurements of turbulent velocity and temperature fields for high Rayleigh number natural convection in an enclosure

ABSTRACT An experimental study of the turbulent velocity and temperature fields for high Rayleigh number natural convecton in an enclosure was conducted. A water filled test cell with one heated vertical wall and one opposite cooled vertical wall was used. All the other walls were thermally insulated. The cell had an aspect ratio of 1. Velocity measurements were taken at Rayleigh number of 3.0×10 10 based on the height of the vertical walls. In addition to the laminar, transitional and turbulent flow regions reported in the literature, a region near the corner with two recirculating flow structure was identified. Mean and fluctuating velocities were measured with laser-Doppler anemometer system. Mean and fluctuating temperatures were measured with fine-wire thermocouples. The concurrent measurement measurements of velocity and temperature provided the much needed experimental data for a better understanding of the underlying physics of turbulent natural convection in an enclosure.

A Weak collision of two natural-convection boundary layers

The collision of two natural-convection boundary layers at the tip of a vertical wedge is used to demonstrate that a double-deck flow structure provides a proper description of the heat-convection mechanism which is shared by many convection problems with sudden geometry changes. The present theory differs from previous work which indicate the existence ofrecirculating flow regions. This difference is due to a failure to recognize that recirculating flow structures can only exist for forced flows, but not for natural-convection boundary layers or wall jets. The present solution is obtained by a proper application of Prandtl's transposition theorem for geometries with finite solid displacements and appropriately matches the upstream natural-convection boundary layers and the downstream thermal plume. The interaction of the local pressure development in the main deck (outer layer) and the displacement of the lower deck (inner layer) removes the singularity associated with the boundary-layer equations at the location where the viscous layers leave the solid surface.