Separated Flow Regimes Research Articles

Neural network based two-phase flow classification in a vertical narrow rectangular channel

Flow Regimes in a vertical narrow rectangular channel of cross-section 20×1cm2 are investigated up-to wispy annular flow using a dense test matrix and double sensor conductivity probe at section mid point. The data from the probe is used to calculate void fraction, velocity of interfaces, and chord length of various flow structures. A five unit self organizing neural network is used to identify various flow regimes by using single point geometrical data of flow structures. Six separate flow regimes are found to exist. A new flow regime is identified and is called rolling-wispy flow. A discussion on boiling crisis is given regarding this flow regime. The resultant flow regime map is compared with various existing maps.

Numerical simulation of flow characteristics in nonprismatic compound channels

ABSTRACT The assessment of the flow characteristics of river systems is a very intricate undertaking in the development of hydraulic models for the purposes of flood control and floodplain management. Therefore, it is essential to use simulation models in order to calibrate and verify the experimental results. In this study, the Hydrologic Engineering Centre's – River Analysis System (HEC-RAS) is used to calibrate and validate the distribution of velocity and shear stress for different converging compound channels. Two separate flow regimes were assessed for validation based on experimental data obtained from converging compound channels with angles of θ = 5°, 9°, and 12.38°. The projected values for two relative depths (β = 0.15 and 0.20) exhibit a similar pattern of variation as the empirical observations and are marginally lower than the recorded values. This suggests that the HEC-RAS model accurately estimates the velocity and shear stress values. The disparity between the simulated and experimental outcomes shows a discrepancy of less than 10%. Hence, the implications of our results suggest that while dealing with nonprismatic rivers, it is advisable to take into account lower values. The used methodology and the outcomes focused on problem-solving might potentially inform the development of flood control infrastructure for nonprismatic watercourses.

Enhanced Aerodynamic Performance of a Two-Dimensional Airfoil Using Moving Boundaries

The effects of moving boundary conditions on the aerodynamic performance of a two-dimensional NACA 0012 airfoil at different angles of attack (α) in a uniform freestream at a Reynolds number of 10,000 are numerically studied. The moving boundary conditions are motivated by inviscid potential flow around the airfoil, which also satisfies the viscous equations of motion. In this study, the wall is moved at the slip velocity of the inviscid flow, or a fraction of it. These prescribed moving boundary conditions are shown to suppress vortex shedding, thus allowing the drag to go toward zero and the lift coefficient toward 2πα over a wide range of angles of attack (α≤20°) even in the separated flow regime. It is shown that moving boundary conditions are effective over a wide range of their strengths with respect to the overall power required (required to overcome aerodynamic drag and move the airfoil boundary). This power is shown to attain a minimum near the inviscid flow moving boundary condition, which is around 10% or less of the power required for the stationary boundary condition. Finally, the effectiveness of the moving boundary conditions is demonstrated for fully turbulent flow regimes at Re=6 million as well.

A modified dual time integration technique for aerodynamic quasi-static and dynamic stall hysteresis

Simulation of the aerodynamic stall phenomenon in both quasi-static and dynamic conditions requires expensive computational resources. The computations become even more costly for static stall hysteresis using an unsteady solver with very slow variation of angle of attack at low reduced frequencies. In an explicit time-marching solver that satisfies the low Courant number condition, that is, [Formula: see text], the computational cost for such simulations becomes prohibitive, especially at higher Reynolds numbers due to the presence of thin-stretched cells with large aspect ratio in the boundary layer. In this paper, a segregated solver method such as the Semi-Implicit Method for Pressure-Linked Equations (SIMPLE) is modified as a dual pseudo-time marching method so that the unsteady problem at each time step is reformulated as a steady state problem. The resulting system of equations in the discretized finite volume formulation is then reduced to zero or near-zero residuals using available convergence acceleration methods such as local time stepping, multi-grid acceleration and residual smoothing. The performance and accuracy of the implemented algorithm was tested for three different airfoils at low to moderate Reynolds numbers in both incompressible and compressible flow conditions covering both attached and separated flow regimes. The results obtained are in close agreement with the published experimental and computational results for both quasi-static and dynamic stall and have demonstrated significant savings in computational time.

Forced Response Due to Vane Stagger Angle Variation in an Axial Compressor

Abstract Recent developments in turbomachinery design require an improved prediction accuracy of blade vibrations to maintain safe operations. This article aims to investigate the accuracy of numerical aeroelastic approaches for the calculation of blade vibrations. For validation, extensive aerodynamic and forced response measurements in an 1.5-stage axial compressor with a blade integrated disk (Blisk) are presented. The excitation intensity of the vibration is controlled by varying the stagger angle of the inlet guide vane (IGV). In addition, a second engine order is imposed by a nonsymmetric circumferential vane angle distribution to simulate a multistage behavior. Experimental validated Reynolds-averaged Navier–Stokes (RANS) simulations in both the frequency and the time domain are compared to assess the prediction accuracy of the numerical approaches. The numerical results agree with the experiments for low and intermediate vane angles. However, at high IGV stagger angles and when exciting multiple engine orders, the inaccuracy in the prediction of flow separation by the RANS simulations leads to an overprediction of vibration amplitudes. This exaggeration becomes even more pronounced in the frequency domain simulations. Time domain methods with a time lag formulation tend to be efficient and more accurate approaches for large separated flow regimes.

Modeling and Control of Dynamic Stall Loads on a Smart Airfoil at Low Reynolds Number

This article describes the development and testing of a modified, semi-empirical ONERA dynamic stall model for an airfoil with a trailing edge flap—a “smart airfoil”—pitching at reduced frequencies up to 0.1. The Reynolds number is 105. The model reconstructs the load fluctuations associated with the shedding of multiple dynamic stall vortices (DSVs) in a time-marching solution, which makes it suitable for real-time control of a trailing edge flap (TEF). No other model captures the effect of the DSVs on the aerodynamic loads on smart airfoils. The model was refined and tuned for force measurements on a smart NACA 643-618 airfoil model that was pitching with an inactive TEF and was validated against the measurements when the TEF was activated. A substantial laminar separation bubble can develop on this airfoil, which is challenging for modelers of the unsteady response. A closed-loop controller was designed offline in SIMULINK, and the output of the controller was applied to the TEF in a wind tunnel. The results indicated that the model has a comparable accuracy for predicting loads with the active TEF compared to inactive TEF loads. In the fully separated flow regime, the controller performed worse when dealing with the development of the laminar separation bubble and DSVs.

Dynamic Stall on High-Lift Airfoil 30P30N in Ground Proximity

Computational prediction of stall aerodynamics in free air and in close proximity to the ground considering the 30P30N three-element high-lift configuration is carried out based on CFD simulations using the OpenFOAM code and Fluent software. Both the attached and separated flow regimes are simulated using the Reynolds Averaged Navier-Stokes (RANS) equations closed with the Spalart-Allamaras (SA) turbulence model for static conditions and pitch oscillations at Reynolds number, Re = 5 x 106 and Mach number, M = 0.2. The effects of closeness to the ground and dynamic stall are investigated and the reduction in the lift force in close proximity to the ground is discussed.

Оптимізація теплообміну робочих поверхонь на основі локального контролю теплофізичних параметрів

The progressive trend of increasing the efficiency and operational reliability of equipment requires continuous improvement of methods for monitoring and managing work processes. One of the most promising methods for studying transport processes occurring in difficult conditions is physical modeling. The object of this research is heat-exchange prismatic surfaces, which are typical for many technical applications. The purpose of the work is to develop methods for optimizing heat transfer of working surfaces based on local control of thermophysical parameters in characteristic zones of the working environment. The studies were carried out in the ADS-1 aerodynamic stand using the methods of heat and mass analogy and hot-wire anemometry. The arrangement included four rows of prismatic elements. The essence of the local approach is to determine the average surface heat transfer coefficient from the local velocity, measured above each prismatic element, which makes it possible to estimate the spatial temperature heterogeneity of a specific arrangement and take measures to change the temperature in the desired direction by maneuvering the location of the elements. The proposed approach is fundamentally different from another widespread approach, which we called the channel one, in which information about the velocity and temperature fields is ignored, and the influence of the configuration and size of elements and their location on the hydrodynamic structure of the flow is insufficiently taken into account. The local approach allows, firstly, to more accurately diagnose the types of flow in the boundary layer of a streamlined element, determining laminar, turbulent, pseudolaminar, quasiturbulent, transition and separated flow regimes. Second, on the basis of a set of statistical data, make the transition to an arbitrarily specified arrangement of elements and thereby increase the accuracy of determining the temperature state of individual prismatic elements and the entire arrangement as a whole. Equations of similarity are proposed for calculating the local heat transfer of each face of the prism separately and the average surface heat transfer depending on the geometric and operating parameters. On the basis of the recommendations received, the thermal state of the elements of a specific arrangement was evaluated and measures were developed to improve it through targeted rearrangements of the elements.

Direct Numerical Simulation of Interaction between Transient Flow and Blade Structure in a Modern Low-Pressure Turbine

A feature of a modern aeronautical Low-Pressure Turbine (LPT) is the high blade loadings with complex, transient and separated flow regimes. Most existing research have focused only on analysing the transient flow and flow separation in such turbines. The aerodynamics of a modern LPT, however, can be significantly influenced by the interaction between the unsteady flow field and the blade structure motion in a complex non-linear fashion which could lead to aeroelastic instabilities such as flutter. Therefore, the understanding of the mechanism of the interaction between the flow field unsteadiness and the blade structure in a modern LPT is essential to examine the vibration stress levels to ensure the blade mechanical integrity. The novelty of this paper, first and foremost, is using a high-fidelity direct numerical simulation method to explore the mechanism of flutter and forced response in a modern LPT, T106A turbine, and to study the effects of various sources of unsteadiness on the aeroelastic instabilities of the blade. Secondly, this paper investigates and identifies the adequate working ranges of the harmonic balance method, which has been widely used for the aeromechanical analysis of turbomachines, on predicting the behaviour of the highly unsteady flow due to fluid-structure interaction in an LPT. Another emphasis of this paper is the determination of the capability of the Unsteady Reynolds Averaged Navier–Stokes (URANS) model for the aeroelasticity analysis of an LPT involving the highly unsteady flow. This paper will bridge a key gap in the knowledge of aeroelasticity modelling and analysis of modern LPTs.

The presence of the “locking effect” when the fluid flows through the cylindrical throttle channels

A study was made for the continuous flow of the working fluid through a cylindrical throttle channel (so-called “locking effect”). The flow of the fluid into the atmosphere and the change in absolute pressure at the inlet to the throttle channel ranging between 0.221 MPa and 0.24 MPa are studied. It was found that the pressure in the compressed section inside the throttle channel remains almost constant and equal to 4 kPa. A comparison is made of the experimental values of the flow rate determined for the same absolute pressure difference Δp in the separated and unseparated flow regimes. Knowing that the value of the flow coefficient μ in the zone of the existence of the “locking effect” can be determined from known data for the process of fluid outflow from holes with a sharp edge in a thin wall. Also, similar phenomena occur when fluid flows through small holes and slots in the guide, control and regulatory equipment of various hydraulic systems.

From droplets to waves: periodic instability patterns in highly viscous microfluidic flows

We experimentally study the transition from droplet to wave regimes in microfluidic liquid–liquid multiphase flows having large differences in viscosity. A unified approach based on periodic pattern analysis is employed to study relationships between dispersed and separated flow regimes, including dripping, jetting, capillary waves, inertial waves and core–annular flows over a wide range of flow rates and viscosity contrasts. We examine the morphology and dynamics of each flow regime based on wavelength, frequency and velocity of repeating unit cells to elucidate their connections and to develop predictive capabilities based on dimensionless control parameters. We demonstrate in particular that pattern selection is contingent upon the propagation velocity of droplets and waves at the transition. We also investigate microfluidic wave breaking phenomena with the formation of ligaments and droplets from wave crests in both capillary and inertial wave regimes. This work expands conventional multiphase flow regimes observed in microchannels and shows new routes to disperse highly viscous materials using interfacial waves dynamics in confined microsystems.

On the Periodically Unsteady Interaction of Wakes, Secondary Flow Development, and Boundary Layer Flow in An Annular Low-Pressure Turbine Cascade: An Experimental Investigation

The experimental results reported in this contribution address the time-dependent impact of periodically unsteady wakes on the development of profile and end wall boundary layers and consequently on the secondary flow system. Experimental investigations are conducted on an annular 1.5 stage axial turbine rig at Ruhr-Universität Bochum’s Chair of Thermal Turbomachines and Aeroengines. The object under investigation is a modified T106 profile low-pressure turbine (LPT) stator row at a representative exit flow Reynolds number of 200,000. By making use of an annular geometry instead of a linear cascade, the influence of curvilinear end walls, nonuniform, increasing pitch across the span and radial flow migration can be represented. Incoming wakes are generated by a variable-speed driven rotor equipped with cylindrical bars. Special emphasis is put on the wake-induced recurrent formation, suppression, weakening, and displacement of individual vortices and separated flow regimes. For this, based on a comprehensive set of time-resolved measurement data, the interaction of impinging bar wakes and boundary layer flow and thus separation and its periodic manipulation along the passage end walls and on the blade suction surface are studied within the frequency domain.

Analysis of Pressure Pulsations on the Wall in a Flat Diffuser Channel in Unseparated and Separated Flow Regimes

The paper presents the results of experimental studies of pressure pulsations on the wall of a flat diffuser channel in unseparated and separated flow regimes. The dependences of the root-mean-square values of pressure pulsation amplitudes along the wall and the frequency responses for different flow regimes in the channel are given. It is shown that the maximum of the pulsation motion near the wall takes place in the beginning of the diffuser section and corresponds to the preseparation regime. The enhancement of the pulsating motion in the near-wall zone is related to the response of the flow to the pressure gradient, which is determined by the channel geometry, and is aimed at maintaining the initial unseparated flow in the boundary layer.

Validation of the SST-HL turbulence model for separated flows and flows around airfoils

Testing of the modification of the SST turbulence model (SST-HL) developed for improvement of prediction of flow around airfoils was carried out for different flows. Although the SST-HL model is slightly worse than the original SST model for turbulent separated flows the agreement of the SST-HL model with the experimental data is quite satisfactory. For flow around airfoils the SST-HL demonstrates superiority over the SST model for separated flow regimes near stall conditions.

Effect of Thickness-to-Chord Ratio on Flow Structure of a Low Swept Delta Wing

The effect of thickness-to-chord () ratio on flow structure of a delta wing with sweep angle of 35 deg is characterized in a low-speed wind tunnel using laser-illuminated smoke visualization, particle image velocimetry, and surface pressure measurements. Four different ratios varying from 4.75 to 19% are tested at angles of attack, 4, 6, 8, and 10 deg, for Reynolds numbers and . The results indicate that the effect of thickness-to-chord ratio on flow structure is quite substantial, such that, as the wing thickness increases, the flow structure transforms from leading edge vortex to three-dimensional separated flow regime. The wing with lowest ratio of 4.75% has pronounced surface separation at significantly higher angle of attack compared with the wing with highest ratio, which indicates that lowest ratio wing might be more resistive to the stall condition. However, considering the low angles of attack where all wings experience leading edge vortex structure, the strength of the vortex structure increases as the ratio increases, which might suggest a better vortex-induced lift performance with high ratio wing at low angles of attack.

Formation of Branched and Chained Alginate Microfibers Using Theta-Glass Capillaries.

This study proposes a microfluidic spinning method to form alginate microfibers with branched and chained structures by controlling two streams of a sodium alginate solution extruded from a theta-glass capillary (a double-compartmented glass capillary). The two streams have three flow regimes: (i) a combined flow regime (single-threaded stream), (ii) a separated flow regime (double-threaded stream), and (iii) a chained flow regime (stream of repeating single- and double-threaded streams). The flow rate of the sodium alginate solution and the tip diameter of the theta-glass capillary are the two parameters which decide the flow regime. By controlling the two parameters, we form branched (a Y-shaped structure composed of thick parent fiber and permanently divided two thin fibers) and chained (a repeating structure of single- and double-threaded fibers with constant frequency) alginate microfibers with various dimensions. Furthermore, we demonstrate the applicability of the alginate microfibers as sacrificial templates for the formation of chain-shaped microchannels with two inlets. Such microchannels could mimic the structure of blood vessels and are applicable for the research fields of fluidics including hemodynamics.

Mechanistic model to predict frequency and amplitude of Density Wave Oscillations in vertical upflow boiling

Modeling of two-phase flow transient behavior and instabilities has traditionally been one of the more challenging endeavors in heat transfer research due to the need to distinguish between a wide range of instability modes systems can manifest depending on differences in operating conditions, as well as the difficulty in experimentally determining key characteristics of these phenomena. This study presents a new mechanistic model for Density Wave Oscillations (DWOs) in vertical upflow boiling using conclusions drawn from analysis of flow visualization images and transient experimental results as a basis from which to begin modeling. Counter to many prior studies attributing DWOs to feedback effects between flow rate, pressure drop, and flow enthalpy causing oscillations in position of the bulk boiling boundary, the present instability mode stems primarily from body force acting on liquid and vapor phases in a separated flow regime leading to liquid accumulation in the near-inlet region of the test section, which eventually departs and moves along the channel, acting to re-wet liquid film along the channel walls and re-establish annular, co-current flow. This process was modeled by dividing the test section into three distinct control volumes and solving transient conservation equations for each, yielding predictions of frequencies at which this process occurs as well as amplitude of associated pressure oscillations. Values for these parameters were validated against an experimental database of 236 FC-72 points and show the model provides good predictive accuracy and capably captures the influence of parametric changes to operating conditions.

Self-noise and directivity of simple airfoils during stall: An experimental comparison

Noise measurements of NACA 0012, NACA 0021 and flat plate airfoils are obtained at a Reynolds number of 96,000, at angles of attack ranging from −30° to 30°. As the airfoils enter a separated flow regime the strength of the dipolar noise due to shed vorticity increases. This noise source grows in strength over a much smaller range of angles as the thickness of the airfoils is increased, causing the sound pressure level of the NACA 0021 airfoil in this frequency range to increase considerably more sharply than the other profiles. In addition the NACA 0021 test model displays noise behaviour that is consistent with an unstable transition between prestall and stalled flow states, compared to transitions involving intermediate flow states observed for the NACA 0012 and flat plate airfoils.

Dynamic unified RANS-LES simulations of high Reynolds number separated flows

The development of hybrid RANS-LES methods is seen to be a very promising approach to enable efficient simulations of high Reynolds number turbulent flows involving flow separation. To contribute to further advances, we present a new, theoretically well based, dynamic hybrid RANS-LES method, referred to as DLUM. It is applied to a high Reynolds number flow involving both attached and separated flow regimes: a periodic hill flow is simulated at a Reynolds number of 37 000. Its performance is compared to pure LES, pure RANS, other hybrid RANS-LES (given by DLUM modifications), and experimental observations. It is shown that the use of this computational method offers huge cost reductions (which scale with Re/200, Re refers to the Reynolds number) of very high Reynolds number flow simulations compared to LES, it is much more accurate than RANS, and more accurate than LES, which is not fully resolved. In particular, this conclusion does also apply to the comparison of DLUM and pure LES simulations on rather coarse grids, which are often simply required to deal with simulations of very high Reynolds number flows: the DLUM provides mean velocity fields which are hardly affected by the grid, whereas LES velocity fields reveal significant shortcomings. We identified the reason for the superior performance of our new dynamic hybrid RANS-LES method compared to LES: it is the model’s ability to respond to a changing resolution with adequate turbulent viscosity changes by ensuring simultaneously a physically correct turbulence length scale specification under the presence of interacting RANS and LES modes.

Experimental investigation of viscous oil-water flows in pipeline

Pipelines encounter high pressure losses during transport of high viscous crudes, especially at lower temperatures present in deep-waters. With emphasis on flow regimes and pressure drop of viscous oil-water flows, an experimental study was conducted on oil-water two phase flow in a 27.86mm pipeline, with oils of viscosity 30cP and 300cP.High resolution visualization studies, using high speed camera capture, were conducted to study flow patterns of oil-water two phase flows, that were further developed into flow regime maps. Visualization of the oil-water flow regimes reveals different layers of stratification for separated flow regimes with the existence of a near stationary oil layer on the top for heavy oil(300cP). While light oil(30cP)-water flow patterns were comparable to flow pattern maps found in literature, heavy oil-water flow patterns showed considerable deviations. The flow regime transitions have been found to occur at lower superficial velocities for heavy oil, than for light oil-water flows. Pressure drop characteristics of the viscous mixture flows were also investigated and compared with existing experimental results in literature. The studies reveal that phase inversion point occurs at higher phase fractions for heavy oils as opposed to light oils.