Characteristics Of Transitional Flow Research Articles

Direct cumulant lattice Boltzmann simulations of transitional flow in gyroidal structures including experimental validation

In this work, transitional flow characteristics in gyroidal structures are investigated. For specified flow rates corresponding to a range of Reynolds numbers from 400 to 7000, the pressure drop between inlet and outlet is determined. The experimentally obtained pressure drops are computed numerically with the cumulant lattice Boltzmann method which serves as a solver for the weakly compressible flow. Different grid resolutions are used for the simulations in order to investigate grid convergence. In order to create an accurate geometric representation of the setup, we use a newly developed formalism to describe gyroids with finite thickness based on an implicit volume representation. This representation avoids the expensive and inaccurate triangulation of the gyroid structure. Consequently, it avoids the problems encountered with mesh-based representations by providing a compact, local, and therefore inherently parallel representation for geometric computations such as classification of nodes and the computation of subgrid distances.

Convergence of Phase-Averaged, Transitional Flow in an Abdominal Aortic Aneurysmal Model.

Abdominal aortic aneurysm can exhibit transitional flow characteristics in laminar flow regimes. To report transitional flow characteristics, we examined the convergence of phase-averaged solutions by executing blood flow simulations of a patient-specific abdominal aortic aneurysmal model for 257 cardiac cycles with periodic, pulsatile boundary conditions. The phase-averaged solutions were computed by averaging the solutions over various numbers of cardiac cycles and compared against the ones averaged over 124 cycles. The phase-averaged solutions reported small differences when they were averaged over a large number of cardiac cycles. The instantaneous solutions, however, failed to exhibit fluctuations reported in the phase-averaged solutions. To study transitional blood flows in the aneurysmal region, we need to report phase-averaged solutions as they exhibit nonperiodic, disturbed flow characteristics. Additionally, when reporting phase-averaged solutions, it is preferred to compute an average over a large number of cardiac cycles to be able to represent flow structures of the converged phase-averaged solutions.

Flow diagnosis in variable bottom hole pressure multi-well horizontal pad with well interference using rate/pressure transient analysis

Horizontal multi-well pads are frequently used in unconventional reservoirs. Along with infill wells and hydraulic fracturing, interference between multiple multi-fractured horizontal wells (MFHWs) has become a major concern. The current rate transient analysis (RTA) makes the assumption that the unconventional formation contains a single MFHW. This study introduces a novel multi-MFHW solution and associated analysis methodology for analyzing the performance of targeted well rates in a multi-MFHW system.The constant bottom-hole pressure (BHP) condition and the Laplace transform can be used to obtain multi-MFHW solutions for transient flow. We investigated interference between various fractures and MFHWs using the superposition of various constant BHP solutions. The variable BHP of the targeted well is calculated using a variable dimensionless BHP function in the Laplace domain without performing any convolution or deconvolution. The proposed method is rigorously validated using a commercial numerical simulator for cases involving offset MFHWs and multi-MFHW with variable BHP. With this multi-MFHW analysis, we can analyze a target well in the pad using the total material balance of the multi-MFHW system. Offset well interference frequently occurs following the onset of infinite-acting radial flow (IARF) in the target well's hydraulic fracture. It results in an increase in the pressure derivative curves for elliptical flow and IARF, as well as the rate-normalized pressure (RNP) derivative. Inverse semi-log derivatives exhibit the inverse trend. The proposed deviation pressure integral and RNP can be used to diagnose the flow region caused by the offset well's flow rate in a unique manner, displaying the horizontal line, V-shaped dip, and unit slope, respectively, during IARF, cross flow, and boundary-dominated flow (BDF). Sensitivity analysis of well spacing demonstrates that as well spacing increases, the “transition flow” between wells transitions from elliptical to formation linear flow and can exhibit transitional flow characteristics in more common cases.

Implementation of the transition model for high order discontinuous Galerkin method with hybrid discretization strategy

The γ-Reθt⌢ transition model has been applied within the framework of high order discontinuous Galerkin (DG) discretization of Reynolds-averaged Navier-Stokes (RANS) equations. A hybrid discretization strategy that takes advantage of all available information from DG method is suggested for the implementation of the transition model. Some techniques for the spatial and temporal discretization are utilized to enhance the effectiveness of the method. The resulting approaches achieve high accuracy of transition prediction and meanwhile do not increase notable computational challenge when compared to DG discretization of fully coupled RANS/transition system. We carry out both steady and unsteady transitional simulations to examine the performance of the designed method. Numerical results have demonstrated that the present approaches are capable of capturing the transition flow characteristics as well as improving transition prediction accuracy with high order DG schemes.

Numerical identification of separation bubble in an ultra-high-lift turbine cascade using URANS simulation and proper orthogonal decomposition

The flow-field inside a gas turbine engine, especially in the low-pressure turbine, is very complicated as it is normally accompanied by unsteady flow structures, strong and rapidly changing pressure gradients, intermittent transition of boundary layer, and flow separation and reattachment, especially during off-design performance. In this article, flow separation and reattachment on the suction side of an ultra-high-lift low-pressure turbine blade is studied and characterized using 3D Unsteady Reynolds-Averaged Navier-Stokes (URANS) equations. For turbulence modeling, transitional-SST method (γ-Reθ) is adopted. The simulations are performed at the exit Reynolds numbers of 200,000 and 60,000, and at a constant isentropic exit Mach number of 0.4. The shape and extent of the separation bubble are primarily dependent on large vortical structures due to the Kelvin-Helmholtz instability and spanwise vortex tube shedding. Therefore, a better prediction of these phenomena could result in a more realistic separation bubble identification and consequently more accurate profile loss assessment. In order to better capture the transitional flow characteristics, which are not often readily available from conventional computational fluid dynamics simulations, the method of Proper Orthogonal Decomposition (POD) is used in this study. Non-coherent structures in the main flow, such as separation bubble, are investigated and studied. The POD modes of pressure-field are analyzed to clarify the generation of spanwise vortex tubes after separation point. In the higher Reynolds number, low-energy small-scale structures in the separation zone and downstream of the trailing edge are observed from the POD analysis. In the lower Reynolds number, high-energy large-scale structures shed from the separated shear layer are identified, which are responsible for increasing turbulent kinetic energy as well as increasing profile losses. This study also shows that the combination of URANS and POD can successfully be used to identify the separation bubble.

Investigation of flow through the two-stage orifice

ABSTRACTIn the field of engineering, understanding the flow characteristics of an orifice is key to accurately controlling pressure or flow; however, the transitional flow characteristics of the two-stage orifice has not been studied. The value of the discharge coefficient parameterizing the two-stage orifice flow equation is primarily based on that of the single classic orifice; however, this assumption leads to significant errors, because of the different numbers in the sudden contraction structure. Through theoretical derivation, the flow equations difference between the two-stage orifice and single classic orifice was compared and analyzed. Combing theoretical analysis, Computational Fluid Dynamics (CFD) simulation and experimental measurement, the transitional flow characteristics of the two-stage orifice were investigated using mineral oil. The comparison results of the three parts are essentially consistent. In this paper, the discharge coefficient and the pressure drop over the two-stage orifice were primarily focused on Reynolds numbers between 900 and 1700. In comparison with the single orifice, the results show that under the same dimensions (d = 3 mm), the discharge coefficient of the two-stage orifice is greater, and the transitional flow state of the two-stage orifice is different from that of the single orifice. Additionally, the pressure drop of the two-stage orifice is less than that of the single orifice under the same volume flow rate. The prestage cylindrical hole slows the changing trend of the flow field parameters, and the second-stage orifice with smaller diameter is mainly responsible for the pressure loss. The current research provides important support for accurate pressure and flow control of the two-stage orifice, and the role of the prestage cylindrical hole can also be a good reference in the structure design of micro-orifices.

Transition flow regime on stepped spillways: air–water flow characteristics and step-cavity fluctuations

Stepped spillways are man-made hydraulic structures designed to control the release of flow and to achieve a high energy dissipation. The flow pattern for a given stepped chute geometry can be distinguished into different regimes. Herein, the transition flow regime occurs at a range of intermediate discharges and is characterised by strong hydrodynamic fluctuations and intense splashing next to the free-surface. Up to date, only minimal experimental data is available for the transition flow. As this flow regime is likely to occur on stepped spillways designed for skimming flow operation, a knowledge of the transition flow characteristics is important to ensure safe operation. The present article investigates the hydraulics of the transition flow regime on a laboratory spillway, presenting a detailed characterisation of air–water flow properties and an image-based analysis of pool depth fluctuations within successive step-cavities. The results show two different void fraction and turbulence intensity profiles, indicating the existence of an upper and a lower transition flow sub-regime. The image-based analysis suggests the presence of a rapidly and a gradually varied flow region downstream of the inception point for both sub-regimes, whereas full equilibrium flow was not reached in the physical model. Overall, the study contributes towards improving the characterisation of the transition flow by assembling analytical solutions for different two-phase flow parameters, including void fraction, interfacial velocity and step-cavity pool height.

Investigation on liquid flow characteristics in microtubes

The fundamental understanding and prediction of liquid flow characteristics in microscale are important to control the performance of microfluidic devices. However, fundamental questions about liquid flow characteristics in microscale have not been settled yet and systematical investigation is needed. A systematical investigation on liquid flow characteristics through microtubes with diameters varying from 44.5–1011 μm and relative roughness in the range 0.02–4.32% in the Reynolds number range 29–11,644 was performed in this work, using water as working fluid. Experimental results indicated that early transition occurred when the diameter was smaller than 1000 μm, the transitional flow characteristics for smooth microtubes differed from rough microtubes and the friction factor in turbulent region for rough microtubes was larger than conventional theory. Moreover, a parameter α was proposed to describe the characteristic of microtube. The characteristic parameter was used to calculate the critical Reynolds number and the friction factor in turbulent region for microscale. © 2014 American Institute of Chemical Engineers AIChE J, 61: 718–735, 2015

Measurements and modeling of open-channel flows with finite semi-rigid vegetation patches

The hydrodynamics of flows through a finite length semi-rigid vegetation patch (VP) were investigated experimentally and numerically. Detailed measurements have been carried out to determine the spatial variation of velocity and turbulence profiles within the VP. The measurement results show that an intrusion region exists in which the peak Reynolds stress remains near the bed. The velocity profile is invariant within the downstream part of the VP while the Reynolds stress profile requires a longer distance to attain the spatially invariant state. Higher vegetation density leads to a shorter adjustment length of the transition region, and a higher turbulence level within the VP. The vegetation density used in the present study permits the passing through of water and causes the peak Reynolds stress and turbulence kinetic energy each the maximum at the downstream end of the patch. A 3D Reynolds-averaged Navier–Stokes model incorporating the Spalart–Allmaras turbulence closure was employed subsequently to replicate the flow development within the VP. The model reproduced transitional flow characteristics well and the results are in good agreement with the experimental data. Additional numerical experiments show that the adjustment length can be scaled by the water depth, mean velocity and maximum shear stress. Empirical equations of the adjustment lengths for mean velocity and Reynolds stress were derived with coefficients quantified from the numerical simulation results.

Transitional flow analysis in the carotid artery bifurcation by proper orthogonal decomposition and particle image velocimetry

Blood flow instabilities in the carotid artery bifurcation have been highly correlated to clot formation and mobilization resulting in ischemic stroke. In this work, PIV-measured flow velocities in normal and stenosed carotid artery bifurcation models were analyzed by means of proper orthogonal decomposition (POD). Through POD analysis, transition to more complex flow was visualized and quantified for increasing stenosis severity. While no evidence of transitional flow was seen in the normal model, the 50%-stenosed model started to show characteristics of transitional flow, which became highly evident in the 70% model, with greatest manifestation during the systolic phase of the cardiac cycle. By means of a model comparison, we demonstrate two quantitative measures of the flow complexity through the power-law decay slope of the energy spectrum and the global entropy. The more complex flow in the 70%-stenosed model showed a flatter slope of energy decay (−0.91 compared to −1.34 for 50% stenosis) and higher entropy values (0.26 compared to 0.17). Finally, the minimum temporal resolution required for POD analysis of carotid artery flow was found to be 100Hz when determined through a more typical energy-mode convergence test, as compared to 400Hz based on global entropy values.

The Computational Modeling of Transitional Flow through a Transonic Linear Turbine: Comparative Performance of Various Turbulence Models

Aero-thermal characteristics of transitional flow through a highly loaded transonic linear turbine are studied at the design incidence using computational methods. The three-dimensional compressible turbulent flow through a turbine inlet guide vane is simulated using finite-volume method-based fluid flow solutions using both Reynolds-averaged Navier Stokes (RANS) equations-based turbulence models, and a dynamic large eddy simulation (LES) approach which is based on a Smagorinsky-Lilly subgrid scale (SGS) model. The calculations are made for exit isentropic Mach numbers ranging from 0.7 to 1.1, and for Reynolds numbers ranging from 5 × 105 to 2 × 106. The numerical results obtained from different turbulence model simulations are compared with each other, and with the experimental data available in the open literature in terms of integrated flow parameters, such as pressure and heat transfer coefficients for varying exit Mach and Reynolds numbers. The present numerical approach indicates a guide line that the present LES approach can be a reliable choice for turbomachinery flows.

Experimental Study on the Helical Flow Field in a Concentric Annulus with Rotating Inner Cylinders

This experimental study concerns the characteristics of a transitional flow in a concentric annulus with a diameter ratio of 0.52, whose outer cylinder is stationary and inner one rotating. The pressure drops and skin-friction coefficients have been measured for the fully developed flow of water and that of glycerine-water solution (44%) at a inner cylinder rotational speed of rpm, respectively. The transitional flow has been examined by the measurement of pressure drops and the visualization of flow field, to reveal the relation of the Reynolds and Rossby numbers with the skin-friction coefficients and to understand the flow instability mechanism. The present results show that the skin-friction coefficients have the significant relation with the Rossby numbers, only for laminar regime. The occurrence of transition has been checked by the gradient changes of pressure drops and skin-friction coefficients with respect to the Reynolds numbers. The increasing rate of skin-friction coefficient due to the rotation is uniform for laminar flow regime, whereas it is suddenly reduced for transitional flow regime and, then, it is gradually declined for turbulent flow regime. Consequently, the critical (axial-flow) Reynolds number decreases as the rotational speed increases. Thus, the rotation of inner cylinder promotes the early occurrence of transition due to the excitation of taylor vortices

Numerical Prediction of Unsteady Transitional Boundary Layer Flows due to Rotor-Stator Interaction(II)-Characteristics of Unsteady Transitional Boundary Layer Flow-

A Navier-Stokes code with a modified low Reynolds number k-.epsilon. turbulence model was used to study the unsteady transitional boundary layer flow due to rotor-stator interaction. The modification, proposed by Launder, to improve prediction of stagnation flows was incorporated to the low Reynolds number k-.epsilon. turbulence model by Fan-Lakshminarayana-Barnett. Numerical solution is shown to capture well the calmed laminar flow as well as the wake induced transitional strip due to rotor-stator interaction and shows improvement, in terms of onset of transition and its length, over previous Euler/boundary layer solution. The turbulent kinetic energy shows local maximum along the upstream rotor wake in the wake induced transitional strip and this characteristics is observed untill the end of transition. The wake induced strip also shown apparent even in the laminar sublayer as the upstream rotor wake penetrates inside the boundary layer.