Flow In Curved Channel Research Articles

FLEW: A DNS Solver for Compressible Flows in Generalized Curvilinear Coordinates

We present FLEW, an in-house high-fidelity solver for direct numerical simulation (DNS) of turbulent compressible flows over arbitrary shaped geometries. FLEW solves the Navier–Stokes equations written in a generalized curvilinear coordinate system, in which the surface coordinates are non-orthogonal, whereas the third axis is normal to the surface. Spatial discretization relies on high-order finite-difference schemes. The convective terms are discretized using an hybrid approach, combining the near-zero numerical dissipation provided by central approximations with the robustness of weighted essentially non-oscillatory (WENO) schemes, required to capture shock waves. Central schemes are stabilized using a skew-symmetric-like splitting of convective derivatives, endowing the solver with the energy-preserving property in the inviscid limit. The maximum order of accuracy is eighth for central schemes (also used for viscous terms discretization) and seventh for WENO. The code is oriented to modern high-performance computing (HPC) platforms thanks to message passing interface (MPI) parallelization and the ability to run on graphics processing unit (GPU) architectures. Reliability, accuracy and robustness of the code are assessed in the low-subsonic, transonic and supersonic regimes. We present the results of several benchmarks, namely the inviscid Taylor–Green vortex, turbulent curved channel flow, transonic laminar flow over a NACA 0012 airfoil and turbulent supersonic ramp flow. The results for all configurations proved to be in excellent agreement with previous studies.

Effect of Rigid Vegetation Arrangement on the Mixed Layer of Curved Channel Flow

Curved channels and aquatic vegetation are commonly present in the riverine environment. In this study, the effects of vegetation density and distribution on the hydrodynamic characteristics of a mixed layer developed over a 180-degree curved channel were investigated through flume experiments. Wooden sticks were used to simulate rigid vegetation distributed along the half side of the channel, and a 200 Hz acoustic Doppler velocimeter (ADV) was employed to measure the three-dimensional instantaneous velocity at five selected cross sections along the curved channel. Experimental results show that the vegetation covering the half of the channel significantly affects the hydrodynamic structure of the curved channel flow, and the unequal vegetation resistance induces the K-H instability at the vegetation and non-vegetation interface, resulting in a standard hyperbolic tangent function of streamwise velocity distribution along the lateral direction. The influence of curve position on turbulence kinetic energy is far greater than that of vegetation density and vegetation distribution. The peak value of turbulent kinetic energy is comprehensively affected by vegetation density and distribution, and the peak position of turbulent kinetic energy at the interface is changed by different vegetation distribution. The combined effect of the curve and the partly covered vegetation increases the mixing between the water bodies, enhancing turbulent kinetic energy, and vegetation along the concave bank plays a more significant role. For turbulent bursting, the inward and outward interactions are mainly bursting events in the vegetation area, while ejections and sweeps are dominant in the non-vegetation area. However, the critical vegetation condition to initiate large-scale coherent structure (LSS) in the mixed layer and the influence of flexible vegetation need to be further studied in the future.

Three-Dimensional Hydrostatic Curved Channel Flow Simulations Using Non-Staggered Triangular Grids

Non-staggered triangular grids have many advantages in performing river or ocean modeling with the finite-volume method. However, horizontal divergence errors may occur, especially in large-scale hydrostatic calculations with centrifugal acceleration. This paper proposes an unstructured finite-volume method with a filtered scheme to mitigate the divergence noise and avoid further influencing the velocities and water elevation. In hydrostatic pressure calculations, we apply the proposed method to three-dimensional curved channel flows. Approximations reduce the numerical errors after filtering the horizontal divergence operator, and the approximation is second-order accurate. Numerical results for the channel flow accurately calculate the velocity profile and surface elevation at different Froude numbers. Moreover, secondary flow features such as the vortex pattern and its movement along the channel sections are also well captured.

Numerical simulation for peristalsis of Sisko nanofluid in curved channel with double-diffusive convection

A theoretical study has been developed to explore peristaltically deformable curved channel flow of Sisko nanofluid with double-diffusive (thermal and concentration) convection. Such consideration supports the flow of fluid in biological vessels and arteries to transport the food contents, oxygen, waste products, heat, blood and several other nutrients to the body. The flow stream is regulated and restricted in laminar region by considering long wavelength (d≪λ→∞) and low Reynolds number (Re→0) in terms of velocity, temperature, concentration and nanoparticle volume fraction fields. Moreover the mathematical expressions are listed for mixed convection and viscous dissipation aspects. The outcomes of influential dynamic parameters are sketched as graphical flow patterns with focus towards double-diffusive convection. Computational technique NDSolve in Mathematica is employed. The results for classical Newtonian fluid are captured as a special case of this analysis. The upshots of this investigation can be supportive for an enhancement of gastrointestinal movements and pumping in several engineering devices.

Darcy–Brinkman Flow in a Corrugated Curved Channel

A theoretical analysis of a viscous flow in a corrugated curved channel enclosing a porous medium is carried out. The alignment of the corrugations of the outer and the inner curved walls is taken arbitrarily, and the corrugations are considered to be sinusoidal in nature with periodicity. The flow problem is described by Darcy–Brinkman model, derived in the curvilinear coordinates. The effects of the channel curvature, the wall corrugations and the medium permeability are studied through the boundary perturbation technique, for small corrugation amplitude. A substantial effect of the porous medium on the flow is observed when compared to that of the flow in a corrugated curved channel with clear conduit, especially for low permeability medium. Flow enhancement is found to take place for small corrugation wavenumbers, and maximum augmentation is realized for the completely out-of-phase alignment of the two corrugated curved walls. However, the flow reduces for large enough wavenumbers, and the alignment of corrugated curved walls eventually becomes irrelevant, with no influence on the flow. For low permeability medium, the results also show no effect of the wall alignment on the flow. In general, the effect of the channel curvature on the corrugated curved channel flow is discussed relative to a corrugated straight channel flow to demonstrate the implications of the wall geometry enclosing the porous medium.

Numerical thermal‐hydraulic performance investigations in turbulent curved channel flow with horseshoe baffles

AbstractA numerical work is performed to investigate the thermal‐hydraulic performance in a curved channel of a journal bearing equipped with oblique horseshoe baffles. Water, a working fluid, is passed through the curved channel at a constant temperature condition of 358 K. The effects of different parameters of baffles, that is, attack angle (α = 45°, 60°, and 90°) and the number of baffles (NB = 9 and 13 baffles), are examined. Influences of design parameters on heat transfer and friction performances are studied and displayed in terms of the Nusselt number, the friction factor, the Nusselt number enhancement ratio, and the thermal‐hydraulic performance factor (THPF). The numerical simulations present the flow structures of the tested channel in terms of velocity, isotherms, turbulent kinetic energy, and vorticity contours. The numerical results reveal that the adopted geometry of the curved channel with baffles yields a significant enhancement of heat transfer rate over the plain channel (without baffles), which is approximately 2.5 to 3.8 times. Also, the results show that the best condition to achieve maximum heat transfer is at angle α = 90°, NB = 13, and Re = 5000, compared with other conditions. Furthermore, the maximum THPF of the curved channel using baffles is 4.4 at the same condition. The results confirmed that the geometry of the baffles inside the curved channel has a remarkable impact on heat transfer improvement, accompanied by a reasonable increase in friction losses.

Two-point correlation and integral scale of spatially advancing curved channel flow at friction-velocity-based Reynolds number 550

Two-point correlation and integral scale of curved channel turbulent flow were examined for onset and growth of large-scale vortex. Away from the two walls of the straight channel, the two-point correlation of streamwise velocity has large negative values around local minimum at y/δ = 1.6 and those of radial and spanwise components take small negative values at the same area. The large-scale motion in the straight channel is thus indicated to have conspicuous streamwise component and weak secondary motion. In the curved channel, two-point correlation of radial velocity decreased around the minimum away from the wall (y/δ = 1.6) and large negative values extended to the whole channel width. It is thus indicated that the large-scale vortex originated from the large-scale structures with conspicuous streamwise component in the outer-wall side, grew and expanded to the full channel width of the curved part. The integral scale integrating two-point correlation conditionally on the negative values greatly decreased to denote the onset and growth of the large-scale motion quantitatively. The local minima of two-point correlation for radial velocity were used as indicator of the large-scale vortex and near-wall small-scale vortex. The magnitude of it at the channel center area was as low as one thirds of those of near-wall area in the straight channel, and however increased and surpassed those of near-wall area in the curved channel. Therefore, large-scale vortices were weaker than the small-scale motion in the straight part of the channel, and they grew and surpassed the small-scale motion in strength in the curved part of the channel.

曲面流路を流下する水流の不安定性制御に関する研究

曲面流路を流下する水流の不安定性制御に関する研究

A comparative study between stirred dead end and circular flow in microfiltration of China clay suspensions

A well-defined comparative study between stirred dead end and circular crossflow for microfiltration of china clay suspensions has been undertaken. The comparisons have been made with respect to convective mass transfer coefficients, permeation and rejection rates, and energy consumption. Similar operating and hydrodynamic conditions were implemented for the comparison. According to our experimental data the circular crossflow module was proven to perform better as compared with the stirred dead end system due to the higher mass transfer coefficients, higher permeation rates and lower energy consumption. The mass transfer coefficients observed are comparable to those previously found in vortex flow filtration and dead end flow filtration. The presence of Dean vortices in the circular crossflow module promotes flow instabilities in the curved channel flow path which reduce the concentration polarization effect during the filtration process. The concentration polarization effect however deteriorated due to solute build up (high solute concentration at the membrane surface) and decrease of the shear stress, i.e., the particle lift forces on the membrane surface. This resulted in deposition of particles on the membrane surface. In terms of energy consumption, for the same energy cost the limiting flux reached in circular crossflow was found to be higher than in the stirred dead end unit.

Secondary flow structure and thermal behaviour of immiscible two-phase fluid flow in curved channels

This paper investigates the behaviour of two non-mixing fluids flowing in a heated curved channel. In simulating this immiscible fluid flow, a multi-phase model based on VOF approach is developed for steady, turbulent, incompressible fluid flow through curved channel. The analysis examines the influence of centrifugal forces arising from channel curvature on the phase distribution and flow patterns, and their impact on the thermal characteristics. Unique flow features are identified through the interactions between the fluid phases and vortex structures of the immiscible flow in curved channels. The convective heat transfer in curved channel is appraised in relation to the behaviour of fluid components and curved channel flow parameters. Finally, the study presents a Second-Law irreversibility analysis whereby a thermal optimisation technique is developed for immiscible fluid flow through heated curved channels.

Relaxation of Spatially Advancing Coherent Structures in a Turbulent Curved Channel Flow

A direct numerical simulation is made for the incompressible turbulent flow in the 180 deg curved channel with a long straight portion connected to its exit port. An examination is made for how the organized coherent vortex grows and decays in the curved channel: the radius ratio of 0.92, the aspect ratio of 7.2, and the succeeding straight section length of 75 times the channel half width. The 1552 × 91 × 128 ( = 18,427,136) grids are allocated to the computational domain. The frictional-velocity-based Reynolds number is kept at 150 to resolve the long domain including curved and straight regions. In contrast to that the coherent vortex grows along the concave wall, the vortex remains strong in the convex-wall side after the curvature accompanying a tail of the small-scale turbulence near the convex wall. The dissimilarity between the onset and disappearing of the coherent vortex essentially comes from the mean pressure gradient, which aids or averts the near-wall fluid oppositely between the curvature inlet and the exit. The mean flow is decelerated near the inlet of the convex wall to destabilize the flow and to trigger the onset of the coherent vortex. Contrary, the mean flow is accelerated near the exit of the convex wall to weaken the coherent vortex, and is decelerated near the exit of the concave wall to enhance the turbulence. Therefore, the turbulence enhancement and attenuation occurs oppositely between the inlet and exit of the curvature, and the coherent vortex draws a wake in the convex-side rather than the concave-side where it starts.

Mitigating horizontal divergence “checker-board” oscillations on unstructured triangular C-grids for nonlinear hydrostatic and nonhydrostatic flows

A complication of finite-volume triangular C-grid methods is the numerical emergence of horizontal divergence errors that lead to grid-scale oscillations in vertical velocity. Nonlinear feedback via advection of momentum can lead to numerical instability in velocity modes via positive feedback with spurious vertical velocities induced by horizontal divergence truncation error. Existing strategies to mitigate divergence errors such as direct divergence averaging and increased diffusion do not completely mitigate horizontal vertical velocity oscillations. We present a novel elliptic filtering approach to mitigate this spurious error and more accurately represent vertical velocities via improved calculation of horizontal divergences. These results are applied to laminar curved channel flows, demonstrating the applicability of the method to reproduce secondary flow features.

Al2O3나노입자가 젤(Gel) 추진제의 곡관 유동특성에 미치는 연구

Curved duct channel flow characteristics for non-Newtonian gel fluid is investigated. A simulant gel propellant mixed by Water, Carbopol 941 and NaOH solution has been chosen to analyze the gel propellant flow behavior. Rheological data have been measured prior to the flow analysis where water-gel propellant and water-gel propellant with nano particles are both used. The critical Dean number examined by the numerical simulation in the U-shape duct flow reveals that although water-gel-nano propellants have higher apparent viscosity, the critical Dean number do show no notable difference for both the two gel propellant. It is found that the power-law index may be a dominant parameter in determining the critical Dean number and that the gel with particles addition may be more vulnerable to Dean instability.

Calculation for Circulating Velocity in Curved Channel

The hydraulic characteristics of curved channel flow are one of the most important subjects to hydraulic researchers and engineers. Among them the magnitude of circulation velocity and its calculation method is of great interests. Experimentation of generalized curved channel was carried out to explore the flow velocity parameters and circulation velocity characteristics by adopting the ADV to measure and analyze the 3D velocity datum. Theoretic formula was established for calculating the circulating velocity in the curved channel. Comparison with the results calculated from the formulas of other researchers the formula proposed in the paper is relatively quite good. The paper provides a basis to explore the hydraulic characteristics of the curved channel profoundly.

Analysis of Curved-Cooling-Channel Flow and Heat Transfer in Rocket Engines

DOI: 10.2514/1.B34163 Coolant-flow modeling in regeneratively cooled rocket engines fed with turbomachinery is a challenging task because of the high wall-temperature gradient, the high Reynolds number, the high aspect ratio of the channel cross section,andthecurvedgeometry.Inthepresentstudy,tobettercomprehendtheroleofthethrust-chambershapeof a rocket engine on the heat exchange, computations of supercritical hydrogen flow in single- and double-curvature channels are carried out. In particular, a parametric numerical analysis of the flow in an asymmetrically heated rectangular channel with a high aspect ratio and various radii of curvature is performed by means of a Reynoldsaveraged Navier–Stokes solver for real fluids, which is validated against experimental data of heated and curvedchannel flow taken from open literature. Results permit the effect of curvature on global heat transfer coefficient, pressure loss, and bulk temperature increase to be quantified.

Study on Three-Dimensional Turbulence Characteristics of Curved Channel Flow

The paper presents an experimental study on three-dimensional turbulence characteristics of curved channel flow by Doppler meter .Measured the time-average velocity and fluctuating intensity and so on. Analysised turbulence characteristics of curved channel flow by the experimental data,and also compared the flow distribution of vertical、horizontal fluctuating intensity,get the conclusion.

Energy transient growth in curved channel flow

Transient growth of perturbations due to non-normality for the curved channel Poiseuille flow (CCPF) is presented. The study covers a wide range of the radii ratios η as well as axial and azimuthal modes, with the purpose of complementing the results of linear stability for this flow with a study of the optimal linear growth possible in the linearly stable parameter regions. For the wide-gap case, the transient growth of streamwise-azimuthal modes that grow most is of low level and suppressed by curvature. It is also found that as curved channel flow approaches the flow in a straight channel enough, both the normal and non-nor- mal stability characteristics are almost identical to that of plane Poiseuille flow. The modulation of the basic circular Poiseuille flow by the presence of azimuthal streaks resulting from the significant growth of initial perturbations can be clearly visualized. For the transition region between the wide-gap case and the narrow-gap case, the sensitivity of eigenvalues is shown to be closely related to the magnitude of transient growth, which is tuned by curvature in a smooth way.

Discussion of “Modeling the Evolution of Incised Streams. II: Streambank Erosion” by Eddy J. Langendoen and Andrew Simon

The authors are to be congratulated on having successfully implemented a conceptual model for two bank failure mechanisms into their numerical model CONservational Channel Evolution and Pollutant Transport System CONCEPTS and on having validated it against long-term field data of streambank erosion. A comparison of measured and simulated cross sections at the study site indicates reasonable behavior of their numerical model of bank stability for streambank failure. As noted by the authors, CONCEPTS does not predict the increased shear stress along an outer riverbank caused by the helical flow pattern within a bend. Curved channel flow is associated with a helical motion—often termed “secondary flow”— generating a transverse velocity component in addition to the main streamwise velocity. Yalin and da Silva 2001 mention the prominence of the secondary flow that progressively decreases as the width-to-depth ratio B / h of a curve increases. The authors’ treatment of the missing helical motion within their field validation is, however, questionable. The increased shear stresses at the bend apex due to secondary flow are accounted for by a reduction of the critical shear stress. However, the range of critical shear stresses used, between 1 Pa up- and downstream of the bend and 8 Pa at the bend apex, is surprising. This is nearly a one-order-of-magnitude difference, while the magnitude of the secondary flow velocity is typically some 10% of the streamwise velocity magnitude Blanckaert and de Vriend 2004. The range of the selected critical shear stresses appears therefore to be extremely large. This is also reflected by the results presented. Cross sections with a small critical shear stress of 1 Pa underlie stronger and temporally increased streambank retreat than those considered using a higher critical shear stress of 4 Pa. The discusser therefore questions the sensitivity of the authors’ approach relative to the selected model to account for secondary flow. Were simulations also made with a larger or smaller ratio of critical shear stresses at cross sections along a typical bend? Finally, the discusser would like to ask the authors how the critical shear stresses can be selected in terms of bend hydraulics, topography and geotechnical data?

Passive microfluidic devices for plasma extraction from whole human blood

Our goal is to analyze and compare different continuous microfluidic principles dedicated to plasma extraction from hardly diluted human blood for lab-on-chip applications. First, the strengths and weaknesses of various emerging passive microfluidic methods (microfiltration- and centrifugation-based methods) were analyzed. Various devices were designed, microfabricated and tested with beads or blood. Filtration may be efficient, but with a high sample dilution, low flow rate and optimized geometry. Due to fast cell clogging, this remains a short-term solution. Separation effects resulting from centrifugal acceleration in curved channel flows are hindered by Dean vortices and anyhow are not pronounced with blood. An innovative device is then proposed and investigated experimentally. This is based on the lateral migration of red cells and the resulting cell-free layer, which is used to supply geometric singularities (an ear-cavity or a corner-edge) and locally enhance the clear plasma region. A maximum extraction of 10.7% is obtained for 1/20 diluted blood, injected at 100 μL/min in the corner-edge design.

Higher order critical flow condition in curved streamline flow

A new extended Bernoulli energy equation, which is valid for moderately curved channel flows, is presented. From this generalized energy equation, a higher order critical flow condition valid for curved streamline flow was developed. As a practical application, the flow over a circular-crested weir was studied. The proposed new critical flow theory improves the results of a previous lower order theory up to a dimensionless specific energy of 2, thus obtaining accurate results in situations in which the velocity is non-uniform, the pressure non-hydrostatic, and the conventional critical flow theory not valid.