We perform RANS-VOF simulation of density-stratified, fully developed air-water flow in a 2D channel. The flow is completely specified by the (common) driving pressure gradient down the channel and by the fill factor (relative height of the heavier phase to the total height of the channel). Varying the pressure gradient and fill factor results in different flow combinations: namely, laminar air/laminar water, turbulent air/laminar water, turbulent air/turbulent water, laminar air/turbulent water. The focus of the study is the near-interface interaction when either or both phases are turbulent. The RANS-VOF equations are solved on a 2D channel with periodic inlet/outlet boundary conditions. For a fixed fill factor, the Reynolds numbers of each phase varies monotonically with driving pressure gradient. However, at fixed pressure gradient, the fluid Reynolds numbers are non-monotonic at high fill factor. This Reynolds number phase diagram is important in understanding the laminar and turbulent regimes of each phase. The mean velocity is axial (down the channel) and exhibits a dip below the free surface whenever one of the phases is turbulent. As expected, the diagonal Reynolds stresses <u’u’>, <v’v’>, are strongly coupled between the phases, however coupling in <u’v’> is less pronounced.

The simulation models used in the design process of modern turbo-machines are becoming increasingly complex. Nevertheless, the steady-state RANS approach is still the mostly used method for CFD computations. However, detailed flow information is more and more required for further improving the performance and extend the operating range. Turbulence modeling has becoming therefore a key issue in this context. The increased computer power already available would enable the use of more sophisticated turbulence models than standard two equation ones, such as SST k-omega and k-epsilon. These, despite their shortcomings, are still predominant in the field, since more advanced models often lead to numerical instabilities in the simulations. The most important shortcomings can be related to either boundary layer effects or mixing process in the channels. In order to improve the predictions considering boundary layer effects, like impingement or large pressure gradients in flow direction, various derivations of four equations models were investigated. Using an additional transport equation for the wall normal Reynolds-stress component and an elliptic equation for near-wall effects, they improve the results for this kind of flows. Considering the accurate prediction of mixing processes, like (1) the interaction of tip-clearance vortices with the main flow or (2) off-design conditions, the focus was oriented to the anisotropy present in the turbulent structures. Standard models are often not sufficient to predict accurately vortices, which can have a huge impact on the performance, since based on the assumption of isotropic turbulence. Accordingly, they tend to dissipate and diffuse the vortices too quickly. Improved models, which take the anisotropic nature of the Reynolds-stresses into account, can help in this context. The models can thereby introduce the additional anisotropy via an explicit algebraic expression, or model directly the transport equations for the Reynolds-stresses. In order to improve the predictions using advanced turbulence models a particularly robust framework based on a pressure-based fully coupled approach was used. The goal of this work is the development and testing of improved models for the application in turbo-machinery. The focus lies thereby on near-wall behavior and mixing / vortex dissipation. The assessment of the models is exemplarily used on the centrifugal compressor open-case Radiver with vaned diffuser.

In this paper, we use a CFD analysis of a simplified, 2D geometry to study the ability of mako shark denticles to mitigate flow separation. We represent the viscous sublayer below a turbulent boundary layer streak as a Couette flow. Incipient separation is simulated by balancing upper wall velocity and adverse pressure gradient to achieve zero net mass flow, and we add various denticle geometries to study their effects. Each modeled denticle protrudes at an angle from 15° to 85° and sublayer blockage ratio from 0.05 to 0.85. Through variation of fluid properties and boundary conditions, we show that the anti-flow-reversal abilities of a single, bristled shark denticle are independent of Reynolds number, and we investigate the effect of the denticle at cases other than zero net mass flux. Based on these results, we create a new relationship to predict separation inhibition. These conclusions are highly generalizable and represent previously undiscovered universal behavior.

The present paper illustrates a new method for solving the two-dimensional turbulent fluid flow, for an incompressible fluid, near solid walls. Equations of energy conservation and momentum for thin layers have been used and solved with a slip velocity implemented in the Navier-Stokes equations. As a first step, a pseudo-laminar analysis was made. In parallel, some other models for turbulence were analyzed, to estimate the most appropriate model in conformity with the obtained experimental results. The numerical simulation was performed to investigate the effects of the Reynolds number on the flow characteristics, over a two-dimensional rectangular body and a curved shape surface. It was used the CFD, near the solid surfaces with a strong adverse pressure gradient. The viscous layer is expressed as an integral solution, imposing the wall shear stress as a boundary condition on solid surfaces. The velocity and shear stress are deduced from the Navier-Stokes equations as long as the convergence is reached. Numerical modeling was tested for Reynolds numbers ranging from 2040 to 13000. Firstly, the theoretical model for the flow equations and velocity distribution is described. Some details are presented for estimating as better as possible the wall velocity and the shear stresses. Numerical modeling, with the FEM model, is based on the Galerkin formulation. Some aspects referring to the boundary conditions, for the analyzed cases are also mentioned. The flow is restricted at a central vortex, considered the primary vortex. In turbulent boundary layer, for the analyzed applications were not considered the axial vortices. Some aspects referring to the flow domain and the grid generation are mentioned in a dedicated paragraph. For numerical modeling, firstly was adopted the non-viscous solution, as the initial condition. It depends on the available information concerning the shapes geometry, the curvature radius, and the flow conditions for the considered problems. For the first approximation, was used the integral boundary-layer method, which can provide a proper solution. The advantage consists of reducing the number of iteration to reach the numerically final settlement. If it will be used the integral method, will be provided only an approximate distribution for the edge velocity. Variation of the reattachment length of vortices over the analyzed solid bodies is correlated with the experimental results from the literature. Exclusive analysis of the selected data shows that the turbulent values correlated directly to the body length-b are more accurate than those referring to the dimensionless rapport δ/b. From the analyzed models, the K-ε model presents better agreement with experimental data, also for the negative values of the Reynolds shear stress in the re-attachment point. Finally, some conclusions and references are mentioned.

The use of solid biomass as a primary energy source for cooking is common to nearly half of the world’s population. Household air pollution as a byproduct of biomass combustion creates powerful negative health impacts related to air quality and a strong influence on our global radiative balance. Despite efforts to improve biomass-fueled cooking technology, many current designs still fail to meet WHO guidelines for air quality and consume excessive fuel. One promising method to improve in both of these areas is through introduction of forced primary or secondary air to the combustion process to increase turbulence, mixing, and velocity. Incorporating computational fluid dynamics to the design process for this forced draft air flow can provide insights into the complex and interconnected thermophysical relationships which, otherwise, would require extensive experimentation. The objective of this work is to provide a preliminary computational fluid dynamics study of a secondary air forced draft biomass cookstove. Thermal efficiency and emissions concentrations are investigated relative to various combinations of secondary air flow rates and injection angles. The results from the case study suggest that thermal efficiency of the cookstove is a function of secondary air injection angle, with optimal angle being a function of the specific air-fuel ratio. Additionally, a design trade-off is evident when comparing the pollutant concentration data and thermal efficiency data. Lastly, analysis of the computational results suggests that large pressure gradients about secondary air vortices in the combustion chamber lead to improved thermal efficiency and more complete combustion. The continued development of this work into an open-source computational fluid dynamics tool is underway.

Temporally varying turbulent flows are of considerable interest in complex engineering problems such as combustion, hydrodynamics, and hemodynamics. These types of flows are often associated with complex flow physics such as varying mean pressure gradients, interactions of different scales of motion, and complex boundary layer separations. Hybrid Reynolds-averaged Navier-Stokes (RANS)/Large-Eddy Simulation (LES) methods have recently shown promise for accurate and computationally efficient simulation of these flows. One such method is the dyanamic hybrid RANS-LES (DHRL) model which has been demonstrated for numerous statistically stationary turbulent flows. More recently, it has been shown that Exponential Time-Averaging (ETA) and Dynamic Time Filtering (DTF) methods for obtaining resolved flow statistics have significantly improved the predictive capabilities of the Dynamic Hybrid RANS-LES (DHRL) model performance for a non-stationary turbulent flows with periodically time-varying statistics. However, for non-periodic temporally evolving flows with monotonically varying statistics, a more suitable alternative is desired. In this study, the performance of the Dynamic Hybrid RANS-LES (DHRL) model with a double exponential dynamic time filtering (DDTF) methodology is evaluated against a Reynolds-Averaged Navier-Stokes (RANS) model, a conventional Hybrid RANS-LES (HRL) model, implicit LES, and the DHRL model with DTF for a pulsating channel and a temporally-varying turbulent mixing layer. Model performance is evaluated based on comparisons to existing experimental and Direct Numerical Simulation (DNS) results. A comprehensive analysis of the results highlights key similarities and differences between the models and indicates that the use of a double exponential DTF technique improves the accuracy of the baseline DHRL model. It is concluded that the DDTF is a useful alternative to simulate unsteady non-periodic temporally evolving turbulent flows.

Friction drag reduction effect of a passive blowing on a Clark-Y airfoil is investigated. The passive blowing is conducted by the pressure difference on a wing surface between suction and blowing regions. The suction and the blowing regions are respectively set around the leading edge and the rear part of the upper surface. The Reynolds number based on the chord length is 0.65 × 10 6 and 1.54 × 10 6 . The angle of attack is set to 0° and 6°. The mean velocity profiles on the blowing region and the downstream are shifted away from the wall by the passive blowing. This behavior qualitatively suggests local reduction of skin friction on the wing surface. As a result of the quantitative assessment, which takes into account the effects of pressure gradient and the roughness of the wall, the local friction drag reduction effect of passive blowing is estimated to reach 4%–23%.

Unsteady turbulent wall bounded flows can produce complex flow physics including temporally varying mean pressure gradients, intermittent regions of high turbulence intensity, and interaction of different scales of motion. As a representative example, pulsating channel flow presents significant challenges for newly developed and existing turbulence models in computational fluid dynamics (CFD) simulations. The present study investigates the performance of the Dynamic Hybrid RANS-LES (DHRL) model with a newly proposed dynamic time filtering (DTF) technique, compared against an industry standard Reynolds-Averaged Navier-Stokes (RANS) model, Monotonically Integrated Large Eddy Simulation (MILES), and two conventional Hybrid RANS-LES (HRL) models. Model performance is evaluated based on comparison to previously documented Large Eddy Simulation (LES) results. Simulations are performed for a fully developed flow in a channel with time-periodic driving pressure gradient. Results highlight the relative merits of each model type and indicate that the use of a dynamic time filtering technique improves the accuracy of the DHRL model when compared to a static time filtering technique. A comprehensive evaluation of the results suggests that the DHRL-DTF method provides the most consistently accurate reproduction of the time-dependent mean flow characteristics for all models investigated.

A rotorcraft main-rotor blade experiences a broad range of the Mach number in high-speed forward flights or rapid maneuvering conditions. Near the boundary of the flight envelope, the rotor blade often encounters severe dynamic stall which limits the overall aerodynamic performance. Of interest here is effects of the Mach number on the dynamic stall. A rotor airfoil, VR-12, is computationally investigated for both static and dynamic stall conditions with varying the Mach number from 0.2 to 0.4. For the small enough Mach number 0.2, both static and dynamic stall are significantly influenced by flow separation due to adverse pressure gradient. For the highest Mach number 0.4, compressibility near the leading edge is no longer negligible, forming a shock there. The shock-induced separation occurs near the leading edge, which dominates both the static and dynamic stall at the high Mach number. At the intermediate Mach number 0.3, it is observed that both adverse pressure gradient and the shock affect the stall. Current computations are conducted with the unsteady Reynolds-averaged Navier Stokes approach with the Spalart-Allmaras model. Numerical results are compared to relevant wind-tunnel test data.

Temporally developing direct numerical simulations (T-DNS) are performed for bypass transition of a zero pressure gradient flat plate boundary layer to understand the interplay between pressure-strain terms and flow instability mechanisms, and to propose and validate a phenomenological hypothesis for the identification of a robust transition onset marker for use in transition-sensitive Reynolds-averaged Navier-Stokes (RANS) computational fluid dynamics (CFD) simulations. Results show that transition initiates at a location where the slow pressure-strain term becomes more dominant than the rapid term in the pre-transitional boundary layer region. The slow term is responsible for the transfer of turbulence energy from the streamwise component to other components, most importantly the wall-normal. The relative magnitudes of the slow and rapid terms can potentially provide a basis for the development of physically meaningful large-scale parameters that can be used as transition onset markers for Reynolds averaged Navier-Stokes (RANS) simulations.

Numerical modeling of turbulent mixing is complicated not only by the natural complexity of the flow physics, but by the model sensitivity to the user specified conditions. When aiming to quantify the level of trust in a given model, a validation experiment is often performed to provide data against which confidence comparisons can be made, ultimately evaluating the adequacy of the model assumptions. In the nuclear community, a number of promising Generation IV reactor concepts have been proposed, each requiring high fidelity modeling capabilities to accurately assess safety related issues. The experiment described in this paper is intended to provide a much needed validation data set to assess the use of computational fluid dynamics (CFD) in a complex turbulent mixing scenario relevant to the prismatic very high temperature reactor (VHTR) concept. Over the course of the VHTR’s operation, the reactor’s graphite moderated hexagonal fuel blocks shrink from neutron damage, forming interstitial gaps between adjacent blocks. A significant percentage of the coolant can flow through these gaps, having a substantial impact on the thermal-hydraulic conditions in the core. An experimental facility is presented that uses air as a simulant fluid and includes a unit cell representation of the hexagonal blocks, which accounts for both the intended circular channels and secondary rectangular slot features induced by the bypass gaps. The outlet of the unit cell consists of a collated jet consisting of a central round jet surrounded by three slot jets at relative 120° angles to one another issuing into a stagnant domain. A preliminary test case is proposed in which the collated jet is set to an isothermal and iso-velocity condition. Constant temperature anemometry (CTA) and constant current anemometry (CCA) measurements serve to capture the velocity and temperature inlet quantities (IQs). Particle image velocimetry (PIV) measurements provide the appropriate system response quantities (SRQs), yielding insight into the mean and fluctuating components of the 2-D velocity field. Results are presented in the range of 0–8 diameters downstream of the inlet to the test section. The collated jet inlet region yields velocity profiles that are heavily influenced by opposing pressure gradients between the neighboring round and slot regions. As a result, the velocity peaks found in this area are neither in the centerline of the round jet nor that of the slot, but are towards the outer edge of each. With increasing downstream distance, the collated jet is found to exhibit a more classical round jet profile. The inlet region of the collated jet is thus of particular interest to future modeling efforts to more accurately depict the lower plenum behavior and transition to a self-similar profile downstream. Proper uncertainty quantification is also presented, and aids in assessing the integrity of the experimental results for future CFD validation.

Nowadays, mini unmanned aerial vehicles (MUAVs) and micro air vehicles (MAVs) are not only beneficially used as aviation models but also as modern drones for military missions and other civilian applications. Hence, research and development of propulsion sources for MUAVs and MAVs dynamically increase with a future trend of high performance, but low energy consumption. Certainly, using micro and ultra-small-size gas turbine is a good option for the propulsion source. To achieve ideal flight of MUAVs and MAVs powered by micro and ultra-small-size gas turbines under this trend, understanding of flow phenomena at wide ranges of Reynolds number is essential. This research presents a 2D numerical study of characteristics of laminar flow separation and the trailing-edge vortex on a turbine vane at extra-low Reynolds numbers (Res) i.e. Re = 1800 and 3600, and three rotational angles (α) i.e. α = 0°, 15° and 30° using immersed boundary method (IBM). With this method, the problem of incompressible flow is addressed by a sharp interface IBM. Numerical results indicate that IBM can characterize phenomena of laminar separation flow, which usually happens on the turbine airfoil when laminar boundary layer cannot overcome adverse pressure gradients and viscous effects. To our current knowledge, this may be the first research to study flow behavior at such low Res for gas turbine vanes using IBM. Even though it is now not common to operate micro and ultra-small-size gas turbines under these conditions, it is important to know how aerodynamic performance may be if micro and ultra-small-size gas turbines need to run under such conditions in the near future.

Interaction between surge behavior and internal flow field under coexisting phenomena of surge and rotating stall was experimentally investigated. In the experiment, the tank pressure of the compressor during surge was measured to detect the effect of the back-pressure fluctuation on the change in the internal flow field. Furthermore, the rotating stall in the compressor was investigated to define the influence of an unsteady internal flow field change on the surge behavior. From the tank pressure measurements, the amplitude of the tank pressure fluctuation was found to vary depending on the cycle. A larger maximal value for the tank pressure fluctuation led to a higher flow rate where the stall inception occurred. This difference in the flow rate indicated that the stall was induced by a severe adverse pressure gradient in the compressor. Then, the absolute rate of change in the flow coefficient was increased by both a large decrease in the compressor back pressure and performance degradation from stalling. In a case where the rate of decline in the flow rate was large, the scale of the stall cell developed up to a deep stall according to the movement of the operating point. Thus, a large trajectory for the surge cycle was selected, where the unsteady operating point went through the deep stall region. This development in the scale of the stall cell suggested to be influenced by the instability of the inner flow field caused by the rapid change in the flow rate.

We perform a careful nondimensional analysis of the turbulent boundary layer equations in order to bring out, without assuming any self-similar behaviour, a consistent set of nondimensional parameters characterizing the outer region of turbulent boundary layers with arbitrary pressure gradients. These nondimensional parameters are a pressure gradient parameter, a Reynolds number (different from commonly used ones) and an inertial parameter. They are obtained without assuming a priori the outer length and velocity scales. They represent the ratio of the magnitudes of two types of forces in the outer region, using the Reynolds shear stress gradient (apparent turbulent force) as the reference force: inertia to apparent turbulent forces for the inertial parameter, pressure to apparent turbulent forces for the pressure gradient parameter and apparent turbulent to viscous forces for the Reynolds number. We determine under what conditions they retain their meaning, depending on the outer velocity scale that is considered, with the help of seven boundary layer databases. We find the impressive result that if the Zagarola-Smits velocity is used as the outer velocity scale, the streamwise evolution of the three ratios of forces in the outer region can be accurately followed with these non-dimensional parameters in all these flows — not just the order of magnitude of these ratios. This cannot be achieved with three other outer velocity scales commonly used for pressure gradient turbulent boundary layers. Consequently, the three new nondimensional parameters, when expressed with the Zagarola-Smits velocity, can be used to follow — in a global sense — the streamwise evolution of the stream-wise mean momentum balance in the outer region. This study provides a clear and consistent framework for the analysis of the outer region of adverse-pressure-gradient turbulent boundary layers.

The Eulerian methods are susceptible to generate the nonphysical spurious currents in the multiphase flow simulations near the interfaces. This paper presents a new Eulerian method to accurately simulate the velocity fields, especially near the multiphase flow interfaces and prevents the numerical results from generating the nonphysical currents. A Eulerian central difference finite-volume scheme equipped with the suitable numerical dissipation terms is used to simulate incompressible multiphase flows. The interface is captured by Flux Corrected Transport-Volume of Fluid method (FCT-VOF). Increasing the accuracy near the sharp gradients, such as interface, the conservative form of incompressible Navier-Stokes equations is solved to locally conserve the properties. The main feature of this algorithm is its ability to control the pressure gradient oscillation and also spurious currents near the interface; two common problems in multiphase flow simulations, and as a result improves the accuracy of the simulation by artificial numerical dissipations. The results show the FCT-VOF is able to precisely calculate the interface, and the numerical dissipation terms are the powerful device to prevent the spurious currents.

Recently, the number of gas turbine combined cycle plants is rapidly increasing in substitution of nuclear power plants. The turbine inlet temperature (TIT) is being constantly increased in order to achieve higher efficiency. Therefore, the improvement of the cooling technology for high temperature gas turbine blades is one of the most important issue to be solved. In a gas turbine, the main flow impinging at the leading edge of the turbine blade generates a so called horseshoe vortex by the interaction of its boundary layer and generated pressure gradient at the leading edge. The pressure surface leg of this horseshoe vortex crosses the passage and reaches the blade suction surface, driven by the pressure gradient existing between two consecutive blades. In addition, this pressure gradient generates a crossflow along the endwall. This all results into a very complex flow field in proximity of the endwall. For this reason, burnouts tend to occur at a specific position in the vicinity of the leading edge. In this research, a methodology to cool the endwall of the turbine blade by means of film cooling jets from the blade surface is proposed. The cooling performance and heat transfer coefficient distribution is investigated using the transient thermography method. CFD analysis is also conducted to know the phenomena occurring at the end wall and calculate the heat transfer distribution.

Recent experimental and modeling studies have indicated that turbulence and cavitation behaviors within a realistic fuel injector have significant effects on the liquid atomization and spray processes. In addition to the breakup process induced by aerodynamic force at the liquid/gas interface, the effects of flow characteristics including turbulence and cavitation inside the injector nozzle on atomization have been shown to be important. The cavitation within the injector is complicated by the turbulent flow under large pressure gradient and geometry of the injector orifice. We have previously developed the “T-blob” and “T-TAB” model, for liquid fuel primary and secondary breakup predictions respectively, to account for liquid turbulence effects within the injector. The objective of this study is to further account for the cavitation effect in the atomization process of a cylindrical liquid jet. In the primary breakup model, the level of the turbulence effect on the liquid breakup depends on the characteristic scales and the initial flow conditions. These scales are further modified to include the cavitation effect. The drop size formed is estimated based on the energy distribution among wave, turbulence and cavitation modes. This paper describes theoretical development of the current model. Both non-evaporating and evaporating spray cases will be investigated to validate the newly developed cavitation-induced atomization model.

Three-dimensional flow structures of sheet-to-cloud transition have attracted a lot of attentions in the past years. Cloud cavitating flow around a slender cylinder with the petals shaped section was investigated by experimental and numerical methods. The experiment was performed in a launching system on the basis of the SHPB technology with a high-speed camera. The numerical method was established with large-eddy simulation (LES) and volume of fraction (VOF) approach. Numerical results agree well with experimental data. Interesting phenomena of three-dimensional evolution are observed, especially in which the U-type shedding of cavities are generated repeatedly in the channels along the inner region. Analysis of the unsteady evolution of cavitating flow is performed, and emphasis is placed on the study of the relationship between flow structures around the ridges and channels. Results demonstrate that the pressure gradient variation in the tangential direction of the cylinder is essential for the unsteady development of the cavity including re-entry jet and local shedding. Interactions between the cavity and vortex are also analyzed, which indicates that the U-type cavity/vortex shedding is jointly induced by the re-entry jet and tangential vortex.

Internal velocity distribution is an important content of flow structure and reveals the gas loss mechanism for supercavitating flows. Considering the three-phase momentum interactions and the water-vapor mass transport, the water-gas-vapor multi-fluid model is established to simulate ventilated supercavitating flows at high speed in the frame of the nonhomogeneous multiphase flows theory. Based on the model, the gas velocity field inside supercavity is studied. In the case of supercavitating flows around disk cavitator, two vortex cores are formed in the longitudinal plane under the actions of the adverse pressure gradient in the tail and the viscous friction on cavity surface, and are symmetrically distributed about the longitudinal axis. Most inner regions in the cavity cross section are occupied by circulation flows, where the velocity is in the opposite direction of incoming flows and decreases in the radial direction. When passing the vortex center, the velocity changes direction and increases in the radial direction. Part of gas departs to wake flows from the outermost regions close to the section boundary. The results confirm Spurk’s assumption for gas entrainment in detail. It is also found that the gas velocity distribution in the cross section through vortex cores does not depend on cavitation number. Supercavitating vehicle has the similar internal velocity distribution and gas loss mechanism. Due to the added viscous effect of the enveloped body, there are multiple axisymmetrical distributed vortices inside the cavity. The relative distance between the vortex core and the cavity wall increases downstream. Computations of ventilated supercavitating flows at different Reynolds numbers show that the gas leakage is decreasing with increasing Reynolds number for a given cavitation number. This study deepens the understanding of gas loss for ventilated supercavity at high speed, and lays a foundation for further refinement of the dynamic model of the maneuvering ventilated supercavity and the control of ventilated supercavitating flows.

In this paper, a mathematical model has been developed to analyze the combined electroosmotic and pressure driven flow of power law fluids in a micro channel in the presence of Joule heating effects. The effects of Navier slip boundary condition and thermal radiation is examined for effective heat transfer in a hydrophobic microchannel. The analytical treatment has been performed for fluid flow and heat transfer effects in terms of flow governing parameters. This study highlights the effect of channel height to the electric double layer thickness and observed the flow variation due to heat transfer effect with the available scientific data. For a pure EOF, velocity slip have more significant role to get a maximum flow rate as expected. For both pseudo-plastic and dilatent fluids Nusselt number is decreased with the increment of the hydrophobic parameter and dimensionless pressure gradient where as increment in Joule heating effect enhance the heat transfer rate.