Progress in Computational Fluid Dynamics, An International Journal (32 papers in press)
DNS of wake from perforated plates: aspect ratio effects
by Abhinav Singh, Vagesh Narasimhamurthy
Abstract: Direct numerical simulations of flow over a thin perforated plate with square holes placed normal to the free-stream is carried out at Reynolds number 250 (based on plate width, d, and inflow velocity, Uo). The effect of the plate aspect ratio L/d (where, L is the plate length) is studied by varying the spanwise length L as 1d, 3d, 6d and 12d. The hydrodynamic forces acting on the plate and the shedding frequency match for all the cases, though the wake dynamics is clearly distinct between the lower and upper L/d cases. In the low L/d cases, the wake is fairly coherent along the span and has reduced three-dimensionality. The flow here is undergoing wake-transition, where the wavelength of the secondary instability is about 1d. Further, the jet flow emanating from the perforation holes exhibit in-phase oscillation along the span. Another interesting feature here is the presence of energetic subharmonics, where the wake is experiencing period-doubling bifurcations. In the high L/d cases, the wake exhibits higher incoherence in the form of intermittent vortex-dislocations and incoherent oscillations of the perforation jets, which promotes flow three-dimensionality. The present study further indicates that a critical aspect ratio of about 6 is required for the simulations to be free from numerical effects and to predict the flow physics accurately.
Keywords: DNS; spanwise aspect ratio; bluff-body; period-doubling cascade.
A simple and efficient two-dimensional hydrodynamic model for unsteady flow simulation in undulating bathymetry
by Anupal Baruah
Abstract: Two-dimensional modeling in a river having undulating bathymetry is challenging in terms of numerical stability, accuracy, and robustness. The main reason behind the model stability is the oscillations in the solutions due to the nonlinear and hyperbolic form of the flow equations. Sometimes it is observed that using primitive variables as temporal derivatives in conservation equation is a source of model instability, especially in a braided river. In this work, the surface form of two-dimensional hydrodynamic equations of shallow water is solved using a five-point TVD Mac-Cormack predictor-corrector scheme. The water surface gradient is used as a gravitational force component in the source term to overcome the possible error that may occur during bed slope discretization in the case of undulating bathymetry. Model is then applied over a wide range of problems including irregular bed, dam breach flow, and mix flow with a hydraulic jump. Model outputs are validated with the available experimental data.
Keywords: TVD; Complex topography; mix flow; water surface slope; two-dimensional shallow water equation; unsteady flow.
Parametric study of nanofluid jet impingement cooling of protruding heaters with porous medium
by Chinmayee Behera, Shantanu Pramanik
Abstract: Abstract: In the present study fluid flow and thermal behavior of a nanofluid jet in connection with forced convection cooling of an array of discrete protruded heaters fixed on an impingement wall and embedded in a porous layer has been investigated for various channel heights (H/L), Darcy Numbers (Da), porosities (ε) and nanoparticle volume fraction (ϕ). The numerical simulations are performed using Al2O3-water based nanofluids with nanoparticles having a diameter of 30 nm. The numerical investigation has been performed using commercial CFD software ANSYS-FLUENT (17.0) implementing LTE model simulating flow through the porous layer. The value of ϕ has been varied at 0%, 2%, 4% and 6% for each case of H/L ratios (0.5 and 1), Da (10-3, 10-4 and 10-5) and porosities (0.5, 0.7 and 0.9), respectively. Increase in percentage of ϕ, with other parameter fixed, has always resulted in improved heat transfer. Maximum value of Nuavg is observed at ϕ=6%. For H/L=0.5 and 1.0, maximum increment in average Nusselt number (Nuavg) has been found to be 33.3% and 15.38%, respectively, along the fourth heater for Da=10-2, ϕ=6% and ε=0.5. Maximum increment in Nuavg has been found to be 15.7% along the first heater for Da=10-4, on the contrary, the minimum value of the increment of 0.3% is observed along the second heater at Da=10-5 in both case for a fixed value of ϕ=6%. However, the effect of variation in porosity variation (0.5, 0.7 and 0.9) on enhancement of heat transfer is not discernible except on the fourth heater where increment of Nuavg is 33.3% at ε=0.5 and ϕ=6%.
Keywords: Nanoparticle; impinging jet; porous layer; discrete heaters,LTE model; single phase.
CFD-based Prediction of Hydrogen Deflagration in Semi-open Ducts
by Eilidh Seville, Helen Scott, Iain Waddell, Eero Immonen, Bingzhi Li
Abstract: Explosion of hydrogen-air mixture in an enclosure produces high local pressure levels, potentially resulting in personnel and property losses. Design of explosion venting systems for mitigating this explosion risk typically involves the use of computational methods. The purpose of this article is to present an unsteady RANS CFD model for simulating hydrogen deflagration in the high flame speed regime in semi-open ducts. This is a relevant scenario from the practical point of view, but with only few previously reported successful modeling attempts. The model proposed herein has been implemented in ANSYS Fluent 19 R2, and is compared to a well-known experiment reported in the literature. The results show a good agreement of the flame speed prediction up to almost 500 m/s, and improved accuracy, relative to existing models, for higher flame speeds. A validation study presented in this paper shows that the model is also robust with respect to mesh resolution as long as the time step in the simulations is small enough (order of 10^-5 s). As a relatively lightweight alternative to computationally intensive Large-Eddy Simulation models, the proposed modeling approach is expected to be beneficial for practical power system design concerns.
Keywords: CFD; Hydrogen; Deflagration; Semi-open ducts; Flame speed; Explosion; Safety.
Numerical Investigation of Atomization Using a Hybrid Eulerian-Lagrangian Solver
by Botond Pál, Dirk Roekaerts, Barry Zandbergen
Abstract: This study investigates the potential of a newly released multi-phase solver to simulate atomization in an air-blast type atomizer. The "VOF-to-DPM" solver was used to simulate primary and secondary atomization in an air-blast atomizer with a coaxial injector-like geometry. The solver uses a hybrid Eulerian/Eulerian-Lagrangian formulation with geometric transition criteria between the two models. The conducted study assumed isothermal, non-reacting flow at room temperature. The primary focus was predicting Sauter Mean Diameter and droplet velocity data at a sampling plane downstream of the injection site. The results showed that the solver is able to produce the expected data and to predict trends similar to those found in experimental measurements. The accuracy of the produced droplet diameters was roughly a factor 2 off compared to experiment. This is attributed primarily to mesh resolution. It was concluded that the solver has the potential to predict atomization at a reasonable computational cost, but further study is needed to confirm its full capabilities.
Keywords: Atomization; Ansys Fluent; Multi-phase flow; Spray Formation; Volume of Fluid.
Simplified Level Set Method Coupled to Stabilized Finite Element Flow Solver for Moving Boundaries
by Ahmed Mohamed, Mohamed Abdulrahman, Amr Guaily
Abstract: A simplified version of the level set method (SLSM) is proposed for interfaces moving with prescribed motion e.g., deployment of a spoiler. The core of the proposed method is the signed distance function which is constructed by direct computation for all vertices in the mesh. The main advantage of the proposed algorithm is that it avoids the need for re-meshing and its associated problems as well as it could be used equally with all types of grids. The SLSM is successfully coupled to the finite element method flow solver, and the resulting hybrid solver is used to study the flow field during the spoiler deployment of a NACA0012 airfoil. Finally, two algorithms are proposed for force calculations over moving interfaces with new definitions for the smeared Dirac-Delta and Heaviside functions. At the moment of spoiler deployment, an abrupt rise in the lift force is observed and interpreted. The results assure the robustness of the proposed hybrid solver.
Keywords: Simplified Level Set Method; Fluid-Solid Interaction; Spoiler Deployment; Lift and Drag Computation.
Computational Analysis of Mixing Characteristics of Port-Injected BioCNG with Air for Different Designs of Intake Manifold
by Akash Chandrabhan Chandekar, Biplab Kumar Debnath
Abstract: The bioCNG with 90% of methane makes a potential renewable alternative of CNG for the application in dual-fuel diesel engine. The bioCNG is to be inducted in the manifold through port injection. The work is aimed to find the optimum port length and port injector diameter. The objective is to mix bioCNG homogenously with air, before entering the engine. This would create an opportunity for major replacement of diesel to reduce the running cost of the vehicle. The two injector diameters (7 and 10 mm) and four port lengths (0, 100, 150 and 200 mm) are investigated. The manifold has a diameter of 28.5 mm and the port injector is inclined at 35? to the manifold axis. The geometries are simulated in ANSYS Workbench 19.2, followed by analysis in Tecplot. The results show that the port length of 200 mm with an injector diameter of 7 mm delivered homogeneous mixture for the given dual-fuel engine.
Keywords: bioCNG; dual-fuel; intake manifold; port fuel injector; helicity; mass fraction of CH4; homogeneous mixing.
Computational study of sloshing in spherical tanks and the effect of using annular baffle over slosh force frequency and damping
by Habib Ahmad, Ammar Ahmed, Ismail Baila, Ajmal Shah
Abstract: Partially filled liquid containers are subjected to sloshing when acted upon by an external force or acceleration. The sloshing liquid applies varying forces on walls of the container that have to be included in its design. In this study, the CFD method was validated against experimental data from existing literature. Afterward, the impact of the liquid fill level in the container, initial perturbation velocity, and variation in time step size over the frequency and damping of sloshing force was studied. Moreover, a novel idea of introducing an annular baffle in spherical tanks has been proposed to minimize the effects of sloshing on the container. The damping effect of the annular baffle in a spherical container was analyzed using a validated CFD method. The result comparison of the two cases shows a significant increase in the damping of slosh force by the introduction of an annular baffle.
Keywords: Sloshing; Spherical tanks; Slosh damping; CFD; Annular baffle.
A Computational and Experimental Study on Aerodynamics of Motor-driven Propellers Using Thrust Stand and Rotating Cup Anemometer
by Zaid Siddiqi, Jin Wook Lee
Abstract: The aims of this study are: (1) Develop a cost-effective experimental set up that incorporates a commercially available thrust-stand and rotating cup anemometer; (2) Analyze rotor performance at rotational speeds ranging from 6,000 PRM to 14,000 RPM; (3) Use the Multiple Reference Frame (MRF) approach to develop Computational Fluid Dynamics (CFD) simulations that provide a good agreement with experimental results; (4) to assess the efficacy of using a rotating cup anemometer to measure propeller downwash velocity. A 6-inch propeller is paired with a 2600 KV motor. Overall rotor efficiency peaks near 7,200 RPM and beyond 9,300 RPM; it declines rapidly as the mechanical and electrical power output increases. To assess the thrust and downwash velocity, two turbulence models are used in steady state: k-? Realizable and the k-? SST. The k-? SST shows good overall agreements for thrust, while the k-? Realizable provides a more accurate modeling of the downwash velocity. Based on the CFD results, the rotating cup anemometer is found to be unsuitable for measuring propeller downwash velocities.
Keywords: Unmanned Aerial Vehicle; Computational Fluid Dynamics; Multiple Reference Frame; Thrust; Thrust-stand; Anemometer.
The behavior of time-independent non-Newtonian fluids in an impact problem of a wedge using smoothed particle hydrodynamics method
by Jafar Gerdabi, Amir Hossein Nikseresht, Mohammad A. Esmaeili-Sikarudi
Abstract: The impact of solid objects on non-Newtonian fluids may occur in many industrial applications such as liquid armors or moving speedboats on muddy water. So far wedge impact on a Newtonian fluid surface has been simulated through various Eulerian and Lagrangian numerical methods while there are few studies on how non-Newtonian fluids may behave under such impacts. This paper aims to investigate the behavior of time-independent non-Newtonian fluids in a 2DOF (2 Degrees Of Freedom) wedge impact problem using a numerical Lagrangian in-house code which is based on the Weakly Compressible Smoothed Particle Hydrodynamics (WCSPH) method. At first, a dam break test case is simulated using non-Newtonian fluid models to validate both the generated code and the method. Afterward, wedge impact on a Newtonian fluid is simulated and its results are compared with Boundary Element Method (BEM) and experimental results. Finally, wedge impact on various non-Newtonian fluids is simulated and the resulted force exerted on the wedge, pressure coefficient, wedge velocity, and fluid free surface profile are compared with each other and with the Newtonian fluid results. The results show that in general, the vertical force exerted on the wedge by the non-Newtonian dilatant and Herschel-Bulkley fluids is bigger than Newtonian and other non-Newtonian fluids. The percentage increase in the maximum vertical force when non-Newtonian fluids instead of Newtonian fluids are used is greater in the case of the lighter wedge versus the heavier one. Also, the dilatant fluids behave differently after wedge impact and it is interesting to know that after the impact there is no formation of the fluid jet.
Keywords: WCSPH; Non-Newtonian fluids; Artificial viscosity; Density correction; Repulsive force; Impact.
Computational Study of Combined Effects of Tip Leakage Jets and Wetness on the Performance of Steam Turbine Exhaust System using Actuator Disc Model
by Sreeja Sadasivan, Senthil Kumar Arumugam, Mahesh C. Aggarwal
Abstract: A numerical model to study the individual and combined effects of the tip clearance jets and the presence of wetness on the performance of the steam turbine exhaust hood has been developed. The Eulerian- Eulerian wet steam flow equations along with mass, momentum, energy, and k-epsilon model equations are solved in the present study. An increase in inlet wetness level improves the pressure recovery capacity of the steam turbine exhaust hood by reducing the total pressure loss. The presence of moisture causes a turbulence attenuation which in turn reduces the vortex strength. The combined effect of the tip leakage jets and the presence of inlet wetness improves the performance of the exhaust system further. However, the presence of a non-uniform condenser pressure gradient caused by the temperature variation of the cooling water flow increases the level of complexity of the flow in the exhaust system deteriorating the performance.
Keywords: Actuator-disc model; Wetness; Steam turbine exhaust hood; Turbulence damping; Rotor-tip leakage.
Numerical investigation on shear flow and boiling heat transfer on shell-side of LNG spiral wound heat exchanger
by Zhiyong Wu, Ying Fan, Weihua Cai, Yiqiang Jiang
Abstract: A numerical model was developed to investigate the shear flow and boiling heat transfer on shell-side of spiral wound heat exchanger used for natural gas liquefaction. Thermal ratio of phase transition (?) as a important parameter in simulation was studied, the error on defining?proposed in reference was illuminated. After?was anew defined, the value of ? was appropriately selected under different working condition, and the major factors governing?were analyzed. Simulation results were consistent with experimental data, the simulation deviations of both frictional pressure drop and heat transfer were within
Keywords: Spiral wound heat exchanger; shell side; shear flow; boiling.
Wake interaction studies of flow past tandem circular cylinders for different diameter and gap ratios
by R. Rajita Shenoi, Neeraj Paul Manelil, Thirumalachari Sundararajan, Shaligram Tiwari
Abstract: Three-dimensional computations are carried out to study the flow past tandem stationary cylinders at moderate Reynolds number (Re = 2000). Wake structures are analyzed with the help of instantaneous vorticity contours and hydrodynamic characteristics are reported using drag and lift coefficient plots. Effects of diameter ratio and gap ratio on wake structure as well as its dynamic characteristics are presented. Cross recurrence analysis is performed to study the coupled interactions between cylinder wakes. In particular, conditions for the in-phase synchronization, out-of-phase coupling or non-synchronous coupling of the wakes behind the tandem cylinders are identified. Qualitative results obtained from cross recurrencernplots are quantified using parameters such as recurrence rate, determinism and averaged diagonal length. These results give deep insight into wake synchronizations between the cylinders. In-phase or out-of-phase wake oscillations can lead to catastrophic structural failures and predicting such conditions can be a useful design tool.
Keywords: Tandem circular cylinders; diameter ratio; inter-cylinder spacing; dynamic wake characteristics; recurrence plots; synchronization.
Coupled natural convection and radiation in a cubic cavity filled with an air - H_2O mixture in the presence of a heated obstacle
by Xuan Bach NGUYEN, Didier SAURY, Denis LEMONNIER
Abstract: Natural convection in a cubical cavity with a hot obstacle located on its floor is investigated. The inner fluid is a semi transparent mixture of dry air and water vapor, which creates a coupled convective and radiative transport within the fluid. The conservation equations are solved by a finite volume method and the radiative transfer equation by using the discrete ordinates method. The radiative properties of the mixture are accounted for by a spectral line weighted sum of gray gases model associated to the rank correlated approach. It was observed that the volume radiation has a strong influence on the thermal and dynamic fields. The nearly vertical stratification of the temperature field around the plume is broken. Radiation also accelerates the boundary layers near the lateral surfaces and the ceiling and the floor of the enclosure. The total heat transfer is decreased due to both the reduction in convective process near the vertical walls and the attenuation by radiation.
Keywords: Heat transfer; Convection; Radiation; Numerical method.
NUMERICAL SIMULATION OF THE INFLUENCE OF FURNACE GEOMETRY ON ITS PERFORMANCE IN A TANGENTIALLY FIRED PULVERISED-COAL SUPERCRITICAL BOILER
by Sankar G, Balasubramanian K R, Chandra Sekhar Dhannina, Santhosh Kumar D
Abstract: The influence of furnace geometry on furnace performance was numerically investigated and validated for a tangentially fired pulverised coal boiler. Six simulation cases were studied by varying the plan area of the furnace; three cases were based on a rectangular furnace and three cases were based on a square furnace. Performance characteristics that were investigated include variation of wall heat flux, Furnace Exit Gas Temperature (FEGT) and NOx concentrations at the outlet of the furnace. The simulations revealed that, with a 20% increase in plan area, a decrease in FEGT was observed, which corresponded to a drop of approximately 30
Keywords: Heat flux distribution; Tangentially fired boiler; NOx emission levels; Furnace geometry; Supercritical boiler; Computational fluid dynamics (CFD).
Solid Rocket Motor Interior Ballistics Fluid-Solid Interaction Simulation Using Level Set Method for 2D Grains
by Hossam Alqaleiby, Aly Hashem, Mohamed Tosson, Amr Guaily
Abstract: The flow inside a solid rocket motor is considered taking into account the fluid-solid interaction between the combustion gases and the burning grain surface. An unsteady flow model is presented for the flow variables while the time-dependent burning surface is captured using the level set method. Then a hybrid model is presented by coupling the flow model with the interface capturing technique. The proposed hybrid model is successfully tested against previously published experimental and numerical data. The use of the level set method enables the flow model to consider complex grain shapes like the star shapes. The proposed hybrid model is used to produce the pressure-time curves for cross-shaped grain and a star grain with five points.
Keywords: Fluid-solid interaction; Level set method; Finite element method; Internal ballistics.
Chebyshev spectral collocation method for MHD duct flow under slip condition
by Canan Bozkaya, Önder Türk
Abstract: The magnetohydrodynamic problem of a fully developed flow of an incompressible and electrically conducting fluid is solved numerically by a Chebyshev spectral collocation method in a square duct with walls of variable electric conductivities under a slip condition for velocity. The flow is driven by a constant pressure gradient under the effect of an externally applied oblique magnetic field. The efficiency of the method that is implemented in the physical space on preassigned collocation points is exploited to discretise the governing equations. The corroboration and validation of the proposed technique are carried out by means of a case study with published results substantiating that its implementation results in satisfactorily good agreements. Novel results are presented graphically, and the combined effects of the most characteristic magnetohydrodynamic flow parameters such as the slip length, conductivity parameter, and Hartmann number on the velocity and induced magnetic field are investigated.
Keywords: MHD flow; rectangular duct; slip condition; variable conductivity; Chebyshev spectral collocation.
CFD based Optimization of Base Pressure Behavior on Suddenly Expanded Flows at Supersonic Mach numbers
by Jaimon Dennis Quadros, Sher Afghan Khan, Prashanth T.
Abstract: In this work, a suddenly expanded flow process is modeled for determining base pressure characteristics. The base pressure developed in a suddenly expanded flow process majorly depends on the pertinent selection of Mach number (M), nozzle pressure ratio (NPR), area ratio (AR), and length to diameter ratio (L/D). Numerical analysis of the flow process was carried out using the computational fluid dynamics (CFD) technique. The input-output test cases for CFD analysis were developed as per two statistical methods, namely central composite design (CCD) and Box-Behnken design (BBD). The CFD results were validated by experiments that were conducted by using a nozzle and expanded duct. The input-output relationship obtained from the BBD was found to be statistically adequate and yielded better prediction accuracy. The BBD response model was used for generating data that trained the recurrent and backpropagation neural networks. The recurrent neural network outperformed both the backpropagation neural network and Box-Behnken design.
Furthermore, to assess the right range of conditions for maximizing base pressure, the genetic algorithm (GA), desirability function approach (DFA), and particle swarm optimization (PSO) techniques were implemented. The performance of PSO and GA techniques was found to be better, as they carried out search operations in many directions at multi-dimensional space simultaneously. All the models and optimization techniques were compared, and final comments are drawn.
Keywords: base pressure; CFD; Response surface methodology (RSM); neural networks; optimization.
On the use of adaptive relaxation times in lattice Boltzmann methods
by Simon Marié
Abstract: The lattice Boltzmann collision model with multiple relaxation times is modified to make the relaxation rates dependent on the shear stress. With these adaptive relaxation times, the optimal choice made by the theoretical development of MRT does not hold anymore. However, it is shown that the numerical properties of the collision model remains close to the MRT model. In particular, it is shown that the adaptive model recover the BGK properties in terms of acoustical dissipation but keeps the numerical stability of MRT when shear stress becomes high. Then the adaptive model is studied in terms of stability and accuracy on the Taylor-Green vortex case, and the acoustic properties are tested on the sound radiated by a square cylinder.
Keywords: Lattice Boltzmann method; multiple relaxation times; Adaptive relaxationrntimes; Taylor Green Vortex; aerodynamic sound.
Experimental and Numerical Investigation of Co-Axial Rotor Interaction to Thrust
by Ahmet Soydan, Hurkan Sahin, Baris Bicer, Sebnem Sariozkan, Mehmet Sahin
Abstract: The experimental and numerical computational investigation of co-axial rotor performance has been increased over the past decade in order to understand complex interactions in coaxial-rotor flows to improve design of unmanned-aerial vehicles. Nevertheless, the issues related rotor aerodynamic performance, wake interactions, etc. are not well understood. In the current work, aerodynamic interactions in co-axial rotor have been investigated with both experimental and numerical methods in hover flight by varying tip diameters, rpm, axial distance etc. In order to calculate the co-axial thrust efficiency, in-house test bench has been created. On the numerical side, the three-dimensional unsteady Navier-Stokes equation is solved using a pressure based, segregated, compressible and time-accurate solver of OpenFOAM. A sliding mesh interface procedure is utilized to link rotating regions and SST k-$omega$ model is employed for the turbulence modelling. The computational results indicate relatively good agreement with in-house experimental data.
Keywords: Multi-rotor; Coaxial rotor; OpenFOAM; RANS; Sliding mesh; Hover flight.
Numerical Study of Characteristics of Confined Diffusion Flames of Synthetic Gases in Coflow and Inverse Coflow Configurations
by Mohd. Ibrahim, S. Muthu Kumaran, Vasudevan Raghavan
Abstract: Comprehensive numerical predictions of diffusion flame characteristics of practical synthetic gas (syngas) fuels in coflow (CDF) and inverse coflow (IDF) configurations with stoichiometric amount of coflow air have been compared. A numerical model with variable thermo-physical properties, multi-component diffusion, Soret diffusion, detailed chemical kinetics mechanism and an optically thin radiation model has been used. Effect of hydrogen content in fuel, which controls the reactivity, on flame characteristics has been revealed systematically. Higher peak flame temperature is observed in IDF than in CDF. However, its radially averaged temperature peak is lower in IDF than that in CDF within the flame zone. Higher rate of radial oxygen transport is seen in IDF, resulting in lesser flame radius. Combustion efficiency is higher in IDF than in CDF, for syngas fuels with intermediate reactivity. Understanding the characteristics of these flames will lead to better design of multi-slot burners using mixture of syngas fuels.
Keywords: Synthetic gas; hydrogen content; inverse coflow diffusion flames; detailed chemical kinetics; multi-component diffusion; flame structure; numerical model; combustion efficiency; thermo-physical properties; optically-thin radiation model.
Effect of cross-confinement on unsteady wake characteristics of circular cylinder immersed in shear flow
by Prashant Kumar, Shaligram Tiwari
Abstract: Present study investigates effect of cross-confinement on three-dimensional unsteady wake characteristics in flow past circular cylinder mounted on a surface using OpenFOAM. Aspect ratio (ratio of height to diameter of the cylinder) is kept fixed equal to four. Temporal wake behaviour has been characterised at fixed Reynolds number equal to 200 using Hilbert Huang transformation of transverse velocity signals. Frequency and energy distributions of component signals have been illustrated with the help of Hilbert spectra. Variation in frequency of vortex shedding with the change in shear intensity and top confinement has been illustrated in marginal spectra. Quantification of nonlinear fluctuations in the wake has been presented in terms of degree of stationarity. Spatial and temporal evolutions of vortex modes (frequency and growth rate) have been examined using recently developed snapshot-based technique 'dynamic mode decomposition (DMD)'. Periodic behaviour of the wake has been illustrated using recurrence plot (RP) and different quantification estimates are presented based on patterns that appear in the RP.
Keywords: Hilbert spectra; marginal spectra; degree of stationarity; dynamic mode decomposition; DMD; recurrence plot.
Effect of ground on the shape optimisation of a symmetric aerofoil at low angles of attack
by Dennis Joseph, Ajith Kumar Arumugham-Achari, Jithin P. Narayanan
Abstract: Numerical investigation on the aerodynamic characteristics of an optimised NACA0012 aerofoil in-ground effect (IGE) has been performed. Gradient-based shape optimisation was carried out using the ANSYS® 19.0 Adjoint Solver to augment lift over drag ratio (L/D) by at least 10%, at various heights and angles of attack. SST k-ω turbulence model was chosen for the simulations, after its validation for out-of-ground effect (OGE) and performing wind tunnel tests for IGE. While the desired target of 10% increase in the performance parameter was easily achieved through optimisation at low angles of attack (α < 6°), the frozen turbulence assumption in Adjoint Solver limited large shape alterations at higher angles of attack. Upper surface of the aerofoil had larger changes from original camber when compared to the lower surface. Also, the optimised profiles had significant modifications towards x/c ≥ 0.8. This signifies the suitability of trailing edge morphing for such applications.
Keywords: aerodynamics; shape optimisation; wing-in-ground effect; ANSYS® Adjoint Solver; NACA0012.
Numerical simulation of viscoelastic blood flow with hematocrit variation in an arterial segment with two aneurysms
by Ahmed Elhanafy, Ahmed Elsaid, Amr Guaily
Abstract: In this study, a viscoelastic model with variable viscosity and relaxation-time is proposed for the simulation of the blood flow in an arterial segment with two aneurysms. The Quemada model is adopted to model both the shear rate-dependent viscosity and hematocrit variation. Available experimental data for the shear rate-dependent relaxation-time of the blood, in a certain range, are fitted and used. The arterial segment with aneurysms is considered as a rigid axisymmetric thin tube with two balloon expansions. The stabilised finite element method with the discrete elastic viscous stress splitting (DEVSS) method is used to solve the governing equations to overcome the numerical instabilities. Numerical results including velocity profiles, shear rate distribution and viscosity contours are obtained under different values of red blood cells (RBCs) concentrations. The results indicate that the hematocrit variation has a significant effect on the flow regimes and hemodynamics factors such as the wall shear stress (WSS). Hence, the shear thinning property should not be ignored for blood flow simulations.
Keywords: abdominal aortic aneurysms; AAAs; blood viscoelasticity; DEVSS method; hematocrit variation; wall shear stress; WSS.
The Galerkin reliable scheme for the numerical analysis of the Burgers'-Fisher equation
by Pius W.M. Chin
Abstract: We consider in this paper, the non-standard finite difference method in the time variable combined with the Galerkin method in the space variables. We use this to study the Burgers'-Fisher equation which is one of the most important nonlinear partial differential equation appearing in various applications such as in fluid dynamics. Existence and uniqueness of the solution of the problem is determined for a given small data in the space L∞[(0, T); L2(Ω)] ∩ L2[(0, T); H 1 0(Ω) ]. The numerical scheme of the problem is designed using the said combination. The proposed scheme is successfully implemented by firstly establishing the stability of the numerical scheme and secondly by determining the estimate for the optimal convergence rate of the numerical solution of the scheme in both the L2 as well as H1-norms. Furthermore, we show that the numerical solution of the scheme preserves the decaying properties of the exact solution of the problem and moreover, the numerical experiments with the help of an example are presented to justify the validity of the results.
Keywords: Burgers'-Fisher equations; fluid dynamics; nonlinear equation; non-standard finite difference method; Galerkin method; optimal rate of convergence.
Experimental and numerical study of upstream slope stability in an earth dam reservoir under rapid drawdown conditions
by Mansour Pakmanesh, Seyed Habib Mousavi Jahromi, Amir Khosrojerdi, Hossein Hassanpour Darvishi, Hossein Babazadeh
Abstract: The rapid water level drawdown of the dam reservoirs is one of important factors of earth dam stabilities. This phenomenon might occur over the lifetime of the earth dams and levees. In order to investigate the effect of the hydraulic conductivity on the rapid drawdown of water level, an experimental model was constructed in experimental flume. By obtaining the hydraulic parameters of the materials, the flow through this model was modelled by SEEP/W software. After validation of hydraulic conductivity with the numerical model, the results were compared with the observed data. Finally, a good agreement is observed between the experiments and predictions. Furthermore, the saturated and unsaturated simulations indicate that the unsaturated model has a much better agreement with the experimental model. It is found that the unsaturated model is more convenient for the simulation flow through the homogenous earth dam than the saturated model.
Keywords: slope stability; rapid drawdown; RDD; earth dam; drainage prediction; SEEP/W.
Numerical analysis on the development of vortex structure in 90° pipe bend
by Prasun Dutta, Nityananda Nandi
Abstract: In the present paper single-phase turbulent flow through 90° pipe bend with different curvature ratio (Rc/D) is numerically studied. Simulations are accomplished for turbulent flow conditions under high Reynolds number (Re) ranging from 1 × 105 to 10 × 105 and for three different curvature ratio conditions: 1) low curvature ratio (Rc/D = 1); 2) medium curvature ratio (Rc/D = 2 and 3); 3) high curvature ratio (Rc/D = 4 and 5). Numerical simulations are performed by solving unsteady Reynolds averaged Navier-Stokes (U-RANS) equations using k-ε turbulence model. Development of a complex three-dimensional flow pattern with different vortices with their development in space is investigated through flow visualisation by numerical simulation. The numerical results indicate that with respect to both Reynolds number and curvature ratio, there is a complex change in flow pattern regarding the appearance, development and disappearance of several types of Dean vortices. In general, this paper provides numerical results on the development of different vortices and their evolution in space with a tentative map for 90° pipe bend with different Reynolds number and curvature ratios.
Keywords: 90° pipe bend; curvature ratio; Dean vortices; k-ε turbulence model; turbulent flow; secondary flow.
Numerical simulation of the unsteady aerodynamic loads on the tail fin in the vortex breakdown flow
by Andrey Epikhin
Abstract: This work presents numerical simulation results of the flow around an airbrake-tail fin system of an aircraft. In this research, airbrake location and deflection angles effects on the unsteady aerodynamic loads acting on the tail fin that leads to the buffeting have been studied. The main flow field structures and their transformation features are investigated. The area of the maximal pressure applied to the tail fin has been determined. Analysing the obtained data during a series of numerical simulations allows to conclude that aerodynamic loads on the tail fin increase by five times. This conclusion has been made for the considered configuration of the airbrake-tail fin system when the airbrake deflection angle grows bigger. Change in the distance between the airbrake and the tail fin does not significantly affect the force loads. The efficiency of using the airbrake under different operating conditions has been estimated.
Keywords: vortex flow aerodynamics; airbrake; tail fin; buffeting; unsteady loads; OpenFOAM.
Numerical simulations of the transverse injection and mixing with a Mach 3 supersonic cross-flow
by Sagarika Iyyer, V. Babu
Abstract: Numerical simulations of the transverse injection of helium (surrogate for hydrogen) into a Mach 3 cross-flow through circular and wedge injectors have been carried out. Three-dimensional, compressible, steady Favre-averaged Navier-Stokes equations have been solved using the SST k-ω turbulence model. Flow conditions of the injectors are matched to isolate the effect of injector geometry. Mach number contours inside the injector reveal the flow at the exit of the injector to be supersonic and not sonic, as it is usually assumed to be. Oil flow visualisation shows flow features such as a bow shock and separated flow regions around the injectors. Shadowgraphs reveal two parallel shock waves originating from the injection location as well as the thickening of the boundary later downstream of the injector. Helium mass fraction contours on axial planes show the injectant plume and its spreading in the vertical and spanwise (lateral) direction. Performance metrics such as penetration height and degree of mixing of the injectors have been compared. Results show that the circular injector has better mixing performance when compared to the wedge injector for the operating conditions considered. Results from the present calculations show reasonable agreement with experimental data from an earlier study.
Keywords: scramjet; supersonic combustion; simulations; injection; mixing.
Numerical investigation on the effect of sister holes on film cooling performance with Barchan-dune shaped shells
by Mustapha Grine, Khadidja Boualem, Zineddine Ahmed Dellil, Abbès Azzi
Abstract: The film cooling is the traditional and efficient technique that is used to adopt the turbine cooling process, a new configuration that combines sister holes with Barchan-dune shaped shells (SH-BDS) is used in this study to improve the film cooling effectiveness laterally and at spanwise direction. The investigations are performed for high blowing ratios, M = 1 and 1.5. Three-dimensional Reynolds Averaged Navier-Stokes (RANS) equations with the RNG k-ε model are solved to simulate and analysis the thermal behaviour of this new configuration. The validation shows good agreement and almost all flow structures are well reproduced. It is found that the new configuration provides a significant change in the film cooling effectiveness results. Moreover, the counter-rotating vortex pairs structure (CRV) is nearly vanished. From this contribution, it is well demonstrated that this new technique (SH-BDS) has a great influence on the cooling process of the turbine and hence on its effective functioning.
Keywords: SH-BDS; BDS; sister holes; CRV; anti-CRV; film cooling.
Characteristics and soot formation in laminar liquefied petroleum gas flames in air crossflow
by S. Muthu Kumaran, Vasudevan Raghavan
Abstract: Crossflow non-premixed LPG-air flames display stability issues at higher air velocities. They also emit notable amount of soot. To improve the flame stability, obstacles such as backward facing steps, are used. In this work, laminar LPG-air crossflow flames are studied using a comprehensive numerical model incorporating multi-component mass transfer with thermal diffusion, diffusion energy source, sub-models for soot formation, its oxidation and radiation losses due to gas and soot. For the cases without backward facing step (baseline cases), the flame is unstable after a given air velocity. Flames are much stable over an increased air velocity, when a backward facing step is used. In these cases, the net soot production is relatively higher when compared to baseline cases as a result of increased residence time in the wake of the step. The location of the step from the fuel injection point also has its effects on soot formation.
Keywords: backward facing step; multi-component mass transfer; soot formation; gas radiation; soot radiation; numerical model; kinetic mechanism; crossflow; liquefied petroleum gas; LPG.
A computational investigation of olive oil residues combustion in a vertical furnace
by Mariem Bayoudh, Hazem Touati, Hmaied Ben N'Ticha
Abstract: The paper presents a numerical investigation of combustion of pulverised olive waste and its impregnated samples. Olive oil by-products, mainly olive solid waste (OSW) and olive mill wastewater (OMWW) mixtures that are called impregnated solid waste (ISW) are investigated for different mixing ratios. The investigation is carried out by an in-house two-dimensional CFD code based on the finite volume method, developed for this purpose. The model is first validated by comparisons to the experimental data and predictions of other authors. Subsequently, it is applied to the cases of present interest, predicting the efficiency of the thermal process, species and temperature distributions. The combustion behaviour of ISW is compared with that of the original OSW under the same conditions. The findings of the present study shows that a beneficial effect of using ISW is observed, as the addition of OMWW leads to a decrease of carbon oxides production. Furthermore, the OMWW addition does not have a negative effect on their firing quality since the different samples exhibit quite close temperature profiles. Obtained results are important for the investigation the possibility of using olive residues as an alternative energy source in olive producing countries.
Keywords: biomass; olive solid waste; OSW; olive mill wastewater; OMWW; kinetic model; turbulence; carbon oxide emissions; impregnated solid waste; ISW.