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Despite advances in turbulence modelling, the Smagorinsky model remains a popular choice for large eddy simulation (LES) due to its simplicity and ease of use. The dissipation in turbulence energy that the model introduces, is proportional to the Smagorinsky constant, of which many different values have been proposed. These values have been derived for certain simulated test-cases while using a specific set of numerical schemes, to obtain the correct dissipation in energy simply because an incorrect value of the Smagorinsky constant would lead to an incorrect dissipation. However, it is important to bear in mind that numerical codes may suffer from numerical or artificial dissipation, which occurs spuriously through a combination of spatio-temporal and iterative errors. The latter can be controlled through more iterations, the former however, depends on the grid resolution and the time step. Recent research suggests that a complete energy-conserving (EC) spatio-temporal discretisation guarantees zero numerical dissipation for any grid resolution and time step. Therefore, using an EC scheme would ensure that dissipation occurs primarily through the Smagorinsky model (and errors in its implementation) than through the discretisation of the Navier-Stokes (NS) equations. To evaluate the efficacy of these schemes for engineering applications, the article first discusses the use of an EC temporal discretisation as regards to accuracy and computational effort, to ascertain whether EC time advancement is advantageous or not. It was noticed that a simple non-EC explicit method with a smaller time step not only reduces the numerical dissipation to an acceptable level but is computationally cheaper than an implicit-EC scheme for wide range of time steps. Secondly, in terms of spatial discretisation on uniform grids (popular in LES), a simple central-difference scheme is as accurate as an EC spatial discretisation. Finally, following the removal of numerical dissipation with an, Wind Energy, Aerodynamics

Despite advances in turbulence modelling, the Smagorinsky model remains a popular choice for large eddy simulation (LES) due to its simplicity and ease of use. The dissipation in turbulence energy that the model introduces, is proportional to the Smagorinsky constant, of which many different values have been proposed. These values have been derived for certain simulated test-cases while using a specific set of numerical schemes, to obtain the correct dissipation in energy simply because an incorrect value of the Smagorinsky constant would lead to an incorrect dissipation. However, it is important to bear in mind that numerical codes may suffer from numerical or artificial dissipation, which occurs spuriously through a combination of spatio-temporal and iterative errors. The latter can be controlled through more iterations, the former however, depends on the grid resolution and the time step. Recent research suggests that a complete energy-conserving (EC) spatio-temporal discretisation guarantees zero numerical dissipation for any grid resolution and time step. Therefore, using an EC scheme would ensure that dissipation occurs primarily through the Smagorinsky model (and errors in its implementation) than through the discretisation of the Navier-Stokes (NS) equations. To evaluate the efficacy of these schemes for engineering applications, the article first discusses the use of an EC temporal discretisation as regards to accuracy and computational effort, to ascertain whether EC time advancement is advantageous or not. It was noticed that a simple non-EC explicit method with a smaller time step not only reduces the numerical dissipation to an acceptable level but is computationally cheaper than an implicit-EC scheme for wide range of time steps. Secondly, in terms of spatial discretisation on uniform grids (popular in LES), a simple central-difference scheme is as accurate as an EC spatial discretisation. Finally, following the removal of numerical dissipation with an, Wind Energy, Aerodynamics

Aerodynamics, Control & Simulation

Vortex generators (VGs) are a widely used means of flow control, and predictions of their influence are vital for efficient designs. However, accurate CFD simulations of their effect on the flow field by means of a body fitted mesh are computationally expensive. Therefore the BAY and jBAY models, which represent the effect of VGs on the flow using source terms in the momentum equations, are popular in industry. In this contribution we examine the ability of the BAY and jBAY model to provide accurate flow field results by looking at boundary layer properties close behind VGs. The results are compared with both body fitted mesh and other source term model RANS simulations of 3D incompressible flows, over flat plate and airfoil geometries. We show the influence of mesh resolution and domain of application on the accuracy of the models and investigate the influence of the source term on the generated flow field. Our results demonstrate the grid dependence of the models and indicate the presence of model errors. Furthermore we find that the total applied force has a larger influence on both the intensity and shape of the created vortex than the distribution of the source term over the cells., Aerodynamics, Education and Student Affairs

This paper presents the experimental and numerical study on MEXICO wind turbine blades. Previous work by other researchers shows that large deviations exist in the loads comparison between numerical predictions and experimental data for the rotating MEXICO wind turbine. To reduce complexities and uncertainties, a non-rotating experimental campaign has been carried out on MEXICO blades Delft University of Technology. In this new measurement, quasi-2D aerodynamic characteristics of MEXICO blades on three spanwise sections are measured at different inflow velocities and angles of attack. Additionally, RANS simulations are performed with OpenFOAM-2.1.1 to compare numerical results against measured data. The comparison and analysis of aerodynamic loads on the blade, where three different airfoil families and geometrical transition regions are used, show that for attached flow condition, RANS computation predicts excellent pressure distribution on the NACA airfoil section (r=R D 0.92) and good agreement is observed on the DU (r=R D 0.35) and RISØ (r=R D 0.60) airfoil sections. Unexpected aerodynamic characteristics are observed at the intermediate transition regions connecting the RISØ and DU airfoils, where sudden lift force drop is found at the radial position r=R D 0.55. Through numerical flow visualization, large-scale vortical structures are observed on the suction side of the blade near the mid-span. Moreover, counter-rotating vortices are generated behind the blade at locations where unexpected loads occurs. Consequently, the RISØ airfoil could not give expected 2D aerodynamic characteristics because of upwash/downwash effects induced by these counter-rotating vortices, which make 3D effects play an important role in numerical modeling when calculating the aerodynamic loads for MEXICO rotor., Wind Energy, Aerodynamics

This paper presents the experimental and numerical study on MEXICO wind turbine blades. Previous work by other researchers shows that large deviations exist in the loads comparison between numerical predictions and experimental data for the rotating MEXICO wind turbine. To reduce complexities and uncertainties, a non-rotating experimental campaign has been carried out on MEXICO blades Delft University of Technology. In this new measurement, quasi-2D aerodynamic characteristics of MEXICO blades on three spanwise sections are measured at different inflow velocities and angles of attack. Additionally, RANS simulations are performed with OpenFOAM-2.1.1 to compare numerical results against measured data. The comparison and analysis of aerodynamic loads on the blade, where three different airfoil families and geometrical transition regions are used, show that for attached flow condition, RANS computation predicts excellent pressure distribution on the NACA airfoil section (r=R D 0.92) and good agreement is observed on the DU (r=R D 0.35) and RISØ (r=R D 0.60) airfoil sections. Unexpected aerodynamic characteristics are observed at the intermediate transition regions connecting the RISØ and DU airfoils, where sudden lift force drop is found at the radial position r=R D 0.55. Through numerical flow visualization, large-scale vortical structures are observed on the suction side of the blade near the mid-span. Moreover, counter-rotating vortices are generated behind the blade at locations where unexpected loads occurs. Consequently, the RISØ airfoil could not give expected 2D aerodynamic characteristics because of upwash/downwash effects induced by these counter-rotating vortices, which make 3D effects play an important role in numerical modeling when calculating the aerodynamic loads for MEXICO rotor., Wind Energy, Aerodynamics

Vortex generators (VGs) are a widely used means of flow control, and predictions of their influence are vital for efficient designs. However, accurate CFD simulations of their effect on the flow field by means of a body fitted mesh are computationally expensive. Therefore the BAY and jBAY models, which represent the effect of VGs on the flow using source terms in the momentum equations, are popular in industry. In this contribution we examine the ability of the BAY and jBAY model to provide accurate flow field results by looking at boundary layer properties close behind VGs. The results are compared with both body fitted mesh and other source term model RANS simulations of 3D incompressible flows, over flat plate and airfoil geometries. We show the influence of mesh resolution and domain of application on the accuracy of the models and investigate the influence of the source term on the generated flow field. Our results demonstrate the grid dependence of the models and indicate the presence of model errors. Furthermore we find that the total applied force has a larger influence on both the intensity and shape of the created vortex than the distribution of the source term over the cells., Aerodynamics, Education and Student Affairs

Aerodynamics, Control & Simulation

In this article, we endeavour to find a fast solver for finite volume discretizations for compressible unsteady viscous flows. Thereby, we concentrate on comparing the efficiency of important classes of time integration schemes, namely time adaptive Rosenbrock, singly diagonally implicit (SDIRK) and explicit first stage singly diagonally implicit Runge-Kutta (ESDIRK) methods. To make the comparison fair, efficient equation system solvers need to be chosen and a smart choice of tolerances is needed. This is determined from the tolerance TOL that steers time adaptivity. For implicit Runge-Kutta methods, the solver is given by preconditioned inexact Jacobian-free Newton-Krylov (JFNK) and for Rosenbrock, it is preconditioned Jacobian-free GMRES. To specify the tolerances in there, we suggest a simple strategy of using TOL/100 that is a good compromise between stability and computational effort. Numerical experiments for different test cases show that the fourth order Rosenbrock method RODASP and the fourth order ESDIRK method ESDIRK4 are best for fine tolerances, with RODASP being the most robust scheme., Aerodynamics, Science Centre & Programmering

High computational cost associated with the high fidelity wake models such as RANS or LES serves as a primary bottleneck to perform a direct high fidelity wind farm layout optimization (WFLO) using accurate CFD based wake models. Therefore, a surrogate based multi-fidelity WFLO methodology (SWFLO) is proposed. The surrogate model is built using an SBO method referred as manifold mapping (MM). As a verification, optimization of spacing between two staggered wind turbines was performed using the proposed surrogate based methodology and the performance was compared with that of direct optimization using high fidelity model. Significant reduction in computational cost was achieved using MM: a maximum computational cost reduction of 65%, while arriving at the same optima as that of direct high fidelity optimization. The similarity between the response of models, the number of mapping points and its position, highly influences the computational efficiency of the proposed method. As a proof of concept, realistic WFLO of a small 7-turbine wind farm is performed using the proposed surrogate based methodology. Two variants of Jensen wake model with different decay coefficients were used as the fine and coarse model. The proposed SWFLO method arrived at the same optima as that of the fine model with very less number of fine model simulations., Aerodynamics, Aerospace Engineering

In this article, we endeavour to find a fast solver for finite volume discretizations for compressible unsteady viscous flows. Thereby, we concentrate on comparing the efficiency of important classes of time integration schemes, namely time adaptive Rosenbrock, singly diagonally implicit (SDIRK) and explicit first stage singly diagonally implicit Runge-Kutta (ESDIRK) methods. To make the comparison fair, efficient equation system solvers need to be chosen and a smart choice of tolerances is needed. This is determined from the tolerance TOL that steers time adaptivity. For implicit Runge-Kutta methods, the solver is given by preconditioned inexact Jacobian-free Newton-Krylov (JFNK) and for Rosenbrock, it is preconditioned Jacobian-free GMRES. To specify the tolerances in there, we suggest a simple strategy of using TOL/100 that is a good compromise between stability and computational effort. Numerical experiments for different test cases show that the fourth order Rosenbrock method RODASP and the fourth order ESDIRK method ESDIRK4 are best for fine tolerances, with RODASP being the most robust scheme., Aerodynamics, Science Centre & Programmering

High computational cost associated with the high fidelity wake models such as RANS or LES serves as a primary bottleneck to perform a direct high fidelity wind farm layout optimization (WFLO) using accurate CFD based wake models. Therefore, a surrogate based multi-fidelity WFLO methodology (SWFLO) is proposed. The surrogate model is built using an SBO method referred as manifold mapping (MM). As a verification, optimization of spacing between two staggered wind turbines was performed using the proposed surrogate based methodology and the performance was compared with that of direct optimization using high fidelity model. Significant reduction in computational cost was achieved using MM: a maximum computational cost reduction of 65%, while arriving at the same optima as that of direct high fidelity optimization. The similarity between the response of models, the number of mapping points and its position, highly influences the computational efficiency of the proposed method. As a proof of concept, realistic WFLO of a small 7-turbine wind farm is performed using the proposed surrogate based methodology. Two variants of Jensen wake model with different decay coefficients were used as the fine and coarse model. The proposed SWFLO method arrived at the same optima as that of the fine model with very less number of fine model simulations., Aerodynamics, Aerospace Engineering

We present an integrated in-situ visualization approach for partitioned multi-physics simulation of fluid-structure interaction. The simulation itself is treated as a black box and only the information at the fluid-structure interface is considered, and communicated between the fluid and solid solvers with a separate coupling tool. The visualization of the interface data is performed in conjunction with the fluid solver. Furthermore, we present new visualization techniques for the analysis of the interrelation of the two solvers , with emphasis on the involved error due to discretization in space and time and the reconstruction. Our visualization approach also enables the investigation of these errors with respect of their mutual influence on the two simulation codes and their space-time discretization. For efficient interactive visualization, we employ the concept of explorable spatiotemporal images, which also enables finite-time temporal navigation in an in-situ context. We demonstrate our overall approach and its utility by means of a fluid-structure simulation using OpenFOAM that is coupled by the preCICE software layer., Aerodynamics, Wind Energy & Propulsion, Aerospace Engineering

A numerical investigation is performed to address the flexing effect on the propulsion performance of flapping wing particularly on the counter-flapping wings of the biplane configuration. A Reynolds number of 10,000 is considered in the present study which corresponds to the flight regime of most existing flapping wing micro air vehicles. The computation involves solving the compressible unsteady Reynolds- averaged Native-Stokes equation using an inhouse developed code. The flapping motion is incorporated by an efficient deforming overset grid technique which allows multiple flexible bodies to be embedded into the flow field. Results show that the biplane wing with counter-flapping configuration has a better propulsive performance in comparison to a single flapping wing. A low-pressure regime between the two wings during the outstroke produces more thrust, while the counter-flapping motion can also generate a surfeit momentum rushing in to the wake. The more flexible wing can produce more thrust while less power is required thus owning a better propulsive performance., Aerodynamics, Wind Energy & Propulsion, Aerospace Engineering

The flow around a beveled flat plate model with an asymmetric 25 degrees trailing edge with three rounding radii is analyzed using a Navier-Stokes based open source software package OpenFOAM in order to predict the aeroacoustic properties of the models. A Large Eddy Simulation with a dynamic Smagorinsky and implicit model are used as closure model for the flow solver, and are compared regarding their aeroacoustic performance. Velocity coherence and pressure correlation is determined in spanwise direction. The acoustic far field spectrum is obtained by solving Curle’s analogy in frequency domain as a post-processing step., Aerodynamics, Aerospace Engineering

Multi-physics simulations, such as fluid-structure-acoustics interaction (FSA), require a high performance computing environment in order to perform the simulation in a reasonable amount of computation time. Currently used coupling methods use a staggered execution of the fluid and solid solver [6], which leads to inherent load imbalances. In [12] a new coupling scheme based on a quasi-Newton method is proposed for fluidstructure interaction which coupled the fluid and solid solver in parallel. The quasi-Newton method requires approximately the same number of coupling iterations per time step compared to a staggered coupling approach, resulting in a better load balance when running in a parallel environment. This contribution investigates the scalability limit and load-balancing for a strongly coupled fluid-structure interaction problem, and also for a fluid-structure-acoustics interaction problem. The acoustic far field of the fluid-structure-acoustics interaction problem is loosely coupled with the flow field., Aerodynamics, Wind Energy & Propulsion, Aerospace Engineering

A numerical investigation is performed to address the flexing effect on the propulsion performance of flapping wing particularly on the counter-flapping wings of the biplane configuration. A Reynolds number of 10,000 is considered in the present study which corresponds to the flight regime of most existing flapping wing micro air vehicles. The computation involves solving the compressible unsteady Reynoldsaveraged Native-Stokes equation using an inhouse developed code. The flapping motion is incorporated by an efficient deforming overset grid technique which allows multiple flexible bodies to be embedded into the flow field. Results show that the biplane wing with counterflapping configuration has a better propulsive performance in comparison to a single flapping wing. A low-pressure regime between the two wings during the outstroke produces more thrust, while the counter-flapping motion can also generate a surfeit momentum rushing in to the wake. The more flexible wing can produce more thrust while less power is required thus owning a better propulsive performance., Aerodynamics

Background Solid bio-wastes (or organic residues) are worldwide produced in high amount and increasingly considered bioenergy containers rather than waste products. A complete bioprocess from recalcitrant solid wastes to methane (SW2M) via anaerobic digestion (AD) is believed to be a sustainable way to utilize solid bio-wastes. However, the complex and recalcitrance of these organic solids make the hydrolysis process inefficient and thus a rate-limiting step to many AD technologies. Effort has been made to enhance the hydrolysis efficiency, but a comprehensive assessment over a complete flow scheme of SW2M is rare. Results In this study, it comes to reality of a complete scheme for SW2M. A novel process to efficiently convert organic residues into methane is proposed, which proved to be more favorable compared to conventional methods. Brewers’ spent grain (BSG) and pig manure (PM) were used to test the feasibility and efficiency. BSG and PM were enzymatically pre-hydrolyzed and solubilized, after which the hydrolysates were anaerobically digested using different bioreactor designs, including expanded granular sludge bed (EGSB), continuously stirred tank reactor (CSTR), and sequencing batch reactor (SBR). High organic loading rates (OLRs), reaching 19 and 21 kgCOD?·?m?3?·?day?1 were achieved for the EGSBs, fed with BSG and PM, respectively, which were five to seven times higher than those obtained with direct digestion of the raw materials via CSTR or SBR. About 56% and 45% organic proportion of the BSG and PM can be eventually converted to methane. Conclusions This study proves that complex organic solids, such as cellulose, hemicellulose, proteins, and lipids can be efficiently hydrolyzed, yielding easy biodegradable/bio-convertible influents for the subsequent anaerobic digestion step. Although the economical advantage might not be clear, the current approach represents an efficient way for industrial-scale treatment of organic residues with a small footprint and, Water Management, Civil Engineering and Geosciences

We present an integrated in-situ visualization approach for partitioned multi-physics simulation of fluid-structure interaction. The simulation itself is treated as a black box and only the information at the fluid-structure interface is considered, and communicated between the fluid and solid solvers with a separate coupling tool. The visualization of the interface data is performed in conjunction with the fluid solver. Furthermore, we present new visualization techniques for the analysis of the interrelation of the two solvers , with emphasis on the involved error due to discretization in space and time and the reconstruction. Our visualization approach also enables the investigation of these errors with respect of their mutual influence on the two simulation codes and their space-time discretization. For efficient interactive visualization, we employ the concept of explorable spatiotemporal images, which also enables finite-time temporal navigation in an in-situ context. We demonstrate our overall approach and its utility by means of a fluid-structure simulation using OpenFOAM that is coupled by the preCICE software layer., Aerodynamics, Wind Energy & Propulsion, Aerospace Engineering

A numerical investigation is performed to address the flexing effect on the propulsion performance of flapping wing particularly on the counter-flapping wings of the biplane configuration. A Reynolds number of 10,000 is considered in the present study which corresponds to the flight regime of most existing flapping wing micro air vehicles. The computation involves solving the compressible unsteady Reynolds- averaged Native-Stokes equation using an inhouse developed code. The flapping motion is incorporated by an efficient deforming overset grid technique which allows multiple flexible bodies to be embedded into the flow field. Results show that the biplane wing with counter-flapping configuration has a better propulsive performance in comparison to a single flapping wing. A low-pressure regime between the two wings during the outstroke produces more thrust, while the counter-flapping motion can also generate a surfeit momentum rushing in to the wake. The more flexible wing can produce more thrust while less power is required thus owning a better propulsive performance., Aerodynamics, Wind Energy & Propulsion, Aerospace Engineering

Multi-physics simulations, such as fluid-structure-acoustics interaction (FSA), require a high performance computing environment in order to perform the simulation in a reasonable amount of computation time. Currently used coupling methods use a staggered execution of the fluid and solid solver [6], which leads to inherent load imbalances. In [12] a new coupling scheme based on a quasi-Newton method is proposed for fluidstructure interaction which coupled the fluid and solid solver in parallel. The quasi-Newton method requires approximately the same number of coupling iterations per time step compared to a staggered coupling approach, resulting in a better load balance when running in a parallel environment. This contribution investigates the scalability limit and load-balancing for a strongly coupled fluid-structure interaction problem, and also for a fluid-structure-acoustics interaction problem. The acoustic far field of the fluid-structure-acoustics interaction problem is loosely coupled with the flow field., Aerodynamics, Wind Energy & Propulsion, Aerospace Engineering

A numerical investigation is performed to address the flexing effect on the propulsion performance of flapping wing particularly on the counter-flapping wings of the biplane configuration. A Reynolds number of 10,000 is considered in the present study which corresponds to the flight regime of most existing flapping wing micro air vehicles. The computation involves solving the compressible unsteady Reynoldsaveraged Native-Stokes equation using an inhouse developed code. The flapping motion is incorporated by an efficient deforming overset grid technique which allows multiple flexible bodies to be embedded into the flow field. Results show that the biplane wing with counterflapping configuration has a better propulsive performance in comparison to a single flapping wing. A low-pressure regime between the two wings during the outstroke produces more thrust, while the counter-flapping motion can also generate a surfeit momentum rushing in to the wake. The more flexible wing can produce more thrust while less power is required thus owning a better propulsive performance., Aerodynamics

The flow around a beveled flat plate model with an asymmetric 25 degrees trailing edge with three rounding radii is analyzed using a Navier-Stokes based open source software package OpenFOAM in order to predict the aeroacoustic properties of the models. A Large Eddy Simulation with a dynamic Smagorinsky and implicit model are used as closure model for the flow solver, and are compared regarding their aeroacoustic performance. Velocity coherence and pressure correlation is determined in spanwise direction. The acoustic far field spectrum is obtained by solving Curle’s analogy in frequency domain as a post-processing step., Aerodynamics, Aerospace Engineering

Background Solid bio-wastes (or organic residues) are worldwide produced in high amount and increasingly considered bioenergy containers rather than waste products. A complete bioprocess from recalcitrant solid wastes to methane (SW2M) via anaerobic digestion (AD) is believed to be a sustainable way to utilize solid bio-wastes. However, the complex and recalcitrance of these organic solids make the hydrolysis process inefficient and thus a rate-limiting step to many AD technologies. Effort has been made to enhance the hydrolysis efficiency, but a comprehensive assessment over a complete flow scheme of SW2M is rare. Results In this study, it comes to reality of a complete scheme for SW2M. A novel process to efficiently convert organic residues into methane is proposed, which proved to be more favorable compared to conventional methods. Brewers’ spent grain (BSG) and pig manure (PM) were used to test the feasibility and efficiency. BSG and PM were enzymatically pre-hydrolyzed and solubilized, after which the hydrolysates were anaerobically digested using different bioreactor designs, including expanded granular sludge bed (EGSB), continuously stirred tank reactor (CSTR), and sequencing batch reactor (SBR). High organic loading rates (OLRs), reaching 19 and 21 kgCOD?·?m?3?·?day?1 were achieved for the EGSBs, fed with BSG and PM, respectively, which were five to seven times higher than those obtained with direct digestion of the raw materials via CSTR or SBR. About 56% and 45% organic proportion of the BSG and PM can be eventually converted to methane. Conclusions This study proves that complex organic solids, such as cellulose, hemicellulose, proteins, and lipids can be efficiently hydrolyzed, yielding easy biodegradable/bio-convertible influents for the subsequent anaerobic digestion step. Although the economical advantage might not be clear, the current approach represents an efficient way for industrial-scale treatment of organic residues with a small footprint and, Water Management, Civil Engineering and Geosciences

A three-dimensional simulation of hovering flapping wings was performed using an immersed noundary method. This was done to investigate the effects of chordwise wing deformation on three important unsteady aerodynamic mechanisms found in flapping flight, namely Leading Edge Vortex (LEV) shedding, wake capture and clap and fling. A wing was modeled as a flat plate, flapping close to a symmetry plane. Three different deforming chords were defined, a rigid case, a case with maximum deformation at the trailing edge and increased angle of attack (AoA) near the leading edge, and a case with the maximum deformation in the center of the chord and decreased AoA near the leading edge. All cases had zero deformation at the wing root and maximal deformation at the wing tip. A higher AoA near the leading edge resulted in faster LEV buildup and faster buildup of lift. No shedding of the LEV was observed in any of the cases even when deformation caused a high AoA near the leading edge. A distinct dip in lift buildup was observed and shown to be caused by the interaction between the previously shed vortex and the newly developing LEV. This interaction occurred faster when the AoA at the leading edge was increased, and slower when the angle of attack was decreased. Moving the wing closer to the symmetry plane had a positive effect on the cycle average value of CL. This positive effect was reduced however by the earlier interaction between the LEV and the previously shed vortex.

A three-dimensional simulation of hovering flapping wings was performed using an immersed boundary method. This was done to investigate the effects of chordwise wing deformation on three important unsteady aerodynamic mechanisms found in flapping flight, namely Leading Edge Vortex (LEV) shedding, wake capture and clap and fling. A wing was modeled as a flat plate, flapping close to a symmetry plane. Three different deforming chords were defined, a rigid case, a case with maximum deformation at the trailing edge and increased angle of attack (AoA) near the leading edge, and a case with the maximum deformation in the center of the chord and decreased AoA near the leading edge. All cases had zero deformation at the wing root and maximal deformation at the wing tip. A higher AoA near the leading edge resulted in faster LEV buildup and faster buildup of lift. No shedding of the LEV was observed in any of the cases even when deformation caused a high AoA near the leading edge. A distinct dip in lift buildup was observed and shown to be caused by the interaction between the previously shed vortex and the newly developing LEV. This interaction occurred faster when the AoA at the leading edge was increased, and slower when the angle of attack was decreased. Moving the wing closer to the symmetry plane had a positive effect on the cycle average value of CL. This positive effect was reduced however by the earlier interaction between the LEV and the previously shed vortex., Aerodynamics, Wind Energy & Propulsion, Aerospace Engineering

This paper is the starting point for the development of a multi-fidelity modeling approach for vortex generators (VG) arrays, where a fully resolved VG model will be coupled with an approximate model in order to improve both accuracy and flexibility without increasing the required computational cost. As a first step thereto, an analysis of the ability of the BAY-model to simulate incompressible flows around a VG on a flat plate has been performed. Results are presented for several turbulence models and using different cell selection approaches, where comparison is made with fully resolved VG results. In addition to a coarse uniform mesh, the BAY-model has been evaluated on a densely gridded mesh in order to distinguish between model and mesh related discrepancies., Aerodynamics, Wind Energy & Propulsion, Aerospace Engineering

Strongly coupled partitioned fluid-structure interaction problems require multiple coupling iterations per time step. The fluid domain and the structure domain are solved multiple times in each time step such that the kinematic and dynamic interface conditions on the fluid-structure interface are satisfied. Quasi-Newton methods have been successfully applied in case the fluid and structure solvers are considered as black boxes, i.e. only input and output information of the solvers are used by the coupling techniques. In this contribution a computationally inexpensive low-fidelity model is combined with a high-fidelity model in order to accelerate the convergence of the high-fidelity model. This is achieved by applying the manifold mapping algorithm on the fluid-structure interaction problem in order to minimize the fluid-structure interface residual. Originating from multi-fidelity optimization, the manifold mapping algorithm is applied for the first time in a simulation context, instead of an optimization context. The manifold mapping algorithm is applied on a standard fluid-structure interaction benchmark, namely the cylinder flap FSI3 case. A reduction of 48% in terms of high fidelity iterations is achieved compared with the inverse least squares Quasi-Newton algorithm, resulting in a 42% decrease in computational costs., Aerodynamics, Wind Energy & Propulsion, Aerospace Engineering

A Large Eddy Simulation (LES) with four different closure models are analyzed in OpenFOAM, an open source Computional Fluid Dynamics (CFD) package and validated for the determination of the streamwise and spanwise coherence length of the pressure field below a turbulent boundary layer at low Reynolds numbers. Matching results are found for outer scaling mean and fluctuating velocity data as well as for the pressure spectrum data. The coherence function shows a similar decay with respect to various literature studies. An exponential fit is applied to determine the coherence length. Agreement within one displacement thickness error in streamwise and spanwise direction is found for the coherence length with semi-empirical data. The spanwise coherence length is considerably smaller than the streamwise coherence length, but indicates a clear peak in the low frequency regime originating from large coherent structures with relatively small amplitudes., Aerodynamics, Wind Energy & Propulsion, Aerospace Engineering

This paper is the starting point for the development of a multi-fidelity modeling approach for vortex generators (VG) arrays, where a fully resolved VG model will be coupled with an approximate model in order to improve both accuracy and flexibility without increasing the required computational cost. As a first step thereto, an analysis of the ability of the BAY-model to simulate incompressible flows around a VG on a flat plate has been performed. Results are presented for several turbulence models and using different cell selection approaches, where comparison is made with fully resolved VG results. In addition to a coarse uniform mesh, the BAY-model has been evaluated on a densely gridded mesh in order to distinguish between model and mesh related discrepancies., Aerodynamics, Wind Energy & Propulsion, Aerospace Engineering

A three-dimensional simulation of hovering flapping wings was performed using an immersed noundary method. This was done to investigate the effects of chordwise wing deformation on three important unsteady aerodynamic mechanisms found in flapping flight, namely Leading Edge Vortex (LEV) shedding, wake capture and clap and fling. A wing was modeled as a flat plate, flapping close to a symmetry plane. Three different deforming chords were defined, a rigid case, a case with maximum deformation at the trailing edge and increased angle of attack (AoA) near the leading edge, and a case with the maximum deformation in the center of the chord and decreased AoA near the leading edge. All cases had zero deformation at the wing root and maximal deformation at the wing tip. A higher AoA near the leading edge resulted in faster LEV buildup and faster buildup of lift. No shedding of the LEV was observed in any of the cases even when deformation caused a high AoA near the leading edge. A distinct dip in lift buildup was observed and shown to be caused by the interaction between the previously shed vortex and the newly developing LEV. This interaction occurred faster when the AoA at the leading edge was increased, and slower when the angle of attack was decreased. Moving the wing closer to the symmetry plane had a positive effect on the cycle average value of CL. This positive effect was reduced however by the earlier interaction between the LEV and the previously shed vortex.

A three-dimensional simulation of hovering flapping wings was performed using an immersed boundary method. This was done to investigate the effects of chordwise wing deformation on three important unsteady aerodynamic mechanisms found in flapping flight, namely Leading Edge Vortex (LEV) shedding, wake capture and clap and fling. A wing was modeled as a flat plate, flapping close to a symmetry plane. Three different deforming chords were defined, a rigid case, a case with maximum deformation at the trailing edge and increased angle of attack (AoA) near the leading edge, and a case with the maximum deformation in the center of the chord and decreased AoA near the leading edge. All cases had zero deformation at the wing root and maximal deformation at the wing tip. A higher AoA near the leading edge resulted in faster LEV buildup and faster buildup of lift. No shedding of the LEV was observed in any of the cases even when deformation caused a high AoA near the leading edge. A distinct dip in lift buildup was observed and shown to be caused by the interaction between the previously shed vortex and the newly developing LEV. This interaction occurred faster when the AoA at the leading edge was increased, and slower when the angle of attack was decreased. Moving the wing closer to the symmetry plane had a positive effect on the cycle average value of CL. This positive effect was reduced however by the earlier interaction between the LEV and the previously shed vortex., Aerodynamics, Wind Energy & Propulsion, Aerospace Engineering

Strongly coupled partitioned fluid-structure interaction problems require multiple coupling iterations per time step. The fluid domain and the structure domain are solved multiple times in each time step such that the kinematic and dynamic interface conditions on the fluid-structure interface are satisfied. Quasi-Newton methods have been successfully applied in case the fluid and structure solvers are considered as black boxes, i.e. only input and output information of the solvers are used by the coupling techniques. In this contribution a computationally inexpensive low-fidelity model is combined with a high-fidelity model in order to accelerate the convergence of the high-fidelity model. This is achieved by applying the manifold mapping algorithm on the fluid-structure interaction problem in order to minimize the fluid-structure interface residual. Originating from multi-fidelity optimization, the manifold mapping algorithm is applied for the first time in a simulation context, instead of an optimization context. The manifold mapping algorithm is applied on a standard fluid-structure interaction benchmark, namely the cylinder flap FSI3 case. A reduction of 48% in terms of high fidelity iterations is achieved compared with the inverse least squares Quasi-Newton algorithm, resulting in a 42% decrease in computational costs., Aerodynamics, Wind Energy & Propulsion, Aerospace Engineering

A Large Eddy Simulation (LES) with four different closure models are analyzed in OpenFOAM, an open source Computional Fluid Dynamics (CFD) package and validated for the determination of the streamwise and spanwise coherence length of the pressure field below a turbulent boundary layer at low Reynolds numbers. Matching results are found for outer scaling mean and fluctuating velocity data as well as for the pressure spectrum data. The coherence function shows a similar decay with respect to various literature studies. An exponential fit is applied to determine the coherence length. Agreement within one displacement thickness error in streamwise and spanwise direction is found for the coherence length with semi-empirical data. The spanwise coherence length is considerably smaller than the streamwise coherence length, but indicates a clear peak in the low frequency regime originating from large coherent structures with relatively small amplitudes., Aerodynamics, Wind Energy & Propulsion, Aerospace Engineering

The numerical simulation of a “X-wing” type biplane flapping wings, has been performed in 3D using the Immersed Boundary Method (IBM). This “X-wing” type flapping configuration draws its inspiration from Delfly [1], a family of ornithopters developed by the Delft University of Technology, as shown in Figure 1. The unique “X-wing” design features a biplane flapping wings where two sets of wings were are placed above each other moving in counter phase. On comparison with configurations using a single pair of wings or two sets of wings in tandem, experiments showed that the “X-wing” configuration gives lower power requirement and zero rocking amplitude, which is a beneficial property for a Flapping Micro Aerial Vehicle (FMAV) to be used as a camera platform., Aerodynamics, Wind Energy & Propulsion, Aerospace Engineering

This contribution compares the efficiency of Rosenbrock time integration schemes with ESDIRK schemes, applicable to unsteady flow and fluid-structure interaction simulations. Compared to non-linear ESDIRK schemes, the linear implicit Rosenbrock-Wanner schemes require subsequent solution of the same linear systems with different right hand sides. By solving the linear systems with the iterative solver GMRES, the preconditioner can be reused for the subsequent stages of the Rosenbrock-Wanner scheme. Unsteady flow simulations show a gain in computational efficiency of approximately factor three to five in comparison with ESDIRK., Aerodynamics, Wind Energy & Propulsion, Aerospace Engineering

Aerodynamics, Wind Energy & Propulsion, Aerospace Engineering

The numerical simulation of a “X-wing” type biplane flapping wings, has been performed in 3D using the Immersed Boundary Method (IBM). This “X-wing” type flapping configuration draws its inspiration from Delfly [1], a family of ornithopters developed by the Delft University of Technology, as shown in Figure 1. The unique “X-wing” design features a biplane flapping wings where two sets of wings were are placed above each other moving in counter phase. On comparison with configurations using a single pair of wings or two sets of wings in tandem, experiments showed that the “X-wing” configuration gives lower power requirement and zero rocking amplitude, which is a beneficial property for a Flapping Micro Aerial Vehicle (FMAV) to be used as a camera platform., Aerodynamics, Wind Energy & Propulsion, Aerospace Engineering

This contribution compares the efficiency of Rosenbrock time integration schemes with ESDIRK schemes, applicable to unsteady flow and fluid-structure interaction simulations. Compared to non-linear ESDIRK schemes, the linear implicit Rosenbrock-Wanner schemes require subsequent solution of the same linear systems with different right hand sides. By solving the linear systems with the iterative solver GMRES, the preconditioner can be reused for the subsequent stages of the Rosenbrock-Wanner scheme. Unsteady flow simulations show a gain in computational efficiency of approximately factor three to five in comparison with ESDIRK., Aerodynamics, Wind Energy & Propulsion, Aerospace Engineering

Aerodynamics, Wind Energy & Propulsion, Aerospace Engineering

Thermal interaction of fluids and solids, or conjugate heat transfer (CHT), is encountered in many engineering applications. Since time-accurate computations of such coupled problems can be computationally expensive, we consider loosely-coupled and strongly- coupled solution algorithms in which higher order multi-stage Runge-Kutta schemes are employed for time integration. The higher order time integration schemes have the potential to improve the computational efficiency at arriving at a certain accuracy relative to the traditionally used 1st and 2nd order implicit schemes. The spatial coupling between the subdomains is realized using Dirichlet-Neumann interface conditions and the coupled domains are solved in a sequential manner at each stage (Block Gauss-Seidel). In this paper, the stability of two partitioned algorithms is analyzed by considering a one dimensional model problem. The model problem consists of two thermally coupled domains where the governing equation within each subdomain is unsteady linear heat conduction. In the loosely-coupled approach, a family of multi-stage IMEX schemes is used for time integration. By observing similarities between the second stage of the IMEX schemes and the ? scheme with ? = 0.5 (Crank-Nicolson), the stability of the partitioned algorithm in which the Crank-Nicolson scheme is used for time integration is first analyzed by applying the stability theory of Godunov-Ryabenkii. The stability of the IMEX schemes is next investigated by numerically solving the model problem and comparing the results to the conclusions of the stability analysis for the Crank-Nicolson scheme. Due to partly explicit nature of the IMEX schemes, the loosely-coupled algorithm becomes unstable for sufficiently large Fourier numbers (similar to the Crank-Nicolson scheme). When the ratio of the thermal effusivities of the coupled domains is much smaller than unity, time step restriction due to stability is sufficiently weak that computations can be perf, Aerdynamics, Wind Energy and Propulsion, Aerospace Engineering

Numerical simulations have been performed to examine the interference effects between an upstream flapping biplane airfoil arrangement and a downstream stationary tail at a Reynolds number of 1000, which is around the regime of small flapping micro aerial vehicles. The objective is to investigate the effect of the relative distance and angle of attack between the airfoils and its tail on the overall propulsive efficiency, thrust and lift. An immersed boundary method Navier-Stokes solver is used for the simulation. Results show that overall efficiency and average thrust per airfoil can be increased up to 17% and 126% respectively when the top and bottom airfoils come into contact during flapping. When placing the tail at a strategic position, the overall configuration generates much higher lift, although at the expense of decreased efficiency and thrust. Increasing the angle of attack of the tail also helps to increase the lift. Analysis of the vorticity plots reveals the interaction between the vortices and the airfoils and the reason behind the high thrust and lift. The results obtained from this study can be used to optimize the performance of small flapping MAVs., Aerodynamics, Wind Energy & Propulsion, Aerospace Engineering

The focus of this paper is on acceleration of strong partitioned coupling algorithms for fluid-structure interaction. Strong partitioned coupling requires the solution of a coupled problem at each time step during the simulation. Hereto, an interface residual is defined such that the kinematic and dynamic interface conditions on the fluid-structure interface are satisfied when it amounts to zero. Subsequently, the coupled problem is formulated as a minimization problem of the interface residual which can efficiently be performed using Newton’s method. However, Newton’s method cannot be applied when the fluid and structure solvers are considered black-boxes since the Jacobian of the interface residual is not available. For this reason, Quasi-Newton methods were developed that approximate either the Jacobian or the inverse Jacobian of the interface residual directly from input/output information. In this contribution we present a new algorithm that uses a technique from multifidelity optimization – called space-mapping – to efficiently perform the minimization of the interface residual. The space-mapping technique exploits a computationally inexpensive lowfidelity model in order to accelerate an expensive high-fidelity model using black-box information only. The space-mapping algorithm is applied to the supersonic panel flutter problem in order to demonstrate its effectiveness. The speedup – defined with respect to a Quasi-Newton algorithm – is found to be 1-1.5 for typical time step sizes. It is expected that higher speedups can be obtained when problems are considered that require strong coupling as the time step decreases, e.g. due to the added mass effect when the structure is in interaction with an incompressible fluid., Aerodynamics, Wind Energy and Propulsion, Aerospace Engineering

Noting that time-accurate computations of the unsteady incompressible Navier-Stokes (INS) equations can be computationally expensive, a family of higher order implicit multi-stage time integration schemes (namely ESDIRK) is used for advancing the solution to the unsteady INS in time. The higher order time integration schemes have the potential to decrease the computational cost of obtaining engineering levels of accuracy relative to the traditionally used 2nd order implicit schemes. The finite volume method is used for spatial discretization, and co-located arrangement of the primitive variables is considered. Furthermore, an iterated PISO algorithm is used to solve the incompressible Navier-Stokes equations. By using a temporally consistent Rhie-Chow interpolation, higher order temporal accuracy in solving the INS equations on co-located grids is achieved. For a two-dimensional lid driven cavity test case, the temporal convergence of the solution is investigated, with the third and fourth order ESDIRK schemes for time integration. The results demonstrate the temporal consistency and temporal order preservation of the algorithm., Aerodynamics, Wind Energy and Propulsion, Aerospace Engineering

Thermal interaction of fluids and solids, or conjugate heat transfer (CHT), is encountered in many engineering applications. Since time-accurate computations of such coupled problems can be computationally expensive, we consider loosely-coupled and strongly- coupled solution algorithms in which higher order multi-stage Runge-Kutta schemes are employed for time integration. The higher order time integration schemes have the potential to improve the computational efficiency at arriving at a certain accuracy relative to the traditionally used 1st and 2nd order implicit schemes. The spatial coupling between the subdomains is realized using Dirichlet-Neumann interface conditions and the coupled domains are solved in a sequential manner at each stage (Block Gauss-Seidel). In this paper, the stability of two partitioned algorithms is analyzed by considering a one dimensional model problem. The model problem consists of two thermally coupled domains where the governing equation within each subdomain is unsteady linear heat conduction. In the loosely-coupled approach, a family of multi-stage IMEX schemes is used for time integration. By observing similarities between the second stage of the IMEX schemes and the ? scheme with ? = 0.5 (Crank-Nicolson), the stability of the partitioned algorithm in which the Crank-Nicolson scheme is used for time integration is first analyzed by applying the stability theory of Godunov-Ryabenkii. The stability of the IMEX schemes is next investigated by numerically solving the model problem and comparing the results to the conclusions of the stability analysis for the Crank-Nicolson scheme. Due to partly explicit nature of the IMEX schemes, the loosely-coupled algorithm becomes unstable for sufficiently large Fourier numbers (similar to the Crank-Nicolson scheme). When the ratio of the thermal effusivities of the coupled domains is much smaller than unity, time step restriction due to stability is sufficiently weak that computations can be perf, Aerdynamics, Wind Energy and Propulsion, Aerospace Engineering

Numerical simulations have been performed to examine the interference effects between an upstream flapping biplane airfoil arrangement and a downstream stationary tail at a Reynolds number of 1000, which is around the regime of small flapping micro aerial vehicles. The objective is to investigate the effect of the relative distance and angle of attack between the airfoils and its tail on the overall propulsive efficiency, thrust and lift. An immersed boundary method Navier-Stokes solver is used for the simulation. Results show that overall efficiency and average thrust per airfoil can be increased up to 17% and 126% respectively when the top and bottom airfoils come into contact during flapping. When placing the tail at a strategic position, the overall configuration generates much higher lift, although at the expense of decreased efficiency and thrust. Increasing the angle of attack of the tail also helps to increase the lift. Analysis of the vorticity plots reveals the interaction between the vortices and the airfoils and the reason behind the high thrust and lift. The results obtained from this study can be used to optimize the performance of small flapping MAVs., Aerodynamics, Wind Energy & Propulsion, Aerospace Engineering

The focus of this paper is on acceleration of strong partitioned coupling algorithms for fluid-structure interaction. Strong partitioned coupling requires the solution of a coupled problem at each time step during the simulation. Hereto, an interface residual is defined such that the kinematic and dynamic interface conditions on the fluid-structure interface are satisfied when it amounts to zero. Subsequently, the coupled problem is formulated as a minimization problem of the interface residual which can efficiently be performed using Newton’s method. However, Newton’s method cannot be applied when the fluid and structure solvers are considered black-boxes since the Jacobian of the interface residual is not available. For this reason, Quasi-Newton methods were developed that approximate either the Jacobian or the inverse Jacobian of the interface residual directly from input/output information. In this contribution we present a new algorithm that uses a technique from multifidelity optimization – called space-mapping – to efficiently perform the minimization of the interface residual. The space-mapping technique exploits a computationally inexpensive lowfidelity model in order to accelerate an expensive high-fidelity model using black-box information only. The space-mapping algorithm is applied to the supersonic panel flutter problem in order to demonstrate its effectiveness. The speedup – defined with respect to a Quasi-Newton algorithm – is found to be 1-1.5 for typical time step sizes. It is expected that higher speedups can be obtained when problems are considered that require strong coupling as the time step decreases, e.g. due to the added mass effect when the structure is in interaction with an incompressible fluid., Aerodynamics, Wind Energy and Propulsion, Aerospace Engineering

We perform a fully stochastic analysis of an experiment in aerodynamics. Given estimated uncertainties on the principle input parameters of the experiment, including uncertainties on the shape of the model, we apply uncertainty propagation methods to a suitable CFD model of the experimental setup. Thereby we predict the stochastic response of the measurements due to the experimental uncertainties. To reduce the variance of these uncertainties a Bayesian updating technique is employed in which the uncertain parameters are treated as calibration parameters, with priors taken as the original uncertainty estimates. Imprecise measurements of aerodynamic forces are used as observational data. Motivation and a concrete application come from a wind-tunnel experiment whose parameters and model geometry have substantial uncertainty. In this case the uncertainty was a consequence of a poorly constructed model in the pre-measurement phase. These methodological uncertainties lead to substantial uncertainties in the measurement of forces. Imprecise geometry measurements from multiple sources are used to create an improved stochastic model of the geometry. Calibration against lift and moment data then gives us estimates of the remaining parameters. The effectiveness of the procedure is demonstrated by prediction of drag with uncertainty., Aerodynamics, Wind Energy & Propulsion, Aerospace Engineering

Noting that time-accurate computations of the unsteady incompressible Navier-Stokes (INS) equations can be computationally expensive, a family of higher order implicit multi-stage time integration schemes (namely ESDIRK) is used for advancing the solution to the unsteady INS in time. The higher order time integration schemes have the potential to decrease the computational cost of obtaining engineering levels of accuracy relative to the traditionally used 2nd order implicit schemes. The finite volume method is used for spatial discretization, and co-located arrangement of the primitive variables is considered. Furthermore, an iterated PISO algorithm is used to solve the incompressible Navier-Stokes equations. By using a temporally consistent Rhie-Chow interpolation, higher order temporal accuracy in solving the INS equations on co-located grids is achieved. For a two-dimensional lid driven cavity test case, the temporal convergence of the solution is investigated, with the third and fourth order ESDIRK schemes for time integration. The results demonstrate the temporal consistency and temporal order preservation of the algorithm., Aerodynamics, Wind Energy and Propulsion, Aerospace Engineering

Numerical simulations have been performed to examine the interference effects between an upstream flapping airfoil and a downstream stationary airfoil in a tandem configuration at a Reynolds number of 1000, which is around the regime of small flapping micro aerial vehicles. The objective is to investigate the effect of the distance of the tail and its angle of attack on the overall propulsive efficiency, thrust and lift. An immersed boundary method Navier-Stokes solver is used for the simulation. Results show that efficiency and average thrust can be increased up to 10% and 25% respectively when a stationary airfoil is placed downstream. The simulations reveal how the vortex-shedding pattern of the airfoils are affected by the interaction between them. As the angle of attack of this airfoil increases from 0 to 45o, high lift is generated at the expense of rapidly decreasing efficiency and thrust. The results are not very sensitive to the shape of the airfoil; similar results are obtained with a flat plate airfoil. Lastly, a simple optimization study is performed to obtain the configuration which gave the best performance based on the range of parameters studied. The results obtained from this study can be used to optimize the performance of small flapping MAVs.