Dealing with numerical noise in CFD-based design optimization

Optimization problems in many industrial applications are very hard to solve. Many examples of them can be found in the design of aeronautical systems. In this field, the designer is frequently faced with the problem of considering not only a single design objective, but several of them, i.e., the designer needs to solve a Multi-Objective Optimization Problem (MOP). In aeronautical systems design, aerodynamics plays a key role in aircraft design, as well as in the design of propulsion system components, such as turbine engines. Thus, aerodynamic shape optimization is a crucial task, and has been extensively studied and developed. Multi-Objective Evolutionary Algorithms (MOEAs) have gained popularity in recent years as optimization methods in this area, mainly because of their simplicity, their ease of use and their suitability to be coupled to specialized numerical simulation tools. In this chapter, we will review some of the most relevant research on the use of MOEAs to solve multi-objective and/or multi-disciplinary aerodynamic shape optimization problems. In this review, we will highlight some of the benefits and drawbacks of the use of MOEAs, as compared to traditional design optimization methods. In the second part of the chapter, we will present a case study on the application of MOEAs for the solution of a multi-objective aerodynamic shape optimization problem.

Flow and heat transfer in an aero-engine compressor disk cavity with radial inflow has been studied using computational fluid dynamics (CFD), large eddy simulation (LES), and coupled fluid/solid modeling. Standalone CFD investigations were conducted using a set of popular turbulence models along with 0.2 deg axisymmetric and a 22.5 deg discrete sector CFD models. The overall agreement between the CFD predictions is good, and solutions are comparable to an established integral method solution in the major part of the cavity. The LES simulation demonstrates that flow unsteadiness in the cavity due to the unstable thermal stratification is largely suppressed by the radial inflow. Steady flow CFD modeling using the axisymmetric sector model and the Spalart–Allmaras turbulence model was coupled with a finite element (FE) thermal model of the rotating cavity. Good agreement was obtained between the coupled solution and rig test data in terms of metal temperature. Analysis confirms that using a small radial bleed flow in compressor cavities is effective in reducing thermal response times for the compressor disks and that this could be applied in management of compressor blade clearance.

The Meshless Finite Volume Method (MFVM) is developed for solving elasto-static problems, through a new Meshless Local Petrov-Galerkin (MLPG) approach. In this MLPG mixed approach, both the strains as well as displacements are interpolated, at randomly distributed points in the domain, through lo- cal meshless interpolation schemes such as the mov- ing least squares(MLS) or radial basis functions(RBF). The nodal values of strains are expressed in terms of the independently interpolated nodal values of displace- ments, by simply enforcing the strain-displacement rela- tionships directly by collocation at the nodal points. The MLPG local weak form is then written for the equilib- rium equations over the local sub-domains, by using the nodal strains as the independent variables. By taking the Heaviside function as the test function, the local domain integration is avoided; this leads to a Meshless Finite Vol- ume Method, which is a counterpart to the mesh-based finite volume method that is popular in computational fluid dynamics. The present approach eliminates the ex- pensive process of directly differentiating the MLS inter- polations for displacements in the entire domain, to find the strains, especially in 3D cases. Numerical examples are included to demonstrate the advantages of the present methods: (i) lower-order polynomial basis can be used in the MLS interpolations; (ii) smaller support sizes can be used in the MLPG approach; and (iii) higher accuracies and computational efficiencies are obtained. keyword: Meshless Local Petrov-Galerkin approach (MLPG), Finite Volume Methods, Mixed Methods, Ra- dial Basis Functions (RBF), and Moving Least Squares (MLS).

A variable fidelity concept is introduced in the proper orthogonal decomposition (POD) based re-parameterization approach to efficiently solve multi-objective aerodynamic shape optimizations. The re-parameterization approach enables to extract dominant shape deformation modes from a database of good airfoil designs and to reduce the number of design variables. Our proposed variable fidelity approach is realized by utilizing low-fidelity functional evaluations to select the good airfoil designs. The validity of the proposed approach is investigated in a multi-objective aerodynamic shape optimization problem of 2D airfoil.

Maintaining a standardized microclimate in the poultry house is one of the main factors. The quality of the output ultimately depends on the quality indicators of the air parameters. Keeping a bird requires great efforts and technological solutions. In connection with this study, there is an improvement of the microclimate system in the air environment of the poultry house due to the inclusion of exhaust fans on the rear end wall in an unconventional way. Computational Fluid Dynamics (CFD) using ANSYS Fluent is a powerful tool for predicting the microclimate system in the poultry house as an alternative to experimental studies. The CFD model was performed on the Navier-Stokes equations for convective flows. The calculations use the Discrete Ordinates radiation model and the Spalart-Allmaras turbulence model. The calculations were made with an air consumption of 21.5 kg/s. The temperature of the outside air is assumed equal to +3 ºС and the thermal radiation parameter is entered. For air removal, Munters EM50 1.5 HP exhaust fans are used in a total number of 4 pcs. Inflow valves Wlotpowietrza 3000-VFG with a total number of 80 pcs., which are placed at a height of 0.21 m from the ceiling (1-8 valves) and 0.81 m for 9-40 valves. According to the results of CFD modeling of hydrodynamics and heat and mass exchange processes, it was concluded that changing the angle of the spoiler by 73o allows air to be supplied to the center of the poultry house. The upper air layers near the ceiling and near the side wall have a slightly higher temperature. This is accompanied by radiation from the sun and ranges from +21.5 to +24.5 ℃. Since the bird is a source of heat, and in combination with radiation, the air in the room is partially heated. In the center of the room along the entire height, the temperature reaches +15-16 ℃. At the same time, the pressure drop of the inlet valves is 70 Pa, which allows the exhaust fan to be fully supplied. The air velocity at the inlet of the inflow valves is 11.54 m/s. The average air speed at a height of 0.7 m from the floor level is 0.5 m/s, the temperature is 16.55 ℃. Thus, the presented scientific research can be used in the future to develop new ventilation systems for poultry houses.

Expedited constrained multi-objective aerodynamic shape optimization by means of physics-based surrogates

Aerodynamic shape optimisation technology is presented, comprising an efficient variable fidelity shape parameterisation method, an efficient andhigh quality mesh deformation scheme, and a parallel optimisation algorithm. The objective of the research presented here is the comparison of truly three-dimensional optimisations of aircraft wings in both aerodynamic and aeroelastic environments. The novel shape parameterisation technique allows various fidelities of design parameters, ranging from detailed surface changes to novel truly three-dimensional planform adjustments. An efficient interpolation scheme, using radial basis functions, transfers domain element movements into direct deformations of the design surface and corresponding CFD mesh, thus allowing total independence from the grid generation package and type (structured or unstructured). Optimisation is independent from the CFD flow solver by obtaining sensitivity information for an advanced parallel gradient-based optimiser by finite-differences. This ‘wrap-around’ optimisation technique is applied to a modern large transport aircraft wing in the cruise flight condition for minimum drag with stringent constraints in lift, volume, and two root moments. The objective of all optimisations is aerodynamic, however the static aeroelastic deflection provided by an aeroelastic solver will give that particular optimisation increased accuracy and real world relevance. The result of a constrained inviscid aerodynamic optimisation is presented and has a significant reduction in drag when compared to the initial wing with no violation of any constraints. The shape parameterisation method demonstrates that only a low number of design variables are necessary to achieve innovative planform and surface geometries with dramatically improved performance. Computational fluid dynamics (CFD) methods are now commonplace in aerospace industries, and at the forefront of analysis capabilities, providing a fast and effective method of predicting a design’s aerodynamic performance. However, with ever increasing complexity of designs, engineers can often struggle to interpret the intricacies of the CFD results sufficiently to be able to manually alter the geometry to improve performance. Hence, there has been an increase in demand for intelligent and automatic shape optimisation schemes. This requires combining geometry control methods with numerical optimisation algorithms, to provide a mechanism to mathematically seek improved and optimum designs, using CFD as the analysis tool. Optimisation requires consideration of three issues, each of which have numerous solutions: shape parameterisation including CFD surface and volume mesh deformation, computation of design variable derivatives, and effective use of these derivatives to improve design. Geometry parameterisation is critical for effective shape optimisation. This is the method of representing the design surface, and defines the degrees of freedom in which the geometry can be altered and, ideally, this should be linked with an effective method of deforming the CFD surface and volume mesh in a corresponding fashion. Parameterising complex shapes is a problem that remains a serious obstacle to both manual and automatic CFD-based optimisation. A wide variety of shape control and morphing methods have been developed, but many do not allow sufficiently

Natural laminar-flow wings are one of the most promising technologies for reducing fuel burn and emissions for commercial aviation. However, there is a lack of tools for performing shape optimization of wings based on computational fluid dynamics considering laminar-to-turbulent transition. To address this need, we develop a discrete adjoint-based optimization framework where transition is modeled. The core of this framework is a Reynolds-averaged Navier–Stokes solver that is coupled with a simplified method to predict Tollmien–Schlichting and laminar separation-induced transition that consists of a laminar boundary-layer code and a database method for flow stability analysis. The transition prediction is integrated with a Spalart–Allmaras turbulence model through a smoothed intermittency function, which makes it suitable for gradient-based optimization. A coupled-adjoint approach that uses transpose Jacobian-vector products derived via automatic differentiation computes the transition prediction derivatives. Lift-constrained drag minimizations of airfoils for a single-point design and a multipoint design problem are performed. The results show that the optimizer successfully reduces the drag coefficient by increasing the extent of laminar flow. The multipoint optimization formulation produces an airfoil with a significant amount of laminar flow that is maintained at several flight conditions. The proposed methods make it possible to perform aerodynamic shape optimization considering laminar-to-turbulent transition in airfoil optimization.

We report a broadband digital electro-optical switch, based upon a multi-stage Mach-Zehnder lattice design in silicon-on-insulator. A digital switching response is demonstrated, engineered through apodization of the coupling coefficients between stages. The digital switching behavior results in crosstalk lower than -15 dB for drive-voltage noise levels in excess of 300 mV(pp), which exceeds the noise tolerance of a conventional single-stage Mach-Zehnder switch by more than six-fold. In addition, the digital design enables a larger maximum 'on'-state extinction (below -26 dB) and lower 'on'-state free-carrier-induced insertion loss (less than 0.45 dB) than that of the single-stage switch. The noise-tolerant, low-crosstalk switch can thus play a key role within CMOS-integrated reconfigurable optical networks operating under noisy on-chip conditions.

ABSTRACTThere have been many previous noise-related studies on liquefied natural gas (LNG) facilities in the United States; however, noise control of these facilities using a top-down approach has not been explored in detail. Most studies have demonstrated noise compliance to applicable standards by focusing on a combination of treatments and specifications, with less consideration on control technology feasibility, ranking, and cost-effectiveness. The Federal Energy Regulatory Commission (FERC) prohibits natural gas facilities from emitting day-night noise levels in excess of 55 dB(A) (equivalent to 24-hr continuous level of 49 dB(A)) at nearby receivers. A case study was conducted to evaluate a top-down approach to reduce noise at a typical LNG peak-shaving facility under normal operating conditions, accounting for technical feasibility, control effectiveness, and cost implications. A modeling approach (International Organization for Standardization standard ISO 9613-2) was used to predict and evaluate the facility’s noise reduction potential. The study found that the strategy could achieve feasible and environmentally effective reductions up to 11 dB(A) at 500 m from the facility by first identifying source groups with highest-emitting sources and then targeting major noise source contributors per group. This approach is cost-effective because the FERC noise goals can still be achieved by avoiding unnecessary control costs associated with lower-ranked sources. The study identified the following four source groups as the highest noise emitters: (1) liquefaction and instrument air, (2) boil-off gas (BOG) compression, (3) glycol water system (air coolers), and (4) pretreatment. Of all the treatments evaluated, installation of enhanced silencers for gas turbine (GT) package—as well as construction of an acoustical building for the BOG compressors and drivers—resulted in the greatest noise reduction at nearby receivers. The study notes that incremental treatment costs presented in this paper are approximate estimates that may vary depending on factors such as facility size and region.Implications: This study assessed potential noise reductions associated with implementing a top–down noise control strategy on a typical LNG peak-shaving facility. The study determined the top–down noise control strategy could achieve feasible and environmentally effective reductions up to 11 decibels at receivers within 500 m from the facility’s center. As LNG suppliers need to support potential supply disruptions, some regions of the US, including New England and Gulf Coast with projected increase in LNG exports and growing needs from power sector, may find information in this study useful with regard to evaluating and prioritizing noise reduction potential of their LNG peak-shaving facilities.

Exposure to noise in recreational settings is a major public health concern. Under the Ontario Occupational Health and Safety Act, workers shall not be exposed to noise levels in excess of 85 dB over an 8-h period. It is generally accepted that the public’s exposure to noise be held to this same standard. The current pilot study explored whether the public is being exposed to excessive noise in indoor aquatic facilities across the Greater Toronto Area and, if so, at what levels. To our knowledge, no other study has explored this topic. Eight separate facilities were assessed using sound level meters to capture LAeq, LApeak, and LAS > 85 dB exceedance/duration. Public perception of noise was determined using self-administered questionnaires. LAeq ranged from 73.6 dB–82.3 dB and LApeak ranged from 102.2 dB–122 dB. Although the LAeq did not exceed the legislated standard, LApeak values were high. Noise levels were mainly associated with the type of activity in the pool and whistle blowing. Overall, the public were indifferent to the noise levels, despite the LApeak being quite high. Further studies are recommended to explore this topic, with a particular emphasis on comparison of different activities within the same facility.

In this paper, the flow field around and the heat transfer characteristics of a high kinetic energy projectile with eight symmetrically-placed fins is analysed using computational fluid dynamics. Tests were performed at a Mach number of 5.4 using a noncontiguous grid and the Spalart-Allmaras turbulence model. The CFD results captured the three-dimensional recirculation vortex above the fin, and were used to relate the surface temperatures to the flow patterns. The numerical predictions agree qualitatively with other CFD studies and available field data.

In this paper, transient computational fluid dynamics (CFD) simulations of a straight-bladed Darrieus type vertical axis wind turbine were performed by means of an in-house CFD code. The Spalart-Allmaras turbulence model was implemented in the numerical code for the turbulence. Particular emphasis was placed on effect of interaction between vortices and blades on the aerodynamic performance of the simulated turbine at different tip speed and solidity ratios. The obtained results suggested that vortices were shed from previous blade passages and the close encounter of a rotor blade with these vortices can have a considerable impact on power coefficient of the simulated turbine during operation at different tip speed ratios. As a result, possible reasons for the changes in the behavior of this type of turbine due to the variation of tip speed ratio and solidity were proposed.

More than 40% of Koreans are exposed to excessive noise levels which can lead to serious health effects, community annoyance and sleep disturbance. Government policy and management of the noise are necessary to solve these problems. In this paper, some projects carried out to mitigate the excessive noise levels under the support of the Ministry of Environment in Korea have been introduced. They include the projects such as making a long-term plan in the field of noise management, amendment of the existing legislation and noise measurement methods, establishment of noise map directive, construction of traffic noise monitoring system, and management of low frequency noise.

The up-to-date design of turbomachinery involves the use of three dimensional computational fluid dynamic analyses to match the challenge of the ever increasing speed of product development. Recently, the application of these analyses to turbomachinery, which in past years was confined to specialist fields, has become widespread in many industrial applications. Hence, besides the teaching of traditional methods for turbomachinery design, an important part of modern engineering education is to produce graduates with advanced skills in Computational Fluid Dynamics (CFD) techniques applied to turbomachinery design. Moreover, at this time, these skills should either match the needs of research laboratories or be compatible with industrial needs, since three dimensional numerical calculation is also coming to be seen as a feasible tool in ordinary applications. For these reasons, CFD courses for the training of graduate students have been developed. In this paper, the methodology followed and the work carried out by the students of the course in Fluid Dynamic Design of Turbomachinery held at the University of Ferrara is presented. In particular, three levels of investigation are taken into consideration and presented: (i) focus on the fundamentals to achieve a basic knowledge of a design process based on numerical simulation; (ii) extension to higher level of in-depth analyses of the specific models which could be used in every phase of a numerical simulation; (iii) insight into the specificity of the main thermodynamic and fluid dynamic characteristics of turbomachinery. To achieve this, the numerical analysis of a simple but exhaustive geometry of a centrifugal pump has been carried out and the results obtained are analyzed. In particular, special emphasis is devoted to: (i) the comparison among the numerical models which can be chosen throughout the simulations of turbomachinery (type of grid, turbulence models, rotor/stator interface models) and (ii) the analysis of some of the most important fluid dynamic phenomena, such as, in this case, velocity profiles and jet-and-wake structure.