Jin-Hyuk Kim

Mechanical degradation of dilute polymer solutions under turbulent flow

The degradation of high molecular weight polystyrene under turbulent flow was investigated using a rotating disk apparatus for three different solvent systems at a maximum polymer concentration of 150 ppm by weight. The drag reduction efficiency decreases with time due to the mechanical degradation of the polymer molecules, and the extent of the degradation was found to be a function of the solubility parameter of the solvents. A theoretical model for molecular degradation provides excellent agreement with our experimental data.

Optimization of Vane Diffuser in a Mixed-Flow Pump for High Efficiency Design

This paper presents an optimization procedure for high-efficiency design of a mixed-flow pump. Optimization techniques based on a weighted-average surrogate model are used to optimize a vane diffuser of a mixed-flow pump. Validation of the numerical results is performed through experimental data for head, power and efficiency. Three-level full factorial design is used to generate nine design points within the design space. Three-dimensional Reynoldsaveraged Navier-Stokes equations with the shear stress transport turbulence model are discretized by using finite volume approximation and solved on hexahedral grids to evaluate the efficiency as the objective function. In order to reduce pressure loss in the vane diffuser, two variables defining the straight vane length ratio and the diffusion area ratio are selected as design variables in the present optimization. As the results of the design optimization, the efficiency at the design flow coefficient is improved by 7.05% and the off-design efficiencies are also improved in comparison with the reference design.

Analysis and Optimization of a Vaned Diffuser in a Mixed Flow Pump to Improve Hydrodynamic Performance

Hydrodynamic analysis and an optimization of a vaned diffuser in a mixed-flow pump are performed in this work. Numerical analysis is carried out by solving three-dimensional Reynolds-averaged Navier-Stokes equations using the shear stress transport turbulence model. A validation of numerical results is conducted by comparison with experimental data for the head, power, and efficiency. An optimization process based on a radial basis neural network model is performed with four design variables that define the straight vane length ratio, the diffusion area ratio, the angle at the diffuser vane tip, and the distance ratio between the impeller blade trailing edge and the diffuser vane leading edge. Efficiency as a hydrodynamic performance parameter is selected as the objective function for optimization. The objective function is numerically assessed at design points selected by Latin hypercube sampling in the design space. The optimization yielded a maximum increase in efficiency of 9.75% at the design flow coefficient compared to a reference design. The performance curve for efficiency was also enhanced in the high flow rate region. Detailed internal flow fields between the reference and optimum designs are analyzed and discussed.

Surrogate Modeling for Optimization of a Centrifugal Compressor Impeller

Abstract This paper presents a procedure for the design optimization of a centrifugal compressor. The centrifugal compressor consists of a centrifugal impeller, vaneless diffuser and volute. And, optimization techniques based on the radial basis neural network method are used to optimize the impeller of a centrifugal compressor. The Latin-hypercube sampling of design-of-experiments is used to generate the thirty design points within design spaces. Three-dimensional Reynolds-averaged Navier-Stokes equations with the shear stress transport turbulence model are discretized by using finite volume approximations and solved on hexahedral grids to evaluate the objective function of the total-to-total pressure ratio. Four variables defining the impeller hub and shroud contours are selected as design variables in this optimization. The results of optimization show that the total-to-total pressure ratio of the optimized shape at the design flow coefficient is enhanced by 2.46% and the total-to-total pressure ratios at the off-design points are also improved significantly by the design optimization.

Axial-Flow Ventilation Fan Design Through Multi-Objective Optimization to Enhance Aerodynamic Performance

This paper presents an optimization procedure for axial-flow ventilation fan design through a hybrid multiobjective evolutionary algorithm (MOEA) coupled with a response surface approximation (RSA) surrogate model. Numerical analysis of a preliminary fan design is conducted by solving three-dimensional (3-D) Reynolds-averaged Navier-Stokes (RANS) equations with the shear stress transport (SST) turbulence model. The multiobjective optimization processes are performed twice to understand the coupled effects of diverse variables. The first multiobjective optimization process is conducted with three design variables defining stagger angles at the hub, mid-span, and tip, and the second is conducted with five design variables defining hub-to-tip ratio, hub cap installation distance, hub cap ratio, and the stagger angles at the mid-span and tip. Two aerodynamic performance parameters, the total efficiency and total pressure rise, are selected as the objective functions for optimization. These objective functions are numerically assessed through 3-D RANS analysis at design points sampled by Latin hypercube sampling in the design space. The optimization yields a maximum increase in efficiency of 1.8% and a 31.4% improvement in the pressure rise. The off-design performance is also improved in most of the optimum designs except in the region of low flow rate.

Loss Recovery Scheme for TCP Using MAC MIB over Wireless Access Networks

In the wireless network, if the TCP cannot identify the wireless error, it overly reduces the transmission rate by unnecessarily cutting the congestion window down. Therefore, to improve TCP goodput by accurately detecting wireless loss, we propose the MAC MIB based loss recovery scheme that can differentiate between congestion loss and wireless loss without intervention of any intermediate node. The simulation results show that the goodput is substantially enhanced compared to existing schemes in the wireless access network environment, where errors tend to occur in burst due to the intrinsic wireless link characteristics.

Optimization of the Aerodynamic and Aeroacoustic Performance of an Axial-Flow Fan

A multidisciplinary optimization to simultaneously enhance the aerodynamic and aeroacoustic performance of an axial-flow fan was performed. Flow analysis through the axial-flow fan was conducted by solving three-dimensional steady and unsteady Reynolds-averaged Navier–Stokes equations with the shear-stress transport turbulence model. Starting with the results for the unsteady flow, aeroacoustic analysis was performed by solving the Ffowcs Williams–Hawkings equations. A single-objective optimization for high-efficiency design was carried out before the multi-objective optimization. The single-objective optimization was conducted using a weighted average surrogate model with five design variables defining the hub-to-tip ratio, hubcap installation distance, hubcap ratio, and angle distributions at the midspan and blade tip. The objective function (i.e., the efficiency) was evaluated at the design points, sampled by Latin hypercube sampling in the design space, to construct the surrogate model. Then, multi-ob...

Design Optimization of a Centrifugal Compressor Impeller Using Radial Basis Neural Network Method

This paper presents a procedure for design optimization of a centrifugal compressor. The centrifugal compressor consists of a centrifugal impeller, vaneless diffuser and volute. And, optimization techniques based on radial basis neural network method are used to optimize the impeller of the centrifugal compressor. Latin hypercube sampling of design of experiments is used to generate thirty design points within design spaces. Three-dimensional Reynolds-averaged Navier-Stokes equations with shear stress transport turbulence model are discretized by using finite volume approximations and solved on hexahedral grids to evaluate the objective function of an isentropic efficiency. Four variables defining impeller hub and shroud contours are selected as design variables in this optimization. The results of optimization show that the isentropic efficiency of the optimized shape at the design flow coefficient is enhanced by 1.0% and the efficiencies at the off-design points are also improved significantly by the design optimization.Copyright

Design Optimization of Circumferential Casing Grooves for a Transonic Axial Compressor to Enhance Stall Margin

This paper presents a design optimization of an axial compressor with NASA Rotor 37 and five circumferential casing grooves for enhancement of stall margin. Three-dimensional Reynolds-averaged Navier-Stokes equations with the shear stress transport turbulence model are discretized by finite volume approximations and solved on hexahedral grids for the flow analyses. The validation of the numerical results is performed in comparison with experimental data for pressure ratio and adiabatic efficiency. The Latin-hypercube sampling as design-of-experiments is used to generate the twelve design points within the design space. A stall margin parameter is considered as an objective function with two design variables defining the geometry of the circumferential casing grooves. The radial basis neural network method employed as a surrogate model for the design optimization of the circumferential casing grooves is trained on the numerical solutions by carrying out leave-one-out cross-validation for the data set. The results show that the stall margin of the optimum shape is enhanced considerably by the design optimization compared to the cases with smooth casing and the reference grooves.Copyright

Numerical Investigation on Aerodynamic Performance of a Centrifugal Fan with Splitter Blades

This paper presents a numerical investigation on the aerodynamic performance according to the application of splitter blades in an impeller of a centrifugal fan used for a refuse collection system. Numerical analysis of a centrifugal fan was carried out by solving three-dimensional Reynolds-averaged Navier-Stokes equations with the shear stress transport turbulence model. A validation of numerical results was conducted by comparison with experimental data for the pressure and efficiency. From analyses of the internal flow field of the reference fan, the losses by the reverse-flows were observed in the region of the blade passage. In order to reduce these losses and enhance fan performance, two splitter blades were applied evenly between the main blades, and centrifugal impellers having the different numbers of the main blades were tested with their application. Throughout the numerical analyses of the centrifugal fan with splitter blades, it was found that the reverse-flow regions in the blade passage can be reduced by controlling the main blade numbers with splitter blades. The application of splitter blades in a centrifugal fan leads to significant improvement in the overall fan performance.