Joo-Sung Maeng

SHAPE OPTIMIZATION OF CUTOFF IN A MULTIBLADE FAN/SCROLL SYSTEM USING RESPONSE SURFACE METHODOLOGY

This article presents a new method that overcomes three-dimensional effects by using two-dimensional computational fluid dynamics (CFD) and response surface methodology (RSM). The method is applied to shape optimization of cutoff. The distributions of velocity and pressure obtained by two-dimensional CFD analysis are compared with those of three-dimensional CFD analysis and experimental results. RSM with central composite designs is used to obtain an approximated volume flow rate in terms of the angle and curvature radius of cutoff. The optimal angle and radius of cutoff are determined as 72.4° and 0.092 times the outer diameter of the impeller, respectively.

COMPUTATION OF SLIP FLOW IN MICROCHANNELS USING LANGMUIR SLIP CONDITION

To analyze microscale slip flows, a new simulation method is proposed that combines the Navier-Stokes solution with a new slip model called the Langmuir slip condition. The proposed method is applied to a gaseous microchannel flow model. The Langmuir slip model can simulate the velocity slip effect at the wall that comes from rarefaction and the compressibility effect of the microscale gases. The Langmuir slip model solution is analogous to the results of the well-known Maxwell slip condition. As the Knudsen number becomes higher, the Maxwell slip condition decreases pressure nonlinearity, while the Langmuir slip condition increases it slightly. Increased nonlinearity is more compatible with experimental results. The Langmuir slip condition does not require a cumbersome process in either calculation of streamwise velocity gradient at the wall or calibration of empirical accommodation coefficient. The proposed method using the Langmuir slip condition is proved to be an efficient, practical, and accurate tool in predicting microscale slip flow.

Thickness Effect on the Thrust Generation of Heaving Elliptic Airfoils

The influence of the Reynolds number on the propulsive characteristics of insect wings is investigated by focusing on the aerodynamic transition from drag to thrust. The effect of both the thickness ratio and the Reynolds number on the thrust generation of elliptic airfoils is investigated using a lattice Boltzmann method. Three Reynolds numbers (Re = 50,100, and 185) and two Strouhal numbers (Sr = 0.2 and 0.4) are treated, while the thickness ratio is varied from 0.05 to 1.0. For all investigated Reynolds numbers, it is found that the airfoils oscillating at Sr = 0.2 do not produce thrust. For Sr = 0.4, it is found that thrust is produced at Re = 185. It is found that an airfoil of approximately 10% thickness produces maximum thrust. Thus, it can be said that, for thrust generation, there exists critical Reynolds and Strouhal numbers, and the thickness ratio is also a crucial parameter. By investigating the vortex pattern, velocity profiles, and vorticity intensities of the vortex cores behind the oscillating airfoils, it is found that 1) mushroom vortex patterns indicative of thrust do not completely guarantee the thrust generation, and 2) a phase lag between the thrust-force indicating vortex pattern and the resulting thrust production is observed. The present investigation shows that thrust-producing airfoils should produce strong vortices, enough to overcome the momentum deficit due to the boundary layer. Present results are obtained within the limitation of the laminar flow assumption.

A NODE-CENTERED PRESSURE-BASED METHOD FOR ALL SPEED FLOWS ON UNSTRUCTURED GRIDS

This article proposes a node-centered pressure-based method for predicti ng flows at all speeds on unstructured grids. The compressible version of the SIMPLE algorithm is extended to unstructured grids. For memory and computing time efficiency of arbitrary cell topologies, a node-centered scheme with edge-based data structure is applied to the segregated unstructur ed-grid algorithm. Convection terms are discretized using the second-order scheme with a deferred-correction approach. Diffusion-term discretization is based on the structured-grid analogy, which can be easily adapted to hybrid unstructured grid solvers. By using the proposed approach, three test cases in the continuum regime are computed. The demonstration of this method is extended to a slip flow problem that has low Reynolds number and compressibility effect. Predictions are compared to other results, which show that the proposed method can improve efficiency in memory usage and computing time without losing any accuracy even in mixed-element grids.

A modeling approach to energy savings of flying Canada geese using computational fluid dynamics

A flapping flight mechanism of the Canada goose (Branta canadensis) was estimated using a two-jointed arm model in unsteady aerodynamic performance to examine how much energy can be saved in migration. Computational fluid dynamics (CFD) was used to evaluate airflow fields around the wing and in the wake. From the distributions of velocity and pressure on the wing, it was found that about 15% of goose flight energy could be saved by drag reduction from changing the morphology of the wing. From the airflow field in the wake, it was found that a pair of three-dimensional spiral flapping advantage vortices (FAV) was alternately generated. We quantitatively deduced that the optimal depth (the distance along the flight path between birds) was around 4m from the wing tip of a goose ahead, and optimal wing tip spacing (WTS, the distance between wing tips of adjacent birds perpendicular to the flight path) ranged between 0 and -0.40m in the spanwise section. It was found that a goose behind can save about 16% of its energy by induced power from FAV in V-formation. The phase difference of flapping between the goose ahead and behind was estimated at around 90.7° to take full aerodynamic benefit caused by FAV.

APPLICATION OF THE GROWTH-STRAIN METHOD FOR SHAPE OPTIMIZATION OF FLOW SYSTEMS

In this study, the growth-strain method was used for shape optimization of flow systems. It optimizes a shape by making a distributed parameter such as dissipation energy uniform in a flow system. In order to overcome the instability that occurred in the numerical analysis by the growth-strain method, the equation of bulk strain has been modified. And the distributed parameters were variously established in this study. By comparing the optimized shapes with the known optimal shapes for two flow systems, it is confirmed that the modified growth-strain method is very efficient and practical in shape optimization of flow systems.

The Effect of Karman Vortex for Mixing in a Micro-channel with an Oscillating Micro-stirrer

In order to consider the effect of Karman vortex for mixing, mixing indices are calculated for 4 models of micro channel flows driven from the combinations of a circular cylinder and a oscillating stirrer. And their results are compared to that of a simple straight micro channel flow(model I). The mixing rate is improved 5.5 times by Karman vortex (model II) and 11.0 times by the stirrer(model III) respectively. In case of successive mixing by the cylinder and the stirrer(model IV), of shortening the channel length for the complete mixing as well as 1.37 times improvement of mixing efficiency then model III. And then, variation of mixing indices are much stable comparing with the others. Thus, it is found that the Karman vortex plays a good role as a pre-mixing method. The D2Q9 Lattice Boltzmann methods are used.

A Study on Mixing Enhancement by Rotating and Oscillating Stirrers in the Micro Channel

The mixing effect is studied by comparing rotating and oscillating stirrers in the micro channel. The cases of Re

Parameter Optimization of a Micro-Static Mixer Using Successive Response Surface Method

In this study, parameter optimization of micro-static mixer with a cantilever beam was accomplished for maximizing the mixing efficiency by using successive response surface approximations. Variables were chosen as the length of cantilever beam and the angle between horizontal and the cantilever beam. Sequential approximate optimization method was used to deal with both highly nonlinear and non-smooth characteristics of flow field in a micro-static mixer. Shape optimization problem of a micro-static mixer can be divided into a series of simple subproblems. Approximation to solve the subproblems was performed by response surface approximation, which does not require the sensitivity analysis. To verify the reliability of approximated objective function and the accuracy of it, ANOVA analysis and variables selection method were implemented, respectively. It was verified that successive response surface approximation worked very well and the mixing efficiency was improved very much comparing with the initial shape of a micro-static mixer.

Some Relations Between the Geometric Parameters and Internal Flow Field Characteristics in Multiblade Fan/Scroll System

This paper describes that the size of inactive zone can be directly applied to design multiblade fan/scroll system. From the experimental studies using a five hole pitot tube and smoke test, it is found that the size of inactive zone has linear relations with the mean velocity of impeller inlet and cut-off angle gives a great influences to the fan efficiency. For the practical design, a function related with geometric parameters(i.e. inner radius, cord length, cut-off clearance and cut-off angle) of fan/scroll system is suggested. By using these formulas, the size and distribution of inactive zone can be predicted without the measurements through the full domain, it can be possible to use them to know the efficiency improvement for new model designed.