Lavi Rizki Zuhal

The Structural Response of Cylindrical Shells to Internal Shock Loading

The internal shock loading of cylindrical shells can be represented as a step load advancing at constant speed. Several analytical models are available to calculate the structural response of shells to this type of loading. These models show that the speed of the shock wave is an important parameter. In fact, for a linear model of a shell of infinite length, the amplitude of the radial deflection becomes unbounded when the speed of the shock wave is equal to a critical velocity. It is evident that simple (static) design formulas are no longer accurate in this case. The present paper deals with a numerical and experimental study on the structural response of a thin aluminum cylindrical shell to shock loading. Transient finite element calculations were carried out for a range of shock speeds. The results were compared to experimental results obtained with the GALCIT 6-in. shock tube facility. Both the experimental and the numerical results show an increase in amplitude near the critical velocity, as predicted by simple steady-state models for shells of infinite length. However, the finite length of the shell results in some transient phenomena. These phenomena are related to the reflection of structural waves and the development of the deflection profile when the shock wave enters the shell.

Resolving multi objective stock portfolio optimization problem using genetic algorithm

Portfolio optimization is an important research field in modern finance. The most important characteristic within this optimization problem is the risk and the returns. Modern portfolio theory provides a well-developed paradigm to form a portfolio with the highest expected return for a given level of risk tolerance. Multi objective portfolio optimization problem is the portfolio selection process that result highest expected return and smallest identified risk among the various financial assets.

Near Field Dynamics of Wing Tip Vortices

An experimental investigation has been conducted to study the formation and near-field dynamics of a wing tip vortex. Stereoscopic PIV (SPIV), which allows instantaneous measurements of all three components of velocity within a planar slice, was used to measure velocity fields behind the wing. The present study has found that the wing sheds multiple vortices of opposite signs. Farther downstream, the vortices, with opposite sense of rotation, break up into smaller vortices which satellite around the tip vortex. At least one relatively strong satellite vortex appears in some of the instantaneous fields. The study found that the interaction of the tip vortex and satellite vortices give rise to the unsteady motion of the wing tip vortex.

Global sensitivity analysis via multi-fidelity polynomial chaos expansion

The presence of uncertainties are inevitable in engineering design and analysis, where failure in understanding their effects might lead to the structural or functional failure of the systems. The role of global sensitivity analysis in this aspect is to quantify and rank the effects of input random variables and their combinations to the variance of the random output. In problems where the use of expensive computer simulations are required, metamodels are widely used to speed up the process of global sensitivity analysis. In this paper, a multi-fidelity framework for global sensitivity analysis using polynomial chaos expansion (PCE) is presented. The goal is to accelerate the computation of Sobol sensitivity indices when the deterministic simulation is expensive and simulations with multiple levels of fidelity are available. This is especially useful in cases where a partial differential equation solver computer code is utilized to solve engineering problems. The multi-fidelity PCE is constructed by combining the low-fidelity and correction PCE. Following this step, the Sobol indices are computed using this combined PCE. The PCE coefficients for both low-fidelity and correction PCE are computed with spectral projection technique and sparse grid integration. In order to demonstrate the capability of the proposed method for sensitivity analysis, several simulations are conducted. On the aerodynamic example, the multi-fidelity approach is able to obtain an accurate value of Sobol indices with 36.66% computational cost compared to the standard single-fidelity PCE for a nearly similar accuracy.

Framework for Robust Optimization Combining Surrogate Model, Memetic Algorithm, and Uncertainty Quantification

In this paper, our main concern is to solve expensive robust optimization with moderate to high dimensionality of the decision variables under the constraint of limited computational budget. For this, we propose a local-surrogate based multi-objective memetic algorithm to solve the optimization problem coupled with uncertainty quantification method to calculate the robustness values. The robust optimization framework was applied to two aerodynamic cases to assess its capability on real world problems. Result on subsonic airfoil shows that the surrogate-based optimizer can produce non-dominated solutions with better quality than the non-surrogate optimizer. It also successfully solved the transonic airfoil optimization problem and found a strong tradeoff between the mean and standard deviation of lift-to-drag ratio.

Smooth Particle Hydrodynamics (SPH) for Simulating 2D Elastodynamics Problems

SPH is a Lagrangian based computational method that can be used for solving fluids and solid mechanics problems. In this work, SPH is utilized to solve the two dimensional Navier’s equation for linear elastodynamics problems. The SPH technique computes the discrete particle properties using a smoothing distribution function, which takes into account the effect of neighboring particles. To investigate the performance of the developed method in solving solid mechanics problems, the problem of plate bending was simulated. The results show good agreement between the simulation and analytical solution. Additionally, the study found an indication that the current method of enforcing boundary conditions produces boundary effects at locations where the plate is attached to the wall and at the end of the plate.

Numerical Simulation of 2D Flow around Deforming Fish-Like Body

In computational fluid dynamics (CFD), the physical domain is usually discretized by using mesh/grid, cells, nodes or particles generation. Although there are many advantages, these methods are required to have high computational storage/time cost, especially for solving the complex, deforming, and moving flows/bodies. Hence, we developed the vortex-in-cell (VIC) algorithm which is hybrid combination of grid-based and mesh-free method. VIC method, which was originally developed to simulate incompressible single-phase flows, becomes a very promising alternative for simulating complex flows. In addition, to simulate flows over deforming geometries, we utilized an immersed boundary method for enforcing the boundary condition. VIC interpolates the particle strength to an underlying mesh. VIC method has the advantage that the Poisson inversion can be accomplished by Fast Fourier Transform (FFT) techniques, this accelerates the computational time and provides accurate results. Here we consider an anguilliform swimming motion based on lateral displacement of the “backbone” which describes a complex, deforming, and moving body. “Flow over an Impulsively Started Circular Cylinder” problem are also simulated to validate the developed numerical method.

Evolutionary Algorithm Based Multi-Objective Aerodynamics Optimization Method for Low Reynolds Number Airfoil

This paper presents the development of multi-objective population-based optimization method, called Non-dominated Sorting Genetic Algorithm II (NSGA-II), to optimize the aerodynamic characteristic of a low Reynolds number airfoil. The optimization is performed by changing the shape of the airfoil to obtain geometry with the best aerodynamic characteristics. The results of the study show that the developed optimization tool, coupled with modified PARSEC parameterization, has yielded optimum airfoils with better aerodynamic characteristics compared to original airfoil. Additionally, it is found that the developed method has better performance compared to similar methods found in literature.

Flutter Speed Determination of Two Degree of Freedom Model Using Discrete Vortex Method

This paper describes a fluid-structure interaction methodology to determine flutter speed of a two degree of freedom model. Fluid properties are calculated using a specially developed mesh-free computational fluid dynamics (CFD) method called Discrete Vortex Method (DVM). The acquired unsteady aerodynamic loads from DVM are used to calculate the flutter derivatives. The results are then used to predict the maximum allowable wind speed before flutter occurs. To test the methodology, a flutter instability analysis of a long span bridge with two degree of freedom (heaving and pitching) is performed. It is found that results obtained using the current methods are in good agreement with those obtained in previous studies.

Flow Field around Asymmetric Flapping Flat Plate Optimized using Micro Genetic Algorithm

Asymmetric flapping flight motion employs inclined stroke plane to support the hoverers weight by taking advantage of the drag force. The strategy reduces the energy cost of hoverer and thus the optimal motion of this asymmetric flapping motion is an interesting subject to be studied. To reveal this optimal motion, a heuristic optimization method based on micro genetic algorithm coupled with experiment to calculate fitness function was used to optimize the drag force of flapping flat plate with constraint on profile power. Particle flow visualization method is employed on the final optimal solution to reveal the flow field and to discover the important fluid phenomenon. Result shows that the attachment of large leading edge vortex to the plate is essential to maintain high force production during downstroke phase. Interaction of flow field from previous stroke and current stroke also creates an advantage to enhance force production in upstroke phase. Basic philosophy of typical asymmetric hovering flapping motion is also discovered in the optimal solution where the downstroke acts as primary force generator whereas the upstroke serves as a motion to return the wing to its original position and only produces small amount of drag force with small profile power.