Sharif Digital Repository

This article reviews the oil-soluble polymeric drag-reducing agents (DRAs) with an outlook to their large-scale application in petroleum industry. A general discussion on mechanism of the DR phenomenon and mechanical degradation of the drag reducers in turbulent flow is offered with an emphasis on the molecular parameters. Furthermore, low charge density associating polymers, as a new class of oil-soluble flow improvers, are described  

A shock control bump (SCB) is a flow control method which uses local small deformations in a flexible wing surface to considerably reduce the strength of shock waves and the resulting wave drag in transonic flows. Most of the reported research is devoted to optimization in a single flow condition. Here, we have used a multi-point adjoint optimization scheme to optimize shape and location of the SCB. Practically, this introduces transonic airfoils equipped with the SCB which are simultaneously optimized for different off-design transonic flight conditions. Here, we use this optimization algorithm to enhance and optimize the performance of SCBs in two benchmark airfoils, i.e., RAE-2822 and... 

Flow hydrodynamic effects and film cooling effectiveness of placing a coolant port (upstream jet) just upstream of the main cooling jet were numerically investigated. The upstream jet was added such that the total cooling cross section (cross sections of the main and upstream jets) remains constant, in comparison to the case of ordinary cooling jet. The finite volume method and the unsteady SIMPLE algorithm on a multi- block non-uniform staggered grid arrangement were applied. The large eddy simulation (LES) approach with the one equation subgrid scale model was used. The jet to cross flow velocity ratio (for both of the main and the upstream jets) is 0.5 and the cross flow Reynolds number... 

A shock control bump (SCB) is a flow control method that uses local small deformations in a flexible wing surface to considerably reduce the strength of shock waves and the resulting wave drag in transonic flows. Most of the reported research is devoted to optimization in a single flow condition. Here, we have used a multi-point adjoint optimization scheme to optimize shape and location of the SCB. Practically, this introduces transonic airfoils equipped with the SCB that are simultaneously optimized for different off-design transonic flight conditions. Here, we use this optimization algorithm to enhance and optimize the performance of SCBs in two benchmark airfoils, i.e., RAE-2822 and... 

The most effective approach to drag reduction is to concentrate on the components that make up the largest percentage of the overall drag. Small improvements on large quantities can become in fact remarkable aerodynamic improvements. Our experience shows that the use of light material in constructing human-powered airplanes and unmanned-air-vehicles UAVs has a few side effects on the aerodynamic characteristics of their wings. One important side effect is the unwanted deflection on wing shell. It is because of high flexibility and low solidity of the light material, which covers the wing skeleton. The created curvature has direct impact on the separation phenomenon occurred over the wing in... 

Trim control mechanisms such as interceptors and trim flaps have been widely used in recent years in highspeed crafts for ride and trim control. In spite of their extensive application, a few studies investigating the impact of interceptors on planing craft performance, have been published. In the present study, the impact of interceptors on planing crafts hydrodynamic quality is investigated through application of an experimental method. Two scaled-down models of high-speed planing mono-hull and catamaran are tested with and without interceptors in calm water at different heights of the interceptors to investigate the effect of interceptors on drag reduction of the models. The first one is... 

Devising effective methods to reduce drag forces is of great interest as these methods could prevent the wastage of fuel and decrease carbon emission and the global warming rate. Since the invention of Liquid-Infused Surfaces (LISs) in 2011, numerous investigations have been conducted to study their capability in various applications. Due to recirculation or drawing of the penetrated liquid within the surface structure, these surfaces acquire a slippiness property. The conducted investigations showed that these slippery surfaces have great potential for reducing drag forces, whether in a laminar or turbulent flow. In the present work, we first briefly elucidate common drag reduction methods,... 

In this research, effects of superhydrophobic nano-coating on frictional drag force have been investigated. The result of this study could be considered to be used as a method in applications concerned with fuel consumption reduction, less CO2 emission and environmental problems as well as speed increase; while, its significance can also be of great use in marine applications. A rotating disc apparatus was used as the experimental set-up to compare the frictional drag force on an aluminum disc with TiO2 superhydrophobic nano-coating and a smooth coatless aluminum disc. The superhydrophobic nano-coating was prepared using sol-gel method and was shown to be able to produce a contact angle of... 

There are different ways in order to achieve higher velocity in underwater vehicles. One of these methods is using a body with special form. This paper presents a towing tank based experimental study on drag forces for different Reynolds Numbers of a special underwater model. This paper investigates drag force and drag coefficient in a different flow direction over the model. Obtained experimental results in towing tank are explained. Computational Fluid Dynamic (CFD) simulation also is performed using commercial CFD software package FLUENT 6.3.26. There is a significant decrease in drag coefficient of model moving with small diameter at upstream  

This paper empirically examines the hydrodynamic performances of squat submarines under the resistance and wave tests beside numerical investigation of pressure drag reduction techniques. Despite vast information about the operation of the streamlined fluid vessels, there is not much information about the geometries and hydrodynamic behaviors of squat vessels with L/D ratios below four. This study experimentally investigates the impacts of various relative depths and flow inclinations, intending to find drag, heave, and sway forces at the velocities of 0.5, 1.0, 1.5, 2.0, and 2.5-m/s. A one-tenth scaled model of a squat submarine is examined under the resistance and wave train scenarios as...