جستجوی مقالات مرتبط با کلیدواژه « نیروی پسا » در نشریات گروه « فیزیک »

A hovercraft is a dual-purpose vessel capable of traversing both water and land surfaces with minimal thrust, operating by generating a cushion of compressed air. This type of vessel comprises numerous components, many of which are unique and not utilized in other crafts. Consequently, accurately estimating the drag force on hovercraft presents a greater challenge compared to conventional vessels. In this research, first identifies and describes the various types of drag forces acting on a hovercraft. These include wave-making drag from the air cushion, aerodynamic profile drag, aerodynamic momentum drag, skirt drag, and the momentum drag force associated with air escaping from the front and rear skirts in calm water. In wavy conditions, additional forces such as wave-making drag, momentum drag, aerodynamic drag, residual drag, and wave motion drag are also considered. For each scenario, the relevant analytical relationships were presented. Subsequently, the total drag force acting on the hovercraft under specific conditions for both calm water and sea state 3 are calculated. The results indicate that the hump drag force at a speed of approximately 10 knots is approximately 30 kN in calm water and 36.2 kN in sea state 3. Furthermore, an appropriate propeller for the hovercraft is designed that can generate sufficient propulsion force to overcome the drag. The study also includes calculations of the propeller’s propulsion force in relation to the vehicle’s speed.

Using transverse steps is a way to reduce friction resistance and also increase the velocity of stepped planing hull vessels. In stepped planing hull vessels, by increasing the velocity and entry to the planing regime and finally separation fluid flow from the step, the wetted area decreases and as a result, the drag force will decrease. In this research, an appropriate analytical method is presented to study the hydrodynamic characteristics of the stepped planing hull vessels and the results of this procedure are validated with experimental data of a specified stepped planing hull vessel for three step heights of 2, 4 and 6% of width. In the mentioned approach, using available semi-empirical relations for the geometric dimensions of the produced waves at heel of the stepped planing hull vessels, the Savitsky method is developed. The results reveal that the developed method has a much better performance compared to the Savitsky method and has an acceptable agreement with the experimental data. After validating the proposed analytical method, the effect of geometric parameters including step height, center of mass location and transvers step position on the hydrodynamic behavior of the stepped planing hull vessel is examined and the optimal value of the mentioned parameters are achieved.

Nature and living organisms provide an abundance of creative designs that can be used to solve many technical and engineering problems. Fast swimming shark skin is covered with low drag microstructures which is a good source of inspiration from nature for numerous engineering applications. The main objective of this research is to explore the abilities of additive manufacturing methods and to use 3D printing technology to fabricate drag-reducing riblet surfaces. Thus, the shark skin imitated riblet surfaces have been designed and fabricated in micron scale by using a yellow resin specially-formulated for 3D printers and the DLP technique. Then, a closed channel made of Plexiglas with its bottom surface covered with a riblet surface has been designed and constructed to evaluate the ability of these surfaces in reducing the pressure drop. An engineering system including a pump, rotameter, water tank, connecting tubes, differential pressure transmitter, power supply and monitor has been installed to measure the amount of pressure drop across the channel for various riblet surfaces. The tests have been performed for different Reynolds numbers and various riblet surfaces, and the pressure drop results have been compared with empirical data for a channel with simple sample. The findings indicate the ability of riblet surfaces in reducing the pressure drop across the channel.

We calculated the drag force for asymptotically Lifshitz space times in (d+ 2)-dimensions with the arbitrary dynamical exponent z. We find that at zero and finite temperature, the drag force has a non-zero value. Using the drag force calculations, we investigate the DC conductivity of the strange metals.