جستجوی مقالات مرتبط با کلیدواژه « U » در نشریات گروه « مکانیک »

Linear viscoelastic constitutive laws, such as hyperelasticity with the Prony series, are commonly used in commercial software to simulate polymer materials. However, these models are not accurate regarding large strain problems despite performing well for small strain problems. To gather experimental data for soft adhesives, various shear modes were employed, including monotonic, creep, and low-cycle tests using single-lap shear specimens. These tests were conducted on optically clear adhesives (OCAs). Initially, the validity range for linear viscoelasticity was established, revealing the inability to predict large strains accurately using this approach. Subsequently, the three-network viscoplastic (TNV) model parameters were calibrated experimentally under large strains. The calibration procedures took advantage of variations in loading modes, enhancing the precision and improving the accuracy of the constitutive models. For calibration purposes, it is recommended to utilize the low-cycle loading-unloading test as it offers a suitable and cost-effective means of precision. This approach provides a cost-effective way to accurately predict material behavior, owing to the variations in loading modes. Finally, the characteristic model was used to evaluate the results through the finite element method. The results showed that the proposed model accurately predicts stress values, energy dissipation, and energy loss due to softening

This research aims to experimentally investigate the energy absorption of thin-walled lattice tubes with a square section. The walls of the thin-walled tube are made in the form of a lattice with three types of cells: re-entrant auxetic, semi-re-entrant, and conventional honeycomb structures, and the material of the specimens is considered 304 stainless steels. All three types of lattice cells are produced by the rotary laser cutting method on a conventional tube and are axially compressed by a universal test machine under quasi-static loading at a 5 mm/min velocity. The test evaluation parameters are initial maximum force, mean crushing force, crushing force efficiency, energy absorption, and specific energy absorption. The results show that the thin-walled tube with the re-entrant auxetic structure has more specific energy absorption than the other two structures. The specific energy absorption of this structure is 25% higher than the conventional honeycomb structure. The semi-re-entrant structure has more energy absorption and mean crushing force than the other structures. The crushing force efficiency in the conventional honeycomb structure is higher than that of the other two structures, which has a value of 85%.

Superhydrophobic surfaces have gained significant attention as a promising approach for drag reduction of submerged objects. Accurate evaluation and prediction of drag reduction induced by these surfaces require expensive experimental measurements, numerical simulations, or the development of reliable models and correlations. In this paper, a model is proposed for calculating the skin friction coefficient and drag reduction of superhydrophobic flat surfaces. Utilizing previous data on the skin friction coefficient of flat surfaces under no-slip boundary conditions, a model is developed to estimate the skin friction reduction and skin friction coefficient of these surfaces after applying superhydrophobic coatings. The validity of the model is verified by comparing its results with those of computational fluid dynamics (CFD) simulations of flow over a flat plate at different velocities. The results of the model and simulations indicate that for inlet velocities of 1, 5, and 25 m/s and a slip length of 50 μm, drag reductions of 15%, 41%, and 77%, respectively, are expected. Additionally, the skin friction reduction increases with increasing flow Reynolds number. The developed model is validated for flat surfaces and its ability to accurately estimate the skin friction coefficient and drag force of these surfaces is thoroughly examined. However, further investigations are required to assess the model's validity for curved surfaces and variable slip lengths.

The main approach in the study of fluid flow instabilities is the theory of linear stability, which is based on linearizing the governing equations and finding unstable eigenvalues. In many flows, like shear flows, the results of linear stability theory fail to match most experiments. In a linear system, even if all the eigenvalues are stable, the perturbations can lead to instability, if the eigenfunctions are not orthogonal. The transient features of these non-normal dynamical systems, can be described with low-dimensional structures, i.e. a few modes. It is possible to suppress the asymptotic and transient growth by identification of time-dependent modes. In this paper, a method of order reduction based on optimally time-dependent modes has been implemented. This method identifies the growth behavior of disturbances in short and long times. Also, a control algorithm based on the above method has been implemented to stabilize the growth of disturbances. The DNS solution of the flow and the implementation of the reduction and control algorithms is based on the NEKTAR++ open-source solver. At first problem, to validate the solution method, the order reduction and control algorithm has been implemented on the flow over a cylinder with Re=50. At second problem, for the first time, the control algorithm is implemented on the flow over a cylinder subjected to persistent time-varying disturbances. The results show that by applying a control force, the Von-Karman vortices are stabilized and a constant lift is obtained and body vibrations are cancelled.