Papers by Keyword: Unsteady

Abstract: The subject of unsteady convective flow with non-linear thermal radiation has become an important issue of research, due to its implications in advanced energy conversion systems operating at high temperature, solar energy technology and chemical process at high operation temperature. Due to the importance of this issue, a time dependent incompressible viscous fluid flow, heat and mass transfer over a curved stretching surface has been numerically analysed by taking into account the heat flux due to concentration gradient and mass flux due to temperature gradient. Together with this the Rosseland approximation is being employed for the nonlinear thermal radiation impact in presence of thermal slip. With the aid of non-dimensional variables and the corresponding physical boundary conditions, the leading nonlinear momentum, energy, and species equations are converted into a set of coupled ordinary differential equations. These equations are then resolved using the MATLAB bvp4c solver. The stability of the numerical technique has been verified and compared with available literatures. The resultant parameters of engineering interest and the boundary layer flow field parameters and have been presented using tables and graphically plots. The study concludes that for lesser curvature parameter (0.5≤K≤0.7) the surface drag force, heat and mass transfer rates can improve by about 9.59%, 2.87% and 1.67% each respectively. The presence of the temperature ratio parameter and the non-linear thermal radiation are found to greatly influence the temperature profile and the heat transfer rate of the system. Results show that the heat transfer rate improves by about 24.39% and 16.66% for varying non-linear thermal radiation (1≤Rd≤1.5) and temperature ratio parameter (1.2≤θ_w≤1.4) respectively. Results obtained also show that improving the thermal slip parameter (0.4≤L≤0.6) can reduce heat transfer rate by about 13.62% and reduce the surface temperature profile.

Abstract: This study considers the unsteady magnetohydrodynamic slip flow past a permeable stretching cylinder by taking into account the Soret and Dufour effects. Using similarity transformations, the partial differential equations governing the flow, heat, and mass transfers are transformed into a system of ordinary differential equations. These equations are numerically solved for a variety of governing parameter values using the boundary value problem solved package, bvp4c, which is available in the MATLAB software. The outcomes of the governing parameters on the skin friction coefficient, Nusselt and Sherwood numbers are also examined. Upon observation, the unsteadiness parameter influences a positive growth on the momentum, thermal and concentration boundary layers. Slip parameter can be enhanced to improve the friction drag force about 28% and the rate of cooling around 3%. Also, larger effects of Dufour leads to around 9% decay in heat transfer rate and Soret effect to cause around 11% drop in mass transfer rate. Comparison with existing results show excellent agreement which justifies the reliability of the obtained results.

Abstract: The development of boundary layer flow problem which is involve of nanofluid for improve thermal efficiency in many applications is gaining momentum as most of base fluid are not very good thermal conductor. The enhancement of thermal conductivity can be improving when the magnetic fields is presented. Also, by using this material may encountered another problem such as separation flow. This research is study about magnetic fields effect on unsteady mixed convection flow on nanofluid past a sphere with heat flux as its boundary condition. This problem will consider its mathematical model where it can help to predict the solution in experimental method. The mathematical method that being used is Keller-box because of its ability to solve non-linear equation. The nanoparticles that being choose are copper and titanium oxide with water as based fluid. There will have three equation which is mass, momentum, and energy equation. All equations are transform into non-similarity ordinary differential equations by using appropriate variables. The effect of magnetic parameter on velocity and temperature profiles as well as skin friction and Nusselt number are studied. The results show that volume fraction affected the heat-transfer rate, and skin friction coefficient. This study also shows that magnetic parameter affected the separation times which is decelerated the flow.

Abstract: This paper presents a numerical study on the flow dynamics and heat transfer behaviour of unsteady conjugate natural convection boundary layers (CNCBLs) in a partitioned, air filled square cavity. An unsteady two-dimensional multigrid-assisted solver is developed in the C#.NET programming language on stretched Cartesian meshes. The finite volume method is used to discretise the governing equations. To solve the coupled pressure and velocity, the SIMPLE algorithm is used, and to increase simulation accuracy the Adam-Bashforth, QUICK and central difference schemes are employed for time, convection, and diffusion terms respectively. The Poisson pressure equation is solved through the use of the multigrid method. The developed code is used to model CNCBLs which typically require a large amount of simulation time. The numerical results provide detailed descriptions of unsteady CNCBLs and associated heat transfer behaviour over a wide range of Ra, such as the thermal and viscous boundary layer thicknesses, temperature and velocity distributions, and maximum velocities within the CNCBLs.

Abstract: A numerical investigate of unsteady (rotor 37) based on dual time step is carried out .Results were used to identify the existence of an interface between the approach fluid and the tip-leakage flow. This paper discusses mechanism of spike stall.

Abstract: The unsteady flow parameters in control stage of partial admission are analyzed in details with full 3-D numerical simulation. The full annulus structure of air turbine in partial admission is modeled due to the unsymmetrical geometry. The partial admission is accomplished through the inlet blocked using segmental arc. The unsteady surface pressure changes of eight blades in the transition regions which demonstrate the power output ability are presented. That the entropy rise associated with the losses at different cross mainly caused by mixing losses and flow separation in partial admission is analyzed to estimate the efficiency distribution.

Abstract: 2nd order implicit format is implemented in the Navier-Stokes code to deal with instantaneous item unsteady flows. Three simulations are made to testify the method on flow control. First, the external flow fields of synthetic jets are simulated, the mean velocity on the center line, the jet width and velocity distribution are compared well with experimental results. Secondly, the flow fields of synthetic jet in a crossflow are simulated, orifice slot, the mean velocity on the center line and velocity distribution are compared well with experimental results. Finally, the flow control experiments on separation of airfoil are simulated, control methods include steady suction and synthetic jets. Both methods show their ability to favorably effect the flow separation, shortening the length of separation bubble and improving the pressure levels in separation areas in different degrees.

Abstract: In this paper, unsteady MHD flow of heat and mass transfer of Cu-water and TiO₂-water nanofluids over stretching sheet with a non-uniform heat/source/sink considering viscous dissipation and chemical reaction is investigated. The governing partial differential equations with the corresponding boundary conditions are transformed to a system of non-linear ordinary differential equations and solved using Keller box method. The velocity, temperature and concentration profiles are obtained and the influences of various relevant parameters, namely the magnetic parameter M, Prandtl number Pr, Eckert number Ec, Schmidt number Le , chemical reaction parameter K,unsteadiness parameter S and the Soret number Sr on velocity, temperature and concentration profiles are discussed. The skin-friction coefficient–f''(0), heat transfer coefficient –θ'(0) and mass transfer coefficient –φ'(0) are presented in tables. A comparison with published results is also presented and found in good agreement. Keywords: MHD; Keller box method; unsteady; nanofluid; non-uniform heat/source/sink; chemical reaction; viscous dissipation.

Abstract: To be able to more accurate analysis aerodynamic characteristic variation under yaw of NREL Phase VI wind turbine. The influence of yaw on the aerodynamic characteristic by using a three-dimensional unsteady CFD methods based sliding mesh in this paper. To flow, the overall performance and Cp of the three-dimensional flow numerical simulation were compared with experimental results with the same wind speeds to confirm the reliability of the model at a given axial wind speed conditions. In the yaw conditions, the effect of yaw on the aerodynamic performance of wind turbine blades, changing law and reveal the aerodynamic performance is in the flow details and rules in different azimuth.

Abstract: The numerical simulation method of active flow control technology was studied in this paper. The simplified mathematical model of the active flow control is established with unsteady velocity boundary condition at the specific location of model surface. The reliability of flow control model was verified by standard cases of CFDVAL2004, and the capability of capturing micro-jet flow characteristics for Spalart-Allmaras (SA) and Menter’s Shear Stress Transport (SST) turbulence model were analyzed. The results showed that the accuracy of SA turbulence model is better than the SST model, and flow control model meet the accuracy requirements for numerical simulation method.