Engineering topics
Publications and source records attributed to Biringen, S..
The Chebyshev matrix collocation method is applied to obtain the spatial modes of the Orr-Sommerfeld equation for Poiseuille flow and the Blausius boundary layer. The problem is linearized by the companion matrix technique for semi-infinite domain using a mapping transformation. The method can be easily adapted to problems with different boundary conditions requiring different transformations.
In this numerical study the effects of a modulated gravitational field on three-dimensional Rayleigh-Benard convection with heating from above or from below is investigated. The full, nonlinear, time-dependent, Boussinesq Navier-Stokes equations and the energy equation are solved by a semiimplicit, pseudo-spectral procedure. This study has been motivated by the need to better understand the effects of vibration (G-Jitter) on fluids systems especially in the low gravity environment.
A Chebyshev matrix collocation method is outlined for the solution of the Orr-Sommerfeld equation for the Blausius boundary layer. User information is provided for FORTRAN program ORRBL which solves the equation by the QR method.
The time-dependent three-dimensional incompressible Navier-Stokes equations are used to study the mechanisms and developing structures comprising the later stages of the transition to turbulence in plane channel flow. Computer animated flow visualizatons are employed using three-dimensional vortex lines and other graphical representations. The flow visualizations clearly show the development of organized structures such as vortex loops, horse-show vortices, and vortex rings. It is noted that the development of these structures is similar to the bursting process found in turbulent channel flow.
The reduction-to-periodicity method using the pseudo-spectral Fast Fourier Transform (FFT) technique is applied to the solution of nonperiodic problems including the two-dimensional Navier-Stokes equations. The accuracy of the method is demonstrated by calculating derivatives of given functions, one- and two-dimensional convective-diffusive problems, and by comparing the relative errors due to the FFT method with seocnd order Finite Difference Methods (FDM). Finally, the two-dimensional Navier-Stokes equations are solved by a fractional step procedure using both the FFT and the FDM methods for the driven cavity flow and the backward facing step problems. Comparisons of these solutions provide a realistic assessment of the FFT method indicating its range of applicability.
Pressure boundary conditions satisfying the normal momentum equation at solid boundaries with second-order accuracy are developed. Implementation of these conditions in an explicit numerical procedure for the two-dimensional incompressible Navier-Stokes equations enables convergent and accurate solutions for the driven cavity problem provided that the integral constraint of the Neumann boundary condtions is satisfied.
Results are presented from a numerical simulation of transition control in plane channel and boundary layer flows. The analysis is based on a pseudo-spectral/finite difference semi-implicit solution procedure employed to numerically integrate the time-dependent, three-dimensional, incompressible Navier-Stokes equations in a doubly periodic domain. In the channel flow, it was found that the active periodic suction/blowing method was effective in controlling strongly three-dimensional disturbances. In the boundary layer, the preliminary analysis indicated that in the early stages, passive control by suction is as effective as active control to suppress instabilities. The current work is focused on a detailed comparison of active and passive control by suction/blowing in the boundary layer.
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Three-dimensional visualization of flwo field structures in transitional plane channel flow obtained from a numerical simulation are presented at two Reynolds numbers. It is revealed that at the one-spike stage, independent of Reynolds number, the flow is characterized by a multi-layer vortex system. In the upper layer, total vorticity vector plots indicate a high-shear layer dominated by spanwise vorticity, whereas the middle layer (corresponding approximately to the critical layer) forms a vortex loop (alpha-vortex) consisting of strong streamwise and spanwise vorticity components. At the three-spike stage, the breakdown of the vortical structure proceeds in a manner similar to frontal relaminarization of turbulent spots. This process is accompanied by intense vortex lift-up activity near the wall which seems to originate at the legs of the vortex loop trailing the high-shear layer. Finally, at the five-spike stage, it is shown that high-vorticity regions (vortical structures) develop into horseshow eddies in planes inclined to the main flow direction.
In this work the applicability of the combined bulk convection and gradient transport hypotheses for modeling turbulent diffusion is investigated. The resulting model equation, namely the one-equation model, is solved for free-shear flows by an implicit finite-difference method. Results indicate that significant improvements over previous models can be achieved with this new formulation of turbulent diffusion for both heat and momentum transport.
An attempt is made to formulate a more accurate and physically plausible diffusion model, which will include the effects of large eddy motion on turbulent transport, by incorporating bulk convection and gradient diffusion. The two-equation model of turbulence thus obtained is used in self-similar form to calculate axisymmetric and plane two-dimensional jets.
The applicability of active control of transition by period suction-blowing is investigated via direct numerical simulations of the Navier-Stokes equations. The time-evolution of finite-amplitude disturbances in plane channel flow is compared in detail with and without control. The analysis indicates that for relatively small three-dimensional amplitudes, a two-dimensional control effectively reduces disturbance growth rates even for linearly unstable Reynolds numbers. After the flow goes through secondary instability, three-dimensional control seems necessary to stabilize the flow. An investigation of the temperature field suggests that passive temperature contamination is operative to reflect the vorticity dynamics during transition.
This paper involves a numerical simulation of the final stages of transition to turbulence in plane channel flow at a Reynolds number of 1500. Three-dimensional incompressible Navier-Stokes equations are numerically integrated to obtain the time evolution of two- and three-dimensional finite-amplitude disturbances. Computations are performed on the CYBER-203 vector processor for a 32 x 51 x 32 grid. Solutions indicate the existence of structures similar to those observed in the laboratory and characteristics of the various stages of transition that lead to final breakdown. In particular, evidence points to the formation of a upside-down-V-shaped vortex and the subsequent system of horseshoe vortices inclined to the main flow direction as the primary elements of transition. Details of the resulting flow field after breakdown indicate the evolution of streaklike formations found in turbulent flows. Although the flow field does approach a steady state (turbulent channel flow), the introduction of subgrid-scale terms seems necessary to obtain fully developed turbulence statistics.
A numerical study is conducted to investigate a new method of transition control by periodic suction-blowing. It is shown that significant reduction in the amplitudes of two- and three-dimensional finite-amplitude disturbances can be obtained by the application of this method to transition in plane channel flow.
Inlet flowfields for airbreathing missiles are calculated by a two-dimensional computational method. A supersonic freestream is assumed to allow the forebody calculation to be uncoupled from the inlet calculation. The inlet calculation employs an implicit, time-marching, finite difference procedure to solve the Euler equations formulated in body-fitted coordinates. The method can be used for a flowfield with both subsonic and supersonic regions and is found to converge rapidly for supercritical inlet operation. For subcritical inlet operation, however, convergence to steady state is slow.
Results obtained from a numerical simulation of the final stages of transition to turbulence in plane channel flow are described. Three dimensional, incompressible Navier-Stokes equations are numerically integrated to obtain the time evolution of two and three dimensional finite amplitude disturbances. Computations are performed on CYBER-203 vector processor for a 32x51x32 grid. Results are presented for no-slip boundary conditions at the solid walls as well as for periodic suction blowing to simulate active control of transition by mass transfer. Solutions indicate that the method is capable of simulating the complex character of vorticity dynamics during the various stages of transition and final breakdown. In particular, evidence points to the formation of a lambda-shape vortex and the subsequent system of horseshoe vortices inclined to the main flow direction as the main elements of transition. Calculations involving periodic suction-blowing indicate that interference with a wave of suitable phase and amplitude reduces the disturbance growth rates.
A numerical simulation of the final stages of transition in plane channel flow is presented. Investigation of various flow field quantities provides some new insight into the breakdown stage and the formation of spot-like packets of turbulence.
A numerical simulation of the final stages of transition to turbulence in plane channel flow is reported. Three dimensional, incompressible Navier-Stokes equations are numerically integrated to obtain the time-evolution of two and three dimensional finite amplitude disturbances. Computations are performed on the CYBER-203 vector processor for a 32x51x32 grid. Results are presented for no-slip boundary conditions at the solid walls as well as for periodic suction-blowing to simulate active control of transition by mass transfer. Solutions indicate that the method is capable of simulating the complex character of vorticity dynamics during the various stages of transition and final breakdown. In particular, evidence points to the formation of a lambda-shape vortex and the subsequent system of horseshoe vortices inclined to the main flow direction as the main elements of transition. Calculations involving suction-blowing indicate that interference with a wave of suitable phase and amplitude reduces the disturbance growth rates.