Turbulence models have long been developed and examined for their accuracy and stability in variety of environments. While many flows work with excited turbulence intensity, models have rarely been tested to explore whether their accuracy withstands with augmented free stream turbulence intensity or decline in reasonable solutions. In present study the turbulent intensity of the air, moving parallel to a flat plate is increased from 0.4 to 6.6% for the whole flow, downstream to the screen. Three popular turbulence models are examined by investigating the turbulence penetration into flow field as well as into turbulent boundary layers over the flat plate. Results of numerical solutions for Standard k-e, Realizable k-e and finally two equations Shear Stress Transport k-w model are compared to experimental measurements and results are discussed. Results of variation of free stream turbulence intensity from flow field out of boundary layer, in addition, streamwise mean velocity, streamwise rms velocity and skin friction coefficient from boundary layer are investigated. Conclusion is made that despite restrictions of these turbulent models specially in predicting flow near a turbulent/non-turbulent interface, they have acceptable performance in both low and high intensity turbulent flows.

Turbulence schemes have long been developed and examined for their accuracy and stability in a variety of environments. While many industrial flows are highly turbulent, models have rarely been tested to explore whether their accuracy withstands such augmented free-stream turbulence intensity or declines to an erroneous solution. In the present study, the turbulence intensity of an air flow stream, moving parallel to a flat plate is augmented by the means of locating a grid screen at a point at which Rex=2.5×105 and the effect on the flow and the near-wall boundary is studied. At this cross section, the turbulence intensity is augmented from 0.4% to 6.6% to flow downstream. Wind tunnel measurements provide reference bases to validate the numerical results for velocity fluctuations in the main stream and at the near-wall. Numerically, four of the most popular turbulence models are examined, namely the one-equation Spalart-Almaras, the two equation Standard k-e, the two equation Shear Stress Transport k-w and the anisotropy multi equation Reynolds Stress Models (RSM). The resulting solutions for the domain are compared to experimental measurements and then the results are discussed. The conclusion is made that, despite the accuracy that these turbulence models are believed to have, even for some difficult flow field, they fail to handle high intensity turbulence flows. Turbulence models provide a better approach in experiments when the turbulence intensity is at about 2% and/or when the Reynolds number is high.

Paulhausen integral techniques together with Johnson, Redford and Jungho correlations are used in predicting transition onset, separation point position and penetration of free stream turbulence into transitional boundary layer in a diffusing flow. Free stream turbulence intensities are set to variety of values covering both linear and non-linear disturbances. In addition, diffuser half angle opening is assumed to vary in ranges not more than that bringing separation point into transitional zone. Results show that the free stream turbulence intensity and positive pressure gradient have both direct effects to bring transition onset closer to indifference stability point. Separation point, however, moves closest towards diffuser's inlet point as pressure gradient increases and moves away as FSTI increase. FSTI effects on near wall turbulence intensity, primarily causing its increase to a maximum and later approaching to a limit for the rest of boundary layer length.

An experimental investigation was carried out to study the turbulent flow over a flat plate in a subsonic wind tunnel. The enhanced level of turbulence was generated by five wicker grids with square meshes, and different parameters (diameter of the grid rod d = 0. 3 to 3 mm and the grid mesh size M = 1 to 30 mm). The velocity of the flow was measured by means of a 1D hot-wire probe, suitable for measurements in a boundary layer. The main aim of the investigation was to explore the influence of the free stream turbulence length scale on the onset of laminar-turbulent bypass transition in a boundary layer on a flat plate. For this purpose, several transition correlations were presented, including intensity and length scales of turbulence, both at the leading edge of a plate and at the onset of transition. The paper ends with an attempt to create a correlation, which takes into account a simultaneous impact of turbulence intensity and turbulence scale on the boundary layer transition. To assess the isotropy of turbulence, the skewness factor of the flow velocity distribution was determined. Also several longitudinal scales of turbulence were determined and compared (integral scale, dissipation scale, Taylor microscale and Kolmogorov scale) for different grids and different velocities of the mean flow U = 4, 6, 10, 15, 20 m/s.

The prediction and control of the laminar-turbulent transition is crucial to the designs of vehicles, turbines, etc. The initial condition of transition depends on the exciting process of boundary-layer instability, which is the key to implement its prediction and control. The current researches confirm that the exciting process of boundary-layer instability, namely receptivity, is affected not only by different types of free-stream disturbances and shape parameters of surface roughness elements, but also by the pressure gradient of mean flow. Hence, we study the effect of pressure-gradient on local excitation of boundary-layer instability under the interaction of the low-level, isotropic free-stream turbulence and micro surface roughness in this work. The numerical results reveal the pressure-gradient effect on the receptive process and the group speed of excited wave packets in the Falkner-Skan boundary layer. The favorable/adverse pressure gradients (FPG/APG) are found to be able to promote/suppress the excitation and subsequent evolution of Tollmien–Schlichting (T-S) waves. Then the relations of the pressure gradient with the amplitude, growth rate, wave number, phase speed and shape function of excited T-S waves are studied.

Inlet performance is an important field in aerodynamic design of aerial vehicle engines. This study focuses on numerical investigation of Mach number effects on a supersonic axisymmetric mixed compression inlet performance. For this purpose, a density based finite volume CFD code has been developed. A structured multi-block grid and an explicit time discretization of Reynolds averaged Navier-Stokes (RANS) equations have been used. Furthermore, Roe’s approximated Riemann solver has been utilized for computing inviscid flux vectors. Also, the monotone upstream centered schemes for conservation laws (MUSCL) extrapolation with Van Albada limiter have been used to obtain second order accuracy. In addition, Spalart-Allmaras one-equation turbulence model has been used to close the governing equations. The code is validated in three test cases by comparing numerical results against experimental data. Finally, the code has been utilized for numerical simulation of a specific supersonic mixed compression inlet. The effects of free stream Mach number on performance parameters, including mass flow ratio (MFR), drag coefficient, total pressure recovery (TPR), and flow distortion (FD) have been discussed and investigated. Results show that increase in Mach number, leads to decrease in TPR and drag coefficient; however, MFR and FD increase. Also, FD variations with respect to other performance parameters are significant, such that increase in Mach number from 1.8 to 2.2 leads to more than 100% FD increment while increase in MFR is less than 10%. By using this code it will be possible to design, performance parametric study, and geometrical optimization of axisymmetric supersonic inlet.

Steady, transverse boundary layer flow and heat transfer caused by an exponentially stretching cylinder of constant radius immersed in an uniform flow of an incompressible, viscous nanoliquid are considered in the present study. The paper discusses a systematic procedure of obtaining a local similarity transformation that reduces the governing partial differential equations into ordinary differential equations. Power series solution is then obtained for velocity, temperature and nanoparticle concentration distributions using the uni-variate differential transform method. Help is sought from Domb-Sykes plots in making a decision on the minimum number of terms required in the power series expansion to ensure convergence. Radius of convergence is quite naturally suggested by these plots. Pad ´ e approximants are then appropriately decided upon to increase the radius of convergence. The algorithm used succeeds in capturing boundary effects, free stream flow effects and nanoparticle effects on flow and heat transfer. An important finding of the paper is the prediction of accelerated cooling of the stretching cylinder due to the nanoparticles in the cooling liquid. In having a desirable property for the extruding cylinder nanoliquid coolant seems an attractive proposition.

Oscillation or rotation rates change flow behavior and wake patterns over and around cylinders. In this study, laminar fluid flow over a rotational cylinder with triangular cross section is studied numerically. Because of rotating geometry, dynamic mesh is used for the numerical solution and simulations are done for 50, 100, 150, 200 Reynolds numbers and 1, 2 and 3 rotation rates and effects of rotation rate and Reynolds numbers on wake and flow patterns are investigated. Investigating average Lift, drag and moment coefficients express that increasing in Reynolds number and rotation rate lead to lower lift and drag coefficients, while moment coefficient has a gradual growth similar to the circular cylinder case. However, the dependency on the Reynolds number is higher in a way that by increasing the rotation rate from 1 to 3 the change in the coefficients is at least 40 percent more than changes of coefficients due to tripling the Reynolds number.