Amirfarhang Mehdizadeh

A discrete model for the apparent viscosity of polydisperse suspensions including maximum packing fraction

Based on the notion of a construction process consisting of the stepwise addition of particles to the pure fluid, a discrete model for the apparent viscosity as well as for the maximum packing fraction of polydisperse suspensions of spherical, noncolloidal particles is derived. The model connects the approaches by Bruggeman and Farris and is valid for large size ratios of consecutive particle classes during the construction process, appearing to be the first model consistently describing polydisperse volume fractions and maximum packing fraction within a single approach. In that context, the consistent inclusion of the maximum packing fraction into effective medium models is discussed. Furthermore, new generalized forms of the well-known Quemada and Krieger–Dougherty equations allowing for the choice of a second-order Taylor coefficient for the volume fraction (ϕ2-coefficient), found by asymptotic matching, are proposed. The model for the maximum packing fraction as well as the complete viscosity model is...

Analytical and numerical investigations of laminar and turbulent Poiseuille–Ekman flow at different rotation rates

Laminar and turbulent Poiseuille–Ekman flows at different rotation rates have been investigated by means of analytical and numerical approaches. A series of direct numerical simulations (DNSs) with various rotation rates (Ro2=0–1.82) for Reynolds number Reτ0=180 based on the friction velocity in the nonrotating case has been conducted. Both (laminar and turbulent) flow states are highly sensitive to the rotation. Even a small rotation rate can reduce the mean streamwise velocity and induce a very strong flow in the spanwise direction, which, after attaining a maximum, decreases by further increasing the rotation rate. It has been further observed that turbulence is damped by increasing the rotation rate and at about Ro2=0.145 a transition from the fully turbulent to a quasilaminar state occurs. In this region Reynolds number is only large enough to sustain some perturbations and the mean velocity profiles have inflection points. The instability of the turbulent shear stress is probably the main reason for...

Comparative Assessment of Hybrid LES/RANS Models in Turbulent Flows Separating from Smooth Surfaces

Several hybrid LES/RANS (LES - Large-Eddy Simulation; RANS - Reynolds- Averaged Navier-Stokes) models have been assessed in computations of separated flows over smooth-contoured, wall-mounted hills, including flow control. The models considered include DES (Detached Eddy Simulation; Spalart et al. 1997, Travin et al. 2002), DDES (Delayed DES; Spalart et al. 2006), a zonal hybrid LES/RANS scheme (HLR; Jakirlic et al. 2006) and an Instability-Sensitized (IS) k-e model. We report on models performance in the two configurations: periodic flow over a symmetric 2-D hill at moderate Reynolds number (Re b =10595; LES: Frohlich et al.,2005; Breuer et al., 2005) and flow over a 2-D hump at high Re number( Re c ≈ 106,Exp.: Greenblatt et al., 2004). In the latter case the separation was controlled by steady suction through a narrow opening at the natural separation line in addition to the baseline flow. The computational results obtained confirm a crucial role of the LES/RANS interface treatment.

Mixed convection and thermodynamic irreversibilities in MHD nanofluid stagnation-point flows over a cylinder embedded in porous media

The impingement of CuO-water nanofluid flows upon a cylinder subject to a uniform magnetic field with constant surface temperature and embedded in porous media is investigated for the first time in literature. The surface of the cylinder can feature uniform or non-uniform mass transpiration and is hotter than the incoming nanofluid flow. The gravitational effects are taken into account and the three-dimensional governing equations of mixed convection in curved porous media, under magnetohydrodynamic effects, are reduced to those solvable by a finite difference scheme. Through varying a mixed convection parameter, the situations dominated by forced, mixed and free convection are examined systematically. The numerical solutions of these equations reveal the flow velocity and temperature fields as well as the Nusselt number and induced shear stress. These are then used to calculate the rate of entropy generation within the system by viscous and heat transfer irreversibilities. The results show that Nusselt number increases with increasing the concentration of nanoparticles, while it slightly deceases through intensifying the magnetic parameter. Non-uniform transpiration is shown to strongly affect the average rate of heat transfer. Importantly, it is demonstrated that the specific mode of heat convection can majorly influence the intensity of entropy generation and that the irreversibilities are much larger under natural convection compared to those in mixed and forced convection. Calculation of Bejan number shows that this is due to more pronounced relative contribution of viscous irreversibilities when free convection effects dominate the mixed convection process.

A new formulation of scale-adaptive simulation approach to predict complex wall-bounded shear flows

A new formulation of scale-adaptive simulation (SAS) approach for complex wall-bounded shear flows is presented. This approach makes use of a unique modelling representation and requires moderate computational costs. Based on the Rotta original transport equation for turbulence integral length scale, the suggested model is able to resolve unsteady turbulent structures with sufficient spatial resolution. Similar to the classical SAS-turbulence model (SST–SAS) proposed by Menter etxa0al. [5] that represents an improvement of the proven turbulence model for unsteady calculations, the new model appears advantageous where classical LES (large eddy simulation) or hybrid LES–RANS models are too expensive. However, in its zonal formulation, the new model provides accurate predictions in (1) so-called stable flows (e.g. channel flow) in which the classical SAS () model will not be able to switch from Reynolds-averaged Navier–Stokes (RANS) to scale-resolving simulations without an explicit introduction of synthetic t...

Analysis of Scale Adaptive Approaches Based on the Rotta Transport Equation

A zonal formulation of the scale adaptive simulation (SAS) approach for wall bounded shear flows based on the Rotta’s transport equation for integral length scale is contrasted with the (SST-SAS) model of Menter and Egorov (Flow Turbul Combust 85(1):113–138, 2010) with local triggering (seamless formulation). It is known that the SAS approach does not trigger to a scale resolving mode in attached/mildly separated flows even if grid supports the transition Menter et al. (4th Symposium on Hybrid RANS-LES Methods, Beijing, China, September, 2011). This work addresses the question whether a zonal formulation of SAS ((k-varepsilon ) formulation along with different norm for second derivative of velocity) could improve the triggering process from URANS to LES-like mode in attached/mildly separated flows. In order to study the effects of different formulations, both models were applied to different flow configurations ranging from fully attached to strongly separated, including stationary streamwise-homogeneous turbulent channel flow, flow over an S809 airfoil and swirling flow through a sudden expansion. We find that, in both formulations, even when grid is sufficiently fine to resolve the integral scale motions, the simulation only transitions to scale-resolving mode when the base URANS flow is naturally unstable.

Magnetohydrodynamics, Natural Convection, and Entropy Generation of CuO–Water Nanofluid in an I-Shape Enclosure—A Numerical Study

This paper presents a numerical study of the magnetohydrodynamics, natural convection, and thermodynamic irreversibilities in an I-shape enclosure, filled with CuO-water nanofluid and subject to a uniform magnetic field. The lateral walls of the enclosure are maintained at different but constant temperatures, while the top and bottom surfaces are adiabatic. The Brownian motion of the nanoparticles is taken into account and an extensive parametric study is conducted. This involves the variation of Rayleigh and Hartmann numbers, and the concentration of nanoparticles and also the geometrical specifications of the enclosure. Further, the behaviors of streamlines and isotherms under varying parameters are visualized. Unlike that in other configurations, the rate of heat transfer in the I-shaped enclosure appears to be highly location dependent and convection from particular surfaces dominates the heat transfer process. It is shown that interactions between the magnetic field and natural convection currents in the investigated enclosure can lead to some peculiarities in the thermal behavior of the system. The results also demonstrate that different parts of the enclosure may feature significantly different levels of heat transfer sensitivity to the applied magnetic field. Further, the analysis of entropy generation indicates that the irreversibility of the system is a strong function of the geometrical parameters and that the variations in these parameters can minimize the total generation of entropy. This study clearly shows that ignoring the exact shape of the enclosure may result in major errors in the prediction of heat transfer and second law performances of the system.

Effects of Near Wall Modeling in the Improved-Delayed-Detached-Eddy-Simulation (IDDES) Methodology

The present study aims to assess the effects of two different underlying RANS models on overall behavior of the IDDES methodology when applied to different flow configurations ranging from fully attached (plane channel flow) to separated flows (periodic hill flow). This includes investigating prediction accuracy of first and second order statistics, response to grid refinement, grey area dynamics and triggering mechanism. Further, several criteria have been investigated to assess reliability and quality of the methodology when operating in scale resolving mode. It turns out that irrespective of the near wall modeling strategy, the IDDES methodology does not satisfy all criteria required to make this methodology reliable when applied to various flow configurations at different Reynolds numbers with different grid resolutions. Further, it is found that using more advanced underlying RANS model to improve prediction accuracy of the near wall dynamics results in extension of the grey area, which may delay the transition to scale resolving mode. This systematic study for attached and separated flows suggests that the shortcomings of IDDES methodology mostly lie in inaccurate prediction of the dynamics inside the grey area and demands further investigation in this direction to make this methodology capable of dealing with different flow situations reliably.

Heat transfer analysis of unsteady MHD rotating graphene oxide water nanofluid flow

Abstract In this study, the Runge Kuta numerical method (RK4), was used to find an accurate solution for the velocity and temperature fields of an unsteady flow of grapheme oxide nanofluid, in a rotating system, within a channel with lower porous wall in presence of imposed external uniform magnetic field. First of all, a local similarity solution for the transformed governing equations was achieved. The nonlinear partial differential equations (PDEs) and boundary conditions were changed to dimensionless form. Thereafter, the numerical approach was employed for obtaining the solution of resulted coupled nonlinear ordinary differential equations. The effect of various parameters including Eckert number, Magnetic parameter and etc. on temperature and velocity distributions were investigated. Finally, the entropy generation rate was calculated. To study the accuracy of solutions, RVIM –analytical method- was used for validation procedures.

Assessment of ability of RANS based turbulence models to predict the mould filling process and oxide film formation in Aluminum casting

Mould filling operations involve usually turbulent flow processes. Up to now a reliable evaluation of the effects of turbulence on the mold filling process along with the oxide film formation and disintegration does not exist. This paper aims to assess the ability of RANS based turbulence models in mould filling process in Rectangular Runner (RR runner) configuration in aluminum casting. For this purpose, three types of turbulence models have been considered for their specific capability throughout the literature. Different numerical simulations have been performed using various boundary conditions and pouring rates. It has been established that the k − ∊ − v2 model is able to predict better the flow dynamics during the filling process. Based on the stability concept of the interface, a modified expression for the total amount of oxide films formed during this process has been introduced. This expression has been used to find an optimal pouring rate which led to 0.5m/s for RR runner under the investigated conditions.