This study is motivated by the need to devise means to enhance heat transfer in configurations, like the back step, that appear in certain types of MEMS that involve fluid flow and that are not very efficient from the thermal transfer point of view. In particular, the work described in this paper studies the effect that a prescribed flow pulsation (defined by two control parameters: velocity pulsation frequency and pressure gradient amplitude at the inlet section) has on the heat transfer rate behind a backward facing step in the unsteady laminar 2-D regime. The working fluid that we have considered is water with temperature dependent viscosity and thermal conductivity. We have found that, for inlet pressure gradients that avoid flow reversal at both the upstream and downstream boundary conditions, the timeaveraged Nusselt number behind the step depends on the two above mentioned control parameters and is always larger than in the steady-state case. At Reynolds 100 and pulsating at the resonance frequency, the maximum time-averaged Nusselt number in the horizontal wall region located behind the step whose length is four times the step height is 55% larger than in the steady-case. Away from the resonant pulsation frequency, the time-averaged Nusselt number smoothly decreases and approaches its steady-state value.

No abstract

No abstract

CFD predictions have been carried out to study the aerodynamic behavior of wind turbine airfoils with distributed roughness over their surface. Wind turbines blades work in variable roughness surface conditions during their operational life, new or washed blades with very low roughness levels and blades that are contaminated by insects, dirt, dust or erosion. The existence of roughness over the blade surface generates a performance loss in the airfoil aerodynamics which understanding and accurate prediction is very important for wind turbine blade designers. In this paper, CFD calculations using the in-house compressible code WMB for several Reynolds numbers and roughness sizes are presented. Numerical data are compared to OSU experimental results for the NREL S809 wind turbine airfoil with locally distributed roughness over the airfoil leading edge. The study is completed with computations for the NACA0012 airfoil and a study of the boundary layer evolution with distributed roughness over the airfoil surface. WMB (Wind Multi Block) is a CFD method developed and validated by CENER and the University of Liverpool for wind turbine aerodynamics analysis (2D and 3D). It is capable of analysing compressible, RANS or URANS equations. In this study RANS equations have been solved. In addition, distributed roughness can be simulated using WMB. WMB has the capacity to simulate distributed roughness elements spread over a chosen area of the airfoil surface (upper or lower area, the whole airfoil or only an isolated zone). Nevertheless, this work is focused on airfoils with leading edge roughness calculations with turbulent flows. Roughness is included in WMB using two different approaches: Hellsten-Laine model and Knopp et al model. In Hellsten-Laine model the boundary condition for the specific dissipation rate is modified to account for the roughness layer that replaces the viscous sublayer. On the other hand in Knopp model a law of the wall is used to obtain values for k (turbulent kinetic energy) and ω at the wall. The main goal of this work is to calculate airfoil aerodynamics when roughness elements are distributed over its surface for different types of airfoils at high Reynolds numbers and including a sensitivity study to roughness parameters. Special attention will be paid to stall area prediction. In this work global airfoil magnitudes and local boundary layer magnitudes are studied numerically with WMB and compared with experiments. The final conclusion obtained from the study presented in this paper is that WMB is a valid tool to study airfoil aerodynamics when there are rough elements distributed over its surface. Nomenclatureα the angle of attack [degrees] c airfoil chord [m] Cd total drag coefficient = Drag 1 2 .ρ∞.V 2 ∞ .S ref [−] Cdp pressure drag coefficient (measured through pressure taps) [−] Cdw wake drag coefficient (measured with wake rake) [−] Cl lift coefficient = Lif t 1 2 .ρ∞.V 2 ∞ .S ref [−] Cm moment coefficient = M oment 1 2 .ρ∞.V 2 ∞ .S ref .length [−] Cp pressure coefficient = P −P∞ 1 2 .ρ∞....

PurposeThe purpose of this paper is to present numerical study on the behaviour of 2D unsteady incompressible laminar wakes behind square cylinders.Design/methodology/approachThe numerical method that has been developed is based on a finite point formulation characterised by its weak connectivity requirements. This formulation allows for a patched unstructured approach to computational domain modelling that is of interest for industrial applications. Time evolution of pressure is computed by using a pseudo‐compressibility relaxation model that is based on physical considerations.FindingsThis model is characterised by the fact that no sub‐iterations on a numerical pseudo‐time are required so that computational efficiency is increased. Algorithm stability requires the use of second and fourth order artificial viscosity operators that effectively change the order of the equations. A discussion is included regarding the boundary conditions for these operators that do not influence vortex shedding behaviour.Research limitations/implicationsBearing in mind the industrial drive (MEMS design) that the authors have in mind, solver validation has been addressed at two levels: global coefficients (lift, drag and Strouhal number) were compared with those published in the specialised literature, while local velocity and rms profiles were compared with those obtained after performing a specific low velocity wind tunnel testing campaign (Reynolds numbers in the range from 110 to 268).Practical implicationsA sensitivity analysis of the results obtained is presented and it shows that the solver numerical robustness makes it amenable for project oriented applications.Originality/valueThe formulation being presented is competitive and could be considered as a potential alternative to other approaches.