چكيده به لاتين
Abstract:
In this thesis, drag reduction phenomena by hydrophobic coating is studied experimentally and with computer simulation. The lack of accurate theory for explanation of physics of drag reduction by hydrophobic coating was motivated to do this project. so in this investigation some efforts were done to present a suitable numerical simulation and some apparatus design for experimental study to clarify the effect of hydrophobic coating on drag reduction.
In experimental part some apparatuses like radiative oven for coating curing and some coating chambers are designed. In addition for measuring dynamic sliding angle equipment is built so we can measure sliding angel in some way except photography. Besides these, for research on effect of hydrophobic coating on drag reduction in flow field a microchannel was design.
With these set up and some other instruments and preprocessing method the surfaces with high non wettability characteristic are produced. The results show that preprocessing and proper absolute humidity lead to superhydrophobic surfaces with high stability and durability.
These surfaces are used in microchannel to investigate drag reduction effect. In this microchannel pressure drop is measured in two cases, one without coating and the other with superhydrophobic coating.in constant mass flow rate pressure drop decreases in coating case more than 30%.
Also in experimental study we tried to predict the rate of drag reduction in turbulent flow and external flow field. So we use an assumption that if surfaces follow the same procedure to produce and experience the same shear rate in the flow filed, they can have the same slip length. In this way ,by computing slip length in internal flow in microchannel we can calculate the slip velocity and drag reduction in turbulent flow or increasing the velocity for external flow.It should be mentioned that for exact predication more experimental research and more accurate instrument are needed.
In the computational simulation the effect of superhydrophobic surfaces and their characteristic on the drag reduction was studied .for this purpose a model is used which is capable to simulate the different condition of these surfaces. The results confirm that this model can predict the influence of hydrophobicity on turbulence structure and vorticity filed .In this research this general model is used with some numerical method rather than direct numerical simulation.
The other thing that is studied in numerical part is using contact layer as a wall function.Unlike other wall functions, the Contact Layer technique is sensitive to pressure gradient and surface curvature and has no experimental constants in its formulation. The total number of grid points in the simulation of turbulent flow with this wall model is reduced up to70%., while the flow statistics is not changed significantly.
Key words: Superhydrophobic Surfaces, Frictional Drag, Large Eddy Simulation, Microchannel
