Articles written in Sadhana

    • A comparative study of sand-blasted and electro-discharge-machined surfaces of steel substrates


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      Sand blasting is a common process to prepare steel surfaces prior to thermal spray coating application to obtain better coating adhesion. Die-sinking electro-discharge machining (EDM) is a non-conventional machining process that also produces rough surfaces. In this study, steel (EN 31) surfaces are prepared by both methods to obtain the same average roughness (Ra) of 3, 5 and 7μm. The prepared surfaces are studied and compared to investigate whether the rough EDMed surface is suitable for applying thermally sprayed Ni–5Al coating on it or not. XRD and scanning electron microscopy analysis of the samples are carried out. Nanohardness behaviour of the samples is also studied. Failure in obtaining well-adhered D-gun-sprayed Ni–5Al coating on EDMed surface is due to the presence of hard cementite and austenite phases on the surface. It isconcluded that for thermal spraying, adhesion of coating material on substrate cannot be achieved without proper metallurgical compatibility. Also, for thermally sprayed Ni–5Al coating application on steel substrate, grit blasting method is the suitable process for substrate preparation.

    • Receptivity and sensitivity analysis of Jeffery-Hamel flow


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      In local stability framework, receptivity and sensitivity analysis for Jeffery-Hamel (JH) flow for converging and diverging angles are presented here. The frequencies are pointed out, upon which the internal eigenfrequencyof the system resonates with that of external forcing frequencies. This resonance is often characterized as a starting step of disturbance growth of internal disturbances influenced from external environment disturbance. Identifying and avoiding such frequencies in external disturbance environment apriori, can certainly help delay in transition process. A mathematical model as a harmonically driven input-output system is formulated (through resolvent norm) to quantify the amplification of energy and identifying the resonant external frequencies of the system. Sensitivity analysis is also mapped by resolvent norm by highlighting the most sensitive eigenvalues in the pseudospectrumof the system. Numerical simulation is done for small angles of converging and diverging JH flow, for which parallel flow assumptions are also valid. For numerical discretization, Chebyshev spectral method is utilized. The wall normal direction were discretized at Chebyshev collocation points in order to achieve higher accuracy.We have studied three different cases for near critical Reynolds number values. In 2D diverging JH flow case (at wavenumbers kx = 1:66, kz = 0) with near critical Reynolds no = 250 and diverging angle α = 1°, theresonant peak is observed at ω = 0:7653. For 3D diverging JH flow case (at wavenumbers kx = 0, kz = 1) with Reynolds no = 250 and diverging angle α = 1°, the resonance occurs at ω = 0:0102, having comparatively higher peak. Whereas for converging JH flow (α = -0:005°), Re = 9000, kx = 0 and kz = 2, the eigen-frequency ω = 0:0102 resonates with that of external frequency, with even higher magnitude as compared with that of both diverging JH flow cases. The JH flows are accompanied with single resonant peak, as compared with that of plane Poiseuille flow. This qualitatively links the inherited better stability of the JH flows as compared to the plane Poiseuille flow. These resonances could induce the starting step leading to transition in the flow.

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