Dipankar Sahoo

Bio: Dipankar Sahoo is an academic researcher from Booz Allen Hamilton. The author has contributed to research in topics: Boundary layer & Diesel engine. The author has an hindex of 13, co-authored 33 publications receiving 445 citations. Previous affiliations of Dipankar Sahoo include Tenneco & Princeton University.

Experiments on the structure and scaling of hypersonic turbulent boundary layers

Abstract: Particle image velocimetry and filtered Rayleigh scattering experiments were performed over a range of Reynolds numbers to study the scaling and structure of a smooth, flat-plate turbulent boundary layer with a free stream Mach number of 7.5. The measurements indicate few, if any, dynamic differences due to Mach number. Mean and fluctuating streamwise velocities in the outer layer show strong similarity to incompressible flows at comparable Reynolds numbers when scaled according to van Driest and Morkovin. In addition, correlation lengths and structure angles based on velocity statistics were found to be less sensitive to compressibility than indicated by previous studies based on density fields or mass-weighted statistics, suggesting that the density and velocity fields obey different scaling. Finally, the boundary layer displays uniform momentum zones, with the number of these zones similar to incompressible boundary layers at comparable Reynolds numbers.

Mixed-convective cooling of an isothermal hot surface by confined slot jet impingement

Abstract: The flow and heat transfer characteristics in the cooling of a heated surface by impinging slot jets have been investigated numerically. Computations are done for vertically downward-directed two-dimensional slot jets impinging on a hot isothermal surface at the bottom and confined by a parallel adiabatic surface on top. Some computations are also performed where the jet is vertically upward, with an impingement plate at the top. The principal objective of this study is to investigate the associated heat transfer process in the mixed-convective regime. The computed flow patterns and isotherms for various domain aspect ratios (4–10) and for a range of jet exit Reynolds numbers (100–500) and Richardson numbers (0–10) are analyzed to understand the mixed-convection heat transfer phenomena. The local and average Nusselt numbers and skin friction coefficients at the hot surface for various conditions are presented. It is observed that for a given domain aspect ratio and Richardson number, the average Nusselt n...

Direct numerical simulation of hypersonic turbulent boundary layers. Part 3. Effect of Mach number

Abstract: In this paper, we perform direct numerical simulations (DNS) of turbulent boundary layers with nominal free-stream Mach number ranging from 0.3 to 12. The main objective is to assess the scalings with respect to the mean and turbulence behaviours as well as the possible breakdown of the weak compressibility hypothesis for turbulent boundary layers at high Mach numbers (M > 5). We find that many of the scaling relations, such as the van Driest transformation for mean velocity, Walz's relation, Morkovin's scaling and the strong Reynolds analogy, which are derived based on the weak compressibility hypothesis, remain valid for the range of free-stream Mach numbers considered. The explicit dilatation terms such as pressure dilatation and dilatational dissipation remain small for the present Mach number range, and the pressure–strain correlation and the anisotropy of the Reynolds stress tensor are insensitive to the free-stream Mach number. The possible effects of intrinsic compressibility are reflected by the increase in the fluctuations of thermodynamic quantities (p′rms/pw, ρ′rms/ρ, T′rms/T) and turbulence Mach numbers (Mt, M′rms), the existence of shocklets, the modification of turbulence structures (near-wall streaks and large-scale motions) and the variation in the onset of intermittency.

Development of a Reduced Primary Reference Fuel Mechanism for Internal Combustion Engine Combustion Simulations

Abstract: A reduced PRF mechanism was proposed for combustion simulations of PRF and diesel/gasoline fuels based on the latest LLNL mechanism. The reduced PRF mechanism consists of 73 species and 296 reactions. The major reaction pathways of the detailed mechanism were mostly retained in the reduced mechanism, which ensures its predictive capability, the ability to be extended to other fuels, and the high computational efficiency of the reduced mechanism. The important reaction pathways and reactions in the reduced mechanism are identified and discussed. Furthermore, the reaction rates of two reactions, HO2 + OH = HO2 + O2 and HO2 + HO2 = H2O2 + O2, in the hydrogen submechanism are discussed and updated. The reduced mechanism was validated with measured ignition delays, laminar flame speeds, premixed flame species concentrations, jet stirred reactor and shock tube species profiles, and PRF fuel HCCI and PPCI combustion and diesel/gasoline direct injection spray combustion data. The reduced mechanism predicts well t...

Post injections for soot reduction in diesel engines: A review of current understanding

Abstract: This work is a technical review of past research and a synthesis of current understanding of post injections for soot reduction in diesel engines. A post injection, which is a short injection after a longer main injection, is an in-cylinder tool to reduce engine-out soot to meet pollutant emissions standards while maintaining efficiency, and potentially to reduce or eliminate exhaust aftertreatment. A sprawling literature on post injections documents the effects of post injections on engine-out soot with variations in many engine operational parameters. Explanations of how post injections lead to engineout soot reduction vary and are sometimes inconsistent or contradictory, in part because supporting fundamental experimental or modeling data are often not available. In this paper, we review the available data describing the efficacy of post-injections and highlight several candidate in-cylinder mechanisms that may control their efficacy. We first discuss three in-cylinder mechanisms that have been frequently proposed to explain how post injections reduce engine-out soot. Thereafter, to provide a foundation for interpretation of past research, we briefly review basic soot formation and oxidation chemistry, and soot/fluid processes in fuel sprays and engine flows. Next, we provide a comprehensive overview of the literature on the efficacy of post-injections for soot reduction as a function of engine operational parameters including injection duration and dwell, exhaust-gas recirculation, load, boost, speed, swirl, and spray targeting. We conclude by identifying major remaining research questions that need to be addressed to help achieve a design-level understanding of the mechanisms of soot reduction by post injections. CITATION: O'Connor, J. and Musculus, M., \"Post Injections for Soot Reduction in Diesel Engines: A Review of Current Understanding,\" SAE Int. J. Engines 6(1):2013, doi:10.4271/2013-01-0917. ____________________________________ THIS DOCUMENT IS PROTECTED BY U.S. AND INTERNATIONAL COPYRIGHT. It may not be reproduced, stored in a retrieval system, distributed or transmitted, in whole or in part, in any form or by any means. Downloaded from SAE International by Jacqueline O'Connor, Tuesday, April 16, 2013 09:51:00 AM

Unsteady flow past an airfoil pitching at a constant rate

Abstract: The unsteady flow past a NACA 0012 airfoil that is undertaking a constant-rate pitching up motion is investigated experimentally by the PIDV technique in a water towing tank. The Reynolds number is 5000, based upon the airfoil's chord and the free-stream velocity. The airfoil is pitching impulsively from 0 to 30 deg. with a dimensionless pitch rate alpha of 0.131. Instantaneous velocity and associated vorticity data have been acquired over the entire flow field. The primary vortex dominates the flow behavior after it separates from the leading edge of the airfoil. Complete stall emerges after this vortex detaches from the airfoil and triggers the shedding of a counter-rotating vortex near the trailing edge. A parallel computational study using the discrete vortex, random walk approximation has also been conducted. In general, the computational results agree very well with the experiment.