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Using the Force: Applications and Implications of Turbulence Forcing Terms in Direct Numerical Simulations

Citation

Dhandapani, Chandru (2019) Using the Force: Applications and Implications of Turbulence Forcing Terms in Direct Numerical Simulations. Dissertation (Ph.D.), California Institute of Technology. doi:10.7907/FH31-4468. https://resolver.caltech.edu/CaltechTHESIS:06102019-185605511

Abstract

Most energy requirements of modern life can be fulfilled by renewable energy sources, but it is impossible in the near future to provide an alternative energy source to combustion for airplanes. That being said, combustion in aviation can be made more sustainable by using alternative jet fuels, which are made from renewable sources like agricultural wastes, solid wastes, oils, and sugars. These alternative fuels can be used in commercial flights only after a long certification process by the Federal Aviation Agency (FAA) and ASTM International. Unfortunately, in over 50 years of fuel research, only five fuels have been certified. This research project aims to speed up the certification process with quicker testing of alternative fuels. Engine testing and even laboratory testing require large amounts of time and fuel. Simulations can make the process much more efficient, but accurately simulating highly turbulent flames in such complex geometries would need large amounts of computational resources. The goal of this thesis is to create an efficient computational framework, that can replicate different engine-like turbulent flow conditions in simple geometries with numerical tractability.

The central idea is to decompose the flow field into ensemble mean and fluctuating quantities. The simulations then resolve only the fluctuations using simple computational domains, while emulating the effect of the mean flow using "forcing" terms. These forcing terms are calculated first for incompressible turbulence, and this method is later extended to turbulent reacting flows. In incompressible turbulence, Direct Numerical Simulations (DNS) performed on simple triply periodic cubic domains reasonably capture the statistically stationary shear turbulence, that is observed in free shear flows. The simulations are also performed in cuboidal domains, that are longer in one direction and with an inflow/outflow along it. Both changes are observed to not have a significant impact on the turbulence statistics. Finally, shear convection is applied to the turbulence simulations with inflow/outflow, which has a significant impact on the turbulence. These simulations accurately capture the turbulence anisotropy in free-shear flows.

The study is extended to DNS of highly turbulent n-heptane-air flames performed under different flow conditions. Turbulent flames involve two-way coupling between fluid mechanics and combustion. The effects of the flame on the turbulence and the impact of the turbulent flow conditions on the flame behavior are analyzed. The focus is placed on the effects of turbulence production, shear convection, and pressure gradients. The anisotropy produced in the turbulence due to the different flow conditions and the flame are also compared and contrasted. While the global behavior and flow anisotropy were affected by these conditions, the local chemistry effects were unaffected, and depend only on the laminar flame properties and turbulence intensity. These findings can help predict turbulent flame behavior, and can expedite the search and testing of sustainable alternatives to conventional jet fuels.

Item Type:Thesis (Dissertation (Ph.D.))
Subject Keywords:Turbulence,Simulations, DNS, Reacting flows, Chemistry, Fluid mechanics
Degree Grantor:California Institute of Technology
Division:Engineering and Applied Science
Major Option:Aeronautics
Awards:Graduate Deans’ Award, 2019.
Thesis Availability:Public (worldwide access)
Research Advisor(s):
  • Blanquart, Guillaume
Group:Resnick Sustainability Institute
Thesis Committee:
  • Meiron, Daniel I. (chair)
  • Pullin, Dale Ian
  • Colonius, Timothy E.
  • Blanquart, Guillaume
Defense Date:17 May 2019
Non-Caltech Author Email:chandrudiitm (AT) gmail.com
Funders:
Funding AgencyGrant Number
Resnick Sustainability InstituteUNSPECIFIED
Air Force Office of Scientific Research (AFOSR)UNSPECIFIED
Record Number:CaltechTHESIS:06102019-185605511
Persistent URL:https://resolver.caltech.edu/CaltechTHESIS:06102019-185605511
DOI:10.7907/FH31-4468
ORCID:
AuthorORCID
Dhandapani, Chandru0000-0002-7319-557X
Default Usage Policy:No commercial reproduction, distribution, display or performance rights in this work are provided.
ID Code:11737
Collection:CaltechTHESIS
Deposited By: Chandru Dhandapani
Deposited On:12 Jun 2019 18:06
Last Modified:04 Oct 2019 00:26

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