Research Focus

Each year we will be reporting on some of the current research of one of our faculty members.

Professor Krishnan Mahesh has been an Assistant Professor in AEM since Fall 2000. Professor Mahesh and his research group use supercomputers to unravel the mysteries of the “last unsolved problem in classical physics’’— turbulent fluid motion.

Turbulence features prominently in a wide range of applications in nature and technology. Turbulence increases the drag on airplanes, improves mixing and reduces pollution inside engines, transports nutrients in the oceans, makes jet exhaust noisy, and causes buffeting inside computer disk-drives. Despite its considerable importance, our ability to control or even predict turbulent flows is very limited. This difficulty was expressed in 1932 by British scientist, Sir Horace Lamb as follows: “I am an old man now, and when I die and go to heaven, there are two matters on which I hope enlightenment. One is quantum electro-dynamics and the other is turbulence of fluids. And about the former, I am really rather optimistic.”

Simulation of flow inside a Pratt & Whitney gas-turbine combustor. The speed of the fluid is shown in color; red corresponds to high speeds and blue regions have slow moving fluid. (Click image to view the animation, 2MB AVI format.)

Lamb’s comments are relevant even today. The equations that govern turbulent flows are well-known, and are called the Navier-Stokes equations. Numerically solving these equations to simulate turbulence is extremely challenging. This is because turbulent flows are very chaotic; they consist of eddying motions of various sizes and shapes. A complete description of a turbulent flow would require that all these eddies be represented on the computational grid. This is practical only for the simplest flows; for most flows, representing all eddies requires an impossibly large number of computational elements.

As a result, the first experimentally validated turbulence simulation was only performed about twenty years ago. Presently, simulations of turbulence have two important limitations — they are restricted to very simple geometries, and to very low Reynolds numbers (defined as the ratio of inertial to viscous forces; airplane wings fly at Reynolds numbers of approximately 106, while presently simulations are performed around 103). As a result, they are not directly applicable to the complex flows encountered in practice. The numerically generated data have largely been used to develop simpler models for practical use. However, there is considerable evidence that these models lack the accuracy to predict many critically important flows; e.g. pollutant formation in engines, airplane jet exhaust noise, and the drag on aircraft at high speeds.

Simulation of a turbulent jet flow.

Contours of velocity immediately adjacent to the wall of a turbulent channel.

Professor Mahesh’s research aims to overcome this obstacle. His group is developing numerical methods and models that are for the first time, allowing three-dimensional, unsteady turbulent simulations to be used as predictive tools for engineering flows in complex configurations. Moreover, the high fidelity of their simulations allow fundamental turbulence data to be obtained in configurations that are beyond the reach of current simulation methods. This work involves the development of novel numerical algorithms, turbulence models and large-scale simulation.

By recognizing that turbulence is inherently non-linear, Professor Mahesh has recently developed a novel simulation method that mimics the relevant non-linear properties of turbulent flows. The result is an algorithm that is accurate enough to compute the eddying motions in turbulent flows, and robust enough to handle complex engineering geometries. In collaboration with colleagues at Stanford University, Professor Mahesh’s approach has made possible three-dimensional unsteady simulations in the exceedingly complex geometry of a Pratt & Whitney gas-turbine combustor. These simulations are the first in a configuration with this level of complexity and were reported as one of the Science Successes in 2002 by the National Partnership for Advanced Computing Infrastructure (www.npaci.edu/successes/2002_jet.html).

Professor Mahesh’s research is currently supported by the Department of Energy, the National Science Foundation and the Office of Naval Research. His group of graduate and undergraduate students study how dilution jets inside modern gas-turbine combustors mix cold air with combustion products (Mr. S. Muppidi), the mixing properties of turbulent jets (Mr. P. Babu), turbulent compressible flows (Mr. Y. Hou), marine propeller crashback - a procedure where the propeller suddenly reverses direction (Mr. A. Dande), near-wall turbulence (Mr. P. Lommel) and the effect of cacti geometry on their resistance to high winds (Mr. J. Graham).

| AEM Home | Institute of Technology |

| Academics | Research | People | Information | Contact AEM |