# AEM faculty spotlight:

*Krishnan Mahesh *

In the event of an emergency, a submarine may reverse its
propellers. While traditional methods can accurately predict the constant,
forward movement of fluid around this propeller, that sudden reversal creates a
mess beyond what many methods can handle. This is where Krishnan Mahesh, a
professor in the Department of Aerospace Engineering and Mechanics, comes in.
Utilizing numerical simulations, Mahesh can predict highly turbulent,
seemingly-random flow in exceedingly complex situations. These methods help the
navy design more efficient propellers, help industry better understand fuel
combustion in engines, and more.

Below, Professor Mahesh explains a bit
about numerical simulations and how his simulation methods are impacting the
field of fluid dynamics.

Krishnan Mahesh

Krishnan Mahesh

**Firstly, what are numerical methods?**

If you
want to predict something, you have to have equations that describe its
behavior. So if I were to place a leaf on a river and watch it, I notice
it’s basically being carried by the flow. If I create an equation for
that, it would be called a governing equation. To compute transport from one
point to another, you lay a grid down and then numerically represent the
governing equation on this grid. You then have numerical theory which lets you
approximate the spatial gradients and integrate values over time, and this
constitutes a numerical method.

**What is different between
your method and traditional methods of calculation?**

Generally
speaking, what we do is conduct research into turbulent flows with an emphasis
on complex physics. Traditionally, high fidelity simulations for these problems
have been restricted to simple geometries. We are now able to perform three
dimensional, high fidelity simulations with practical applications. These
models succeed where traditional models fail.

**What are those sorts
of problems?**

As an example, in modern engines, if you want to
reduce Nitrous oxide gases you use dilution jets to cool the combustion
products. How do these jets mix with the combustion products? How does this
process depend on the shape and distribution of the dilution holes, and how
does the velocity of the jets affect this process? To better understand these
issues, we simulate and study flows ranging from just a single jet to an entire
combustor. In doing so, we both advance the state of the art in
simulating the real-life engineering flow, as well as answer fundamental
questions about the basic physics. What allows us to do this is the fidelity of
our numerical tools. Other problems that we work on at present include:
developing strategies to control turbulent mixing, modeling how a laser
interacts with turbulent air, simulating how underwater flows cavitate, and
shock waves and combustion in high speed flows. Our interest in high-speed
flows is driven by scramjets.

**What is difficult about flow in a
scramjet engine?**

If you look at a typical flow, it could be Mach
15 around the vehicle, but inside the scramjet it’s about one-third of
that. The flow comes in, shock waves form, those shocks hit fluid near walls
and when that happens, you have a mess. Similarly when you inject fuel
into a high-speed airstream, the fuel stream breaks down, mixes and burns in a
very complicated manner. What we are doing is developing our brand of
numerical methods for high speed flows.

**Why are you interested in
turbulent flow?**

I guess I am interested because of its presence
everywhere; if you look at waves in the ocean, sandstorms, flow over airplane
wings, sound produced by helicopter rotors, flow inside engines, it’s all
turbulent flow. Seeing it everywhere and seeing that it is not a problem
that’s going to go away anytime soon makes it both challenging and
intriguing.

**Is there one big question that needs solving in terms
of turbulence?**

I guess the one big question would be, 'is there a
complete solution to the turbulence problem?’ Meaning, can we
theoretically predict the instantaneous velocities, pressure, chemical
species, temperature, etc. for any turbulent flow? We are so far away from this
that the question almost seems philosophical. But what I do see as happening is
that simulations are increasingly playing an important role in practical
engineering. The question I ask is, “Can we develop simulation tools and
flow models that will allow the scientist as well as the designer to develop
next-generation devices and next-generation physical understanding?”

*Last Modified:
Tuesday, 22-Mar-2016 10:46:50 CDT
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