[meteorite-list] Understanding Turbulence in the Fast Lane - Mach 10 and Beyond

From: Ron Baalke <baalke_at_meteoritecentral.com>
Date: Mon Mar 21 13:24:57 2005
Message-ID: <200503162211.j2GMBmk17762_at_zagami.jpl.nasa.gov>

UNDERSTANDING TURBULENCE IN THE FAST LANE - MACH 10 AND BEYOND
>From Ed Stiles, UA College of Engineering, 520-621-3754
March 16, 2005
 
Although NASA's X-43A and other hypersonic airplanes use air-breathing
engines and fly much like 747s, there's a big difference between ripping air
at Mach 10 (around 7,000 mph) and cruising through it at 350 mph.
 
These differences are even more pronounced when hypersonic aircraft sip
rarified air at 100,000 feet, while commercial airliners gulp the much
thicker stuff at 30,000.
 
Aero-thermodynamic heating is a very big deal at Mach 10. The critical point
comes where air changes from flowing smoothly across a surface ? laminar
flow - to when it becomes chaotic - turbulent flow.
 
Aero-thermodynamic heating largely determines the engine size, weight,
choice of materials and overall size in hypersonic airplanes. So engineers
would like to have a much better understanding of what triggers turbulence
and how they can control it at hypersonic speeds.
 
Air goes from laminar to turbulent at what engineers call the "boundary
layer." They understand how this happens at slower speeds, but they're still
grappling with which factors influence it at hypersonic speeds.
 
---------------------------
Contact Information:
Anatoli Tumin
Associate Professor
Aerospace and Mechanical Engineering
tumin_at_email.arizona.edu
 
Related Web Sites:
Anatoli Tumin's Homepage
http://www.ame.arizona.edu/faculty/tumin/tumin.php
 
NASA X-43A Page
http://www.nasa.gov/missions/research/x43-main.html
---------------------------
  
University of Arizona Associate Professor Anatoli Tumin, of Aerospace and
Mechanical Engineering (AME), is among those studying the problem and has
developed a model that predicts the surface roughness effects on the
transition from laminar to turbulent flow at hypersonic speeds.
 
His theory has a lot to do with partial differential equations,
Navier-Stokes equations and other brain-taxing mathematics that Tumin and
Applied Math Ph.D. student Eric Forgoston have grappled with during the past
couple of years.
 
"In principle, the theory tells us what the optimal perturbations are that
will lead to turbulent flow," Tumin said. "Now we can explore different
geometries for roughness elements to see which are best. We can explore how
to space them and where we should position them."
 
The researchers will soon run a supercomputer simulation to compare their
theory with what actually happens when air flows across a roughened surface
at hypersonic speeds.
 
Currently, these simulations guzzle tens of hours of supercomputing time.
But if Tumin's theory is correct, engineers will soon get the same results
from their office laptops.
 
Tumin is working with Research Assistant Professor Simone Zuccher, of UA
AME, to develop a software package that will allow designers to do this
laptop-style analysis. The software will help them predict when and where
the transitions from laminar to turbulent flow occur in engines and on
surfaces operating at hypersonic speeds.
 
"We developed our theory and arrived at what is called the 'transient growth
mechanism,'" Tumin said. "The airflow is stable, but there are some tiny
disturbances within it that can grow downstream. We can generate these
downstream, streamwise vortices (spiraling flows) by using the correct
amount of roughness in the right places. We can do this at an engine inlet,
for instance, in order to trip the boundary layer and to have stable engine
performance."
 
"If we can understand the laminar-turbulent transition mechanism, we can
predict the transition point accurately," Tumin said. "This is important for
heat protection, where you want laminar flow. Otherwise, you need to add a
lot of weight for thermal insulation because you have to assume turbulent
flow at the surface when you do your design calculations. Similarly, engine
designers would like to have a quick transition to turbulence to have a
turbulent flow at an engine inlet."
 
Ultimately, better understanding the transition to turbulence at hypersonic
speeds will allow designers to build lighter, faster, more efficient
airplanes capable of traveling at even higher speeds of Mach 15 or more.
 
Received on Wed 16 Mar 2005 05:11:48 PM PST