PhD Candidate: Rogier Giepman
Department: AWEP
Section:
Aerodynamics
Supervisors: B. van Oudheusden &
F. Schrijer
Promoter:
F. Scarano
Start date: 01-12-2011
Funding:
FP7
Flow Control for Oblique
Shock Wave Reflections
Background
Turbulent interaction controlled by microramp vortex generators
Shock wave / boundary layer interactions can be
encountered on transonic wings, on the turbine blades of
jet engines, in the inlet of a supersonic jet and in many
more situations. Because of their importance, they have
been investigated for over decades now, but still the
underlying processes are not fully understood and
controlling these interactions remains difficult. However,
new advances in experimental (Particle Image Velocimetry
/ Infrared Thermography) and numerical techniques (Large
Eddy Simulations), make it possible to resolve more details
than ever before and provide us with very detailed
information of the interaction process.
Supersonic mixed-compression intake
To investigate the effects of transition on the interaction,
one first has to determine the transition location itself,
which is not trivial in supersonic flows with boundary layers
less than 1 mm thick. Since, a turbulent boundary layer
has a higher rate of heat transfer than a laminar boundary
layer, this part of the plate will cool down faster in the
tunnel. This can be visualized by means of infrared
thermography and to increase the contrast, the plate is
heated to about 65 degrees before the run.
M<1
M>1
Effects of boundary layer transition on the
oblique shock wave reflection
Interaction topology
Before run
Particle Image Velocimetry
SR71 Blackbird jet intake
z
centre
25%
50%
U/U∞
My focus is on the oblique shock wave reflections occurring
in supersonic jet intakes. The shock poses a strong adverse
gradient and can cause the boundary layer to separate. A
separated boundary layer not only increases the drag of
the engine, but also reduces its performance and can in a
worst case scenario cause the engine to unstart.
Flow control strategies are therefore being developed to
reduce the amount of separation by stabilizing the
incoming boundary layer. Within my project two cases can
be distinguished:
x
Transition strips
The temperature distribution, however, provides a delayed
representation of the transition location. Techniques were
therefore developed to convert the temperature
distributions into a heat flux distribution, which does show
the instantaneous transition location. This delivers us the
transition location xT as a function of the temperature at
the transition location TT. A linear trend can be observed
between the two variables. The colder the plate, the more
downstream the transition location.
Along the centre line
• Laminar / transitional boundary layer: To reduce the
drag contribution of jet intakes it is beneficial to keep
the boundary layer laminar for as long as possible.
Laminar boundary layers are, however, also prone to
separation and are not suitable for passing through
shock waves. The question is therefore where and
how to trip the laminar boundary layer into a turbulent
state.
U/U∞
Sonic line
Separation bubble
• Turbulent boundary layer: Turbulent boundary layers
can also separate when the shock wave is strong
enough or when the flow has to pass through a series of
shocks. Micro-ramp vortex generators offer a promising
flow control device within this context. They create two
streamwise vortices that transport high-momentum fluid
towards the wall, creating a fuller and more stable
velocity profile.
U’/U∞
Aerospace Engineering
My focus
t=17 s
Mounting a micro-ramp (MR) upstream of the
interaction:
•
•
Micro-ramp vortex generator
•
The micro-ramp is most effective along its centre
line, at 50% span the flow behaves virtually as if
there is no ramp
On average there is no separation taking place
along the centre line. Instantaneously there still is,
however a factor of 4 less than without a ramp
Maximum turbulence intensity levels along the
centre line are reduced by 10-15%
Progress and Objectives
The experiments on the micro-ramp vortex generators are
completed and the challenge is now to analyze all the data.
The advantages of micro-ramps are clear in terms of
separation probabilities and turbulence intensities. The
exact working principles are, however, not yet fully
understood. I am currently working on correlating flow
features that enter the interaction with the instantaneous
amount of separation. If the working mechanism of these
ramps is better understood, then it should also be possible
to design a more efficient ramp.
Regarding the effects of transition on the interaction, the
basic elements are now in place. The models have been
designed, were mounted in the tunnel and tested
successfully. Methodologies were developed for processing
the infrared thermography data and at the moment I am
carrying out the first PIV experiments. Since the boundary
layer is very thin, difficulties are expected with the seeding
conditions, the time separation of the images and
reflections coming from the model. These challenges will
be dealt with in the coming 4 months.
Publications
- R.H.M. Giepman, G. de Giovanni, F.F.J. Schrijer and B.W. van Oudheusden, (2012) “Experimental Investigation into the Effects of Micro-Ramps on the Separation Bubble of Shock Wave /
Boundary Layer Interactions”, 20th International Shock Interaction Symposium, Stockholm, August 2012