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Experimental Investigations of Hypersonic Boundary Layer Transition on a Re-entry Capsule
von Syed Raza Christopher AliAbstract
In general, a capsule boundary layer flow is strongly accelerated and has a
favorable pressure gradient. Thus, the boundary layer is stable against superimposed
freestream disturbances, and modal instabilities can only occur
at significantly higher Re/l, which normally cannot be operated in conventional
facilities. Capsules goes for transition as far as the boundary layer
suffers from a critical amount of disturbance energy in the form of freestream
noise, impacts of particles or roughness-induced disturbance energy. Therefore,
this thesis aims at a systematic parametric scanning of possible flow
parameters influencing the boundary layer transition on a capsule.
In the Hypersonic Ludwieg Tube in Braunschweig (HLB), different versions
of the Apollo capsule model are operated. Measurements taken with a
coated capsule surface provide two sets of IR data. These prove an essential
role of distributed surface roughness. The surface roughness characterization
confirmed different mean surface roughness heights with respect to the
Ra parameter, which is in the range of several micrometers. The noise level
of the incoming freestream is assumed to be another important parameter
in the transition process.
Model instabilities can be successfully detected by surface sensors located
close behind a single roughness element. With regard to spatial distribution
predicted by stability calculations, the experimental methods can be
used to check the spanwise and the streamwise extent of so-called high-speed
streaks originating from the vortex system near the disturbance element.
Unfortunately, the sampled data do not allow the calculation of amplification
factors for the observed instabilities since the sensors also measure too
high surface pressure caused by tunnel noise, which is negatively amplified
when passing the shock and entering the boundary layer.
The examination of deterministically distributed surface roughness takes
into account the previous investigations with IR-measurement technique.
Moreover, the stochastically distributed roughness mimics the most realistic
state of the re-entry capsule in flight. Flush-mounted heat flux sensors
identified the transition that only occurs with subcritical ordered roughness
elements if they are exposed to sufficient freestream noise. Transition experiments
with stochastically distributed roughness help to experimentally
substantiate the promising method of predicting the transition for blunt
bodies with roughness. These measurements are carried out without any
significant influence of tunnel noise as the roughness heights dominated the
transition process. The roughness applied on the entire surface shows good
agreement with the available correlation. Since the roughness location and
spatial extent are varied, the agreement with the correlation decreases, and
the limitation of the correlation is demonstrated.
In general, a capsule boundary layer flow is strongly accelerated and has a
favorable pressure gradient. Thus, the boundary layer is stable against superimposed
freestream disturbances, and modal instabilities can only occur
at significantly higher Re/l, which normally cannot be operated in conventional
facilities. Capsules goes for transition as far as the boundary layer
suffers from a critical amount of disturbance energy in the form of freestream
noise, impacts of particles or roughness-induced disturbance energy. Therefore,
this thesis aims at a systematic parametric scanning of possible flow
parameters influencing the boundary layer transition on a capsule.
In the Hypersonic Ludwieg Tube in Braunschweig (HLB), different versions
of the Apollo capsule model are operated. Measurements taken with a
coated capsule surface provide two sets of IR data. These prove an essential
role of distributed surface roughness. The surface roughness characterization
confirmed different mean surface roughness heights with respect to the
Ra parameter, which is in the range of several micrometers. The noise level
of the incoming freestream is assumed to be another important parameter
in the transition process.
Model instabilities can be successfully detected by surface sensors located
close behind a single roughness element. With regard to spatial distribution
predicted by stability calculations, the experimental methods can be
used to check the spanwise and the streamwise extent of so-called high-speed
streaks originating from the vortex system near the disturbance element.
Unfortunately, the sampled data do not allow the calculation of amplification
factors for the observed instabilities since the sensors also measure too
high surface pressure caused by tunnel noise, which is negatively amplified
when passing the shock and entering the boundary layer.
The examination of deterministically distributed surface roughness takes
into account the previous investigations with IR-measurement technique.
Moreover, the stochastically distributed roughness mimics the most realistic
state of the re-entry capsule in flight. Flush-mounted heat flux sensors
identified the transition that only occurs with subcritical ordered roughness
elements if they are exposed to sufficient freestream noise. Transition experiments
with stochastically distributed roughness help to experimentally
substantiate the promising method of predicting the transition for blunt
bodies with roughness. These measurements are carried out without any
significant influence of tunnel noise as the roughness heights dominated the
transition process. The roughness applied on the entire surface shows good
agreement with the available correlation. Since the roughness location and
spatial extent are varied, the agreement with the correlation decreases, and
the limitation of the correlation is demonstrated.