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Physics and Control of Unstart in Supersonic
Inlet-Isolator Configurations

Ryan Henderson, Nicole Quindara, Justin L. Wagner, Agustin Valdivia,  K. Bulent Yuceil, Noel T. Clemens and David S. Dolling



The dual-mode engine concept allows an engine to act as a ramjet at lower supersonic flight Mach numbers and then transition to a scramjet at higher supersonic to hypersonic flight Mach numbers.  In dual-mode engines, the inlet and isolator provide the compression necessary for combustion.  If the combustion pressure becomes larger than the maximum that can be provided by the inlet / isolator combination, the process of inlet unstart will occur.  This transient process can result in large pressure loads, high drag and loss of combustion.  The physics of unstart are not well understood.  At our laboratory, unstart has been investigated using time resolved Schlieren combined with fast response pressure measurements. 

The approach is to use a simplified inlet / isolator model in a Mach 5 flow as shown in the schematic and photo below.  The model is mounted to the test section floor and has an aspect ratio (width to height) of 2.  Plexiglass isolator sidewalls provide optical access.  A flap at the rear of the model provides simulated combustion pressure rises.

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The figure below shows the pressure time history of the most downstream transducer (T1 above) during an unstart event.  The first pressure rise corresponds to unstart and the subsequent pressure peaks and valleys are a result of an oscillatory unstarted flow seen to occur with a frequency of about 124 Hz.

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Schlieren images with corresponding instantaneous pressure distributions during the transient unstart process are shown below.  The flow is from left to right and the white border represents the swept inlet entrance followed by the inlet / isolator floor line.  From top to bottom, the three images are at unstart times of 0, 4.38 and 7.38 ms.  The flow is visible in the isolator section of the model.  Initially at 0 ms, the model contains an oblique shock generated from the inlet ramp followed by three reflected shocks.  The three reflected shocks can be seen in the isolator.  A shock system at the rear of the isolator due to the flap can also be seen.  At 4.38 ms, the flap shock or unstart shock system has moved upstream of the isolator streamwise center location.  At 7.38 ms, the unstart shock system is near the entrance of the isolator and the isolator flow appears to be more separated.  A more complete picture of the unstart process can be seen with the 8 kHz Schlieren and simultaneous pressure distribution Unstart Process Video.

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After unstart, an oscillatory unstarted flow is always observed.  This complex flow appears to be driven by different mechanisms as demonstrated with the 8 kHz Oscillatory Unstarted Flow Video.

Ongoing Work

Ongoing work includes simultaneous PIV and pressure measurements for further understanding of the physics of unstart.  In addition, passive and active control techniques using vortex generators and vortex generator jets are being investigated.  This work is supported by the Air Force Office of Scientific Research under the Multidisciplinary Research Initiative (MURI) program.

 

Publications:

 

An Experimental Investigation of Supersonic Inlet Unstart, Wagner, J.L., Yuceil, K.B., Valdivia, A., Clemens, N.T., and Dolling, D.S., AIAA 2007-4352, 37th Fluid Dynamics Conference  & Exhibit, Jun. 2007, Miami, Fl, U.S.A



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