Computation of flows in supersonic wind-tunnels

Int. J. Numer. Meth. Fluids

2006

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SUMMARYNumerical simulation of a two-dimensional mixed compression supersonic inlet is carried out by solving unsteady compressible Euler equations via a stabilized ÿnite element method. The geometry of the inlet is similar to the one used by Anderson and Wong for experimental investigation for Mach 3 ow. The computations are capable of simulating the start-up problems associated with the inlet. The critical back pressure for the successful operation of the inlet is computed. The e ect of inlet back pressure on the total pressure recovery and the ow distortion level is analysed. Contrary to the popular belief, it is found that in addition to the throat to inlet capture area ratio, the ramp geometry close to the throat plays an important role in the start-up dynamics. It is demonstrated via simulations that, everything else being same, the geometries of ramp upstream of the throat that are associated with a curvature higher than a certain threshold, result in unstarting the intake.

Section: The Governing Equations Letmentioning

confidence: 99%

Section: Introductionmentioning

confidence: 99%

Euler flow in a supersonic mixed-compression inlet

Jain

Int. J. Numer. Meth. Fluids

2006

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“…(12) are the shock-capturing terms that stabilize the computations in the presence of sharp gradients [15]. The stabilization coefficients τ and δ are the ones that were used in our earlier work [16,17,[23][24][25] and similar to those employed by Aliabadi and Tezduyar [26]. The application of this formulation to compute various internal and external flows can be found in our earlier papers [16,17,[23][24][25]27].…”

Section: A Flow Equationsmentioning

confidence: 99%

Flow in a Y-Intake at Supersonic Speeds

Kotteda

Journal of Propulsion and Power

2016

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Numerical simulation of the flow in a twin intake via solution to unsteady viscous compressible equations using a stabilized finite-element method has been carried out for various sideslip angles and backpressure ratios. Various flow regimes that are possible in the entire parameter space are identified. They are classified on the basis of shock structure and their locations and the unsteadiness in the flow. Buzz instability is observed in the intake at low system mass rates for nonzero sideslip angles. The onset of buzz is identified by zero mass flow rate in the leeward duct. It is associated with asymmetric movement of the bow shocks upstream of the duct entry. The onset of buzz is accompanied with a loss in static and total pressure recovery and increase in flow distortion. The effect of initial condition is studied. The flow in a certain regime is sensitive to the time history of the backpressure. Hysteresis in the flow as well as in the onset of buzz are observed. The effect of bleed is studied. The amount of bleed and the regions on which it is applied are varied to understand their effect on the performance of the intake. The onset of buzz is delayed with application of bleed.

“…The second series of element-level integrals in the formulation given by Equation (8) are the shock-capturing terms that stabilize the computations in the presence of sharp gradients [23]. The stabilization coefficients and s are the ones that were used in our earlier work [24,25,30] and similar to those employed by Aliabadi and Tezduyar [31]. They are defined as…”

Section: Finite-element Formulationmentioning

confidence: 99%

Viscous flow in a mixed compression intake

Kotteda

Numerical Methods in Fluids

2010

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Numerical simulation of the flow in a two-dimensional mixed compression intake is carried out by solving unsteady viscous compressible equations using a stabilized finite element method. The effect of bleed in starting/unstarting of the intake and controlling the buzz instability is investigated in detail. Higher bleed leads to an increase in the ability of the intake to sustain larger back-pressure for stable operation. The amount of bleed and its location is varied to understand its effect on the performance of the intake. Two kinds of unsteady oscillations are observed: 'little' and 'big' buzz. The frequency of the both kinds of buzz oscillations is found to be super-harmonic of the fundamental acoustic frequency of intake modeled as an open-closed organ pipe. The frequency as well as amplitudes of the big buzz cycles is larger than those of the little buzz. The little-and big-buzz are found to occur for low-and high-subcritical state of the intake and are very similar to Ferri and Dailey criteria, respectively. Buzz is eliminated when relatively high bleed is implemented, both, upstream and downstream of the throat. The effect of rate of change of back-pressure on the start/unstart of the intake is investigated. Two situations are considered. The first case is that of an intake where the back-pressure remains below the critical value. It is found that the intake remains started if the change in back-pressure is gradual. However, it unstarts if the back-pressure is changed relatively rapidly. The second set of simulations is an attempt to model the situation where the back-pressure at the exit of the intake exceeds the critical value and a logic is incorporated in the feed back loop of the fuel modulation to start the intake. Low rate of change of pressure is unsuccessful in starting the intake. Relatively high rates result in either a quick starting of the intake or a slow unstart. and forth in the convergent portion leading to high unsteadiness in the flow. It involves periodic filling and discharge of the plenum chamber, complex shock-boundary layer interaction, shear layer/slip stream-boundary layer interaction, transient shock movement and flow separation. It adversely affects the mass flow entering the engine and may lead to combustion instability, engine surge and flame out. It can also lead to deterioration of the performance of propulsion system, thus causing catastrophic loss in thrust.The buzz instability was first reported by Oswatitsch [3]. The tests were conducted for supersonic missile flights for Mach number in the range of 2.5-3.0. He observed buzz during his experiments, but disregarded it on the basis that a sub-critical range of operation is impractical and, therefore, not important. Later (1945)(1946)(1947)(1948)(1949)(1950)(1951)(1952)(1953)(1954)(1955), buzz was observed in many of the extensive experiments that were conducted at NASA, Langley on external compression inlets. Ferri and Nucci [4] conducted detailed experiments on an axisymmetrical external compression air intake. The occur...