High-speed unsteady flows around spiked-blunt bodies

2009 ◽  
Vol 632 ◽  
pp. 69-96 ◽  
Author(s):  
ARGYRIS G. PANARAS ◽  
DIMITRIS DRIKAKIS
Keyword(s):  
Flow Field ◽  
Unsteady Flow ◽  
High Speed ◽  
Initial Conditions ◽  
Separation Bubble ◽  
Spike Length ◽  
Initial Flow ◽  
Flow Development ◽  
Stable Conditions ◽  
Blunt Bodies

This paper presents a detailed investigation of unsteady supersonic and hypersonic flows around spiked-blunt bodies, including the investigation of the effects of the flow field initialization on the flow results. Past experimental research has shown that if the geometry of a spiked-blunt body is such that a shock formation consisting of an oblique foreshock and a bow aftershock appears, then the flow may be unsteady. The unsteady flow is characterized by periodic radial inflation and collapse of the conical separation bubble formed around the spike (pulsation). Beyond a certain spike length the flow is ‘stable’, i.e. steady or mildly oscillating in the radial direction. Both unsteady and ‘stable’ conditions have been reported when increasing or decreasing the spike length during an experimental test and, additionally, hysteresis effects have been observed. The present study reveals that for certain geometries the numerically simulated flow depends strongly on the assumed initial flow field, including the occurrence of bifurcations due to inherent hysteresis effects and the appearance of unsteady flow modes. Computations using several different configurations reveal that the transient (initial) flow development corresponds to a nearly inviscid flow field characterized by a foreshock–aftershock interaction. When the flow is pulsating, the further flow development is not sensitive to initial conditions, whereas for an oscillating or almost ‘steady’ flow, the flow development depends strongly on the assumed initial flow field.

Critical microjets in collapsing cavities

1995 ◽  
Vol 290 ◽  
pp. 183-201 ◽  
Author(s):  
Michael S. Longuet-Higgins ◽  
Hasan Oguz
Keyword(s):  
Flow Field ◽  
Spherical Harmonic ◽  
Velocity Potential ◽  
Initial Conditions ◽  
Numerical Study ◽  
Critical Flow ◽  
Jet Formation ◽  
Initial Flow ◽  
The Family ◽  
Self Similar

Inward microjets are commonly observed in collapsing cavities, but here we show that jets with exceptionally high velocities and accelerations occur in certain critical flows dividing jet formation from bubble pinch-off. An example of the phenomenon occurs in the family of flows which evolve from a certain class of initial conditions: the initial flow field is that due to a moving point sink within the cavity.A numerical study of the critical flow shows that in the neighbourhood of microjet formation the flow is self-similar. The local accelerations, velocities and distances scale as tβ-2, tβ-1 and tβ respectively, where β = 0.575. The velocity potential is approximately a spherical harmonic of degree ¼.

scholarly journals Bubble pinch-off in turbulence

2019 ◽  
Vol 116 (51) ◽  
pp. 25412-25417 ◽  
Author(s):  
Daniel J. Ruth ◽  
Wouter Mostert ◽  
Stéphane Perrard ◽  
Luc Deike
Keyword(s):  
Flow Field ◽  
Initial Conditions ◽  
The Self ◽  
Similar Process ◽  
Linear Perturbation ◽  
Bubble Shape ◽  
Complex Flow ◽  
Experimental Conditions ◽  
Initial Flow ◽  
Self Similar

Although bubble pinch-off is an archetype of a dynamical system evolving toward a singularity, it has always been described in idealized theoretical and experimental conditions. Here, we consider bubble pinch-off in a turbulent flow representative of natural conditions in the presence of strong and random perturbations, combining laboratory experiments, numerical simulations, and theoretical modeling. We show that the turbulence sets the initial conditions for pinch-off, namely the initial bubble shape and flow field, but after the pinch-off starts, the turbulent time at the neck scale becomes much slower than the pinching dynamics: The turbulence freezes. We show that the average neck size,d¯, can be described byd¯∼(t−t0)α, wheret0is the pinch-off or singularity time andα≈0.5, in close agreement with the axisymmetric theory with no initial flow. While frozen, the turbulence can influence the pinch-off through the initial conditions. Neck shape oscillations described by a quasi–2-dimensional (quasi-2D) linear perturbation model are observed as are persistent eccentricities of the neck, which are related to the complex flow field induced by the deformed bubble shape. When turbulent stresses are less able to be counteracted by surface tension, a 3-dimensional (3D) kink-like structure develops in the neck, causingd¯to escape its self-similar decrease. We identify the geometric controlling parameter that governs the appearance of these kink-like interfacial structures, which drive the collapse out of the self-similar route, governing both the likelihood of escaping the self-similar process and the time and length scale at which it occurs.

Numerical Investigation on Unsteady Flow Field of the Forward-Facing Cavity and Opposing Jet Combine Thermal Protection System

2014 ◽  
Vol 1022 ◽  
pp. 372-375 ◽  
Author(s):  
Hai Bo Lu
Keyword(s):  
Flow Field ◽  
Unsteady Flow ◽  
Stream Flow ◽  
High Speed ◽  
Free Stream ◽  
Thermal Protection ◽  
Periodic Variation ◽  
Thermal Protection System ◽  
Protection System ◽  
Stream Condition

In this paper, the unsteady flow field formed by the hemisphere nose-tip with forward-facing cavity and opposing jet combined thermal protection system (TPS) under high speed free stream flow (Ma4.98) was investigated numerically. The periodic variation of the drag coefficient and the flow field of the nose-tip were obtained. The numerical results show that a suitable cooperation between the opposing jet stream condition and the physical dimension of the cavity is necessary.

Influence of an Adaptive Blade System on Unsteady Flow Conditions in a 2D Compressor Cascade

2017 ◽  
Author(s):  
Julija Peter ◽  
David Konstantin Tilcher ◽  
Robert Meyer ◽  
Paul Uwe Thamsen
Keyword(s):  
Phase Shift ◽  
Flow Field ◽  
Unsteady Flow ◽  
High Speed ◽  
Optical Measurement ◽  
Water Channel ◽  
Tip Clearance ◽  
Flow Conditions ◽  
Compressor Cascade ◽  
Flow Angle

The flow field inside a compressor is characterized by highly unsteady flow effects. Consequently, the performance of a compressor is significantly influenced by the complex flow field. Especially at off-design conditions, flow separation and tip clearance flow cause vortex structures and thus increased losses. The objective of this paper is to give an insight into the effect mechanism of the movable stator vanes as an adaptive system to affect unsteady flow conditions. The experiments were conducted in a stator cascade in a water channel at a Reynolds number of Re = 500 000. Inlet guide vanes with movable flaps were used to simulate the periodic variation of the inlet flow angle. As parameters, the mean stagger angle of the stator cascade as well as the phase shift between the sinusoidal movement of the stator and the inlet guide cascade were varied. By using the optical measurement technique High-Speed Particle Image Velocimetry (HS-PIV), the flow fields upstream and downstream of the stator cascade were captured. Overall, the results revealed that the loss coefficient is strongly dependent on the phase shift between the inlet guide cascade and the stator cascade. Using certain phase shifts, a reduction in losses of up to 20% was achieved by the movable stator cascade.

Meshing Method of Calculating Water-Entry Flow Field at High Speed

2013 ◽  
Vol 300-301 ◽  
pp. 1144-1147
Author(s):  
Zhu Zhu ◽  
Xu Long Yuan ◽  
Ya Dong Wang ◽  
Yun Ju Yan
Keyword(s):  
Numerical Simulation ◽  
Flow Field ◽  
Unsteady Flow ◽  
High Speed ◽  
Great Influence ◽  
Water Entry ◽  
Unsteady Process ◽  
Natural Cavitation ◽  
Multi Phase

An important part of the numerical simulation is the grid which the quality has great influence on the calculation precision, and also the influence often is crucial factor in most of situation. Water-entry at high speed is a complex unsteady process, and its numerical simulation needs to take consider of natural cavitation as well as rotation of the underwater body. In this paper, a new meshing method was given with using the Layering, Smoothing and Remeshing for calculating the unsteady flow field. Numerical simulation shows that the mesh given in this paper has better quality, and can be used to calculate the multi-phase mode of water-entry at the high speed.

Numerical Study of Oscillation Mechanism in Underexpanded Jet Impinging on Plate

1998 ◽  
Vol 120 (3) ◽  
pp. 477-481 ◽  
Author(s):  
Y. Sakakibara ◽  
J. Iwamoto
Keyword(s):  
Flow Field ◽  
Unsteady Flow ◽  
Flat Plate ◽  
Numerical Study ◽  
Separation Bubble ◽  
Underexpanded Jet ◽  
Oscillation Mechanism ◽  
Pressure Changes ◽  
The Waves

The mechanism of the oscillatory phenomena of an underexpanded jet impinging on a flat plate is studied numerically. Pressure changes generated in the flow field near the plate propagate radially in the surrounding region of the jet. The configuration of the jet boundary is changed by them and so, the waves forming the underexpanded jet are displaced when they are reflected from the jet boundary. And then, the pressure disturbances return to the region near the plate. Unsteady flow with repetition of growth and decay of the separation bubble on the plate is also found under certain conditions.

Hot-Wire Measurements in a High Speed Counter Rotating Turbo Fan Rig

2010 ◽  
Author(s):  
Robert Meyer ◽  
Karsten Knobloch ◽  
Johannes Linden
Keyword(s):  
Spectral Analysis ◽  
Flow Field ◽  
Unsteady Flow ◽  
Rotor Blade ◽  
High Speed ◽  
Flow Structures ◽  
Hot Wire ◽  
Rotating Blades ◽  
Mean Vectors ◽  
German Aerospace

The German Aerospace Center (DLR) has measured the unsteady flow field of a high speed Contra-Rotating Turbo Fan (CRTF). In order to perform the measurement, hot-wire probes were placed upstream of the first rotor, between both rotors and downstream of the second rotor. With the DLR hot-wire measuring technique, unsteady flow structures (e.g. the wake of each individual rotor blade) were measured and analyzed in detail. The averaged mean vectors as well as the fluctuation components of the velocity were determined. The calculated local turbulence distribution of the flow downstream of the rotating blades shows an increase of the turbulence intensity of 3% in the wake. With the phase-locked averaging and the spectral analysis an upstream effect of the rotor 2 was verified.

Experimental and Numerical Investigation of Cavitation Regimes and Its Effects on Drag Characteristics of Wedge Shaped Bodies

2007 ◽  
Author(s):  
M. Osta ◽  
H. Mansouri ◽  
A. M. Razmi ◽  
M. A. Amini
Keyword(s):  
High Pressure ◽  
Flow Field ◽  
Unsteady Flow ◽  
Incompressible Flow ◽  
High Speed ◽  
Numerical Models ◽  
High Speed Camera ◽  
Cavitation Inception ◽  
Cavitation Phenomenon ◽  
Two Bodies

Cavitation phenomenon is defined as the process of rupturing any liquid by a decrease in pressure at nearly constant temperature. The cavities driven by the flow in a region of high pressure will implode and generate high pressure pulses leading eventually to erosion and vibration. But in supercavitation the bubbles produced by cavitation combine to form a large, stable bubble region around the supercavitating object. This phenomenon decreases the drag on the supercavitating body. Experimental testsware performed at 2-D unsteady flow for two wedge shaped bodies made before in laboratory and cavitation inception and its development were captured by a high speed camera. Then this cavitation regime around the wedge was studied numerically. In these cases CFD code was developed to simulate the unsteady and incompressible flow based on finite volume, 2D transient, with different boundary conditions. These numerical models which were evaluated experimentally depicted the capabilities of this CFD code to simulate this flow field cavitation inception, its development, and drag force in all cases. In this study we worked on two different geometries. Whether the cavitation is occurred at nose of body or not is worthy and studied by the above mentioned scheme. Moreover, we wanted to find the supercavitation regime and drag reduction for these two bodies.

Numerical Analysis of Aerodynamic Noise of a New High-Speed Train

2013 ◽  
Vol 275-277 ◽  
pp. 681-686 ◽  
Author(s):  
Wei Chen ◽  
Song Ping Wu
Keyword(s):  
Flow Field ◽  
Unsteady Flow ◽  
Body Surface ◽  
High Speed ◽  
Source Model ◽  
Broadband Noise ◽  
Physical Models ◽  
Aerodynamic Noise ◽  
High Speed Train ◽  
Car Body

Along with the raising of the train speed, aerodynamic noise of the high-speed train is generated more and more significantly and their reduction has become one of the key factors to control noise of the high-speed train. Aerodynamic noise radiated from the high-speed train surface was analyzed numerically. The mathematical and physical models of the three dimensional flow field of the high-speed train were established and the external steady and unsteady flow fields of the high-speed train were calculated by using the standard "k-ε" turbulence model and large eddy simulation (LES) respectively. On the basis of the steady flow field, aerodynamic noise sources on the car body surface of the high-speed train are calculated by using the broadband noise source model. On the basis of the unsteady flow field, the time domain characteristics of fluctuating pressures on the car body surface are analyzed. The sound pressure level on the surface pressure demonstrating is calculated and the flow field of some critical parts is analyzed.

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