Studies on Characteristic Frequency and Length Scale of Shock Induced Motion in Transonic Diffuser Using a High Order LES Approach

2015 ◽  
Author(s):  
Debasish Biswas ◽  
Tomohiko Jimbo
Keyword(s):  
Boundary Layer ◽  
Mach Number ◽  
Unsteady Flow ◽  
Separation Bubble ◽  
Pressure Fluctuations ◽  
Flow Characteristics ◽  
Propulsion Systems ◽  
Shock Oscillation ◽  
Shock Location ◽  
Suction Slot

Unsteady transonic flows in diffuser have become increasingly important, because of its application in new propulsion systems. In the development of supersonic inlet, air breathing propulsion systems of aircraft and missiles, detail investigations of these types of flow behavior are very much essential. In these propulsion systems, naturally present self-sustaining oscillations, believed to be equivalent to dynamically distorted flow fields in operational inlets, were found under all operating conditions. The investigations are also relevant to pressure oscillations known to occur in ramjet inlets in response to combustor instabilities. The unsteady aspects of these flows are important because the appearance of undesirable fluctuations generally impose limitation on the inlet performance. Test results of ramjet propulsion systems have shown undesirable high amplitude pressure fluctuations caused by the combustion instability. The pressure fluctuations originated from the combustor extend forward into the inlet and interact with the diffuser flow-field. Depending on different parameters such as the diffuser geometry, the inlet/exit pressure ratio, the flow Mach number, different complicated phenomena may occur. The most important characteristics are the occurrence of shock induced separation, the length of separation region downstream of the shock location, and the oscillation of shock location as well as the oscillation of the whole downstream flow. Sajben experimentally investigated in detail the time mean and unsteady flow characteristics of supercritical transonic diffuser as a function of flow Mach number upstream the shock location and diffuser length. The flows exhibited features similar to those in supersonic inlets of air-breathing propulsion systems of aircraft. A High-order LES turbulence model developed by the author is assessed with experimental data of Sajben on the self-excited shock oscillation phenomena. The whole diffuser model configuration including the suction slot located at certain axial location around the bottom and side walls to remove boundary layer, are included in the present computation model. The time-mean and unsteady flow characteristics in this transonic diffuser as a function of flow Mach number and diffuser length are investigated in detail. The results of study showed that in the case of shock-induced separation flow, the length and thickness of the reverse flow region of the separation-bubble change, as the shock passed through its cycle. The instabilities in the separated layer, the shock /boundary layer interaction, the dynamics of entrainment in the separation bubble, and the interaction of the travelling pressure wave with the pressure fluctuation region caused by the step-like structure of the suction slot play very important role in the shock-oscillation frequency.

EFFECTS OF BOUNDARY-LAYER THICKNESS ON UNSTEADY FLOW CHARACTERISTICS INSIDE OPEN CAVITIES AT SUBSONIC AND TRANSONIC SPEEDS

2012 ◽  
Vol 19 ◽  
pp. 206-213
Author(s):  
DANG-GUO YANG ◽  
JIAN-QIANG LI ◽  
ZHAO-LIN FAN ◽  
XIN-FU LUO
Keyword(s):  
Boundary Layer ◽  
Unsteady Flow ◽  
Layer Thickness ◽  
Boundary Layer Thickness ◽  
Sound Pressure ◽  
Flow Characteristics ◽  
Open Cavity ◽  
Aerodynamic Noise ◽  
Sustained Oscillation ◽  
Cavity Depth

An experimental study was conducted in a 0.6m by 0.6m wind-tunnel to analyze effects of boundary-layer thickness on unsteady flow characteristics inside a rectangular open cavity at subsonic and transonic speeds. The sound pressure level (SPL) distributions at the centerline of the cavity floor and Sound pressure frequency spectrum (SPFS) characteristics on some measurement positions presented herein was obtained with cavity length-to-depth ratio (L/D) of 8 over Mach numbers (Ma) of 0.6 and 1.2 at a Reynolds numbers (Re) of 1.23 × 107 and 2.02 × 107 per meter under different boundary-layer thickness to cavity-depth ratios (δ/D). The experimental angle of attack, yawing and rolling angles were 0°. The results indicate that decrease in δ/D leads to severe flow separation and unsteady pressure fluctuation, which induces increase in SPL at same measurement points inside the cavity at Ma of 0.6. At Ma of 1.2, decrease in δ/D results in enhancing compressible waves. Generally, decrease in δ/D induces more flow self-sustained oscillation frequencies. It also makes severer aerodynamic noise inside the open cavity.

scholarly journals Numerical study of unsteady flow and exciting force for swept turbomachinery blades

2016 ◽  
Vol 20 (suppl. 3) ◽  
pp. 669-676
Author(s):  
Di Zhang ◽  
Ma Jiao-Bin ◽  
Qi Jing
Keyword(s):  
Boundary Layer ◽  
Unsteady Flow ◽  
Pressure Fluctuation ◽  
Numerical Study ◽  
Flow Characteristics ◽  
Aerodynamic Performance ◽  
Rotor Blades ◽  
Stable Operation ◽  
Straight Blade ◽  
Excitation Force

The aerodynamic performance of blade affects the vibration characteristics and stable operation of turbomachinery closely. The aerodynamic performance of turbine stage can be improved by using swept blade. In this paper, the RANS method and the RNG k-? turbulence mode were adopted to investigate the unsteady flow characteristics and excitation force of swept blade stage. According to the results, for the swept blade, the fluid of boundary layer shifts in radial direction due to the influence of geometric construction. It is observed that there is similar wake development for several kinds of stators, and the wake has a notable effect on the boundary layer of the rotor blades. When compared with straight blade, pressure fluctuation of forward-swept blade is decreased while the pressure fluctuation of backward-swept blade is increased. The axial and tangential fundamental frequency excitation force factors of 15?forward-swept blade are 0.139 and 0.052 respectively, which are the least, and all excitation force factors are in the normal range. The excitation factor of the forward-swept blade is decreased compared with straight blade, and the decreasing percentage is closely related to the swept angle. As for backward-swept blades, the situation is the other way around. Additionally, the change of axial excitation factor is more obvious. So the vibration reduction performance of forward-swept blade is better.

scholarly journals Large-eddy simulation of laminar transonic buffet

2018 ◽  
Vol 850 ◽  
pp. 156-178 ◽  
Author(s):  
Julien Dandois ◽  
Ivan Mary ◽  
Vincent Brion
Keyword(s):  
Boundary Layer ◽  
Large Eddy Simulation ◽  
Separation Bubble ◽  
Oscillation Amplitude ◽  
Boundary Layer Separation ◽  
Stream Velocity ◽  
Eddy Simulation ◽  
Large Eddy ◽  
Shock Oscillation ◽  
Transonic Buffet

A large-eddy simulation of laminar transonic buffet on an airfoil at a Mach number $M=0.735$, an angle of attack $\unicode[STIX]{x1D6FC}=4^{\circ }$, a Reynolds number $Re_{c}=3\times 10^{6}$ has been carried out. The boundary layer is laminar up to the shock foot and laminar/turbulent transition occurs in the separation bubble at the shock foot. Contrary to the turbulent case for which wall pressure spectra are characterised by well-marked peaks at low frequencies ($St=f\cdot c/U_{\infty }\simeq 0.06{-}0.07$, where $St$ is the Strouhal number, $f$ the shock oscillation frequency, $c$ the chord length and $U_{\infty }$ the free-stream velocity), in the laminar case, there are also well-marked peaks but at a much higher frequency ($St=1.2$). The shock oscillation amplitude is also lower: 6 % of chord and limited to the shock foot area in the laminar case instead of 20 % with a whole shock oscillation and intermittent boundary layer separation and reattachment in the turbulent case. The analysis of the phase-averaged fields allowed linking of the frequency of the laminar transonic buffet to a separation bubble breathing phenomenon associated with a vortex shedding mechanism. These vortices are convected at $U_{c}/U_{\infty }\simeq 0.4$ (where $U_{c}$ is the convection velocity). The main finding of the present paper is that the higher frequency of the shock oscillation in the laminar regime is due to a different mechanism than in the turbulent one: laminar transonic buffet is due to a separation bubble breathing phenomenon occurring at the shock foot.

Aerodynamic Investigation of a Passive Secondary Jet Exciting a Separation Bubble for a Blunt Flat Plate

2015 ◽  
Author(s):  
Christian Helcig ◽  
Stefan aus der Wiesche ◽  
Stephan Uhkoetter
Keyword(s):  
Boundary Layer ◽  
Laser Doppler Anemometry ◽  
Separation Bubble ◽  
Flow Characteristics ◽  
Numerical Procedure ◽  
Large Eddy Simulations ◽  
Reattachment Length ◽  
Large Eddy ◽  
Low Speed Wind Tunnel ◽  
Aerodynamic Investigation

The aim of this study is to examine the influence of passive jets interacting with the separation region of the flow around a blunt plate. Experimental and numerical analysis are used to measure the velocity within the separation and reattachment region of the blunt plate with different passive jet configurations. A blunt plate was placed in a low speed wind tunnel to conduct Laser-Doppler anemometry (LDA) measurements at Re = 2.06 × 104. For the numerical procedure a dynamical sub-grid model for Large Eddy Simulations (LES) was used. For all configurations the flow characteristics such as the reattachment length were determined to characterize the boundary layer. The passive jets showed a strong influence by interacting with the boundary layer of the blunt plate.

Experimental Study on Pressure Fluctuations in the Boundary Layer of an Axisymmetric Body

2011 ◽  
Vol 66-68 ◽  
pp. 1488-1493
Author(s):  
Hong Xiao ◽  
Chao Gao ◽  
Zhen Kun Ma
Keyword(s):  
Boundary Layer ◽  
Experimental Study ◽  
Mach Number ◽  
Dynamic Pressure ◽  
Pressure Fluctuations ◽  
Axisymmetric Body ◽  
Pressure Measurements ◽  
Fluctuating Pressure ◽  
Frequency Domains ◽  
Dynamic Pressure Measurements

The characteristics of the fluctuating pressure in the boundary layer of an axisymmetric body have been investigated experimentally using dynamic pressure measurements and Schlieren photograghs. Data were acquired at subsonic and super-sonic Mach numbers. The angles of attack ranged from 0° to 5°. Pressure signals were measured simultaneously in several positions along the model and were analyzed both in the time and frequency domains. The Mach number shows the relevant influence on . Furthermore, the pressure fluctuations’ level decreases with the increasing of Mach number except M=1.15. And it is shown that, the location along the axis of the model and the angles of attack have small effect on pressure fluctuations.

Study on Flow Characteristics Downstream of Annular Inlet Guide Vanes

2013 ◽  
Author(s):  
Masanori Kudo ◽  
Koichi Nishibe ◽  
Masayuki Takahashi ◽  
Kotaro Sato ◽  
Yoshinobu Tsujimoto
Keyword(s):  
Unsteady Flow ◽  
Flow Instability ◽  
Velocity Ratio ◽  
Pressure Fluctuations ◽  
Flow Characteristics ◽  
Flow Instabilities ◽  
Guide Vanes ◽  
Inlet Guide Vanes ◽  
Flow Oscillations ◽  
Measured Pressure

The main objectives of the present study are to identify the dominant parameters responsible for the generation of unsteady flow, determine the conditions under which flow oscillations are produced and the relation between the flow characteristics and the number of vanes with identical solidity. The flow instabilities downstream of inlet guide vanes (IGV) are clarified experimentally and by numerical simulation. The conditions for the onset of flow instability, including the number of cells and the oscillation characteristics of the unsteady flow, are discussed based on measured pressure fluctuations and the propagating angular velocity ratio of the instability for various radius ratios (r3/r2). The effectiveness of adjusting the number of vanes to control the flow instabilities is also discussed.

Unsteady Surface Pressures Due to Wake Induced Transition in a Laminar Separation Bubble on a LP Turbine Cascade

2003 ◽  
Author(s):  
Rory Stieger ◽  
David Hollis ◽  
Howard Hodson
Keyword(s):  
Boundary Layer ◽  
Steady Flow ◽  
Coherent Structures ◽  
Separation Bubble ◽  
Pressure Fluctuations ◽  
Separation Point ◽  
Blade Surface ◽  
Turbine Cascade ◽  
Lp Turbine ◽  
Wake Passing

This paper presents unsteady surface pressures measured on the suction surface of a LP turbine cascade that was subject to wake passing from a moving bar wake generator. The surface pressures measured under the laminar boundary layer upstream of the steady flow separation point were found to respond to the wake passing as expected from the kinematics of wake convection. In the region where a separation bubble formed in steady flow, the arrival of the convecting wake produced high frequency, short wavelength, fluctuations in the ensemble averaged blade surface pressure. The peak-to-peak magnitude was 30% of the exit dynamic head. The existence of fluctuations in the ensemble averaged pressure traces indicates that they are deterministic and that they are produced by coherent structures. The onset of the pressure fluctuations was found to lie beneath the convecting wake and the fluctuations were found to convect along the blade surface at half of the local freestream velocity. Measurements performed with the boundary layer tripped ahead of the separation point showed no oscillations in the ensemble average pressure traces indicating that a separating boundary layer is necessary for the generation of the pressure fluctuations. The coherent structures responsible for the large amplitude pressure fluctuations were identified using PIV to be vortices embedded in the boundary layer. It is proposed that these vortices form in the boundary layer as the wake passes over the inflexional velocity profiles of the separating boundary layer and that the rollup of the separated shear layer occurs by an inviscid Kelvin-Helmholtz mechanism.

Cavitation-Induced Unsteady Flow Characteristics in the First Stage of a Centrifugal Charging Pump

2016 ◽  
Vol 139 (1) ◽  
Author(s):  
Fan Zhang ◽  
Shouqi Yuan ◽  
Qiang Fu ◽  
Ji Pei ◽  
Martin Böhle ◽  
...  
Keyword(s):  
Unsteady Flow ◽  
Nuclear Power ◽  
Power Plants ◽  
Pressure Fluctuations ◽  
Flow Characteristics ◽  
Absolute Error ◽  
Impeller Blade ◽  
Frequency Domains ◽  
Vapor Distribution ◽  
Main Factor

Cavitation is the main factor that causes reliability problems in centrifugal charging pumps (CCPs) in nuclear power plants. In this study, the cavitation-induced unsteady flow characteristics of a CPR1000 CCP were investigated by numerical and experimental methods. The vapor distribution in the impeller, velocity fluctuation, and pressure fluctuation results in the time and frequency domains were considered for several typical monitoring points in the impeller and volute. The pressure fluctuations in the impeller occurred at an impeller rotating frequency of fR and its integer harmonics, whereas those in the volute mainly occurred at an impeller blade-passing frequency of fB and its integer harmonics. The absolute error between the simulated and measured NPSHr was 3.6%, and that between the calculated and measured head was 2.9%, validating the simulation of the cavitation performances of a CCP.

Mechanism of drag reduction by a surface trip wire on a sphere

2011 ◽  
Vol 672 ◽  
pp. 411-427 ◽  
Author(s):  
KWANGMIN SON ◽  
JIN CHOI ◽  
WOO-PYUNG JEON ◽  
HAECHEON CHOI
Keyword(s):  
Boundary Layer ◽  
Drag Reduction ◽  
Separation Bubble ◽  
Flow Characteristics ◽  
Reynolds Numbers ◽  
Transition To Turbulence ◽  
Stream Velocity ◽  
Sphere Diameter ◽  
Sphere Surface ◽  
Streamwise Location

The effect of a surface trip wire on the flow around a sphere is experimentally investigated at subcritical Reynolds numbers of Re = 0.5 × 105 – 2.8 × 105 based on the free-stream velocity U∞ and sphere diameter d. By varying the streamwise location (20° – 70° from the stagnation point) and diameter (0.33 × 10−2 < k/d < 1.33 × 10−2) of a trip wire, we measure the drag, surface pressure distribution and boundary layer velocity profiles above the sphere surface, and conduct flow visualization. Depending on the size and streamwise location of the trip wire, three different flow characteristics are observed above the sphere surface. For low Reynolds numbers, the disturbance induced by the trip wire decays downstream and main separation occurs at a streamwise location similar to that of a smooth sphere. As the Reynolds number is increased, laminar separation is delayed farther downstream by the disturbance from the trip wire and the transition to turbulence occurs along the separated shear layer, resulting in the flow reattachment to the sphere surface and thus forming a secondary separation bubble on the sphere surface. Then, the main separation is delayed due to high momentum near the surface and the drag is significantly reduced. When the trip wire produces even larger disturbances through the separation and reattachment right at the trip-wire location for higher Reynolds numbers, the boundary layer flow becomes turbulent soon after the trip-wire location and the main separation is delayed, resulting in drag reduction.

Role of Shock and Boundary Layer Separation on Unsteady Aerodynamic Characteristics of Oscillating Transonic Cascade

2003 ◽  
Author(s):  
Mizuho Aotsuka ◽  
Toshinori Watanabe ◽  
Yasuo Machina
Keyword(s):  
Boundary Layer ◽  
Pressure Fluctuations ◽  
Boundary Layer Separation ◽  
Aerodynamic Characteristics ◽  
Compressor Cascade ◽  
Layer Interaction ◽  
Unsteady Aerodynamic ◽  
Shock Boundary Layer Interaction ◽  
Boundary Layer Interaction ◽  
Shock Oscillation

The unsteady aerodynamic characteristics of an oscillating compressor cascade composed of Double-Circular-Arc airfoil blades were both experimentally and numerically studied under transonic flow conditions. The study aimed at clarifying the role of shock waves and boundary layer separation due to the shock boundary layer interaction on the vibration characteristics of the blades. The measurement of the unsteady aerodynamic moment on the blades was conducted in a transonic linear cascade tunnel using an influence coefficient method. The cascade was composed of seven DCA blades, the central one of which was an oscillating blade in a pitching mode. The unsteady moment was measured on the central blade as well as the two neighboring blades. The behavior of the shock waves was visualized through a schlieren technique. A quasi-three dimensional Navier-Stokes code was developed for the present numerical simulation of the unsteady flow fields around the oscillating blades. A k-ε turbulence model was utilized to adequately simulate the flow separation phenomena caused by the shock-boundary layer interaction. The experimental and numerical results complemented each other and enabled a detailed understanding of the unsteady aerodynamic behavior of the cascade. It was found that the surface pressure fluctuations induced by the shock oscillation were the governing factor for the unsteady aerodynamic moment acting on the blades. Such pressure fluctuations were primarily induced by the movement of impingement point of the shock on the blade surface. During the shock oscillation the separated region caused by the shock boundary layer interaction also oscillated along the blade surface, and induced additional pressure fluctuations. The shock oscillation and the movement of the separated region were found to play the principal role in the unsteady aerodynamic and vibration characteristics of the transonic compressor cascade.

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