Pressure fluctuations due to ‘trapped waves’ in the initial region of compressible jets

2021 ◽  
Vol 931 ◽  
Author(s):  
K.B.M.Q. Zaman ◽  
A.F. Fagan ◽  
P. Upadhyay
Keyword(s):  
Mach Number ◽  
Aspect Ratio ◽  
Pressure Fluctuations ◽  
Major Axis ◽  
Trapped Waves ◽  
Potential Core ◽  
Screech Tone ◽  
Aspect Ratios ◽  
Supersonic Regime ◽  
Different Diameters

An experimental study is conducted on unsteady pressure fluctuations occurring near the nozzle exit and just outside the shear layer of compressible jets. These fluctuations are related to ‘trapped waves’ within the jet's potential core, as investigated and reported recently by other researchers. Round nozzles of three different diameters and rectangular nozzles of various aspect ratios are studied. The fluctuations manifest as a series of peaks in the spectra of the fluctuating pressure. Usually the first peak at the lowest frequency (fundamental) has the highest amplitude and the amplitude decreases progressively for successive peaks at higher frequencies. These ‘trapped wave spectral peaks’ are found to occur with all jets at high subsonic conditions and persist into the supersonic regime. Their characteristics and variations with axial and radial distances, jet Mach number and aspect ratio of the nozzle are documented. For round nozzles, the frequency of the fundamental is found to be independent of the jet's exit boundary layer characteristics and scales with the nozzle diameter. On a Strouhal number (based on diameter) versus jet Mach number plot it is represented by a unique curve. Relative to the fundamental the frequencies of the successive peaks are found to bear the ratios of 5/3, 7/3, 9/3 and so on, at a given Mach number. For rectangular nozzles, the number of peaks observed on the major axis is found to be greater than that observed on the minor axis by a factor approximately equal to the nozzle's aspect ratio; the fundamental is the same on either edge. For all nozzles the onset of screech tones appears as a continuation of the evolution of these peaks; it is as if one of these peaks abruptly increases in amplitude and turns into a screech tone as the jet Mach number is increased.

scholarly journals Aspect Ratio Driven Relationship between Nozzle Internal Flow and Supersonic Jet Mixing

Aerospace ◽  
2021 ◽  
Vol 8 (3) ◽  
pp. 78
Author(s):  
Kalyani Bhide ◽  
Kiran Siddappaji ◽  
Shaaban Abdallah
Keyword(s):  
Boundary Layer ◽  
Aspect Ratio ◽  
Nozzle Exit ◽  
Supersonic Jet ◽  
Internal Flow ◽  
Shear Stresses ◽  
Loss Mechanism ◽  
Potential Core ◽  
Jet Mixing ◽  
Aspect Ratios

This work attempts to connect internal flow to the exit flow and supersonic jet mixing in rectangular nozzles with low to high aspect ratios (AR). A series of low and high aspect ratio rectangular nozzles (design Mach number = 1.5) with sharp throats are numerically investigated using steady state Reynolds-averaged Navier−Stokes (RANS) computational fluid dynamics (CFD) with k-omega shear stress transport (SST) turbulence model. The numerical shadowgraph reveals stronger shocks at low ARs which become weaker with increasing AR due to less flow turning at the throat. Stronger shocks cause more aggressive gradients in the boundary layer resulting in higher wall shear stresses at the throat for low ARs. The boundary layer becomes thick at low ARs creating more aerodynamic blockage. The boundary layer exiting the nozzle transforms into a shear layer and grows thicker in the high AR nozzle with a smaller potential core length. The variation in the boundary layer growth on the minor and major axis is explained and its growth downstream the throat has a significant role in nozzle exit flow characteristics. The loss mechanism throughout the flow is shown as the entropy generated due to viscous dissipation and accounts for supersonic jet mixing. Axis switching phenomenon is also addressed by analyzing the streamwise vorticity fields at various locations downstream from the nozzle exit.

The Effect of Aspect-Ratio on the Development of the Large-Scale Structures in the Three-Dimensional Wall Jet

2005 ◽  
Author(s):  
Joseph W. Hall ◽  
Daniel Ewing
Keyword(s):  
Aspect Ratio ◽  
Large Scale ◽  
Pressure Fluctuations ◽  
Three Dimensional ◽  
Decomposition Analysis ◽  
Wall Pressure ◽  
Symmetric Mode ◽  
Large Scale Structures ◽  
Aspect Ratios ◽  
Scale Structures

The development of the large-scale structures in three-dimensional wall jets exiting rectangular nozzles with aspect-ratios of 1 and 4 was investigated using simultaneous measurements of the fluctuating wall pressure across the jet. The pressure fluctuations in the jets were asymmetric and caused the fluctuating wall pressure to be poorly correlated across the jet centerline. A Proper Orthogonal Decomposition analysis indicated that both the first and second modes make similar contributions to the variance of the fluctuating pressure, and were symmetric and antisymmetric, respectively, and the interplay between these modes caused the asymmetry in the instantaneous pressure fluctuations across the jet centreline. A wavelet analysis of the instantaneously reconstructed pressure fields indicated that the fluctuations were predominantly in two frequency bands near the jet centerline, but were only contained in one band on the outer lateral edges of the jet, indicating there were two different large-scale motions present. The development of large-scale structures in the two jets initially differed in the intermediate field with the antisymmetric mode being more prominent in the square jet and the symmetric mode being more prominent in the larger aspect-ratio jet. Further downstream, the symmetric mode was more prominent in both jets.

Straight Channel Diffuser Performance at High Inlet Mach Numbers

1969 ◽  
Vol 91 (3) ◽  
pp. 397-412 ◽  
Author(s):  
P. W. Runstadler ◽  
R. C. Dean
Keyword(s):  
Mach Number ◽  
Aspect Ratio ◽  
Pressure Recovery ◽  
Straight Channel ◽  
Single Plane ◽  
Aspect Ratios ◽  
Straight Wall ◽  
Throat Width ◽  
Mach Numbers

Measurements have been made of the pressure recovery of straight wall, single plane divergence diffusers with inlet Mach numbers between 0.2 and choking (0.2 ≤ Mt < 1.0). In contrast to the widely held assertion in the literature, there is no “critical” inlet subsonic Mach number above which pressure recovery decreases drastically. Two aspect ratios, AS = 0.25 and 1.0, have been studied for a range of length-to-throat-width ratios L/W1 and divergence angles 2θ around the regions of peak recovery. Diffuser performance maps are given showing pressure recovery Cp as a function of diffuser geometry for fixed values of throat Mach number Mt, throat blockage B, and aspect ratio AS. Significant changes in the location and magnitude of pressure recovery do occur with variations in Mt, B, and AS. The importance to the designer of a knowledge of how diffuser performance depends upon geometric and diffuser inlet parameters is discussed.

Low Aspect Ratio Turbines

1964 ◽  
Vol 86 (1) ◽  
pp. 13-16 ◽  
Author(s):  
Gunnar O. Ohlsson
Keyword(s):  
Reynolds Number ◽  
Mach Number ◽  
Aspect Ratio ◽  
Axial Distance ◽  
Mass Rate ◽  
Low Aspect Ratio ◽  
Turbine Efficiency ◽  
Aspect Ratios

Four different axial, impulse turbines with extremely low aspect ratios (between 0.07 and 0.70) were tested over wide ranges of pressure and speed ratios. The influence on mass rate of flow and efficiency of Reynolds number and axial distance between stator and rotor is given. Stator and rotor efficiency, Mach number, and flow angles, as well as other quantities, are obtained by means of a wheel with axial outlet. Semiempirical formulas are given for turbine efficiency, stator efficiency, and rotor efficiency as functions of aspect ratio.

scholarly journals Biglobal instabilities of compressible open-cavity flows

2017 ◽  
Vol 826 ◽  
pp. 270-301 ◽  
Author(s):  
Yiyang Sun ◽  
Kunihiko Taira ◽  
Louis N. Cattafesta ◽  
Lawrence S. Ukeiley
Keyword(s):  
Stability Analysis ◽  
Mach Number ◽  
Control Strategies ◽  
Pressure Fluctuations ◽  
Open Cavity ◽  
Nonlinear Flow ◽  
Cavity Flows ◽  
Strong Nonlinearity ◽  
Aspect Ratios ◽  
Subsonic Regime

The stability characteristics of compressible spanwise-periodic open-cavity flows are investigated with direct numerical simulation (DNS) and biglobal stability analysis for rectangular cavities with aspect ratios of $L/D=2$ and 6. This study examines the behaviour of instabilities with respect to stable and unstable steady states in the laminar regime for subsonic as well as transonic conditions where compressibility plays an important role. It is observed that an increase in Mach number destabilizes the flow in the subsonic regime and stabilizes the flow in the transonic regime. Biglobal stability analysis for spanwise-periodic flows over rectangular cavities with large aspect ratio is closely examined in this study due to its importance in aerodynamic applications. Moreover, biglobal stability analysis is conducted to extract two-dimensional (2-D) and 3-D eigenmodes for prescribed spanwise wavelengths $\unicode[STIX]{x1D706}/D$ about the 2-D steady state. The properties of 2-D eigenmodes agree well with those observed in the 2-D nonlinear simulations. In the analysis of 3-D eigenmodes, it is found that an increase of Mach number stabilizes dominant 3-D eigenmodes. For a short cavity with $L/D=2$, the 3-D eigenmodes primarily stem from centrifugal instabilities. For a long cavity with $L/D=6$, other types of eigenmodes appear whose structures extend from the aft-region to the mid-region of the cavity, in addition to the centrifugal instability mode located in the rear part of the cavity. A selected number of 3-D DNS are performed at $M_{\infty }=0.6$ for cavities with $L/D=2$ and 6. For $L/D=2$, the properties of 3-D structures present in the 3-D nonlinear flow correspond closely to those obtained from linear stability analysis. However, for $L/D=6$, the 3-D eigenmodes cannot be clearly observed in the 3-D DNS due to the strong nonlinearity that develops over the length of the cavity. In addition, it is noted that three-dimensionality in the flow helps alleviate violent oscillations for the long cavity. The analysis performed in this paper can provide valuable insights for designing effective flow control strategies to suppress undesirable aerodynamic and pressure fluctuations in compressible open-cavity flows.

scholarly journals Wind Tunnel Testing on Start/Unstart Characteristics of Finite Supersonic Biplane Wing

2013 ◽  
Vol 2013 ◽  
pp. 1-10 ◽  
Author(s):  
Hiroshi Yamashita ◽  
Naoshi Kuratani ◽  
Masahito Yonezawa ◽  
Toshihiro Ogawa ◽  
Hiroki Nagai ◽  
...  
Keyword(s):  
Mach Number ◽  
Aspect Ratio ◽  
Three Dimensional ◽  
Flow Characteristics ◽  
Wind Tunnels ◽  
Wind Tunnel Testing ◽  
Ratio Model ◽  
Number Range ◽  
Aspect Ratios ◽  
Schlieren System

This study describes the start/unstart characteristics of a finite and rectangular supersonic biplane wing. Two wing models were tested in wind tunnels with aspect ratios of 0.75 (model A) and 2.5 (model B). The models were composed of a Busemann biplane section. The tests were carried out using supersonic and transonic wind tunnels over a Mach number range of0.3≤M∞≤2.3with angles of attack of 0°, 2°, and 4°. The Schlieren system was used to observe the flow characteristics around the models. The experimental results showed that these models had start/unstart characteristics that differed from those of the Busemann biplane (two dimensional) owing to three-dimensional effects. Models A and B started at lower Mach numbers than the Busemann biplane. The characteristics also varied with aspect ratio: model A (1.3<M∞<1.5) started at a lower Mach number than model B (1.6<M∞<1.8) owing to the lower aspect ratio. Model B was located in the double solution domain for the start/unstart characteristics atM∞=1.7, and model B was in either the start or unstart state atM∞=1.7. Once the state was determined, either state was stable.

scholarly journals The Ice Particle and Aggregate Simulator (IPAS). Part I: Extracting Dimensional Properties of Ice–Ice Aggregates for Microphysical Parameterization

2019 ◽  
Vol 76 (6) ◽  
pp. 1661-1676 ◽  
Author(s):  
Vanessa M. Przybylo ◽  
Kara J. Sulia ◽  
Carl G. Schmitt ◽  
Zachary J. Lebo ◽  
William C. May
Keyword(s):  
Aspect Ratio ◽  
Crystal Size ◽  
Three Dimensional ◽  
Major Axis ◽  
Ice Crystal ◽  
Particle Properties ◽  
Aspect Ratios ◽  
Almost All ◽  
Cloud Ice ◽  
Microphysical Parameterization

Abstract Aggregation, the process by which two or more ice particles attach to each other, is typically observed in clouds that span a range of temperatures and is influenced by the crystal shape (habit). In this study, the resulting characteristics of ice–ice two-monomer aggregation is investigated, which is expected to improve microphysical parameterizations through more precise aggregate characteristics and in turn better predict the rate of aggregation and snow development. A systematic way to determine the aspect ratio of the aggregate was developed, which takes into account the expected falling orientations, overlap of each monomer, and any contact angle that may form through so-called constrained randomization. Distributions were used to obtain the most frequent aspect ratio, major axis, and minor axis of aggregated particles with respect to the monomer aspect ratio. Simulations were completed using the Ice Particle and Aggregate Simulator (IPAS), a model that uses predefined three-dimensional geometries, (e.g., hexagonal prisms) to simulate ice crystal aggregation and allows for variation in crystal size, shape, number, and falling orientation. In this study, after collection in a theoretical grid space, detailed information is extracted from the particles to determine the properties of aggregates. It was found that almost all monomer aspect ratios aggregate to less extreme aggregate aspect ratios at nearly the same rate. Newly formed aggregate properties are amenable to implementation into more sophisticated bulk microphysical models designed to predict and evolve particle properties, which is crucial in realistically evolving cloud ice mass distribution and for representing the collection process.

Effects of Mach Number and Secondary Flows on Ultra-Low Aspect Ratio Radial Outflow Turbine Cascade Aerodynamics

2020 ◽  
Author(s):  
Aki Grönman ◽  
Jonna Tiainen ◽  
Antti Uusitalo
Keyword(s):  
Mach Number ◽  
Aspect Ratio ◽  
Turbine Cascade ◽  
Penetration Length ◽  
Axial Turbine ◽  
Low Aspect Ratio ◽  
Aspect Ratios ◽  
Flow Phenomena ◽  
Axial Turbines ◽  
Radial Outflow

Abstract Radial outflow turbines are an alternative for axial turbines for example in heat recovery applications. They are, however, also often characterized by ultra-low aspect ratios. In these designs, the secondary losses dominate the overall loss share, and under a certain aspect ratio, the secondary structures from the hub and shroud begin to interact. This interaction causes a decrease in aerodynamic performance. Previous studies have suggested that the general flow phenomena between radial outflow and axial turbines could share several similarities due to observed trends in performance prediction. The blade outlet Mach number is known to affect the spanwise positions of the secondary vortices in axial turbine blading and therefore, its effect is also tested here for an ultra-low aspect ratio radial outflow turbine cascade. In addition, there are currently no cascade level experimental data publicly available, and the suitability of axial turbine loss correlations under these conditions remains an open question. From this background, the current study presents an experimental, numerical, and loss correlation analysis of the effects of an isentropic Mach number in a radial outflow turbine cascade. An experimental campaign is used to validate the numerical model both quantitatively and qualitatively. In addition, the validity of the axial turbine loss correlation is extended to ultra-low aspect ratios by introducing a new variable called penetration length. The main findings are: 1. The flow phenomena do not differ significantly from what has been observed with axial turbines, 2. The effect of penetration length calculation method on the loss breakdown is relatively low, and 3. With ultra-low aspect ratio radial outflow turbines, the loss breakdown is markedly changed when the extended Benner’s approach is employed.

The Effect of Mach Number and Aspect Ratio on the Interfacial Characteristics of a Submerged Rectangular Gas Jet

2010 ◽  
Author(s):  
Chris Weiland ◽  
Pavlos Vlachos
Keyword(s):  
Mach Number ◽  
Aspect Ratio ◽  
Gas Jet ◽  
Non Invasive ◽  
Gas Jets ◽  
Aspect Ratios ◽  
Instability Growth ◽  
Logarithmic Decay ◽  
The Stability ◽  
Geometric Length

Gas jets formed by rectangular nozzles submerged in water were studied using a non-invasive photographic technique which allowed simultaneous measurements of the entire interface. Three aspect ratios were considered corresponding to 2, 10, and 20 with all nozzles sharing a common width. As far as the authors know this study represents the first time the effects of aspect ratio and Mach number on a submerged gas jet have been studied. The results indicate aspect ratio and Mach number play a large role in dictating both the unsteadiness of the interface and the penetration of the gas jet into the surrounding liquid medium. The jet pinch-off is shown to have a logarithmic decay with increasing Mach number and when appropriately scaled by the total viewing length and a geometric length scale (LQ) is relatively constant across all aspect ratio nozzles. The location of pinch-off is also a function of aspect ratio, with the subsonic aspect ratio 2 nozzles showing maximum pinch-off at y/LQ ≈ 23–26 while sonic and supersonic Mach numbers have peaks over the range y/LQ ≈ 11–14. The AR 10 and 20 nozzles show no dependence on Mach number with the maximum number of pinch-off events observed over the interval y/LQ ≈ 3–5. Jet spreading which is indicative of liquid entrainment is also shown to increase with Mach number and aspect ratio. The jet penetration also increases with increasing Mach number and aspect ratio. The spatial instability growth rate was deduced from the downstream evolution of the interfacial unsteadiness and it is shown that the nozzle with aspect ratio of 2 follows a different trend than the aspect ratio 10 and 20 nozzles, suggesting a fundamentally different mechanism dominates the stability of large aspect ratio rectangular gas jets.

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