Cavitation about a jet in crossflow

2015 ◽  
Vol 768 ◽  
pp. 141-174 ◽  
Author(s):  
P. A. Brandner ◽  
B. W. Pearce ◽  
K. L. de Graaf
Keyword(s):  
Boundary Layer ◽  
Shear Layer ◽  
High Speed ◽  
Velocity Ratio ◽  
Cavitation Number ◽  
Variable Pressure ◽  
Lower Velocity ◽  
Freestream Velocity ◽  
Jet In Crossflow ◽  
Cavity Closure

Cavitation occurrence about a jet in crossflow is investigated experimentally in a variable-pressure water tunnel using still and high-speed photography. The 0.012 m diameter jet is injected on the centreplane of a 0.6 m square test section at jet to freestream velocity ratios ranging from 0.2 to 1.6, corresponding to jet-velocity-based Reynolds numbers of $25\times 10^{3}$ to $160\times 10^{3}$ respectively. Measurements were made at a fixed freestream-based Reynolds number, for which the ratio of the undisturbed boundary layer thickness to jet diameter is 1.18. The cavitation number was varied from inception (up to about 10) down to 0.1. Inception is investigated acoustically for bounding cases of high and low susceptibility to phase change. The influence of velocity ratio and cavitation number on cavity topology and geometry are quantified from the photography. High-speed photographic recordings made at 6 kHz provide insight into cavity dynamics, and derived time series of spatially averaged pixel intensities enable frequency analysis of coherent phenomena. Cavitation inception was found to occur in the high-shear regions either side of the exiting jet and to be of an intermittent nature, increasing in occurrence and duration from 0 to 100 % probability with decreasing cavitation number or increasing jet to freestream velocity ratio. The frequency and duration of individual events strongly depends on the cavitation nuclei supply within the approaching boundary layer. Macroscopic cavitation develops downstream of the jet with reduction of the cavitation number beyond inception, the length of which has a power-law dependence on the cavitation number and a linear dependence on the jet to freestream velocity ratio. The cavity closure develops a re-entrant jet with increase in length forming a standing wave within the cavity. For sufficiently low cavitation numbers the projection of the re-entrant jet fluid no longer reaches the cavity leading edge, analogous to supercavitation forming about solid cavitators. Hairpin-shaped vortices are coherently shed from the cavity closure via mechanisms of shear-layer roll-up similar to those shed from protuberances and jets in crossflow in single-phase flows. These vortices are shed at an apparently constant frequency, independent of the jet to freestream velocity ratio but decreasing in frequency with reducing cavitation number and cavity volume growth. Highly coherent cavitating vortices form along the leading part of the cavity due to instability of the jet upstream shear layer for jet to freestream velocity ratios greater than about 0.8. These vortices are cancelled and condense as they approach the trailing edge in the shear layer of opposing vorticity associated with the cavity closure and the hairpin vortex formation. For lower velocity ratios, where there is decreased jet penetration, the jet upstream shear velocity gradient reverses and vortices of the opposite sense form, randomly modulated by boundary layer turbulence.

Blunt Trailing Edge Shear Wake Development With Turbulent In-Flow Boundary Layers

2005 ◽  
Author(s):  
Dhanalakshmi Challa ◽  
Joe Klewicki
Keyword(s):  
Boundary Layer ◽  
Shear Layer ◽  
Boundary Layers ◽  
High Speed ◽  
Velocity Ratio ◽  
Free Shear Layer ◽  
Low Speed ◽  
Axial Pressure Gradient ◽  
Layer Flow ◽  
Good Agreement

Experiments are conducted to explore the structural mechanisms involved in the post-separation evolution of a wall-bounded to a free-shear turbulent flow. At the upstream, both the boundary layers are turbulent. Experiments were conducted in a two-stream shear-layer tunnel, under a zero axial pressure gradient shear-wake configuration. A velocity ratio near 2 was explored. Profile data were collected with a single wire probe at various locations downstream of the blunt separation lip. With this set of measurements, mean profile, axial intensity and measures of profile evolution indicate that the predominant shift from turbulent boundary layer to free shear-layer like behavior occurs between the downstream locations x/θ = 13.7 & 27.4, where θ is the upstream momentum deficit thickness on the low-speed stream. The shear wake width is observed to be nominally constant with the downstream position. Axial velocity spectra show that the transition from boundary layer flow to shear flow occurs earlier in high-speed stream when compared to low speed stream. Strouhal number, Sto, of initial vortex rollup based on initial momentum thickness was found to be 0.034, which is in very good agreement with the existing literature. Other measures are in good agreement with linear stability considerations found in the literature.

Effect of velocity ratio on the interaction between plasma synthetic jets and turbulent cross-flow

2019 ◽  
Vol 865 ◽  
pp. 928-962 ◽  
Author(s):  
Haohua Zong ◽  
Marios Kotsonis
Keyword(s):  
Boundary Layer ◽  
High Speed ◽  
Velocity Ratio ◽  
Vortex Pair ◽  
Synthetic Jets ◽  
Low Energy ◽  
Vortex Rings ◽  
Wall Region ◽  
Mean Velocity ◽  
Counter Rotating Vortex Pair

Plasma synthetic jet actuators (PSJAs) are particularly suited for high-Reynolds-number, high-speed flow control due to their unique capability of generating supersonic pulsed jets at high frequency (${>}5$  kHz). Different from conventional synthetic jets driven by oscillating piezoelectric diaphragms, the exit-velocity variation of plasma synthetic jets (PSJs) within one period is significantly asymmetric, with ingestion being relatively weaker (less than $20~\text{m}~\text{s}^{-1}$) and longer than ejection. In this study, high-speed phase-locked particle image velocimetry is employed to investigate the interaction between PSJAs (round exit orifice, diameter 2 mm) and a turbulent boundary layer at constant Strouhal number (0.02) and increasing mean velocity ratio ($r$, defined as the ratio of the time-mean velocity over the ejection phase to the free-stream velocity). Two distinct operational regimes are identified for all the tested cases, separated by a transition velocity ratio, lying between $r=0.7$ and $r=1.0$. At large velocity and stroke ratios (first regime, representative case $r=1.6$), vortex rings are followed by a trailing jet column and tilt downstream initially. This downstream tilting is transformed into upstream tilting after the pinch-off of the trailing jet column. The moment of this transformation relative to the discharge advances with decreasing velocity ratio. Shear-layer vortices (SVs) and a hanging vortex pair (HVP) are identified in the windward and leeward sides of the jet body, respectively. The HVP is initially erect and evolves into an inclined primary counter-rotating vortex pair ($p$-CVP) which branches from the middle of the front vortex ring and extends to the near-wall region. The two legs of the $p$-CVP are bridged by SVs, and a secondary counter-rotating vortex pair ($s$-CVP) is induced underneath these two legs. At low velocity and stroke ratios (second regime, representative case $r=0.7$), the trailing jet column and $p$-CVP are absent. Vortex rings always tilt upstream, and the pitching angle increases monotonically with time. An $s$-CVP in the near-wall region is induced directly by the two longitudinal edges of the ring. Inspection of spanwise planes ($yz$-plane) reveals that boundary-layer energization is realized by the downwash effect of either vortex rings or $p$-CVP. In addition, in the streamwise symmetry plane, the increasing wall shear stress is attributed to the removal of low-energy flow by ingestion. The downwash effect of the $s$-CVP does not benefit boundary-layer energization, as the flow swept to the wall is of low energy.

Study on Flow Fields of Boundary-Layer Separation and Hydraulic Jump during Rundown Motion of Shoaling Solitary Wave

2015 ◽  
Vol 09 (05) ◽  
pp. 1540002 ◽  
Author(s):  
Chang Lin ◽  
Ming-Jer Kao ◽  
Guang-Wei Tzeng ◽  
Wei-Ying Wong ◽  
James Yang ◽  
...  
Keyword(s):  
Boundary Layer ◽  
Free Surface ◽  
Solitary Wave ◽  
Shear Layer ◽  
Flow Separation ◽  
Hydraulic Jump ◽  
High Speed ◽  
Flow Fields ◽  
Main Stream ◽  
Separated Shear Layer

The characteristics of flow fields for a complete evolution of the non-breaking solitary wave, having a wave-height to water-depth ratio of 0.363 and propagating over a 1:5 sloping bottom, are investigated experimentally. This study mainly focuses on the occurrences of both flow separation on the boundary layer under an adverse pressure gradient and subsequent hydraulic jump with the abrupt rising of free surface during rundown motion of the shoaling wave, together with emphasis on the evolution of vortex structures underlying the separated shear layer and hydraulic jump. A flow visualization technique with particle trajectory method and a high-speed particle image velocimetry (HSPIV) system with a high-speed digital camera were used. Based on the instantaneous flow images visualized and/or the ensemble-averaged velocity fields measured, the following interesting features, which are unknown up-to-date, are presented and discussed in this study: (1) Flow bifurcation occurring on both offshore and onshore sides of the explicit demarcation curve and the stagnation point during runup motion; (2) The dependence of the diffuser-like flow field, being changed from the supercritical flow in the shallower region to the subcritical flow in the deeper counterpart, on the Froude number during the early and middle stages of rundown motion; (3) The positions and times for the occurrences of the incipient flow separation and the sudden rising of free surface of the hydraulic jump; (4) The associated movement and evolution of vortex structures under the separated shear layer, the hydraulic jump and/or the high-speed external main stream of the retreated flow; and (5) The entrainment of air bubbles from the free surface into the external main stream of the retreated flow.

Flow Characteristics of a Low Reynolds Number Jet in Crossflow with an Obstacle Block

2016 ◽  
Vol 9 (1) ◽  
pp. 37-46 ◽  
Author(s):  
Jianlong Chang ◽  
Xudong Shao ◽  
Xiao Hu ◽  
Shuangbiao Zhang
Keyword(s):  
Reynolds Number ◽  
Low Reynolds Number ◽  
Velocity Ratio ◽  
Flow Characteristics ◽  
Vortex Pair ◽  
Lower Velocity ◽  
Eddy Simulation ◽  
Jet In Crossflow ◽  
Crossflow Velocity ◽  
Low Reynolds

The jet in crossflow at very low Reynolds number (Re=100) with and without block is performed by means of large eddy simulation for the jet-to-crossflow velocity ratios (r) ranging from 1 to 3, and the corresponding flow characteristics are compared. The results show that the time-averaged particle trajectories of the jet are slightly changed if a block is presented, and the mixed vortices are weakened. The existence of the block also can accelerate the formation of stable counter-rotating vortex pair. At lower velocity ratio (r=1), the block has little effect on the jet in crossflow with a symmetrically positive and negative kidney shaped vortices. As the velocity ratio increases, the effect of block not only can generate an asymmetry of positive and negative kidney shaped vortices, but also it can reinforce the interaction between the positive and negative vortices in the jet in crossflow. The effect of block on the temperature field is also analyzed in detail.

High-speed boundary-layer transition induced by a discrete roughness element

2013 ◽  
Vol 729 ◽  
pp. 524-562 ◽  
Author(s):  
Prahladh S. Iyer ◽  
Krishnan Mahesh
Keyword(s):  
Boundary Layer ◽  
Shear Layer ◽  
High Speed ◽  
Roughness Element ◽  
Transition Process ◽  
Significant Rise ◽  
Boundary Layer Transition ◽  
Streamwise Vortices ◽  
Layer Transition ◽  
Mach Numbers

AbstractDirect numerical simulation (DNS) is used to study laminar to turbulent transition induced by a discrete hemispherical roughness element in a high-speed laminar boundary layer. The simulations are performed under conditions matching the experiments of Danehy et al. (AIAA Paper 2009–394, 2009) for free-stream Mach numbers of 3.37, 5.26 and 8.23. It is observed that the Mach 8.23 flow remains laminar downstream of the roughness, while the lower Mach numbers undergo transition. The Mach 3.37 flow undergoes transition closer to the bump when compared with Mach 5.26, in agreement with experimental observations. Transition is accompanied by an increase in ${C}_{f} $ and ${C}_{h} $ (Stanton number). Even for the case that did not undergo transition (Mach 8.23), streamwise vortices induced by the roughness cause a significant rise in ${C}_{f} $ until 20$D$ downstream. The mean van Driest transformed velocity and Reynolds stress for Mach 3.37 and 5.26 show good agreement with available data. Temporal spectra of pressure for Mach 3.37 show that frequencies in the range of 10–1000 kHz are dominant. The transition process involves the following key elements: upon interaction with the roughness element, the boundary layer separates to form a series of spanwise vortices upstream of the roughness and a separation shear layer. The system of spanwise vortices wrap around the roughness element in the form of horseshoe/necklace vortices to yield a system of counter-rotating streamwise vortices downstream of the element. These vortices are located beneath the separation shear layer and perturb it, which results in the formation of trains of hairpin-shaped vortices further downstream of the roughness for the cases that undergo transition. These hairpins spread in the span with increasing downstream distance and the flow increasingly resembles a fully developed turbulent boundary layer. A local Reynolds number based on the wall properties is seen to correlate with the onset of transition for the cases considered.

Effect of Upstream Shear on Flow and Heat (Mass) Transfer Over a Flat Plate

2008 ◽  
Author(s):  
Kalyanjit Ghosh ◽  
R. J. Goldstein
Keyword(s):  
Boundary Layer ◽  
Mass Transfer ◽  
Reynolds Number ◽  
Turbulent Boundary Layer ◽  
Flow Direction ◽  
Velocity Ratio ◽  
Leading Edge ◽  
Surface Velocity ◽  
Freestream Velocity ◽  
Inner Region

A parametric study is conducted to investigate the effect of wall shear on a two-dimensional turbulent boundary layer. The shear is imparted by a moving belt, flush with the wall, translating in the flow direction. Velocity and mass transfer experiments have been performed for four surface-to-freestream velocity ratios (0, 0.38, 0.52, 0.65) with a Reynolds number based on the momentum thickness between 770 and 1776. The velocity data indicate that the location of the ‘virtual origin’ of the turbulent boundary layer ‘moves’ downstream towards the trailing edge of the belt with increasing surface velocity. The highest velocity ratio represents a case which is responsible for the removal of the inner region of the boundary layer. Mass transfer measurements downstream of the belt show the presence of a local minimum in the variation of the Stanton vs. Reynolds number for the highest velocity ratio. Downstream of this minimum, approximately 1 cm from the leading edge of the mass transfer plate, the characteristics of the turbulent boundary layer are restored and the data fall back on the empirical variation of the Stanton number with Reynolds number.

An Experimental Study of Spark Anemometry for In-Cylinder Velocity Measurements

2008 ◽  
Vol 130 (4) ◽  
Author(s):  
D. P. Gardiner ◽  
G. Wang ◽  
M. F. Bardon ◽  
M. LaViolette ◽  
W. D. Allan
Keyword(s):  
Experimental Study ◽  
Flow Velocity ◽  
High Speed ◽  
Spark Ignition Engine ◽  
Constant Current ◽  
Velocity Measurements ◽  
Lower Velocity ◽  
Freestream Velocity ◽  
Channel Shape ◽  
Spark Channel

It has been demonstrated by previous researchers that an approximate value of the bulk flow velocity through the spark plug gap of a running spark ignition engine may be deduced from the voltage and current wave forms of the spark. The technique has become known as spark anemometry and offers a robust means of velocity sensing for engine combustion chambers and other high temperature environments. This paper describes an experimental study aimed at improving performance of spark anemometry as an engine research tool. Bench tests were conducted using flow provided by a calibrated nozzle apparatus discharging to atmospheric pressure. While earlier studies had relied upon assumptions about the shape of the stretching spark channel to relate the spark voltage to the flow velocity, the actual spark channel shape was documented using high-speed video in the present study. A programmable ignition system was used to generate well-controlled constant current discharges. The spark anemometry apparatus was then tested in a light duty automotive engine. Results from the image analysis of the spark channel shape undertaken in the present study have shown that the spark kernel moves at a velocity of less than that of the freestream gas velocity. A lower velocity threshold exists below, which there is no response from the spark. It is possible to obtain a consistent, nearly linear relationship between the first derivative of the sustaining voltage of a constant current spark and the freestream velocity if the velocity falls within certain limits. The engine tests revealed a great deal of cycle-to-cycle variation in the in-cylinder velocity measurements. Instances where the spark restrikes occur during the cycle must also be recognized in order to avoid false velocity indications.

Predicted Effects of Tangential Slot Injection on Turbulent Boundary Layer Flow over a Wide Speed Range

1979 ◽  
Vol 101 (4) ◽  
pp. 699-704 ◽  
Author(s):  
A. M. Cary ◽  
D. M. Bushnell ◽  
J. N. Hefner
Keyword(s):  
Shear Layer ◽  
High Speed ◽  
Turbulence Modeling ◽  
Near Field ◽  
Velocity Ratio ◽  
Anomalous Behavior ◽  
Numerical Code ◽  
Stream Velocity ◽  
Wall Region ◽  
Wide Range

Paper describes a numerical calculation method using eddy viscosity/mixing length concepts for tangential slot injection (wall-wake) flows; application of the method over a wide range of flow conditions indicates increased accuracy compared to previous work. Predictions from the numerical code were in good agreement with experiment (velocity profile, skin friction, and effectiveness data) for low and high speed flows. To achieve improved accuracy, improvements in the turbulence modeling, compared to previous research, were necessary for the imbedded shear layer region in the near field and for the wall region near shear layer impingement. Anomalous behavior was noted for far field experimental velocity profiles in low speed flow when the slot-to-free stream velocity ratio was near one.

Cavitation noise and inception as influenced by boundary-layer development on a hydrofoil

1977 ◽  
Vol 80 (4) ◽  
pp. 617-640 ◽  
Author(s):  
William K. Blake ◽  
M. J. Wolpert ◽  
F. E. Geib
Keyword(s):  
Boundary Layer ◽  
High Speed ◽  
Leading Edge ◽  
Variable Pressure ◽  
Frequency Range ◽  
Cavitation Noise ◽  
Two Phase ◽  
Boundary Layer Development ◽  
Pressure Water ◽  
Layer Development

This paper describes measurements of noise from two-phase flow over hydrofoils. The experiments were performed in a variable-pressure water tunnel which was acoustically calibrated so that sound power levels could be deduced from the sound measurements. It is partially reverberant in the frequency range of interest.Cavitation was generated on a hydrofoil in the presence of either a separated laminar boundary layer or a fully turbulent attached boundary layer. The turbulent boundary layer was formed downstream of a trip which was positioned near the leading edge. High-speed photographs show the patterns of cavitation which were obtained in each case. The noise is shown to depend on the type of cavitation produced; and for each type, the dependence on speed and cavitation index has been determined. Dimensionless spectral densities of the sound are shown for each type of flow.

Evaluation of Reattaching Shear-Layer in Compressible Turbulent Flows: A Large Eddy Simulation Approach

2020 ◽  
Author(s):  
Rozie Zangeneh
Keyword(s):  
Boundary Layer ◽  
Large Eddy Simulation ◽  
Shear Layer ◽  
Turbulent Flows ◽  
High Speed ◽  
High Fidelity ◽  
Free Shear Layer ◽  
Eddy Simulation ◽  
Shock Unsteadiness ◽  
Large Eddy

Abstract The boundary-layer separation and subsequent reattachment due to the free shear-layer and Shockwave interaction have a significant impact on the aerothermal design of supersonic aerospace systems. This problem is prevalent in high-speed flights and can significantly affect the skin friction, aerodynamic loads, and heat transfer. In recent years, considerable progress has been achieved in the prediction of turbulent compressible flows using high-fidelity models. However, the prediction of reattaching free shear-layer and shockwave interactions still needs to be modified for accurate predictivity. The objective of this study is to investigate the ability of a new computational fluid dynamics model to predict these critical flow phenomena accurately. The new high-fidelity model is based on a collocated central scheme, which has the advantage of being a Riemann free solver, and therefore easy to implement on unstructured grids. It is developed to capture any discontinuities at shocks while it is able to capture broadband spatial and temporal variations in turbulent flows with minimal dissipation and dispersion. Large Eddy Simulation is performed on a compression corner at a Mach number of 2.92 and a high Reynolds number. The geometry of the model is specifically designed to isolate the reattachment process of a high-speed separated flow. To examine the accuracy of the predicted results, results of velocity profiles in the free shear-layer, boundary layer development, turbulent fluctuations, and pressure are compared to an experimental effort by Princeton. Excellent agreement is observed, and it is recommended that the model can be used to investigate the physics of the shock unsteadiness due to interaction with a free shear-layer.

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