Shock wave–boundary layer interaction in supersonic flow over a forward-facing step

2016 ◽  
Vol 807 ◽  
pp. 258-302 ◽  
Author(s):  
Jayaprakash N. Murugan ◽  
Raghuraman N. Govardhan
Keyword(s):  
Boundary Layer ◽  
Reverse Flow ◽  
Separation Bubble ◽  
Velocity Fields ◽  
Recirculation Region ◽  
Velocity Data ◽  
Flow Area ◽  
Piv Measurements ◽  
Shock Position ◽  
Forward Facing Step

We study in the present work a Mach 2.5 flow over a forward-facing step. The focus of the work is the flow ahead of the step, in particular, the unsteady interactions between the shock, the boundary layer and the separation bubble. The primary geometrical parameter in the problem is the ratio of the step height to the incoming boundary layer thickness, $h/\unicode[STIX]{x1D6FF}$, which is kept fixed at 2. Results are presented from detailed particle image velocimetry (PIV) measurements in two orthogonal planes to obtain a reasonable picture of the whole flow field. The mean velocity field in the central cross-stream or wall-normal ($x$–$y$) plane shows that the incoming boundary layer separates upstream of the step forming a large separation bubble ahead of the step, which can be relatively well resolved in PIV measurements compared to the compression ramp cases. Wall pressure fluctuation spectra close to the separation location show a dominant frequency ($f$) that is two orders of magnitude smaller than the characteristic frequency of the incoming boundary layer ($U_{\infty }/\unicode[STIX]{x1D6FF}$), consistent with low-frequency motions of the shock that have received a lot of recent attention ($U_{\infty }$ $=$ free-stream velocity, $\unicode[STIX]{x1D6FF}$ $=$ boundary layer thickness). PIV measurements in the wall-normal plane show large variations in shock position with time. The shock position measured from velocity data outside the boundary layer is found to be well correlated with the reverse flow area ahead of the step, and weakly correlated to structures in the incoming boundary layer. In contrast, the shock foot, determined from velocity data within the boundary layer, is found to be well correlated to the low- and high-speed streaks in the incoming boundary layer, in addition to the reverse flow area ahead of the step. Instantaneous velocity fields in the spanwise ($x$–$z$) plane parallel to the lower wall show that the shock is broadly two-dimensional with small spanwise ripples, while the recirculation region has very large spanwise variations. The spanwise-averaged shock location is found to be well correlated to the most upstream location of the recirculation region over a spanwise length ($x_{r,min}^{sp}$). Instantaneous velocity fields show that when some part of the recirculation region is far upstream, the corresponding nearly two-dimensional shock is also far upstream. On the other hand, when $x_{r,min}^{sp}$ is relatively downstream, the resulting shock is also found to be downstream. Hence, the present results suggest that for the forward-facing step configuration, the large-scale streamwise motions of the shock are mainly correlated to the most upstream point of the recirculation region, which has large spanwise variations.

The Effects of Upstream Wall Roughness On the Spatio-Temporal Characteristics of Flow Separations Induced by a Forward-Facing Step

2021 ◽  
Author(s):  
Sedem Kumahor ◽  
Xingjun Fang ◽  
Mark F. Tachie
Keyword(s):  
Boundary Layer ◽  
Reverse Flow ◽  
Rough Wall ◽  
Step Height ◽  
Stream Velocity ◽  
Wall Roughness ◽  
Mean Flow ◽  
Flow Area ◽  
Separation Bubbles ◽  
Forward Facing Step

Abstract Separating and reattaching turbulent flows induced by a forward-facing step submerged in thick oncoming turbulent boundary layers (TBL) developed over smooth and rough upstream walls were investigated using time-resolved particle image velocimetry. The examined upstream walls resulted in smooth, transitionally rough and fully rough wall conditions. The upstream boundary layer thicknesses were 4.3 and 6.7 times the step height in the smooth and rough wall cases, respectively. The Reynolds number based on the step height and free-stream velocity was 7800. The effects of upstream wall roughness on the mean flow characteristics, Reynolds stresses defined in both Cartesian and curvilinear coordinate systems as well as the unsteadiness of the turbulent separation bubbles were critically examined. The results show that upstream wall roughness increases the boundary layer thickness and turbulence intensity and consequently, promotes early mean flow reattachment over the step. Distinct regions of significantly elevated vertical Reynolds normal stress and Reynolds shear stress were observed upstream of the step in the fully rough wall case compared to the smooth wall case. Proper orthogonal decomposition (POD) and the reverse flow area over the step were employed to investigate the unsteadiness of the separation bubbles. The first POD mode coefficient and the reverse flow area over the step were well correlated and exhibited the same dominant frequency.

scholarly journals Retrieval of Urban Boundary Layer Structures from Doppler Lidar Data. Part I: Accuracy Assessment

2008 ◽  
Vol 65 (1) ◽  
pp. 3-20 ◽  
Author(s):  
Quanxin Xia ◽  
Ching-Long Lin ◽  
Ronald Calhoun ◽  
Rob K. Newsom
Keyword(s):  
Boundary Layer ◽  
Radial Velocity ◽  
Eddy Viscosity ◽  
Velocity Fields ◽  
Urban Boundary Layer ◽  
Flow Structures ◽  
Lidar Data ◽  
Velocity Data ◽  
Beam Direction ◽  
The Cross

Abstract Two coherent Doppler lidars from the U.S. Army Research Laboratory (ARL) and Arizona State University (ASU) were deployed in the Joint Urban 2003 atmospheric dispersion field experiment (JU2003) held in Oklahoma City, Oklahoma. The dual-lidar data were used to evaluate the accuracy of a four-dimensional variational data assimilation (4DVAR) method and to identify the coherent flow structures in the urban boundary layer. The objectives of the study are threefold. The first objective is to examine the effect of eddy viscosity models on the quality of retrieved velocity data. The second objective is to determine the fidelity of single-lidar 4DVAR and evaluate the difference between single- and dual-lidar retrievals. The third objective is to inspect flow structures above some geospatial features on the land surface. It is found that the approach of treating eddy viscosity as part of the control variables yields better results than the approach of prescribing viscosity. The ARL single-lidar 4DVAR is able to retrieve radial velocity fields with an accuracy of 98% in the along-beam direction and 80%–90% in the cross-beam direction. For the dual-lidar 4DVAR, the accuracy of retrieved radial velocity in the ARL cross-beam direction improves to 90%–94% of the ASU radial velocity data. By using the dual-lidar-retrieved data as a reference, the single-lidar 4DVAR is able to recover fluctuating velocity fields with 70%–80% accuracy in the along-beam direction and 60%–70% accuracy in the cross-beam direction. Large-scale convective roll structures are found in the vicinity of the downtown airport and parks. Vortical structures are identified near the business district. Strong up- and downdrafts are also found above a cluster of restaurants.

CFD Simulations and PIV Measurements for Optimization of Flow Pattern in a Sieve Plate Extraction Column

2011 ◽  
Vol 391-392 ◽  
pp. 1464-1468
Author(s):  
Chang Chun Duan ◽  
Chun Jiang Liu ◽  
Xi Gang Yuan
Keyword(s):  
Flow Pattern ◽  
Reverse Flow ◽  
Continuous Phase ◽  
Extraction Column ◽  
Sieve Plate ◽  
Cfd Simulations ◽  
Flow Area ◽  
Piv Measurements ◽  
Image Velocimetry ◽  
Computational Fluid Dynamics Cfd

Present work deals with the optimization for flow pattern of continuous phase in a sieve plate extraction column using both computational fluid dynamics (CFD) simulations and particle image velocimetry (PIV) measurements. Firstly single-phase simulation was conducted for the traditional column and it was found that there was a very large reverse flow area between every two plates. Then step by step, by changing the downcomer structure, consisting of inclining downcomers, adding baffles, slotting downcomers and baffles and adjusting the number and size of slots, the reverse flow area was decreased and thereby the flow pattern of continuous phase was optimized. Finally, an optimal flow pattern was obtained with reverse flow area greatly reduced. In order to prove the validity of the simulation results, PIV experiments of two columns were carried out and it was found that the results of simulations and experiments are in good agreement.

Influence of Oscillating Boundary Layer Suction on Aerodynamic Performance in Highly-Loaded Compressor Cascades

2018 ◽  
Author(s):  
Xu Hao ◽  
Liu Bao ◽  
Cai Le ◽  
Zhou Xun ◽  
Wang Songtao ◽  
...  
Keyword(s):  
Boundary Layer ◽  
Vortex Shedding ◽  
Large Scale ◽  
Reverse Flow ◽  
Separation Bubble ◽  
Flow Region ◽  
Flow Fields ◽  
Separation Flow ◽  
Oscillating Boundary ◽  
Pod Analysis

Vortex structures of the separation flow fields in compressor cascades controlled by the boundary layer oscillating suction (BLOS) are numerically investigated. The proper orthogonal decomposition (POD) method is adopted to present the variation of characteristics owned by large-scale vortices. It is found that unsteady perturbation re-organizes the aspirated flow fields and, if in a proper situation, reduces the loss furthermore. Through POD analysis, variations of vortical structures are described. The results turn out that the periodic perturbation leads to a vortex shedding process with the same frequency as the excitation. The reason of loss reduction could be summarized by actuated vortices enhancing the momentum of the stagnated fluid in the reverse flow region as well as decreasing the frequencies of vortex shedding. Finally, 3-D numerical results turn out that the oscillation can transform the stable corner separation bubble to vortex rings shedding downstream and hence improve cascade performance.

scholarly journals Turbulent separation upstream of a forward-facing step

2013 ◽  
Vol 724 ◽  
pp. 284-304 ◽  
Author(s):  
D. S. Pearson ◽  
P. J. Goulart ◽  
B. Ganapathisubramani
Keyword(s):  
Boundary Layer ◽  
Reverse Flow ◽  
Three Dimensional ◽  
Steady Increase ◽  
Separation Region ◽  
Two Dimensional ◽  
Low Velocity ◽  
Time Resolved ◽  
Displacement Thickness ◽  
Forward Facing Step

AbstractThe turbulent flow over a forward-facing step is studied using two-dimensional time-resolved particle image velocimetry. The structure and behaviour of the separation region in front of the step is investigated using conditional averages based on the area of reverse flow present. The relation between the position of the upstream separation and the two-dimensional shape of the separation region is presented. It is shown that when of ‘closed’ form, the separation region can become unstable resulting in the ejection of fluid over the corner of the step. The separation region is shown to grow simultaneously in both the wall-normal and streamwise directions, to a point where the maximum extent of the upstream position of separation is limited by the accompanying transfer of mass over the step corner. The conditional averages are traced backwards in time to identify the average behaviour of the boundary-layer displacement thickness leading up to such events. It is shown that these ejections are preceded by the convection of low-velocity regions from upstream, resulting in a three-dimensional interaction within the separation region. The size of the low-velocity regions, and the time scale at which the separation region fluctuates, is shown to be consistent with the large boundary layer structures observed in the literature. Instances of a highly suppressed separation region are accompanied by a steady increase in velocity in the upstream boundary layer.

Separation-Bubble-Transition Measurements on a Low-Re Airfoil Using Particle Image Velocimetry

2005 ◽  
Author(s):  
B. R. McAuliffe ◽  
M. I. Yaras
Keyword(s):  
Reynolds Number ◽  
Particle Image Velocimetry ◽  
Large Scale ◽  
Particle Image ◽  
Reverse Flow ◽  
Separation Bubble ◽  
Low Reynolds Number ◽  
Piv Measurements ◽  
Image Velocimetry ◽  
Low Reynolds

This paper presents experimental results on separation-bubble transition at low Reynolds number and low freestream turbulence, measured on an airfoil using particle image velocimetry (PIV). The two-dimensional PIV measurements have been performed over the suction surface of a low-Reynolds-number airfoil in a water tow-tank facility. Reynolds numbers, based on airfoil chord length and towing speed, of 40,000 and 65,000 have been examined at various angles of incidence, providing a range of streamwise pressure distributions and transitional separation-bubble geometries. The types of bubbles observed range from a short and thick bubble with separation near the leading edge of the airfoil, to a long and thin bubble with separation far downstream of the suction peak. The PIV measurements facilitate visualization of the vortex dynamics associated with separation-bubble transition. The growth of instability waves within the separated shear layer and eventual breakdown into turbulence is documented through the instantaneous vector fields. For all cases examined, large-scale vortex shedding and multiple reverse-flow zones are observed in the reattachment region. A technique for estimating the location of transition onset based on statistical turbulence quantities is presented, and comparisons are made to existing transition models.

The Effect of Surface Roughness on Laminar Separated Boundary Layers

2013 ◽  
Author(s):  
Mark P. Simens ◽  
Ayse G. Gungor
Keyword(s):  
Boundary Layer ◽  
Separation Bubble ◽  
Roughness Element ◽  
Turbine Blades ◽  
Recirculation Region ◽  
Immersed Boundary ◽  
Spanwise Direction ◽  
Roughness Effects ◽  
Laminar Separation ◽  
Turbulent Transition

Roughness effects on a laminar separation bubble, formed on a flat plate boundary layer due to a strong adverse pressure gradient similar to those encountered on the suction side of typical low-pressure turbine blades, are studied by direct numerical simulation. The discrete roughness elements that have a uniform height in the spanwise direction and ones that have a height that is a function of the spanwise coordinate are modeled using the immersed boundary method. The location and the size of the roughness element are varied to study the effects on boundary development and turbulent transition, and it was found that the size of the separation bubble can be controlled by positioning the roughness element away from the separation bubble. Roughnesses that have a height that varies in a periodic manner in the spanwise direction have a big influence on the separation bubble. The separation point is moved downstream due to the accelerated flow in the openings in the roughness element, which also prevents the formation of the recirculation region after the roughness element. The reattachment point is moved upstream, while the height of the separation bubble is reduced. These numerical experiments indicate that laminar separation and turbulent transition, are mainly affected by the type, the height, and the location of the roughness element. Finally a comparison between the individual influence of wakes and roughness on the separation is made. It is found that the transition of the separated boundary layer with wakes occurs at almost the same streamwise location as that induced by the three-dimensional roughness element.

Experimental Study of the Fence Wake Manipulation Using Pulsating Jet

2009 ◽  
Author(s):  
In-Su Kang ◽  
Young-Ho Choi ◽  
Chel-Woo Park ◽  
Hyoung-Bum Kim
Keyword(s):  
Boundary Layer ◽  
Experimental Study ◽  
Separation Bubble ◽  
Water Channel ◽  
Separated Flow ◽  
Flow Region ◽  
Velocity Fields ◽  
Circulating Water ◽  
Pulsating Jet ◽  
Upstream Flow

In this study, we experimentally investigated the effect of rear-located pulsating jet to reduce the separated flow region behind the vertical fence. The separation bubble behind the fence is the representative feature of fence wake. Control of fence wake can be used for various purposes such as the reduction of drag, increasing or decreasing the mixing, etc. The vertical fence was submerged in the turbulent boundary layer in the circulating water channel. Reynolds number based on the fence height and upstream flow velocity was 3000. The parameters used for controlling the pulsating jet included the frequency, jet speed and distance between the fence and slit nozzle. In addition, we investigated the effect of continuous jet on the fence wake. Phase averaged DPIV method was applied to measure the instantaneous velocity fields around the fence. And the obtained results were compared with those of uncontrolled fence flow. The obtained results quantitatively show the decrease of reattachment region brought by the pulsating jet. And the specific jet condition which were effective to reduce the separation bubble behind the fence were found.

scholarly journals Dynamic linear response of a shock/turbulent-boundary-layer interaction using constrained perturbations

2018 ◽  
Vol 840 ◽  
pp. 291-341 ◽  
Author(s):  
Michael C. Adler ◽  
Datta V. Gaitonde
Keyword(s):  
Boundary Layer ◽  
Turbulent Boundary Layer ◽  
Linear Response ◽  
Separation Bubble ◽  
Low Frequency ◽  
Linear Dynamics ◽  
Local Linear ◽  
Flow Features ◽  
Shock Position ◽  
Shock Motion

Comprehensive experimental and computational investigations have revealed possible mechanisms underlying low-frequency unsteadiness observed in spanwise homogeneous shock-wave/turbulent-boundary-layer interactions (STBLI). In the present work, we extend this understanding by examining the dynamic linear response of a moderately separated Mach 2.3 STBLI to small perturbations. The statistically stationary linear response is analysed to identify potential time-local and time-mean linear tendencies present in the unsteady base flow: these provide insight into the selective amplification properties of the flow at various points in the limit cycle, as well as asymmetry and restoring mechanisms in the dynamics of the separation bubble. The numerical technique uses the synchronized large-eddy simulation method, previously developed for free shear flows, significantly extended to include a linear constraint necessary for wall-bounded flows. The results demonstrate that the STBLI fosters a global absolute linear instability corresponding to a time-mean linear tendency for upstream shock motion. The absolute instability is maintained through constructive feedback of perturbations through the recirculation: it is self-sustaining and insensitive to external forcing. The dynamics are characterized for key frequency bands corresponding to high–mid-frequency Kelvin–Helmholtz shedding along the separated shear layer $(St_{L}\sim 0.5)$, low–mid-frequency oscillations of the separation bubble $(St_{L}\sim 0.1)$ and low-frequency large-scale bubble breathing and shock motion $(St_{L}\sim 0.03)$, where the Strouhal number is based on the nominal length of the separation bubble, $L$: $St_{L}=fL/U_{\infty }$. A band-pass filtering decomposition isolates the dynamic flow features and linear responses associated with these mechanisms. For example, in the low-frequency band, extreme shock displacements are shown to correlate with time-local linear tendencies toward more moderate displacements, indicating a restoring mechanism in the linear dynamics. However, a disparity between the linearly stable shock position and the mean shock position leads to an observed asymmetry in the low-frequency shock motion cycle, in which upstream motion occurs more rapidly than downstream motion. This is explained through competing linear and nonlinear (mass depletion through shedding) mechanisms and discussed in the context of an oscillator model. The analysis successfully illustrates how time-local linear dynamics sustain several key unsteady broadband flow features in a causal manner.

On the internal flow of a ventilated supercavity

2019 ◽  
Vol 862 ◽  
pp. 1135-1165 ◽  
Author(s):  
Yue Wu ◽  
Yun Liu ◽  
Siyao Shao ◽  
Jiarong Hong
Keyword(s):  
Boundary Layer ◽  
Reverse Flow ◽  
Internal Flow ◽  
Internal Boundary Layer ◽  
Internal Boundary ◽  
Water Tunnel ◽  
Piv Measurements ◽  
Gas Leakage ◽  
Ventilated Supercavity ◽  
Vortex Tubes

This study presents an experimental investigation on the internal flow of a ventilated supercavity using fog flow visualization and particle image velocimetry (PIV) measurements. The ventilated supercavity is generated on a backward-facing cavitator and studied in the high-speed water tunnel at St. Anthony Falls Laboratory. Fog particles are introduced into the supercavity through the ventilation line, and then illuminated by a laser sheet for flow visualizations and PIV measurements. The experiments are performed on the supercavities with two closure types, i.e. the re-entrant jet (RJ) and the twin vortex (TV), under the same water tunnel flow condition but different ventilation rates. The flow visualization revealed three distinct regions within the supercavity, including the ventilation influence region near the cavitator, the extended internal boundary layer along the liquid–gas interface and the reverse flow region occupying a large centre portion of the supercavity. The streamwise and vertical extent of the ventilation influence region, the streamwise growth of the internal boundary layer and the reverse flow within the supercavity are then quantified through PIV flow measurements. Compared to the RJ case, the results indicate that the TV supercavity yields a longer vertical extent of the ventilation influence region, a thinner internal boundary layer and a stronger reverse flow. The internal flow results suggest that at the upstream of the location of the maximum cavity diameter, the gas enters the forward flow (including the internal boundary layer and the forward moving portion of the ventilation influence region) from the reverse flow, while at the downstream of that location, the gas is stripped from the internal boundary layer and enters the reverse flow due to the increasing adverse pressure gradient in the streamwise direction. The above results are combined with visualization results of the supercavity geometry and closure patterns to further explain the influence of gas leakage mechanisms on cavity growth and closure transition. Specifically, visualization of the cavity geometry change during the RJ to TV supercavity transition indicates external flow separation associated with a critical incline angle of the bottom liquid–gas interface at the closure contributes to the onset of RJ closure. The closure visualization shows the coexistence of the toroidal vortex and twin-vortex tubes for the RJ supercavity leads to two gas leakage mechanisms: one associated with the shedding of toroidal vortices ($Q_{RJ}$) and the other due to the gas entrained by the internal boundary layer and leaking from the twin-vortex tubes ($Q_{TV}$). For the RJ supercavity, with increasing ventilation input, due to the reduction of $Q_{RJ}$, the supercavity needs to elongate to increase the gas entrained by the internal boundary layer (i.e. $Q_{TV}$) to balance the ventilation increase. The elongation of the supercavity leads to reduced flow separation, and eventually a transition to the TV supercavity with ventilation above a critical value. For the TV supercavity, $Q_{RJ}$ is absent. An increase of ventilation input can be balanced by the increase of $Q_{TV}$ associated with the widening of the twin-vortex tubes, and therefore, no appreciable elongation of cavity length is observed.

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