scholarly journals Fluorescent PIV using Atomized Liquid Particles

2021 ◽  
Vol 1 (1) ◽  
Author(s):  
Adit S. Acharya ◽  
K. Todd Lowe ◽  
Wing F. Ng
Keyword(s):  
Boundary Layer ◽  
Scattered Light ◽  
Adverse Pressure Gradient ◽  
Mie Scattering ◽  
Fluorescence Yield ◽  
Surface Measurement ◽  
Fluorescent Light ◽  
Near Surface ◽  
Freestream Velocity ◽  
Forward Facing Step

It is shown that aerosolized fluorescent particles generated using a Venturi-type atomizer, from a solution of fluorescent Kiton Red 620 dye in a water/glycol fluid, provide effective flow seeding for fluorescent PIV. The atomized liquid particles were found to be of acceptable size for PIV purposes, with 92% of detected particles by number concentration measuring < 1 μm in diameter. A PIV application was conducted in a wind tunnel (freestream velocity U∞ = 27 m/s), using the particles for measurement of the boundary layer flow approaching a forward-facing step (approach boundary layer momentum thickness Reynolds number of Reθ = 5930), to identify potential benefits in near-wall regions normally affected by unwanted laser reflections from tunnel surfaces. Particles were generated from solutions with dye molar concentrations of 2.5 × 10−3 and 1.0 × 10−2 mol/L, and PIV images were obtained for both elastic Mie scattering and filtered, Stokes-shifted fluorescent light. Raw images indicate that the fluorescence yield of the 1.0 × 10−2 mol/L solution provides PIV images with high contrast, even in the near-surface regions where Mie scattering images are highly affected by surface reflections. Boundary layer profiles are processed in the adverse pressure gradient region leading up to the forward-facing step, where the fluorescent PIV performed comparably to the most optimized Mie scattering PIV; both obtained data as near to the wall as 30 μm, or 2 viscous wall units in our flow of interest. These results indicate that the new seeding method holds excellent promise for near-surface measurement applications with more complicated three-dimensional geometries, where it is impossible to arrange PIV cameras to reject surface-scattered light.

scholarly journals Instability and Transition of a Boundary Layer over a Backward-Facing Step

Fluids ◽  
2022 ◽  
Vol 7 (1) ◽  
pp. 35
Author(s):  
Ming Teng ◽  
Ugo Piomelli
Keyword(s):  
Boundary Layer ◽  
Pressure Gradient ◽  
Velocity Distribution ◽  
Adverse Pressure Gradient ◽  
Transition Process ◽  
Transition To Turbulence ◽  
Recirculation Region ◽  
Backward Facing Step ◽  
Freestream Velocity ◽  
Displacement Thickness

The development of secondary instabilities in a boundary layer over a backward-facing step is investigated numerically. Two step heights are considered, h/δo*=0.5 and 1.0 (where δo* is the displacement thickness at the step location), in addition to a reference flat-plate case. A case with a realistic freestream-velocity distribution is also examined. A controlled K-type transition is initiated using a narrow ribbon upstream of the step, which generates small and monochromatic perturbations by periodic blowing and suction. A well-resolved direct numerical simulation is performed. The step height and the imposed freestream-velocity distribution exert a significant influence on the transition process. The results for the h/δo*=1.0 case exhibit a rapid transition primarily due to the Kelvin–Helmholtz instability downstream of step; non-linear interactions already occur within the recirculation region, and the initial symmetry and periodicity of the flow are lost by the middle stage of transition. In contrast, case h/δo*=0.5 presents a transition road map in which transition occurs far downstream of the step, and the flow remains spatially symmetric and temporally periodic until the late stage of transition. A realistic freestream-velocity distribution (which induces an adverse pressure gradient) advances the onset of transition to turbulence, but does not fundamentally modify the flow features observed in the zero-pressure gradient case. Considering the budgets of the perturbation kinetic energy, both the step and the induced pressure-gradient increase, rather than modify, the energy transfer.

Effect of FreeStream Velocity Definition on Boundary Layer Thickness and Losses in Centrifugal Compressors

2018 ◽  
Vol 140 (5) ◽  
Author(s):  
Jonna Tiainen ◽  
Ahti Jaatinen-Värri ◽  
Aki Grönman ◽  
Teemu Turunen-Saaresti ◽  
Jari Backman
Keyword(s):  
Boundary Layer ◽  
Layer Thickness ◽  
Centrifugal Compressor ◽  
Boundary Layer Thickness ◽  
Adverse Pressure Gradient ◽  
Compressor Blade ◽  
Wake Flow ◽  
Centrifugal Compressors ◽  
Freestream Velocity ◽  
Common Assumption

The estimation of boundary layer losses requires the accurate specification of the freestream velocity, which is not straightforward in centrifugal compressor blade passages. This challenge stems from the jet-wake flow structure, where the freestream velocity between the blades cannot be clearly specified. In addition, the relative velocity decreases due to adverse pressure gradient. Therefore, the common assumption of a single freestream velocity over the blade surface might not be valid in centrifugal compressors. Generally in turbomachinery, the losses in the blade cascade boundary layers are estimated, e.g., with different loss coefficients, but they often rely on the assumption of a uniform flow field between the blades. To give guidelines for the estimation of the mentioned losses in highly distorted centrifugal compressor flow fields, this paper discusses the difficulties in the calculation of the boundary layer thickness in the compressor blade passages, compares different freestream velocity definitions, and demonstrates their effect on estimated boundary layer losses. Additionally, a hybrid method is proposed to overcome the challenges of defining a boundary layer in centrifugal compressors.

Combined Effects of Wakes and Pulsed Vortex Generator Jet Flow Control on Boundary Layer Separation on a Very High Lift Low Pressure Turbine Airfoil

2011 ◽  
Author(s):  
Ralph J. Volino
Keyword(s):  
Boundary Layer ◽  
Vortex Generator ◽  
Adverse Pressure Gradient ◽  
Boundary Layer Separation ◽  
Suction Surface ◽  
High Lift ◽  
Freestream Velocity ◽  
Low Pressure Turbine ◽  
Wake Passing ◽  
Very High

Boundary layer separation control with pulsed vortex generator jets (VGJs) has been studied on a very high lift, low-pressure turbine airfoil in the presence of unsteady wakes. Experiments were done under low (0.6%) and high (4%) freestream turbulence conditions on a linear cascade in a low speed wind tunnel. Cases were considered at Reynolds numbers (based on the suction surface length and the nominal exit velocity from the cascade) of 25,000 and 50,000. Wakes were produced from moving rods upstream of the cascade with flow coefficient 1.13 and rod spacing equal 2 blade pitches, resulting in a dimensionless wake passing frequency F = fLj-te/Uave = 0.14, where f is the frequency, Lj-te is the length of the adverse pressure gradient region on the suction surface, and Uave is the average freestream velocity. The VGJs were injected at the beginning of the adverse pressure gradient region on the suction surface with maximum jet velocity in each pulse equal to the local freestream velocity and a jet duty cycle of 10%. Several different timings of the VGJs with respect to the wakes were considered. Pressure surveys on the airfoil surface and downstream total pressure loss surveys were documented. Instantaneous velocity profile measurements were acquired in the suction surface boundary layer and downstream of the cascade. In cases without VGJs, the boundary layer momentarily reattached in response to the wake passing, but separated between wakes. The VGJs also caused reattachment, and if the VGJ pulsing frequency was sufficiently high, separation was largely suppressed for the full wake passing cycle. The timing of the VGJs with respect to the wakes was not very important. The jet pulsing frequency needed for separation control was about the same as found previously in cases without wakes. The background freestream turbulence effect was negligible in the presence of the larger wake and VGJ disturbances.

scholarly journals Quantifying the Hygroscopic Growth of Marine Boundary Layer Aerosols by Satellite-Based and Buoy Observations

2015 ◽  
Vol 72 (3) ◽  
pp. 1063-1074 ◽  
Author(s):  
Tao Luo ◽  
Renmin Yuan ◽  
Zhien Wang ◽  
Damao Zhang
Keyword(s):  
Boundary Layer ◽  
Climate Models ◽  
Marine Boundary Layer ◽  
High Surface ◽  
Surface Wind ◽  
Mie Scattering ◽  
Hygroscopic Growth ◽  
Near Surface ◽  
Humidity Effects ◽  
Relationship Of

Abstract In this study, collocated satellite and buoy observations as well as satellite observations over an extended region during 2006–10 were used to quantify the humidity effects on marine boundary layer (MBL) aerosols. Although the near-surface aerosol size increases with increasing near-surface relative humidity (RH), the influence of RH decreases with increasing height and is mainly limited to the lower well-mixed layer. In addition, the size changes of MBL aerosols with RH are different for low and high surface wind () conditions as revealed by observations and Mie scattering calculations, which may be related to different dominant processes (i.e., the hygroscopic growth process during low wind and the evaporation process during sea salt production during high wind). These different hygroscopic processes under the different conditions, together with the MBL processes, control the behaviors of the MBL aerosol optical depth () with RH. In particular, under high conditions, the MBL stratifications effects can overwhelm the humidity effects, resulting in a weak relationship of MBL on RH. Under low conditions, the stronger hygroscopic growth can overwhelm the MBL stratification effects and enhance the MBL with increasing RH. These results are important to evaluate and to improve MBL aerosols simulations in climate models.

scholarly journals Thermal Submeso Motions in the Nocturnal Stable Boundary Layer. Part 2: Generating Mechanisms and Implications

2021 ◽  
Author(s):  
Lena Pfister ◽  
Karl Lapo ◽  
Larry Mahrt ◽  
Christoph K. Thomas
Keyword(s):  
Boundary Layer ◽  
Stable Boundary Layer ◽  
Side Wall ◽  
Cold Pool ◽  
Continuous Data ◽  
Valley Bottom ◽  
Near Surface ◽  
Cold Air ◽  
Stable Boundary ◽  
Surface Temperatures

AbstractIn the stable boundary layer, thermal submesofronts (TSFs) are detected during the Shallow Cold Pool experiment in the Colorado plains, Colorado, USA in 2012. The topography induces TSFs by forming two different air layers converging on the valley-side wall while being stacked vertically above the valley bottom. The warm-air layer is mechanically generated by lee turbulence that consistently elevates near-surface temperatures, while the cold-air layer is thermodynamically driven by radiative cooling and the corresponding cold-air drainage decreases near-surface temperatures. The semi-stationary TSFs can only be detected, tracked, and investigated in detail when using fibre-optic distributed sensing (FODS), as point observations miss TSFs most of the time. Neither the occurrence of TSFs nor the characteristics of each air layer are connected to a specific wind or thermal regime. However, each air layer is characterized by a specific relationship between the wind speed and the friction velocity. Accordingly, a single threshold separating different flow regimes within the boundary layer is an oversimplification, especially during the occurrence of TSFs. No local forcings or their combination could predict the occurrence of TSFs except that they are less likely to occur during stronger near-surface or synoptic-scale flow. While classical conceptualizations and techniques of the boundary layer fail in describing the formation of TSFs, the use of spatially continuous data obtained from FODS provide new insights. Future studies need to incorporate spatially continuous data in the horizontal and vertical planes, in addition to classic sensor networks of sonic anemometry and thermohygrometers to fully characterize and describe boundary-layer phenomena.

Numerical Study of Boundary Layers With Reverse Wedge Flows Over a Semi-Infinite Flat Plate

2009 ◽  
Vol 77 (2) ◽  
Author(s):  
R. Ahmad ◽  
K. Naeem ◽  
Waqar Ahmed Khan
Keyword(s):  
Boundary Layer ◽  
Flat Plate ◽  
Numerical Study ◽  
Boundary Layer Equation ◽  
Wedge Angle ◽  
Adverse Pressure Gradient ◽  
Problem Solution ◽  
Weighted Residual Method ◽  
Boundary Layer Problem ◽  
Layer Problem

This paper presents the classical approximation scheme to investigate the velocity profile associated with the Falkner–Skan boundary-layer problem. Solution of the boundary-layer equation is obtained for a model problem in which the flow field contains a substantial region of strongly reversed flow. The problem investigates the flow of a viscous liquid past a semi-infinite flat plate against an adverse pressure gradient. Optimized results for the dimensionless velocity profiles of reverse wedge flow are presented graphically for different values of wedge angle parameter β taken from 0≤β≤2.5. Weighted residual method (WRM) is used for determining the solution of nonlinear boundary-layer problem. Finally, for β=0 the results of WRM are compared with the results of homotopy perturbation method.

scholarly journals Inference of Marine Atmospheric Boundary Layer Moisture and Temperature Structure Using Airborne Lidar and Infrared Radiometer Data

1998 ◽  
Vol 37 (3) ◽  
pp. 308-324 ◽  
Author(s):  
Stephen P. Palm ◽  
Denise Hagan ◽  
Geary Schwemmer ◽  
S. H. Melfi
Keyword(s):  
Boundary Layer ◽  
Atmospheric Boundary Layer ◽  
Potential Temperature ◽  
Airborne Lidar ◽  
Temperature Structure ◽  
Marine Atmospheric Boundary Layer ◽  
Space Technology ◽  
Near Surface ◽  
Surface Moisture ◽  
Infrared Radiometer

Abstract A new technique for retrieving near-surface moisture and profiles of mixing ratio and potential temperature through the depth of the marine atmospheric boundary layer (MABL) using airborne lidar and multichannel infrared radiometer data is presented. Data gathered during an extended field campaign over the Atlantic Ocean in support of the Lidar In-space Technology Experiment are used to generate 16 moisture and temperature retrievals that are then compared with dropsonde measurements. The technique utilizes lidar-derived statistics on the height of cumulus clouds that frequently cap the MABL to estimate the lifting condensation level. Combining this information with radiometer-derived sea surface temperature measurements, an estimate of the near-surface moisture can be obtained to an accuracy of about 0.8 g kg−1. Lidar-derived statistics on convective plume height and coverage within the MABL are then used to infer the profiles of potential temperature and moisture with a vertical resolution of 20 m. The rms accuracy of derived MABL average moisture and potential temperature is better than 1 g kg−1 and 1°C, respectively. The method relies on the presence of a cumulus-capped MABL, and it was found that the conditions necessary for use of the technique occurred roughly 75% of the time. The synergy of simple aerosol backscatter lidar and infrared radiometer data also shows promise for the retrieval of MABL moisture and temperature from space.

scholarly journals Observed Boundary Layer Wind Structure and Balance in the Hurricane Core. Part I: Hurricane Georges

2006 ◽  
Vol 63 (9) ◽  
pp. 2169-2193 ◽  
Author(s):  
Jeffrey D. Kepert
Keyword(s):  
Boundary Layer ◽  
Wind Speed ◽  
Tropical Cyclone ◽  
Radial Profile ◽  
Lower Altitude ◽  
Near Surface ◽  
Wind Structure ◽  
The Right ◽  
Tropical Cyclone Boundary Layer ◽  
Upper Boundary

Abstract The GPS dropsonde allows observations at unprecedentedly high horizontal and vertical resolution, and of very high accuracy, within the tropical cyclone boundary layer. These data are used to document the boundary layer wind field of the core of Hurricane Georges (1998) when it was close to its maximum intensity. The spatial variability of the boundary layer wind structure is found to agree very well with the theoretical predictions in the works of Kepert and Wang. In particular, the ratio of the near-surface wind speed to that above the boundary layer is found to increase inward toward the radius of maximum winds and to be larger to the left of the track than to the right, while the low-level wind maximum is both more marked and at lower altitude on the left of the storm track than on the right. However, the expected supergradient flow in the upper boundary layer is not found, with the winds being diagnosed as close to gradient balance. The tropical cyclone boundary layer model of Kepert and Wang is used to simulate the boundary layer flow in Hurricane Georges. The simulated wind profiles are in good agreement with the observations, and the asymmetries are well captured. In addition, it is found that the modeled flow in the upper boundary layer at the eyewall is barely supergradient, in contrast to previously studied cases. It is argued that this lack of supergradient flow is a consequence of the particular radial structure in Georges, which had a comparatively slow decrease of wind speed with radius outside the eyewall. This radial profile leads to a relatively weak gradient of inertial stability near the eyewall and a strong gradient at larger radii, and hence the tropical cyclone boundary layer dynamics described by Kepert and Wang can produce only marginally supergradient flow near the radius of maximum winds. The lack of supergradient flow, diagnosed from the observational analysis, is thus attributed to the large-scale structure of this particular storm. A companion paper presents a similar analysis for Hurricane Mitch (1998), with contrasting results.

Sign in / Sign up

Export Citation Format

Share Document