Comparison of Flying-Hot-Wire and Stationary-Hot-Wire Measurements of Flow Over a Backward-Facing Step

1999 ◽  
Vol 121 (2) ◽  
pp. 441-445 ◽  
Author(s):  
O. O. Badran ◽  
H. H. Bruun
Keyword(s):  
Reverse Flow ◽  
Separation Bubble ◽  
Separated Flow ◽  
Leading Edge ◽  
Flow Region ◽  
Hot Wire ◽  
Backward Facing Step ◽  
Mean Velocity ◽  
Hot Wire Anemometry ◽  
Blunt Leading Edge

This paper is concerned with measurements of the flow field in the separated flow region behind a backward-facing step. The main instrument used in this research was Flying X Hot-Wire Anemometry (FHWA). Stationary (single normal) Hot-Wire Anemometry (SHWA) was also used. Comparative measurements between the SHW probe and the FHW system were conducted downstream of the step (step height H = 120 mm) and results are presented for axial locations of 1H and 2H. Two step configurations were considered; (i) a blunt leading edge with flow underneath (Case I) and (ii) a blunt leading edge with no flow underneath (Case II). It is observed from the results presented that the two Hot-Wire methods produce significantly different mean velocity and turbulence results inside the separation bubble. In particular, the SHWA method cannot detect the reverse flow velocity direction, while the Flying Hot-Wire clearly identifies the existing reverse flow. Also, in the shear flow region, the results presented indicate that measurements with a SHW probe must be treated with great caution.

scholarly journals Numerical investigation of the effect of airfoil thickness on onset of dynamic stall

2019 ◽  
Vol 870 ◽  
pp. 870-900 ◽  
Author(s):  
Anupam Sharma ◽  
Miguel Visbal
Keyword(s):  
Reverse Flow ◽  
Separation Bubble ◽  
Time Integration ◽  
Angle Of Attack ◽  
Leading Edge ◽  
Flow Region ◽  
Dynamic Stall ◽  
Implicit Time ◽  
Gradual Change ◽  
Dynamic Simulations

Effect of airfoil thickness on onset of dynamic stall is investigated using large eddy simulations at chord-based Reynolds number of 200 000. Four symmetric NACA airfoils of thickness-to-chord ratios of 9 %, 12 %, 15 % and 18 % are studied. The three-dimensional Navier–Stokes solver, FDL3DI is used with a sixth-order compact finite difference scheme for spatial discretization, second-order implicit time integration and discriminating filters to remove unresolved wavenumbers. A constant-rate pitch-up manoeuver is studied with the pitching axis located at the airfoil quarter chord. Simulations are performed in two steps. In the first step, the airfoil is kept static at a prescribed angle of attack ($=4^{\circ }$). In the second step, a ramp function is used to smoothly increase the pitch rate from zero to the selected value and then the pitch rate is held constant until the angle of attack goes past the lift-stall point. The solver is verified against experiments for flow over a static NACA 0012 airfoil. Static simulation results of all airfoil geometries are also compared against XFOIL predictions with a generally favourable agreement. FDL3DI predicts two-stage transition for thin airfoils (9 % and 12 %), which is not observed in the XFOIL results. The dynamic simulations show that the onset of dynamic stall is marked by the bursting of the laminar separation bubble (LSB) in all the cases. However, for the thickest airfoil tested, the reverse flow region spreads over most of the airfoil and reaches the LSB location immediately before the LSB bursts and dynamic stall begins, suggesting that the stall could be triggered by the separated turbulent boundary layer. The results suggest that the boundary between different classifications of dynamic stall, particularly leading edge stall versus trailing edge stall, is blurred. The dynamic-stall onset mechanism changes gradually from one to the other with a gradual change in some parameters, in this case, airfoil thickness.

Laser-Doppler-Velocimetry Measurements in a Cascade of Compressor Blades at Stall

1998 ◽  
Vol 120 (1) ◽  
pp. 170-178 ◽  
Author(s):  
G. V. Hobson ◽  
A. J. H. Williams ◽  
H. J. Ganaim Rickel
Keyword(s):  
Flow Visualization ◽  
Laser Doppler Velocimetry ◽  
Reverse Flow ◽  
Separation Bubble ◽  
Laser Sheet ◽  
Leading Edge ◽  
Flow Region ◽  
Laser Doppler ◽  
Laser Doppler Velocimeter ◽  
Doppler Velocimetry

Compressor stall was simulated in the Low-Speed Cascade Wind Tunnel at the Turbopropulsion Laboratory of the Naval Postgraduate School. The test blades were of controlled-diffusion design with a solidity of 1.67, and stalling occurred at 10 deg of incidence above the design inlet air angle. All measurements were taken at a flow Reynolds number, based on chord length, of 700,000. Laser-sheet flow visualization techniques showed that the stalling process was unsteady and occurred over the whole cascade. Detailed laser-Doppler-velocimetry measurements over the suction side of the blades showed regions of continuous and intermittent reverse flow. The measurements of the continuous reverse flow region at the leading edge were the first data of their kind in the leading edge separation bubble. The regions of intermittent reverse flow, measured with laser-Doppler velocimeter, corresponded to the flow visualization studies. Blade surface pressure measurements showed a decrease in normal force on the blade, as would be expected at stall. Data are presented in a form that characterizes the unsteady positive and negative velocities about their mean, for both the continuous reverse flow regions and the intermittent reverse flow regions.

Laser-Doppler Velocimetry Measurements in a Cascade of Compressor Blades at Stall

1996 ◽  
Author(s):  
Garth V. Hobson ◽  
Andrew J. H. Williams ◽  
Humberto J. Ganaim Rickel
Keyword(s):  
Flow Visualization ◽  
Laser Doppler Velocimetry ◽  
Reverse Flow ◽  
Separation Bubble ◽  
Laser Sheet ◽  
Leading Edge ◽  
Flow Region ◽  
Laser Doppler ◽  
Laser Doppler Velocimeter ◽  
Doppler Velocimetry

Compressor stall was simulated in the Low Speed Cascade Wind Tunnel at the Turbopropulsion Laboratory. The test blades were of controlled-diffusion design with a solidity of 1.67, and stalling occurred at 10 degrees of incidence above the design inlet air angle. All measurements were taken at a flow Reynolds number, based on chord length, of 700 000. Laser-sheet flow visualization techniques showed that the stalling process was unsteady and occurred over the whole cascade. Detailed laser-Doppler-velocimetry measurements over the suction side of the blades showed regions of continuous and intermittent reverse flow. The measurements of the continuous reverse flow region at the leading edge were the first data of their kind in the leading edge separation bubble. The regions of intermittent reverse flow, measured with laser Doppler velocimeter, corresponded to the flow visualization studies. Blade surface pressure measurements showed a decrease in normal force on the blade as would be expected at stall. Data is presented in a form which characterizes the unsteady positive and negative velocities about their mean, for both the continuous reverse flow regions and the intermittent reverse flow regions.

Experimental Study on the Internal Separated and Inverse Flow Between the Blades

2013 ◽  
Author(s):  
Hiroki Furuta ◽  
Osamu Terashima ◽  
Yasuhiko Sakai ◽  
Kouji Nagata ◽  
Shunsuke Ishiguro ◽  
...  
Keyword(s):  
Kinetic Energy ◽  
Energy Loss ◽  
Test Section ◽  
Velocity Fluctuation ◽  
Separation Bubble ◽  
Flow Direction ◽  
Separated Flow ◽  
Leading Edge ◽  
Hot Wire ◽  
Blade Thickness

The objective of this study is to investigate an internal separated flow between the blades of turbo machinery. Flow separation often causes undesirable phenomena such as an increase of the total pressure loss and a vibration of the machine. Therefore, understanding the characteristics of the separated flow in detail is very important for optimizing the machine to decrease the energy loss. In general, the separated flow involves a reverse flow near the solid wall in the separation bubble and its reattachment of the further downstream location. Hence, a typical hot-wire sensor is not useful for measuring the internal separated flow between the blades because it can detect only the magnitude of the flow velocity, not the flow direction. Based on this background, a self-developed tandem-type hot-wire sensor, by which both the magnitude and the flow direction can be detected, is used to measure the velocity field between the blades in this study. The tandem-type hot-wire sensor consists of two I-type hot-wire sensors and a small insulated elliptical cylinder placed between them. A calibration test is first conducted to validate its performance. Subsequently, the separated flow between the blades is measured with the tandem-type hot-wire sensor. The experimental apparatus consists of a closed-type test section which is connected to the nozzle exit of a blowout-type wind tunnel. In this test section, a sample of blade is set up. In this study, experiments are conducted with three kinds of blades with the different shapes (i.e., experiments are performed under three different conditions): a constant blade thickness from the leading edge to the trailing edge (Blade 1), a constant blade thickness from the leading edge to trailing edge but a rounded leading edge (Blade 2) and a thin blade thickness at the leading and trailing edges (Blade 3). In addition, the unsteady internal separated flow between the blades is also investigated by large-eddy simulation (LES) whose validity was fully confirmed in the previous study. The flow field and dissipation rate of the turbulent kinetic energy obtained by the simulation are discussed. Experimental and numerical results show that Blade 3 has a smallest separation bubble around the leading edge than that of Blade 1 and 2, and shows a smallest root mean square (RMS) value for the velocity fluctuation near the reattachment point than them. These differences of the size of separation bubble and velocity fluctuation were considered to be resulted in a decrease of the kinetic energy loss in the test section with Blade 3. Therefore, it can be concluded that the non-uniform thickness of the blade causes the decrease of the energy loss around the blade.

Characteristics of a separated flow past a semicircular leading-edge airfoil model under different imposed pressure gradient

2021 ◽  
pp. 095441002110212
Author(s):  
K Anand ◽  
KT Ganesh
Keyword(s):  
Boundary Layer ◽  
Particle Image Velocimetry ◽  
Pressure Gradient ◽  
Particle Image ◽  
Separation Bubble ◽  
Separated Flow ◽  
Leading Edge ◽  
Field Measurements ◽  
Bench Mark ◽  
Image Velocimetry

The effect of pressure gradient on a separated boundary layer past the leading edge of an airfoil model is studied experimentally using electronically scanned pressure (ESP) and particle image velocimetry (PIV) for a Reynolds number ( Re) of 25,000, based on leading-edge diameter ( D). The features of the boundary layer in the region of separation and its development past the reattachment location are examined for three cases of β (−30°, 0°, and +30°). The bubble parameters such as the onset of separation and transition and the reattachment location are identified from the averaged data obtained from pressure and velocity measurements. Surface pressure measurements obtained from ESP show a surge in wall static pressure for β = −30° (flap deflected up), while it goes down for β = +30° (flap deflected down) compared to the fundamental case, β = 0°. Particle image velocimetry results show that the roll up of the shear layer past the onset of separation is early for β = +30°, owing to higher amplification of background disturbances compared to β = 0° and −30°. Downstream to transition location, the instantaneous field measurements reveal a stretched, disoriented, and at instances bigger vortices for β = +30°, whereas a regular, periodically shed vortices, keeping their identity past the reattachment location, is observed for β = 0° and −30°. Above all, this study presents a new insight on the features of a separation bubble receiving a disturbance from the downstream end of the model, and these results may serve as a bench mark for future studies over an airfoil under similar environment.

scholarly journals On the structure of the self-sustaining cycle in separating and reattaching flows

2018 ◽  
Vol 857 ◽  
pp. 907-936 ◽  
Author(s):  
A. Cimarelli ◽  
A. Leonforte ◽  
D. Angeli
Keyword(s):  
Shear Layer ◽  
Rectangular Plate ◽  
Large Scale ◽  
Reverse Flow ◽  
Leading Edge ◽  
Streamwise Velocity ◽  
The Self ◽  
Small Scale ◽  
Spectral Evolution ◽  
Mean Velocity

The separating and reattaching flows and the wake of a finite rectangular plate are studied by means of direct numerical simulation data. The large amount of information provided by the numerical approach is exploited here to address the multi-scale features of the flow and to assess the self-sustaining mechanisms that form the basis of the main unsteadinesses of the flows. We first analyse the statistically dominant flow structures by means of three-dimensional spatial correlation functions. The developed flow is found to be statistically dominated by quasi-streamwise vortices and streamwise velocity streaks as a result of flow motions induced by hairpin-like structures. On the other hand, the reverse flow within the separated region is found to be characterized by spanwise vortices. We then study the spectral properties of the flow. Given the strongly inhomogeneous nature of the flow, the spectral analysis has been conducted along two selected streamtraces of the mean velocity field. This approach allows us to study the spectral evolution of the flow along its paths. Two well-separated characteristic scales are identified in the near-wall reverse flow and in the leading-edge shear layer. The first is recognized to represent trains of small-scale structures triggering the leading-edge shear layer, whereas the second is found to be related to a very large-scale phenomenon that embraces the entire flow field. A picture of the self-sustaining mechanisms of the flow is then derived. It is shown that very-large-scale fluctuations of the pressure field alternate between promoting and suppressing the reverse flow within the separation region. Driven by these large-scale dynamics, packages of small-scale motions trigger the leading-edge shear layers, which in turn created them, alternating in the top and bottom sides of the rectangular plate with a relatively long period of inversion, thus closing the self-sustaining cycle.

Time-Dependent Laminar Backward-Facing Step Flow in a Two-Dimensional Duct

1988 ◽  
Vol 110 (3) ◽  
pp. 289-296 ◽  
Author(s):  
F. Durst ◽  
J. C. F. Pereira
Keyword(s):  
Reynolds Number ◽  
Steady State ◽  
Separated Flow ◽  
Flow Region ◽  
Reynolds Numbers ◽  
Steady State Flow ◽  
Backward Facing Step ◽  
Step Flow ◽  
Recirculating Flow ◽  
State Flow

This paper presents results of numerical studies of the impulsively starting backward-facing step flow with the step being mounted in a plane, two-dimensional duct. Results are presented for Reynolds numbers of Re = 10; 368 and 648 and for the last two Reynolds numbers comparisons are given between experimental and numerical results obtained for the final steady state flow conditions. In the computational scheme, the convective terms in the momentum equations are approximated by a 13-point quadratic upstream weighted finite-difference scheme and a fully implicit first order forward differencing scheme is used to discretize the temporal derivatives. The computations show that for the higher Reynolds numbers, the flow starts to separate on the lower and upper corners of the step yielding two disconnected recirculating flow regions for some time after the flow has been impulsively started. As time progresses, these two separated flow regions connect up and a single recirculating flow region emerges. This separated flow region stays attached to the step, grows in size and approaches, for the time t → ∞, the dimensions measured and predicted for the separation region for steady laminar backward-facing flow. For the Reynolds number Re = 10 the separation starts at the bottom of the backward-facing step and the separation region enlarges with time until the steady state flow pattern is reached. At the channel wall opposite to the step and for Reynolds number Re = 368, a separated flow region is observed and it is shown to occur for some finite time period of the developing, impulsively started backward-facing step flow. Its dimensions change with time and reduce to zero before the steady state flow pattern is reached. For the higher Reynolds number Re = 648, the secondary separated flow region opposite to the wall is also present and it is shown to remain present for t → ∞. Two kinds of the inlet conditions were considered; the inlet mean flow was assumed to be constant in a first study and was assumed to increase with time in a second one. The predicted flow field for t → ∞ turned out to be identical for both cases. They were also identical to the flow field predicted for steady, backward-facing step flow using the same numerical grid as for the time-dependent predictions.

Laminar Mixed Convection Adjacent to Three-Dimensional Backward-Facing Step

2001 ◽  
Vol 124 (1) ◽  
pp. 209-213 ◽  
Author(s):  
A. Li and ◽  
B. F. Armaly
Keyword(s):  
Nusselt Number ◽  
Mixed Convection ◽  
Reverse Flow ◽  
Three Dimensional ◽  
Flow Region ◽  
The Other ◽  
Backward Facing Step ◽  
Number Range ◽  
Grashof Number ◽  
Recirculation Flow

Simulations of three-dimensional laminar buoyancy-assisting mixed convection adjacent to a backward-facing step in a vertical rectangular duct are presented to demonstrate the influence of Grashof number on the distributions of the Nusselt number, and the reverse flow regions that develop adjacent to the duct’s walls. The Reynolds number, and duct’s geometry are kept constant: heat flux at the wall downstream from the step is kept uniform but its magnitude varied to cover a Grashof number range of 0–4000; all the other walls in the duct are kept at adiabatic condition; and the flow, upstream of the step, is treated as fully developed and isothermal. Increasing the Grashof number results in increasing the Nusselt number; the size of the secondary recirculation flow region adjacent to the stepped wall; the size of the reverse flow region adjacent to the sidewall and the flat wall; and the spanwise flow from the sidewall toward the center of the duct. On the other hand, the size of the primary recirculation flow region adjacent to the stepped wall decreases and detaches partially from the heated stepped wall as the Grashof number increases. Details are presented and discussed.

The reduction and elimination of a closed separation region by free-stream turbulence

2001 ◽  
Vol 446 ◽  
pp. 271-308 ◽  
Author(s):  
M. KALTER ◽  
H. H. FERNHOLZ
Keyword(s):  
Boundary Layer ◽  
Pressure Gradient ◽  
Free Stream ◽  
Reverse Flow ◽  
Flow Region ◽  
Adverse Pressure Gradient ◽  
Turbulence Structure ◽  
Hot Wire ◽  
Free Stream Turbulence ◽  
The Mean

This paper is an extension of an experimental investigation by Alving & Fernholz (1996). In the present experiments the effects of free-stream turbulence were investigated on a boundary layer with an adverse pressure gradient and a closed reverse-flow region. By adding free-stream turbulence the mean reverse-flow region was shortened or completely eliminated and this was used to control the size of the separation bubble. The turbulence intensity was varied between 0.2% and 6% using upstream grids while the turbulence length scale was on the order of the boundary layer thickness. Mean and fluctuating velocities as well as spectra were measured by means of hot-wire and laser-Doppler anemometry and wall shear stress by wall pulsed-wire and wall hot-wire probes.Free-stream turbulence had a small effect on the boundary layer in the mild adverse-pressure-gradient region but in the vicinity of separation and along the reverse-flow region mean velocity profiles, skin friction and turbulence structure were strongly affected. Downstream of the mean or instantaneous reverse-flow regions highly disturbed boundary layers developed in a nominally zero pressure gradient and converged to a similar turbulence structure in all three cases at the end of the test section. This state was, however, still very different from that in a canonical boundary layer.

DNS of Expansion Ratio Effects on Three-Dimensional Unsteady Separated Flow and Heat Transfer Around a Downward Step

2005 ◽  
Author(s):  
Aya Kito ◽  
Kazuaki Sugawara ◽  
Hiroyuki Yoshikawa ◽  
Terukazu Ota
Keyword(s):  
Heat Transfer ◽  
Rectangular Channel ◽  
Three Dimensional ◽  
Side Wall ◽  
Separated Flow ◽  
Flow Region ◽  
Expansion Ratio ◽  
Mean Velocity ◽  
Flow And Heat Transfer ◽  
Channel Expansion

The direct numerical simulation methodology was employed to analyze the unsteady features of a three-dimensional separated flow and heat transfer around a downward step in a rectangular channel, and to clarify systematically the channel expansion ratio effects upon them. Numerical calculations were carried out using the finite difference method. The Reynolds number Re based on the mean velocity at inlet and the step height was varied from 300 to 1000. The channel expansion ratio ER is 1.5, 2.0 and 3.0 under a step aspect ratio of 36.0. It is found that the flow is steady upto Re = 500 but becomes sensibly unsteady at Re = 700 for all the three expansion ratios. In the case of ER = 2.0, the separated shear layer is most unstable. In the case of ER = 1.5, the longitudinal vortices formed near the side walls of channel are strongest. Nusselt number reaches its maximum in the reattachment flow region and also in the neighborhood of the side wall, and their locations depend greatly upon ER and Re.

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