scholarly journals Decomposition of the mean friction drag on an NACA4412 airfoil under uniform blowing/suction

2021 ◽  
Vol 932 ◽  
Author(s):  
Yitong Fan ◽  
Marco Atzori ◽  
Ricardo Vinuesa ◽  
Davide Gatti ◽  
Philipp Schlatter ◽  
...  
Keyword(s):  
Large Scale ◽  
Control Strategies ◽  
Reference Conditions ◽  
Adverse Pressure Gradient ◽  
Suction Side ◽  
Reynolds Numbers ◽  
Friction Drag ◽  
Mode Decomposition ◽  
Control Scheme ◽  
The Mean

The application of drag-control strategies on canonical wall-bounded turbulence, such as periodic channel and zero- or adverse-pressure-gradient boundary layers, raises the question on how to distinguish consistently the origin of control effects under different reference conditions. We employ the RD identity (Renard & Deck, J. Fluid Mech., vol. 790, 2016, pp. 339–367) to decompose the mean friction drag and investigate the control effects of uniform blowing and suction applied to an NACA4412 airfoil at chord Reynolds numbers $Re_c=200\,000$ and $400\,000$ . The connection of the drag reduction/increase by using blowing/suction with the turbulence statistics (including viscous dissipation, turbulence kinetic energy production and spatial growth of the flow) across the boundary layer, subjected to adverse or favourable pressure gradients, is examined. We found that the inner and outer peaks of the contributions associated with the friction-drag generation show good scaling with either inner or outer units, respectively. They are also independent of the Reynolds number, control scheme and intensity of the blowing/suction. The small- and large-scale structures are separated with an adaptive scale-decomposition method, namely the empirical mode decomposition (EMD), which aims to analyse the scale-specific contribution of turbulent motions to friction-drag generation. Results unveil that blowing on the suction side of the airfoil is able to enhance the contribution of large-scale motions and to suppress that of small scales; however, suction behaves contrarily. The contributions related to cross-scale interactions remain almost unchanged with different control strategies.

Structure of Turbulent Flows Over Forward Facing Steps With Adverse Pressure Gradient

2016 ◽  
Vol 138 (11) ◽  
Author(s):  
Hassan Iftekhar ◽  
Martin Agelin-Chaab
Keyword(s):  
Reynolds Number ◽  
Pressure Gradient ◽  
Turbulent Flows ◽  
Large Scale ◽  
Adverse Pressure Gradient ◽  
Reynolds Numbers ◽  
Step Height ◽  
Reattachment Length ◽  
Mean Velocity ◽  
The Mean

This paper reports an experimental study on the effects of adverse pressure gradient (APG) and Reynolds number on turbulent flows over a forward facing step (FFS) by employing three APGs and three Reynolds numbers. A particle image velocimetry (PIV) technique was used to conduct velocity measurements at several locations downstream, and the flow statistics up to 68 step heights are reported. The step height was maintained at 6 mm, and the Reynolds numbers based on the step height and freestream mean velocity were 1600, 3200, and 4800. The mean reattachment length increases with the increase in Reynolds number without the APG whereas the mean reattachment length remains constant for increasing APG. The proper orthogonal decomposition (POD) results confirmed that higher Reynolds numbers caused the large-scale structures to be more defined and organized close to the step surface.

LDV and PIV Velocity Results in a Thermosiphon

2003 ◽  
Author(s):  
J. Kulman ◽  
D. Gray ◽  
S. Sivanagere ◽  
S. Guffey
Keyword(s):  
Large Scale ◽  
Single Phase ◽  
Flow Characteristics ◽  
Reynolds Numbers ◽  
Flow Patterns ◽  
Laser Doppler Velocimeter ◽  
Mean Velocity ◽  
Developed Turbulence ◽  
The Mean ◽  
Good Agreement

Heat transfer and flow characteristics have been determined for a single-phase rectangular loop thermosiphon. The plane of the loop was vertical, and tests were performed with in-plane tilt angles ranging from 3.6° CW to 4.2° CCW. Velocity profiles were measured in one vertical leg of the loop using both a single-component Laser Doppler Velocimeter (LDV), and a commercial Particle Image Velocimeter (PIV) system. The LDV data and PIV data were found to be in good agreement. The measured average velocities were approximately 2–2.5 cm/s at an average heating rate of 70 W, and were independent of tilt angle. Significant RMS fluctuations of 10–20% of the mean velocity were observed in the test section, in spite of the laminar or transitional Reynolds numbers (order of 700, based on the hydraulic diameter). These fluctuations have been attributed to vortex shedding from the upstream temperature probes and mitre bends, rather than to fully developed turbulence. Animations of the PIV data clearly show these large scale unsteady flow patterns. Multiple steady state flow patterns were not observed.

Experimental Study of Flow Inside a Centrifugal Fan Using Magnetic Resonance Velocimetry

2019 ◽  
Author(s):  
Davis W. Hoffman ◽  
Laura Villafañe ◽  
Christopher J. Elkins ◽  
John K. Eaton
Keyword(s):  
Three Dimensional ◽  
Leading Edge ◽  
Suction Side ◽  
Reynolds Numbers ◽  
Centrifugal Fan ◽  
Mean Flow ◽  
Magnetic Resonance Velocimetry ◽  
Flow Features ◽  
Outlet Flow ◽  
The Mean

Abstract Three-dimensional, three-component time-averaged velocity fields have been measured within a low-speed centrifugal fan with forward curved blades. The model investigated is representative of fans commonly used in automotive HVAC applications. The flow was analyzed at two Reynolds numbers for the same ratio of blade rotational speed to outlet flow velocity. The flow patterns inside the volute were found to have weak sensitivity to Reynolds number. A pair of counter-rotating vortices evolve circumferentially within the volute with positive and negative helicity in the upper and lower regions, respectively. Measurements have been further extended to capture phase-resolved flow features by synchronizing the data acquisition with the blade passing frequency. The mean flow field through each blade passage is presented including the jet-wake structure extending from the blade and the separation zone on the suction side of the blade leading edge.

A supersonic turbulent boundary layer in an adverse pressure gradient

1990 ◽  
Vol 211 ◽  
pp. 285-307 ◽  
Author(s):  
Emerick M. Fernando ◽  
Alexander J. Smits
Keyword(s):  
Boundary Layer ◽  
Turbulent Boundary Layer ◽  
Pressure Gradient ◽  
Pressure Distribution ◽  
Large Scale ◽  
Adverse Pressure Gradient ◽  
Large Scale Structures ◽  
Streamline Curvature ◽  
The Mean ◽  
Scale Structures

This investigation describes the effects of an adverse pressure gradient on a flat plate supersonic turbulent boundary layer (Mf ≈ 2.9, βx ≈ 5.8, Reθ, ref ≈ 75600). Single normal hot wires and crossed wires were used to study the Reynolds stress behaviour, and the features of the large-scale structures in the boundary layer were investigated by measuring space–time correlations in the normal and spanwise directions. Both the mean flow and the turbulence were strongly affected by the pressure gradient. However, the turbulent stress ratios showed much less variation than the stresses, and the essential nature of the large-scale structures was unaffected by the pressure gradient. The wall pressure distribution in the current experiment was designed to match the pressure distribution on a previously studied curved-wall model where streamline curvature acted in combination with bulk compression. The addition of streamline curvature affects the turbulence strongly, although its influence on the mean velocity field is less pronounced and the modifications to the skin-friction distribution seem to follow the empirical correlations developed by Bradshaw (1974) reasonably well.

Vortex-Excited Response of Large-Scale Cylinders in Sheared Flow

1988 ◽  
Vol 110 (3) ◽  
pp. 272-277 ◽  
Author(s):  
J. A. Humphries ◽  
D. H. Walker
Keyword(s):  
Large Scale ◽  
Shear Velocity ◽  
Reynolds Numbers ◽  
Hydrodynamic Drag ◽  
Flow Induced Vibration ◽  
Sheared Flow ◽  
Drag Forces ◽  
Linear Shear ◽  
Series Of Experiments ◽  
The Mean

A series of experiments were performed to measure the vortex-excited response of a 0.168-m-dia slender circular cylinder in a range of linear shear velocity profiles. Reynolds numbers of up to 2.5 × 105 were achieved. The results clearly showed that regular large-amplitude cylinder vibrations occurred well within the critical drag transition region. It was found that increasing the linear shear profile decreased the peak amplitude response but broadened the range of lock-on over which large oscillations occurred. The flow-induced vibration of the cylinder caused amplification of the mean hydrodynamic drag forces acting on the cylinder when compared with those expected for a similar rigid cylinder.

scholarly journals A predictive inner–outer model for streamwise turbulence statistics in wall-bounded flows

2011 ◽  
Vol 681 ◽  
pp. 537-566 ◽  
Author(s):  
ROMAIN MATHIS ◽  
NICHOLAS HUTCHINS ◽  
IVAN MARUSIC
Keyword(s):  
Pressure Gradient ◽  
Turbulent Flows ◽  
Large Scale ◽  
Adverse Pressure Gradient ◽  
Reynolds Numbers ◽  
Streamwise Velocity ◽  
Gradient Flows ◽  
Streamwise Pressure Gradient ◽  
Logarithmic Region ◽  
Inner Region

A model is proposed with which the statistics of the fluctuating streamwise velocity in the inner region of wall-bounded turbulent flows are predicted from a measured large-scale velocity signature from an outer position in the logarithmic region of the flow. Results, including spectra and all moments up to sixth order, are shown and compared to experimental data for zero-pressure-gradient flows over a large range of Reynolds numbers. The model uses universal time-series and constants that were empirically determined from zero-pressure-gradient boundary layer data. In order to test the applicability of these for other flows, the model is also applied to channel, pipe and adverse-pressure-gradient flows. The results support the concept of a universal inner region that is modified through a modulation and superposition of the large-scale outer motions, which are specific to the geometry or imposed streamwise pressure gradient acting on the flow.

Turbulence Structures of Leading Edge Film Cooling Jets

2000 ◽  
Author(s):  
Sabine Ardey ◽  
Stefan Wolff ◽  
Leonhard Fottner
Keyword(s):  
Film Cooling ◽  
Large Scale ◽  
Leading Edge ◽  
Adverse Pressure Gradient ◽  
Suction Side ◽  
Pressure Side ◽  
Reynolds Stresses ◽  
Mean Flow ◽  
Turbulence Structures ◽  
Injection Zone

For a better understanding of the turbulence structures attached to film cooling jets, mean flow velocities and turbulent fluctuations were measured by means of 3D hot wire anemometry in the injection zone of a linear, large scale, high pressure turbine cascade with leading edge film cooling. Near the stagnation point, the blades are equipped with one row of film cooling holes each on the suction and pressure side. Two basically different coolant jet situations are compared: On the suction side the jet features the ordinary kidney vortex. On the pressure side, the jet separates completely from the blade surface since it is located above a large recirculation zone created by a locally adverse pressure gradient and a flow separation near the pressure side injection. The measured Reynolds stresses were analyzed with regard to turbulence production and diffusion. The Bousinesque Hypothesis was tested and could not be confirmed. It was found that the turbulence is highly anisotropic. In addition to the brief description of the experimental set up and the acquired data, given in this paper, the complete information are published as a test case (Ardey and Fottner, 1998) that is directly accessible via internet at: http://www.unibw-muenchen.de/campus/LRT12/welcome.html

scholarly journals Scale interactions in turbulent rotating planar Couette flow: insight through the Reynolds stress transport

2019 ◽  
Vol 879 ◽  
pp. 255-295 ◽  
Author(s):  
Takuya Kawata ◽  
P. Henrik Alfredsson
Keyword(s):  
Couette Flow ◽  
Reynolds Stress ◽  
Coriolis Force ◽  
Large Scale ◽  
Reynolds Numbers ◽  
Wall Structures ◽  
Rotation Numbers ◽  
Small Scales ◽  
The Mean ◽  
Scale Turbulence

In turbulent planar Couette flow under anticyclonic spanwise system rotation, large-scale roll-cell structures arise due to a Coriolis-force-induced instability. The structures are superimposed on smaller-scale turbulence, and with increasing angular velocity ($\unicode[STIX]{x1D6FA}_{z}$) such roll cells dominate the flow field and small-scale turbulence is instead suppressed in a certain rotation number range $0<Ro\lesssim 0.1$ ($Ro=2\unicode[STIX]{x1D6FA}_{z}h/U_{w}$, where $h$ is the channel half-width, $U_{w}$ the wall velocity). At low rotation numbers around $Ro\approx 0.02$ both large-scale roll cells and smaller-scale turbulence coexist. In the present study, we investigate interaction between these structures through a scale-by-scale analysis of the Reynolds stress transport. We show that at low rotation numbers $Ro\approx 0.01$ the turbulence productions by the mean flow gradient and the Coriolis force occur at different scales and thereby the turbulent energy distribution over a wide range of scales is maintained. On the other hand at higher rotation numbers $Ro\gtrsim 0.05$, a zero-absolute-vorticity state is established and production of small scales from the mean shear disappears although large-scale turbulence production is maintained through the Coriolis force. At high enough Reynolds numbers, where scale separation between the near-wall structures and the roll cells is relatively distinct, transition between these different $Ro$ regimes is found to occur rather abruptly around $Ro\approx 0.02$, resulting in a non-monotonic behaviour of the wall shear stress as a function of $Ro$. It is also shown that at such an intermediate rotation number the roll cells interact with smaller scales by moving near-wall structures towards the core region of the channel, by which the Reynolds stress is transported from relatively small scales near the wall towards larger scales in the channel centre. Such Reynolds stress transport by scale interaction becomes increasingly significant as the Reynolds number increases, and results in a reversed mean velocity gradient at the channel centre at high enough Reynolds numbers.

scholarly journals Turbulent Taylor–Couette flow with stationary inner cylinder

2016 ◽  
Vol 799 ◽  
Author(s):  
R. Ostilla-Mónico ◽  
R. Verzicco ◽  
D. Lohse
Keyword(s):  
Angular Momentum ◽  
Large Scale ◽  
Outer Cylinder ◽  
Reynolds Numbers ◽  
Law Of The Wall ◽  
Large Scale Structures ◽  
Large Gradient ◽  
The Mean ◽  
Taylor Couette ◽  
Cylinder Rotation

A series of direct numerical simulations were performed of Taylor–Couette (TC) flow, the flow between two coaxial cylinders, with the outer cylinder rotating and the inner one fixed. Three cases were considered, where the Reynolds number of the outer cylinder was $Re_{o}=5.5\times 10^{4}$, $Re_{o}=1.1\times 10^{5}$ and $Re_{o}=2.2\times 10^{5}$. The ratio of radii ${\it\eta}=r_{i}/r_{o}$ was fixed to ${\it\eta}=0.909$ to mitigate the effects of curvature. Axially periodic boundary conditions were used, with the aspect ratio of vertical periodicity ${\it\Gamma}$ fixed to ${\it\Gamma}=2.09$. Being linearly stable, TC flow with outer cylinder rotation is known to have very different behaviour than TC flow with pure inner cylinder rotation. Here, we find that the flow nonetheless becomes turbulent, but the torque required to drive the cylinders and level of velocity fluctuations was found to be smaller than those for pure inner cylinder rotation at comparable Reynolds numbers. The mean angular momentum profiles showed a large gradient in the bulk, instead of the constant angular momentum profiles of pure inner cylinder rotation. The near-wall mean and fluctuation velocity profiles were found to coincide only very close to the wall, showing large deviations from both pure inner cylinder rotation profiles and the classic von Karman law of the wall elsewhere. Finally, transport of angular velocity was found to occur mainly through intermittent bursts, and not through wall-attached large-scale structures as is the case for pure inner cylinder rotation.

scholarly journals Wavy Walls, a Passive Way to Control the Transition to Turbulence. Detailed Simulation and Physical Explanation

Energies ◽  
2021 ◽  
Vol 14 (13) ◽  
pp. 3937
Author(s):  
Andrés Mateo-Gabín ◽  
Miguel Chávez ◽  
Jesús Garicano-Mena ◽  
Eusebio Valero
Keyword(s):  
Control Strategies ◽  
Reynolds Numbers ◽  
Dynamic Mode Decomposition ◽  
Transition To Turbulence ◽  
Dynamic Mode ◽  
Mode Decomposition ◽  
Detailed Simulation ◽  
Walled Channel ◽  
Laminar And Turbulent Regimes ◽  
Careful Design

Inducing spanwise motions in the vicinity of solid boundaries alters the energy, mass and/or momentum transfer. Under some conditions, these motions are such that drag is reduced and/or transition to turbulence is delayed. There are several possibilities to induce those spanwise motions, be it through active imposition a predefined velocity distribution at the walls or by careful design of the wall shape, which corresponds to passive control.In this contribution, we investigate the effect that wavy walls might have on delaying transition to turbulence. Direct Numerical Simulation of both planar and wavy-walled channel flows at laminar and turbulent regimes are conducted. A pseudo laminar regime that remains stable until a Reynolds number 20% higher that the critical is found for the wavy-walled simulations. Dynamic Mode Decomposition applied to the simulation data reveals that in these configurations, modes with wavelength and frequency compatible with the surface undulation pattern appear. We explain and visualize the appearance of these modes. At higher Reynolds numbers we show that these modes remain present but are not dominant anymore. This work is an initial demonstration that flow control strategies that trigger underlying stable modes can keep or conduct the flow to new configurations more stable than the original one.

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