Experimental analysis of radially resolved  dynamic inflow effects due to pitch steps

Abstract. The dynamic inflow effect denotes the unsteady aerodynamic response to fast changes in rotor loading due to a gradual adaption of the wake. This does lead to load overshoots. The objective of the paper was to increase the understanding of that effect based on pitch step experiments on a 1.8 m diameter model wind turbine, which are performed in the large open jet wind tunnel of ForWind – University of Oldenburg. The flow in the rotor plane is measured with a 2D laser Doppler anemometer, and the dynamic wake induction factor transients in axial and tangential direction are extracted. Further, integral load measurements with strain gauges and hot-wire measurements in the near and close far wake are performed. The results show a clear gradual decay of the axial induction factors after a pitch step, giving the first direct experimental evidence of dynamic inflow due to pitch steps. Two engineering models are fitted to the induction factor transients to further investigate the relevant time constants of the dynamic inflow process. The radial dependency of the axial induction time constants as well as the dependency on the pitch direction is discussed. It is confirmed that the nature of the dynamic inflow decay is better described by two rather than only one time constant. The dynamic changes in wake radius are connected to the radial dependency of the axial induction transients. In conclusion, the comparative discussion of inductions, wake deployment and loads facilitate an improved physical understanding of the dynamic inflow process for wind turbines. Furthermore, these measurements provide a new detailed validation case for dynamic inflow models and other types of simulations.

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Experimental analysis of radially resolved dynamic inflow effects due to pitch steps

10.5194/wes-2021-70 ◽

2021 ◽

Author(s):

Frederik Berger ◽

David Onnen ◽

J. Gerard Schepers ◽

Martin Kühn

Keyword(s):

Tangential Direction ◽

Strain Gauges ◽

Hot Wire ◽

Dynamic Changes ◽

Time Constants ◽

Unsteady Aerodynamic ◽

Pitch Direction ◽

Engineering Models ◽

Gradual Decay ◽

Far Wake

Abstract. The dynamic inflow effect denotes the unsteady aerodynamic response to fast changes in rotor loading due to a gradual adaption of the wake. This does lead to load overshoots. The objective of the paper was to increase the understanding of that effect based on pitch step experiments on a 1.8 m diameter model wind turbine, which we performed in the large open jet wind tunnel of ForWind – University of Oldenburg. We measured the flow in the rotor plane with a 2D Laser Doppler Anemometer and were able to extract the dynamic wake induction factor transients in axial and tangential direction. Further, integral load measurements with strain gauges and hot wire measurements in the near and close far wake were performed. Our results show a clear gradual decay of the axial induction factors after a pitch step, giving the first direct experimental evidence of dynamic inflow due to pitch steps. We fitted two engineering models to the induction factor transients to further investigate the relevant time constants of the dynamic inflow process.We discussed the radial dependency of the axial induction time constants as well as the dependency on the pitch direction. We confirmed that the nature of the dynamic inflow decay is better described by two rather than only one time constant. The dynamic changes in wake radius were connected to the radial dependency of the axial induction transients. In conclusion, the comparative discussion of inductions, wake deployment and loads facilitated the improved physical understanding of the dynamic inflow process for wind turbines. Furthermore, these measurements provide a new detailed validation case for dynamic inflow models and other types of simulations.

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Wavelet Analysis of Memory Effects on Turbulence Structures in a Far Wake

Volume 1: Fora, Parts A and B ◽

10.1115/fedsm2002-31115 ◽

2002 ◽

Author(s):

Hui Li ◽

Yu Zhou ◽

Masahiro Takei ◽

Yoshifuru Saito ◽

Kiyoshi Horii

Keyword(s):

Reynolds Shear Stress ◽

Initial Condition ◽

Streamwise Direction ◽

Hot Wire ◽

Cylinder Axis ◽

Orthogonal Wavelet ◽

Turbulence Structures ◽

Streamwise Distance ◽

Far Wake ◽

Orthogonal Wavelet Transform

Initial condition effects in a self-preserving plane wake were investigated for two wake generators, i.e. a triangular cylinder and a screen of 50% solidity. Two orthogonal arrays of sixteen X-wires, eight in the (x, y)-plane, i.e. the plane of mean shear, and eight in the (x, z)-plane, which is parallel to both the cylinder axis and the streamwise direction, are used to simultaneously obtain velocity data in the experimental investigation. Measurements were made at x/h (x is the streamwise distance downstream of the cylinder and h is the height of the wake generator) = 330 for the triangular cylinder and 220 for the screen. A two-dimensional orthogonal wavelet transform is used to analyze the measured hot-wire data. This technique enables the turbulence structures of various scales to be separated and characterized. Discernible differences are observed between the two wake generators in the turbulence structures of large- down to intermediate-scales. The differences are quantified in terms of the Reynolds shear stress, kinetic energy and root mean square vorticity values.

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Corrections for the response time and delay of mass spectrometers

Journal of Applied Physiology ◽

10.1152/jappl.1981.51.6.1417 ◽

1981 ◽

Vol 51 (6) ◽

pp. 1417-1422 ◽

Cited By ~ 22

Author(s):

R. Arieli ◽

H. D. Van Liew

Keyword(s):

Response Time ◽

Main Part ◽

Good Resolution ◽

Response Curves ◽

Dynamic Changes ◽

Mass Spectrometers ◽

Time Constants ◽

Input Event ◽

Time Correction ◽

The Difference

Computations based on mass spectrometer (MS) outputs will be incorrect unless the delay for drawing the sample into the instrument and response time of the instrument are accounted for. When we changed concentration abruptly in two different mass spectrometers, the responses were sigmoid shaped not exponential, and time constants derived from the main part of the response curves were 43–60 ms; single-exponent corrections using these values caused the corrected waveform to overshoot. For a better correction, we used a two-exponent correction, C2 = Co + (Y1 + Y2) (d2C0/dt) + Y1Y2 (d2C0/dt2), where C0 is MS output as a function of time t, C2 is corrected concentration, and Y1 and Y2 are time constants. Assumption of a third exponent was of little value. For a successful correction Y1 must be smaller than a measured one-exponent time constant. We used two-thirds of the measured value for Y1 and then calculated Y2 from the once-corrected response. The second-order correction approximates a square output in response to a square input. To deal with delay time in a way that would give good resolution of dynamic changes and also be compatible with our response-time correction, we corrected for the difference between time of the input event and time that the output reaches 20% of full response. We validated our methods by integrations of amounts of gases drawn into and out of a syringe and in human breaths.

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Entrainment vortices and interfacial intermittent turbulent bulges in a plane turbulent wake

Journal of Fluid Mechanics ◽

10.1017/s0022112002001672 ◽

2002 ◽

Vol 469 ◽

pp. 49-70 ◽

Cited By ~ 6

Author(s):

GREGORY A. KOPP ◽

FRANCESC GIRALT ◽

JAMES F. KEFFER

Keyword(s):

Phenomenological Models ◽

Coherent Structures ◽

Large Scale ◽

Horseshoe Vortex ◽

Turbulent Wake ◽

Wake Region ◽

Hot Wire ◽

Helmholtz Instability ◽

Far Wake ◽

Plane Turbulent Wake

Hot-wire measurements were made simultaneously in two homogeneous ‘horizontal’ planes in the far-wake region of a cylinder. A technique developed using hot-wire data to identify the spatial characteristics of the large-scale bulges at the interface between the internal turbulent motions and the external irrotational flow was used to unambiguously relate these outer intermittent bulges to the inner coherent structures. It was found that a turbulent bulge is made up of a combination of a horseshoe vortex (whose legs form one double-roller eddy) and the straining region present just upstream of this structure. The approach also allowed the evaluation of the two most prominent phenomenological models for the entrainment mechanism in the far-wake region: the Kelvin–Helmholtz instability and Townsend's growth–decay cycle. It was found that the decaying and re-forming of the bulges and entrainment structures is not likely to occur. Rather, the evidence is that the large-scale bulges remain coherent for long streamwise distances in equilibrium with the overall similarity of the flow.

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Ultrastructural Cytopiasmic Changes of Adrenal Cortex During Pregnancy

Proceedings, annual meeting, Electron Microscopy Society of America ◽

10.1017/s0424820100063408 ◽

1969 ◽

Vol 27 ◽

pp. 298-299

Author(s):

T. M. Murad ◽

Karen Israel ◽

Jack C. Geer

Keyword(s):

Adrenal Gland ◽

Steroid Hormones ◽

Adrenal Cortex ◽

Lipid Droplets ◽

Plasma Cholesterol ◽

Adrenal Steroids ◽

Dynamic Changes ◽

Cortical Cells ◽

Acetyl Coenzyme A ◽

Adrenal Cortical Cells

Adrenal steroids are normally synthesized from acetyl coenzyme A via cholesterol. Cholesterol is also shown to enter the adrenal gland and to be localized in the lipid droplets of the adrenal cortical cells. Both pregnenolone and progesterone act as intermediates in the conversion of cholesterol into steroid hormones. During pregnancy an increased level of plasma cholesterol is known to be associated with an increase of the adrenal corticoid and progesterone. The present study is designed to demonstrate whether the adrenal cortical cells show any dynamic changes during pregnancy.

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