Editing Measurement of Near-Bed Velocity and Bottom Shear Stress by Ferrofluids

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Z. You, B. Yin, and G. Huo, “Direct Measurement of Wave-Induced Bottom Shear Stress Under Irregular Waves,” in Advances in Water Resources and Hydraulic Engineering, Springer, 2009, pp. 1213–1218.
 
Z. You, B. Yin, and G. Huo, “Direct Measurement of Wave-Induced Bottom Shear Stress Under Irregular Waves,” in Advances in Water Resources and Hydraulic Engineering, Springer, 2009, pp. 1213–1218.
 
</ref>
 
</ref>
. The system allows to resolve shear stress of O(1 mNm<sup>-2</sup>). However, one of the main problems while using shear plates is the tradeoff between sensor spatial resolution and the sensor capability to measure small forces.  
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. The system allows to resolve shear stress of O(1 mNm−2). However, one of the main problems while using shear plates is the tradeoff between sensor spatial resolution and the sensor capability to measure small forces.  
  
 
Another class of instruments is the one of thermal sensors, to which hot film anemometers belong  
 
Another class of instruments is the one of thermal sensors, to which hot film anemometers belong  
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<figure id="fig:Ferrofluidspike" >
 
<figure id="fig:Ferrofluidspike" >
[[Image:UC FERR 02.png|thumb|center|650px|'''<xr id="fig:Ferrofluidspike" />''': Ferrofluid spike (adapted from <ref name = "Musumeci2018"/>): (I) hydrostatic conditions; (II) dynamic conditions (mean velocity U = 0.18ms<sup>-1</sup>; velocity at the ferrofluid tip UFF = 0.11ms<sup>-1</sup>, bottom shear stress in the flow direction τ x0 = 0.096Nm<sup>-2</sup>); (II) dynamic conditions (mean velocity U = 0.26ms<sup>-1</sup>; velocity at the ferrofluid tip UFF = 0.19ms<sup>-1</sup>, bottom shear stress in the flow direction τ x0 = 0.200Nm<sup>-2</sup>).]]
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[[Image:UC FERR 02.png|thumb|center|650px|'''<xr id="fig:Ferrofluidspike" />''': Ferrofluid spike (adapted from <ref name = "Musumeci2018"/>): (I) hydrostatic conditions; (II) dynamic conditions (mean velocity U = 0.18ms<sup>-1</sup>; velocity at the ferrofluid tip UFF = 0.11ms<sup>-1</sup>, bottom shear stress in the flow direction τ x0 = 0.096Nm−2); (II) dynamic conditions (mean velocity U = 0.26ms<sup>-1</sup>; velocity at the ferrofluid tip UFF = 0.19ms<sup>-1</sup>, bottom shear stress in the flow direction τ x0 = 0.200Nm<sup>-2</sup>).]]
 
</figure>
 
</figure>
  
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The results of the experimental campaign can be resumed as follows:
 
The results of the experimental campaign can be resumed as follows:
* the lower limit of system working range is equal to 0.08 Nm<sup>-2</sup> for both currents and regular waves;  
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* the lower limit of system working range is equal to 0.08 Nm-2 for both currents and regular waves;  
* the upper limit is equal to 0.2 Nm<sup>-2</sup> for steady currents and 0.4 Nm<sup>-2</sup> in the presence of waves;
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* the upper limit is equal to 0.2 Nm-2 for steady currents and 0.4 Nm-2 in the presence of waves;
 
* in the presence of steady flow larger gains can be used without introducing significant noise in the measurements (gain of the conditioning circuit equal to 277 is considered in all the steady current tests); in the experimental wave conditions, the optimal value of the gain of the conditioning circuit is equal to 11.6;
 
* in the presence of steady flow larger gains can be used without introducing significant noise in the measurements (gain of the conditioning circuit equal to 277 is considered in all the steady current tests); in the experimental wave conditions, the optimal value of the gain of the conditioning circuit is equal to 11.6;
 
* in steady current conditions, sensitivity of the system increases as the intensity of the magnetic field increases and the measurement errors are larger if the magnetic field is smaller;
 
* in steady current conditions, sensitivity of the system increases as the intensity of the magnetic field increases and the measurement errors are larger if the magnetic field is smaller;

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